title: Cation clock permits distinction between the mechanisms of α- and β-O- and β-C-glycosylation in the mannopyranose series: evidence for the existence of a mannopyranosyl oxocarbenium ion.
abstract: The use of a cationic cyclization reaction as a probe of the glycosylation mechanism has been developed and applied to the 4,6-O-benzylidene-protected mannopyranoside system. Cyclization results in the formation of both cis- and trans-fused tricyclic systems, invoking an intermediate glycosyl oxocarbenium ion reacting through a boat conformation. Competition reactions with isopropanol and trimethyl(methallyl)silane are interpreted as indicating that β-O-mannosylation proceeds via an associative S(N)2-like mechanism, whereas α-O-mannosylation and β-C-mannosylation are dissociative and S(N)1-like. Relative rate constants for reactions going via a common intermediate can be estimated.
authors.affiliation: Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles, CNRS, 1 Avenue de la Terrasse, 91190 Gif-sur-Yvette, France.
authors.forename: Min
authors.initials: M
authors.lastname: Huang
authors.forename: Pascal
authors.initials: P
authors.lastname: Retailleau
authors.forename: Luis
authors.initials: L
authors.lastname: Bohé
authors.forename: David
authors.initials: D
authors.lastname: Crich
mesh_terms: D002236:Carbohydrate Conformation; D002412:Cations / Q000737:chemistry; D018360:Crystallography, X-Ray; D003500:Cyclization; D006031:Glycosylation; D007477:Ions / Q000138:chemical synthesis / Q000737:chemistry; D008358:Mannose / Q000138:chemical synthesis* / Q000737:chemistry; D008958:Models, Molecular
chemical_list: D002412:Cations; D007477:Ions; D008358:Mannose