Synthesis of the tetrasaccharide repeating unit of the K-antigen from Klebsiella type 57

Starting from d-galactose and d-mannose two disaccharide blocks, namely 2-(trimethylsilyl)ethyl 2,3,4,6-tetra-O-benzyl-α-d-mannopyranosyl-(1→4)-6-O-(4-methoxybenzyl)-β-d-galactopyranoside and ethyl 2,3,4,6-tetra-O-benzyl-α-d-mannopyranosyl-(1→3)-4-O-acetyl-2,6-di-O-benzoyl-1-thio-β-d-galactopyranoside, were synthesized which were then allowed to react, in the presence of dimethyl(methylthio)sulfonium triflate to give a tetrasaccharide derivative. This compound was converted to 2-(trimethylsilyl)ethyl 2,3,4,6-tetra-O-benzyl-α-d-mannopyranosyl-(1→3)-4-O-acetyl-2,6-di-O-benzoyl-β-d-galactopyranosyl-(1→3)-[2,3,4,6-tetra-O-benzyl-α-d-mannopyranosyl-(1→4)]-tert-butyl (2-O-acetyl-β-d-galactopyranosid)uronate which, on treatment with sodium methoxide followed by hydrogenolysis, afforded the methyl [2-(trimethylsilyl)ethyl glycosid]uronate of the tetrasaccharide repeating unit of the K-antigen from Klebsiella type 57.