Anomeric stereochemistry oligosaccharide synthesis

Notes and discussion. This procedure is used for the stereoselective formation of a, 2-trans configured fluoride. The use of the orthoester precursor allows differentiation of the 0-2, which is a strategic manipulation that is often useful in oligosaccharide synthesis. The product also has participatory acetyl protection of 0-2, which may be used to direct the stereochemistry of its glycosylation reaction. The direct treatment with DAST has the advantage that only a single anomer is formed, compared to the anomeric mixture which results from the two step process of hydrolysis to the hemiacetal followed by DAST treatment. 

Among all the synthetic methods developed for glycosidic bond formation, the trichloroacetimidate method developed by Richard Schmidt and coworkers [18] is probably the most popular and widely used for the synthesis of complex oligosaccharides. Glycosyl trichloroacetimidates can be readily prepared by a base-catalyzed addition of the anomeric hydroxy group to trichloroacetonitrile. The stereochemistry can be influenced by the addition of different bases the use of K2C03 favors formation... 

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