Methionine-enabled peptide modification through late-stage Pd-catalyzed β-C(sp3)-H olefination/cyclization

We present a method for site-selective diversification of peptides via Pd-catalyzed β-C(sp3)-H olefination/cyclization. In this protocol, the native methionine residue acts as a directing group, enabling site-specific olefination/cyclization of peptides. This chemistry demonstrates broad substrate scope, offering a versatile tool for peptide ligation.

Backbone-Enabled and Ester Groups Switched δ-C(sp2)-H Amination/Fluorination: Cyclic Dipeptides Synthesis

An efficient and concise strategy for the synthesis of cyclic dipeptides via Pd-catalyzed site-selective δ-C(sp2)-H amination/fluorination and N-to-C cyclization is disclosed. The backbone amides within the dipeptides serves as endogenous directing groups, while the desired products were switched by the C-terminal ester group. This chemistry presents a novel and robust alternative to construct cyclodipeptide fragments.

Directed C(sp3)-H Arylation of Free α-Aminophosphonates: Dual Models Exploration via Palladium Catalysis

In this report, we present the dual activation models for transient directing group-directed and amino-self-directed Pd-catalyzed α-aminophosphonate side-chain C(sp3)-H arylation. Both strategies showed facile, efficient, and single regioselectivity in the reaction between free α-aminophosphonates and aryl iodides. Furthermore, the modification of amino and late-stage functionalization of the C(sp3)-P bond from products indicates potential applications for α-aminophosphonates.

Postassembly Modification of Peptides by Histidine-Directed β-C(sp3)-H Arylation of Alanine at the Internal Positions: Overcoming the Inhibitory Effect of Peptide Bonds

Peptide modification by C(sp3)-H functionalization of residues at the internal positions remains underdeveloped due to the inhibitory effect of backbone amides. In this study, using histidine (His) as an endogenous directing group, we developed a novel method for the β-C(sp3)-H functionalization of alanine (Ala) at diverse positions of peptides. Through this approach, a wide range of linear peptides were modified on the side-chain of Ala adjacent to His to afford the functionalized peptides in moderate to good yield and excellent position selectivity. Furthermore, conjugation of peptides with functional molecules such as glucuronide, oleanolic acid, dipeptide, and fluorophore derivatives was achieved.

Therapeutic stapled peptides: Efficacy and molecular targets

Peptide stapling, by employing a stable, preformed alpha-helical conformation, results in the production of peptides with improved membrane permeability and enhanced proteolytic stability, compared to the original peptides, and provides an effective solution to accelerate the rapid development of peptide drugs. Various reviews present peptide stapling chemistries, anchoring residues and one- or two-component cyclization, however, therapeutic stapled peptides have not been systematically summarized, especially focusing on various disease-related targets. This review highlights the latest advances in therapeutic peptide drug development facilitated by the application of stapling technology, including different stapling techniques, synthetic accessibility, applicability to biological targets, potential for solving biological problems, as well as the current status of development. Stapled peptides as therapeutic drug candidates have been classified and analysed mainly by receptor- and ligand-based stapled peptide design against various diseases, including cancer, infectious diseases, inflammation, and diabetes. This review is expected to provide a comprehensive reference for the rational design of stapled peptides for different diseases and targets to facilitate the development of therapeutic peptides with enhanced pharmacokinetic and biological properties.

Cobalt- or rhodium-catalyzed synthesis of 1,2-dihydrophosphete oxides via C-H activation and formal phosphoryl migration

A highly stereo- and chemoselective intermolecular coupling of diverse heterocycles with dialkynylphosphine oxides has been realized via cobalt/rhodium-catalyzed C-H bond activation. This protocol provides an efficient synthetic entry to functionalized 1,2-dihydrophosphete oxides in excellent yields via the merger of C-H bond activation and formal 1,2-migration of the phosphoryl group. Compared with traditional methods of synthesis of 1,2-dihydrophosphetes that predominantly relied on stoichiometric metal reagents, this catalytic system features high efficiency, a relatively short reaction time, atom-economy, and operational simplicity. Photophysical properties of selected 1,2-dihydrophosphete oxides are also disclosed.

On-Resin Photochemical Decarboxylative Arylation of Peptides

Here we describe the application of photochemical decarboxylative arylation as a late-stage functionalization reaction for peptides. The reaction uses redox-active esters of aspartic acid and glutamic acid on the solid phase to provide analogues of aromatic amino acids. By using aryl bromides as arylation reagents, a wide variety of amino acids can be accessed without having to synthesize them individually in solution. The reaction is compatible with proteinogenic amino acids and was used to perform a structure-activity relationship study of a PRMT5 binding peptide.

Aggregation-Induced Enhanced Emission of Tetraphenylethene-phenylalanine Hybrids: Synthesis and Characterization

Aggregation-induced emitting (AIE) luminophores are sensitive and easy-to-handle types of probes that allow driving a stimulus-responsive off/on optical tool through the manipulation of the aggregation behavior. In this work, tetraphenylethene (TPE)-phenylalanine derivatives, characterized by strong aggregation-induced luminescence, were obtained through Suzuki-Miyaura cross-coupling reactions. The reaction proved to be straightforwardly applicable in the single amino acid synthesis as well as in the late-stage peptide functionalization by means of both the classical solution-phase reaction and solid-phase synthesis. A comprehensive structural and analytical investigation highlighted the features driving the self-assembly process and its relationship to AIE efficiency. In particular, we showed that the simple slight (asymmetric) extension of the TPE π-systems results in more efficient and brighter emissions, with respect to the simple TPE system itself.

Pd(II)-Catalyzed β-C(sp3)-H Alkynylation of Alanine in Di- and Tripeptides with Asn as an Endogenous Directing Group

The introduction of alkyne moieties into peptides remains in demand as it represents a promising approach for further structural diversification of peptides. Herein, we describe the Pd(II)-catalyzed C(sp3)-H alkynylation of Ala-Asn-embedded di- and tripeptides using Asn as the endogenous lead group. In addition, a key building block for the glycopeptide Tyc4PG-14 and Tyc4PG-15 was produced by our methodology.

Backbone-enabled modification of peptides with benzoquinone via palladium-catalyzed δ-C(sp2)-H functionalization

Backbone-enabled site-selective modification of peptides with benzoquinone via Pd-catalyzed δ-C(sp2)-H functionalization has been achieved. The amide groups of peptides serve as internal directional groups, facilitating C-H functionalization through a kinetically less favored six-membered palladacycle. This methodology presents novel opportunities for the late-stage site-selective diversification of peptides.

Picolinamide-assisted ortho-C-H functionalization of pyrenylglycine derivatives using aryl iodides

Chemical transformations involving the pyrenylglycine motif (an unnatural amino acid) and practical methods toward it are seldom known. This work aimed at developing a method for synthesizing novel pyrenylglycine (pyrene-based glycine) unnatural amino acid derivatives. To realize this, initially, a new pyrenylglycine substrate possessing the picolinamide moiety was assembled via the Ugi multicomponent reaction. The picolinamide moiety linked to amine substrates is a well-known bidentate directing group for accomplishing the site-selective γ-C-H functionalization of amines. Subsequently, it was aimed at using a Pd(II)-catalyzed bidentate directing group-aided γ-C-H arylation strategy for generating a wide range of unprecedented examples of C(2)-H arylated pyrenylglycines. Accordingly, pyrenylglycine possessing the picolinamide moiety was subjected to Pd(II)-catalyzed C(2)-H arylation in the non-K-region to afford a library of C(2)-arylated pyrenylglycines (π-extended pyrenes). Additionally, pyrenylglycine-based small peptides were assembled using C(2)-arylated pyrenylglycines. The X-ray structure of a representative compound was obtained, which corroborated the structure of pyrenylglycine and the regioselectivity of C(2)-H arylation of the pyrene in the non-K-region.

The Recent Advances in Iron-Catalyzed C(sp3 )-H Functionalization

The use of iron as a core metal in catalysis has become a research topic of interest over the last few decades. The reasons are clear. Iron is the most abundant transition metal on Earth's crust and it is widely distributed across the world. It has been extracted and processed since the dawn of civilization. All these features render iron a noncontaminant, biocompatible, nontoxic, and inexpensive metal and therefore it constitutes the perfect candidate to replace noble metals (rhodium, palladium, platinum, iridium, etc.). Moreover, direct C-H functionalization is one of the most efficient strategies by which to introduce new functional groups into small organic molecules. The majority of organic compounds contain C(sp3 )-H bonds. Given the enormous importance of organic molecules in so many aspects of existence, the utilization and bioactivity of C(sp3 )-H bonds are of the utmost importance. This review sheds light on the substrate scope, selectivity, benefits, and limitations of iron catalysts for direct C(sp3 )-H bond activations. An overview of the use of iron catalysis in C(sp3 )-H activation protocols is summarized herein up to 2022.

Promiscuous Enzymes for Residue-Specific Peptide and Protein Late-Stage Functionalization

The late-stage functionalization of peptides and proteins holds significant promise for drug discovery and facilitates bioorthogonal chemistry. This selective functionalization leads to innovative advances in in vitro and in vivo biological research. However, it is a challenging endeavor to selectively target a certain amino acid or position in the presence of other residues containing reactive groups. Biocatalysis has emerged as a powerful tool for selective, efficient, and economical modifications of molecules. Enzymes that have the ability to modify multiple complex substrates or selectively install nonnative handles have wide applications. Herein, we highlight enzymes with broad substrate tolerance that have been demonstrated to modify a specific amino acid residue in simple or complex peptides and/or proteins at late-stage. The different substrates accepted by these enzymes are mentioned together with the reported downstream bioorthogonal reactions that have benefited from the enzymatic selective modifications.

Metal-Free Late-Stage Alkylation of Tryptophan and Tryptophan-Containing Peptides with 1,3-Dithiane Derivatives

Late-stage diversification of structurally complex peptides has enormous potential for drug discovery and molecular imaging. We report a simple, metal-free, late-stage reductive C2 alkylation of tryptophan and tryptophan-containing peptides using readily available 1,3-dithianes. This alkylation protocol has a wide substrate scope and an excellent tolerance for reactive functional groups.

Site-Selective Modification of Peptides via Late-Stage Pd-Catalyzed Tandem Reaction of Phenylalanine with Benzoquinone

An efficient and straightforward approach for site-selective functionalization of phenylalanine and phenylalanine-containing peptide via a Pd-catalyzed tandem reaction has been developed. The robust method underwent dual C-H activation, including C-C coupling with benzoquinone and intramolecular C-N cyclization, providing a feasible and rapid synthetic route to incorporate 4-benzoquinone-indoline fragments into peptides.

An update on late-stage functionalization in today's drug discovery

INTRODUCTION

Late-stage functionalization (LSF) allows for the introduction of new chemical groups toward the end of a synthetic sequence, which means new molecules can be rapidly accessed without laborious de novo chemical synthesis. Over the last decade, medicinal chemists have begun to implement LSF strategies into their drug discovery programs, affording benefits such as efficient access to diverse libraries to explore structure-activity relationships and the improvement of physicochemical and pharmacokinetic properties.

AREAS COVERED

An overview of the key advancements in LSF methodology development from 2019 to 2022 and their applicability to drug discovery is provided. In addition, several examples from both academia and industry where LSF methodologies have been applied by medicinal chemists to their drug discovery programs are presented.

EXPERT OPINION

Utilization of LSF by medicinal chemists is on the rise, both in academia and in industry. The maturation of the LSF field to produce methodologies bearing increased regioselectivity, scope, and functional group tolerance is envisaged to narrow the gap between methodology development and medicinal chemistry research. The authors predict that the sheer versatility of these techniques in facilitating challenging chemical transformations of bioactive molecules will continue to increase the efficiency of the drug discovery process.

Palladium-Catalyzed C(sp2)-H Arylation of Peptides Directed by Aspartic Acid

Herein, we report a palladium-catalyzed C(sp²)-H di- or monoarylation of short peptides containing N-terminal benzamide groups using aspartic acid (Asp) as an endogenous directing group. This strategy has the following merits: a broad substrate scope, selective diarylation of peptides, and gram-scale synthesis. Furthermore, this strategy can be successfully utilized to synthesize peptide-peptide conjugates.

The recent advances in cobalt-catalyzed C(sp3)-H functionalization reactions

Over the past decades, reactions involving C-H functionalization have become a hot theme in organic transformations because they have a lot of potential for the streamlined synthesis of complex molecules. C(sp³)-H bonds are present in most organic species. Since organic molecules have massive significance in various aspects of life, the exploitation and functionalization of C(sp³)-H bonds hold enormous importance. In recent years, the first-row transition metal-catalyzed direct and selective functionalization of C-H bonds has emerged as a simple and environmentally friendly synthetic method due to its low cost, unique reactivity profiles and easy availability. Therefore, research advancements are being made to conceive catalytic systems that foster direct C(sp³)-H functionalization under benign reaction conditions. Cobalt-based catalysts offer mild and convenient reaction conditions at a reasonable expense compared to conventional 2^nd and 3^rd-row transition metal catalysts. Consequently, the probing of Co-based catalysts for C(sp³)-H functionalization is one of the hot topics from the outlook of an organic chemist. This review primarily focuses on the literature from 2018 to 2022 and sheds light on the substrate scope, selectivity, benefits and limitations of cobalt catalysts for organic transformations.

Synthesis of maleimide-braced peptide macrocycles and their potential anti-SARS-CoV-2 mechanisms

Macrocycles often exhibit good biological properties and potential druggability, which lead to versatile applications in the pharmaceutical industry. Herein, we report a highly efficient and practical methodology for the functionalization and macrocyclization of Trp and Trp-containing peptides via Pd(II)-catalyzed C-H alkenylation at the Trp C4 position. This method provides direct access to C4 maleimide-decorated Trp-containing peptidomimetics and maleimide-braced 17- to 30-membered peptide macrocycles. In particular, these unique macrocycles revealed low micro- to sub-micromolar EC₅₀ values with promising anti-SARS-CoV-2 activities. Further explorations with computational methodologies and experimental validations indicated that these macrocycles exert antiviral effects through binding with the N protein of SARS-CoV-2.

Post-Assembly Modification of Head-to-Tail Cyclic Peptides by Methionine-Directed β-C(sp3)-H Arylation

Peptide modification by C(sp³)-H functionalization of internal residues remains a major challenge due to the inhibitory effect of peptide bonds. In this work, we developed a methionine-directed β-C(sp³)-H arylation method for internal alanine functionalization. By tuning the σ_C-C bond rotation of internal Ala through head-to-tail cyclization, we overcame the inhibitory effect and functionalized a wide range of head-to-tail cyclic peptides with aryl iodides with excellent position selectivity.

Selective Stepwise Arylation of Unprotected Peptides by PtIV Complexes

LPt^IVF(Aryl) complexes bearing a bulky bidentate 2‐[bis(adamant‐1‐yl)phosphino]phenoxide ligand (L) demonstrate excellent reactivity and selectivity in the arylation of X−H (X=S, N) bonds of amino acid residues in unprotected peptides under mild, including aqueous, conditions. Stepwise addition of these complexes allowed a convenient one‐pot introduction of different aromatic groups in the X−H bonds of Cys and N terminus. Pt^IV reagents can also be used to further arylate N−H bonds in Lys and Trp providing access to peptides bearing multiple aromatic groups.

Visible-Light-Promoted Cross Dehydrogenative/Decarboxylative Coupling Cascades of Glycine Ester Derivatives and β-Keto Acids

A visible-light-induced dehydrogenative/decarboxylative coupling reaction of arylglycine derivatives and β-keto acids is described. This photocatalyst- and additive-free protocol can be applied in the efficient synthesis of γ-keto glycine derivatives under ambient conditions. Further uses of this methodology and a plausible mechanism are also demonstrated.

Selective Labeling of Peptides with o-Carboranes via Manganese(I)-Catalyzed C-H Activation

A robust method for the selective labeling of peptides via manganese(I) catalysis was devised to achieve the C-2 alkenylation of tryptophan containing peptides with 1-ethynyl-o-carboranes. The manganese-catalyzed C-H activation was accomplished with high catalytic efficiency, and featured low toxicity, high functional group tolerance and excellent E-stereoselectivity. This approach unravels a promising tool for the assembly of o-carborane with structurally complex peptides of relevance to applications in boron neutron capture therapy.

Diastereoselective palladium-catalyzed C(sp3)-H cyanomethylation of amino acid and carboxylic acid derivatives

In this study, we report an efficient protocol for Pd-catalyzed methylene β-C(sp³)-H cyanomethylation of 8-aminoquinoline-directed α-amino acids using inexpensive chloroacetonitrile. Iodoacetonitrile generated in situ from chloroacetonitrile reacts with methylene C(sp³)-H bonds of α-amino acids with excellent diastereoselectivity, enabling access to a wide range of important γ-cyano-α-amino acids. Our protocol works well with different amino acid and carboxylic acid derivatives with good chemical yields and high functional group tolerance.

Construction of carbazole-based unnatural amino acid scaffolds via Pd(II)-catalyzed C(sp3)-H functionalization

We report the synthesis of carbazole-based unnatural α-amino acid and non-α-amino acid derivatives via a Pd(II)-catalyzed bidentate directing group 8-aminoquinoline-aided β-C(sp³)-H activation/functionalization method. Various N-phthaloyl, DL-, L- and D-carboxamides derived from their corresponding α-amino acids, non-α-amino acids and aliphatic carboxamides were subjected to the β-C(sp³)-H functionalization with 3-iodocarbazoles in the presence of a Pd(II) catalyst to afford the corresponding carbazole moiety installed unnatural amino acid derivatives and aliphatic carboxamides. Carbazole motif-containing racemic (DL) and enantiopure (L and D) amino acid derivatives including phenylalanine, norvaline, leucine, norleucine and 2-aminooctanoic acid with anti-stereochemistry and various non-α-amino acid derivatives including GABA have been synthesized. Removal of the 8-aminoquinoline directing group, deprotection of the phthalimide moiety and the preparation of carbazole amino acid derivatives containing free amino- and carboxylate groups are shown. The carbazole motif is prevalent in alkaloids and biologically active molecules and functional materials. Thus, this work on the synthesis of carbazole-based unnatural amino acid derivatives would enrich the libraries of unnatural amino acid derivatives and carbazoles.

4-Aminobenzotriazole (ABTA) as a Removable Directing Group for Palladium-Catalyzed Aerobic Oxidative C-H Olefination

4-Aminobenzotriazole (ABTA) was applied as an effective removable directing group (DG) in Pd-catalyzed C-H activation for the first time. Compared with the widely applied pyridine and quinoline analogs, ABTA showed significantly improved reactivity, achieving aerobic oxidative C-H olefination in excellent yields (up to 95% vs <50% with other reported DGs under identical conditions). Using this new strategy, macrocyclization was achieved to give cyclic peptides in good yields with easy ABTA removal under mild conditions, highlighting the promising potential of this new DG.

On-Resin Synthesis of Linear Aryl Thioether Containing Peptides and in-Solution Cyclization via Cysteine SNAr Reaction

Cyclic peptides represent one of the most promising therapeutic agents in drug discovery due to their good affinity and selectivity. Herein, an on-resin synthesis of aryl thioether containing peptides and a concise cyclization strategy via chemoselective cysteine SNAr reaction was developed. The arylation group could be incorporated into a series of amino acids and used for standard SPPS and peptides cyclization. Constructed cyclic peptides showed increased cellular uptakes compared to their linear peptides.

Diastereoselective palladium-catalyzed functionalization of prochiral C(sp3)-H bonds of aliphatic and alicyclic compounds

We highlight the reported developments of the palladium-catalyzed C-H activation and functionalization of the inactive/unreactive prochiral C(sp³)-H bonds of aliphatic and alicyclic compounds. There exist numerous classical methods for generating contiguous stereogenic centers in a compound with a high degree of stereocontrol. Along similar lines, the Pd(II)-catalyzed, directing group-aided functionalization of inactive prochiral/diastereotopic C(sp³)-H bonds have been exploited to accomplish the stereoselective construction of stereo-arrays in organic compounds. We present a concise discussion on how specific strategies consisting of Pd(II)-catalyzed, directing group-aided C(sp³)-H functionalization have been utilized to generate two or more stereogenic centers in aliphatic and alicyclic compounds.

Rh(III)-Catalyzed mild straightforward synthesis of quinoline-braced cyclophane macrocycles via migratory insertion

An efficient Rh(III)-catalyzed straightforward strategy is developed for the synthesis of quinoline braced cyclophane macrocycles via methyl (sp³) C-H functionalization. The method is mild, simple and regioselective with various ring sizes and has good functional group tolerance. The method proceeds via C8-methyl metalation, metal-carbene formation and a subsequent migratory insertion. High dilution is not necessary for this macrocyclization and the only byproduct is nitrogen. A preliminary investigation shows that the C-H metalation step is the rate-determining step.

Late-stage construction of stapled peptides through Fujiwara-Moritani reaction between tryptophan and olefins

Herein, the first example of a palladium-catalyzed Fujiwara-Moritani reaction for olefination of tryptophan (Trp) residues, free from directing groups, was presented. The developed reaction proceeds efficiently for peptide modification, ligation and peptide stapling.

Synthesis of tryptophan-containing 2,5-diketopiperazines via sequential C-H activation: total syntheses of tryprostatin A, maremycins A and B

Indole 2,5-diketopiperazines (DKPs) are an important type of metabolic cyclic dipeptides containing a tryptophan (Trp) unit possessing a range of interesting biological activities. The intriguing structural features and divergent activities have stimulated tremendous efforts towards their efficient synthesis. Herein, we report the development of a unified strategy for the synthesis of three Trp-containing DKPs, namely tryprostatin A, and maremycins A and B, via a sequential C–H activation strategy. The key Trp skeletons were synthesized from the inexpensive, readily available alanine via a Pd(ii)-catalyzed β-methyl C(sp³)–H monoarylation. A subsequent C2-selective prenylation of the resulting 6-OMe-Trp by Pd/norbornene-promoted C–H activation led to the total synthesis of tryprostatin A in 12 linear steps from alanine with 25% overall yield. Meanwhile, total syntheses of maremycins A and B were successfully accomplished using a sequential Pd-catalyzed methylene C(sp³)–H methylation as the key step in 15 linear steps from alanine.

Indole 2,5-diketopiperazines (DKPs) are an important type of metabolic cyclic dipeptides containing a tryptophan (Trp) unit possessing a range of interesting biological activities.

Late-Stage C-H Acylation of Tyrosine-Containing Oligopeptides with Alcohols

The selective tagging of amino acids within a peptide framework while using atom-economical C-H counterparts poses an unmet challenge within peptide chemistry. Herein, we report a novel Pd-catalyzed late-stage C-H acylation of a collection of Tyr-containing peptides with alcohols. This water-compatible labeling technique is distinguished by its reliable scalability and features the use of ethanol as a renewable feedstock for the assembly of a variety of peptidomimetics.

Macrocyclization strategies for cyclic peptides and peptidomimetics

Peptides are a growing therapeutic class due to their unique spatial characteristics that can target traditionally "undruggable" protein-protein interactions and surfaces. Despite their advantages, peptides must overcome several key shortcomings to be considered as drug leads, including their high conformational flexibility and susceptibility to proteolytic cleavage. As a general approach for overcoming these challenges, macrocyclization of a linear peptide can usually improve these characteristics. Their synthetic accessibility makes peptide macrocycles very attractive, though traditional synthetic methods for macrocyclization can be challenging for peptides, especially for head-to-tail cyclization. This review provides an updated summary of the available macrocyclization chemistries, such as traditional lactam formation, azide-alkyne cycloadditions, ring-closing metathesis as well as unconventional cyclization reactions, and it is structured according to the obtained functional groups. Keeping peptide chemistry and screening in mind, the focus is given to reactions applicable in solution, on solid supports, and compatible with contemporary screening methods.

Sustainable manganese catalysis for late-stage C-H functionalization of bioactive structural motifs

The late-stage C–H functionalization of bioactive structural motifs is a powerful synthetic strategy for accessing advanced agrochemicals, bioimaging materials, and drug candidates, among other complex molecules. While traditional late-stage diversification relies on the use of precious transition metals, the utilization of 3d transition metals is an emerging approach in organic synthesis. Among the 3d metals, manganese catalysts have gained increasing attention for late-stage diversification due to the sustainability, cost-effectiveness, ease of operation, and reduced toxicity. Herein, we summarize recent manganese-catalyzed late-stage C–H functionalization reactions of biologically active small molecules and complex peptides.

Atroposelective synthesis of N-aryl peptoid atropisomers via a palladium(ii)-catalyzed asymmetric C-H alkynylation strategy

The introduction of chirality into peptoids is an important strategy to determine a discrete and robust secondary structure. However, the lack of an efficient strategy for the synthesis of structurally diverse chiral peptoids has hampered the studies. Herein, we report the efficient synthesis of a wide variety of N-aryl peptoid atropisomers in good yields with excellent enantioselectivities (up to 99% yield and 99% ee) by palladium-catalyzed asymmetric C–H alkynylation. The inexpensive and commercially available l-pyroglutamic acid was used as an efficient chiral ligand. The exceptional compatibility of the C–H alkynylation with various peptoid oligomers renders this procedure valuable for peptoid modifications. Computational studies suggested that the amino acid ligand distortion controls the enantioselectivity in the Pd/l-pGlu-catalyzed C–H bond activation step.

The introduction of chirality into peptoids is an important strategy to determine a discrete and robust secondary structure.

Progress and perspectives on directing group-assisted palladium-catalysed C-H functionalisation of amino acids and peptides

Peptide modifications can unlock a variety of compounds with structural diversity and abundant biological activity. In nature, peptide modifications, such as functionalisation at the side-chain position of amino acids, are performed using post-translational modification enzymes or incorporation of unnatural amino acids. However, accessing these modifications remains a challenge for organic chemists. During the past decades, selective C-H activation/functionalisation has attracted considerable attention in synthetic organic chemistry as a pathway to peptide modification. Various directing group strategies have been discovered that assist selective C-H activation. In particular, bidentate directing groups that enable tuneable and reversible coordination are now recognised as one of the most efficient methods for the site-selective C-H activation and functionalisation of numerous families of organic compounds. Synthetic peptide chemists have harnessed bidentate directing group strategies for selective functionalisation of the β- and γ-positions of amino acids. This method has been expanded and recognised as an effective device for the late stage macrocyclisation and total synthesis of complex peptide natural products. In this review, we discuss various β-, γ-, and δ-C(sp³)-H bond functionalisation reactions of amino acids for the formation of C-X bonds with the aid of directing groups and their application in late-stage macrocyclisation and the total synthesis of complex peptide natural products.

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.

Recent Advances in Late-Stage Construction of Stapled Peptides via C-H Activation

Stapled peptides have been widely applied in many fields, including pharmaceutical chemistry, diagnostic reagents, and materials science. However, most traditional stapled peptide preparation methods rely on prefunctionalizations, which limit the diversity of stapled peptides. Recently, the emergence of late-stage transition metal-catalyzed C-H activation in amino acids and peptides has attracted wide interest due to its robustness and applicability for peptide stapling. In this review, we summarize the methods for late-stage construction of stapled peptides via transition metal-catalyzed C-H activation.