STEREOSELECTIVE SYNTHESIS OF 2-DEOXY-BETA-GLYCOSIDES FROM GLYCAL PRECURSORS .1. STEREOCHEMISTRY OF THE REACTIONS OF D-GLUCAL DERIVATIVES WITH PHENYLSULFENYL CHLORIDE AND PHENYLSELENENYL CHLORIDE

The stereoselectivity of the reactions of D-glucal derivatives with Ph SCl and PhSeCl is dependent on the presence of an electronegative hete roatom substituent at C(6) and the nature of the functionality at C(4) . The C(6)-substituent influences the conformational preferences of th e D-glucal derivatives, and greatest stereoselectivity is obtained wit h those glycals that preferentially exist in the inverted H-5(4) half- chair conformation 28b. A polar substituent at C(4) increases the sele ctivity by stabilizing the episulfonium/episelenonium ion intermediate s 31b and 33. (C) 1997 Elsevier Science Ltd.