Selective deprotection of 2',6'-di-O-benzyl-2,3:5,6:3',4'-tri-O-isopropylidenelactose dimethyl acetal

Synthesis of a Lewis b hexasaccharide thioglycoside donor and its use towards an extended mucin core Tn heptasaccharide structure and a photoreactive biotinylated serine linked hexasaccharide

Investigation into Heliobacter pylori binding to Lewis b (Le^b) antigens through the blood group antigen binding adhesion protein (BabA) requires structurally well-defined tools. A Le^b hexasaccharide thioglycoside donor was chemically prepared through a linear approach starting from D-lactose. This donor can be used to attach reducing end linkers providing a range of options for conjugation techniques or to further extend the oligosaccharide structure. To evaluate its efficiency as a donor, it was coupled to a 6-OH GalNAc acceptor, producing an extended Le^b-containing Tn mucin core structure in 84% yield, and to L-serine in 72% yield. The latter compound was subsequently functionalized with a photolabile diazirine linker and biotin, creating a Le^b hexasaccharide structure-function tool suitable for lectin tagging interaction studies. This donor opens a wide range of possibilities for conjugation of Le^b structures to produce a variety of chemical biology tools to assist in the study of these interactions.

Investigation on the Synthesis of Shigella flexneri Specific Oligosaccharides Using Disaccharides as Potential Transglucosylase Acceptor Substrates

Chemo-enzymatic strategies hold great potential for the development of stereo- and regioselective syntheses of structurally defined bioactive oligosaccharides. Herein, we illustrate the potential of the appropriate combination of a planned chemo-enzymatic pathway and an engineered biocatalyst for the multistep synthesis of an important decasaccharide for vaccine development. We report the stepwise investigation, which led to an efficient chemical conversion of allyl α-d-glucopyranosyl-(1→4)-α-l-rhamnopyranosyl-(1→3)-2-deoxy-2-trichloroacetamido-β-d-glucopyranoside, the product of site-specific enzymatic α-d-glucosylation of a lightly protected non-natural disaccharide acceptor, into a pentasaccharide building block suitable for chain elongation at both ends. Successful differentiation between hydroxyl groups features the selective acylation of primary alcohols and acetalation of a cis-vicinal diol, followed by a controlled per-O-benzylation step. Moreover, we describe the successful use of the pentasaccharide intermediate in the [5 + 5] synthesis of an aminoethyl aglycon-equipped decasaccharide, corresponding to a dimer of the basic repeating unit from the O-specific polysaccharide of Shigella flexneri 2a, a major cause of bacillary dysentery. Four analogues of the disaccharide acceptor were synthesized and evaluated to reach a larger repertoire of O-glucosylation patterns encountered among S. flexneri type-specific polysaccharides. New insights on the potential and limitations of planned chemo-enzymatic pathways in oligosaccharide synthesis are provided.

Chemical transformation of lactose into 4-O-beta-D-galactopyranosyl-D-glucuronic acid (pseudolactobiouronic acid) and some derivatives thereof

The selective oxidation of the primary alcoholic function of the reducing unit of lactose was achieved in good overall yield (67%) starting from 2',6'-di-O-benzyl-2,3:3',4'-di-O-isopropylidenelactose dimethyl acetal (1) through a simple multi-step procedure based on the selective acetylation of OH-5 of 1 (methoxyisopropylation, acetylation, de-methoxyisopropylation) followed by a two-step oxidation at C-6 (TPAP-NMO then TEMPO-NaOCl) and finally, complete removal of the protecting groups.