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

Generalizing arene C-H alkylations by radical-radical cross-coupling

The efficient and modular diversification of molecular scaffolds, particularly for the synthesis of diverse molecular libraries, remains a notable challenge in drug optimization campaigns1-3. The late-stage introduction of alkyl fragments is especially desirable due to the high sp3 character and structural versatility of these motifs4. Given their prevalence in molecular frameworks, C(sp2)-H bonds serve as attractive targets for diversification, although this process often requires difficult prefunctionalization or lengthy de novo syntheses. Traditionally, direct alkylations of arenes are achieved by using Friedel-Crafts reaction conditions with strong Brønsted or Lewis acids5,6. However, these methods suffer from poor functional group tolerance and low selectivity, limiting their broad implementation in late-stage functionalization and drug optimization campaigns. Here we report the application of a new strategy for the selective coupling of differently hybridized radical species, which we term 'dynamic orbital selection'. This mechanistic model overcomes common limitations of Friedel-Crafts alkylations via the in situ formation of two distinct radical species, which are subsequently differentiated by a copper-based catalyst on the basis of their respective binding properties. As a result, we demonstrate here a general and highly modular reaction for the direct alkylation of native arene C-H bonds using abundant and benign alcohols and carboxylic acids as the alkylating agents. Ultimately, this solution overcomes the synthetic challenges associated with the introduction of complex alkyl groups into highly sophisticated drug scaffolds in a late-stage fashion, thereby granting access to vast new chemical space. Based on the generality of the underlying coupling mechanism, 'dynamic orbital selection' is expected to be a broadly applicable coupling platform for further challenging transformations involving two distinct radical species.

Stereoselective P(III)-Glycosylation for the Preparation of Phosphinated Sugars

Most of the reported work focus on the development of O-, N-, C- and S-glycosylation methods. However, no study explores P(III)-glycosylation reaction. Herein we describe a convenient protocol to realize P(III)-glycosylation process. A simple β-phosphino ester is adopted as P(III)-transfer reagent for this new type of glycosylation via a nucleophilic substitution and release strategy. Diverse phosphine units are introduced to the anomeric center of various sugars efficiently and with excellent stereoselectivity. The value of this method is showcased by the prepared P(III)-sugars as novel linkers in bioactive molecule conjugation, new chiral ligands in metal-catalyzed asymmetric allylic substitutions and organocatalysts. Preliminary mechanistic studies corroborated the designed P(III)-transfer process.

Stereoretentive Conversion to C-Glycosides from S-Glycosides via Ligand-Coupling on Sulfur(IV)

A novel strategy is reported for the stereoselective synthesis of C(sp2)-C(sp3) C-glycosides, which converts heteroaryl S-glycosides into heteroaryl C-glycosides with retention of configuration through a sequential process involving oxidation and Grignard reagent attack. The new method involves the generation of a S(IV) intermediate, followed by ligand coupling of the glycosyl and heteroaryl groups to yield heteroaryl C-glycosides. The diverse heteroaryl C-glycosides were achieved with good efficiency.

Ruthenium-catalyzed three-component tandem remote C-H functionalization of naphthalenes: modular and concise synthesis of multifunctional naphthalenes

The prevalence of naphthalene compounds in biologically active natural products, organic ligands and approved drugs has motivated investigators to develop efficient strategies for their selective synthesis. C-H functionalization of naphthalene has been frequently deployed, but mainly involves two-component reactions, while multiple-component C-H functionalization for the synthesis of naphthalene compounds has thus far proven elusive. Herein, we disclose a versatile three-component protocol for the modular synthesis of multifunctional naphthalenes from readily available simple naphthalenes, olefins and alkyl bromides via P(iii)-assisted ruthenium-catalyzed remote C-H functionalization. This protocol not only tolerates various functional groups, but can be applied to many natural product and drug derivatives, and can involve a three-component reaction with two different bioactive molecules. Mechanism studies indicated that the utilization of tertiary phosphines as auxiliary groups is the key to achieving the three-component free-radical reaction.

Pd-Catalyzed Stereospecific Glycosyl Cross-Coupling of Reversed Anomeric Stannanes for Modular Synthesis of Nonclassical C-Glycosides

Nonclassical C-glycosides, distinguished by their unique glycosidic bond connection mode, represent a promising avenue for the development of carbohydrate-based drugs. However, the accessibility of nonclassical C-glycosides hinders broader investigations into their structural features and modes of action. Herein, we present the first example of Pd-catalyzed stereospecific glycosylation of nonclassical anomeric stannanes with aryl or vinyl halides. This method furnishes desired nonclassical aryl and vinyl C-glycosides in good to excellent yields, while allowing for exclusive control of nonclassical anomeric configuration. Of significant note is the demonstration of the generality and practicality of this nonclassical C-glycosylation approach across more than 50 examples, encompassing various protected and unprotected saccharides, deoxy sugars, oligopeptides, and complex molecules. Furthermore, biological evaluation indicates that nonclassical C-glycosylation modifications of drug molecules can positively impact their biological activity. Additionally, extensive computational studies are conducted to elucidate the rationale behind differences in reaction reactivity, unveiling a transmetalation transition state containing silver (Ag) within a six-membered ring. Given its remarkable controllability, predictability, and consistently high chemical selectivity and stereospecificity regarding nonclassical anomeric carbon and Z/E configuration, the method outlined in this study offers a unique solution to the longstanding challenge of accessing nonclassical C-glycosides with exclusive stereocontrol.

Anomeric oxyacetamide assisted site-selective C-2 arylation and its application in O/S glycosylation of 2,3 eno-pyranoside

Herein we developed a palladium-catalyzed coupling of 2,3-enopyranose with arylboronic acid using a removable oxyacetamide directing group, which provides an efficient method for the synthesis of C-2 aryl sugars. The synthesized products were subsequently utilized as glycosyl donors in O/S glycosylation, enabling regio- and stereoselective production of 1,2-disubstituted branched sugars.

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.

Ruthenium(II)-Catalyzed Remote C-H Alkylation of Arenes Using Diverse N-Directing Groups through Aziridine Ring Opening

An efficient approach for the remote C-H alkylation of arenes, employing a variety of N-directing groups is described. This method facilitates the straightforward synthesis of valuable phenylethylamine derivatives by exclusively cleaving the benzylic C-N bond in aziridines. Furthermore, these products can easily remove the protecting groups, resulting in a variety of meta-substituted compounds, such as amines and ketones, which hold significance in synthetic chemistry.

meta-C-H functionalization of phenylethyl and benzylic alcohol derivatives via Pd/NBE relay catalysis

The transition metal-catalyzed meta-C-H functionalization of alcohols and their hydroxylamine derivatives remains underdeveloped. Herein, we report an efficient meta-C-H arylation of both phenylethyl and benzylic alcohols and their hydroxylamine derivatives using a readily removable oxime ether directing group. Using electronically activated 2-carbomethoxynorbornene as the transient mediator and 3-trifluoromethyl-2-pyridone as the enabling ligand, this reaction features a broad substrate scope and good functional group tolerance. More importantly, with this oxime-directed meta-C-H functionalization, this method provides a dual approach for efficient access to both meta-substituted alcohols and hydroxylamines using two sets of simple deprotection conditions. This protocol leads to the efficient synthesis of bioactive compounds possessing promising reactivities for the treatment of pulmonary fibrosis.

Synthesis of C-Alkyl Glycosides from Alkyl Bromides and Glycosyl Carboxylic Acids via Ni/Photoredox Dual Catalysis

C-Alkyl glycosides, an important class of C-glycosides, are widely found in various drugs and natural products. The synthesis of C-alkyl glycosides has attracted considerable attention. Herein, we developed a Ni/photoredox catalyzed decarboxylative C(sp3)-C(sp3) coupling reaction of stable glycosylcarboxylic acids with simple aliphatic bromides to generate C-alkyl glycosides. The method successfully linked several functional molecular fragments (natural products or drugs) to a sugar moiety, showing the extensive application prospects of this transformation. Controlled experiments and DFT calculations demonstrated that the reaction pathway contains a free radical process, and a possible mechanism is proposed.

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.

Synthesis of C-Oligosaccharides via Ni-Catalyzed Reductive Hydroglycosylation

C-Oligosaccharides are metabolically stable surrogates of native glycans containing O/N/S-glycosidic linkages and thus have therapeutic potential. Here we report a straightforward approach to the synthesis of vinyl C-linked oligosaccharides via the Ni-catalyzed reductive hydroglycosylation of alkynyl glycosides with glycosyl bromides.

Diversification of Glycosyl Compounds via Glycosyl Radicals

Glycosyl radical functionalization is one of the central topics in synthetic carbohydrate chemistry. Recent advances in metal-catalyzed cross-coupling chemistry and metallaphotoredox catalysis provided powerful platforms for glycosyl radical diversification. In particular, the discovery of new glycosyl radical precursors in conjunction with these advanced reaction technologies have significantly expanded the space for glycosyl compound synthesis. In this Review, we highlight the most recent progress in this area starting from 2021, and the reports included will be categorized based on different reaction types for better clarity.

Transition-Metal-Catalyzed C-H Bond Activation for the Formation of C-C Bonds in Complex Molecules

Site-predictable and chemoselective C-H bond functionalization reactions offer synthetically powerful strategies for the step-economic diversification of both feedstock and fine chemicals. Many transition-metal-catalyzed methods have emerged for the selective activation and functionalization of C-H bonds. However, challenges of regio- and chemoselectivity have emerged with application to highly complex molecules bearing significant functional group density and diversity. As molecular complexity increases within molecular structures the risks of catalyst intolerance and limited applicability grow with the number of functional groups and potentially Lewis basic heteroatoms. Given the abundance of C-H bonds within highly complex and already diversified molecules such as pharmaceuticals, natural products, and materials, design and selection of reaction conditions and tolerant catalysts has proved critical for successful direct functionalization. As such, innovations within transition-metal-catalyzed C-H bond functionalization for the direct formation of carbon-carbon bonds have been discovered and developed to overcome these challenges and limitations. This review highlights progress made for the direct metal-catalyzed C-C bond forming reactions including alkylation, methylation, arylation, and olefination of C-H bonds within complex targets.