*Review* **The Synthesis and Biological Applications of the 1,2,3-Dithiazole Scaffold**

**Andreas S. Kalogirou <sup>1</sup> , Hans J. Oh <sup>2</sup> and Christopher R. M. Asquith 2,3,\***


**Abstract:** The 1,2,3-dithiazole is an underappreciated scaffold in medicinal chemistry despite possessing a wide variety of nascent pharmacological activities. The scaffold has a potential wealth of opportunities within these activities and further afield. The 1,2,3-dithiazole scaffold has already been reported as an antifungal, herbicide, antibacterial, anticancer agent, antiviral, antifibrotic, and is a melanin and Arabidopsis gibberellin 2-oxidase inhibitor. These structure activity relationships are discussed in detail, along with insights and future directions. The review also highlights selected synthetic strategies developed towards the 1,2,3-dithiazole scaffold, how these are integrated to accessibility of chemical space, and to the prism of current and future biological activities.

**Keywords:** antibacterial; anticancer; antifibrotic; antifungal; antimicrobial; antiviral; appel salt; 1,2,3-dithiazole; disulfide bridge; herbicidal

### **1. Introduction**

The 1,2,3-dithiazole core is a five membered heterocycle containing two sulfur atoms and one nitrogen atom. Despite the fact that the 1,2,3-dithiazole is not present in nature, similar to many other heterocycles, it does have a broad range of interesting biological activities. The 1,2,3-dithiazole moiety was first synthesized in 1957 by G. Schindler et al. [1]. This was followed two decades later by a report by J. E. Moore on behalf of Chevron Research Co. (San Ramon, CA, USA) where it showcased antifungal and herbicidal activity [2,3]. In 1985, Appel et al. reported the synthesis of 4,5-dichloro-1,2,3-dithiazolium chloride **1** (Appel's salt), a precursor which allowed access to the 1,2,3-dithiazole core within a single step [4,5].

The synthesis of Appel salt **1** acted as a catalyst to the field and granted access to many 1,2,3-dithiazole derivatives, and to other heterocycles incorporating sulfur and nitrogen atoms [6–11]. The subsequent synthetic reports focused on transformations on the C5 position [6–11]. However, one of the key synthetic interests beyond expanding the scope of 5-substituted 1,2,3-dithiazoles was the limited reactivity of the C4 position. Several different approaches were used to address this C4 reactively issue, including intramolecular cyclization [6] using a multi-step oxime pathway [12,13], or more recently, direct reactions [14], all of which expanded the chemical space around the 1,2,3-dithiazole. Some of these approaches have been covered in past reviews around the chemistry of 1,2,3-dithiazoles [6–11] (Figure 1).

**Figure 1.** Appel salt (**1**) and other general 1,2,3-dithazoles structures **2**–**6**.

**Citation:** Kalogirou, A.S.; Oh, H.J.; Asquith, C.R.M. The Synthesis and Biological Applications of the 1,2,3-Dithiazole Scaffold. *Molecules* **2023**, *28*, 3193. https://doi.org/ 10.3390/molecules28073193

Academic Editors: Joseph Sloop and Bartolo Gabriele

Received: 7 March 2023 Revised: 28 March 2023 Accepted: 31 March 2023 Published: 3 April 2023

**Copyright:** © 2023 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/).

Despite the remaining synthetic challenges, the 1,2,3-dithiazole scaffold has already been reported as an antifungal [2], herbicide [2], antibacterial [15], anticancer agent [16], antiviral [17], antifibrotic [18], and as a melanin [19] and Arabidopsis gibberellin 2-oxidase [20] inhibitor. While there is a wide range of existing biology, there are a wealth of opportunities for expansion, including broader application toward cystine reactive sites [21–25]. In this review, we are primarily focused on the impact of: (1) The chemistry limiting the chemical space, and hence, limiting the biology; (2) The chemistry impacting the biology observed; and (3) How chemistry could be applied to new biology. The chemistry, biology, structure activity relationships, and future directions of research in 1,2,3-dithiazoles are all outlined below.

#### **2. 1,2,3-Dithiazoles Synthesis Overview**

### *2.1. Early Years before Appel Salt*

Early work on the synthesis of 1,2,3-dithiazoles used cyanothioformamides as starting materials. Treatment of a variety of arylcyanothioformamides **7** with sulfur dichloride at 0–25 ◦C gave a number of *N*-aryl-5*H*-1,2,3-dithiazol-5-imines **4** (Scheme 1) [2]. The initial reaction yielded the corresponding hydrochloride salts, which could be converted to the free base by refluxing in a toluene solution.

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**Scheme 1.** Synthesis of *N*-aryl-5*H*-1,2,3-dithiazol-5-imines **4** from arylcyanothioformamides **7**.

Interestingly, the *N*-aryl-5*H*-1,2,3-dithiazol-5-imines **4** can be degraded to the respective cyanothioformamides **7** by thiophilic ring cleavage after reaction with triphenylphosphine or sodium hydroxide [4,26], oxidative ring cleavage after reaction with *m*-CPBA [27], or by reductive ring cleavage after reaction with sodium cyanoborohydride [28] (Scheme 2).

**Scheme 2.** Degradation of *N*-aryl-5*H*-1,2,3-dithiazol-5-imines **4** to cyanothioformamides **7**.

#### *2.2. Discovery of Appel Salt and Applications*

A significant discovery in the chemistry of 1,2,3-dithiazoles was the synthesis of 4,5-dichloro-1,2,3-dithiazolium chloride **1** (Appel salt) by Appel et al. in 1985, which was readily prepared from chloroacetonitrile and disulfur dichloride [4,29] (Scheme 3). Appel salt **1** was subsequently used as an important reagent for the preparation of other 4-chloro-5*H*-1,2,3-dithiazoles with the most reactive site being the electrophilic C-5 position [4,9,27].

**Scheme 3.** Synthesis of Appel salt and transformation to 4-chloro-5*H*-1,2,3-dithiazolylidenes **3**.

Appel salt **1** can condense with active methylenes, such as acetonitrile derivatives [4,30,31], diketones, ketoesters, and others [32], to give 4-chloro-5*H*-1,2,3-dithiazolylidenes **3** (Scheme 3).

The condensation of Appel salt with hydrogen sulfide [4] afforded dithiazole-5-thione **6** in 69% yield (Scheme 4). The reaction with oxygen nucleophiles are also common with NaNO<sup>3</sup> [4], sulfoxides [33], or formic acid [34] all acting as the source of oxygen to give 4-chloro-5*H*-1,2,3-dithiazol-5-one **5** in good yields (Scheme 4). Furthermore, the reaction with other carboxylic acids [35] at −78 ◦C and subsequent treatment with alcohols gave esters **8** in medium to good yields (Scheme 4).

**Scheme 4.** Reactions of Appel salt **1** with oxygen and sulfur nucleophiles.

The condensation of Appel salt **1** with primary anilines is well studied [4,5,15,36] and typically occurs by treatment with 1 equiv. of the aniline in the presence of pyridine (2 equiv.) as the base to give, in most cases, good yields of *N*-aryl-5*H*-1,2,3-dithiazol-5 imines **4** (Scheme 5).

**Scheme 5.** Synthesis of *N*-aryl-5*H*-1,2,3-dithiazol-5-imines **4** from Appel salt **1**.

Some limitations of this chemistry appear when using heterocyclic arylamines, such as aminopyridines. A recent study by Koutentis et al. highlighted that the reactions of the three isomeric aminopyridines with Appel salt **1** gave very different yields based on the position of the amino group. The 2-, 3- and 4-aminopyridines gave 69%, 24%, and 1% yields of the desired 1,2,3-dithiazole, respectively [37] (Scheme 6). Koutentis et al. suggested the low yield of 4-aminopyridine is likely attributed to the reduced nucleophilicity of the primary amine due to a contribution of its zwitterionic resonance form. The low reactivity of the amine leads to complex reaction mixtures due to side reactions.

**Scheme 6.** Reaction of Appel salt **1** with aminopyridines.

#### *2.3. Reactivity of C-4 and the Displacement of the Chloride*

The less reactive C4 chlorine of neutral 5*H*-1,2,3-dithiazoles cannot be directly substituted by nucleophiles. However, utilizing an ANRORC-(Addition of the Nucleophile, Ring

Opening, and Ring Closure)-style mechanism, nucleophilic substitution can occur on the C4 chlorine of the 1,2,3-dithazole. An example of this is where the *N*-Aryl-5*H*-1,2,3-dithiazol-5 imines **4** react with an excess of dialkylamines to give 4-aminodithiazoles **9** in variable yields (Scheme 7). The reaction was found to proceed *via* an ANRORC-style mechanism [38,39] involving ring opening by nucleophilic attack on the S2 position to yield disulfides **10** and subsequent recyclization after amine addition on the cyano group [40]. In another report by Koutentis et al. [14], DABCO was reacted with neutral 5*H*-1,2,3-dithiazoles **4**–**6** to give *N*-(2-chloroethyl)piperazines **11** in good yields (Scheme 7). The chloroethyl group originating from chloride attack on the intermediate quaternary ammonium salt formed by the displacement of the C4 chloride by DABCO.

**Scheme 7.** Displacement of the C4 chlorine of neutral 5*H*-1,2,3-dithiazoles.

#### *2.4. Alternatives beyond Appel Salt Chemistry*

A different way to access both monocyclic and ring fused 1,2,3-dithiazoles is by the reaction of oximes with disulfur dichloride. An example of the synthesis of a ring fused dithiazole is the reaction of benzoindenone oxime **12** to give dithiazole **13** in 81% yield [41,42] (Scheme 8). Acetophenone oximes **14** were reacted with disulfur dichloride to yield dithiazolium chlorides **2**, which were subsequently converted to either imines **15**, thiones **16**, or ketone **17** [13] (Scheme 8). Insights in the mechanism of the oxime to dithiazole transformation were given by Hafner et al. [12], who isolated the dithiazole *N*-oxide, which is the intermediate in this reaction.

**Scheme 8.** Synthesis of 1,2,3-dithiazoles from oximes.

#### *2.5. Reactivity of 1,2,3-Dithiazoles*

Neutral 1,2,3-dithiazoles can also be transformed to a plethora of other heterocycles, often substituted by a cyano group originating from the imidoyl chloride of the starting

material using thermal or reactions with thiophiles. An interesting example of an ANRORCstyle mechanism leading to a ring transformation was the reaction of (*Z*)-*N*-(4-chloro-5*H*-1,2,3-dithiazol-5-ylidene)-1*H*-pyrazol-5-amines **4d** with diethylamine that results in disulfide intermediates **18**. Subsequent treatment with concentrated sulfuric acid gave 1,2,4-dithiazines **19** in good yields [43] (Scheme 9).

**Scheme 9.** Synthesis of 1,2,4-dithiazines **19**.

In another example, the pyrazoleimino dithiazoles **20** were converted to 4-methoxypyrazolo[3,4-*d*]pyrimidines **21** in medium to good yields by treatment with sodium methoxide in methanol [16] (Scheme 10). The transformation occurs after addition of the methoxide on the nitrile followed by cyclisation onto the dithiazole C5 position that fragments losing S<sup>2</sup> and chloride to give the final pyrimidine **21**.

**Scheme 10.** Synthesis of 4-methoxy-pyrazolo[3,4-*d*]pyrimidines **21**.

A similar example of ring transformations is that of 2-aminobenzyl alcohol dithiazoleimines **4e** to 1,3-benzoxazines **22** and 1,3-benzothiazines **23 [44]**. Treatment of imines **4e** with sodium hydride in THF gave mixtures of benzoxazines **22** and benzothiazines **23**, with the former as the main products (Scheme 11). The formation of the former involves deprotonation of the alcohol and cyclisation of the alkoxide onto the dithiazole C5 position. Subsequent fragmentation with loss of S<sup>2</sup> and chloride gave the final benzoxazine **22**. Alternatively, treatment of imines **4e** with Ph3P gave exclusively benzothiazines **23** in good yields (Scheme 11). Thiophilic attack on S1 ring opens the dithiazole ring and a second attack by Ph3P gives the intermediate alkene **24** that cyclizes to benzothiazine **23**.

**Scheme 11.** Synthesis of 1,3-benzoxazines **22** and 1,3-benzothiazines **23**.

1,2,3-Dithiazole derivatives can also be converted to mercaptoacetonitriles by the removal of the S1 atom. One example of this are the 3-(1,2,3-dithiazolylidene)indololin-2 ones **25** reacting with sodium hydride (2 equiv.) to yield the mercaptoacetonitrile products **26** in medium to good yields [45] (Scheme 12).

**Scheme 12.** Conversion of dithiazoles **25** to mercaptoacetonitriles **26**.

Perhaps the most unstable 1,2,3-dithiazole is Appel salt itself, which, while relatively stable at ca. 20 ◦C under a desiccant, in its absence, Appel salt has a tendency to react with moisture. One study by Koutentis et al. revealed that simple stirring in wet MeCN gave elemental sulfur, dithiazole-5-thione **6**, dithiazol-5-one **5,** and thiazol-5-one **27** [46] (Scheme 13), assisting other scientists working with Appel salt, to identify these products. Interestingly, other dithiazolium salts have also been prepared with increased stability and lower sensitivity to moisture. A series of perchlorate salts of 1,2,3-dithiazoles were prepared by the anion exchange with perchloric acid allowing for more detailed characterization and study of the 1,2,3-dithiazole [29].

**Scheme 13.** Degradation of Appel salt **1** in wet MeCN.

In another study by Rakitin et al., 4-substituted 5*H*-1,2,3-dithiazoles **16** and **17** were converted to 1,2,5-thiadiazoles **28** and **29** by treatment with primary amines [47] (Scheme 14). Mechanistically, the reaction occurs by addition of the amine to the C5 position followed by ring opening of the C-S bond and subsequent ring closing by loss of hydrogen sulfide.

**Scheme 14.** Transformation of dithiazoles **16**–**17** to thiadiazines **28**–**29**.

To summarize, 1,2,3-dithiazoles can be converted to other heterocyclic or ring opened derivatives. The six most common mechanisms involved in the transformations of 1,2,3 dithiazoles to other systems are shown below (Scheme 15). These mechanisms begin *via* a nucleophile assisted ring opening of the dithiazole to disulfide intermediates that then can react either intermolecular or intramolecular with other nucleophiles *via* the six paths presented.

**Scheme 15.** Overview of the mechanisms of the reactions of 1,2,3-dithiazoles.

#### **3. 1,2,3-Dithiazoles in Medicinal Chemistry**

*3.1. Antimicrobial Activities of 1,2,3-Dithiazoles, including Antifungal, Herbicidal, and Antibacterial*

The first report of biological activity using the 1,2,3-dithiazole scaffold was published in a patent filed by J. E. Moore in 1977 on behalf of Chevron Research Co. [2,3]. The patent disclosed a series of novel 1,2,3-dithiazoles afforded in a 2–3 step sequence from *N*-aryl cyanothioformamide and sulfur dichloride. The main application of these compounds was the controlling of various fungal infections, leaf blights, invasive plant species, and mites.

First, the tomato early blight organism, *Alternaria solani conidia* was tested against 6- to 7-week-old tomato plate seedlings. The tomato plants were sprayed with 250 ppm solutions of a 1,2,3-dithiazole library. This resulted in the identification of (Z)-4-((4-chloro-5*H*-1,2,3-dithiazol-5-ylidene)amino)benzonitrile (**30**) with a 90% reduction compared with non-treatment. The 2,4-dichloro analogue **31** had weaker activity, with a reduction of just over half of the infection (Figure 1). Next, the tomato late blight organism, *Phytophthora infestans conidia* was tested against seedlings of 5 to 6 weeks old using the same procedure. The 4-cyano analogue **30** was found to afford 97% protection, while the 2-(4-nitrophenoxy) analogue **32** showed an 80% reduction (Figure 2). Then, the celery late blight organism *septoria api* was tested using 11-week-old plants. The 4-cyano analogue **30** afforded less protection at just over 60%, while several other analogues showed improvements, including 2-fluoro **33** and 3-(4-trifluoro, 2-cyanopenoxy) **34** analogues, both reported with 80% protection (Figure 2).

A series of halogenated analogues **35**–**39** were then identified as active against the powdery mildew pathogen *Erysiphe polygoni* using bean seedlings with well-developed primary leaves. The (Z)-4-chloro-*N*-(4-chloro-2-methylphenyl)-5*H*-1,2,3-dithiazol-5-imine analogue (**35**) along with the corresponding 3-chloro **36** showed 100% protection at 250 ppm. The corresponding 5-chloro **37** and 4-bromo **38** both showed a small reduction in efficacy, 10% and 1%, respectively, while the 3,5-dichloro **39** was only net 76% effective (Figure 3).

**Figure 2.** Antifungicidal activities of early 1,2,3-dithiazole derivatives at a concentration of 250 ppm.

**Figure 3.** Antifungicidal activities of 1,2,3-dithiazoles against powdery mildew (250 ppm) and *Botriytis cinerea* (40 ppm).

Initial screening was also carried out against necrotrophic fungus *Botrytis cinerea* on the well-developed primary leaves of a 4–6-week-old horsebean plant at a lower concentration (40 ppm). Only 1,2,3-dithiazole **35** was demonstrated to be effective with 92% inhibition (Figure 3). However, after this initial result, screening was carried out on a broader panel of fungal (Figure 4) and herbicidal strains (Figure 5). The fungal panel included *Botrytis cinerea*, *Rhizoctonia solani*, *Fusarium moniloforma*, *Phythium ultimum,* and *Aspergillus niger*. The compounds **30**, **33**, and **39**–**48** were tested at 500 ppm and fungicidal activities were measured by the zone of inhibited mycelia growth (Figure 4). Interestingly, the unsubstituted phenyl analogue (*Z*)-4-chloro-*N*-phenyl-5*H*-1,2,3-dithiazol-5-imine (**40**) was active on *Botrytis cinerea* at 0.33 µg/cm<sup>2</sup> . The addition of a 4-position methyl in analogue **41** reduced the activity against *Botrytis cinerea* by over 2-fold, but increased the activity against *Rhizoctonia solani* and *Fusarium moniloforma*. The 2-position methyl analogue **42** showed a profile switch showing activity only against *Aspergillus niger* (0.98 µg/cm<sup>2</sup> ). The 2,4,6-trimethyl analogue **43** also only retained activity against on strain *Rhizoctonia solani* (0.98 µg/cm<sup>2</sup> ). The original 4-chloro, 2-methyl analogue **35** showed activity against *Rhizoctonia solani* (0.63 µg/cm<sup>2</sup> ), but the dose dependent *Botrytis cinerea* data was not reported. The removal of the methyl group to afford the 4-chloro analogue **44** increased the activity against *Rhizoctonia solani* by 2-fold and showed commensurate activity against *Phythium ultimum* and 3-fold weaker activity against *Aspergillus niger*. The addition of a second chloro in the 3-position in analogue **45** was unfavored with only activity against *Aspergillus niger* retained. When the 4-chloro is removed to afford **46**, the activity is switched again with potency only demonstrated for *Rhizoctonia solani* at the same level as **43**. Addition of a second choro at the 5-position to afford **47** has same activity profile as **46**. The 4-cyano analogue **30** showed good activity against *Rhizoctonia solani* (0.60 µg/cm<sup>2</sup> ). The 2-fluoro analogue **33**, while having a slightly weaker potency, did show activity against 4 out of 5 of the fungal panel, only excluding *Botrytis cinerea.* The final two analogues identified in this series, 3-(4-nitrophenoxy) **47** and

4-(4-nitrophenoxy) **48**, both showed activity against only *Rhizoctonia solani* with analogue **48** having a 2-fold improvement over **47** at (0.45 µg/cm<sup>2</sup> ).

**Figure 4.** Antifungicidal activities of 1,2,3-dithiazoles, values are amounts required for mycelia inhibition, micrograms/cm<sup>2</sup> for 99% control of fungal growth.

The 1,2,3-dithiazoles were then screened at 33 ppm on a herbal panel that included wild oats (*Awena fatua*), watergrass (*Echinochloa crusgall*), crabgrass (*Digitaria sanguinalis*), mustard (*Brassica arversis*), pigweed (*Amaranthus retroflexus*), and lambsquarter (*Cheropodium album*) (Figure 5). The first analogue (*Z*)-4-chloro-*N*-(*p*-tolyl)-5*H*-1,2,3-dithiazol-5-imine (**41**), showed good efficacy against *Amaranthus retroflexus* (90%) and total control of *Brassica arvensis*. Switching to the 4-fluoro analogue **49** increased coverage across all strains tested, including *Avena fatua* (40%), which was only weakly inhibited across the series and total control of *Amaranthus retroflexus.* The 4-chloro analogue **44** was 3-fold less effective against *Digitaria sanguinalis* and *Avena fatua*. The addition of a 2-position chloro **50** decreased strain coverage, but did mean total control of *Brassica arvensis* in addition to *Amaranthus retroflexus*, with additional high efficacy against *Chenopodium Album* (95%). The original 2-methyl 4-chloro analogue **35**, while still showing efficacy across several strains, did not offer total or near total control for any of the strains tested. The 2-chloro analogue **50** showed total control for *Chenopodium album* and *Brassica arvensis* and near total for *Amaranthus retroflexus* (93%). However, 2-chloro **50** had a limited effect on *Digitaria sanguinalis* and *Echinochloa crusgalli*, with no impact on *Avena fatua*. The 3,5-dichloro analogue **39** demonstrated good efficacy

against most strains, including total control of *Amaranthus retroflexus*, *Chenopdium album, Brassica arvensis,* and some activity against *Avena fatua* (35%). The 2-methyl, 5-chloro analogue **51** offered the highest efficacy across the series on *Avena fatua* (45%), total control *of Amaranthus retroflexus* and *Brassica arvensis*, with near total control of *Chenopodium album* (95%). The 3,4-dichloro analogue **45** had a potent but narrower band of activity with total control of *Amaranthus retroflexus*, *Chenopdium album* and *Brassica arvensis*, but weaker activity on the other three strains (30–55%). The 3-bromo analogue **38** has a similar profile to the 4-methyl **41**, while the 2-naphthyl analogue **52** was the most potent in the screening for *Echinochloa crusgalli* (90%) and offered good control over *Amaranthus retroflexus* (85%) and total control over *Brassica arvensis*.

**Figure 5.** Herbicidal activities of early 1,2,3-dithiazole derivatives tested at 33 ppm.

In order to test for other pests, pinto bean leaves were treated with two spotted mites (*Tetramuchus urticae*). The mites were then allowed to lay eggs on the leaves, and after 48 h, the leaves were treated with 40 ppm of the test compound (Figure 6). A series of halogenated phenyl-5*H*-1,2,3-dithiazol-5-imines were identified with activity against both *Tetramuchus urticae* and their eggs. The 3,5-dichloro analogue **39** showed a high degree of control with 90% of mites and 85% of eggs suppressed. This increased to almost total control with the 3,4-dichloro **45**. Interestingly, the 2,4-dichloro analogue **31** demonstrated total mite control but had no effect on the eggs. The mono-substituted 2-chloro analogue had a similar profile with no effect on the mite eggs, but only 70% effective control of the mite. The 4-chloro, 2-methyl analogue **35** showed complete egg control and almost complete mite control (94%). The switch to the bromo **38** showed a similar profile, but with 70% mite control. The 2-methyl substituted match pair analogues 3-chloro **56** and 5-chloro **52** both demonstrated a high level of mite and egg control with the 3-position preferred.

**Figure 6.** Mite (*Tetranychus urticae*) control activities of early 1,2,3-dithiazole derivatives at 40 ppm.

Subsequent to the work reported by Chevron Research Co., in 1980, a brief patent was filed by Appel, R. et al. on behalf of Bayer AG on the use of 1,2,3-dithiazoles as antifungals specifically against *Trichophyton Mentagrophytes* [48]. This was followed up by another brief patent in 1984 by Mayer R. et al. on behalf of Dresden University of Technology (Technische Universität Dresden) on the use of *N*-arylcyanothioformamides derived from 1,2,3-dithiazoles as herbicides and crop protection agents [49].

The 1,2,3-dithazoles chemical space and synthesis progressed as outlined in Section 2.2 during the late 1980s and early 1990s. However, it was not until 1996 when Pons et al. disclosed a focused series of *N*-arylimino-1,2,3-dithiazoles and related *N*-arylcyanothioformamides before further biology was elucidated [15]. The unsubstituted aromatic compound **40** and the 2-methoxy analogue **54** were shown to have potent activity on several bacteria strains (Figure 7). Compound **40** had an MIC of 16 µg/mL on *S. aureus*, *E. faecalis*, and *L. monocyotogenes*, while 2-methoxy **54** had the same level of potency, but only on *E. faecalis* and *L. monocyotogenes*. Interestingly, all the *N*-arylcyanothioformamides analogues tested were ineffective, highlighting the need for the 1,2,3-dithiazole ring.

**Figure 7.** Report on a small panel of 1,2,3-dithiazoles highlighted some nascent antibacterial activity on the dithiazole scaffold.

This work was extended in a subsequent report by Pons et al. [50], where a focused library of 1,2,3-dithiazoles and related analogues were screened on a series of fungal targets. The 1,2,3-dithiazoles were the only compounds that showed antifungal activity, with most potent analogues identified as unsubstituted aromatic **40**, the 2-methoxy **54**, and 4-methoxy analogue **55** (Figure 8). These three most potent analogues all had an MIC of 16 µg/mL on *C. albicans*, *C. glabrata*, *C. tropicalis*, *L. orientalis,* and an MIC of 8 µg/mL on *C. neoformans*.

**Figure 8.** Report on a small panel of 1,2,3-dithiazoles highlighted some nascent antifungal activity on the dithiazole scaffold.

This was followed by a patent filed in 1997 by Joseph, R. W. et al. on behalf of Rohm & Haas Co. [51], a company specializing in the manufacture of coatings. The disclosed innovation involved the use of 1,2,3-dithiazoles to rapidly inhibit microbial and algae growth for industrial applications. These included paints, coatings, treatments, and textiles, among others. The effective amount applied was between 0.1 to 300 ppm, with three main exemplar 1,2,3-dithiazoles highlighted (Figure 9). This included 4-chloro-5*H*-1,2,3-dithiazol-5-one (**5**) with potent antibacterial properties against *R. Rubra TSB* (MIC = 7.5 ppm) and *E. Coli M9G* (MIC = 19 ppm), with potent algae inhibition of *Chlorella*, *Scenedesmus,* and *Anabaena* (all MIC = 3.9 ppm) and *Phormidium* (MIC = 7.8 ppm). In addition to **5,** the 2-chloro analogue **52** was reported to have potent activity against *R. Rubra TSB* (MIC = 7.5 ppm) and good activity against *E. Coli M9G* (MIC = 32 ppm) and *A. Niger TSB* (MIC = 50 ppm). The 4-nitro analogue **56** also performed well with both *E. Coli M9G* and *A. Niger TSB* having an MIC or 50 ppm. The activity reported between **5** and **52** on *E. Coli M9G* is the first evidence of activity against a Gram-negative bacterium. The company also provided data with time of addition experiments showing that 5 and 10 ppm of **5** are effective at 1 h, whereas 10 ppm of methylene *bis*thiocyanate (MBT), a known commercial antimicrobial compound, is not effective until 24 h.

Subsequently in 1998, more detailed screening and structure activity relationships (SAR) were published from Pons et al. related to the antimicrobial properties of the 1,2,3 dithiazole scaffold [52,53]. These two studies tested activity against bacteria: *S. aureus*, *E. faecalis*, *S. pyogenes*, and *L. monocytogenes*, and fungi: *C. albicans*, *C. glabrata*, *C. tropicalis,* and *I. orientalis*. This screening supported earlier work on the 1,2,3-dithiazole scaffold, and broadened the scope of this inhibition to several new fungal and bacteria strains (Figure 10). The compounds showed antibacterial activity against Gram-positive bacteria, but as previously described [15], there was no activity against Gram-negative bacteria.

**Figure 9.** Rohm & Haas Co. filed a patent for industrial applications around three 1,2,3-dithiazoles for antibacterial and antialgae properties.

The unsubstituted analogue **40** was a direct repeat of all activities previously demonstrated with antibacterial *S. aureus*; *E. faecalis*; and *L. monocyotogenes* (all MIC = 16 µg/mL); and antifungal *C. albicans*, *C. glabrata*; *C. tropicalis*; and *L. orientalis* (all MIC = 16 µg/mL). All of the highlighted compounds (**40**, **54**, **57–66**) had *C. albicans* activity at MIC = 16 µg/mL. The 2-cyano analogue **57** had activity (MIC = 16 µg/mL) across all fungal strains tested but had limited antibacterial effects. Switching to the 2-methylester **58** narrowed the antifungal activity. However, the 2-methoxy **54** had good broad spectrum antimicrobial activity hitting 7 out of the 8 strains tested. The introduction of a second methoxy group in the 5-position to afford (*Z*)-4-chloro-*N*-(2,5-dimethoxyphenyl)-5*H*-1,2,3-dithiazol-5-imine (**59**) increased the

potency (MIC = 4 µg/mL) on *C. glabrata*, while maintaining antifungal coverage. Moving the 2-position methoxy to the 4-position in analogue **60** maintained the antifungal coverage but lost the 4-fold boost seen against *C. glabrata* with **59**. The (*Z*)-(4-chloro-2-((4-chloro-5*H*-1,2,3-dithiazol-5-ylidene)amino)phenyl)methanol (**61**) analogue showed potency against *E. faecalis*, *C. glabrata*, *C. albicans,* and *L. orientalis* (all MIC = 16 µg/mL); in addition to demonstrating a tolerability for more diverse substitution patterns. *Molecules* **2023**, *28*, x FOR PEER REVIEW 36 of 36

**Figure 10.** Results of a focused investigation of antibacterial and antifungal activities of selected 1,2,3-dithiazoles [52,53].

A switch to fused heterocycles including quinolines and naphthalene was maintained rather than increased overall potency and coverage. The quinolin-6-yl substituted analogue **62** showed potency against *C. glabrata* and *C. albicans* (both MIC = 16 µg/mL), while the quinolin-5-yl **63** was only active against the *C. albicans*. The naphthalen-1-yl **65** and hydroxy substituted naphthalen-1-yl **66** had the same profile with coverage against all four bacteria tested (*S. aureus*, *E. faecalis*, *S. pyogenes*, and *L. monocyotogenes* all MIC = 16 µg/mL) and *C. albicans*. The hydroxy substitution of **65** to afford **66** did not provide any potency advantage, but did demonstrate there was an ability to alter physicochemical properties without affecting potency.

Access to a series of new substituted 5-phenylimino, 5-thieno, or 5-oxo-1,2,3-dithiazoles was reported in 2009 by Rakitin et al. [13] (synthesis discussed in Section 2.4). A series of sixteen compounds were screened against four fungi strains: *C. albicans*, *C. glabrata*, *C. tropicalis*, and *I. orientalis*; four Gram-negative bacteria strains: *E. coli*, *P. aeruginosa*, *K. pneumoniae*, and *S. Typhimurium;* and four Gram-positive bacteria strains: *E. faecalis*, *S. aureus*, *B. cereus*, and *L. inocua*. The result of this screening were some compounds with limited activity and 4-(pyridin-2-yl)-5*H*-1,2,3-dithiazole-5-thione (**67**), which was active against bacteria *L. inocua* (MIC = 16 µg/mL) and fungi *C. galbrata* (MIC = 8 µg/mL) (Figure 11). These results opened up additional chemical space to potentially further investigate the 1,2,3-dithiazole SAR.

**Figure 11.** The 4-(pyridin-2-yl)-5*H*-1,2,3-dithiazole-5-thione (**67**) was the only compound with potent antimicrobial activity from the C4 substituted analogue library.

In 2012, a patent was filed by Benting et al. on behalf of Bayer Cropscience AG focusing on phytopathogenic antifungal crop protection aspects of heteroaromatic substituted 1,2,3-dithiazole analogues [54]. Heteroaromatic substitution patterns had until the point been largely neglected, due in part to electron deficient amines affording lower yields (as in the case of [37]). A library of heteroaromatic 1,2,3-dithiazole derivative were screened against a series of different fungi strains. These included tomato (*Phytophtora*), cucumber (*Sphaerotheca*), apples (*Venturiatest*), tomato (*Alternia*), beans (*Botyrtis*), wheat (*Leptosphaeria nodorum*), wheat (*Septoria tritici*), and rice (*Pyricularia*). Only compounds that showed inhibition above 70% at the respective concentration tested were reported. The results were broadly clustered into three groups, small 5-membered heterocyles, pyrimidine, and 3-pyridyl substituted heterocycles and 2-pyridyl substituted heterocycles. There was broad SAR tolerability for *Phytophtora* (Figure 12), *Sphaerotheca* (Figure 13), *Venturiatest* (Figure 14), *Alternia* (Figure 15), and *Botyrtis* (Figure 16). While with *Leptosphaeria nodorum* (Figure 17), *Septoria tritici* (Figure 18), and *Pyricularia* (Figure 19) the SAR narrowed considerably.

The chemical space around *Phytophtora* inhibition included a broad array of 1,2,3 dithiazol-5-imines **4a** and **68–90** (Figure 12). While most analogues reported were potent; only the 4-methyl 2-pyridyl **87** and 3-methoxy 2-pyridyl **90** achieved total control of the fungi. In the case of *Sphaerotheca*, many compounds demonstrated very high degrees of antifungal control, including **4a**, **4b**, **68–70**, **73**, **77**, **79–81**, **86–87,** and **91–104** (Figure 13), while ten compounds showed complete control including the unsubstituted analogues 2-pyridyl **4a** and 4-methyl 2-pyridyl **91**.

**Figure 12.** *Phytophtora* (tomato) preventive; ≥70% efficacy at concentration of 1500 ppm.

The *Venturiatest* fungi appears to be easier to target as the reported dose is 6-fold less (250 ppm vs. 1500 ppm), and most reported compounds **4a**, **68**–**69**, **75**–**76**, **79**–**81**, **84**, **86**–**87**, **90**–**91**, **95**–**97**, and **105**–**111** having high potency with a mixture of substitution patterns offering total control (Figure 14). These included isoxazoles **68** and **105**, 1,2,5-oxadiazole **91** and a series of seven substituted 2-pyridyl analogues including **4a**, **80**–**81**, **84**, **96**–**97**, and **101** (Figure 14). The *Alternia* fungi appears to be more difficult to effectively target as, while the compounds **4a**, **68**–**69**, **75**–**76**, **79**–**81**, **84**, **87**, **90**–**91**, **95**–**97**, **105,** and **107**–**111** were similar to the inhibitors identified for *Venturiatest,* none reached total control of *Alternia*. The most potent compounds, pyrazole **69**, unsubstituted 2-pyridyl **4a,** and 3-fluoro 2-pyridyl **80** all had 96% control at 250 ppm (Figure 15). The *Botyrtis* fungi was also not completely controlled by the active compounds **4a**, **68**–**69**, **75**–**76**, **81**, **84**, **91**, **95**–**97**, **105**, **107,** and **110**, despite a high level of potency. The SAR around *Botyrtis* was considerably narrower with roughly half the number of earlier analogues reported (Figure 16), despite the higher 500ppm concentration tested. The pyrazole analogue **69** was able to potently inhibit *Botyrtis* infection to 99%, while several other analogues also had potent inhibition (>95%).

Only five 1,2,3-dithiazoles were reported to be active against *Leptosphaeria nodorum* (Figure 17) and the need for an increased concentration of test compound to 1000 ppm, potentially highlighting that *Leptosphaeria nodorum* is more difficult to target. The five compounds reported were all 2-pyridyl substituted **80**, **84**, **87**, **90**, and **96**, but only (*Z*)-4 chloro-*N*-(6-methoxypyridin-2-yl)-5*H*-1,2,3-dithiazol-5-imine (**90**) had total control of the *Leptosphaeria nodorum* infection.

The fungi *Septoria tritici* had a similar profile to *Leptosphaeria nodorum*, with only five potent compounds reported: **84**, **87**, **98**, **109**, and **112** (Figure 18). The most potent four of the five compounds reported were 2-pyridyl substituted **84**, **87**, **98,** and **112**. The 4-methyl 2-pyridyl **87** and 3-methoxy 2-pyridyl **112** were the most potent, with 100% control of *Septoria tritici* infection.

**Figure 13.** *Sphaerotheca* (cucumber) preventive; ≥70% efficacy at concentration of 1500 ppm.

Interestingly, the final set of results of inhibitors against *Pyricularia* revealed only two highly active compounds. These two compounds, (*Z*)-4-chloro-*N*-(6-methoxypyridin-2 yl)-5*H*-1,2,3-dithiazol-5-imine (**90**) and (*Z*)-4-chloro-*N*-(isoxazol-3-yl)-5*H*-1,2,3-dithiazol-5-imine (**68**), were both able to control 100% of the *Pyricularia* infection even at the lower concentration of 250 ppm. The lack of further SAR may (or may not) indicate that, while two highly active compounds are reported this infection was the most difficult to treat.

More recently, a 2020 study by our group reported a set of 1,2,3-dithiazoles and matched pair 1,2,3-thiaselenazoles as antimicrobials [55]. The rare 1,2,3-thiaselenazoles were synthesized by sulfur extrusion and selenium insertion into 1,2,3-dithiazoles [55,56]. This work was part of the Community for Antimicrobial Drug Discovery (CO-ADD) project to develop new lead compounds for priority targets with an unmet clinical need [57]. The compounds were screened against *S. aureus*, *A. baumannii*, *C. albicans,* and *C. neoformans var. grubii.* with a toxicity counter screen in HEK293 cells and an additional hemolysis assay (Hc10) (Figure 20). These strains are considered by the World Health Organization (WHO) to be the highest priority to develop novel antibiotics for control of these bacteria and fungi [58].

**Figure 14.** *Venturiatest* (apples) preventive; ≥70% efficacy at concentration of 250 ppm.

**Figure 15.** *Alternia* (tomatoes) preventive ≥70% efficacy at concentration of 250 ppm.

**Figure 16.** *Botyrtis* (beans) preventive ≥70% efficacy at concentration of 500 ppm.

**Figure 17.** *Leptosphaeria nodorum* (wheat) preventive ≥70% efficacy at concentration of 1000 ppm.

**Figure 18.** *Septoria tritici* (wheat) preventive ≥70% efficacy at concentration of 1000 ppm.

**Figure 19.** *Pyricularia* (rice) preventive ≥80% efficacy at concentration of 250 ppm.

The compounds **113**–**120** demonstrated potency against several of the strains tested, with the 1,2,3-thiaselenazoles tending to be more active (Figure 20). The 4,5,6-trichlorocyclopenta[*d*][1,2,3]thiaselenazole (**113**) demonstrated potent activity against Gram-positive bacteria *S. aureus* (MIC = ≤0.25 µg/mL), Gram-negative bacteria *A. baumannii* (MIC = ≤0.25 µg/mL) along with antifungal activity against *C. albicans* and *C. neofromans* (both MIC ≤0.25 µg/mL). The trichoro analogue **113** had some toxicity (CC<sup>50</sup> = 0.52 µM), whereas both the 4-cyano **114**/**115** and 4-ethylester **116**/**117** 1,2,3-dithiazole/1,2,3-thiaselenazole matched pair analogues showed limited to no toxicity (all CC<sup>50</sup> = >32 µM, apart from **117** = CC<sup>50</sup> = 7 µM). The 4-cyano analogues **114**/**115** were both active against *C. albicans* and *C. neofromans* (both MIC ≤0.25 µg/mL); however, the 1,2,3-thiaselenazole also had activity against *S. aureus* (MIC = ≤0.25 µg/mL). This activity trend was matched exactly by the 4-ethylester analogues **116**/**117**. The 4,5,6-trichlorobenzo[6,7]cyclohepta [1,2-*d*][1,2,3]thiaselenazole (**118**) analogue matched the profile of **115** and **117** albeit with some toxicity (CC<sup>50</sup> = 0.48 µM). Interestingly, the activity profiles of 8-chloroindeno[1,2-*d*][1,2,3]thiaselenazole (**119**) and benzo[*b*][1,2,3] thiaselenazolo[5,4-*e*][1,4]oxazine (**120**) were similar with antifungal activity against *C. albicans* and *C. neofromans* (all MIC = ≤0.25 µg/mL, apart from **120**, *C. neofromans* = 2 µg/mL). Taken together these results demonstrate an ability for the 1,2,3-dithiazole/ 1,2,3-thiaselenazole to inhibit a broad range of challenging and clinically relevant bacteria and fungi [55,58].

**Figure 20.** Summary of the most active antifungal 1,2,3-dithiazoles and 1,2,3-thiaselenazoles from the 2020 study by our group [55].

#### *3.2. Antiviral Activities of 1,2,3-Dithiazoles*

The first antiviral activities on the 1,2,3-dithazole scaffold were reported in 2016 by Hilton et al. [17]. A series of 5-thien HIV infection. The rationale of using the 1,2,3-dithiazoles to target the nucleocapsid protein was that it could potentially act as ao-, 5-oxo-, and 5-imino-1,2,3-dithiazole derivatives were screened against Feline Immunodeficiency Virus (FIV) as a model for zinc ejector by utilizing the disulfide bridge [59–62]. The compounds were tested for antiviral effects in a feline lymphoid cell line (FL-4) and tested for toxicity using Crandell-Rees feline kidney (CrFK) cells (Figure 21). The four highlighted compounds, **121–124**, were the most potent antivirals with the largest toxicity window (ratio of FL-4/CrFK). The 4-phenyl-5*H*-1,2,3-dithiazole-5-thione (**121**) had an excellent ratio of activity vs. toxicity (>4000) and potency of EC<sup>50</sup> = 23 nM. The 4-(4-fluorophenyl)-5*H*-1,2,3-dithiazol-5-one (**122**) was equipotent to **121** with a small amount of toxicity at higher concentrations (CC<sup>50</sup> = 64 µM). The 4-methoxy analogue **123** had a drop of almost 8-fold in potency, with the ethyl (Z)-5-(phenylimino)- 5*H*-1,2,3-dithiazole-4-carboxylate (**124**) analogue had an almost 3-fold drop, both with a comparable toxicity profile.

The proposed mechanism of action was modelled on previously experimental reports (Figure 22) [59–62]. Zinc ejection from nucleocapsid protein starts with Zn2+ coordinated to cysteine thiol(ate)s reacting with the disulfide of the 1,2,3-dithiazole core to generate a transient intermediate disulfide. This complex then rearranges to form an intramolecular protein disulfide, which has a consequent reduction in zinc ion affinity. This results in the zinc being ejected from the protein in a similar mechanism as previously reported for the HIF1alpha/P300 interaction triple zinc finger [59]. To indirectly prove the mechanism in addition to computational modelling,

a disrupted disulfide bridge of analogue **121**, compound **125** was synthesized (Figure 23) [47], demonstrating that the disulfide was required for activity.

**Figure 22.** Proposed redox mechanism for 1,2,3-dithiazoles mediated zinc ejection of the FIV nucleocapsid protein. (**A**) Summary of reaction; (**B**) Detailed reaction pathway analysis.

**Figure 23.** Direct comparison of **121** against **125** with the disrupted 1,2,5-thiadiazole-3(2*H*)-thione ring system.

This idea was followed up in 2019 by our group [63], investigating the same inhibitors later reported as antimicrobials [55]. The key rationale behind this subsequent work was the further investigation of the disulfide bridge involvement on antiviral efficacy with a matched pair side by side comparison between the 1,2,3-dithiazoles and the 1,2,3-thiaselenazole scaffold. Where the weaker S-Se vs. S-S bond should assist in increasing the antiviral efficacy. This followed on from a previous report of a successful selenide isosteric replacement to several literature nucleocapsid protein inhibitors, including DIBA-4 to DISeBA-4 HIV inhibitors, resulting in good potency and only a very limited associated toxicity [64]. The antiviral efficacy of the 1,2,3-dithiazole scaffold was tested using FL-4 cells, but in this study an additional toxicity assay was preformed directly on the FL-4 cells (Figure 24). The 8-phenylindeno[1,2-*d*][1,2,3]dithiazole (**126**) was only weakly active, while the selenium analogue **127** demonstrated a 10-fold boost in potency to EC<sup>50</sup> = 0.26 µM with only limited toxicity. The ethyl 5,6-dichlorocyclopenta[*d*][1,2,3]dithiazole-4-carboxylate (**116**) had a similar profile with an EC<sup>50</sup> = 0.26 µM, while the selenium analogue **117** was almost 4-fold more potent. The difference between the benzo[*b*][1,2,3]dithiazolo [1,4]oxazine (**128**) and selenium analogue **120** was even more pronounced with an almost 17-fold increase in potency. These results highlight the advantages of including selenium in the 1,2,3-dithiazole scaffold.

**Figure 24.** Summary active antiviral matched pair 1,2,3-dithiazoles and 1,2,3-thiaselenazoles.

More recently, a further extension of investigation of the 1,2,3-dithiazoles in 2022 by our group evaluated a further series of 1,2,3-dithiazoles against FIV as a model for HIV infection [36]. The rationale of this investigation was to find a tractable series of 1,2,3-dithiazoles with consistently high potency and lower toxicity to further advance the scaffold. The antiviral screening was performed using FL-4 cells, with a direct toxicity assay on FL-4 cells in addition to CrFK and feline embryo cell line (FEA) cells (Figures 25–27).

**Figure 25.** Initial hit compounds from the 1,2,3-dithiazole library.

**Figure 26.** 2-Pyridyl substituted 1,2,3-dithiazoles active against FIV.

**Figure 27.** Most potent compound **133** active agiants FIV in the 2022 study by our group [36].

The initial hit compounds from the 1,2,3-dithiazole library yielded a series of 4-position substituted phenyl analogues **30** and **129–131** with a range of activities EC<sup>50</sup> = 0.26–0.48 µM and limited toxicity (Figure 25). Another trend observed within the series was activity across a number of 2-pyridyl substituted analogues **4a**, **90**, **112** and **132**, at a similar level to the earlier analogues but with a divergent SAR profile (Figure 26). The most promising compound identified in this work was the pyrazole (*Z*)-4-chloro-*N*-(3-methyl-1*H*-pyrazol-5-yl)-5*H*-1,2,3-dithiazol-5-imine (**133**) that showed good antiviral potency EC<sup>50</sup> = 0.083 µM with very limited toxicity (Figure 27).

The proposed mechanism of action on the nucleocapsid protein of this 4-chloro-1,2,3 dithiazol-5-imine series is different to the C4 substituted version previously reported (Figure 22). The DFT calculations and previous ANRORC-style rearrangement reported on this scaffold suggest that there will be a ring opening and chloride elimination. An outline mechanism would be a Zn2+-coordinating cysteine thiol(ate) reacts with 2-S of the 1,2,3-dithiazole core mediated by water to generate a transient trisulfide. This is then followed by a rearrangement to a more thermodynamically stable cyano functionality, resulting in the loss of HCl and water from the system. The, disulfide then rearranges to form an intramolecular protein disulfide with consequent reduction in zinc ion affinity. The zinc ion is then ejected to form a stable complex, with or without adducts (Figure 28). In addition to the literature rearrangement examples [38–40], we also provided extensive computational modelling to support the mechanistic rationale provided.

**Figure 28.** Proposed mechanism of action of the 4-choro-1,2,3-dithiazole series.

#### *3.3. Anticancer Activities of 1,2,3-Dithiazoles*

Initial reports of anticancer activity with the 1,2,3-dithiazole scaffold were reported in 2002 by Baraldi et al. [16]. A set of ten 1,2,3-dithiazoles were prepared and screened across multiple antimicrobial and anticancer therapeutic targets. Several of the compounds **134**–**136** showed low signal digit micromolar potency against the leukemia cell lines L1210 and K562 (Figure 29). While the overall SAR within the series was flat, this first phenotypic report showed tractable activity across both cell lines. The antibacterial screen showed limited activity, but the antifungal screening identified **135** and **136** as having some activity against *Aspergillus niger* at MIC<sup>50</sup> = 10 µM. This further supports the overall tractability of this scaffold as an antifungal.

**Figure 29.** Initial 1,2,3-dithiazoles reported with anticancer activity in 2002 by Baraldi et al. [16].

The earlier 2009 study reported in Section 3.1 by Rakitin et al. also screened the series of C-4 substituted dithiazoles against two breast cancer cell lines, MCF7 and MDA-MB-231 [13]. Limited activity was observed on the MDA-MB-231 cell line across the series. However, (*Z*)-4-(4-nitrophenyl)-*N*-phenyl-5*H*-1,2,3-dithiazol-5-imine (**137**) and the benzofuran-2-yl analogue **138** showed 50% growth inhibition after 72 h at approximately 10 µM (Figure 30). These results lay the groundwork to expand the chemical space to further investigate the anticancer 1,2,3-dithiazole SAR.

**Figure 30.** Substituted C4 1,2,3-dithiazoles with activity against breast cancer.

Subsequent to these phenotypic reports of 1,2,3-dithiazoles as anticancer compounds in various cell lines, a just over forty compound 1,2,3-dithiazole library was screened by Indiveri et al. against a transporter target over-expressed in various cancers, the glutamineamino acid transporter ASCT2 in 2012 [65]. Interactions with scaffold proteins and posttranslational modifications regulate the stability, trafficking, and transport activity of ASCT2 [66]. The expression of ASCT2 has been shown to increase in cells with rapid proliferation, including stem cells and inflammation, this enables delivery of the increased glutamine requirements [67]. This same mechanism can be hijacked by cancer promoting pathways to fulfill glutamine demand and facilitate rapid growth by over-expression of ASCT2 [68]. In addition to being described as an anticancer target, ASCT2 also has the ability to traffic virions to infect human cells [69]. A series of 1,2,3-dithiazoles were

synthesized and evaluated as transporter inhibitors. While many compounds were in-active at 30 µM, six compounds showed activity at IC<sup>50</sup> = ~10 µM or below (Figure 31). These compounds potently inhibited the glutamine/glutamine transport catalyzed by ASCT2.

**Figure 31.** Anticancer 1,2,3-dithiazoles ASCT2 transport inhibitors.

The inhibition was shown to be non-competitive. The inhibition was also reversed by addition of dithiothreitol (DTE), indicating the reaction with protein Cys formed adducts, indicating that the reaction was likely going *via* an ANRORC-style rearrangement. Modelling, including molecular and quantum mechanical studies (MM and QM, respectively) and Frontier Orbital Theory (FOT) on 1,2,3-dithiazole models showed pathway (ii) was more likely, which is also supported by previous reports on the 1,2,3-dithiazole (Figure 32) [38–40].

**Figure 32.** Two mechanisms are proposed, with nucleophilic attack at S2 to likely be preferred.

The ASCT2 report was followed up by a screening of just over fifty 1,2,3-dithiazoles by Indiveri et al. against the LAT1 transporter in 2017 [70]. ASCT2 and LAT1 are both amino acid transporters that are overexpressed in cancer [71]. Subsequently, a number of inhibitors have been reported against both ASCT2 and LAT1, with one LAT1 inhibitor JPH203 used in a recent phase 1 clinical trial [72]. The results of the library screen were eight compounds with inhibition of >90% at 100 µM. The two most potent compounds were **144** and **145** with an IC<sup>50</sup> = <1 µM (Figure 33).

**Figure 33.** Anticancer 1,2,3-dithiazoles targeting transporter protein LAT1 (SLC7A5).

The inhibition kinetics, performed on the two best inhibitors (**144** and **145**), indicated a mixed type of inhibition with respect to the substrate. The inhibition of LAT1 was still present after removal of the compounds from the reaction mixture, indicating irreversible binding. However, this effect could be reversed by the addition of dithioerythritol, a S-S reducing agent, which supports the rationale of the formation of disulfide(s) bonds between the compounds and LAT1. Molecular modelling of **144** and **145** on a homology model of LAT1, highlighted the interaction with the substrate binding site and the formation of a covalent bond with the residue C407. This was further supported by a more detailed study reported in 2021 by Marino et al., which also highlighted the need for a molecule of water in the reactive pathway [73].

More recently, an extension of the phenotypic reports of 1,2,3-dithiazoles as anticancer agents was published in 2021 by our group [74]. A library of just under forty 1,2,3-dithiazole analogues were screened on a series of cancer cell lines including breast, bladder, prostate, pancreatic, chordoma and lung; with a skin fibroblast cell line as a non-specific toxicity control (Figures 34–36).

**Figure 34.** Initial screening highlighted results from the 2021 cancer panel by our group [74].

**Figure 35.** 5-membered heteroatomic analogues highlighted results from the 2021 cancer panel by our group [74].

**Figure 36.** C4 substituted 1,2,3-dithazole analogues highlighted results from the 2021 cancer panel by our group [74].

Initial results were encouraging (Figure 34) with (*Z*)-4-(4-bromophenyl)-*N*-phenyl-5*H*-1,2,3-dithiazol-5-imine (**130**) and the corresponding 3-position bromo analogue **54** demonstrated potency against breast cancer cell line MCF7 (IC<sup>50</sup> = 11 and 6.7 µM, respectfully). This was followed by the identification of (*Z*)-*N*-(4-((4-chloro-5*H*-1,2,3-dithiazol-5 ylidene)amino)phenyl)pyrimidine-2-sulfonamide (**146**) with good activity against breast cancer (MCF7—IC<sup>50</sup> = 3.0 µM) and no observed toxicity (WS1—IC<sup>50</sup> = >100 µM). Interestingly, the (*Z*)-*N*-(4-(benzyloxy)phenyl)-4-chloro-5*H*-1,2,3-dithiazol-5-imine (**147**) analogue

showed a preference for bladder cancer inhibition (5637—IC<sup>50</sup> = 13 µM) with no observed toxicity (WS1—IC<sup>50</sup> = >100 µM).

This was followed by screening a small focused 5-membered heteroatomic compounds, which identified a trend of potency against prostate cancer (Figure 35). (*Z*)-4-chloro-*N*-(thiazol-2-yl)-5*H*-1,2,3-dithiazol-5-imine (**75**) and the two pyrazoles (**148** and **149**) all showed activity against prostate cancer (DU145—IC<sup>50</sup> = 8–11 µM). The thiazole analogue **75** also showed activity against the chordoma cell line (U-CH1—IC<sup>50</sup> = 10 µM), albeit with some limited toxicity (WS1—IC<sup>50</sup> = 24 µM). The (*Z*)-4-chloro-*N*-(3-methyl-1-phenyl-1*H*pyrazol-5-yl)-5*H*-1,2,3-dithiazol-5-imine (**149**) also showed low single digit micromolar activity against bladder cancer (5637—IC<sup>50</sup> = 2.1 µM); unfortunately, this was coupled with some associated toxicity (WS1—IC<sup>50</sup> = 15 µM).

Subsequent to this screening, a series of C-4 substituted analogues were evaluated resulting in a trend of activity against breast cancer (MCF7) (Figure 36) [14]. The most potent compounds **150–153** (IC<sup>50</sup> = 2–10 µM) did not show any defining SAR characteristics. In addition to this activity, (*Z*)-5-bromo-2-((4-(4-(2-chloroethyl)piperazin-1-yl)-5*H*-1,2,3 dithiazol-5-ylidene)amino)benzonitrile (**151**) demonstrated good potency against bladder cancer (5637—IC<sup>50</sup> = 8.0 µM), while 4-(4-(2-(methyl(phenyl)amino)ethyl)piperazin-1-yl)- 5*H*-1,2,3-dithiazol-5-one (**153**) was potent against prostate cancer (DU145—IC<sup>50</sup> = 4.4 µM), albeit with some observed toxicity (WS1—IC<sup>50</sup> = 20 µM) in the case of **153**. Interestingly, the 4-chloro-1,2,3-dithiazole of the earlier analogues was not required for activity suggesting there may be multiple mechanisms of action.

#### *3.4. Other Biological Applications*

#### 3.4.1. Melanin Synthesis Inhibitors

In 2015, a phenotypic screen was carried out using *Xenopus laevis* embryos by Skourides et al. [19]. This led to the identification of a series of 1,2,3-dithiazoles, which caused loss of pigmentation in melanophores and the retinal pigment epithelium (RPE) of developing embryos (Figure 37). This effect was independent of the developmental stage of initial exposure and was reversible. While the target was not elucidated, SAR of the series indicated that the presence of the mesmerically electron-donating methoxy group was important for pigment loss. Compounds with inductive and/or mesmerically electron-withdrawing groups had no effect on pigment loss.

**Figure 37.** 1,2,3-dithiazoles demonstrating In vivo pigment loss in *Xenopus laevis* embryos.

The (*Z*)-4-chloro-*N*-(4-methoxyphenyl)-5*H*-1,2,3-dithiazol-5-imine (**154**) analogue demonstrated complete pigment loss at 10 µM and moderate at 5 µM. Extension of the methoxy to propyloxy **155** or butyloxy **156** reduced the potency, with the addition of a methyl group in the 2-position had the same effect. The formation of a 3,4-fused methyl catacol **157** increased potency, but did not match the activity of the 4-position methoxy **154**. The extension to form that benzyloxy analogue **158** did boost the potency of **154**, resulting in **158** having complete pigment loss at 5 µM.

Skourides et al. extensively investigated the structural features driving the phenotypic effects observed with **159**. An analogue of **159** was synthesized in two steps via the oxime route (Scheme 8) [12,13], where the 4-chlorine substituent was replaced with a phenyl group to give compound **160** (Figure 38). A second analogue of **159** was furnished where the nitrogen of the 1,2,3-dithiazole was replaced with a chlorocarbon in one step from Boberg salt [75,76] to give compound **161** (Figure 38) [75,77].

**Figure 38.** Direct comparison of 1,2,3-dithiazole **159** with disrupted analogues **160** and **161** in In vivo pigment loss in *Xenopus laevis* embryos.

The replacement of the 4-chlorine substituent yielded compound **160**, which showed no phenotypic affect. This supports the idea of an ANRORC-style ring opening mechanism, as the chlorine is a good nucleofuge that facilities the ring opening mechanism [40]. The second analogue **161**, showed some mild activity at 5 µM, while at 10 µM, mild toxicity and developmental defects were observed. This again pointed towards a ring opening ARONOC style mechanism, but more work needs to be done to establish the exact mechanism of action [19].

#### 3.4.2. Antifibrotic Collagen Specific Chaperone hsp47 Inhibitor

Other activities of 1,2,3-dithiazoles include hit compound methyl 6-chloro-3*H*-benzo[*d*] dithiazole-4-carboxylate 2-oxide (**162**), which was reported twice, once in 2005 [18] and the second in 2010 [19]. These reports were both high-throughput screens of the compound library, one from Maybridge Chemical Co., Cornwall, U.K. and the other unspecified.

In 2005, Ananthanarayanan et al. screened a Maybridge compound library against Heat shock protein 47 (Hsp47), which, at the time, had no known inhibitors. Hsp47 is a collagen-specific molecular chaperone whose activity has been implicated in the pathogenesis of fibrotic diseases. The regulation of both Hsp47 and collagen expression has been implicated in several different disease indications where changes in the collagen expression are found. These diseases include fibrotic diseases of the liver [78], kidney [79], lung [80], and skin [81], in addition to atherosclerosis [82] and cancer [83]. The screen resulted in a primary hit rate of 0.2%, with 4 out of 2080 compounds being shown to be inhibitors of Hsp47. Secondary screening confirmed **162** (Figure 39), as the most potent compound (IC<sup>50</sup> = 3.1 µM).

**Figure 39.** Hsp47 inhibitor methyl 6-chloro-3*H*-benzo[*d*][1,2,3]dithiazole-4-carboxylate 2-oxide (**162**).

#### 3.4.3. Arabidopsis Gibberellin 2-Oxidase Inhibitors

In 2010, screening a commercial library of starting points against to Arabidopsis gibberellin 2-oxidases identified compound **162** (Figure 40) [20]. The screening aimed to identify an inhibitor that could both promote Arabidopsis seed germination and seedling growth. Compound **162** was able to do both, without having broad spectrum activity similar to Prohexadione (PHX), which is a broad-spectrum inhibitor of all three 2-oxoglutarate dependent dioxygenase's (2ODD) that were involved in Gibberellin (GA) production (GA 2-oxidase (GA2oxs), GA 3-oxidase (GA3oxs), and GA20-oxidase (GA20oxs)) [84,85]. The 1,2,3-dithiazole **162** was shown to have inhibition GA2oxs with a high degree of specificity, but not on other 2ODDs. The selective inhibition of GA2oxs activity could potentially lead to the delay of GA catabolism in plants, and hence, extend the life of endogenous GA.

**Figure 40.** GA2oxs inhibitor methyl 6-chloro-3*H*-benzo[*d*][1,2,3]dithiazole-4-carboxylate 2-oxide (**162**).

#### **4. Summary and Overview**

The initial observation of the 1,2,3-dithiazole salt 4,5-dichloro-1,2,3-dithiazolium chloride in 1957 [1], was followed by detailed characterization in 1985 [4], and came to be known as Appel salt (**1**) post-1990s [5]. Appel salt (**1**) allowed for one-step access to a range of different chemistries to furnish a wide scope of 5-substituted-1,2,3-dithiazole derivatives [4,5,9,15,27,30–37]. While several additional methods also exist to access C4 substituted derivatives, the main screening has been done on 4-chloro derivatives until more recently [6,12–14,40]. However, synthetic challenges remain, including expanding the chemical space including effective synthesis of *N*-alkyl-5*H*-1,2,3-dithiazol-5-imine analogues, and effective access to 4-pyridyl analogues in good yields [37].

The first screening was carried out by Chevron Research Co. in 1977 [2,3], this relatively detailed study has been the foundation of the phenotypic biology observed on this scaffold. It described detailed work on a series of herbicidal effects and anti-mite efficacy, in addition to antifungal activities. This work was followed up in the late 1990s and 2000s by a series of groups extending the understanding of the antifungal and antibacterial SAR scope of the 1,2,3-dithiazole scaffold [15,50–53]. Interestingly, a patent in 1997 by Rohm & Haas Co. [51] highlighted a potential coating application for the 1,2,3-dithiazole with the discovery of potent antialgae and Gram-negative bacteria inhibition. In 2012, a patent filed by Bayer Cropscience AG presented a much broader library of heteroaromatic derivatives [54], highlighting a wider range of antifungal activities, with high degrees of control of commercially important fungi for crop protection. More recently, the antifungal and antibacterial screening has focused on clinically relevant hospital derived infections with good efficacy [86], in part aided by a series of matched pair 1,2,3-thiaselenazoles [55].

More recently, several other phenotypic observations have been reported. These include antiviral efficacy against FIV as a model for HIV, where modelling and mechanistic rationale point to cystine containing nucleocapsid protein (NCp) as the target for the 1,2,3dithiazole [17,36,63]. Anticancer effects against a broad range of cancer cell lines have also been reported with limited off-target toxicity [13,16,74]. These were also supported by modelling and mechanistic rationale highlighting ASCT2 [65] and LAT1 [70] as potential targets responsible. This rationale has been further supported by a series of mechanism of action experiments [65,70,73]. dithiazole [17,36,63]. Anticancer effects against a broad range of cancer cell lines have also been reported with limited off-target toxicity [13,16,74]. These were also supported by modelling and mechanistic rationale highlighting ASCT2 [65] and LAT1 [70] as potential targets responsible. This rationale has been further supported by a series of mechanism of action experiments [65,70,73]. In addition to these reports, a series of other studies also highlighted other activities

More recently, several other phenotypic observations have been reported. These include antiviral efficacy against FIV as a model for HIV, where modelling and mechanistic rationale point to cystine containing nucleocapsid protein (NCp) as the target for the 1,2,3-

scaffold. It described detailed work on a series of herbicidal effects and anti-mite efficacy, in addition to antifungal activities. This work was followed up in the late 1990s and 2000s by a series of groups extending the understanding of the antifungal and antibacterial SAR scope of the 1,2,3-dithiazole scaffold [15,50–53]. Interestingly, a patent in 1997 by Rohm & Haas Co. [51] highlighted a potential coating application for the 1,2,3-dithiazole with the discovery of potent antialgae and Gram-negative bacteria inhibition. In 2012, a patent filed by Bayer Cropscience AG presented a much broader library of heteroaromatic derivatives [54], highlighting a wider range of antifungal activities, with high degrees of control of commercially important fungi for crop protection. More recently, the antifungal and antibacterial screening has focused on clinically relevant hospital derived infections with good efficacy [86], in part aided by a series of matched pair 1,2,3-thiaselenazoles [55].

*Molecules* **2023**, *28*, x FOR PEER REVIEW 30 of 34

In addition to these reports, a series of other studies also highlighted other activities of the 1,2,3-dithiazole scaffold. These included an anti-melanin phenotype in *Xenopus laevis* embryos, where active potent (>5 µM) non-toxic compounds were identified in an in vivo model [19]. An ANRONC style mechanism of action was proposed supported by a series of chemical modifications to the scaffold [38–40]. Finally, two reports of high-throughput screens identified hit compounds against antifibrotic collagen specific chaperone hsp47 [18] and Arabidopsis Gibberellin 2-Oxidase [20]. of the 1,2,3-dithiazole scaffold. These included an anti-melanin phenotype in *Xenopus laevis* embryos, where active potent (>5 μM) non-toxic compounds were identified in an *in vivo* model [19]. An ANRONC style mechanism of action was proposed supported by a series of chemical modifications to the scaffold [38–40]. Finally, two reports of highthroughput screens identified hit compounds against antifibrotic collagen specific chaperone hsp47 [18] and Arabidopsis Gibberellin 2-Oxidase [20].

The full potential of the 1,2,3-dithiazole scaffold has yet to be realized. Key areas of biological activities have been identified with preliminary work in the literature showing encouraging results. These included activities as antifungal [2], herbicidal [2], antibacterial [15], anticancer [16], antiviral [17], antifibrotic [18], and being a melanin [19] and Arabidopsis gibberellin 2-oxidases [20] inhibitors. These results provide a prospective to the versatility as to what is possible with this scaffold. In addition to these interesting reported biology applications, there are potentially significant untapped chemical biology opportunities towards targeting cystine reactive sites [21–25]; using the ANRORC-style 1,2,3-dithiazole chemistry as a latent functionality (Figure 41). The full potential of the 1,2,3-dithiazole scaffold has yet to be realized. Key areas of biological activities have been identified with preliminary work in the literature showing encouraging results. These included activities as antifungal [2], herbicidal [2], antibacterial [15], anticancer [16], antiviral [17], antifibrotic [18], and being a melanin [19] and Arabidopsis gibberellin 2-oxidases [20] inhibitors. These results provide a prospective to the versatility as to what is possible with this scaffold. In addition to these interesting reported biology applications, there are potentially significant untapped chemical biology opportunities towards targeting cystine reactive sites [21–25]; using the ANRORC-style 1,2,3 dithiazole chemistry as a latent functionality (Figure 41).

**Figure 41.** The 1,2,3-dithiazole as a latent cystine reactive functionality. **Figure 41.** The 1,2,3-dithiazole as a latent cystine reactive functionality.

#### **5. Conclusions 5. Conclusions**

Taken together, the chemistry and biology of the 1,2,3-dithiazoles chemotype has shown a lot of exciting potential. The ANRORC-style rearrangements potentially affording a new route for potential chemical tools and relative cystine within proteins pockets, while the sub-micro molar phenotypic potencies against a series of diverse targets demonstrate potential for further development. Many of these diseases and pathogens have limited treatment options and need new therapies with novel mechanisms of action. The Taken together, the chemistry and biology of the 1,2,3-dithiazoles chemotype has shown a lot of exciting potential. The ANRORC-style rearrangements potentially affording a new route for potential chemical tools and relative cystine within proteins pockets, while the sub-micro molar phenotypic potencies against a series of diverse targets demonstrate potential for further development. Many of these diseases and pathogens have limited treatment options and need new therapies with novel mechanisms of action. The identification of starting points and defined SAR provides the foundation to define a medicinal chemistry trajectory towards optimized inhibitors and potential new treatments for a broad range of diseases.

**Author Contributions:** Conceptualization, C.R.M.A. and A.S.K.; formal analysis, C.R.M.A. and A.S.K.; investigation, C.R.M.A., H.J.O. and A.S.K.; writing—original draft preparation, C.R.M.A. and A.S.K.; writing—review and editing, C.R.M.A. and A.S.K. 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.

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

**Sample Availability:** Not applicable.

### **References**


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*Review* 

#### *Review* **Advances in the Synthesis of Heteroaromatic Hybrid Chalcones Ajay Mallia \* and Joseph Sloop**

**Ajay Mallia \* and Joseph Sloop** Department of Chemistry, School of Science & Technology, Georgia Gwinnett College, 1000 University Center Lane, Lawrenceville, GA 30043, USA; jsloop@ggc.edu

> Department of Chemistry, School of Science & Technology, Georgia Gwinnett College, 1000 University Center Lane, Lawrenceville, GA 30043, USA **Abstract:** Chalcones continue to occupy a venerated status as scaffolds for the construction of a

> **\*** Correspondence: amallia@ggc.edu variety of heterocyclic molecules with medicinal and industrial properties. Syntheses of hybrid chal-

**\*** Correspondence: amallia@ggc.edu

**Abstract:** Chalcones continue to occupy a venerated status as scaffolds for the construction of a variety of heterocyclic molecules with medicinal and industrial properties. Syntheses of hybrid chalcones featuring heteroaromatic components, especially those methods utilizing green chemistry principles, are important additions to the preparative methodologies for this valuable class of molecules. This review outlines the advances made in the last few decades toward the incorporation of heteroaromatic components in the construction of hybrid chalcones and highlights examples of environmentally responsible processes employed in their preparation. cones featuring heteroaromatic components, especially those methods utilizing green chemistry principles, are important additions to the preparative methodologies for this valuable class of molecules. This review outlines the advances made in the last few decades toward the incorporation of heteroaromatic components in the construction of hybrid chalcones and highlights examples of environmentally responsible processes employed in their preparation. **Keywords:** chalcone; heteroaromatic; hybrid chalcone

**Keywords:** chalcone; heteroaromatic; hybrid chalcone

#### **1. Introduction 1. Introduction**  The chalcone class of enones has been a privileged scaffold in organic synthesis for

The chalcone class of enones has been a privileged scaffold in organic synthesis for more than a century. Kostanecki and Tambor are credited with the first reported preparation of E-1,3-diphenylprop-2-en-1-one and coined the term "chalcone" in 1899 [1]. Figure 1 shows the structure of E-chalcone, the most energetically favorable stereoisomer, as well as the sterically encumbered and less common Z-chalcone, both of which contain benzene rings at C<sup>1</sup> and C<sup>3</sup> joined by a three-carbon α,β-unsaturated ketone unit. The absolute configuration of solid chalcone stereochemistry obtained during synthesis can often be determined with X-ray crystallography [2,3]. more than a century. Kostanecki and Tambor are credited with the first reported preparation of E-1,3-diphenylprop-2-en-1-one and coined the term "chalcone" in 1899 [1]. Figure 1 shows the structure of E-chalcone, the most energetically favorable stereoisomer, as well as the sterically encumbered and less common Z-chalcone, both of which contain benzene rings at C1 and C3 joined by a three-carbon α,β-unsaturated ketone unit. The absolute configuration of solid chalcone stereochemistry obtained during synthesis can often be determined with X-ray crystallography [2,3].

**Figure 1.** Chalcone structure and stereochemistry. **Figure 1.** Chalcone structure and stereochemistry.

By convention, the aromatic ring attached to C1 is designated as ring A while the aromatic ring attached to C3 is designated as ring B. For the purposes of this review, we will adhere to the conventional ring designations in describing preparations of heteroar-By convention, the aromatic ring attached to C<sup>1</sup> is designated as ring A while the aromatic ring attached to C<sup>3</sup> is designated as ring B. For the purposes of this review, we will adhere to the conventional ring designations in describing preparations of heteroaromatic hybrid chalcones.

omatic hybrid chalcones. The utility of chalcones both as a pharmacophore and as a scaffold in the synthesis of a wide variety of heterocycles ranging from pyrazoles, isoxazoles, triazoles, barbituric acid derivatives, etc. has been investigated thoroughly over the years, with numerous research articles as well as several reviews appearing in the last decade describing the current chalcone synthetic strategies, the heterocycles derived from them, and the bioactivity The utility of chalcones both as a pharmacophore and as a scaffold in the synthesis of a wide variety of heterocycles ranging from pyrazoles, isoxazoles, triazoles, barbituric acid derivatives, etc. has been investigated thoroughly over the years, with numerous research articles as well as several reviews appearing in the last decade describing the current chalcone synthetic strategies, the heterocycles derived from them, and the bioactivity and pharmaceutical uses of these compounds [4–13]. Within that context, the preparation of more highly functionalized chalcones that contain heteroaromatic components has been an area of intense research over the last decade [10,14–30].

**Citation:** Mallia, A.; Sloop, J. Advances in the Synthesis of Heteroaromatic Hybrid Chalcones. *Molecules* **2023**, *28*, 3201. https:// doi.org/10.3390/molecules28073201 Heteroaromatic Hybrid Chalcones. *Molecules* **2023**, *27*, x. https://doi.org/10.3390/xxxxx Academic Editor: Alexander F.

Academic Editor: Alexander F. Khlebnikov Khlebnikov Received: 12 February 2023

**Citation:** Mallia, A.; Sloop, J. Advances in the Synthesis of

Received: 12 February 2023 Revised: 18 March 2023 Accepted: 21 March 2023 Published: 4 April 2023 Revised: 18 March 2023 Accepted: 21 March 2023 Published: 4 April 2023

**Copyright:** © 2023 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/). censee 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/).

Research has established that heteroaromatic hybrid chalcones themselves possess broad medicinal value as anticancer [16,19,23], antimicrobial [11,20,23,28], antifungal [16], anti-tuberculosis [25] and anti-inflammatory agents [22] as well as having other important pharmacological functions [9,10], agrochemical utility as photosynthesis inhibitors [18] and industrial use as photoinitiators in 3D printing [17]. Figure 2 shows a representative selection of heteroaromatic hybrid chalcone pharmacophores and industrially important compounds. broad medicinal value as anticancer [16,19,23], antimicrobial [11,20,23,28], antifungal [16], anti-tuberculosis [25] and anti-inflammatory agents [22] as well as having other important pharmacological functions [9,10], agrochemical utility as photosynthesis inhibitors [18] and industrial use as photoinitiators in 3D printing [17]. Figure 2 shows a representative selection of heteroaromatic hybrid chalcone pharmacophores and industrially important compounds.

and pharmaceutical uses of these compounds [4–13]. Within that context, the preparation of more highly functionalized chalcones that contain heteroaromatic components has

Research has established that heteroaromatic hybrid chalcones themselves possess

*Molecules* **2023**, *27*, x FOR PEER REVIEW 2 of 28

been an area of intense research over the last decade [10,30].

**Figure 2.** Medicinally and industrially important heteroaromatic hybrid chalcones. **Figure 2.** Medicinally and industrially important heteroaromatic hybrid chalcones.

Synthetic methodologies to prepare hybrid chalcones have developed rapidly over the last two decades. To the best of our knowledge, no reviews have been found that focus on heteroaromatic chalcone synthesis and the green synthesis methods employed to prepare them. This review will focus on the construction of heteroaromatic hybrid chalcones with the Claisen–Schmidt condensation, 1,3-dipolar additions, ring-opening reactions, 3+2 annulations and Wittig reactions. The review will discuss four different heteroaromatic hybrid chalcone types: A-ring and B-ring-substituted mono-heteroaromatic hybrid chalcones, hybrid chalcones possessing heteroaromatic moieties on both the A and B rings, and the synthesis strategies used to prepare heteroaromatic bis chalcone hybrids. Herein, we also detail the green methods that have been employed to prepare these hybrid chalcones including microwave irradiation, sonication, ball milling, continuous flow re-Synthetic methodologies to prepare hybrid chalcones have developed rapidly over the last two decades. To the best of our knowledge, no reviews have been found that focus on heteroaromatic chalcone synthesis and the green synthesis methods employed to prepare them. This review will focus on the construction of heteroaromatic hybrid chalcones with the Claisen–Schmidt condensation, 1,3-dipolar additions, ring-opening reactions, 3+2 annulations and Wittig reactions. The review will discuss four different heteroaromatic hybrid chalcone types: A-ring and B-ring-substituted mono-heteroaromatic hybrid chalcones, hybrid chalcones possessing heteroaromatic moieties on both the A and B rings, and the synthesis strategies used to prepare heteroaromatic bis chalcone hybrids. Herein, we also detail the green methods that have been employed to prepare these hybrid chalcones including microwave irradiation, sonication, ball milling, continuous flow reactions, the use of benign solvents, solvent-free/solid-state processes and nanocatalysis. See Figure 3.

actions, the use of benign solvents, solvent-free/solid-state processes and nanocatalysis.

See Figure 3.

**Figure 3.** Heteroaromatic hybrid chalcone construction. **Figure 3.** Heteroaromatic hybrid chalcone construction.

#### **2. A-Ring Heteroaromatic Hybrid Chalcone Synthesis 2. A-Ring Heteroaromatic Hybrid Chalcone Synthesis**

This section catalogues several representative conventional and green processes by which hybrid chalcones bearing a heteroaromatic species at ring A may be prepared. Heteroaromatic components of the chalcone products include a variety of single-ring (furan, pyrrole, thiazole, thiophene, pyridine, pyrimidine) and fused-ring (indole, benzimidazole, benzothiazole, benzofuran, pyrazolopyridine, quinoline) systems. This section catalogues several representative conventional and green processes by which hybrid chalcones bearing a heteroaromatic species at ring A may be prepared. Heteroaromatic components of the chalcone products include a variety of single-ring (furan, pyrrole, thiazole, thiophene, pyridine, pyrimidine) and fused-ring (indole, benzimidazole, benzothiazole, benzofuran, pyrazolopyridine, quinoline) systems.

#### *2.1. Claisen–Schmidt Condensations 2.1. Claisen–Schmidt Condensations*

The Claisen–Schmidt (C-S) condensation has been widely used to prepare chalcones for many years. This reaction, which can be catalyzed by acids or bases, offers mild conditions that tolerate a wide scope of functionality in both the ketone donors and aldehyde The Claisen–Schmidt (C-S) condensation has been widely used to prepare chalcones for many years. This reaction, which can be catalyzed by acids or bases, offers mild conditions that tolerate a wide scope of functionality in both the ketone donors and aldehyde acceptors.

#### acceptors. 2.1.1. Base-Catalyzed C-S Condensations

2.1.1. Base-Catalyzed C-S Condensations The hydroxide bases KOH, NaOH and to a lesser extent Ba(OH)2 are the bases used to promote the condensations depicted below in Schemes 1–12. These bases may be introduced to the reaction medium as dilute or concentrated aqueous solutions or as solids. Ethanol or methanol are the solvents of choice in most reactions depicted herein. The reaction temperatures vary from 0 °C to those obtained by refluxing the alcoholic solvents. The hydroxide bases KOH, NaOH and to a lesser extent Ba(OH)<sup>2</sup> are the bases used to promote the condensations depicted below in Schemes 1–12. These bases may be introduced to the reaction medium as dilute or concentrated aqueous solutions or as solids. Ethanol or methanol are the solvents of choice in most reactions depicted herein. The reaction temperatures vary from 0 ◦C to those obtained by refluxing the alcoholic solvents. The reaction times range from less than a minute in the case of selected microwave-mediated reactions and can extend to 72 h for the conventional condensations.

The reaction times range from less than a minute in the case of selected microwave-medi-

In our first entry, three room-temperature C-S preparations of pyrrolyl chalcone **3** are presented that have differing reaction times and different base concentrations. Sweeting et al. (Scheme 1a) used strongly basic conditions (60% aqueous KOH) and centrifugation mixing to prepare the pyrrolyl chalcone **3** in a modest yield. The low yield is likely attributed to the short reaction time. [31] Robinson et al. reported that increasing the reaction time ([32], Scheme 1b) using NaOH (aq) in ethanol increased the yield of the pyrrolyl chalcone. Using 20 mol % NaOH (aq) in ethanol, Song et al. obtained a 91% yield in the preparation of the chalcone (Scheme 1c). [33] Lokeshwari's team (Scheme 2a) and Liu's group prepared furyl chalone **5** in an 87% yield using 0.1 mol % KOH (aq) in 4 h, while Liu's group (Scheme 2b) obtained equally high yields with 20 mol% NaOH (aq) in 6 h [34,35]. Robinson et al. (Scheme 3) condensed 2-acetylfuran and 2-acetyl-5-methylfuran with assorted benzaldehydes at room temperature en route to the twelve furyl chalcones

In our first entry, three room-temperature C-S preparations of pyrrolyl chalcone **3** are presented that have differing reaction times and different base concentrations. Sweeting et al. (Scheme 1a) used strongly basic conditions (60% aqueous KOH) and centrifugation mixing to prepare the pyrrolyl chalcone **3** in a modest yield. The low yield is likely attributed to the short reaction time. [31] Robinson et al. reported that increasing the reaction time ([32], Scheme 1b) using NaOH (aq) in ethanol increased the yield of the pyrrolyl chalcone. Using 20 mol % NaOH (aq) in ethanol, Song et al. obtained a 91% yield in the preparation of the chalcone (Scheme 1c). [33] Lokeshwari's team (Scheme 2a) and Liu's group prepared furyl chalone **5** in an 87% yield using 0.1 mol % KOH (aq) in 4 h, while Liu's group (Scheme 2b) obtained equally high yields with 20 mol% NaOH (aq) in 6 h [34,35]. Robinson et al. (Scheme 3) condensed 2-acetylfuran and 2-acetyl-5-methylfuran with assorted benzaldehydes at room temperature en route to the twelve furyl chalcones

In our first entry, three room-temperature C-S preparations of pyrrolyl chalcone **3** are presented that have differing reaction times and different base concentrations. Sweeting et al. (Scheme 1a) used strongly basic conditions (60% aqueous KOH) and centrifugation mixing to prepare the pyrrolyl chalcone **3** in a modest yield. The low yield is likely attributed to the short reaction time. [31] Robinson et al. reported that increasing the reaction time ([32], Scheme 1b) using NaOH (aq) in ethanol increased the yield of the pyrrolyl chalcone. Using 20 mol % NaOH (aq) in ethanol, Song et al. obtained a 91% yield in the preparation of the chalcone (Scheme 1c). [33] Lokeshwari's team (Scheme 2a) and Liu's group prepared furyl chalone **5** in an 87% yield using 0.1 mol % KOH (aq) in 4 h, while Liu's group (Scheme 2b) obtained equally high yields with 20 mol% NaOH (aq) in 6 h [34,35]. Robinson et al. (Scheme 3) condensed 2-acetylfuran and 2-acetyl-5-methylfuran with assorted benzaldehydes at room temperature en route to the twelve furyl chalcones

In our first entry, three room-temperature C-S preparations of pyrrolyl chalcone **3** are presented that have differing reaction times and different base concentrations. Sweeting et al. (Scheme 1a) used strongly basic conditions (60% aqueous KOH) and centrifugation mixing to prepare the pyrrolyl chalcone **3** in a modest yield. The low yield is likely attributed to the short reaction time. [31] Robinson et al. reported that increasing the reaction time ([32], Scheme 1b) using NaOH (aq) in ethanol increased the yield of the pyrrolyl chalcone. Using 20 mol % NaOH (aq) in ethanol, Song et al. obtained a 91% yield in the preparation of the chalcone (Scheme 1c). [33] Lokeshwari's team (Scheme 2a) and Liu's group prepared furyl chalone **5** in an 87% yield using 0.1 mol % KOH (aq) in 4 h, while Liu's group (Scheme 2b) obtained equally high yields with 20 mol% NaOH (aq) in 6 h [34,35]. Robinson et al. (Scheme 3) condensed 2-acetylfuran and 2-acetyl-5-methylfuran with assorted benzaldehydes at room temperature en route to the twelve furyl chalcones

The reaction times range from less than a minute in the case of selected microwave-medi-

The reaction times range from less than a minute in the case of selected microwave-medi-

The reaction times range from less than a minute in the case of selected microwave-medi-

ated reactions and can extend to 72 h for the conventional condensations.

ated reactions and can extend to 72 h for the conventional condensations.

ated reactions and can extend to 72 h for the conventional condensations.

ated reactions and can extend to 72 h for the conventional condensations.

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**Scheme 2.** Synthesis of furyl chalcone. **Scheme 2.** Synthesis of furyl chalcone. **Scheme 2.** Synthesis of furyl chalcone. **Scheme 2.** Synthesis of furyl chalcone.

**Scheme 2.** Synthesis of furyl chalcone.

**8** in modest to medium yields [36].

**8** in modest to medium yields [36].

**8** in modest to medium yields [36].

**8** in modest to medium yields [36].

Parveen et al. reported a nearly quantitative conversion for the room-temperature C-**Scheme 3.** Synthesis of furyl chalcone derivatives. **Scheme 3.** Synthesis of furyl chalcone derivatives. **Scheme 3.** Synthesis of furyl chalcone derivatives. in ethanol to the thienyl chalcone **10** [37]. Sunduru et al. reported the preparation of pyridyl chalcone derivatives **13** by con-

**Scheme 4.** Synthesis of thienyl chalcone. in yields ranging from 67 to 76% (Scheme 5).

Sinha and coworkers (Scheme 6) used similar conditions to synthesize eighteen 1,3-

Zhao et al. (Scheme 7) used reflux conditions to achieve yields in excess of 60% for

the small series of fused-ring indolyl chalcones **18** [40]. In two separate publications, Hsieh and coworkers used base-catalyzed C-S condensations to prepare indolyl (Scheme 8, [41]),

thiazolyl and benzothiazolyl hybrid chalcones (Scheme 9, [42]).

**Scheme 5.** Synthesis of pyridyl chalcone. **Scheme 5.** Synthesis of pyridyl chalcone.

**Scheme 6.** Synthesis of thiazolyl chalcones.

**Scheme 7.** Synthesis of indolyl chalcones.

thiazolylchalcones **16** in very good overall yields [39].

Sunduru et al. reported the preparation of pyridyl chalcone derivatives **13** by condensing 4-acetylpyridine with the respective aromatic aldehyde (Scheme 5) [38]. In this reaction, one equivalent of 4-acetylpyridine was added dropwise to a cooled methanolic solution containing 10% aqueous NaOH. Then, one equivalent of aldehyde was added slowly at 0 °C. After workup and recrystallization, the pyridyl chalcones were obtained

Sunduru et al. reported the preparation of pyridyl chalcone derivatives **13** by condensing 4-acetylpyridine with the respective aromatic aldehyde (Scheme 5) [38]. In this reaction, one equivalent of 4-acetylpyridine was added dropwise to a cooled methanolic solution containing 10% aqueous NaOH. Then, one equivalent of aldehyde was added slowly at 0 °C. After workup and recrystallization, the pyridyl chalcones were obtained

Sinha and coworkers (Scheme 6) used similar conditions to synthesize eighteen 1,3-

Sinha and coworkers (Scheme 6) used similar conditions to synthesize eighteen 1,3-

**Scheme 4.** Synthesis of thienyl chalcone.

**Scheme 4.** Synthesis of thienyl chalcone.

*Molecules* **2023**, *27*, x FOR PEER REVIEW 5 of 28

in yields ranging from 67 to 76% (Scheme 5).

in yields ranging from 67 to 76% (Scheme 5).

**Scheme 5.** Synthesis of pyridyl chalcone.

**Scheme 5.** Synthesis of pyridyl chalcone.

thiazolylchalcones **16** in very good overall yields [39].

thiazolylchalcones **16** in very good overall yields [39].

**Scheme 6.** Synthesis of thiazolyl chalcones. **Scheme 6.** Synthesis of thiazolyl chalcones. thiazolyl and benzothiazolyl hybrid chalcones (Scheme 9, [42]).

**Scheme 7. Scheme 7.**  Synthesis of indolyl chalcones. Synthesis of indolyl chalcones.

**Scheme 8.** Synthesis of benzimidazolyl chalcones. **Scheme 8.** Synthesis of benzimidazolyl chalcones. **Scheme 8.** Synthesis of benzimidazolyl chalcones.

Saito's team used 5% KOH in ethanol at room temperature to prepare a series of

Saito's team used 5% KOH in ethanol at room temperature to prepare a series of

Grigoropoulou's team found barium hydroxide octahydrate effective in promoting the condensation of both single- and fused-ring heteroaromatic ketones with dehydroabietic acid methyl ester en route to sixteen hybrid chalcones in good overall yields

Grigoropoulou's team found barium hydroxide octahydrate effective in promoting the condensation of both single- and fused-ring heteroaromatic ketones with dehydroabietic acid methyl ester en route to sixteen hybrid chalcones in good overall yields

functionalized benzofuran hybrid chalcones in yields as high as 97% (Scheme 10) [43].

functionalized benzofuran hybrid chalcones in yields as high as 97% (Scheme 10) [43].

**Scheme 9.** Synthesis of thiazolyl and benzothiazolyl chalcones. **Scheme 9.** Synthesis of thiazolyl and benzothiazolyl chalcones. **Scheme 9.** Synthesis of thiazolyl and benzothiazolyl chalcones.

**Scheme 10.** Synthesis of benzofuryl chalcones.

**Scheme 10.** Synthesis of benzofuryl chalcones.

(Scheme 11) [44].

(Scheme 11) [44].

0.3 *M* NaOH in MeOH 0 oC RT 5 min overnight

(15-25%)

0.3 *M* NaOH in MeOH 0 oC RT 5 min overnight

(17-61%)

Saito's team used 5% KOH in ethanol at room temperature to prepare a series of

functionalized benzofuran hybrid chalcones in yields as high as 97% (Scheme 10) [43].

**<sup>21</sup> <sup>15</sup> 22 a-c**

O

N S

**a**: R = 4-MeO, 24% **b**: R = 4-Me2N, 25% **c**: R = -O2NC6H4, 15%

N S

> **a**: R = 4-MeO, 61% **b**: R = 4-Me2N, 17% **c**: R = 4-O2NC6H4, 42%

**24 a-c**

O

R

R

**Scheme 10.** Synthesis of benzofuryl chalcones. **Scheme 10.** Synthesis of benzofuryl chalcones.

**Scheme 8.** Synthesis of benzimidazolyl chalcones.

+ CHO

+ CHO

**15**

**Scheme 9.** Synthesis of thiazolyl and benzothiazolyl chalcones.

R

R

N

N

S O

**23**

S O

**Scheme 11.** Synthesis of heteroaromatic dehydroabietic acid-chalcone hybrids. **Scheme 11.** Synthesis of heteroaromatic dehydroabietic acid-chalcone hybrids. yield (Scheme 12).

persed in water at 4 °C and after workup the pyridyl chalcone **31** was obtained in a good yield (Scheme 12). **Scheme 12.** Green synthesis of pyridyl chalcone. **Scheme 12.** Green synthesis of pyridyl chalcone.

**Scheme 12.** Green synthesis of pyridyl chalcone. Jianga et al. showed that the condensation of 2-acetylfuran or 2-acetylthiophene and benzaldehyde using 2 mol% NaOH (aq) gave (E)-1-(Furan-2-yl)-3-phenylprop-2-en-1-one or (E)-1-(thiophen-2-yl)-3-phenylprop-2-en-1-one at room temperature in nearly quantitative yields (Scheme 13) [46]. Jianga et al. showed that the condensation of 2-acetylfuran or 2-acetylthiophene and benzaldehyde using 2 mol% NaOH (aq) gave (E)-1-(Furan-2-yl)-3-phenylprop-2-en-1-one or (E)-1-(thiophen-2-yl)-3-phenylprop-2-en-1-one at room temperature in nearly quantitative yields (Scheme 13) [46]. **Scheme 13.** Green synthesis of furyl and thienyl chalcones. In our first entry, three room-temperature C-S preparations of pyrrolyl chalcone **3** are presented that have differing reaction times and different base concentrations. Sweeting et al. (Scheme 1a) used strongly basic conditions (60% aqueous KOH) and centrifugation mixing to prepare the pyrrolyl chalcone **3** in a modest yield. The low yield is likely attributed to the short reaction time. Ref. [31] Robinson et al. reported that increasing the reaction time ([32], Scheme 1b) using NaOH (aq) in ethanol increased the yield of the pyrrolyl chalcone. Using 20 mol % NaOH (aq) in ethanol, Song et al. obtained a 91% yield in the preparation of the chalcone (Scheme 1c). Ref. [33] Lokeshwari's team (Scheme 2a) and Liu's group prepared furyl chalone **5** in an 87% yield using 0.1 mol % KOH (aq) in 4 h, while Liu's group (Scheme 2b) obtained equally high yields with 20 mol% NaOH (aq) in 6 h [34,35]. Robinson et al. (Scheme 3) condensed 2-acetylfuran and 2-acetyl-5-methylfuran with assorted benzaldehydes at room temperature en route to the twelve furyl chalcones **8** in modest to medium yields [36].

Ritter et al. (Scheme 14) used 2-acetylthiophene **9** and assorted benzaldehydes in glycerin solvent to prepare seven 2-thienochalcones **10** and **32a–f** in very good yields [47].

**Scheme 13.** Green synthesis of furyl and thienyl chalcones.

**Scheme 14.** Green synthesis of 2-thienyl chalcones.

**Scheme 14.** Green synthesis of 2-thienyl chalcones.

Parveen et al. reported a nearly quantitative conversion for the room-temperature C-S condensation of 2-acetylthiophene and benzaldehyde using aqueous KOH (Scheme 4) in ethanol to the thienyl chalcone **10** [37].

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Sunduru et al. reported the preparation of pyridyl chalcone derivatives **13** by condensing 4-acetylpyridine with the respective aromatic aldehyde (Scheme 5) [38]. In this reaction, one equivalent of 4-acetylpyridine was added dropwise to a cooled methanolic solution containing 10% aqueous NaOH. Then, one equivalent of aldehyde was added slowly at 0 ◦C. After workup and recrystallization, the pyridyl chalcones were obtained in yields ranging from 67 to 76% (Scheme 5).

Sinha and coworkers (Scheme 6) used similar conditions to synthesize eighteen 1,3-thiazolylchalcones **16** in very good overall yields [39].

Zhao et al. (Scheme 7) used reflux conditions to achieve yields in excess of 60% for the small series of fused-ring indolyl chalcones **18** [40]. In two separate publications, Hsieh and coworkers used base-catalyzed C-S condensations to prepare indolyl (Scheme 8, [41]), thiazolyl and benzothiazolyl hybrid chalcones (Scheme 9, [42]). **Scheme 11.** Synthesis of heteroaromatic dehydroabietic acid-chalcone hybrids. Base-catalyzed C-S condensations have also been demonstrated using green principles. These processes include the use of benign solvents including water and microwave **Scheme 11.** Synthesis of heteroaromatic dehydroabietic acid-chalcone hybrids. Base-catalyzed C-S condensations have also been demonstrated using green principles. These processes include the use of benign solvents including water and microwave

Saito's team used 5% KOH in ethanol at room temperature to prepare a series of functionalized benzofuran hybrid chalcones in yields as high as 97% (Scheme 10) [43]. irradiation. Mubofu and Engberts reported a C-S condensation reaction of 2-acetylpyridine and benzaldehyde using 10% NaOH (Scheme 12) [45]. The reagents were finely disirradiation. Mubofu and Engberts reported a C-S condensation reaction of 2-acetylpyridine and benzaldehyde using 10% NaOH (Scheme 12) [45]. The reagents were finely dis-

Grigoropoulou's team found barium hydroxide octahydrate effective in promoting the condensation of both single- and fused-ring heteroaromatic ketones with dehydroabietic acid methyl ester en route to sixteen hybrid chalcones in good overall yields (Scheme 11) [44]. persed in water at 4 °C and after workup the pyridyl chalcone **31** was obtained in a good yield (Scheme 12). persed in water at 4 °C and after workup the pyridyl chalcone **31** was obtained in a good yield (Scheme 12).

Base-catalyzed C-S condensations have also been demonstrated using green principles. These processes include the use of benign solvents including water and microwave irradiation. Mubofu and Engberts reported a C-S condensation reaction of 2-acetylpyridine and benzaldehyde using 10% NaOH (Scheme 12) [45]. The reagents were finely dispersed in water at 4 ◦C and after workup the pyridyl chalcone **31** was obtained in a good yield (Scheme 12). **Scheme 12.** Green synthesis of pyridyl chalcone. **Scheme 12.** Green synthesis of pyridyl chalcone. Jianga et al. showed that the condensation of 2-acetylfuran or 2-acetylthiophene and

Jianga et al. showed that the condensation of 2-acetylfuran or 2-acetylthiophene and benzaldehyde using 2 mol% NaOH (aq) gave (E)-1-(Furan-2-yl)-3-phenylprop-2-en-1-one or (E)-1-(thiophen-2-yl)-3-phenylprop-2-en-1-one at room temperature in nearly quantitative yields (Scheme 13) [46]. Jianga et al. showed that the condensation of 2-acetylfuran or 2-acetylthiophene and benzaldehyde using 2 mol% NaOH (aq) gave (E)-1-(Furan-2-yl)-3-phenylprop-2-en-1-one or (E)-1-(thiophen-2-yl)-3-phenylprop-2-en-1-one at room temperature in nearly quantitative yields (Scheme 13) [46]. benzaldehyde using 2 mol% NaOH (aq) gave (E)-1-(Furan-2-yl)-3-phenylprop-2-en-1-one or (E)-1-(thiophen-2-yl)-3-phenylprop-2-en-1-one at room temperature in nearly quantitative yields (Scheme 13) [46].

**Scheme 13.** Green synthesis of furyl and thienyl chalcones. **Scheme 13.** Green synthesis of furyl and thienyl chalcones. **Scheme 13.** Green synthesis of furyl and thienyl chalcones.

Ritter et al. (Scheme 14) used 2-acetylthiophene **9** and assorted benzaldehydes in glycerin solvent to prepare seven 2-thienochalcones **10** and **32a–f** in very good yields [47]. Ritter et al. (Scheme 14) used 2-acetylthiophene **9** and assorted benzaldehydes in glycerin solvent to prepare seven 2-thienochalcones **10** and **32a–f** in very good yields [47]. Ritter et al. (Scheme 14) used 2-acetylthiophene **9** and assorted benzaldehydes in glycerin solvent to prepare seven 2-thienochalcones **10** and **32a–f** in very good yields [47].

Khan and Asiri (Scheme 15) showed that 3-acetylthiophene **33** underwent a microwavemediated C-S condensation with several benzaldehydes in less than a minute to give thienyl chalcones **34a–f** in yields exceeding 82% [48].

give thienyl chalcones **34a–f** in yields exceeding 82% [48].

give thienyl chalcones **34a–f** in yields exceeding 82% [48].

give thienyl chalcones **34a–f** in yields exceeding 82% [48].

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**Scheme 15.** Microwave synthesis of 3-thienyl chalcones. **Scheme 15.** Microwave synthesis of 3-thienyl chalcones. **Scheme 15.** Microwave synthesis of 3-thienyl chalcones. **Scheme 15.** Microwave synthesis of 3-thienyl chalcones.

Sarveswari and Vijayakumar (Scheme 16) conducted a comparative study of conventional and microwave processes in which four examples of highly substituted quinolinyl hybrid chalcones **36a–d** were prepared [49]. Both processes gave the desired chalcones in yields greater than 75%. Particularly noteworthy is the fact that the microwave reaction time is 1/144 of the conventional reaction time. Sarveswari and Vijayakumar (Scheme 16) conducted a comparative study of conventional and microwave processes in which four examples of highly substituted quinolinyl hybrid chalcones **36a–d** were prepared [49]. Both processes gave the desired chalcones in yields greater than 75%. Particularly noteworthy is the fact that the microwave reaction time is 1/144 of the conventional reaction time. Sarveswari and Vijayakumar (Scheme 16) conducted a comparative study of conventional and microwave processes in which four examples of highly substituted quinolinyl hybrid chalcones **36a–d** were prepared [49]. Both processes gave the desired chalcones in yields greater than 75%. Particularly noteworthy is the fact that the microwave reaction time is 1/144 of the conventional reaction time. Sarveswari and Vijayakumar (Scheme 16) conducted a comparative study of conventional and microwave processes in which four examples of highly substituted quinolinyl hybrid chalcones **36a–d** were prepared [49]. Both processes gave the desired chalcones in yields greater than 75%. Particularly noteworthy is the fact that the microwave reaction time is 1/144 of the conventional reaction time.

Khan and Asiri (Scheme 15) showed that 3-acetylthiophene **33** underwent a microwave-mediated C-S condensation with several benzaldehydes in less than a minute to

Khan and Asiri (Scheme 15) showed that 3-acetylthiophene **33** underwent a microwave-mediated C-S condensation with several benzaldehydes in less than a minute to

Khan and Asiri (Scheme 15) showed that 3-acetylthiophene **33** underwent a microwave-mediated C-S condensation with several benzaldehydes in less than a minute to

**Scheme 16.** Synthesis of quinolinyl chalcones. **Scheme 16.** Synthesis of quinolinyl chalcones. **Scheme 16.** Synthesis of quinolinyl chalcones. **Scheme 16.** Synthesis of quinolinyl chalcones.

Polo et al. demonstrated that sonochemical mediation was very effective in preparing a series of pyrazolopyridyl hybrid chalcones **38a–e** (Scheme 17) in high yields that compare favorably with conventional base-catalyzed C-S condensations [50]. Polo et al. demonstrated that sonochemical mediation was very effective in preparing a series of pyrazolopyridyl hybrid chalcones **38a–e** (Scheme 17) in high yields that compare favorably with conventional base-catalyzed C-S condensations [50]. Polo et al. demonstrated that sonochemical mediation was very effective in preparing a series of pyrazolopyridyl hybrid chalcones **38a–e** (Scheme 17) in high yields that compare favorably with conventional base-catalyzed C-S condensations [50]. Polo et al. demonstrated that sonochemical mediation was very effective in preparing a series of pyrazolopyridyl hybrid chalcones **38a–e** (Scheme 17) in high yields that compare favorably with conventional base-catalyzed C-S condensations [50].

**Scheme 17.** Sonochemical synthesis of pyrazolopyridyl chalcones. **Scheme 17.** Sonochemical synthesis of pyrazolopyridyl chalcones. **Scheme 17.** Sonochemical synthesis of pyrazolopyridyl chalcones. **Scheme 17.** Sonochemical synthesis of pyrazolopyridyl chalcones.

2.1.2. Acid-Catalyzed C-S Condensations 2.1.2. Acid-Catalyzed C-S Condensations

2.1.2. Acid-Catalyzed C-S Condensations In the recent literature, Adnan et al. showed that *p*-toluenesulfonic acid (PTSA) effectively catalyzed the condensation of 2-acetylthiophene (**9**) and *p*-tolualdehyde (**2**) in a green solventless process in which the reactants were ground in a warm mortar and pestle 2.1.2. Acid-Catalyzed C-S Condensations In the recent literature, Adnan et al. showed that *p*-toluenesulfonic acid (PTSA) effectively catalyzed the condensation of 2-acetylthiophene (**9**) and *p*-tolualdehyde (**2**) in a green solventless process in which the reactants were ground in a warm mortar and pestle In the recent literature, Adnan et al. showed that *p*-toluenesulfonic acid (PTSA) effectively catalyzed the condensation of 2-acetylthiophene (**9**) and *p*-tolualdehyde (**2**) in a green solventless process in which the reactants were ground in a warm mortar and pestle for 4 min to give the thienyl chalcone **32e** in a very good yield [13]. See Scheme 18. In the recent literature, Adnan et al. showed that *p*-toluenesulfonic acid (PTSA) effectively catalyzed the condensation of 2-acetylthiophene (**9**) and *p*-tolualdehyde (**2**) in a green solventless process in which the reactants were ground in a warm mortar and pestle for 4 min to give the thienyl chalcone **32e** in a very good yield [13]. See Scheme 18. *Molecules* **2023**, *27*, x FOR PEER REVIEW 9 of 28

> Shaik et al. reported an acid-catalyzed condensation reaction of 2,4-dimethyl-5 acetylthiazole with 2,4-difluorobenzaldehyde to prepare (E)-1-(2′,4′-dimethyl)-(5-

> Our final installment of A-ring hybrid chalcone synthesis is an interesting green coupling reaction between a series of arylacetylene derivatives (**42a–j**) and various pyridine and benzopyridine carboxaldehydes (Scheme 20). Yadav's group showed that a copperbased silica-coated magnetic nanocatalyst (Cu@DBM@ASMNPs) used in conjunction with a piperidine base was very effective in preparing ten hybrid chalcones in yields ranging from 49 to 94% [51]. A noteworthy feature of this reaction was the ability to recover the catalyst via a magnet. The catalyst was reported to be efficient for up to seven reaction

> > R2

O

**f:** R1 = t-Bu, R2 = 2-(4-Clpyridyl), 56% **g**: R1 = Me, R2 = 2-(4-Clpyridyl), 49% **h**: R1 = F, R2 = 2-(4-Clpyridyl), 71% **i**: R1 = H, R2 = 2-(5,6-benzopyridyl), 75% **j**: R1 = MeO, R2=2-(4-Clpyridyl), 90%

**Scheme 18.** PTSA-catalyzed synthesis of thienyl chalcone. **Scheme 18.** PTSA-catalyzed synthesis of thienyl chalcone.

**Scheme 19.** Acid-catalyzed synthesis of thiazolyl chalcone.

<sup>+</sup> R2 CHO Cu@DBM@ASMNPs

**a**: R1 = H, R2 = 2-pyridyl, 94% **b**: R1 = t-Bu, R2 = 2-pyridyl, 90% **c**: R1 = Me, R2 = 2-pyridyl, 76% **d**: R1 = F, R2 = 2-pyridyl, 55% **e**: R1 = H, R2 = 2-(4-Clpyridyl), 88%

**3. B-Ring Heteroaromatic Hybrid Chalcone Synthesis** 

piperidine, neat, N2

R1 **42 2** (49-94%) **43a-j**

**Scheme 20.** Cu-based nanocatalyzed A3 synthesis of pyridyl- and benzopyridyl chalcones.

This section catalogues selected conventional and green processes by which hybrid chalcones containing a heteroaromatic component at ring B may be prepared. In addition, examples of tandem ring-opening dipolar additions to obtain ring B heteroaromatic substituted chalcones are presented. The heteroaromatic components of the chalcone products highlighted in this section include a variety of single-ring (furan, pyrrole, pyrazole, thiazole, thiophene, pyridine) and fused-ring (indole, benzimidazole, benzothiazole, benzofuran, quinoline, imidazo [1,2-a]pyrimidine or imidazo [1,2-a]pyridine, quinoxaline,

As in the preceding section, Claisen–Schmidt (C-S) condensation has been widely used to prepare B-ring heteroaromatic chalcones. This reaction, which can be catalyzed

*2.2. Non C-S Condensations*

cycles.

R1

carbazole) systems.

*3.1. Claisen–Schmidt Condensations*

Shaik et al. reported an acid-catalyzed condensation reaction of 2,4-dimethyl-5-acetylthiazole with 2,4-difluorobenzaldehyde to prepare (E)-1-(20 ,40 -dimethyl)- (5-acetylthiazole)-(2,4"-difluorophenyl)-prop-2-en-1-one (Scheme 19) [23]. Shaik et al. reported an acid-catalyzed condensation reaction of 2,4-dimethyl-5 acetylthiazole with 2,4-difluorobenzaldehyde to prepare (E)-1-(2′,4′-dimethyl)-(5 acetylthiazole)-(2,4″-difluorophenyl)-prop-2-en-1-one (Scheme 19) [23]. acetylthiazole)-(2,4″-difluorophenyl)-prop-2-en-1-one (Scheme 19) [23].

Shaik et al. reported an acid-catalyzed condensation reaction of 2,4-dimethyl-5 acetylthiazole with 2,4-difluorobenzaldehyde to prepare (E)-1-(2′,4′-dimethyl)-(5-

**Scheme 19.** Acid-catalyzed synthesis of thiazolyl chalcone. **Scheme 19.** Acid-catalyzed synthesis of thiazolyl chalcone. *2.2. Non C-S Condensations*

**Scheme 18.** PTSA-catalyzed synthesis of thienyl chalcone.

**Scheme 18.** PTSA-catalyzed synthesis of thienyl chalcone.

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*Molecules* **2023**, *27*, x FOR PEER REVIEW 9 of 28

#### *2.2. Non C-S Condensations 2.2. Non C-S Condensations* Our final installment of A-ring hybrid chalcone synthesis is an interesting green cou-

Our final installment of A-ring hybrid chalcone synthesis is an interesting green coupling reaction between a series of arylacetylene derivatives (**42a–j**) and various pyridine and benzopyridine carboxaldehydes (Scheme 20). Yadav's group showed that a copperbased silica-coated magnetic nanocatalyst (Cu@DBM@ASMNPs) used in conjunction with a piperidine base was very effective in preparing ten hybrid chalcones in yields ranging from 49 to 94% [51]. A noteworthy feature of this reaction was the ability to recover the catalyst via a magnet. The catalyst was reported to be efficient for up to seven reaction Our final installment of A-ring hybrid chalcone synthesis is an interesting green coupling reaction between a series of arylacetylene derivatives (**42a–j**) and various pyridine and benzopyridine carboxaldehydes (Scheme 20). Yadav's group showed that a copperbased silica-coated magnetic nanocatalyst (Cu@DBM@ASMNPs) used in conjunction with a piperidine base was very effective in preparing ten hybrid chalcones in yields ranging from 49 to 94% [51]. A noteworthy feature of this reaction was the ability to recover the catalyst via a magnet. The catalyst was reported to be efficient for up to seven reaction cycles. pling reaction between a series of arylacetylene derivatives (**42a–j**) and various pyridine and benzopyridine carboxaldehydes (Scheme 20). Yadav's group showed that a copperbased silica-coated magnetic nanocatalyst (Cu@DBM@ASMNPs) used in conjunction with a piperidine base was very effective in preparing ten hybrid chalcones in yields ranging from 49 to 94% [51]. A noteworthy feature of this reaction was the ability to recover the catalyst via a magnet. The catalyst was reported to be efficient for up to seven reaction cycles.

**d**: R1 = F, R2 = 2-pyridyl, 55% **e**: R1 = H, R2 = 2-(4-Clpyridyl), 88% **i**: R1 = H, R2 = 2-(5,6-benzopyridyl), 75% **j**: R1 = MeO, R2=2-(4-Clpyridyl), 90% **Scheme 20.** Cu-based nanocatalyzed A3 synthesis of pyridyl- and benzopyridyl chalcones. **Scheme 20.** Cu-based nanocatalyzed A<sup>3</sup> synthesis of pyridyl- and benzopyridyl chalcones.

#### **Scheme 20.** Cu-based nanocatalyzed A3 synthesis of pyridyl- and benzopyridyl chalcones. **3. B-Ring Heteroaromatic Hybrid Chalcone Synthesis 3. B-Ring Heteroaromatic Hybrid Chalcone Synthesis**

**3. B-Ring Heteroaromatic Hybrid Chalcone Synthesis**  This section catalogues selected conventional and green processes by which hybrid chalcones containing a heteroaromatic component at ring B may be prepared. In addition, examples of tandem ring-opening dipolar additions to obtain ring B heteroaromatic substituted chalcones are presented. The heteroaromatic components of the chalcone products highlighted in this section include a variety of single-ring (furan, pyrrole, pyrazole, thiazole, thiophene, pyridine) and fused-ring (indole, benzimidazole, benzothiazole, ben-This section catalogues selected conventional and green processes by which hybrid chalcones containing a heteroaromatic component at ring B may be prepared. In addition, examples of tandem ring-opening dipolar additions to obtain ring B heteroaromatic substituted chalcones are presented. The heteroaromatic components of the chalcone products highlighted in this section include a variety of single-ring (furan, pyrrole, pyrazole, thiazole, thiophene, pyridine) and fused-ring (indole, benzimidazole, benzothiazole, benzofuran, quinoline, imidazo [1,2-a]pyrimidine or imidazo [1,2-a]pyridine, quinoxaline, carbazole) systems. This section catalogues selected conventional and green processes by which hybrid chalcones containing a heteroaromatic component at ring B may be prepared. In addition, examples of tandem ring-opening dipolar additions to obtain ring B heteroaromatic substituted chalcones are presented. The heteroaromatic components of the chalcone products highlighted in this section include a variety of single-ring (furan, pyrrole, pyrazole, thiazole, thiophene, pyridine) and fused-ring (indole, benzimidazole, benzothiazole, benzofuran, quinoline, imidazo [1,2-a]pyrimidine or imidazo [1,2-a]pyridine, quinoxaline, carbazole) systems.

#### zofuran, quinoline, imidazo [1,2-a]pyrimidine or imidazo [1,2-a]pyridine, quinoxaline, carbazole) systems. *3.1. Claisen–Schmidt Condensations*

*3.1. Claisen–Schmidt Condensations* As in the preceding section, Claisen–Schmidt (C-S) condensation has been widely used to prepare B-ring heteroaromatic chalcones. This reaction, which can be catalyzed *3.1. Claisen–Schmidt Condensations* As in the preceding section, Claisen–Schmidt (C-S) condensation has been widely used to prepare B-ring heteroaromatic chalcones. This reaction, which can be catalyzed As in the preceding section, Claisen–Schmidt (C-S) condensation has been widely used to prepare B-ring heteroaromatic chalcones. This reaction, which can be catalyzed by bases or acids, offers mild conditions that tolerate a wide scope of functionality in both the ketone donors and aldehyde acceptors.

#### Base-Catalyzed C-S Condensations

In the preparations shown below, NaOH and KOH are the bases of choice. Shown in Scheme 21, Li et al. used dilute aqueous KOH to prepare pyrrolyl chalcone (**46**) in a very good yield. Using mild conditions, Robinson et al. (Scheme 22) condensed acetophenones **47** and furfural derivatives **48** to prepare five furyl chalcones (**49a–e**) that show promise as monoamine oxidase inhibitors in low to medium yields [36].

as monoamine oxidase inhibitors in low to medium yields [36].

as monoamine oxidase inhibitors in low to medium yields [36].

as monoamine oxidase inhibitors in low to medium yields [36].

by bases or acids, offers mild conditions that tolerate a wide scope of functionality in both

by bases or acids, offers mild conditions that tolerate a wide scope of functionality in both

In the preparations shown below, NaOH and KOH are the bases of choice. Shown in Scheme 21, Li et al. used dilute aqueous KOH to prepare pyrrolyl chalcone (**46**) in a very good yield. Using mild conditions, Robinson et al. (Scheme 22) condensed acetophenones **47** and furfural derivatives **48** to prepare five furyl chalcones (**49a–e**) that show promise

by bases or acids, offers mild conditions that tolerate a wide scope of functionality in both

In the preparations shown below, NaOH and KOH are the bases of choice. Shown in Scheme 21, Li et al. used dilute aqueous KOH to prepare pyrrolyl chalcone (**46**) in a very good yield. Using mild conditions, Robinson et al. (Scheme 22) condensed acetophenones **47** and furfural derivatives **48** to prepare five furyl chalcones (**49a–e**) that show promise

In the preparations shown below, NaOH and KOH are the bases of choice. Shown in Scheme 21, Li et al. used dilute aqueous KOH to prepare pyrrolyl chalcone (**46**) in a very good yield. Using mild conditions, Robinson et al. (Scheme 22) condensed acetophenones **47** and furfural derivatives **48** to prepare five furyl chalcones (**49a–e**) that show promise

**Scheme 21.** Pyrrolyl chalcone synthesis. **Scheme 21.** Pyrrolyl chalcone synthesis. **Scheme 21.** Pyrrolyl chalcone synthesis.

the ketone donors and aldehyde acceptors.

the ketone donors and aldehyde acceptors.

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Base-Catalyzed C-S Condensations

the ketone donors and aldehyde acceptors.

Base-Catalyzed C-S Condensations

Base-Catalyzed C-S Condensations

**Scheme 22.** Furyl chalcone synthesis. **Scheme 22.** Furyl chalcone synthesis.

**Scheme 22.** Furyl chalcone synthesis. In Scheme 23, Fu and coworkers reacted 1,2,3-triazole-substituted acetophenones **50** with furfural **48** and thiophene-2-carbaldehyde **51** in ethanolic KOH for 3 h to prepare hybrid chalcones **52a** and **52b** in satisfactory yields. Condensation of **50** and pyridine In Scheme 23, Fu and coworkers reacted 1,2,3-triazole-substituted acetophenones **50** with furfural **48** and thiophene-2-carbaldehyde **51** in ethanolic KOH for 3 h to prepare hybrid chalcones **52a** and **52b** in satisfactory yields. Condensation of **50** and pyridine carbaldehydes **53a–b** under the same conditions provided eight additional pyridyl hybrid In Scheme 23, Fu and coworkers reacted 1,2,3-triazole-substituted acetophenones **50** with furfural **48** and thiophene-2-carbaldehyde **51** in ethanolic KOH for 3 h to prepare hybrid chalcones **52a** and **52b** in satisfactory yields. Condensation of **50** and pyridine carbaldehydes **53a–b** under the same conditions provided eight additional pyridyl hybrid chalcone examples **54a–h** in yields ranging from 50 to 79% [52]. In Scheme 23, Fu and coworkers reacted 1,2,3-triazole-substituted acetophenones **50** with furfural **48** and thiophene-2-carbaldehyde **51** in ethanolic KOH for 3 h to prepare hybrid chalcones **52a** and **52b** in satisfactory yields. Condensation of **50** and pyridine carbaldehydes **53a–b** under the same conditions provided eight additional pyridyl hybrid chalcone examples **54a–h** in yields ranging from 50 to 79% [52].

Gadhave and Uphade demonstrated the satisfactory condensation of 4-morpholinoacetophenone **55** with 4-pyrazolocarbaldehydes **56** conducted at room temperature, which provided five examples of 4-pyrazolylchalcones **57** [53]. See Scheme 24.

which provided five examples of 4-pyrazolylchalcones **57** [53]. See Scheme 24.

which provided five examples of 4-pyrazolylchalcones **57** [53]. See Scheme 24.

**Scheme 24.** Pyrazolyl chalcone synthesis. **Scheme 24.** Pyrazolyl chalcone synthesis. An interesting study conducted by Mallik and associates involves the preparation of

**Scheme 23.** Furyl, thienyl and pyridyl chalcone synthesis.

**Scheme 23.** Furyl, thienyl and pyridyl chalcone synthesis.

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An interesting study conducted by Mallik and associates involves the preparation of pyrrole-substituted hybrid chalcones from the C-S condensation of several acetophenones **58** and 2-formylpyrrole **44** under different molar ratios of **58**:**44** [54]. As Scheme 25 shows, the desired product **59** predominated when the reactant molar ratios were 1:1, but when the ratio was lowered to 1:2, a nearly equal proportion of the product mixture was found to be the heteroaromatic ketone **60**. Upon increasing the molar proportion of **58** to four times that of **44**, ketone **60** was the major product. The authors propose an interesting mechanism by which **60** is formed—a twin aldol addition—intramolecular cyclization-An interesting study conducted by Mallik and associates involves the preparation of pyrrole-substituted hybrid chalcones from the C-S condensation of several acetophenones **58** and 2-formylpyrrole **44** under different molar ratios of **58**:**44** [54]. As Scheme 25 shows, the desired product **59** predominated when the reactant molar ratios were 1:1, but when the ratio was lowered to 1:2, a nearly equal proportion of the product mixture was found to be the heteroaromatic ketone **60**. Upon increasing the molar proportion of **58** to four times that of **44**, ketone **60** was the major product. The authors propose an interesting mechanism by which **60** is formed—a twin aldol addition—intramolecular cyclization-dehydration. pyrrole-substituted hybrid chalcones from the C-S condensation of several acetophenones **58** and 2-formylpyrrole **44** under different molar ratios of **58**:**44** [54]. As Scheme 25 shows, the desired product **59** predominated when the reactant molar ratios were 1:1, but when the ratio was lowered to 1:2, a nearly equal proportion of the product mixture was found to be the heteroaromatic ketone **60**. Upon increasing the molar proportion of **58** to four times that of **44**, ketone **60** was the major product. The authors propose an interesting mechanism by which **60** is formed—a twin aldol addition—intramolecular cyclizationdehydration.

Gadhave and Uphade demonstrated the satisfactory condensation of 4-morpholinoacetophenone **55** with 4-pyrazolocarbaldehydes **56** conducted at room temperature,

R

Gadhave and Uphade demonstrated the satisfactory condensation of 4-morpholinoacetophenone **55** with 4-pyrazolocarbaldehydes **56** conducted at room temperature,


**Scheme 25.** Pyrrolyl chalcone synthesis. **Scheme 25.** Pyrrolyl chalcone synthesis.

[40].

**Scheme 25.** Pyrrolyl chalcone synthesis. Fused-ring heteroaromatic aldehydes have also been successfully condensed with various acetophenones to prepare B-ring hybrid chalcones under typical C-S reaction con-Fused-ring heteroaromatic aldehydes have also been successfully condensed with various acetophenones to prepare B-ring hybrid chalcones under typical C-S reaction conditions. Zhao et al. prepared indole hybrid chalcones **63a–e** (Scheme 26) from assorted acetophenones and N-methylindolycarbaldehydes **62** in yields ranging from 60 to 90% Fused-ring heteroaromatic aldehydes have also been successfully condensed with various acetophenones to prepare B-ring hybrid chalcones under typical C-S reaction conditions. Zhao et al. prepared indole hybrid chalcones **63a–e** (Scheme 26) from assorted acetophenones and N-methylindolycarbaldehydes **62** in yields ranging from 60 to 90% [40]. *Molecules* **2023**, *27*, x FOR PEER REVIEW 12 of 28

ditions. Zhao et al. prepared indole hybrid chalcones **63a–e** (Scheme 26) from assorted

**Scheme 26.** Indolyl chalcone synthesis. **Scheme 26.** Indolyl chalcone synthesis.

See Scheme 29.

Bandgar and coworkers (Scheme 27) synthesized a diverse library of carbazole hybrid chalcones **66** [30], while Bindu's team condensed acetophenone derivatives with quinoline carboxaldehdes **68** under mild C-S conditions (Scheme 28) to prepare eight ex-Bandgar and coworkers (Scheme 27) synthesized a diverse library of carbazole hybrid chalcones **66** [30], while Bindu's team condensed acetophenone derivatives with quinoline carboxaldehdes **68** under mild C-S conditions (Scheme 28) to prepare eight examples of

amples of B-ring-substituted quinolinoid hybrid chalcones **68a–h** [55]. Abonia et al. prepared the chromen-4-one—quinoline hybrid chalcone **71** under similar conditions [56].

Desai and coworkers used mild C-S reaction conditions to prepare a series of thirteen quinoxalinyl hybrid chalcones **73a–m** in yields ranging from 60 to 95%, as shown in

**Scheme 27.** Carbazolyl hybrid chalcone synthesis.

**Scheme 28.** Quinolinyl hybrid chalcone synthesis.

Scheme 29 [24].

B-ring-substituted quinolinoid hybrid chalcones **68a–h** [55]. Abonia et al. prepared the chromen-4-one—quinoline hybrid chalcone **71** under similar conditions [56]. See Scheme 29. pared the chromen-4-one—quinoline hybrid chalcone **71** under similar conditions [56]. See Scheme 29. pared the chromen-4-one—quinoline hybrid chalcone **71** under similar conditions [56]. See Scheme 29.

Bandgar and coworkers (Scheme 27) synthesized a diverse library of carbazole hybrid chalcones **66** [30], while Bindu's team condensed acetophenone derivatives with quinoline carboxaldehdes **68** under mild C-S conditions (Scheme 28) to prepare eight examples of B-ring-substituted quinolinoid hybrid chalcones **68a–h** [55]. Abonia et al. pre-

Bandgar and coworkers (Scheme 27) synthesized a diverse library of carbazole hybrid chalcones **66** [30], while Bindu's team condensed acetophenone derivatives with quinoline carboxaldehdes **68** under mild C-S conditions (Scheme 28) to prepare eight examples of B-ring-substituted quinolinoid hybrid chalcones **68a–h** [55]. Abonia et al. pre-

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**Scheme 27.** Carbazolyl hybrid chalcone synthesis. **Scheme 27.** Carbazolyl hybrid chalcone synthesis. **Scheme 27.** Carbazolyl hybrid chalcone synthesis.

**Scheme 26.** Indolyl chalcone synthesis.

**Scheme 26.** Indolyl chalcone synthesis.

**Scheme 28.** Quinolinyl hybrid chalcone synthesis. **Scheme 28.** Quinolinyl hybrid chalcone synthesis. **Scheme 28.** Quinolinyl hybrid chalcone synthesis.

**Scheme 29.** Quinoxalinyl hybrid chalcone synthesis. **Scheme 29.** Quinoxalinyl hybrid chalcone synthesis.

picts the scope of this work.

In a study of microtubule polymerization inhibition, Sun et al. synthesized a library of fused-ring heteroaromatic chalcones featuring indoles, benzofurans, dibenzofurans, benzothiophenes, dibenzothiophenes, and benzimidazoles [57]. See Figure 4. Of particu-Desai and coworkers used mild C-S reaction conditions to prepare a series of thirteen quinoxalinyl hybrid chalcones **73a–m** in yields ranging from 60 to 95%, as shown in Scheme 29 [24].

lar note were the numerous methods used in the preparation of these hybrid chalcones, which included both base-promoted processes (piperidine, NaOH, KOH, NaOMe, Cs2CO3 and NaH) in methanolic and ethanolic solvents, Lewis acid catalysis (BF3•etherate) in dioxane solvent and Brønsted (glacial acetic acid) acid catalysis in toluene. Scheme 30 de-

**Figure 4.** Hybrid chalcone heteroaromatic components prepared by Sun et al.

In a study of microtubule polymerization inhibition, Sun et al. synthesized a library of fused-ring heteroaromatic chalcones featuring indoles, benzofurans, dibenzofurans, benzothiophenes, dibenzothiophenes, and benzimidazoles [57]. See Figure 4. Of particular note were the numerous methods used in the preparation of these hybrid chalcones, which included both base-promoted processes (piperidine, NaOH, KOH, NaOMe, Cs2CO<sup>3</sup> and NaH) in methanolic and ethanolic solvents, Lewis acid catalysis (BF3•etherate) in dioxane solvent and Brønsted (glacial acetic acid) acid catalysis in toluene. Scheme 30 depicts the scope of this work. In a study of microtubule polymerization inhibition, Sun et al. synthesized a library of fused-ring heteroaromatic chalcones featuring indoles, benzofurans, dibenzofurans, benzothiophenes, dibenzothiophenes, and benzimidazoles [57]. See Figure 4. Of particular note were the numerous methods used in the preparation of these hybrid chalcones, which included both base-promoted processes (piperidine, NaOH, KOH, NaOMe, Cs2CO3 and NaH) in methanolic and ethanolic solvents, Lewis acid catalysis (BF3•etherate) in dioxane solvent and Brønsted (glacial acetic acid) acid catalysis in toluene. Scheme 30 depicts the scope of this work.

N

**h**: Ar = 3-HOC6H4, 80% **i**: Ar = 3-BrC6H4, 95% **j**: Ar = 3-H2NC6H4, 75% **k**: Ar = 4-O2NC6H4, 60% **l**: Ar = 3-O2NC6H4, 60% **m**: Ar = naphthyl, 90%

N

Ar

O

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**a**: Ar = Ph, 70% **b**: Ar = 4-HOC6H4, 78% **c**: Ar = 4-H2NC6H4, 75% **d**: Ar = 4-BrC6H2, 80% **e**: Ar = 4-MeOC6H4, 85% **f**: Ar = 4-ClC6H4, 75% **g**: Ar = 4-FC6H4, 80%

**61 72 73a-m**

20% aq KOH, EtOH, RT, 2–4 h

N

**Scheme 29.** Quinoxalinyl hybrid chalcone synthesis.

OHC N

+

O

R1

**Figure 4. Figure 4.**  Hybrid chalcone heteroaromatic components prepared by Sun et al. Hybrid chalcone heteroaromatic components prepared by Sun et al.

Base-catalyzed C-S condensations that employ green chemistry principles to produce B-ring-substituted hybrid chalcones have also been successfully conducted. See Scheme 31. These processes include the use of benign solvents, solvent-free reactions, microwave irradiation, ultrasound and ball milling. For example, Ashok's group compared a typical base-catalyzed C-S condensation of **83** and **84** with a solvent-free, microwave-mediated process to prepare a series of carbazolyl hybrid chalcones **85** [58]. The yields for the shortduration microwave-mediated reactions exceeded those of the lengthy conventional C-S reactions in every case. Bhatt et al. prepared the furyl chalcone **87** using both conventional C-S and ultrasound processes to condense furfural **48** and 2,4-dihydroxyacetophenone **86** [59]. The effectiveness of sonication is evident—a 10% increase in yield in 1/20 the reaction time. Jadhava's team used PEG-400 as a benign solvent to mediate the condensation of 4-fluoroacetophenone 84 and a series of pyrazole carbaldehydes **85** en route to eight fluorinated pyrazolyl hybrid chalcones 86 [60]. Kudlickova and coworkers employed a mechanochemical ball-milling process to prepare a series of indoylchalcones **92** in yields ranging from 28 to 79% in only 30 min [61]. Nimmala's group used a solventless process to condense various acetophenones and imidazo [1,2-a]pyrimidine **93** or imidazo [1,2 a]pyridine **95** en route to hybrid chalcones **94a–f** and **96a–f**, respectively, in very good yields [62]. Joshi and Saglani employed ultrasound to assist in the condensation of the fused-ring ketone **97** and a series of quinoline carbaldehydes **98** to prepare the quinolinyl hybrid chalcones **99** [63].

**Scheme 30.** N, O, S Fused-ring heteroaromatic hybrid chalcone synthesis. **Scheme 30.** N, O, S Fused-ring heteroaromatic hybrid chalcone synthesis.

#### *3.2. Non C-S Condensations*

chalcones **99** [63].

Base-catalyzed C-S condensations that employ green chemistry principles to produce B-ring-substituted hybrid chalcones have also been successfully conducted. See Scheme 31. These processes include the use of benign solvents, solvent-free reactions, microwave irradiation, ultrasound and ball milling. For example, Ashok's group compared a typical base-catalyzed C-S condensation of **83** and **84** with a solvent-free, microwave-mediated process to prepare a series of carbazolyl hybrid chalcones **85** [58]. The yields for the short-The final entries describing ring-B-substituted heteroaromatic hybrid chalcones feature unique tandem reactions involving pyrylium tetrafluoroborate derivatives. Devi and colleagues conducted a very interesting examination of a single-pot, base-mediated, tandem-ring-opening, 1,3-dipolar addition reaction between several electron withdrawing group (EWG)-substituted diazo compounds **101** with tri-substituted pyrylium salts **100**, producing an extensive array of pyrazole hybrid chalcones **102** in moderate to high yields, as shown in Scheme 32 [64].

duration microwave-mediated reactions exceeded those of the lengthy conventional C-S reactions in every case. Bhatt et al. prepared the furyl chalcone **87** using both conventional C-S and ultrasound processes to condense furfural **48** and 2,4-dihydroxyacetophenone **86** [59]. The effectiveness of sonication is evident—a 10% increase in yield in 1/20 the reaction Tan and Wang leveraged a similar pyrilium ring-opening strategy in a single-pot 3+2 reductive annulation with benzil derivatives **103** to prepare a comprehensive library of tetra-substituted Furano chalcones **105a–ii** in yields as high as 70% [65]. See Scheme 33. A noteworthy observation in both works was the finding that *Z*-chalcone derivatives were the major or sole product in all instances.

time. Jadhava's team used PEG-400 as a benign solvent to mediate the condensation of 4 fluoroacetophenone 84 and a series of pyrazole carbaldehydes **85** en route to eight fluorinated pyrazolyl hybrid chalcones 86 [60]. Kudlickova and coworkers employed a mechanochemical ball-milling process to prepare a series of indoylchalcones **92** in yields ranging

dine **95** en route to hybrid chalcones **94a–f** and **96a–f**, respectively, in very good yields [62]. Joshi and Saglani employed ultrasound to assist in the condensation of the fusedring ketone **97** and a series of quinoline carbaldehydes **98** to prepare the quinolinyl hybrid

**Scheme 31.** *Cont.*

**Scheme 31.** Green syntheses of B-ring heteroaromatic hybrid chalcones. **Scheme 31.** Green syntheses of B-ring heteroaromatic hybrid chalcones. as shown in Scheme 32 [64].

**Scheme 31.** Green syntheses of B-ring heteroaromatic hybrid chalcones.

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**Scheme 32.** Synthesis of pyrazole hybrid Z-chalcones via a pyrilium*.*ring-opening dipolar addi-**Scheme 32.** Synthesis of pyrazole hybrid Z-chalcones via a pyrilium.ring-opening dipolar addition.

tion.

**Scheme 33.** Synthesis of furanyl hybrid Z-chalcones via pyrilium ring-opening benzil-derivative reductive 3+2 annulation. **Scheme 33.** Synthesis of furanyl hybrid Z-chalcones via pyrilium ring-opening benzil-derivative reductive 3+2 annulation.

credibly diverse array of chalcones produced that feature 21 different heteroaromatic A–

As noted in the preceding sections, the Claisen–Schmidt (C-S) condensation is the most common method used to prepare A–B ring heteroaromatic chalcones. This reaction, which can be catalyzed by bases or acids, offers mild conditions that tolerate a wide scope

In most instances, NaOH and KOH are the most widely used bases. Sweeting's group synthesized and obtained an X-ray crystal structure for the pyrrolyl–thienyl hybrid chalcone **106** as part of a chalcone solubility and stability study [30]. See Scheme 34. While the use of centrifuging to mix the reagents is of interest, the low yield is likely attributable to the limited reaction time of 30 min. Sinha and coworkers prepared two thiazolyl–furyl hybrid chalcones in high yields (Scheme 35) while investigating potential ant-lipoxygen-

B ring-substituted groups on the hybrid chalcones shown in Schemes 34–46.

*4.1. Claisen–Schmidt Condensations*

ase agents [37].

4.1.1. Base-Catalyzed C-S Condensations

**Scheme 34***.* Synthesis of pyrrolyl–thienyl hybrid chalcones.

**4. A–B Ring Dual Heteroaromatic Hybrid Chalcone Synthesis** 

of functionality in both the ketone donors and aldehyde acceptors.

#### **4. A–B Ring Dual Heteroaromatic Hybrid Chalcone Synthesis** use of centrifuging to mix the reagents is of interest, the low yield is likely attributable to

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4.1.1. Base-Catalyzed C-S Condensations

reductive 3+2 annulation.

*4.1. Claisen–Schmidt Condensations*

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This section catalogues selected processes by which hybrid chalcones bearing a heteroaromatic species at both rings A and B may be prepared. Of particular note is the incredibly diverse array of chalcones produced that feature 21 different heteroaromatic A–B ring-substituted groups on the hybrid chalcones shown in Schemes 34–46. the limited reaction time of 30 min. Sinha and coworkers prepared two thiazolyl–furyl hybrid chalcones in high yields (Scheme 35) while investigating potential ant-lipoxygenase agents [37]. **Scheme 35.** Synthesis of thiazolyl–furyl hybrid chalcones. **Scheme 35.** Synthesis of thiazolyl–furyl hybrid chalcones. Fused-ring A–B hybrid chalcone examples have also been successfully prepared un-**Scheme 35.** Synthesis of thiazolyl–furyl hybrid chalcones. Fused-ring A–B hybrid chalcone examples have also been successfully prepared un-

of functionality in both the ketone donors and aldehyde acceptors.

**Scheme 33.** Synthesis of furanyl hybrid Z-chalcones via pyrilium ring-opening benzil-derivative

This section catalogues selected processes by which hybrid chalcones bearing a het-

As noted in the preceding sections, the Claisen–Schmidt (C-S) condensation is the

In most instances, NaOH and KOH are the most widely used bases. Sweeting's group

synthesized and obtained an X-ray crystal structure for the pyrrolyl–thienyl hybrid chalcone **106** as part of a chalcone solubility and stability study [30]. See Scheme 34. While the

most common method used to prepare A–B ring heteroaromatic chalcones. This reaction, which can be catalyzed by bases or acids, offers mild conditions that tolerate a wide scope

eroaromatic species at both rings A and B may be prepared. Of particular note is the incredibly diverse array of chalcones produced that feature 21 different heteroaromatic A–

B ring-substituted groups on the hybrid chalcones shown in Schemes 34–46.

**4. A–B Ring Dual Heteroaromatic Hybrid Chalcone Synthesis** 

**Scheme 34***.* Synthesis of pyrrolyl–thienyl hybrid chalcones. **Scheme 34.** Synthesis of pyrrolyl–thienyl hybrid chalcones. hyde were condensed in 20% KOH, the unusual pyrrolizinyl–pyrrolyl chalcone **112** was formed in modest yield (32%), accompanied by the acetylpyrrolizine **113** (17%) [53]. See formed in modest yield (32%), accompanied by the acetylpyrrolizine **113** (17%) [53]. See

**Scheme 35.** Synthesis of thiazolyl–furyl hybrid chalcones. **Scheme 35.** Synthesis of thiazolyl–furyl hybrid chalcones. See Scheme 39. See Scheme 39.

hyde were condensed in 20% KOH, the unusual pyrrolizinyl–pyrrolyl chalcone **112** was **Scheme 36.** Synthesis of pyridyl– and thienyl–carbazole hybrid chalcones. **Scheme 36.** Synthesis of pyridyl– and thienyl–carbazole hybrid chalcones. **Scheme 36.** Synthesis of pyridyl– and thienyl–carbazole hybrid chalcones. **Scheme 36.** Synthesis of pyridyl– and thienyl–carbazole hybrid chalcones.

carbaldehyde **115** gave rise to an array of hybrid chalcones **118** in moderate yields [18].

work to include condensations of 4-heteroaromatic acetophenones **117** with pyrazine **Scheme 37.** Synthesis of pyridyl–quinoxazolyl hybrid chalcone. **Scheme 37.** Synthesis of pyridyl–quinoxazolyl hybrid chalcone. **Scheme 37.** Synthesis of pyridyl–quinoxazolyl hybrid chalcone. **Scheme 37.** Synthesis of pyridyl–quinoxazolyl hybrid chalcone.

**Scheme 38.** Synthesis of pyrrole–[(2-pyrrolyl)-3H-pyrrolizinyl] hybrid chalcone. **Scheme 38.** Synthesis of pyrrole–[(2-pyrrolyl)-3H-pyrrolizinyl] hybrid chalcone. **Scheme 38. Scheme 38.**  Synthesis of pyrrole–[(2-pyrrolyl)-3H-pyrrolizinyl] hybrid chalcone. Synthesis of pyrrole–[(2-pyrrolyl)-3H-pyrrolizinyl] hybrid chalcone.

**Scheme 36.** Synthesis of pyridyl– and thienyl–carbazole hybrid chalcones.

**Scheme 38.** Synthesis of pyrrole–[(2-pyrrolyl)-3H-pyrrolizinyl] hybrid chalcone.

min. See Scheme 40. Moreover, in pursuit of suitable chalcones that have antimicrobial

and microwave processes [27]. The yields reported were as high as 90% after only 3 min

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*Molecules* **2023**, *27*, x FOR PEER REVIEW 19 of 28

**Scheme 39.** Synthesis of pyrazinyl hybrid chalcones. **Scheme 39.** Synthesis of pyrazinyl hybrid chalcones. of irradiation. See Scheme 41. properties, Usta's team prepared two pyrrole–pyridyl chalcones using both conventional

and microwave processes [27]. The yields reported were as high as 90% after only 3 min of irradiation. See Scheme 41. **Scheme 40.** Synthesis of furyl–triazolyl hybrid chalcones. **Scheme 40.** Synthesis of furyl–triazolyl hybrid chalcones. **Scheme 40.** Synthesis of furyl–triazolyl hybrid chalcones.

**Scheme 41. Scheme 41.**  Synthesis of pyrrolyl–pyridyl hybrid chalcones. Synthesis of pyrrolyl–pyridyl hybrid chalcones.

**Scheme 41.** Synthesis of pyrrolyl–pyridyl hybrid chalcones.

**Scheme 40.** Synthesis of furyl–triazolyl hybrid chalcones.

**Scheme 41.** Synthesis of pyrrolyl–pyridyl hybrid chalcones.

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Several syntheses of A–B ring heteroaromatic chalcones having fused-ring systems

Several syntheses of A–B ring heteroaromatic chalcones having fused-ring systems have also been reported. Khan and Asiri prepared two hybrid chalcones and tested them for antibacterial activity, a thienyl–pyrazole chalcone as well as a thienyl–carbazolyl chalcone using a microwave oven [46]. See Scheme 42. The base-catalyzed process, completed in only 45 s, provided the chalcones in 89–90%. Quinolinyl chalcones, such as those prepared by Sarveswari and Vijayakumar in Scheme 43, have also shown promise as antibacterial and antifungal agents [47]. Again, yields for the short-duration, microwave-mediated process was on par with or exceeded those obtained by the conventional C-S reac-

have also been reported. Khan and Asiri prepared two hybrid chalcones and tested them for antibacterial activity, a thienyl–pyrazole chalcone as well as a thienyl–carbazolyl chalcone using a microwave oven [46]. See Scheme 42. The base-catalyzed process, completed in only 45 s, provided the chalcones in 89–90%. Quinolinyl chalcones, such as those prepared by Sarveswari and Vijayakumar in Scheme 43, have also shown promise as antibacterial and antifungal agents [47]. Again, yields for the short-duration, microwave-mediated process was on par with or exceeded those obtained by the conventional C-S reac-

**Scheme 42.** Synthesis of thienyl–pyrazolyl/carbazolyl hybrid chalcones. **Scheme 42.** Synthesis of thienyl–pyrazolyl/carbazolyl hybrid chalcones. **Scheme 42.** Synthesis of thienyl–pyrazolyl/carbazolyl hybrid chalcones.

tions conducted in their comparative study.

tions conducted in their comparative study.

**Scheme 43.** Synthesis of quinolinyl–pyridyl/thienyl hybrid chalcones. **Scheme 43.** Synthesis of quinolinyl–pyridyl/thienyl hybrid chalcones. **Scheme 43.** Synthesis of quinolinyl–pyridyl/thienyl hybrid chalcones.

In Scheme 45, Kumar et al. employed piperidine base to catalyze the microwave-mediated condensation of indoles **131** and **132** en route to a large array of highly differentially functionalized twin indolyl hybrid chalcones **133** [67]. The yields reported were excellent,

Our final entry in this section is a green, solid-state, acid-catalyzed condensation of 2-acetylthiophene **9** and the thienyl carboxaldehyde **51** conducted by Adnan and associates, which produced the twin thienyl chalcone **134** in an excellent yield [13]. See Scheme

This section catalogues several processes by which heteroaromatic bis chalcone hybrids bearing two or more heteroaromatic species have been prepared. The reactions feature both heteroaromatic donors and acceptors as the linker unit in the bis hybrid chalcone systems. Conventional and green condensations as well as a unique Wittig preparation

**Scheme 44.** Synthesis of pyrazolopyridyl–heteroaryl hybrid chalcones. **Scheme 44.** Synthesis of pyrazolopyridyl–heteroaryl hybrid chalcones.

ranging from 72 to 92%, especially given the reaction time of 5 min.

**Scheme 45.** Synthesis of twin indolyl hybrid chalcones.

**Scheme 46.** Synthesis of twin thienyl hybrid chalcone.

**5. Heteroaromatic Bis Chalcone Hybrid Synthesis** 

46.

are discussed.

In Scheme 45, Kumar et al. employed piperidine base to catalyze the microwave-mediated condensation of indoles **131** and **132** en route to a large array of highly differentially functionalized twin indolyl hybrid chalcones **133** [67]. The yields reported were excellent,

In Scheme 45, Kumar et al. employed piperidine base to catalyze the microwave-mediated condensation of indoles **131** and **132** en route to a large array of highly differentially functionalized twin indolyl hybrid chalcones **133** [67]. The yields reported were excellent,

**Scheme 45.** Synthesis of twin indolyl hybrid chalcones. **Scheme 45.** Synthesis of twin indolyl hybrid chalcones. 46.

**Scheme 44.** Synthesis of pyrazolopyridyl–heteroaryl hybrid chalcones.

**Scheme 44.** Synthesis of pyrazolopyridyl–heteroaryl hybrid chalcones.

ranging from 72 to 92%, especially given the reaction time of 5 min.

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ranging from 72 to 92%, especially given the reaction time of 5 min.

**Scheme 46.** Synthesis of twin thienyl hybrid chalcone. **Scheme 46.** Synthesis of twin thienyl hybrid chalcone.

#### *4.1. Claisen–Schmidt Condensations*

**Scheme 46.** Synthesis of twin thienyl hybrid chalcone. **5. Heteroaromatic Bis Chalcone Hybrid Synthesis**  This section catalogues several processes by which heteroaromatic bis chalcone hy-**5. Heteroaromatic Bis Chalcone Hybrid Synthesis**  This section catalogues several processes by which heteroaromatic bis chalcone hybrids bearing two or more heteroaromatic species have been prepared. The reactions feature both heteroaromatic donors and acceptors as the linker unit in the bis hybrid chalcone As noted in the preceding sections, the Claisen–Schmidt (C-S) condensation is the most common method used to prepare A–B ring heteroaromatic chalcones. This reaction, which can be catalyzed by bases or acids, offers mild conditions that tolerate a wide scope of functionality in both the ketone donors and aldehyde acceptors.

#### brids bearing two or more heteroaromatic species have been prepared. The reactions feasystems. Conventional and green condensations as well as a unique Wittig preparation are discussed. 4.1.1. Base-Catalyzed C-S Condensations

ture both heteroaromatic donors and acceptors as the linker unit in the bis hybrid chalcone systems. Conventional and green condensations as well as a unique Wittig preparation are discussed. In most instances, NaOH and KOH are the most widely used bases. Sweeting's group synthesized and obtained an X-ray crystal structure for the pyrrolyl–thienyl hybrid chalcone **106** as part of a chalcone solubility and stability study [30]. See Scheme 34. While the use of centrifuging to mix the reagents is of interest, the low yield is likely attributable to the limited reaction time of 30 min. Sinha and coworkers prepared two thiazolyl–furyl hybrid chalcones in high yields (Scheme 35) while investigating potential ant-lipoxygenase agents [37].

Fused-ring A–B hybrid chalcone examples have also been successfully prepared under very mild, base-catalyzed C-S conditions. Bandgar's team prepared the pyridyl and thienyl–carbazolyl heteroaromatic hybrid chalcones **108–109** in very good yields (Scheme 36) [29]. While investigating ACP reductase inhibition, Desai's group prepared the pyridyl/quinoxazolyl chalcone **110** in a good yield as shown in Scheme 37 [23]. Mallik et al. found that when one equivalent of acetone and four equivalents of 2-pyrrole carbaldehyde were condensed in 20% KOH, the unusual pyrrolizinyl–pyrrolyl chalcone **112** was formed in modest yield (32%), accompanied by the acetylpyrrolizine **113** (17%) [53]. See Scheme 38. This finding is complementary to the work shown in Scheme 25 in which similar pyrrolizine products were formed. In an examination of chalcones with potential anticancer properties, Bukhari prepared a diverse set of furyl-, thienyl-, benzofuryl, and benzothienyl-1,4-pyrazinyl chalcones **116** in yields ranging from 42 to 75%. Extending that work to include condensations of 4-heteroaromatic acetophenones **117** with pyrazine carbaldehyde **115** gave rise to an array of hybrid chalcones **118** in moderate yields [18]. See Scheme 39.

#### 4.1.2. Green C-S Condensations

The recent literature reports a number of green, base-promoted C-S condensations used to prepare A–B ring heteroaromatic hybrid chalcones. While studying potential antimicrobial agents, Kumar et al. synthesized ten furyl-triazolyl chalcones **120a–j** via a continuous-flow reactor [66]. Of note are the exceptional yields (84–90%) obtained in only 15 min. See Scheme 40. Moreover, in pursuit of suitable chalcones that have antimicrobial properties, Usta's team prepared two pyrrole–pyridyl chalcones using both conventional and microwave processes [27]. The yields reported were as high as 90% after only 3 min of irradiation. See Scheme 41.

Several syntheses of A–B ring heteroaromatic chalcones having fused-ring systems have also been reported. Khan and Asiri prepared two hybrid chalcones and tested them for antibacterial activity, a thienyl–pyrazole chalcone as well as a thienyl–carbazolyl chalcone using a microwave oven [46]. See Scheme 42. The base-catalyzed process, completed in only 45 s, provided the chalcones in 89–90%. Quinolinyl chalcones, such as those prepared by Sarveswari and Vijayakumar in Scheme 43, have also shown promise as antibacterial and antifungal agents [47]. Again, yields for the short-duration, microwave-mediated process was on par with or exceeded those obtained by the conventional C-S reactions conducted in their comparative study.

Acetylated pyrazolo pyridines **37** and **128** were condensed with five heteroaryl aldehydes by Polo et al. under both ultrasonic and conventional conditions to prepare interesting A–B ring hybrid chalcones substituted with furyl, pyridyl, imidazolyl and quinolinyl groups [48]. See Scheme 44. Chalcone series **38** was part of a larger study discussed earlier in the review (Scheme 17). Yields for the short-duration ultrasound-assisted condensation met or exceeded those obtained by the conventional, base-promoted C-S condensations performed by the group.

In Scheme 45, Kumar et al. employed piperidine base to catalyze the microwavemediated condensation of indoles **131** and **132** en route to a large array of highly differentially functionalized twin indolyl hybrid chalcones **133** [67]. The yields reported were excellent, ranging from 72 to 92%, especially given the reaction time of 5 min.

Our final entry in this section is a green, solid-state, acid-catalyzed condensation of 2-acetylthiophene **9** and the thienyl carboxaldehyde **51** conducted by Adnan and associates, which produced the twin thienyl chalcone **134** in an excellent yield [13]. See Scheme 46.

#### **5. Heteroaromatic Bis Chalcone Hybrid Synthesis**

This section catalogues several processes by which heteroaromatic bis chalcone hybrids bearing two or more heteroaromatic species have been prepared. The reactions feature both heteroaromatic donors and acceptors as the linker unit in the bis hybrid chalcone systems. Conventional and green condensations as well as a unique Wittig preparation are discussed.

#### *5.1. Claisen–Schmidt Condensations*

The Claisen–Schmidt (C-S) condensation is the most widely used method to prepare heteroaromatic bis chalcone hybrids. In this section, we present base-promoted condensations that tolerate a wide scope of functionality in both the bis-ketone donors and bis-aldehyde acceptors.

#### 5.1.1. Base-Catalyzed C-S Condensations

As seen in the previous sections, NaOH and KOH are the most widely used bases. Methanol and ethanol are the solvents of choice in these condensations. In the first entry of bis hybrid chalcone preparation (Scheme 47), Alidmat et al. prepared three examples of mono- and dichlorinated bis-thienyl chalcones with potential as anticancer agents [68]. Of note is the one-pot preparation of the non-symmetric bis hybrid chalcone **138** from the condensation of 4-formylbenzaldehyde **135** (1 mole) and equimolar quantities of acetylthio-

phenes **136** and **137**. In contrast, the condensation of **135** (1 mole) with two moles of **136** or **137** resulted in the symmetric bis hybrid chalcones **139** or **141**, respectively. ophenes **136** and **137**. In contrast, the condensation of **135** (1 mole) with two moles of **136**  or **137** resulted in the symmetric bis hybrid chalcones **139** or **141**, respectively.

The Claisen–Schmidt (C-S) condensation is the most widely used method to prepare heteroaromatic bis chalcone hybrids. In this section, we present base-promoted condensations that tolerate a wide scope of functionality in both the bis-ketone donors and bis-

As seen in the previous sections, NaOH and KOH are the most widely used bases. Methanol and ethanol are the solvents of choice in these condensations. In the first entry of bis hybrid chalcone preparation (Scheme 47), Alidmat et al. prepared three examples of mono- and dichlorinated bis-thienyl chalcones with potential as anticancer agents [68]. Of note is the one-pot preparation of the non-symmetric bis hybrid chalcone **138** from the condensation of 4-formylbenzaldehyde **135** (1 mole) and equimolar quantities of acetylthi-

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*5.1. Claisen–Schmidt Condensations*

5.1.1. Base-Catalyzed C-S Condensations

aldehyde acceptors.

**Scheme 47.** Synthesis of bis thienyl hybrid chalcones. **Scheme 47.** Synthesis of bis thienyl hybrid chalcones.

While investigating photoinitiators with applications in 3D/4D printing, Chen's group prepared several bis hybrid chalcones that show promise as light-sensitive photoinitiators. See Scheme 48. 4,4*′*-diacetylbiphenyl **142** was condensed with 2-formylthiophene under mild, base-promoted conditions to synthesize the bis thienyl biphenyl chalcone **143** in a good yield [17]. Under the same reaction conditions, 2,6-diacetylpyridine **144** was condensed with several substituted benzaldehydes **145** en route to three pyridyl bis aryl hybrid chalcones **146a**–**c** in yields ranging from 58 to 86%. While investigating photoinitiators with applications in 3D/4D printing, Chen's group prepared several bis hybrid chalcones that show promise as light-sensitive photoinitiators. See Scheme 48. 4,40 -diacetylbiphenyl **142** was condensed with 2-formylthiophene under mild, base-promoted conditions to synthesize the bis thienyl biphenyl chalcone **143** in a good yield [17]. Under the same reaction conditions, 2,6-diacetylpyridine **144** was condensed with several substituted benzaldehydes **145** en route to three pyridyl bis aryl hybrid chalcones **146a**–**c** in yields ranging from 58 to 86%. *Molecules* **2023**, *27*, x FOR PEER REVIEW 23 of 28

**Scheme 48.** Synthesis of biphenyl bis thienyl and pyridyl bis aryl hybrid chalcones. **Scheme 48.** Synthesis of biphenyl bis thienyl and pyridyl bis aryl hybrid chalcones.

NaOH

While investigating lung cancer cell growth inhibitors, Zhao et al. prepared the indole bis phenyl chalcone **148** by condensing 1,2-diacetyl-3-methylindole **147** with benzaldehyde in 60% yield [54]. See Scheme 49. While investigating lung cancer cell growth inhibitors, Zhao et al. prepared the indole bis phenyl chalcone **148** by condensing 1,2-diacetyl-3-methylindole **147** with benzaldehyde in 60% yield [54]. See Scheme 49.

N

O

Presented in Schemes 50 and 51 are green methods used to prepare bis heteroaromatic chalcones. Asir and coworkers used sonochemical mediation to prepare examples of bis thienyl and bis furyl hybrid chalcones **150a–b**. The reaction time of 5 min was sufficient to give product yields in excess of 70%. [69] In a study of the anti-inflammatory activity of 3,4-bis-chalcone-N-arylpyrazoles, Abdel-Aziz et al. prepared eight examples of assorted aryl- and heteroaryl-substituted chalcone pyrazoles **152** using an aqueous KOH/EtOH medium at 60 °C and microwave irradiation [70]. The total reaction time reported was only four minutes to achieve yields ranging from 70 to 93%. Analogous conventional C-S condensations were also carried out over a 12 h period; the yields obtained

O

were about 75–85% of those obtained with *μ*wave mediation.

**Scheme 50.** Sonochemical synthesis of bis thienyl and bis furyl hybrid chalcones.

**Scheme 49.** Synthesis of indolyl bis aryl hybrid chalcones.

CHO + EtOH, 2–4 h

N

O O

**Scheme 48.** Synthesis of biphenyl bis thienyl and pyridyl bis aryl hybrid chalcones.

**Scheme 48.** Synthesis of biphenyl bis thienyl and pyridyl bis aryl hybrid chalcones.

*Molecules* **2023**, *27*, x FOR PEER REVIEW 23 of 28

While investigating lung cancer cell growth inhibitors, Zhao et al. prepared the indole bis phenyl chalcone **148** by condensing 1,2-diacetyl-3-methylindole **147** with benzal-

Presented in Schemes 50 and 51 are green methods used to prepare bis heteroaro-

While investigating lung cancer cell growth inhibitors, Zhao et al. prepared the indole bis phenyl chalcone **148** by condensing 1,2-diacetyl-3-methylindole **147** with benzal-

**Scheme 49.** Synthesis of indolyl bis aryl hybrid chalcones. **Scheme 49.** Synthesis of indolyl bis aryl hybrid chalcones.

dehyde in 60% yield [54]. See Scheme 49.

dehyde in 60% yield [54]. See Scheme 49.

Presented in Schemes 50 and 51 are green methods used to prepare bis heteroaromatic chalcones. Asir and coworkers used sonochemical mediation to prepare examples of bis thienyl and bis furyl hybrid chalcones **150a–b**. The reaction time of 5 min was sufficient to give product yields in excess of 70%. [69] In a study of the anti-inflammatory activity of 3,4-bis-chalcone-N-arylpyrazoles, Abdel-Aziz et al. prepared eight examples of assorted aryl- and heteroaryl-substituted chalcone pyrazoles **152** using an aqueous KOH/EtOH medium at 60 °C and microwave irradiation [70]. The total reaction time reported was only four minutes to achieve yields ranging from 70 to 93%. Analogous conventional C-S condensations were also carried out over a 12 h period; the yields obtained Presented in Schemes 50 and 51 are green methods used to prepare bis heteroaromatic chalcones. Asir and coworkers used sonochemical mediation to prepare examples of bis thienyl and bis furyl hybrid chalcones **150a–b**. The reaction time of 5 min was sufficient to give product yields in excess of 70%. [69] In a study of the anti-inflammatory activity of 3,4-bis-chalcone-N-arylpyrazoles, Abdel-Aziz et al. prepared eight examples of assorted aryl- and heteroaryl-substituted chalcone pyrazoles **152** using an aqueous KOH/EtOH medium at 60 ◦C and microwave irradiation [70]. The total reaction time reported was only four minutes to achieve yields ranging from 70 to 93%. Analogous conventional C-S condensations were also carried out over a 12 h period; the yields obtained were about 75–85% of those obtained with *µ*wave mediation. matic chalcones. Asir and coworkers used sonochemical mediation to prepare examples of bis thienyl and bis furyl hybrid chalcones **150a–b**. The reaction time of 5 min was sufficient to give product yields in excess of 70%. [69] In a study of the anti-inflammatory activity of 3,4-bis-chalcone-N-arylpyrazoles, Abdel-Aziz et al. prepared eight examples of assorted aryl- and heteroaryl-substituted chalcone pyrazoles **152** using an aqueous KOH/EtOH medium at 60 °C and microwave irradiation [70]. The total reaction time reported was only four minutes to achieve yields ranging from 70 to 93%. Analogous conventional C-S condensations were also carried out over a 12 h period; the yields obtained were about 75–85% of those obtained with *μ*wave mediation.

**Scheme 50. Scheme 50.**  Sonochemical synthesis of bis thienyl and bis furyl hybrid chalcones. Sonochemical synthesis of bis thienyl and bis furyl hybrid chalcones.

**Scheme 51.** Microwave-mediated synthesis of bis aryl/heteroaryl chalcone pyrazoles. **Scheme 51.** Microwave-mediated synthesis of bis aryl/heteroaryl chalcone pyrazoles.

#### 5.1.2. Non C-S Condensations 5.1.2. Non C-S Condensations

nylethylene linker units.

Our final installment for the bis hybrid chalcone section is an early example published by Saikachi and Muto in 1971 [71]. Their work, shown in Scheme 52, which focused on the preparation and utility of bisphosphoranes in oligimerization studies, exemplified how the bis-Wittig reagents **153**, **155** and **157** could be successfully coupled with furan or thienylcarbaldehydes to provide a series of bis heteroaromatic chalcones **154**, **156** and **158** in yields ranging from 45 to 99%. This work was unique in providing the bis hybrid chalcone system with benzene, biphenyl, diphenyl ether, diphenylmethylene, and diphe-Our final installment for the bis hybrid chalcone section is an early example published by Saikachi and Muto in 1971 [71]. Their work, shown in Scheme 52, which focused on the preparation and utility of bisphosphoranes in oligimerization studies, exemplified how the bis-Wittig reagents **153**, **155** and **157** could be successfully coupled with furan or thienylcarbaldehydes to provide a series of bis heteroaromatic chalcones **154**, **156** and **158** in yields ranging from 45 to 99%. This work was unique in providing the bis hybrid chalcone system with benzene, biphenyl, diphenyl ether, diphenylmethylene, and diphenylethylene linker units.

**Scheme 52.** Wittig synthesis of bis thienyl and bis furyl hybrid chalcones.

This review of the preparation of heteroaromatic hybrid chalcones gives a robust accounting of more than 50 historic and current synthetic processes leading to more than 430 different hybrid chalcone examples that include single-ring and multi-ring heteroaromatic moieties. We have shown that the venerable Claisen–Schmidt reaction, by far the most common condensation method discussed herein, has been successfully used in ei-

**6. Conclusions and Future Directions** 

**Scheme 51.** Microwave-mediated synthesis of bis aryl/heteroaryl chalcone pyrazoles.

Our final installment for the bis hybrid chalcone section is an early example published by Saikachi and Muto in 1971 [71]. Their work, shown in Scheme 52, which focused on the preparation and utility of bisphosphoranes in oligimerization studies, exemplified how the bis-Wittig reagents **153**, **155** and **157** could be successfully coupled with furan or thienylcarbaldehydes to provide a series of bis heteroaromatic chalcones **154**, **156** and **158** in yields ranging from 45 to 99%. This work was unique in providing the bis hybrid chalcone system with benzene, biphenyl, diphenyl ether, diphenylmethylene, and diphe-

**Scheme 52.** Wittig synthesis of bis thienyl and bis furyl hybrid chalcones. **Scheme 52.** Wittig synthesis of bis thienyl and bis furyl hybrid chalcones.

#### **6. Conclusions and Future Directions 6. Conclusions and Future Directions**

5.1.2. Non C-S Condensations

nylethylene linker units.

This review of the preparation of heteroaromatic hybrid chalcones gives a robust accounting of more than 50 historic and current synthetic processes leading to more than 430 different hybrid chalcone examples that include single-ring and multi-ring heteroaromatic moieties. We have shown that the venerable Claisen–Schmidt reaction, by far the most common condensation method discussed herein, has been successfully used in ei-This review of the preparation of heteroaromatic hybrid chalcones gives a robust accounting of more than 50 historic and current synthetic processes leading to more than 430 different hybrid chalcone examples that include single-ring and multi-ring heteroaromatic moieties. We have shown that the venerable Claisen–Schmidt reaction, by far the most common condensation method discussed herein, has been successfully used in either base-promoted or acid-catalyzed processes en route to heteroaromatic hybrid chalcones. We note that variations in the base or acid identity, solution concentration and physical state often make direct comparisons of the yields challenging. Also discussed has been the wide array of reaction conditions, such as the temperature and reaction time, which likewise impact the overall yield. Finally, the topology and electronic reactivity of the ketone donors and aldehyde acceptors likely modulate the product stereochemistry and yields as well.

Additionally, this review has provided the reader with an appreciation of alternative methods used to prepare these hybrid chalcones. Presented in our review are metalcatalyzed coupling reactions, cycloadditions, ring-opening processes and Wittig reactions that enable the formation of more than 75 hybrid chalcone examples.

A key thrust of this review has been to highlight the application of green chemistry methods in heteroaromatic hybrid chalcone synthesis. From the use of benign/renewable solvents and solvent-free and solid-state processes, researchers have demonstrated the ability to minimize waste streams. Through the use of sonochemical, mechanochemical, microwave irradiation, continuous-flow reactions and nanocatalytic methods, scientists minimize the reagent costs, reaction times and energy expenditure while optimizing the yields. Taken together, the important advances in green method uses noted herein portend well for future investigations of heteroaromatic chalcone synthesis.

**Author Contributions:** Conceptualization, A.M. and J.S.; writing—original draft preparation, A.M. and J.S.; writing—review and editing, A.M. and J.S.; visualization, A.M. and J.S. 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.

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

#### **References**


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