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

One-Pot Stereoselective Synthesis of C-Glycosyl Amino Acids with Vicinal Stereocenters

A palladium-catalyzed cascade coupling between glycals and racemic α-nitroesters enables the stereoselective construction of C-glycosyl amino acids bearing two contiguous stereocenters, achieving dual stereocontrol (exclusive β-C1, up to 10:1 diastereomeric ratio at C7). This one-pot protocol at room temperature accommodates diverse substrates (25 examples). Mechanistic studies demonstrate that decarboxylation-induced C4-oxyanion directs 1,4-syn selectivity, while steric modulation governs C7 stereochemistry. Synthetic versatility is evidenced by gram-scale synthesis and late-stage functionalization to trihydroxy C-glycosides, 2,3-dideoxy sugars, and 4-O-amino acid conjugates.

Synthesis of C4-Acyl Glycosides by Cross-Ketonization: The Importance of Precise Temperature Control in Photochemistry

We describe herein a method for the synthesis of reversed C-acyl glycosides by direct cross-ketonization of linear carboxylic acids with carboxy sugars under metallaphotoredox conditions. This reaction proves to be highly sensitive to reaction temperature, with the reproducibility and yield of ketonization highly dependent upon precise temperature control. Analysis of the reaction mechanism suggests this sensitivity reflects the need to match the rate of Ni catalytic turnover with the rate of photocatalytic decarboxylation of the carboxy sugar. The insights gained from this study have important implications for the study of all metallaphotoredox reactions as well as other photoreactions that involve dual catalytic cycles.

Dual Photoredox and Copper-Catalyzed Asymmetric Remote C(sp3)-H Alkylation of Hydroxamic Acid Derivatives with Glycine Derivatives

Dual photoredox and copper-catalyzed remote asymmetric C(sp)3-H alkylation of hydroxamic acid derivatives with glycine derivatives via a 1,5-hydrogen transfer (1,5-HAT) process has been realized. The reaction was characterized by redox-neutral and mild conditions, good yields, excellent enantioselectivity, and broad substrate scope. This protocol provides a straightforward and efficient strategy to prepare highly valuable enantioenriched noncanonical α-amino acids. Moreover, the potential synthetic value of this reaction was demonstrated in late-stage asymmetric alkylation of dipeptides with a high diastereomeric ratio.

Visible-Light-Induced Glycosylation/Annulation of 2-Isocyanobiaryls and Glycosyl NHP Esters to Access Nonclassical Heteroaryl C-Glycosides

A photoredox-promoted cascade glycosylation/cyclization reaction of 2-isocyanobiaryls and glycosyl NHP esters was established for the synthesis of nonclassical heteroaryl C-glycosides. This methodology is characterized by an exceedingly simple reaction system, high diastereoselectivity, and good functional group tolerance. In contrast to traditional strategies, this innovative approach circumvents the need for high temperature, transition metal, and photocatalyst, offering an environmentally friendly and efficient protocol.

Photoinduced Regiodivergent and Enantioselective Cross-Coupling of Glycine Derivatives with Hydrocarbon Feedstocks

Regiodivergent asymmetric synthesis represents a transformative strategy for the efficient generation of structurally diverse chiral products from a single set of starting materials, significantly enriching their enantiomeric composition. However, the design of radical-mediated regiodivergent and enantioselective reactions that can accommodate a wide range of functional groups and substrates has posed significant challenges. The obstacles primarily lie in switching the regioselectivity and achieving high enantiodiscrimination, especially when dealing with high-energy intermediates. To address these issues, we have developed a new catalytic system that integrates photoinduced hydrogen atom transfer (HAT) and chiral copper catalysis, involving the fine-tuning of chiral ligands, additives, and other reaction parameters. The strategy facilitates regiodivergent and enantioselective cross-couplings between N-aryl glycine ester/amide derivatives and abundant hydrocarbon feedstocks through strong C(sp3)-H bond activation. This approach allows for the controlled and stereoselective formation of C(sp3)-C(sp3) and C(sp3)-N bonds, yielding a rich variety of C- or N-alkylated glycine esters and amides with commendable yields (up to 92% yield), exclusive regioselectivities (typically >20:1 rr), and high enantioselectivities (up to 96% ee). Our methodology not only provides a promising avenue for the stereoselective incorporation of alkyl functionalities onto specific sites of biologically significant molecules but also offers a practical approach for regioselectivity switching while simultaneously achieving high asymmetric induction within photochemical reactions.

Glycosyl Radical-Based Synthesis of C-Alkyl Glycosides Bearing a Cyclopropane via a Deoxygenative Giese Addition-Reduction-Cyclization Cascade

We have developed a glycosyl radical-based synthesis of C-alkyl glycosides through a deoxygenative Giese addition-reduction-cyclization cascade, in which readily available 1-hydroxy carbohydrates serve as precursors for glycosyl radicals and aryl alkenes function as radical acceptors. This reaction not only provides an effective method for accessing a previously underexplored class of functionalized cyclopropanes but also enhances the application of Giese addition in the synthesis of C-alkyl glycosides by derivatizing the radical intermediate generated through polar cyclization to yield a cyclopropane.

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.

Stabilized Carbon-Centered Radical-Mediated Carbosulfenylation of Styrenes: Modular Synthesis of Sulfur-Containing Glycine and Peptide Derivatives

Sulfur-containing amino acids and peptides play critical roles in organisms. Thiol-ene reactions between the thiol residues of L-cysteine and the alkenyl fragments in the designed coupling partners serve as primary tools for constructing C─S bonds in the synthesis of unnatural sulfur-containing amino acid derivatives. These reactions are favored due to the preference for hydrogen transfer from thiol to β-sulfanyl carbon radical intermediates. In this paper, the study proposes utilizing carbon-centered radicals stabilized by the capto-dative effect, generated under photocatalytic conditions from N-aryl glycine derivatives. The aim is to compete with the thiol hydrogen, enabling radical C─C bond formation with β-sulfanyl carbon radicals. This protocol is robust in the presence of air and water, offers significant potential as a modular and efficient platform for synthesizing sulfur-containing amino acids and modifying peptides, particularly with abundant disulfides and styrenes.

Synthesis of Nonclassical Heteroaryl C-Glycosides via Decarboxylative C-H Glycosylation

A photoredox-promoted decarboxylative C-H glycosylation for the synthesis of nonclassical heteroaryl C-glycosides is reported. This methodology is characterized by an exceedingly simple reaction system, high diastereoselectivity, and good functional group tolerance. Moreover, the operational procedure is simple, and the gram-scale reaction highlights the practical applicability of this protocol.

Visible-light-induced synthesis of heteroaryl C-glycosides via decarboxylative C-H glycosylation

A photoredox promoted decarboxylative C-H glycosylation has been developed for the synthesis of heteroaryl C-glycosides. This methodology is characterized by its exceedingly simple reaction system, high diastereoselectivity and good functional group tolerance. Moreover, this innovative approach circumvents the need for high temperatures, transition metals, and photocatalysts, providing an environmentally friendly, straightforward, and efficient protocol.

Diastereoselective access to C,C-glycosyl amino acids via iron-catalyzed, auxiliary-enabled MHAT coupling

Access to C,C-glycosyl amino acids as a novel class of glycomimetics is reported by means of radical generation, intermolecular addition and stereoselective reduction via a metal-induced hydrogen atom transfer (MHAT) sequence. The 'matched' coupling of exo-D-glycals with an enantiopure dehydroalanine bearing a (R)-configured benzyl oxazolidinone enables a singular case of two-fold diastereocontrol under iron catalysis. In the common exo-D-glucal series, the nature of the C-2 substituent was found to play a key role from both reactivity and stereocontrol aspects.

Copper-Catalyzed Asymmetric Remote C(sp3)-H Alkylation of N-Fluorocarboxamides with Glycine Derivatives and Peptides

Saturated hydrocarbon bonds are ubiquitous in organic molecules; to date, the selective functionalization of C(sp3)-H bonds continues to pose a notorious difficulty, thereby garnering significant attention from the synthetic chemistry community. During the past several decades, a wide array of powerful new methodologies has been developed to enantioselectively modify C(sp3)-H bonds that is successfully applied in asymmetric formation of diverse bonds, including C-C, C-N, and C-O bonds; nevertheless, the asymmetric C(sp3)-H alkylation is elusive and, therefore, far less explored. In this work, we report a direct and robust strategy to construct highly valuable enantioenriched unnatural α-amino acid (α-AA) cognates and peptides by a copper-catalyzed enantioselective remote C(sp3)-H alkylation of N-fluorocarboxamides and readily accessible glycine esters under ambient conditions. The key to success lies in the optically active Cu catalyst generated through the coordination of glycine derivatives to enantiopure bisphosphine/Cu(I) species, which is beneficial to the single electronic reduction of N-fluorocarboxamides and the subsequent stereodetermining alkylation. More importantly, all types (primary, secondary, tertiary, and even α-oxy) of δ-C(sp3)-H bonds could be site- and stereospecifically activated by the kinetically favored 1,5-hydrogen atom transfer (1,5-HAT) step.

Visible Light-Driven Flexible Synthesis of α-Alkylated Glycine Derivatives Catalyzed by Reusable Covalent Organic Frameworks

Herein, we report a heterogeneous visible light-driven preparation of α-alkylated glycine derivatives. This approach employed a β-ketoenamine-linked covalent organic framework (2D-COF-4) as the heterogeneous photocatalyst and N-hydroxy phthalimide (NHPI) esters as the alkyl radical sources. Numerous glycine derivatives, including dipeptides, were precisely and efficiently alkylated under visible light-driven reaction conditions. Based on the excellent photoactivity and organic reaction compatibility of 2D-COF-4, this alkylation could proceed flexibly in a green solvent (ethanol) without any other additives. The photocatalyst and phthalimide were fruitfully recycled with a simple workup procedure, revealing a high ecoscale value and low environmental factor (E-factor).

Transformations of carbohydrate derivatives enabled by photocatalysis and visible light photochemistry

This review article highlights the diverse ways in which recent developments in the areas of photocatalysis and visible light photochemistry are impacting synthetic carbohydrate chemistry. The major topics covered are photocatalytic glycosylations, generation of radicals at the anomeric position, transformations involving radical formation at non-anomeric positions, additions to glycals, processes initiated by photocatalytic hydrogen atom transfer from sugars, and functional group interconversions at OH and SH groups. Factors influencing stereo- and site-selectivity in these processes, along with mechanistic aspects, are discussed.

Cobalt(II)-Catalyzed Selective Three-Component Oxyalkylation of N-Aryl Glycinates: A Route to CF3-Labeled Threonine Analogues

Glycinates, protected enols, and an electrophilic trifluoromethylating reagent were employed to assemble CF3-labeled threonine analogues through a radical addition cascade. To suppress the competing oxidation of the oxyalkyl radical intermediate, various redox catalysts were evaluated and Co(II) exhibited supreme selectivity control with a proper counterion. A series of glycinate and related peptides were thus successfully modified under Co-catalysis. Mechanistic studies revealed that a N-aryl glycinate could be preferentially oxidized by cobalt over the oxyalkyl radical to generate an imine intermediate, and the key to this selectivity could be ascribed to the prechelation of glycinate, as well as a weakly basic carboxylate counterion.

Visible-light-mediated synthesis of non-anomeric S-aryl glycosides via a photoactive electron-donor-acceptor complex

A visible-light-mediated glycosylation reaction between glycosyl redox-active esters and disulfides has been reported, through which a series of S-aryl glycosides were obtained in good yields with satisfactory stereoselectivity. The preliminary mechanistic studies revealed that this transformation proceeded via an EDA complex. Moreover, the potential application value was demonstrated in the late-stage functionalisation of drug molecules and a gram-scale experiment.

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.

Photoinduced C(sp3)-H Functionalization of Glycine Derivatives: Preparation of Unnatural α-Amino Acids and Late-Stage Modification of Peptides

ConspectusPeptides are essential components of living systems and contribute to critical biological processes, such as cell proliferation, immune defense, tumor formation, and differentiation. Therefore, peptides have attracted considerable attention as targets for the development of therapeutic products. The incorporation of unnatural amino acid residues into peptides can considerably impact peptide immunogenicity, toxicity, side effects, water solubility, action duration, and distribution and enhance the peptides' druggability. Typically, the direct modification of natural amino acids is a practical and effective approach for promptly obtaining unnatural amino acids. However, selective functionalization of multiple C(sp³)-H bonds with comparable chemical reactivities in the peptide side chains remains a formidable challenge. Furthermore, chemical modifications aimed at highly reactive (nucleophilic and aromatic) groups on peptide side chains can interfere with the biological activity of peptides.In recent years, the rapid advancement of photoinduced radical reactions has made photoredox radical-radical cross-coupling a practical approach for constructing C(sp³)-C(sp³) bonds under mild conditions. Glycine, a naturally occurring amino acid and the fundamental skeleton of all α-amino acids, provides a basis for the alkylated modification of its own α-C(sp³)-H bond under mild conditions. This Account describes our recent research endeavors for systematically investigating the photocatalytic α-C(sp³)-H alkylation of glycine derivatives via radical-radical coupling between N-aryl glycinate-derived radicals and various alkyl radicals. In 2018, we disclosed the photoinduced Cu-catalyzed decarboxylative α-C(sp³)-H alkylation of glycine derivatives. Subsequently, we developed a catalyst-free method for alkylating glycine derivatives and glycine residues in peptides via electron donor-acceptor (EDA)-complex-promoted single electron transfer. Moreover, we developed a photoinduced method for the radical alkylation of N-aryl glycinate α-C(sp³)-H bonds using unactivated alkyl chlorides (iodides) under photocatalyst-free conditions. Notably, by building on racemic alkylations of glycine derivatives and glycine-residue-containing peptides, we recently stereoselectively alkylated the N-aryl glycinate α-C(sp³)-H bond using a dual-functional Cu catalyst generated in situ for synthesizing a series of unnatural chiral α-amino and C-glycoamino acids.We have developed a series of methods for synthesizing unnatural amino acids through the α-C(sp³)-H alkylation of glycine derivatives using photoredox-promoted radical coupling as a key strategy. These methods are efficient and versatile and can be used for the late-stage modification of peptides in various contexts. Our work builds on the fundamental importance of glycine as the basic scaffold of all α-amino acids and highlights the potential of radical-based chemistry for developing chemical transformations in peptide synthesis. These findings have broad implications for chemical biology and may open doors for discovering peptide drugs and developing therapeutics.

Stereoselective alkyl C-glycosylation of glycosyl esters via anomeric C-O bond homolysis: efficient access to C-glycosyl amino acids and C-glycosyl peptides

C-Glycosyl peptides possess excellent metabolic stability and therapeutic properties and thus play critical roles in biological studies as well as drug discoveries. However, the limited accessibility of C-glycosyl amino acids has significantly hindered the broader research of their structural features and mode of action. Herein, for the first time we disclose a novel visible-light-driven radical conjugate addition of 1,4-dihydropyridine (DHP)-derived glycosyl esters with dehydroalanine derivatives, generating C-glycosyl amino acids and C-glycosyl peptides in good yields with excellent stereoselectivities. Redox-active glycosyl esters, as readily accessible and bench-stable radical precursors, could be easily converted to glycosyl radicals via anomeric C(sp³)-O bond homolysis under mild conditions. Importantly, the generality and practicality of this transformation were fully demonstrated in >40 examples including 2-dexosugars, oligosaccharides, oligopeptides, and complex drug molecules. Given its mild reaction conditions, robust sugar scope, and high anomeric control and diastereoselectivity, the method presented herein could find widespread utility in the preparation of C(sp³)-linked sugar-based peptidomimetics.