A Serendipitous Formation of a Cysteine-bridged Disaccharide
N-acetyl-L-cysteine bearing free carboxylic acid and sulfhydryl groups was glycosylated with 1,2,3,4,6-Penta-O-acetyl-β- D-glucopyranoside in the presence of SnCl4 as a promoter to give the S-glycosylated cysteine in 64%yield. However, when excess donor was used, a previously unreported cysteine-bridged disaccharide was isolated in 54 % yield. The acetamido group on cysteine, which lowers the pKa of the carboxylic acid group of the amino acid, plays no role in the formation of the bridged disaccharide since 3-mercaptopropionic acid reacts in a similar manner to give the 3-mercaptopropionic acid-bridged disaccharide in 52 % yield.
KEYWORDS: Glycopeptides, glycosylation, bridged-disaccharides.
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