Synthesis of two phosphate-containing "heptasaccharide" fragments of the capsular polysaccharides of Streptococcus pneumoniae types 6A and 6B

Article abstract of DOI:10.1016/0008-6215(91)84143-3

The "heptasaccharides" O-α-D-galactopyranosyl-(1->3)-O-α-D-glucopyranosyl-(1->3)-α,β-L-rhamnopyranose 2''-3)-O-α-D-glucopyranosyl-(1->3)-O-α-L-rhamnopyranosyl-(1->3)-D-ribit-5-yl sodium phosphate> (25) and O-α-D-galactopyranosyl-(1->3)-O-α-D-glucopyranosyl-(1->3)-α,β-L-rhamnopyranose 2''-3)-O-α-D-glucopyranosyl-(1->3)-O-α-L-rhamnopyranosyl-(1->4)-D-ribit-5-yl sodium phosphate> (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-RibOH-(5-P->>n; 6A X = 3, 6B X = 4>, respectively, have been synthesised. 2,4-Di-O-acetyl-3-O-<2,4,6-tri-O-acetyl-3-O-(2,3,4,6-tetra-O-acetyl-α-D-galactopyranosyl)-α-D-glucopyranosyl>-α-L-rhamnopyranosyl trichloroacetimidate (13) was coupled with 5-O-allyloxycarbonyl-1,2,4-tri-O-benzyl-D-ribitol (10), using trimethylsilyl triflate as a promotor (->14), and deallyloxycarbonylation (->15) and conversion into the corresponding triethylammonium phosphonate then gave 16.Condensation of 16 with 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) followed by oxidation and deprotection afforded 25. 5-O-Allyl-1-O-allyloxycarbonyl-2,3-di-O-benzyl-D-ribitol (12) was coupled with 13, using trimethylsilyl triflate as a promoter, the resulting tetrasaccharide-alditol derivative 17 was deallyloxycarbonylated (->18), acetylated (->19), and deallylated (->20), and the product was converted into the triethylammonium phosphonate derivative 21.Condensation of 21 with 22 followed by oxidation and deprotection afforded 27.