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Article

Synthesis of Purine-1,4,7,10-Tetraazacyclododecane Conjugate and Its Complexation Modes with Copper(II)

by
Aleksejs Burcevs
1,
Gediminas Jonusauskas
2,
Irina Novosjolova
1,* and
Māris Turks
1,*
1
Institute of Chemistry and Chemical Technology, Faculty of Natural Sciences and Technology, Riga Technical University, P. Valdena Str. 3, LV-1048 Riga, Latvia
2
Laboratoire Ondes et Matière d’Aquitaine, Bordeaux University, UMR CNRS 5798, 351 Cours de la Libération, 33405 Talence, France
*
Authors to whom correspondence should be addressed.
Molecules 2025, 30(7), 1612; https://doi.org/10.3390/molecules30071612
Submission received: 3 March 2025 / Revised: 21 March 2025 / Accepted: 2 April 2025 / Published: 4 April 2025
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Abstract

:
Purine-1,4,7,10-tetraazacyclododecane (cyclen) conjugate was designed to study its Cu2+ ions complexation capability. Several synthetic approaches were tested to achieve the target compound. The optimal approach involved stepwise modifications of purine N9, C8, and C6 positions that, in nine consecutive steps, provided purine–cyclen conjugate. The synthetic sequence involved Mitsunobu-type alkylation at N9 and iodination at C8, followed by Stille, SNAr, CuAAC, and alkylation reactions. The designed purine–cyclen conjugate is able to complex Cu2+ ions in both the cyclen part and between the purine N7 and triazole N2 positions. The complexation pattern and equilibrium were studied using the NMR titration technique in MeCN-d3 and absorption spectra.

Graphical Abstract

1. Introduction

Purines have a broad range of applications in medicine and materials science due to their wide biological [1,2,3,4] and photophysical [5,6,7,8,9,10,11,12] properties. Therefore, the importance of the development of new synthetic approaches towards intended substitution into purine is high.
In this study, we designed purine–cyclen conjugate 1 as a potential photocatalyst (Figure 1). The incorporation of redox-active transition metals into organic light-absorbing molecular structures makes it possible to use light to change the oxidation state of metal ions. Cyclen derivative is an attractive moiety for derivatization due to its great ability to complex such metal ions as Cu2+, Ni2+, Pd2+, and Zn2+ [13,14,15,16,17,18]. The photoinitiated electron transfer may be easily controlled and directed toward a site containing metal ions. Moreover, the use of available molecular building blocks using well-developed synthetic methods makes the molecular construction easily adapted for specific metal ions and targeted chemical reactions. The ON/OFF switching possibility of the catalytically active state largely increases the flexibility in the control of the reaction.
Our previous work [19] inspired us to create a new structural design of the photocatalyst based on the purine core. Instead of using benzophenone with its complicated excited state behavior, including unwanted side reactions, the ground state association between dimethylaniline electron donor and strongly absorbing antenna, bearing purine, will ensure efficient light absorption and electron transfer between them without wasting the absorbed energy. The oxidation potentials of all molecular fragments are known to be optimal for an electron transfer until copper(II) is bound by cyclen. The formed copper(I) may be used for a wealth of chemical reactions, including the generation of H2O2, which possesses high reactivity towards biological structures and may also be employed for wastewater treatment using only sunlight.
Over the years, a wide range of methods for the derivatization of purines have been developed. Four positions in the purine core can be modified: C2, C6, C8, and N7 or N9. Derivatization of the C positions of the purine cycle is usually done in metal-catalyzed cross-coupling or SNAr reactions (Scheme 1). In SNAr reactions, halogens are the most common leaving groups in the purine, but there are reports about sulfonyl groups [20], azides [21], and C-N bonded 1,2,3-triazoles [22] acting as leaving groups as well. Various O- [23,24], N- [25,26,27], and S- [20,23,28] nucleophiles have been introduced to the purine ring via SNAr reactions. On the other hand, the most widespread cross-coupling reactions within the purine core are Suzuki-Miyaura [29,30,31], Negishi [32,33], Stille [34,35], and Sonogashira [36,37,38] reactions. In cases of di- or trihalogenated purines, reactions occur chemo- and regioselectively [39,40], with the purine C6 position being the most reactive. By choosing different halogens at the purine core, it is possible to control reaction occurrence at less reactive positions with the introduction of the iodine there. It is worth noting that the proton at the purine C8 position is the most acidic, making it the primary target of deprotonation and C-H activation reactions [41,42,43].
In derivatizations of the N9 position of purine, a mixture of products is frequently formed due to the purine imidazole ring N7/N9 position tautomerism. The most used reactions for the introduction of the substituents to the N9 position are alkylation, Mitsunobu, and copper-catalyzed cross-coupling reactions [41,42,44]. Alkylation reactions with the formation of N7 product as a major product are challenging, requiring specific substrates and additives, like alkyl halides in the presence of Grignard reagents [45], glycosyl donors or tert-butyl halides with bis(trimethylsilyl)acetamide, TiCl4 or SnCl4 [46,47]. The most certain way of acquiring N7 purine derivatives is purine de novo synthesis from the corresponding pyrimidines [48] or imidazoles [49].
In addition, purine derivatives can complex metal ions and be used in medicine and as metal ion sensors. Cisplatin (cis-diamminedichloroplatinum(II)) is a known anticancer drug where the metal binds with the N7-positions of guanine bases and thus activates signal transduction pathways and apoptosis [50]. In 1971, 6-amino-, 6-chloro-, 6-hydroxy- and 6-mercaptopurines were studied for such metals as Mn2+, Ca2+, Mg2+, Zn2+, Ni2+, Co2+, and Cu2+ complexation in water, and it was proposed that the 1:1 complexes formed and metal coordinated to N7 of purine and to substituent at C6 position [51].
Purine–metal complexes are often studied by fluorescence because, due to the formation of complexes, fluorescence changes. Known purine-based derivatives are used as sensors for Hg2+ and Pd2+ ion detection in aqueous media [52,53] and for Cu2+ in cells [54,55]; upon complexation, they quench the fluorescence. Recently, there were published reports about purine nucleoside complexes for Cu2+ ions [56,57]. On the other hand, purine Schiff base conjugate forms complexes with Al3+ and increases the fluorescence [58]. 1,2,3-Triazolyl pyridine, isoquinoline, and ferrocene derivatives form complexes with Pd2+, Cu2+, Ni2+, Ru2+, Au+, and Ag+, but there is only one example in the literature of 2,6-bistriazolylpurine derivatives, which can serve as ratiometric fluorescence cation sensors, especially for the Zn2+ and Ca2+ ions [59].
The formation constant for tetramethylated cyclen and Cu2+ complex reaches logK 18.4 [60]. For triazole–pyridine–triazole conjugate with tridentate coordination of Cu2+ ions complex with logK 19.69 forms, and the complex is protonated (logβ[CuHL2]3+ [61]. On the other hand, 6-aminopurine forms a complex with Cu2+ ions with coordination to the N7 position and amino group at the C6 position and a formation constant logK 8.94 [62]. 3-Methyl-5-pyridin-2-yl-1,2,4-triazole forms complexes with Cu2+ ions in metal-to-ligand ratios 1:1 and 1:3, and their complex formation logK are 10.35 and 17.82, respectively [63]. Based on this information, the competitive complexation between cyclen moiety Cu2+ ions and between the N7 position of purine and the N2 position of triazole and Cu2+ ions may take place. In addition, the most structurally related compounds to the target compound 1 reported in the literature are purine–copper–cyclen complexes, where purine and cyclen moieties are separate ligands and are not linked within a single molecule. In these complexes, copper ions coordinate with the nitrogen atoms of the cyclen and purine N7, N9, or N3 positions. However, binding constants for these interactions have not been reported. Notably, these studies are focused on the chelating properties of the complexes rather than their potential photocatalytic applications [64,65].
Hence, we report here the design and synthesis of the purine derivative, which contains all the foreseen necessary substituents for metal complexation along with light absorption and electron transfer from the organic ligand to the metal center.

2. Results and Discussion

The proposed structure consists of a purine core and three main moieties A, B, and C, which may be added in various order. The retrosynthetic analysis of the proposed structure 1 offers possible approaches toward the introduction of target building blocks (Scheme 2). Moiety A makes it possible to construct via copper-catalyzed azide–alkyne cycloaddition reaction (CuAAC) between the azido group containing purine derivative 2 and alkyne 3. Thus, the azidation of 6-chloropurines is well-known and results in high to quantitative yields [66,67], while corresponding alkyne 3 requires a separate synthetic approach. Derivatization of the purine C8 position is possible to achieve via a palladium-catalyzed cross-coupling reaction. We decided to use the Stille reaction conditions due to the easy, one-step synthesis of the stannane 5 from commercially available 4-bromo-N,N-dimethylaniline [68]. The introduction of moiety C is proposed to be achieved in two ways—either via the Mitsunobu reaction between 9H-purine 6 and cyclen derivative 7 or using the SN2 reaction between substituted 9-(2-iodoethyl)-9H-purine 8 and cyclen derivative 9.
In general, we have explored several synthetic routes toward target compound 1. The first synthetic route started with SNAr reaction between 6-chloro-9-tetrahydropyranyl purine 10 [69] and sodium azide, resulting in 6-azidopurine derivative 11 in a 70% yield (Scheme 3). Next, we envisioned two approaches for the construction of moiety A at the C6 position of purine. The first approach included the CuAAC reaction of compound 11 with 2-(prop-2-yn-1-yloxy)ethan-1-ol (12) [70], followed by the derivatization of the free hydroxyl group of an obtained product 13. While the CuAAC reaction successfully provided triazole derivative 13 in a 63% yield, the further derivatizations with 4-bromo-N,N-dimethylaniline (14) via copper-catalyzed Ullmann-type reactions were unsuccessful [71,72,73]. For this transformation, we applied previously reported reagent systems of KOH/CuI/1,10-phenanthroline/n-Bu4NI at 100 °C [71], CuI/3,4,7,8-tetramethyl-1,10-phenanthroline/Cs2CO3 at 110 °C [72] and CuI/K3PO4/8-hydroxyquinoline at 110 °C [73], but did not achieve any conversion. For the second approach, building block 3 was obtained in 4 steps from 1-bromo-4-nitrobenzene (16) and used in CuAAC reaction with compound 11, resulting in a 56% yield of desired product 15. While deprotection of the tetrahydropyranyl group at the purine N9 position of compound 15 was easily achieved in AcOH/H2O/THF mixture at 50 °C temperature overnight, further attempts to derivatize N9 position of this compound using the Mitsunobu (DIAD/PPh3/2-bromoethanol or 2-chloroethanol at 0 → 60 °C) or alkylation (NaH/1,2-dibromoethane at 0 → 100 °C) reaction procedures were not fruitful. The explanation involves the electron-poor nature of the purine system and the reduced reactivity of the N9 position after the introduction of the triazolyl moiety A. This encouraged us to design the second approach toward the desired compound 1.
Next, we decided to introduce the THP-protected oxyethyl linker at the N9 position prior to derivatization of the C6 position with moiety A, keeping in mind that the cyclen part should be introduced later due to the excellent complexation ability of cyclen with transition metals [16], which would make CuAAC and cross-coupling reactions impossible. Thereby, the second synthetic route started with a Mitsunobu reaction between 6-chloropurine (20) and 2-(cyclohexyloxy)ethan-1-ol (21) [74], yielding product 22 in a 60% yield (Scheme 4). Next, the sequence of SNAr and CuAAC reactions was used toward compound 24. The following bromination attempt of the C8 purine position with NBS resulted in phenyl ring bromination instead, providing product 25 with a 59% yield. The use of LDA and other deprotonation reagents (NaH and n-BuLi) led to the deprotonation of triazole instead of the purine C8 position, so we concluded that the introduction of halogen to the C8 position and moiety B should be done prior to the derivatization of the C6 position.
The final synthetic route started with the iodination of previously obtained compound 22 using the LDA and I2 reagent system (Scheme 5). After that, a Stille cross-coupling reaction between N,N-dimethyl-4-(tributylstannyl)aniline (5) and purine derivative 26 resulted in C8 derivatized product 27 in a 52% yield. Next, the SNAr and CuAAC reaction sequence was performed to achieve compound 29. Further, THP group cleavage with the following mesylation step resulted in almost quantitative yields in both transformations. In the first attempts, cyclen derivative 9, which was previously obtained in three steps using literature methods [75], did not react with acquired mesylate 31 in the SN2 reaction. Thus, we decided to substitute the mesylate group with bromide and then perform an alkylation reaction with cyclen derivative 9. The best conversion toward target product 1 in this reaction was 20%, with a 6% isolated yield due to the difficulties with purification stemming from the similar polarity of by-products. In addition, this transformation preferred the E2 mechanism, forming elimination product 32 as a main by-product with 57% isolated yield.
With target product 1 in hand, we did preliminary studies using 1H NMR titration experiments (in the Supplementary Materials) to determine the complexation ability of purine–cyclen conjugate toward metal ions. Purine-cyclen conjugate 1 was titrated in MeCN-d3 with Cu2+ ions using Cu(ClO4)2·6H2O as a Cu2+ source and benzene as an internal standard. The corresponding spectra were taken after each 0.1 eq. addition of Cu2+ ions (Figure 2).
The shifts of the signals correspond to protons from triazole at the C6 position of purine, phenyl ring at the C8 position of purine, and cyclen at the N9 position of purine. The 1H NMR spectra show relatively complex behavior of the Cu2+ binding process by compound 1. The most important feature representing an expected Cu2+ binding in cyclen is shown by a thick arrow at the values 2.0–2.5 ppm. Starting from approximately 0.7 equivalents of metal ion, all 1H NMR signals of cyclen assemble into a single peak at approximately 2.2 ppm, which does not shift at higher concentrations of Cu2+. This fact indicates that metal ion is coordinated inside the cyclen ring in a unique configuration, and all four cyclen nitrogens are equally involved in metal ion binding. An additional metal binding site between triazole and purine nitrogens is also occupied by Cu2+, as is seen by triazole proton shift at approximately 9.0–9.2 ppm. The proton shifts of the other aromatic fragments of compound 1 indicate the rearrangements of electron density over the molecule during the complexation process. Based on NMR titration experiments, the possible equivalence points were determined at 0.35 and 0.50 equivalents, which corresponded to M1:L3 and M1:L2 complexes, respectively (Figure 3 and Figure 4). The competitive complexation happened between the cyclen part and the triazole N2 and purine N7 positions.
Due to the paramagnetic nature of the copper, the performance and analysis of NMR experiments and obtaining spectra at higher metal ion concentrations turn out to be difficult. Thus, the complex formation of compound 1 with Cu2+ and the UV–spectrophotometric titration was performed in acetonitrile (ACN) at room temperature, and it confirmed the result obtained by 1H NMR titration. The initial concentration of 1 was 5 × 10−5 mol/L. The ratio of compound 1 to Cu2+ was varied by adding aliquots of a solution of copper(II) perchlorate with 0.125 × 10−6 mol/L concentration.
The variation of absorption spectra upon adding Cu2+ into the solution of 1 is quite complex, yet the step-by-step analysis of absorption spectra crossed with the data of NMR titration provides a realistic picture of the composition and geometry of the formed complexes (Figure 4, Figure 5 and Figure 6). The increase of the intensity of spectral feature at approximately 290 nm indicates the increase of the concentration of Cu2+ ions in the ACN solution. According to the literature [76], this absorption band may be attributed to the ligand-to-metal charge transfer (LMCT) transition between ACN and copper ion. At the concentrations corresponding to metal–ligand ratio below 1:1, the main spectral changes occur at the 450–800 nm spectral range (Figure 5a). The absorption bands observed may be attributed to the d-d transition of Cu2+ ions coordinated by cyclen nitrogens [15].
It is important to note that the main absorption band (including the vibronic replicas) located at 370 nm remains at the same position; only the intensity varies within 30% (Figure 5b). Such behavior may suggest that the π-conjugated electronic system of N,N-dimethylaniline-purine fragment is not affected by a metal ion. The main coordination site for Cu2+ is located inside cyclen until the metal–ligand ratio reaches 1:1; the ligands in excess are most probably oriented with triazole-purine fragment pointing towards Cu2+ inside cyclen and forming a lossy 1:3 and 1:2 complexes. After the metal–ligand ratio exceeds 1:1, the 370 nm absorption band starts to decay, causing a simultaneous rise of a new absorption band at 410 nm, which reaches its maximum at a metal–ligand ratio equal to 2:1. The formation of a new red-shifted absorption band is a typical spectroscopic signature of coordination of metal ion by an acceptor in donor-acceptor compounds; in our situation, it corresponds to the coordination of the second Cu2+ by triazole-purine fragment. Additionally, selected absorption spectra, together with complex compositions and supposed geometries, as well as types of vertical transitions involved [76], are presented in Figure 6.

3. Materials and Methods

3.1. General Information

Compounds 5, 9, 10, 12, and 21 were prepared according to the procedures outlined in the literature [68,69,70,74,75]. Compounds 17–19 were prepared using synthetic procedures described in this work, and their 1H spectra match the literature [77,78,79].
Commercial grade reagents and solvents were purchased from Sigma-Aldrich (Burlington, MA, USA), Fluorochem (Hadfield, UK) or BLD Pharm (Reinbek, Germany) at the highest commercial quality and used without further purification.
The absorption spectra were recorded using the UV-VIS-NIR absorption spectrometer Varian Cary 5000 (Agilent Technologies, Santa Clara, CA, USA). Overall, more than 200 titration steps were provided to obtain the full picture of the complexation process.

3.2. Characterization Data for Products

3.2.1. 2-(4-Nitrophenoxy)ethan-1-ol (17)

KOH (26.7 g, 0.48 mol, 4.0 eq.) was added in portions to abs. ethylene glycol (200 mL) at 0 °C, and the solution was stirred for 30 min at 0 °C. Then, the reaction mixture was warmed up to 25 °C and stirred until KOH was fully dissolved. Meanwhile, 1-bromo-4-nitrobenzene (16) (24.0 g, 0.12 mol, 1.0 eq.) was dissolved in abs. DMSO (200 mL) was then added to the KOH solution and stirred at 50 °C for 30 min. An additional abs. DMSO (40 mL) was added until everything was dissolved, and the reaction was stirred overnight at 50 °C for 16 h. The final reaction mixture was poured into cold water (1.5 L), and the formed precipitate was filtered (13.53 g of 17). Filtrate was extracted with DCM (3 × 100 mL). The organic phase was washed with brine (100 mL), dried over anhydrous Na2SO4, filtered, evaporated in a vacuum, and then lyophilized from the remaining DMSO. After lyophilization, the solid was dissolved in a minimum amount of DCM, added to the cold hexane (100 mL), and the resulting precipitate was filtered (6.4 g of 17). Beige amorphous solid, yield: 19.93 g, 92%. 1H-NMR spectrum is consistent with the literature [77].

3.2.2. 2-(4-Aminophenoxy)ethan-1-ol (18)

Compound 17 (13.5 g, 73.77 mmol, 1.0 eq.) and 10% palladium on carbon (1.35 g) were suspended in MeOH (300 mL) and H2 gas was bubbled through for 6 h at 25 °C. The reaction was controlled with HPLC. The reaction mixture was filtered through a celite pad and evaporated, providing product 18 (yield: 11.12 g, 99%) as a brown oil. HPLC: tR = 1.47 min, eluent E1. 1H-NMR spectrum is consistent with the literature [78].

3.2.3. 2-(4-(Dimethylamino)phenoxy)ethan-1-ol (19)

Compound 18 (11.12 g, 72.7 mmol, 1.0 eq.) and NaBH4 (16.5 g, 436.2 mmol, 6.0 eq.) were suspended in abs. THF (250 mL) and then slowly added to the mixture of H2SO4 (3M, 28.8 mL) and H2CO (37%, 21.6 mL) at 0 °C. The resulting mixture was left to stir for 2 h at 25 °C, then 4 h at 50 °C. The reaction mixture was basified with 2M KOH solution till pH = 11 and extracted with DCM (2 × 100 mL). The organic phase was washed with brine (2 × 100 mL), dried over anhydrous Na2SO4, filtered and evaporated. Silica gel column chromatography (EtOH/Tol, gradient 0% → 7%) provided product 19 (yield: 9.16 g, 70%) as a brown amorphous solid. Rf = 0.73 (Tol/EtOH = 5:1). 1H-NMR spectrum is consistent with the literature [79].

3.2.4. N,N-Dimethyl-4-(2-(prop-2-yn-1-yloxy)ethoxy)aniline (3)

To the solution of compound 19 (9.16 g, 50.5 mmol, 1.0 eq.) in abs. THF (100 mL) at 0 °C, 60% NaH (3.03 g, 75.8 mmol, 1.5 eq.) was added in small portions over 30 min, then warmed until 25 °C. 80 wt% propargyl bromide in toluene (8.18 mL, ρ = 1.38 g/cm3, 75.9 mmol, 1.5 eq.) was added, and the reaction was left to stir for 16 h at 25 °C. The reaction mixture was quenched with water (50 mL) and extracted with DCM (2 × 100 mL); the organic phase was washed with brine (100 mL), dried over anhydrous Na2SO4, filtered, and evaporated. Silica gel column chromatography (EtOH/Tol, gradient 0% → 20%) provided product 3 (yield: 8.73 g, 79%) as a brown amorphous solid. Rf = 0.67 (EtOH/Tol = 5:1). HPLC: tR = 1.52 min, eluent E1. IR (KBr) ν (cm−1): 2923, 2852, 1513, 1455, 1352, 1242, 1104, 1035, 947, 817, 704. 1H-NMR (500 MHz, CDCl3) δ (ppm): 6.87 (d, 2H, 3J = 9.0 Hz, 2 × H-Ar), 6.73 (d, 2H, 3J = 9.0 Hz, 2 × H-Ar), 4.27 (s, 2H, (-CH2-)), 4.10 (t, 2H, 3J = 4.4 Hz, (-CH2-)), 3.87 (t, 2H, 3J = 4.4 Hz, (-CH2-)), 2.87 (s, 6H, 2 × (-CH3)), 2.45 (s, 1H, CH(alkynyl)). 13C NMR (126 MHz, CDCl3) δ (ppm): 151.1, 146.2, 115.9, 114.8, 79.7, 74.8, 68.6, 68.1, 58.7, 41.8. HRMS (ESI) m/z: [M + H]+ Calculated for C13H18NO2 220.1332; Found 220.1330 (0.91 ppm).

3.2.5. 6-Azido-9-(Tetrahydro-2H-pyran-2-yl)-9H-purine (11)

To a solution of 10 [69] (7.0 g, 29.33 mmol, 1.0 eq.) in DMF (240 mL) NaN3 (3.81 g, 58.66 mmol, 2 eq.) was added, and the reaction mixture was stirred at 50 °C for 3 h. Then, it was evaporated and dried in a vacuum. The resulting solid was suspended in DCM (100 mL) and washed with 5% aqueous LiCl solution (2 × 100 mL), water (2 × 200 mL), and brine (100 mL). The organic phase was dried over anhydrous Na2SO4, filtered, and evaporated, providing compound 11 (yield: 5.06 g, 70%) as a beige amorphous solid. HPLC: tR = 3.00 min, eluent E1. IR (KBr) ν (cm−1): 2917, 2852, 2134, 1640, 1374, 1345, 1228, 1083, 1043, 967, 915. 1H-NMR (500 MHz, DMSO-d6) δ (ppm): 10.12 (s, 1H, H-C(2)), 8.87 (s, 1H, H-C(8)), 5.90 (d, 1H, 3J = 11.2 Hz, Ha-C), 4.05 (d, 1H, 3J = 11.2 Hz, He-C), 3.77 (td, 1H, 2J = 3J = 11.2 Hz, 3J = 4.1 Hz, He-C), 2.42–2.35 (qd, 1H, 2J = 3J = 11.2 Hz, 3J = 4.1 Hz, Hb-C), 2.09–1.98 (m, 2H, Hb-C, Hc-C), 1.87–1.74 (m, 1H, Hc-C), 1.68–1.58 (m, 2H, (-CH2-)). 13C-NMR (126 MHz, DMSO-d6) δ (ppm): 145.4, 142.5, 141.5, 136.0, 119.9, 82.0, 67.8, 30.0, 24.4, 22.2. HRMS (ESI) m/z: [M + H]+ Calculated for C10H12N7O 246.1098; Found 246.1093 (2.03 ppm).

3.2.6. 2-({1-[9-(Tetrahydro-2H-pyran-2-yl)-9H-purin-6-yl]-1H-1,2,3-triazol-4-yl}methoxy)ethan-1-ol (13)

General method A for CuAAC reactions: To a solution of 11 (1.0 g, 4.08 mmol, 1.0 eq.), CuI (0.14 g, 0.73 mmol, 0.18 eq.), AcOH (0.26 mL, ρ = 1.05 g/cm3, 4.49 mmol, 1.1 eq.) and Et3N (0.63 mL, ρ = 0.73 g/cm3, 8.16 mmol, 2 eq.) in DCM (40 mL) 2-(prop-2-yn-1-yloxy)ethanol (0.82 g, 0.90 mmol, 1.5 eq.) (12) [70] was added and the reaction mixture was stirred isolated from the daylight for 1 h at 25 °C. Then, the reaction mixture was poured into water (50 mL) and extracted with DCM (3 × 50 mL). The combined organic phase was washed with aqueous NaHS (2 × 30 mL) and brine (50 mL), dried over anhydrous Na2SO4, filtered, and evaporated. Silica gel column chromatography (DCM/EtOH, gradient 0% → 15%) provided product 13 (yield: 881 mg, 63%) as a white amorphous solid. Rf = (0.48 DCM/EtOH = 10:1). HPLC: tR = 2.16 min, eluent E1. IR (KBr) ν (cm−1): 3370, 3111, 2924, 2862, 1616, 1574, 1462, 1335, 1212, 1082, 1014, 908, 822. 1H-NMR (500 MHz, CDCl3) δ 9.07 (s, 1H, H-C(triazole)), 8.92 (s, 1H, H-C(2)), 8.42 (s, 1H, H-C(8)), 5.86 (dd, 1H, 2J = 10.5 Hz, 3J = 2.5 Hz, Ha-C), 4.84 (s, 2H, (-CH2-)), 4.23–4.18 (m, 1H, He-C), 3.84–3.75 (m, 3H, He-C, (-CH2-)), 3.74–3.70 (m, 2H, (-CH2-)), 2.45 (s, 1H, (-OH)), 2.26–2.17 (m, 1H, Hb-C), 2.14–2.01 (m, 2H, Hb-C, Hc-C), 1.87–1.73 (m, 2H, Hc-C, Hd-C), 1.73–1.64 (m, 1H, Hd-C). 13C-NMR (126 MHz, CDCl3) δ (ppm): 153.8, 152.3, 145.8, 144.9, 144.1, 123.2, 122.9, 82.6, 72.1, 69.1, 64.5, 62.0, 32.1, 24.9, 22.8. HRMS (ESI) m/z: [M + H]+ Calculated for C15H20N7O3 346.1622; Found 346.1613 (2.6 ppm).

3.2.7. N,N-Dimethyl-4-[2-({1-[9-(tetrahydro-2H-pyran-2-yl)-9H-purin-6-yl]-1H-1,2,3-triazol-4-yl}methoxy)ethoxy]aniline (15)

Prepared according to general method A: Compound 11 (85 mg, 0.35 mmol, 1.0 eq.), CuI (12 mg, 0.06 mmol, 0.18 eq.), AcOH (23 µL, ρ = 1.05 g/cm3, 0.39 mmol, 1.1 eq.), Et3N (54 µL, ρ = 0.73 g/cm3, 0.39 mmol, 1.1 eq.) and N,N-dimethyl-4-(2-(prop-2-yn-1-yloxy)ethoxy)aniline (115 mg, 0.52 mmol, 1.5 eq.), DCM (4 mL), 25 °C, 3 h. Silica gel column chromatography (Tol/EtOH, gradient 0% → 8%) provided product 15 (yield: 90 mg, 56%) as a gray amorphous solid. Rf = 0.49 (Tol/EtOH = 5:1). HPLC: tR = 1.93 min, eluent E1. IR (KBr) ν (cm−1): 2939, 2869, 2801, 1614, 1523, 1456, 1231, 1209, 1083, 1032, 910, 803. 1H-NMR (500 MHz, CDCl3) δ 9.11 (s, 1H, H-C(triazole)), 8.92 (s, 1H, H-C(2)), 8.42 (s, 1H, H-C(8)), 6.85 (d, 2H, 3J = 6.5 Hz, 2 × H-C(Ar)), 6.68 (d, 2H, 3J = 6.5 Hz, 2 × H-C(Ar)), 5.85 (d, 1H, 3J = 10.5 Hz, Ha-C), 4.90 (s, 2H, (-CH2-)), 4.20 (d, 1H, 3J = 10.5 Hz, He-C), 4.14–4.05 (m, 2H, (-CH2-)), 3.95–3.88 (m, 2H, (-CH2-)), 3.80 (t, 1H, 3J = 10.5 Hz, He-C), 2.83 (s, 6H, 2 × (-CH3)), 2.23–2.16 (m, 1H, Hb-C), 2.13–2.01 (m, 2H, Hb-C, Hc-C), 1.87–1.72 (m, 2H, Hc-C, Hd-C), 1.71–1.64 (m, 1H, Hd-C). 13C-NMR (126 MHz, CDCl3) δ (ppm): 153.8, 152.3, 151.0, 146.0, 145.9, 144.8, 144.0, 123.3, 122.8, 115.8, 114.7, 82.6, 69.4, 69.0, 68.2, 64.7, 41.8, 31.9, 24.9, 22.7. HRMS (ESI) m/z: [M + H]+ Calculated for C23H29N8O3 465.2357; Found 465.2355 (0.43 ppm).

3.2.8. 6-Chloro-9-{2-[(Tetrahydro-2H-pyran-2-yl)oxy]ethyl}-9H-purine (22)

To a suspension of 6-chloropurine (20) (4.8 g, 31.15 mmol, 1.1 eq.), PPh3 (9.65 g, 36.82 mmol, 1.3 eq.) and 2-((tetrahydro-2H-pyran-2-yl)oxy)ethanol (21) [74] (4.14 g, 28.32 mmol, 1.0 eq.) in dry THF (200 mL) at 0 °C DIAD (6.12 mL, ρ = 1.03 g/cm3, 31.15 mmol, 1.1 eq.) was added slowly over 1 h. Then, the reaction mixture was left to stir for 16 h at 25 °C and then filtered. The filtrate was evaporated. Reverse phase silica gel column chromatography (MeOH/H2O, gradient 0% → 70%) provided product 22 (yield: 4.82 g, 55%) as a pale yellow oil. Rf = 0.84 (DCM/EtOH = 10:1). HPLC: tR = 2.15 min, eluent E1. IR (KBr) ν (cm−1): 2941, 2867, 1593, 1560, 1334, 1197, 1123, 1034, 933, 856. 1H-NMR (500 MHz, CDCl3) δ (ppm): 8.73 (s, 1H, H-C(2)), 8.28 (s, 1H, H-C(8)), 4.56–4.45 (m, 3H, Ha-C, (-CH2-)), 4.11–4.05 (m, 1H, Hf-C), 3.77–3.71 (m, 1H, Hf-C), 3.61–3.54 (m, 1H, He-C), 3.44–3.38 (m, 1H, He-C), 1.77–1.62 (m, 2H, Hb-C, Hc-C), 1.57–1.45 (m, 4H, Hb-C, Hc-C, (-CH2-)). 13C-NMR (126 MHz, CDCl3) δ (ppm): 152.0, 151.9, 151.0, 146.5, 131.6, 99.3, 65.2, 62.6, 44.4, 30.5, 25.3, 19.5. HRMS (ESI) m/z: [M + H]+ Calculated for C12H16ClN4O2 283.0956; Found 283.0950 (2.12 ppm).

3.2.9. 6-Azido-9-{2-[(Tetrahydro-2H-pyran-2-yl)oxy]ethyl}-9H-purine (23)

To a solution of 22 (0.72 g, 2.54 mmol, 1.0 eq.) in DMF (14 mL) NaN3 (0.33 g, 5.08 mmol, 2 eq.) was added and the reaction mixture was stirred at 50 °C for 16 h. Then, it was evaporated and dried in a vacuum. The resulting solid was suspended in DCM (20 mL) and washed with 5% aqueous LiCl solution (2 × 20 mL), water (2 × 20 mL), and brine (20 mL). The organic phase was dried over anhydrous Na2SO4, filtered, and evaporated, providing compound 23 (yield: 0.73 g, 99%) as a white amorphous solid. HPLC: tR = 2.94 min, eluent E1. IR (KBr) ν (cm−1): 2985, 2938, 2185, 1636, 1376, 1345, 1256, 1125, 1071, 981, 872. 1H-NMR (500 MHz, DMSO-d6) δ (ppm): 10.11 (s, 1H, H-C(2)), 8.63 (s, 1H, H-C(8)), 4.68–4.55 (m, 3H, Ha-C, (-CH2-)), 4.02 (ddd, 1H, 2J = 10.9 Hz, 3J = 6.5, 4.1 Hz, Hf-C), 3.84 (ddd, 1H, 2J = 10.9 Hz, 3J = 6.5, 4.1 Hz, Hf-C), 3.44 (ddd, 1H, 2J = 11.2 Hz, 3J = 8.4, 3.0 Hz, He-C), 3.32–3.29 (m, 1H, He-C), 1.64–1.48 (m, 2H, Hb-C, Hc-C), 1.45–1.31 (m, 4H, Hb-C, Hc-C, (-CH2-)). 13C NMR (126 MHz, DMSO-d6) δ (ppm): 145.4, 144.7, 142.3, 135.7, 119.6, 97.6, 64.6, 61.1, 44.3, 29.9, 24.8, 18.8. HRMS (ESI) m/z: [M + H]+ Calculated for C12H16N7O2 290.1360; Found 290.1350 (3.45 ppm).

3.2.10. N,N-Dimethyl-4-(2-{[1-(9-{2-[(Tetrahydro-2H-pyran-2-yl)oxy]ethyl}-9H-purin-6-yl)-1H-1,2,3-triazol-4-yl]methoxy}ethoxy)aniline (24)

Prepared according to general method A: Compound 23 (100 mg, 0.35 mmol, 1.0 eq.), CuI (12 mg, 0.06 mmol, 0.16 eq.), AcOH (21 µL, ρ = 1.05 g/cm3, 0.38 mmol, 1.1 eq.), Et3N (53 µL, ρ = 0.73 g/cm3, 0.38 mmol, 1.1 eq.) and N,N-dimethyl-4-(2-(prop-2-yn-1-yloxy)ethoxy)aniline (151 mg, 0.69 mmol, 2.0 eq.), DCM (3 mL), 25 °C, 1 h. Silica gel column chromatography (DCM/EtOH, gradient 0% → 5%) provided product 24 (yield: 81 mg, 46%) as a pale brown amorphous solid. Rf = 0.53 (DCM/EtOH = 20:1). HPLC: tR = 5.03 min, eluent E2. IR (KBr) ν (cm−1): 2919, 2868, 1605, 1513, 1331, 1234, 1111, 1032, 845, 823. 1H-NMR (500 MHz, CDCl3) δ (ppm): 9.11 (s, 1H, H-C(triazole)), 8.91 (s, 1H, H-C(2)), 8.39 (s, 1H, H-C(8)), 6.86 (d, 2H, 3J = 9.2 Hz, 2 × H-C(Ar)), 6.70 (d, 2H, 3J = 9.2 Hz, 2 × H-C(Ar)), 4.90 (s, 2H, (-CH2-)), 4.64–4.51 (m, 3H, Ha-C, (-CH2-)), 4.17–4.12 (m, 1H, Hf-C), 4.11 (dd, 2H, 3J = 6.0, 4.0 Hz, (-CH2-)), 3.92 (dd, 2H, 3J = 6.0, 4.0 Hz, (-CH2-)), 3.78 (ddd, 1H, 2J = 10.7 Hz, 3J = 7.1, 3.4 Hz, Hf-C), 3.62–3.56 (m, 1H, He-C), 3.46–3.39 (m, 1H, He-C), 2.83 (s, 6H, 2 × (-CH3)), 1.80–1.64 (m, 2H, Hb-C, Hc-C), 1.59–1.45 (m, 4H, Hb-C, Hc-C, (-CH2-)). 13C-NMR (126 MHz, CDCl3) δ (ppm): 154.6, 152.1, 151.1, 147.3, 146.0, 145.9, 144.7, 123.3, 122.6, 115.8, 114.8, 99.3, 69.4, 68.2, 65.2, 64.8, 62.6, 44.4, 41.8, 30.5, 25.2, 19.4. HRMS (ESI) m/z: [M + H]+ Calculated for C25H33N8O4 509.2619; Found 509.2624 (0.98 ppm).

3.2.11. 2-Bromo-N,N-Dimethyl-4-{2-[(1-(9-{2-[(Tetrahydro-2H-pyran-2-yl)oxy]ethyl}-9H-purin-6-yl)-1H-1,2,3-triazol-4-yl)methoxy]ethoxy}aniline (25)

To a solution of 24 (100 mg, 0.2 mmol, 1.0 eq.) in CHCl3 (3 mL) at 0 °C NBS (37 mg, 0.21 mmol, 1.05 eq.) was slowly added. The reaction mixture was left to stir for 1 h at 0 °C, then diluted with CHCl3 to 20 mL in total, washed with water (3 × 50 mL) and brine (20 mL), then evaporated and dried in a vacuum. Silica gel column chromatography (DCM/EtOH, gradient 0% → 10%) provided product 25 (yield: 68 mg, 59%) as a brown oil. Rf = 0.85 (DCM/EtOH = 10:1). HPLC: tR = 5.37 min, eluent E2. IR (KBr) ν (cm−1): 2940, 2867, 1604, 1575, 1331, 1223, 1122, 1031, 991, 850. 1H-NMR (500 MHz, CDCl3) δ (ppm): 9.14 (s, 1H, H-C(triazole)), 8.94 (s, 1H, H-C(2)), 8.42 (s, 1H, H-C(8)), 7.20 (d, 1H, 4J = 2.8 Hz, H-C(Ar)), 7.02 (d, 1H, 3J = 8.9 Hz, H-C(Ar)), 6.85 (dd, 1H, 3J = 8.9 Hz, 4J = 2.8 Hz, H-C(Ar)), 4.91 (s, 2H, (-CH2-)), 4.66–4.51 (m, 3H, Ha-C, (-CH2-)), 4.17–4.13 (m, 1H, Hf-C), 4.12 (dd, 2H, 3J = 5.7, 3.6 Hz, (-CH2-)), 3.94 (dd, 2H, 3J = 5.7, 3.6 Hz, (-CH2-)), 3.79 (ddd, 1H, 2J = 10.7 Hz, 3J = 7.1, 3.3 Hz, Hf-C), 3.65–3.58 (m, 1H, He-C), 3.47–3.40 (m, 1H, He-C), 2.71 (s, 6H, 2 × (-CH3)), 1.81–1.69 (m, 2H, Hb-C, Hc-C), 1.59–1.44 (m, 4H, Hb-C, Hc-C, (-CH2-)). 13C-NMR (126 MHz, CDCl3) δ (ppm): 155.1, 154.6, 152.2, 147.4, 145.9, 145.7, 144.8, 123.3, 122.6, 121.1, 120.2, 115.9, 114.3, 99.4, 69.1, 68.1, 65.3, 64.8, 62.7, 44.9, 44.4, 30.5, 25.3, 19.5. HRMS (ESI) m/z: [M + H]+ Calculated for C25H32BrN8O4 587.1724; Found 587.1707 (2.90 ppm).

3.2.12. 6-Chloro-8-iodo-9-{2-[(Tetrahydro-2H-pyran-2-yl)oxy]ethyl}-9H-purine (26)

To dry, degassed THF (96 mL) at −78 °C DIPEA (3.71 mL, ρ = 0.72 g/cm3, 26.31 mmol, 1.55 eq.) and n-BuLi (10.38 mL, 2.29 M, 23.77 mmol, 1.4 eq.) were added and left to stir for 30 min at −78 °C. Compound 22 (4.8 g, 16.98 mmol, 1.0 eq.) was dissolved in dry, degassed THF (48 mL), slowly added to the solution of LDA at −78 °C, and left to stir for 1 h at that temperature. I2 (21.5 g, 84.89 mmol, 5.0 eq.) was dissolved in dry, degassed THF (20 mL) and slowly added to the reaction mixture until it became solid. Then, the reaction was warmed up to room temperature, and saturated aqueous Na2S2O3·5H2O solution was added until the aqueous phase became colorless. The reaction mixture was washed with EtOAc (3 × 150 mL), and the combined organic phase was washed with brine (100 mL) and dried over anhydrous Na2SO4, filtered, and evaporated. Silica gel column chromatography (DCM/EtOH, gradient 0% → 5%) provided product 26 (yield: 6.34 g, 91%) as a pale brown amorphous solid. Rf = 0.65 (DCM/EtOH = 20:1). HPLC: tR = 4.66 min, eluent E1. IR (KBr) ν (cm−1): 2940, 2865, 1587, 1558, 1454, 1321, 1150, 1123, 1068, 1037, 948, 866. 1H-NMR (500 MHz, CDCl3) δ (ppm): 8.65 (s, 1H, H-C(2)), 4.58–4.41 (m, 3H, Ha-C, (-CH2-)), 4.10 (ddd, 1H, 2J = 11.1 Hz, 3J = 6.8, 4.5 Hz, Hf-C), 3.81 (ddd, 1H, 2J = 11.1 Hz, 3J = 6.8, 4.5 Hz, Hf-C), 3.48 (td, 1H, 2J = 11.4 Hz, 3J = 3.3 Hz, He-C), 3.41–3.34 (m, 1H, He-C), 1.73–1.57 (m, 2H, Hb-C, Hc-C), 1.54–1.40 (m, 4H, Hb-C, Hc-C, (-CH2-)). 13C-NMR (126 MHz, CDCl3) δ (ppm): 153.1, 151.9, 149.3, 134.0, 109.1, 98.5, 64.3, 62.0, 46.6, 30.2, 25.3, 19.0. HRMS (ESI) m/z: [M + H]+ Calculated for C12H15ClIN4O2 408.9923; Found 408.9913 (2.45 ppm).

3.2.13. 4-(6-Chloro-9-{2-[(Tetrahydro-2H-pyran-2-yl)oxy]ethyl}-9H-purin-8-yl)-N,N-Dimethylaniline (27)

In the pressure flask a solution of compound 26 (825 mg, 2.02 mmol, 1.0 eq.), N,N-dimethyl-4-(tributylstannyl)aniline [68] (911 mg, 2.22 mmol, 1.1 eq.) and Pd(PPh3)4 (116 mg, 0.1 mmol, 0.05 eq.) in dry toluene (25 mL) was stirred at 110 °C for 16 h. Then, it was evaporated and dried in a vacuum. Silica gel column chromatography (DCM/EtOH, gradient 0% → 25%) provided product 27 (yield: 420 mg, 52%) as a pale orange amorphous solid. Rf = 0.60 (DCM/EtOH = 20:1). HPLC: tR = 5.89 min, eluent E1. IR (KBr) ν (cm−1): 2930, 2863, 1609, 1479, 1365, 1343, 1125, 1037, 978, 948, 819. 1H-NMR (500 MHz, CDCl3) δ (ppm): 8.67 (s, 1H, H-C(2)), 7.91 (d, 2H, 3J = 8.6 Hz, 2 × H-C(Ar)), 6.78 (d, 2H, 3J = 8.6 Hz, 2 × H-C(Ar)), 4.65–4.55 (m, 2H, (-CH2-)), 4.53–4.48 (m, 1H, Ha-C), 4.26–4.16 (m, 1H, Hf-C), 4.00–3.90 (m, 1H, Hf-C), 3.56–3.47 (m, 1H, He-C), 3.43–3.35 (m, 1H, He-C), 3.07 (s, 6H, 2 × (-CH3)), 1.75–1.59 (m, 2H, Hb-C, Hc-C), 1.55–1.40 (m, 4H, Hb-C, Hc-C, (-CH2-)). 13C-NMR (126 MHz, CDCl3) δ (ppm): 157.8, 154.3, 152.0, 150.8, 148.7, 131.8, 131.2, 115.6, 111.8, 98.8, 64.7, 62.0, 44.7, 40.3, 30.3, 25.3, 19.2. HRMS (ESI) m/z: [M + H]+ Calculated for C20H25ClN5O2 402.1691; Found 402.1685 (1.49 ppm).

3.2.14. 4-(6-Azido-9-{2-[(Tetrahydro-2H-pyran-2-yl)oxy]ethyl}-9H-purin-8-yl)-N,N-Dimethylaniline (28)

To a solution of 27 (2.84 g, 7.07 mmol, 1.0 eq.) in DMF (140 mL) NaN3 (1.40 g, 21.22 mmol, 3.0 eq.) was added and the reaction mixture was stirred at 60 °C for 16 h. Then, the reaction mixture was poured into cold water (1 L), and the precipitate was filtered and washed with water (0.5 L), providing compound 28 (yield: 2.69 g, 93%) as a pale pink amorphous solid. HPLC: tR = 4.66 min, eluent E1. IR (KBr) ν (cm−1): 2939, 2886, 2025, 1608, 1478, 1344, 1202, 1126, 1073, 977, 874, 821. 1H-NMR (500 MHz, DMSO-d6) δ (ppm): 10.08 (s, 1H, H-C(2)), 7.86 (d, 2H, 3J = 8.7 Hz, 2 × H-C(Ar)), 6.86 (d, 2H, 3J = 8.7 Hz, 2 × H-C(Ar)), 4.48–4.45 (m, 2H, (-CH2-)), 4.48–4.43 (m, 1H, Ha-C), 4.06 (ddd, 1H, 2J = 11.2 Hz, 3J = 6.7, 4.7 Hz, Hf-C), 3.86 (ddd, 1H, 2J = 11.2 Hz, 3J = 6.7, 4.7 Hz, Hf-C), 3.35–3.30 (m, 1H, He-C), 3.26–3.22 (m, 1H, He-C), 3.02 (s, 6H, 2 × (-CH3)), 1.47–1.40 (m, 2H, Hb-C, Hc-C), 1.37–1.24 (m, 4H, Hb-C, Hc-C, (-CH2-)). 13C-NMR (126 MHz, DMSO-d6) δ (ppm): 154.6, 151.4, 144.9, 144.0, 134.4, 130.3, 119.6, 115.6, 111.6, 97.5, 64.2, 60.8, 44.7, 39.72, 29.7, 24.7, 18.6. HRMS (ESI) m/z: [M + H]+ Calculated for C20H25N8O2 409.2095; Found 409.2097 (0.49 ppm).

3.2.15. 4-[6-(4-({2-[4-(Dimethylamino)phenoxy]ethoxy)methyl}-1H-1,2,3-triazol-1-yl)-9-{2-[(Tetrahydro-2H-pyran-2-yl)oxy]ethyl}-9H-purin-8-yl]-N,N-Dimethylaniline (29)

Prepared according to general method A: Compound 28 (500 mg, 1.22 mmol, 1.0 eq.), CuI (40 mg, 0.20 mmol, 0.16 eq.), AcOH (77 µL, ρ = 1.05 g/cm3, 1.35 mmol, 1.1 eq.), Et3N (187 µL, ρ = 0.73 g/cm3, 1.35 mmol, 1.1 eq.) and N,N-dimethyl-4-(2-(prop-2-yn-1-yloxy)ethoxy)aniline (540 mg, 2.45 mmol, 2.0 eq.), DCM (20 mL), 25 °C, 16 h. Silica gel column chromatography (DCM/EtOH, gradient 0% → 3%) provided product 29 (yield: 420 mg, 55%) as a brown amorphous solid. Rf = 0.43 (DCM/EtOH = 20:1). HPLC: tR = 4.15 min, eluent E1. IR (KBr) ν (cm−1): 2923, 2855, 1606, 1512, 1457, 1338, 1243, 1118, 1032, 816, 797. 1H-NMR (500 MHz, CDCl3) δ (ppm): 9.40 (s, 1H, H-C(triazole)), 8.88 (s, 1H, H-C(2)), 7.96 (d, 2H, 3J = 8.6 Hz, 2 × H-C(Ar)), 6.85 (d, 2H, 3J = 8.6 Hz, 2 × H-C(Ar)), 6.78 (d, 2H, 3J = 8.6 Hz, 2 × H-C(Ar)), 6.67 (d, 2H, 3J = 8.6 Hz, 2 × H-C(Ar)), 4.91 (s, 2H, (-CH2-)), 4.72–4.62 (m, 2H, (-CH2-)), 4.54–4.49 (m, 1H, Ha-C), 4.25 (ddd, 1H, 2J = 11.0 Hz, 3J = 5.6, 5.2 Hz, Hf-C), 4.10 (t, 2H, 3J = 4.7 Hz, (-CH2-)), 3.98 (ddd, 1H, 2J = 11.0 Hz, 3J = 5.6, 5.2 Hz, Hf-C), 3.92(t, 2H, 3J = 4.7 Hz, (-CH2-)), 3.55–3.48 (m, 1H, He-C), 3.40–3.34 (m, 1H, He-C), 3.07 (s, 6H, 2 × (-CH3)), 2.82 (s, 6H, 2 × (-CH3)), 1.64–1.54 (m, 2H, Hb-C, Hc-C), 1.51–1.36 (m, 4H, Hb-C, Hc-C, (-CH2-)). 13C NMR (126 MHz, CDCl3) δ 158.4, 157.0, 152.1, 151.1, 151.0, 146.0, 145.8, 142.9, 131.2, 124.5, 122.8, 115.8, 115.5, 114.8, 111.7, 98.8, 69.1, 68.2, 64.68, 64.67, 62.0, 44.7, 41.8, 40.2, 30.3, 25.3, 19.1. HRMS (ESI) m/z: [M + H]+ Calculated for C33H42N9O4 628.3354; Found 628.3383 (4.62 ppm).

3.2.16. 2-{6-[4-({2-[4-(Dimethylamino)phenoxy]ethoxy}methyl)-1H-1,2,3-triazol-1-yl]-8-[4-(Dimethylamino)phenyl]-9H-purin-9-yl}ethan-1-ol (30)

To the mixture of compound 29 (815 mg, 1.30 mmol, 1.0 eq.) in 52 mL of MeOH/H2O/DCM mixture (10:2:1) TFA (0.99 mL, ρ = 1.49 g/cm3, 13.0 mmol, 10.0 eq.) was added at 25 °C. Then, the reaction was left stirring for 2 h, evaporated, dissolved in DCM (10 mL), and washed with aqueous NaHCO3 solution (10 mL) and brine (10 mL). The organic phase was dried over anhydrous Na2SO4, filtered, and evaporated, providing product 30 (yield: 689 mg, 98%) as a dark yellow amorphous solid. Rf = 0.68 (DCM/EtOH = 10:1). HPLC: tR = 6.32 min, eluent E2. IR (KBr) ν (cm−1): 3339, 2873, 1606, 1513, 1483, 1338, 1229, 1200, 1033, 950, 819. 1H-NMR (500 MHz, CDCl3) δ (ppm): 9.28 (s, 1H, H-C(triazole)), 8.78 (s, 1H, H-C(2)), 7.81 (d, 2H, 3J = 8.8 Hz, 2 × H-C(Ar)), 6.85 (d, 2H, 3J = 8.6 Hz, 2 × H-C(Ar)), 6.75 (d, 2H, 3J = 8.8 Hz, 2 × H-C(Ar)), 6.68 (d, 2H, 3J = 8.6 Hz, 2 × H-C(Ar)), 4.89 (s, 2H, (-CH2-)), 4.55 (t, 2H, 3J = 5.0 Hz, (-CH2-)), 4.22 (t, 2H, 3J = 5.0 Hz, (-CH2-)), 4.10 (t, 2H, 3J = 4.7 Hz, (-CH2-)), 3.92 (t, 2H, 3J = 4.7 Hz, (-CH2-)), 3.06 (s, 6H, 2 × (-CH3)), 2.82 (s, 6H, 2 × (-CH3)). 13C-NMR (126 MHz, CDCl3) δ (ppm): 158.6, 156.8, 152.1, 151.3, 150.6, 146.0, 145.9, 142.7, 131.4, 124.2, 122.7, 115.8, 115.0, 114.9, 111.8, 69.3, 68.2, 64.7, 61.2, 48.2, 42.0, 40.2. HRMS (ESI) m/z: [M + H]+ Calculated for C28H34N9O3 544.2779; Found 544.2805 (4.78 ppm).

3.2.17. 2-{6-[4-({2-[4-(Dimethylamino)phenoxy]ethoxy}methyl)-1H-1,2,3-triazol-1-yl]-8-[4-(Dimethylamino)phenyl]-9H-purin-9-yl}ethyl Methanesulfonate (31)

To a solution of compound 30 (689 mg, 1.27 mmol, 1.0 eq.) in DCM (5 mL), Et3N (176 µL, ρ = 0.73 g/cm3, 1.27 mmol, 1.0 eq.) was added. The reaction mixture was cooled to 0 °C. MsCl (0.11 mL, ρ = 1.48 g/cm3, 1.46 mmol, 1.15 eq.) was slowly added dropwise to the reaction mixture, and it was left to stir for 1 h at 25 °C. Then, the reaction mixture was washed with H2O (2 × 10 mL) and brine (10 mL). The organic phase was dried over anhydrous Na2SO4, filtered, and poured in cold hexane (200 mL). The resulting precipitate was filtered, providing product 31 (yield: 772 mg, 98%) as a dark orange amorphous solid. HPLC: tR = 5.34 min, eluent E2. IR (KBr) ν (cm−1): 2929, 2868, 1608, 1514, 1477, 1339, 1221, 1170, 1038, 898, 810. 1H-NMR (500 MHz, CDCl3) δ (ppm): 9.34 (s, 1H, H-C(triazole)), 8.86 (s, 1H, H-C(2)), 7.76 (d, 2H, 3J = 8.4 Hz, 2 × H-C(Ar)), 6.86 (d, 2H, 3J = 8.8 Hz, 2 × H-C(Ar)), 6.82 (d, 2H, 3J = 8.8 Hz, 2 × H-C(Ar)), 6.77 (d, 2H, 3J = 8.4 Hz, 2 × H-C(Ar)), 4.89 (s, 2H, (-CH2-)), 4.84–4.77 (m, 2H, (-CH2-)), 4.74–4.68 (m, 2H, (-CH2-)), 4.15–4.07 (m, 2H, (-CH2-)), 3.95–4.89 (m, 2H, (-CH2-)), 3.06 (s, 6H, 2 × (-CH3)), 2.87 (s, 6H, 2 × (-CH3)), 2.84 (s, 3H, (-CH3)). 13C NMR (126 MHz, CDCl3) δ (ppm): 157.9, 156.8, 152.7, 152.2, 151.2, 145.8, 144.1, 143.1, 130.8, 124.4, 122.8, 116.1, 115.9, 114.6, 111.9, 69.1, 68.1, 65.6, 64.6, 43.8, 42.8, 40.2, 37.6. HRMS (ESI) m/z: [M + H]+ Calculated for C29H36N9O5S 622.2555; Found 625.2549 (0.94 ppm).

3.2.18. 4-(6-(4-((2-(4-(Dimethylamino)phenoxy)ethoxy)methyl)-1H-1,2,3-triazol-1-yl)-9-(2-(4,7,10-trimethyl-1,4,7,10-tetraazacyclododecan-1-yl)ethyl)-9H-purin-8-yl)-N,N-Dimethylaniline (1)

A suspension of compound 31 (770 mg, 1.24 mmol, 1.0 eq.) and LiBr (215 mg, 2.48 mmol, 2.0 eq.) in THF (16 mL) was stirred in a pressure flask at 70 °C for 3 h. Then, the reaction mixture was evaporated, suspended in DCM (10 mL), and washed with water (2 × 10 mL) and brine (10 mL). The organic phase was dried over anhydrous Na2SO4, filtered, and evaporated. 1,4,7-Trimethyl-1,4,7,10-tetraazacyclododecane [75] (2.66 g, 12.4 mmol, 10 eq.) was added to the precipitate, and this mixture was dissolved in abs. MeCN (60 mL) and left to stir for 72 h at 25 °C. Silica gel column chromatography (DCM/EtOH, gradient 0% → 5%) provided by-product 32 (yield: 370 mg, 57%) as a yellow amorphous solid. Reverse phase silica gel column chromatography (MeOH/H2O, gradient 0% → 70%) with the following preparative HPLC column chromatography (that used TFA as an additive) was used to separate impurities from the product. Then, it was dissolved in DCM (10 mL), washed with sat. aqueous NaHCO3 solution (10 mL) and brine (10 mL), and evaporated. Next, it was dissolved in water (10 mL), filtered, and lyophilized, providing product 1 (yield: 56 mg, 6%) as a yellow amorphous solid. Rf = 0.1 (DCM/EtOH = 20:1). HPLC: tR = 4.38 min, eluent E2. IR (KBr) ν(cm−1): 2924, 2852, 1667, 1607, 1585, 1464, 1338, 1259, 1066, 1033, 796. 1H-NMR (500 MHz, CDCl3) δ 9.41 (ppm) (s, 1H, H-C(triazole)), 8.89 (s, 1H, H-C(2)), 7.83 (d, 2H, 3J = 8.8 Hz, 2 × H-C(Ar)), 6.86 (d, 2H, 3J = 9.0 Hz, 2 × H-C(Ar)), 6.80 (d, 2H, 3J = 8.8 Hz, 2 × H-C(Ar)), 6.67 (d, 2H, 3J = 9.0 Hz, 2 × H-C(Ar)), 4.92 (s, 2H, (-CH2-)), 4.59 (t, 2H, 3J = 6.8 Hz, (-CH2-)), 4.11 (t, 2H, 3J = 4.8 Hz, (-CH2-)), 3.92 (t, 2H, 3J = 4.8 Hz, (-CH2-)), 3.07 (s, 6H, 2 × (CH3)), 2.82 (s, 6H, 2 × (CH3)), 2.77 (t, 2H, 3J = 6.8 Hz, (-CH2-)), 2.65–2.61 (m, 4H, 2 × (-CH2-)), 2.32–2.26 (m, 12H, 6 × (-CH2-)), 2.16 (s, 3H, (-CH3)), 2.11 (s, 6H, 2 × (-CH3)). 1H-NMR (500 MHz, MeCN-d3) δ 9.27 (ppm) (s, 1H, H(1)), 8.84 (s, 1H, H(2)), 7.85 (d, 2H, 3J = 8.6 Hz, 2 × H(3)), 6.88 (d, 2H, 3J = 8.6 Hz, 2 × H(4)), 6.81 (d, 2H, 3J = 9.1 Hz, 2 × H(5)), 6.66 (d, 2H, 3J = 9.1 Hz, 2 × H(6)), 4.81 (s, 2H, 2 × H(7)), 4.61 (t, 2H, 3J = 6.6 Hz, 2 × H(8)), 4.07 (t, 2H, 3J = 4.6 Hz, 2 × H(9)), 3.86 (t, 2H, 3J = 4.6 Hz, 2 × H(10)), 3.05 (s, 6H, 6 × H(11)), 2.86–2.74 (m, 8H, 6 × H(12), 2 × H(13)), 2.58 – 2.51 (m, 4H, 4 × H(14)), 2.46 – 2.17 (m, 21H, 12 × H(15), 9 × H(16)). 13C NMR (126 MHz, CDCl3) δ (ppm): 157.8, 157.2, 152.1, 151.13, 151.03, 146.0, 145.8, 142.9, 130.7, 124.6, 122.7, 115.9, 115.8, 114.8, 111.9, 69.1, 68.2, 64.7, 56.4, 55.79, 55.75, 54.6, 53.4, 45.0, 44.4, 42.9, 41.8, 40.2. HRMS (ESI) m/z: [M + H]+ Calculated for C39H58N13O2 740.4831; Found 740.4846 (2.03 ppm).

3.2.19. 4-(6-(4-((2-(4-(Dimethylamino)phenoxy)ethoxy)methyl)-1H-1,2,3-triazol-1-yl)-9-vinyl-9H-purin-8-yl)-N,N-Dimethylaniline (32)

Rf = 0.61 (DCM/EtOH = 20:1). HPLC: tR = 6.21 min, eluent E2. IR (KBr) ν (cm−1): 2925, 2865, 1606, 1480, 1342, 1199, 1124, 1036, 1023, 946, 819. 1H NMR (500 MHz, CDCl3) δ 9.39 (s, 1H, H-C(triazole)), 8.95 (s, 1H, H-C(2)), 7.84 (d, 2H, 3J = 8.6 Hz, 2 × H-C(Ar)), 7.07 (dd, 1H, 3J = 15.8, 8.9 Hz, (-CHa)), 6.86 (d, 2H, 3J = 8.6 Hz, 2 × H-C(Ar)), 6.81 (d, 2H, 3J = 8.6 Hz, 2 × H-C(Ar)), 6.68 (d, 2H, 3J = 8.6 Hz, 2 × H-C(Ar)), 6.43 (d, 1H, 2J = 15.8 Hz, (-CHb)), 5.51 (d, 1H, 3J = 8.9 Hz, (-CHc)), 4.93 (s, 2H, (-CH2-)), 4.12 (t, 2H, 3J = 4.8 Hz, (-CH2-)), 3.94 (t, 2H, 3J = 4.8 Hz, (-CH2-)), 3.09 (s, 6H, 2 × (-CH3)), 2.83 (s, 6H, 2 × (-CH3)). 13C NMR (126 MHz, CDCl3) δ (ppm): 156.8, 156.2, 152.3, 151.5, 151.1, 146.0, 145.9, 143.2, 131.6, 128.4, 124.5, 123.4, 115.9, 114.8, 114.7, 111.7, 110.9, 69.2, 68.3, 64.7, 41.8, 40.2. HRMS (ESI) m/z: [M + H]+ Calculated for C28H32N9O2 526.2673; Found 526.2688 (2.85 ppm).

4. Conclusions

We have developed a new synthetic approach toward the proposed purine–cyclen conjugate using the sequence of modifications, which includes Mitsunobu, iodination, Stille, SNAr, CuAAC, and alkylation reactions as main transformations. The optimal sequence of derivatization (positions: N9 → C8 → C6 → N9) was found as the result of several less successful attempts toward the target compound. Purine–cyclen conjugate was obtained in nine steps, starting from commercially available 6-chloropurine. As the main by-product, the N9-vinyl group containing purine–triazole conjugate was obtained, which was formed via the E2 type reaction mechanism between purine alkyl bromide derivative and modified cyclen moiety. Preliminary NMR and UV titration experiments revealed that target purine–cyclen conjugate easily complexes copper(II) ions in the cyclen ring and between triazole N2 and purine N7 positions. Photophysical and photocatalytic properties of the developed purine–cyclen derivatives will be studied in the future and will be reported elsewhere.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules30071612/s1, 1H- and 13C-NMR spectra of compounds 1, 3, 11, 13, 15, 22–32; NMR titration procedure and stacked spectra for the titration experiments of compound 1.

Author Contributions

Synthetic experiments, investigation, methodology, and manuscript draft, A.B.; conceptualization, the idea of the purine–cyclen conjugate, spectrophotometric analysis, and manuscript review, G.J.; conceptualization, supervision, and manuscript review, I.N. and M.T. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge the financial support provided by the Latvia–Lithuania–Taiwan joint grant #LV-LT-TW/2022/9 “Molecular Electronics in Functionalized Purines: Fundamental Study and Applications (MEPS)” from the Latvian Council of Science. I.N. and G.J. acknowledge the PHC Osmosis program project (#50602NB for France, #LV-FR/2024/5 for Latvia) “Purine-based carbenes: synthesis, photophysical and optical studies (PBC)” for financial support of scientific exchanges.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Molecules 30 01612 g001
Figure 1. The structure of purine–cyclen conjugate 1 as a potential photocatalyst.
Figure 1. The structure of purine–cyclen conjugate 1 as a potential photocatalyst.
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Scheme 1. Most used approaches for derivatization of purine at C2, C6, C8, N7, and N9 positions.
Scheme 1. Most used approaches for derivatization of purine at C2, C6, C8, N7, and N9 positions.
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Scheme 2. Retrosynthetic analysis toward purine–cyclen conjugate 1.
Scheme 2. Retrosynthetic analysis toward purine–cyclen conjugate 1.
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Scheme 3. Attempts to introduce moiety A to the 6-chloro-9-tetrahydropyranyl purine 10.
Scheme 3. Attempts to introduce moiety A to the 6-chloro-9-tetrahydropyranyl purine 10.
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Scheme 4. Introduction of THP-protected oxyethyl linker to 6-chloropurine (20), followed by a moiety A and attempts to modify the C8 purine position.
Scheme 4. Introduction of THP-protected oxyethyl linker to 6-chloropurine (20), followed by a moiety A and attempts to modify the C8 purine position.
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Molecules 30 01612 sch005
Scheme 5. The final synthetic route toward target purine–cyclen conjugate 1.
Scheme 5. The final synthetic route toward target purine–cyclen conjugate 1.
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Figure 2. 1H NMR titration spectra of compound 1 with Cu2+ ions in MeCN-d3.
Figure 2. 1H NMR titration spectra of compound 1 with Cu2+ ions in MeCN-d3.
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Figure 3. The change of the chemical shifts of Ar1 and cyclen proton signals in 1H NMR titration experiments for compound 1 with Cu2+ ions in MeCN-d3.
Figure 3. The change of the chemical shifts of Ar1 and cyclen proton signals in 1H NMR titration experiments for compound 1 with Cu2+ ions in MeCN-d3.
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Figure 4. Possible Cu2+ complexes with purine–cyclen conjugate.
Figure 4. Possible Cu2+ complexes with purine–cyclen conjugate.
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Figure 5. (a) The variation of the absorption spectra of complexes going from pure compound 1 until a 1:1 metal–ligand ratio; (b) the absorption spectra of complexes with compositions going from 1:1 until a 2:1 metal–ligand ratio.
Figure 5. (a) The variation of the absorption spectra of complexes going from pure compound 1 until a 1:1 metal–ligand ratio; (b) the absorption spectra of complexes with compositions going from 1:1 until a 2:1 metal–ligand ratio.
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Figure 6. Characteristic absorption spectra extracted from spectrophotometric titration experiment indicating types of vertical transitions, compound 1: Cu2+ complex compositions, and supposed geometries.
Figure 6. Characteristic absorption spectra extracted from spectrophotometric titration experiment indicating types of vertical transitions, compound 1: Cu2+ complex compositions, and supposed geometries.
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Burcevs, A.; Jonusauskas, G.; Novosjolova, I.; Turks, M. Synthesis of Purine-1,4,7,10-Tetraazacyclododecane Conjugate and Its Complexation Modes with Copper(II). Molecules 2025, 30, 1612. https://doi.org/10.3390/molecules30071612

AMA Style

Burcevs A, Jonusauskas G, Novosjolova I, Turks M. Synthesis of Purine-1,4,7,10-Tetraazacyclododecane Conjugate and Its Complexation Modes with Copper(II). Molecules. 2025; 30(7):1612. https://doi.org/10.3390/molecules30071612

Chicago/Turabian Style

Burcevs, Aleksejs, Gediminas Jonusauskas, Irina Novosjolova, and Māris Turks. 2025. "Synthesis of Purine-1,4,7,10-Tetraazacyclododecane Conjugate and Its Complexation Modes with Copper(II)" Molecules 30, no. 7: 1612. https://doi.org/10.3390/molecules30071612

APA Style

Burcevs, A., Jonusauskas, G., Novosjolova, I., & Turks, M. (2025). Synthesis of Purine-1,4,7,10-Tetraazacyclododecane Conjugate and Its Complexation Modes with Copper(II). Molecules, 30(7), 1612. https://doi.org/10.3390/molecules30071612

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