Protecting Groups Effects on Reactivity, Glycosylation Stereoselectivity, and Coupling Efficiency

17 Protecting Groups Effects on Reactivity, Glycosylation Stereoselectivity, and Coupling Efficiency 

Even though the a-bromide is predominant (as it is the more thermodynamically stabilized species) interconversion between a- and P-bromides (C) is considerably faster than glycosylation (especially when lowly nucleophilic alcohols such as glycosyl acceptors are used and a source of bromide ion is present— / situ anomeri-zation protocol ) [6] and glycosylation occurs preferentially on the more reactive p-bromide, resulting in the a-glycoside as the major product. This is one of the earliest examples of a dynamic kinetic resolution. 

The leaving group (X) is initially coordinated by the activating group (M+Y ) to form a catalyst-coordinated ion-pair. Although this pair can be attacked by the alcohol directly, affording products with inverted stereochemistry of the initial 

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