Cationic palladium(II)-catalyzed stereoselective glycosylation with glycosyl trichloroacetimidates

Zinc Tetrafluoroborate Catalyzed Stereo- and Regioselective O-Glycosylation for the Direct Synthesis of β-Glycosides from Armed O-Glycosyl Trichloroacetimidates

Efficient stereo- and regioselective O-glycosylation methods remain essential to capacitate the studies of sugars and sugar derivatives in various disciplines. In this work, we demonstrated an operationally simple and cost-effective strategy for the synthesis of 1,2-trans glycosides by the activation of armed O-glycosyl trichloroacetimidates donor using zinc tetrafluoroborate. This mild, transition metal-free, and scalable approach allowed stereo- and regioselective synthesis of β-glycosides with a wide range of acceptors containing various protecting groups/functionalities. This method is exemplified by synthesizing a branched trisaccharide fragment related to the cell wall O-polysaccharide of E. Coli O27.

β-C-Glycosylation with 2-Oxindole Acceptors via Palladium-Catalyzed Decarboxylative Reactions

This report describes a highly efficient β-selective C-glycosylation of bicyclic galactals with 2-oxindoles through a palladium-catalyzed decarboxylative pathway. A variety of substrates representing both glycosyl donors and acceptors could be transformed in greater than 90% yields under mild reaction conditions. The decarboxylation intermediate of galactal could serve as an efficient base to deprotonate the enol tautomer of 2-oxindole and enhance its nucleophilicity. The β-selective nucleophilic addition at the anomeric center originates from the steric hindrance imposed by the palladium and bulky ligand.

Chemical synthesis of saponins: An update

Saponins, as secondary metabolites in terrestrial plants and marine invertebrate, constitute one of the largest families of natural products. The long history of folk medicinal applications of saponins makes them attractive candidates for innovative drug design and development. Chemical synthesis has become a practical alternative to the availability of the natural saponins and their modified analogs, so as to facilitate SAR studies and the discovery of optimal structures for clinical applications. The recent achievements in the synthesis of these complex saponins reflect the advancements of both steroid/triterpene chemistry and carbohydrate chemistry. This chapter provides an updated review on the chemical synthesis of natural saponins, covering the literature from 2014 to 2020.

Visible-light-induced photoacid catalysis: application in glycosylation with O-glycosyl trichloroacetimidates

The development of visible-light-induced photoacid catalyzed glycosylation is reported. The eosin Y and PhSSPh catalyst system is applied to realize glycosylation with different glycosyl donors upon light irradiation. The reaction shows a broad substrate scope, including both glycosyl donors and acceptors, and highlights the mild nature of the reaction conditions.

Synthesis of Cardiotonic Steroids Oleandrigenin and Rhodexin B

This article describes a concise synthesis of cardiotonic steroids oleandrigenin (7) and its subsequent elaboration into the natural product rhodexin B (2) from the readily available intermediate (8) that could be derived from the commercially available steroids testosterone or DHEA via three-step sequences. These studies feature an expedient installation of the β16-oxidation based on β14-hydroxyl-directed epoxidation and subsequent epoxide rearrangement. The following singlet oxygen oxidation of the C17 furan moiety provides access to oleandrigenin (7) in 12 steps (LLS) and a 3.1% overall yield from 8. The synthetic oleandrigenin (7) was successfully glycosylated with l-rhamnopyranoside-based donor 28 using a Pd(II)-catalyst, and the subsequent deprotection under acidic conditions provided cytotoxic natural product rhodexin B (2) in a 66% yield (two steps).

Palladium(II)-assisted activation of thioglycosides

Described herein is the first example of glycosidation of thioglycosides in the presence of palladium(ii) bromide. While the activation with PdBr2 alone was proven feasible, higher yields and cleaner reactions were achieved when these glycosylations were performed in the presence of propargyl bromide as an additive. Preliminary mechanistic studies suggest that propargyl bromide assists the reaction by creating an ionizing complex, which accelerates the leaving group departure. A variety of thioglycoside donors in reactions with different glycosyl acceptors were investigated to determine the initial scope of this new reaction. Selective and chemoselective activation of thioglycosides over other leaving groups has also been explored.

Efficient and reproducible synthesis of an Fmoc-protected Tn antigen

Glycoconjugate ready for solid-phase-peptide synthesis is scalably accessible using a palladium-mediated glycosylation.

Direct N-glycosylation of tosyl and nosyl carbamates with trichloroacetimidate donors

Acidic sulphonamide reactants act as both catalysts and nucleophiles to afford the desired N-glycofuranosyl sulfonamides stereoselectively.

Iridium-promoted deoxyglycoside synthesis: stereoselectivity and mechanistic insight

Herein, we devised a method for stereoselective O-glycosylation using an Ir(i)-catalyst which enables both hydroalkoxylation and nucleophilic substitution of glycals with varying substituents at the C3 position. In this transformation, 2-deoxy-α-O-glycosides were acquired when glycals equipped with a notoriously poor leaving group at C3 were used; in contrast 2,3-unsaturated-α-O-glycosides were produced from glycals that bear a good leaving group at C3. Mechanistic studies indicate that both reactions proceed via the directing mechanism, through which the acceptor coordinates to the Ir(i) metal in the α-face-coordinated Ir(i)-glycal π-complex and then attacks the glycal that contains the O-glycosidic bond in a syn-addition manner. This protocol exhibits good functional group tolerance and is exemplified with the preparation of a library of oligosaccharides in moderate to high yields and with excellent stereoselectivities.

Ir(i)-catalyzed α-selective O-glycosylation of glycals provided an access to both 2-deoxyglycosides and 2,3-unsaturated glycosides with a broad substrate scope. The underlying rationale of α-selectivity has been illustrated by the DFT study.

Transition metal catalyzed glycosylation reactions - an overview

Carbohydrates are a large class of natural products that play key roles in a number of biological processes such as in cellular communication or disease progression. Carbohydrates are also used as vaccines and pharmaceuticals. Their synthesis through glycosylation reactions is challenging, and often stoichiometric amounts of promoters are required. Transition metal catalyzed glycosylation reactions are far less common, but can have advantages with respect to reaction conditions and selectivity. The review intends to approach the topic from the catalysis and carbohydrate perspective to encourage researchers from both the fields to perform research in the area. The article covers the basics in glycosylation and catalysis chemistry. The catalysts for the reaction can be roughly divided into two groups. In one group, the catalysts serve as Lewis acids. In the other group, the catalysts play a higher sophisticated role, are involved in all elementary steps of the mechanism and remain coordinated to the substrate throughout the whole catalytic cycle. Based on selected examples, the main trends in transition metal catalyzed glycosylation reactions are explained. Lewis acid catalysts tend to require a somewhat higher catalyst load compared to other organometallic catalysts. The reaction conditions such as the temperature and time depend in many cases on the leaving group employed. An outlook is also presented. The article is not meant to be comprehensive; it outlines the most common transition metal catalyzed processes with the intention to bring the catalysis and carbohydrate communities together and to inspire research activities in both areas.

Highly Selective β-Mannosylations and β-Rhamnosylations Catalyzed by Bis-thiourea

We report highly β-selective bis-thioureas-catalyzed 1,2-cis-O-pyranosylations employing easily accessible acetonide-protected donors. A wide variety of alcohol nucleophiles, including complex natural products, glycosides, and amino acids were β-mannosylated and β-rhamnosylated successfully using an operationally simple protocol under mild and neutral conditions. Less nucleophilic acceptors such as phenols were also glycosylated efficiently in excellent yields and with high β-selectivities.

Biomimetic synthesis of nudicaulins I and II, yellow pigments from the Iceland poppy Papaver nudicaule

Indole and the anthocyanin orientalin proceed through a unique cascade sequence that leads to nudicaulins I and II in 92% yield. This biomimetic synthesis confirms the biosynthesis proposal for these structurally unprecedented flavoalkaloids that play a key role in the colour range displayed by the Iceland poppy.

Are Brønsted Acids the True Promoter of Metal-Triflate-Catalyzed Glycosylations? A Mechanistic Probe into 1,2-cis-Aminoglycoside Formation by Nickel Triflate

Metal triflates have been utilized to catalytically facilitate numerous glycosylation reactions under mild conditions. In some methods, the metal triflate system provides stereocontrol during the glycosylation, rather than the nature of protecting groups on the substrate. Despite these advances, the true activating nature of metal triflates remains unclear. Our findings indicated that the in situ generation of trace amounts of triflic acid from metal triflates can be the active catalyst species in the glycosylation. This fact has been mentioned previously in metal triflate-catalyzed glycosylation reactions; however, a thorough study on the subject and its implications on stereoselectivity has yet to be performed. Experimental evidence from control reactions and 19F NMR spectroscopy have been obtained to confirm and quantify the triflic acid released from nickel triflate, for which it is of paramount importance in achieving a stereoselective 1,2-cis-2-amino glycosidic bond formation via a transient anomeric triflate. A putative intermediate resembling that of a glycosyl triflate has been detected using variable temperature NMR (1H and 13C) experiments. These observations, together with density functional theory calculations and a kinetic study, corroborate a mechanism involving triflic acid-catalyzed stereoselective glycosylation with N-substituted trifluoromethylbenzylideneamino protected electrophiles. Specifically, triflic acid facilitates formation of a glycosyl triflate intermediate which then undergoes isomerization from the stable α-anomer to the more reactive β-anomer. Subsequent SN2-like displacement of the reactive anomer by a nucleophile is highly favorable for the production of 1,2-cis-2-aminoglycosides. Although there is a previously reported work regarding glycosyl triflates, none of these reports have been confirmed to come from the counter ion of the metal center. Our work provides supporting evidence for the induction of a glycosyl triflate through the role of triflic acid in metal triflate-catalyzed glycosylation reactions.

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.

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.

2- O- N-Benzylcarbamoyl as a Protecting Group To Promote β-Selective Glycosylation and Its Applications in the Stereoselective Synthesis of Oligosaccharides

This study examines the utility of the N-benzylcarbamoyl (BnCar) protecting group in glycosylation reactions of the parent O-2 protected carbohydrate donor. It was found that the BnCar group imparted exclusively β-selectivity with primary and secondary alcohols. A mechanistic study revealed the activated intermediate to be the glycosyl triflate in a skew conformation, which results in β-selective glycosylation via an S_N2-like pathway. The BnCar group can be readily cleaved using tetrabutylammonium nitrite, without affecting ester and ether protecting groups. Taken together, these results show BnCar to be useful for the synthesis of complex oligosaccharides, an undertaking that requires delicate chemical differentiation of various protecting groups.

Stereocontrolled glycoside synthesis by activation of glycosyl sulfone donors with scandium(iii) triflate

Activation of armed glycosyl sulfone donors, using scandium(iii) triflate under microwave irradiation, provides a selective preparation of α-mannosides.

Sweet Poisons: Synthetic Strategies towards Tutin Glycosides

The polycyclic, polyoxygenated picrotoxane tutin was subjected to various glycosylation reaction conditions in an effort to synthesise β-linked tutin glycosides, recently found in toxic honeys. Cationic palladium-mediated glycosylation of tutin was successful; however, the α-linked tutin tetrabenzyl glucoside was obtained as the major product (5 : 1, α : β). Hydrogenolysis of the benzyl ether protecting groups resulted in concomitant tutin double-bond migration. Epoxide opening and rearrangement were observed upon acetylation of the tutin glucoside.

Utilization of bench-stable and readily available nickel(II) triflate for access to 1,2-cis-2-aminoglycosides

The utilization of substoichiometric amounts of commercially available nickel(II) triflate as an activator in the reagent-controlled glycosylation reaction for the stereoselective construction of biologically relevant targets containing 1,2-cis-2-amino glycosidic linkages is reported. This straightforward and accessible methodology is mild, operationally simple and safe through catalytic activation by readily available Ni(OTf)₂ in comparison to systems employing our previously in-house prepared Ni(4-F-PhCN)₄(OTf)₂. We anticipate that the bench-stable and inexpensive Ni(OTf)₂, coupled with little to no extra laboratory training to set up the glycosylation reaction and no requirement of specialized equipment, should make this methodology be readily adopted by non-carbohydrate specialists. This report further highlights the efficacy of Ni(OTf)₂ to prepare several bioactive motifs, such as blood type A-type V and VI antigens, heparin sulfate disaccharide repeating unit, aminooxy glycosides, and α-GalNAc-Serine conjugate, which cannot be achieved in high yield and α-selectivity utilizing in-house prepared Ni(4-F-PhCN)₄(OTf)₂ catalyst. The newly-developed protocol eliminates the need for the synthesis of Ni(4-F-PhCN)₄(OTf)₂ and is scalable and reproducible. Furthermore, computational simulations in combination with ¹H NMR studies analyzed the effects of various solvents on the intramolecular hydrogen bonding network of tumor-associated mucin Fmoc-protected GalNAc-threonine amino acid antigen derivative, verifying discrepancies found that were previously unreported.

Chemical O-Glycosylations: An Overview

The development of glycobiology relies on the sources of particular oligosaccharides in their purest forms. As the isolation of the oligosaccharide structures from natural sources is not a reliable option for providing samples with homogeneity, chemical means become pertinent. The growing demand for diverse oligosaccharide structures has prompted the advancement of chemical strategies to stitch sugar molecules with precise stereo- and regioselectivity through the formation of glycosidic bonds. This Review will focus on the key developments towards chemical O-glycosylations in the current century. Synthesis of novel glycosyl donors and acceptors and their unique activation for successful glycosylation are discussed. This Review concludes with a summary of recent developments and comments on future prospects.

Stereoselective synthesis of glycosides using (salen)Co catalysts as promoters

The use of (salen)Co catalysts as a new class of bench-stable stereoselective glycosylation promoters of trichloroacetimidate glycosyl donors at room temperature is described. The conditions are practical and do not require the use of molecular sieves with products being isolated in good to high yields.

Tandem Allylboration-Prins Reaction for the Rapid Construction of Substituted Tetrahydropyrans: Application to the Total Synthesis of (-)-Clavosolide A

Tetrahydropyrans are common motifs in natural products and have now been constructed with high stereocontrol through a three‐component allylboration‐Prins reaction sequence. This methodology has been applied to a concise (13 steps) and efficient (14 % overall yield) synthesis of the macrolide (−)‐clavosolide A. The synthesis also features an early stage glycosidation reaction to introduce the xylose moiety and a lithiation‐borylation reaction to attach the cyclopropyl‐containing side chain.

Iron(iii) chloride modulated selective 1,2-trans glycosylation based on glycosyl trichloroacetimidate donors and its application in orthogonal glycosylation

FeCl₃ modulated excellent 1,2-trans selective glycosylations based on trichloroacetimidate glycosyl donors even in the presence of apparently silent C-2 protecting group, along with orthogonal glycosylation reactions are reported.

Studies of Highly-Ordered Heterodiantennary Mannose/Glucose-Functionalized Polymers and Concanavalin A Protein Interactions Using Isothermal Titration Calorimetry

Preparations of the highly ordered monoantennary, homofunctional diantennary, and heterofunctional diantennary neoglycopolymers of α-d-mannose and β-d-glucose residues were achieved via ring-opening metathesis polymerization. Isothermal titration calorimetry measurements of these synthetic neoglycopolymers with Concanavalin A (Con A), revealed that heterofunctional diantennary architectures bearing both α-mannose and nonbinding β-glucose units, poly(Man-Glc), binds to Con A (Ka = 16.1 × 10(6) M(-1)) comparably to homofunctional diantennary neoglycopolymer (Ka = 30 × 10(6) M(-1)) bearing only α-mannose unit, poly(Man-Man). In addition, poly(Man-Glc) neoglycopolymer shows a nearly 5-fold increasing in binding affinity compared to monoantennary neoglycopolymer, poly(Man). Although the exact mechanism for the high binding affinity of poly(Man-Glc) to Con A is unclear, we hypothesize that the α-mannose bound to Con A might facilitate interaction of β-glucose with the extended binding site of Con A due to the close proximity of β-glucose to α-mannose residues in the designed polymerizable scaffold.

Stereocontrolled 1,2-cis glycosylation as the driving force of progress in synthetic carbohydrate chemistry

Recent developments in stereoselective 1,2-cis glycosylation that have emerged during the past decade are surveyed herein.

Recent developments in stereoselective 1,2-cis glycosylation that have emerged during the past decade are surveyed herein. For detailed coverage of the previous achievements in the field the reader is referred to our earlier reviews: A. V. Demchenko, Curr. Org. Chem. , 2003, 7 , 35–79 and Synlett , 2003, 1225–1240.

Reversing the stereoselectivity of a palladium-catalyzed O-glycosylation through an inner-sphere or outer-sphere pathway

An efficient and concise method for the construction of various O-glycosidic bonds by a palladium-catalyzed reaction with a 3-O-picoloyl glucal has been developed. The stereochemistry of the anomeric center derives from either an inner-sphere or outer-sphere pathway. Harder nucleophiles, such as aliphatic alcohols and sodium phenoxides give β-products, and α products result from using softer nucleophiles, such as phenol.

Investigations of scope and mechanism of nickel-catalyzed transformations of glycosyl trichloroacetimidates to glycosyl trichloroacetamides and subsequent, atom-economical, one-step conversion to α-urea-glycosides

The development and mechanistic investigation of a highly stereoselective methodology for preparing α-linked-urea neo-glycoconjugates and pseudo-oligosaccharides is described. This two-step procedure begins with the selective nickel-catalyzed conversion of glycosyl trichloroacetimidates to the corresponding α-trichloroacetamides. The α-selective nature of the conversion is controlled with a cationic nickel(II) catalyst, [Ni(dppe)(OTf)2 ] (dppe=1,2-bis(diphenylphosphino)ethane, OTf=triflate). Mechanistic studies have identified the coordination of the nickel catalyst with the equatorial C2 -ether functionality of the α-glycosyl trichloroacetimidate to be paramount for achieving an α-stereoselective transformation. A cross-over experiment has indicated that the reaction does not proceed in an exclusively intramolecular fashion. The second step in this sequence is the direct conversion of α-glycosyl trichloroacetamide products into the corresponding α-urea glycosides by reacting them with a wide variety of amine nucleophiles in presence of cesium carbonate. Only α-urea-product formation is observed, as the reaction proceeds with complete retention of stereochemical integrity at the anomeric CN bond.

Recent Advances in Transition Metal-Catalyzed Glycosylation

Having access to mild and operationally simple techniques for attaining carbohydrate targets will be necessary to facilitate advancement in biological, medicinal, and pharmacological research. Even with the abundance of elegant reports for generating glycosidic linkages, stereoselective construction of α- and β-oligosaccharides and glycoconjugates is by no means trivial. In an era where expanded awareness of the impact we are having on the environment drives the state-of-the-art, synthetic chemists are tasked with developing cleaner and more efficient reactions for achieving their transformations. This movement imparts the value that prevention of waste is always superior to its treatment or cleanup. This review will highlight recent advancement in this regard by examining strategies that employ transition metal catalysis in the synthesis of oligosaccharides and glycoconjugates. These methods are mild and effective for constructing glycosidic bonds with reduced levels of waste through utilization of sub-stoichiometric amounts of transition metals to promote the glycosylation.

Selective formation of beta-O-aryl glycosides in the absence of the C(2)-ester neighboring group

The development of a general and practical method for the stereoselective synthesis of beta-O-aryl glycosides that exploits the nature of a cationic palladium(II) catalyst, instead of a C(2)-ester directing group, to control the beta-selectivity is described. This beta-glycosylation reaction is highly diastereoselective and requires 2-3 mol % of Pd(CH(3)CN)(4)(BF(4))(2) to activate glycosyl trichloroacetimidate donors at room temperature. The current method has been applied to d-glucose, d-galactose, and d-xylose donors with a nondirecting group incorporated at the C(2)-position to provide the O-aryl glycosides with good to excellent beta-selectivity. In addition, its application is widespread to electron-donating, electron-withdrawing, and hindered phenols. The reaction is likely to proceed through a seven-membered ring intermediate, wherein the palladium catalyst coordinates to both C(1)-trichloroacetimidate nitrogen and C(2)-oxygen of the donor, blocking the alpha-face. As a result, the phenol nucleophile preferentially approaches to the top face of the activated donor, leading to formation of the beta-O-aryl glycoside.