MECHANISM OF REGIOSELECTIVE MITSUNOBU THIOFUNCTIONALIZATION OF PENTOFURANOSES

Triphenylphosphine and diethyl azodicarboxylate react with 1,2-O-isopr opylidene-alpha-D-xylo-(1) and ribofuranose (2) to give six-membered-r ing phosphoranes. Xylofuranose 1 undergoes cyclodehydration to produce oxetane 17 in 85% yield, but ribofuranose 2 gives a pyrazolidine deri vative 19 in 80% yield. Tn the presence of 2-mercaptobenzothiazole, th e desired 5-S-(benzothiazol-2-yl)-5-thio derivatives 3 and 4 were isol ated in 80% yield. P-31 NMR examination of this Mitsunobu thiofunction alization reveals the presence of an alkoxytriphenylphosphonium specie s as the most stable intermediate which reacts with the thio-nucleophi le via S(N)2 in a rate limiting step.