A divergent approach to the synthesis of simplexides and congeners via a late-stage olefin cross-metathesis reaction

Combination of 3-O-Levulinoyl and 6-O-Trifluorobenzoyl Groups Ensures α-Selectivity in Glucosylations: Synthesis of the Oligosaccharides Related to Aspergillus fumigatus α-(1 → 3)-d-Glucan

Stereospecific α-glucosylation of primary and secondary OH-group at carbohydrate acceptors is achieved using glucosyl N-phenyl-trifluoroacetimidate (PTFAI) donor protected with an electron-withdrawing 2,4,5-trifluorobenzoyl (TFB) group at O-6 and the participating levulinoyl (Lev) group at O-3. New factors have been revealed that might explain α-stereoselectivity in the case of TFB and pentafluorobenzoyl (PFB) groups at O-6. They are of conformational nature and confirmed by DFT calculations. The potential of this donor, as well as the orthogonality of TFB and Lev protecting groups, is showcased by the synthesis of α-(1 → 3)-linked pentaglucoside corresponding to Aspergillus fumigatus α-(1 → 3)-d-glucan and of its hexasaccharide derivative, bearing β-glucosamine residue at the non-reducing end.

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 and biological evaluation of S-simplexides and other analogues of simplexide

Simplexides are natural glycolipids isolated from the marine sponge Plakortis simplex, and contain alkyl 4-O-(α-D-glucopyranosyl)-β-D-galactopyranoside. Simplexides can release of cytokines (IL-6) and chemokines (CXCL-8) from human monocytes and cause the expansion of natural killer T-cells (iNKTs) in vitro, with iNKTs contributing to the sustenance of immune homeostasis. Herein, the stereoselective syntheses of S-glycosidic analogues, i.e. S-simplexides, are described. The routes included Lewis acid promoted anomerisation of glycosyl thiols and thioglycolipids, as well as anomeric S-alkylation. Synthesis of O-glycosidic analogues are included. Heptadecanyl O- and S-glycosides as well as the 17-tritriacontyl 4-O-(α-D-glucopyranosyl)-β-D-galactopyranoside, a component of the natural simplexide isolate, all induced IL-6 and CXCL-8 production at both 10 and 30 μg/mL concentrations from PBMCs whereas the two S-simplexides were inactive. It is speculated that the lack of activity for the S-disaccharide analogue could be due to inhibition of cellular α-glucosidase, preventing degradation of the simplex disaccharide to a simpler galactopyranoside, whereas lack of activity for the S-galactolipid analogue could be due to increased conformational flexibility of S-glycosides. On the other hand, simpler unbranched O- and S-glycolipid analogues were active. Natural simplexide, and a synthetic simplexide, the 18-pentatriacontanyl 4-O-(α-D-glucopyranosyl)-β-D-galactopyranoside, were more potent than the new compounds tested.

Chemistry and Biology of Bioactive Glycolipids of Marine Origin

Glycolipids represent a broad class of natural products structurally featured by a glycosidic fragment linked to a lipidic molecule. Despite the large structural variety of these glycoconjugates, they can be classified into three main groups, i.e., glycosphingolipids, glycoglycerolipids, and atypical glycolipids. In the particular case of glycolipids derived from marine sources, an impressive variety in their structural features and biological properties is observed, thus making them prime targets for chemical synthesis. In the present review, we explore the chemistry and biology of this class of compounds.

Design of α-Selective Glycopyranosyl Donors Relying on Remote Anchimeric Assistance

Oligosaccharides have a variety of versatile biological effects, but unlike peptides and oligonucleotides, investigation of the roles of oligosaccharides is not easy. Since biosynthesis of oligosaccharides does not comply with direct genetic control, their isolation from natural sources and biotechnological preparation are complicated, due to the heterogeneous composition of raw carbohydrates. Oligosaccharide synthesis is needed for the establishment or confirmation of the structure of natural glycocompounds. Also, synthetically prepared, defined oligosaccharides and their derivatives are becoming increasingly important tools for many biological and immunological research projects. The key step of oligosaccharide synthesis involves glycosylation, a reaction that builds glycosidic bonds. Usually, construction of 1,2-trans-bonds is easy, and therefore, this reaction can even be included into automated syntheses. At this time, installation of the 1,2-cis-bond remains simultaneously frustrating and compelling. In our and other laboratories, a strategy of α-selective (1,2-cis) glycosylation, relying on remote anchimeric assistance with acyl groups, is studied. The state of the art in this work is summarized in this review.