Abstract Reaction of N-ethoxycarbonylguanidine III with 2, 3, 5-tri-O-benzoyl-β-D-ribosyl isocyanate (II) afforded N-ethoxycarbonylamidinourea IV, which was cyclized by bis (trimethylsilyl) acetamide to the 6-ethoxy derivative VII. Ammono-and methanolysis of intermediate VII yielded 6-amino and 6-methoxy derivatives of 5-azacytidine I and VIII, respectively. Reaction of amidinourea IV with a mixture of chlorotrimethylsilane and ...