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Concept
Chemical glycosylation
Summary
A chemical glycosylation reaction involves the coupling of a glycosyl donor, to a glycosyl acceptor forming a glycoside. If both the donor and acceptor are sugars, then the product is an oligosaccharide. The reaction requires activation with a suitable activating reagent. The reactions often result in a mixture of products due to the creation of a new stereogenic centre at the anomeric position of the glycosyl donor. The formation of a glycosidic linkage allows for the synthesis of complex polysaccharides which may play important roles in biological processes and pathogenesis and therefore having synthetic analogs of these molecules allows for further studies with respect to their biological importance.
The glycosylation reaction involves the coupling of a glycosyl donor and a glycosyl acceptor via initiation using an activator under suitable reaction conditions.
A glycosyl donor is a sugar with a suitable leaving group at the anomeric position. This group, under the reaction conditions, is activated and via the formation of an oxocarbenium is eliminated leaving an electrophilic anomeric carbon.
A glycosyl acceptor is a sugar with an unprotected nucleophilic hydroxyl group which may attack the carbon of the oxocarbenium ion formed during the reaction and allow for the formation of the glycosidic bond.
An activator is commonly a Lewis acid which enables the leaving group at the anomeric position to leave and results in the formation of the oxocarbenium ion.
The formation of a glycosidic linkage results in the formation of a new stereogenic centre and therefore a mixture of products may be expected to result. The linkage formed may either be axial or equatorial (α or β with respect to glucose). To better understand this, the mechanism of a glycosylation reaction must be considered.
The stereochemical outcome of a glycosylation reaction may in certain cases be affected by the type of protecting group employed at position 2 of the glycosyl donor. A participating group, typically one with a carboxyl group present, will predominantly result in the formation of a β-glycoside.
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