Nonnatural amino acid synthesis by using carbon-hydrogen bond functionalization methodology

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.

Palladium-Catalyzed Arylation of C(sp2)-H Bonds and C(sp3)-H Bonds with 4-Amino-benzotriazole as the Bidentate Directing Group

The arylation of C(sp2)-H and C(sp3)-H bonds in carboxylic acids catalyzed by Pd(II) with 4-aminobentriazole as the directing group was investigated. In addition to activation of the C(sp2)-H bond, selective arylation of alkyl carboxylic acids and amino acids in the β position can also be achieved. This strategy involved a 5,5-bicyclic Pd intermediate complex whose structure was determined by X-ray single crystal diffraction analysis. Importantly, the DG (directing group) can be easily removed under mild conditions.

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.

Enantioselective organocatalytic strategies to access noncanonical α-amino acids

Organocatalytic asymmetric synthesis has evolved over the years and continues to attract the interest of many researchers worldwide. Enantiopure noncanonical amino acids (ncAAs) are valuable building blocks in organic synthesis, medicinal chemistry, and chemical biology. They are employed in the elaboration of peptides and proteins with enhanced activities and/or improved properties compared to their natural counterparts, as chiral catalysts, in chiral ligand design, and as chiral building blocks for asymmetric syntheses of complex molecules, including natural products. The linkage of ncAA synthesis and enantioselective organocatalysis, the subject of this perspective, tries to imitate the natural biosynthetic process. Herein, we present contemporary and earlier developments in the field of organocatalytic activation of simple feedstock materials, providing potential ncAAs with diverse side chains, unique three-dimensional structures, and a high degree of functionality. These asymmetric organocatalytic strategies, useful for forging a wide range of C-C, C-H, and C-N bonds and/or combinations thereof, vary from classical name reactions, such as Ugi, Strecker, and Mannich reactions, to the most advanced concepts such as deracemisation, transamination, and carbene N-H insertion. Concurrently, we present some interesting mechanistic studies/models, providing information on the chirality transfer process. Finally, this perspective highlights, through the diversity of the amino acids (AAs) not selected by nature for protein incorporation, the most generic modes of activation, induction, and reactivity commonly used, such as chiral enamine, hydrogen bonding, Brønsted acids/bases, and phase-transfer organocatalysis, reflecting their increasingly important role in organic and applied chemistry.

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.

Palladium-Catalyzed C(sp3)-H Nitrooxylation of Aliphatic Carboxamides with Practical Oxidants

Here we report the palladium-catalyzed β-C(sp3)-H nitrooxylation of aliphatic carboxamides using a modified quinoline auxiliary. Notably, Al(NO3)3·9H2O was used as a nitrate source as well as a practical oxidant. The 5-chloro-8-aminoquinoline auxiliary was nitrated in situ during the reaction, which may enhance its directing ability and help its removal. The reaction has a broad substrate scope with a variety of aliphatic carboxamides. The multiple substituted auxiliary can be easily removed and recovered. Two C-H-insertion palladacycle intermediates were isolated and characterized to elucidate the mechanism.

Palladium-Catalyzed Enantioselective Directed C(sp3)-H Functionalization Using C5-Substituted 8-Aminoquinoline Auxiliaries

8-Aminoquinoline (AQ) has proven to be a highly effective bidentate directing group for palladium-catalyzed C-H functionalization reactions. However, enantiocontrol of AQ-directed C(sp3)-H functionalization reactions has been challenging. Herein, a new protocol is presented for the Pd-catalyzed enantioselective arylation of unactivated β C(sp3)-H bonds of alkyl carboxamides with aryl iodides using a C5-iodinated 8-aminoquinolines (IQ) auxiliary in conjugation with a BINOL ligand. Additionally, a C5-aryl substituted 8-aminoquinoline auxiliary can facilitate enantioselective alkenylation and alkynylation of benzylic C(sp3)-H bonds of 3-arylpropanamides with the corresponding bromide reagents under similar conditions.

Carboxylate-Directed Pd-Catalyzed β-C(sp3)-H Arylation of N-Methyl Alanine Derivatives for Diversification of Bioactive Peptides

This study presents a Pd(II)-catalyzed method for the β-C(sp3)-H arylation of N-Cbz- or N-Fmoc-protected N-methyl alanines, providing ready access to building blocks for N-methylated peptide synthesis. For this transformation, the native carboxylate was exploited as the directing group, attributing its success to the use of a monoprotected amino-pyridine ligand. Its synthetic utility was demonstrated by facile generation of nine analogues of the naturally occurring N-methylated cyclic peptide cycloaspeptide A.

Ligand-Enabled γ-C(sp3)-H Hydroxylation of Free Amines with Aqueous Hydrogen Peroxide

Selective oxidation of the γ-C-H bonds from abundant amine feedstocks via palladium catalysis is a valuable transformation in synthesis and medicinal chemistry. Despite advances on this topic in the past decade, there remain two significant limitations: C-H activation of aliphatic amines requires an exogenous directing group except for sterically hindered α-tertiary amines, and a practical catalytic system for C(sp3)-H hydroxylation using a green oxidant, such as oxygen or aqueous hydrogen peroxide, has not been developed to date. Herein, we report a ligand-enabled selective γ-C(sp3)-H hydroxylation using sustainable aqueous hydrogen peroxide (7.5-10%, w/w). Enabled by a CarboxPyridone ligand, a series of primary amines (1°), piperidines, and morpholines (2°) were hydroxylated at the γ-position with excellent monoselectivity. This method provides an avenue for the synthesis of a wide range of amines, including γ-amino alcohols, β-amino acids, and azetidines. The retention of chirality in the reaction allows rapid access to chiral amines starting from the abundant chiral amine pool.

Palladium-Catalyzed Direct Selanylation of Chalcogenophenes and Arenes Assisted by 2-(Methylthio)amide

The direct and selective conversion of a C-H bond into a C-Se bond remains a significant challenge, which is even more intricate with substrates having an innate regioselectivity under several reaction conditions, such as chalcogenophenes. We overrode their selectivity toward selanylation using palladium, copper, and the 2-(methylthio)amide directing group. This chelation-assisted direct selanylation was also suitable for mono and double ortho functionalization of arenes. The mechanistic studies indicate high-valent Pd(IV) species in the catalytic cycle, a reversible C-H activation step, and Cu(II) as a sequestering agent for organoselenide byproducts.

Regioselective Formal β-Allylation of Carbonyl Compounds Enabled by Cooperative Nickel and Photoredox Catalysis

The Tsuji-Trost reaction between carbonyl compounds and allylic precursors has been widely used in the synthesis of natural products and pharmaceutical compounds. As the α-C-H bond is far more acidic than the β-C-H bond, carbonyl compounds undergo highly regioselective allylation at the α-position and their β-allylation is therefore highly challenging. This innate α-reactivity conversely hampers diversity, especially if the corresponding β-allylation product is targeted. Herein, we present a formal intermolecular β-C-C bond formation reaction of a broad range of aldehydes and ketones with different allyl electrophiles through cooperative nickel and photoredox catalysis. β-Selectivity is achieved via initial transformation of the aldehydes and ketones to their corresponding silyl enol ethers. The overall transformation features mild conditions, excellent regioselectivity, wide functional group tolerance and high reaction efficiency. The introduced facile and regioselective β-allylation of carbonyl compounds proceeding through cooperative catalysis allows the preparation of valuable building blocks that are difficult to access from aldehydes and ketones using existing methodology.

Simplifying the Synthesis of Nonproteinogenic Amino Acids via Palladium-Catalyzed δ-Methyl C-H Olefination of Aliphatic Amines and Amino Acids

Transition metal-catalyzed directing group assisted C-H functionalizations provide a straightforward access to a wide variety of nonproteinogenic amino acids. While altering the side chain of an existing natural amino acids is one way, introducing a functional group to an aliphatic amine to synthesize versatile unnatural amino acids is another exciting avenue. In this work, we explore both the possibilities by the palladium-catalyzed δ-C(sp³)-H olefination of aliphatic amines and amino acids. A diverse substrate scope including sequential difunctionalizations followed by post synthetic transformations were achieved to understand the applicability of the current protocol. An in-depth mechanistic study was carried out to learn the mode of the reaction pathway.

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.

Synthesis of HC-Toxin via Matteson Homologation and C-H Functionalization

A new synthetic route toward host-specific HC-toxin was developed. The HC-toxin belongs to a group of cyclic, tetrapeptide histone deacetylase inhibitors containing the unusual amino acid Aeo. Key steps in the synthesis of this building block include the Matteson homologation to generate the stereogenic centers in the side chain and a C-H functionalization to connect the side chain to a protected alanine.

Synthesis of the Isodityrosine Moiety of Seongsanamide A-D and Its Derivatives

The concise and highly convergent synthesis of the isodityrosine unit of seongsanamide A-D and its derivatives bearing a diaryl ether moiety is described. In this work, the synthetic strategy features palladium-catalyzed C(sp³)-H functionalization and a Cu/ligand-catalyzed coupling reaction. We report a practical protocol for the palladium-catalyzed mono-arylation of β-methyl C(sp³)-H of an alanine derivative bearing a 2-thiomethylaniline auxiliary. The reaction is compatible with a variety of functional groups, providing practical access to numerous β-aryl-α-amino acids; these acids can be converted into various tyrosine and dihydroxyphenylalanine (DOPA) derivatives. Then, a CuI/N,N-dimethylglycine-catalyzed arylation of the already synthesized DOPA derivatives with aryl iodides is described for the synthesis of isodityrosine derivatives.

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.

Total synthesis and biological evaluation of histone deacetylase inhibitor WF-3161

A novel synthesis of the naturally occurring HDAC inhibitor WF-3161 is described. Key steps include the Matteson homologation to generate the stereogenic centres in the side chain, and Pd-catalysed C-H functionalisation to connect the side chain to the peptide backbone. WF-3161 was found to be highly selective for HDAC1, whereas no activity was observed towards HDAC6. High activity was also found against the cancer cell line HL-60.

Late-stage C-H Functionalization of Tryptophan-Containing Peptides with Thianthrenium Salts: Conjugation and Ligation

Bioorthogonal late-stage diversification of structurally complex peptides bears enormous potential for drug discovery and molecular imaging, among other applications. Herein, we report on a palladium-catalyzed C-H arylation of tryptophan-containing peptides with readily accessible and modular arylthianthrenium salts. Under exceedingly mild reaction conditions, the late-stage diversification of structurally complex peptides was accomplished. The tunability and ease of preparation of arylthianthrenium salts allowed the expedient stitching of tryptophan-containing peptides with drug, natural product, and peptidic scaffolds by forging sterically congested biaryl linkages. The robustness of the palladium catalysis regime was reflected by the full tolerance of a plethora of sensitive and coordinating functional groups. Hence, our manifold enabled efficient access to highly decorated, labelled, conjugated, and ligated linear and cyclic peptides.

Synthesis of β-Deuterated Amino Acids via Palladium-Catalyzed H/D Exchange

Despite several synthetic approaches that have been developed for α-deuterated amino acids, the synthesis of β-deuterated amino acids has remained a challenge. Herein, we disclose a palladium catalyzed H/D exchange protocol for a β-deuterated N-protected amino amide, which can be converted to a β-deuterated amino acid simply by removal of protecting groups. This protocol is highly efficient, simply manipulated, and appliable for deuterium-labeling of many amino amides. In addition, deuterium labeling of phenylalanine derivatives was also successful when pivalic acid served as an additive to promote the H/D exchange process.

MPAI-Ligand Accelerated Pd-Catalyzed C(sp3)-H Arylation of Free Aliphatic Acids

Here, we report the development of a class of bifunctional monoprotected amino-imidazoline (MPAI) ligands and their applications in Pd-catalyzed C(sp³)-H arylation of free aliphatic acids. The newly developed MPAI ligand allows the use of 1.0 equiv of aliphatic acids containing an alpha hydrogen for the first time.

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.

Formal β‐C−H Arylation of Aldehydes and Ketones by Cooperative Nickel and Photoredox Catalysis

α‐C−H‐functionalization of ketones and aldehydes has been intensively explored in organic synthesis. The functionalization of unactivated β‐C−H bonds in such carbonyl compounds is less well investigated and developing a general method for their β‐C−H arylation remains challenging. Herein we report a method that uses cooperative nickel and photoredox catalysis for the formal β‐C−H arylation of aldehydes and ketones with (hetero)aryl bromides. The method features mild conditions, remarkable scope and wide functional group tolerance. Importantly, the introduced synthetic strategy also allows the β‐alkenylation, β‐alkynylation and β‐acylation of aldehydes under similar conditions. Mechanistic studies revealed that this transformation proceeds through a single electron oxidation/Ni‐mediated coupling/reductive elimination cascade.

Silver Ions Promoted Palladium-Catalyzed Inactive β-C(sp3)-H Bond Arylation in Batch and Continuous-Flow Conditions

A palladium(II)-catalyzed protocol for inactive β-C(sp³)-H bond functionalization has been first accomplished. The reaction proceeds through five-membered carbocycles for the formation of C-C bonds via the Pd(II)/Pd(IV) cycle. This reaction was carried out with various aryl iodides and benzothiazoles/benzoxazoles/benzimidazoles, which were well-tolerated in this reaction and successfully generated β-C(sp³)-H arylated products. Further implementation of this batch protocol to continuous flow by utilizing a PTFE (polytetrafluoroethylene) capillary reactor enhanced the reaction efficiency and decreased the reaction time (18.4 min) as compared to batch conditions (8 h). Even on the gram scale, the process produced excellent yield with negligible diarylations. Functional group tolerance, a continuous-flow approach, and easy-to-handle reaction conditions make this inactive β-C(sp³)-H bond functionalization protocol very attractive.

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.

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.

Ligand-Enabled C-H Olefination and Lactonization of Benzoic Acids and Phenylacetic Acids via Palladium Catalyst

A novel ligand propan-2-one O-(p-tolylcarbamoyl) oxime (L7) has been developed to promote C(sp²)-H olefination of benzoic acids and phenylacetic acids via a palladium catalyst. With the subsequent lactonization of the olefinated products through 1,4-addition, highly monoselective cyclic lactone products of benzofuranones and benzopyrones were obtained in moderate to excellent yields. The DFT calculation demonstrated that the novel ligand propan-2-one O-(p-tolylcarbamoyl) oxime (L7) could improve the C-H activation reaction to give cyclic lactone products elegantly.

Photocatalytic Redox-Neutral Reaction of γ-Indolyl α-Keto Esters

The direct γ-C(sp3)-H activation of saturated α-keto esters has long been an elusive transformation. We found that photo-redox catalysis IrIII in combination with DABCO as dual hydrogen-bonding donor and organic...

Transamidation and Decarbonylation of N-Phthaloyl-Amino Acid Amides Enabled by Palladium-Catalyzed Selective C-N Bond Cleavage

Amides are important functional synthons that have been widely used in the construction of peptides, natural products, and drugs. The C-N bond cleavage provides the direct method for amide conversion. However, amides, especially secondary amides, tend to be chemically inert due to the resonance of the amide bond. Here, we describe an efficient Pd-catalyzed transamidation and decarbonylation of multiamide structure molecules through C-N bond cleavage with excellent chemoselectivity. The transamidation of secondary amides and the decarbonylation of phthalimide provide meaningful tools for the modification of amino acid derivatives. Moreover, further transformations of azidation and C(sp³)-H monoarylation emphasized the potential utility of this selective C-N bond cleavage method.

Transition-Metal-Catalyzed, Coordination-Assisted Functionalization of Nonactivated C(sp3)-H Bonds

Transition-metal-catalyzed, coordination-assisted C(sp³)-H functionalization has revolutionized synthetic planning over the past few decades as the use of these directing groups has allowed for increased access to many strategic positions in organic molecules. Nonetheless, several challenges remain preeminent, such as the requirement for high temperatures, the difficulty in removing or converting directing groups, and, although many metals provide some reactivity, the difficulty in employing metals outside of palladium. This review aims to give a comprehensive overview of coordination-assisted, transition-metal-catalyzed, direct functionalization of nonactivated C(sp³)-H bonds by covering the literature since 2004 in order to demonstrate the current state-of-the-art methods as well as the current limitations. For clarity, this review has been divided into nine sections by the transition metal catalyst with subdivisions by the type of bond formation. Synthetic applications and reaction mechanism are discussed where appropriate.

Pd-Catalyzed C(sp2)-H olefination: synthesis of N-alkylated isoindolinone scaffolds from aryl amides of amino acid esters

Isoindolinone is a constituent of various natural products and synthetic biologically active compounds. The classical multi-step synthetic methods used to prepare various indolinone derivatives are tedious and challenging. One-pot synthetic methods are attractive and economical. Transition-metal-catalyzed C-H activation is an emerging tool for synthesizing natural products and small organic molecules via reducing the number of synthetic steps necessary. This paper describes the synthesis of N-alkyl-3-methenyl chiral isoindolinone derivatives from aryl amides of L-amino acids and non-activated alkene via Pd-catalyzed C(sp²)-H olefination. Herein, the amino acid residue acts as a directing group for olefination at the aryl ring, and then cyclization occurs at the amide NH. Hence, this methodology could be helpful to transform standard amino acids into respective chiral isoindolinone derivatives.

Advances on the Merger of Electrochemistry and Transition Metal Catalysis for Organic Synthesis

Synthetic organic electrosynthesis has grown in the past few decades by achieving many valuable transformations for synthetic chemists. Although electrocatalysis has been popular for improving selectivity and efficiency in a wide variety of energy-related applications, in the last two decades, there has been much interest in electrocatalysis to develop conceptually novel transformations, selective functionalization, and sustainable reactions. This review discusses recent advances in the combination of electrochemistry and homogeneous transition-metal catalysis for organic synthesis. The enabling transformations, synthetic applications, and mechanistic studies are presented alongside advantages as well as future directions to address the challenges of metal-catalyzed electrosynthesis.

Pd-Catalyzed and ligand-enabled alkene difunctionalization via unactivated C-H bond functionalization

Palladium-catalyzed and ligand-enabled C-H functionalization methods have emerged as a powerful approach for the preparation of therapeutically important motifs and complex natural products. Olefins, owing to their natural abundance, have been extensively employed for the formation of C-C and C-X bonds and the generation of various heterocycles. Traditionally, activated as well as starting materials with preinstalled functional groups, and also halide substrates under transition metal catalysis, have been employed for olefin difunctionalization. However, strategies for employing unactivated C-H bond functionalization to achieve alkene difunctionalization have rarely been explored. A possible solution to this challenge is the application of bulky ligands which enhances the reductive elimination pathway and inhibits β-hydride elimination to selectively yield difunctionalized alkene products. This feature article summarizes the utilization of unreactive C-H bonds in the Pd-catalyzed and ligand-enabled difunctionalization of alkenes.

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.

Radical 1,4-Aryl Migration Enabled Remote Cross-Electrophile Coupling of α-Amino-β-Bromo Acid Esters with Aryl Bromides

We report an unprecedented, efficient nickel-catalysed radical relay for the remote cross-electrophile coupling of β-bromo-α-benzylamino acid esters with aryl bromides via 1,4-aryl migration/arylation cascades. β-Bromo-α-benzylamino acid esters are considered as unique molecular scaffolds allowing for aryl migration reactions, which are conceptually novel variants for the radical Truce-Smiles rearrangement. This reaction enables the formation of two new C(sp³ )-C(sp² ) bonds using a bench-stable Ni/bipyridine/Zn system featuring a broad substrate scope and excellent diastereoselectivity, which provides an effective platform for the remote aryl group migration and arylation of amino acid esters via redox-neutral C(sp³ )-C(sp² ) bond cleavage. Mechanistically, this cascade reaction is accomplished by combining two powerful catalytic cycles consisting of a cross-electrophile coupling and radical 1,4-aryl migration through the generation of C(sp³ )-centred radical intermediates from the homolysis of C(sp³ )-Br bonds and the switching of the transient alkyl radical into a robust α-aminoalkyl radical.

C-H Bond Functionalization of Amines: A Graphical Overview of Diverse Methods

This Graphical Review provides a concise overview of the manifold and mechanistically diverse methods that enable the functionalization of sp3 C-H bonds in amines and their derivatives.

C-H Functionalization of Peptides via Cyclic Aminal Intermediates

Protected dipeptides can be converted into cyclic ketoaminals, which can be subjected to palladium-catalyzed regioselective C-H functionalization. The best results are obtained using the 2-(methylthio)aniline (MTA) directing group, which is superior to the commonly used 8-aminoquinoline (AQ) group. No epimerization of stereogenic centers is observed. Subsequent cleavage of the directing and protecting groups allows the incorporation of a modified dipeptide into larger peptide chains.

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.

Ligand-Enabled δ-C(sp3 )-H Borylation of Aliphatic Amines

Directed C-H functionalization has been realized as a complimentary technique to achieve borylation at a distal position of aliphatic amines. Here, we demonstrated the oxidative borylation at the distal δ-position of aliphatic amines using various borylating agents, a palladium catalyst, and a rightly tuned ligand in the presence of a cheap oxidant. Moreover, an organopalladium δ-C(sp³ )-H-activated intermediate has been isolated and crystallographically characterized to get mechanistic insight.

Regio- and Stereoselective Functionalization Enabled by Bidentate Directing Groups

Chelation-assisted C-H bond and alkene functionalization using bidentate directing groups offers an elegant and versatile approach to overcome regiocontrol issues by allowing the catalyst to come into close proximity with the targeted sites. In this personal account, we highlight our recent works in developing regio- and stereocontrolled functionalizations through transition-metal catalysis enabled by bidentate directing groups. We classify our results into two categories: (1) regioselective alkene functionalization using bidentate directing groups, and (2) asymmetric C-H functionalization using chiral bidentate directing groups. Furthermore, density functional theory studies to elucidate the origin of the regio- and stereoselectivity exerted by bidentate directing groups are discussed.

Advances in Transition-Metal-Catalyzed C–H Bond Oxygenation of Amides

C–O bond formation represents a fundamental chemical transformation in organic synthesis to develop valuably oxygenated (hetero)arenes. Particularly, the direct and regioselective C–H bond oxygenation of privileged amides, using a transition metal catalyst and a mild oxygenating source, is a step-economy and attractive approach. During the last decade, considerable progress has been realized in the direct C–H oxygenation of primary, secondary, and tertiary amides. This Short Review compiles the advances in transition-metal-catalyzed oxygenation of C(sp2)–H and C(sp3)–H bonds on various amides with diverse oxygenation sources. The review is categorized into two different major sections: (i) C(sp2)–H oxygenation and (ii) C(sp3)–H oxygenation. Each section is discussed based on the directing group (monodentate and bidentate) attached to the amide derivatives.

1 Introduction

2 C(sp2)–H Oxygenation

2.1 Monodentate Directed

2.2 Bidentate Directed

3 C(sp3)–H Oxygenation

3.1 Monodentate Directed

3.2 Bidentate Directed

4 Conclusion and Outlook

Synthetic studies toward inducamide C

The alkaloid inducamide C is proposed to contain a very rare benzoxazepine ring. Herein, we report that the benzoxazepine ring in inducamide C is unstable and prone to rearrangement, indicating that structural revision of the natural product may be necessary. In a first-generation synthetic approach, attempts to assemble the benzoxazepine by cyclization of 4-hydroxyinducamide A led to the regioisomeric oxepanoindole, a result of the 4-hydroxyindole (C4-OH) undergoing preferential cyclization instead of the desired chlorosalicylic acid C15-OH. A second-generation approach involved dealkylation of O-isopropylinducamide C, but the same oxepanoindole formed via rearrangement of the proposed inducamide C structure. Computational studies validate preferential formation of the oxepanoindole and the lactone in O-isopropylinducamide C is susceptible to nucleophilic attack. Thus, inducamide C is either highly unstable or in need of structural revision.

A Guide to Directing Group Removal: 8-Aminoquinoline

The use of directing groups allows high levels of selectivity to be achieved in transition metal-catalyzed transformations. Efficient removal of these auxiliaries after successful functionalization, however, can be very challenging. This review provides a critical overview of strategies used for removal of Daugulis' 8-aminoquinoline (2005-2020), one of the most widely used N,N-bidentate directing groups. The limitations of these strategies are discussed and alternative approaches are suggested for challenging substrates. Our aim is to provide a comprehensive end-users' guide for chemists in academia and industry who want to harness the synthetic power of directing groups-and be able to remove them from their final products.