SYNTHETIC OLIGOSACCHARIDES RELATED TO GROUP-B STREPTOCOCCAL POLYSACCHARIDES .5. COMBINED CHEMICAL AND ENZYMATIC-SYNTHESIS OF A PENTASACCHARIDE REPEATING UNIT OF THE CAPSULAR POLYSACCHARIDE OF TYPE-III GROUP-B STREPTOCOCCUS AND ONE-DIMENSIONAL AND 2-DIMENSIONAL NMR SPECTROSCOPIC STUDIES

A combination of chemical and enzymatic methods is described for the synthesis of the branching pentasaccharide 3-OMe-beta-D-Galp-(1 --> 4)-beta-D-Glcp-(1 --> 6)-[alpha-D-Neup5Ac-(2 --> 3)-beta-D-Galp-(1 --> 4)]-beta-D-GlcpNAc-OMe (4). Pentasaccharide 4, represents a complete repeating unit of the capsular polysaccharide of type III group B Streptococci, which will be used as a molecular probe in the study of the fine specificities of carbohydrate-protein interactions. Key intermediate was the alcohol methyl 2-acetamido-3-O-benzyl-2-deoxy-6-O-(4-methoxy-benzyl)-beta-D-glucopyranoside (9), which was beta-D-galactosylated and chemoselectively deprotected to provide the beta-D-Galp-beta-D-GlcpNAc acceptor block 13. Two approaches are described for the synthesis of the beta-D-Galp-(1 --> 4)-beta-D-Glcp donor block 17, which was coupled to acceptor 13 under the agency of trimethylsilyl trifluoromethanesulfonate. Removal of the blocking groups from the fully protected tetrasaccharide 24 by transesterification and hydrogenolysis provided tetrasaccharide 5, which was sialylated enzymatically using a specific rat liver sialyltransferase to provide pentasaccharide 4. Complete assignment of the H-1 and C-13 NMR spectra of the tetra-5 and pentasaccharide 4 is presented, and their carbon spin-lattice relaxation times (T1) are also reported.