Unexpected stereochemical outcome of activated 4,6-O-benzylidene derivatives of the 2-deoxy-2-trichloroacetamido-d-galacto series in glycosylation reactions during the synthesis of a chondroitin 6-sulfate trisaccharide methyl glycoside Original Research A

The synthesis of methyl (β-d-glucopyranosyluronic acid)-(1→3)-(2-acetamido-2-deoxy-6-O-sulfonato-β-d-galactopyranosyl)-(1→4)-(β-d-glucopyranosid)uronate trisodium salt, a chondroitin 6-sulfate trisaccharide derivative, is described. Loss of stereocontrol in glycosylation reactions involving activated 4,6-O-benzylidene derivatives of the 2-deoxy-2-trichloroacetamido-d-galacto series and d-glucuronic acid-derived acceptors was highlighted. This drawback was overcome through the use of phenyl 3,4,6-tri-O-acetyl-2-deoxy-1-thio-2-trichloroacetamido-β-d-galactopyranoside, which afforded the desired β-linked disaccharide derivative in high yield with an excellent stereoselectivity. This later was submitted to acid-catalyzed methanolysis, followed by benzylidenation, and condensed with methyl 2,3,4-tri-O-benzoyl-1-O-trichloroacetimidoyl-α-d-glucopyranuronate to afford the expected trisaccharide derivative. Subsequent transformation of the N-trichloroacetyl group into N-acetyl, mild acid hydrolysis, selective O-sulfonation at C-6 of the amino sugar moiety, and saponification afforded the target molecule as its sodium salt in high yield.