Synthesis of a Pentasaccharide Fragment Related to the Inner Core Region of Rhizobial and Agrobacterial Lipopolysaccharides

Glycosylation of an N-Acetylated Glucosamine Disaccharide Using an Orthogonally Protected 3-Iodo-Kdo Fluoride Donor

Kdo (3-Deoxy-d-manno-oct-2-ulosonic acid) is an essential sugar found in bacterial lipopolysaccharides with significant biomedical relevance. This study introduces an orthogonally protected 3-iodo-Kdo fluoride donor and demonstrates its coupling to a pre-synthesized β-(1→6)-linked N-acetylglucosamine disaccharide acceptor as an example. Nuclear magnetic resonance data indicates the presence of an intraresidue hydrogen bond in the distal glucosamine unit. Two complementary glycosylation approaches are explored with an emphasis on achieving high stereoselectivity and minimizing protecting-group manipulation. The orthogonal protection of 3-iodo Kdo fluoride donor offers insights into tailoring Kdo-based donors for specific biomedical applications. While yields vary depending on the approach, they are sufficient to demonstrate the donor's applicability. These findings enable the design of advanced glycomimetic constructs for therapeutic and vaccine research.

Ph3PO-Modulated Kdo Glycosidation for Stereoselective Synthesis of β-Kdo-Containing Disaccharides

A Ph3PO-modulated β-selective Kdo glycosidation approach is developed for the stereoselective synthesis of β-Kdo glycosides. With the readily available per-O-acetylated Kdo ynenoate as the donor, the glycosylation with a series of alcohols in the presence of Ph3PAuOTf and Ph3PO in toluene at low temperatures afforded the desired Kdo glycosides with good to excellent β-selectivities. Furthermore, the Ph3PO-modulated approach was effectively applied to the synthesis of β-(2→4)- and β-(2→8)-linked Kdo-Kdo disaccharides for further biological studies.

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.

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.

NMR chemical shift prediction and structural elucidation of linker-containing oligo- and polysaccharides using the computer program CASPER

Carbohydrate structures containing alkyl groups as aglycones are useful for investigating enzyme activity and glycan-protein interactions. Moreover, linker-containing oligosaccharides with a spacer group are commonly used to print glycan microarrays or to prepare protein-conjugates as vaccine candidates. The structural accuracy of these synthesized glycans are essential for interpretation of results from biological experiments in which the compounds have been used and NMR spectroscopy can unravel and confirm their structures. An approach for efficient 1H and 13C NMR chemical shift assignments employed a parallel NOAH-10 measurement followed by NMR spin-simulation to refine the 1H NMR chemical shifts, as exemplified for a disaccharide with an azidoethyl group as an aglycone, the NMR chemical shifts of which have been used to enhance the quality of CASPER (http://www.casper.organ.su.se/casper/). The CASPER program has been further developed to aid characterization of linker-containing oligo- and polysaccharides, either by chemical shift prediction for comparison to experimental NMR data or as structural investigation of synthesized glycans based on acquired unassigned NMR data. The ability of CASPER to elucidate structures of linker-containing oligosaccharides is demonstrated and comparisons to assigned or unassigned NMR data show the utility of CASPER in supporting a proposed oligosaccharide structure. Prediction of NMR chemical shifts of an oligosaccharide, corresponding to the repeating unit of an O-antigen polysaccharide, having a linker as an aglycone and a non-natural substituent derivative thereof are presented to exemplify the diversity of structures handled. Furthermore, NMR chemical shift predictions of synthesized polysaccharides, corresponding to bacterial polysaccharides, containing a linker are described showing that in addition to oligosaccharide structures also polysaccharide structures having an aglycone spacer group can be analyzed by CASPER.

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 and Antigenic Evaluation of Inner Core Oligosaccharides from Acinetobacter baumannii Lipopolysaccharide

Acinetobacter baumannii is currently posing a serious threat to global health. Lipopolysaccharide (LPS) is a potent virulence factor of pathogenic Gram-negative bacteria. To explore the antigenic properties of A. baumannii LPS, four Kdo-containing inner core glycans from A. baumannii strain ATCC 17904 were synthesized. A flexible and divergent method based on the use of the orthogonally substituted α-Kdo-(2→5)-Kdo disaccharides was developed. Selective removal of different protecting groups in these key precursors and elongation of sugar chain via α-stereocontrolled coupling with 5,7-O-di-tert-butylsilylene or 5-O-benzoyl protected Kdo thioglycosides and 2-azido-2-deoxyglucosyl thioglycoside allowed efficient assembly of the target molecules. Glycan microarray analysis of sera from infected patients revealed that the 4,5-branched Kdo trimer was a potential antigenic epitope, which is attractive for further immunological research to develop carbohydrate vaccines against A. baumannii.

Rhamnogalacturonan II: Chemical Synthesis of a Substructure Including α-2,3-Linked Kdo*

The synthesis of a fully deprotected Kdo-containing rhamnogalacturonan II pentasaccharide is described. The strategy relies on the preparation of a suitably protected homogalacturonan tetrasaccharide backbone, through a post-glycosylation oxidation approach, and its stereoselective glycosylation with a Kdo fluoride donor.

Recent progress in chemical synthesis of bacterial surface glycans

With the continuing advancement of carbohydrate chemical synthesis, bacterial glycomes have become increasingly attractive and accessible synthetic targets. Although bacteria also produce carbohydrate-containing secondary metabolites, our review here will cover recent chemical synthetic efforts on bacterial surface glycans. The obtained compounds are excellent candidates for the development of improved structurally defined glycoconjugate vaccines to combat bacterial infections. They are also important probes for investigating glycan-protein interactions. Glycosylation strategies applied for the formation of some challenging glycosidic bonds of various uncommon sugars in a number of recently synthesized bacterial surface glycans are highlighted.

Dimethylformamide-Modulated Kdo Glycosylation for Stereoselective Synthesis of α-Kdo Glycosides

A simple and direct DMF-modulated α-selective Kdo glycosylation approach for the stereoselective synthesis of the α-linked Kdo glycosides is developed. Glycosylation of the readily available peracetylated Kdo ortho-hexynylbenzoate with common acceptor alcohols using SPhosAuNTf₂ as a promoter and DMF as a modulating molecule afforded a range of Kdo glycosides with good α-selectivities. Furthermore, the present method is effectively applied in the latent-active synthesis of the α-linked di-Kdo glycoside bearing a linker at the reducing end. Finally, the first observation of a Kdo imidinium ion in the low-temperature NMR provides evidence for the plausible mechanism of the DMF-modulated α-selective Kdo glycosylation.

(2-Ketulosonyl)onate 2,3-O-thionocarbonate donors for the synthesis of KO and KDO α-glycosides and a one-pot glycosylation method for 2-keto acid donors

Bifunctional (2-ketulosonyl)onate thionocarbonates are effective donors for the synthesis of KO and KDO α-glycosides with perfect control in stereoselectivity.

Synthesis of an Undecasaccharide Featuring an Oligomannosidic Heptasaccharide and a Bacterial Kdo-lipid A Backbone for Eliciting Neutralizing Antibodies to Mammalian Oligomannose on the HIV-1 Envelope Spike

Lipooligosaccharides (LOS) from the bacterium Rhizobium radiobacter Rv3 are structurally related to antigenic mammalian oligomannoses on the HIV-1 envelope glycoprotein spike that are targets for broadly neutralizing antibodies. Here, we prepared a hybrid structure of viral and bacterial epitopes as part of a vaccine design strategy to elicit oligomannose-specific HIV-neutralizing antibodies using glycoconjugates based on the Rv3 LOS structure. Starting from a Kdo2GlcNAc2 tetrasaccharide precursor, a central orthogonally protected mannose trichloroacetimidate donor was coupled to OH-5 of the innermost Kdo residue. To assemble larger glycans, the N-acetylamino groups of the glucosamine units were converted to imides to prevent formation of unwanted imidate byproducts. Blockwise coupling of the pentasaccharide acceptor with an α-(1→2)-linked mannotriosyl trichloroacetimidate donor introduced the D1-arm fragment. Glycosylation of O-6 of the central branching mannose with an α-(1→2)-α-(1→6)-linked mannotriosyl trichloroacetimidate donor unit then furnished the undecasaccharide harboring a D3-arm extension. Global deprotection yielded the 3-aminopropyl ligand, which was activated as an isothiocyanate or adipic acid succinimidoyl ester and conjugated to CRM197. However, representative oligomannose-specific HIV-neutralizing antibodies bound the undecasaccharide conjugates poorly. Possible reasons for this outcome are discussed herein along with paths for improvement.

Comparative Antigenicity of Thiourea and Adipic Amide Linked Neoglycoconjugates Containing Modified Oligomannose Epitopes for the Carbohydrate-Specific anti-HIV Antibody 2G12

Novel neoglycoproteins containing oligomannosidic penta- and heptasaccharides as structural variants of oligomannose-type N-glycans found on human immunodeficiency virus type 1 gp120 have been prepared using different conjugation methods. Two series of synthetic ligands equipped with 3-aminopropyl spacer moieties and differing in the anomeric configuration of the reducing mannose residue were activated either as isothiocyanates or as adipic acid succinimidoyl esters and coupled to bovine serum albumin. Coupling efficiency for adipic acid connected neoglycoconjugates was better than for the thiourea-linked derivatives; the latter constructs, however, exhibited higher reactivity toward antibody 2G12, an HIV-neutralizing antibody with exquisite specificity for oligomannose-type glycans. 2G12 binding avidities for the conjugates, as determined by Bio-Layer Interferometry, were mostly higher for the β-linked ligands and, as expected, increased with the numbers of covalently linked glycans, leading to approximate KD values of 10 to 34 nM for optimized ligand-to-BSA ratios. A similar correlation was observed by enzyme-linked immunosorbent assays. In addition, dendrimer-type ligands presenting trimeric oligomannose epitopes were generated by conversion of the amino-spacer group into a terminal azide, followed by triazole formation using "click chemistry". The severe steric bulk of the ligands, however, led to poor efficiency in the coupling step and no increased antibody binding by the resulting neoglycoconjugates, indicating that the low degree of substitution and the spatial orientation of the oligomannose epitopes within these trimeric ligands are not conducive to multivalent 2G12 binding.

Synthesis and Stereocontrolled Equatorially Selective Glycosylation Reactions of a Pseudaminic Acid Donor: Importance of the Side-Chain Conformation and Regioselective Reduction of Azide Protecting Groups

Pseudaminic acid is an amino deoxy sialic acid whose glycosides are essential components of many pathogenic Gram-negative bacterial cell walls including those from Pseudomonas aeruginosa, Vibrio cholerae, Campylobacter jejuni, Campylobacter coli, Vibrio vulnificus, and Pseudoalteromonas distincta. The study of pseudaminic acid glycosides is however hampered by poor availability from nature and the paucity of good synthetic methods and limited to no understanding of the factors controlling stereoselectivity. Conformational analysis of the side chains of various stereoisomeric sialic acids suggested that the side chain of pseudaminic acid would take up the most electron-withdrawing trans, gauche-conformation, as opposed to the gauche, gauche conformation of N-acetyl neuraminic acid and the gauche, trans-conformtion of 7- epi N-acetyl neuraminic acid, leading to the prediction of high equatorial selectivity. This prediction is borne out by the synthesis of a suitably protected pseudaminic acid donor from N-acetyl neuraminic acid in 20 steps and 5% overall yield and by the exquisite equatorial selectivity it displays in coupling reactions with typical glycosyl acceptors. The selectivity of the glycosylation reactions is further buttressed by the development and implementation of conditions for the regioselective release of the two amines from the corresponding azides, such as required for the preparation of the lipopolysaccharides. These findings open the way to the synthesis and study of pseudaminic acid-based bacterial lipopolysaccharides and, importantly in the broader context of glycosylation reactions in general, underline the significant role played by side-chain conformation in the control of reactivity and selectivity.