Nucleosides XII.1 Synthesis of 5‐Modified Isoguanosines and Reinvestigation of 5′‐Deoxy‐N3,5′‐cycloisoguanosine

Abstract

Isoguanosine (3) underwent a coupling reaction with diaryl disulfides in the presence of tri‐n‐butylphosphine when its 6‐amino group was protected by N,N‐dimethylaminomethylidene. The synthesis of 5′‐deoxy‐N3,5′‐cycloisoguanosine (6) and its 2′,3′‐O‐isopropylidene derivative (11) were accomplished in excellent yields from isoguanosines (3 & 10) in the presence of triphenylphospine and carbon tetrachloride in pyridine. Chlorination at the 5′‐position of isoguanosine (3) with thionyl chloride followed by the aqueous base‐promoted cyclization afforded the same product 6. The structures were elucidated by spectroscopic analysis including IR, UV, 1‐D and 2‐D NMR.