Glycosylation reaction mechanisms

Rencurosi A, Lay L, Russo G, Caneva E, Poletti L (2006) NMR evidence for the participation of triflated ionic liquids in glycosylation reaction mechanisms. Carbohydr Res 341 903-908... 

Glycosylation (major activators/promoters, particulars of the reaction mechanism, examples of both 1,2-cis and 1,2-trans glycosylations)... 

Other common approaches to control the stereoselectivity of glycosylation will be discussed in the subsequent sections. In addition to the apparent complexity of the glycosidation process, there are other competing processes that cannot be disregarded. These reactions often cause the compromised yields of the glycosylation products and further complicate the studies of the reaction mechanism. 

This improvement allowed achieving the synthesis of 2-deoxy disaccharide 103 with high stereoselectivity and good yield in short reaction times. The composition of the reaction mixture was found to be practically independent of the configuration of the S-(2-deoxy-D-glucopyranosyl)phosphorothioate. These results seem to provide an evidence that this glycosylation procedure proceeds via the SN1 displacement reaction mechanism. 

In attempts to improve the yields of problematic glycosylation reactions, it has become clear that the mechanism is much more complex than Scheme 3.5 suggests. In essence, the oxycarbenium ion(V) is susceptible to attack by nucleophiles other than the glycosyl acceptor (Scheme 3.6). Two intermediates for which there is experimental evidence are glycosyl triflates(VII)17 and glycosyl sulfenates(IX).18... 

This reaction mechanism could be applicable to the method employing xanthate (6) or phosphite (7) as the glycosyl donor in acetonitrile medium. 

This model makes no assumption as to detailed reaction mechanism, but it does allow for glycosyl-proton interchange with enzyme (as H-X ) to form glycosyl-enzyme intermediates, whether covalently linked or of carbonium-oxonium type (42). In the past few years such glycosyl-enzyme intermediates have been demonstrated for a number of carbo-... 

The position of sugar attachment for most of the common purines is the 9-position. Careful studies, mainly by Shimizu and Miyaki, however, reveal that the actual glycosylation reaction must occur via initial alkylation at N3, followed by a rapid migration to N9 by an intermolecular route.The general mechanism of this intermolecular N3 -> N9 glycosylation can be depicted as shown. 

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