Gold(I)-Catalyzed Glycosylation with Glycosyl o-Alkynylbenzoates as Donors

Palladium-Catalyzed O-Glycosylation through π-π Interactions

A stereodivergent synthesis of β- and α-O-glycosides using 3-O-quinaldoyl glucals was developed by palladium catalysis at 60 and 110 °C respectively. Various alcohols, monosaccharides, and amino acid were glycosylated to form β- and α- products in good yields with high stereoselectivity. Mechanistic studies indicated no classic Pd-N (quinoline) coordination, but π-π stacking interactions promoted the anomeric stereodiversity. The practicality was demonstrated by glycosylating natural products/drugs and synthesizing a complex tetrasaccharide.

Total Synthesis of Nona-decasaccharide Motif from Ganoderma sinense Polysaccharide Enabled by Modular and One-Pot Stereoselective Glycosylation Strategy

Polysaccharides from a medicinal fungus Ganoderma sinense represent important and adjunctive therapeutic agents for treating various diseases, including leucopenia and hematopoietic injury. However, the synthetic accessibility to long, branched, and complicated carbohydrates chains from Ganoderma sinense polysaccharides remains a challenging task in chemical synthesis. Here, we report the modular chemical synthesis of nona-decasaccharide motif from Ganoderma sinense polysaccharide GSPB70-S with diverse biological activities for the first time through one-pot stereoselective glycosylation strategy on the basis of glycosyl ortho-(1-phenyvinyl)benzoates, which not only sped up carbohydrates synthesis but also reduced chemical waste and avoided aglycones transfer issues inherent to one-pot glycosylation on the basis of thioglycosides. The synthetic route also highlights the following key steps: (1) preactivation-based one-pot glycosylation for highly stereoselective constructions of several 1,2-cis-glycosidic linkages, including three α-d-GlcN-(1 → 4) linkages and one α-d-Gal-(1 → 4) bond via the reagent N-methyl-N-phenylformamide modulation; (2) orthogonal one-pot assembly of 1,2-trans-glycosidic linkages in various linear and branched glycans fragments by strategic combinations of glycosyl N-phenyltrifluoroacetimidates, glycosyl ortho-alkynylbenzoates, and glycosyl ortho-(1-phenyvinyl)benzoates; and (3) the final [1 × 4 + 15] Yu glycosylation for efficient assembly of nona-decasaccharide target. Additionally, shorter sequences of 4-mer, 5-mer, and 6-mer are also prepared for structure-activity relationship biological studies. The present work shows that this one-pot stereoselective glycosylation strategy can offer a reliable and effective means to streamline chemical synthesis of long, branched, and complex carbohydrates with many 1,2-cis-glycosidic bonds.

Stereocontrolled Synthesis of Aryl C-Nucleosides under Ambient Conditions

A stereocontrolled synthesis of an aryl C-nucleoside has been developed using D-ribals and arylboronic acids catalyzed by palladium without additional ligands in common solvents under an open-air atmosphere at room temperature. This protocol features very mild conditions, simplicity in operation, exclusive β-stereoselectivity, broad substrate scopes, and good compatibility with reactive amino and hydroxyl groups. The functionalization of unsaturated C-nucleosides and the late-stage glycosylation of natural products/drugs demonstrated the high practicality of this strategy.

Chemical synthesis and functional evaluation of glycopeptides and glycoproteins containing rare glycosyl amino acid linkages

Covering: 1987 to 2023Naturally existing glycoproteins through post-translational protein glycosylation are highly heterogeneous, which not only impedes the structure-function studies, but also hinders the development of their potential medical usage. Chemical synthesis represents one of the most powerful tools to provide the structurally well-defined glycoforms. Being the key step of glycoprotein synthesis, glycosylation usually takes place at serine, threonine, and asparagine residues, leading to the predominant formation of the O- and N-glycans, respectively. However, other amino acid residues containing oxygen, nitrogen, sulfur, and nucleophilic carbon atoms have also been found to be glycosylated. These diverse glycoprotein linkages, occurring from microorganisms to plants and animals, play also pivotal biological roles, such as in cell-cell recognition and communication. The availability of these homogenous rare glycopeptides and glycoproteins can help decipher the glyco-code for developing therapeutic agents. This review highlights the chemical approaches for assembly of the functional glycopeptides and glycoproteins bearing these "rare" carbohydrate-amino acid linkages between saccharide and canonical amino acid residues and their derivatives.

Transition-Metal-Catalyzed Transformations for the Synthesis of Marine Drugs

Transition metal catalysis has contributed to the discovery of novel methodologies and the preparation of natural products, as well as new chances to increase the chemical space in drug discovery programs. In the case of marine drugs, this strategy has been used to achieve selective, sustainable and efficient transformations, which cannot be obtained otherwise. In this perspective, we aim to showcase how a variety of transition metals have provided fruitful couplings in a wide variety of marine drug-like scaffolds over the past few years, by accelerating the production of these valuable molecules.

Reaction Rate and Stereoselectivity Enhancement in Glycosidations with O-Glycosyl Trihaloacetimidate Donors due to Catalysis by a Lewis Acid-Nitrile Cooperative Effect

Activation of O-glycosyl trihaloacetimidate glycosyl donors with AuCl3 as a catalyst and pivalonitrile (tBuCN) as a ligand led to excellent glycosidation results in terms of yield and anomeric selectivity. In this way, various β-d-gluco- and β-d-galactopyranosides were obtained conveniently and efficiently. Experimental studies and density functional theory (DFT) calculations, in order to elucidate the reaction course, support formation of the tBuCN-AuCl2-OR(H)+ AuCl4- complex as a decisive intermediate in the glycosidation event. Proton transfer from this acceptor complex to the imidate nitrogen leads to donor activation. In this way, guided by the C-2 configuration of the glycosyl donor, the alignment of the acceptor complex enforces the stereoselective β-glycoside formation in an intramolecular fashion, thus promoting also a fast reaction course. The high stereocontrol of this novel 'Lewis acid-nitrile cooperative effect' is independent of the glycosyl donor anomeric configuration and without the support of neighboring group or remote group participation. The power of the methodology is shown by a successful glycoalkaloid solamargine synthesis.

Microwave-assisted synthesis of highly sulfated mannuronate glycans as potential inhibitors against SARS-CoV-2

Algae-based marine carbohydrate drugs are typically decorated with negative ion groups such as carboxylate and sulfate groups. However, the precise synthesis of highly sulfated alginates is challenging, thus impeding their structure-activity relationship studies. Herein we achieve a microwave-assisted synthesis of a range of highly sulfated mannuronate glycans with up to 17 sulfation sites by overcoming the incomplete sulfation due to the electrostatic repulsion of crowded polyanionic groups. Although the partially sulfated tetrasaccharide had the highest affinity for the receptor binding domain (RBD) of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant, the fully sulfated octasaccharide showed the most potent interference with the binding of the RBD to angiotensin-converting enzyme 2 (ACE2) and Vero E6 cells, indicating that the sulfated oligosaccharides might inhibit the RBD binding to ACE2 in a length-dependent manner.

A Strain-Promoted Divergent Chemical Steroidation Unveils Potent Anti-Inflammatory Pseudo-Steroidal Glycosides

The development of novel agents with immunoregulatory effects is a keen way to combat the growing threat of inflammatory storms to global health. To synthesize pseudo-steroidal glycosides tethered by ether bonds with promising immunomodulatory potential, we develop herein a highly effective deoxygenative functionalization of a novel steroidal donor (steroidation) facilitated by strain-release, leveraging cost-effective and readily available Sc(OTf)3 catalysis. This transformation produces a transient steroid-3-yl carbocation which readily reacts with O-, C-, N-, S-, and P-nucleophiles to generate structurally diverse steroid derivatives. DFT calculations were performed to shed light on the mechanistic details of the regioselectivity, underlying an acceptor-dependent steroidation mode. This approach can be readily extended to the etherification of sugar alcohols to enable the achievement of a diversity-oriented, pipeline-like synthesis of pseudo-steroidal glycosides in good to excellent yields with complete stereo- and regiospecific control for anti-inflammatory agent discovery. Immunological studies have demonstrated that a meticulously designed cholesteryl disaccharide can significantly suppress interleukin-6 secretion in macrophages, exhibiting up to 99% inhibition rates compared to the negative control. These findings affirm the potential of pseudo-steroidal glycosides as a prospective category of lead agents for the development of novel anti-inflammatory drugs.

Metal-Assisted Carbohydrate Assembly

The sequence-controlled assembly of nucleic acids and amino acids into well-defined superstructures constitutes one of the most revolutionary technologies in modern science. The elaboration of such superstructures from carbohydrates, however, remains elusive and largely unexplored on account of their intrinsic constitutional and configurational complexity, not to mention their inherent conformational flexibility. Here, we report the bottom-up assembly of two classes of hierarchical superstructures that are formed from a highly flexible cyclo-oligosaccharide─namely, cyclofructan-6 (CF-6). The formation of coordinative bonds between the oxygen atoms of CF-6 and alkali metal cations (i) locks a myriad of flexible conformations of CF-6 into a few rigid conformations, (ii) bridges adjacent CF-6 ligands, and (iii) gives rise to the multiple-level assembly of three extended frameworks. The hierarchical superstructures present in these frameworks have been shown to modulate their nanomechanical properties. This research highlights the unique opportunities of constructing convoluted superstructures from carbohydrates and should encourage future endeavors in this underinvestigated field of science.

Expeditious Synthesis of Gwanakoside A and the Chloronaphthol Glycoside Congeners

The synthesis of gwanakoside A, a chlorinated naphthol bis-glycoside, and its analogues was achieved through stepwise chlorination and donor-equivalent controlled regioselective phenol glycosylation with glycosyl N-phenyltrifluoroacetimidates as donors. Gwanakoside A displayed considerable inhibitory effects against various cancer cells and Staphylococcus aureus strains.

Iodosylbenzene-Promoted Glycosylation with Selenoglycosides: Application in One-Pot Glycosylation

A novel method for the glycosylation of selenoglycosides activated by iodosylbenzene was developed. The glycosylation reaction conditions were mild, fast, and efficient, with a high tolerance to diverse protecting groups and a wide substrate scope, which is advantageous for synthesizing complex glycosides. In addition, selenoglycosides were shown to be orthogonal to thioglycosides under the promotion of iodosylbenzene. Notably, a high yield of the poorly reactive glucuronidation reaction product was obtained by acetyl-protected selenoglycoside. Finally, the orthogonal one-pot synthesis of β-(1→6) oligoglucans demonstrated the usefulness of this method in oligosaccharide synthesis.

Direct Synthesis of α- and β-2'-Deoxynucleosides with Stereodirecting Phosphine Oxide via Remote Participation

2'-Deoxynucleosides and analogues play a vital role in drug development, but their preparation remains a significant challenge. Previous studies have focused on β-2'-deoxynucleosides with the natural β-configuration. In fact, their isomeric α-2'-deoxynucleosides also exhibit diverse bioactivities and even better metabolic stability. Herein, we report that both α- and β-2'-deoxynucleosides can be prepared with high yields and stereoselectivity using a remote directing diphenylphosphinoyl (DPP) group. It is particularly efficient to prepare α-2'-deoxynucleosides with an easily accessible 3,5-di-ODPP donor. Instead of acting as a H-bond acceptor on a 2-(diphenylphosphinoyl)acetyl (DPPA) group in our previous studies for syn-facial O-glycosylation, the phosphine oxide moiety here acts as a remote participating group to enable highly antifacial N-glycosylation. This proposed remote participation mechanism is supported by our first characterization of an important 1,5-briged P-heterobicyclic intermediate via variable-temperature NMR spectroscopy. Interestingly, antiproliferative assays led to a α-2'-deoxynucleoside with IC50 values in the low micromole range against central nervous system tumor cell lines SH-SY5Y and LN229, whereas its β-anomer exhibited no inhibition at 100 μM. Furthermore, the DPP group significantly enhanced the antitumor activities by 10 times.

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.

Synthesis of an Immunologically Active Heptamannoside of Mycobacterium tuberculosis by the [Au]/[Ag]-Catalyzed Activation of Ethynylcyclohexyl Glycosyl Carbonate Donor

Tuberculosis (TB) is one of the most dreadful diseases, killing more than 3 million humans annually. M. tuberculosis (MTb) is the causative agent for TB and has a thick and waxy cell wall, making it an attractive target for immunological studies. In this study, a heptamannopyranoside containing 1 → 2 and 1 → 6 α-mannopyranosidic linkages has been explored for the immunological evaluations. The conjugation-ready heptamannopyranoside was synthesized by exploiting the salient features of recently discovered [Au]/[Ag]-glycosidation of ethynylcyclohexyl glycosyl carbonate donors. The glycan was conjugated to the ESAT6, an early secreted protein of MTb for further characterization as a potential subunit vaccine candidate.

Synthesis of the Reducing-end Hexasaccharide Fragment of Marine Lipopolysaccharide Axinelloside A

Chemical synthesis of an orthogonally protected hexasaccharide relevant to the reducing-end half of axinelloside A, a highly sulfated marine lipopolysaccharide, is disclosed. The synthesis features preparation of the scyllo-inositol unit via a Ferrier-type-II rearrangement, construction of the 1,2-cis-glycosidic bonds via remote participation, and concise [2+2+2] assembly via Au(I)-catalyzed glycosylation.

Total synthesis and cytotoxicity evaluation of the spirostanol saponin gitonin

The spirostanol saponin gitonin was efficiently synthesized in 12 steps (longest linear sequence) in 18.5% overall yield from the commercially available isopropyl β-D-1-thiogalactopyranoside (IPTG) and tigogenin. A cascade two-step glycosylation and Schmidt's inverse procedure significantly facilitated the synthesis of gitonin and its derivatives. The cytotoxic activities of gitonin and its structural analogues were evaluated against A549, HepG2, and MCF-7, and most of them exhibited moderate to excellent inhibitory activity. Our study demonstrates that the removal of the β-D-galactopyranosyl residue (attached at C-2 of the glucose unit) from gitonin would not decrease the inhibition activities; however, further cleavage of sugar units could seriously reduce the activities. A bioassay on these cancer cell lines also suggested that the presence of 2α-hydroxy on the aglycone weakened the cytotoxicity of the designed saponin.

An Open-Air Palladium-Catalyzed Stereoselective O-Glycosylation of Glycals via in-situ Generation of gem-Disubstituted Methanols from p-Quinone Methides

We devised a palladium-catalyzed α-stereoselective glycosylation that incorporates oxygen via in-situ generation of gem-disubstituted methanols from p-quinone methides to access 2,3-unsaturated gem-diarylmethyl O-glycosides under open-air atmosphere at room temperature. Advantages of this environmentally friendly strategy include the absence of additives and ligands, using water as the green source of oxygen, mildest, operationally simple, exhibiting a wide functional group tolerance, and compatibility with a variety of glycal progenitors in appreciable yields. A mechanistic study has been verified via H2 18 O labeling, which validates that water (moisture) is a sole source of oxygen.

One-Pot Assembly of Mannose-Capped Lipoarabinomannan Motifs up to 101-Mer from the Mycobacterium tuberculosis Cell Wall

Lipoarabinomannan (LAM) from the Mycobacterium tuberculosis cell envelope represents important targets for the development of new therapeutic agents against tuberculosis, which is a deadly disease that has plagued mankind for a long time. However, the accessibility of long, branched, and complex lipoarabinomannan over 100-mer remains a long-standing challenge. Herein, we report the modular synthesis of mannose-capped lipoarabinomannan 101-mer from the M. tuberculosis cell wall using a one-pot assembly strategy on the basis of glycosyl ortho-(1-phenylvinyl)benzoates (PVB), which not only accelerates the modular synthesis but also precludes the potential problems associated with one-pot glycosylation with thioglycosides. Shorter sequences including 18-mer, 19-mer, and 27-mer are also synthesized for in-depth structure-activity relationship biological studies. Current synthetic routes also highlight the following features: (1) streamlined synthesis of various linear and branched glycans using one-pot orthogonal glycosylation on the combination of glycosyl N-phenyltrifluoroacetimidates, glycosyl ortho-alkynylbenzoates, and glycosyl PVB; (2) highly stereoselective construction of 10 1,2-cis-arabinofuranosyl linkages using 5-O-(2-quinolinecarbonyl)-directing 1,2-cis-arabinofuranosylation via a hydrogen-bond-mediated aglycone delivery strategy; and (3) convergent [(18 + 19) × 2 + 27] one-pot synthesis of the 101-mer LAM polysaccharide. The present work demonstrates that this orthogonal one-pot glycosylation strategy can highly streamline the chemical synthesis of long, branched, and complex polysaccharides.

Use of Novel Homochiral Thioureas Camphor Derived as Asymmetric Organocatalysts in the Stereoselective Formation of Glycosidic Bonds

We synthesized six new camphor-derived homochiral thioureas 1-6, from commercially available (1R)-(-)-camphorquinone. These new compounds 1-6 were evaluated as asymmetric organocatalysts in the stereoselective formation of glycosidic bonds, with 2,3,4,6-tetra-O-benzyl-D-glucopyranosyl and 2,3,4,6-tetra-O-benzyl-D-galactopyranosyl trichloroacetimidates as donors, and several alcohols as glycosyl acceptors, such as methanol, ethanol, 1-propanol, 1-butanol, 1-octanol, iso-propanol, tert-butanol, cyclohexanol, phenol, 1-naphtol, and 2-naphtol. Optimization of the asymmetric glycosylation reaction was achieved by modifying reaction conditions such as solvent, additive, loading of catalyst, temperature, and time of reaction. The best result was obtained with 2,3,4,6-tetra-O-benzyl-D-galactopyranosyl trichloroacetimidates, using 15 mol% of organocatalyst 1, in the presence of 2 equiv of MeOH in solvent-free conditions at room temperature for 1.5 h, affording the glycosidic compound in a 99% yield and 1:73 α:β stereoselectivity; under the same reaction conditions, without using a catalyst, the obtained stereoselectivity was 1:35 α:β. Computational calculations prior to the formation of the products were modeled, using density functional theory, M06-2X/6-31G(d,p) and M06-2X/6-311++G(2d,2p) methods. We observed that the preference for β glycoside formation, through a stereoselective inverted substitution, relies on steric effects and the formation of hydrogen bonds between thiourea 1 and methanol in the complex formed.

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.

Recent Advances in Chemical Synthesis of Structural Domains of Lipopolysaccharides from the Commensal Gut-Associated Microbiota

Lipopolysaccharides from the commensal gut-associated microbiota are interesting biomolecules for the treatment of various inflammatory diseases. Different from pathogenic lipopolysaccharides, commensal lipopolysaccharides have distinct chemical structures and mediate beneficial homeostasis with the immune system of the host. However, the accessibility issues of homogenous and pure commensal lipopolysaccharides hampered the in-depth studies of their functions. In this concept article, we highlight the recent synthesis of lipopolysaccharides from gut-associated lymphoid-tissue-resident Alcaligenes faecalis and Bacteroides vulgatus, which hopes to inspire the more efforts devoting to these fantastic biomolecules.

Recent chemical synthesis of plant polysaccharides

Here, chemical syntheses of long, branched and complex glycans over 10-mer from plants are summarized, which highlights amylopectin 20-mer from starch, 17-mer from carthamus tinctorius, α-glucan 30-mer from Longan, 19-mer from psidium guajava and 11-mer from dendrobium huoshanense. The glycans assembly strategies, protecting groups utilization and glycosylation methods discussed here will inspire the efficient synthesis of diverse complex glycans with many 1,2-cis glycosidic linkages.

Palladium catalysis enables cross-coupling-like SN2-glycosylation of phenols

Despite their importance in life and material sciences, the efficient construction of stereo-defined glycosides remains a challenge. Studies of carbohydrate functions would be advanced if glycosylation methods were as reliable and modular as palladium (Pd)-catalyzed cross-coupling. However, Pd-catalysis excels in forming sp2-hybridized carbon centers whereas glycosylation mostly builds sp3-hybridized C-O linkages. We report a glycosylation platform through Pd-catalyzed SN2 displacement from phenols toward bench-stable, aryl-iodide-containing glycosyl sulfides. The key Pd(II) oxidative addition intermediate diverges from an arylating agent (Csp2 electrophile) to a glycosylating agent (Csp3 electrophile). This method inherits many merits of cross-coupling reactions, including operational simplicity and functional group tolerance. It preserves the SN2 mechanism for various substrates and is amenable to late-stage glycosylation of commercial drugs and natural products.

Facile and stereospecific synthesis of diverse β-N-glycosyl sulfonamide scaffolds via palladium catalysis

Glycosylation is an important strategy to improve the druggability of lead compounds. Here, we present a palladium-catalysed stereospecific N-glycosylation of sulfonamides. This approach stands out with wide substrate scope, high functional group tolerance, and easy scalability, furnishing a broad spectrum of densely functionalized β-N-glycosyl sulfonamides with good efficiency and exceptional regio-/stereoselectivity. Diverse drug-like glycosulfonamido scaffolds have been constructed via a late-stage diversification strategy and various facile synthetic transformations of the products. Collectively, the established protocol provides a valuable tool for efficiently preparing glycosyl sulfonamides to facilitate drug discovery.

Promoter-Controlled Synthesis and Conformational Analysis of Cyclic Mannosides up to a 32-mer

Cyclodextrins are widely used as carriers of small molecules for drug delivery owing to their remarkable host properties and excellent biocompatibility. However, cyclic oligosaccharides with different sizes and shapes are limited. Cycloglycosylation of ultra-large bifunctional saccharide precursors is challenging due to the constrained conformational spaces. Herein we report a promoter-controlled cycloglycosylation approach for the synthesis of cyclic α-(1→6)-linked mannosides up to a 32-mer. Cycloglycosylation of the bifunctional thioglycosides and (Z)-ynenoates was found to be highly dependent on the promoters. In particular, a sufficient amount of a gold(I) complex played a key role in the proper preorganization of the ultra-large cyclic transition state, providing a cyclic 32-mer polymannoside, which represents the largest synthetic cyclic polysaccharide to date. NMR experiments and a computational study revealed that the cyclic 2-mer, 4-mer, 8-mer, 16-mer, and 32-mer mannosides adopted different conformational states and shapes.

Base-Promoted Glycosylation Allows Protecting Group-Free and Stereoselective O-Glycosylation of Carboxylic Acids

Here we report a simple and general method to achieve fully unprotected, stereoselective glycosylation of carboxylic acids, employing bench-stable allyl glycosyl sulfones as donors. Running the glycosylation reaction under basic conditions was crucial for the efficiencies and selectivities. Both the donor activation stage and the glycosidic bond forming stage of the process are compatible with free hydroxyl groups, thereby allowing for the use of fully unprotected glycosyl donors. This transformation is stereoconvergent, occurs under mild and metal-free conditions at ambient temperature with visible light (455 nm) irradiation, and displays remarkable scope with respect to both reaction partners. Many natural products and commercial drugs, including an acid derived from the complex anticancer agent taxol, were efficiently glycosylated. Experimental studies provide insights into the origin of the stereochemical outcome.

Modular and Stereoselective One-Pot Total Synthesis of Icosasaccharide Motif from Cordyceps sinensis EPS-1A Glycan

The first stereoselective one-pot synthesis of the icosasaccharide motif of EPS-1A glycan from Cordyceps sinensis has been achieved. The synthetic approach highlights the following features: (1) merging reagent modulation and remote anchimeric assistance α-glycosylation strategy for the highly stereoselective formation of five and ten continuous 1,2-cis glucosidic bonds; (2) the strategic employment of glycosyl N-phenyltrifluoroacetimidates and glycosyl o-(1-phenylvinyl)benzoates as the matched pair for efficient orthogonal one-pot synthesis; and (3) [11 + 8 + 1] orthogonal one-pot glycosylation for the efficient assembly of the target icosasaccharide.

One-Pot Assembly of the Highly Branched Tetradecasaccharide from Ganoderma lucidum Glycan GLSWA-1 with Immune-Enhancing Activities

The highly branched tetradecasaccharide repeating unit and shorter sequences of GLSWA-1 with immune-enhancing activities from Ganoderma lucidum have been prepared via a one-pot glycan assembly strategy. The synthetic route features (1) orthogonal one-pot glycosylation on the basis of PVB glycosylation to streamline glycan synthesis avoiding such issues as aglycone transfer, (2) one-pot assembly of oligosaccharides with up to four different glycosyl linkages, and (3) modular and convergent [4+5+5] one-pot assembly of the highly branched tetradecasaccharide.

Dynamic Kinetic Stereoselective Glycosylation via RhII and Chiral Phosphoric Acid-Cocatalyzed Carbenoid Insertion to the Anomeric OH Bond for the Synthesis of Glycoconjugates

Chemical synthesis of glycoconjugates is essential for studying the biological functions of carbohydrates. We herein report an efficient approach for the stereoselective synthesis of challenging α-linked glycoconjugates via a RhII /chiral phosphoric acid (CPA)-cocatalyzed dynamic kinetic anomeric O-alkylation of sugar-derived lactols via carbenoid insertion to the anomeric OH bond. Notably, we observed excellent anomeric selectivity, excellent diastereoselectivity, broad substrate scope, and high efficiency for this glycosylation reaction by exploring various parameters of the cocatalytic system. DFT calculations suggested that the anomeric selectivity was mainly determined by steric interactions between the C2-carbon of the carbohydrate and the phenyl group of the metal carbenoid, while π/π interactions with the C2-OBn substituent on the carbohydrate substrate play a significant role for diastereoselectivity at the newly generated stereogenic center.

S-o-(p-Methoxyphenylethynyl)benzyl (SMPEB) Glycosides for Catalytic Glycosylation and Their Application in the Synthesis of Polyporus Umbellatus Polysaccharides

We herein reported a new type of S-o-(p-methoxyphenylethynyl)benzyl donor for a highly efficient glycosylation method. The donor was activated by 10% Tf2O and underwent glycosylation with various acceptors to provide the corresponding glycosides in excellent yield. Furthermore, two repetitive fragments of Polyporus umbellatus polysaccharides (PUPs), isolated from traditional Chinese medicine "Polyporus umbellatus", were prepared by combining the "single-catalyst one-pot" and "latent-active" strategies for the first time for future clear studies on the structure-activity relationship of PUPs.

Stereoselective Synthesis of O-Glycosides with Borate Acceptors

Borate esters have been applied widely as coupling partners in organic synthesis. However, the direct utilization of borate acceptors in O-glycosylation with glycal donors remains underexplored. Herein, we describe a novel O-glycosylation resulting in the formation of 2,3-unsaturated O-glycosides and 2-deoxy O-glycosides mediated by palladium and copper catalysis, respectively. This O-glycosylation method tolerated a broad scope of trialkyl/triaryl borates and various glycals with exclusive stereoselectivities in high yields. All the desired aliphatic/aromatic O-glycosides and 2-deoxy O-glycosides were generated successfully, without the hemiacetal byproducts and O→C rearrangement because of the nature of borate esters. The utility of this strategy was demonstrated by functionalizing the 2,3-unsaturated glycoside products to form saturated β-O-glycosides, 2,3-deoxy O-glycosides, and 2,3-epoxy O-glycosides.

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.

4-Thioribose Analogues of Adenosine Diphosphate Ribose (ADPr) Peptides

Adenosine diphosphate (ADP) ribosylation is an important post-translational modification (PTM) that plays a role in a wide variety of cellular processes. To study the enzymes responsible for the establishment, recognition, and removal of this PTM, stable analogues are invaluable tools. We describe the design and synthesis of a 4-thioribosyl APRr peptide that has been assembled by solid phase synthesis. The key 4-thioribosyl serine building block was obtained in a stereoselective glycosylation reaction using an alkynylbenzoate 4-thioribosyl donor.

Efficient and versatile formation of glycosidic bonds via catalytic strain-release glycosylation with glycosyl ortho-2,2-dimethoxycarbonylcyclopropylbenzoate donors

Catalytic glycosylation is a vital transformation in synthetic carbohydrate chemistry due to its ability to expediate the large-scale oligosaccharide synthesis for glycobiology studies with the consumption of minimal amounts of promoters. Herein we introduce a facile and efficient catalytic glycosylation employing glycosyl ortho-2,2-dimethoxycarbonylcyclopropylbenzoates (CCBz) promoted by a readily accessible and non-toxic Sc(III) catalyst system. The glycosylation reaction involves a novel activation mode of glycosyl esters driven by the ring-strain release of an intramolecularly incorporated donor-acceptor cyclopropane (DAC). The versatile glycosyl CCBz donor enables highly efficient construction of O-, S-, and N-glycosidic bonds under mild conditions, as exemplified by the convenient preparation of the synthetically challenging chitooligosaccharide derivatives. Of note, a gram-scale synthesis of tetrasaccharide corresponding to Lipid IV with modifiable handles is achieved using the catalytic strain-release glycosylation. These attractive features promise this donor to be the prototype for developing next generation of catalytic glycosylation.

β-Glycosylations with 2-Deoxy-2-(2,4-dinitrobenzenesulfonyl)-amino-glucosyl/galactosyl Selenoglycosides: Assembly of Partially N-Acetylated β-(1 → 6)-Oligoglucosaminosides

An efficient protocol has been established for β-glycosylations with 2-deoxy-2-(2,4-dinitrobenzenesulfonyl)amino (2dDNsNH)-glucopyranosyl/galactopyranosyl selenoglycosides using PhSeCl/AgOTf as an activating system. The reaction features highly β-selective glycosylation with a wide range of alcohol acceptors that are either sterically hindered or poorly nucleophilic. Thioglycoside- and selenoglycoside-based alcohols prove to be viable nucleophiles, opening up new opportunities for one-pot construction of oligosaccharides. The power of this approach is highlighted by the efficient assembly of tri-, hexa-, and nonasaccharides composed of β-(1 → 6)-glucosaminosyl residues based on one-pot preparation of a triglucosaminosyl thioglycoside with DNs, phthaloyl, and 2,2,2-trichloroethoxycarbonyl as the protecting groups of amino groups. These glycans are potential antigens for developing glycoconjugate vaccines against microbial infections.

Synthesis and biological activity of ganglioside GM3 analogues with a (S)-CHF-Sialoside linkage and an alkyne tag

The alkyne tag, consisting of only two carbons, is widely used as a bioorthogonal functional group due to its compactness and nonpolar structure, and various probes consisting of lipids bearing an alkyne tag have been developed. Here, we designed and synthesized analogues of ganglioside GM3 bearing an alkyne tag in the fatty acid moiety and evaluated the effect of the alkyne tag on the biological activity. To eliminate the influence of other factors such as degradation of the glycan chain when evaluating biological activity in a cellular environment, we introduced the tag into sialidase-resistant (S)-CHF-linked GM3 analogues developed by our group. The designed analogues were efficiently synthesized by tuning the protecting group of the glucosylsphingosine acceptor. The growth-promoting effect of these analogues on Had-1 cells was dramatically altered depending upon the position of the alkyne tag.

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.

Bioinspired Total Synthesis of Complex Nucleoside Antibiotics A201A, A201D and A201E

Herein, bioinspired total syntheses of A201A, A201D, and A201E based on a previously reported biosynthetic pathway are presented. The challenging 1,2-cis-furanoside, a core structure of the A201 family, was obtained by remote 2-quinolinecarbonyl-assisted glycosylation. We accomplished the total synthesis of A201A and A201E based on the critical 1,2-cis-furanoside moiety through late-stage glycosylation without any interference from basic dimethyl adenosine. We also confirmed the absolute configuration of A201E by total synthesis. This modular synthesis strategy enables efficient preparation of A201 family antibiotics, allowing the study of their structure-activity relationships and mode of action. This study satisfies the increasing demand for developing novel antibiotics inspired by the A201 family.

Stereoselective synthesis of α-glucosides with glucosyl (Z)-Ynenoates as donors

A SPhosAuNTf₂-promoted DMF-modulated glycosylation approach with glycosyl (Z)-ynenoates as donors was developed for highly α-selective synthesis of various linkage types of α-glucans. The substituent groups were also found to play a significant role in the α-selective glucosylation reactions. The glycosylation approach was effectively applied to the stereospecific synthesis of the α-1,6-linked triglucoside.

Synthesis of Nucleosides and Deoxynucleosides via Gold(I)-Catalyzed N-Glycosylation of Glycosyl (Z)-Ynenoates

Nucleoside analogues are widely used as anticancer and antiviral drugs. Here, we develop a highly efficient gold(I)-catalyzed N-glycosylation approach for versatile synthesis of various types of nucleosides and deoxynucleosides with glycosyl (Z)-ynenoates as donors. The wide scope of the N-glycosylation approach was demonstrated by the synthesis of 31 pyrimidine nucleosides and 8 purine nucleosides. Remarkably, the gold(I)-catalyzed N-glycosylation of pyranosyl (Z)-ynenoates with purines was found to be very effective for regioselective synthesis of pyranosyl N9 purine nucleosides. Based on the catalytic N-glycosylation approach, convenient synthesis of two 5'-deoxynucleosides drugs (capecitabine and galocitabine), four 2'-deoxynucleoside drugs (floxuridine, trifluridine, decitabine and cladribine), four 3',5'-dideoxynucleoside analogues, and four 2',5'-dideoxynucleoside analogues was achieved in a collective manner.

Synthesis and conformational analysis of a potentially super-armed glucuronidation donor

The concise synthesis of a potentially “super-armed” glucuronidation donor is reported. The α-anomer was crystallized and analyzed by single crystal X-ray diffraction. The pyranose ring was found to be in a twist-boat conformation in the solid state. To confirm the relevance of this finding for the solution state, and explain the failure of analysis by NMR, DFT calculations were performed. They revealed the twist-boat to be the dominant among a group of several possible conformers at ambient temperature.

Graphical abstract

Chemical Synthesis of a Colanic Acid Hexasaccharide

Chemical synthesis of a hexasaccharide relevant to colanic acid, which bears a high prevalence of polyanionic and acetyl decoration, has been achieved, highlighting stereoselective glycosylation and effective installation of O-acetyl and pyruvate residues.

Tri(N-carbazolyl)phosphine Gold(I) Complexes: Structural and Catalytic Activity Studies

Twelve tri(N-carbazolyl)phosphine gold(I) complexes, bearing both protonated and deuterated aryl phosphorous triamide-type ligands, have been synthesized and characterized. An elusive Au-H(D) interaction between the H(D) atoms of the tri(N-carbazolyl)phosphine ligand at the H-1(D-1) position of the carbazolyl ring and the central gold atom was observed. Complexes 5(H)/5(D) bearing the dibrominated tri(N-carbazolyl)phosphine ligand exhibit isotopic polymorphism, in which two dramatically different crystal-packing modes between the protonated and deuterated forms occur. The catalytic potential of these complexes has been showcased in the gold(I)-catalyzed glycosylation with glycosyl o-alkynylbenzoates as donors, with TON being up to 27 000.