1,3,4,6-Tetra-O-acetyl-2-chloroacetamido-2-deoxy-β-d-glucopyranose as a glycosyl donor in syntheses of oligosaccharides☆

Add time:09/02/2019    Source:sciencedirect.com

1,3,4,6-Tetra-O-acetyl-2-chloroacetamido-2-deoxy-β-d-glucopyranose was tested as a glycosyl donor for oligosaccharide synthesis via a ferric chloride-catalyzed coupling reaction. Glycosyl acceptors tried (6 in all) were O-benzyl-protected d-galactosides having free OH groups at positions 3 and 4, respectively, and similarly protected glycosides of d-glucose and 2-acetamido-2-deoxy-d-glucose unsubstituted on O-4. Existing syntheses of all the acceptors were improved, in four instances by exploitation of Garegg and Hultberg's cyanoborohydride procedure for the conversion 4,6-O-benzylidene →6-O-benzyl [Carbohydr. Res., 93 (1981) c10–c11; 108 (1982) 97–101]. Good to excellent yields of β-linked disaccharides were obtained from the galactoside and glucoside acceptors, but with allyl 2-acetamido-3,6-di-O-benzyl-2-deoxy-a-d-glucopyranoside, stereoselectivity was lost (a:β-ratio 1:2). Allyl and benzyl 2-acetamido-3,6-di-O-benzyl-2-deoxy-β-d-glucopyranosides gave, respectively, the allyl and benzyl β-glycosides of the donor as major products. A mechanism is proposed for this transglycosidation reaction. The N-chloroacetyl groups in the disaccharide products were readily converted into N-acetyl by reduction with zinc-acetic acid.

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