Glycosylations by Nucleophilic Substitutions at the Anomeric Carbon

A phosphite forms the phosphonium salt by interaction with a Lewis acid owing to their basic character. When a glycosyl phosphite is transformed to the phosphonium salt, the resulting phosphonooxy group acts as a strong leaving group in a nucleophilic substitution at the anomeric carbon. Since the early reports on their use... 

If aspartic acid-52 acts as a nucleophile in lysozyme reactions a glycosyl enzyme intermediate will be formed [60]. There is no evidence, kinetic or otherwise, for substituted enzyme intermediates, but rapid breakdown might preclude attainment of detectable concentrations. Formation of a substituted enzyme could explain the observed retention of configuration at the anomeric carbon in transglycosidation reactions, provided backside attack in a subsequent reaction is chemically reasonable. It has therefore been important to attempt to understand the chemistry of acylal hydrolysis so as to assess the properties that would be expected of an acylal intermediate in reactions catalysed by the enzyme. 

A common way for the introdnction of azides into carbohydrates is the nucleophilic replacement of leaving groups by the azide ion. These reactions can be divided into three groups substitutions at the anomeric center leading to glycosyl azides, substitutions at primary, and substitutions at secondary carbon atoms. 

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