title: Stereocontrolled Synthesis of alpha-C-Galactosamine Derivatives via Chelation-Controlled C-Glycosylation.
abstract: The samarium diiodide-promoted reduction of 2-deoxy-2-acetamidogalactosyl pyridyl sulfone alpha-5 with ketones or aldehydes under Barbier conditions led unexpectedly to the stereoselective synthesis of alpha-C-galactosamine derivatives in good yields. With carbonyl substrates, alpha:beta selectivities ranged from 20:1 to 5:1, and with aldehydes a stereoselectivity of approximately 5:1 was observed at C7 in favor of the S-isomer. The stereochemical preference of these C-glycosylation reactions is explained by the intermediacy of an alpha-oriented anomeric glycosyl samarium(III) compound that is stabilized via chelation of the metal ion to the C2-acetamido group.
authors.affiliation: Université Paris-Sud, Laboratoire de Synthèse de Biomolécules, URA CNRS 462, Institut de Chimie Moléculaire, F91405 Orsay Cédex, France.
authors.forename: Dominique
authors.initials: D
authors.lastname: Urban
authors.forename: Troels
authors.initials: T
authors.lastname: Skrydstrup
authors.forename: Jean-Marie
authors.initials: JM
authors.lastname: Beau