Mechanism of regioselective Mitsunobu thiofunctionalization of pentofuranoses

Triphenylphosphine and diethyl azodicarboxylate react with 1,2-O-isopropylidene-alpha-D-xylo-(1) and ribofuranose (2) to give six-membered-ring phosphoranes. Xylofuranose 1 undergoes cyclodehydration to produce oxetane 17 in 85% yield, but ribofuranose 2 gives a pyrazolidine derivative 19 in 80% yield. Tn the presence of 2-mercaptobenzothiazole, the desired 5-S-(benzothiazol-2-yl)-5-thio derivatives 3 and 4 were isolated in 80% yield. P-31 NMR examination of this Mitsunobu thiofunctionalization reveals the presence of an alkoxytriphenylphosphonium species as the most stable intermediate which reacts with the thio-nucleophile via S(N)2 in a rate limiting step.