Item

Abstract

Treatment of the sugar hemiacetals 1, 5, 7, 10, 12 and 14 with ArSSAr (Ar = phenyl, 2-pyridyl, 2-benzothiazolyl) and trialkylphosphanes affords in an S_N2 process the corresponding aryl thioglycosides 3, 6, 8, 9, 11, 13, 15 and 16 in good to excellent yields. The reaction can be performed in a variety of solvents of greatly different polarity and avoids the use of free thiols and Lewis acid catalysts. The promoting phosphane has to carry at least two alkyl groups, with R₃P (R = short, unbranched alkyl group) being best suited in terms of yield and reaction rate. In contrast, triphenylphosphane and phosphites are inactive. The reaction is compatible with a variety of functional groups and distinguishes between the anomeric center and other free hydroxyl groups in the substrates. Thus, thioglycosides from partially protected or unprotected sugars can be readily obtained. Treatment of phenyl thioglycosides 13a and 3a with potassium-graphite laminate (C₈K) in THF affords the corresponding pyranoid or furanoid glycals 17 and 18 by reductive elimination.