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
The 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)