Synthesis of 3- and 4-(β-D-ribofuranosyl)-s-triazolo[2,3-a] pyrimid-7-one,isomers of inosine containing a bridgehead nitrogen atom

The first synthesis of a purine nucleoside analog containing a bridgehead nitrogen atom is here reported. The direct glycosylation of the trimethylsilyl derivative of s-triazolo2,3-a] pyrimid-7-one has been shown to give 3-(β-D-ribofuranosyl)-s-triazolo2,3-a]pyrimid-7-one (V) and 4-(β-D-ribof'uranosyl)-s-lriazolo2,3-α]pyrimid-7-one (VII). The nueleoside V may he considered a close analog of inosine in which the nitrogen N₁ and C₅ of inosine have been interchanged. Bro-minalion of the tri-O-acelyl derivative IV gave, after deblocking, 6-bromo-3-(β-D-ribofurnaosyl)-s-triazolo2,3-a] pyrimid-7-one (IX). Structural assignments of the nucleosides were made on the basis of comparison of the ultraviolet absorption spectral characteristics with 3-methyl-s-triazolo-2,3-a]pyrimid-7-one (XI) and 4-methyl-s-lriazolo2,3-a Jpyrimid-7-one (XII) prepared by a standard procedure from 7-methoxy-s-triazolo(2,3-a] pyrimidine (X).