Stereoselective glycosylations via chiral auxiliaries
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In this study, three generations of chiral auxiliary are synthesized and attached to glucose and galactose. The donors containing these chiral auxiliary are synthesized and glycosylations with a variety of glycosyl acceptors are conducted. The glycosylation results showed that a glycosyl donor substituted with a chiral auxiliary can be employed for the stereoselective introduction of 1, 2-cis glycosides such as ±-glucosides and ±-galactosides. The neighboring group participation by the chiral auxiliary leads to a quasi-stable anomeric sulfonium ion, which due to steric and electronic factors, is formed as a trans-decalin ring system. Displacement of the sulfonium ion by a glycosyl acceptor leads to the stereoselective formation of ±-glycosides. The use of this new method in combination with traditional neighboring group participation by esters to introduce ²-glycosides makes it possible to synthesize a wide variety of complex carbohydrates with complete anomeric control.