Chemistry of 1-alkoxy-1-glycosyl radicals: Formation of beta-mannopyranosides by radical decarboxylation and decarbonylation of manno-heptulosonic acid glycoside derivatives

A method for the preparation of highly enriched beta-mannopyranosides is described. A glycosyl donor 28 is prepared from tetraallyl mannonolactone by standard means and coupled to a number of primary carbohydrate alcohols, resulting in the isolation in excellent yields of axial disaccharides. Following exchange of the allyl groups for acetyl esters, the furan is oxidatively cleaved with catalytic RuO2 and NaIO4 and the resulting acid subjected to the Barton decarboxylation. Coupling of 28 to a secondary alcohol, methyl 2,3-isopropylidene-alpha-L-rhamnopyranoside, resulted in an apparent inversion of anomeric stereochemistry and isolation of an equatorial disaccharide.