1,2,3-Triazoles as leaving groups in SNAr–Arbuzov reactions: synthesis of C6-phosphonated purine derivatives

A new method for C–N bond transformations into C–P bonds was developed using 1,2,3-triazoles as leaving groups in SNAr–Arbuzov reactions. A series of C6-phosphonated 2-triazolylpurine derivatives was synthesized for the first time, with the isolated yields reaching up to 82% in the C–P-bond-forming event. The SNAr–Arbuzov reaction of 2,6-bistriazolylpurines follows the general regioselectivity pattern of the C6-position being more reactive towards substitution, which was unambiguously proved by X-ray analysis of diethyl (9-heptyl-2-(4-phenyl-1H-1,2,3-triazol-1-yl)-9H-purin-6-yl)phosphonate.

On the contrary, only a few examples can be found in the literature where a phosphorus-containing substituent is directly at-tached to the purine ring [12,13]. In 2008, an S N Ar-Arbuzov reaction was developed for 6-chloropurine derivatives under microwave irradiation (Scheme 1) [12]. In 2011, a single example of a C6-phosphonate, B (X = NH 2 ; R 1 = 2'-C-methylribose; R 2 = Et), was synthesized among other compounds as a potential anti-hepatitis C virus agent and showed 19% inhibition at 10 μM in Huh7 cells (Scheme 1) [13]. Additionally, there are a few examples of C8-phosphonate synthesis. They can be obtained by 1) the reaction of a lithiated C8 position with diethyl chlorophosphate (C→D, Scheme 1) [14] and 2) an intermolecular [15] or intramolecular [16] photochemical reaction between 8-bromopurine derivatives and phosphite (E→F and G→H, re-Scheme 1: Structural diversity and synthetic methods of purinylphosphonates. MWI = microwave irradiation; LG = leaving group. spectively, Scheme 1). Further, the synthesis of C8-phosphonates of 7-and 9-deazapurines via C-H phosphonation has been reported [17].
On the other hand, azolylpurines are an important compound class that combines two recognized structural motifs of drug design -purines and azoles. Derivatives of this class are known for their activity against Mycobacterium tuberculosis and also as agonists and antagonists of adenosine receptors [18].
Herein, we describe an extension for S N Ar reactions that makes use of the 1,2,3-triazole leaving group of 2,6-bistriazolylpurines. This led to a discovery of novel C-P bond formations from C-N bonds in S N Ar-Arbuzov reactions (I→J, Scheme 1). The obtained series of compounds combines three structural motifs that are important in terms of medicinal chemistry in one molecule: purine, triazole, and phosphonate.
The S N Ar-Arbuzov reaction between 2,6-dichloropurine derivative 1 and triethylphosphite gave product 2a in 82% yield (Scheme 3) [12]. Next, attempts to substitute the chlorine atom at the purine C2 position were made using either NaN 3 or   3 . Azidation experiments were tried in solvents such as EtOH, MeOH, and MeCN in temperature diapasons up to 100 °C, but no conversion of the staring material 2a (R 1 = Et) was observed. The change of the solvent to DMF or DMSO resulted in the cleavage of one ethyl ester group [25], but still the S N Ar reaction at C2 was not effective. LC-MS analysis of the crude reaction mixtures revealed the presence of the products 7a and 8a (Scheme 3). When the latter mixture was submitted to CuAAC with phenylacetylene (CuI/Et 3 N/AcOH/ EtOH (or DCM), CuSO 4 •5H 2 O/sodium ascorbate/EtOH (or DMF)), no triazole formation at the purine C2 position was observed.
We briefly tried to optimize the Cl→N 3 S N Ar process at the purine C2 position, and that way, the isopropyl phosphonate 2b was also obtained. It is known that both chloride and azide can cleave phosphonate esters [25][26][27][28], but the chloride source would not interfere with the S N Ar process at C2. Hence, we compared the reaction outcome and rates when DMSO-d 6 solutions of the starting materials 2a and 2b were treated either with NaN 3 or NaCl in parallel experiments. The reaction mixtures were directly analyzed by 1 H and 31 P NMR spectroscopy using 1,2,3-trimethoxybenzene as an internal standard (Tables S1 and S2 as well as Figures S1 and S2 in Supporting Information File 1). The reaction between the diethyl phosphonate 2a and NaN 3 gave a mixture of products 3a, 7a, and 8a already after 15 min. A significant amount of the azido monoester 8a (39%) was formed in only 48 h (Scheme 4, Figure 1, and Table S1 in Supporting Information File 1). The cleavage of the ester groups in the presence of NaCl was slower than in the presence of NaN 3 ( Figure 1 and Table S2 in Supporting Information File 1). Further, the cleavage of the sterically bulky isopropyl ester from phosphonate 2b showed a similar pattern: 5% conversation to monoester 7b was observed with NaCl after 48 h (Scheme 4, Figure 1, and Table S2 in Supporting Information File 1), but the reaction with NaN 3 resulted in a mixture of products, which contained 45% of 2-azido monoester 8b (Scheme 4).
Based on the previous observations, we forced the S N Ar reaction of the Cl atom at the C2 position of purine with an excess of NaN 3 , and after chromatographic isolation. We obtained the Scheme 4: Synthesis of phosphonic acid monoesters 3 and 7-9 as well as phosphonic acid 10. Figure 1: Screenings of the rate for the ester group cleavage (conversion determined by NMR spectroscopy) in the reactions between dialkyl (2-chloro-9-heptyl-9H-purin-6-yl)phosphonates 2a and 2b, respectively, with NaN 3 (a, c) and NaCl (b, d). Reaction conditions: DMSO-d 6 , 90 °C.
pure azido-substituted phosphonate monoesters 9a and 9b in 28 and 23% yield, respectively (Scheme 4). The products 9a and 9b were further submitted to CuAAC reactions, but the desired triazole derivatives were not obtained. Further-more, the hydrolysis of the dialkyl ester groups were performed with TMSI [29,30], and phosphonic acid 10 was obtained. The latter was inert to the S N Ar reaction with NaN 3 at C2 (Scheme 4).  S N Ar-Arbuzov reaction between 2,6-bistriazolylpurines and P(OEt) 3 Next, we switched to pathway B (Scheme 2) and prepared 2,6diazidopurine derivative 5 from 2,6-dichloropurine (11) via a Mitsunobu alkylation and S N Ar reaction with NaN 3 (Scheme 5) [22]. 2,6-Bistriazolylpurine derivatives 6a-i were obtained in CuAAC reactions with various alkynes in 35-76% yield ( Table 1). We found that a combination of CuI with an amine buffer system [31][32][33][34][35][36][37] suites substrate 5 better than the previously used CuSO 4 •5H 2 O and sodium ascorbate catalytic system [22]. Most probably, this is due to the solubility issues of the starting material 5 in aqueous solutions, as used in the Cu(II) and ascorbate protocol. In some cases, the use of Et 3 N lowered the yield of 2,6-bistriazolylpurines 6c and 6f-i due to the competing Glaser coupling [38,39] and the reduction of 2,6-diazide 5 by the Cu(I) species [40,41]. The bistriazolyl Scheme 6: S N Ar-Arbuzov reaction between the bistriazolylpurines 6a-i and P(OEt) 3 . derivatives 6a-i were easily crystalized from MeOH, EtOH, or a hexane/EtOH mixture or purified by column chromatography.
The obtained 2,6-bistriazolylpurine derivatives 6a-i were explored as substrates for the S N Ar-Arbuzov reaction with P(OEt) 3 . In attempts to perform the S N Ar-Arbuzov reaction in common laboratory solvents, such as toluene, MeCN, and DCM, and in the presence of 1-20 equiv of P(OEt) 3 , the formation of the desired phosphonates 4 was not observed (Scheme 6). We started an optimization of the reaction conditions using substrate 6d, and reactions in neat phosphite at various temperatures were tried ( Table 2). The conversion of starting material 6d was monitored by HPLC, and after completion, product 4d was precipitated from the reaction mixture by hexane. For entries 1 and 3 in Table 2, an extra purification step by silica gel column chromatography was required. For compound 4d, the optimal reaction conditions were 2 hours in neat P(OEt) 3 at 160 °C.
With the experimental conditions in hand, the S N Ar-Arbuzov reaction between 2,6-bistriazolylpurines 6a-i and P(OEt) 3 provided a library of novel purine phosphonates 4a-i in 27-82% yield ( Table 3). The products 4a, 4d, 4e, and 4i were easily precipitated from hexane left at −20 °C within 10 hours and were then filtered and washed with cold hexane. The product purity, if necessary, was further improved by column chromatography. Some phosphonates, for example, 4b, 4c, and 4f, were reductant to precipitate from hexane and were purified solely by silica gel column chromatography. At the preparative level, the excess of P(OEt) 3 was evaporated under vacuum (5 mbar) over 4-5 hours at 50 °C before further purification. The regioselectivity of the newly developed S N Ar-Arbuzov reaction was unambiguously established by X-ray analysis of the product 4d, which was crystalized from a mixture of hexane and DCM using the slow-evaporation technique (Figure 2). This follows the previously reported regioselective C6-substitution of 2,6-bistriazolylpurines in S N Ar transformations.

Conclusion
We have developed a novel S N Ar-Arbuzov transformation that makes use of 1,2,3-triazole as a leaving group. This has permitted to obtain a novel series of C6-phosphonated 2-triazolylpurine derivatives. It was also demonstrated that there is no alternative S N Ar protocol towards the designed products. The synthetic intermediates, (2-chloro-9H-purin-6-yl)phosphonates, of the alternative pathway are sluggish in substitution reactions with NaN 3 , and the burdensomely obtained (2-azido-9H-purin-6-yl)phosphonates fail to undergo CuAAC reactions. The developed S N Ar-Arbuzov reaction between 2,6-bistriazolylpurine derivatives and trialkyl phosphites is C6-regioselective, as proved by single-crystal X-ray analysis. This is similar to the previously observed substitution pattern in S N Ar reactions of 2,6-bistriazolylpurine derivatives with simple N-and S-nucleophiles.

Experimental General information
Commercially available reagents were used as received. The reactions and the purity of the synthesized compounds were monitored by HPLC and TLC analysis using silica gel 60 F 254 aluminum plates (Merck

General procedures and product characterization
The synthesis and characterization of the starting materials 1 and 5 and of 2,6-bistriazolylpurine derivative 6d have been reported earlier [22].

Supporting Information
Supporting Information File 1 Full experimental procedures and copies of the 1 H, 13