Glycosides, bond formation stereochemistry

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... 

Disaccharides are formed from two monosaccharide units that are connected by a gly-cosidic bond. Different disaccharides result from the combination of different monosaccharides, from the formation of the glycosidic bond using different hydroxy groups on the monosaccharides, or from different stereochemistries of the glycosidic bond. A few of the more important disaccharides are described in the following paragraph. 

The Tsuji-Trost reaction is the Pd-catalyzed allylation of nucleophiles [105] with allylic halides, acetates, carbonates, etc. This transformation proceeds via intermediate allylpalladium complexes (e.g. 110), and typically proceeds with overall retention of stereochemistry. In addition, the trapping of the intermediate allylpalladium complex usually occurs at the least hindered carbon. A representative example of this transformation is shown below in an application to the formation of an 7V-glycosidic bond. Treatment of 2,3-unsaturated hexopyranoside 109 with imidazole in the presence of a Pd(0) catalyst... 

The chemistry of the cellulosic fibers is similar to that of the simple sugars, but more complex, as the stereochemistry of the alcohol and hemiacetal structures is affected by degree of crystallinity, e.g., the -OFI groups on the carbon atoms 2, 3, and 6 have different reactivities. Reaction with mineral acids leads to cleavage of the glycosidic bond and formation of a reducing end (RO-CFI-OFI). 

First, it is difficult to control the stereochemistry of the glyco-sidic bond (a or /3) because the formation of an acetal (from a hemiacetal) proceeds through an Sfjl mechanism a mixture of a- and /3-glycosidic bonds results. In addition, it is difficult to form only the desired 1,4-linkage, because any one of the hydroxyl groups located on carbons 2, 3, 4, and 6 could be used to form the bond. 

The structural elucidation of polysaccharides represents an even greater challenge. The individual monosaccharide subunits that constitute the unknown polysaccharide must first be identified. Then the ring size and position in the polysaccharide sequence must be elucidated for each monosaccharide. Finally, the nature of the gly-cosidic linkages that form the polysaccharide backbone must be defined. Toward this end, it is necessary to establish which hydroxyl group on one monosaccharide is involved in the formation of the acetal or ketal that forms the glycosidic bond to the adjoining monosaccharide the stereochemistry at this anomeric center must also be determined. 

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