General Homologation Strategy for Synthesis of L-glycero- and D-glycero-Heptopyranoses.

A general and stereospecific homologation strategy for the synthesis of heptopyranosides is reported. The strategy employs the Wittig olefination and proline-catalyzed α-aminoxylation to achieve one carbon elongation and stereoselective hydroxylation at the C6 position, respectively. The L-glycero- and D-glycero-heptopyranosides can be obtained with nearly perfect stereoselectivity. Further study reveals the difference in the chemical shift of the C6 proton of L/D-glycero-heptopyranosyl diastereomers, which is found to be useful for assignment of the configuration of heptopyranosides.