Synthesis of orthogonally protected bacterial, rare-sugar and D-glycosamine building blocks

Total Synthesis of the Conjugation-Ready Tetrasaccharide Repeating Unit of Shewanella japonica Type Strain KMM 3299T

Here we report the first total synthesis of the conjugation-ready tetrasaccharide repeating unit of Shewanella japonica type strain KMM 3299T. The presence of rare deoxyamino sugars and installation of three consecutive 1,2-cis glycosidic linkages makes the synthesis formidable. The challenging late-stage oxidation was overcome by using a galacturonate donor. The total synthesis was completed via a longest linear sequence of 22 steps in an overall yield of 3.5% starting from d-mannose.

Chemical Synthesis of a Densely Functionalized Aminodisaccharide from Fusobacterium nucleatum ATCC 23726 O-Antigen

Fusobacterium nucleatum, a colorectal-cancer-associated oncomicrobe, can trigger or accelerate numerous pathologies. We report the first synthesis of a conjugation-ready disaccharide containing six amino groups from F. nucleatum ATCC 23726 O-antigen. Rare 2,3-diamido-d-glucuronic acid amide and 2-acetamido-4-amino-d-fucose were synthesized from d-glucosamine through configuration inversion, nucleophilic substitution, C6 oxidation, and C6 deoxygenation. A judicious choice of protecting groups and reaction conditions enabled the selective installation of N-acetyl, N-propanoyl, N-formyl, and carboxamido groups.

Total Synthesis of Vibrio Cholerae O43 Tetrasaccharide Repeating Unit

Vibrio cholerae is a pathogen responsible for the deadly pandemic - cholera. The glycans present on the surface of various strains of V. cholerae are considered as potential vaccine candidates. The tetrasaccharide repeating unit (RU) of V. cholerae O43 is decorated with less-explored rare deoxy amino sugars like d-quinosamine and d-viosamine, along with a rare amino acid, N-acetyl-l-allothreonine. Herein, we report a detailed account of the total synthesis of V. cholerae O43 tetrasaccharide RU. In our earlier attempt, while a one-pot assembly of trisaccharide was successful, the final coupling with a fully functionalized d-viosamine donor was low yielding. The successful route involved employing the Fmoc-protected d-viosamine building block as a donor and a late-stage amide bond formation of the tetrasaccharide.

Circumventing aglycon transfer en route to the synthesis of pentasaccharide thioglycoside donor for the chain extension of Plesiomonas shigelloides strain 302-73 (serotype O1) repeating unit

Herein we report a chemical synthesis of a pentasaccharide thioglycoside repeating unit of Plesiomonas shigelloides Strain 302-73 (Serotype O1), as a chain extension unit. In our synthetic endeavor we encountered multiple aglycon transfer reactions during glycosylations. This problem was obviated by employing a PMP group as a transient protecting group.

Total Synthesis of a Conjugation-Ready Tetrasaccharide Repeating Unit of Vibrio cholerae O:3 O-antigen Polysaccharide

Herein, we report the first total synthesis of the tetrasaccharide repeating unit of Vibrio cholerae O:3 O-antigen polysaccharide. The highly complex tetrasaccharide contains rare amino sugars such as d-bacillosamine and l-fucosamine, highly labile sugar ascarylose, and higher carbon sugar d-d-heptose. Stereoselective glycosylation of the notoriously reactive ascarylose with d-d-heptose, poor nucleophilicity of the axial C4-OH of l-fucosamine, and amide coupling are the key challenges encountered in the total synthesis, which was completed via a longest linear sequence of 23 steps in 4.2% overall yield.

Reagent-controlled chemo/stereoselective glycosylation of ʟ-fucal to access rare deoxysugars

2,6-Dideoxy sugars constitute an important class of anticancer antibiotics natural products and serve as essential medicinal tools for carbohydrate-based drug discovery and vaccine development. In particular, 2-deoxy ʟ-fucose or ʟ-oliose is a rare sugar and vital structural motif of several potent antifungal and immunosuppressive bioactive molecules. Herein, we devised a reagent-controlled stereo and chemoselective activation of ʟ-fucal, enabling the distinctive glycosylation pathways to access the rare ʟ-oliose and 2,3-unsaturated ʟ-fucoside. The milder oxo-philic Bi(OTf)3 catalyst induced the direct 1,2-addition predominantly, whereas B(C6F5)3 promoted the allylic Ferrier-rearrangement of the enol-ether moiety in ʟ-fucal glycal donor, distinguishing the competitive mechanisms. The reagent-tunable modular approach is highly advantageous, employing greener catalysts and atom-economical transformations, expensive ligand/additive-free, and probed for a diverse range of substrates comprising monosaccharides, amino-acids, bioactive natural products, and drug scaffolds embedded with susceptible or labile functionalities.

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.

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

De Novo Synthetic Approach to 2,4-Diamino-2,4,6-trideoxyhexoses (DATDH): Bacterial and Rare Deoxy-Amino Sugars

A synthetic route to 2,4-diamino-2,4,6-trideoxysugar stereoisomers in 6-7 steps and 22-33% overall yield is described. A key step in this pathway is the carbonyl coupling of d- and l-threoninol or d- and l-allo-threoninol to a phthalimido-allene mediated by chiral iridium-H8-BINAP, which allows for installation of two new chiral centers in one, highly diastereoselective (>20:1 dr) step. This approach provides a more concise, diastereoselective, and versatile method to access these deoxy-amino sugars than is currently available.

Thioglycosides Act as Metabolic Inhibitors of Bacterial Glycan Biosynthesis

Glycans that coat the surface of bacteria are compelling antibiotic targets because they contain distinct monosaccharides that are linked to pathogenesis and are absent in human cells. Disrupting glycan biosynthesis presents a path to inhibiting the ability of a bacterium to infect the host. We previously demonstrated that O-glycosides act as metabolic inhibitors and disrupt bacterial glycan biosynthesis. Inspired by a recent study which showed that thioglycosides (S-glycosides) are 10 times more effective than O-glycosides at inhibiting glycan biosynthesis in mammalian cells, we crafted a panel of S-glycosides based on rare bacterial monosaccharides. The novel thioglycosides altered glycan biosynthesis and fitness in pathogenic bacteria but had no notable effect on glycosylation or growth in beneficial bacteria or mammalian cells. In contrast to findings in mammalian cells, S-glycosides and O-glycosides exhibited comparable potency in bacteria. However, S-glycosides exhibited enhanced selectivity relative to O-glycosides. These novel metabolic inhibitors will allow selective perturbation of the bacterial glycocalyx for functional studies and set the stage to expand our antibiotic arsenal.

Total Synthesis of a Structurally Complex Tetrasaccharide Repeating Unit of Vibrio cholerae O43

Herein we report the first total synthesis of a densely functionalized tetrasaccharide repeating unit of Vibrio cholerae O43, which contains rare deoxy amino sugars d-quinovosamine and d-viosamine attached with the rare amino acid N-acetyl-l-allothreonine. Synthesis of orthogonally protected rare sugars and unnatural amino acid building blocks, stereoselective construction of three consecutive 1,2-cis glycosidic linkages, amide coupling, and the presence of five nitrogen atoms dispersed over four sugar units as well as the carboxylic acid functionality make the total synthesis a formidable task.

Total Synthesis of the Repeating Units of Proteus penneri 26 and Proteus vulgaris TG155 via a Common Disaccharide

Herein, we report the first total synthesis of the trisaccharide and tetrasaccharide repeating units of P. penneri 26 and P. vulgaris TG155, respectively, having a common disaccharide unit, 3-α-l-QuipNAc-(1 → 3)-α-d-GlcpNAc-(1 →. Striking features of the targets are the presence of rare sugar units, l-quinovosamine and l-rhamnosamine, all joined through α-glycosidic linkages. Major challenges in the formation of 1,2-cis glycosidic linkages in the case of d-glucosamine, l-quinovosamine, and d-galactosamine have been addressed.

Total Synthesis and Structural Studies of Zwitterionic Bacteroides fragilis Polysaccharide A1 Fragments

Zwitterionic polysaccharides (ZPSs) are exceptional carbohydrates, carrying both positively charged amine groups and negatively charged carboxylates, that can be loaded onto MHC-II molecules to activate T cells. It remains enigmatic, however, how these polysaccharides bind to these receptors, and to understand the structural features responsible for this "peptide-like" behavior, well-defined ZPS fragments are required in sufficient quantity and quality. We here present the first total synthesis of Bacteroides fragilis PS A1 fragments encompassing up to 12 monosaccharides, representing three repeating units. Key to our successful syntheses has been the incorporation of a C-3,C-6-silylidene-bridged "ring-inverted" galactosamine building block that was designed to act as an apt nucleophile as well as a stereoselective glycosyl donor. Our stereoselective synthesis route is further characterized by a unique protecting group strategy, built on base-labile protecting groups, which has allowed the incorporation of an orthogonal alkyne functionalization handle. Detailed structural studies have revealed that the assembled oligosaccharides take up a bent structure, which translates into a left-handed helix for larger PS A1 polysaccharides, presenting the key positively charged amino groups to the outside of the helix. The availability of the fragments and the insight into their secondary structure will enable detailed interaction studies with binding proteins to unravel the mode of action of these unique oligosaccharides at the atomic level.

Scalable Synthesis of Versatile Rare Deoxyamino Sugar Building Blocks from d-Glucosamine

We report the syntheses of 1,3,4-tri-O-acetyl-2-amino-2,6-dideoxy-β-d-glucopyranose and allyl 2-amino-2,6-dideoxy-β-d-glucopyranoside from d-glucosamine hydrochloride. The potential of these two versatile scaffolds as key intermediates to a diversity of orthogonally protected rare deoxyamino hexopyranosides is exemplified in the context of fucosamine, quinovosamine, and bacillosamine. The critical C-6 deoxygenation step to 2,6-dideoxy aminosugars is performed at an early stage on a precursor featuring an imine moiety or a trifluoroacetamide moiety in place of the 2-amino group, respectively. Robustness and scalability are demonstrated for a combination of protecting groups and incremental chemical modifications that sheds light on the promise of the yet unreported allyl 2,6-dideoxy-2-N-trifluoroacetyl-β-d-glucopyranoside when addressing the feasibility of synthetic zwitterionic oligosaccharides. In particular, allyl 3-O-acetyl-4-azido-2,4,6-trideoxy-2-trifluoroacetamido-β-d-galactopyranoside, an advanced 2-acetamido-4-amino-2,4,6-trideoxy-d-galactopyranose building block, was achieved on the 30 g scale from 1,3,4,6-tetra-O-acetyl-β-d-glucosamine hydrochloride in 50% yield and nine steps, albeit only two chromatography purifications.

Total Synthesis of Trisaccharide Repeating Unit of Staphylococcus aureus Strain M

An efficient total synthesis of a conjugation-ready trisaccharide repeating unit of Staphylococcus aureus strain M is reported here. The main challenges involved in this synthesis are the procurement of rare sugars (d-FucNAc and d-GalNAcA) and installation of consecutive 1,2-cis-glycosidic linkages between them. Stereoselective 1,2-cis glycosylation with the linker acceptor was achieved with easily accessible benzylidene protected d-galactosamine thioglycoside by employing a DMF modulated preactivation glycosylation method. The consecutive 1,2-cis linkages were installed with the help of solvent participation. The carboxylic acid functionality was introduced via postglycosylation oxidation on the disaccharide moiety. The total synthesis of trisaccharide repeating unit was accomplished with the longest linear sequence of 24 steps in 4.5% overall yield.

Total Synthesis of Trisaccharide Repeating Unit of Staphylococcus aureus Type 8 (CP8) Capsular Polysaccharide

Herein, we report a highly efficient total synthesis of Staphylococcus aureus type 8 trisaccharide repeating unit in a lesser number of steps and high stereoselectivity. The complex trisaccharide contains rare amino sugars, viz., d-fucosamine, l-fucosamine, and 2-acetamido d-mannuronic acid. The installation of consecutive sterically hindered 1,2-cis glycosidic linkages, especially β-mannosylation, is the key challenge in this synthesis. The total synthesis of target molecule was completed via a longest linear sequence of 18 steps in 7.1% overall yield.

Sulfamate-Tethered Aza-Wacker Cyclization Strategy for the Syntheses of 2-Amino-2-deoxyhexoses: Preparation of Orthogonally Protected d-Galactosamines

We present a new strategy for the assembly of protected d-galactosamine synthons. Our route uses a sulfamate-tethered aza-Wacker cyclization as a key step and commences from d-erythrono-1,4-lactone. This stands in contrast to most literature syntheses of 2-amino-2-deoxyhexose derivatives, as these generally employ glycals or hexoses as starting materials. This strategy may serve as a template for the assembly of many other 2-amino-2-deoxyhexoses with protection patterns difficult to access by conventional methods.

Chemical tools to study bacterial glycans: a tale from discovery of glycoproteins to disruption of their function

Bacteria coat themselves with a dense array of cell envelope glycans that enhance bacterial fitness and promote survival. Despite the importance of bacterial glycans, their systematic study and perturbation remains challenging. Chemical tools have made important inroads toward understanding and altering bacterial glycans. This review describes how pioneering discoveries from Prof. Carolyn Bertozzi's laboratory inspired our laboratory to develop sugar probes to facilitate the study of bacterial glycans. As described below, we used metabolic glycan labelling to install bioorthogonal reporters into bacterial glycans, ultimately permitting the discovery of a protein glycosylation system, the identification of glycosylation genes, and the development of metabolic glycan inhibitors. Our results have provided an approach to screen bacterial glycans and gain insight into their function, even in the absence of detailed structural information.

Synthetic Zwitterionic Streptococcus pneumoniae Type 1 Oligosaccharides Carrying Labile O-Acetyl Esters

We herein report the first total synthesis of the Streptococcus pneumoniae serotype 1 (Sp1) oligosaccharide, a unique zwitterionic capsular polysaccharide carrying labile O-acetyl esters. The target oligosaccharides, featuring rare α-2,4-diamino-2,4,6-trideoxy galactose (AAT) and α-galacturonic acids, were assembled up to the 9-mer level, in a highly stereoselective manner using trisaccharide building blocks. The lability of the O-acetyl esters imposed a careful deprotection scheme to prevent migration and hydrolysis. The migration was investigated in detail at various pD values using NMR spectroscopy, to show that migration and hydrolysis of the C-3-O-acetyl esters readily takes place under neutral conditions. Structural investigation showed the oligomers to adopt a right-handed helical structure with the acetyl esters exposed on the periphery of the helix in close proximity of the neighboring AAT residues, thereby imposing conformational restrictions on the AATα1-4GalA(3OAc) glycosidic linkages, supporting the helical shape of the polysaccharide, that has been proposed to be critical for its unique biological activity.

Protecting group enabled stereocontrolled approach for rare-sugars talose/gulose via dual-ruthenium catalysis

We herein report a convenient and highly stereocontrolled approach for rare and vital ᴅ-talo and ᴅ-gulo sugars directly from economical ᴅ-galactal through dual ruthenium-catalysis. The stereo-divergent strategy involves Ru(III)Cl₃-catalyzed Ferrier glycosylation of ᴅ-galactal to give 2,3-unsaturated ᴅ-galactopyranoside, further selective functionalization of C-4 and C-6 position with diverse protecting groups and dihydroxylation with Ru(VIII)O₄ generated in situ providing access to talo/gulo isomers. The α-anomeric stereoselectivity and syn-diastereoselectivity in glycosylation-dihydroxylation steps have been predominantly achieved by judicious selection of stereoelectronically diverse protecting groups. The synthetic utility of the dual-ruthenium catalysis was demonstrated for efficiently assembling the ᴅ-talose and/or ᴅ-gulose sugars in natural products and bioactive scaffolds.

Small Carbohydrate Derivatives as Potent Antibiofilm Agents

Biofilm formation by most pathogenic bacteria is considered as one of the key mechanisms associated with virulence and antibiotic resistance. Biofilm-forming bacteria adhere to the surfaces of biological or implant medical devices and create communities within their self-produced extracellular matrix that are difficult to treat by existing antibiotics. There is an urgent need to synthesize and screen structurally diverse molecules for their antibiofilm activity that can remove or minimize the bacterial biofilm. The development of carbohydrate-based small molecules as antibiofilm agents holds a great promise in addressing the problem of the eradication of biofilm-related infections. Owing to their structural diversity and specificity, the sugar scaffolds are valuable entities for developing antibiofilm agents. In this perspective, we discuss the literature pertaining to carbohydrate-based natural antibiofilm agents and provide an overview of the design, activity, and mode of action of potent synthetic carbohydrate-based molecules.

Facile and Scalable Route to Access Rare Deoxy Amino Sugars for Nonulosonic Acid Aldolase Biosynthesis

We presented a facile and scalable route for the synthesis of di-azido sugars via one-pot double inversion of the mono-benzoyl sugars by TBAN₃ and studied the dependency pattern between solvent and protecting groups as well as the configuration of the neighboring and leaving groups. Moreover, we developed a chemical synthetic strategy for pseudaminic acid precursors (11 steps in 49%). Furthermore, we discussed the configuration of nonulosonic acid precursors for specificity of PseI and PmNanA enzymatic synthesis, permitting us to synthesize new nonulosonic acid derivatives for accessing Pse isomers.

Total Synthesis of the Repeating Units of O-Specific Polysaccharide of Pseudomonas chlororaphis subsp. aureofaciens UCM B-306 via One-Pot Glycosylation

Herein we report the first total syntheses of the trisaccharide-repeating units of Pseudomonas chlororaphis subsp. aureofaciens UCM B-306 via a one-pot assembly of the core trisaccharide structure. The rare-sugar-containing trisaccharide-repeating units are comprised of d-bacillosamine, 2-amino-2-deoxy-d-galacturonic acid or amide, and d-rhamnose units linked through three consecutive α-linkages. The total syntheses of two repeating units were completed starting from d-mannose via a longest-linear sequence of 27 steps in 5.8% and 4.4% overall yields, respectively.

Synthesis of Oligosaccharides Resembling the Streptococcus suis Serotype 18 Capsular Polysaccharide as a Basis for Glycoconjugate Vaccine Development

Here we report the first total synthesis of several oligosaccharides resembling the capsular polysaccharide of swine pathogen S. suis serotype 18 repeating unit [→3)-d-GalNAc(α1-3)[d-Glc(β1-2)]-d-GalA4OAc(β1-3)-d-GalNAc(α1-3)-d-BacNAc4NAc(α1→]_n. Access to the pentasaccharide repeating unit antigen proved to be very challenging due to the poor reactivity in the context of the trisaccharide. The challenge was overcome by the creation of a galacturonic acid in a late stage of the synthesis.

General Strategy for the Synthesis of Rare Sugars via Ru(II)-Catalyzed and Boron-Mediated Selective Epimerization of 1,2-trans-Diols to 1,2-cis-Diols

Human glycans are primarily composed of nine common sugar building blocks. On the other hand, several hundred monosaccharides have been discovered in bacteria and most of them are not readily available. The ability to access these rare sugars and the corresponding glycoconjugates can facilitate the studies of various fundamentally important biological processes in bacteria, including interactions between microbiota and the human host. Many rare sugars also exist in a variety of natural products and pharmaceutical reagents with significant biological activities. Although several methods have been developed for the synthesis of rare monosaccharides, most of them involve lengthy steps. Herein, we report an efficient and general strategy that can provide access to rare sugars from commercially available common monosaccharides via a one-step Ru(II)-catalyzed and boron-mediated selective epimerization of 1,2-trans-diols to 1,2-cis-diols. The formation of boronate esters drives the equilibrium toward 1,2-cis-diol products, which can be immediately used for further selective functionalization and glycosylation. The utility of this strategy was demonstrated by the efficient construction of glycoside skeletons in natural products or bioactive compounds.

Total Synthesis of the Repeating Unit of Streptococcus pneumoniae Zwitterionic Polysaccharide Sp1

Reported herein is the total synthesis of the trisaccharide repeating unit of Streptococcus pneumoniae zwitterionic polysaccharide Sp1 containing a rare sugar, 2-acetamido-4-amino-2,4,6-trideoxy-d-galactose (AAT), and three consecutive 1,2-cis-glycosidic linkages. The total synthesis was completed via highly stereoselective glycosylations and late-stage oxidation as key steps involving a longest linear sequence of 21 steps with 4.4% overall yield.

Total Synthesis of the Phosphorylated Zwitterionic Trisaccharide Repeating Unit of Photorhabdus temperata cinerea 3240

Herein, we report the total synthesis of the phosphorylated zwitterionic trisaccharide repeating unit of Photorhabdus temperata subsp. cinerea 3240. The efficient route involves regio- and stereoselective assembly of trisaccharide with rare deoxyamino sugar AAT at the nonreducing end, late stage oxidation, and installation of a phosphate linker on the trisaccharide. The total synthesis was completed via a longest linear sequence of 24 steps in 6.5% overall yield.

Total Synthesis of the All-Rare Sugar-Containing Pentasaccharide Repeating Unit of the O-Polysaccharide of Plesiomonas shigelloides Strain 302-73 (Serotype O1)

First total synthesis of the conjugation-ready pentasaccharide repeating unit of Plesiomonas shigelloides strain 302-73 (serotype O1) is reported. The complex target pentasaccharide is composed of all-rare amino sugars such as orthogonally functionalized d-bacillosamine, l-fucosamine, and l-pneumosamine linked through four consecutive α-linkages. The poor nucleophilicity of axial 4-OH of l-fucosamine and stereoselective glycosylations are the key challenges in the total synthesis, which was completed via a longest linear sequence of 27 steps in 3% overall yield.

Serendipitous one-pot synthesis of chiral dienes from pyranosidic 2,4-bistriflates

Attempted nucleophilic displacements of l-rhamnosyl 2,4-bistriflates led to serendipitous formation of a chiral diene via competing cascade eliminations. The reaction also followed the same pathway with d-rhamnosyl and d-mannosyl 2,4-bistriflates substrates providing access to dienes with opposite stereochemistry. The reaction presumably proceeds through E2 elimination of C2 triflate followed by allylic rearrangement. The easily accessible chiral dienes would be useful in the synthesis of natural products.

Synthesis of L-hexoses: an Update

Over the years, carbohydrates have increasingly become an important class of compounds contributing significantly to the target specific drug discovery and vaccine development. Several oligosaccharides contain L-hexoses that are biologically relevant as therapeutic and diagnostic tools. Since, L-hexoses and deoxy L-hexoses are not readily available in large amount and pure form, attention is drawn towards development of cost effective and high yielding synthetic routes for their procurement. In this review we give an update on the recent developments in strategies for synthesis of L-hexoses and deoxy L-hexoses.

Total Synthesis of the Repeating Unit of Bacteroides fragilis Zwitterionic Polysaccharide A1

Zwitterionic polysaccharides isolated from commensal bacteria are endowed with unique immunological properties and are emerging as immunotherapeutic agents as well as vaccine carriers. Reported herein is a total synthesis of the repeating unit of Bacteroides fragilis zwitterionic polysaccharide A1 (PS A1). The structurally complex tetrasaccharide unit contains a rare sugar 2-acetamido-4-amino-2,4,6-trideoxy-d-galactose (AAT) and two consecutive 1,2-cis glycosidic linkages. The repeating unit was efficiently assembled by rapid synthesis of d-galactosamine and AAT building blocks from cheap and abundant d-mannose via a one-pot S_N2 displacement of 2,4-bistriflates and installation of all of the glycosidic bonds in a highly stereoselective manner. The total synthesis involves a longest linear sequence of 17 steps with 3.47% overall yield.

Exploratory N-Protecting Group Manipulation for the Total Synthesis of Zwitterionic Shigella sonnei Oligosaccharides

Shigella sonnei surface polysaccharides are well-established protective antigens against this major cause of diarrhoeal disease. They also qualify as unique zwitterionic polysaccharides (ZPSs) featuring a disaccharide repeating unit made of two 1,2-trans linked rare aminodeoxy sugars, a 2-acetamido-2-deoxy-l-altruronic acid (l-AltpNAcA) and a 2-acetamido-4-amino-2,4,6-trideoxy-d-galactopyranose (AAT). Herein, the stereoselective synthesis of S. sonnei oligosaccharides comprising two, three and four repeating units is reported for the first time. Several sets of up to seven protecting groups were explored, shedding light on the singular conformational behavior of protected altrosamine and altruronic residues. A disaccharide building block equipped with three distinct N-protecting groups and featuring the uronate moiety already in place was designed to accomplish the iterative high yielding glycosylation at the axial 4-OH of the altruronate component and achieve the challenging full deprotection step. Key to the successful route was the use of a diacetyl strategy whereby the N-acetamido group of the l-AltpNAcA is masked in the form of an imide.

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.

Chemical Synthesis and Immunological Evaluation of a Pentasaccharide Bearing Multiple Rare Sugars as a Potential Anti-pertussis Vaccine

With the infection rate of Bordetella pertussis at a 60-year high, there is an urgent need for new anti-pertussis vaccines. The lipopolysaccharide (LPS) of B. pertussis is an attractive antigen for vaccine development. With the presence of multiple rare sugars and unusual glycosyl linkages, the B. pertussis LPS is a highly challenging synthetic target. In this work, aided by molecular dynamics simulation and modeling, a pertussis-LPS-like pentasaccharide was chemically synthesized for the first time. The pentasaccharide was conjugated with a powerful carrier, bacteriophage Qβ, as a vaccine candidate. Immunization of mice with the conjugate induced robust anti-glycan IgG responses with IgG titers reaching several million enzyme-linked immunosorbent assay (ELISA) units. The antibodies generated were long lasting and boostable and could recognize multiple clinical strains of B. pertussis, highlighting the potential of Qβ-glycan as a new anti-pertussis vaccine.

Synthesis of the pentasaccharide repeating unit of the O-antigen from Enterobacter cloacae C4115 containing the rare α-d-FucNAc

Total synthesis of the pentasaccharide repeating unit associated with the O-antigen of Enterobacter cloacae C4115 is reported. The synthesis of the said oligosaccharide was accomplished through rational protecting group manipulations on commercially available monosaccharides followed by stereoselective glycosylations either by activation of thioglycosides or glycosyl trichloroacetimidates and was found to be productive. Towards the synthesis of the rare sugar unit, α-d-FucNAc in this case, it was established that the methoxymethyl (MOM) group is advantageous over the earlier reported tetrahydro pyran (THP) protection. The effect of MOM-protection was successfully tested for the synthesis of a rare sugar synthon which can serve as a precursor to the rare d-fucosamine residue.

Total Synthesis of the Lipid-Anchor-Attached Core Trisaccharides of Lipoteichoic Acids of Streptococcus pneumoniae and Streptococcus oralis Uo5

Herein we report an efficient total synthesis of lipid-anchor-appended core trisaccharides of lipoteichoic acids of Streptococcus pneumoniae and Streptococcus oralis Uo5. The key features include the expedient synthesis of the rare sugar 2,4,6-trideoxy-2-acetamido-4-amino-d-Galp building block via one-pot sequential S_N2 reactions and the α-selective coupling of d-thioglucoside with the diacyl glycerol acceptor to construct a common disaccharide acceptor, which was utilized in the total synthesis of target molecules 1 and 2.

Metabolic inhibitors of bacterial glycan biosynthesis

The bacterial cell wall is a quintessential drug target due to its critical role in colonization of the host, pathogen survival, and immune evasion. The dense cell wall glycocalyx contains distinctive monosaccharides that are absent from human cells, and proper assembly of monosaccharides into higher-order glycans is critical for bacterial fitness and pathogenesis. However, the systematic study and inhibition of bacterial glycosylation enzymes remains challenging. Bacteria produce glycans containing rare deoxy amino sugars refractory to traditional glycan analysis, complicating the study of bacterial glycans and the creation of glycosylation inhibitors. To ease the study of bacterial glycan function in the absence of detailed structural or enzyme information, we crafted metabolic inhibitors based on rare bacterial monosaccharide scaffolds. Metabolic inhibitors were assessed for their ability to interfere with glycan biosynthesis and fitness in pathogenic and symbiotic bacterial species. Three metabolic inhibitors led to dramatic structural and functional defects in Helicobacter pylori. Strikingly, these inhibitors acted in a bacteria-selective manner. These metabolic inhibitors will provide a platform for systematic study of bacterial glycosylation enzymes not currently possible with existing tools. Moreover, their selectivity will provide a pathway for the development of novel, narrow-spectrum antibiotics to treat infectious disease. Our inhibition approach is general and will expedite the identification of bacterial glycan biosynthesis inhibitors in a range of systems, expanding the glycochemistry toolkit.

Synthesis of trehalose glycolipids

Chemical synthesis of trehalose glycolipids such as DAT, TDM, SL-1, SL-3, and Ac₂SGL from MTb, emmyguyacins from fungi, succinoyl trehalose from rhodococcus, and maradolipids from worms, as well as mycobacterial oligosaccharides is reviewed.

Synthetic, Zwitterionic Sp1 Oligosaccharides Adopt a Helical Structure Crucial for Antibody Interaction

The zwitterionic Streptococcus pneumoniae serotype 1 polysaccharide (Sp1) is an important anchor point for our immune system to act against streptococcal infections. Antibodies can recognize Sp1 saccharides, and it has been postulated that Sp1 can elicit a T-cell-dependent immune reaction as it can be presented by MHC-II molecules. To unravel the molecular mode of action of this unique polysaccharide we here describe the chemical synthesis of a set of Sp1 fragments, ranging from 3 to 12 monosaccharides in length. We outline a unique synthetic approach to overcome the major synthetic challenges associated with the complex Sp1 structure and provide a stereoselective route of synthesis for the oligosaccharide backbone as well as a strategy to introduce the carboxylic acid functions. Molecular dynamics (MD) simulations together with NMR spectroscopy studies reveal that the oligosaccharides take up helical structures with the nona- and dodecasaccharide completing a full helical turn. The 3D structure of the oligosaccharides coincides with the topology required for good interaction with anti-Sp1 antibodies, which has been mapped in detail using STD-NMR. Our study has revealed the Sp1 nona- and dodecasaccharides as promising synthetic antigens, displaying all (3D) structural elements required to mimic the natural polysaccharide and required to unravel the molecular mode of action of these unique zwitterionic polysaccharides.