Stereodivergent Synthesis of Carbasugars from D-Mannose. Syntheses of 5a-Carba-α-D-allose, β-L-Talose, and α-L-Gulose Pentaacetates

A stereodivergent entry to 5a-carba-D- and L-pyranoses from a single precursor is described. The approach is based on the selective deoxygenation of polyoxygenated methylcyclohexane intermediates, readily available from radical cyclization of D-mannose derivatives. This strategy has been applied to the preparation of 5a-carba-α-D-allo-, 5a-carba-β-L-talo-, and 5a-carba-α-L-gulopyranose pentaacetates.