Nucleosides XII.¹ synthesis of 5′-modified isoguanosines and reinvestigation of 5′-deoxy-N³,5′-cycloisoguanosine

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-N ³,5′-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.