*Article* **Synthesis and Luminescent Properties of** *s***-Tetrazine Derivatives Conjugated with the 4***H***-1,2,4-Triazole Ring**

**Anna Maj <sup>1</sup> , Agnieszka Kudelko 1,\* and Marcin Swi ˛atkowski ´ <sup>2</sup>**


**Abstract:** New derivatives obtained by the combination of unique 1,2,4,5-tetrazine and 4*H*-1,2,4 triazole rings have great application potential in many fields. Therefore, two synthetic few-step methodologies, which make use of commercially available 4-cyanobenzoic acid (method A) and ethyl diazoacetate (method B), were applied to produce two groups of the aforementioned heterocyclic conjugates. In both cases, the target compounds were obtained in various combinations, by introducing electron-donating or electron-withdrawing substituents into the terminal rings, together with aromatic or aliphatic substituents on the triazole nitrogen atom. Synthesis of such designed systems made it possible to analyze the influence of individual elements of the structure on the reaction course, as well as the absorption and emission properties. The structure of all products was confirmed by conventional spectroscopic methods, and their luminescent properties were also determined.

**Keywords:** *s*-tetrazine; 4*H*-1,2,4-triazole; pinner reaction

**Citation:** Maj, A.; Kudelko, A.; Swi ˛atkowski, M. Synthesis and ´ Luminescent Properties of *s*-Tetrazine Derivatives Conjugated with the 4*H*-1,2,4-Triazole Ring. *Molecules* **2022**, *27*, 3642. https://doi.org/ 10.3390/molecules27113642

Academic Editor: Joseph Sloop

Received: 11 May 2022 Accepted: 2 June 2022 Published: 6 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

### **1. Introduction**

Over the years, scientists from around the world have been keen to study heterocyclic organic compounds, and nitrogen-rich systems have proven to be particularly valuable. One of the most interesting areas of this research is the synthesis and properties of 1,2,4,5 tetrazine derivatives (*s*-tetrazine). This unique ring contains four nitrogen atoms, which is the maximum content in a stable six-membered system. This specific structure has attracted scientists' attention as an important candidate for high energy density materials (HEDMs, A, Scheme 1), as its thermal decomposition leads to ring opening and the release of a nitrogen molecule [1–3]. The high nitrogen content has also encouraged research into its biological activity (B, Scheme 1), which has resulted in compounds that have anti-tubercular, anti-cancer, or anti-malarial effects [4–6]. Moreover, its high reactivity in Diels–Alder reactions with inverse electron demand determines its application potential in bioorthogonal chemistry (C, Scheme 1) [7–10]. Important features of the *s*-tetrazine ring are its low-energy n→π electronic transitions, which are especially valuable from the point of view of optoelectronics (Scheme 1). It can be used in the production of organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and solar cells. Due to the high electronegativity of nitrogen, the ring in question is also characterized by a high electron deficit, and thus a high electron affinity. Consequently, it is also a promising building block in ambipolar and n-type materials [11,12].

The five-membered compound, which, like *s*-tetrazine, shows high nitrogen content, is 4*H*-1,2,4-triazole. In this case, too, the presence of nitrogen is associated with a high affinity toward biological macromolecules, which results in biological activity, such as the possession of antiviral, anti-migraine, antifungal, anti-cancer, or psychotropic properties, and various commercially available products incorporate 4*H*-1,2,4-triazole rings (E, Scheme 1) [13–16]. Another consequence of the nitrogen atoms is the aforementioned

change in the electron density distribution, and the associated ability to transport electrons, making it an acceptor unit (F, Scheme 1). Therefore, 4*H*-1,2,4-triazole derivatives are often used in the production of blue OLEDs [17–20].

**Scheme 1.** Derivatives of the title heterocycles with great application potential [3,4,7,12,13,17].

Many synthetic methods can be found in the literature for both *s*-tetrazine and 4*H*-1,2,4 triazole derivatives. The five-membered heterocycle is usually obtained from acyclic compounds such as *N*,*N'*-diacylhydrazines, *N*-cyanoguanidine, isothiocyanates, hydrazides, aminoethylidenehydrazones, aldehydes, and semicarbazides [21]. For the six-membered *s*-tetrazine system, the Pinner method is the most popular: cyclization, supported by an activating agent, occurs as a result of the reaction of carbonitriles with hydrazine hydrate. The product of this transformation is the corresponding dihydro derivative that requires oxidation to give the desired ring [22,23]. This approach is distinguished by a wide range of substrates, but also the ability to synthesize both symmetrical and unsymmetrical products. Our research to date proves that, among its other uses, it is perfect for the preparation of complex conjugated systems that contain additional five-membered rings. In recent years, we have successfully synthesized *s*-tetrazine conjugated via a 1,4-phenylene linker with a range of 1,3,4-oxadiazoles, 1,3,4-thiadiazoles and 4*H*-1,2,4-triazoles; however, in the latter case, we have so far only obtained symmetrical systems [24–26]. In a continuation of our research, we decided to use the Pinner method to prepare unsymmetrical ones. Moreover, encouraged by the improvement in the luminescent properties after the introduction of the 4*H*-1,2,4-triazole ring, we found that the directly connected heterocycles could be the basis of very promising products. Therefore, we focused on modifying the methodology used to prepare analogous compounds containing 1,3,4-oxadiazole and 1,3,4-thiadiazole, so as to introduce the triazole ring instead [27]. This study was planned to make it possible not only to obtain new, unknown compounds, but also to analyze the influence of their structure on their absorption and emission properties.

### **2. Results**

#### *2.1. Synthesis*

As already mentioned in previous studies, we obtained a series of symmetric *s*-tetrazine derivatives conjugated via a 1,4-phenylene linker with a 4*H*-1,2,4-triazole ring. For this purpose, it was necessary to prepare appropriate precursors for the Pinner reaction, i.e., carbonitriles containing a five-membered ring (Scheme 2, **6a**–**h**). Initially, from the commercially available 4-cyanobenzoic acid (**1**), we obtained the hydrazide (**2**) in a two-step reaction sequence. The original assumption was to treat it with acid chlorides (**3a**–**d**) in order to obtain diacyl derivatives (**4a**–**d**), and then convert them into the corresponding imidoyl chlorides (**5a**–**d**), which, under the influence of amines, would be cyclized to the assumed products (**6a**–**h**). This approach, however, turned out to be very troublesome due to the formation of the undesirable products **7a**–**d**. This prompted us to change the reaction path by synthesizing other imidoyl chlorides (**8a**–**h**) from the corresponding amides. These intermediates were treated with hydrazide (**3**), resulting in the target precursors (**6a**–**h**) in satisfactory yields [26].

**Scheme 2.** Synthesis of precursors containing a 4*H*-1,2,4-triazole ring (**6a**–**h**) [26].

The presence of the carbonitrile moiety allows the formation of a second heterocycle, which is *s*-tetrazine. Under the conditions of the Pinner method, the treatment of the precursors **6a**–**h** with hydrazine hydrate, in the presence of an activating agent, leads to the formation of unoxidized derivatives of the assumed products **9a**–**l**. One of the popular activating agents is sulfur, with the help of which we have successfully obtained symmetrical *s*-tetrazine derivatives connected via a 1,4-phenylene linker with a 4*H*-1,2,4-triazole ring, and extended systems containing 1,3,4-oxadiazole and 1,3,4-thiadiazole cores [24–26]. Therefore, we also began to research the synthesis of unsymmetrical compounds with the use of this methodology, which allowed us to obtain the product 10a with a yield of 42% (Entry 1, Table 1). In connection with literature reports on the possibility of improving this yield with the use of zinc catalysts [28,29], we attempted to repeat the described transformation with its participation and, as a result, the yield increased to 56% (Entry 2, Table 1). An analogous test was performed for derivatives containing an aliphatic chain attached to the triazole nitrogen atom, instead of an aromatic ring (**10g**). Again, the yield improved from 35% to 50% (Entries 8 and 9, Table 1). These results were an important reason to modify the previously used procedure. Such a modified approach resulted in obtaining a series of unsymmetrical systems containing both electron-donating and electron-withdrawing substituents in the terminal ring. Traces of two symmetrical products were also detected. As in the previous studies, the oxidation was carried out with hydrogen peroxide (Scheme 3).


**Table 1.** The yield of the reaction for the preparation of *s*-tetrazine derivatives conjugated via a 1,4-phenylene linker with a 4*H*-1,2,4-triazole ring (**10a–l**).

**Scheme 3.** Synthesis of *s*-tetrazine derivatives conjugated via a 1,4-phenylene linker with a 4*H*-1,2,4 triazole ring (**10a**–**l**). Reaction conditions: step 1: two precursors (**6a**–**h**, 0.5 mmol of each compound), activating agent (zinc trifluoromethanesulfonate (0.009 g, 5 mol%) or sulfur (0.02 g, 125 mol%), ethanol (25 mL), hydrazine hydrate (hydrazine 64%,0.1 mL), reflux 12 h; step 2: methanol (10 mL), hydrogen peroxide (solution 34.5–36.5%,11 mL), rt, 24 h.

The next step was the synthesis of products in which *s*-tetrazine is directly linked to the 4*H*-1,2,4-triazole ring. As part of our previous research, we had already obtained similar compounds containing 1,3,4-oxadiazole and 1,3,4-thiadiazole, but their synthesis required the use of microwave irradiation [27]. The methodology was based on the use of commercially available ethyl diazoacetate (**11**), which was transformed into a dihydrazide (**12**) in a sequence of several transformations (Scheme 4). The product was then treated with acid chlorides to prepare bisdiacyl derivatives (**13**). In this case, too, we intended to convert these compounds into imidoyl chlorides (**14**), which could then be cyclized to triazoles (**15a**) under the influence of amines. However, the high reactivity of such derivatives again caused serious difficulties. Despite the maximum shortening of the reaction times, which had a beneficial effect in previous studies, the observed undesirable derivatives of 1,3,4-oxadiazole (**16**) were predominantly formed. Additionally, isolation of the desired product from the reaction mixture was extremely problematic and, as a result, only traces of the target compound were obtained.

**Scheme 4.** An attempt to synthesize *s*-tetrazine derivatives directly conjugated to the 4*H*-1,2,4 triazole ring.

Based on the experience of obtaining triazole precursors for the Pinner reaction, where we encountered a similar problem, we decided to use an alternative methodology. For this purpose, the dihydrazide **12** was reacted with a range of imidoyl chlorides (**8a**–**h**) previously obtained from amides (Scheme 5). This approach was effective for both systems containing an aromatic ring (**15a**–**d**) and an aliphatic chain (**15e**–**h**) on the triazole nitrogen atom. In addition, derivatives containing both electron-donating and electron-withdrawing moieties attached to a terminal aromatic ring were obtained. Compared to the unsubstituted products, the electron-withdrawing nitro group showed a decreased yield (Entries 4 and 8, Table 2), while for the electron-donating groups (methoxy and *tert*-butyl) the yield was increased (Entries 2, 3, 6, 7, Table 2). The presence of an aliphatic chain also had a beneficial effect on the reaction yield (Entries 5–8, Table 2).

**Scheme 5.** Synthesis of *s*-tetrazine derivatives directly conjugated to the 4*H*-1,2,4-triazole ring (**15a**–**h**). Reaction conditions: 1,2,4,5-tetrazine-3,6-dicarbohydrazide (**12**, 0.50 g, 2.5 mmol), imidoyl chloride (**8a**–**h**, 5.5 mmol), chloroform (20 mL), reflux, 24 h.


**Table 2.** The yield of the reaction for the preparation of *s*-tetrazine derivatives directly conjugated to the 4*H*-1,2,4-triazole ring (**15a**–**h**).

The structure of all the obtained intermediates and final products was confirmed by <sup>1</sup>H- and <sup>13</sup>C-NMR spectroscopy. Both in the case of systems containing a 1,4-phenylene linker, and with directly conjugated heterocycles, the <sup>13</sup>C-NMR spectra were the most characteristic. The presence of the 4*H*-1,2,4-triazole ring was confirmed by signals above 140 ppm, and the presence of the *s*-tetrazine ring by signals above 160 ppm. The introduction of individual groups to the terminal aromatic ring conditioned the appearance of specific signals for the benzene carbon attached to them: above 160 ppm for the methoxy group, above 150 ppm for the *tert*-butyl group, and above 140 ppm for the nitro group. The lowest shifts corresponded to the carbon atoms of the aliphatic chain (13–45 ppm), the methoxy group (about 55 ppm), and the *tert*-butyl group (30–35 ppm). The <sup>1</sup>H-NMR spectra mainly included aromatic signals. Additionally, the protons of the aliphatic chain (butyl) gave a series of signals in the range of 0.6–4.5 ppm, the methoxy group a peak around 3.8 ppm, and the *tert*-butyl group a peak around 1.3 ppm.

#### *2.2. Luminescent Properties*

UV-Vis and 3D fluorescence spectra were registered for compounds **10a**–**l** and **15a**–**h** (Figures S40–S64, Supplementary Materials). The fluorescence was completely quenched in the case of **15d** and **15h**, due to the presence of two NO<sup>2</sup> groups in their structure. The rest of the compounds exhibited a maximum of one emission. The range of emission wavelengths is 375–412 nm for the **10a**–**l** series (Entries 1–12, Table 3) and 353–375 nm for the **15a**–**h** series (Entries 13–20, Table 3). It shows that the separation of fluorophore moieties by phenyl ring leads to a bathochromic shift of fluorescence. In the tetrazine and triazole derivatives, the n→π\* transitions are a source of fluorescence [30–33]. The location of emission maximum (excitation wavelength—λex and emission wavelength—λem) is dependent on substituents R 1 , R<sup>2</sup> , and R<sup>3</sup> , which indicates that both tetrazine and triazole rings are involved in the orbitals from which the excitation occurs. The influence of substituents on λex and λem is the same as in previously reported symmetrically substituted analogs of the **10a**–**l** series [26]. The R<sup>2</sup> affects the λex, whereas R<sup>1</sup> and R<sup>3</sup> affect the λem. The Ph substituent as R<sup>2</sup> induces the bathochromic shift of λex (Entries 1–6 and 13–16, Table 3) in comparison to *n*-Bu (Entries 7–12 and 17–20, Table 3, red color vs. blue color in Figure S65). In the case of the **15a**–**h** series, which consists of the symmetrically substituted compounds, the λem increases together with the rising electron-donating strength of R<sup>1</sup> (H < *t*-Bu < OCH3), which is typical for tetrazine derivatives [34,35]. A partially similar relationship is observed in the unsymmetrically substituted **10a**–**l** series. Taking into account compounds with the same substituent as one of R <sup>1</sup>/R<sup>3</sup> , e.g., NO2, the λem shifts bathochromically in line with the electron-donating properties of the second R1/R<sup>3</sup> substituent, i.e., H < *t*-Bu < OCH3. However, there are some exceptions to that rule in this series because, compared to compounds containing OCH3/ *t*-Bu and OCH3/NO<sup>2</sup> substituents (**10d** vs. **10e** and **10j** vs. **10k**, Entries 4, 5, 10 and 11, Table 3), those with NO<sup>2</sup> (which is an electron-withdrawing group) unexpectedly possess a larger λem. This shows that the changes in the electron density distribution induced by different substituents in unsymmetrically substituted compounds are difficult to predict, thus inferring their absorption-emission properties based only on a molecular structure can

be misleading. The quantum yield (Φ) is directly related to the fluorescence intensity for the studied compounds (Figure S66). Generally, the compounds with Ph as R<sup>2</sup> exhibit higher Φs than those with *n*-Bu, which is in agreement with previous findings [26]. However, most of the studied compounds are not efficient fluorescent materials, because their Φs do not exceed 0.3 (Table 3). The relatively favorable conjugation occurs only for three compounds, i.e., **10a**, **10b**, and **10d**. It shows that the direct coupling of tetrazine and triazole rings, as well as *n*-Bu as R<sup>2</sup> and NO<sup>2</sup> as R1/R<sup>3</sup> , decreases the population of fluorescent transitions.

**Table 3.** Spectroscopic data for the studied *s*-tetrazine derivatives. λabs—wavelength of absorption maximum directly preceding λem. λex and λem—excitation and emission wavelength at global fluorescence maximum. Stokes shift was calculated as λem − λabs. UV-Vis absorption and 3D fluorescence spectra were registered in dichloromethane solutions (c = 5 <sup>×</sup> <sup>10</sup>−<sup>6</sup> mol/dm<sup>3</sup> ). The quantum yields Φ were determined according to the method described by Brouwer [36] by comparison with two standards: quinine sulphate (qn-SO<sup>4</sup> <sup>2</sup>−) [37] and *trans*,*trans*-1,4-diphenyl-1,3-butadiene (dpb) [38].


Summarizing the current and previous research on *s*-tetrazine derivatives in terms of their Φs, it can be stated that they are moderately efficient fluorescent materials. Most of the investigated tetrazine derivatives exhibit Φ no higher than 0.60, but there are some examples, which achieve Φ close to 1, which shows their great potential to use as functional materials, e.g., in optoelectronic applications. In the case of *s*-tetrazines conjugated via phenylene linkers with different 5-membered rings (Scheme 6, Table 4), the Φ changes approximately according to the following order, Triazole (R <sup>2</sup> <sup>=</sup> *<sup>n</sup>*-Bu) < Oxadiazole <sup>≤</sup> Thiadiazole < Triazole (R<sup>2</sup> = Ph). On the other hand, the analogical order for*s*-tetrazines directly conjugated with the same 5-membered rings is as follows, Triazole (R<sup>2</sup> = *n*-Bu) < Triazole (R<sup>2</sup> = Ph) < Oxadiazole < Thiadiazole (Scheme 7, Table 5). The greatest similarities are between oxadiazoles and thiadiazoles bearing *s*-tetrazine, due to small structural changes resulting from the replacement of oxygen with sulfur (atoms with similar electronic properties). Notably, the separation of tetrazine rings and triazole rings via phenylene linkers is more favorable for the fluorescence efficiency than the direct conjugation of them. This is in agreement with the study on the nature of the absorption–emission properties of tetrazine derivatives, which revealed that fluorescence is dependent on the character of HOMO and HOMO-1 orbitals [34]. Fluorescence occurs when the orbital involved in the excitation has a nonbonding n character, but if it is π orbital, the fluorescence is quenched. In this research, it was found that tetrazine

derivatives directly conjugated with heteroatomic rings did not exhibit fluorescence, while diphenyl s-tetrazine was reported to be weakly fluorescent [34,39]. It showed that the conjugation with phenyl rings allows for the retention of the nonbonding n character of the excited orbitals, whereas the direct conjugation with heteroatomic rings changes its character to the π one.

**Scheme 6.** Structure of *s*-tetrazine derivatives conjugated via phenylene linkers with oxadiazole, thiadiazole, and triazole rings.

**Table 4.** Comparison of the quantum yields of *s*-tetrazine derivatives conjugated via phenylene linkers with oxadiazole [24], thiadiazole [25], and triazole rings (symmetrically substituted from [26], and unsymmetrically substituted from current work).


**Scheme 7.** Structure of *s*-tetrazine derivatives directly conjugated with oxadiazole, thiadiazole, and triazole rings.


**Table 5.** Comparison of the quantum yields of *s*-tetrazine derivatives directly conjugated with oxadiazole [27], thiadiazole [27], and triazole rings (current work).

\* compound was not synthesized.

### **3. Experimental Section**

#### *3.1. General Information*

All reagents were purchased from commercial sources and used without further purification. Melting points were measured on a Stuart SMP3 melting point apparatus (Staffordshire, UK). NMR spectra were recorded at 25 ◦C on an Agilent 400-NMR spectrometer (Agilent Technologies, Waldbronn, Germany) at 400 MHz for <sup>1</sup>H and 100 MHz for <sup>13</sup>C, using CDCl<sup>3</sup> or DMSO as the solvent and TMS as the internal standard. UV-Vis absorption and 3D fluorescence spectra were registered in dichloromethane solutions (c = 5 <sup>×</sup> <sup>10</sup>−<sup>6</sup> mol/dm<sup>3</sup> ) with Jasco V-660 (Jasco Corporation, Tokyo, Japan) and Jasco F-6300 (Jasco Corporation, Tokyo, Japan) spectrometers, respectively. FT-IR spectra were measured between 4000 and 650 cm−<sup>1</sup> on an FT-IR Nicolet 6700 apparatus (Thermo Fischer Scientific, Wesel, Germany) with a Smart iTR accessory. Elemental analyses were performed with a VarioELanalyser (Elementar UK Ltd., Stockport, UK). High-resolution mass spectra were obtained by means of a Waters ACQUITY UPLC/Xevo G2QT instrument (Waters Corporation, Milford, MA, USA). Thin-layer chromatography was performed on silica gel 60 F254 (Merck, Merck KGaA, Darmstadt, Germany) thin-layer chromatography plates using chloroform, chloroform/ethyl acetate (1:1 *v*/*v*), or chloroform/ethyl acetate (5:1 *v*/*v*) as the mobile phases.

#### *3.2. Synthesis and Characterization*

Compounds **6**, **8** and **12** were synthesized according to the literature [26,27].

3.2.1. Synthesis of *s*-Tetrazine Derivatives Coupled via a 1,4-Phenylene Linkage with a 4*H*-1,2,4-Triazole Ring (**10a**–**l**)

Two of substrates (**6a**–**h**, 0.5 mmol of each compound) and zinc trifluoromethanesulfonate (0.009 g, 5 mol%) were suspended in ethanol (25 mL) and hydrazine hydrate (hydrazine 64%, 0.1 mL) was added dropwise. It was heated under reflux for 12 h, then filtered and evaporated on a rotary evaporator. The obtained crude intermediate (**9a**–**l**) was dissolved in methanol (10 mL), hydrogen peroxide was added (hydrogen peroxide solution 34.5−36.5%, 11 mL), and it was stirred at room temperature for 24 h. The resulting mixture was filtered and concentrated on a rotary evaporator. The crude product (**10a**–**l**) was purified by column chromatography using chloroform/ethyl acetate (1:1 *v*/*v*) as the mobile phases.

3-(4-(4,5-Diphenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-6-(4-(5-(4-methoxyphenyl)-4-phenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (**10a**)

The product was obtained as yellow powder (0.20 g, 56%); m.p. 187–188 ◦C. UV (CH2Cl2) λmax (log ε) 257 (4.76), 283 (4.64) nm; IR (ATR) νmax 3064, 2947, 2232, 2187, 2141, 2129, 2098, 1696, 1683, 1609, 1565, 1533, 1494, 1472, 1445, 1256, 1179, 1077, 1019, 991, 972, 932, 848, 790, 772, 751, 730, 713, 699, 678 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 3.79 (s, 3H, OCH3), 6.81 (d, 2H, *J* = 8.0 Hz, Ar), 7.17–7.21 (m, 2H, Ar), 7.30–7.38 (m, 7H, Ar), 7.52–7.58 (m, 12H, Ar), 7.76 (d, 2H, *J* = 12.0 Hz, Ar), 8.11 (d, 2H, *J* = 8.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 55.3, 113.4, 114.1, 117.6, 117.7, 118.1 126.2, 127.7, 127.7, 128.6, 128.8, 129.0, 129.9, 130.1, 130.2, 130.3, 130.3, 130.4, 130.4, 131.1, 132.2, 123.5, 134.2, 134.7, 152.7, 153.0, 155.3, 155.5, 161.0, 169.2, 171.1. Anal. calc. for C43H30N10O: C, 73.49; H, 4.30; N, 19.93. Found: C, 73. 46; H, 4.32; N, 19.91; HRMS (ESI): *m/z* calcd for C43H30N10O + H<sup>+</sup> : 703.2682; found: 703.2684.

3-(4-(5-(4-(*tert*-Butyl)phenyl)-4-phenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-6-(4-(4,5-diphenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (**10b**)

The product was obtained as pink powder (0.19 g, 52%); m.p. 174–175 ◦C. UV (CH2Cl2) λmax (log ε) 284 (4.64) nm; IR (ATR) νmax 3062, 2964, 2868, 2232, 2167, 2155, 2028, 2007, 1966, 1695, 1610, 1527, 1494, 1473, 1435, 1362, 1305, 1269, 1201, 1181, 1156, 1108, 1078, 1019, 973, 932, 850, 837, 790, 773, 749, 730, 699 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 1.28 (s, 9H, C(CH3)3), 7.18 (t, 2H, *J* = 8.0 Hz, Ar), 7.29–7.39 (m, 9H, Ar), 7.48–7.57 (m, 14H, Ar), 7.78 (d, 2H, *J* = 8.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 31.1, 34.8, 113.2, 113.3, 118.1, 118.1, 123.4, 125.5, 126.2, 127.6, 127.7, 128.3, 128.5, 128.8, 128.9, 129.0, 129.0, 130.0, 130.2, 130.2, 131.3, 132.2, 132.4, 134.7, 134.9, 152.9, 153.0, 153.3, 155.5, 155.5, 166.7, 167.6. Anal. calc. for C46H36N10: C, 75.80; H, 4.98; N, 19.22. Found: C, 75.81; H, 4.99; N, 19.20; HRMS (ESI): *m*/*z* calcd for C46H36N<sup>10</sup> + H<sup>+</sup> : 729.3203; found: 729.3202.

3-(4-(4,5-Diphenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-6-(4-(5-(4-nitrophenyl)-4-phenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (**10c**)

The product was obtained as yellow powder (0.18 g, 49%); m.p. 199–200 ◦C. UV (CH2Cl2) λmax (log ε) 293 (4.71) nm; IR (ATR) νmax 3053, 2232, 2172, 2142, 2129, 2003, 1965, 1698, 1608, 1550, 1515, 1494, 1468, 1446, 1428, 1406, 1337, 1317, 1277, 1202, 1181, 1152, 1108, 1078, 1018, 1002, 973, 933, 848, 790, 773, 760, 739, 713, 698, 685 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 7.22 (d, 2H, *J* = 8.0 Hz, Ar), 7.54–7.63 (m, 21H, Ar), 7.88 (d, 2H, *J* = 8.0 Hz, Ar), 8.16 (d, 2H, *J*= 8.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 113.3, 113.8, 117.9, 118.1, 123.8, 126.4, 127.5, 127.7, 128.1, 128.5, 128.8, 129.0 129.0, 129.4, 130.1, 130.2, 130.4, 130.8, 131.3, 132.2, 132.3, 134.2, 134.8, 148.4, 153.0, 153.5, 153.8, 155.5, 163.6, 164.0. Anal. calc. for C42H27N11O2: C, 70.28; H, 3.79; N, 21.47. Found: C, 70.25; H, 3.77; N, 21.45; HRMS (ESI): *m*/*z* calcd for C42H27N11O<sup>2</sup> + H<sup>+</sup> : 718.2427; found: 718.2425.

3-(4-(5-(4-(*tert*-Butyl)phenyl)-4-phenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-6-(4-(5-(4 methoxyphenyl)-4-phenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (**10d**)

The product was obtained as pink powder (0.21 g, 56%); m.p. 159–160 ◦C. UV (CH2Cl2) λmax (log ε) 238 (4.56), 287 (4,62) nm; IR (ATR) νmax 3060, 2966, 2268, 2232, 2172, 2140, 2032, 2003, 1972, 1948, 1911, 1690, 1609, 1565,1531, 1496, 1475, 1459, 1434, 1362, 1305, 1254, 1200, 1175, 1156, 1099, 1076, 1020, 992, 972, 920, 851, 837, 789, 774, 749, 737, 714, 699 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 1.28 (s, 9H, C(CH3)3), 3.79 (s, 3H, OCH3), 6.80 (d, 2H, *J* = 8.0 Hz, Ar), 7.17 (m, 4H, Ar), 7.29–735 (m, 6H, Ar), 7.49–7.58 (m, 14H, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 31.1, 34.8, 55.3, 113.2, 114.0, 118.1, 118.6, 120.2, 123.3, 125.5, 125.7, 127.7, 127.7, 128.3, 129.0, 129.9, 130.2, 130.2, 130.4, 131.3, 132.2, 132.4, 132.5, 132.9, 134.9, 152.9, 153.3, 154.6, 155.4, 155.5, 160.9, 164.0, 154.8. Anal. calc. for C47H38N10O: C, 74.39; H, 5.05; N, 18.46. Found: C, 74.38; H, 5.07; N, 18.44; HRMS (ESI): *m*/*z* calcd for C47H38N10O + H<sup>+</sup> : 759.3308; found: 759.3309.

3-(4-(5-(4-Methoxyphenyl)-4-phenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-6-(4-(5-(4-nitrophenyl)- 4-phenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (**10e**)

The product was obtained as orange powder (0.20 g, 54%); m.p. 189–190 ◦C. UV (CH2Cl2) λmax (log ε) 303 (4.68) nm; IR (ATR) νmax 3073, 2957, 2228, 2175, 2138, 2030, 2014, 1978, 1960, 1697, 1684, 1607, 1577, 1515, 1493, 1472, 1434, 1407, 1337, 1316, 1288, 1253, 1178, 1108, 1068, 1021, 992, 972, 848, 834, 784, 771, 752, 741, 698 cm–1; <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 3.75 (s, 3H, OCH3), 6.92 (d, 2H, *J* = 8.0 Hz, Ar), 7.33 (d, 2H, *J* = 12.0 Hz, Ar), 7.47–7.59 (m, 10H, Ar), 7.67 (d, 4H, *J* = 8.0 Hz, Ar), 7.88 (d, 4H, *J* = 8.0 Hz, Ar), 8.23 (d,

4H, *J* = 8.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 55.2, 112.4, 113.95, 116.5, 118.1, 118.8, 123.7, 127.6, 128.1, 128.9, 129.1, 129.5, 129.7, 129.9, 130.0, 130.2, 130.4, 131.0, 131.5, 132.4, 132.5, 133.9, 134.6, 147.9, 152.6, 153.2, 153.6, 154.7, 160.3, 161.2, 163.3. Anal. calc. for C43H29N11O3: C, 69.07; H, 3.91; N, 20.60. Found: C, 69.09; H, 3.94; N, 20.58; HRMS (ESI): *m*/*z* calcd for C43H29N11O<sup>3</sup> + H<sup>+</sup> : 748.2533; found: 748.2531.

3-(4-(5-(4-(*tert*-Butyl)phenyl)-4-phenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-6-(4-(5-(4 nitrophenyl)-4-phenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (**10f**)

The product was obtained as orange powder (0.20 g, 52%); m.p. 124–125 ◦C. UV (CH2Cl2) λmax (log ε) 292 (4.65) nm; IR (ATR) νmax 3062, 2962, 2229, 2159, 2136, 2127, 2099, 2028, 1989, 1974, 1966, 1700, 1608, 1523, 1498, 1476, 1433, 1407, 1338, 1268, 1200, 1156, 1109, 1075, 1019, 972, 842, 787, 771, 752, 739, 729, 698 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 1.28 (s, 9H, C(CH3)3), 7.23 (d, 4H, *J* = 8.0 Hz, Ar), 7.32 (d, 2H, *J* = 8.0 Hz, Ar), 7.37 (d, 2H, *J* = 8.0 Hz, Ar), 7.48–7.64 (m, 16H, Ar), 8.16 (d, 2H, *J* = 8.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 31.1, 34.9, 113.6, 113.8, 117.9, 118.0, 122.2, 123.8, 125.7, 127.5, 127.8, 128.6, 129.1, 129.2, 129.4, 130.5, 130.5, 130.6, 130.8, 130.9, 132.2, 132.3, 134.2, 134.3, 148.5, 152.9, 153.5, 153.8, 154.0, 155.2, 164.6, 166.0. Anal. calc. for C46H35N11O2: C, 71.40; H, 4.56; N, 19.91. Found: C, 71.42; H, 4.54; N, 19.90; HRMS (ESI): *m*/*z* calcd for C46H35N11O<sup>2</sup> + H<sup>+</sup> : 774.3054; found: 774.3056.

3-(4-(4-Butyl-5-(4-methoxyphenyl)-4*H*-1,2,4-triazol-3-yl)phenyl)-6-(4-(4-butyl-5-phenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (**10g**)

The product was obtained as orange powder (0.16 g, 52%); m.p. 173–174 ◦C. UV (CH2Cl2) λmax (log ε) 242 (4.52) nm; IR (ATR) νmax 3103, 3075, 3053, 2329, 2231, 2175, 2138, 1945, 1695, 1682, 1607, 1566, 1504, 1403, 1317, 1294, 1243, 1176, 1130, 1112, 1052, 1024, 990, 869, 856, 844, 769, 751, 676 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 0.63–0.67 (m, 6H, CH3), 0.91–0.93 (m, 4H CH2), 1.36–1.43 (m, 4H, CH2), 3.87 (s, 3H, OCH3), 3.93–3.96 (m, 4H, CH2), 7.13 (d, 2H, *J* = 8.0 Hz, Ar), 7.73–7.78 (m, 9H, Ar), 7.90 (d, 2H, *J* = 8.0 Hz, Ar), 7.97–8.01 (m, 4H, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 14.2, 14.3, 22.7, 22.8, 29.9, 30.4, 47.6, 47.6, 55.7, 113.7, 114.1, 117.7, 117.8, 128.2, 129.5, 129.7, 130.0, 130.2, 130.4, 131.7, 131.9, 132.3, 133.1, 134.3, 151.9, 152.1, 154.6, 155.4, 162.2, 164.1, 165.1. Anal. calc. for C39H38N10O: C, 70.67; H, 5.78; N, 21.13. Found: C, 70.69; H, 5.75; N, 21.11; HRMS (ESI): *m*/*z* calcd for C39H38N10O + H<sup>+</sup> : 663.3308; found: 663.3309.

3-(4-(4-Butyl-5-(4-(*tert*-butyl)phenyl)-4*H*-1,2,4-triazol-3-yl)phenyl)-6-(4-(4-butyl-5-phenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (**10h**)

The product was obtained as pink powder (0.16 g, 47%); m.p. 90–91 ◦C. UV (CH2Cl2) λmax (log ε) 232 (4.51) nm; IR (ATR) νmax 3316, 3067, 2958, 2932, 2867, 2231, 2193, 2170, 2134, 2034, 1978, 1959, 1721, 1637, 1578, 1541, 1490, 1465, 1395, 1364, 1308, 1275, 1249, 1221, 1178, 1154, 1109, 1074, 1018, 993, 946, 845, 803, 772, 694 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 0.63–0.67 (m, 6H, CH3), 0.92–0.95 (m, 4H CH2), 1.36–1.41 (m, 13H, CH2, C(CH3)3), 3.42–3.46 (m, 4H, CH2), 7.45–7.50 (m, 3H, Ar), 7.53 (d, 2H, *J* = 8.0 Hz, Ar), 7.60 (d, 2H, *J* = 8.0 Hz, Ar), 7.68–7.71 (m, 2H, Ar), 7.74–7.77 (m, 4H, Ar), 7.84 (d, 4H, *J* = 4.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 13.1, 13.8, 19.3, 20.2, 31.2, 31.2, 21.7, 34.9, 44.8, 44.9, 113.9, 114.7, 118.0, 118.1, 124.0, 126.9, 127.8, 128.5, 129.4, 129.5, 129.9, 130.1, 131.3, 132.3, 132.5, 153.7, 153.8, 154.8, 155.3, 156.4, 167.5, 167.6. Anal. calc. for C42H44N10: C, 73.23; H, 6.44; N, 20.33. Found: C, 73.21; H, 6.46; N, 20.32; HRMS (ESI): *m*/*z* calcd for C42H44N<sup>10</sup> + H<sup>+</sup> : 689.3829; found: 689.3827.

3-(4-(4-Butyl-5-(4-nitrophenyl)-4*H*-1,2,4-triazol-3-yl)phenyl)-6-(4-(4-butyl-5-phenyl-4*H*-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (**10i**)

The product was obtained as orange powder (0.15 g, 45%); m.p. 183–184 ◦C. UV (CH2Cl2) λmax (log ε) 236 (4.34) nm; IR (ATR) νmax 3307, 3067, 2958, 2928, 2872, 2231,

2173, 2136, 1697, 1637, 1602, 1578, 1526, 1490, 1466, 1346, 1307, 1248, 1178, 1108, 1074, 1016, 995, 853, 803, 771, 753, 694 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 0.94–0.97 (m, 6H, CH3), 1.36–1.46 (m, 4H CH2), 1.57–1.64 (m, 4H, CH2), 3.43–3.48 (m, 4H, CH2), 7.42 (t, 3H, *J* = 8.0 Hz, Ar), 7.47–7.49 (m, 2H, Ar), 7.74–7.76, m, 8H, Ar), 7.94 (d, 2H, *J* = 12.0 Hz, Ar), 8.24 (d, 2H, *J* = 8.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 13.1, 13.8, 19.3, 20.2, 31.6, 31.7, 39.9, 40.2, 114.4, 114.8, 117.9, 119.0, 123.7, 126.8, 127.7, 128.2, 128.6, 129.5, 129.9, 130.4, 131.4, 132.5, 132.9, 149.5, 151.8, 152.0, 154.8, 155.1, 165.6, 167.7. Anal. calc. for C38H35N11O2: C, 67.34; H, 5.21; N, 22.73. Found: C, 67.33; H, 5.23; N, 22.74; HRMS (ESI): *m*/*z* calcd for C38H35N11O<sup>2</sup> + H<sup>+</sup> : 678.3054; found: 678.3053.

3-(4-(4-Butyl-5-(4-(*tert*-butyl)phenyl)-4*H*-1,2,4-triazol-3-yl)phenyl)-6-(4-(4-butyl-5-(4 methoxyphenyl)-4*H*-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (**10j**)

The product was obtained as pink powder (0.18 g, 51%); m.p. 95–96 ◦C. UV (CH2Cl2) λmax (log ε) 253 (4.58) nm; IR (ATR) νmax 3265, 2957, 2871, 2229, 1632, 1607, 1544, 1504, 1464, 1396, 1365, 1307, 1253, 1222, 1176, 1113, 1031, 978, 918, 841, 772 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 0.65–0.67 (m, 6H, CH3), 0.92–0.96 (m, 4H CH2), 1.37–1.41 (m, 13H, CH2, C(CH3)3), 3.84 (s, 3H, OCH3), 3.86–3.89 (m, 4H, CH2), 7.04 (d, 2H, *J* = 8.0 Hz, Ar), 7.54 (d, 2H, *J* = 8.0 Hz, Ar), 7.61 (d, 2H, *J* = 4.0 Hz, Ar), 7.66–7.74 (m, 8H, Ar), 7.92 (d, 2H, *J* = 8.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 13.2, 13.5, 19.2, 19.7, 31.2, 31.6, 31.8, 34.9, 44.8, 44.9, 55.4, 113.7, 114.6, 117.4, 118.2, 125.4, 126.0, 127.8, 128.6, 128.7, 129.4, 130.5, 131.5, 132.3, 133.0, 153.8, 154.7, 154.8, 155.2, 155.8, 162.0, 167.1, 167.5. Anal. calc. for C43H46N10O: C, 71.84; H, 6.45; N, 19.48. Found: C, 71.86; H, 6.44; N, 19.45; HRMS (ESI): *m*/*z* calcd for C43H46N10O + H<sup>+</sup> : 719.3934; found: 719.3935.

3-(4-(4-Butyl-5-(4-methoxyphenyl)-4*H*-1,2,4-triazol-3-yl)phenyl)-6-(4-(4-butyl-5-(4 nitrophenyl)-4*H*-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (**10k**)

The product was obtained as orange powder (0.17 g, 48%); m.p. 194–195 ◦C. UV (CH2Cl2) λmax (log ε) 257 (4.51) nm; IR (ATR) νmax 2964, 2842, 2228, 2128, 1601, 1578, 1519, 1437, 1308, 1256, 1171, 1105, 1050, 1033, 1021, 919, 837, 762, 747, 727, 686 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 0.65–0.69 (m, 6H, CH3), 0.94–0.99 (m, 4H CH2), 1.38–1.46 (m, 4H, CH2), 3.88–3.95 (m, 7H, CH2, OCH3), 6.92 (d, 2H, *J* = 8.0 Hz, Ar), 7.60 (d, 2H, *J* = 12.0 Hz, Ar), 7.72 (d, 2H, *J* = 8.0 Hz, Ar), 7.81–7.84 (m, 4H, Ar), 7.90–7.93 (m, 4H, Ar), 8.27–8.29 (m, 2H, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 13.8, 13.8, 20.1, 20.2, 31.8, 31.9, 39.8, 40.2, 55.5, 113.7, 114.7, 117.5, 118.2, 124.1, 128.0, 128.6, 129.0, 129.4, 129.6, 130.3, 130.4, 130.7, 133.0, 148.0, 150.8, 150.8, 155.4, 156.2, 160.6, 165.1, 165.8. Anal. calc. for C39H37N11O3: C, 66.18; H, 5.27; N, 21.77. Found: C, 66.15; H, 5.29; N, 21.76; HRMS (ESI): *m*/*z* calcd for C39H37N11O<sup>3</sup> + H<sup>+</sup> : 708.3159; found: 707.3157.

3-(4-(4-Butyl-5-(4-(*tert*-butyl)phenyl)-4*H*-1,2,4-triazol-3-yl)phenyl)-6-(4-(4-butyl-5-(4 nitrophenyl)-4*H*-1,2,4-triazol-3-yl)phenyl)-1,2,4,5-tetrazine (**10l**)

The product was obtained as orange powder (0.17 g, 47%); m.p. 99–100 ◦C. UV (CH2Cl2) λmax (log ε) 239 (4.57) nm; IR (ATR) νmax 3265, 3067, 2958, 2933, 2867, 2230, 2149, 2132, 1636, 1611, 1526, 1501, 1477, 1464, 1395, 1364, 1346, 1304, 1286, 1269, 1200, 1154, 1111, 1016, 977, 841, 773, 751, 710, 693 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 0.64–0.69 (m, 6H, CH3), 1.02–1.05 (m, 4H CH2), 1.38–1.44 (m, 13H, CH2, C(CH3)3), 4.12–4.20 (m, 4H, CH2), 7.55 (d, 2H, *J* = 8.0 Hz, Ar), 7.61 (d, 2H, *J* = 8.0 Hz, Ar), 7.69–7.71 (m, 4H, Ar), 7.84–7.90 (m, 4H, Ar), 7.96 (d, 2H, *J* = 12.0 Hz, Ar), 8.25 (d, 2H, *J* = 12.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 13.1, 13.8, 19.3, 20.2, 31.2, 31.6, 31.8, 35.0, 39.7, 40.2, 114.8, 114.9, 117.9, 118.1, 123.7, 125.5, 126.1, 126.6, 127.7, 128.2, 128.7, 129.3, 132.0, 132.4, 149.4, 151.2, 151.4, 154.8, 155.2, 155.7, 165.7, 167.5. Anal. calc. for C42H43N11O2: C, 68.74; H, 5.91; N, 20.99. Found: C, 68.75; H, 5.94; N, 20.97; HRMS (ESI): *m*/*z* calcd for C42H43N11O<sup>2</sup> + H<sup>+</sup> : 734.3679; found: 734.3678. 3.2.2. Synthesis of *s*-Tetrazine Derivatives Directly Conjugated with a 4*H*-1,2,4-Triazole Ring (**15a**–**h**)

The crude imidoyl chloride (**8a**–**h**, 5.5 mmol) and 1,2,4,5-tetrazine-3,6-dicarbohydrazide (12, 0.50 g, 2.5 mmol) were dissolved in chloroform (20 mL) and heated under reflux for 24 h. The mixture was then cooled to room temperature, filtered, and evaporated on a rotary evaporator. For systems containing an aromatic ring attached to a triazole nitrogen atom (**15a**–**d**) and compound **15h**, residue was washed with a small amount of cold ethanol to produce a pure product. For systems with an aliphatic chain, except compound **15h** (**15e**–**g**), a small amount of ethanol (5 mL) was added, filtered, and the filtrate was evaporated again to give the product as an oil.

#### 3,6-Bis(4,5-diphenyl-4*H*-1,2,4-triazol-3-yl)-1,2,4,5-tetrazine (**15a**)

The product was obtained as brown powder (0.59 g, 45%); m.p. 208–209 ◦C. UV (CH2Cl2) λmax (log ε) 278 (4.48) nm; IR (ATR) νmax 3352, 3061, 1967, 1685, 1596, 1541, 1497, 1466, 1444, 1385, 1317, 1261, 1188, 1074, 1017, 1000, 973, 931, 803, 781, 769, 730, 715, 692 cm–1; <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 7.39–7.45 (m, 12H, Ar), 7.50–7.56 (m, 8H, Ar); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 120.3, 123.6, 127.6, 128.3, 128.4, 128.5, 128.6, 134.2, 154.5, 155.3, 164.1. Anal. calc. for C30H20N10: C, 69.22; H, 3.87; N, 26.91. Found: C, 69.25; H, 3.89; N, 26.90; HRMS (ESI): *m*/*z* calcd for C30H20N<sup>10</sup> + H<sup>+</sup> : 521.1951; found: 521.1952.

#### 3,6-Bis(5-(4-methoxyphenyl)-4-phenyl-4*H*-1,2,4-triazol-3-yl)-1,2,4,5-tetrazine (**15b**)

The product was obtained as yellow powder (0.99 g, 68%); m.p. 217–218 ◦C. UV (CH2Cl2) λmax (log ε) 256 (4.56) nm; IR (ATR) νmax 3308, 3212, 2938, 2840, 2038, 1712, 1697, 1686, 1604, 1578, 1551, 1535, 1512, 1458, 1432, 1363, 1318, 1307, 1276, 1252, 1172, 1105, 1073, 1020, 916, 887, 851, 832, 795, 771, 741, 697 cm–1; <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 3.83 (s, 6H, OCH3), 7.04 (d, 4H, *J* = 8.0 Hz, Ar), 7.27–7.51 (m, 10H, Ar), 7.92 (d, 4H, *J* = 12.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 55.5, 113.9, 120.3, 122.1, 127.7, 129.3, 130.1, 131.1, 153.8, 155.4, 163.0, 165.4. Anal. calc. for C32H24N10O2: C, 66.20; H, 4.17; N, 24.12. Found: C, 66.21; H, 4.19; N, 24.11; HRMS (ESI): *m/z* calcd for C32H24N10O<sup>2</sup> + H<sup>+</sup> : 581.2162; found: 581.2160.

#### 3,6-Bis(5-(4-(*tert*-butyl)phenyl)-4-phenyl-4*H*-1,2,4-triazol-3-yl)-1,2,4,5-tetrazine(**15c**)

The product was obtained as orange powder (0.93 g, 59%); m.p. 198–199 ◦C. UV (CH2Cl2) λmax (log ε) 276 (4.51) nm; IR (ATR) νmax 3058, 2957, 2866, 2238, 2184, 2174, 2019, 1982, 1958, 1697, 1596, 1541, 1495, 1466, 1439, 1394, 1363, 1316, 1269, 1201, 1112, 1076, 1017, 963, 915, 841, 751, 731, 711, 692 cm–1; <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 1.22 (s, 18H, C(CH3)3), 7.14–7.19 (m, 4H, Ar), 7.24(d, 4H, *J* = 8.0 Hz, Ar), 7.47–7.53 (m, 10H, Ar); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 29.8, 33.5, 119.2, 124.4, 126.8, 127.0, 127.2, 127.8, 132.9, 151.8, 153.0, 153.3, 163.1. Anal. calc. for C38H36N10: C, 72.13; H, 5.73; N, 22.14. Found: C, 72.11; H, 5.76; N, 22.12; HRMS (ESI): *m/z* calcd for C38H36N<sup>10</sup> + H<sup>+</sup> : 633.3203; found: 633.3204.

### 3,6-Bis(5-(4-nitrophenyl)-4-phenyl-4*H*-1,2,4-triazol-3-yl)-1,2,4,5-tetrazine (**15d**)

The product was obtained as orange powder (0.61 g, 40%); m.p. 172–173 ◦C. UV (CH2Cl2) λmax (log ε) 298 (4.50) nm; IR (ATR) νmax 3064, 2851, 2206, 2166, 2030, 1983, 1948, 1698, 1653, 1598, 1576, 1520, 1494, 1441, 1343, 1205, 1178, 1108, 1075, 1014, 965, 919, 853, 756, 708, 692 cm–1; <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 7.61–7.69 (m, 6H, Ar), 7.80(d, 4H, *J* = 8.0 Hz, Ar), 8.21 (d, 4H, *J* = 8.0 Hz, Ar), 8.37 (d, 4H, *J* = 8.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 120.5, 123.5, 123.7, 124.1, 128.7, 129.2, 138.7, 149.1, 153.2, 153.4, 163.8. Anal. calc. for C30H18N12O4: C, 59.02; H, 2.97; N, 27.53. Found: C, 59.03; H, 2.99; N, 27.51; HRMS (ESI): *m*/*z* calcd for C30H18N12O<sup>4</sup> + H<sup>+</sup> : 611.1652; found: 611.1650.

### 3,6-Bis(4-butyl-5-phenyl-4*H*-1,2,4-triazol-3-yl)-1,2,4,5-tetrazine (**15e**)

The product was obtained as brown oil (0.67 g, 56%). UV (CH2Cl2) λmax (log ε) 257 (4.45) nm; IR (ATR) νmax 3264, 2957, 2932, 2873, 2212, 2165, 1636, 1541, 1491, 1449, 1378, 1308, 1220, 1157, 1113, 1074, 1026, 930, 802, 772, 694 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ

0.92 (t, 6H, *J* = 8.0 Hz, CH3), 1.37 (sextet, 4H, *J* = 8.0 Hz, CH2), 1.63 (quintet, 4H, *J* = 8.0 Hz, CH2), 3.48 (t, 4H, *J* = 8.0 Hz, CH2), 7.39 (t, 4H, *J* = 8.0 Hz, Ar), 7.51 (t, 2H, *J* = 8.0 Hz, Ar), 7.85 (d, 4H, *J* = 8.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 13.7, 20.1, 31.0, 41.3, 128.1, 128.6, 130.9, 132.8, 149.3, 153.8, 169.8. Anal. calc. for C26H28N10: C, 64.98; H, 5.87; N, 29.15. Found: C, 64.96; H, 5.88; N, 29.17; HRMS (ESI): *m*/*z* calcd for C26H28N<sup>10</sup> + H<sup>+</sup> : 481.2577; found: 481.2578.

#### 3,6-Bis(4-butyl-5-(4-methoxyphenyl)-4*H*-1,2,4-triazol-3-yl)-1,2,4,5-tetrazine (**15f**)

The product was obtained as brown oil (1.05 g, 78%). UV (CH2Cl2) λmax (log ε) 253 (4.57) nm; IR (ATR) νmax 3299, 2957, 2932, 2872, 2213, 2151, 1697, 1608, 1577, 1541, 1506, 1464, 1440, 1365, 1295, 1251, 1176, 1112, 1027, 971, 837, 801, 770 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 0.95 (t, 6H, *J* = 8.0 Hz, CH3), 1.40 (sextet, 4H, *J* = 8.0 Hz, CH2), 1.59 (quintet, 4H, *J* = 8.0 Hz, CH2), 4.44 (t, 4H, *J* = 8.0 Hz, CH2), 3.84 (s, 6H, OCH3), 6.90 (d, 4H, *J* = 8.0 Hz, Ar), 7.74 (d, 4H, *J* = 12.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 13.9, 20.3, 31.9, 39.8, 55.5, 113.8, 114.6, 128.7, 144.6, 155.0, 161.3, 167.1. Anal. calc. for C28H32N10O2: C, 62.21; H, 5.97; N, 25.91. Found: C, 62.24; H, 5.99; N, 25.90; HRMS (ESI): *m*/*z* calcd for C28H32N10O<sup>2</sup> + H<sup>+</sup> : 541.2788; found: 541.2789.

#### 3,6-Bis(4-butyl-5-(4-(*tert*-butyl)phenyl)-4*H*-1,2,4-triazol-3-yl)-1,2,4,5-tetrazine (**15g**)

The product was obtained as brown oil (1.08 g, 73%). UV (CH2Cl2) λmax (log ε) 259 (4.18) nm; IR (ATR) νmax 3265, 2958, 2867, 2240, 2212, 2170, 2049, 1978, 1958, 1698, 1612, 1541, 1504, 1464, 1363, 1302, 1254, 1219, 1177, 1114, 1024, 924, 839, 771, 751 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 0.94 (t, 6H, *J* = 8.0 Hz, CH3), 1,35–1,38 (m, 22H, CH2, C(CH3)3), 1.58 (quintet, 4H, *J* = 8.0 Hz, CH2), 3.72 (t, 4H, *J* = 8.0 Hz, CH2), 7.42 (d, 4H, *J* = 8.0 Hz, Ar), 7.69 (d, 4H, *J* = 8.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 13.9, 20.3, 31.3, 31.9, 35.1, 39.8, 125.5, 126.2, 126.8, 145.4, 154,8, 155.0, 167.5. Anal. calc. for C34H44N10: C, 68.89; H, 7.48; N, 23.63. Found: C, 68.87; H, 7.49; N, 23.65; HRMS (ESI): *m*/*z* calcd for C34H44N<sup>10</sup> + H<sup>+</sup> : 593.3829; found: 593.3827.

#### 3,6-Bis(4-butyl-5-(4-nitrophenyl)-4*H*-1,2,4-triazol-3-yl)-1,2,4,5-tetrazine (**15h**)

The product was obtained as yellow powder (0.60 g, 42%); m.p. 103–104 ◦C. UV (CH2Cl2) λmax (log ε) 269 (4.11) nm; IR (ATR) νmax 3303, 3110, 2938, 2864, 2167, 2142, 2038, 2029, 2004, 1949, 1635, 1599, 1518, 1481, 1466, 1422, 1343, 1317, 1294, 1255, 1181, 1153, 1132, 1108, 1011, 973, 938, 868, 855, 841, 762, 723, 710, 691 cm–1; <sup>1</sup>H-NMR (400 MHz, CDCl3): δ 0.97 (t, 6H, *J* = 8.0 Hz, CH3), 1.43 (sextet, 4H, *J* = 8.0 Hz, CH2), 1.63 (quintet, 4H, *J* = 8.0 Hz, CH2), 3.49 (t, 4H, *J* = 8.0 Hz, CH2), 7.93 (d, 4H, *J* = 12.0 Hz, Ar), 8.28 (d, 4H, *J* = 12.0 Hz, Ar); <sup>13</sup>C-NMR (100 MHz, CDCl3): δ 13.9, 20.3, 31.7, 40.3, 123.9, 128.2, 130.2, 140.6, 145.1, 149.6, 165.6. Anal. calc. for C26H26N12O4: C, 54.73; H, 4.59; N, 29.46. Found: C, 54.71; H, 4.58; N, 29.47; HRMS (ESI): *m*/*z* calcd for C26H26N12O<sup>4</sup> + H<sup>+</sup> : 571.2278; found: 571.2277.

#### **4. Conclusions**

Two effective methodologies for the synthesis of extended systems containing 1,2,4,5 tetrazine and 4*H*-1,2,4-triazole have been presented. The first methodology, comprising the Pinner reaction of carbonitriles bearing a 4*H*-1,2,4-triazole scaffold, is useful for obtaining unsymmetrical derivatives with heterocycles connected via a 1,4-phenylene linker. The second procedure, which makes use of imidoyl chloride and *s*-tetrazine-3,6-dicarbohydrazide, has proven to be successful for symmetrical systems with directly conjugated rings. In both cases, the approach leads to the desired products in satisfactory yields, regardless of the nature of the substituents attached to the terminal rings, as well as the type of groups on the triazole nitrogen atom. The obtained compounds exhibit mainly violet luminescence in CH2Cl<sup>2</sup> solution. Their absorption–emission properties are directly related to the compound structure. The spectroscopic investigation revealed the dependency between the electron-donating strength of substituents and the emission wavelength, as well as

the relationship between the quantum yield and the separation or direct conjunction of fluorophore moieties (tetrazine and triazole rings).

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/molecules27113642/s1. Copies of the <sup>1</sup>H-NMR, <sup>13</sup>C-NMR, UV-Vis and fluorescent spectra of the title compounds are available in the online Supplementary Materials.

**Author Contributions:** A.M. and A.K. conceived and designed the experiments, performed the experiments and analyzed the data. M.S. performed emission measurements. A.M. and A.K. wrote ´ the manuscript with the help of M.S. All authors have read and agreed to the published version of ´ the manuscript.

**Funding:** The synthetic part of the project was financially supported by The Silesian University of Technology (Gliwice, Poland) Grant No. 04/050/RGP20/0115 and 04/050/BKM22/0147, BKM-608/RCh5/2022.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds **10a**–**l**, **15a**–**h** are available from the authors.

#### **References**


#### *Review* **Syntheses and Applications of 1,2,3-Triazole-Fused Pyrazines and Pyridazines and Pyridazines Gavin R. Hoffman and Allen M. Schoffstall \***

**Gavin R. Hoffman and Allen M. Schoffstall \*** Department of Chemistry and Biochemistry, University of Colorado Colorado Springs,

> Department of Chemistry and Biochemistry, University of Colorado Colorado Springs, Colorado Springs, CO 80918, USA; ghoffman@uccs.edu **\*** Correspondence: aschoffs@uccs.edu; Tel.: +1-719-255-3479

**\*** Correspondence: aschoffs@uccs.edu; Tel.: +1-719-255-3479 **Abstract:** Pyrazines and pyridazines fused to 1,2,3-triazoles comprise a set of heterocycles obtained

Colorado Springs, CO 80918, USA; ghoffman@uccs.edu

**Abstract:** Pyrazines and pyridazines fused to 1,2,3-triazoles comprise a set of heterocycles obtained through a variety of synthetic routes. Two typical modes of constructing these heterocyclic ring systems are cyclizing a heterocyclic diamine with a nitrite or reacting hydrazine hydrate with dicarbonyl 1,2,3-triazoles. Several unique methods are known, particularly for the synthesis of 1,2,3-triazolo[1,5-*a*]pyrazines and their benzo-fused quinoxaline and quinoxalinone-containing analogs. Recent applications detail the use of these heterocycles in medicinal chemistry (c-Met inhibition or GABA<sup>A</sup> modulating activity) as fluorescent probes and as structural units of polymers. through a variety of synthetic routes. Two typical modes of constructing these heterocyclic ring systems are cyclizing a heterocyclic diamine with a nitrite or reacting hydrazine hydrate with dicarbonyl 1,2,3-triazoles. Several unique methods are known, particularly for the synthesis of 1,2,3-triazolo[1,5-*a*]pyrazines and their benzo-fused quinoxaline and quinoxalinone-containing analogs. Recent applications detail the use of these heterocycles in medicinal chemistry (c-Met inhibition or GABAA modulating activity) as fluorescent probes and as structural units of polymers.

**Keywords:** synthesis; 1,2,3-triazole; fused 1,2,3-triazole; 1,2,3-triazolo[4,5-*b*]pyrazine; 1,2,3 triazolo[4,5-*c*]pyridazine; 1,2,3-triazolo[4,5-*d*]pyridazine; 1,2,3-triazolo[1,5-*a*]pyrazine; 1,2,3 triazolo[1,5-*b*]pyridazine; triazolopyrazine; triazolopyridazine; practical applications zolo[4,5-*c*]pyridazine; 1,2,3-triazolo[4,5-*d*]pyridazine; 1,2,3-triazolo[1,5-*a*]pyrazine; 1,2,3-triazolo[1,5-*b*]pyridazine; triazolopyrazine; triazolopyridazine; practical applications

**Keywords:** synthesis; 1,2,3-triazole; fused 1,2,3-triazole; 1,2,3-triazolo[4,5-*b*]pyrazine; 1,2,3-tria-

#### **1. Introduction 1. Introduction**

Within the 1,2,3-triazole-fused pyrazines and pyridazines, a series of congeners exists depending on whether a nitrogen atom occupies a position at the ring fusion (Figure 1). Within the 1,2,3-triazole-fused pyrazines and pyridazines, a series of congeners exists depending on whether a nitrogen atom occupies a position at the ring fusion (Figure 1).

**Figure 1.** Structures of 1,2,3-triazole-fused pyrazines and pyridazines: 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine (**2**); 1*H*-1,2,3-triazolo[4,5-*c*]pyridazine (**4**); 1*H*-1,2,3-triazolo[4,5-*d*]pyridazine (**6**); 1,2,3-triazolo[1,5-*a*]pyrazine (**8**); 1,2,3-triazolo[1,5-*b*]pyridazine (**10**); common precursors **1, 3, 5, 7**, **9**. **Figure 1.** Structures of 1,2,3-triazole-fused pyrazines and pyridazines: 1*H*-1,2,3-triazolo[4,5 *b*]pyrazine (**2**); 1*H*-1,2,3-triazolo[4,5-*c*]pyridazine (**4**); 1*H*-1,2,3-triazolo[4,5-*d*]pyridazine (**6**); 1,2,3 triazolo[1,5-*a*]pyrazine (**8**); 1,2,3-triazolo[1,5-*b*]pyridazine (**10**); common precursors **1**, **3**, **5**, **7**, **9**.

**Citation:** Hoffman, G.R.;

Schoffstall, A.M. Syntheses and Applications of 1,2,3-Triazole-Fused Pyrazines and Pyridazines. *Molecules* **2022**, *27*, 4681. https://doi.org/ 10.3390/molecules27154681 **Citation:** Hoffman, G.R.; Schoffstall, A.M. Syntheses and Applications of 1,2,3-Triazole-Fused Pyrazines and

Academic Editor: Joseph Sloop Pyridazines. *Molecules* **2022**, *27*, x.

Received: 21 June 2022 Accepted: 19 July 2022 Published: 22 July 2022 https://doi.org/10.3390/xxxxx Academic Editor: Joseph Sloop

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Received: 21 June 2022 Accepted: 19 July 2022 Published: 22 July 2022 **Publisher's Note:** MDPI stays neu-

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). tional affiliations. **Copyright:** © 2022 by the authors. Submitted for possible open access publication under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://cre-

ativecommons.org/licenses/by/4.0/).

We became interested in structures containing heterocyclic nuclei **2**, **4**, **6**, **8** and **10** following reports detailing potent mesenchymal–epithelial transition factor (c-Met) protein kinase inhibition, such as the current clinical candidate Savolitinib [1] (Figure 2, Structure A) and specifically those containing substructures **2** and **8** [1,2]. In addition to c-Met inhibition, structures containing these heterocyclic nuclei have shown GABA<sup>A</sup> allosteric modulating activity [3] (Figure 2, Structure B), have been incorporated into polymers for use in solar cells [4,5] (Figure 2, Structure C), and have demonstrated *β*-secretase 1 (BACE-1) inhibition [6] (Figure 2, Structure D). Their piperazine derivatives have demonstrated potent PDP-IV inhibition [7]. We became interested in structures containing heterocyclic nuclei **2**, **4**, **6**, **8** and **10**  following reports detailing potent mesenchymal–epithelial transition factor (c-Met) protein kinase inhibition, such as the current clinical candidate Savolitinib [1] (Figure 2, Structure A) and specifically those containing substructures **2** and **8** [1,2]. In addition to c-Met inhibition, structures containing these heterocyclic nuclei have shown GABAA allosteric modulating activity [3] (Figure 2, Structure B), have been incorporated into polymers for use in solar cells [4,5] (Figure 2, Structure C), and have demonstrated -secretase 1 (BACE-1) inhibition [6] (Figure 2, Structure D). Their piperazine derivatives have demonstrated potent PDP-IV inhibition [7].

**Figure 2.** Examples of useful structures containing 1,2,3-triazole-fused pyrazines and pyridazines: (**A**) c-Met inhibitor Savolitinib [1], containing a 1,2,3-triazolo[4,5-*b*]pyrazine, (**B**) a compound with GAGAA allosteric modulating activity containing a 1,2,3-triazolo[4,5-*c*]pyridazine [3], (**C**) a 1,2,3 triazolo[4,5-*d*]pyridazine derivative used in polymers for solar cells [4,5], and (**D**), a 1,2,3-triazolo[1,5-*a*]pyrazine derivative with BACE-1 inhibitory activity [6]. **Figure 2.** Examples of useful structures containing 1,2,3-triazole-fused pyrazines and pyridazines: (**A**) c-Met inhibitor Savolitinib [1], containing a 1,2,3-triazolo[4,5-*b*]pyrazine, (**B**) a compound with GAGA<sup>A</sup> allosteric modulating activity containing a 1,2,3-triazolo[4,5-*c*]pyridazine [3], (**C**) a 1,2,3-triazolo[4,5-*d*]pyridazine derivative used in polymers for solar cells [4,5], and (**D**), a 1,2,3-triazolo[1,5-*a*]pyrazine derivative with BACE-1 inhibitory activity [6].

Emphasis in this review is placed on the more common derivatives of **2** and **8**. In comparison to the heterocyclic scaffolds outlined in Figure 2, derivatives of **4**, **6** and **10** are less common in the literature. Among fused heterocycles containing the more well-known fused 1,2,4-triazoles, both 1,2,4-triazolo[1,5-*a*]pyrimidines [8] and 1,2,4-triazolo[4,3-*a*]pyrazines [9] have been recently reviewed. Kumar and coworkers [10] surveyed 1,2,3-triazoles fused to various rings, both aromatic and non-aromatic. In the present review, we address approaches to the synthesis of 1,2,3-triazole-fused pyrazines and pyridazines and their related congeners, while setting two limitations: Emphasis in this review is placed on the more common derivatives of **2** and **8**. In comparison to the heterocyclic scaffolds outlined in Figure 2, derivatives of **4**, **6** and **10** are less common in the literature. Among fused heterocycles containing the more well-known fused 1,2,4-triazoles, both 1,2,4-triazolo[1,5-*a*]pyrimidines [8] and 1,2,4-triazolo[4,3-*a*]pyrazines [9] have been recently reviewed. Kumar and coworkers [10] surveyed 1,2,3-triazoles fused to various rings, both aromatic and non-aromatic. In the present review, we address approaches to the synthesis of 1,2,3-triazole-fused pyrazines and pyridazines and their related congeners, while setting two limitations:

1. This review covers synthetic methods of preparing structures containing fused heterocycles **2**, **4**, **6**, **8**, **10** (Figure 1). Tricyclic and tetracyclic congeners containing these heterocycles are included. heterocycles are included. 2. 1,2,3-Triazolopyrimidines do not appear in this review. They have received attention in the literature on purine chemistry [11–13].

1. This review covers synthetic methods of preparing structures containing fused heterocycles **2**, **4**, **6**, **8**, **10** (Figure 1). Tricyclic and tetracyclic congeners containing these

*Molecules* **2022**, *27*, x FOR PEER REVIEW 3 of 29

2. 1,2,3-Triazolopyrimidines do not appear in this review. They have received attention in the literature on purine chemistry [11–13]. 1,2,3-Triazolopyrimidines, which form the core structure of 8-azapurines, 8 azaadenines, and 8-azaguanines, have been well-studied and reviewed [13–15] owing to

1,2,3-Triazolopyrimidines, which form the core structure of 8-azapurines, 8-azaadenines, and 8-azaguanines, have been well-studied and reviewed [13–15] owing to their similarity to the respective nucleobases. With both scope and limitations in place, this review addresses synthetic approaches to the 1,2,3-triazolodiazine family: 1,2,3-triazolo[4,5 *b*]pyrazine, 1,2,3-triazolo[4,5-*c*]pyridazine, 1,2,3-triazolo[4,5-*d*]pyridazine, 1,2,3-triazolo[1,5 *a*]pyrazine, and 1,2,3-triazolo[1,5-*b*]pyridazine. The literature covered includes articles published since the most recent review of each type of compound, or earlier if no review exists. Reports are covered until the spring of 2022 and exclude tetrahydro-derivatives. their similarity to the respective nucleobases. With both scope and limitations in place, this review addresses synthetic approaches to the 1,2,3-triazolodiazine family: 1,2,3-triazolo[4,5-*b*]pyrazine, 1,2,3-triazolo[4,5-*c*]pyridazine, 1,2,3-triazolo[4,5-*d*]pyridazine, 1,2,3 triazolo[1,5-*a*]pyrazine, and 1,2,3-triazolo[1,5-*b*]pyridazine. The literature covered includes articles published since the most recent review of each type of compound, or earlier if no review exists. Reports are covered until the spring of 2022 and exclude tetrahydroderivatives.

#### **2. Synthetic Approaches 2. Synthetic Approaches**

This overview of synthetic methods is organized according to the type of heterocycle. In the case of 1*H*-1,2,3-triazolo[1,5-*a*]pyrazines, methods are subdivided into pyrazines and benzopyrazines. Reaction times are included along with solvents, catalysts, and other reagents in most examples. Commercial availability of precursors is emphasized where applicable. This overview of synthetic methods is organized according to the type of heterocycle. In the case of 1*H*-1,2,3-triazolo[1,5-*a*]pyrazines, methods are subdivided into pyrazines and benzopyrazines. Reaction times are included along with solvents, catalysts, and other reagents in most examples. Commercial availability of precursors is emphasized where applicable.

#### *2.1. Syntheses of 1H-1,2,3-Triazolo[4,5-b]pyrazines 2.1. Syntheses of 1H-1,2,3-triazolo[4,5-b]pyrazines*

One of the first reported preparations of a 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine came from Lovelette and coworkers [16], who utilized condensation of a 4,5-diamino-1,2,3-triazole, **14**, and a 1,2-dicarbonyl compound **15** (Scheme 1) to give the desired triazolopyrazines **16** in yields in the range 30–35%. A useful precursor, 4,5-diamino-1,2,3-1*H*-triazole **14**, was prepared by reacting carbamate **13** with a strong base. This carbamate was readily prepared from the carbonyl azide by refluxing in ethanol. The carbonyl azide can be prepared from benzyl azide **11**, ethyl cyanoacetate **12**, and sodium ethoxide, all commercially available starting materials. One of the first reported preparations of a 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine came from Lovelette and coworkers [16], who utilized condensation of a 4,5-diamino-1,2,3-triazole, **14**, and a 1,2-dicarbonyl compound **15** (Scheme 1) to give the desired triazolopyrazines **16** in yields in the range 30–35%. A useful precursor, 4,5-diamino-1,2,3-1*H*-triazole **14**, was prepared by reacting carbamate **13** with a strong base. This carbamate was readily prepared from the carbonyl azide by refluxing in ethanol. The carbonyl azide can be prepared from benzyl azide **11**, ethyl cyanoacetate **12**, and sodium ethoxide, all commercially available starting materials.

**Scheme 1.** One of the first reported syntheses of 1*H*-1,2,3-triazolo[4,5-*b*]pyrazines, **16**. (R1, R2 = alkyl, aryl, H.) **Scheme 1.** One of the first reported syntheses of 1*H*-1,2,3-triazolo[4,5-*b*]pyrazines, **16**. (R<sup>1</sup> , R<sup>2</sup> = alkyl, aryl, H.)

O

Dicarbonyl compounds included glyoxal (R<sup>1</sup> = R<sup>2</sup> = H), benzil (R<sup>1</sup> = R<sup>2</sup> = Ph), and others. This was one of the first reports of 1,2,3-triazole-fused pyrazines, highlighted within a study of fused 1,2,3-triazoles. This method offers three-point diversity, one from the triazole substituent, and the other two from the respective dicarbonyl substituents. Despite this, a potential drawback lay in the restriction to a symmetrically substituted 1,2-dicarbonyl species to avoid mixtures of isomers. Indeed, the authors noted the two condensation products using an asymmetrically substituted diketone, where R<sup>1</sup> = CH<sup>3</sup> and R<sup>2</sup> = H, as being indistinguishable. a study of fused 1,2,3-triazoles. This method offers three-point diversity, one from the triazole substituent, and the other two from the respective dicarbonyl substituents. Despite this, a potential drawback lay in the restriction to a symmetrically substituted 1,2-dicarbonyl species to avoid mixtures of isomers. Indeed, the authors noted the two condensation products using an asymmetrically substituted diketone, where R1 = CH3 and R2 = H, as being indistinguishable. Monge and coworkers [17] prepared benzo-fused 1*H*-1,2,3-triazolo[4,5-*b*]pyrazines a study of fused 1,2,3-triazoles. This method offers three-point diversity, one from the triazole substituent, and the other two from the respective dicarbonyl substituents. Despite this, a potential drawback lay the restriction to a symmetrically substituted 1,2-dicarbonyl species to avoid mixtures of isomers. Indeed, the authors noted the two condensation products using an asymmetrically substituted diketone, where R1 = CH3 and R2 = H, as being indistinguishable. Monge and coworkers [17] prepared benzo-fused 1*H*-1,2,3-triazolo[4,5-*b*]pyrazines through the acid-catalyzed cyclization of 2-azido-3-cyanoquinoxaline, **18**, obtained from

Dicarbonyl compounds included glyoxal (R1 = R2 = H), benzil (R1 = R2 = Ph), and others. This was one of the first reports of 1,2,3-triazole-fused pyrazines, highlighted within

Dicarbonyl compounds included glyoxal (R1 = R2 = H), benzil (R1 = R2 = Ph), and others. This was one of the first reports of 1,2,3-triazole-fused pyrazines, highlighted within

Monge and coworkers [17] prepared benzo-fused 1*H*-1,2,3-triazolo[4,5-*b*]pyrazines through the acid-catalyzed cyclization of 2-azido-3-cyanoquinoxaline, **18**, obtained from 2-chloro-3-cyanoquinoxaline **17**, yielding 1-hydroxy-1*H*-1,2,3-triazolo[4,5-*b*]quinoxaline **19** (Scheme 2) in 52% yield. Though uncommon, acid-catalyzed cyclization of *ortho*-substituted azidocyanoaryl species may represent an underutilized method of obtaining structures with the 1,2,3-triazolo[4,5-*b*]pyrazine core. Despite this, the use of costly starting materials hinders wider applicability. through the acid-catalyzed cyclization of 2-azido-3-cyanoquinoxaline, **18**, obtained from 2-chloro-3-cyanoquinoxaline **17**, yielding 1-hydroxy-1*H*-1,2,3-triazolo[4,5-*b*]quinoxaline **19** (Scheme 2) in 52% yield. Though uncommon, acid-catalyzed cyclization of *ortho*-substituted azidocyanoaryl species may represent an underutilized method of obtaining structures with the 1,2,3-triazolo[4,5-*b*]pyrazine core. Despite this, the use of costly starting materials hinders wider applicability. 2-chloro-3-cyanoquinoxaline **17**, yielding 1-hydroxy-1*H*-1,2,3-triazolo[4,5-*b*]quinoxaline **19** (Scheme 2) in 52% yield. Though uncommon, acid-catalyzed cyclization of *ortho*-substituted azidocyanoaryl species may represent an underutilized method of obtaining structures with the 1,2,3-triazolo[4,5-*b*]pyrazine core. Despite this, the use of costly starting materials hinders wider applicability.

*Molecules* **2022**, *27*, x FOR PEER REVIEW 4 of 29

*Molecules* **2022**, *27*, x FOR PEER REVIEW 4 of 29

**Scheme 2.** Conversion of 2-chloro-3-cyanoquinoxaline **17** to 2-azido-3-cyanoquinoxaline **18** and benzo-fused 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine **19**. **Scheme 2.** Conversion of 2-chloro-3-cyanoquinoxaline **17** to 2-azido-3-cyanoquinoxaline **18** and benzo-fused 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine **19**. benzo-fused 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine **19**.

**Scheme 2.** Conversion of 2-chloro-3-cyanoquinoxaline **17** to 2-azido-3-cyanoquinoxaline **18** and

Unexpectedly, Starchenkov and coworkers [18] determined that, upon treatment of diamine **20** with trifluoroacetic anhydride (TFAA) and HNO3 and proceeding via intermediate **21**,triazolopyrazine N-oxide **22** was formed (Scheme 3). This was one of the first reports of the preparation of a fused 1,2,3-triazole 2-N-oxide, namely [1,2,5]oxadiazolo[3,4- Unexpectedly, Starchenkov and coworkers [18] determined that, upon treatment of diamine **20** with trifluoroacetic anhydride (TFAA) and HNO<sup>3</sup> and proceeding via intermediate **21**, triazolopyrazine N-oxide **22** was formed (Scheme 3). This was one of the first reports of the preparation of a fused 1,2,3-triazole 2-N-oxide, namely [1,2,5]oxadiazolo[3,4 *b*][1,2,3]triazolo[4,5-*e*]pyrazine-6-oxide **22**, formed in 92% yield. Unexpectedly, Starchenkov and coworkers [18] determined that, upon treatment of diamine **20** with trifluoroacetic anhydride (TFAA) and HNO3 and proceeding via intermediate **21**,triazolopyrazine N-oxide **22** was formed (Scheme 3). This was one of the first reports of the preparation of a fused 1,2,3-triazole 2-N-oxide, namely [1,2,5]oxadiazolo[3,4- ,*b*][1,2,3]triazolo[4,5-*e*]pyrazine-6-oxide **22**, formed in 92% yield.

**Scheme 3.** Formation of triazolopyrazine N-oxide **22** from diaminopyrazine **20**, proceeding via in-**20 22 21 Scheme 3.** Formation of triazolopyrazine N-oxide **22** from diaminopyrazine **20**, proceeding via intermediate **21**. **Scheme 3.** Formation of triazolopyrazine N-oxide **22** from diaminopyrazine **20**, proceeding via intermediate **21**.

termediate **21**. Forming a mesoionic ring system while studying luminescence, Slepukhin and coworkers [19] obtained the 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine core within the azapentalene inner salt **27** in 50% yield after intramolecular cyclization of 8-(benzotriazole-1-yl)tetrazolo[1,5-*a*]pyrazine **25** in refluxing DMF, causing loss of nitrogen via intermediate **26** and formation of 5*H*-pyrazino[2′,3′:4,5][1,2,3]triazol[1,2-*a*]benzotriazol-6-ium, inner salt Forming a mesoionic ring system while studying luminescence, Slepukhin and coworkers [19] obtained the 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine core within the azapentalene inner salt **27** in 50% yield after intramolecular cyclization of 8-(benzotriazole-1-yl)tetrazolo[1,5-*a*]pyrazine **25** in refluxing DMF, causing loss of nitrogen via intermediate **26** and formation of 5*H*-pyrazino[2′,3′:4,5][1,2,3]triazol[1,2-*a*]benzotriazol-6-ium, inner salt **27** (Scheme 4). Pyrazine **25** was prepared in 48% yield by nucleophilic aromatic substitution of chloride by the benzotriazolyl ion after deprotonation of 1*H*-1,2,3-benzotriazole **24** Forming a mesoionic ring system while studying luminescence, Slepukhin and coworkers [19] obtained the 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine core within the azapentalene inner salt **27** in 50% yield after intramolecular cyclization of 8-(benzotriazole-1-yl)tetrazolo[1,5 *a*]pyrazine **25** in refluxing DMF, causing loss of nitrogen via intermediate **26** and formation of 5*H*-pyrazino[20 ,30 :4,5][1,2,3]triazol[1,2-*a*]benzotriazol-6-ium, inner salt **27** (Scheme 4). Pyrazine **25** was prepared in 48% yield by nucleophilic aromatic substitution of chloride by the benzotriazolyl ion after deprotonation of 1*H*-1,2,3-benzotriazole **24** by carbonate.

**27** (Scheme 4). Pyrazine **25** was prepared in 48% yield by nucleophilic aromatic substitution of chloride by the benzotriazolyl ion after deprotonation of 1*H*-1,2,3-benzotriazole **24**

by carbonate.

by carbonate.

*Molecules* **2022**, *27*, x FOR PEER REVIEW 5 of 29

**Scheme 4.** Synthesis of an azapentalene, 5*H*-pyrazino[2′,3′:4,5][1,2,3]triazolo[1,2-*a*]benzotriazol-6 ium, inner salt **27**, after intramolecular cyclization of **25** and loss of nitrogen via intermediate **26**. **Scheme 4.** Synthesis of an azapentalene, 5*H*-pyrazino[20 ,30 :4,5][1,2,3]triazolo[1,2-*a*]benzotriazol-6-ium, inner salt **27**, after intramolecular cyclization of **25** and loss of nitrogen via intermediate **26**. ium, inner salt **27**, after intramolecular cyclization of **25** and loss of nitrogen via intermediate **26**. Azapentalenes, containing the 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine nucleus, have gained

Azapentalenes, containing the 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine nucleus, have gained attention for their useful properties, such as in luminescence and complexation [19]. Compounds of this type have demonstrated low toxicity, high solubility, and other properties desirable as potential fluorescence probes [20]. This intramolecular approach has remained popular in obtaining various substituted azapentalenes, another example being that of Nyffenegger and coworkers [21]. Here, the azapentalene, 5*H*-pyrazolo[1′,2′:1,2][1,2,3]triazolo[4,5-*b*]pyrazin-6-ium, inner salt, **31**, was obtained in yields up to 85% via cyclization with loss of nitrogen after amination of 2-azido-3-chloropyrazine, **28**, with either pyrazole **29** or 1,2,4-triazole affording 2-azido-3-(1*H*-pyrazol-1-yl)pyrazine **30** (Scheme 5). Other derivatives using nitro-substituted pyrazoles were formed in yields in the range 63–97%. This method offers convenience in that a precursor to **28**, 2,3-dichloropyrazine, is commercially Azapentalenes, containing the 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine nucleus, have gained attention for their useful properties, such as in luminescence and complexation [19]. Compounds of this type have demonstrated low toxicity, high solubility, and other properties desirable as potential fluorescence probes [20]. This intramolecular approach has remained popular in obtaining various substituted azapentalenes, another example being that of Nyffenegger and coworkers [21]. Here, the azapentalene, 5*H*-pyrazolo[10 ,20 :1,2][1,2,3]triazolo[4,5 *b*]pyrazin-6-ium, inner salt, **31**, was obtained in yields up to 85% via cyclization with loss of nitrogen after amination of 2-azido-3-chloropyrazine, **28**, with either pyrazole **29** or 1,2,4 triazole affording 2-azido-3-(1*H*-pyrazol-1-yl)pyrazine **30** (Scheme 5). Other derivatives using nitro-substituted pyrazoles were formed in yields in the range 63–97%. This method offers convenience in that a precursor to **28**, 2,3-dichloropyrazine, is commercially available. attention for their useful properties, such as in luminescence and complexation [19]. Compounds of this type have demonstrated low toxicity, high solubility, and other properties desirable as potential fluorescence probes [20]. This intramolecular approach has remained popular in obtaining various substituted azapentalenes, another example being that of Nyffenegger and coworkers [21]. Here, the azapentalene, 5*H*-pyrazolo[1′,2′:1,2][1,2,3]triazolo[4,5-*b*]pyrazin-6-ium, inner salt, **31**, was obtained in yields up to 85% via cyclization with loss of nitrogen after amination of 2-azido-3-chloropyrazine, **28**, with either pyrazole **29** or 1,2,4-triazole affording 2-azido-3-(1*H*-pyrazol-1-yl)pyrazine **30** (Scheme 5). Other derivatives using nitro-substituted pyrazoles were formed in yields in the range 63–97%. This method offers convenience in that a precursor to **28**, 2,3-dichloropyrazine, is commercially available.

**Scheme 5.** Synthesis of the azapentalene **31**, from pyrazine **30**, derived from 2-azido-3-chloropyra-**28 30 31 Scheme 5.** Synthesis of the azapentalene **31**, from pyrazine **30**, derived from 2-azido-3-chloropyrazine **28**. **Scheme 5.** Synthesis of the azapentalene **31**, from pyrazine **30**, derived from 2-azido-3-chloropyrazine **28**.

zine **28**. Notably, in addition to having an azido group substituted *ortho* to the pyrazole of **30**  [20,21], reports have also made use of the respective amine via ring closure by displacement of an N-iodonium intermediate by an adjacent nitrogen atom of the attached pyrazole to form azapentalenes [22,23]. Compounds of this type have been thoroughly characterized via NMR spectroscopy [24]. A Pfizer patent [25] filed in 2007 detailed the use of either isoamyl nitrite in DMF or NaNO2 in aqueous acetic acid, after first aminating commercially available 2-amino-3,5-dibromopyrazine **32** in the presence of a sterically hin-Notably, in addition to having an azido group substituted *ortho* to the pyrazole of **30**  [20,21], reports have also made use of the respective amine via ring closure by displacement of an N-iodonium intermediate by an adjacent nitrogen atom of the attached pyrazole to form azapentalenes [22,23]. Compounds of this type have been thoroughly characterized via NMR spectroscopy [24]. A Pfizer patent [25] filed in 2007 detailed the use of either isoamyl nitrite in DMF or NaNO2 in aqueous acetic acid, after first aminating commercially available 2-amino-3,5-dibromopyrazine **32** in the presence of a sterically hindered base, *N,N*-diisopropylethylamine (DIPEA), then treating diaminopyrazine **33** with Notably, in addition to having an azido group substituted *ortho* to the pyrazole of **30** [20,21], reports have also made use of the respective amine via ring closure by displacement of an N-iodonium intermediate by an adjacent nitrogen atom of the attached pyrazole to form azapentalenes [22,23]. Compounds of this type have been thoroughly characterized via NMR spectroscopy [24]. A Pfizer patent [25] filed in 2007 detailed the use of either isoamyl nitrite in DMF or NaNO<sup>2</sup> in aqueous acetic acid, after first aminating commercially available 2-amino-3,5-dibromopyrazine **32** in the presence of a sterically hindered base, *N,N*-diisopropylethylamine (DIPEA), then treating diaminopyrazine **33** with nitrite to form 3,5-disubstituted 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine **34** (Scheme 6).

dered base, *N,N*-diisopropylethylamine (DIPEA), then treating diaminopyrazine **33** with

nitrite to form 3,5-disubstituted 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine **34** (Scheme 6).

nitrite to form 3,5-disubstituted 1*H*-1,2,3-triazolo[4,5-*b*]pyrazine **34** (Scheme 6).

**Scheme 6.** Amination of 2-amino-3,5-dibromopyrazine **32** to form diaminopyrazine **33**, followed by cyclization to form triazolo[4,5-*b*]pyrazine **34**. (R = alkyl or aryl.) **Scheme 6.** Amination of 2-amino-3,5-dibromopyrazine **32** to form diaminopyrazine **33**, followed by cyclization to form triazolo[4,5-*b*]pyrazine **34**. (R = alkyl or aryl.)

The use of nitrite for triazole cyclization, via the nitrosonium ion, has also been reported by Ye and coworkers [26,27], Cui and coworkers [2] (who were cited in the original patent [25]), and others. Thottempudi and coworkers [28] used a combination of TFAA/HNO3 as an in situ nitronium source, giving a triazole 2-N-oxide, while Jia and coworkers [1], and others [29,30] used nitrosonium generated from nitrite. Both syntheses offer straightforward introduction of the triazole based on the amine chosen during amination. They also have the advantage of short reaction times and little or no required purification. Likely owing to these benefits, cyclization using nitrite to generate nitrosonium ion, such as in **33** to **34** (Scheme 6), continues to dominate reports in the literature. Indeed, the reaction of various diazinyl diamines with nitrite represents a central theme throughout the discussion of syntheses of 1*H*-1,2,3-triazole-fused pyrazines and pyridazines. The use of nitrite for triazole cyclization, via the nitrosonium ion, has also been reported by Ye and coworkers [26,27], Cui and coworkers [2] (who were cited in the original patent [25]), and others. Thottempudi and coworkers [28] used a combination of TFAA/HNO<sup>3</sup> as an in situ nitronium source, giving a triazole 2-N-oxide, while Jia and coworkers [1], and others [29,30] used nitrosonium generated from nitrite. Both syntheses offer straightforward introduction of the triazole based on the amine chosen during amination. They also have the advantage of short reaction times and little or no required purification. Likely owing to these benefits, cyclization using nitrite to generate nitrosonium ion, such as in **33** to **34** (Scheme 6), continues to dominate reports in the literature. Indeed, the reaction of various diazinyl diamines with nitrite represents a central theme throughout the discussion of syntheses of 1*H*-1,2,3-triazole-fused pyrazines and pyridazines.

#### *2.2. Syntheses of 1,2,3-Triazolo[1,5-a]pyrazines 2.2. Syntheses of 1,2,3-Triazolo[1,5-a]pyrazines*

More well-known than 1,2,3-triazolo[4,5-*b*]pyrazines are the fused [1,5-*a*]pyrazine derivatives. While benzo[*b*]pyrazines (i.e., quinoxalines) are not commonly encountered as part of 1,2,3-triazolo[4,5-*b*]pyrazines, they are widespread in the literature in compounds containing the 1,2,3-triazolo[1,5-*a*]pyrazine nucleus. Therefore, this section is organized into the syntheses of benzo-fused structures (e.g., 1,2,3-triazolo[1,5-*a*]pyrazines containing quinoxaline or quinoxalinone), and those that are bicyclic 1,2,3-triazolo[1,5 *a*]pyrazines. A recent brief review of 4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-*a*]pyrazines has been published [31]. An earlier review detailed aspects of the chemistry of 1,2,3-triazolo[1,5-*a*]pyrazines [32]. A review on the synthesis of triazoloquinazolines also appeared More well-known than 1,2,3-triazolo[4,5-*b*]pyrazines are the fused [1,5-*a*]pyrazine derivatives. While benzo[*b*]pyrazines (i.e., quinoxalines) are not commonly encountered as part of 1,2,3-triazolo[4,5-*b*]pyrazines, they are widespread in the literature in compounds containing the 1,2,3-triazolo[1,5-*a*]pyrazine nucleus. Therefore, this section is organized into the syntheses of benzo-fused structures (e.g., 1,2,3-triazolo[1,5-*a*]pyrazines containing quinoxaline or quinoxalinone), and those that are bicyclic 1,2,3-triazolo[1,5 *a*]pyrazines. A recent brief review of 4,5,6,7-tetrahydro[1–3]triazolo[1,5-*a*]pyrazines has been published [31]. An earlier review detailed aspects of the chemistry of 1,2,3-triazolo[1,5 *a*]pyrazines [32]. A review on the synthesis of triazoloquinazolines also appeared in 2016 [33].

#### in 2016 [33]. 2.2.1. Syntheses of Bicyclic 1,2,3-Triazolo[1,5-a]pyrazines

2.2.1. Syntheses of Bicyclic 1,2,3-Triazolo[1,5-a]pyrazines The first method of synthesizing a 1,2,3-triazolo[1,5-*a*]pyrazine by Wentrup [34] was, at the time, the synthesis of a novel purine isomer. Wentrup utilized the thermolysis of 5- (2-pyrazinyl)tetrazole **36** (400 °C, 10−5 Torr), affording **38**, 1,2,3-triazolo[1,5-*a*]pyrazine in 20% yield proceeding via diazo intermediate **37**. The precursor 2-(2*H*-tetrazol-5-yl)pyrazine **36** was readily prepared from 2-cyanopyrazine, **35**, upon treatment with hydrazoic acid generated in situ from ammonium chloride and sodium azide (Scheme 7). This method, while suffering from harsh reaction conditions and poor yields, was the first utilizing intramolecular cyclization of diazo intermediates in the formation of 1,2,3-tria-The first method of synthesizing a 1,2,3-triazolo[1,5-*a*]pyrazine by Wentrup [34] was, at the time, the synthesis of a novel purine isomer. Wentrup utilized the thermolysis of 5-(2-pyrazinyl)tetrazole **36** (400 ◦C, 10−<sup>5</sup> Torr), affording **38**, 1,2,3-triazolo[1,5-*a*]pyrazine in 20% yield proceeding via diazo intermediate **37**. The precursor 2-(2*H*-tetrazol-5-yl)pyrazine **36** was readily prepared from 2-cyanopyrazine, **35**, upon treatment with hydrazoic acid generated in situ from ammonium chloride and sodium azide (Scheme 7). This method, while suffering from harsh reaction conditions and poor yields, was the first utilizing intramolecular cyclization of diazo intermediates in the formation of 1,2,3-triazolo[1,5 *a*]pyrazines. Lead tetraacetate oxidation of the hydrazone of pyrazine-2-carbaldehyde similarly gave **38** in 75% yield [35]. *Molecules* **2022**, *27*, x FOR PEER REVIEW 7 of 29

zolo[1,5-*a*]pyrazines. Lead tetraacetate oxidation of the hydrazone of pyrazine-2-carbal-

**Scheme 7.** Conversion of 2-cyanopyrazine **35** to 1,2,3-triazolo[1,5-*a*]pyrazine **38** via tetrazolylpyrazine **36. Scheme 7.** Conversion of 2-cyanopyrazine **35** to 1,2,3-triazolo[1,5-*a*]pyrazine **38** via tetrazolylpyrazine **36**.

[36] afforded 1-(4-bromophenyl)-3-methyl-1,2,3-triazolo[1,5-*a*]pyrazinium tetrafluoroborates **31** in 55% yield (when R1 = *p*-chlorophenyl) and 81% yield (when R1 = CH3) after reaction of 4-bromophenylhydrazones **39** (prepared from the respective 2-pyrazinyl ketone) with tribromophenol bromine (TBB) and NH4BF4 (Scheme 8). When R1 = CH3, the yield of **40** was 81%. Interestingly, after treatment of **40** with pyrrolidine in methanol, the ring-opened 2-aza-1,3-butadienes can be valuable starting materials for other conversions. For example, a ring-opened triazolyl-2-aza-1,3-butadiene was converted to a fused pyridine after treatment with N-phenylmaleinimide, or an imidazoline when treated with tosyl

**Scheme 8.** Intramolecular cyclization of a 4-bromophenylhydrazone **39** forming triazolopyrazinium

BF4

Methods have been reported for the preparation of 1,2,3-triazolo[1,5-*a*]pyrazinones. In work by Nein and coworkers [37,38], the reaction of 5-hydroxy-*N*-diphenyl-1*H*-1,2,3 triazole-4-carboxamide **41** with chloroacetonitrile in DMF and base gave the alkylated product **42**, which, after refluxing in sodium ethoxide, gave 6-amino-4-oxo-2,5-diphenyl-4,5-dihydro-2*H*-1,2,3-triazolo[1,5-*a*]pyrazinium-5-olate **43** in 80% yield (Scheme 9). They proposed the geometry of 3-phenacyl- and 3-cyanomethyl derivatives of triazolium-5 olates indicated interaction of the carboxamide nitrogen at position 4 of the triazole with cyano groups, which was then confirmed experimentally after obtaining the desired

<sup>N</sup> <sup>N</sup> N

R1

Br

In addition to syntheses of neutral compounds of this type, several reports have ap-

Br

**39 40**

(i) TBB (ii) NH4BF4 <sup>N</sup>

azide [36].

N

N R1 <sup>H</sup> N

N

tetrafluoroborate **40**. (R1 = alkyl or aryl.)

mesoionic **42** [37].

N

N

N

zine **36.**

NH4Cl + NaN3

HN3 DMF, 100 °C

> In addition to syntheses of neutral compounds of this type, several reports have appeared for the preparation of fused pyrazinium salts. A method by Beres and coworkers [36] afforded 1-(4-bromophenyl)-3-methyl-1,2,3-triazolo[1,5-*a*]pyrazinium tetrafluoroborates **31** in 55% yield (when R<sup>1</sup> = *p*-chlorophenyl) and 81% yield (when R<sup>1</sup> = CH3) after reaction of 4-bromophenylhydrazones **39** (prepared from the respective 2-pyrazinyl ketone) with tribromophenol bromine (TBB) and NH4BF<sup>4</sup> (Scheme 8). When R<sup>1</sup> = CH3, the yield of **40** was 81%. Interestingly, after treatment of **40** with pyrrolidine in methanol, the ring-opened 2-aza-1,3-butadienes can be valuable starting materials for other conversions. For example, a ring-opened triazolyl-2-aza-1,3-butadiene was converted to a fused pyridine after treatment with N-phenylmaleinimide, or an imidazoline when treated with tosyl azide [36]. peared for the preparation of fused pyrazinium salts. A method by Beres and coworkers [36] afforded 1-(4-bromophenyl)-3-methyl-1,2,3-triazolo[1,5-*a*]pyrazinium tetrafluoroborates **31** in 55% yield (when R1 = *p*-chlorophenyl) and 81% yield (when R1 = CH3) after reaction of 4-bromophenylhydrazones **39** (prepared from the respective 2-pyrazinyl ketone) with tribromophenol bromine (TBB) and NH4BF4 (Scheme 8). When R1 = CH3, the yield of **40** was 81%. Interestingly, after treatment of **40** with pyrrolidine in methanol, the ring-opened 2-aza-1,3-butadienes can be valuable starting materials for other conversions. For example, a ring-opened triazolyl-2-aza-1,3-butadiene was converted to a fused pyridine after treatment with N-phenylmaleinimide, or an imidazoline when treated with tosyl azide [36].

> **Scheme 7.** Conversion of 2-cyanopyrazine **35** to 1,2,3-triazolo[1,5-*a*]pyrazine **38** via tetrazolylpyra-

In addition to syntheses of neutral compounds of this type, several reports have ap-

N

N

<sup>N</sup> <sup>N</sup>

<sup>N</sup> <sup>N</sup>

*Molecules* **2022**, *27*, x FOR PEER REVIEW 7 of 29

N2

<sup>N</sup> <sup>N</sup> <sup>N</sup>

**35 36 37 38**

N

N N N NH

400 °C, 10-5 Torr

**Scheme 8.** Intramolecular cyclization of a 4-bromophenylhydrazone **39** forming triazolopyrazinium tetrafluoroborate **40**. (R1 = alkyl or aryl.) **Scheme 8.** Intramolecular cyclization of a 4-bromophenylhydrazone **39** forming triazolopyrazinium tetrafluoroborate **40**. (R<sup>1</sup> = alkyl or aryl.)

Methods have been reported for the preparation of 1,2,3-triazolo[1,5-*a*]pyrazinones. In work by Nein and coworkers [37,38], the reaction of 5-hydroxy-*N*-diphenyl-1*H*-1,2,3 triazole-4-carboxamide **41** with chloroacetonitrile in DMF and base gave the alkylated product **42**, which, after refluxing in sodium ethoxide, gave 6-amino-4-oxo-2,5-diphenyl-4,5-dihydro-2*H*-1,2,3-triazolo[1,5-*a*]pyrazinium-5-olate **43** in 80% yield (Scheme 9). They proposed the geometry of 3-phenacyl- and 3-cyanomethyl derivatives of triazolium-5 olates indicated interaction of the carboxamide nitrogen at position 4 of the triazole with cyano groups, which was then confirmed experimentally after obtaining the desired mesoionic **42** [37]. Methods have been reported for the preparation of 1,2,3-triazolo[1,5-*a*]pyrazinones. In work by Nein and coworkers [37,38], the reaction of 5-hydroxy-*N*-diphenyl-1*H*-1,2,3 triazole-4-carboxamide **41** with chloroacetonitrile in DMF and base gave the alkylated product **42**, which, after refluxing in sodium ethoxide, gave 6-amino-4-oxo-2,5-diphenyl-4,5-dihydro-2*H*-1,2,3-triazolo[1,5-*a*]pyrazinium-5-olate **43** in 80% yield (Scheme 9). They proposed the geometry of 3-phenacyl- and 3-cyanomethyl derivatives of triazolium-5 olates indicated interaction of the carboxamide nitrogen at position 4 of the triazole with cyano groups, which was then confirmed experimentally after obtaining the desired mesoionic **42** [37]. *Molecules* **2022**, *27*, x FOR PEER REVIEW 8 of 29

**Scheme 9.** Intramolecular cyclization of a triazolium salt **42**, forming zwitterionic **43**. **Scheme 9.** Intramolecular cyclization of a triazolium salt **42**, forming zwitterionic **43**.

form 1,2,3-triazolo[1,5-*a*]-1,2,4-triazolo[5,1-*c*]pyrazines in 55–65% yield.

EtOH

Similarly involving reaction of triazolium olates such as **42**, during a synthesis of 1,2,5-triazepines by Savel'eva and coworkers [39], [1,5-*a*]triazolopyrazines were formed as byproducts (5–7%) from the intramolecular cyclization of 1-amino-3-(*p*-phenacyl)-4-{[2-

.

<sup>N</sup> <sup>N</sup> N

NC

H

**46**

<sup>N</sup> CN NH2

AcOH

CN

R1OOC

R1OOC

PhOCHN

PhOCHN

CN

Isoamyl nitrite,

[40] took a novel approach for the reaction of 4-(ethoxymethylene)-2-phenyloxazol-5(4*H*) one **44** with commercially available diaminomaleonitrile **45**, forming adduct **46** which, after conversion to triazole **47** with nitrite, afforded the substituted 1,2,3-triazolo[1,5-*a*]pyrazine **48** (Scheme 10). Later, derivatives of **48** such as ethyl 4-amino-3-cyano-1,2,3-triazolo[1,5-*a*]pyrazine-6-carboxylate were further reacted by Trcek and coworkers [41] to

**Scheme 10.** Reaction of 4-(ethoxymethylene)-2-phenyloxazol-5(4*H*)-one **44** and diaminomaleonitrile

Raghavendra and coworkers [42] reported a triazolopyrazine synthesis employing solid-phase polystyrene *p*-toluenesulfonyl hydrazide, a common carbonyl scavenging

**48 47**

EtOH/AcOH, *or* TEA

**45** forming an adduct **46**, which led to triazole **47** and pyrazine **48**. (R1 = alkyl.)

<sup>N</sup> <sup>N</sup> N

N

R1OOC

N

<sup>O</sup> OEt

+

**44 45**

O

Ph

PhOCHN CN

H2N NH2

NC CN

Similarly involving reaction of triazolium olates such as **42**, during a synthesis of 1,2,5-triazepines by Savel'eva and coworkers [39], [1,5-*a*]triazolopyrazines were formed as byproducts (5–7%) from the intramolecular cyclization of 1-amino-3-(*p*-phenacyl)-4-{[2-(1 methylethylidene)hydrazino]carbonyl}-[1,2,3]-triazolium-5-olates. Jug and coworkers [40] took a novel approach for the reaction of 4-(ethoxymethylene)-2-phenyloxazol-5(4*H*)-one **44** with commercially available diaminomaleonitrile **45**, forming adduct **46** which, after conversion to triazole **47** with nitrite, afforded the substituted 1,2,3-triazolo[1,5-*a*]pyrazine **48** (Scheme 10). Later, derivatives of **48** such as ethyl 4-amino-3-cyano-1,2,3-triazolo[1,5 *a*]pyrazine-6-carboxylate were further reacted by Trcek and coworkers [41] to form 1,2,3 triazolo[1,5-*a*]-1,2,4-triazolo[5,1-*c*]pyrazines in 55–65% yield. Similarly involving reaction of triazolium olates such as **42**, during a synthesis of 1,2,5-triazepines by Savel'eva and coworkers [39], [1,5-*a*]triazolopyrazines were formed as byproducts (5–7%) from the intramolecular cyclization of 1-amino-3-(*p*-phenacyl)-4-{[2- (1-methylethylidene)hydrazino]carbonyl}-[1,2,3]-triazolium-5-olates. Jug and coworkers [40] took a novel approach for the reaction of 4-(ethoxymethylene)-2-phenyloxazol-5(4*H*) one **44** with commercially available diaminomaleonitrile **45**, forming adduct **46** which, after conversion to triazole **47** with nitrite, afforded the substituted 1,2,3-triazolo[1,5-*a*]pyrazine **48** (Scheme 10). Later, derivatives of **48** such as ethyl 4-amino-3-cyano-1,2,3-triazolo[1,5-*a*]pyrazine-6-carboxylate were further reacted by Trcek and coworkers [41] to form 1,2,3-triazolo[1,5-*a*]-1,2,4-triazolo[5,1-*c*]pyrazines in 55–65% yield.

N

H2N

100 °C <sup>N</sup> <sup>N</sup>

N H O

CN

<sup>N</sup> <sup>N</sup> N

**43**

O O

5 h

N

NaOEt EtOH, reflux,

O

**Scheme 9.** Intramolecular cyclization of a triazolium salt **42**, forming zwitterionic **43**.

Na2CO3, DMF,

**41 42**

*Molecules* **2022**, *27*, x FOR PEER REVIEW 8 of 29

O ClCH2CN

<sup>N</sup> <sup>N</sup> N

N H OH

**Scheme 10.** Reaction of 4-(ethoxymethylene)-2-phenyloxazol-5(4*H*)-one **44** and diaminomaleonitrile **45** forming an adduct **46**, which led to triazole **47** and pyrazine **48**. (R1 = alkyl.) **Scheme 10.** Reaction of 4-(ethoxymethylene)-2-phenyloxazol-5(4*H*)-one **44** and diaminomaleonitrile **45** forming an adduct **46**, which led to triazole **47** and pyrazine **48**. (R<sup>1</sup> = alkyl.)

.

Raghavendra and coworkers [42] reported a triazolopyrazine synthesis employing solid-phase polystyrene *p*-toluenesulfonyl hydrazide, a common carbonyl scavenging Raghavendra and coworkers [42] reported a triazolopyrazine synthesis employing solid-phase polystyrene *p*-toluenesulfonyl hydrazide, a common carbonyl scavenging resin. After reaction of the polystyrene *p*-toluenesulfonyl hydrazide **49** with an acetylpyrazine **50** in the presence of 5% TiCl<sup>4</sup> in MeOH, hydrazone **51** was obtained. Reaction of **51** with morpholine gave the desired 1,2,3-triazolo[1,5-*a*]pyrazines, **52** (Scheme 11), in yields ranging from 33–62%. This regiospecific, traceless protocol represented the first solid-phase assisted synthesis of a triazolopyrazine and was also used for the synthesis of several non-fused 1,2,3-triazoles in the same report in yields up to 60%. resin. After reaction of the polystyrene *p*-toluenesulfonyl hydrazide **49** with an acetylpyrazine **50** in the presence of 5% TiCl4 in MeOH, hydrazone **51** was obtained. Reaction of **51** with morpholine gave the desired 1,2,3-triazolo[1,5-*a*]pyrazines, **52** (Scheme 11), in yields ranging from 33–62%. This regiospecific, traceless protocol represented the first solid-phase assisted synthesis of a triazolopyrazine and was also used for the synthesis of several non-fused 1,2,3-triazoles in the same report in yields up to 60%.

*Molecules* **2022**, *27*, x FOR PEER REVIEW 9 of 29

**Scheme 11.** Intramolecular cyclization of hydrazone **51**, derived from tosylhydrazide **49**, in the formation of disubstituted 1,2,3-triazolo[1,5-*a*]pyrazines **52**. (R1, R2 = alkyl.) **Scheme 11.** Intramolecular cyclization of hydrazone **51**, derived from tosylhydrazide **49**, in the formation of disubstituted 1,2,3-triazolo[1,5-*a*]pyrazines **52**. (R<sup>1</sup> , R<sup>2</sup> = alkyl.)

zolo[1,5-*a*]pyrazines. These authors verified that the one-pot reaction gave cycloaddition of the alkyne and azide first, followed by reaction of the amine with the ketone [44].

R2

N

O

R3 R4

<sup>N</sup> <sup>N</sup> N

R1

**Scheme 12.** Cycloaddition of propiolamide **53** and displacement of iodide to form triazolopipera-

One of the first reported preparations of a 1,2,3-triazolo[1,5-*a*]pyrazine by Kauer and coworkers [45] started with dimethyl l-(*o*-nitrophenyl)-l*H*-triazole-4,5-dicarboxylate **59**, and upon treatment with tributyl phosphine in refluxing toluene, afforded methyl 4-methoxy-1,2,3-triazolo[3,4-*a*]quinoxaline-3-carboxylate **60** (Scheme 13). Triazole **59** was readily prepared from *o*-azidonitrobenzene **57** (which in turn was prepared from *o*-chloronitrobenzene **55** or *o*-aminonitrobenzene **56**) and dimethyl acetylenedicarboxylate **58** in CHCl3.

Copper-catalyzed [3 + 2] cycloaddition of propiolamide **53**, followed by halide displacement to form a fused product, was utilized in the synthesis of saturated derivatives

CuI (10 mol%), NaN3 DMSO, 110 °C, 4-6 h

2.2.2. Syntheses of Benzo-Fused 1,2,3-Triazolo[1,5-a]pyrazines

**53 54**

zine **54**. (R1, R2, R3, R4 = alkyl or aryl.)

N O

I

R2

R1

R3

R4

<sup>S</sup> <sup>N</sup> H NH2

5% TiCl4, MeOH, 12 h, r.t.

R1 R2

**50**

N

N

O

O O

= solid-phase polystyrene

commercially available

Copper-catalyzed [3 + 2] cycloaddition of propiolamide **53**, followed by halide displacement to form a fused product, was utilized in the synthesis of saturated derivatives of 1,2,3-triazolo[1,5-*a*]pyrazine (i.e., triazolopiperazines) **54** in 80% yield [43] (Scheme 12). Koguchi and coworkers used ynones and β-amino azides to afford 6,7-dihydro-1,2,3 triazolo[1,5-*a*]pyrazines. These authors verified that the one-pot reaction gave cycloaddition of the alkyne and azide first, followed by reaction of the amine with the ketone [44]. of 1,2,3-triazolo[1,5-*a*]pyrazine (i.e., triazolopiperazines) **54** in 80% yield [43] (Scheme 12). Koguchi and coworkers used ynones and β-amino azides to afford 6,7-dihydro-1,2,3-triazolo[1,5-*a*]pyrazines. These authors verified that the one-pot reaction gave cycloaddition of the alkyne and azide first, followed by reaction of the amine with the ketone [44].

mation of disubstituted 1,2,3-triazolo[1,5-*a*]pyrazines **52**. (R1, R2 = alkyl.)

*Molecules* **2022**, *27*, x FOR PEER REVIEW 9 of 29

several non-fused 1,2,3-triazoles in the same report in yields up to 60%.

resin. After reaction of the polystyrene *p*-toluenesulfonyl hydrazide **49** with an acetylpyrazine **50** in the presence of 5% TiCl4 in MeOH, hydrazone **51** was obtained. Reaction of **51** with morpholine gave the desired 1,2,3-triazolo[1,5-*a*]pyrazines, **52** (Scheme 11), in yields ranging from 33–62%. This regiospecific, traceless protocol represented the first solid-phase assisted synthesis of a triazolopyrazine and was also used for the synthesis of

**Scheme 11.** Intramolecular cyclization of hydrazone **51**, derived from tosylhydrazide **49**, in the for-

<sup>S</sup> <sup>N</sup> H N

R1

N

N

morpholine

95 °C, 4 h

N

R2

<sup>N</sup> <sup>N</sup>

N

R1

R2

O O

**49 51 52**

placement to form a fused product, was utilized in the synthesis of saturated derivatives

oxy-1,2,3-triazolo[3,4-*a*]quinoxaline-3-carboxylate **60** (Scheme 13). Triazole **59** was readily

Copper-catalyzed [3 + 2] cycloaddition of propiolamide **53**, followed by halide dis-

**Scheme 12.** Cycloaddition of propiolamide **53** and displacement of iodide to form triazolopiperazine **54**. (R1, R2, R3, R4 = alkyl or aryl.) **Scheme 12.** Cycloaddition of propiolamide **53** and displacement of iodide to form triazolopiperazine **54**. (R<sup>1</sup> , R<sup>2</sup> , R<sup>3</sup> , R<sup>4</sup> = alkyl or aryl.)

#### 2.2.2. Syntheses of Benzo-Fused 1,2,3-Triazolo[1,5-a]pyrazines

2.2.2. Syntheses of Benzo-Fused 1,2,3-Triazolo[1,5-a]pyrazines One of the first reported preparations of a 1,2,3-triazolo[1,5-*a*]pyrazine by Kauer and coworkers [45] started with dimethyl l-(*o*-nitrophenyl)-l*H*-triazole-4,5-dicarboxylate **59**, and upon treatment with tributyl phosphine in refluxing toluene, afforded methyl 4-meth-One of the first reported preparations of a 1,2,3-triazolo[1,5-*a*]pyrazine by Kauer and coworkers [45] started with dimethyl l-(*o*-nitrophenyl)-l*H*-triazole-4,5-dicarboxylate **59**, and upon treatment with tributyl phosphine in refluxing toluene, afforded methyl 4-methoxy-1,2,3-triazolo[3,4-*a*]quinoxaline-3-carboxylate **60** (Scheme 13). Triazole **59** was readily prepared from *o*-azidonitrobenzene **57** (which in turn was prepared from *o*-chloronitrobenzene **55** or *o*-aminonitrobenzene **56**) and dimethyl acetylenedicarboxylate **58** in CHCl3. *Molecules* **2022**, *27*, x FOR PEER REVIEW 10 of 29

**Scheme 13.** Treatment of triazolyl-*o*-nitrobenzene **59** with tributyl phosphine, PBu3, resulting in 1,2,3-triazolo[3,4-*a*]quinoxaline **60**. , resulting in 1,2,3-triazolo[3,4-*a*]quinoxaline **60**.

Through a different approach, Cue and coworkers [46] accessed 1,2,3-triazolo[l,5 *a*]quinoxaline N-oxides **62** in yields ranging from 52–70% by cyclization of quinoxaline-3 carboxaldehyde-1-oxide-*p*-toluenesulfonylhydrazone **61** (Scheme 14). The starting sulfonylhydrazone **61** was prepared by reaction of a 3-substituted quinoxaline N-oxide with *p*-toluenesulfonylhydrazide [45]. Isoamyl nitrite **Scheme 13.** Treatment of triazolyl-*o*-nitrobenzene **59** with tributyl phosphine, PBu3 Through a different approach, Cue and coworkers [46] accessed 1,2,3-triazolo[l,5 *a*]quinoxaline N-oxides **62** in yields ranging from 52–70% by cyclization of quinoxaline-3-carboxaldehyde-1-oxide-*p*-toluenesulfonylhydrazone **61** (Scheme 14). The starting sulfonylhydrazone **61** was prepared by reaction of a 3-substituted quinoxaline N-oxide with *p*-toluenesulfonylhydrazide [45].

N

O

NaOMe

1 h

ClCCO2H,

**63 64**

O O

N

N N N

CO2Me

N N

R2

R1

**Scheme 14.** Synthesis of substituted 1,2,3-triazolo[l,5-*a*]quinoxaline N-oxides, **62**, via intramolecular

**Scheme 15.** Synthesis of 1,2,3-triazole-fused quinoxalinones, using a diamine in which one amine is

0 °C, 12 h X

For the intramolecular cyclization of *ortho*-substituted amines to prepare 1,2,3-triazoles using nitrite, as is commonly reported for non-fused derivatives [2,25], Ager and coworkers [47] illustrated that the amines used in cyclization do not need to be primary. Through reaction of a secondary amine within a ring and a primary amine **63** with isoamyl nitrite in chloroacetic acid, they obtained 1,2,3-triazoles fused to both lactones and lactams, **64**, in yields in the range 54–76% (Scheme 15). In the case of lactams, 1,2,3-triazolo-

cyclization of sulfonylhydrazone **61**. (R1, R2 = H, methyl.)

quinoxalinones were formed.

<sup>N</sup> CO2Me NH2

X

X = NH, O

H

secondary.

N

O

N

N H <sup>N</sup> <sup>S</sup> O O

R2

R1

N3

NO2

NH2

NH2

NO2

NO2

O

**57 58**

O

R1

**57 58**

O

O

+ O

O

+ O

Cl

NO2

Cl

NO2

*or*

**55 56**

N3

commercially available precursors

*or*

**55 56**

commercially available precursors

NO2

1,2,3-triazolo[3,4-*a*]quinoxaline **60**.

*p*-toluenesulfonylhydrazide [45].

O

O

1,2,3-triazolo[3,4-*a*]quinoxaline **60**.

*p*-toluenesulfonylhydrazide [45].

R2

H

**Scheme 14.** Synthesis of substituted 1,2,3-triazolo[l,5-*a*]quinoxaline N-oxides, **62**, via intramolecular cyclization of sulfonylhydrazone **61**. (R1, R2 = H, methyl.) **Scheme 14.** Synthesis of substituted 1,2,3-triazolo[l,5-*a*]quinoxaline N-oxides, **62**, via intramolecular cyclization of sulfonylhydrazone **61**. (R<sup>1</sup> , R<sup>2</sup> = H, methyl.) For the intramolecular cyclization of *ortho*-substituted amines to prepare 1,2,3-tria-

**Scheme 13.** Treatment of triazolyl-*o*-nitrobenzene **59** with tributyl phosphine, PBu3, resulting in

CO2Me

CO2Me

**Scheme 13.** Treatment of triazolyl-*o*-nitrobenzene **59** with tributyl phosphine, PBu3, resulting in

NO2

NO2

N N N

N N N

*Molecules* **2022**, *27*, x FOR PEER REVIEW 10 of 29

Through a different approach, Cue and coworkers [46] accessed 1,2,3-triazolo[l,5 *a*]quinoxaline N-oxides **62** in yields ranging from 52–70% by cyclization of quinoxaline-3 carboxaldehyde-1-oxide-*p*-toluenesulfonylhydrazone **61** (Scheme 14). The starting sulfonylhydrazone **61** was prepared by reaction of a 3-substituted quinoxaline N-oxide with

O

R1

R2

Through a different approach, Cue and coworkers [46] accessed 1,2,3-triazolo[l,5 *a*]quinoxaline N-oxides **62** in yields ranging from 52–70% by cyclization of quinoxaline-3 carboxaldehyde-1-oxide-*p*-toluenesulfonylhydrazone **61** (Scheme 14). The starting sulfonylhydrazone **61** was prepared by reaction of a 3-substituted quinoxaline N-oxide with

**59 60**

toluene,

toluene,

**59 60**

reflux N

reflux N

N N N

N N N

OMe

CO2Me

CO2Me

OMe

CO2Me PBu3

CO2Me PBu3

For the intramolecular cyclization of *ortho*-substituted amines to prepare 1,2,3-triazoles using nitrite, as is commonly reported for non-fused derivatives [2,25], Ager and coworkers [47] illustrated that the amines used in cyclization do not need to be primary. Through reaction of a secondary amine within a ring and a primary amine **63** with isoamyl nitrite in chloroacetic acid, they obtained 1,2,3-triazoles fused to both lactones and lactams, **64**, in yields in the range 54–76% (Scheme 15). In the case of lactams, 1,2,3-triazolo-For the intramolecular cyclization of *ortho*-substituted amines to prepare 1,2,3-triazoles using nitrite, as is commonly reported for non-fused derivatives [2,25], Ager and coworkers [47] illustrated that the amines used in cyclization do not need to be primary. Through reaction of a secondary amine within a ring and a primary amine **63** with isoamyl nitrite in chloroacetic acid, they obtained 1,2,3-triazoles fused to both lactones and lactams, **64**, in yields in the range 54–76% (Scheme 15). In the case of lactams, 1,2,3-triazoloquinoxalinones were formed. zoles using nitrite, as is commonly reported for non-fused derivatives [2,25], Ager and coworkers [47] illustrated that the amines used in cyclization do not need to be primary. Through reaction of a secondary amine within a ring and a primary amine **63** with isoamyl nitrite in chloroacetic acid, they obtained 1,2,3-triazoles fused to both lactones and lactams, **64**, in yields in the range 54–76% (Scheme 15). In the case of lactams, 1,2,3-triazoloquinoxalinones were formed.

**Scheme 15.** Synthesis of 1,2,3-triazole-fused quinoxalinones, using a diamine in which one amine is **Scheme 15.** Synthesis of 1,2,3-triazole-fused quinoxalinones, using a diamine in which one amine is secondary. **Scheme 15.** Synthesis of 1,2,3-triazole-fused quinoxalinones, using a diamine in which one amine is secondary. Synthesizing compounds of the same type, Bertelli, and coworkers [48] first formed

secondary. Synthesizing compounds of the same type, Bertelli, and coworkers [48] first formed a triazole diester on a ring *ortho* to a nitro group, **65**, which was intramolecularly cyclized to form ethyl 4,5-dihydro-4-oxo-[1,2,3]triazolo[1,5-*a*]quinoxaline-3-carboxylate, **66** (Scheme 16). This reaction was conducted by hydrogenation with a 10% Pd/C catalyst or by reaction with FeCl<sup>3</sup> and Fe powder. Biagi and coworkers [49] cyclized the triazole diester into 1,2,3-triazoloquinoxalinone **66** with 10% Pd/C in ethanol in an excellent 98% yield. Shen and coworkers further modified the ester group of **66** to prepare a derivative suitable for biological testing [50]. a triazole diester on a ring *ortho* to a nitro group, **65**, which was intramolecularly cyclized to form ethyl 4,5-dihydro-4-oxo-[1,2,3]triazolo[1,5-*a*]quinoxaline-3-carboxylate, **66** (Scheme 16). This reaction was conducted by hydrogenation with a 10% Pd/C catalyst or by reaction with FeCl3 and Fe powder. Biagi and coworkers [49] cyclized the triazole diester into 1,2,3-triazoloquinoxalinone **66** with 10% Pd/C in ethanol in an excellent 98% yield. Shen and coworkers further modified the ester group of **66** to prepare a derivative suitable for biological testing [50].

CO2Li N

(i) R2-Ar-NH2, NaNO2, aq. HCl

(ii) pyridine, Ac2O

**67 68**

**Scheme 16.** Synthesis of 1,2,3-triazole-fused quinoxalinone **66**, using a pre-formed 1,2,3-triazole **65**  in the presence of iron catalysts. **Scheme 16.** Synthesis of 1,2,3-triazole-fused quinoxalinone **66**, using a pre-formed 1,2,3-triazole **65** in the presence of iron catalysts.

Abbott and coworkers [51] prepared 1,2,3-triazoloquinoxalines in an analogous manner, but opted for use of an amide instead of a nitro group, giving mesoionic 1,2,3-triazoles

NHCOR1

reflux

O

R2

N N

with *p*-toluenesulfonic acid (*p*-TSA) in refluxing toluene in yields in the range 16–59%.

**Scheme 17.** Synthesis of mesoionic 1,2,3-triazoloquinoxalines **69** from *ortho*-substituted 1,2,3-tria-

Saha and coworkers [52] used the intramolecular cyclization of *ortho*-substituted anilines with tethered 1,2,3-triazoles, **72**, a Pictet–Spengler reaction, to form 1,2,3-triazoloquinoxalines **73** in yields in the range 61–70% (Scheme 18). This sequence offers two-point

N

**69**

N

N N

O

R1

R2

*p*-TSA, toluene,

H N

NHCOR1

zolobenzamides **68**. (R1, R2 = alkyl or aryl.)

Abbott and coworkers [51] prepared 1,2,3-triazoloquinoxalines in an analogous manner, but opted for use of an amide instead of a nitro group, giving mesoionic 1,2,3-triazoles **68**, which were derived from the lithium salt of [2-(acetylamino)phenyl]amino acetic acid **67** (Scheme 17). A series of 1,2,3-triazoloquinoxalines, **69**, was synthesized after cyclization with *p*-toluenesulfonic acid (*p*-TSA) in refluxing toluene in yields in the range 16–59%. ner, but opted for use of an amide instead of a nitro group, giving mesoionic 1,2,3-triazoles **68**, which were derived from the lithium salt of [2-(acetylamino)phenyl]amino acetic acid **67** (Scheme 17). A series of 1,2,3-triazoloquinoxalines, **69**, was synthesized after cyclization with *p*-toluenesulfonic acid (*p*-TSA) in refluxing toluene in yields in the range 16–59%.

**Scheme 16.** Synthesis of 1,2,3-triazole-fused quinoxalinone **66**, using a pre-formed 1,2,3-triazole **65** 

Abbott and coworkers [51] prepared 1,2,3-triazoloquinoxalines in an analogous man-

N H

N

N N

O

CO2Et

*Molecules* **2022**, *27*, x FOR PEER REVIEW 11 of 29

5 mol% FeCl3, Fe powder

EtOH, reflux, 4 h

**65 66**

suitable for biological testing [50].

CO2Et

in the presence of iron catalysts.

CO2Et

NO2

N N N

Synthesizing compounds of the same type, Bertelli, and coworkers [48] first formed a triazole diester on a ring *ortho* to a nitro group, **65**, which was intramolecularly cyclized to form ethyl 4,5-dihydro-4-oxo-[1,2,3]triazolo[1,5-*a*]quinoxaline-3-carboxylate, **66** (Scheme 16). This reaction was conducted by hydrogenation with a 10% Pd/C catalyst or by reaction with FeCl3 and Fe powder. Biagi and coworkers [49] cyclized the triazole diester into 1,2,3-triazoloquinoxalinone **66** with 10% Pd/C in ethanol in an excellent 98% yield. Shen and coworkers further modified the ester group of **66** to prepare a derivative

**Scheme 17.** Synthesis of mesoionic 1,2,3-triazoloquinoxalines **69** from *ortho*-substituted 1,2,3-triazolobenzamides **68**. (R1, R2 = alkyl or aryl.) **Scheme 17.** Synthesis of mesoionic 1,2,3-triazoloquinoxalines **69** from *ortho*-substituted 1,2,3 triazolobenzamides **68**. (R<sup>1</sup> , R<sup>2</sup> = alkyl or aryl.)

Saha and coworkers [52] used the intramolecular cyclization of *ortho*-substituted anilines with tethered 1,2,3-triazoles, **72**, a Pictet–Spengler reaction, to form 1,2,3-triazoloquinoxalines **73** in yields in the range 61–70% (Scheme 18). This sequence offers two-point Saha and coworkers [52] used the intramolecular cyclization of *ortho*-substituted anilines with tethered 1,2,3-triazoles, **72**, a Pictet–Spengler reaction, to form 1,2,3-triazoloquinoxalines **73** in yields in the range 61–70% (Scheme 18). This sequence offers two-point diversity: one from **72**, and the other from an aryl aldehyde **73**. The prerequisite triazole **72** was conveniently prepared from readily available starting materials, including *o*-fluoronitrobenzene **70**, phenylacetylene **71**, and sodium azide. *Molecules* **2022**, *27*, x FOR PEER REVIEW 12 of 29 diversity: one from **72**, and the other from an aryl aldehyde **73**. The prerequisite triazole **72** was conveniently prepared from readily available starting materials, including *o*-fluoronitrobenzene **70**, phenylacetylene **71**, and sodium azide.

**Scheme 18.** 3-Phenyl-4-*p*-tolyl-1,2,3-triazolo[1,5-*a*]quinoxalines **74** synthesized from *ortho*-substituted 2-(4-phenyl-[1,2,3]triazol-1-yl)anilines **72**. (R1, R2 = alkyl or aryl.) **77 78 Scheme 18.** 3-Phenyl-4-*p*-tolyl-1,2,3-triazolo[1,5-*a*]quinoxalines **74** synthesized from *ortho*-substituted 2-(4-phenyl-[1,2,3]triazol-1-yl)anilines **72**. (R<sup>1</sup> , R<sup>2</sup> = alkyl or aryl.)

**Scheme 19.** Synthesis of 3-phenyl-4-(trifluoromethyl)-1,2,3-triazolo[1,5-*a*]quinoxaline **76** from N-(*o*-

triazoles including tetrazoles, pyrazoles, and imidazoles in yields as high as 80%.

PhI(OCOCy)2 (1.5 eq.)

 1 mol% [*fac*-Ir(ppy)3], 26W fluorescent bulb, DMF

**Scheme 20.** Photoredox approach for the synthesis of a 1,2,3-triazoloquinoxaline **78**. (Cy = cyclo-

Using photoredox catalysis, He and coworkers [54] used [*fac*-Ir(ppy)3] as a photocatalyst to afford the corresponding 1,2,3-triazoloquinoxaline **78** from isonitrile **77** in 60% yield (Scheme 20). Due to poor solubility of the catalyst, ACN resulted in decreased yields compared to DMF. This work is a rare example of free-radical generation of 1,2,3-triazolefused ring systems, as cyclohexyl radicals are proposed to have formed from phenyliodine(III)dicarboxylate. The radicals yield isonitrile carbon radicals, followed by reaction with carbon 5 of the triazole. Various fused rings were synthesized in addition to 1,2,3-

N

CF3

N

Cy

N N N

N N N

Chen and coworkers [53] used a novel approach for the synthesis of 4-(trifluoromethyl)-1,2,3-triazolo[1,5-*a*]quinoxaline **76** via cascade reactions of N-(*o*-haloaryl)alkynylimine **75** with sodium azide in the presence of copper iodide and L-proline (Scheme 19). Among a series of amine-containing catalysts, L-proline resulted in a 98% isolated

F3C 10 mol% CuI,

haloaryl)alkynylimine **75**.

<sup>N</sup> <sup>C</sup>

N N N

I

N

hexyl, fac = facial, ppy = 2-phenylpyridine).

yields, and higher percentages of the uncyclized imine product.

20 mol% L-proline, DMSO, r.t.

**75 76**

NaN3

F

+ + NaN3

commercially available precursors

F

NO2

NO2

+ + NaN3

I

N

R1

commercially available precursors

Chen and coworkers [53] used a novel approach for the synthesis of 4-(trifluoromethyl)- 1,2,3-triazolo[1,5-*a*]quinoxaline **76** via cascade reactions of N-(*o*-haloaryl)alkynylimine **75** with sodium azide in the presence of copper iodide and L-proline (Scheme 19). Among a series of amine-containing catalysts, L-proline resulted in a 98% isolated yield, while tetramethylethylenediamine and *N*,*N*0 -dimethylethylenediamine gave lower yields, and higher percentages of the uncyclized imine product. thyl)-1,2,3-triazolo[1,5-*a*]quinoxaline **76** via cascade reactions of N-(*o*-haloaryl)alkynylimine **75** with sodium azide in the presence of copper iodide and L-proline (Scheme 19). Among a series of amine-containing catalysts, L-proline resulted in a 98% isolated yield, while tetramethylethylenediamine and *N*,*N'*-dimethylethylenediamine gave lower yields, and higher percentages of the uncyclized imine product. yields, and higher percentages of the uncyclized imine product. NaN3 N N N

*Molecules* **2022**, *27*, x FOR PEER REVIEW 12 of 29

diversity: one from **72**, and the other from an aryl aldehyde **73**. The prerequisite triazole **72** was conveniently prepared from readily available starting materials, including *o*-fluo-

ronitrobenzene **70**, phenylacetylene **71**, and sodium azide.

N

R1

tuted 2-(4-phenyl-[1,2,3]triazol-1-yl)anilines **72**. (R1, R2 = alkyl or aryl.)

*Molecules* **2022**, *27*, x FOR PEER REVIEW 12 of 29

ronitrobenzene **70**, phenylacetylene **71**, and sodium azide.

N

N N

NH2

**72**

**70 71 74**

N N

**Scheme 18.** 3-Phenyl-4-*p*-tolyl-1,2,3-triazolo[1,5-*a*]quinoxalines **74** synthesized from *ortho*-substi-

NH2

Chen and coworkers [53] used a novel approach for the synthesis of 4-(trifluorome-

**72**

thyl)-1,2,3-triazolo[1,5-*a*]quinoxaline **76** via cascade reactions of N-(*o*-haloaryl)alkynylimine **75** with sodium azide in the presence of copper iodide and L-proline (Scheme

**70 71 74**

tuted 2-(4-phenyl-[1,2,3]triazol-1-yl)anilines **72**. (R1, R2 = alkyl or aryl.)

yield, while tetramethylethylenediamine and *N*,*N'*-dimethylethylenediamine gave lower

diversity: one from **72**, and the other from an aryl aldehyde **73**. The prerequisite triazole **72** was conveniently prepared from readily available starting materials, including *o*-fluo-

**73**

O

*p*-TsOH, toluene, reflux, 12 h

R2

**Scheme 18.** 3-Phenyl-4-*p*-tolyl-1,2,3-triazolo[1,5-*a*]quinoxalines **74** synthesized from *ortho*-substi-

Chen and coworkers [53] used a novel approach for the synthesis of 4-(trifluorome-

*p*-TsOH, toluene, reflux, 12 h

**73**

R1

R2

O

R1

N

N

N N

N

R2

N N N

R2

**Scheme 19.** Synthesis of 3-phenyl-4-(trifluoromethyl)-1,2,3-triazolo[1,5-*a*]quinoxaline **76** from N-(*o*haloaryl)alkynylimine **75**. **Scheme 19.** Synthesis of 3-phenyl-4-(trifluoromethyl)-1,2,3-triazolo[1,5-*a*]quinoxaline **76** from N-(*o*haloaryl)alkynylimine **75**. Using photoredox catalysis, He and coworkers [54] used [*fac*-Ir(ppy)3] as a photocatalyst to afford the corresponding 1,2,3-triazoloquinoxaline **78** from isonitrile **77** in 60% yield

Using photoredox catalysis, He and coworkers [54] used [*fac*-Ir(ppy)3] as a photocatalyst to afford the corresponding 1,2,3-triazoloquinoxaline **78** from isonitrile **77** in 60% yield (Scheme 20). Due to poor solubility of the catalyst, ACN resulted in decreased yields compared to DMF. This work is a rare example of free-radical generation of 1,2,3-triazolefused ring systems, as cyclohexyl radicals are proposed to have formed from phenyliodine(III)dicarboxylate. The radicals yield isonitrile carbon radicals, followed by reaction with carbon 5 of the triazole. Various fused rings were synthesized in addition to 1,2,3- Using photoredox catalysis, He and coworkers [54] used [*fac*-Ir(ppy)3] as a photocatalyst to afford the corresponding 1,2,3-triazoloquinoxaline **78** from isonitrile **77** in 60% yield (Scheme 20). Due to poor solubility of the catalyst, ACN resulted in decreased yields compared to DMF. This work is a rare example of free-radical generation of 1,2,3 triazole-fused ring systems, as cyclohexyl radicals are proposed to have formed from phenyliodine(III)dicarboxylate. The radicals yield isonitrile carbon radicals, followed by reaction with carbon 5 of the triazole. Various fused rings were synthesized in addition to 1,2,3-triazoles including tetrazoles, pyrazoles, and imidazoles in yields as high as 80%. (Scheme 20). Due to poor solubility of the catalyst, ACN resulted in decreased yields compared to DMF. This work is a rare example of free-radical generation of 1,2,3-triazolefused ring systems, as cyclohexyl radicals are proposed to have formed from phenyliodine(III)dicarboxylate. The radicals yield isonitrile carbon radicals, followed by reaction with carbon 5 of the triazole. Various fused rings were synthesized in addition to 1,2,3 triazoles including tetrazoles, pyrazoles, and imidazoles in yields as high as 80%.

**Scheme 20.** Photoredox approach for the synthesis of a 1,2,3-triazoloquinoxaline **78**. (Cy = cyclohexyl, fac = facial, ppy = 2-phenylpyridine). **Scheme 20.** Photoredox approach for the synthesis of a 1,2,3-triazoloquinoxaline **78**. (Cy = cyclohexyl, fac = facial, ppy = 2-phenylpyridine). **Scheme 20.** Photoredox approach for the synthesis of a 1,2,3-triazoloquinoxaline **78**. (Cy = cyclohexyl, fac = facial, ppy = 2-phenylpyridine).

> In the presence of Cu(OAc)<sup>2</sup> and base in DMSO/THF, Li and coworkers [55] reported an efficient one-pot synthesis of 1,2,3-triazolo[1,5-*a*]quinoxalines **81** from 1-azido-2 isocyanoarenes **79** in yields in the range 40–84% (Scheme 21). They outlined the option of using terminal acetylenes **80** or substituted acetaldehydes **82**, the former being cyclized into **81** in one step (in yields ranging from 40–83%), and the latter forming uncyclized triazole **83,** which was annulated using Togni's reagent II and tetra-*n*-butylammonium iodide (TBAI), forming **84**, or phenylboronic acid, forming **85**. Derivatives of **84** were prepared in yields in the range 26–78%, and one synthesis of **85** yielded 86%.

in the range 26–78%, and one synthesis of **85** yielded 86%.

line derivatives via Rh(II)-catalyzed carbenoid insertion reactions [55].

**Scheme 21.** Copper-catalyzed synthesis of 1,2,3-triazolo[1,5-*a*]quinoxalines **81** from 1-azido-2-isocyanoarenes **79**. Annulation of intermediate **83** with 1) Togni's reagent II and catalytic TBAI forming **84**, or 2) phenylboronic acid and a manganese catalyst forming **85**. (R1, R2 = alkyl or aryl.) **Scheme 21.** Copper-catalyzed synthesis of 1,2,3-triazolo[1,5-*a*]quinoxalines **81** from 1-azido-2 isocyanoarenes **79**. Annulation of intermediate **83** with 1) Togni's reagent II and catalytic TBAI forming **84**, or 2) phenylboronic acid and a manganese catalyst forming **85**. (R<sup>1</sup> , R<sup>2</sup> = alkyl or aryl.)

In the presence of Cu(OAc)2 and base in DMSO/THF, Li and coworkers [55] reported an efficient one-pot synthesis of 1,2,3-triazolo[1,5-*a*]quinoxalines **81** from 1-azido-2-isocyanoarenes **79** in yields in the range 40–84% (Scheme 21). They outlined the option of using terminal acetylenes **80** or substituted acetaldehydes **82**, the former being cyclized into **81** in one step (in yields ranging from 40–83%), and the latter forming uncyclized triazole **83,** which was annulated using Togni's reagent II and tetra-*n*-butylammonium iodide (TBAI), forming **84**, or phenylboronic acid, forming **85**. Derivatives of **84** were prepared in yields

Owing to the versatility of intermediate **83**, many functionalized 1,2,3-triazoloquinoxalines were prepared, and indeed, Li and coworkers reported several compounds containing the 1,2,3-triazolo[1,5-*a*]quinoxaline core with a variety of functionalities. Additionally, in this report, the fused products were further reacted into diversified quinoxa-

Employing a Pd-catalyzed intramolecular cyclization of triazole **86**, Kotovshchikov and coworkers [56] synthesized 3-butyl-[1,2,3]triazolo[1,5-*a*]quinoxalin-4(5*H*)-one **87** in 77% yield. As this reaction was conducted under CO (1 atm), the carbonyl carbon of the quinoxalinone was introduced by Pd-catalyzed insertion of CO (Scheme 22). Owing to the versatility of intermediate **83**, many functionalized 1,2,3-triazoloquinoxalines were prepared, and indeed, Li and coworkers reported several compounds containing the 1,2,3-triazolo[1,5-*a*]quinoxaline core with a variety of functionalities. Additionally, in this report, the fused products were further reacted into diversified quinoxaline derivatives via Rh(II)-catalyzed carbenoid insertion reactions [55].

Employing a Pd-catalyzed intramolecular cyclization of triazole **86**, Kotovshchikov and coworkers [56] synthesized 3-butyl-[1,2,3]triazolo[1,5-*a*]quinoxalin-4(5*H*)-one **87** in 77% yield. As this reaction was conducted under CO (1 atm), the carbonyl carbon of the quinoxalinone was introduced by Pd-catalyzed insertion of CO (Scheme 22). *Molecules* **2022**, *27*, x FOR PEER REVIEW 14 of 29

**Scheme 22.** Pd-catalyzed synthesis of quinoxalinone **87** from *ortho*-substituted aniline **86**. **Scheme 22.** Pd-catalyzed synthesis of quinoxalinone **87** from *ortho*-substituted aniline **86**.

Xiao and coworkers [43] and Chen and coworkers [57] used in situ conversion of Npropargyl-N-(2-iodoaryl)amides **88** to azides, which underwent 1,3-dipolar cycloaddition with the adjacent alkyne to form substituted 1,2,3-triazolo[1,5-*a*]quinoxalines **89** (Scheme 23) in yields in the range 58–91%. Chen and coworkers suggested that cycloaddition might occur first. The sequence was conducted in the presence of DIPEA and 1,2-dimethyleth-Xiao and coworkers [43] and Chen and coworkers [57] used in situ conversion of N-propargyl-N-(2-iodoaryl)amides **88** to azides, which underwent 1,3-dipolar cycloaddition with the adjacent alkyne to form substituted 1,2,3-triazolo[1,5-*a*]quinoxalines **89** (Scheme 23) in yields in the range 58–91%. Chen and coworkers suggested that cycloaddition might occur first. The sequence was conducted in the presence of DIPEA and 1,2-dimethylethylenediamine (DMEDA).

**Scheme 23.** Intramolecular cyclization of N-propargyl-N-(2-iodoaryl)amides **88**, yielding 1,2,3-tria-

Preparative thermolysis of tetrazoloquinoxaline **90** proceeded by loss of nitrogen through diazo intermediate **91** and then to 1,2,3-triazolo[1,5-*a*]quinoxaline **92** in 67% yield (Scheme 24) [58]. Using a ring-closure method similar to that used by both Raghavendra and coworkers [42] and Cue and coworkers [46], Vogel and Lippmann [59] developed a route to derivatives of **92** in 47–89% yield via conversion from tosylhydrazones **93** using

N

N

N

R

N

**93**

H <sup>N</sup> <sup>S</sup> O O

N

R

NaOMe, MeOH

N N N R2

N N N

R1 R4

zoloquinoxalines **89** after in situ conversion of **88** to the azide. (R1, R2, R3, R4 = alkyl or aryl.)

165 °C 89 h

base (Bamford-Stevens conditions) or, in certain cases, heat (Scheme 24).

N2

N

**90 91 92**

N

**88 89**

**Scheme 24.** Cyclization methods for preparing 1,2,3-triazolo[1,5-*a*]quinoxalines.

NaN3, CuI, DIPEA DMEDA

DMF, 50 °C

ylenediamine (DMEDA).

I

R2

R4

R1

R3 N

N

<sup>N</sup> <sup>N</sup>

HN N

N

N2

R = H (ref. 58)

R = O-alkyl, O-aryl, NH2 (ref. 59)

heat

O

ylenediamine (DMEDA).

ylenediamine (DMEDA).

N

N

N N

N N

NH2

NH2

I

I

*n*-Bu

*n*-Bu

**Scheme 22.** Pd-catalyzed synthesis of quinoxalinone **87** from *ortho*-substituted aniline **86**.

**Scheme 22.** Pd-catalyzed synthesis of quinoxalinone **87** from *ortho*-substituted aniline **86**.

5 mol% Pd(OAc)2 CO (1 atm)

5 mol% Pd(OAc)2 CO (1 atm)

*Molecules* **2022**, *27*, x FOR PEER REVIEW 14 of 29

MeOH, TEA, 50 °C, 17 h

MeOH, TEA, 50 °C, 17 h

**86 87**

**86 87**

Xiao and coworkers [43] and Chen and coworkers [57] used in situ conversion of Npropargyl-N-(2-iodoaryl)amides **88** to azides, which underwent 1,3-dipolar cycloaddition with the adjacent alkyne to form substituted 1,2,3-triazolo[1,5-*a*]quinoxalines **89** (Scheme 23) in yields in the range 58–91%. Chen and coworkers suggested that cycloaddition might occur first. The sequence was conducted in the presence of DIPEA and 1,2-dimethyleth-

Xiao and coworkers [43] and Chen and coworkers [57] used in situ conversion of Npropargyl-N-(2-iodoaryl)amides **88** to azides, which underwent 1,3-dipolar cycloaddition with the adjacent alkyne to form substituted 1,2,3-triazolo[1,5-*a*]quinoxalines **89** (Scheme 23) in yields in the range 58–91%. Chen and coworkers suggested that cycloaddition might occur first. The sequence was conducted in the presence of DIPEA and 1,2-dimethyleth-

O

O

*n*-Bu

*n*-Bu

N H

N H

N N N

N N N

**Scheme 23.** Intramolecular cyclization of N-propargyl-N-(2-iodoaryl)amides **88**, yielding 1,2,3-triazoloquinoxalines **89** after in situ conversion of **88** to the azide. (R1, R2, R3, R4 = alkyl or aryl.) **Scheme 23.** Intramolecular cyclization of N-propargyl-N-(2-iodoaryl)amides **88**, yielding 1,2,3 triazoloquinoxalines **89** after in situ conversion of **88** to the azide. (R<sup>1</sup> , R<sup>2</sup> , R<sup>3</sup> , R<sup>4</sup> = alkyl or aryl.) **Scheme 23.** Intramolecular cyclization of N-propargyl-N-(2-iodoaryl)amides **88**, yielding 1,2,3-triazoloquinoxalines **89** after in situ conversion of **88** to the azide. (R1, R2, R3, R4 = alkyl or aryl.)

Preparative thermolysis of tetrazoloquinoxaline **90** proceeded by loss of nitrogen through diazo intermediate **91** and then to 1,2,3-triazolo[1,5-*a*]quinoxaline **92** in 67% yield (Scheme 24) [58]. Using a ring-closure method similar to that used by both Raghavendra and coworkers [42] and Cue and coworkers [46], Vogel and Lippmann [59] developed a route to derivatives of **92** in 47–89% yield via conversion from tosylhydrazones **93** using base (Bamford-Stevens conditions) or, in certain cases, heat (Scheme 24). Preparative thermolysis of tetrazoloquinoxaline **90** proceeded by loss of nitrogen through diazo intermediate **91** and then to 1,2,3-triazolo[1,5-*a*]quinoxaline **92** in 67% yield (Scheme 24) [58]. Using a ring-closure method similar to that used by both Raghavendra and coworkers [42] and Cue and coworkers [46], Vogel and Lippmann [59] developed a route to derivatives of **92** in 47–89% yield via conversion from tosylhydrazones **93** using base (Bamford-Stevens conditions) or, in certain cases, heat (Scheme 24). Preparative thermolysis of tetrazoloquinoxaline **90** proceeded by loss of nitrogen through diazo intermediate **91** and then to 1,2,3-triazolo[1,5-*a*]quinoxaline **92** in 67% yield (Scheme 24) [58]. Using a ring-closure method similar to that used by both Raghavendra and coworkers [42] and Cue and coworkers [46], Vogel and Lippmann [59] developed a route to derivatives of **92** in 47–89% yield via conversion from tosylhydrazones **93** using base (Bamford-Stevens conditions) or, in certain cases, heat (Scheme 24).

**Scheme 24.** Cyclization methods for preparing 1,2,3-triazolo[1,5-*a*]quinoxalines. **Scheme 24.** Cyclization methods for preparing 1,2,3-triazolo[1,5-*a*]quinoxalines. **Scheme 24.** Cyclization methods for preparing 1,2,3-triazolo[1,5-*a*]quinoxalines.

Overall, there exist diverse methods for the synthesis of both bicyclic 1,2,3-triazolo[1,5 *a*]pyrazines and 1,2,3-triazolo[1,5-*a*]quinoxalines.

#### *2.3. Syntheses of 1H-1,2,3-Triazolo[4,5-d]pyridazines*

Livi and coworkers [60] reviewed syntheses of this heterocyclic system covering reports prior to 1996. Another review on condensed 1,2,3-triazoles appeared in 2008, which includes synthesis of 1*H*-1,2,3-triazolo[4,5-*d*]pyridazines [32]. Here, we summarize both older and newer reports. A common theme in the literature regarding the synthesis of 1*H-*1,2,3-triazolo[4,5-*d*]pyridazines is the reaction of 1,2,3-triazole dicarbonyl species with hydrazine hydrate. This yields a diacylhydrazide, which can be cyclized with either high heat or acid. One of the first examples (Scheme 25) is from Fournier and Miller [61], who used 2-(4,5-dibenzoyl-1*H*-1,2,3-triazol-1-ylmethyl)-3,4,6-trimethylhydroquinone diacetate and hydrazine hydrate in ethanol to form 4,5-diphenyl-1*H-*1,2,3-triazolo[4,5-*d*]pyridazine. In a comparable manner, Erichomovitch [62] used triazole diesters **94** to obtain diacylhydrazides **95**, which were heated to form 1*H-*1,2,3-triazolo[4,5-*d*]pyridazines **96** in 80% yield with loss of hydrazine.

**Scheme 25.** Intramolecular cyclization of diacylhydrazide **95**, forming 1,2,3-triazolo[4,5-*d*]pyridazine **96** upon high heat with loss of hydrazine. (R1 = alkyl.) **Scheme 25.** Intramolecular cyclization of diacylhydrazide **95**, forming 1,2,3-triazolo[4,5-*d*]pyridazine **96** upon high heat with loss of hydrazine. (R<sup>1</sup> = alkyl.) **Scheme 25.** Intramolecular cyclization of diacylhydrazide **95**, forming 1,2,3-triazolo[4,5-*d*]pyridazine **96** upon high heat with loss of hydrazine. (R1 = alkyl.)

N N

Overall, there exist diverse methods for the synthesis of both bicyclic 1,2,3-tria-

Overall, there exist diverse methods for the synthesis of both bicyclic 1,2,3-tria-

Livi and coworkers [60] reviewed syntheses of this heterocyclic system covering reports prior to 1996. Another review on condensed 1,2,3-triazoles appeared in 2008, which includes synthesis of 1*H*-1,2,3-triazolo[4,5-*d*]pyridazines [32]. Here, we summarize both older and newer reports. A common theme in the literature regarding the synthesis of 1*H-*1,2,3-triazolo[4,5-*d*]pyridazines is the reaction of 1,2,3-triazole dicarbonyl species with hydrazine hydrate. This yields a diacylhydrazide, which can be cyclized with either high heat or acid. One of the first examples (Scheme 25) is from Fournier and Miller [61], who used 2-(4,5-dibenzoyl-1*H*-1,2,3-triazol-1-ylmethyl)-3,4,6-trimethylhydroquinone diacetate and hydrazine hydrate in ethanol to form 4,5-diphenyl-1*H-*1,2,3-triazolo[4,5-*d*]pyridazine. In a comparable manner, Erichomovitch [62] used triazole diesters **94** to obtain diacylhydrazides **95**, which were heated to form 1*H-*1,2,3-triazolo[4,5-*d*]pyridazines **96** in

Livi and coworkers [60] reviewed syntheses of this heterocyclic system covering reports prior to 1996. Another review on condensed 1,2,3-triazoles appeared in 2008, which includes synthesis of 1*H*-1,2,3-triazolo[4,5-*d*]pyridazines [32]. Here, we summarize both older and newer reports. A common theme in the literature regarding the synthesis of 1*H-*1,2,3-triazolo[4,5-*d*]pyridazines is the reaction of 1,2,3-triazole dicarbonyl species with hydrazine hydrate. This yields a diacylhydrazide, which can be cyclized with either high heat or acid. One of the first examples (Scheme 25) is from Fournier and Miller [61], who used 2-(4,5-dibenzoyl-1*H*-1,2,3-triazol-1-ylmethyl)-3,4,6-trimethylhydroquinone diacetate and hydrazine hydrate in ethanol to form 4,5-diphenyl-1*H-*1,2,3-triazolo[4,5-*d*]pyridazine. In a comparable manner, Erichomovitch [62] used triazole diesters **94** to obtain diacylhydrazides **95**, which were heated to form 1*H-*1,2,3-triazolo[4,5-*d*]pyridazines **96** in

zolo[1,5-*a*]pyrazines and 1,2,3-triazolo[1,5-*a*]quinoxalines.

zolo[1,5-*a*]pyrazines and 1,2,3-triazolo[1,5-*a*]quinoxalines.

*Molecules* **2022**, *27*, x FOR PEER REVIEW 15 of 29

*2.3. Syntheses of 1H-1,2,3-triazolo[4,5-d]pyridazines* 

*2.3. Syntheses of 1H-1,2,3-triazolo[4,5-d]pyridazines* 

80% yield with loss of hydrazine.

80% yield with loss of hydrazine.

Janietz and coworkers [63] developed a scheme that proceeded through dichlorotriazole **97**, which, after conversion to a dinitrone and subsequent treatment with acid, afforded the dialdehyde **98**, which cyclized to form the desired 1*H*-1,2,3-triazolo[4,5-*d*]pyridazine **99** after treatment with hydrazine (Scheme 26). Janietz and coworkers [63] developed a scheme that proceeded through dichlorotriazole **97**, which, after conversion to a dinitrone and subsequent treatment with acid, afforded the dialdehyde **98**, which cyclized to form the desired 1*H*-1,2,3-triazolo[4,5-*d*]pyridazine **99** after treatment with hydrazine (Scheme 26). Janietz and coworkers [63] developed a scheme that proceeded through dichlorotriazole **97**, which, after conversion to a dinitrone and subsequent treatment with acid, afforded the dialdehyde **98**, which cyclized to form the desired 1*H*-1,2,3-triazolo[4,5-*d*]pyridazine **99** after treatment with hydrazine (Scheme 26).

**Scheme 26.** Synthesis of substituted 1,2,3-triazolo[4,5-*d*]pyridazines **99** from 4,5-dichloromethyltriazoles **97**, proceeding through dialdehyde **98**. (R = aryl.) **Scheme 26.** Synthesis of substituted 1,2,3-triazolo[4,5-*d*]pyridazines **99** from 4,5-dichloromethyltriazoles **97**, proceeding through dialdehyde **98**. (R = aryl.) **Scheme 26.** Synthesis of substituted 1,2,3-triazolo[4,5-*d*]pyridazines **99** from 4,5-dichloromethyltriazoles **97**, proceeding through dialdehyde **98**. (R = aryl.)

Reports of forming 1,2,3-triazolo[4,5-*d*]pyridazones or pyridazines using this method include those of Gilchrist [64,65], Milhelcic [66], Ramesh [67], Theocharis [68], Bussolari [69], Biagi [70–72], Abu-Orabi [73], Ramanaiah [74], Bankowska [75], and others [5,76–78]. Martin and Castle [79] used ring closure by nitrosonium ion in their treatment of a Reports of forming 1,2,3-triazolo[4,5-*d*]pyridazones or pyridazines using this method include those of Gilchrist [64,65], Milhelcic [66], Ramesh [67], Theocharis [68], Bussolari [69], Biagi [70–72], Abu-Orabi [73], Ramanaiah [74], Bankowska [75], and others [5,76–78]. Reports of forming 1,2,3-triazolo[4,5-*d*]pyridazones or pyridazines using this method include those of Gilchrist [64,65], Milhelcic [66], Ramesh [67], Theocharis [68], Bussolari [69], Biagi [70–72], Abu-Orabi [73], Ramanaiah [74], Bankowska [75], and others [5,76–78].

4,5-diamino-6-pyridazinone **101** in forming 3,5-dihydro-4*H-*1,2,3-triazolo[4,5-*d*]pyridazin-4-one **102** in 91% yield (Scheme 27). Commercially available 4,5-dichloro-3(2*H*)-pyridazinone **100** was converted to **101** in three steps. Similar methods of reacting substituted diaminopyridazines with nitrite have been conducted by Yanai [80] (conversion of **103** to **104** in Scheme 27), Chen [81], Draper [82], and Mataka [83]. Martin and Castle [79] used ring closure by nitrosonium ion in their treatment of a 4,5-diamino-6-pyridazinone **101** in forming 3,5-dihydro-4*H-*1,2,3-triazolo[4,5-*d*]pyridazin-4-one **102** in 91% yield (Scheme 27). Commercially available 4,5-dichloro-3(2*H*)-pyridazinone **100** was converted to **101** in three steps. Similar methods of reacting substituted diaminopyridazines with nitrite have been conducted by Yanai [80] (conversion of **103** to **104** in Scheme 27), Chen [81], Draper [82], and Mataka [83]. Martin and Castle [79] used ring closure by nitrosonium ion in their treatment of a 4,5 diamino-6-pyridazinone **101** in forming 3,5-dihydro-4*H-*1,2,3-triazolo[4,5-*d*]pyridazin-4 one **102** in 91% yield (Scheme 27). Commercially available 4,5-dichloro-3(2*H*)-pyridazinone **100** was converted to **101** in three steps. Similar methods of reacting substituted diaminopyridazines with nitrite have been conducted by Yanai [80] (conversion of **103** to **104** in Scheme 27), Chen [81], Draper [82], and Mataka [83]. *Molecules* **2022**, *27*, x FOR PEER REVIEW 16 of 29

HN <sup>N</sup>

O

N N R1 NO2

**Scheme 27.** Formation of 3,5-dihydro-4*H-*1,2,3-triazolo[4,5-*d*]pyridaz-4-one **102** upon treatment of 4,5-diamino-6-pyridazone **101** with nitrite, and a similar reaction of diaminopyridazine **103** cyclizing to **104** with nitrite. (R1 = H, O-alkyl, SH, SCH3, OH, NH2, R2 = H, CH3, O-alkyl, OH, Cl.) **Scheme 27.** Formation of 3,5-dihydro-4*H-*1,2,3-triazolo[4,5-*d*]pyridaz-4-one **102** upon treatment of 4,5-diamino-6-pyridazone **101** with nitrite, and a similar reaction of diaminopyridazine **103** cyclizing to **104** with nitrite. (R<sup>1</sup> = H, O-alkyl, SH, SCH<sup>3</sup> , OH, NH<sup>2</sup> , R<sup>2</sup> = H, CH<sup>3</sup> , O-alkyl, OH, Cl.)

Smolyar and coworkers [84] reported a novel synthesis of a 1*H-*1,2,3-triazolo[4,5-

hydrazine hydrate (Scheme 28). They reported that after heating for 3–4 h, at 140 °C, the desired pyrazinone was obtained in 86% yield with no chromatography required. 5-Nitropyridin-2(1*H*)-ones fused with benzene and pyridine were also studied in this report.

**Scheme 28.** Cyclotransformation of 1,2,3-triazole-fused lactams **105** to 1,5-dihydro-1,7-dimethyl-1,5- 4*H*-1,2,3-triazolo[4,5-*d*]pyridazin-4-ones **106** in the presence of excess hydrazine hydrate and high

A number of methods exist for the preparation of molecules containing the 1,2,3-triazolo[4,5-*d*]pyridazine core, the majority of which involve the treatment of 1,2,3-triazole dicarbonyl species with hydrazine hydrate followed by acid or heat-promoted cyclization,

N N R1 CH3

(via ring-opening/ring-closing)

Despite being reported as early as 1949 by Schofield and coworkers [85] in their study of cinnolines, 1,2,3-triazolo[1,5-*b*]pyridazines remain rare in the literature, in part owing to few methods available for their synthesis. While synthesizing azepinones, Evans and coworkers [86] instead serendipitously obtained 3,6-diphenyl-1,2,3-triazolo[1,5-*b*]pyridazine **108**. This was obtained from the intramolecular cyclization of diketo-oxime **107**  (Scheme 29) after refluxing in HCl. This gave up to 22% of a pyrazinylhydrazone byproduct. A similar method in the same report used HOAc, but this resulted in poor yields

N

HN <sup>N</sup>

O

heat. (R1 = methyl, ethyl, butyl, cyclohexyl, and (CH2)3NMe2.)

NH2NH2 (20 eq.) 140 °C, 3-5 h

**105 106**

or the cyclization of a diaminopyridazine with nitrite.

*2.4. Syntheses of 1,2,3-Triazolo[1,5-b]pyridazines* 

(about 15%) and up to three products.

<sup>N</sup> <sup>N</sup> R1

<sup>N</sup> <sup>N</sup> R1

O

(i) NH2NH2 (ii) Pb(OAc)4

N

R2

<sup>N</sup> <sup>N</sup> <sup>N</sup>

**115**

R1

H N

H2N

R2 <sup>+</sup> BrMg

EtOH reflux, 3 h

**112**

R2

**109 110**

Smolyar and coworkers [84] reported a novel synthesis of a 1*H-*1,2,3-triazolo[4,5 *d*]pyridaz-4-one, **106** by a ring-opening/ring-closing "cyclotransformation" involving treatment of 1*H-*1,2,3-triazole-fused 5-nitropyridin-2(1*H*)-ones **105** with a large excess of hydrazine hydrate (Scheme 28). They reported that after heating for 3–4 h, at 140 ◦C, the desired pyrazinone was obtained in 86% yield with no chromatography required. 5- Nitropyridin-2(1*H*)-ones fused with benzene and pyridine were also studied in this report. Smolyar and coworkers [84] reported a novel synthesis of a 1*H-*1,2,3-triazolo[4,5 *d*]pyridaz-4-one, **106** by a ring-opening/ring-closing "cyclotransformation" involving treatment of 1*H-*1,2,3-triazole-fused 5-nitropyridin-2(1*H*)-ones **105** with a large excess of hydrazine hydrate (Scheme 28). They reported that after heating for 3–4 h, at 140 °C, the desired pyrazinone was obtained in 86% yield with no chromatography required. 5-Nitropyridin-2(1*H*)-ones fused with benzene and pyridine were also studied in this report.

ing to **104** with nitrite. (R1 = H, O-alkyl, SH, SCH3, OH, NH2, R2 = H, CH3, O-alkyl, OH, Cl.)

**Scheme 27.** Formation of 3,5-dihydro-4*H-*1,2,3-triazolo[4,5-*d*]pyridaz-4-one **102** upon treatment of 4,5-diamino-6-pyridazone **101** with nitrite, and a similar reaction of diaminopyridazine **103** cycliz-

**103 104**

NaNO2 AcOH

NH2

NH2

N

<sup>N</sup> <sup>N</sup>

R2

R1

aq. H2SO4

NaNO2, H2O

**101 102**

N N

HN N O

N H

N N

Ph

*Molecules* **2022**, *27*, x FOR PEER REVIEW 16 of 29

N N

HN N O

**100**

commercially available precursor

Cl

Cl

R1

3 steps

NH2

HN N O

NH2

R2

**Scheme 28.** Cyclotransformation of 1,2,3-triazole-fused lactams **105** to 1,5-dihydro-1,7-dimethyl-1,5- 4*H*-1,2,3-triazolo[4,5-*d*]pyridazin-4-ones **106** in the presence of excess hydrazine hydrate and high heat. (R1 = methyl, ethyl, butyl, cyclohexyl, and (CH2)3NMe2.) **Scheme 28.** Cyclotransformation of 1,2,3-triazole-fused lactams **105** to 1,5-dihydro-1,7-dimethyl-1,5- 4*H*-1,2,3-triazolo[4,5-*d*]pyridazin-4-ones **106** in the presence of excess hydrazine hydrate and high heat. (R<sup>1</sup> = methyl, ethyl, butyl, cyclohexyl, and (CH<sup>2</sup> )3NMe<sup>2</sup> .)

A number of methods exist for the preparation of molecules containing the 1,2,3-triazolo[4,5-*d*]pyridazine core, the majority of which involve the treatment of 1,2,3-triazole dicarbonyl species with hydrazine hydrate followed by acid or heat-promoted cyclization, or the cyclization of a diaminopyridazine with nitrite. A number of methods exist for the preparation of molecules containing the 1,2,3 triazolo[4,5-*d*]pyridazine core, the majority of which involve the treatment of 1,2,3-triazole dicarbonyl species with hydrazine hydrate followed by acid or heat-promoted cyclization, or the cyclization of a diaminopyridazine with nitrite.

#### *2.4. Syntheses of 1,2,3-Triazolo[1,5-b]pyridazines*

*2.4. Syntheses of 1,2,3-Triazolo[1,5-b]pyridazines*  Despite being reported as early as 1949 by Schofield and coworkers [85] in their study of cinnolines, 1,2,3-triazolo[1,5-*b*]pyridazines remain rare in the literature, in part owing to few methods available for their synthesis. While synthesizing azepinones, Evans and coworkers [86] instead serendipitously obtained 3,6-diphenyl-1,2,3-triazolo[1,5-*b*]pyridazine **108**. This was obtained from the intramolecular cyclization of diketo-oxime **107**  (Scheme 29) after refluxing in HCl. This gave up to 22% of a pyrazinylhydrazone byproduct. A similar method in the same report used HOAc, but this resulted in poor yields Despite being reported as early as 1949 by Schofield and coworkers [85] in their study of cinnolines, 1,2,3-triazolo[1,5-*b*]pyridazines remain rare in the literature, in part owing to few methods available for their synthesis. While synthesizing azepinones, Evans and coworkers [86] instead serendipitously obtained 3,6-diphenyl-1,2,3-triazolo[1,5 *b*]pyridazine **108**. This was obtained from the intramolecular cyclization of diketo-oxime **107** (Scheme 29) after refluxing in HCl. This gave up to 22% of a pyrazinylhydrazone byproduct. A similar method in the same report used HOAc, but this resulted in poor yields (about 15%) and up to three products. *Molecules* **2022**, *27*, x FOR PEER REVIEW 17 of 29

**Scheme 29.** Formation of a 1,2,3-triazolo[1,5-*b*]pyridazine **108** after the intramolecular cyclization of oxime **107**. **Scheme 29.** Formation of a 1,2,3-triazolo[1,5-*b*]pyridazine **108** after the intramolecular cyclization of oxime **107**.

A fluoroborate salt was prepared by Riedl and coworkers [87] in a manner similar to that of Beres and coworkers [36]. The acyl-substituted pyridazine, **111**, after treatment with *p*-bromophenyl hydrazine hydrochloride **112** gave the hydrazone **113**. Tribromophenol bromine (TBP) in DCM afforded the desired ring-closed product **114** in 67% yield (Scheme 30). The initial bromide salt was converted to the fluoroborate salt with 40% fluoroboric acid in ACN. Ketone **111** was prepared by the same group via reaction of a commercially available 3-cyanopyridizine **109** with *p*-chlorophenylmagnesium bromide **110**, also synthesized from commercially available *p*-chlorobromobenzene and Mg. This A fluoroborate salt was prepared by Riedl and coworkers [87] in a manner similar to that of Beres and coworkers [36]. The acyl-substituted pyridazine, **111**, after treatment with *p*-bromophenyl hydrazine hydrochloride **112** gave the hydrazone **113**. Tribromophenol bromine (TBP) in DCM afforded the desired ring-closed product **114** in 67% yield (Scheme 30). The initial bromide salt was converted to the fluoroborate salt with 40% fluoroboric acid in ACN. Ketone **111** was prepared by the same group via reaction of a commercially available 3-cyanopyridizine **109** with *p*-chlorophenylmagnesium bromide **110**, also synthesized from commercially available *p*-chlorobromobenzene and Mg. This

was followed by acidic workup to afford the desired ketone. Compounds of this type were also prepared by Vasko and coworkers [88] using a similar method, which gave a 27%

tramolecular oxidative ring closure of a hydrazone derived from **111** to afford the neutral 1,2,3-triazolo[1,5-*b*]pyridazine **115** [89]. Kvaskoff and coworkers employed MnO2 as an oxidant using a similar procedure [35,89,90], where purification by sublimation afforded

**Scheme 30.** Cyclization of hydrazone **113**, derived from acylpyridazine **111**, to afford the 1,2,3-triazolo[1,5-*b*]pyridazinium salt **114**, or 1,2,3-triazolo[1,5-*b*]pyridazine **115**. (R1, R2 = alkyl or aryl.)

Br

(i) TBP, DCM (ii) 40% HBF4, ACN

<sup>N</sup> <sup>N</sup> <sup>N</sup>

BF4

R1

N

Br

R2

the desired product **115** (where R1 = R2 = H) in 71% yield.

precursors

N

HN

**111 113 114**

R2

<sup>N</sup> <sup>N</sup> R1

Br

CN commercially available

oxime **107**.

O

N

OH

O

was followed by acidic workup to afford the desired ketone. Compounds of this type were also prepared by Vasko and coworkers [88] using a similar method, which gave a 27% yield. A third method for the synthesis of 1,2,3-triazolo[1,5-*b*]pyridazines consisted of intramolecular oxidative ring closure of a hydrazone derived from **111** to afford the neutral 1,2,3-triazolo[1,5-*b*]pyridazine **115** [89]. Kvaskoff and coworkers employed MnO<sup>2</sup> as an oxidant using a similar procedure [35,89,90], where purification by sublimation afforded the desired product **115** (where R<sup>1</sup> = R<sup>2</sup> = H) in 71% yield. was followed by acidic workup to afford the desired ketone. Compounds of this type were also prepared by Vasko and coworkers [88] using a similar method, which gave a 27% yield. A third method for the synthesis of 1,2,3-triazolo[1,5-*b*]pyridazines consisted of intramolecular oxidative ring closure of a hydrazone derived from **111** to afford the neutral 1,2,3-triazolo[1,5-*b*]pyridazine **115** [89]. Kvaskoff and coworkers employed MnO2 as an oxidant using a similar procedure [35,89,90], where purification by sublimation afforded the desired product **115** (where R1 = R2 = H) in 71% yield.

**Scheme 29.** Formation of a 1,2,3-triazolo[1,5-*b*]pyridazine **108** after the intramolecular cyclization of

HCl, reflux, 2 h

**107 108**

NH2NH2

A fluoroborate salt was prepared by Riedl and coworkers [87] in a manner similar to that of Beres and coworkers [36]. The acyl-substituted pyridazine, **111**, after treatment with *p*-bromophenyl hydrazine hydrochloride **112** gave the hydrazone **113**. Tribromophenol bromine (TBP) in DCM afforded the desired ring-closed product **114** in 67% yield (Scheme 30). The initial bromide salt was converted to the fluoroborate salt with 40% fluoroboric acid in ACN. Ketone **111** was prepared by the same group via reaction of a commercially available 3-cyanopyridizine **109** with *p*-chlorophenylmagnesium bromide **110**, also synthesized from commercially available *p*-chlorobromobenzene and Mg. This

<sup>N</sup> <sup>N</sup> <sup>N</sup>

N

*2.5. Syntheses of 1,2,3-Triazolo[4,5-c]pyridazines* 

*Molecules* **2022**, *27*, x FOR PEER REVIEW 17 of 29

**Scheme 30.** Cyclization of hydrazone **113**, derived from acylpyridazine **111**, to afford the 1,2,3-triazolo[1,5-*b*]pyridazinium salt **114**, or 1,2,3-triazolo[1,5-*b*]pyridazine **115**. (R1, R2 = alkyl or aryl.) **Scheme 30.** Cyclization of hydrazone **113**, derived from acylpyridazine **111**, to afford the 1,2,3 triazolo[1,5-*b*]pyridazinium salt **114**, or 1,2,3-triazolo[1,5-*b*]pyridazine **115**. (R<sup>1</sup> , R<sup>2</sup> = alkyl or aryl.) More prevalent in the literature than 1,2,3-triazolo[1,5-*b*]pyridazines but still uncom-

#### *2.5. Syntheses of 1,2,3-Triazolo[4,5-c]pyridazines* mon are the 1,2,3-triazolo[4,5-*c*]pyridazines. One of the first reports of such a compound

More prevalent in the literature than 1,2,3-triazolo[1,5-*b*]pyridazines but still uncommon are the 1,2,3-triazolo[4,5-*c*]pyridazines. One of the first reports of such a compound came from Gerhardt and coworkers [91], whereas in previous reports, nitrite was used to cyclize 5-chloro-3,4-diaminopyridazine **116** to afford 7-chloro-3*H*-1,2,3-triazolo[4,5 *c*]pyridazine **117** (Scheme 31) in 83% yield. Nitrite in the presence of an acid catalyst has been used for the synthesis of this heterocyclic ring system from the respective diaminopyridazines in other reports by Murakami [92], Lunt [93], Ramanaiah [74], and Owen [3]. came from Gerhardt and coworkers [91], whereas in previous reports, nitrite was used to cyclize 5-chloro-3,4-diaminopyridazine **116** to afford 7-chloro-3*H*-1,2,3-triazolo[4,5-*c*]pyridazine **117** (Scheme 31) in 83% yield. Nitrite in the presence of an acid catalyst has been used for the synthesis of this heterocyclic ring system from the respective diaminopyridazines in other reports by Murakami [92], Lunt [93], Ramanaiah [74], and Owen [3].

**Scheme 31.** Cyclization of diaminopyridazine **116** to give the triazolo[4,5-*c*]pyridazine **117**. **Scheme 31.** Cyclization of diaminopyridazine **116** to give the triazolo[4,5-*c*]pyridazine **117**.

(as opposed to other cyclizations, which have two amine groups present). For example, 4- (3,4-dimethoxyphenyl)-1-phenyl-1*H*-1,2,3-triazol-5-amine **118** was reacted with sodium nitrite and glacial acetic acid to give the desired 3-(4-chlorophenyl)-7,8-dimethoxy-3*H*- [1,2,3]triazolo[4,5-*c*]cinnoline **119** in 35% yield (Scheme 32). Yields may have been low compared to other nitrite cyclizations due to the formation of a C-N bond directly with a

O

**Scheme 32.** Cyclization of aminotriazole **118** to give triazolo[4,5-*c*]cinnoline **119**.

**118 119**

(i) NaH, DMF, r.t. (ii) PhI(OAc)2 (2 eq.)

24 h

**120 121**

NaNO2, AcOH

15 min

Daniel and coworkers [22] formed tricyclic ylides **121** in 65% yield by oxidative cy-

<sup>N</sup> <sup>N</sup> <sup>N</sup>

<sup>N</sup> <sup>N</sup> <sup>N</sup>

F3C N

N

O

N

Cl

N

clization of the respective *ortho*-substituted amino pyridazine **120** (Scheme 33). Unfortunately, compounds containing the 1,2,3-triazolo[4,5-*c*]pyridazine nucleus remain rare in

the literature, and little is known of their biological or pharmacological properties.

**Scheme 33.** Cyclization of aminotriazole **120** into 1,2,3-triazolo[4,5-*c*]pyridazinium ylide **121**.

carbon of an aromatic ring.

O

<sup>N</sup> <sup>N</sup>

F3C <sup>N</sup>

N

NH2

O

H2N <sup>N</sup>

N

Cl

N

In a report by Pokhodylo and coworkers [94], nitrite was used in the synthesis of a substituted 1,2,3-triazolo[4,5-*c*]pyridazine despite only having one amine group present (as opposed to other cyclizations, which have two amine groups present). For example, 4-(3,4-dimethoxyphenyl)-1-phenyl-1*H*-1,2,3-triazol-5-amine **118** was reacted with sodium nitrite and glacial acetic acid to give the desired 3-(4-chlorophenyl)-7,8-dimethoxy-3*H*- [1,2,3]triazolo[4,5-*c*]cinnoline **119** in 35% yield (Scheme 32). Yields may have been low compared to other nitrite cyclizations due to the formation of a C-N bond directly with a carbon of an aromatic ring. substituted 1,2,3-triazolo[4,5-*c*]pyridazine despite only having one amine group present (as opposed to other cyclizations, which have two amine groups present). For example, 4- (3,4-dimethoxyphenyl)-1-phenyl-1*H*-1,2,3-triazol-5-amine **118** was reacted with sodium nitrite and glacial acetic acid to give the desired 3-(4-chlorophenyl)-7,8-dimethoxy-3*H*- [1,2,3]triazolo[4,5-*c*]cinnoline **119** in 35% yield (Scheme 32). Yields may have been low compared to other nitrite cyclizations due to the formation of a C-N bond directly with a carbon of an aromatic ring. (3,4-dimethoxyphenyl)-1-phenyl-1*H*-1,2,3-triazol-5-amine **118** was reacted with sodium nitrite and glacial acetic acid to give the desired 3-(4-chlorophenyl)-7,8-dimethoxy-3*H*- [1,2,3]triazolo[4,5-*c*]cinnoline **119** in 35% yield (Scheme 32). Yields may have been low compared to other nitrite cyclizations due to the formation of a C-N bond directly with a carbon of an aromatic ring. O O

**Scheme 31.** Cyclization of diaminopyridazine **116** to give the triazolo[4,5-*c*]pyridazine **117**.

N H

N N

N H

N N

**Scheme 31.** Cyclization of diaminopyridazine **116** to give the triazolo[4,5-*c*]pyridazine **117**.

<sup>N</sup> <sup>N</sup>

Cl

Cl

<sup>N</sup> <sup>N</sup>

In a report by Pokhodylo and coworkers [94], nitrite was used in the synthesis of a

In a report by Pokhodylo and coworkers [94], nitrite was used in the synthesis of a substituted 1,2,3-triazolo[4,5-*c*]pyridazine despite only having one amine group present (as opposed to other cyclizations, which have two amine groups present). For example, 4-

*Molecules* **2022**, *27*, x FOR PEER REVIEW 18 of 29

*Molecules* **2022**, *27*, x FOR PEER REVIEW 18 of 29

More prevalent in the literature than 1,2,3-triazolo[1,5-*b*]pyridazines but still uncommon are the 1,2,3-triazolo[4,5-*c*]pyridazines. One of the first reports of such a compound came from Gerhardt and coworkers [91], whereas in previous reports, nitrite was used to cyclize 5-chloro-3,4-diaminopyridazine **116** to afford 7-chloro-3*H*-1,2,3-triazolo[4,5-*c*]pyridazine **117** (Scheme 31) in 83% yield. Nitrite in the presence of an acid catalyst has been used for the synthesis of this heterocyclic ring system from the respective diaminopyridazines in other reports by Murakami [92], Lunt [93], Ramanaiah [74], and Owen [3].

More prevalent in the literature than 1,2,3-triazolo[1,5-*b*]pyridazines but still uncommon are the 1,2,3-triazolo[4,5-*c*]pyridazines. One of the first reports of such a compound came from Gerhardt and coworkers [91], whereas in previous reports, nitrite was used to cyclize 5-chloro-3,4-diaminopyridazine **116** to afford 7-chloro-3*H*-1,2,3-triazolo[4,5-*c*]pyridazine **117** (Scheme 31) in 83% yield. Nitrite in the presence of an acid catalyst has been used for the synthesis of this heterocyclic ring system from the respective diaminopyridazines in other reports by Murakami [92], Lunt [93], Ramanaiah [74], and Owen [3].

*2.5. Syntheses of 1,2,3-Triazolo[4,5-c]pyridazines* 

*2.5. Syntheses of 1,2,3-Triazolo[4,5-c]pyridazines* 

aq. H2SO4 NaNO2

**116 117**

aq. H2SO4

NaNO2

**116 117**

<sup>N</sup> <sup>N</sup>

<sup>N</sup> <sup>N</sup>

Cl

Cl

NH2

NH2

NH2

NH2

**Scheme 32.** Cyclization of aminotriazole **118** to give triazolo[4,5-*c*]cinnoline **119**. **Scheme 32.** Cyclization of aminotriazole **118** to give triazolo[4,5-*c*]cinnoline **119**. Daniel and coworkers [22] formed tricyclic ylides **121** in 65% yield by oxidative cy-

Daniel and coworkers [22] formed tricyclic ylides **121** in 65% yield by oxidative cyclization of the respective *ortho*-substituted amino pyridazine **120** (Scheme 33). Unfortunately, compounds containing the 1,2,3-triazolo[4,5-*c*]pyridazine nucleus remain rare in Daniel and coworkers [22] formed tricyclic ylides **121** in 65% yield by oxidative cyclization of the respective *ortho*-substituted amino pyridazine **120** (Scheme 33). Unfortunately, compounds containing the 1,2,3-triazolo[4,5-*c*]pyridazine nucleus remain rare in the literature, and little is known of their biological or pharmacological properties. clization of the respective *ortho*-substituted amino pyridazine **120** (Scheme 33). Unfortunately, compounds containing the 1,2,3-triazolo[4,5-*c*]pyridazine nucleus remain rare in the literature, and little is known of their biological or pharmacological properties.

**120 121 Scheme 33.** Cyclization of aminotriazole **120** into 1,2,3-triazolo[4,5-*c*]pyridazinium ylide **121**. **Scheme 33.** Cyclization of aminotriazole **120** into 1,2,3-triazolo[4,5-*c*]pyridazinium ylide **121**.

#### **Scheme 33.** Cyclization of aminotriazole **120** into 1,2,3-triazolo[4,5-*c*]pyridazinium ylide **121**. **3. Applications**

Recent applications of the aforementioned heterocyclic systems, covering both medicinal and non-medicinal topics, are discussed in the following section.

### *3.1. Applications of 1H-1,2,3-Triazolo[4,5-b]pyrazines*

In the last decade, 1*H*-1,2,3-triazolo[4,5-*b*]pyrazines have garnered an interest within the field of medicinal chemistry for serving as the scaffold of selective c-Met inhibitors. Medicinal studies of 1*H*-1,2,3-triazolo[4,5-*b*]pyrazines have extended well into the patent literature, with one patent even exploring antiviral efficacy against SARS-CoV-2 [95]. The first notable report of physiological activity came from Cui and coworkers [2], who reported the discovery of PF-04217903, a 1,2,3-triazolo[4,5-*b*]pyrazine that demonstrated potent (IC<sup>50</sup> = 0.005 µM) and selective inhibition of over 200 c-Met kinases [2]. This heterocyclic scaffold in general gave rise to derivatives (altering substituents at the 2 and 6 ring positions) with potent inhibition, of which PF-04217903 was the best. This compound was selected as a preclinical candidate for the treatment of cancer [96].

**3. Applications** 

Later, using PF-04217903 as a reference, Jia, and coworkers [1] reported the discovery of a compound now known as Savolitinib (Figure 3). This compound, also an exquisite c-Met inhibitor with an equal IC<sup>50</sup> of 0.005 µM, demonstrated favorable pharmacokinetic properties in mice [1]. Savolitinib possessed equal potency. Having recently passed phase II clinical trials for the treatment of metastatic non-small cell lung cancer, papillary and clear cell renal cell carcinoma, gastric cancer, and colorectal cancer, Savolitinib has been granted conditional approval for use in China at the time of this review [97]. A review of c-Met inhibitors in non-small cell lung cancer has recently appeared [98]. Met inhibitor with an equal IC50 of 0.005 µM, demonstrated favorable pharmacokinetic properties in mice [1]. Savolitinib possessed equal potency. Having recently passed phase II clinical trials for the treatment of metastatic non-small cell lung cancer, papillary and clear cell renal cell carcinoma, gastric cancer, and colorectal cancer, Savolitinib has been granted conditional approval for use in China at the time of this review [97]. A review of c-Met inhibitors in non-small cell lung cancer has recently appeared [98].

Recent applications of the aforementioned heterocyclic systems, covering both me-

In the last decade, 1*H*-1,2,3-triazolo[4,5-*b*]pyrazines have garnered an interest within

Later, using PF-04217903 as a reference, Jia, and coworkers [1] reported the discovery

of a compound now known as Savolitinib (Figure 3). This compound, also an exquisite c-

the field of medicinal chemistry for serving as the scaffold of selective c-Met inhibitors. Medicinal studies of 1*H*-1,2,3-triazolo[4,5-*b*]pyrazines have extended well into the patent literature, with one patent even exploring antiviral efficacy against SARS-CoV-2 [95]. The first notable report of physiological activity came from Cui and coworkers [2], who reported the discovery of PF-04217903, a 1,2,3-triazolo[4,5-*b*]pyrazine that demonstrated potent (IC50 = 0.005 µM) and selective inhibition of over 200 c-Met kinases [2]. This heterocyclic scaffold in general gave rise to derivatives (altering substituents at the 2 and 6 ring positions) with potent inhibition, of which PF-04217903 was the best. This compound was

c-Met Ki = 0.004 µM c-Met cell IC50 = 0.005 µM

dye.

O2N

N

N N N

N

O2N

zinotriazapentalene scaffold [20].

Probes exhibiting fluorescence following nitroreductase reduction

*3.2. Applications of 1H-1,2,3-triazolo[4,5-c]pyridazines* 

N

N N N

N

*Molecules* **2022**, *27*, x FOR PEER REVIEW 19 of 29

*3.1. Applications of 1H-1,2,3-triazolo[4,5-b]pyrazines* 

dicinal and non-medicinal topics, are discussed in the following section.

c-Met cell IC50 = 0.005 µM

**Figure 3.** Two potent and selective c-Met inhibitors containing the 1,2,3-triazolo[4,5-*b*]pyrazine core: PF-04217903 and Savolitinib. **Figure 3.** Two potent and selective c-Met inhibitors containing the 1,2,3-triazolo[4,5-*b*]pyrazine core: PF-04217903 and Savolitinib.

Sirbu and coworkers [20] recently reported a novel class of small molecules containing the 1,2,3-triazolo[4,5-*b*]pyrazine scaffold with excellent properties for use as versatile fluorescent probes in optical imaging (Figure 4). Specifically, a phenyl ester derivative was used to dye HeLa cells in epifluorescence microscopy. Compared to commercially available LysoTracker Green DND-26, the tested triazolopyrazine derivative demonstrated comparable properties. In addition, it showed low cytotoxicity when evaluated in Alamar Blue assay (>95% cell viability up to 170 µM) and showed high solubility with a variety of Sirbu and coworkers [20] recently reported a novel class of small molecules containing the 1,2,3-triazolo[4,5-*b*]pyrazine scaffold with excellent properties for use as versatile fluorescent probes in optical imaging (Figure 4). Specifically, a phenyl ester derivative was used to dye HeLa cells in epifluorescence microscopy. Compared to commercially available LysoTracker Green DND-26, the tested triazolopyrazine derivative demonstrated comparable properties. In addition, it showed low cytotoxicity when evaluated in Alamar Blue assay (>95% cell viability up to 170 µM) and showed high solubility with a variety of desirable characteristics. A phenyl ester derivative, when evaluated as a dye in HeLa cells, showed high photostability and low cytotoxicity [20]. *Molecules* **2022**, *27*, x FOR PEER REVIEW 20 of 29

**Figure 4.** A novel class of small molecule fluorescent probes developed by Sirbu and coworkers [20] for use in optical and/or cellular imaging. **Figure 4.** A novel class of small molecule fluorescent probes developed by Sirbu and coworkers [20] for use in optical and/or cellular imaging.

Intriguingly, another application lay in the monitoring of hypoxic regions within tumor cells. This was explored by Janczy-Cempa and coworkers [23], who looked at the

had very weak fluorescence in normoxic regions, but their reduction by nitroreductases led to a 15-fold increase in intensity in hypoxic regions. This was evaluated using the human melanoma cell line A2058. In contrast to the fluorescence probes developed by Sirbu and coworkers [20], probes in this study had substitutions on the pyrazine ring as opposed to the triazole-fused pyrazole. While additional work is still to be done, this report demonstrates the potential for these highly conjugated compounds to be useful in biomedical monitoring. Legentil and coworkers [99] obtained compounds similar to the structure on the right in Figure 5 in yields as high as 79%, which were used to develop a luminescence layered double-hydroxide filter. This material was dispersed into a polymer for use as a

**Figure 5.** Compounds containing 1,2,3-triazolo[4,5-*b*]pyrazines based on the nitro-pyra-

demonstrating a wide range of favorable characteristics as fluorescent probes.

Overall, applications of compounds containing 1,2,3-triazolo[4,5-*b*]pyrazines in the current literature are focused on c-Met inhibition (i.e., the treatment of distinct types of cancers), and optical and/or cellular imaging, with triazapentalene-type molecules

HO2C

N

Demonstrated photoluminescence properties

N N N

N

After being initially evaluated by Gerhardt and coworkers [91] as potential purine antagonists, 1*H*-1,2,3-triazolo[4,5-*c*]pyridazines have since found broader interest within medicinal chemistry. In a report by Owen and coworkers [3], a 1*H*-1,2,3-triazolo[4,5-*c*]pyridazine was found to have GABAA modulating activity during a structure–activity relationship study of the respective imidazolopyridazine. Compounds containing the 1,2,3 triazolo[4,5-*c*]pyridazine scaffold have been investigated in the patent literature for the

Intriguingly, another application lay in the monitoring of hypoxic regions within tumor cells. This was explored by Janczy-Cempa and coworkers [23], who looked at the fluorescent products produced after reduction of nitrotriazolopyrazine probes by nitroreductases (enzymes often overexpressed in tumor regions). Both probes studied (Figure 5) had very weak fluorescence in normoxic regions, but their reduction by nitroreductases led to a 15-fold increase in intensity in hypoxic regions. This was evaluated using the human melanoma cell line A2058. In contrast to the fluorescence probes developed by Sirbu and coworkers [20], probes in this study had substitutions on the pyrazine ring as opposed to the triazole-fused pyrazole. While additional work is still to be done, this report demonstrates the potential for these highly conjugated compounds to be useful in biomedical monitoring. Legentil and coworkers [99] obtained compounds similar to the structure on the right in Figure 5 in yields as high as 79%, which were used to develop a luminescence layered double-hydroxide filter. This material was dispersed into a polymer for use as a dye. mor cells. This was explored by Janczy-Cempa and coworkers [23], who looked at the fluorescent products produced after reduction of nitrotriazolopyrazine probes by nitroreductases (enzymes often overexpressed in tumor regions). Both probes studied (Figure 5) had very weak fluorescence in normoxic regions, but their reduction by nitroreductases led to a 15-fold increase in intensity in hypoxic regions. This was evaluated using the human melanoma cell line A2058. In contrast to the fluorescence probes developed by Sirbu and coworkers [20], probes in this study had substitutions on the pyrazine ring as opposed to the triazole-fused pyrazole. While additional work is still to be done, this report demonstrates the potential for these highly conjugated compounds to be useful in biomedical monitoring. Legentil and coworkers [99] obtained compounds similar to the structure on the right in Figure 5 in yields as high as 79%, which were used to develop a luminescence layered double-hydroxide filter. This material was dispersed into a polymer for use as a dye.

**Figure 4.** A novel class of small molecule fluorescent probes developed by Sirbu and coworkers [20]

N N N

O

O

N

N

Intriguingly, another application lay in the monitoring of hypoxic regions within tu-

>95% cell viability up to 170 µM, high photostability, low cytotoxicity

*Molecules* **2022**, *27*, x FOR PEER REVIEW 20 of 29

for use in optical and/or cellular imaging.

Ar

N

N N N

Substructure

N

Probes exhibiting fluorescence following nitroreductase reduction Demonstrated photoluminescence properties

**Figure 5.** Compounds containing 1,2,3-triazolo[4,5-*b*]pyrazines based on the nitro-pyrazinotriazapentalene scaffold [20]. **Figure 5.** Compounds containing 1,2,3-triazolo[4,5-*b*]pyrazines based on the nitro-pyrazinotriazapentalene scaffold [20].

Overall, applications of compounds containing 1,2,3-triazolo[4,5-*b*]pyrazines in the current literature are focused on c-Met inhibition (i.e., the treatment of distinct types of cancers), and optical and/or cellular imaging, with triazapentalene-type molecules demonstrating a wide range of favorable characteristics as fluorescent probes. Overall, applications of compounds containing 1,2,3-triazolo[4,5-*b*]pyrazines in the current literature are focused on c-Met inhibition (i.e., the treatment of distinct types of cancers), and optical and/or cellular imaging, with triazapentalene-type molecules demonstrating a wide range of favorable characteristics as fluorescent probes.

#### *3.2. Applications of 1H-1,2,3-Triazolo[4,5-c]pyridazines*

*3.2. Applications of 1H-1,2,3-triazolo[4,5-c]pyridazines*  After being initially evaluated by Gerhardt and coworkers [91] as potential purine antagonists, 1*H*-1,2,3-triazolo[4,5-*c*]pyridazines have since found broader interest within medicinal chemistry. In a report by Owen and coworkers [3], a 1*H*-1,2,3-triazolo[4,5-*c*]pyridazine was found to have GABAA modulating activity during a structure–activity relationship study of the respective imidazolopyridazine. Compounds containing the 1,2,3 triazolo[4,5-*c*]pyridazine scaffold have been investigated in the patent literature for the After being initially evaluated by Gerhardt and coworkers [91] as potential purine antagonists, 1*H*-1,2,3-triazolo[4,5-*c*]pyridazines have since found broader interest within medicinal chemistry. In a report by Owen and coworkers [3], a 1*H*-1,2,3-triazolo[4,5 *c*]pyridazine was found to have GABA<sup>A</sup> modulating activity during a structure–activity relationship study of the respective imidazolopyridazine. Compounds containing the 1,2,3-triazolo[4,5-*c*]pyridazine scaffold have been investigated in the patent literature for the treatment of Huntington's disease [100] and as modulators of Janus-family kinaserelated diseases [101].

Other recent patents have been filed regarding fused pyridazines with herbicidal activity, of which 1,2,3-triazolo[4,5-*c*]pyridazine is included [102]. In another recent patent, compounds of this type were implicated in controlling unwanted plant growth [103].

Reports of compounds containing the 1,2,3-triazolo[4,5-*c*]pyridazine scaffold are uncommon in the current literature beyond synthetic reports and patents. Undoubtedly, there is still work to be done in exploring the potential applications of this unique heterocyclic system.

#### *3.3. Applications of 1H-1,2,3-Triazolo[4,5-d]pyridazines 3.3. Applications of 1H-1,2,3-triazolo[4,5-d]pyridazines*

diseases [101].

cyclic system.

*Molecules* **2022**, *27*, x FOR PEER REVIEW 21 of 29

In a recent development, Li, and coworkers [4] outlined a series of triazole-based structures for the construction of conjugated polymers for solar cells. In addition to demonstrating desirable properties as units incorporated into polymers (Figure 6), their reported synthetic route uses affordable, commercially available starting materials and produces units compatible with other monomers. Structures containing 1,2,3-triazolo[4,5-*d*]pyridazine components offer a privileged, conjugated unit for the construction of polymers owing in part to the convenient para substitution of the pyridazine ring and perpendicular N2 substitution of the triazole ring. In a recent development, Li, and coworkers [4] outlined a series of triazole-based structures for the construction of conjugated polymers for solar cells. In addition to demonstrating desirable properties as units incorporated into polymers (Figure 6), their reported synthetic route uses affordable, commercially available starting materials and produces units compatible with other monomers. Structures containing 1,2,3-triazolo[4,5 *d*]pyridazine components offer a privileged, conjugated unit for the construction of polymers owing in part to the convenient para substitution of the pyridazine ring and perpendicular N2 substitution of the triazole ring.

treatment of Huntington's disease [100] and as modulators of Janus-family kinase-related

Other recent patents have been filed regarding fused pyridazines with herbicidal activity, of which 1,2,3-triazolo[4,5-*c*]pyridazine is included [102]. In another recent patent, compounds of this type were implicated in controlling unwanted plant growth [103].

Reports of compounds containing the 1,2,3-triazolo[4,5-*c*]pyridazine scaffold are uncommon in the current literature beyond synthetic reports and patents. Undoubtedly, there is still work to be done in exploring the potential applications of this unique hetero-

**Figure 6.** Structures containing the 1,2,3-triazolo[4,5-*d*]pyridazine-based monomer, *m*-TAZ, used to construct highly conjugated TAZ-based polymers. **Figure 6.** Structures containing the 1,2,3-triazolo[4,5-*d*]pyridazine-based monomer, *m*-TAZ, used to construct highly conjugated TAZ-based polymers.

Another notable outcome of the study of 1,2,3-triazolo[4,5-*d*]pyridazines was that from Biagi and coworkers [104], who reported compounds of this type with high selectivity for the A1 receptor subtype in radioligand binding assays at bovine brain adenosine A1 and A2A receptors. The most potent compound contained a 4-amino-substituted 7-hydroxy-1,2,3-triazolo[4,5-*d*]pyridazine, and after substitution of the hydroxyl group for a chlorine, affinity decreased and suggested a hydrogen-bond donating substituent at position 7 was critical for binding affinity. Another notable outcome of the study of 1,2,3-triazolo[4,5-*d*]pyridazines was that from Biagi and coworkers [104], who reported compounds of this type with high selectivity for the A<sup>1</sup> receptor subtype in radioligand binding assays at bovine brain adenosine A<sup>1</sup> and A2A receptors. The most potent compound contained a 4-amino-substituted 7-hydroxy-1,2,3-triazolo[4,5-*d*]pyridazine, and after substitution of the hydroxyl group for a chlorine, affinity decreased and suggested a hydrogen-bond donating substituent at position 7 was critical for binding affinity.

#### *3.4. Applications of 1,2,3-Triazolo[1,5-a]pyrazines 3.4. Applications of 1,2,3-Triazolo[1,5-a]pyrazines*

Among applications of compounds containing the 1,2,3-triazolo[1,5-*a*]pyrazine unit are those of benzo-fused 1,2,3-triazoloquinoxalines and saturated 1,2,3-triazole-fused piperidines. In a recent report by Pérez Morales and coworkers [105], a 1,2,3- Among applications of compounds containing the 1,2,3-triazolo[1,5-*a*]pyrazine unit arethose of benzo-fused 1,2,3-triazoloquinoxalines and saturated 1,2,3-triazole-fused piperidines. In a recent report by Pérez Morales and coworkers [105], a 1,2,3-triazoloquinoxalinone (Structure A, Figure 7) was identified via high-throughput screening as inducing expression of Rgg2/3-regulated genes in the presence of short hydrophobic pheromones at low concentrations. This work stemmed from interest in the Rgg2/3 quorum sensing circuit of the pathogen *Streptococcus pyogenes*, with the objective of manipulating and inhibiting the bacteria. After analyzing its mode of action, it was determined this compound directly uncompetitively inhibited recombinant PepO in vitro, and induced quorum sensing signaling by stabilizing short hydrophobic pheromones.

triazoloquinoxalinone (Structure A, Figure 7) was identified via high-throughput screening as inducing expression of Rgg2/3-regulated genes in the presence of short hydrophobic pheromones at low concentrations. This work stemmed from interest in the Rgg2/3 quorum sensing circuit of the pathogen *Streptococcus pyogenes*, with the objective of manipulating and inhibiting the bacteria. After analyzing its mode of action, it was determined this compound directly uncompetitively inhibited recombinant PepO in vitro, and

induced quorum sensing signaling by stabilizing short hydrophobic pheromones.

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}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}

**Figure 7.** Compounds containing congeners of the 1,2,3-triazolo[1,5-*a*]pyrazine core with a diverse set of biological activities: (**A**) an inducer of Rgg2/3-related genes of the human pathogen *Streptococcus pyogenes* [105]*,* (**B**) a potent DPP-IV inhibitor evaluated for the treatment of type II diabetes [7], and (**C**), an identified BACE-1 inhibitor [6]. **Figure 7.** Compounds containing congeners of the 1,2,3-triazolo[1,5-*a*]pyrazine core with a diverse set of biological activities: (**A**) an inducer of Rgg2/3-related genes of the human pathogen *Streptococcus pyogenes* [105]*,* (**B**) a potent DPP-IV inhibitor evaluated for the treatment of type II diabetes [7], and (**C**), an identified BACE-1 inhibitor [6].

Based on the antidiabetic 1,2,4-triazolopiperazine-containing drug Sitagliptin (brand name Januvia), Shan and coworkers [7] identified a dipeptidyl peptidase (DPP) IV inhibitor containing a 1,2,3-triazolopiperazine (Structure B, Figure 7) for use in the treatment of type II diabetes. Based on the antidiabetic 1,2,4-triazolopiperazine-containing drug Sitagliptin (brand name Januvia), Shan and coworkers [7] identified a dipeptidyl peptidase (DPP) IV inhibitor containing a 1,2,3-triazolopiperazine (Structure B, Figure 7) for use in the treatment of type II diabetes.

Partially saturated 1,2,3-triazolo[1,5-*a*]pyrazines have demonstrated BACE-1 inhibition, an enzyme implicated in the formation of amyloid beta in Alzheimer's disease. Oehlrich and coworkers [6] identified one such candidate, (*R*)-*N*-(3-(4-amino-6-methyl-6,7-dihydro-[1,2,3]triazolo[1,5-*a*]pyrazin-6-yl)-4-fluorophenyl)-5-cyanopicolinamide, (Structure C, Figure 7). This demonstrated an inhibition of the BACE-1 enzyme of pIC50 = Partially saturated 1,2,3-triazolo[1,5-*a*]pyrazines have demonstrated BACE-1 inhibition, an enzyme implicated in the formation of amyloid beta in Alzheimer's disease. Oehlrich and coworkers [6] identified one such candidate, (*R*)-*N*-(3-(4-amino-6-methyl-6,7 dihydro-[1,2,3]triazolo[1,5-*a*]pyrazin-6-yl)-4-fluorophenyl)-5-cyanopicolinamide, (Structure C, Figure 7). This demonstrated an inhibition of the BACE-1 enzyme of pIC<sup>50</sup> = 8.70.

8.70. These reports, while not exhaustive, demonstrate recent applications of compounds containing the 1,2,3-triazolo[1,5-*a*]pyrazine scaffold or congeners thereof. Particularly These reports, while not exhaustive, demonstrate recent applications of compounds containing the 1,2,3-triazolo[1,5-*a*]pyrazine scaffold or congeners thereof. Particularly prominent in the literature are benzo-fused and piperazine-containing analogs.

#### prominent in the literature are benzo-fused and piperazine-containing analogs. *3.5. Applications of 1,2,3-Triazolo[1,5-b]pyridazines*

There are no reported applications of compounds containing the 1,2,3-triazolo[1,5 *b*]pyridazine ring system, and little regarding its physiological and/or pharmacological effects are known. Aside from one recent patent [106] regarding immunoregulatory functions, additional applications remain scarce at the time of this review.

#### **4. Conclusions**

In reviewing synthetic approaches to and reported applications of members of the 1,2,3 triazolodiazine family of fused bicyclic heterocycles, the following conclusions can be drawn regarding the most common synthetic methods and applications in the present literature:


The potential for new synthetic contributions is considerable for the triazole-fused pyrazines and pyridazines. Given the diversity of synthetic methods summarized in this review, new contributions that could be most beneficial are new routes to some of the precursors of the fused systems. In many of the reports cited, the starting materials are either not available commercially or are very expensive. For example, some diamino pyrazines are available as unsubstituted compounds or as halogenated derivatives, but all are USD 500–1000 per gram. Future studies of methods employing additional intramolecular cycloadditions leading to 1,2,3-triazolo[1,5-*a*]pyrazine derivatives would appear to

have potential. Work on synthesis of the 1*H*-1,2,3-triazolo[4,5-*c*]pyridazines and the 1,2,3 triazolo[1,5-*b*]pyridazines would be welcome for these less frequently studied areas.

Overall, diverse methods exist for the preparation of 1,2,3-triazole-fused diazines, spanning the last seven decades with numerous reports in the last five years. Currently, drugs containing these ring systems remain scarce with only a handful of exceptions, particularly containing either the 1,2,3-triazolo[4,5-*b*]pyrazine or 1,2,3-triazolo[4,5-*c*]pyridazine scaffold. Applications of the aforementioned types of compounds span from medicinal chemistry into the development of dyes, probes, and inhibitors of enzymes implicated in various diseases. Despite this, there lies underrealized and exciting potential for employing triazolopyrazines and triazolopyridazines as diverse substrates in the generation of novel molecules with a wide array of applications.

**Author Contributions:** Conceptualization: G.R.H., A.M.S. (these authors contributed equally); writing—original draft preparation: G.R.H.; preparation of graphics: G.R.H.; validation: A.M.S.; writing—review and editing: G.R.H., A.M.S. (these authors contributed equally). All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** G.R.H. would like to acknowledge his grandparents, Bob and Shirley Hoffman, for their support and encouragement throughout the research and writing process.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of compounds are not available from the authors.

#### **References**

