Synthesis of thio-linked disaccharides by 1-->2 intramolecular thioglycosyl migration: oxacarbenium versus episulfonium ion intermediates.
The conversion of 1,1'-thio-linked glucopyranosyl alpha-D-mannopyranosides to 1,2-thio-linked methyl sophorosides or methyl kojibiosides is described. The method involves the 1-->2-migration of the thioglucopyranosyl portion of the nonreducing disaccharide with inversion of configuration at C-2 of the mannopyranose ring and concomitant formation of the methyl glucopyranoside. The thioglucosyl migration does not occur when electron-withdrawing benzoate protecting groups are present. The rearrangement occurs with retention of configuration in the migrating thioglucoside but the methyl glycoside is formed as a mixture of alpha- and beta-isomers. This is attributed to a mechanism involving an oxacarbenium-ion intermediate rather than an episulfonium-ion intermediate. The relevance of this work to recent theoretical predictions concerning the relative stability of such intermediates is discussed.
Johnston
B D
BD
Department of Chemistry, Simon Fraser University, Burnaby, BC, Canada.
Pinto
B M
BM
D002236:Carbohydrate Conformation; D004187:Disaccharides / Q000138:chemical synthesis* / Q000737:chemistry; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D010084:Oxidation-Reduction; D013816:Thermodynamics; D013865:Thioglycosides / Q000138:chemical synthesis* / Q000737:chemistry
D004187:Disaccharides; D013865:Thioglycosides