1,2,3-Triazoles as leaving groups: SNAr reactions of 2,6-bistriazolylpurines with O- and C-nucleophiles

A new approach was designed for the synthesis of C6-substituted 2-triazolylpurine derivatives. A series of substituted products was obtained in SNAr reactions between 2,6-bistriazolylpurine derivatives and O- and C-nucleophiles under mild conditions. The products were isolated in yields up to 87%. The developed C–O and C–C bond forming reactions clearly show the ability of the 1,2,3-triazolyl ring at the C6 position of purine to act as leaving group.

Azolylpurine derivatives are important due to their potential as drug candidates. They can be used as agonists and antagonists of adenosine receptors [58,[64][65][66] and against Mycobacterium tuberculosis [60]. They also show useful fluorescent properties [11,[67][68][69] and can be used as metal ion sensors [70]. Therefore, it is important to develop novel methods towards this type of derivatives. To date two approaches have been used to obtain 6-substituted 2-triazolylpurine derivatives (Scheme 1). According to the pathway A, firstly a selected substituent is introduced at the C6 position of the purine ring using S N Ar reactions (Ia→II, Scheme 1). If purine contains identical leaving groups at C2 and C6 positions the reactivity order in its S N Ar reactions is C6 > C2 [71,72]. Also transition metal catalyzed reactions can be used for C6 functionalization of purine [73][74][75][76] or alkylation of inosine or guanosine derivatives (Ib→II, Scheme 1) [30,36]. In the next step, azide can be introduced either by a second S N Ar reaction on the C2-halo derivative or by diazotization/azidation at C2. Then, the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction provides the target product IV (Scheme 1, pathway A) [59][60][61]. Pathway B is designed on the basis of our group investigations on the synthesis of 2,6-bistriazolylpurine derivatives and their application in reactions with N-, S-and P-nucleophiles making use of regioselective S N Ar reactions at C(6) (V→VI→IV, Scheme 1) [11,14,62,63,77,78]. The main advantage of pathway B is a straightforward access to 2,6-diazidopurines V and 2,6-bistriazolylpurines VI due to excellent nucleophilic properties of the azide ion and wellestablished CuAAC reaction. Pathway B also avoids performing of an S N Ar process on partially deactivated purines as the introduced nucleophiles are mostly seen as electron-donating substituents (e.g., R 2 N-, RS-, RO-).
Herein, we report a synthetic extension of this methodology. We have found that the pronounced leaving group character of 1,2,3-triazoles makes 2,6-bistriazolylpurines excellent substrates for S N Ar reactions with O-and C-nucleophiles.

Results and Discussion
Synthesis of 2,6-bistriazolylpurine derivatives and their reactions with O-nucleophiles The 2,6-diazidopurine derivatives 1a and 1b as strategic starting materials and 2,6-bistriazolylpurine derivatives 2a-c were obtained in the synthetic procedures developed by us before [11,14,67]. The CuAAC reaction was performed between diazide derivatives 1a and 1b and phenylacetylene or methyl propiolate (Scheme 2). S N Ar reactions between bistriazolylpurine derivatives and O-nucleophiles were first performed on N9-alkylated bistriazole 2c. The reactions were carried out with primary and secondary alcohols in the presence of NaH in DMF. The developed transformation required only nearly equimolar loading of an alcohol and a base, and products 3a-f were obtained in yields up to 83% (Scheme 3). In most cases the full conversion of the starting material was reached in 15-30 min at room temperature, which clearly showed the excellent leaving group ability of the triazolyl ring. These S N Ar reactions can also be performed in DMSO or DMF in the presence of K 2 CO 3 , but the completion of these transformations requires heating the reaction mixtures up to 60 °C for 24 h. An S N Ar reaction with a non-trivial alcohol was demonstrated on the example of 2',3'-O-isopropylideneuridine and product 3f was isolated after 21 h of heating at 50 °C in 82% yield. It should be noted that tertiary alcohols (e.g., t-BuOH) were inert in S N Ar reactions with 2,6-bistriazolylpurines and their attempted reactions resulted in an unidentifiable mixture of byproducts.
The following experiments were performed on 2,6-bistriazolylpurine nucleoside 2b in MeOH, EtOH and PrOH used as solvents and nucleophiles in the presence of NaH (5.0 equiv). The excess of base and alcohol was required due to the cleavage of acetyl protecting groups. Products 3g-i were obtained in yields of up to 79% (Scheme 4). Furthermore, purification of the products 3g-i was complicated due to their poor solubility in organic solvents. The C6 regioselectivity of S N Ar reactions was proved by 13 C NMR comparison of the products 3a-i with similar compounds from literature [61].
Intriguingly, we were able to conserve the acetate protecting groups in product 3j, when the S N Ar reaction was performed in the presence of DBU used as base. The artificial dinucleotide analogue 3j was obtained in 25% isolated yield.
were sufficiently mild to maintain the acetyl protecting groups in product 4a. Also hydrolysis of 2c into 4b proceeded under mild conditions and only gentle warming to 50 °C was required.
2,6-Bistriazolylpurine derivatives in S N Ar reactions with C-nucleophiles Next, S N Ar reactions between 2,6-bistriazolylpurine 2c and C-nucleophiles offered an easy way for the C-N bond transformation into a C-C bond. Compounds containing electron-withdrawing groups such as malonitrile, dimedone, ethyl cyanoacetate and diethyl malonate were used as C-nucleophiles. Transformations were performed in DMF in the presence of NaH and the products were obtained in high yields (Scheme 6). The lower yield of compound 5d was obtained due to the ethyl ester hydrolysis and subsequent decarboxylation. Such side reactions were also observed for similar compounds in literature [79,80].
As a limitation of the method we have found that 2,6-bistriazolylpurine 2c was inert to S N Ar reactions with deprotonated acetylacetone and diphenylmethane. Even there are reports on S N Ar reactions of acetylacetone with purines and pyrimidines [56,80], in our hands only polymerization of acetylacetone was observed. On the other hand, the diphenylmethane anion (pK a 32; DMSO [81]) apparently is too basic and deprotonates purine C(8)-H, thus suspending the S N Ar process.
The structures of C6-substituted products 5a-d were elucidated by NMR and IR analysis. These compounds can exist as either C-H acids (A) or N-H acids (B), but dimedone conjugate 5b may possess also an enol form C (Figure 1).
During the structural studies of cyano group containing products 5a and 5c the cross signals for the C(2'')-H system were not found using HSQC spectra, excluding the existence of C-H tautomeric forms A. In addition, IR analysis (KBr tablet) indicated absorption bands of cyano groups at 2205 and 2170 cm −1 for product 5a and at 2205 cm −1 for product 5c. These results differ from the absorption in the range of 2260-2240 cm −1 , which would be characteristic for a cyano group attached to sp 3hybridized carbon [82]. On the other hand, 13 C NMR shifts of the C(2'') position of purine-malononitrile conjugate 5a and ethyl cyanoacetate-purine conjugate 5c were 40.9 and 61.7 ppm, respectively. This range does not fully correspond to the theoretical values 80-140 ppm, expected for the Csp 2 atom of the N-H form B. In compound 5c the N-H form 5cB is possibly the major tautomer in CDCl 3 solution as it is stabilized via an intramolecular hydrogen bond. This is supported by a smaller deviation of the C(2'') chemical shift value (61.7 ppm) in comparison to the theoretical shifts for a Csp 2 centre. Similar structural analogues are known in the literature [54,[83][84][85] but their structural analysis was incomplete. As the aforementioned experiments did not determine preference for tautomer A or B of compound 5a, it was analysed in its deprotonated form C (CD 3 OD/D 2 O/NaOD). Interestingly, that the 13 C NMR spectrum of 5a in basic medium revealed a similar chemical shift for carbon C(2'') (40.9 ppm) as in neutral CD 3 OD.
The 13 C NMR analysis of purine-dimedone conjugate 5b revealed two downfield shifts of 194.1 and 185.3 ppm. It showed that the structure is not symmetrical and corresponds to either tautomer structure B or C in CDCl 3 solution with a theo- retical preference for enol form C. Finally, the structure of C-H tautomer 5dA was proved by its HSQC spectrum, in which a cross peak clearly indicated the C(2'')-H system.

Conclusion
The S N Ar reactivity of 2,6-bis(1,2,3-triazol-1-yl)purine derivatives was extended with their substitution with O-and C-nucleophiles. The reactions proceeded under transition metal free conditions and revealed excellent C6 selectivity. The developed synthetic approach provided O-adducts with 25-83% yields and C-adducts with 67-87% yields. The methodology demonstrated the leaving group ability of the 1,2,3-triazolyl substituent at the C6 position of the purine ring.

Experimental
General information 1 H and 13 C NMR spectra were recorded with a Bruker Avance 300 or a Bruker Avance 500 spectrometer, at 300 and 75.5 MHz or 500 and 125.7 MHz, respectively. The proton signals for residual non-deuterated solvents (δ 7.26 for CDCl 3 , δ 2.50 for DMSO-d 6 , δ 3.31 for CD 3 OD) and the carbon signals (δ 77.1 for CDCl 3 , δ 39.5 for DMSO-d 6 , δ 49.0 for CD 3 OD) were used as an internal reference for 1 H and 13 C NMR spectra, respectively. Coupling constants are reported in Hz. Chemical shifts of signals are given in ppm and multiplicities are assigned as follows: s -singlet, d -doublet, t -triplet, m -multiplet, brsbroad singlet, tq -triplet of quartets.
Analytical thin-layer chromatography (TLC) was performed on Merck 60 Å silica gel F 254 plates. Column chromatography was performed on Merck 40-60 µm 60 Å silica gel. Yields of products refer to chromatographically and spectroscopically homogeneous materials. The solvents used in the reactions were dried with standard drying agents and freshly distilled prior to use. Commercial reagents were used as received.

General procedures and product characterization
Synthesis of compounds 1a,b and 2a-c and their characterization are described earlier [11,14,67].

Supporting Information
Supporting Information File 1 Full experimental procedures and copies of 1 H, 13 C and 1 H, 13 C HSQC NMR spectra.