Synergistic Glycosylation as Key to the Chemical Synthesis of an Outer Core Octasaccharide ofHelicobacter pylori

Chemical Synthesis of an Orthogonally Protected 5,7-Diamino-3,5,7,9-tetradeoxy-d-glycero-l-gluco-2-nonulosonic Acid from N-Acetylneuraminic Acid

Bacterial nonulosonic acids (NulOs), which feature a nine-carbon backbone, are associated with the biological functions of bacterial glycans. Here, an orthogonally protected 5-amino-7-azido-3,5,7,9-tetradeoxy-d-glycero-l-gluco-2-nonulosonic acid related to Fusobacterium nucleatum ATCC 23726 NulO was synthesized from N-acetylneuraminic acid with sequential performance of C5,7 azidation, C9 deoxygenation, C4 epimerization, and N5,7 differentiation. The C5 azido group in the obtained 5,7-diazido-NulO can be regioselectively reduced to differentiate the two amino groups.

Parametric Analysis of Donor Activation for Glycosylation Reactions

The chemical synthesis of complex oligosaccharides relies on efficient and highly reproducible glycosylation reactions. The outcome of a glycosylation is contingent upon several environmental factors, such as temperature, acidity, the presence of residual moisture, as well as the steric, electronic, and conformational aspects of the reactants. Each glycosylation proceeds rapidly and with a high yield within a rather narrow temperature range. For better control over glycosylations and to ensure fast and reliable reactions, a systematic analysis of 18 glycosyl donors revealed the effect of reagent concentration, water content, protecting groups, and structure of the glycosyl donors on the activation temperature. With these insights, we parametrize the first step of the glycosylation reaction to be executed reliably and efficiently.

Michael F, Cox A, Sauvageau J. Optimization of the Synthesis and Conjugation of the Methyl Rhamnan Tip of Pseudomonas aeruginosa A-Band Polysaccharide and Immunogenicity Evaluation for the Continued Development of a Potential Glycoconjugate Vaccine

Pseudomonas aeruginosa is an antimicrobial-resistant bacterium that has no vaccine approved for human use. Additionally, it has been identified by the World Health Organization as a priority pathogen for novel vaccines and therapeutic development. We previously developed a synthetic mimic of the A-band polysaccharide tip that showed promise in terms of immunogenicity for use as a glycoconjugate vaccine. In this current manuscript, we improve upon the previous work to continue the development of this glycoconjugate vaccine. Herein, we report a higher-yielding synthesis of mimics containing a handle and a spacer that improved conjugation efficiency, resulting in better carbohydrate-to-protein ratios and also good immunogenicity of these conjugates in mice and rabbits. The data suggested that perhaps only a tetrasaccharide was required to induce an immune response capable of recognizing whole cells of P. aeruginosa.

Chemical Synthesis of a Branched Nonasaccharide Fragment from Helicobacter pylori Lipopolysaccharide

A chemical synthesis of a unique nanosaccharide fragment from Helicobacter pylori lipopolysaccharide was achieved via a convergent glycosylation method. Challenges involved in the synthesis include the highly stereoselective construction of β-3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) and two 1,2-cis-glycosidic linkages, as well as the formation of a branched 2,7-disubstituted heptose subunit. Hydrogen-bond mediated aglycone delivery strategy and benzoyl-directing remote participation effect were employed, respectively, for the efficient generation of the desired β-Kdo glycoside and 1,2-cis-α-l-fucoside/d-glucoside. Moreover, the key branched framework was successfully established through a [(7 + 1) + 1] assembly approach involving the stepwise glycosylation of the heptasaccharide alcohol with two monosaccharide donors. The synthesized 1 containing a propylamine linker at the reducing end can be covalently bound to a carrier protein for further immunological studies.

Combination of 3-O-Levulinoyl and 6-O-Trifluorobenzoyl Groups Ensures α-Selectivity in Glucosylations: Synthesis of the Oligosaccharides Related to Aspergillus fumigatus α-(1 → 3)-d-Glucan

Stereospecific α-glucosylation of primary and secondary OH-group at carbohydrate acceptors is achieved using glucosyl N-phenyl-trifluoroacetimidate (PTFAI) donor protected with an electron-withdrawing 2,4,5-trifluorobenzoyl (TFB) group at O-6 and the participating levulinoyl (Lev) group at O-3. New factors have been revealed that might explain α-stereoselectivity in the case of TFB and pentafluorobenzoyl (PFB) groups at O-6. They are of conformational nature and confirmed by DFT calculations. The potential of this donor, as well as the orthogonality of TFB and Lev protecting groups, is showcased by the synthesis of α-(1 → 3)-linked pentaglucoside corresponding to Aspergillus fumigatus α-(1 → 3)-d-glucan and of its hexasaccharide derivative, bearing β-glucosamine residue at the non-reducing end.

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.

Influence of Side Chain Conformation on the Activity of Glycosidase Inhibitors

Substrate side chain conformation impacts reactivity during glycosylation and glycoside hydrolysis and is restricted by many glycosidases and glycosyltransferases during catalysis. We show that the side chains of gluco and manno iminosugars can be restricted to predominant conformations by strategic installation of a methyl group. Glycosidase inhibition studies reveal that iminosugars with the gauche,gauche side chain conformations are 6- to 10-fold more potent than isosteric compounds with the gauche,trans conformation; a manno-configured iminosugar with the gauche,gauche conformation is a 27-fold better inhibitor than 1-deoxymannojirimycin. The results are discussed in terms of the energetic benefits of preorganization, particularly when in synergy with favorable hydrophobic interactions. The demonstration that inhibitor side chain preorganization can favorably impact glycosidase inhibition paves the way for improved inhibitor design through conformational preorganization.

The Influence of the Electron Density in Acyl Protecting Groups on the Selectivity of Galactose Formation

The stereoselective formation of 1,2-cis-glycosidic bonds is a major bottleneck in the synthesis of carbohydrates. We here investigate how the electron density in acyl protecting groups influences the stereoselectivity by fine-tuning the efficiency of remote participation. Electron-rich C4-pivaloylated galactose building blocks show an unprecedented α-selectivity. The trifluoroacetylated counterpart with electron-withdrawing groups, on the other hand, exhibits a lower selectivity. Cryogenic infrared spectroscopy in helium nanodroplets and density functional theory calculations revealed the existence of dioxolenium-type intermediates for this reaction, which suggests that remote participation of the pivaloyl protecting group is the origin of the high α-selectivity of the pivaloylated building blocks. According to these findings, an α-selective galactose building block for glycosynthesis is developed based on rational considerations and is subsequently employed in automated glycan assembly exhibiting complete stereoselectivity. Based on the obtained selectivities in the glycosylation reactions and the results from infrared spectroscopy and density functional theory, we suggest a mechanism by which these reactions could proceed.

Chemical Synthesis of the Highly Sterically Hindered Core Undecasaccharide of Helicobacter pylori Lipopolysaccharide for Antigenicity Evaluation with Human Serum

Helicobacter pylori, listed as a human carcinogen by the Department of Health and Human Services, colonizes the gastric mucosa of more than half of the world's population. The individuals infected with H. pylori have a high risk to develop chronic gastritis, peptic ulcers, and even gastric cancer. The conserved core structure of H. pylori lipopolysaccharide (LPS) has been regarded as a promising candidate structure for development of a glycoconjugate vaccine targeting multiple serotypes. Here, we report a total synthesis of the core undecasaccharide of H. pylori LPS and its subunit antigens. The match and mismatch between the glycosyl donor and acceptor caused by the inert hydroxyl groups were addressed by a judicious choice of orthogonal protection strategies and glycosylation conditions. A combination of acyl remote participation and solvent effects has been applied for selective formation of the five 1,2-cis-glucosidic bonds. The high steric hindrance induced by the high carbon sugars and trinacriform architecture required that the core undecasaccharide was synthesized through a finely tuned linear assembly [2 + (1 + (3 + (1 + (1 + 3))))] rather than convergent strategies. An antigenicity evaluation using glycan microarrays showed that an α-(1 → 6)-glucan trisaccharide is recognized by IgG antibodies in sera of H. pylori-infected patients. The phosphate group of the inner core trisaccharide key epitope is very important for IgG recognition. These findings are an important step toward designing carbohydrate-based vaccines against H. pylori.

Glycan Assembly Strategy: From Concept to Application

Glycans have been hot topics in recent years due to their exhibition of numerous biological activities. However, the heterogeneity of their natural source and the complexity of their chemical synthesis impede the progress in their biological research. Thus, the development of glycan assembly strategies to acquire plenty of structurally well-defined glycans is an important issue in carbohydrate chemistry. In this review, the latest advances in glycan assembly strategies from concepts to their applications in carbohydrate synthesis, including chemical and enzymatic/chemo-enzymatic approaches, as well as solution-phase and solid-phase/tag-assisted synthesis, are summarized. Furthermore, the automated glycan assembly techniques are also outlined.

Stereoselective Synthesis of the I-L Fragment of the Pacific Ciguatoxins

The I-L ring system found in all the Pacific ciguatoxins has been prepared from a tricyclic precursor in a highly stereoselective manner. Subtle differences in the reactivity of the enones present in the seven- and eight-membered rings of the tricyclic ether starting material have been exploited to allow selective protection of the enone in the eight-membered ring. Subsequent distereoselective allylation of the seven-membered ring has been accomplished by a palladium-mediated Tsuji-Trost reaction. The K-ring methyl and hydroxyl groups have been installed in a highly stereoselective manner by sequential conjugate reduction and enolate oxidation reactions. Ring L has been constructed by a use of a novel relay ring-closing metathesis reaction to complete the tetracyclic framework, which possesses the functionality necessary for elaboration of rings I and L and the introduction of ring M.

Chemical Synthesis Elucidates the Key Antigenic Epitope of the Autism-Related Bacterium Clostridium bolteae Capsular Octadecasaccharide

The gut pathogen Clostridium bolteae has been associated with the onset of autism spectrum disorder (ASD). To create vaccines against C. bolteae, it is important to identify exact protective epitopes of the immunologically active capsular polysaccharide (CPS). Here, a series of C. bolteae CPS glycans, up to an octadecasaccharide, was prepared. Key to achieving the total syntheses is a [2+2] coupling strategy based on a β-d-Rhap-(1→3)-α-d-Manp repeating unit that in turn was accessed by a stereoselective β-d-rhamnosylation. The 4,6-O-benzylidene-induced conformational locking is a powerful strategy for forming a β-d-mannose-type glycoside. An indirect strategy based on C2 epimerization of β-d-quinovoside was efficiently achieved by Swern oxidation and borohydride reduction. Sequential glycosylation, and regioselective and global deprotection produced the disaccharide and tetrasaccharide, up to the octadecasaccharide. Glycan microarray analysis of sera from rabbits immunized with inactivated C. bolteae bacteria revealed a humoral immune response to the di- and tetrasaccharide, but none of the longer sequences. The tetrasaccharide may be a key motif for designing glycoconjugate vaccines against C. bolteae.

Total Synthesis of Tri-, Hexa- and Heptasaccharidic Substructures of the O-Polysaccharide of Providencia rustigianii O34

A general and efficient strategy for synthesis of tri-, hexa- and heptasaccharidic substructures of the lipopolysaccharide of Providencia rustigianii O34 is described. For the heptasaccharide seven different building blocks were employed. Special features of the structures are an α-linked galactosamine and the two embedded α-fucose units, which are either branched at positions-3 and -4 or further linked at their 2-position. Convergent strategies focused on [4+3], [3+4], and [4+2+1] couplings. Whereas the [4+3] and [3+4] coupling strategies failed the [4+2+1] strategy was successful. As monosaccharidic building blocks trichloroacetimidates and phosphates were employed. Global deprotection of the fully protected structures was achieved by Birch reaction.

Conjugation of Synthetic Trisaccharide of Staphylococcus aureus Type 8 Capsular Polysaccharide Elicits Antibodies Recognizing Intact Bacterium

Staphylococcus aureus causes a wide range of life-threatening diseases. One of the powerful approaches for prevention and treatment is to develop an efficient vaccine as antibiotic resistance greatly increases. S. aureus type 8 capsular polysaccharide (CP8) has shown great potential in vaccine development. An understanding of the immunogenicity of CP8 trisaccharide repeating unit is valuable for epitope-focused vaccine design and cost-efficient vaccine production. We report the chemical synthesis of conjugation-ready CP8 trisaccharide 1 bearing an amine linker, which effectively served for immunological evaluation. The trisaccharide 1-CRM197 conjugate elicited a robust immunoglobulin G (IgG) immune response in mice. Both serum antibodies and prepared monoclonal antibodies recognized S. aureus strain, demonstrating that synthetic trisaccharide 1 can be an efficient antigen for vaccine development.

Total synthesis of theHelicobacter pyloriserotype O2 O-antigen α-(1 → 2)- and α-(1 → 3)-linked oligoglucosides

Unique α-(1 → 2)- and α-(1 → 3)-linked oligoglucosides from theH. pyloriserotype O2 O-antigen were synthesized with exclusive α-selectivity using remote participation effects.

Stereoselective Thioconjugation by Photoinduced Thiol-ene Coupling Reactions of Hexo- and Pentopyranosyl d- and l-Glycals at Low-Temperature-Reactivity and Stereoselectivity Study

A comprehensive optimization and mechanistic study on the photoinduced hydrothiolation of different d- and l- hexo- and pentoglycals with various thiols was performed, at the temperature range of RT to -120 °C. Addition of thiols onto 2-substituted hexoglycals proceeded with complete 1,2-cis-α-stereoselectivity in all cases. Hydrothiolation of 2-substituted pentoglycals resulted in mixtures of 1,2-cis-α- and -β-thioglycosides of varying ratio depending on the configuration of the reactants. Hydrothiolation of unsubstituted glycals at -80 °C proceeded with excellent yields and, except for galactal, provided the axially C2-S-linked isomers with high selectivity. Cooling was always beneficial to the efficacy, increased the yields and in most cases significantly raised the stereoselectivity. The suggested mechanism explains the different conformational preferences of the intermediate carbon-centered radicals, which is a crucial factor in the stereoselectivity of the reactions.