Synthesis of methyl 3-[3-(2-O-α-L-rhamnopyranosyl-α-L-rhamnopyranosyloxy)decanoyloxy]decanoate, a rhamnolipid from Pseudomonas aeruginosa

Total synthesis, isolation, surfactant properties, and biological evaluation of ananatosides and related macrodilactone-containing rhamnolipids

Rhamnolipids are a specific class of microbial surfactants, which hold great biotechnological and therapeutic potential. However, their exploitation at the industrial level is hampered because they are mainly produced by the opportunistic pathogen Pseudomonas aeruginosa. The non-human pathogenic bacterium Pantoea ananatis is an alternative producer of rhamnolipid-like metabolites containing glucose instead of rhamnose residues. Herein, we present the isolation, structural characterization, and total synthesis of ananatoside A, a 15-membered macrodilactone-containing glucolipid, and ananatoside B, its open-chain congener, from organic extracts of P. ananatis. Ananatoside A was synthesized through three alternative pathways involving either an intramolecular glycosylation, a chemical macrolactonization or a direct enzymatic transformation from ananatoside B. A series of diasteroisomerically pure (1→2), (1→3), and (1→4)-macrolactonized rhamnolipids were also synthesized through intramolecular glycosylation and their anomeric configurations as well as ring conformations were solved using molecular modeling in tandem with NMR studies. We show that ananatoside B is a more potent surfactant than its macrolide counterpart. We present evidence that macrolactonization of rhamnolipids enhances their cytotoxic and hemolytic potential, pointing towards a mechanism involving the formation of pores into the lipidic cell membrane. Lastly, we demonstrate that ananatoside A and ananatoside B as well as synthetic macrolactonized rhamnolipids can be perceived by the plant immune system, and that this sensing is more pronounced for a macrolide featuring a rhamnose moiety in its native 1 C 4 conformation. Altogether our results suggest that macrolactonization of glycolipids can dramatically interfere with their surfactant properties and biological activity.

Multigram synthesis of an orthogonally-protected pentasaccharide for use as a glycan precursor in a Shigella flexneri 3a conjugate vaccine: application to a ready-for-conjugation decasaccharide

Fine-tuned catalytic processes facilitating regio- and stereoselective conversions for the large-scale synthesis of a pentasaccharide and its oligomerization into ready-for-conjugation haptens.

Synthesis of Rhamnolipid Derivatives Containing Ester Isosteres

Rhamnolipids are biosurfactants with many applications, arising from their inherent biological activity and their potential as bioremediation agents. Herein, we report the synthesis of four rhamnolipid derivatives in which the ester linkage connecting the two lipid chains in the natural compound is replaced with amide, ketone, ether, or hydrocarbon functional groups. Such compounds are anticipated to have enhanced hydrolytic stability and thus be useful probes of rhamnolipid-mediated biology and biotechnology.

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.

Synthesis of α-1,2- and α-1,3-linked di-rhamnolipids for biological studies

For a detailed examination of the interaction of rhamnose containing derivatives with recombinant horseshoe crab plasma lectin (rHPL), two di-rhamno-di-lipids (an α-1,2- and an α-1,3-linked) were synthesized via a new simple method. The N-iodosuccinimide/triflic acid mediated glycosylation of the methyl (R)-3-hydroxydecanoate with phenyl-1-thio-rhamnobioside donors afforded the mono-lipid disaccharides. Removal of the methyl ester group followed by esterification of the mono-lipids with a second (R)-3-hydroxydecanoate unit resulted in fully protected di-lipid derivatives, transformation of which into the target compounds was accomplished in two steps. This method allows the synthesis of both regioisomers in only 6 steps starting from the corresponding free disaccharides. Both synthetic di-rhamnolipids were biologically active for lectin binding differential binding preference between two isomeric di-rhamno-di-lipids. The rHPL lectin favours the α-1,3-linked di-rhamno-di-lipids over its α-1,2-linked regioisomer.

Glycosylation studies on conformationally restricted 3,5-O-(di-tert-butylsilylene)-D-galactofuranosyl trichloroacetimidate donors for 1,2-cis α-D-galactofuranosylation

Conformationally restricted 3,5-O-di-tert-butylsilylene-D-galactofuranosyl trichloroacetimidate donors were synthesized from allyl α-D-galactofuranoside for the construction of 1,2-cis α-D-galactofuranosyl linkages. Glycosylation reactions were performed with several acceptors, including D-galactono-1,4-lactone, D-rhamnopyranosyl, and D-mannopyranosyl derivatives. The influence of the temperature and the reaction solvents was evaluated, as well as the 6-O-substitution pattern of the donor. The higher α-selectivities were obtained at -78 °C in diethyl ether as solvent. 6-O-Acetyl substitution on constrained donor increased the α-selectivity compared to the 6-O-benzyl substitution. Almost no selectivities were observed in the non-participating solvent CH(2)Cl(2). In contrast, ethereal solvents enhanced the α-selectivity suggesting a participating effect in the reaction intermediate.

Influence of the solvent in low temperature glycosylations with O-(2,3,5,6-tetra-O-benzyl-β-D-galactofuranosyl) trichloroacetimidate for 1,2-cis α-D-galactofuranosylation

Glycosylation studies for the construction of 1,2-cis α-linkages with O-(2,3,5,6-tetra-O-benzyl-β-D-galactofuranosyl) trichloroacetimidate (1) and several acceptors, including D-mannosyl and l-rhamnosyl derivatives were performed. The reactions were conducted at low temperatures using CH(2)Cl(2), Et(2)O, and acetonitrile as solvents. A non-participating solvent such as CH(2)Cl(2) at -78°C, favored the α-D-configuration. In contrast, acetonitrile strongly favored the β-D-configuration, whereas no selectivities were observed with Et(2)O. The use of thiophene as an additive did not enhance the α-D-selectivity as in the pyranose counterpart. Although selectivities strongly depended on the acceptor, trichloroacetimidate 1 constitutes a valuable donor for the synthesis of α-D-Galf-(1→2)-l-Rha and α-D-Galf-(1→6)-D-Man. As these motifs are present in pathogenic microorganisms, these procedures described here are useful for the straightforward synthesis of natural oligosaccharides.

Blockwise Approach to Fragments of the O-Specific Polysaccharide of Shigella flexneri Serotype 2a: Convergent Synthesis of a Decasaccharide Representative of a Dimer of the Branched Repeating Unit1

The D'A'B'(E')C'DAB(E)C decasaccharide representative of a dimer of a frame-shifted pentasaccharide repeating unit of the O-specific polysaccharide of Shigella flexneri 2a was synthesized as its methyl glycoside by condensing a pentasaccharide donor (D'A'B'(E')C') and a pentasaccharide acceptor (DAB(E)C-OMe). Several convergent routes to these two building blocks, involving either the AB linkage or the BC linkage as the disconnection site, were evaluated in comparison to the linear strategy. The latter was preferred. It is based on the use of the trichloroacetimidate chemistry. The target branched oligosaccharide was designed to probe the recognition at the molecular level of the natural polysaccharide by protective monoclonal antibodies.

Preparation of synthetic glycoconjugates as potential vaccines against Shigella flexneri serotype 2a disease

The synthesis of three neoglycopeptides incorporating carbohydrate haptens, differing in length, covalently linked to a non natural universal T helper peptide is disclosed. They were synthesized according to a blockwise strategy based on the condensation of appropriate di-, tri-, and tetrasaccharide trichloroacetimidate donors onto an azidoethyl 2-acetamido-2-deoxybeta-D-glucopyranoside acceptor. Use of thiol-maleimide coupling chemistry allowed site-selective efficient conjugation.