Stereodivergent synthesis of 5a-carba-hexopyranoses from carbohydrates via 6- exo- dig radical cyclization: preparation of 5a-carba-β- d-manno-, α- d-allo-, β- l-talo- and α- l-gulopyranose pentaacetates from d-mannose

Abstract

Four carbasugars, 5a-carba-β- d-manno-, α- d-allo-, β- l-talo- and α- l-gulopyranose pentaacetates, have been prepared in a stereodivergent manner from d-mannose. Alkynyl derivatives of 2,3:4,6-di- O-isopropylidene- d-mannopyranose, which are prepared by homologation at C-1, by reaction with phenyl acetylide, undergo a 6- exo- dig radical cyclization, from a radical located at C-5, to yield a mixture of highly functionalized cyclohexanes. Some of these compounds, after transformation of their exocyclic double bond in a hydroxy function, were correlated with polyhydroxylated cyclohexanes, which were then selectively deoxygenated either at position C-4 or C-5a (carbohydrate numbering) to afford carbasugars of the d- or l- series.