Stable Alkynyl Glycosyl Carbonates: Catalytic Anomeric Activation and Synthesis of a Tridecasaccharide Reminiscent ofMycobacterium tuberculosisCell Wall Lipoarabinomannan

Synthesis of Glycolipid Library Reminiscent of Mycobacterial Cell Wall Motif A and Its Significance by Microscopy

The cell wall of Mycobacterium tuberculosis (MTb) is distinguished by its unique glycolipid composition, notably mycolyl arabinogalactan, which features Araf, Galf, and mycolic acid. The terminal portion of arabinogalactan (AG) is motif A, a pentaarabinofuranoside characterized by two β-1,2 linkages and one each of α-1,5 and α-1,3 linkages. The C5 positions of motif A are esterified with mycolic acids, further enriched by unusual cyclopropanes. In this study, we synthesized a diverse array of MTb cell wall glycolipids inspired by motif A and explored their self-assembly behavior using microscopy. This collection of glycolipids includes variations with α or β-(1→2, 3, or 5) Araf linkages in di-, tri-, tetra-, and penta-arabinofuranosides, esterified with saturated, doubly unsaturated, or cyclopropanated long-chain fatty acids. The TEM analysis of the resulting self-assemblies revealed that subtle modifications in the anomeric linkages, length of the glycan, and the presence or absence of cyclopropanes impacted the morphology of the self-assembled structures.

Expedient synthesis of mucin-type oligosaccharide analogues using alkynyl glycosyl carbonate donors

Access to mucin-type O-glycans as tools to help further our understanding of the biological role of these complex molecules is of great importance. Herein we report the development of an efficient and practical synthetic strategy for the synthesis of 1-azidopropyl functionalised di- and trisaccharide mucin analogues using alkynyl glycosyl carbonate donors that can be activated using gold(I)/silver(I) salts. The reactions are high yielding and stereoselective, which allows us to streamline the overall process by telescoping several steps without the need for purification after each step.

Shelf-Stable 3-Thiocresyl-Prop-1-Enyl (TCP) Glycosides in Oligosaccharide Synthesis

Transformation of the allyl glycosides to a glycosylation-active 3-thiocresyl-prop-1-enyl (TCP) glycosides is reported herein, in an effort to devise a shelf-stable, active vinyl glycoside. Remote electrophilic activation with an iodonium source results in the formation of a glycosylation-active moiety. The efficiency of the TCP glycoside donor is demonstrated through the synthesis of a panel of di- and trisaccharides of pyranosides and furanosides, including globo-trioside Gb3 antigen, wherein a disarmed TCP glycoside is converted facile to an armed TCP glycosyl donor, further ascertaining the tolerance towards protecting group manipulations. In addition, the synthetic utility of the TCP glycoside donor is demonstrated through the syntheses of biologically relevant tri- and hexasaccharide oligomannans, thereby establishing the potential of this new TCP glycosyl donor for oligosaccharide synthesis. Mechanistic studies reveal the remote activation of the thioether moiety by promoters, as evidenced through identification of the thioether and acrolein by-products arising from the reaction. Further, the applicability of TCP glycosides as glycosyl acceptors is also verified in glycosylations involving glycosyl halide donors. The facile isomerization of allyl glycoside, remarkable shelf-stability, tolerance towards protecting groups and applicability as a glycosyl donor, as well as glycosyl acceptor offer key advantages to TCP glycosides.

Gold(I)-Catalyzed 2-Deoxy-β-glycosylation via 1,2-Alkyl/Arylthio Migration: Synthesis of Velutinoside A Pentasaccharide

2-Deoxy-β-glycosides are essential components of natural products and pharmaceuticals; however, the corresponding 2-deoxy-β-glycosidic bonds are challenging to chemically construct. Herein, we describe an efficient catalytic protocol for synthesizing 2-deoxy-β-glycosides via either IPrAuNTf2-catalyzed activation of a unique 1,2-trans-positioned C2-S-propargyl xanthate (OSPX) leaving group or (PhO)3PAuNTf2-catalyzed activation of a 1,2-trans-C2-ortho-alkynylbenzoate (OABz) substituent of the corresponding thioglycosides. These activation processes trigger 1,2-alkyl/arylthio-migration glycosylation, enabling the synthesis of structurally diverse 2-deoxy-β-glycosides under mild reaction conditions. The power of this strategy is demonstrated by the first synthesis of the pentasaccharide chain corresponding to velutinoside A, which features gold(I)-catalyzed construction of four successive β-l-oleandrosidic bonds in both a convergent and a one-pot glycosylation manner. Mechanistic studies, including control experiments and deuterium-labeling experiments, emphasize the crucial role of the OSPX and the involvement of the gold(I)-activated C≡C triple bond during the glycosylation process. The low-temperature NMR experiments unveiled a unique dual-coordination pattern of the gold(I) catalyst to the thiocarbonyl group and the alkynyl group of the OSPX, initiating a 5-exo-dig cyclization process. Furthermore, density functional theory (DFT) simulations reveal the ligand-induced match-mismatch effect between leaving groups OSPX and OABz and gold catalysts IPrAuNTf2 and (PhO)3PAuNTf2. The DFT simulations also suggest that the formation of 2-deoxy-β-glycosidic bonds occurs via the bottom-face attack of the acceptor to the oxocarbenium intermediate, which adopts a 4H3 half-chair conformation, leading to an energetically favored, 4C1-conformed intermediate Dβ that is stabilized by a hydrogen bonding interaction.

1-(2'-Hydroxy-2'-Methylpropionyl)Glycoside as a Versatile Glycosyl Donor for O-/C-Glycosylation

Herein, we developed a Lewis acid-mediated O-glycosylation and C-glycosylation protocol using stable glycosyl 2'-hydroxy-2'-methylpropionates as donors. These glycosylation reactions reached completion within 1 h at room temperature. The practicality of this protocol is characterized by their straightforward operation and efficient applicability to various substrates, including both disarmed and armed glycosyl donors, through the remote activation of easily accessible TMSOTf.

[Au]/[Ag]-Catalyzed Glycosidation of Ethynylcyclohexyl Glycosyl Carbonates Enables Direct Stereoselective Synthesis of 2-Azido-2-deoxy-β-mannopyranosides

We describe the [Au]/[Ag]-catalyzed activation of ethynylcyclohexyl 2-azido-4,6-O-benzylidene mannosyl carbonates for β-mannosamine linkage preparation in high yield and selectivity in a temperature- and concentration-dependent manner. VT-NMR studies reveal an anomeric triflate intermediate generated in the absence of an acceptor alcohol that is stable up to -10 °C. The generality of the protocol was illustrated by successful application to a series of acceptors and by synthesis of a fully protected Streptococcus pneumoniae type 19F capsular trisaccharide.

[Au]/[Ag]-catalysed synthesis of non-hydrolysable C-glycosides

Many C-glycosides are found in natural products, drugs and small molecular probes. Herein, we report the synthesis of C-glycosides by the [Au]/[Ag]-catalysed activation of ethynylcyclohexyl glycosyl carbonate donors. This mild, catalytic, fast and high yielding protocol enables the synthesis of a diverse array of C-glycosides that were otherwise challenging to synthesize.

Development of Glycosylation Protocols Using Glycosyl N-Phenylethynyl Pyrrole-2-carboxylates as Donors

We herein introduce glycosyl N-phenylethynyl pyrrole-2-carboxylates (PEPCs) as novel and highly efficient glycosyl donors. The unique inclusion of the pyrrole group, serving as both an electron-donating group and an ortho-tethering scaffold, imparts exceptional shelf stability while retaining reactivity for glycosylation reactions. PEPC donors exhibit broad utility for both O- and N-glycosylation across a variety of substrates. Additionally, their versatility and efficiency were further demonstrated in a one-pot saccharide synthesis, showcasing their potential for diverse synthetic applications.

Synthesis of Salmonella enteritidis Antigenic Tetrasaccharide Repeating Unit by Employing Cationic Gold(I)-Catalyzed Glycosylation Involving Glycosyl N-1,1-Dimethylpropargyl Carbamate Donors

Synthesis of an antigenic tetrasaccharide repeating unit of the O-polysaccharide of Salmonella enteritidis lipopolysaccharide has been accomplished. Those four monosaccharides were assembled stereoselectively by employing our recently developed cationic gold(I)-catalyzed glycosylation methodology involving various glycosyl N-1,1-dimethylpropargyl carbamate donors. The newly formed α-anomeric stereochemical configuration was controlled by the axial C2-OBz of the glycosyl donors via anchimeric assistance.

Total Synthesis of Conjugation-Ready Hyperbranched Pentasaccharide of Clavibacter phaseoli VKM Ac-2641T by [Au]/[Ag] Catalysis

Infections by Clavibacter spp. cause an economic burden to farmers. The components present in the cell wall glycopolymers (CWGs) are important for studying the host-pathogen interactions, colonization, and infection. A pentasaccharide containing a hyperbranched Ribf-, Galf-, and Manp- has recently been identified. Herein, we describe the first total synthesis of the conjugation-ready hyperbranched pentasaccharide of C. phaseoli VKM Ac-2641T using the [Au]/[Ag]-catalyzed glycosidation chemistry of ethynylcyclohexyl carbonate glycosyl donors. The pentasaccharide was synthesized in a highly convergent fashion from readily accessible monosaccharide building blocks.

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.

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.

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.

Synthesis of N-Glycosides by Silver-Assisted Gold Catalysis

The synthesis of N-glycosides from stable glycosyl donors in a catalytic fashion is still challenging, though they exist ubiquitously in DNA, RNA, glycoproteins, and other biological molecules. Herein, silver-assisted gold-catalyzed activation of alkynyl glycosyl carbonate donors is shown to be a versatile approach for the synthesis of purine and pyrimidine nucleosides, asparagine glycosides and quinolin-2-one N-glycosides. Thus synthesized nucleosides were subjected to the oxidation-reduction sequence for the conversion of Ribf- into Araf- nucleosides, giving access to nucleosides that are otherwise difficult to synthesize. Furthermore, the protocol is demonstrated to be suitable for the synthesis of 2'-modified nucleosides in a facile manner. Direct attachment of an asparagine-containing dipeptide to the glucopyranose and subsequent extrapolation to afford the dipeptide disaccharide unit of chloroviruses is yet another facet of this endeavor.

Large-Scale Synthesis of Man9GlcNAc2 High-Mannose Glycan and the Effect of the Glycan Core on Multivalent Recognition by HIV Antibody 2G12

Access to homogeneous high-mannose glycans in high-mg quantities is necessary for carbohydrate-based HIV vaccine development research. We have used directed evolution to design highly antigenic oligomannose clusters that are recognized in low-nM affinity by HIV antibodies. Herein we report an optimized large-scale synthesis of Man9GlcNAc2 including improved building block synthesis and a fully stereoselective 5 + 6 coupling, yielding 290 mg of glycan. We then use this glycan to study the effect of the GlcNAc2 core on the antigenicity of an evolved 2G12-binding glycopeptide, 10F2.

The Astounding World of Glycans from Giant Viruses

Viruses are a heterogeneous ensemble of entities, all sharing the need for a suitable host to replicate. They are extremely diverse, varying in morphology, size, nature, and complexity of their genomic content. Typically, viruses use host-encoded glycosyltransferases and glycosidases to add and remove sugar residues from their glycoproteins. Thus, the structure of the glycans on the viral proteins have, to date, typically been considered to mimick those of the host. However, the more recently discovered large and giant viruses differ from this paradigm. At least some of these viruses code for an (almost) autonomous glycosylation pathway. These viral genes include those that encode the production of activated sugars, glycosyltransferases, and other enzymes able to manipulate sugars at various levels. This review focuses on large and giant viruses that produce carbohydrate-processing enzymes. A brief description of those harboring these features at the genomic level will be discussed, followed by the achievements reached with regard to the elucidation of the glycan structures, the activity of the proteins able to manipulate sugars, and the organic synthesis of some of these virus-encoded glycans. During this progression, we will also comment on many of the challenging questions on this subject that remain to be addressed.

Cationic gold(I)-catalyzed glycosylation with glycosyl N-1,1-dimethylpropargyl carbamate donors

A mild and efficient cationic gold(I)-catalyzed O-glycosylation methodology involving the use of bench-stable glycosyl N-1,1-dimethylpropargyl carbamate donors has been developed. In the presence of 1-2 mol% [tris(2,4-di-tert-butylphenyl)phosphite]gold(I) chloride and 5 mol% silver triflate, both "armed" and "disarmed" glycosyl N-1,1-dimethylpropargyl carbamate donors react with various sugar acceptors at room temperature to afford the corresponding glycosides in good to excellent yields. These glycosyl N-1,1-dimethylpropargyl carbamates are found to be orthogonal to regular phenyl thioglycoside donors. The utilization of this method has been demonstrated in the synthesis of a trisaccharide.

Insights into the Activation of Alkyne-Installed Glycosyl Donors with Dual Acidic Metal Catalysts: Reaction Pathway, Influencing Factors, and Enlightenment for Glycosylation

The activation of alkyne-installed glycosyl donors with dual acidic metal catalysts were studied. Lewis and/or π acidity-activated pathways were observed for alkynyl carbonate-, ester-, and ether-type donors, and π acidity-promoted reaction mode afforded higher efficiency and yields. The activation mode for a certain metal catalyst is determined by the nature of catalysts itself, protecting groups on sugar rings, type of sugars, and structure of aglycones. The discovery gives us valuable insights into the glycosylation of alkyne-containing donors.

Glycosyl o-[1-(p-MeO-Phenyl)vinyl]benzoates (PMPVB) as Easily Accessible, Stable, and Reactive Glycosyl Donors for O-, S-, and C-Glycosylations under Brønsted Acid Catalysis

Methods suitable for the synthesis of both O- and S-glycosylations are relatively rare because commonly used promoters like halonium sources or gold catalysts are incompatible with thiols as nucleophiles. Here, we present (p-MeO)phenylvinylbenzoates (PMPVB) as easily accessible, stable, and reactive alkene-based glycosyl donors that can be activated with catalytic amounts of a Brønsted acid. This activation protocol not only allows us to synthesize O-glycosides but also can successfully provide S- and C-linked glycosides. The armed and disarmed donors lead to product formation in 5 min, showcasing the high reactivity of the donors. Competitive experiments show that the PMPVB donors are much more reactive than the corresponding PVB donors even under NIS/TMSOTf conditions, whereas PVB donors are not reactive enough to be efficiently activated under Brønsted acid conditions. The potential of the catalytic glycosylation protocol has also been showcased by synthesizing trisaccharides. The Brønsted acid activation of PMPVB donors also allows access to C-glycosides in a stereoselective fashion. The easy accessibility of the donor aglycon on a multigram scale in just two steps makes the PMPVB donors highly attractive alternatives.

Glycosyl 3-Phenyl-4-pentenoates as Versatile Glycosyl Donors: Reactivity and Their Application in One-Pot Oligosaccharide Assemblies

Both glycoconjugates and oligosaccharides are important biomolecules having significant roles in several biological processes, and a new strategy for their synthesis is crucial. Here, we report a versatile N-iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf) promoted glycosidation approach with shelf-stable 3-phenyl-4-pentenoate glycosyl as a donor for the efficient synthesis of O/C-glycosides with free alcohols, silylated alcohols, and C-type nucleophile acceptors in good to excellent yields. The mild activation conditions and outstanding reactivity of phenyl substituted pentenoate donors analogous to 4-pentenoate glycosyl donors enhance their applicability to various one-pot strategies for the synthesis of oligosaccharides, such as single-catalyst one-pot and acceptor reactivity-controlled one-pot strategies.

Glycosyl Vinylogous Carbonates as Glycosyl Donors by Metal-Free Activation

Synthesis of glycoconjugates employs a glycosylation reaction wherein an electrophile and a nucleophile known as a glycosyl donor and an aglycon, respectively, are involved. Glycosyl donors often contain a leaving group at the anomeric carbon that upon reaction with activator(s) results in a highly reactive electrophilic species reported as an oxycarbenium ion contact pair that will then be attacked by the aglycon. Therefore, identification of the correct glycosyl donor and activation protocol is essential for the synthesis of all glycoconjugates. Recently identified [Au]/[Ag]-catalyzed activation of ethynylcyclohexyl glycosyl carbonates is one such versatile method for the synthesis of glycosides, oligosaccharides, and glycoconjugates. In this work, stable glycosyl vinylogous carbonates were identified to undergo glycosidation in the presence of a sub-stoichiometric quantity of TfOH. The reaction is fast and suitable for donors containing both C2-ethers and C2-esters. Donors positioned with C2-ethers resulted in anomeric mixtures with greater selectivity toward 1,2-cis glycosides, whereas those with C2-esters gave 1,2-trans selective glycosides. The versatility of the method is demonstrated by conducting the glycosylation with more than 25 substrates. Furthermore, the utility of the glycosyl vinylogous carbonate donors is demonstrated with the successful synthesis of the branched pentaarabinofuranoside moiety of the Mycobacterium tuberculosis cell wall.

Silver-assisted gold-catalyzed solid phase synthesis of linear and branched oligosaccharides

Unlike solid phase synthesis of peptides, synthesis of oligosaccharides by solid phase methods is lagging behind owing to inherent challenges faced while executing glycosidations. In this communication, silver-assisted gold-catalyzed glycosidations are found to be excellent for solid phase oligosaccharide synthesis. Glycosidations under catalytic conditions, one time coupling with four equivalents of donor, reactions in less than 30 min at 25 °C and on-resin deprotection of silyl and benzoate protecting groups are the salient features. Photocleaved glycans possess a protected amino functionality that can be utilized for bioconjugation. The versatility of this approach is established by synthesizing linear and branched pentaarabinofuranosides.

Glycan Assembly Strategy: From Concept to Application

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

Recent chemical syntheses of bacteria related oligosaccharides using modern expeditious approaches

Apart from some essential and crucial roles in life processes carbohydrates also are involved in a few detrimental courses of action related to human health, like infections by pathogenic microbes, cancer metastasis, transplanted tissue rejection, etc. Regarding management of pathogenesis by microbes, keeping in mind of multi drug-resistant bacteria and epidemic or endemic incidents, preventive measure by vaccination is the best pathway as also recommended by the WHO; by vaccination, eradication of bacterial diseases is also possible. Although some valid vaccines based on attenuated bacterial cells or isolated pure polysaccharide-antigens or the corresponding conjugates thereof are available in the market for prevention of several bacterial diseases, but these are not devoid of some disadvantages also. In order to develop improved conjugate T-cell dependent vaccines oligosaccharides related to bacterial antigens are synthesized and converted to the corresponding carrier protein conjugates. Marketed Cuban Quimi-Hib is such a vaccine being used since 2004 to resist Haemophilus influenza b infections. During nearly the past two decades research is going on worldwide for improved synthesis of bacteria related oligosaccharides or polysaccharides towards development of such semisynthetic or synthetic glycoconjugate vaccines. The present dissertation is an endeavour to encompass the recent syntheses of several pathogenic bacterial oligosaccharides or polysaccharides, made during the past ten-eleven years with special reference to modern expeditious syntheses.

Azide-Functionalized Derivatives of the Virulence-Associated Sugar Pseudaminic Acid: Chiral Pool Synthesis and Labeling of Bacteria

Pseudaminic acid (Pse) is a significant prokaryotic monosaccharide found in important Gram-negative and Gram-positive bacteria. This unique sugar serves as a component of cell-surface-associated glycans or glycoproteins and is associated with their virulence. We report the synthesis of azidoacetamido-functionalized Pse derivatives as part of a search for Pse-derived metabolic labeling reagents. The synthesis was initiated with d-glucose (Glc), which served as a cost-effective chiral pool starting material. Key synthetic steps involve the conversion of C1 of Glc into the terminal methyl group of Pse, and inverting deoxyaminations at C3 and C5 of Glc followed by backbone elongation with a three-carbon unit using the Barbier reaction. Metabolic labeling experiments revealed that, of the four Pse derivatives, ester-protected C5 azidoacetamido-Pse successfully labeled cells of Pse-expressing Gram-positive and Gram-negative strains. No labeling was observed in cells of non-Pse-expressing strains. The ester-protected and C5 azidoacetamido-functionalized Pse is thus a useful reagent for the identification of bacteria expressing this unique virulence-associated nonulosonic acid.

NIS/TMSOTf-Promoted Glycosidation of Glycosyl ortho-Hexynylbenzoates for Versatile Synthesis of O-Glycosides and Nucleosides

Glycosidation plays a pivotal role in the synthesis of O-glycosides and nucleosides that mediate a diverse range of biological processes. However, efficient glycosidation approach for the synthesis of both O-glycosides and nucleosides remains challenging in terms of glycosidation yields, mild reaction conditions, readily available glycosyl donors, and cheap promoters. Here, we report a versatile N-iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf)-promoted glycosidation approach with glycosyl ortho-hexynylbenzoates as donors for the highly efficient synthesis of O-glycosides and nucleosides. The glycosidation approach highlights the merits of mild reaction conditions, cheap promoters, extremely wide substrate scope, and good to excellent yields. Notably, the glycosidation approach performs very well in the construction of a series of challenging O- and N-glycosidic linkages. The glycosidation approach is then applied to the efficient synthesis of oligosaccharides via the one-pot strategy and the stepwise strategy. On the basis of the isolation and characterization of the departure species derived from the leaving group, a plausible mechanism of NIS/TMSOTf-promoted glycosidation of glycosyl ortho-hexynylbenzoates is proposed.

Silver-assisted gold-catalyzed formal synthesis of the anticoagulant Fondaparinux pentasaccharide

Clinically approved anti-coagulant Fondaparinux is safe since it has zero contamination problems often associated with animal based heparins. Fondaparinux is a synthetic pentasaccharide based on the antithrombin-binding domain of Heparin sulfate and contains glucosamine, glucuronic acid and iduronic acid in its sequence. Here, we show the formal synthesis of Fondaparinux pentasaccharide by performing all glycosidations in a catalytic fashion for the first time to the best of our knowledge. Designer monosaccharides were synthesized avoiding harsh reaction conditions or reagents. Further, those were subjected to reciprocal donor-acceptor selectivity studies to guide [Au]/[Ag]-catalytic glycosidations for assembling the pentasaccharide in a highly convergent [3 + 2] or [3 + 1 + 1] manner. Catalytic and mild activation during glycosidations that produce desired glycosides exclusively, scalable route to the synthesis of unnatural and expensive iduronic acid, minimal number of steps and facile purifications, shared use of functionalized building blocks and excellent process efficiency are the salient features.

N-Heterocyclic silylene/germylene ligands in Au(i) catalysis

Cationic Au(i) complexes (2, 5 and 8) supported by N-heterocyclic carbene, silylene and germylene ligands were prepared and their potential as catalysts in glycosidation chemistry has been evaluated. Insights into the mechanism are provided using DFT studies. Practical application of them as catalysts was achieved by the synthesis of the branched pentamannan core of the HIV-gp120 envelope under mild conditions.

Defining the Scope of the Acid-Catalyzed Glycosidation of Glycosyl Bromides

Following the recent discovery that traditional silver(I) oxide-promoted glycosidations of glycosyl bromides (Koenigs-Knorr reaction) can be greatly accelerated in the presence of catalytic TMSOTf, reported herein is a dedicated study of all major aspects of this reaction. A thorough investigation of numerous silver salts and careful refinement of the reaction conditions led to an improved mechanistic understanding. This, in turn, led to a significant reduction in the amount of silver salt required for these glycosylations. The progress of this reaction can be monitored by naked eye, and the completion of the reaction can be judged by the disappearance of characteristic dark color of Ag2 O. Further evidence on higher reactivity of benzoylated α-bromides in comparison to that of their benzylated counterparts has been acquired.

Glycosylation with ulosonates under Mitsunobu conditions: scope and limitations

Mitsunobu reactions on highly hindered tertiary type hydroxy groups of ulosonates: from 47 NuH compounds O-, N-, and S-nucleophiles gave the corresponding ulosidonates in 30–78% yields, while C-nucleophiles were unreactive.

Facile Synthesis of Sugar Lactols via Bromine-Mediated Oxidation of Thioglycosides

Synthesis of a variety of sugar lactols (hemiacetals) has been accomplished in moderate to excellent yields by using bromine-mediated oxidation of thioglycosides. It was found that acetonitrile is the optimal solvent for this oxidation reaction. This approach involving bromine as oxidant is superior to that using N-bromosuccimide (NBS) which produces byproduct succinimide often difficult to separate from the lactol products.

Stable Benzylic (1-Ethynylcyclohexanyl)carbonates Protect Hydroxyl Moieties by the Synergistic Action of [Au]/[Ag] Catalytic System

Chemical syntheses of oligosaccharides and glycosides call utilization of many protecting groups that can be installed or deprotected without affecting other functional groups present. Benzyl ethers are routinely used in the synthesis of glycans as they can be subjected to hydrogenolysis under neutral conditions. However, installation of benzyl ethers is often carried out under strong basic conditions using benzyl halides. Many a times, strongly basic conditions will be detrimental for some of the other sensitive functionalities (e.g., esters). Later introduced reagents such as benzyl trichloroacetimidate and BnOTf are not shelf-stable, and hence, a new method is highly desirable. Taking a cue from the [Au]/[Ag]-catalyzed glycosidations, we have identified a method that enables protection of hydroxyl groups as benzyl, p-methoxybenzyl, or naphthylenemethyl ethers using easily accessible and stable carbonate reagent. A number of saccharide-derived alcohols were subjected to the benzylation successfully using a catalytic amount of gold phosphite and silver triflate. Furthermore, the protocol is suitable for even protecting menthol, cholesterol, serine, disaccharide OH, and furanosyl-derived alcohol easily. The often-utilized olefins and benzoates, as well as benzylidene-, silyl-, Troc-, and Fmoc-protecting groups do not get affected during the newly identified protocol. Regioselective protection and one-pot installation of benzyl and p-methoxybenzyl ethers are demonstrated.

Gold(III)-Catalyzed Glycosylation using Phenylpropiolate Glycosides: Phenylpropiolic Acid, An Easily Separable and Reusable Leaving Group

An efficient and operationally simple gold(III)-catalyzed glycosylation protocol was developed using newly synthesized benchtop stable phenylpropiolate glycosyl (PPG) donors. Gold(III)-catalyzed activation of PPGs proceeds well with various carbohydrate and noncarbohydrate-based glycosyl acceptors and leads to their corresponding O/ N-glycosides in good to excellent yields with regeneration of reusable and easily separable phenylpropiolic acid. Differentially protected PPGs reacted well under the optimized reaction conditions. In particular, good anomeric selectivity was observed with mannosyl and rhamnosyl PPG donors. A preliminary mechanistic study reveals that the presence of a triple bond adjacent to the ester group is essential for activation, and PPG-based donor shows higher reactivity than analogous acetate and benzoate donors.

The logic of translating chemical knowledge into machine-processable forms: a modern playground for physical-organic chemistry

With renewed interest and significant progress in computer-assisted synthetic planning, it is essential to codify the logic that should be followed when translating organic synthetic knowledge into reaction rules understandable to the machine.

Gold-catalyzed synthesis of α-D-glucosides using an o-ethynylphenyl β-D-1-thioglucoside donor

A gold-catalyzed glucosylation method using an o-ethynylphenyl β-D-1-thioglucoside as donor is described. The reaction proceeds in a mostly SN2 pathway. A series of α-D-glucosides are obtained in good yields and with up to 19:1 α-selectivity.

Automated Chemical Oligosaccharide Synthesis: Novel Approach to Traditional Challenges

Advances in carbohydrate chemistry have certainly made common oligosaccharides much more accessible. However, many current methods still rely heavily upon specialized knowledge of carbohydrate chemistry. The application of automated technologies to chemical and life science applications such as genomics and proteomics represents a vibrant field. These automated technologies also present opportunities for their application to organic synthesis, including that of the synthesis of oligosaccharides. However, application of automated methods to the synthesis of carbohydrates is an underdeveloped area as compared to other classes of biomolecules. The overarching goal of this review article is to present the advances that have been made at the interface of carbohydrate chemistry and automated technology.

Homogeneous Catalysis: A Powerful Technology for the Modification of Important Biomolecules

Homogeneous catalysis plays an important and ubiquitous role in the synthesis of simple and complex molecules, including drug compounds, natural products, and agrochemicals. In recent years, the wide-reaching importance of homogeneous catalysis has made it an indispensable tool for the modification of biomolecules, such as carbohydrates (sugars), amino acids, peptides, nucleosides, nucleotides, and steroids. Such a synthetic strategy offers several advantages, which have led to the development of new molecules of biological relevance at a rapid rate relative to the number of available synthetic methods. Given the powerful nature of homogeneous catalysis in effecting these synthetic transformations, this Focus Review has been compiled to highlight these important developments.

Silver-catalyzed stereoselective formation of glycosides using glycosyl ynenoates as donors

A silver-catalyzed glycosylation reaction employing readily accessible and stable glycosyl ynenoates is developed. This reaction is mostly high yielding and exhibits varying levels of stereoinversion at the anomeric position. Compared to established and versatile Yu's gold catalysis, this chemistry features the use of substantially cheaper AgNTf₂.

O-Glycosylation Enabled by N-(Glycosyloxy)acetamides

A novel glycosylation protocol has been established by using N-(glycosyloxy)acetamides as glycosyl donors. The N-oxyacetamide leaving group in donors could be rapidly activated in the presence of Cu(OTf)₂ or SnCl₄ under microwave irradiation. This glycosylation process afforded the coupled products in high yields, and the reaction enjoyed a broad substrate scope, even for disarmed donors and hindered acceptors. The easy availability of the donors, the high stability of N-(glycosyloxy)acetamides, and the small leaving group make this method very practical.