A serendipitous formation of a cysteine-bridged disaccharide : research article

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.