Syntheses of oligomers of the capsular polysaccharide of the Haemophilus influenzae type b bacteria

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.

A modular synthetic route to size-defined immunogenic Haemophilus influenzae b antigens is key to the identification of an octasaccharide lead vaccine candidate

A Haemophilus influenzae b vaccine lead antigen was identified by the immunological evaluation of chemically precisely defined capsular polysaccharide repeating unit oligosaccharides.

The first glycoconjugate vaccine using isolated glycans was licensed to protect children from Haemophilus influenzae serotype b (Hib) infections. Subsequently, the first semisynthetic glycoconjugate vaccine using a mixture of antigens derived by polymerization targeted the same pathogen. Still, a detailed understanding concerning the correlation between oligosaccharide chain length and the immune response towards the polyribosyl-ribitol-phosphate (PRP) capsular polysaccharide that surrounds Hib remains elusive. The design of semisynthetic and synthetic Hib vaccines critically depends on the identification of the minimally protective epitope. Here, we demonstrate that an octasaccharide antigen containing four repeating disaccharide units resembles PRP polysaccharide in terms of immunogenicity and recognition by anti-Hib antibodies. Key to this discovery was the development of a modular synthesis that enabled access to oligosaccharides up to decamers. Glycan arrays containing the synthetic oligosaccharides were used to analyze anti-PRP sera for antibodies. Conjugates of the synthetic antigens and the carrier protein CRM197, which is used in licensed vaccines, were employed in immunization studies in rabbits.

Determination of the configuration of ribitol in the C-polysaccharide of Streptococcus pneumoniae using a synthetic approach

The C-polysaccharide is an antigen common to all known serotypes of Streptococcus pneumoniae bacteria and is a potential target for vaccine preparation. The final uncertainty in the structure of its repeating unit, a pentasaccharide phosphate containing two phosphorylcholine side chains, has been resolved by determining the configuration of ribitol. Assignment of configuration was performed by synthesis of the two trisaccharide phosphate fragments that have either D- or L-ribitol at their centers and comparison of their 1H and 13C NMR spectral data with that of the natural polysaccharide. The syntheses employed common synthons added in different orders to an asymmetrically substituted chiral ribitol derivative to obtain opposite chiralities in the ribitol segments. The data for the trisaccharide containing D-ribitol was almost identical to that of the natural material while that for the trisaccharide containing L-ribitol differed significantly. In particular, the chemical shift differences between the two protons of the primary carbons of ribitol units directly attached to the β-2-acetamido-2-deoxy-galactopyranosyl residue were 0.10, 0.12, and 0.33 ppm, in the natural polysaccharide, the D-ribitol-containing trisaccharide, and the L-ribitol-containing trisaccharide, respectively. The average difference between the 13C NMR chemical shifts of corresponding ribitol carbons from the natural polysaccharide and the D-ribitol-containing trisaccharide phosphate was 0.034 ppm. This evidence indicates that ribitol in the C-polysaccharide has the D-configuration and that a very similar mixture of conformations in the ribitol portions is present for the natural polysaccharide and the D-ribitol-containing trisaccharide phosphate.Key words: C-polysaccharide, Streptococcus pneumoniae, ribitol, dibutylstannylene acetal, determination of configuration.