Stereoretentive C(sp3)–S Cross-Coupling

Stereoselective Synthesis of β-S-Glycosides via Palladium Catalysis

S-Glycosides are more resistant to enzymatic and chemical hydrolysis and exhibit higher metabolic stability than common O-glycosides, demonstrating their widespread application in biological research and drug development. In particular, β-S-glycosides are used as antirheumatic, anticancer, and antidiabetic drugs in clinical practice. However, the stereoselective synthesis of β-S-glycosides is still highly challenging. Herein, we report an effective β-S-glycosylation using 3-O-trichloroacetimidoyl glycal and thiols under mild conditions. The C3-imidate is designed to guide Pd to form a complex with glucal from the upper face, followed by Pd-S (thiols) coordination to realize β-stereoselectivity. This method demonstrates excellent compatibility with a broad scope of various thiol acceptors and glycal donors with yields up to 87% and a β/α ratio of up to 20:1. The present β-S-glycosylation strategy is used for late-stage functionalization of drugs/natural products such as estrone, zingerone, and thymol. Overall, this novel and simple operation approach provides a general and practical strategy for the construction of β-thioglycosides, which holds high potential in drug discovery and development.

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.

Preparing glycosyl benzothiazoles from 2-isocyanoaryl thioethers and glycosyl radicals under thermal conditions

Herein, we report a method for preparing glycosyl benzothiazoles via radical cascade cyclization, in which glycosyl radicals are generated from readily available and bench-stable allyl glycosyl sulfones. This cascade reaction proceeds under simple conditions and tolerates a broad substrate scope in high yield with excellent stereoselectivity. Mechanistic studies support that the reactions proceed via the intermediacy of imidoyl radicals, which attack the appended sulfide unit by a SH2 process to forge the thiazole ring.

Electrochemical nickel-catalyzed cross-coupling of glycosyl thiols with preactivated phenols and ketones

An efficient electrochemical nickel-catalyzed cross-coupling reaction has been reported here for the synthesis of S-glycosides from preactivated phenols and ketones under mild conditions. Various glycosyl thiols, including unprotected sugar, and a diverse range of aryl/alkenyl triflates, including some complex biorelevant phenols and ketones, were well tolerated in this method.

Photoredox Nickel-Catalysed Stille Cross-Coupling Reactions

The Stille cross-coupling reaction is one of the most common strategies for the construction of C-C bonds. Despite notable strides in the advancement of the Stille reaction, persistent challenges persist in hindering its greener evolution. These challenges encompass multiple facets, such as the high cost of precious metals and ligands, the demand for various additives, and the slow reaction rate. In comparison to the dominant palladium-catalysed Stille reactions, cost-effective nickel-catalysed systems lag behind, and enantioconvergent Stille reactions of racemic stannanes remain undeveloped. Herein, we present a pioneering instance of nickel-catalysed enantioconvergent Stille cross-coupling reactions of racemic stannane reagents, resulting in the formation of C-C bonds in good to high yields with excellent stereoselectivity. This strategy provides a practical, scalable, and operationally straightforward method for the synthesis of C(sp3 )-C(sp3 ), C(sp3 )-C(sp2 ), and C(sp3 )-C(sp) bonds under exceptionally mild conditions (without additives and bases, ambient temperature). The innovative use of synergistic photoredox/nickel catalysis enables a novel single-electron transmetalation process of stannane reagents, providing a new research paradigm of Stille reactions.

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.

New syntheses of thiosaccharides utilizing substitution reactions

Novel synthetic methods published since 2005 affording carbohydrates containing sulfur atom(s) are reviewed. The review is divided to subchapters based on the position of sulfur atom(s) in the sugar molecule. Only those methods that take advantage of substitution are discussed.

A Robust Copper-Catalyzed Cross-Coupling of Glycosyl Thiosulfonate and Boronic Acids Enables the Construction of Thioglycosides

An efficient and stereoretentive copper-catalyzed cross-coupling of glycosyl thiosulfonate and boronic acid for the construction of thioglycosides is described. The good functional group compatibility of this method allows the preparation of many bioactive aryl/alkenyl thioglycosides, including the hSGLT1 inhibitor.

Regio- and Stereoselective Organocatalyzed Relay Glycosylations To Synthesize 2-Amino-2-deoxy-1,3-dithioglycosides

Herein, we describe a novel methodology for the regio- and stereoselective convergent synthesis of 2-amino-2-deoxy-dithioglycosides via one-pot relay glycosylation of 3-O-acetyl-2-nitroglucal donors. This unique organo-catalysis relay glycosylation features excellent site- and stereoselectivity, good to excellent yields, mild reaction conditions, and broad substrate scope. 2-Amino-2-deoxy-glucosides/mannosides bearing 1,3-dithio-linkages were efficiently obtained from 3-O-acetyl-2-nitroglucal donors in both stepwise and one-pot glycosylation protocols. The dithiolated O-antigen of E. coli serogroup 64 was successfully synthesized using this newly developed method.

Utilizing Reusable Catalyst Phosphotungstic Acid for the Synthesis of Thioglycoside from Per-O-acetyl Saccharides with Microwave-Assisted and De-O-acetylation with Methanol

Traditional methods for synthesizing complex oligosaccharides currently developed are not efficient, requiring a new glycosylation methodology. Herein, using phosphotungstic acid (PTA) as a catalyst has demonstrated to be a simple possibility for carbohydrate synthesis. The methodology is engineered into a PTA-catalyzed thioglycoside preparation under microwave conditions and de-O-acetylation of carbohydrates. These easier operations and convenient protocols display a wide substrate scope. Moreover, both methods can be developed into a one-pot reaction for the efficient synthesis of carbohydrate analogues.

Nonbasic Synthesis of Thioethers via Nickel-Catalyzed Reductive Thiolation Utilizing NBS-Like N-Thioimides as Electrophilic Sulfur Donors

The nonbasic synthesis of unsymmetrical thioethers via nickel-catalyzed reductive thiolation between aryl(hetero) iodides and N-thioimides is illustrated. N-Bromosuccinimide (NBS)-like N-thioimides were found quite reactive toward thiolation with carbon electrophiles, and a series of structurally varied thioethers were successfully prepared under mild reaction conditions. The transformation was featured with the new application of the NBS-like reagents, good functional group tolerance, and late-stage modification of biologically active scaffolds, thus providing an expeditious and efficient platform to construct polyfunctional thioethers.

Generation and Use of Glycosyl Radicals under Acidic Conditions: Glycosyl Sulfinates as Precursors

We herein report a method that enables the generation of glycosyl radicals under highly acidic conditions. Key to the success is the design and use of glycosyl sulfinates as radical precursors, which are bench-stable solids and can be readily prepared from commercial starting materials. This development allows the installation of glycosyl units onto pyridine rings directly by the Minisci reaction. We further demonstrate the utility of this method in the late-stage modification of complex drug molecules, including the anticancer agent camptothecin. Experimental studies provide insight into the reaction mechanism.

Atom-Economic Synthesis of Unsymmetrical gem-Diarylmethylthio/Seleno Glycosides via Base Mediated C(O)-S/Se Bond Cleavage and Acyl Transfer Approach of Glycosylthio/Selenoacetates

Herein, we invented the Cs₂CO₃-mediated atom economic method that streamlines the scission of the C(O)-S/Se bond involving the in situ generation of an anomeric thiolate/selenolate anion, which reacted with p-QMs to yield novel unsymmetrical gem-diarylmethylthio/seleno glycosides while retaining the anomeric stereochemistry. Notably, the key features of this protocol involve unprecedented long-range acyl transfer (from S/Se to O), thus affording acylation of the final product which is not yet reported by classical methods. This straightforward protocol offers a mild, short reaction time, synthetically simple approach, and compatibility with 8 types of sugar along with phenylthio/benzylseleno esters.

Copper-Catalyzed C2- or C3-Thioglycosylation of Indoles with N-(Thioglycosides)succinimides: An Effective Strategy for the Total Synthesis of Isatindigotindolosides

Isatindigotindolosides, indoles containing a 1-S-β-glucopyranosyl unit at position C2, show promising bioactivity. Here, we report a copper-catalyzed C2- or C3-thioglycosylation of indoles with N-(thioglycosides)succinimides to construct indole alkaloid glucosides. This reaction is widely tolerant of functional groups, as various indoles and thioglycosides are suitable. It also provides a reliable method for performing late-stage modifications of natural products, such as gramine and melatonin. Total syntheses of isatindigotindolosides I and II were successfully accomplished using the C2-thioglycosylation reaction as a key step.

Efficient Aerobic Oxidative Coupling of Methyl Heteroarenes with Indoles

Direct oxidative coupling of inert C(sp³ )-H bond has been a great challenge. Herein, an environmentally friendly aerobic oxidative coupling of α-methyl substituted N-heteroarenes with indoles is reported. A variety of diheteroaryl ketones were prepared in good yields (up to 72 %). This protocol features simple operation and broad substrates scope (26 examples). Significantly, a plausible mechanism about catalytic cycle was proposed, and two key intermediates were confirmed by high resolution mass spectrometry.

Stereoselective Synthesis of 2-Deoxythiosugars from Glycals

2-deoxythiosugars are more stable than 2-deoxysugars occurring broadly in bioactive natural products and pharmaceutical agents. An effective and direct methodology to stereoselectively synthesize α-2-deoxythioglycosides catalyzed by AgOTf has been developed. Various alkyl thiols and thiophenols were explored and the desired products were formed in good yields with excellent α-selectivity. This method was further applied to the syntheses of S-linked disaccharides and late-stage 2-deoxyglycosylation of estrogen, L-menthol, and zingerone thiols successfully.

A detailed theoretical investigation to unravel the molecular mechanism of the ligand-free copper-catalyzed Suzuki cross-coupling reaction

The Suzuki-Miyaura coupling (SMC) represents a very efficacious method for constructing C-C bonds in organic synthesis. The ligand-free variants of SMC have been grabbing attention these days. Despite this momentousness, the mechanistic details of the ligand-free variants are scant in the literature. Herein, we have carried out a detailed mechanistic investigation into the ligand-free Cu-catalyzed SMC of unsaturated organic halides with aryl boronic acid with the aid of density functional theory (DFT) calculations employing the conductor-like polarizable continuum model (CPCM) method. The present study elucidates that in the absence of ancillary ligands on the metal, the substrates, base, and solvent molecules could act as pseudo-ancillary ligands to facilitate the cross-coupling reaction. The investigation further revealed that unsaturated halides like alkynyl halides/vinyl halides could act as good ancillary ligands for copper by forming a Cu-π intermediate and promoting a facile transmetalation process. However, regarding the oxidative addition and reductive elimination steps, a concerted pathway is observed contrary to Pd catalyzed Suzuki coupling, owing to the instability of Cu(III) species and the favourability of Csp²-Csp bond formation. In the whole set of mechanisms explored, oxidative addition/oxidative nucleophilic substitution was the rate-determining step in all the cases. A thermodynamically stable π-coordinated intermediate species where the substrate and base molecule are coordinated to the metal center is identified as the rate-determining species for the ligand-free Suzuki cross-coupling reaction. The presence of the aforesaid intermediate increases the energy span and consequently the activation barrier for the rate-determining step. This study unveiled a theoretical rationale for the high-temperature requirement in the ligand-free Cu-catalyzed SMC reaction.

Visible-Light-Mediated Stadler-Ziegler Arylation of Thiosugars with Anilines

Here, we report a one-pot Stadler-Ziegler reaction toward the synthesis of 1-thioglycosides in good yield from commercially available anilines and (un)protected 1-glycosyl thiols. This simple and mild approach employs the photoredox catalyst [Ru(bpy)₃](PF₆)₂ under visible light.

Boron-Promoted Umpolung Reaction of Sulfonyl Chlorides for the Stereospecific Synthesis of Thioglycosides via Reductive Deoxygenation Coupling Reactions

S-Glycosides have broad biological activities and serve as stable mimics of natural O-glycoside counterparts and thus are of great therapeutic potential. Herein we disclose an efficient method for the stereospecific synthesis of 1-thioglycosides via a boron-promoted reductive deoxygenation coupling reaction from readily accessible sulfonyl chlorides and glycosyl bromides. Our protocol features mild conditions and excellent functional group tolerance and stereoselectivity. The translational potential of this metal-free approach is demonstrated by the late-stage glycodiversification of natural products and drug molecules.

Diastereoselective Synthesis of Thioglycosides via Pd-Catalyzed Allylic Rearrangement

Stereoselective glycosylation is challenging in carbohydrate chemistry. Herein, stereoselective thioglycosylation of glycals via palladium-catalyzed allylic rearrangement yields various substituents on α-isomer thioglycosides. Two comprehensive series of aryl and benzyl thioglycosides were obtained via a combination of thiosulfates with glycals derived from glucose, arabinose, galactose, and rhamnose. Furthermore, diosgenyl α-l-rhamnoside and isoquercitrin achieved selectivity via stereospecific [2,3]-sigma rearrangements of α-sulfoxide-rhamnoside and α-sulfoxide-glucoside, respectively.

Electrochemical Deoxygenative Thiolation of Preactivated Alcohols and Ketones

This work describes an electrochemically promoted nickel-catalyzed deoxygenative thiolation of alcohols and ketones under mild conditions. Excellent substrate tolerance and good chemical yields can be achieved by graphene/nickel foam electrodes in an undivided cell. Further study to gain mechanistic insight into this electrochemical cross-coupling has been carried out.

Light-Promoted and Tertiary-Amine-Assisted Hydroxysulfenylation of Alkenes: Selective and Direct One-Pot Synthesis of β-Hydroxysulfides

A light-promoted and tertiary-amine-assisted strategy for efficient hydroxysulfenylation of both electron-rich and electron-deficient alkenes with thiophenols to selectively and directly access β-hydroxysulfides in one pot is described herein. In contrast to the previously reported thiol-oxygen co-oxidation reactions, this simple and sustainable approach features mild reaction conditions, high efficiency, and excellent functional group tolerance.

Organometallic AlaM Reagents for Umpolung Peptide Diversification

Selective modifications of peptides and proteins have emerged as a promising strategy to develop novel mechanistic probes and prepare compounds with translational potentials. Here, we report alanine carbastannatranes AlaSn as a universal synthon in various C-C and C-heteroatom bond-forming reactions. These reagents are compatible with peptide manipulation techniques and can undergo chemoselective conjugation in minutes when promoted by Pd(0). Despite their increased nucleophilicity and propensity to transfer the alkyl group, C(sp3)-C(sp2) coupling with AlaSn can be accomplished at room temperature under buffered conditions (pH 6.5-8.5). We also show that AlaSn can be easily transformed into several canonical L- and D-amino acids in arylation, acylation, and etherification reactions. Furthermore, AlaSn can partake in macrocyclizations exemplified by the synthesis of medium size cyclic peptides with various topologies. Taken together, metalated alanine AlaSn demonstrates unparalleled scope and represents a new type of umpolung reagents suitable for structure-activity relationship studies and peptide diversification.

Nonenzymatic Stereoselective S-Glycosylation of Polypeptides and Proteins

Here we report a nonenzymatic glycosylation reaction that builds axial S-glycosidic bonds under biorelevant conditions. This strategy is enabled by the design and use of allyl glycosyl sulfones as precursors to glycosyl radicals and exploits the exceptional functional group tolerance of radical processes. Our method introduces a variety of unprotected glycosyl units to the cysteine residues of peptides in a highly selective fashion. Through developing the second-generation protocol, we applied our method in the direct glycosylation of complex polypeptides and proteins. Computational studies were performed to elucidate the reaction mechanism.

Modular Synthesis of Aryl Thio/Selenoglycosides via the Catellani Strategy

We described a novel palladium-catalyzed domino procedure for the preparation of (hetero)aryl thio/selenoglycosides. Readily available (hetero)aryl iodides and easily accessible 1-thiosugars/1-selenosugars are utilized as the substrates. Meanwhile, 10 types of sugars are quite compatible with this reaction with good regio- and stereoselectivity, high efficiency, and broad applicability (up to 89%, 53 examples). This method enables the straightforward formation of the C(sp²)-S/Se bond of (hetero)aryl thio/selenoglycosides.

Transition-metal-free synthesis of aryl 1-thioglycosides with arynes at room temperature

A mild, convenient and transition-metal-free protocol for the synthesis of aryl 1-thioglycosides is presented via arynes generated in situ combined with glycosyl thiols in the presence of TBAF(tBuOH)₄. The methodology provides a general and efficient way to prepare a series of functionalized thioglycosides in good to excellent yields with a perfect control of the anomeric configuration at room temperature. In addition, the reaction conditions tolerate a variety of the pentoses and hexoses, and the reaction also performs smoothly on protected monosaccharides and disaccharides.

Rhenium(V)-catalyzed synthesis of 1,1'-2-deoxy thioglycosides

As stable glycomimetics, thioglycosides are important tools for the investigation of biological processes and discovery of new drugs. In this note, we report a ReOCl₃(SMe₂)(OPPh₃) catalyzed coupling reaction between β-glycosyl thiols (1-thio sugars) and glycals for the preparation of 1,1'-α,β-2-deoxy thioglycosides, which are glycomimetics of natural trehalose and 2-deoxy glycosides. Furthermore, an S-linked trisaccharide was successfully obtained by successive employment of the Re(V) catalyzed thioglycosylation protocol.

Light-Mediated Cross-Coupling of Anomeric Trifluoroborates

Stereoselective reactions at the anomeric carbon constitute the cornerstone of preparative carbohydrate chemistry. Here, we report stereoselective C-arylation and etherification reactions of anomeric trifluoroborates derived from BMIDA esters. These reactions are characterized by high anomeric selectivities for 2-deoxysugars and broad substrate scope (24 examples), including disaccharides and trifluoroborates with free hydroxyl groups. Taken together, this new class of carbohydrate reagents adds the palette of anomeric nucleophile reagents suitable for efficient installation of C-C bonds.

Generation of Glycosyl Radicals from Glycosyl Sulfoxides and Its Use in the Synthesis of C-linked Glycoconjugates

We here report glycosyl sulfoxides appended with an aryl iodide moiety as readily available, air and moisture stable precursors to glycosyl radicals. These glycosyl sulfoxides could be converted to glycosyl radicals by way of a rapid and efficient intramolecular radical substitution event. The use of this type of precursors enabled the synthesis of various complex C-linked glycoconjugates under mild conditions. This reaction could be performed in aqueous media and is amenable to the synthesis of glycopeptidomimetics and carbohydrate-DNA conjugates.

Direct, stereoselective thioglycosylation enabled by an organophotoredox radical strategy

While strategies involving a 2e- transfer pathway have dictated glycosylation development, the direct glycosylation of readily accessible glycosyl donors as radical precursors is particularly appealing because of high radical anomeric selectivity and atom- and step-economy. However, the development of the radical process has been challenging owing to notorious competing reduction, elimination and/or SN side reactions of commonly used, labile glycosyl donors. Here we introduce an organophotocatalytic strategy through which glycosyl bromides can be efficiently converted into corresponding anomeric radicals by photoredox mediated HAT catalysis without a transition metal or a directing group and achieve highly anomeric selectivity. The power of this platform has been demonstrated by the mild reaction conditions enabling the synthesis of challenging α-1,2-cis-thioglycosides, the tolerance of various functional groups and the broad substrate scope for both common pentoses and hexoses. Furthermore, this general approach is compatible with both sp2 and sp3 sulfur electrophiles and late-stage glycodiversification for a total of 50 substrates probed.

Ligand-Free Copper(I)-Mediated Cross-Coupling Reactions of Organostannanes with Sulfur Electrophiles

The synthesis of aryl thioether through the cross-coupling of C-S bond is a highly attractive area of research due to the prevalence of aryl thioether in bioactive natural products, functional materials, agrochemicals, and pharmaceutically active compounds. Herein, we report a ligand-free Cu(I) mediated electrophilic thiolation of organostannanes with sulfur electrophiles. A selective transfer of alkyl groups was achieved in reactions with alkyl carbastannatranes affording congested thioethers. This study offers a unified method to access diaryl and aryl alkyl thioethers and was demonstrated in the context of late-stage modifications..

Catalytic and Photochemical Strategies to Stabilized Radicals Based on Anomeric Nucleophiles

Carbohydrates, one of the three primary macromolecules of living organisms, play significant roles in various biological processes such as intercellular communication, cell recognition, and immune activity. While the majority of established methods for the installation of carbohydrates through the anomeric carbon rely on nucleophilic displacement, anomeric radicals represent an attractive alternative because of their functional group compatibility and high anomeric selectivities. Herein, we demonstrate that anomeric nucleophiles such as C1 stannanes can be converted into anomeric radicals by merging Cu(I) catalysis with blue light irradiation to achieve highly stereoselective C(sp³)-S cross-coupling reactions. Mechanistic studies and DFT calculations revealed that the C-S bond-forming step occurs via the transfer of the anomeric radical directly to a sulfur electrophile bound to Cu(II) species. This pathway complements a radical chain observed for photochemical metal-free conditions where a disulfide initiator can be activated by a Lewis base additive. Both strategies utilize anomeric nucleophiles as efficient radical donors and achieve a switch from an ionic to a radical pathway. Taken together, the stability of glycosyl nucleophiles, a broad substrate scope, and high anomeric selectivities observed for the thermal and photochemical protocols make this novel C-S cross coupling a practical tool for late-stage glycodiversification of bioactive natural products and drug candidates.

Reactivity of a formal Cu(iii)-alkyl species toward aniline: a DFT investigation

In this work, through theoretical investigation of the reactivity between a formal Cu(iii)-alkyl species and aniline, we demonstrated the possibility of a S_N2-like C–N coupling mechanism.

Facile triflic acid-catalyzed α-1,2-cis-thio glycosylations: scope and application to the synthesis of S-linked oligosaccharides, glycolipids, sublancin glycopeptides, and TN/TF antigens

Studies of S-linked glycoconjugates have attracted growing interest because of their enhanced chemical stability and enzymatic resistance over O-glycoside counterparts.

Studies of S-linked glycoconjugates have attracted growing interest because of their enhanced chemical stability and enzymatic resistance over O-glycoside counterparts. We here report a facile approach to access α-1,2-cis-S-linked glycosides using triflic acid as a catalyst to promote the glycosylation of a series of thiols with d-glucosamine, galactosamine, glucose, and galactose electrophiles. This method is broadly applicable for the stereoselective synthesis of S-linked glycopeptides, oligosaccharides and glycolipids in high yield and excellent α-selectivity. Many of the synthetic limitations associated with the preparation of these S-linked products are overcome by this catalytic method.