Synthesis of a selectively protected trisaccharide building block of the capsular polysaccharide of streptococcus pneumoniae types 6A and 6B

4-Methoxybenzyl 2,4-di-O-benzyl-3-O-[2,4,6-tri-O-benzyl-3-O-(3,4,6-tri-O-benzyl-α-d-galactopyranosyl)-α-d- glucopyranosyl]-α-l-rhamnopyranoside (22), a building block for the α-d-Galp-(1→3)-α-d-Glcp-(1→3)-α-l-Rhap fragment 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-RibOH-(5-P_n (6A, X = 3; 6B, X = 4) has been synthesised. Ethyl 3-O-allyl-2,4,6-tri-O-benzyl-1-thio-β-d-glucopyranoside was coupled with 4-methoxybenzyl 2,4-di-O-benzyl-α-l-rhamnopyranoside in ether, using methyl triflate as promoter. The resulting α-d-Glcp-(1→3)-α-l-Rhap derivative was deallylated with KOBu^t in N,N-dimethylformamide followed by 0.1m HCl in 9:1 acetone-water. The product was coupled with 3,4,6-tri-O-acetyl-2-O-allyl-α,β-d-galactopyranosyl trichloroacetimidate in ether, using trimethylsilyl triflate, to yield 19. Deacetylation, benzylation, and deallylation then gave 22.