Radical cyclization studies directed toward the synthesis of BMS-200475 ‘entecavir’: the carbocyclic core

Two routes are presented for the conversion of d-diacetone glucose (5a) into a protected carbocyclic core of BMS-200475 (Entecavir). The reduction of two terminal epoxides with Cp₂TiCl to form carbon radicals and their cyclizations with a terminal acetylene and an α,β-unsaturated ester lead ultimately to allylic alcohol 11a, a candidate for Mitsunobu coupling with guanine.