Chemical Synthesis and Antigenic Evaluation of Inner Core Oligosaccharides from Acinetobacter baumannii Lipopolysaccharide

Additive-assisted synthesis of α-Kdo glycosides with peracetylated glycosyl ynenoate as a donor

A DMF-modulated glycosylation approach for the stereoselective synthesis of α-Kdo glycosides with readily accessible peracetylated Kdo ynenoate as a donor was described. By utilizing this approach, we completed the synthesis of various linkage types of Kdo-Kdo disaccharides and the α-Kdo-containing protected trisaccharide variant relevant to the lipopolysaccharide of Coxiella burnetii strain Nine Mile.

Serum antibody screening using glycan arrays

Humans and other animals produce a diverse collection of antibodies, many of which bind to carbohydrate chains, referred to as glycans. These anti-glycan antibodies are a critical part of our immune systems' defenses. Whether induced by vaccination or natural exposure to a pathogen, anti-glycan antibodies can provide protection against infections and cancers. Alternatively, when an immune response goes awry, antibodies that recognize self-glycans can mediate autoimmune diseases. In any case, serum anti-glycan antibodies provide a rich source of information about a patient's overall health, vaccination history, and disease status. Glycan microarrays provide a high-throughput platform to rapidly interrogate serum anti-glycan antibodies and identify new biomarkers for a variety of conditions. In addition, glycan microarrays enable detailed analysis of the immune system's response to vaccines and other treatments. Herein we review applications of glycan microarray technology for serum anti-glycan antibody profiling.

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.

Stereocontrolled Synthesis of α-3-Deoxy-d-manno-oct-2-ulosonic Acid (α-Kdo) Glycosides Using C3-p-Tolylthio-Substituted Kdo Donors: Access to Highly Branched Kdo Oligosaccharides

3-Deoxy-d-manno-oct-2-ulosonic acid (Kdo) is an eight-carbon monosaccharide found widely in bacterial lipopolysaccharides (LPSs) and capsule polysaccharides (CPSs). We developed an indirect method for the stereoselective synthesis of α-Kdo glycosides with a C3-p-tolylthio-substituted Kdo phosphite donor. The presence of the p-tolylthio group enhanced the reactivity, suppressed the formation of elimination by-products (2,3-enes), and provided complete α-stereocontrol. A variety of Kdo α-glycosides were synthesized by our method in excellent yields (up to 98 %). After glycosylation, the p-tolylthio group can be efficiently removed by free-radical reduction. Subsequently, the orthogonality of the phosphite donor and thioglycoside donor was demonstrated by the one-pot synthesis of a trisaccharide in Helicobacter pylori and Neisseria meningitidis LPS. Moreover, an efficient total synthesis route to the challenging 4,5-branched Kdo trisaccharide in LPSs from several A. baumannii strains was highlighted. To demonstrate the high reactivity of our approach further, the highly crowded 4,5,7,8-branched Kdo pentasaccharide was synthesized as a model molecule for the first time. Additionally, the reaction mechanism was investigated by DFT calculations.

Total Synthesis of Conjugation-Ready Tetrasaccharide Repeating Units of a Multidrug-Resistant Pathogen Acinetobacter baumannii Strain 34 and O5

Herein, we report the first total synthesis of conjugation-ready tetrasaccharide repeating units of Acinetobacter baumannii strain 34 and O5 comprising a common disaccharide motif [α-l-FucpNAc-(1→4)-α-d-GalpNAcA]. The installation of 1,2-cis linkages employing a disarmed 2-azido-d-galacturonic acid derivative as the donor is addressed here. The synthesis of the tetrasaccharide repeating units of A. baumannii strain 34 and O5 is accomplished via the longest linear sequences of 19 steps in 9.8% and 21 steps in 8.4% overall yields, respectively.

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

Herein, we described the first synthesis of the pentasaccharide and decasaccharide of the A. baumannii ATCC 17961 O-antigen for developing a synthetic carbohydrate-based vaccine against A. baumannii infection. The efficient synthesis of the rare sugar 2,3-diacetamido-glucuronate was achieved using our recently introduced organocatalytic glycosylation method. We found, for the first time, that long-range levulinoyl group participation via a hydrogen bond can result in a significantly improved β-selectivity in glycosylations. This solves the stereoselectivity problem of highly branched galactose acceptors. The proposed mechanism was supported by control experiments and DFT computations. Benefiting from the long-range levulinoyl group participation strategy, the pentasaccharide donor and acceptor were obtained via an efficient [2+1+2] one-pot glycosylation method and were used for the target decasaccharide synthesis.

Orthogonal Glycosylation with Phosphate Acceptors for Expeditious Synthesis of Bacterial Inner Core Oligosaccharides

We report a practical one-pot glycosylation strategy for synthesis of bacterial inner core oligosaccharides that composed of unavailable L-glycero-D-manno and D-glycero-D-manno-heptopyranose components. The glycosylation method features a new orthogonal glycosylation procedure; whereby a phosphate acceptor is coupled with a thioglycosyl donor producing a disaccharide phosphate, which can be engaged in another orthogonal glycosylation procedure to couple with a thioglycosyl acceptor. The phosphate acceptors used in above one-pot procedure are directly prepared from thioglycosyl acceptors via the in-situ phosphorylation. Such phosphate acceptor preparation protocol eliminates the traditional protection and deprotection procedures. Based on the new one-pot glycosylation strategy, two partial inner core structures of Yersinia pestis lipopolysaccharide and Haemophilus ducreyi lipooligosaccharide were acquired.

A Bioactive Synthetic Outer-Core Oligosaccharide Derived from a Klebsiella pneumonia Lipopolysaccharide for Bacteria Recognition

There is an urgent need for new treatment options for carbapenem-resistant Klebsiella pneumoniae (K. pneumoniae), which is a common cause of life-threatening hospital- and community-acquired infections. Prophylactic or therapeutic vaccination may offer an approach to control these infections, however, none has yet been approved for human use. Here, we report the chemical synthesis of an outer core tetra- and pentasaccharide derived from the lipopolysaccharide of K. pneumoniae. The oligosaccharides were equipped with an aminopentyl linker, which facilitated conjugation to the carrier proteins CRM197 and BSA. Mice immunized with the glycoconjugate vaccine candidates elicited antibodies that recognized isolated LPS as well as various strains of K. pneumoniae. The successful preparation of the oligosaccharides relied on the selection of monosaccharide building blocks equipped with orthogonal hydroxyl and amino protecting groups. It allowed the differentiation of three types of amines of the target compounds and the installation of a crowded 4,5-branched Kdo moiety.

Chemical synthesis of oligosaccharides and their application in new drug research

Oligosaccharides are the ubiquitous molecules of life. In order to translate human bioglycosylation into clinical applications, homogeneous samples of oligosaccharides and glycoconjugates can be obtained by chemical, enzymatic or other biological methods for systematic studies. However, the structural complexity and diversity of glycans and their conjugates present a major challenge for the synthesis of such molecules. This review summarizes the chemical synthesis methods of oligosaccharides, the application of oligosaccharides in the field of medicinal chemistry according to their related biological activities, and shows the great prospect of oligosaccharides in the field of pharmaceutical chemistry.

ortho-Methoxycarbonylethynylphenyl Thioglycosides (MCEPTs): Versatile Glycosyl Donors Enabled by Electron-Withdrawing Substituents and Catalyzed by Gold(I) or Cu(II) Complexes

With easily accessible and operator-friendly reagents, shelf-stable ortho-methoxycarbonylethynylphenyl thioglycosides were efficiently prepared. Based on these MCEPT glycoside donors, a novel glycosylation protocol featuring mild and catalytic promotion conditions with Au(I) or Cu(II) complexes, expanded substrate scope encompassing challenging donors and acceptors and clinically used pharmaceuticals, and versatility in various strategies for highly efficient synthesis of glycosides has been established. The practicality of the MCEPT glycosylation protocol was fully exhibited by highly efficient and scalable synthesis of surface polysaccharide subunits of Acinetobacter baumannii via latent-active, reagent-controlled divergent orthogonal one-pot and orthogonal one-pot strategies. The underlying reaction mechanism was investigated systematically through control reactions, leading to the isolation and characterization of the vital catalyst species in MCEPT glycosylation, the benzothiophen-3-yl-gold(I) complex. Based on the results obtained both from control reactions and from studies leading to the glycosylation protocol establishment, an operative mechanism was proposed and the effect of the vital catalyst species reactivity on the results of metal-catalyzed alkyne-containing donor-involved glycosylation was disclosed. Moreover, the mechanism for C-glycosylation side product formation from ortho-(substituted)ethynylphenyl thioglycoside donors with electron-donating substituents was also illuminated.

Total Synthesis of a Pentasaccharide O-Glycan from Acinetobacter baumannii

Acinetobacter baumannii is a Gram-negative bacteria associated with drug resistance and infection in healthcare settings. An understanding of both the biological roles and antigenicity of surface molecules of this organism may provide an important step in the prevention and treatment of infection through vaccination or the development of monoclonal antibodies. With this in mind, we have performed the multistep synthesis of a conjugation-ready pentasaccharide O-glycan from A. baumannii with a longest linear synthetic sequence of 19 steps. This target is particularly relevant due to its role in both fitness and virulence across an apparently broad range of clinically relevant strains. Synthetic challenges include formulating an effective protecting group scheme as well as the installation of a particularly difficult glycosidic linkage between the anomeric position of a 2,3-diacetamido-2,3-dideoxy-D-glucuronic acid and the 4-position of D-galactose.

Chemical Synthesis of the Nonreducing Hexasaccharide Fragment of Axinelloside A Based on a Stepwise Glycosylation Approach

An expedient synthesis of the nonreducing hexasaccharide fragment of axinelloside A has been completed via a linear stepwise glycosylation approach. Challenges involved in the synthesis include the highly stereoselective construction of five consecutive 1,2-cis-glycosidic linkages and the formation of a sterically crowded 2,3-disubstituted l-fucoside subunit. Protecting group-directing glycosylation strategies such as the remote participation effect of the benzoyl substituent and the stereocontrolling effect of the 4,6-O-benzylidene group were employed for the synthesis of the desired 1,2-cis-glycosidic linkages. Moreover, the 2,3-branched l-fucoside framework was established through a 3-O and then 2-O glycosylation sequence in which the 3-hydroxyl group of the core l-fucose unit was glycosylated first and then the 2-hydroxyl. The synthetic hexasaccharide is properly protected, so it can be employed as a precursor to synthesize its natural form.

Antibiotics and Carbohydrate-Containing Drugs Targeting Bacterial Cell Envelopes: An Overview

Certain bacteria constitute a threat to humans due to their ability to escape host defenses as they easily develop drug resistance. Bacteria are classified into gram-positive and gram-negative according to the composition of the cell membrane structure. Gram-negative bacteria have an additional outer membrane (OM) that is not present in their gram-positive counterpart; the latter instead hold a thicker peptidoglycan (PG) layer. This review covers the main structural and functional properties of cell wall polysaccharides (CWPs) and PG. Drugs targeting CWPs are discussed, both noncarbohydrate-related (β-lactams, fosfomycin, and lipopeptides) and carbohydrate-related (glycopeptides and lipoglycopeptides). Bacterial resistance to these drugs continues to evolve, which calls for novel antibacterial approaches to be developed. The use of carbohydrate-based vaccines as a valid strategy to prevent bacterial infections is also addressed.