A new method for the activation of thioglycosides was developed by use of benzeneselenenyl triflate (PhSeOTf), which, upon reaction with either a primary or secondary sugar HO-group, afforded Oglycosides under extremely mild reaction conditions. The reaction was applicable to various I-thiohexosides 2-6 as well as to a 2-thioglycoside 20 derived from N-acetylneuraminic acid (NeuAc).
INTRODUCTIQN
The search for new glycosylation reactions has recently become a subject of active research', especially since the biomedical potential of glycoconjugates has been recog-nized2. These molecules often carry complex glycan chains composed of a variety of sugar residues with diverse types of interglycosidic linkages. Because of such complexity, it seems to be extremely difficult to find a general solution, based on a single concept, which is applicable to all of the situations encountered in oligosaccharide synthesis. However, recent investigations have revealed the promising features of novel glycosyl donors such as fluorides3, trichloroacetimidates", and 1 -thioglycosidesS", among other compounds. These substances are relatively stable and can be prepared under mild conditions. Such properties make these methods clearly distinct from the conventional KoenigsKnorr type process'6, which usually requires the preparation of sensitive halides under rather harsh conditions. Furthermore, improvement in yield and stereoselectivity has quite often been achieved using such modern techniques.
Among these, 1 -thioglycosides possess an obvious advantage, particularly in view of synthetic strategy, because they are particularly insensitive to usual protectiondeprotection conditions except catalytic hydrogenolysis. In addition, the activation of I-thioglycosides is possible under specific neutral conditions which do not affect most other functional groups. Accordingly, an S-glycosidic linkage can be regarded as a * Part of a series on "Novel Approaches to Glycoside Synthesis". For previous papers in the series, see Ref. 18.