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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‐triazolo[2,3‐a] pyrimid‐7‐one has been shown to give 3‐(β‐D‐ribofuranosyl)‐s‐triazolo[2,3‐a]pyrimid‐7‐one (V) and 4‐(β‐D‐ribof'uranosyl)‐s‐lriazolo[2,3‐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‐triazolo[2,3‐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‐lriazolo[2,3‐a Jpyrimid‐7‐one (XII) prepared by a standard procedure from 7‐methoxy‐s‐triazolo(2,3‐a] pyrimidine (X).

Bei der Kondensation des Triazolopyrimidin‐silyläthers (I) mit dem Ribofuranosylbromid (II) entstehen die beiden isomeren Kondensate (IIIa) und (IVa), aus denen die Derivate (IIIb) und (IIIc) sowie (IVb) und (IVc) erhalten werden können.