A Stereodivergent Approach to 5a‐Carba‐α‐D‐gluco‐, ‐α‐D‐galacto and ‐β‐L‐gulopyranose Pentaacetates from D‐Mannose, Based on 6‐exo‐dig Radical Cyclization and Barton−McCombie Radical Deoxygenation

Abstract

AbstractThe three carbasugars, 5a‐carba‐α‐D‐gluco‐, ‐α‐D‐galacto and ‐β‐L‐gulopyranose pentaacetates 42, 35 and 28 respectively, have been prepared in a stereodivergent manner from D‐mannose. Alkynyl derivatives of 2,3:4,6‐di‐O‐isopropylidene‐D‐mannopyranose, which are homologated at C‐1 by reaction with trimethylsilylacetylide, undergo a 6‐exo‐dig radical cyclization, from a radical located at C‐5, to yield a mixture of highly functionalized exo‐methylenecyclohexanes. Ozonolysis of the exocyclic double bond in the latter generated cyclohexanones which, upon stereoselective reduction of the carbonyl moiety followed by site‐selective deoxygenation either at position C‐4 or C‐5a (parent carbohydrate numbering), afforded the title carbasugars. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)