Glycosidation stereochemistry

Rolf D, Benneck JA, Gray GR (1983) Reductive cleavage of glycosides. Stereochemistry of trapping of cyclic oxonium ions. J Carbohydr Chem 2 373-383... 

Pvranoid Compounds.- Glese has published two important reviews on the use of glycosyl radicals in the synthesis of -glycosides and -disaccharides. " Methods for the determination of -glycoside stereochemistry, based on Jc-i,h-i values, have been reported. ... 

Maltose obtained by the hydrolysis of starch and cellobiose by the hydrolysis of cellulose are isomenc disaccharides In both maltose and cellobiose two d glucopyra nose units are joined by a glycosidic bond between C 1 of one unit and C 4 of the other The two are diastereomers differing only m the stereochemistry at the anomeric carbon of the glycoside bond maltose is an a glycoside cellobiose is a (3 glycoside... 

The stereochemistry and points of connection of glycosidic bonds are commonly designated by symbols such as a(l 4) for maltose and (3(1 4) for cellobiose a and (3 designate the stereochemistry at the anomeric position the numerals specify the ring car bons involved... 

The small change in stereochemistry between cellulose and amylose creates a large difference in their overall shape and in their properties. Some of this difference can be seen in the strorcture of a short portion of fflnylose in Figure 25.9. The presence of the a-glycosidic linkages imparts a twist to the fflnylose chain. Where the main chain is roughly linear- in cellulose, it is helical in anylose. Attractive forces between chains are weaker in fflnylose, and fflnylose does not form the same kind of strong fibers that cellulose does. 

Stereochemistry of hydrolysis of glycosidic linkage by three Aspergillus polygalacturonases... 

In the carbohydrate chemistry arena, the Tsuji-Trost reaction has been applied to construct N-glycosidic bonds [53]. In the presence of Pd2(dba>3, the reaction of 2,3-unsaturated hexopyranoside 68 and imidazole afforded N-glycopyranoside 69 regiospecifically at the anomeric center with retention of configuration. In terms of the stereochemistry, the oxidative addition of allylic substrate 68 to Pd(0) formed the jc-allyl complex with inversion of configuration, then nucleophilic attack by imidazole proceeded with another inversion of the configuration. Therefore, the overall stereochemical outcome is retention of configuration. 

Sn2 reactions of glycosyl iodides have proven especially advantageous in the synthesis of 2-deoxy P-O-aryl-D-glycosides. This is a challenging linkage to make, as there is no neighboring group to participate. Sometimes, stereochemistry is... 

Alternatively, the use of an auxiliary with an (f )-stereochemistry at the phenyl-2-(phenylsulfanyl)ethyl moiety will lead to the formation of a 1,2-traro-glycoside because in this case the trans-decalin system will exert unfavourable steric interactions. The proposed mechanism has been supported by computational studies.39... 

Crich and Sun propose that path b predominates when the glycosyl acceptor is present during activation of the sulfoxide. However, they have evidence to suggest that when the sulfoxide is activated, prior to addition of the acceptor, the glycosyl triflate forms within minutes.17 This has important consequences in terms of the stereochemistry of the glycosidic linkage. 

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