Synthesis of structural elements of the capsular polysaccharides of Streptococcus pneumoniae types 6A and 6B

O-α-d-Glucopyranosyl-(1→3)-α,β-l-rhamnopyranose (15), O-α-d-galactopyranosyl-(1→3)-O-α-d-glucopyranosyl-(1→3)-α,β-l-rhamnopyranose (17), O-α-d-galactopyranosyl-(1→3)-O-α-d-glucopyranosyl-(1→3)-O-α-l-rhamnopyranosyl-(1→3)-d-ribitol (23), and O-α-d-galactopyranosyl-(1→3)-O-α-d-glucopyranosyl-(1→3)-O-α-l-rhamnopyranosyl-(1→4)-d-ribitol (27), which are structural elements of the capsular polysaccharides of Streptococcus pneumoniae types 6A and 6B {[→2)-α-d-Galp-(1→3)-α-d-Glcp-(1→3)-α-l-Rhap-(1→X)-d-Rib-ol-(5-P→]_n; 6AXX = 3, 6B X =4}, have been synthesised. Ethyl 3-O-allyl-2,4,6-tri-O-benzyl-1-thio-β-d-glucopyranoside (3) was coupled with benzyl 2,4-di-O-benzyl-α-l-rhamnopyranoside (4), and subsequent deallylation (→14) and debenzylation gave 15. Condensation of 14 with ethyl 2,3,4,6-tetra-O-benzyl-1-thio-β-d-galactopyranoside (2) followed by debenzylation gave 17. Acetylation of 17 followed by removal of AcO-1, conversion into the imidate, coupling with 1,2,4,5-tetra-O-benzyl-d-ribitol (11), deacetylation, and debenzylation gave 23. Coupling of the imidate with 1-O-allyloxycarbonyl-2,3,5-tri-O-benzyl-d-ribitol (12) followed by deallyloxycarbonylation, deacetylation, and debenzylation yielded 27.