STEREOSELECTIVE SYNTHESIS OF 2-DEOXY-BETA-GLYCOSIDES FROM GLYCAL PRECURSORS .2. STEREOCHEMISTRY OF GLYCOSIDATION REACTIONS OF 2-THIOPHENYL-ALPHA-D-GLUCO-PYRANOSYL AND 2-SELENOPHENYL-ALPHA-D-GLUCO-PYRANOSYL DONORS

We have demonstrated that ophenyl-1-trichloroacetimido-alpha-D-glucopy ranose 11b is the most efficient and selective donor for use in the sy nthesis of 2-deoxy-beta-glycosides of the series of glycosyl donors ex amined. Unlike the 2-selenophenyl substituted donors 8 which proved to be configurationally unstable under standard TMS-OTf promoted glycosy lation conditions, giving rise to alpha-manno glycosides 14, 17 and 20 from beta-gluco donors 8, the 2-thiophenyl substituted donors 9 and 1 1 appeared to be completely configurationally stable (at C(2)). The ma in problem with imidates 11 is that the stereoselectivity of their rea ctions with alcohols is substrate dependent, with best selectivity for the desired beta-glycosides 35 and 42 being obtained with the least s terically hindered alcohols. The fact that the alpha-glycosides 36 and 43 comprise up to 20-50% of the product in glycosidation reactions of hindered secondary alcohols supports the thesis that the reaction ste reoselectivity is not governed by the intermediacy of episulfonium ion s (47 and 47'), but rather that substitution reactions of oxonium ions 46 and its conformationally inverted isomer 46' play a dominant role. (C) 1997 Published by Elsevier Science Ltd.