Nucleosides XII.1 Synthesis of 5Modified Isoguanosines and Reinvestigation of 5DeoxyN3,5cycloisoguanosine

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 -deoxyN,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 basepromoted cyclization afforded the same product 6. The structures were elucidated by spectroscopic analysis including IR, UV, 1-D and 2-D NMR.