β-1,4-Galactosyltransferase-catalyzed synthesis of the branched tetrasaccharide repeating unit of Streptococcus pneumoniae type 14

Abstract

A chemoenzymatic approach is described towards the branched tetrasaccharide repeating unit, beta-d-Galp- (1->4)-beta-d-Glcp-(1->6)-[beta-d-Galp-(1->4)]-beta-d-GlcpNAc, of Streptococcus pneumoniae type 14 in a form suitable for conjugation. The linear trisaccharide acceptor, beta-d-Galp-(1->4)-beta-d-Glcp-(1->6)-beta-d-GlcpNAc-(1->O)CH2CH-CH2, was synthesized by coupling of peracetylated lactosyl trichloroacetimidate to a suitably protected glucosamine building block and subsequent deprotection steps. The obtained derivative was found to be a good acceptor for bovine milk beta-1,4-galactosyltransferase, and the resulting branched tetrasaccharide beta-allyl glycoside was isolated and characterized by NMR spectroscopy and FAB mass spectrometry. Reaction of the anomeric allyl function with cysteamine under UV-irradiation gave the beta-aminoethylthio-extended glycoside suitable for further coupling of the tetrasaccharide to protein carriers