Carbonates glycosylations, nucleophilic substitution

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... 

In a different approach to the preparation of glycosides, the nucleophilic substitution takes place at the carbon atom of the aglycone rather than at the anomeric carbon (Scheme 4.53). In contrast to the methods discussed in Section 4.3, there is no scission of the glycosyl-oxygen bond in these reactions. Instead, the R-X bond is cleaved. 

Formation of the glycosidic C—O bond by a nucleophilic substitution in which an OH group of the glycosyl acceptor acts as the nucleophile toward the anomeric carbon of the glycosyl donor. 

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. 

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. 

C-Glycosylation reactions of glycosyl-halide, -ester and -ether with carbon nucleophiles are divided into two types the Sn2 type of substitution with carbon nucleophiles such as Grig-nard reagents, and addition of carbon nucleophiles to oxocarbenium cations generated from... 

Thioethers and selenoethers can be displaced with various nucleophiles after methylation by MeOTf. This has been used extensively in the carbohydrate synthesis field (see the section on glycosylation). A phenylselenide moiety was substituted stere-ospecifically by an alkyl stannane following MeOTf activation in the construction of a C—C bond between two quaternary carbon centers (eq 20). ... 

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