Synthesis of the bis-spiroacetal moiety of spirolides B and D

An enantioselective synthesis of the bis-spiroacetal fragment of spirolides B and D is reported. The carbon framework was constructed via Barbier reaction of dihydropyran 3 with aldehyde 4, followed by a double oxidative radical cyclization to construct the bis-spiroacetal. A silyl-modified Prins cyclization and enantioselective crotylation successfully installed the stereocenters in the cyclization precursor. The initial unsaturated bis-spiroacetals 2a-d underwent equilibration during epoxidation to trans-epoxide 14 that was converted to a tertiary alcohol. [reaction: see text]