Synthesis and NMR assignment of two repeating units (decasaccharide) of the type III group B Streptococcus capsular polysaccharide and its 13C-labeled and N-propionyl substituted sialic acid analogues

For the purpose of carrying out a comprehensive investigation into the nature of the conformational epitope of the type III group B Streptococcus polysaccharide, combined chemical and enzymatic methods were applied to the synthesis of three decasaccharide probes, namely β-d-Glc-(1 → 6)[α-NeuR-(2 → 3)-β-d-Gal-(1 → 4)]-β-d-GlcNAc-(1 → 3)-β-d-Gal-(1 → 4)-β-d-Glc-(1 → 6)[α-NeuR-(2 → 3)-β-d-Gal-(1 → 4)]-β-d-GlcNAc-(1 → 3)-β-d-Gal-OMe (22NeuR = NeuAc; 23NeuR = NeuAc with 8% 13C-labeling; 24NeuR = NeuPr). The precursor core octasaccharide 21 was chemically synthesized from trisaccharide donor 11 and pentasaccharide acceptor 19 by block condensation. Sialylation of 21 with α-(2 → 3)-sialyltransferase and CMP-NeuAc afforded 22. In the presence of CMP-sialic acid synthetase and α-(2 → 3)-sialyltransferase, 21 was sialylated with sialic acid derivatives (8% 13C-labeled, or N-propionyl substituted) to give 23 and 24, respectively. Complete assignments of the 1H and 13C NMR spectra of compounds 21, 22 (23), and 24 are also presented.