Nucleosides. 121. Improved and General Synthesis of 2′-Deoxy C-Nucleosides. Synthesis of 5-(2-Deoxy-β-D-erythro-pentofuranosyl)uracil, -1-methyluracil, -1,3-dimethyluracil, and -isocytosine

5-(2-Deoxy-²-D-erythro-pentoniranosyl)-l,3-dimethyluracil (6a), -1-methyluracil (6b), -uracil (6c), and -isocytosine (6d) were synthesized. Compounds 6b−d are C-nucleoside isosteres of thymidine, 2′-deoxyuridine, and 2′-deoxycytidine, respectively. 1,3-Dimethylpseudouridine (la), 1-methylpseudouridine (lb), pseudouridine (lc), and pseudoisocytidine (1d) were treated with 1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane in pyridine to afford the corresponding 3′,5′-tetraisopropyldisiloxanyl derivatives 2 which were converted into the respective 2′-O-[(imidazol-1-yl)thiocarbonyl] C-nucleosides 3. Compounds 3a,b were converted directly into the corresponding 2′-deoxy β-C-nucleosides 5a,b exclusively by reduction with n-Bu₃SnH. For the synthesis of 2′-deoxy β-C-nucleosides 5c,d, the intermediates 3c,d were trimethylsilylated prior to n-Bu₃SnH treatment. Deprotection of 5a−d was effected by treatment with n-Bu₄NF, and the corresponding free 2′-deoxy β-C-nucleosides 6a−d were obtained in good yields.