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

Abstract

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-Pn (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 KOBut in N,N-dimethylformamide followed by 0.1 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.