β-1,4-Galactosyltransferase-catalyzed Synthesis of the Branched Tetrasaccharide Repeating Unit of Streptococcus pneumoniae Type 14 Original Research Article

A chemoenzymatic approach is described towards the branched tetrasaccharide repeating unit, β-d-Galp-(1→4)-β-d-Glcp-(1→6)-[β-d-Galp-(1→4)]-β-d-GlcpNAc, of Streptococcus pneumoniae type 14 in a form suitable for conjugation. The linear trisaccharide acceptor, β-d-Galp-(1→4)-β-d-Glcp-(1→6)-β-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 β-1,4-galactosyltransferase, and the resulting branched tetrasaccharide β-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 β-aminoethylthio-extended glycoside suitable for further coupling of the tetrasaccharide to protein carriers.