Use of the methylsulfenyl cation as an activator for glycosylation reactions with alkyl (aryl) 1-thioglycopyranosides: synthesis of methyl O-(2-acetamido-2- deoxy-beta-D-glucopyranosyl)-(1----6)-O-alpha-D-glucopyranosyl-(1----2) -alpha-D -glucopyranoside, a derivative of the core trisaccharide of E. coli K12.

Methylsulfenyl bromide (MSB) and methylsulfenyl trifluoromethanesulfonate (MST) have been used to prepare 1,2-cis-linked disaccharides. Ethyl (phenyl) 1-thio-beta-D-gluco- and galacto-pyranosides having non-participating (benzyloxy) protecting groups were used as the donors. The alpha beta-ratio of the products depended on the promoter and conditions of reaction. Intimate ion-pairs, formed initially, may be responsible for the steric outcome of the glycosylations. Thus, with ethyl 2,3,4,6-tetra-O-benzyl-alpha-D-mannopyranoside as a donor, moderate quantities of the beta-linked disaccharide could be produced using MSB as the activator. The synthesis of the title trisaccharide glycoside that contains 1,2-cis and 1,2-trans-linkages is described.