A general stereodivergent strategy for the preparation of carbasugars. Syntheses of 5a-carba-α-d-glucose, α-d-galactose, and β-l-gulose pentaacetates from d-mannose

A stereodivergent approach to 5a-carba-d- and l-pyranoses has been applied to the preparation of 5a-carba-α-d-gluco-, 5a-carba-α-d-galacto-, and 5a-carba-β-l-gulopyranose pentaacetates. The strategy, by which a single precursor can be transformed into three different carbasugars, features a stereoselective reduction followed by deoxygenation of a key polyoxygenated methylcyclohexanone intermediate. The latter is readily available by 6-exo-dig radical cyclization of a d-mannose derivative.