Ultrasonic Agitation Accelerates cis-Glycosylation with Heterogeneous Promoters

Abstract

Ultrasonic agitation increases the yield of glycosylations with donors such as 3,4,6-tri-O-acetyl-2-azido-2-deoxy-α-D-galactopyranosyl chloride using the heterogeneous promoters silver zeolite, cadmium zeolite or a mixture of silver perchlorate and silver carbonate on celite. The stereospecificity of the glycosylation depends on the nature of the alcohol to be glycosylated, the nature of the solid support and the solvent. Glycosylation catalyzed by silver zeolite in toluene solutions of the donor 3,4,6-tri-O-acetyl-2-deoxy-2-phtalimido-β-D-glucopyranosyl bromide, that usually produce trans-β-glycosides, yield cis-glycosides with unreactive alcohols. In these unreactive alcohols, e.g. methyl 2-O-benzoyl-4,6-O-benzilidene-β-D-galactopyranoside and benzyl 3,4,6-tri-O-benzyl-α-D-mannopyranoside, the hydroxyls to be glycosylated are hydrogen-bonded to a cis vicinal oxygen. This feature was evident in their AM1 minimized conformations and was indicated by the solution J(OH,CH) 1H NMR coupling constants.