Convenient construction of a variety of glycosidic linkages using a universal glucosyl donor

Chemical synthesis of the pentasaccharide repeating unit of the O-specific polysaccharide from Ruminococcus gnavus

Microorganisms present in the guts are the causative agents for various diseases in humans. More and more studies are correlating such diseases and the responsible microorganism. The Gram-positive bacterium Ruminococcus gnavus (R. gnavus) has been identified to be responsible for symptoms of Crohn's disease. R. gnavas produces a glucorhamnan polysaccharide and it is postulated that this polysaccharide induces inflammatory response through toll-like receptor 4 (TLR4). The current manuscript describes the chemical synthesis of the pentasaccharide repeating unit of the O-polysachharide from R. gnavus. The major challenge associated with this particular synthesis is the presence of two consecutive 1,2-cis glucose units. The target oligosaccharide is achieved through a linear strategy from commercially available sugars through rational protecting group manipulation. 1,2-cis glycosylation of glucose through remote participation of acyl group at the 6-O position is used successfully with excellent yield. In both cases, sole 1,2-cis products are obtained at -20 °C through the activation of thioglycosides.

Chemical Synthesis of β-l-Rhamnose Containing the Pentasaccharide Repeating Unit of the O-Specific Polysaccharide from a Halophilic Bacterium Halomonas ventosae RU5S2EL in the Form of Its 2-Aminoethyl Glycoside

Total synthesis of the pentasaccharide repeating unit of the OPS from Halomonas ventosae RU5S2EL is accomplished through a [3+2] block strategy. Picoloyl-induced hydrogen-bond-assisted aglycon delivery (HAD) is used for two consecutive 1,2-cis-l-rhamnosylations, and remote participation is used for α-selective glucosylation. The choice of 2-aminoethyl glycoside at the reducing end is opted for, leaving the scope for further glycoconjugate formation without hampering the reducing-end stereochemistry.

Chemical synthesis of the pentasaccharide repeating unit of the O-specific polysaccharide from Escherichia coli O132 in the form of its 2-aminoethyl glycoside

The total chemical synthesis of the pentasaccharide repeating unit of the O-polysaccharide from E. coli O132 is accomplished in the form of its 2-aminoethyl glycoside. The 2-aminoethyl glycoside is particularly important as it allows further glycoconjugate formation utilizing the terminal amine without affecting the stereochemistry of the reducing end. The target was achieved through a [3 + 2] strategy where the required monosaccharide building blocks are prepared from commercially available sugars through rational protecting group manipulation. The NIS-mediated activation of thioglycosides was used extensively for the glycosylation reactions throughout.

1,2- trans Glycosylation via Neighboring Group Participation of 2- O-Alkoxymethyl Groups: Application to One-Pot Oligosaccharide Synthesis

The use of 2- O-alkoxymethyl groups as effective stereodirecting substituents for the construction of 1,2- trans glycosidic linkages is reported. The observed stereoselectivity arises from the intramolecular formation of a five-membered cyclic architecture between the 2- O-alkoxymethyl substituent and the oxocarbenium ion, which provides the expected facial selectivity. Furthermore, the observed stereocontrol and the extremely high reactivity of 2- O-alkoxymethyl-protected donors allowed development of a one-pot sequential glycosylation strategy that should become a powerful tool for the assembly of oligosaccharides.

Application of the superarmed glycosyl donor to chemoselective oligosaccharide synthesis

Recently, we discovered a novel method for "superarming" glycosyl donors. Herein, this concept has been exemplified in one-pot oligosaccharide syntheses, whereby the superarmed glycosyl donor was chemoselectively activated over traditional "armed" and disarmed glycosyl acceptors. Direct side-by-side comparison of the reactivities of the classic armed and superarmed glycosyl donors further validates the credibility of the novel concept.