Palladium-catalyzed C-H glycosylation and retro Diels-Alder tandem reaction via structurally modified norbornadienes (smNBDs)

Practical synthesis of C-aryl glycosides via redox-neutral Borono-Catellani reaction

Herein, we describe a practical Borono-Catellani strategy for the efficient synthesis of C-aryl glycosides, with readily available arylboronic esters and glycosyl chlorides as the building blocks. It features mild reaction conditions, excellent diastereoselectivities, and good functional group tolerance. A diverse array of highly decorated C-(hetero)aryl glycosides are obtained in a convergent and redox-neutral manner.

Visible-light-promoted direct desulfurization of glycosyl thiols to access C-glycosides

C-Glycosides are essential for the study of biological processes and the development of carbohydrate-based drugs. Despite the tremendous hurdles, glycochemists have often fantasized about the efficient, highly stereoselective synthesis of C-glycosides with the shortest steps under mild conditions. Herein, we report a desulfurative radical protocol to synthesize C-alkyl glycosides and coumarin C-glycosides under visible-light induced conditions without the need of an extra photocatalyst, in which stable and readily available glycosyl thiols that could be readily obtained from native sugars are activated in situ by pentafluoropyridine. The benefits of this procedure include high stereoselectivity, broad substrate scope, and easy handling. Mechanistic studies indicate that the in situ produced tetrafluoropyridyl S-glycosides form key electron donor-acceptor (EDA) complexes with Hantzsch ester (for C-alkyl glycosides) or Et3N (for coumarin C-glycosides), which, upon irradiation with visible light, trigger a cascade of glycosyl radical processes to access C-glycosides smoothly.

A switch strategy for the synthesis of C4-ethylamine indole and C7-aminoindoline via controllable carbon elimination

Controllable β-carbon elimination to extrude norbornene remains a long-standing challenge in palladium and norbornene chemistry. Herein, this manuscript describes a switchable synthesis of biologically active C4-ethylaminoindole and C7-aminoindoline scaffolds by controlling β-carbon elimination, utilizing aziridine as a C-H ethylamination reagent through a C-N bond cleavage reaction. Furthermore, the protecting groups of the product can be easily removed, offering an unusual method for the synthesis of dopamine receptor agonists.

Electrochemical Glycosylation via Halogen-Atom-Transfer for C-Glycoside Assembly

Glycosyl donor activation emerged as an enabling technology for anomeric functionalization, but aimed primarily at O-glycosylation. In contrast, we herein disclose mechanistically distinct electrochemical glycosyl bromide donor activations via halogen-atom transfer and anomeric C-glycosylation. The anomeric radical addition to alkenes led to C-alkyl glycoside synthesis under precious metal-free reaction conditions from readily available glycosyl bromides. The robustness of our e-XAT strategy was further mirrored by C-aryl and C-acyl glycosides assembly through nickela-electrocatalysis. Our approach provides an orthogonal strategy for glycosyl donor activation with expedient scope, hence representing a general method for direct C-glycosides assembly.

Direct Synthesis of C4-Acyl Indoles via C-H Acylation

The direct synthesis of C4-acyl indoles deprived of C2 and C3 substituents has proven to be challenging, with scarce efficient synthetic routes being reported. Herein, we disclose a highly site-selective palladium-catalyzed C-H acylation for the construction of C4-acyl indoles via a Catellani-Lautens cyclization strategy. In addition, we systematically studied the ortho C-H acylation mechanism of iodoaniline through density functional theory (DFT) calculations and combined experimental results to elucidate the principle of high chemoselectivity brought by triazine benzoate as an acylation reagent.

Stereoselective assembly of C-oligosaccharides via modular difunctionalization of glycals

C-oligosaccharides are found in natural products and drug molecules. Despite the considerable progress made during the last decades, modular and stereoselective synthesis of C-oligosaccharides continues to be challenging and underdeveloped compared to the synthesis technology of O-oligosaccharides. Herein, we design a distinct strategy for the stereoselective and efficient synthesis of C-oligosaccharides via palladium-catalyzed nondirected C1-H glycosylation/C2-alkenylation, cyanation, and alkynylation of 2-iodoglycals with glycosyl chloride donors while realizing the difunctionalization of 2-iodoglycals. The catalysis approach tolerates various functional groups, including derivatives of marketed drugs and natural products. Notably, the obtained C-oligosaccharides can be further transformed into various C-glycosides while fully conserving the stereochemistry. The results of density functional theory (DFT) calculations support oxidative addition mechanism of alkenyl-norbornyl-palladacycle (ANP) intermediate with α-mannofuranose chloride and the high stereoselectivity of glycosylation is due to steric hindrance.

Palladium(II)-Catalyzed Heck Coupling: Direct Stereoselective Synthesis of C-Aryl Glycosides from Nonactivated Glycals and Thianthrenium Salts

Here, we report an efficient Pd(II)-catalyzed Heck coupling reaction utilizing modular and readily available thianthrenium salts. The tunability and ease of thianthrenium salts facilitated the integration of glycals with drugs, natural products, and peptides. This method allows the incorporation of diverse glycals into structurally varied aglycon components without directing groups or prefunctionalization and provides a practical method for synthesizing C-aryl glycosides, offering a new avenue for the production of complex glycosides with potential applications.

Mild method for the synthesis of α-glycosyl chlorides: A convenient protocol for quick one-pot glycosylation

A simple and efficient protocol for the preparation of α-glycosyl chlorides within 15-30 min is described which employs a stable, cheap, and commercially available Trichloroisocyanuric acid (TCCA) as non-toxic chlorinating agent along with PPh3. This process involved a wide range of substrate scope and is well-suited with labile hydroxyl protecting groups such as benzyl, acetyl, benzoyl, isopropylidene, benzylidene, and TBDPS (tert-butyldiphenylsilyl) groups. This process is operationally simple, mild conditions and obtained good yields with excellent α selectivity. Moreover, a multi-catalyst one-pot glycosylation can be carried out to transform the glycosyl hemiacetals directly to a various O-glycosides in high overall yields without the need for separation or purification of the α-glycosyl chloride donors.

Base/Solvent Controlled Divergent Synthesis of Norbornane-Fused Dihydrophenanthrenes and Triphenylenes via Palladium Catalyst

A base/solvent controlled divergent synthesis for the construction of polycyclic hydrocarbons has been developed. In this process, norbornene through a reagent role leads to the synthesis of norbornane-fused dihydrophenanthrenes, which are essential due to their biological activities. Amazingly, by switching solvent and base, the role of norbornene becomes limited to a mediator/catalyst; therefore, it is removed from the final scaffold, and triphenylenes are regioselectively synthesized. Additionally, by removing norbornene from the reaction conditions, a different path leading to synthesis of unsymmetrically substituted triphenylenes with exceptional regioselectivity is established. This reaction includes a rare domino decarboxylation/C-H activation/annulation in a chemo- and regioselective manner.

Stereoselective Synthesis of Indolyl-C-glycosides Enabled by Sequential Aminopalladation and Heck Glycosylation of 2-Alkynylanilines with Glycals

An efficient and general approach for the synthesis of indolyl-C-glycosides via aminopalladation and subsequent Heck-type glycosylation of easily available 2-alkynylanilines and glycals has been developed. This protocol features excellent stereoselectivity, a broad substrate scope, and mild reaction conditions. In addition, 2,3-pseudoglycals also successfully participated in this cascade reaction, affording C2/C3-branched indolyl glycosides with high regio-/stereoselectivity. The utility of this protocol was also demonstrated by a large-scale reaction and diversified synthetic transformations of the desired products.

C-H Glycosylation of Native Carboxylic Acids: Discovery of Antidiabetic SGLT-2 Inhibitors

C-Glycosides are critical motifs embedded in many bioactive natural products. The inert C-glycosides are privileged structures for developing therapeutic agents owing to their high chemical and metabolic stability. Despite the comprehensive strategies and tactics established in the past few decades, highly efficient C-glycoside syntheses via C-C coupling with excellent regio-, chemo-, and stereoselectivity are still needed. Here, we report the efficient Pd-catalyzed glycosylation of C-H bonds promoted by weak coordination with native carboxylic acids without external directing groups to install various glycals to the structurally diverse aglycon parts. Mechanistic evidence points to the participation of a glycal radical donor in the C-H coupling reaction. The method has been applied to a wide range of substrates (over 60 examples), including many marketed drug molecules. Natural product- or drug-like scaffolds with compelling bioactivities have been constructed using a late-stage diversification strategy. Remarkably, a new potent sodium-glucose cotransporter-2 inhibitor with antidiabetic potential has been discovered, and the pharmacokinetic/pharmacodynamic profiles of drug molecules have been changed using our C-H glycosylation approach. The method developed here provides a powerful tool for efficiently synthesizing C-glycosides to facilitate drug discovery.

Chemical synthesis of oligosaccharides and their application in new drug research

Oligosaccharides are the ubiquitous molecules of life. In order to translate human bioglycosylation into clinical applications, homogeneous samples of oligosaccharides and glycoconjugates can be obtained by chemical, enzymatic or other biological methods for systematic studies. However, the structural complexity and diversity of glycans and their conjugates present a major challenge for the synthesis of such molecules. This review summarizes the chemical synthesis methods of oligosaccharides, the application of oligosaccharides in the field of medicinal chemistry according to their related biological activities, and shows the great prospect of oligosaccharides in the field of pharmaceutical chemistry.

Synthesis of C4-Aminated Carbazoles and Their Derivatives via Pd/NBE Chemistry

Carbazole, as one of the most important organic frameworks, has been used in optoelectronic materials and biochemistry. However, the synthesis of C4-substituted carbazole has always been an unsolved problem. This report describes the one-step synthesis of C4-aminated carbazoles and their derivatives through the series reaction of C-H amination and arylation. The substrate scope is wide. C4-Amino carbazoles substituted by C2, C6, C7, and C8 methyl groups, especially carbazole derivatives of fused rings, pyridine, and dibenzofuran, can be synthesized.

Remote C-H Glycosylation by Ruthenium(II) Catalysis: Modular Assembly of meta-C-Aryl Glycosides

The prevalence of C-aryl glycosides in biologically active natural products and approved drugs has long motivated the development of efficient strategies for their selective synthesis. Cross-couplings have been frequently used, but largely relied on palladium catalyst with prefunctionalized substrates, while ruthenium-catalyzed C-aryl glycoside preparation has thus far proven elusive. Herein, we disclose a versatile ruthenium(II)-catalyzed meta-C-H glycosylation to access meta-C-aryl glycosides from readily available glycosyl halide donors. The robustness of the ruthenium catalysis was reflected by mild reaction conditions, outstanding levels of anomeric selectivity and exclusive meta-site-selectivity.

Visible-Light-Induced Copper-Catalyzed Asymmetric C(sp3)-C(sp3)-H Glycosylation: Access to C-Glycopeptides

Herein, a practical and highly efficient method for visible-light-induced copper-catalyzed N-aminoquinoline-directed asymmetric C(sp³)-C(sp³)-H glycosylation was reported. At the same time, C(sp³)-C(sp³)-H glycosylation of nondeoxysugars with amino acids to construct C-glycopeptides was achieved. This approach promoted the synthesis of various C-glycopeptides and provided a new model for the synthesis of C-glycoamino acids.

Ruthenium-Catalyzed Pyridine-Directed Aryl C-H Glycosylation with Glycosyl Chlorides

Metal-catalyzed C-H glycosylation reactions with glycosyl chloride donors have emerged as a useful strategy for the synthesis of C-glycosides. Previously, palladium and nickel complexes were reported to catalyze C-H glycosylation reactions using amide-linked bidentate auxiliaries. Herein, a ruthenium-catalyzed ortho C-H glycosylation reaction of arenes with various glycosyl chloride donors using a monodentate pyridine directing group is developed. Preliminary mechanistic studies indicated that two-electron oxidative addition and reductive elimination of ruthenocycle intermediate led to the glycosylation products.

One-Pot Stereoselective Synthesis of 2,3-Diglycosylindoles and Tryptophan-C-glycosides via Palladium-Catalyzed C-H Glycosylation of Indole and Tryptophan

We described a novel palladium-catalyzed C-H glycosylation of indole or tryptophan for a one-pot stereoselective synthesis of 2,3-diglycosylindoles and tryptophan-C-glycosides. In this strategy, the use of air and base-free and ligand-free conditions provided a highly efficient route to construct C-glycosides. The method can be applied to a wide range of cost-effective and convenient glycosyl chloride donors. Mechanistic studies indicated that the indole 2,3-diglycosylation sequence was C3 and then C2.