Highly β-selective O-glucosidation due to the restricted twist-boat conformation

[GRAPHICS] Ethyl 1-thio-2,3,4,6-tetrakis-O-triisopropylsilyl-beta-D-glucopyranoside, ethyl 6-O-benzyl-1-thio-2,3,4-tris-O-triisopropylsilyl-beta-D-glucopyranoside, and ethyl 6-O-pivaloyl-1-thio-2,3,4-tris-O-triisopropylsilyl-beta-D-glucopyranoside induced highly beta-selective O-glucosidations. Among them, the 6-O-pivaloylated substrate provided the best selectivity up to alpha/beta = 3:97 with cyclohexylmethanol, and the substrate was used for glucosidations with secondary and tertiary alcohols in a highly beta-selective manner. The selectivity would be caused by the twist-boat conformation of the pyranose; this is the first beta-selective O-glucosidation based on conformational control of the pyranose ring.