Synthesis of two linear PADRE conjugates bearing a deca- or pentadecasaccharide B epitope as potential synthetic vaccines against Shigella flexneri serotype 2a infection

The blockwise synthesis of the 2-aminoethyl glycosides of a deca- and a pentadecasaccharide made of two and three repeating units, respectively, of the Shigella flexneri serotype 2a specific polysaccharide is reported. The strategy relies on trifluoromethanesulfonic acid mediated glycosylation of a pentasaccharide building block acting as a glycosyl donor and a potential glycoside acceptor. Both targets were made available in amounts large enough for their subsequent conversion into glycoconjugates. Indeed, efficient elongation of the spacer through an acetylthioacetyl moiety and subsequent conjugation onto a Pan HLA DR-binding epitope (PADRE) T-cell-universal peptide resulted in two fully synthetic neoglycopeptides, which will be evaluated as potential vaccines against S. flexneri serotype 2a infections.