Peptide Late-Stage Diversifications by Rhodium-Catalyzed Tryptophan C7 Amidation

Late-Stage Stitching Enabled by Palladium-Catalyzed Tryptophan C4 Amination: Peptide Ligation and Cyclodimerization

Here, we report on methods for late-stage peptide diversification through palladium-catalyzed site-selective C(sp2)-H amination of tryptophan residues at the C4 position, utilizing tryptophan-amine cross-links. Our strategy enables practical access to C-N bonds, facilitating the construction of cyclopeptides via late-stage cyclodimerization of structurally complex peptides, which poses significant challenges for organic synthesis. The synthetic utility of this protocol is demonstrated through the synthesis of 30- to 38-membered macrocyclic peptides.

Dioxazolones as electrophilic amide sources in copper-catalyzed and -mediated transformations

Over the past decade, dioxazolones have been widely used as N-acylamide sources in amidation processes of challenging substrates, typically employing precious transition metals. However, these catalytic systems often present several challenges associated with cost, toxicity, stability, and recyclability. Among the 3d transition metals, copper catalysts have been gaining increasing attention owing to their abundance, cost-effectiveness, and sustainability. Recently, these catalytic systems have been applied to the chemical transformation of dioxazolones, conferring a convenient protocol towards amidated products. This review highlights recent advancements in the synthetic transformations of dioxazolones, with particular examples of copper salts.

Tryptophan-specific modification and diversification of peptides and proteins

In spite of being the second-lowest abundant proteinogenic amino acid, approximately 90% of proteins contain at least one tryptophan residue. Hence, the chemoselective functionalization of tryptophan residue can provide access to site-selective bioconjugation of almost all known proteins. With the increase in the utility of bioconjugated proteins and peptides as drugs and therapeutic agents, the development of smart protocols to fabricate and modulate biomolecules has flourished. This review provides a comprehensive summary of the latest advances in tryptophan-specific modification and diversification of peptides and proteins that exhibit significant applications in proteomics, protein engineering, living cell imaging, drug discovery, etc. The article also highlights literature gaps and new opportunities for the sake of future innovation in the field.

Triflic Acid-Mediated Chemoselective Indole C2-Heteroarylation of Peptide Tryptophan Residues by Triazine

Peptide modification provides opportunities to afford peptides with designed functions. Among the proteogenic amino acids, tryptophan represents an ideal and attractive target for peptide modification because of the exclusive chemical reactivity of its unique indole structure. Herein, we reported an indole C2 position-selective and transition-metal-free modification approach for indole derivatives and tryptophan-containing peptides by triazine derivatives via triflic acid activation and that the incorporated functional group could act as an orthogonal handle for further bioconjugation via an inverse electron demand Diels-Alder reaction.

Recent Progress on Transition Metal Catalyzed Macrocyclizations Based on C-H Bond Activation at Heterocyclic Scaffolds

Macrocycles are essential in protein-protein interactions and the preferential intake of bioactive scaffolds. Macrocycles are commonly synthesized by late-stage macrolactonizations, macrolactamizations, transition metal-catalyzed ring-closing metathesis, S-S bond-forming reactions, and copper-catalyzed alkyne-azide cycloaddition. Recently, transition metal-catalyzed C-H activation strategies have gained significant interest among chemists to synthesize macrocycles. This article provides a comprehensive overview of the transition metal-catalyzed macrocyclization via C-H bond functionalization of heterocycle-containing peptides, annulations, and heterocycle-ring construction through direct C-H bond functionalization. In the first part, palladium salt catalyzed coupling with indolyl C(sp3)-H and C(sp2)-H bonds for macrocyclization is reported. The second part summarizes rhodium-catalyzed macrocyclizations via site-selective C-H bond functionalization. Earth-abundant, less toxic 3d metal salt Mn-catalyzed cyclizations are reported in the latter part. This summary is expected to spark interest in emerging methods of macrocycle production among organic synthesis and chemical biology practitioners, helping to develop the discipline. We hope that this mini-review will also inspire synthetic chemists to explore new and broadly applicable C-C bond-forming strategies for macrocyclization via intramolecular C-H activation.

Late-stage peptide modification and macrocyclization enabled by tertiary amine catalyzed tryptophan allylation

Late-stage modification of peptides could potentially endow peptides with significant bioactivity and physicochemical properties, and thereby provide novel opportunities for peptide pharmaceutical studies. Since tryptophan (Trp) bears a unique indole ring residue and plays various critical functional roles in peptides, the modification methods for tryptophan were preliminarily developed with considerable progress via transition-metal mediated C-H activation. Herein, we report an unprecedented tertiary amine catalyzed peptide allylation via the SN2'-SN2' pathway between the N1 position of the indole ring of Trp and Morita-Baylis-Hillman (MBH) carbonates. Using this method that proceeds under mild conditions, we demonstrated an extremely broad scope of Trp-containing peptides and MBH carbonates to prepare a series of peptide conjugates and cyclic peptides. The reaction is amenable to either solid-phase (on resin) or solution-phase conditions. In addition, the modified peptides can be further conjugated with other biomolecules at Trp, providing a new handle for bioconjugation.

Fluorescent coumarin-alkynes for labeling of amino acids and peptides via manganese(I)-catalyzed C-H alkenylation

The late-stage fluorescent labeling of structurally complex peptides bears immense potential for molecular imaging. Herein, we report on a manganese(I)-catalyzed peptide C-H alkenylation under exceedingly mild conditions with natural fluorophores as coumarin- and chromone-derivatives. The robustness and efficiency of the manganese(I) catalysis regime was reflected by a broad functional group tolerance and low catalyst loading in a resource- and atom-economical fashion.

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.

Electrochemistry-enabled residue-specific modification of peptides and proteins

Selective chemical reactions at precise amino acid residues of peptides and proteins have become an exploding field of research in the last few decades. With the emerging utility of bioconjugated peptides and proteins as drug leads and therapeutic agents, the design of smart protocols to modulate and conjugate biomolecules has become necessary. During this modification, the most important concern of biochemists is to keep intact the structural integrity of the biomolecules. Hence, a soft and selective biocompatible reaction environment is necessary. Electrochemistry, a mild and elegant tunable reaction platform to synthesize complex molecules while avoiding harsh and toxic chemicals, can provide such a reaction condition. However, this strategy is yet to be fully exploited in the field of selective modification of polypeptides. With this possibility, the use of electrochemistry as a reaction toolbox in peptide and protein chemistry is flourishing day by day. Unfortunately, there is no suitable review article summarizing the residue-specific modification of biomolecules. The present review provides a comprehensive summary of the latest manifested electrochemical approaches for the modulation of five redox-active amino acid residues, namely cysteine, tyrosine, tryptophan, histidine and methionine, found in peptides and proteins. The article also highlights the incredible potential of electrochemistry for the regio- as well as chemoselective bioconjugation strategy of biomolecules.

Recent Advances in Bioorthogonal Ligation and Bioconjugation

The desire to create biomolecules modified with functionalities that go beyond nature's toolbox has resulted in the development of biocompatible and selective methodologies and reagents, each with different scope and limitations. In this overview, we highlight recent advances in the field of bioconjugation from 2016 to 2023. First, (metal-mediated) protein functionalization by exploiting the specific reactivity of amino acids will be discussed, followed by novel bioorthogonal reagents for bioconjugation of modified biomolecules.

Electrochemical Late-Stage Functionalization

Late-stage functionalization (LSF) constitutes a powerful strategy for the assembly or diversification of novel molecular entities with improved physicochemical or biological activities. LSF can thus greatly accelerate the development of medicinally relevant compounds, crop protecting agents, and functional materials. Electrochemical molecular synthesis has emerged as an environmentally friendly platform for the transformation of organic compounds. Over the past decade, electrochemical late-stage functionalization (eLSF) has gained major momentum, which is summarized herein up to February 2023.

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.

Modular synthesis of clickable peptides via late-stage maleimidation on C(7)-H tryptophan

Cyclic peptides have attracted tremendous attention in the pharmaceutical industry owing to their excellent cell penetrability, stability, thermostability, and drug-like properties. However, the currently available facile methodologies for creating such peptides are rather limited. Herein, we report an efficient and direct peptide cyclization via rhodium(III)-catalyzed C(7)-H maleimidation. Notably, this catalytical system has excellent regioselectivity and high tolerance of functional groups which enable late-stage cyclization of peptides. This architecture of cyclic peptides exhibits higher bioactivity than its parent linear peptides. Moreover, the Trp-substituted maleimide displays excellent reactivity toward Michael addition, indicating its potential as a click functional group for applications in chemical biology and medicinal chemistry. As a proof of principle, RGD-GFLG-DOX, which is a peptide-drug-conjugate, is constructed and it displays a strong binding affinity and high antiproliferative activity toward integrin-αvβ₃ overexpressed cancer cell lines. The proposed strategy for rapid preparation of stapled peptides would be a robust tool for creating peptide-drug conjugates.

Late-stage peptide labeling with near-infrared fluorogenic nitrobenzodiazoles by manganese-catalyzed C-H activation

Late-stage diversification of structurally complex amino acids and peptides provides tremendous potential for drug discovery and molecular imaging. Specifically, labeling peptides with fluorescent tags is one of the most important methods for visualizing their mode of operation. Despite major recent advances in the field, direct molecular peptide labeling by C-H activation is largely limited to dyes with relatively short emission wavelengths, leading to high background signals and poor signal-to-noise ratios. In sharp contrast, here we report on the fluorescent labeling of peptides catalyzed by non-toxic manganese(i) via C(sp²)-H alkenylation in chemo- and site-selective manners, providing modular access to novel near-infrared (NIR) nitrobenzodiazole-based peptide fluorogenic probes.

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.

Modular Synthesis of Unnatural Peptides via Rh(III)-Catalyzed Diastereoselective Three-Component Carboamidation Reaction

Herein we report a modular peptide ligation methodology that couples dioxazolones, arylboronic acids, and acrylamides to construct amide bonds in a diastereoselective manner under mild conditions, facilitated by Rh(III) catalysis. By converting the C-terminus of one peptide into a dioxazolone and the N-terminus of a second peptide into an acrylamide, the two pieces can be bridged by an arylboronic acid to construct unnatural phenylalanine, tyrosine, and tryptophan residues at the junction point with diastereoselectivity for their corresponding d-stereocenters. The reaction exhibits excellent functional group tolerance with a large substrate scope and is compatible with a wide array of protected amino acid residues that are utilized in Fmoc solid phase peptide synthesis. The methodology is applied to the synthesis of six diastereomeric proteasome inhibitor analogs, as well as the ligation of two 10-mer oligopeptides to construct a 21-mer polypeptide with an unnatural phenylalanine residue at the center.

Iron-catalyzed dual decarboxylative coupling of α-amino acids and dioxazolones under visible-light to access amide derivatives

An iron-catalyzed decarboxylative C-N coupling of α-amino acids with dioxazolones is described herein to synthesize amide derivatives under visible-light. The desired products can be given in good to excellent yields under simple, mild, and oxidant-free conditions. This protocol provides a practical route for the transformation of α-amino acids to the corresponding amides. Computational studies were carried out to shed light on the mechanism of this reaction.

Selective editing of a peptide skeleton via C-N bond formation at N-terminal aliphatic side chains

The applications of peptides and peptidomimetics have been demonstrated in the fields of therapeutics, diagnostics, and chemical biology. Strategies for the direct late-stage modification of peptides and peptidomimetics are highly desirable in modern drug discovery. Transition-metal-catalyzed C-H functionalization is emerging as a powerful strategy for late-stage peptide modification that is able to construct functional groups or increase skeletal diversity. However, the installation of directing groups is necessary to control the site selectivity. In this work, we describe a transition metal-free strategy for late-stage peptide modification. In this strategy, a linear aliphatic side chain at the peptide N-terminus is cyclized to deliver a proline skeleton via site-selective δ-C(sp3)-H functionalization under visible light. Natural and unnatural amino acids are demonstrated as suitable substrates with the transformations proceeding with excellent regio- and stereo-selectivity.

Palladium Metallaphotoredox-Catalyzed 2-Arylation of Indole Derivatives

Given that biaryl motifs are found in many useful molecules, including pesticides, pharmaceuticals, functional materials, and polymers, the development of methods for their construction is important. Herein, we report a two-step method for C(sp²)-H/C(sp²)-H cross-coupling reactions to synthesize 2-arylindole derivatives by combining palladium catalysis and photocatalysis. This mild, dual-catalysis method showed good functional group tolerance and a wide substrate scope and could be used for late-stage functionalization of oligopeptides, drugs, and natural products.

Cobalt(II)-Catalyzed C-H and N-H Functionalization of 1-Arylpyrazolidinones with Dioxazolones as Bifunctional Synthons

Dioxazolone has been attractive as an important synthon for a direct C-H amidation through a nitrene intermediate or Curtius rearrangement to form the isocyanate. However, the combination of two reaction models of dioxazolone has not been reported. Herein, a cobalt-catalyzed C-H and N-H functionalization of 1-arylpyrazolidinones with dioxazolones was developed. The dioxazolones acted as an amidated and carboxamidated reagent. Three C-N bonds were formed in a "one-pot" manner, which promoted the requirement of synthetic diversity.

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.

Photoinduced Transition-Metal-Free Chan-Evans-Lam-Type Coupling: Dual Photoexcitation Mode with Halide Anion Effect

Herein, we report a photoinduced transition-metal-free C(aryl)-N bond formation between 2,4,6-tri(aryl)boroxines or arylboronic acids as an aryl source and 1,4,2-dioxazol-5-ones (dioxazolones) as an amide coupling partner. Chloride anion, either generated in situ by photodissociation of chlorinated solvent molecules or added separately as an additive, was found to play a critical cooperative role, thereby giving convenient access to a wide range of synthetically versatile N-arylamides under mild photo conditions. The synthetic virtue of this transition-metal-free Chan-Evans-Lam-type coupling was demonstrated by large-scale reactions, synthesis of 15N-labeled arylamides, and applicability toward biologically relevant compounds. On the basis of mechanistic investigations, two distinctive photoexcitations are proposed to function in the current process, in which the first excitation involving chloro-boron adduct facilitates the transition-metal-free activation of dioxazolones by single electron transfer (SET), and the second one enables the otherwise-inoperative 1,2-aryl migration of the thus-formed N-chloroamido-borate adduct.

Directed C-H Functionalization of C3-Aldehyde, Ketone, and Acid/Ester-Substituted Free (NH) Indoles with Iodoarenes via a Palladium Catalyst System

Pd(II)-catalyzed C-H arylations of free (NH) indoles including different carbonyl directing groups on C3-position with aryl iodides are demonstrated. Importantly, the reactions are carried out using the same catalyst system without any additional transient directing group (TDG). In this study, the formyl group as a directing group gave the C4-arylated indoles versus C2-arylation. Using this catalyst system, C-H functionalization of 3-acetylindoles provided domino C4-arylation/3,2-carbonyl migration products. This transformation involves the unusual migration of the acetyl group to the C2-position following C4-arylation in one pot. Meanwhile, migration of the acetyl group could be simply controlled and N-protected 3-acetylindoles afforded C4-arylation products without migration of the acetyl group. Functionalization of indole-3-carboxylic acid (or methyl ester) with aryl iodides using the present Pd(II)-catalyst system resulted in decarboxylation followed by the formation of C2-arylated indoles. Based on the control experiments and the literature, plausible mechanisms are proposed. The synthetic utilities of these acetylindole derivatives have also been demonstrated. Remarkably, C4-arylated acetylindoles have allowed the construction of functionalized pityiacitrin (a natural product).

Proline selective labeling via on-site construction of naphthoxazole (NapOx)

Chemoselective construction of naphthoxazoles (NapOx) via a three-component annulation reaction enables proline selective labeling of peptides in solution or in solid-phase synthesis. The fluorogenic peptides possess low cytotoxicity, efficient cell membrane permeability and excellent bioimaging potential for biomedical applications.

Macrocyclization of bioactive peptides with internal thiazole motifs via palladium-catalyzed C-H olefination

Peptides containing thiazole fragments represent a large group of bioactive compounds with potential medicinal applications. However, methods for efficient synthesis of these compounds with structural diversity are limited. Herein, we report a method for modification and macrocyclization of thiazole-containing peptides through palladium-catalyzed δ-C(sp²)-H olefination. In this protocol, the thiazole and neighboring amide bonds act as directing groups, which allows site-specific olefination of phenylalanine, tryptophan and tyrosine residues. This chemistry exhibits broad substrate scope and provides facile access to peptide-peptide conjugates and peptide macrocycles. Our results highlight the potency and applicability of thiazole motifs in promoting Pd-catalyzed functionalization of peptides.

Looking deep into C-H functionalization: the synthesis and application of cyclopentadienyl and related metal catalysts

Metal catalyzed C-H functionalization offers a versatile platform for methodology development and a wide variety of reactions now exist for the chemo- and site-selective functionalization of organic molecules. Cyclopentadienyl-metal (CpM) complexes of transition metals and their correlative analogues have found widespread application in this area, and herein we highlight several key applications of commonly used transition-metal Cp-type catalysts. In addition, an understanding of transition metal Cp-type catalyst synthesis is important, particularly where modifications to the catalyst structure are required for different applications, and a summary of this aspect is given.

Catalytic Atroposelective C7 Functionalisation of Indolines and Indoles

Axially chiral atropisomeric compounds are widely applied in asymmetric catalysis and medicinal chemistry. In particular, axially chiral indole- and indoline-based frameworks have been recognised as important heterobiaryl classes because they are the core units of bioactive natural alkaloids, chiral ligands and bioactive compounds. Among them, the synthesis of C7-substituted indole biaryls and the analogous indoline derivatives is particularly challenging, and methods for their efficient synthesis are in high demand. Transition-metal catalysis is considered one of the most efficient methods to construct atropisomers. Here, we report the enantioselective synthesis of C7-indolino- and C7-indolo biaryl atropisomers by means of C-H functionalisation catalysed by chiral RhJasCp complexes.

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.

Visible-Light Mediated Tryptophan Modification in Oligopeptides Employing Acylsilanes

A method for the selective tryptophan modification and labelling of tryptophan-containing peptides is described. Photoirradiation of acylsilanes generates reactive siloxycarbenes which undergo H-N-insertion into the indole moiety of tryptophan to give stable silyl protected hemiaminals. This method is successfully applied to chemically modify various tryptophan containing oligopeptides. The method enables the selective introduction of alkynes to peptides that are eligible for further alkyne-azide click chemistry. In addition, the dansyl fluorophore can be conjugated to a peptide using this approach.

Toolbox for Distal C-H Bond Functionalizations in Organic Molecules

Transition metal catalyzed C-H activation has developed a contemporary approach to the omnipresent area of retrosynthetic disconnection. Scientific researchers have been tempted to take the help of this methodology to plan their synthetic discourses. This paradigm shift has helped in the development of industrial units as well, making the synthesis of natural products and pharmaceutical drugs step-economical. In the vast zone of C-H bond activation, the functionalization of proximal C-H bonds has gained utmost popularity. Unlike the activation of proximal C-H bonds, the distal C-H functionalization is more strenuous and requires distinctly specialized techniques. In this review, we have compiled various methods adopted to functionalize distal C-H bonds, mechanistic insights within each of these procedures, and the scope of the methodology. With this review, we give a complete overview of the expeditious progress the distal C-H activation has made in the field of synthetic organic chemistry while also highlighting its pitfalls, thus leaving the field open for further synthetic modifications.

Site-selective C-H functionalization to access the arene backbone of indoles and quinolines

The site-selective C-H bond functionalization of heteroarenes can eventually provide chemists with great techniques for editing and building complex molecular scaffolds. During the past decade, benzo-fused N-heterocycles such as indoles and quinolines have been among the most widely investigated organic templates. Early developments have led to site-selective C-H bond functionalization on the pyrrole and pyridine cores of indoles and quinolines; however, C-H functionalization on the benzenoid ring has remained a great challenge in catalysis. In this review, we elaborate on recent developments in the highly challenging functionalization of C-H bonds on the less-reactive benzenoid core of indoles and quinolines. These findings are mainly described as selective directing group assisted strategies, remote C-H functionalization techniques and their reaction mechanisms. The underlying principle in each strategy is elucidated, which aims to facilitate the design of a more advanced structure of heterocycles based on bioactive molecules, synthetic drugs, and material aspects. Moreover, the challenges and perspectives for catalytic C-H functionalization to access the arene backbone of indoles and quinolines are also proposed in the conclusion section.

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.

Sulfur-Directed C7-Selective Alkenylation of Indoles under Rhodium Catalysis

Regioselective direct functionalization of an indole benzenoid fragment has been a significant challenge because of its inherently lower reactivity. In this report, we introduce a Rh-catalyzed C₇-selective alkenylation of indole derivatives using a new sulfur directing group N-SCy. A notable feature of this system is that the directing group is readily installed to the indoles and easily removed after the catalysis under mild conditions.

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.

Iridium-catalyzed regioselective C-H sulfonamidation of 1,2,4-thiadiazoles with sulfonyl azides in water

We have developed a regioselective C-N cross-coupling of 1,2,4-thiadiazoles with sulfonyl azides through iridium catalysis in water. This method tactically linked the 1,2,4-thiadiazoles and sulfonamides together, and the novel molecules increased the diversity of 1,2,4-thiadiazoles which may have potential applications.

Palladium-Catalyzed Remote C-H Phosphonylation of Indoles at the C4 and C6 Positions by a Radical Approach

Palladium-catalyzed direct C-H activation of indole benzenoid moiety has been achieved in the past decade. However, palladium-catalyzed remote C-H activation of indoles is rare. Herein, we report a challenging palladium-catalyzed remote C4-H phosphonylation of indoles by a radical approach. The method provides access to a series of C4-phosphonylated indoles, including tryptophan and tryptophan-containing dipeptides, which are typically inaccessible by direct C4-H activation due to its heavy reliance on C3 directing groups. Notably, unexpected C6-phosphonylated indoles were obtained through blocking of the C4 position. The preliminary mechanistic studies indicated that the reactions may proceed via a C7-palladacycle/remote-activation process. Based on the strategy, examples of remote C4-H difluoromethylation with BrCF₂ COOEt are also presented, suggesting that the strategy may offer a general blueprint for other cross-couplings.

Post-synthetic functionalization of tryptophan protected peptide sequences through indole (C-2) photocatalytic alkylation

We report a selective, mild, and efficient C-H functionalization of tryptophan and tryptophan-containing peptides with activated α-bromo-carbonyl compounds under visible-light irradiation. The protocol efficiency is outlined by the wide substrate scope and excellent tolerance of sensitive functional groups present in the amino acid side chains. The method can be successfully extended to access pharmaco-peptide conjugate scaffolds.

Mechanism-Guided Development of Transition-Metal-Catalyzed C-N Bond-Forming Reactions Using Dioxazolones as the Versatile Amidating Source

Catalytic reactions that construct carbon-nitrogen bonds are one of central themes in both synthetic and medicinal chemistry since the obtainable nitrogen-containing motifs are commonly encountered in natural products and have also seen a growing prominence as key structural features in marketed drugs and preclinical candidates. Pd-catalyzed cross-couplings, such as Buchwald-Hartwig amination, are at the forefront of such synthetic methods in practical settings. However, they require prefunctionalized substrates such as (hetero)aryl halides that must be prepared independently, often by multiple operations. One emerging way to circumvent these preparatory steps and directly convert ubiquitous C-H bonds into valuable C-N bonds is catalytic C-H amination, which allows synthetic chemists to devise shorter and more efficient retrosynthetic schemes. The past two decades have witnessed considerable progress in expanding the repertoire of this strategy, especially by identifying effective amino group precursors. In this context, dioxazolones have experienced a dramatic resurgence in recent years as a versatile nitrogen source in combination with transition-metal catalyst systems that facilitate decarboxylation to access key metal-acylnitrenoid intermediates. In addition to their high robustness and easy accessibility from abundant carboxylic acids, the unique reactivity of the transient intermediates in the amido group transfer has led to a fruitful journey for mild and efficient C-H amidation reactions.This Account summarizes our recent contributions to the development of C-N bond-forming reactions using dioxazolones as effective nitrenoid precursors, which are categorized into two subsets according to their mechanistic differences: inner- versus outer-sphere pathways. The first section describes how we could unveil the synthetic potential of dioxazolones in the realm of the inner-sphere C-H amidation, where we demonstrated that dioxazolones serve not only as manageable alternatives to acyl azides but also as highly efficient reagents to significantly reduce the catalyst loading and temperature. Taking advantage of the mild conditions in combination with group 9 Cp*M complexes (M = Rh, Ir, Co) or isoelectronic Ru species, we have dramatically expanded the accessible synthetic scope. Mechanistic investigations revealed that the putative metal-nitrenoid species is involved as a key intermediate during catalysis, which leads to facile C-N bond formation. On the basis of the mechanistic underpinning, we have succeeded in developing novel catalytic platforms that harness the intermediacy of metal-nitrenoids to explore C-H insertion chemistry via an outer-sphere pathway. Indeed, the tailored catalysts were capable of suppressing the competitive Curtius-type decomposition, thus granting access to versatile lactam products. We have further repurposed the catalytic systems upon modification of chelating ligands and also the identity of the transition metal to achieve three goals: (i) addressing selectivity issues to control the regio-, chemo-, and enantioselectivities, (ii) developing sustainable catalysis by first-low metals, and (iii) navigating chemical space for (di)functionalization of alkenes/alkynes. Together with our own research efforts, highlighted herein are some important relevant advances by other groups. We finally conclude with a brief overview with an eye toward further developments.

Enantioselective Catalytic C-H Amidations: An Highlight

The crucial role played by compounds bearing amide functions, not only in biological processes but also in several fields of chemistry, life polymers and material sciences, has brought about many significant discoveries and innovative approaches for their chemical synthesis. Indeed, a plethora of strategies has been developed to reach such moieties. Amides within chiral molecules are often associated with biological activity especially in life sciences and medicinal chemistry. In most of these cases, their synthesis requires extensive rethinking methodologies. In the very last years (2019–2020), enantioselective C-H functionalization has appeared as a straightforward alternative to reach chiral amides. Therein, an overview on these transformations within this timeframe is going to be given.

From C4 to C7: Innovative Strategies for Site-Selective Functionalization of Indole C-H Bonds

The widespread presence of hydrocarbons makes C-H functionalization an attractive alternative to traditional cross-coupling methods. As indole is an important heteroarene in a plethora of natural products and pharmaceuticals, C-H functionalization of indole moieties has emerged as one of the most important topics in this field. Due to the presence of multiple C-H bonds in indoles, site selectivity is a long-standing challenge. Much effort has been devoted to the C-H functionalization of indoles at the C3 or C2 position, while accessing the benzene core (from C4 to C7) is considerably more challenging.This Account summarizes our recent efforts toward site-selective C-H functionalization of indoles at the benzene core based on innovative strategies. A common method to solve the issue involves the development of directing groups (DGs). Our early studies establish that the installation of the N-P(O)^tBu₂ group at the N position can produce C7 and C6 arylation products using palladium and copper catalysts, respectively. The developed system can also be extended to direct arylation of indoles at the C5 and C4 positions by installing a pivaloyl group at the C3 position. Further investigation of indoles bearing N-P^tBu₂ groups shows a more diverse reactivity for C-H functionalizations at the C7 position, including arylation, olefination, acylation, alkylation, silylation, and carbonylation with different coupling partners. Compared to the P(V) DG, the P(III) group can be easily attached to the indole substrates and detached from the products. However, these attractive reactions rely mostly on precious metal catalysts with ligands; this requirement can be a significant limitation, particularly for large-scale syntheses and the necessity of removal of toxic trace metals in pharmaceutical products. We have also uncovered a general strategy for chelation-assisted aromatic C-H borylation just using simple BBr₃ under mild conditions, in which the installation of pivaloyl groups at the N1 or C3 position of indoles can selectively deliver the boron species to the unfavorable C7 or C4 positions and allow subsequent C-H borylation without any metal. This transition-metal-free strategy can be extended to synthesize C7 and C4 hydroxylated indoles by boron-mediated directed C-H hydroxylation under mild reaction conditions and with broad functional group compatibility.In this Account, we describe our contributions to this topic since 2015. These studies provide efficient and attractive methods for the divergent synthesis of valuable substituted indoles and insights into the exploration of new strategies for the site-selective C-H functionalization and directives for other important heteroarenes.