Lifetime of the glucosyl oxocarbenium ion and stereoselectivity in the glycosidation of phenols with 1,2-anhydro-3,4,6-tri-O-methyl-alpha-D-glucopyranose

The glycosidation of para substitued phenols with 1,2-anhydro-3,4,6-tri-O-methyl-alpha-D-glucopyranose. ZnCl2 catalysed, gives predominantly the corresponding a-anomers. In the presence of an amino base, 1.1,3,3-tetramethylguanidine. however, enhances the beta-selectivity, which becomes practically complete when the reaction is carried out under basic conditions, by addition of K2CO3 and 18-crown-6. A rationalisation based on the lifetime of the oxocarbenium ion intermediate and on the nucleophilicity of the phenols is proposed. (C) 1997 Elsevier Science Ltd.