Copper-catalyzed asymmetric C(sp3)-H cyanoalkylation of glycine derivatives and peptides

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.

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.

Copper-Catalyzed Asymmetric C(sp3)-H Benzylation: Stereoselective Synthesis of Unnatural Aromatic Amino Acids

The general synthesis of chiral unnatural aromatic amino acids has rarely been reported. We herein describe a visible light-promoted copper-catalyzed enantioselective C(sp3)-H benzylation of glycine derivatives. The method demonstrated compatibility in coupling various N-hydroxyphthalimide (NHP) esters derived from aromatic acids with glycine derivatives, providing a general protocol for synthesizing analogues of phenylalanine, tryptophan, and tyrosine. This protocol features mild conditions, high enantioselectivity, excellent functional group tolerance, and important impacts on the development of peptide drugs.

Visible-Light-Driven Synthesis of N-Alkyl α-Amino Acid Derivatives from Unactivated Alkyl Bromides and In Situ Generated Imines

One-pot, multicomponent reactions are known for their green and efficient nature. We report a novel three-component reaction of alkyl amines, alkyl glyoxylates, and unactivated alkyl bromides under visible-light-induced palladium catalysis, yielding N-alkyl unnatural α-amino acid derivatives. This method offers mild conditions, broad substrate scope, and excellent functional group tolerance without requiring stoichiometric organometallic reagents. The approach has promising applications in protein engineering and drug discovery.

Photoinduced C(sp3)-H Bicyclopentylation Enabled by an Electron Donor-Acceptor Complex-Mediated Chemoselective Three-Component Radical Relay

The photoredox electron donor-acceptor (EDA) complex-mediated radical coupling reaction has gained prominence in the field of organic synthesis, finding widespread application in two-component coupling reactions. However, EDA complex-promoted multi-component reactions are not well developed with only a limited number of examples have been reported. Herein, we report a photoinduced and EDA complex-promoted highly chemoselective three-component radical arylalkylation of [1.1.1]propellane, which allows the direct functionalization of C(sp3)-H with bicyclo[1.1.1]pentanes (BCP)-aryl groups under mild conditions. A variety of unnatural α-amino acids, featuring structurally diversified 1,3-disubstituted BCP moieties, were synthesized in a single-step process. Notably, leveraging the high tension release of [1.1.1]propellane, the highly unstable transient aryl radical undergoes rapid conversion into a relatively stable tertiary alkyl transient radical, and consequently, the competing side-reaction of two-component coupling was entirely suppressed. The strategic use of this transient radical conversion approach would be useful for the design of diverse EDA complex-mediated multi-component reactions. It is noteworthy that the highly chemoselective late-stage incorporation of the 1,3-disubstituted BCP pharmacophores into peptides was achieved both in liquid-phase and solid-phase reactions. This advancement is anticipated to have significant application potential in the future development of peptide drugs.

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.

Visible-light-induced C-H alkylation of 2-amino-1,4-naphthoquinones

Simple and practical strategies for visible-light-induced C-H alkylation of 2-amino-1,4-naphthoquinones with cyclobutanone oxime esters and hydroxamic acid derivatives have been developed under mild and redox-neutral conditions. These two reactions can be carried out at room temperature and obtain a variety of 2-amino-1,4-naphthoquinone derivatives with cyano and amide groups. Moreover, the cyanoalkylation reaction of 2-amino-1,4-naphthoquinones can proceed smoothly in the absence of photocatalysts.

Photocatalytic redox-neutral α-C(sp3)-H pyridination of glycine derivatives and N-arylamines with cyanopyridines

A photo-induced α-C(sp3)-H decyanative pyridination of N-arylglycine derivatives with cyanopyridines was developed. This reaction was performed under organic photocatalytic and redox-neutral conditions via a radical-radical cross-coupling process. Besides, the protocol was also suitable for the C(sp3)-H pyridination of N-aryl tetrahydroisoquinolines as well as benzylamines.

Asymmetric Cross-Coupling of Aldehydes with Diverse Carbonyl or Iminyl Compounds by Photoredox-Mediated Cobalt Catalysis

The asymmetric cross-coupling of unsaturated bonds, hampered by their comparable polarity and reactivity, as well as the scarcity of efficient catalytic systems capable of diastereo- and enantiocontrol, presents a significant hurdle in organic synthesis. In this study, we introduce a highly adaptable photochemical cobalt catalysis framework that facilitates chemo- and stereoselective reductive cross-couplings between common aldehydes with a broad array of carbonyl and iminyl compounds, including N-acylhydrazones, aryl ketones, aldehydes, and α-keto esters. Our methodology hinges on a synergistic mechanism driven by photoredox-induced single-electron reduction and subsequent radical-radical coupling, all precisely guided by a chiral cobalt catalyst. Various optically enriched β-amino alcohols and unsymmetrical 1,2-diol derivatives (80 examples) have been synthesized with good yields (up to 90% yield) and high stereoselectivities (up to >20:1 dr, 99% ee). Of particular note, this approach accomplishes unattainable photochemical asymmetric transformations of aldehydes with disparate carbonyl partners without reliance on any external photosensitizer, thereby further emphasizing its versatility and cost-efficiency.

Organo-Photoredox Catalyzed gem-Difluoroallylation of Glycine and Glycine Residue in Peptides

An organo-photoredox catalyzed gem-difluoroallylation of glycine with α-trifluoromethyl alkenes via direct C(sp3)-H functionalization of glycine and C-F bond activation of α-trifluoromethyl alkenes has been described. As a consequence, a broad range of gem-difluoroalkene-containing unnatural amino acids are afforded in moderate to excellent yields. This reaction exhibits multiple merits such as readily available starting materials, broad substrate scope, and mild reaction conditions. The feasibility of this reaction has been highlighted by the late-stage modification of several peptides as well as the improved in vitro antifungal activity of compound 3v toward Valsa mali compared to that with commercial azoxystrobin.

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.

A Chiral Titanocene Complex as Regiodivergent Photoredox Catalyst: Synthetic Scope and Mechanism of Catalyst Generation

We describe a combined synthetic, spectroscopic, and computational study of a chiral titanocene complex as a regiodivergent photoredox catalyst (PRC). With Kagan's complex catCl2 either monoprotected 1,3-diols or 1,4-diols can be obtained in high selectivity from a common epoxide substrate in a regiodivergent epoxide opening depending on which enantiomer of the catalyst is employed. Due to the catalyst-controlled regioselectivity of ring opening and the broader substrate scope, the PRC with catCl2 is also a highly attractive branching point for diversity-oriented synthesis. The photochemical processes of cat(NCS)2, a suitable model for catCl2, were probed by time-correlated single-photon counting. The photoexcited complex displays a thermally activated delayed fluorescence as a result of a singlet-triplet equilibration, S1 ⇄ T1, via intersystem crossing and recrossing. Its triplet state is quenched by electron transfer to the T1 state. Computational and cyclic voltammetry studies highlight the importance of our sulfonamide additive. By bonding to sulfonamide additives, chloride abstraction from [catCl2]- is facilitated, and catalyst deactivation by coordination of the sulfonamide group is circumvented.

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.