Visible-Light Photoredox Synthesis of Chiral α-Selenoamino Acids

Recent Advances in Photoinduced Modification of Amino Acids, Peptides, and Proteins

The chemical modification of biopolymers like peptides and proteins is a key technology to access vaccines and pharmaceuticals. Similarly, the tunable derivatization of individual amino acids is important as they are key building blocks of biomolecules, bioactive natural products, synthetic polymers, and innovative materials. The high diversity of functional groups present in amino acid-based molecules represents a significant challenge for their selective derivatization Recently, visible light-mediated transformations have emerged as a powerful strategy for achieving chemoselective biomolecule modification. This technique offers numerous advantages over other methods, including a higher selectivity, mild reaction conditions and high functional-group tolerance. This review provides an overview of the most recent methods covering the photoinduced modification for single amino acids and site-selective functionalization in peptides and proteins under mild and even biocompatible conditions. Future challenges and perspectives are discussed beyond the diverse types of photocatalytic transformations that are currently available.

Switchable Decarboxylation by Energy- or Electron-Transfer Photocatalysis

Kolbe dimerization and Hofer-Moest reactions are well-investigated carboxylic acid transformations, wherein new carbon-carbon and carbon-heteroatom bonds are constructed via electrochemical decarboxylation. These transformations can be switched by choosing an electrode that allows control of the reactive intermediate, such as carbon radical or carbocation. However, the requirement of a high current density diminishes the functional group compatibility with these electrochemical reactions. Here, we demonstrate the photocatalytic decarboxylative transformation of activated carboxylic acids in a switchable and functional group-compatible manner. We discovered that switching between Kolbe-type or Hofer-Moest-type reactions can be accomplished with suitable photocatalysts by controlling the reaction pathways: energy transfer (EnT) and single-electron transfer (SET). The EnT pathway promoted by an organo-photocatalyst yielded 1,2-diarylethane from arylacetic acids, whereas the ruthenium photoredox catalyst allows the construction of an ester scaffold with two arylmethyl moieties via the SET pathway. The resulting radical intermediates were coupled to olefins to realize multicomponent reactions. Consequently, four different products were selectively obtained from a simple carboxylic acid. This discovery offers new opportunities for selectively synthesizing multiple products via switchable reactions using identical substrates with minimal cost and effort.

HTE- and AI-assisted development of DHP-catalyzed decarboxylative selenation

1,4-Dihydropyridine (DHP) derivatives play key roles in biology, but are rarely used as catalysts in synthesis. Here, we developed a DHP derivative-catalyzed decarboxylative selenation reaction that showed a broad substrate scope, with the assistance of high-throughput experimentation (HTE) and artificial intelligence (AI). The AI-based model could identify the key structural features and give accurate prediction of unseen reactions (R² = 0.89, RMSE = 9.0%, and MAE = 6.3%). Our work not only developed the catalytic applications of DHP derivatives, but also demonstrated the power of the combination of HTE and AI to advance chemical synthesis.

Synthesis and Application Dichalcogenides as Radical Reagents with Photochemical Technology

Dichalcogenides (disulfides and diselenides), as reactants for organic transformations, are important and widely used because of their potential to react with nucleophiles, electrophilic reagents, and radical precursors. In recent years, in combination with photochemical technology, the application of dichalcogenides as stable radical reagents has opened up a new route to the synthesis of various sulfur- and selenium-containing compounds. In this paper, synthetic strategies for disulfides and diselenides and their applications with photochemical technology are reviewed: (i) Cyclization of dichalcogenides with alkenes and alkynes; (ii) direct selenylation/sulfuration of C-H/C-C/C-N bonds; (iii) visible-light-enabled seleno- and sulfur-bifunctionalization of alkenes/alkynes; and (iv) Direct construction of the C(sp)-S bond. In addition, the scopes, limitations, and mechanisms of some reactions are also described.

Deaminative Arylation and Alkenyaltion of Aliphatic Tertiary Amines with Aryl and Alkenylboronic Acids via Nitrogen Ylides

Transition-metal-catalyzed Suzuki-Miyaura coupling has significantly advanced C-C bond formation and has been well recognized in organic synthesis, pharmaceuticals, materials science and other fields. In this rapid development, cross coupling without transition metal catalyst is a big challenge in this field, and using widely existing tertiary amines as electrophiles to directly couple with boronic acids has great hurdles yet significant application prospects. Herein, we report an efficient and general deaminative arylation and alkenylation of tertiary amines (propargyl amines, allyl amines and 1H-indol-3-yl methane amines) with ary and alkenylboronic acids enabled by difluorocarbene under transition-metal-free conditions. Preliminary mechanism experiments suggest that in situ formed difluoromethyl quaternary amine salt, nitrogen ylide and tetracoordinate boron species are the key intermediates, the subsequent 1,2-metallate shift and protodeboronation complete the new coupling reaction.

Photocatalyzed Oxidative Decarboxylation Forming Aminovinylcysteine Containing Peptides

The formation of (2S,3S)-S-[(Z)-aminovinyl]-3-methyl-D-cysteine (AviMeCys) substructures was developed based on the photocatalyzed-oxidative decarboxylation of lanthionine-bearing peptides. The decarboxylative selenoetherification of the N-hydroxyphthalimide ester, generated in situ, proceeded under mild conditions at −40 °C in the presence of 1 mol% of eosin Y-Na2 as a photocatalyst and the Hantzsch ester. The following β-elimination of the corresponding N,Se-acetal was operated in a one-pot operation, led to AviMeCys substructures found in natural products in moderate to good yields. The sulfide-bridged motif, and also the carbamate-type protecting groups, such as Cbz, Teoc, Boc and Fmoc groups, were tolerant under the reaction conditions.

Decarboxylative Side-Chain Functionalization of Aspartic/Glutamic Acids Using Two-Molecule Photoredox Catalysts

The side-chain functionalization of aspartic/glutamic acid derivatives through photoinduced decarboxylation was achieved by using organic two-molecule photoredox catalysts without racemization under mild conditions. A facile process involving the preparation of substrates and photoinduced decarboxylative radical additions can provide easy access to the linked amino acids with carbohydrates and amino acids at the side chain.

Solid-Phase Photochemical Peptide Homologation Cyclization

Forging new C(sp3)-C(sp3) bonds to central positions within a peptide backbone is critical for the development of new therapeutics and chemical probes. Currently, there are no methods for decarboxylating Asp and Glu side chains solid-phase photochemically or using such radicals to form peptide macrocycles. Herein, electron-donor-acceptor complexes between Hantzsch ester and on-resin peptide N-hydroxyphthalimide radical precursors are used to access these radicals, demonstrated with two-carbon homologations and homologation cyclizations of Atosiban and RGDf.

Strategies for the Synthesis of Selenocysteine Derivatives

β-Seleno-α-amino acids, known as selenocysteine (Sec) derivatives, have emerged as important targets because of their role in chemical biology, not only as part of selenoproteins with important redox properties, but also because of their activity as antivirals or metabolites effective in inhibiting carcinogenesis. In addition, there is demand for this type of compounds due to their use in native chemical ligation to construct large peptides. Therefore, this review summarizes the various synthetic methods that have been published to construct Sec derivatives. Most of them involve the generation of the C–Se bond by nucleophilic substitution reactions, but other reactions such as radical or multicomponent strategies are also reported. Of particular importance is the Se-Michael addition of Se-nucleophiles to chiral bicyclic dehydroalanines, in which the stereogenic center is generated under complete stereocontrol.

1 Introduction

2 Previously Reviewed Synthesis of Sec

3 Retrosynthesis of Sec Derivatives

4 Sec Derivatives by Nucleophilic Substitutions

5 Sec Derivatives by Radical Processes

6 Sec Derivatives by 1,4-Conjugate Additions

7 Conclusion

Electroreductive Cross-Coupling of Trifluoromethyl Alkenes and Redox Active Esters for the Synthesis of Gem-Difluoroalkenes

An electroreductive access to gem-difluoroalkenes has been developed through the decarboxylative/defluorinative coupling of N-hydroxyphtalimides esters and α-trifluoromethyl alkenes. The electrolysis is performed under very simple reaction conditions in an undivided cell using cheap carbon graphite electrodes. This metal-free transformation features broad scope with good to excellent yields. Tertiary, secondary as well as primary alkyl radicals could be easily introduced. α-aminoacids L-aspartic and L-glutamic acid-derived redox active esters were good reactive partners furnishing potentially relevant gem-difluoroalkenes. In addition, it has been demonstrated that our electrosynthetic approach toward the synthesis of gem-difluoroalkenes could use an easily prepared Kratitsky salt as alkyl radical precursor via a deaminative/defluorinative carbofunctionalization of trifluoromethylstyrene.

Clarification on the Reactivity of Diaryl Diselenides toward Hexacyclohexyldilead under Light

In this study, the reactivity of organochalcogen compounds toward a representative alkyl-lead bond compound under light was investigated in detail. Under light irradiation, the Cy-Pb bond of Cy6Pb2 (Cy = cyclohexyl) undergoes homolytic cleavage to generate a cyclohexyl radical (Cy•). This radical can be successfully captured by diphenyl diselenide, which exhibits excellent carbon-radical-capturing ability. In the case of (PhS)2 and (PhTe)2, the yields of the corresponding cyclohexyl sulfides and tellurides were lower than that of (PhSe)2. This probably occurred due to the low carbon-radical-capturing ability of (PhS)2 and the high photosensitivity of the cyclohexyl-tellurium bond.

Diorganyl diselenides: a powerful tool for the construction of selenium containing scaffolds

Organoselenium compounds find versatile applications in organic synthesis, materials synthesis, and ligand chemistry. Organoselenium heterocycles are widely studied agents with diverse applications in various biological processes. This review highlights the recent progress in the synthesis of selenium heterocycles using diorganyl diselenides with keen attention on green synthetic approaches, scopes, C-H selanylation, the mechanisms of different reactions and insights into the formation of metal complexes. The C-H selanylation using diorganyl diselenides with different catalysts, bases, transition metals, iodine salts, NIS, hypervalent iodine, and other reagents is summarised. Finally, the diverse binding modes of bis(2/4-pyridyl)diselenide with different metal complexes are also summarised.

Photocatalytic cross-couplings via the cleavage of N-O bonds

N-(Acyloxy)phthalimide and oxime derivatives containing N-O bonds are important chemicals and synthetic intermediates, and visible light photoredox reductions of the N-O bonds provide carbon- or nitrogen-centered radicals for N-(acyloxy)phthalimide derivatives and iminyl radicals for oxime derivatives. This feature article summarises the recent progress in the visible light photoredox organic reactions, including decarboxylative addition reactions, alkylation, allylation, alkenylation, alkynylation, arylation, heteroarylation and cascade annulation of N-(acyloxy)phthalimide derivatives through the formation of carbon-carbon bonds, decarboxylative borylation, amination, oxygenation, sulfuration, selenylation, fluorination and iodination of N-(acyloxy)phthalimide derivatives through the formation of carbon-heteroatom bonds, and additions to arenes and alkenes, hydrogen atom transfer and the cleavage of α-carbon-carbon bonds via the iminyl radical intermediates for oxime derivatives.

Radical-Based Synthesis and Modification of Amino Acids

Amino acids (AAs) are key structural motifs with widespread applications in organic synthesis, biochemistry, and material sciences. Recently, with the development of milder and more versatile radical-based procedures, the use of strategies relying on radical chemistry for the synthesis and modification of AAs has gained increased attention, as they allow rapid access to libraries of novel unnatural AAs containing a wide range of structural motifs. In this Minireview, we provide a broad overview of the advancements made in this field during the last decade, focusing on methods for the de novo synthesis of α-, β-, and γ-AAs, as well as for the selective derivatisation of canonical and non-canonical α-AAs.

Small peptide diversification through photoredox-catalyzed oxidative C-terminal modification

A photoredox-catalyzed oxidative decarboxylative coupling of small peptides is reported, giving access to a variety of N,O-acetals. They were used as intermediates for the addition of phenols and indoles, leading to novel peptide scaffolds and bioconjugates. Amino acids with nucleophilic side chains, such as serine, threonine, tyrosine and tryptophan, could also be used as partners to access tri- and tetrapeptide derivatives with non-natural cross-linking.

Visible light photocatalysis in the late-stage functionalization of pharmaceutically relevant compounds

The late stage functionalization (LSF) of complex biorelevant compounds is a powerful tool to speed up the identification of structure-activity relationships (SARs) and to optimize ADME profiles. To this end, visible-light photocatalysis offers unique opportunities to achieve smooth and clean functionalization of drugs by unlocking site-specific reactivities under generally mild reaction conditions. This review offers a critical assessment of current literature, pointing out the recent developments in the field while emphasizing the expected future progress and potential applications. Along with paragraphs discussing the visible-light photocatalytic synthetic protocols so far available for LSF of drugs and drug candidates, useful and readily accessible synoptic tables of such transformations, divided by functional groups, will be provided, thus enabling a useful, fast, and easy reference to them.

Fernandes V, Matsuo BT, C. Delgado JA, de Souza WC, Paixão MW. Photoinduced deaminative strategies: Katritzky salts as alkyl radical precursors

Primary amines are one of the most predominant functional groups found in organic molecules. This review covers the most recent developments on photocatalytic deaminative strategies by using Katritzky Salts as alkyl radical reservoirs.

The photoredox alkylarylation of styrenes with alkyl N-hydroxyphthalimide esters and arenes involving C-H functionalization

The In(OTf)₃-promoted three-component photoredox alkylarylation of styrenes with alkyl NHP esters and arenes to access alkylated arene derivatives through C-C bond cleavage and C-H functionalization is reported. By utilizing visible-light photoredox catalysis, alkyl N-hydroxyphthalimide esters serving as alkyl carbon-centered radicals and a wide range of arenes (e.g., indoles, pyrrole, and electron-rich arenes) as nucleophiles were used to enable the introduction of various alkyl groups and aryl groups across the C[double bond, length as m-dash]C bonds with excellent selectivity and functional group tolerance.

Photoredox-catalyzed stereoselective alkylation of enamides with N-hydroxyphthalimide esters via decarboxylative cross-coupling reactions

Stereoselective β-C(sp2)-H alkylation of enamides with redox-active N-hydroxyphthalimide esters via a photoredox-catalyzed decarboxylative cross-coupling reaction is demonstrated. This methodology features operational simplicity, broad substrate scopes, and excellent stereoselectivities and functional group tolerance, affording a diverse array of geometrically defined and synthetically valuable enamides bearing primary, secondary or tertiary alkyl groups in satisfactory yields.

Divergent Conversion of N-Acyl-isoxazol-5(2 H)-ones to Oxazoles and 1,3-Oxazin-6-ones Using Photoredox Catalysis

The fragmentation of N-acyl-isoxazol-5-ones using visible light photoredox catalysis has been disclosed. The catalyst-controlled divergent mechanisms, namely the oxidative and reductive quenching catalytic cycle, are utilized. Various oxazoles and 1,3-oxazin-6-ones are selectively obtained from the same isoxazol-5-one skeleton under mild conditions.

General Cyclopropane Assembly by Enantioselective Transfer of a Redox-Active Carbene to Aliphatic Olefins

Asymmetric cyclopropane synthesis currently requires bespoke strategies, methods, substrates, and reagents, even when targeting similar compounds. This approach slows down discovery and limits available chemical space. Introduced herein is a practical and versatile diazocompound and its performance in the first unified asymmetric synthesis of functionalized cyclopropanes. The redox-active leaving group in this reagent enhances the reactivity and selectivity of geminal carbene transfer. This effect allowed the asymmetric cyclopropanation of various olefins, including unfunctionalized aliphatic alkenes, that enables the three-step total synthesis of (-)-dictyopterene A. This unified synthetic approach delivers high enantioselectivities that are independent of the stereoelectronic properties of the functional groups transferred. Our results demonstrate that orthogonally differentiated diazocompounds are viable and advantageous equivalents of single-carbon chirons.

Photocatalytic Modification of Amino Acids, Peptides, and Proteins

In the last decade, visible‐light photoredox catalysis has emerged as a powerful strategy to enable novel transformations in organic synthesis. Owing to mild reaction conditions (i.e., room temperature, use of visible light) and high functional‐group tolerance, photoredox catalysis could represent an ideal strategy for chemoselective biomolecule modification. Indeed, a recent trend in photoredox catalysis is its application to the development of novel methodologies for amino acid modification. Herein, an up‐to‐date overview of photocatalytic methodologies for the modification of single amino acids, peptides, and proteins is provided. The advantages offered by photoredox catalysis and its suitability in the development of novel biocompatible methodologies are described. In addition, a brief consideration of the current limitations of photocatalytic approaches, as well as future challenges to be addressed, are discussed.

Photoinduced Deaminative Borylation of Alkylamines

An operationally simple deaminative borylation reaction of primary alkylamines has been developed. The formation of electron-donor-acceptor complexes between N-alkylpyridinium salts and bis(catecholato)diboron enables photoinduced single-electron transfer and fragmentation to carbon-centered radicals, which are subsequently borylated. The mild conditions allow a diverse range of readily available alkylamines to be efficiently converted into synthetically valuable alkylboronic esters under catalyst-free conditions.

Olefination of Alkyl Halides with Aldehydes by Merging Visible-Light Photoredox Catalysis and Organophosphorus Chemistry

Carbon-carbon double bond (C=C) formation is a crucial transformation in organic chemistry. Visible-light photoredox catalysis provides economical and sustainable opportunities for the development of novel and peculiar organic reactions. Here we report a method for the olefination of alkyl halides with aldehydes by visible-light photoredox catalysis using triphenylphosphine as a reductive quencher (103 examples). This transformation accommodates a variety of aldehydes including paraformaldehyde; aqueous formaldehyde; 2,2,2-trifluoroacetaldehyde monohydrate; 2,2,2-trifluoro-1-methoxyethanol; and other common aldehydes. The present method exhibits several advantages, including operational simplicity, mild reaction conditions, wide functional group tolerance, and amenability to gram-scale synthesis. We anticipate that it will be widely used in the synthesis of organic molecules, natural products, biological molecules, and polymers.

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General olefination of benzyl halides and bromoacetamides with aldehydes

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Operational simplicity and mild conditions under visible-light photoredox catalysis

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Wide substrate scope and high efficiency

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Amenability to gram-scale synthesis

Access to Alkyl-Substituted Lactone via Photoredox-Catalyzed Alkylation/Lactonization of Unsaturated Carboxylic Acids

An efficient photoredox-catalyzed alkylation/lactonization reaction of unsaturated carboxylic acids by using alkyl N-hydroxyphthalimide esters as alkylation reagents has been developed. Varieties of redox-active esters derived from aliphatic carboxylic acids were proved viable in this method, affording alkyl substituted lactones in moderate to good yields. This redox-neutral procedure features mild conditions and operational simplicity, which provides a new strategy for the synthesis of alkyl substituted lactones.

Visible light photocatalytic decarboxylative monofluoroalkenylation of α-amino acids with gem-difluoroalkenes

α-Amino acids are among the most common biologically active molecules in nature, and their functionalization has attracted much attention. In this communication, a novel, efficient and general visible-light photocatalytic decarboxylative monofluoroalkenylation of N-protected α-amino acids with gem-difluoroalkenes is reported, affording the corresponding α-amino monofluoroalkenes which might find applications in medical chemistry and materials science. The reaction proceeded at room temperature with high efficiency and tolerance of various functional groups.

Copolymerization of Phenylselenide-Substituted Maleimide with Styrene and Its Oxidative Elimination Behavior

Selenium-containing monomer monophenyl maleimide selenide (MSM) was synthesized and copolymerized with styrene (St) using reversible addition-fragmentation chain transfer (RAFT) polymerization. Copolymers with controlled molecular weight and narrow molecular weight distribution were obtained. The structure of the copolymer was characterized by nuclear magnetic resonance, matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrum, Fourier transform infrared spectroscopy (FT-IR) and Ultraviolet⁻visible spectroscopy (UV-vis) spectroscopy. The copolymer can be oxidized by H₂O₂ to form carbon-carbon double bonds within the main chain due to the unique sensitivity of selenide groups in the presence of oxidants. Such structure changing resulted in an interesting concentration-related photoluminescence emission enhancement.