Glycosylation via locked anomeric configuration: stereospecific synthesis of oligosaccharides containing the β-d-mannopyranosyl and β-l-rhamnopyranosyl linkage

cis-1,2-Stannylene acetals of d-mannose and l-rhamnose, formed preferentially from the free sugars treated with dibutyltin oxide, are capable of displacing the trifluoromethanesulfonyl (triflyl) leaving groups in carbohydrates to give, with retention of configuration at the anomeric center in the nucleophile, cis-1,2-linked oligosaccharides. In the case of secondary triflates, the new glycosidic linkage is formed with complete inversion of configuration in the electrophile. Both the reactivity of the electrophile and nucleophilicity of oxygens in the stannylene complex affect the overall outcome of the reaction. From the comparison of results of a number of glycosylations via stannylene acetals, it appears that nucleophilicity of oxygens involved in the cis-1,2-acetals decreases in the order: equatorial anomeric > equatorial non-anomeric > axial anomeric. Consequently, treatment of the stannylene acetal prepared from d-mannose (mainly the cis-1,2-stannylene compound in admixture with a small proportion of the cis-2,3-stannylene acetal) with methyl 2,3,4-tri-O-benzoyl-6-O-trifluoromethanesulfonyl-α-d-glucopyranoside yielded, in addition to the expected β-d-mannopyranoside (major), a product of non-anomeric alkylation at O-3. On the other hand, glycosylation of the stannylene acetal derived from maltose with methyl 2,3,6-tri-O-benzoyl-4-O-trifluoromethanesulfonyl-α-d-galactopyranoside gave almost exclusively a non-glycosidically, (2→4)-linked pseudo-trisaccharide. Combination of the glycosylation via locked anomeric configuration with conventional glycosylations, to yield higher oligosaccharides, is also demonstrated.