Total synthesis of the O-antigen repeating unit of Providencia stuartii O49 serotype through linear and one-pot assemblies

Capsular polysaccharides of pathogenic bacteria have been reported to be effective vaccines against diseases caused by them. Providencia stuartii is a class of enterobacteria of the family Providencia that is responsible for several antibiotic resistant infections, particularly urinary tract infections of patients with prolonged catheterization in hospital settings. Towards the goal of development of vaccine candidates against this pathogen, we herein report the total synthesis of a trisaccharide repeating unit of the O-antigen polysaccharide of the P. stuartii O49 serotype containing the →6)-β-ᴅ-Galp-(1→3)-β-ᴅ-GalpNAc(1→4)-α-ᴅ-Galp(1→ linkage. The synthesis of the trisaccharide repeating unit was carried out first by a linear strategy involving the [1 + (1 + 1 = 2)] assembly, followed by a one-pot synthesis involving [1 + 1 + 1] strategy from the corresponding monosaccharides. The one-pot method provided a higher yield of the protected trisaccharide intermediate (73%) compared to the two step synthesis (66%). The protected trisaccharide was then deprotected and N-acetylated to finally afford the desired trisaccharide repeating unit as its α-p-methoxyphenyl glycoside.

Synthesis of the tetrasaccharide related to the repeating unit of the O-antigen from Azospirillum brasilense Jm125A2 in the form of its 2-aminoethyl glycoside

Total chemical synthesis of the linear tetrasaccharide repeating unit β-D-Glc-(1 → 2)-α-L-Rha-(1 → 3)-α-L-Rha-(1 → 2)-α-L-Rha-CH₂CH₂NH₂ of the O-antigen from Azospirillum brasilense Jm125A2 is accomplished through rational protecting group manipulations of commercially available monosaccharides and stereoselective glycosylations. The target tetrasaccharide in the form of its 2-aminoethyl glycoside is obtained in ∼24% yield over 10 steps following a linear strategy. The structure is particularly suitable for further glycoconjugate formation through the terminal free amine without hampering the reducing end stereochemistry.

Chemical O-Glycosylations: An Overview

The development of glycobiology relies on the sources of particular oligosaccharides in their purest forms. As the isolation of the oligosaccharide structures from natural sources is not a reliable option for providing samples with homogeneity, chemical means become pertinent. The growing demand for diverse oligosaccharide structures has prompted the advancement of chemical strategies to stitch sugar molecules with precise stereo- and regioselectivity through the formation of glycosidic bonds. This Review will focus on the key developments towards chemical O-glycosylations in the current century. Synthesis of novel glycosyl donors and acceptors and their unique activation for successful glycosylation are discussed. This Review concludes with a summary of recent developments and comments on future prospects.

Chemical synthesis of the hexasaccharide related to the repeating unit of the capsular polysaccharide from carbapenem resistant Klebsiella pneumoniae 2796 and 3264

Total synthesis of the hexasaccharide repeating unit of the capsular polysaccharide fromKlebsiella pneumoniae2796 and 3264 following a linear sequential glycosylation strategy.

Convergent chemical synthesis of the pentasaccharide repeating unit of the O-antigen from E. coli O158

Synthesis of the pentasaccharide repeating unit of the O-antigen fromE. coliO158 through a convergent [3 + 2] strategy is reported.

Convergent synthesis of the hexasaccharide related to the repeating unit of the O-antigen from E. coli O120

The hexasaccharide repeating unit of the O-antigen from E. coli O120 has been synthesized in the form of its 4-methoxyphenyl glycoside by a convergent [2 + 4] strategy.

Chemical synthesis of a tetrasaccharide related to the exocellular polysaccharide from Rhodococcus sp. RHA1

Chemical synthesis of the tetrasaccharide related to the exocellular polysaccharide from Rhodococcus sp. RHA1 is reported. The stereoselective glycosylations were achieved by activation of the thioglycoside donors using N-iodosuccinimide in the presence of La(OTf)3 varying temperature per the need of 1,2-cis or 1,2-trans glycosylations. The target tetrasaccharide is reported in the form of its p-methoxyphenyl glycoside that can be cleaved selectively from the per-O-acetylated derivative allowing further glycoconjugate formation using trichloroacetimidate chemistry.

Stereoselective synthesis of a sulfated tetrasaccharide corresponding to a rare sequence in the galactofucan isolated from Sargassum polycystum

The first chemical synthesis of a highly sulfated tetrasaccharide 1, as the rare sequence in the galactofucan isolated from the brown alga Sargassum polycystum, was achieved in a convergent and stereoselective manner. The key features of the synthetic strategy include construction of multiple contiguous 1,2-cis glycosidic bonds and [2 + 2] assembly based on the rationally developed d-galactose building block 6. The synthesized oligosaccharides were fully characterized using a combination of coupled-HSQC and other 2D NMR techniques.