Histidine-Specific Peptide Modification via Visible-Light-Promoted C–H Alkylation

Protein Chemical Modification Using Highly Reactive Species and Spatial Control of Catalytic Reactions

Protein bioconjugation has become an increasingly important research method for introducing artificial functions in to protein with various applications, including therapeutics and biomaterials. Due to its amphiphilic nature, only a few tyrosine residues are exposed on the protein surface. Therefore, tyrosine residue has attracted attention as suitable targets for site-specific modification, and it is the most studied amino acid residue for modification reactions other than lysine and cysteine residues. In this review, we present the progress of our tyrosine chemical modification studies over the past decade. We have developed several different catalytic approaches to selectively modify tyrosine residues using peroxidase, laccase, hemin, and ruthenium photocatalysts. In addition to modifying tyrosine residues by generating radical species through single-electron transfer, we have developed a histidine modification method that utilizes singlet oxygen generated by photosensitizers. These highly reactive chemical species selectively modify proteins in close proximity to the enzyme/catalyst. Taking advantage of the spatially controllable reaction fields, we have developed novel methods for site-specific antibody modification, detecting hotspots of oxidative stress, and target identification of bioactive molecules.

Boron, silicon, nitrogen and sulfur-based contemporary precursors for the generation of alkyl radicals by single electron transfer and their synthetic utilization

Recent developments in the use of boron, silicon, nitrogen and sulfur derivatives in single-electron transfer reactions for the generation of alkyl radicals are described. Photoredox catalyzed, electrochemistry promoted or thermally-induced oxidative and reductive processes are discussed highlighting their synthetic scope and discussing their mechanistic pathways.

Site-Selective Itaconation of Complex Peptides by Photoredox Catalysis

Site-selective peptide functionalization provides a straightforward and cost-effective access to diversify peptides for biological studies. Among many existing non-invasive peptide conjugations methodologies, photoredox catalysis has emerged as one of the powerful approaches for site-specific manipulation on native peptides. Herein, we report a highly N-termini-specific method to rapidly access itaconated peptides and their derivatives through a combination of transamination and photoredox conditions. This strategy exploits the facile reactivity of peptidyl-dihydropyridine in the complex peptide settings, complementing existing approaches for bioconjugations with excellent selectivity under mild conditions. Distinct from conventional methods, this method utilizes the highly reactive carbamoyl radical derived from a peptidyl-dihydropyridine. In addition, this itaconated peptide can be further functionalized as a Michael acceptor to access the corresponding peptide-protein conjugate.

Radical Addition of 4-Hydroxyquinazolines and Alkylation of Quinones by the Electro-Induced Homolysis of 4-Alkyl-1,4-di­hydropyridines

The formation of C(sp3)-centered radicals via the electro-­induced homolysis of 4-alkyl-1,4-dihydropyridines (alkyl-DHPs) is reported. The resulting alkyl radicals reacted with 4-hydroxyquinazolines or quinones to afford 2-alkyldihydroquinazolinones or alkylated quinones. A broad range of alkyl DHPs could be used as versatile radical precursors under electrolysis conditions. This alterative strategy provided a simple and effective pathway for the construction of C(sp2)–C(sp3) and C(sp3)–C(sp3) bonds under mild conditions.

Histidine-specific bioconjugation via visible-light-promoted thioacetal activation

Histidine (His, H) undergoes various post-translational modifications (PTMs) and plays multiple roles in protein interactions and enzyme catalyzed reactions. However, compared with other amino acids such as Lys or Cys, His modification is much less explored. Herein we describe a novel visible-light-driven thioacetal activation reaction which enables facile modification on histidine residues. An efficient addition to histidine imidazole N3 under biocompatible conditions was achieved with an electrophilic thionium intermediate. This method allows chemo-selective modification on peptides and proteins with good conversions and efficient histidine-proteome profiling with cell lysates. 78 histidine containing proteins were for the first time found with significant enrichment, most functioning in metal accumulation in brain related diseases. This facile His modification method greatly expands the chemo-selective toolbox for histidine-targeted protein conjugation and helps to reveal histidine's role in protein functions.

Functionalization of histidine residues in proteins via visible-light-promoted thioacetal activation is reported. ∼2000 proteins with reactive and exposed histidine residues from the MCF7 cell line are characterized using ABPP by this method.

Transition-Metal-Free Alkylation Strategy: A Facile Access of Alkylated Oxindoles via Alkyl Transfer

An efficient transition-metal-free alkylation/cyclization of activated alkenes using Hantzsch ester derivatives as effective alkyl reagents was described. A wide variety of valuable oxindoles were constructed in a single step with...

Synthesis of 1,4-Dihydropyridines and Related Heterocycles by Iodine-Mediated Annulation Reactions of N-Cyclopropyl Enamines

The annulation of N-cyclopropyl enamines to produce 1,4-dihydropyridine (1,4-DHP) derivatives is described. In the presence of molecular iodine (I₂), an N-cyclopropyl enamine substrate undergoes iodination, opening of the cyclopropyl ring, and annulation with a second molecule of the substrate to form the 1,4-DHP product. This reaction is amenable to gram-scale operations under mild reaction conditions with no transition metals being required. Further transformations of the 1,4-DHPs leads to related pyridine and bicyclic frameworks.

Luminol anchors improve the electrochemical-tyrosine-click labelling of proteins

New methods for chemo-selective modifications of peptides and native proteins are important in chemical biology and for the development of therapeutic conjugates. Less abundant and uncharged amino-acid residues are interesting targets to form less heterogeneous conjugates and preserve biological functions. Phenylurazole (PhUr), N-methylphenylurazole (NMePhUr) and N-methylluminol (NMeLum) derivatives were described as tyrosine (Y) anchors after chemical or enzymatic oxidations. Recently, we developed the first electrochemical Y-bioconjugation method coined eY-click to activate PhUr in biocompatible media. In this work, we assessed the limitations, benefits and relative efficiencies of eY-click conjugations performed with a set of PhUr, NMePhUr and NMeLum derivatives. Results evidenced a high efficiency of NMeLum that showed a complete Y-chemoselectivity on polypeptides and biologically relevant proteins after soft electrochemical activation. Side reactions on nucleophilic or heteroaromatic amino-acids such as lysine or tryptophan were never observed during mass spectrometry analysis. Myoglobine, bovine serum albumin, a plant mannosidase, glucose oxidase and the therapeutically relevant antibody trastuzumab were efficiently labelled with a fluorescent probe in a two-step approach combining eY-click and strain-promoted azide–alkyne cyclization (SPAAC). The proteins conserved their structural integrity as observed by circular dichroism and the trastuzumab conjugate showed a similar binding affinity for the natural HER2 ligand as shown by bio-layer interferometry. Compared to our previously described protocol with PhUr, eY-click with NMeLum species showed faster reaction kinetics, higher (complete) Y-chemoselectivity and reactivity, and offers the interesting possibility of the double tagging of solvent-exposed Y.

We assessed the relative efficiencies of tyrosine anchors in the electrochemical conjugation of peptides and proteins. Luminol derivatives showed faster reaction kinetics, complete tyrosine-chemoselectivity, and possible double modification.

Chemical modification of enzymes to improve biocatalytic performance

Improvement in intrinsic enzymatic features is in many instances a prerequisite for the scalable applicability of many industrially important biocatalysts. To this end, various strategies of chemical modification of enzymes are maturing and now considered as a distinct way to improve biocatalytic properties. Traditional chemical modification methods utilize reactivities of amine, carboxylic, thiol and other side chains originating from canonical amino acids. On the other hand, noncanonical amino acid- mediated 'click' (bioorthogoal) chemistry and dehydroalanine (Dha)-mediated modifications have emerged as an alternate and promising ways to modify enzymes for functional enhancement. This review discusses the applications of various chemical modification tools that have been directed towards the improvement of functional properties and/or stability of diverse array of biocatalysts.

Alkenylation and Arylation of Peptides via Ni-Catalyzed Reductive Coupling of α-C-Tosyl Peptides with Csp2 Triflates/Halides

A Ni-catalyzed reductive cross-coupling between α-C-tosyl peptides and Csp² triflates/halides has been developed. This protocol enables the formation of various unnatural di- and tripeptides containing vinyl and aryl side chains, and it expands the applications of Ni-catalyzed reductive cross-coupling in late-stage diversification of peptides.

Recent progress in covalent warheads for in vivo targeting of endogenous proteins

Covalent drugs exert potent and durable activity by chemical modification of the endogenous target protein in vivo. To maximize the pharmacological efficacy while alleviating the risk of toxicity due to nonspecific off-target reactions, current covalent drug discovery focuses on the development of targeted covalent inhibitors (TCIs), wherein a reactive group (warhead) is strategically incorporated onto a reversible ligand of the target protein to facilitate specific covalent engagement. Various aspects of warheads, such as intrinsic reactivity, chemoselectivity, mode of reaction, and reversibility of the covalent engagement, would affect the target selectivity of TCIs. Although TCIs clinically approved to date largely rely on Michael acceptor-type electrophiles for cysteine targeting, a wide array of novel warheads have been devised and tested in TCI development in recent years. In this short review, we provide an overview of recent progress in chemistry for selective covalent targeting of proteins and their applications in TCI designs.

Visible-Light-Mediated Modification and Manipulation of Biomacromolecules

Chemically modified biomacromolecules-i.e., proteins, nucleic acids, glycans, and lipids-have become crucial tools in chemical biology. They are extensively used not only to elucidate cellular processes but also in industrial applications, particularly in the context of biopharmaceuticals. In order to enable maximum scope for optimization, it is pivotal to have a diverse array of biomacromolecule modification methods at one's disposal. Chemistry has driven many significant advances in this area, and especially recently, numerous novel visible-light-induced photochemical approaches have emerged. In these reactions, light serves as an external source of energy, enabling access to highly reactive intermediates under exceedingly mild conditions and with exquisite spatiotemporal control. While UV-induced transformations on biomacromolecules date back decades, visible light has the unmistakable advantage of being considerably more biocompatible, and a spectrum of visible-light-driven methods is now available, chiefly for proteins and nucleic acids. This review will discuss modifications of native functional groups (FGs), including functionalization, labeling, and cross-linking techniques as well as the utility of oxidative degradation mediated by photochemically generated reactive oxygen species. Furthermore, transformations at non-native, bioorthogonal FGs on biomacromolecules will be addressed, including photoclick chemistry and DNA-encoded library synthesis as well as methods that allow manipulation of the activity of a biomacromolecule.

Cooperative Photoredox- and Nickel-Catalyzed Alkylative Cyclization Reactions of Alkynes with 4-Alkyl-1,4-dihydropyridines

Cooperative photoredox- and nickel-catalyzed alkylative cyclization reactions of iodoalkynes with 4-alkyl-1,4-dihydropyridines as alkylation reagents under visible light irradiation have been achieved to afford the corresponding alkylated cyclopentylidenes in good to high yields. Introduction of substituents at the propargylic position of iodoalkynes has led to the stereoselective formation of E-isomers. The present reaction system provides a novel synthetic method for alkylative cyclization reactions of both terminal and internal alkynes with cooperative photoredox and nickel catalysis.

Visible-Light-Promoted Biomimetic Reductive Functionalization of Quaternary Benzophenanthridine Alkaloids

Reduction of an iminium C═N double bond is the most important phase I metabolism process associated with the cytotoxic property of quaternary benzophenanthridine alkaloids (QBAs). Inspired by the light-mediated reduction of QBAs with nicotinamide adenine dinucleotide, a visible-light-promoted reductive functionalization reaction of QBAs is reported in this study. C4-Alkyl-1,4-dihydropyridines (DHPs) enable the direct reductive alkylation of QBA independently, serving as both single-electron-transfer reductant reagents under irradiation with 455 nm blue light in the absence of photocatalysts and additional additives. Our protocol can be further applied to the semisynthesis of natural 6-substituted dihydrobenzophenanthridine derivatives such as O-acetyl maclekarpine E.

Nonenzymatic Stereoselective S-Glycosylation of Polypeptides and Proteins

Here we report a nonenzymatic glycosylation reaction that builds axial S-glycosidic bonds under biorelevant conditions. This strategy is enabled by the design and use of allyl glycosyl sulfones as precursors to glycosyl radicals and exploits the exceptional functional group tolerance of radical processes. Our method introduces a variety of unprotected glycosyl units to the cysteine residues of peptides in a highly selective fashion. Through developing the second-generation protocol, we applied our method in the direct glycosylation of complex polypeptides and proteins. Computational studies were performed to elucidate the reaction mechanism.

Synthesis of C-Mannosylated Glycopeptides Enabled by Ni-Catalyzed Photoreductive Cross-Coupling Reactions

The biological functions of tryptophan C-mannosylation are poorly understood, in part, due to a dearth of methods for preparing pure glycopeptides and glycoproteins with this modification. To address this issue, efficient and scalable methods are required for installing this protein modification. Here, we describe unique Ni-catalyzed cross-coupling conditions that utilize photocatalysis or a Hantzsch ester photoreductant to couple glycosyl halides with (hetero)aryl bromides, thereby enabling the α-C-mannosylation of 2-bromo-tryptophan, peptides thereof, and (hetero)aryl bromides more generally. We also report that 2-(α-d-mannopyranosyl)-L-tryptophan undergoes facile anomerization in the presence of acid: something that must be considered when preparing and handling peptides with this modification. These developments enabled the first automated solid-phase peptide syntheses of C-mannosylated glycopeptides, which we used to map the epitope of an antibody, as well as providing the first verified synthesis of Carmo-HrTH-I, a C-mannosylated insect hormone. To complement this approach, we also performed late-stage tryptophan C-mannosylation on a diverse array of peptides, demonstrating the broad scope and utility of this methodology for preparing glycopeptides.

Combining flavin photocatalysis with parallel synthesis: a general platform to optimize peptides with non-proteinogenic amino acids

Most peptide drugs contain non-proteinogenic amino acids (NPAAs), born out through extensive structure-activity relationship (SAR) studies using solid-phase peptide synthesis (SPPS). Synthetically laborious and expensive to manufacture, NPAAs also can have poor coupling efficiencies allowing only a small fraction to be sampled by conventional SPPS. To gain general access to NPAA-containing peptides, we developed a first-generation platform that merges contemporary flavin photocatalysis with parallel synthesis to simultaneously make, purify, quantify, and even test up to 96 single-NPAA peptide variants via the unique combination of boronic acids and a dehydroalanine residue in a peptide. We showcase the power of our newly minted platform to introduce NPAAs of diverse chemotypes-aliphatic, aromatic, heteroaromatic-directly into peptides, including 15 entirely new residues, and to evolve a simple proteinogenic peptide into an unnatural inhibitor of thrombin by non-classical peptide SAR.

Late-stage C–H functionalization offers new opportunities in drug discovery

Over the past decade, the landscape of molecular synthesis has gained major impetus by the introduction of late-stage functionalization (LSF) methodologies. C-H functionalization approaches, particularly, set the stage for new retrosynthetic disconnections, while leading to improvements in resource economy. A variety of innovative techniques have been successfully applied to the C-H diversification of pharmaceuticals, and these key developments have enabled medicinal chemists to integrate LSF strategies in their drug discovery programmes. This Review highlights the significant advances achieved in the late-stage C-H functionalization of drugs and drug-like compounds, and showcases how the implementation of these modern strategies allows increased efficiency in the drug discovery process. Representative examples are examined and classified by mechanistic patterns involving directed or innate C-H functionalization, as well as emerging reaction manifolds, such as electrosynthesis and biocatalysis, among others. Structurally complex bioactive entities beyond small molecules are also covered, including diversification in the new modalities sphere. The challenges and limitations of current LSF methods are critically assessed, and avenues for future improvements of this rapidly expanding field are discussed. We, hereby, aim to provide a toolbox for chemists in academia as well as industrial practitioners, and introduce guiding principles for the application of LSF strategies to access new molecules of interest.

Visible-Light-Driven Redox Neutral Direct C-H Amination of Glycine Derivatives and Peptides with N-Acyloxyphthalimides

A room temperature, visible-light-promoted and redox neutral direct C-H amination of glycine and peptides has been firstly accomplished by using N-acyloxyphthalimide or -succinimide as nitrogen-radical precursor. The present strategy provides ways to introduce functionalities such as N-acyloxyphthalimide or -succinimide specifically to terminal glycine segment of peptides. Herein, mild conditions and high functional-group tolerance allow the preparation of non-natural α-amino acids and modification of corresponding peptides in this way.

Ynonylation of Acyl Radicals by Electroinduced Homolysis of 4-Acyl-1,4-dihydropyridines

Herein we report the conversion of 4-acyl-1,4-dihydropyridines (DHPs) into ynones under electrochemical conditions. The reaction proceeds via the homolysis of acyl-DHP under electron activation. The resulting acyl radicals react with hypervalent iodine(III) reagents to form the target ynones or ynamides in acceptable yields. This mild reaction condition allows wider functionality tolerance that includes halides, carboxylates, or alkenes. The synthetic utility of this methodology is further demonstrated by the late-stage modification of complex molecules.

Application of tert-Butyl Disulfide-Protected Amino Acids for the Fmoc Solid-Phase Synthesis of Lactam Cyclic Peptides under Mild Metal-Free Conditions

Lactam cyclic peptides are a class of interesting and pharmaceutically active molecules, but their previous syntheses have required the use of heavy metals and/or forcing conditions. Here, we describe the efficient application of the previously reported tert-butyl disulfide-protected amino acids and their use in the efficient, solid-phase synthesis of a series of lactam cyclic peptides under mild, metal-free conditions.

Post-Assembly Modification of Peptides by Ligand-Enabled β-C(sp3)-H Arylation of Alanine at the C-Terminus: Overcoming the Inhibition Effect of Peptide Bonds

Postassembly modification of peptides via C(sp³)-H functionalization on aliphatic side chains provides a straightforward approach to access functionalized peptides as therapeutics. However, C(sp³)-H functionalization of C-terminal residues remains underdeveloped due to the inhibition effect of secondary amides in the backbone. Herein, we report a ligand-enabled, bidentate auxiliary-assisted β-C(sp³)-H arylation method, which is well tolerant of secondary amides. A wide range of peptides (tri- to dodecapeptides) underwent position-specific modification of alanine at the C-terminus.

Development and application of novel electrophilic warheads in target identification and drug discovery

Nucleophilic amino acids play important roles in maintenance of protein structure and function, covalent modification of such amino acid residues by therapeutic agents is an efficient way to treat human diseases. Most of current clinical drugs are structurally limited to α,β-unsaturated amide as an electrophilic warhead. To alleviate this issue, many novel electrophiles have been developed in recent years that can covalently bind to different amino acid residues and provides a unique way to interrogate proteins, including "undruggable" targets. With an activity-based protein profiling (ABPP) approach, the activity and functionality of a protein and its binding sites can be assessed. This facilitates an understanding of protein function, and contributes to the discovery of new druggable targets and lead compounds. Meanwhile, many novel inhibitors bearing new reactive warhead were developed and displayed remarkable pharmaceutical properties. In this perspective, we have reviewed the recent remarkable progress of novel electrophiles and their applications in target identification and drug discovery.

Visible-light-promoted photocatalyst-free alkylation and acylation of benzothiazoles

Herein we report a protocol for the visible-light-mediated alkylation/acylation reaction of benzothiazoles. Alkyl/acyl substituted Hantzsch esters are easily prepared and rationally used as radical precursors. In the presence of BF3·Et2O and Na2S2O8, various benzothiazole derivatives were readily obtained in good yields. Our user-friendly protocol can proceed by simple irradiation with blue LEDs (λ = 465 nm) and without the assistance of external photocatalysts. The reaction is also characterized by mild conditions and scalability, thus offering an alternative and efficient tool for the synthesis of 2-functionalized benzothiazoles.

Brønsted acid catalyzed radical addition to quinone methides

A fundamental quest for alkyl radical generation under mild conditions through photoinduced Brønsted acid catalysis is addressed. The optimized protocol does not require any organic dyes or transition metal photocatalyst. Under blue light irradiation with diphenyl phosphate as a catalyst and dihydropyridine derivatives as a radical source, functionalized arylmethane derivatives are obtained in high yield.

Late Stage Phosphotyrosine Mimetic Functionalization of Peptides Employing Metallaphotoredox Catalysis

Access to phosphotyrosine (pTyr) mimetics requires multistep syntheses, and therefore late stage incorporation of these mimetics into peptides is not feasible. Here, we develop and employ metallaphotoredox catalysis using 4-halogenated phenylalanine to afford a variety of protected pTyr mimetics in one step. This methodology was shown to be tolerant of common protecting groups and applicable to the late stage pTyr mimetic modification of protected and unprotected peptides, and peptides of biological relevance.

Proximity Histidine Labeling by Umpolung Strategy Using Singlet Oxygen

While electrophilic reagents for histidine labeling have been developed, we report an umpolung strategy for histidine functionalization. A nucleophilic small molecule, 1-methyl-4-arylurazole, selectively labeled histidine under singlet oxygen (¹O₂) generation conditions. Rapid histidine labeling can be applied for instant protein labeling. Utilizing the short diffusion distance of ¹O₂ and a technique to localize the ¹O₂ generator, a photocatalyst in close proximity to the ligand-binding site, we demonstrated antibody Fc-selective labeling on magnetic beads functionalized with a ruthenium photocatalyst and Fc ligand, ApA. Three histidine residues located around the ApA binding site were identified as labeling sites by liquid chromatography-mass spectrometry analysis. This result suggests that ¹O₂-mediated histidine labeling can be applied to a proximity labeling reaction on the nanometer scale.

Interrogating biological systems using visible-light-powered catalysis

Light-powered catalysis has found broad utility as a chemical transformation strategy, with widespread impact on energy, environment, drug discovery and human health. A noteworthy application impacting human health is light-induced sensitization of cofactors for photodynamic therapy in cancer treatment. The clinical adoption of this photosensitization approach has inspired the search for other photochemical methods, such as photoredox catalysis, to influence biological discovery. Over the past decade, light-mediated catalysis has enabled the discovery of valuable synthetic transformations, propelling it to become a highly utilized chemical synthesis strategy. The reaction components required to achieve a photoredox reaction are identical to photosensitization (catalyst, light source and substrate), making it ideally suited for probing biological environments. In this Review, we discuss the therapeutic application of photosensitization and advancements made in developing next-generation catalysts. We then highlight emerging uses of photoredox catalytic methods for protein bioconjugation and probing complex cellular environments in living cells.

Construction of Peptide Macrocycles via Radical-Mediated Intramolecular C-H Alkylations

Enzyme-catalyzed radical-mediated C-H functionalization reactions allow nature to create natural products of unusual three-dimensional structures from simple linear peptide precursors. In comparison, chemist's ability to harness radical C-H functionalization reactions for synthesis of complex peptides remains limited. In this work, new methods have been developed to construct peptide macrocycles via radical-mediated intramolecular C-H alkylation reactions under photoredox catalysis. Linear peptide precursors equipped with a C-terminal N-(acyloxy)phthalimide ester can cyclize with the α C-H bond of N-terminal glycine or aryl C-H bond of N-heteroarene capping units in high yield and selectivity under mild conditions. The strategy uses the C-H cyclization step to incorporate lysine, homolysine, and various heteroarene-derived amino acid linchpins into peptide macrocycles, enabling convergent and flexible synthesis of complex peptide macrocycles from simple building blocks.

nSite-Selective, Chemical Modification of Protein at Aromatic Side Chain and Their Emergent Applications

Site-selective chemical modification of protein side chain has probed enormous opportunities in the fundamental understanding of cellular biology and therapeutic applications. Primarily, in the field of biopharmaceutical where formulation of bioconjugates is found to be potential medicine than an individual constituent. In this regard, Lysine and Cysteine are the most widely used endogenous amino acid for these purposes. Recently, the aromatic side chain residues (Trp, Tyr, and His) that are low abundant in protein have gained more attention in therapeutic applications due to their advantages of chemical reactivity and specificity. This review discusses the site-selective bioconjugation methods for aromatic side chains (Trp, Tyr and His) and highlights the developed strategies in the last three years, along with their applications. Also, the review highlights the prevalent methods published earlier. We have examined that metal-catalyzed and photocatalytic reactions are gaining more attention for bioconjugation, though their practical operation is under development. The review has been summarized with the future perspective of protein and peptide conjugations contemplating therapeutic applications and challenges.

Direct excitation strategy for radical generation in organic synthesis

This tutorial review encompasses the radical generation based on classical methods and photoredox catalysis. It will also focus on radical generation only demanding visible-light, which involves EDA complex and direct photo-excitation strategy.

Photocatalytic methods for amino acid modification

This tutorial review introduces photocatalysis for amino acid modification and summarises recent advances in the field.

Late-Stage Alkylation of N-Containing Heteroarenes Enabled by Homolysis of Alkyl-1,4-dihydropyridines under Blue LED Irradiation

Alkylated heteroarenes are widely found in bioactive molecules and pharmaceuticals. Therefore, there is great interest in developing a chemoselective alkylation of heteroarenes under mild conditions, particularly during a late-stage functionalization step for the purpose of rapid derivatization. Herein, we introduce an efficient visible-light-promoted C–H alkylation of nitrogen-containing heteroarenes by using C4-alkyl 1,4-dihydropyridines (DHPs) as radical precursors at ambient temperatures. A broad scope of heteroarenes, such as 4-hydroxyquinazoline and its derivatives, including those bearing electron-donating or electron-withdrawing groups, can be successfully alkylated in good yields by using various C4-alkyl DHPs.

Single electron transfer-based peptide/protein bioconjugations driven by biocompatible energy input

Bioconjugation reactions play a central facilitating role in engendering modified peptides and proteins. Early progress in this area was inhibited by challenges such as the limited range of substrates and the relatively poor biocompatibility of bioconjugation reagents. However, the recent developments in visible-light induced photoredox catalysis and electrochemical catalysis reactions have permitted significant novel reactivities to be developed in the field of synthetic and bioconjugation chemistry. This perspective describes recent advances in the use of biocompatible energy input for the modification of peptides and proteins mainly, via the single electron transfer (SET) process, as well as key future developments in this area.

Recent Advances in Rapid Synthesis of Non-proteinogenic Amino Acids from Proteinogenic Amino Acids Derivatives via Direct Photo-Mediated C-H Functionalization

Non-proteinogenic amino acids have attracted tremendous interest for their essential applications in the realm of biology and chemistry. Recently, rising C-H functionalization has been considered an alternative powerful method for the direct synthesis of non-proteinogenic amino acids. Meanwhile, photochemistry has become popular for its predominant advantages of mild conditions and conservation of energy. Therefore, C-H functionalization and photochemistry have been merged to synthesize diverse non-proteinogenic amino acids in a mild and environmentally friendly way. In this review, the recent developments in the photo-mediated C-H functionalization of proteinogenic amino acids derivatives for the rapid synthesis of versatile non-proteinogenic amino acids are presented. Moreover, postulated mechanisms are also described wherever needed.

Late-Stage Alkylation of N-Containing Heteroarenes Enabled by Homolysis of Alkyl-1,4-dihydropyridines under Blue LED Irradiation

Alkylated heteroarenes are widely found in bioactive molecules and pharmaceuticals. Therefore, there is great interest in developing a chemoselective alkylation of heteroarenes under mild conditions, particularly during a late-stage functionalization step for the purpose of rapid derivatization. Herein, we introduce an efficient visible-light-promoted C–H alkylation of nitrogen-containing heteroarenes by using C4-alkyl 1,4-dihydropyridines (DHPs) as radical precursors at ambient temperatures. A broad scope of heteroarenes, such as 4-hydroxyquinazoline and its derivatives, including those bearing electron-donating or electron-withdrawing groups, can be successfully alkylated in good yields by using various C4-alkyl DHPs.