MIMICS OF THE STRUCTURAL ELEMENTS OF TYPE-III GROUP-B STREPTOCOCCUS CAPSULAR POLYSACCHARIDE .1. SYNTHESIS OF A CARBOXYLATE-CONTAINING PENTASACCHARIDE

A carboxylate-containing pentasaccharide, methyl D-glucopyranosyl)-(1- ->3)-beta-D-galactopyranoside (27) was synthesized by block condensati on of suitably protected donors and accepters. Phenyl -deoxy-2-phthali mido-1-thio-beta-D-glucopyranoside (17) was condensed with methyl 2,4, 6-tri-O-benzyl-beta-D-galactopyranoside (4) to afford a disaccharide, methyl 1-->3)-2,4,6-tri-O-benzyl-beta-D-galactopyranoside (18). Remova l of chloroacetyl groups gave 4,6-diol, methyl 1-->3)-2,4,6-tri-O-benz yl-beta-D-galactopyranoside (19), in which the primary hydroxy group ( 6-OH) was then selectively chloroacetylated to give methyl 1-->3)-2,4, 6-tri-O-benzyl-beta-D-galactopyranoside (20). This acceptor was then c oupled with 1-(methoxycarbonyl)ethyl]-alpha-D-galactopyranosyl trichlo roacetimidate (14) to afford a trisaccharide, methyl 1-->3)-2,4,6-tri- O-benzyl-beta-D-galactopyranoside (21). Removal of the 6-O-chloroacety l group in 21 gave 22, which was coupled with ranosyl)-2,3,6-tri-O-ace tyl-alpha-D-glucopyranosyl trichloroacetimidate (23) to yield protecte d pentasaccharide 24. Standard procedures were used to remove acetyl g roups and the phthalimido group, followed by N-acetylation, and debenz ylation to yield pentasaccharide 27 and a hydrazide by-product (28) in a 5: 1 ratio, respectively. Compound 27 contains a complete repeating unit of the capsular polysaccharide of type III group B Streptococcus in which terminal sialic acid is replaced by an (S)-1-carboxyethyl gr oup.