Pent-4-enyl glycosides, on treatment with halonium ions, become chemospecifically activated so that coupling with partially protected monosaccharides can be effected, leading to the in situ formation of disaccharides.Glycosidation, i. e. conversion of a glycose (1) into a glycoside (4), is of crucial importance to all phases of carbohydrate chemistry. When the 'alcohol' reactant is a simple alkyl derivative, excellent protection is provided for the delicate anomeric centre through formation of an alkyl glycoside, e.g. (4a). Where the alcohol is a protected monosaccharide, the product is a disaccharide, e.g. (4b). In either case, the anomeric centre of (1) must be converted into an electrophile, and this requires activation by development of a good leaving group, and its subsequent ejection to generate the cyclic oxonium ion (3).1 These tandem stages can be accomplished in situ [(l) + (2a) + = (3)], as in the Fischer glycosidation where simple alkanols, used in generous excess with acidic catalysts, bring about a one-pot transformation, (1) -+ (4a).2 In an alternative, more versatile strategy, the anomeric centre is activated by prior formation of a stable glycosyl derivative (Zb), from which the ion (3) is generated under controlled conditions.3 This option is particularly appropriate where the alcohol is a precious commodity, e.g. a protected sugar alcohol 1'30" activation
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