Stereoselective Synthesis of the O-antigen of A. baumannii ATCC 17961 Using Long-Range Levulinoyl Group Participation

The effect of neighbouring group participation and possible long range remote group participation in O-glycosylation

Stereoselective glycosylations are one of the most challenging tasks of synthetic glycochemists. The protecting building blocks on the glycosides contribute significantly in attaining the required stereochemistry of the resulting glycosides. Strategic installation of suitable protecting groups in the C-2 position, vicinal to the anomeric carbon, renders neighbouring group participation, whereas protecting groups in the distal C-3, C-4, and C-6 positions are often claimed to exhibit remote group participation with the anomeric carbon. Neighbouring group participation and remote group participation are being widely studied to help the glycochemists design the synthetic protocols for multistep synthesis of complex oligosaccharides and in turn, standardise the process of the glycosylation towards a particular stereochemical output. While neighbouring group participation has been quite effective in achieving the required stereochemistry of the produced glycosides, remote participation exhibits comparatively less efficacy in achieving complete stereoselectivity in the glycosylation reactions. Remote participation is a still highly debated topic in the scientific community. However, implementing the participating role of the remote groups in glycosylation reactions is widely practised to achieve better stereocontrol and to facilitate the formation of synthetically challenging glycosidic linkages.

Chemoenzymatic Synthesis of Phosphosaccharides from Haemophilus parasuis Strains ER-6P (Serovar 15) and Nagasaki (Serovar 5)

Herein, we describe a chemoenzymatic and diversity-oriented approach for the first syntheses of octasaccharide repeating units of the capsular polysaccharides of Haemophilus parasuis serovar 15 and serovar 5. The synthetic method features efficient enzymatic assembly of sialyl galactose or N-acetyl-galactosamine building blocks, highly stereoselective chemical construction of α-type H-phosphonate, and the β-stereospecific 1,3-glycosylation reaction of a rare sugar donor.

A Ligand-Controlled Approach Enabling Gold(I)-Catalyzed Stereoinvertive Glycosylation with Primal Glycosyl ortho-Alkynylbenzoate Donors

A diarylurea-containing phosphine ligand-modulated stereoinvertive O-glycosylation with primal furanosyl and pyranosyl ortho-alkynylbenzoate (ABz) donors under gold(I) catalysis is disclosed. Both α- and β-configured glycosides could be obtained from the corresponding stereochemically pure β- and α-glycosyl donors with high yields and good to excellent stereoselectivities, respectively. This method accommodates a variety of glycosyl donors and alcoholic acceptors, leading to both 1,2-cis and 1,2-trans glycosidic linkages, and has been applied to the convenient preparation of a series of linear arabinan glycans. Mechanistic investigations reveal that the counteranion could bridge the diarylurea residue on the phosphine ligand with the alcoholic acceptor via hydrogen bond interactions, thereby permitting stereoinvertive displacement at the anomeric position.

Zinc(ii)-mediated stereoselective construction of 1,2-cis 2-azido-2-deoxy glycosidic linkage: assembly of Acinetobacter baumannii K48 capsular pentasaccharide derivative

The capsular polysaccharide (CPS) is a major virulence factor of the pathogenic Acinetobacter baumannii and a promising target for vaccine development. However, the synthesis of the 1,2-cis-2-amino-2-deoxyglycoside core of CPS remains challenging to date. Here we develop a highly α-selective ZnI2-mediated 1,2-cis 2-azido-2-deoxy chemical glycosylation strategy using 2-azido-2-deoxy glucosyl donors equipped with various 4,6-O-tethered groups. Among them the tetraisopropyldisiloxane (TIPDS)-protected 2-azido-2-deoxy-d-glucosyl donor afforded predominantly α-glycoside (α : β = >20 : 1) in maximum yield. This novel approach applies to a wide acceptor substrate scope, including various aliphatic alcohols, sugar alcohols, and natural products. We demonstrated the versatility and effectiveness of this strategy by the synthesis of A. baumannii K48 capsular pentasaccharide repeating fragments, employing the developed reaction as the key step for constructing the 1,2-cis 2-azido-2-deoxy glycosidic linkage. The reaction mechanism was explored with combined experimental variable-temperature NMR (VT-NMR) studies and mass spectroscopy (MS) analysis, and theoretical density functional theory calculations, which suggested the formation of covalent α-C1GlcN-iodide intermediate in equilibrium with separated oxocarbenium-counter ion pair, followed by an SN1-like α-nucleophilic attack most likely from separated ion pairs by the ZnI2-activated acceptor complex under the influence of the 2-azido gauche effect.

Chemical Synthesis and Antigenicity Evaluation of an Aminoglycoside Trisaccharide Repeating Unit of Pseudomonas aeruginosa Serotype O5 O-Antigen Containing a Rare Dimeric-ManpN3NA

Pseudomonas aeruginosa bacteria are becoming increasingly resistant against multiple antibiotics. Therefore, the development of vaccines to prevent infections with these bacteria is an urgent medical need. While the immunological activity of lipopolysaccharide O-antigens in P. aeruginosa is well-known, the specific protective epitopes remain unidentified. Herein, we present the first chemical synthesis of highly functionalized aminoglycoside trisaccharide 1 and its acetamido derivative 2 found in the P. aeruginosa serotype O5 O-antigen. The synthesis of the trisaccharide targets is based on balancing the reactivity of disaccharide acceptors and monosaccharide donors. Glycosylations were analyzed by quantifying the reactivity of the hydroxyl group of the disaccharide acceptor using the orbital-weighted Fukui function and dual descriptor. The stereoselective formation of 1,2-cis-α-fucosylamine linkages was achieved through a combination of remote acyl participation and reagent modulation. The simultaneous SN2 substitution of azide groups at C2' and C2″ enabled the efficient synthesis of 1,2-cis-β-linkages for both 2,3-diamino-D-mannuronic acids. Through a strategic orthogonal modification, the five amino groups on target trisaccharide 1 were equipped with a rare acetamidino (Am) and four acetyl (Ac) groups. Glycan microarray analyses of sera from patients infected with P. aeruginosa indicated that trisaccharides 1 and 2 are key antigenic epitopes of the serotype O5 O-antigen. The acetamidino group is not an essential determinant of antibody binding. The β-D-ManpNAc3NAcA residue is a key motif for the antigenicity of serotype O5 O-antigen. These findings serve as a foundation for the development of glycoconjugate vaccines targeting P. aeruginosa serotype O5.

Recent chemical synthesis and immunological evaluation of glycans related to bacterial lipopolysaccharides

O-Antigens and core oligosaccharides from bacterial lipopolysaccharides (LPS) are often structurally unique and immunologically active, have become attractive targets in the development of antibacterial vaccines. Structurally well-defined and pure oligosaccharides can be used in identifying protective epitopes of the carbohydrate antigens, which is important for the design of an effective vaccine. Here, the recent progress on chemical synthesis and immunological evaluation of glycans related to O-antigens and core oligosaccharides from bacterial LPS are summarized.

Chemical Synthesis of the Trisaccharide Repeating Unit of the O-Antigen of Fusobacterium nucleatum ATCC 51191

Herein, the trisaccharide repeating unit of Fusobacterium nucleatum ssp. animalis ATCC 51191, which is used to develop oncomicrobial vaccines, was efficiently synthesized for the first time. The synthetic approach featured the following: (i) construction of the 1,2-cis-glycosidic linkage using the large steric hindrance of a phthalimide group at C4 of fucosamine; (ii) synthesis of the trisaccharide via a linear [2 + 1] glycosylation strategy; and (iii) installation of l-alanine using hexafluorophosphate azabenzotriazole tetramethyl uronium as a promoter.