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.
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
CHIAPPE, CINZIA;
1997-01-01
Abstract
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.File in questo prodotto:
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