Highly diastereoselective radical cyclization of a glucose-derived enol ether radical cation/phosphate anion pair

Abstract

Diastereomeric 3- O-allyl-4,6- O-benzylidene-2- O-(diphenylphosphatoxy) β- d- gluco- and β- d -manno-pyranosyl phenylselenides were prepared and subjected to treatment with tributyltin hydride and AIBN. The gluco-compound undergoes smooth radical cyclization to a single diastereomeric product in high yield whereas the manno-isomer reacts only reluctantly to give a complex mixture. This difference in behavior is interpreted as arising from the formation of two different contact radical ion pairs in which the phosphate group shields opposite faces of the enol ether (glycal) radical cation.