Synthesis of a 2,3,4-Triglycosylated Rhamnoside Fragment of Rhamnogalacturonan-II Side Chain A Using a Late Stage Oxidation Approach

Mild and General Protocol for Selective Deacetylation of Acetyl/Benzoyl-Protected Carbohydrates

Herein, we report a mild and general protocol for chemoselective deacetylation of mixed acetyl- and benzoyl-protected carbohydrates under mild acidic conditions. The protocol allows quick access to partially protected carbohydrates, which serve as versatile synthetic intermediates during the total synthesis of various mono- and oligosaccharide targets. The applicability of the developed protocol was successfully demonstrated on a range of carbohydrate substrates of various configurations and substitution patterns featuring functionalized aliphatic and aromatic aglycones. The protocol has shown excellent compatibility with the widely used O-anomeric protecting groups, prespacer aglycones, and thioglycoside glycosyl donors.

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.

TEMPO-mediated late stage photochemical hydroxylation of biaryl sulfonium salts

The late stage photochemical hydroxylation of biaryl sulfonium salts was enabled with a TEMPO derivative as a simple oxygen source, in metal free conditions. The scope and mechanism of this exceptionally simple synthetic method, which constructs important arylated phenols from aromatic C-H bonds, are herein discussed.

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.

Structural and functional analyses of glycoside hydrolase 138 enzymes targeting chain A galacturonic acid in the complex pectin rhamnogalacturonan II

The metabolism of carbohydrate polymers drives microbial diversity in the human gut microbiome. The selection pressures in this environment have spurred the evolution of a complex reservoir of microbial genes encoding carbohydrate-active enzymes (CAZymes). Previously, we have shown that the human gut bacterium Bacteroides thetaiotaomicron (Bt) can depolymerize the most structurally complex glycan, the plant pectin rhamnogalacturonan II (RGII), commonly found in the human diet. Previous investigation of the RGII-degrading apparatus in Bt identified BT0997 as a new CAZyme family, classified as glycoside hydrolase 138 (GH138). The mechanism of substrate recognition by GH138, however, remains unclear. Here, using synthetic substrates and biochemical assays, we show that BT0997 targets the d-galacturonic acid-α-1,2-l-rhamnose linkage in chain A of RGII and that it absolutely requires the presence of a second d-galacturonic acid side chain (linked β-1,3 to l-rhamnose) for activity. NMR analysis revealed that BT0997 operates through a double displacement retaining mechanism. We also report the crystal structure of a BT0997 homolog, BPA0997 from Bacteroides paurosaccharolyticus, in complex with ligands at 1.6 Å resolution. The structure disclosed that the enzyme comprises four domains, including a catalytic TIM (α/β)8 barrel. Characterization of several BT0997 variants identified Glu-294 and Glu-361 as the catalytic acid/base and nucleophile, respectively, and we observed a chloride ion close to the active site. The three-dimensional structure and bioinformatic analysis revealed that two arginines, Arg-332 and Arg-521, are key specificity determinants of BT0997 in targeting d-galacturonic acid residues. In summary, our study reports the first structural and mechanistic analyses of GH138 enzymes.

Synthesis of the Highly Branched Hexasaccharide Core of Chlorella Virus N-Linked Glycans

Chlorella viruses produce N-linked glycoproteins with carbohydrate moieties that differ in structure from all other N-linked glycans. In addition, unlike most viruses, these organisms do not hijack the biosynthetic machinery of the host to make glycocoproteins; instead, they produce their own carbohydrate-processing enzymes. A better understanding of the function and assembly of these fascinating and structurally-unprecedented glycans requires access to probe molecules. This work describes the first synthesis of a chlorella virus N-linked glycan, a highly branched hexasaccharide that contains the pentasaccharide present in all of the >15 structures reported to date. The target molecule includes a glucosyl-asparagine linkage and a "hyperbranched" fucose residue in which all of the hydroxyl groups are glycosylated. Both convergent and linear approaches were investigated with the latter being successful in providing the target in 16 steps and 13 % overall yield.

Effect of temperature modulations on TEMPO-mediated regioselective oxidation of unprotected carbohydrates and nucleosides

Regioselective oxidation of unprotected and partially protected oligosaccharides is a much sought-after goal. Herein, we report a notable improvement in the efficiency of TEMPO-catalyzed oxidation by modulating the temperature of the reaction. Mono-, di-, and tri-saccharides are oxidized regioselectively in yields of 75 to 92%. The present method is simple to implement and is also applicable for selective oxidations of other mono- and poly-hydroxy compounds including unprotected and partially protected nucleosides.

Synthetic plant glycans

For more than a century the primary carbon source for the production of fuels, chemicals and many materials has been fossil resources. Recently, plant polysaccharides from non-food biomass have emerged as a promising renewable alternative that may displace a significant fraction of petroleum-derived products. As a food source, plant polysaccharides can provide beneficial effects on the human immune system in the form of dietary fiber. Despite the strong impact of plant glycans on society and human health, their chemical synthesis remains largely unexplored compared to the synthesis of mammalian and bacterial glycans. Synthetic glycans such as described in this review provide an important toolbox for studying the role of carbohydrates in plant biology and their interaction with human health.

Complex pectin metabolism by gut bacteria reveals novel catalytic functions

The metabolism of carbohydrate polymers drives microbial diversity in the human gut microbiota. It is unclear, however, whether bacterial consortia or single organisms are required to depolymerize highly complex glycans. Here we show that the gut bacterium Bacteroides thetaiotaomicron uses the most structurally complex glycan known: the plant pectic polysaccharide rhamnogalacturonan-II, cleaving all but 1 of its 21 distinct glycosidic linkages. The deconstruction of rhamnogalacturonan-II side chains and backbone are coordinated to overcome steric constraints, and the degradation involves previously undiscovered enzyme families and catalytic activities. The degradation system informs revision of the current structural model of rhamnogalacturonan-II and highlights how individual gut bacteria orchestrate manifold enzymes to metabolize the most challenging glycan in the human diet.

Boronic esters as protective groups in carbohydrate chemistry: processes for acylation, silylation and alkylation of glycoside-derived boronates

Boronic esters are employed in streamlined protocols for protection, functionalization and deprotection of glycosides, avoiding isolation and purification of intermediates.

Protection against Experimental Melioidosis with a Synthetic manno-Heptopyranose Hexasaccharide Glycoconjugate

Melioidosis is an emerging infectious disease caused by Burkholderia pseudomallei and is associated with high morbidity and mortality rates in endemic areas. Antibiotic treatment is protracted and not always successful; even with appropriate therapy, up to 40% of individuals presenting with melioidosis in Thailand succumb to infection. In these circumstances, an effective vaccine has the potential to have a dramatic impact on both the scale and the severity of disease. Currently, no vaccines are licensed for human use. A leading vaccine candidate is the capsular polysaccharide consisting of a homopolymer of unbranched 1→3 linked 2-O-acetyl-6-deoxy-β-d-manno-heptopyranose. Here, we present the chemical synthesis of this challenging antigen using a novel modular disaccharide assembly approach. The resulting hexasaccharide was coupled to the nontoxic H_c domain of tetanus toxin as a carrier protein to promote recruitment of T-cell help and provide a scaffold for antigen display. Mice immunized with the glycoconjugate developed IgM and IgG responses capable of recognizing native capsule, and were protected against infection with over 120 × LD₅₀ of B. pseudomallei strain K96243. This is the first report of the chemical synthesis of an immunologically relevant and protective hexasaccharide fragment of the capsular polysaccharide of B. pseudomallei and serves as the rational starting point for the development of an effective licensed vaccine for this emerging infectious disease.

Synthesis of oligosaccharide fragments of the rhamnogalacturonan of Nerium indicum

Three trisaccharides, one pentasaccharide, and one heptasaccharide, namely α-D-GalA-(1→2)-α-L-Rha-(1→4)-β-D-GalA-OC3H7 (1), α-L-Rha-(1→4)-α-D-GalA-(1→4)-β-D-GalA-OC3H7 (2), α-D-GalA-(1→4)-α-D-GalA-(1→2)-α-L-Rha-OC3H7 (3), α-D-GalA-(1→2)-α-L-Rha-(1→4)-α-D-GalA-(1→2)-α-L-Rha-(1→4)-β-D-GalA-OC3H7 (4), and α-D-GalA-(1→2)-α-L-Rha-(1→4)-α-D-GalA-(1→2)-α-L-Rha-(1→4)-α-D-GalA-(1→2)-α-L-Rha-(1→4)-β-D-GalA-OC3H7 (5), which are relevant to the fragments of the rhamnogalacturonan of Nerium indicum, were concisely synthesized. The syntheses feature highly stereoselective formation of the α-D-GalA-linkage with GalA N-phenyltrifluoroacetimidates as donors.

Carbohydrate microarrays in plant science

Almost all plant cells are surrounded by glycan-rich cell walls, which form much of the plant body and collectively are the largest source of biomass on earth. Plants use polysaccharides for support, defense, signaling, cell adhesion, and as energy storage, and many plant glycans are also important industrially and nutritionally. Understanding the biological roles of plant glycans and the effective exploitation of their useful properties requires a detailed understanding of their structures, occurrence, and molecular interactions. Microarray technology has revolutionized the massively high-throughput analysis of nucleotides, proteins, and increasingly carbohydrates. Using microarrays, the abundance of and interactions between hundreds and thousands of molecules can be assessed simultaneously using very small amounts of analytes. Here we show that carbohydrate microarrays are multifunctional tools for plant research and can be used to map glycan populations across large numbers of samples to screen antibodies, carbohydrate binding proteins, and carbohydrate binding modules and to investigate enzyme activities.

Synthesis of apiose-containing oligosaccharide fragments of the plant cell wall: fragments of rhamnogalacturonan-II side chains A and B, and apiogalacturonan

Fragments of pectic polysaccharides rhamnogalacturonan-II (RG-II) and apiogalacturonan were synthesised using p-tolylthio apiofuranoside derivatives as key building blocks. Apiofuranose thioglycosides can be conveniently prepared by cyclization of the corresponding dithioacetals possessing a 2,3-O-isopropylidene group, which is required for preservation of the correct (3R) configuration of the apiofuranose ring. The remarkable stability of this protecting group in apiofuranose derivatives requires its replacement with a more reactive protecting group, such as a benzylidene acetal which was used in the synthesis of trisaccharide β-Rhap-(1→3')-β-Apif-(1→2)-α-GalAp-OMe. The X-ray crystal structure of the protected precursor of this trisaccharide has been elucidated.

Uronic acids in oligosaccharide and glycoconjugate synthesis

This chapter describes the assembly of uronic acid containing oligosaccharides and glycoconjugates. Two strategies are available to access these target molecules, namely a pre-glycosylation oxidation approach, in which uronic acid building blocks are used, and a post-glycosylation oxidation strategy, which employs an oxidation step after the assembly of the oligosaccharide chain. Because uronic acid building blocks are generally considered to be less reactive than their non-oxidized counterparts, the latter approach has found most application in carbohydrate synthesis. With the aid of selected examples of recent syntheses of biologically relevant oligosaccharides and glycoconjugates, the reactivity of different uronic acid building blocks is evaluated. From these examples it is apparent that the generally assumed low reactivity of uronic acids does not a priori rule out an efficient assembly of these target compounds. Besides influencing the reactivity of a given pyranoside, the C-5 carboxylic acid function can also have a profound effect on the stereochemical course of a glycosylation reaction, which can be exploited in the stereoselective formation of glycosidic bonds.

Silver(I) oxide-mediated regioselective 2-monoacylation in 3-O-benzyl-α-l-rhamnopyranosides and application in synthesis of a protected tetrasaccharide fragment of potent cytotoxic saponins gleditsiosides C and D

The axial 2-hydroxyl group of methyl and allyl 3-O-benzyl-alpha-L-rhamnopyranosides was selectively acylated in 56-78% yields by reaction with 1.1 equiv of acyl chloride in the presence of 1.5 equiv of silver(I) oxide. Use of the method permitted a convenient synthesis of a protected tetrasaccharide fragment of triterpene saponins gleditsiosides C and D.

A facile method for oxidation of primary alcohols to carboxylic acids and its application in glycosaminoglycan syntheses

A convenient two-step, one-pot procedure was developed for the conversion of primary alcohols to carboxylic acids. The alcohol was first treated with NaOCl and TEMPO under phase-transfer conditions, followed by NaClO2 oxidation in one pot. This reaction is applicable to a wide range of alcohols and the mild reaction conditions are compatible with many sensitive functional groups, including electron-rich aromatic rings, acid-labile isopropylidene ketal and glycosidic linkages, and oxidation-prone thioacetal, p-methoxybenzyl, and allyl moieties. Several glycosaminoglycans such as heparin, chondroitin, and hyaluronic acid oligosaccharides have been synthesized in high yields by using this new oxidation protocol.

Highly efficient syntheses of hyaluronic acid oligosaccharides

Highly efficient syntheses of hyaluronic acid oligosaccharides have been accomplished through the pre-activation based iterative one-pot strategy. A series of oligosaccharides ranging from di- to hexasaccharides were rapidly assembled using only near stoichiometric amounts of the building blocks without aglycon adjustment or purifications of intermediate oligosaccharides. Deprotection and oxidation protocols were developed for protective group removal and oxidation-state adjustment. The availability of such structurally well defined synthetic hyaluronic acid oligosaccharides will greatly facilitate the establishment of detailed structure-function relationships.

Direct oxidation of sugar nucleotides to the corresponding uronic acids: TEMPO and platinum-based procedures

The direct oxidation of UDP-alpha-d-glucose and UDP-N-acetyl-alpha-d-glucosamine to the corresponding uronic acids was explored using either TEMPO or platinum-catalysed oxidation with molecular oxygen. Whilst TEMPO-based procedures gave rise to substantial over-oxidation and/or degradation of UDP-glucose, oxidation of UDP-N-acetyl-glucosamine to UDP-N-acetyl-glucosaminuronic acid was achieved with >90% conversion and ca. 65% isolated yield using a platinum-catalysed procedure.