Synthesis of C-Nucleosides by Ring Transformation of 1, 3-Oxazine Derivatives

Treatment of the β-D-ribofuranosyl cyanide 5 with sodium methoxide, followed by reaction with diketene, gave the spiro 1, 3-oxazine 6, which underwent the ring transformation with ammonia and phenylhydrazine to give the protected pyrimidine (8) and 1, 2, 4-triazole (13) C-nucleosides, respectively. Deprotection of 8 with 90% trifluoroacetic acid gave the pyrimidine C-nucleoside 9. Passage of hydrogen chloride gas over a solution of the cyanide 1 and benzyl hydrosulfide gave the fully protected ribofuranosylthioformimidate 15. Treatment of 15 with triethylamine, followed by reaction with diketene gave the dihydrofuran derivative 16. Compound 16, on treatment with ammonia, hydroxylamine, and phenylhydrazine, was transformed into the corresponding furan derivatives 17-19.