Palladium-Catalyzed Stereospecific C-Glycosylation of Glycals with Vinylogous Acceptors

Sugar functionalized coumarin motifs: Synthesis and applications

Sugars are vital biomolecules widely found in nature, playing an indispensable role in a plethora of biological processes. Similarly, coumarins are heterocycles with an effective pharmacophore skeleton, making them crucial in drug design and development. Coupling carbohydrate moieties to the small biologically active molecules creates a vast library of glycoconjugates with impressive structural diversity. The potential of coumarin glycosides is being extensively explored due to their broad spectrum of applications, including antibacterial, anticancer, and anticoagulant properties, etc. This review highlights various chemical methodologies for synthesizing diverse coumarin glycohybrids with distinct linkages and explores their immense biological potential, making a significant contribution to the field of organic synthesis.

Stereoselective Synthesis of Glycosides via Tsuji-Trost Type Glycosylation Using 3,4-Carbonate Galactals

Pd-catalyzed stereoselective glycosylations using unsaturated sugar derivatives, glycals, have been successfully achieved in recent years. This review focuses on approaches to control the stereoselectivities of glycosides via π-allyl intermediates that mimic the Tsuji-Trost asymmetric allylic alkylation reactions, enabling stereoselectivity control through rational design. In the reaction process, zwitterionic Pd-π-allyl complexes, formed after the oxidative addition and decarboxylation, play a crucial role in increasing reactivities and enhancing the stereoselectivities of α- and β-glycosides. We summarized recently developed Tsuji-Trost type glycosylations using 3,4-carbonate galactals, featuring high efficiency, exclusive stereoselectivities, and a broad reaction scope including O-, N-, S-, and C-glycosylations.

Palladium-Catalyzed Regio- and Stereoselective Glycosylation of Azole Heterocycles Enables Access to Diverse Heterocyclic N-Glycosides

An efficient and practical glycosylation platform for synthesizing N-glycosides by leveraging palladium catalysis is disclosed. This approach enables facile access to diverse heterocyclic N-glycosides with excellent regio- and stereoselectivities and high site selectivity of multiple N atoms. The reaction exhibits a broad substrate scope (65 examples), high functional group tolerance, and easy scalability. Its synthetic utility is demonstrated through late-stage functionalization of pharmaceutically relevant molecules and various diastereoselective transformations of the glycoside products. Overall, our method provides a handy tool for efficient and stereocontrolled synthesis of valuable N-glycosylated heterocycles.

Copper-Catalyzed Dual Remote Asymmetric Vinylogous Alkynylallylic Substitution of Yne-Allylic Esters with Coumarins

Chiral coumarins and their derivatives are ubiquitous structural motifs found in an array of biologically and therapeutically active natural products and drugs. Herein, a highly enantioselective dual remote copper-catalyzed vinylogous alkynylallylic substitution of yne-allylic esters with coumarins has been developed. The practicality of this method is exemplified by the use of readily available starting materials; mild reaction conditions; excellent regio-, enantio-, and stereoselectivities; and the very broad substrate scope (67 examples), while the scalability and further applications of this method are illustrated by the gram-scale reaction and the series of derivations of the products.

Developments in the stereoselective synthesis of benzopyran, benzopyrone and flavonoid based natural product analogues using C-glycosides as an intrinsic chiral synthon

Stereoselective synthesis is essential for propelling mainstream academia toward a relentless pursuit of novel and cutting-edge strategies for constructing molecules with unparalleled precision. Naturally derived benzopyrans, benzopyrones, and flavonoids are an essentially prominent group of oxa-heterocycles, highly significant targets in medicinal chemistry owing to their extensive abundance in biologically active natural products and pharmaceuticals. The molecular complexity and stereoselectivity induced by heterocycles embedded with C-glycosides have attracted considerable interest and emerged as a fascinating area of research for synthetic organic chemists. This present article emphasizes the existing growths in the strategies involving the diastereoselective synthesis of C-glycosylated benzopyrans, benzopyrones, and flavonoids using naturally acquired glycones as chiral synthons.

Photocatalyzed Perfluoroalkylation of Endoglycals

The visible light-induced perfluoroalkyl (RF) radical reactions on peracetylglycals derived from hexoses and pentoses (galactal, glucal, arabinal, and xylal derivatives) were investigated. Various photocatalysts and perfluoroalkyl iodides (RF-I) were employed as sources of RF radicals with LEDs as the irradiation source. Particularly noteworthy was the use of an Iridium photocatalyst, Ir[dF(CF3)ppy]2(dtbpy))PF6, which yielded two distinct product types when applied to glucal. On the one hand, the 2-RF-substituted glucal was formed, a trend observed even when utilizing organic dyes as photocatalysts. On the other hand, the unexpected addition product, namely the 1-RF-2-iodo-α-manno-configured C-glycosyl derivative, was also obtained, as a result of a highly regioselective addition reaction of the RF moiety into the anomeric carbon, followed by attachment of the iodine atom on C-2 in axial disposition. This result contrasted with other radical reactions carried out on 2-unsubstituted glycals, where the incipient radical adds to C-2, generating a stabilized 1-glycosyl radical. The photocatalyzed radical perfluoroalkylations of peracetyl glycals derived from galactose, arabinose, and xylose all afforded the 2-RF-substituted glycals in good yields as a result of the expected vinylic substitution reaction. Mechanistic studies revealed that the 1-RF-2-iodo-α-manno-configured C-glycosyl derivatives arise from a radical chain reaction, whereas the 2-RF-substituted glycals proceed from inefficient chain processes.

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.

Synthesis of sugar enones and their use as powerful synthetic precursors of thiodisaccharides

Monosaccharide derivatives having a double bond conjugated to a carbonyl (sugar enones or enuloses) are relevant synthetic tools. They are also suitable starting materials, or versatile intermediates, for the synthesis of a wide variety of natural or synthetic compounds with a broad spectrum of biological and pharmacological activities. The preparation of enones is mainly focused on the search for more efficient and diastereoselective synthetic methodologies. The usefulness of enuloses relies on the diverse reaction possibilities offered by alkene and carbonyl double bonds, which are prone to undergo varied reactions such as halogenation, nitration, epoxidation, reduction, addition, etc. The addition of thiol groups that led to sulfur glycomimetics, such as thiooligosaccharides, is particularly relevant. Therefore, the synthesis of enuloses and the Michael addition of sulfur nucleophiles to give thiosugars or thiodisaccharides are discussed here. Chemical modifications of the conjugate addition products to afford biologically active compounds are also reported.

Reagent-Controlled Regiodivergent Annulations of Achmatowicz Products with Vinylogous Nucleophiles: Synthesis of Bicyclic Cyclopenta[b]pyrans and 8-Oxabicyclo[3.2.1]octane Derivatives

Two reagent-controlled regiodivergent annulation protocols for Achmatowicz products with vinylogous nucleophiles have been developed, which furnished a series of bicyclic cyclopenta[b]pyrans and 8-oxabicyclo[3.2.1]octane derivatives in 28-90% yields. Plausible mechanisms were proposed to involve either Pd-catalyzed Tsuji-Trost allyl-allyl coupling and concomitant Michael cyclization or quinine-promoted cascade stepwise [5 + 2] cycloaddition and intramolecular Michael cyclization.

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.

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.

Palladium catalyzed decarboxylative β-C-glycosylation of glycals with oxazol-5-(4H)-ones as acceptors

Palladium catalyzed decarboxylative glycosylation of bicyclic glycals affords a series of C-glycosylated oxazol-5-(4H)-ones with high efficiency and exquisite chemo- and stereoselectivity at the anomeric center under mild reaction conditions.

Advances in Pd-catalyzed C-C bond formation in carbohydrates and their applications in the synthesis of natural products and medicinally relevant molecules

Advances in the Pd-catalyzed synthesis of C-glycosides and branched sugars are summarized herein and the strategies are categorized based on named reactions or types of sugar moieties involved in the reactions. These include cross-coupling reactions, C-H activations, and carbonylative cross-coupling reactions. Applications of Pd-catalyzed C-glycosylation reactions are discussed in the synthesis of natural products and biologically active molecules such as bergenin, papulacandin D, and SGLT2-inhibitors. Important mechanistic cycles are drawn and the mechanisms for how Pd-activates the sugar moieties for various coupling partners are discussed. The directing group-assisted C-glycosylation and some intramolecular C-H activation reactions are also included.

Stereoselective Synthesis of C-Vinyl Glycosides via Palladium-Catalyzed C-H Glycosylation of Alkenes

C-vinyl glycosides are an important class of carbohydrates and pose a unique synthetic challenge. A new strategy has been developed for stereoselective synthesis of C-vinyl glycosides via Pd-catalyzed directed C-H glycosylation of alkenes with glycosyl chloride donors using an easily removable bidentate auxiliary. Both the γ C-H bond of allylamines and the δ C-H bond of homoallyl amine substrates can be glycosylated in high efficiency and with excellent regio- and stereoselectivity. The resulting C-vinyl glycosides can be further converted to a variety of C-alkyl glycosides with high stereospecificity. These reactions offer a broadly applicable method to streamline the synthesis of complex C-vinyl glycosides from easily accessible starting materials.

Convergent Palladium-Catalyzed Stereospecific Arginine Glycosylation Using Glycals

A stereospecific convergent peptide arginine glycosylation method is reported for the first time. A recently discovered arginine glycosylation invigorated the interests of arginine modification, which has been challenging, because of the inertness of the guanidino side chain. The approach renders the arginine glycoside construction convergently. Catalyzed by palladium complex, glycals modify arginine guanidino groups in one step with high functional group tolerance at ambient temperature. The glycosylated products may be converted to glycopeptide analogues in few steps.

Modular and Stereoselective Synthesis of C-Aryl Glycosides via Catellani Reaction

In this work, we describe a Catellani-type C-H glycosylation to provide rapid access to various highly decorated α-C-(hetero)aryl glycosides in a modular and stereoselective manner (>90 examples). The termination step is flexible, which is demonstrated by ipso-Heck reaction, hydrogenation, Suzuki coupling, and Sonogashira coupling. Application of this methodology has been showcased by preparing glycoside-pharmacophore conjugates and a dapagliflozin analogue. Notably, the technology developed herein represents an unprecedented example of Catellani-type alkylation involving an S_N1 pathway.

A sustainable innovation for the tandem synthesis of sugar-containing coumarin derivatives catalyzed by lipozyme TL IM from Thermomyces lanuginosus in continuous-flow microreactors

We developed an efficient and environmentally friendly two-step tandem methodology for the synthesis of sugar-containing coumarin derivatives catalyzed by lipozyme TL IM from Thermomyces lanuginosus in continuous-flow microreactors. Compared to those observed for other methods, the salient features of this work including green reaction conditions, short residence time (50 min), and catalysts are more readily available and the biocatalysis reaction process is efficient and easy to control. This two-step tandem synthesis of coumarin derivatives using the continuous-flow technology is a proof of concept that opens the use of enzymatic microreactors in coumarin derivative biotransformations.

An effective and environmentally friendly two-step tandem protocol for the synthesis of sugar-containing coumarin derivatives catalyzed by lipozyme TL IM in continuous-flow microreactors has been developed.

Late-stage C(sp2)-H and C(sp3)-H glycosylation of C-aryl/alkyl glycopeptides: mechanistic insights and fluorescence labeling

C(sp³)–H and C(sp²)–H glycosylations of structurally complex amino acids and peptides were accomplished through the assistance of triazole peptide-isosteres. The palladium-catalyzed peptide–saccharide conjugation provided modular access to structurally complex C-alkyl glycoamino acids, glycopeptides and C-aryl glycosides, while enabling the assembly of fluorescent-labeled glycoamino acids. The C–H activation approach represents an expedient and efficient strategy for peptide late-stage diversification in a programmable as well as chemo-, regio-, and diastereo-selective fashion.

C–H glycosylations of complex amino acids and peptides were accomplished through the assistance of triazole peptide-isosteres. The palladium-catalyzed glycosylation provided access to complex C-glycosides and fluorescent-labeled glycoamino acids.

Open-Air Stereoselective Construction of C-Aryl Glycosides

A new methodology of stereoselective C-glycosylation has been developed with 3,4-O-carbonate glycals and boronic acids, catalyzed by 1,2-bis(phenylsulfinyl)ethane palladium(II) acetate under open-air conditions at room temperature. This mild method is simple in operation, wide in substrate range, and tolerant in alcoholic/phenolic hydroxyl and amino groups. High to excellent yields were observed for all substrates tested, with the driving force mainly contributed by decarboxylation. Meanwhile, the high 1,4-trans-selectivity was achieved by steric effects as proposed.

Regio- and diastereoselective Pd-catalyzed synthesis of C2-aryl glycosides

A highly regio- and diastereoselective Pd-catalyzed direct arylation reaction of 2,3-glycals is reported.