Direct Regioselective C-H Cyanation of Purines

Abstract

A direct regioselective C-H cyanation of purines was developed through a sequential triflic anhydride activation, nucleophilic cyanation with TMSCN, followed by a process of base-mediated elimination of triflous acid (CF₃SO₂H). In most cases, the direct C-H cyanation occurred on the electron-rich imidazole motif of purines, affording 8-cyanated purine derivatives in moderate to excellent yields. Various functional groups, including allyl, alkynyl, ketone, ester, nitro et al. were tolerated and acted as a C8 directing group. The electron-donating 6-diethylamino, as C2-directing group substituent, can switch the regioselectivity of purine from 8- to 2-position, enabling the synthesis of 8- and 2-cyano 6-dialkylaminopurines from corresponding 6-chloropurine in different reaction order. Further functional manipulations of the cyano group allow the conversions of 8-cyanopurines to corresponding purine amides, imidates, imidothioates, imidamides, oxazolines, and isothiazoles.

1. Introduction

Purine is a fundamental motif in DNA and RNA nucleic acids, and a primary heterocyclic framework in pharmaceuticals and medicinal chemistry. Purine derivatives bearing a cyano group on their framework have received a great deal of attention due to their biological activities such as antimalarial activity [1]. In addition, they also serve as T. brucei’s cysteine protease inhibitors to cure the Human African trypanosomiasis [2]. The cyanation of purines is generally derived from the corresponding purine halides, with 6-chloropurines as the most useful one, via either an S_N2Ar process with KCN [3]/Bu₄NCN [4] (Scheme 1a) or palladium catalyzed cross-coupling with Zn(CN)₂ [5,6,7]. The cyanation of less reactive 2-chloropurines required harsh reaction conditions [2,8,9] or a transition-metal (TM) catalysis (Scheme 1b) [10,11]. Similarly, 8-cyanopurines were prepared from the corresponding 8-halopurines through a 3/4-step traditional protocol involving swapping the bromine to a more electronegative fluorine [12] or sulfonyl group [13,14], or a transition metal (TM)-catalyzed cross coupling (Scheme 1c) [15,16]. Generally, the cyanation of purines required multiple-step synthesis, with extremely toxic agent (KCN or Bu₄CN) or the TM catalysis. Owing to the fact that 6- and 2-chloropurines are commercially available and easily obtained from adenine and guanine, we thus planned to develop an expedient and highly regioselective C⁸-H cyanation protocol through the triflic anhydride activation on the electron-rich imidazole motif of purines.

As is generally known, purine consists of an electron-deficient pyrimidine and an electron-rich imidazole partner (Scheme 1d). In other previous work and our own work, when purines were exposed to nucleophilic reagents such as Grignard reagents [17,18,19,20] and nucleophilic radical agents (Minisci reaction) [21,22,23,24,25,26], the regioselectivity of the reaction predominantly lay at the electron-deficient 6-position [12,13,14,15,16]. In contrast, when the electrophilic bromine was introduced, a C⁸-brominated purine derivative was obtained (Scheme 1c). To facilitate cyanide-type nucleophilic attack at the 8-position, the polarity of electron-rich imidazole motif should be reversed. In our previous work, we found that trifluoroacetic acid was critical to activate the pyrimidine core of the purine via protonation, facilitating the C⁶-arylation of purines with arylboronic acid through a Minisci-type reaction [24]. The research also revealed that with the equivalents of trifluoroacetic acid increased, a small amount of 2,8-biarylated purines was found in the transformation [24]. We thus envisaged that with a suitable activator, the polarity of the purine core would be reversed, that is the electrophilic site might switch from the originally electron-deficient pyrimidine to the electron-rich imidazole motif. After a series of investigations on Lewis acids, triflic anhydride (Tf₂O) was found to be suitable for this polarity inversion. This interesting development was succeeded in the application of the C⁸ or C²-cyanation with trimethylsilyl cyanide (TMSCN) through a one-pot procedure (Scheme 1e).

2. Results and Discussion

In our model reaction, 9-benzylpurine (1a), holding three reactive sites at 2-, 6-, and 8-positions of the purine skeletal, was explored first in the presence of TFA (trifluoroacetic acid), a reagent used in our previous work [24], but no reaction occurred. A strong acid TfOH and its alternative TMSOTf cannot promote the reaction either (Scheme 1, entry 2). A typical Lewis acid BF₃^.OEt₂ was investigated but the reaction was intact (entry 3). Tf₂O is a useful electrophile to activate 6-membered pyridine compounds, as reported by Corey’s [27], McNally’s [28], and Dixon’s groups [29]. We were pleased to discover that the reaction proceeded smoothly to afford the desired 8-cyanoproduct 2a, whose structure was verified by both ¹H NMR with the signal loss of 8-H at 8.0 ppm and single crystal XRD (CCDC: 2217776; Table 1). Further solvent evaluation showed that the reaction did not occur in THF or MeCN but reacted smoothly in toluene and haloalkane solvents (entries 5 and 6). Various bases, including pyridine (Py), Et₃N, N-methylmorpholine (NMM), 1,5-diazabicyclo [4.3.0]non-5-ene (DBN), 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU), and 1,4-diazabicyclo [2.2.2]octane (DABCO), were investigated, but DBU provided the best result (Table 1, entries 11~15). The reaction time, temperature, and the equivalents of reactants were finally screened, with the detailed information listed in Supplementary Materials (SI, page 3).

Armed with the optimized conditions, we embarked on the task of investigating on the substrate scope. 6-Chloropurines are the most widely used substrates for the construction of various other substituted purine derivatives. With respect to this, various 9-alkyl 6-chloropurines (1) were investigated (Scheme 2). 9-Propyl and more sterically hindered isopropyl purines (1b and 1c) provided the desired 8-cyano products 2b and 2c in 88% and 75% of yields, respectively. Substrates bearing allyl (1d) and propynal (1e), were tolerated under current conditions. 9-Benzyl (1f) and its derivatives with electron-donating methyl (1g), methoxy (1i), and electron-withdrawing nitro (1h) all caused the corresponding 8-cyano products in satisfactory to excellent yields, but electron-rich substrates obviously displayed better reactivity than the electron-deficient one (2g, 2h vs. 2i). Moreover, the electrophilic ketone and ester substituents (1j and 1k) were also tolerated under current conditions, delivering the desired 2j and 2k in 79% and 63% of yields, respectively. N⁹-Arylpurines are highly suitable in this transformation. 6-Chloro-N-phenylpurine provided cyanated product 2l in 94% of the yield, while substituted purines, including N-para-methyl 1m, methoxy 1n, methylthio 1o, bromo 1p, and N-meta-chloro 1q all produced the desired products 2m~2q in 76~94% of yields.

We also briefly investigated the purines bearing other substituents besides the chlorine (Scheme 3). It is worth noting that the reactive intermediates 3 were unstable, sensitive, and generally difficult to isolate. Nevertheless, the key intermediate 3r was isolated when 2,6-dichloropurine (1r) was exploited. Moreover, 3r was transformed into the desired 2r in 68% of the yield by treatment with 1 equiv. of DBU (Scheme 3a). The yield of 2r was increased to 72% when 1.5 equiv. of DBU was added. Moreover, 6-benzylthio purine 1s afforded the desired 8-cyanated product 2s in 97% of the yield, whereas 6-methoxy purine 1t caused the corresponding 2t in only 45% of the yield, with most of the substrate remaining unreactive (Scheme 3b,c). Another interesting result was obtained from the reaction of 6-p-tolyl purine 1u, in which a mixture of 8-monocyanated product 2u (23%) and 2,8-dicyanoated 4u (42%) were generated (Scheme 3d). The results demonstrated that the electron-donating alkoxy and aryl substituents might enhance the electron density of the pyrimidine motif of the purines, causing the decreasing regioselectivity over the imidazole motif. To verify our hypothesis, we introduced a strong electron-donating diethylamino (Et₂N) on the 6-position of purine (1v) but the reaction of 1v was relatively complicated. After careful isolation and structural identification, a purine derivative 2v’ bearing the nitrile on its 2-position, rather than common 8-position, was obtained albeit in only 16% of the yield (Scheme 3e). None of the desired 8-cyanopurine 2v was traced by the TLC and ¹H NMR of the reaction system. Obviously, the strong donating diethylamino completely switched the reactivity of the purine skeletal, enabling the pyrimidine motif to be more reactive than the imidazole counterpart. To obtain the desired 8-cyano-6-diethylaminopurine 2v, the direct amination reaction of 6-chloro-8-cyanopurine 2f and diethylamine (Scheme 3f) was performed in the presence of Et₃N in EtOH, providing the desired 2v in 83% of the yield. The ¹H NMR of 8-cyanopurine 2v and 2-cyanopurine 2v’ are totally different (Scheme 3g), as well as the ¹³C NMR shown in SI (page S25–S28). It is worth mentioning that the alkyl carbons in Et₂N in both 2v and 2v’ are hard to emerge especially the 2v’ in which 1250 scans (~80 min) were tested while the peak appearances were broad and weak (much weaker than the ipso aromatic carbons). The HSQC test of 2v’ was thus performed for further verification of the alkyl carbons in Et₂N (Figure 1). Interestingly, when heating the reaction system, an imidaminopurine derivative 5f was obtained in 33% of the yield from a Et₂NH/EtOH solution at 90 °C through a base-mediated Pinner-type reaction [30]. This yield was further optimized to 91% with pure Et₂NH as solution at 60 °C (Scheme 3h). The results demonstrated that besides the 6-chloro reactive site of purine, the nitrile group is also a highly potential electrophilic reactive position.

As building blocks in purine chemistry, 6-chloropurines can be converted into various purine derivatives (Scheme 4). Gram-scale reactions with 1f were carried out, affording 2f in 1.03 g (76%, 5 mmol scale) and 1.38 g (73%, 7 mmol scale) (Scheme 2-2f). A Sonogashira coupling product 6f was obtained in 52% of the yield from the reaction of 2f with p-methoxyphenylacetylene under the palladium catalysis [31]. Similarly, a Suzuki–Miyaura coupling with p-tolylboronic acid delivered 2u in 28% of the yield, along with 2f recovered in 26% of the yield [32]. The reaction of 1f with sodium methoxide provided a complicated mixture, but it reacted smoothly to cause the desired 6-methoxypurine 2t in the presence of CuBr (10 mol%) [33]. Moving forward, the alkylthiolation with benzyl thiol produced the desired 6-benzylthio purine 2s in 24% of the yield and an extra thiol to nitrile adduct, imidothioate 5s, similar to the adduct from Et₂NH (Scheme 4) in 54% of the yield. The results are similar to our previous discovery, in which an interesting adduct was formed from the addition of potassium thioacetate (KSAc) to acetonitrile (MeCN) [34].

The further derivation of the nitrile group was performed with 9-benzyl-6-benzylthio-8-cyano purine (1s) as an example. A 6 mmol Gram-scaled reaction was carried out, from which 1.60 g of desired 2s was obtained in 74% of the yield (Scheme 5). Treatment of 1s with benzylthiol (BnSH) at r.t. afforded the desired adduct 5s in 78% of the yield (Scheme 5). The result indicated that the cyano group on the purine skeleton was more reactive than the common arylnitriles. We thus explored its reaction with sodium methoxide (NaOMe), which produced the corresponding adduct methyl carbimidate 6s in 80% of the yield. Generally, carbimidate was commonly unstable but herein 6s along with 5s were generated and could be separated through the column chromatography, probably due to the stabilization effect by the intramolecular hydrogen bonding with the nitrogen in the purine scaffold. As that carbimidate was also a general intermediate in the synthesis of oxazoline [35], we thus reacted 6s with 2-amino-2-methylpropan-1-ol in toluene. After an overnight Dean–Stark reflux (~10 h), oxazoline 7s was obtained in 70% of the yield. The transformation of 2s to 7s was also carried out through a one-pot process, delivering a total yield of 75%. The hydrolysis of a nitrile to an amide is a classic transformation. However, the direct hydrolysis of 2s in aqueous NaOH solution failed due to its poor solubility in water, with all starting materials floating on the water surface. The addition of EtOH can disperse 2s into the solution, but only ethyl carbimidate homolog 6s’ was obtained with NaOH in EtOH/H₂O (7:3). Finally, the desired amide 8s was attained in a solution of NaOH in THF/H₂O. It is worth noting that the isolation of 5s, 6s, 6s’, and 8s were quite convenient because all adducts directly precipitated from their solutions and only filtration and drying were required in the work-up process. Very recently, Yang and coworkers reported a highly effective diastereoselective (3 + 2) transannulations of 1,2,3-thiadiazoles with cyanoepoxides to afford the corresponding isothiazoles under the catalysis of [Rh(COD)Cl]₂ and DPPF [36]. We also developed the transannulation of 1,2,3-thiazoles with α,β-unsaturated nitriles [37]. Finally, referring to Yang’s conditions, the reaction of 2s with thiadiazole 9 caused the desired isothiazole 10s in a quantitative yield.

The proposed reaction mechanism is portrayed in Scheme 6. Triflylation of the most nucleophilic nitrogen (N-7) of the purines 1 activates the electron-rich imidazole motif and enables it to be subjected to the subsequent nucleophilic attack by TMSCN, in this way the sensitive and unstable key adducts 3 are formed. We believe that the base-mediated elimination of triflous acid (CF₃SO₂H) from 3 furnishes the formal C-H cyanation products 2, and the transformation of intermediate 3r in Scheme 3 solidly supports this suggested mechanistic insight.

3. Materials and Methods

3.1. General Information

Moisture sensitive reactions were carried out under the nitrogen atmosphere with flame-dried apparatus. The solvents, THF, PhCl, PhMe, CHCl₃, used in Table 1 and the DCE used under the standard conditions, were freshly distilled by referring the standard dehydration process. Other solvents used in Scheme 4, Scheme 5 and Scheme 6, besides DCE, were used directly from the commercial source. Melting points were obtained on a MP-500 melting point apparatus and are uncorrected. TLC were performed on silica gel GF254 plates, the plates of which were visualized under UV light. ¹H and ¹³C-NMR spectra were recorded on a 400 MHz Bruker spectrometer in CDCl₃ with TMS as an internal standard and the chemical shifts (δ) are reported in parts per million (ppm). HRMS measurements were carried out on an Agilent LC/MSD ESI-TOF mass spectrometer. Petroleum ether (PE, 60–90 °C) and ethyl acetate (EtOAc) were used for column chromatography.

All purine substrates were synthesized according to our previous work [24,38].

3.2. General Procedure for the 8-Cyanation of N-Benzylpurine, 6-Chloropurines and 6-Alkoxy, Alkylthio, and Aryl Purines 2

To a frame-dried 10 mL vial was added solid purine (0.2 mmol), and the vial was capped with a septum followed by the N₂ protection. Anhydrous 1,2-dichloroethane (2.0 mL) was then injected and the reaction was cooled in an ice-water bath. Triflic anhydride (0.24 mmol, 40 μL) was then injected dropwise, and the reaction system was stirred at 0 °C for 15 min followed by the injection of trimethylsilyl cyanide (TMSCN; 0.4 mmol, 50 μL). The system was stirred at 84 °C for 3 h. After addition of 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU, 39 μL, 1.3 mmol) the mixture was stirred at 84 °C for another 6 h. After cooling to r.t., the reaction was quenched with 1.0 mL of saturated NaHCO₃ solution and the organic phase was separated. The aqueous phase was then extracted with 3.0 mL of DCE. The combined organic phase was dried over anhydrous Na₂SO₄. After removing the solvent in vacuo, the residue was purified by silica gel column chromatography with a mixture PE and EtOAc as the eluent.

The Gram-scale reactions of 2f and 2s were scaled up proportionally.

3.2.1. 9-Benzyl-9H-purine-8-carbonitrile (2a)

Colorless crystals, mp: 178–180 °C. [CCDC: 2217776] yield: 26 mg, 54%. R_f = 0.12 (PE/EtOAc = 5/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 9.26 (s, 1H, C6-H in purine), 9.17 (s, 2H, C2-H in purine), 7.48–7.45 (m, 2H, ArH), 7.37–7.32 (m, 3H, ArH), and 5.61 (s, 2H, NCH₂). ¹³C NMR (101 MHz, CDCl₃) δ 155.4, 151.3, 150.2, 133.8, 133.0, 129.2, 129.1, 128.7, 128.4, 110.2, and 47.8. HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₃H₁₀N₅⁺, 236.0931; found: 236.0923.

3.2.2. 6-Chloro-9-propyl-9H-purine-8-carbonitrile (2b)

Colorless crystals, mp: 81–83 °C, yield: 39 mg, 88%. R_f = 0.75 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.89 (s, 1H, C2-H in purine), 4.46 (t, J = 7.2 Hz, 2H, NCH₂), 2.03 (sext, J = 7.2 Hz, 2H, CH₂), and 1.00 (t, J = 7.2 Hz, 3H, CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 154.4, 153.9, 150.9, 131.1, 129.1, 109.7, 46.9, 23.2, and 11.0. HRMS (ESI): m/z [M + H]⁺ calcd. for C₉H₉ClN₅⁺, 222.0541; found: 222.0548.

3.2.3. 6-Chloro-9-isopropyl-9H-purine-8-carbonitrile (2c)

Colorless crystals, mp: 112–113 °C, yield: 33 mg, 75%, R_f = 0.65 (PE/EtOAc = 3/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.86 (s, 1H, C2-H in purine), 5.15 (sept, J = 6.8 Hz, 1H, NCH₂), and 1.80 (d, J = 6.8 Hz, 6H, 2CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 153.9, 153.8, 150.6, 131.4, 127.9, 110.2, 51.3, and 21.6. HRMS (ESI): m/z [M + H]⁺ calcd. for C₉H₉ClN₅⁺, 222.0541; found: 222.0537.

3.2.4. 9-Allyl-6-chloro-9h-purine-8-carbonitrile (2d)

Colorless crystals, mp: 108–110 °C, yield: 36 mg, 82%. R_f = 0.50 (PE/EtOAc = 3/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.90 (s, 1H, C2-H in purine), 6.03 (ddt, J = 17.2, 10.4, 6.0, 17.2 Hz, 1H, CH), 5.42 (d, J = 10.0 Hz, 1H, 1H in CH₂), 5.32 (d, J = 17.2 Hz, 1H, 1H in CH₂), and 5.09 (d, J = 6.0 Hz, 2H, NCH₂). ¹³C NMR (101 MHz, CDCl₃) δ 154.6, 153.9, 150.7, 131.0, 129.4, 128.8, 121.2, 109.5, and 46.9. HRMS (ESI): m/z [M + H]⁺ calcd. for C₉H₇ClN₅⁺, 220.0384; found: 222.0368.

3.2.5. 6-Chloro-9-(propargyl)-9H-purine-8-carbonitrile (2e)

Colorless crystals, mp: 134–136 °C, yield: 38 mg, 87%, R_f = 0.77 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.93 (s, 1H, C2-H in purine), 5.25 (d, J = 2.4 Hz, 2H, NCH₂), and 2.57 (t, J = 2.4 Hz, 1H, CH). ¹³C NMR (101 MHz, CDCl₃) δ 154.9, 154.1, 150.3, 130.8, 128.3, 109.1, 76.1, 74.0, and 33.9. HRMS (ESI): m/z [M + H]⁺ calcd. for C₉H₅ClN₅⁺, 218.0228; found: 218.0235.

3.2.6. 9-Benzyl-6-chloro-9H-purine-8-carbonitrile (2f)

Colorless crystals, mp: 102–104 °C, yield: 46 mg, 85%. R_f = 0.51 (PE/EtOAc = 3/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.94 (s, 1H, C2-H in purine), 7.49–7.46 (m, 2H, ArH), 7.39–7.36 (m, 3H, ArH), and 5.62 (s, 2H, NCH₂). ¹³C NMR (101 MHz, CDCl₃) δ 154.7, 153.9, 150.8, 133.4, 131.0, 129.35, 129.33, 128.6, 128.5, 109.9, and 48.6. HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₃H₉ClN₅⁺, 270.0541; found: 270.0550.

3.2.7. 6-Chloro-9-(4-methylbenzyl)-9h-purine-8-carbonitrile (2g)

Colorless crystals, mp: 91–92 °C. Yield: 49 mg, 86%. R_f = 0.62 (PE/EtOAc = 3/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.93 (s, 1H, C2-H in purine), 7.36 (d, J = 8.0 Hz, 2H, ArH), 7.16 (d, J = 8.0 Hz, 2H, ArH), 5.58 (s, 2H, NCH₂), and 2.32 (s, 3H, CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 154.6, 153.8, 150.7, 139.4, 131.0, 130.5, 129.9, 128.6, 128.5, 109.9, 48.4, and 21.1. HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₄H₁₁ClN₅⁺, 284.0697; found: 284.0707.

3.2.8. 6-Chloro-9-(4-nitrobenzyl)-9H-purine-8-carbonitrile (2h)

Light yellow crystals, mp: 217–220 °C. Yield: 40 mg, 64%. R_f = 0.60 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, DMSO-d₆) δ 9.04 (s, 1H, C2-H in purine), 8.22 (d, J = 8.8 Hz, 2H, ArH), 7.65 (d, J = 8.8 Hz, 2H, ArH), and 5.85 (s, 2H, NCH₂). ¹³C NMR (101 MHz, DMSO-d₆) δ 154.7, 152.0, 151.1, 147.4, 141.7, 130.4 129.4, 129.0, 123.9, 110.2, and 47.1. HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₃H₈ClN₅⁺, 315.0392; found: 315.0401.

3.2.9. 6-Chloro-9-(4-methoxybenzyl)-9H-purine-8-carbonitrile (2i)

Colorless crystals, mp: 158–160 °C. Yield: 55 mg, 92%, R_f = 0.81 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.94 (s, 1H, C2-H in purine), 7.44 (d, J = 8.8 Hz, 2H, ArH), 6.88 (d, J = 8.8 Hz, 2H, ArH), 5.56 (s, 2H, NCH₂), and 3.78 (s, 3H, OCH₃). ¹³C NMR (101 MHz, CDCl₃) δ 160.3, 154.6, 153.9, 150.7, 131.1, 130.2, 128.6, 125.6, 114.6, 110.0, 55.3, and 48.2. HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₄H₁₁ClN₅O⁺, 300.0647; found: 300.0640.

3.2.10. Ethyl-2-(6-chloro-8-cyano-9H-purin-9-yl)acetate (2j)

Colorless crystals, mp: 108–109 °C. Yield: 42 mg, 79%. R_f = 0.40 (PE/EtOAc = 3/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.88 (s, 1H, C2-H in purine), 5.20 (s, 2H, NCH₂), 4.31 (q, J = 7.2 Hz, 2H, OCH₂), and 1.33 (t, J = 7.2 Hz, 3H, CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 165.0, 154.8, 154.0, 150.9, 130.8, 129.3, 109.2, 63.3, 44.7, and 14.0. HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₀H₉ClN₅O₂⁺, 266.0439; found: 266.0432.

3.2.11. 6-Chloro-9-(2-(4-chlorophenyl)-2-oxoethyl)-9H-purine-8-carbonitrile (2k)

Colorless crystals, mp: 204–207 °C. Yield: 42 mg, 63%. R_f = 0.65 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.85 (s, 1H, C2-H in purine), 8.01 (d, J = 8.8 Hz, 2H, ArH), 7.59 (d, J = 8.8 Hz, 2H, ArH), and 5.87 (s, 2H, NCH₂). ¹³C NMR (101 MHz, CDCl₃) δ 187.5, 154.7, 154.1, 151.1, 142.0, 131.5, 131.0, 129.8, 129.7, 109.4, and 49.6. HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₄H₈Cl₂N₅O⁺, 332.0100; found: 332.0097.

3.2.12. 6-Chloro-9-phenyl-9H-purine-8-carbonitrile (2l)

Colorless crystals, mp: 158–161 °C. Yield: 48 mg, 94%. R_f = 0.85 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.91 (s, 1H, C2-H in purine), 7.65–7.70 (m, 3H, ArH), and 7.59–7.62 (m, 2H, ArH). ¹³C NMR (101 MHz, CDCl₃) δ 155.2, 154.2, 151.1, 131.4, 131.1, 130.9, 130.3, 128.7, 125.9, and 109.7. HRMS (ESI): m/z [M+H]⁺ calcd. for C₁₂H₇ClN₅⁺, 256.0384; found: 256.0371.

3.2.13. 6-Chloro-9-(p-tolyl)-9H-purine-8-carbonitrile (2m)

Colorless crystals, mp: 120–123 °C, yield: 46 mg, 85%. R_f = 0.80 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.90 (s, 1H, C2-H in purine), 7.47 (s, 4H, ArH), and 2.50 (s, 3H, CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 155.2, 154.2, 151.2, 141.4, 131.1, 130.9, 128.9, 128.8, 125.7, 109.7, and 21.3. HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₃H₉ClN₅⁺, 270.0541; found: 270.0544.

3.2.14. 6-Chloro-9-(4-methoxyphenyl)-9H-purine-8-carbonitrile (2n)

Colorless crystals, mp: 122–125 °C. Yield: 48 mg, 84%. R_f = 0.75 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.88 (s, 1H, C2-H in purine), 7.49 (d, J = 8.8 Hz, 2H, ArH), 7.14 (d, J = 8.8 Hz, 2H, ArH), and 3.91 (s, 3H, CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 161.2, 155.1, 154.1, 151.3, 130.9, 129.0, 127.3, 123.8, 115.4, 109.7, and 55.7. HRMS (ESI): m/z [M+H]⁺ calcd. for C₁₃H₉ClN₅O⁺, 286.0490; found: 286.0499.

3.2.15. 6-Chloro-9-(4-(methylthio) phenyl)-9H-purine-8-carbonitrile (2o)

Yellow crystals, mp: 149–151 °C. Yield: 51 mg, 85%. R_f = 0.80 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.90 (s, 1H, C2-H in purine), 7.51 (d, J = 8.8 Hz, 2H, ArH), 7.47 (d, J = 8.8 Hz, 2H, ArH), and 2.57 (s, 3H, CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 155.2, 154.3, 151.1, 143.3, 131.1, 128.7, 127.7, 127.0, 126.0, 109.7, and 15.2. HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₃H₉ClN₅S⁺, 302.0262; found: 302.0281.

3.2.16. 9-(4-bromophenyl)-6-chloro-9H-purine-8-carbonitrile (2p)

Colorless crystals, mp: 165–167 °C, yield: 51 mg, 76%. R_f = 0.85 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.92 (s, 1H, C2-H in purine), 7.84–7.81 (m, 2H, ArH), and 7.52–7.26 (m, 2H, ArH). ¹³C NMR (101 MHz, CDCl₃) δ 155.4, 154.5, 150.9, 133.6, 131.1, 130.3, 128.3, 127.3, 125.2, and 109.5. HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₂H₆BrClN₅⁺, 333.9490; found: 333.9504.

3.2.17. 6-Chloro-9-(3-chlorophenyl)-9H-purine-8-carbonitrile (2q)

Colorless crystals, mp: 139–141 °C. Yield: 48 mg, 83%. R_f = 0.85 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.93 (s, 1H, C2-H in purine), 7.65–7.61 (m, 3H, ArH), and 7.54–7.51 (m, 1H, ArH). ¹³C NMR (101 MHz, CDCl₃) δ 155.4, 154.5, 151.0, 136.1, 132.3, 131.3, 131.2, 131.1, 128.3, 126.3, 124.1, and 109.5. HRMS (ESI): m/z [M + H]⁺ calcd. for C₁₂H₆Cl₂N₅⁺, 289.9995; found: 290.0001.

3.2.18. 9-Benzyl-2,6-dichloro-9h-purine-8-carbonitrile (2r) and 9-Benzyl-2,6-dichloro-7-((trifluoromethyl)sulfonyl)-8,9-dihydro-7H-purine-8-carbonitrile (3r)

2r: Colorless crystals, mp: 141–143 °C. Yield: 116 mg, 38%. R_f = 0.65 (PE/EtOAc = 5/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 7.49–7.38 (m, 5H, ArH), and 5.58 (s, 2H, NCH₂). ¹³C NMR (101 MHz, CDCl₃) δ 156.3, 154.8, 151.9, 133.0, 130.1, 129.5, 129.4, 129.1, 128.6, 109.6, and 48.8. HRMS (ESI): m/z [M+H]⁺ calcd. for C₁₃H₈Cl₂N₅⁺, [M + H]⁺ 304.0152, found: 304.0155.

3r: Colorless oil, yield: 93 mg, 31%. R_f = 0.72 (PE/EtOAc = 5/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 7.50–7.33 (m, 5H, ArH), 6.12 (s, 1H, CH), 5.32 (d, J = 15.2 Hz, 1H, NCH), and 4.29 (d, J = 15.2 Hz, 1H, NCH). ¹³C NMR (101 MHz, CDCl₃) δ 161.2, 158.1, 145.7, 131.3, 129.74, 129.67, 128.47, 118.9 (q, J = 319.7 Hz, CF₃), 117.3, 110.6, 68.4, and 47.1. ¹⁹F NMR (376 MHz, CDCl₃) δ -74.13. HRMS (ESI) calcd. for C₁₄H₉Cl₂F₃N₅O₂S⁺ [M + H]⁺ 437.9801, found: 437.9796.

3.2.19. 9-Benzyl-6-(benzylthio)-9H-purine-8-carbonitrile (2s)

Colorless crystals, mp: 111–113 °C. Yield: 69 mg, 97%. R_f = 0.90 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.87 (s, 1H, C2-H in purine), 7.45–7.43 (m, 4H, ArH), 7.27–7.35 (m, 6H, ArH), 5.55 (s, 2H, NCH₂), and 4.66 (s, 2H, SCH₂). ¹³C NMR (101 MHz, CDCl₃) δ 164.3, 154.4, 148.0, 136.7, 134.0, 130.6, 129.2, 129.1, 129.0, 128.6, 128.4, 127.5, 125.9, 110.4, 48.0, and 33.0. HRMS (ESI) calcd. for C₂₀H₁₆N₅S⁺ [M+H]⁺, 358.1121, found: 358.1131.

3.2.20. 9-Benzyl-6-methoxy-9H-purine-8-carbonitrile (2t)

Colorless crystals, mp: 138–141 °C. Yield: 24 mg, 45%. R_f = 0.80 (PE/EtOAc = 1/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.70 (s, 1H, C2-H in purine), 7.46–7.44 (m, 2H, ArH), 7.38–7.33 (m, 3H, ArH), 5.57 (s, 2H, NCH₂), and 4.22 (s, 3H, OCH₃). ¹³C NMR (101 MHz, CDCl₃) δ 162.2, 155.2, 151.2, 134.1, 129.2, 129.0, 128.4, 125.8, 121.8, 110.5, 54.8, and 48.1. HRMS (ESI) calcd. for C₁₄H₁₂N₅O⁺ [M+H]⁺, 266.1036, found: 266.1039.

3.2.21. 9-Benzyl-6-(p-tolyl)-9H-purine-8-carbonitrile (2u) and 9-Benzyl-6-(p-tolyl)-9H-purine-2,8-dicarbonitrile (4u)

2u: Colorless crystals, mp: 138–140 °C. Yield: 25 mg, 23%. R_f = 0.19 (PE/EtOAc = 30/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 9.14 (s, 1H, C2-H in purine), 8.70 (d, J = 8.0 Hz, 2H, ArH), 7.51–7.48 (m, 2H, ArH), 7.38–7.35 (m, 5H, ArH), 5.63 (s, 2H, NCH₂), and 2.46 (s, 3H, CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 157.5, 155.2, 151.4, 142.8, 134.2, 131.9, 130.2, 130.1, 129.6, 129.2, 129.0, 128.5, 127.5, 110.7, 47.8, and 21.7. HRMS (ESI) calcd. for C₂₀H₁₆N₅⁺ [M + H]⁺ 326.1400, found: 326.1397.

4u: Colorless crystals, mp: 38–40 °C. Yield: 46 mg, 42%. R_f = 0.25 (PE/EtOAc = 30/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.73 (d, J = 8.4 Hz, 2H, ArH), 7.53–7.51 (m, 2H, ArH), 7.41–7.36 (m, 5H, PhH), and 5.63 (s, 2H, NCH₂), 2.47 (s, 3H, CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 158.4, 151.2, 144.4, 140.0, 133.4, 130.9, 130.6, 130.3, 129.9, 129.8, 129.5, 129.4, 128.7, 116.1, 110.0, 48.5, and 21.8. HRMS (ESI) calcd. for C₂₁H₁₅N₆⁺ [M + H]⁺ 351.1353, found 351.1349.

3.2.22. 9-Benzyl-6-(diethylamino)-9H-purine-2-carbonitrile (2v’)

2v’: Colorless crystals, mp: 88–90 °C. Yield: 10 mg, 16%, R_f = 0.25 (PE/EtOAc = 5/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 7.79 (s, 1H, C8-H in purine), 7.39–7.32 (m, 5H, ArH), 5.34 (s, 2H, ArCH₂), 4.19 (br s, 2H, NCH₂), 3.76 (br s, 2H, NCH₂), and 1.27 (t, J = 7.2 Hz, 6H, CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 153.5, 150.0, 139.9, 137.8, 135.2, 129.1, 128.5, 128.0, 120.8, 117.2, 47.3, 43.9, 43.0, 14.1, and 12.8. HRMS (ESI) calcd. for C₁₇H₁₉N₆⁺ [M + H]⁺ 307.1666, found 307.1670. [Note: The CH₂ near the nitrogen atom was shown as a broad singlet in ¹H NMR and ¹³C NMR because of both part double bond character in the amidine functionality and thus restricted rotation and the electric quadrupole of nitrogen-14. This structure was further verified by HSQC in Figure 1].

3.3. Synthesis of 9-Benzyl-6-(diethylamino)-9H-purine-8-carbonitrile (2v) and 9-Benzyl-6-(diethylamino)-N,N-diethyl-9H-purine-8-carboximidamide (5f)

Synthesis of 2v: To a 10 mL vial were added purine 2f (53.9 mg, 0.2 mmol), ethanol (2.0 mL), Et₂NH (62 μL, 0.6 mmol), and Et₃N (82 μL, 0.6 mmol) in order. The reaction mixture was stirred at room temperature for 2 h. The reaction system was filtered, and the filter residue was washed with EtOH/H₂O followed by the infrared lamp drying to provide 2v directly as white solids (51 mg, 83%). Mp: 94–96 °C. R_f = 0.60 (PE/EtOAc = 3/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.43 (s, 1H, C2-H in purine), 7.45–7.31 (m, 5H, PhH), 5.48 (s, 2H, NCH₂), 4.15 (br s, 2H, 1 CH₂ in NEt₂), 3.78 (br s, 2H, 1 CH₂ in NEt₂), and 1.28 (t, J = 6.9 Hz, 6H, 2CH₃).¹³C NMR (101 MHz, CDCl₃) δ 155.4, 154.1, 150.4, 134.8, 129.0, 128.6, 128.3, 121.7, 120.2, 111.4, 47.3, 44.1, 43.0, 13.8, and 12.6. ¹H NMR (400 MHz, DMSO-d₆) δ 8.39 (s, 1H, C2-H in purine), 7.36–7.30 (m, 5H, ArH), 5.52 (s, 2H, ArCH₂), 4.09 (br s, 2H, NCH₂), 3.73 (br s, 2H, NCH₂), and 1.20 (br s, 6H, CH₃). ¹³C NMR (101 MHz, DMSO-d₆) δ 155.7, 153.9, 150.7, 135.9, 129.4, 128.7, 128.0, 122.4, 119.8, 111.9, 47.2, 44.00, 42.8, 14.2, and 12.9. HRMS (ESI) calcd. for C₁₇H₁₉N₆⁺ [M + H]⁺ 307.1666, found: 367.1687. [Note: The CH₂ near the nitrogen atom was shown as a broad singlet in ¹H NMR and ¹³C NMR because of both part double bond character in the amidine functionality and thus restricted rotation and the electric quadrupole of nitrogen-14].

Synthesis of 5f: To a 10 mL vial were added purine 2f (53.9 mg, 0.2 mmol) and Et₂NH (1 mL). The reaction mixture was then heated at 60 °C for 3 h under stirring. After cooling to room temperature, the solvent was removed in vacuo and the residue was purified through column chromatography with DCM/MeOH (25:1, v/v) as the eluent to obtain 5f as colorless crystals (69 mg, 91%). Mp: 138–140 °C. R_f = 0.20 (DCM/MeOH = 25/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 8.42 (s, 1H, C2-H in purine), 7.28–6.92 (m, 5H, ArH), 6.92 (br s, 1H, NH), 5.46 (s, 2H, NCH₂), 3.97 (br s, 4H, NCH₂), 3.09 (br s, 4H, NCH₂), 1.28 (t, J = 7.2 Hz, 6H, CH₃), and 1.08 (t, J = 7.2 Hz, 6H, CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 157.2, 153.9, 153.3, 151.0, 141.6, 136.3, 128.6, 128.2, 128.0, 118.4, 46.3, 43.0, 42.3, 13.5, and 12.8. HRMS (ESI) calcd. for C₂₁H₃₀N₇⁺ [M + H]⁺ 380.2557, found: 380.2590. [Note: The CH₂ near the nitrogen atom was shown as a broad singlet in ¹H NMR and ¹³C NMR because of both part double bond character in the amidine functionality and thus restricted rotation and the electric quadrupole of nitrogen-14].

3.4. Synthesis of 9-Benzyl-6-((4-methoxyphenyl)ethynyl)-9H-purine-8-carbonitrile (6f)

To a 10 mL vial were added 2f (53.94 mg, 0.2 mmol), CuI (3.44 mg, 0.018 mmol), and (PPh₃)₂PdCl₂ (4.2 mg, 0.006 mmol) and the whole system was exchanged with N₂. Toluene (1 mL) and Et₃N (0.028 mL, 0.2 mmol) were then injected in 5 min at below 25 °C. The whole system was then stirred overnight (~11 h) at room temperature. After filtering with the diatomite, the filtrate was dissolved in 5 mL of EtOAc, then the resulting solution was washed with saturated NH₄Cl and NaCl solution. The organic phase was collected, dried over anhydrous Na₂SO₄, and filtered. After removing the solvent in vacuo, the residue was purified by silica gel column chromatography with PE and EtOAc (5:1, v/v) as the eluent to afford the product 6f as yellow crystals. Yield: 38 mg, 52%, mp: 141–143 °C. R_f = 0.41 (PE/EtOAc = 3/1, v/v). ¹H NMR (400 MHz, CDCl₃) δ 9.10 (s, 1H, C2-H in purine), 7.70 (d, J = 8.8 Hz, 2H, ArH), 7.47 (dd, J = 7.5, 2.2 Hz, 2H, ArH), 7.41–7.33 (m, 3H, ArH), 6.93 (d, J = 8.8 Hz, 2H, ArH), 5.62 (s, 2H, NCH₂), and 3.86 (s, 3H, OCH₃). ¹³C NMR (101 MHz, CDCl₃) δ 161.5, 155.4, 150.4, 145.1, 134.8, 133.9, 133.1, 129.3, 129.1, 128.4, 128.3, 114.3, 112.7, 110.3, 102.2, 83.8, 55.4, and 48.0. HRMS (ESI) calcd. for C₂₂H₁₆N₅O⁺ [M + H]⁺ 366.1350, found: 366.1356.

3.5. Synthesis of 2u from 2f

To a 10 mL reaction tube were added 2f (53.94 mg, 0.2 mmol), p-tolylboronic acid (33 mg, 0.24 mmol), Pd(PPh₃)₄ (15 mg, 0.01 mmol) and K₃PO₄ (212 mg, 1 mmol) and then a mixed solvent of DME:H₂O = 10:1 (2 mL) was injected. The reaction system was stirred at 90 °C for 5 h. After cooling to room temperature, the reaction system was filtered, and the filtrate was dried over anhydrous Na₂SO₄. After removing the solvent in vacuo, the residue was purified by silica gel column chromatography with PE and EtOAc (10:1, v/v) as the eluent, to yield the 2u as colorless crystals, 19 mg, 28%.

3.6. Synthesis of 2t from 2f

To a 10 mL vial were added 2f (53.9 mg, 0.2 mmol), NaOMe (8.0 mg, 0.3 mmol), and CuBr (3.0 mg, 0.02 mmol) in DMF (2 mL). The vial was capped and heated at 110 °C for 5 h. After cooling to room temperature, EtOAc (5 mL) was added and the whole system was washed with brine (5 mL × 3). The aqueous phase was then extracted with EtOAc (5 mL). The organic phase was collected and dried over anhydrous Na₂SO₄. After removing the solvent in vacuo, the residue was purified by silica gel column chromatography with PE and EtOAc (5:1, v/v) as the eluent to afford the 2t as colorless crystals, 17 mg, 32%.

3.7. Synthesis of 2s and Benzyl 9-Benzyl-6-(benzylthio)-2-cyano-9H-purine-8-carbimidothioate (5s) from 2f

In a 10 mL vial, 2f (53.94 mg, 0.2 mmol) was dissolved into 2 mL of ethanol, and then benzylthiol (35 μL, 0.3 mmol) and Et₃N (22 μL, 0.22 mol) were added in order. The resulting mixture was stirred at room temperature for 30 min. After removal of the solvent in vacuo, the residue was separated by column chromatography with PE and EtOAc (10:1, v/v) as the eluent to provide 2s as colorless crystals (18 mg, 24%) and 5s as colorless crystals (39 mg, 54%). 5s: R_f = 0.45 (PE/EtOAc = 5/1, v/v), mp: 110–112 °C. ¹H NMR (400 MHz, CDCl₃) δ 10.23 (s, 1H, NH), 8.78 (s, 1H, C2-H in purine), 7.46–7.44 (m, 2H, ArH), 7.37–7.17 (m, 13H, ArH), 5.94 (s, 2H, NCH₂), 4.67 (s, 2H, SCH₂), and 4.04 (s, 2H, SCH₂). ¹³C NMR (101 MHz, CDCl₃) δ 163.8, 162.0, 153.0, 150.4, 146.5, 137.2, 136.5, 134.1, 130.0, 129.1, 128.9, 128.6, 128.4, 127.8, 127.6, 127.5, 127.2, 47.3, 34.0, and 33.0. HRMS (ESI) calcd. for C₂₇H₂₄N₅S₂⁺ [M + H]⁺ 482.1468, found: 482.1468.

3.8. Synthesis of 5s from 2s

To a 10 mL vial were added purine 2s (71.49 mg, 0.2 mmol), ethanol (2.0 mL), benzyl thiol (27 μL, 0.24 mmol), and Et₃N (23 μL, 0.24 mmol) in order. The reaction mixture was then stirred at room temperature for 1 h. After filtration and washing with H₂O, the residue was dried under infrared lamp to provide 5s (75 mg, 78%) as a white solid.

3.9. Synthesis of Methyl 9-Benzyl-6-(benzylthio)-9H-purine-8-carbimidate (6s) from 2s

In a dry flask 2s (178.73 mg, 0.5 mmol) and NaOMe (2.7 mg, 0.05 mmol) were dissolved in anhydrous methanol (5 mL). The reaction mixture was stirred at room temperature for 4 h. The reaction mixture was filtered, and the filter residue was washed with H₂O followed by the infrared lamp drying to provide 6s directly as white solids (155 mg, 80%). R_f = 0.15 (PE/EtOAc = 5/1, v/v), mp: 105–106 °C. ¹H NMR (400 MHz, CDCl₃) δ 9.14 (s, 1H, NH), 8.81 (s, 1H, C2-H in purine), 7.48–7.46 (m, 2H, ArH), 7.33–7.14 (m, 8H, ArH), 5.74 (s, 2H, NCH₂), 4.68 (s, 2H, SCH₂), and 3.95 (s, 3H, Ome). ¹³C NMR (101 MHz, CDCl₃) δ 161.9, 159.1, 153.1, 150.1, 142.8, 137.1, 136.4, 129.9, 129.1, 128.7, 128.5, 127.9, 127.4, 127.0, 53.8, 48.0, and 32.9. HRMS (ESI) calcd. for C₂₁H₂₀N₅OS⁺ [M + H]⁺ 390.1383, found: 390.1383.

3.10. Synthesis of 2-(9-Benzyl-6-(benzylthio)-9H-purin-8-yl)-4,4-dimethyl-4,5-dihydrooxazole (7s) from 6s, and One-Pot Procedure from 2s

Procedure from 6s: 6s (155 mg,0.4 mmol), 2-amino-2-methylpropan-1-ol (38 μL, 0.4 mmol), TsOH·H₂O (7.6 mg, 0.04 mmol) were dissolved in toluene (5 mL). The reaction mixture was refluxed with a Dean–Stark apparatus for 12 h. After the reaction was cooled to room temperature, the solvent was removed in vacuo and the residue was purified through a silica gel flash column chromatography (PE/EtOAc = 6/1, v/v) to afford 7s as a white solid (120 mg, 70%). R_f = 0.35 (PE/EtOAc = 5/1, v/v), mp: 105–107 °C. ¹H NMR (400 MHz, CDCl₃) δ 8.81 (s, 1H, C2-H in purine), 7.46–7.44 (m, 2H, ArH), 7.33–7.23 (m, 8H, ArH), 6.01 (s, 2H, NCH₂), 4.65 (s, 2H, SCH₂), 4.12 (s, 2H, OCH₂), and 1.36 (s, 6H, 2CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 162.4, 153.8, 153.0, 150.0, 140.3, 137.3, 136.4, 130.3, 129.1, 128.43, 128.37, 128.1, 127.8, 127.2, 78.7, 69.2, 47.2, 32.8, and 28.2. HRMS (ESI) calcd. for C₂₄H₂₄N₅OS⁺ [M + H]⁺ 430.1696, found: 430.1696.

Procedure from 2s: 2s (178.73 mg, 0.5 mmol) was dissolved in anhydrous methanol (5 mL) in a flame-dried flask equipped with a magnetic stir bar. NaOMe (2.7 mg, 0.05 mmol) was added, and the reaction mixture was stirred at room temperature for 4 h. The solvent was removed in vacuo and toluene (5 mL) was added along with 2-amino-2-methylpropan-1-ol (47.5 μL, 0.5 mmol) and TsOH·H₂O (9.5 mg, 0.05 mmol). The reaction system was refluxed with a Dean–Stark apparatus overnight (~12 h). After cooling to r. t., the solvent was removed in vacuo and the residue was purified through silica gel column chromatography (PE/EtOAc = 6/1, v/v) to afford 7s as a white solid (162 mg, 75%).

3.11. Synthesis of Ethyl 9-Benzyl-6-(benzylthio)-9H-purine-8-carbimidate (6s’) from 2s

In a vial 2s (71.49 mg, 0.2 mmol) and NaOH (1.0 mg, 0.02 mmol) were dissolved in a mixture of EtOH/H₂O = 7/3 (2 mL). The resulting solution was stirred at room temperature for 4 h. After filtration and washing with water, the solid was dried under the infrared lamp to afford 6s’ as a white solid (68 mg, 84%). R_f = 0.2 (PE/EtOAc = 5/1, v/v), mp: 56–58 °C. ¹H NMR (400 MHz, CDCl₃) δ 9.13 (s, 1H, NH), 8.79 (s, 1H, C2-H in purine), 7.48–7.46 (m, 2H, ArH), 7.33–7.09 (m, 8H, ArH), 5.78 (s, 2H, NCH₂), 4.68 (s, 2H, SCH₂), 4.38 (q, J = 7.2 Hz, 2H, OCH₂), and 1.30 (t, J = 7.2 Hz, 3H, CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 161.8, 159.4, 153.0, 150.2, 143.0, 137.1, 136.4, 129.9, 129.1, 128.7, 128.5, 127.8, 127.3, 126.6, 63.0, 47.9, 33.0, and 13.9. HRMS (ESI) calcd. for C₂₂H₂₂N₅OS⁺ [M + H]⁺ 404.1540, found: 404.1546.

3.12. Synthesis of 9-Benzyl-6-(benzylthio)-9H-purine-8-carboxamide (8s) from 2s

In a vial 2s (71.49 mg, 0.2 mmol) and NaOH (1.0 mg, 0.02 mmol) were dissolved in a mixture of THF/H₂O = 7/3 (2 mL) and the reaction mixture was stirred at room temperature for 4 h. The reaction system was filtered, and filter residue was washed with H₂O followed by the drying under infrared lamp to provide 8s as white solids (62 mg, 83%). R_f = 0.15 (PE/EtOAc = 5/1, v/v), mp: 184–186 °C. ¹H NMR (400 MHz, DMSO-d₆) δ 8.86 (s, 1H, C2-H in purine), 8.37 (s, 1H in NH₂), 8.04 (s, 1H in NH₂), 7.48–7.46 (m, 2H, ArH), 7.33–7.23 (m, 8H, ArH), 5.86 (s, 2H, NCH₂), and 4.70 (s, 2H, SCH₂). ¹³C NMR (101 MHz, DMSO-d₆) δ 161.4, 160.7, 153.3, 150.3, 144.3, 138.1, 137.4, 129.5, 129.4, 129.00, 128.97, 128.1, 127.8, 127.7, 47.4, and 32.34. HRMS (ESI) calcd. for C₂₀H₁₈N₅OS⁺ [M + H]⁺ 376.1227, found: 376.1237.

3.13. Synthesis of Dimethyl 3-(9-Benzyl-6-(benzylthio)-9H-purin-8-yl)isothiazole-4,5-dicarboxylate (10s) from 2s

To a pre-dried reaction tube were sequentially added 2s (143 mg, 0.4 mmol), dimethyl 1,2,3-thiadiazole-4,5-dicarboxylate (40.4 mg, 0.2 mmol), [Rh(COD)Cl]₂ (5 mg, 0.01 mmol), DPPF (13 mg, 0.024 mmol), and CsI (6 mg, 0.01 mmol). The reaction tube was evacuated (<1 mmHg) and refilled with nitrogen three times. Anhydrous chlorobenzene (2 mL) was injected via a syringe. The reaction tube was placed in a metal module pre-heated to 130 °C and stirred at 130 °C for 2 h. After cooling to r. t., the solution was purified by column chromatography on silica gel (PE/EtOAc = 8/1, v/v) to provide 10s as colorless crystals (106 mg, 99%). R_f = 0.35 (PE/EtOAc = 5/1, v/v), mp: 119–120 °C. ¹H NMR (400 MHz, CDCl₃) δ 8.79 (s, 1H, C2-H in purine), 7.48–7.46 (m, 2H, ArH), 7.34–7.20 (m, 8H, ArH), 6.07 (s, 2H, NCH₂), 4.67 (s, 2H, SCH₂), 4.09 (s, 3H, CH₃), and 3.95 (s, 3H, CH₃). ¹³C NMR (101 MHz, CDCl₃) δ 163.8, 161.7, 158.7, 155.6, 154.5, 152.8, 150.2, 143.7, 137.2, 136.1, 136.0, 130.3, 129.2, 128.5, 127.8, 127.4, 127.3, 53.7, 53.3, 47.4, and 32.9. HRMS (ESI) calcd. for C₂₆H₂₂N₅O₄S₂⁺ [M + H]⁺ 532.1108, found: 532.1108.

4. Conclusions

A direct C-H cyanation of purines was developed from corresponding purines with TMSCN via sequential Tf₂O activation, nucleophilic addition, and a base-mediated detrifluoromethanesulfination. The current transformation showed a good tolerance of variety of functional groups, including allyl, alkynyl, ketone, ester, nitro, etc. The cyanation occurred highly regioselectively at the 8-position of purines with various substituents, including electron-withdrawing 6-chloro, aryl, and electron-donating alkoxy and alkylthio groups. Strong electron-donating diethylamino can reverse the regioselectivity of purine, and 2-cyano-6-diethylamino purine is obtained. Therefore, both 8- and 2-cyano 6-diethylaminopurines can be obtained from corresponding 6-chloropurine from differently substituted purines. Moreover, 6-chloro-8-cyanopurines are also versatile substrates for the further transformations because both of 6-chloro and nitrile are good electrophilic sites for various nucleophiles. For the further synthetic applications of nitriles, cyanopurines were easily converted into numerous derivatives, such as amide, oxazolines, isothiazoles, imidates, imidothioates, and imidamides.

Author Contributions

Conceptualization, N.C.; methodology, L.L., J.H., N.C. and J.X.; investigation, L.L. (Table 1, Scheme 2, Scheme 3a–c), J.H. (Scheme 3d–h, Scheme 4, Scheme 5), X.S. and Y.F. (raw materials); validation, N.C., J.H. and L.L.; data curation, J.H. and L.L.; resources, H.D., J.X. and N.C.; writing—original draft preparation, N.C.; writing—review and editing, J.X.; visualization, L.L., J.H. and N.C.; supervision, N.C.; project administration, N.C.; funding acquisition, N.C. and J.X. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We sincerely thanks Zhanhui Yang for his valuable suggestion.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Sample Availability

Not applicable.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figure, Schemes and Table

Scheme 1. Cyanation of purines.

Scheme 2. Substrates scope with 6-chloropurines.

Scheme 3. Substrates scope with other purines. The reactions in Scheme (a–h) were carried out in 0.2 mmol scale under standard condition. Scheme (g): the 1H NMR comparation of 2v and 2v’.

Figure 1. The 13C NMR (101 MHz; 164 scans and 1250 scans) and the HSQC of 2v’ (the structure identification of the broad and weak carbons in Et2N). The sample was prepared from 10 mg of 2v in 0.5 mL CDCl3.

Scheme 4. Gram-scale synthesis of 2f and its derivation.

Scheme 5. Gram-scale synthesis of 2s and its synthetical application.

Scheme 6. Reaction mechanism of the 8-cyanation of purines.

Table 1

Optimization of the cyanation of purine 1a.

[Image omitted. Please see PDF.]
Entry	Variation from the Standard Conditions	2a (%) a
1	-	54
2	TFA, TfOH, or TMSOTf instead of Tf2O	0
3	BF3.OEt2 or HBF4 instead of Tf2O	0
4	Ac2O, Ts2O, or Boc2o instead of Tf2O	0
5	THF or MeCN as the solvent	0
6	DCM as the solvent	30
7	PhCl as the solvent	47
8	PhMe as the solvent	49
9	CHCl3 as the solvent	50
10	Py instead of DBU	40
11	Et3N instead of DBU	40
12	DABCO instead of DBU	26
13	N-Methylmorpholine (NMM) instead of DBU	47
14	Quinuclidine instead of DBU	26
15	DBN instead of DBU	41

^a Yield of isolated 2a.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28030914/s1, X-ray crystal data, and copies of ¹H-NMR and ¹³C-NMR spectra of unknown compounds are included in the Supporting Information.

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