title: Stereoselective C-glycoside formation with 2-O-benzyl-4,6-O-benzylidene protected 3-deoxy gluco- and mannopyranoside donors: comparison with O-glycoside formation.
abstract: Unlike alcohols, the reaction of C-nucleophiles with 2-O-benzyl-4,6-O-benzylidene-protected 3-deoxy-gluco- and mannopyranosyl thioglycosides is highly stereoselective providing the α-C-glycosides in the gluco-series and the β-C-glycosides in the manno-series. Conformational analysis of nucleophilic attack of putative intermediate glycosyl oxocarbenium ions suggests that the observed selectivities for C-glycoside formation can be explained by preferential attack on the opposite face of the oxocarbenium to the C2-H2 bond and that eclipsing interactions with this bond are the main stereodetermining factor. It is argued that the steric interactions in the attack of alcohols (sp(3)-hybridized O) and of typical carbon-based nucleophiles (sp(2) C) on oxocarbenium ions are very different, with the former being less severe, and thus that there is no a priori reason to expect O- and C-glycosylation to exhibit parallel stereoselectivities for attack on a given oxocarbenium ion.
authors.affiliation: Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, Michigan 48202, USA.
authors.forename: Myriame
authors.initials: M
authors.lastname: Moumé-Pymbock
authors.forename: David
authors.initials: D
authors.lastname: Crich
mesh_terms: D001597:Benzylidene Compounds / Q000737:chemistry*; D006027:Glycosides / Q000138:chemical synthesis* / Q000737:chemistry*; D008968:Molecular Conformation; D013237:Stereoisomerism
chemical_list: D001597:Benzylidene Compounds; D006027:Glycosides