2025 Volume 73 Issue 4 Pages 369-373
A concise approach is presented for preparing 4ʹ-modified thymidines from oxime imidates using readily generated 4ʹ-carbon radicals. This method produces 4ʹ-modified thymidines from natural thymidine using the Mitsunobu reaction, the protection of 3ʹ-hydroxy group (when necessary), and 1,5-hydrogen atom transfer (1,5-HAT)/intermolecular 1,4-addition with electron-deficient olefins. Moreover, using a one-pot synthesis involving 1,5-HAT/intermolecular 1,4-addition, followed by the hydrolysis of the imidate intermediate under basic conditions, 4′-modified thymidine was diastereoselectively isolated. This is because the 4′-isomer transferred to the water layer in the work-up process.
Nucleosides with a substituent at the C4′ position have been developed not only as antiviral drugs1) against hepatitis B virus (HBV), human immunodeficiency virus (HIV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), but also as building blocks to produce oligonucleotide therapeutics.2,3) The synthesis of 4′-modified nucleosides generally entails either i) introducing an electrophile into a nucleoside 5ʹ-aldehyde (Chart 1a) or ii) adding a nucleophile to an epoxide or iodonium prepared from a 4′-exo-olefin4) (Chart 1b). The former method has diasterochemical issues such as providing the l-lyxo isomer as a major product (Chart 1a, left arrow), and requiring the regioselective conversion of two primary hydroxy groups (Chart 1a, right arrow). Although the latter method allows the introduction of heteroatom nucleophiles at the C4ʹ position of nucleosides, the introduction of carbon nucleophiles is limited (Chart 1b). As another approach, Meanwell and colleagues recently reported a five-steps synthesis of 4ʹ-modified ribonucleosides from non-nucleosidic 2,2-dimethoxyacetoaldehyde.5)
The synthesis of 4ʹ-modified nucleosides using carbon radicals generated at the C4ʹ position is also an attractive strategy; however, preparing the radical precursors is generally tedious.6–10) We also recently developed a novel method for the rapid and facile generation of 4ʹ-carbon radicals via the 1,5-hydrogen atom transfer (1,5-HAT) of iminyl radicals formed by the single-electron reduction of oxime imidates, which was applied to synthesis of 2ʹ-O,4ʹ-C- and 3ʹ-O,4ʹ-C-bridged nucleosides via intramolecular radical cyclization11) (Chart 1c). Therefore, we aimed at the concise synthesis of 4ʹ-modified thymidines from oxime imidates via a photoredox-catalyzed 1,5-HAT/intermolecular 1,4-addition process (Chart 1d). Herein, the details are described.
Initially, we investigated the Mitsunobu reaction of thymidine 1 with N-phenoxy-4-(trifluoromethyl)benzamide 2 to prepare the oxime imidate of thymidine 3 (Table 1). As the pKa of N-alkoxybenzamide in dimethyl sulfoxide (DMSO) is approximately 14,12) the Mitsunobu reaction of 1 and 2 was carried out using n-Bu3P and N,N,N,N-tetramethylazodicarboxamide (TMAD); however, no reaction was observed (Table 1, Entry 1). By contrast, although the combination of Ph3P and diisopropyl azodicarboxylate (DIAD) furnished the desired oxime imidate 3 in a 36% yield, 8% of bis-oxime imidate 4, 11% of olefin 5, and 30% of thymidine 1 were also found (Table 1, Entry 2). Because by-products 4 and 5 were generated via the further reaction of 3, DIAD was added dropwise at a slow rate using a syringe pump to suppress the heat produced by the reaction. Thus, the yield of 3 was improved to 84% by avoiding the formation of 4 and 5 (Table 1, Entry 3).
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| Entry | Reagents | Temp. | Yield |
| 1 | n-Bu3P (1.2 equiv.), TMAD (1.2 equiv.) | 0 °C to r.t. | No reaction |
| 2a) | Ph3P (1.2 equiv.), DIAD (1.2 equiv.) | 0 °C to r.t. | 3: 36%, 4: 8%, 5: 11% |
| 3b) | Ph3P (1.2 equiv.), DIAD (1.2 equiv.) | r.t. | 3: 84% |
a) 30% of 1 was recovered. b) DIAD was added using a syringe pump over 1 h at room temperature (r.t.). equivalent (equiv.).
Next, we examined the 1,5-HAT/intermolecular 1,4-addition reaction with easy-to-handle 3ʹ-O-tert-butyldimethylsilyl (TBS)-protected oxime imidate 6a based on our previous report11) (Table 2). In the presence of phenyl vinyl sulfone 7a and fac-Ir(ppy)3 as the radical acceptor and photocatalyst, respectively, 6a in MeCN was irradiated with blue LED (448 nm). After completion of the radical reaction, imidate intermediate A was hydrolyzed using NaOH aq. to obtain 4ʹ-modified thymidine 8a with the d-ribo configuration. However, 8a was produced in a low yield of 13%, and 9 was formed in a 17% yield (Entry 1). This result indicates that the further 1,4-addition of 8a to 7a proceeded under basic conditions. Therefore, the hydrolysis of A under acidic conditions was explored. Although the addition of silica gel in MeOH afforded the desired product 8a in a 24% yield, without the production of 9, a large amount of unhydrolyzed imidate intermediate A remained [HRMS of A (ESI-TOF) m/z: Calcd for C32H40F3N3NaO7SSi [M + Na]+ 718.2206. Found 718.2205.] (Entry 2). When the hydrolysis time was prolonged from 1 to 24 h, 8a and 8b were obtained in 43 and 10% yields, respectively (Entry 3), and the structures of these were determined by nuclear Overhauser effect spectroscopy (NOESY) correlations (Supplementary Charts S1, S2). This result suggested that the radical reaction preferentially produces the desired product 8a. In addition, the l-lyxo isomer 8b was not observed (Entry 2), indicating that the hydrolysis of l-lyxo A is slower than that of d-ribo A. Upon using AcOH instead of the silica gel, the hydrolysis proceeded in a shorter time to afford 8a and 8b in 48 and 15% yields, respectively (Entry 4). Decreasing the amount of radical acceptor 7a from 10 to 5 equiv. led to a slight improvement in the yield (Entry 5). A further decrease in the amount of 7a to 2.5 equiv. caused a decrease in the yield of 8a (Entry 6).
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|---|---|---|---|---|
| Entry | Hydrolysis | 7a | Time | Yield |
| 1 | NaOH (10 equiv.), THF/H2O = 5 : 1 | 10 equiv. | 1.5 h | 8a: 13%, 9: 17% |
| 2a) | Silica gel, MeOH | 10 equiv. | 1 h | 8a: 24% |
| 3 | Silica gel, MeOH | 10 equiv. | 24 h | 8a: 43%, 8b: 10% |
| 4 | 5% AcOH in MeOH | 10 equiv. | 1.5 h | 8a: 48%, 8b: 15% |
| 5 | 5% AcOH in MeOH | 5 equiv. | 1.5 h | 8a: 51%, 8b: 15% |
| 6 | 5% AcOH in MeOH | 2.5 equiv. | 1.5 h | 8a: 43%, 8b: 14% |
a) Imidate intermediate A remained.
The effect of the protecting group on the 3ʹ-hydroxy group was then examined (Chart 2). Under optimized conditions, tert-butyldiphenylsilyl (TBDPS)-protected 6b efficiently underwent the 1,5-HAT/intermolecular 1,4-addition reaction to afford 4ʹ-modified thymidine 10 with good diastereoselectivity (d-ribo : l-lyxo = 5.5 : 1). By contrast, the reaction of protection-free 3 resulted in lower diastereoselectivity (d-ribo : l-lyxo = 2.5 : 1) than those obtained by the reaction of TBS- and TBDPS-protected 6a and 6b. The investigation of the 3′-hydroxy protecting groups revealed that the introduction of bulky substituents improves the diastereoselectivity. Interestingly, the 1,4-addition of the 4′-carbon radical preferentially occurred on the pro-R face, that is the same side as the 3′-hydroxy group, resulting in the opposite diastereoselectivity than that of the 4′-carbon radical generated by deformylation, as previously reported by our group.13) Therefore, the diastereoselectivity in the reaction via the generation of 4′-carbon radicals was found to strongly depend on the presence or absence of a hydroxymethyl group, although the details are unclear.
Next, certain electron-deficient olefins were used as radical acceptors (Chart 3). Although the yields of the 1,5-HAT/intermolecular 1,4-addition reaction of 6b with acrylonitrile 7b, methyl acrylate 7c, and dimethyl vinylphosphonate 7d were generally low, the desired 4ʹ-modified thymidines, 12a–14a were preferentially obtained. Unfortunately, p-(trifluoromethyl) styrene 7e produced a complex mixture that could not be used as a radical acceptor. Therefore, the reactivity of the 4′-carbon radical is governed by the electrophilicity of the olefins, and phenyl vinyl sulfone 7a with the highest electrophilicity among the olefins used in this study14) provided the best result, although Michael acceptor-type olefins could be applied in this reaction.
a) 14a and 14b were obtained as an inseparable mixture.
Surprisingly, when imidate intermediate A was hydrolyzed under basic conditions, l-lyxo isomer 8b was not observed (Table 1, Entry 1). To comprehensively investigate this reaction, the hydrolysis of 8b was carried out (Chart 4). The hydrolysis of 8b with NaOH for 1.5 h resulted in 16 and 11b, which were produced by migration and removal of the TBS group, respectively. Prolonging the reaction time to 24 h produced only 11b in an 83% yield. By contrast, no conversion of d-ribo isomer 8a was observed under the same basic conditions. Therefore, the 1,5-HAT/intermolecular 1,4-addition reaction, followed by hydrolysis under basic conditions might allow the isolation of 4′-modified thymidine with a d-ribo configuration in a highly diastereoselective manner, probably because diol 11b, that is the l-lyxo isomer, could be removed via aqueous extraction.
A method for obtaining the d-ribo isomer as the sole product is valuable, because the diastereomers at the C4′ position are often difficult to separate by column chromatography. Thus, we finally attempted whether only 4′-modified thymidine with d-ribo configuration could be readily obtained (Chart 5). The introduction of phenyl vinyl sulfone 7a at the C4′ position of oxime imidate 6a was carried out using the optimized photoredox conditions. MeNH2 was then added to suppress the production of 5′-O-alkylated by-product 9 by trapping excess 7a. The hydrolysis of the imidate intermediate using NaOH aq. followed by aqueous extraction afforded 4′-modified thymidine 8a in a 51% yield as a sole isomer.
We present a novel synthetic method to concisely produce 4′-modified thymidine. In the key reaction, 1,5-HAT of iminyl radicals, which are formed by the single-electron reduction of oxime imidates, readily generated 4′-carbon radicals via direct C–H activation. The 1,4-addition of the resulting radicals to electron-deficient olefins, followed by hydrolysis of the imidate intermediates under acidic conditions preferentially afforded 4′-modified thymidines with the d-ribo configuration. Moreover, the 1,5-HAT/intermolecular 1,4-addition, followed by basic hydrolysis could allow the isolation of 4′-modified thymidine with d-ribo configuration in a diastereoselective manner.
This research was partially supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant (JP22K06542).
The authors declare no conflict of interest.
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