Synthesis of the trisaccharide component of the repeating unit of the capsular polysaccharide of Streptococcus pneumoniae type 19F

Acetolysis of methyl 4-O-(2-acetamido-3,4,6-tri-O-benzyl-2-deoxy-β-D-mannopyranosyl)-2,3,6-triO-benzyl-α-D-glucopyranoside (3) followed by treatment with hydrogen bromide–acetic acid afforded 4-O-(2-acetamido-6-O-acetyl-3,4-di-O-benzyl-2-deoxy-β-D-mannopyranosyl)-6-O-acetyl-2-O-benzyl-α-D-glucopyranosyl bromide (5). Compound (5) by Hg(CN)2–HgBr2 promoted condensation with benzyl 3,4-di-O-benzyl-α-L-rhamnopyranoside (6) provided the glycosylation product (22% yield) exclusively in the α-configuration. Hydrogenolysis of the benzyl protecting groups of the obtained trisaccharide and O-deacetylation of the 6′ and 6″ positions eventually furnished O-(2-acetamido-2-deoxy-β-D-mannopyranosyl)-(1→4)-O-α-D-glucopyranosyl-(1→2)-α,β-L-rhamnopyranose (1), the trisaccharide component of the repeating unit of Streptococcus pneumoniae type 19F.