Palladium-Catalyzed Carbonylative Cyclization of Amines via γ-C(sp3)–H Activation: Late-Stage Diversification of Amino Acids and Peptides

Palladium-Catalyzed Carbonylative Cyclization of 1-Alkynyl-2-iodo-d-glucal

Cascade reactions are important synthetic tools for the synthesis of heterocyclic molecules, particularly those catalyzed by palladium. Herein, we report a palladium-catalyzed aminocarbonylative cyclization of new 1-alkynyl-2-iodo-d-glucals, which undergo a tandem carbonylative cyclization in the presence of various amine nucleophiles. A broad range of aromatic and aliphatic amines were applied as coupling partners, resulting in the selective and high-yield synthesis of glycosides fused to pyridinones. A plausible mechanism is proposed, proceeding via a tandem palladium aminocarbonylation followed by a palladium-catalyzed endo-dig cyclization.

Pd-catalyzed regioselective activation of C(sp2)-H and C(sp3)-H bonds

Differentiating between two highly similar C-H bonds in a given molecule remains a fundamental challenge in synthetic organic chemistry. Directing group assisted strategies for the functionalisation of proximal C-H bonds has been known for the last few decades. However, distal C-H bond functionalisation is strenuous and requires distinctly specialised techniques. In this review, we summarise the advancement in Pd-catalysed distal C(sp2)-H and C(sp3)-H bond activation through various redox manifolds including Pd(0)/Pd(II), Pd(II)/Pd(IV) and Pd(II)/Pd(0). Distal C-H functionalisation, where a Pd-catalyst is directly involved in the C-H activation step, either through assistance of an external directing group or directed by an inherent functionality or functional group incorporated at the site of the Pd-C bond is covered. The purpose of this review is to portray the current state of art in Pd-catalysed distal C(sp2)-H and C(sp3)-H functionalisation reactions, their mechanism and application in the late-stage functionalisation of medicinal compounds along with highlighting its limitations, thus leaving the field open for further synthetic adjustment.

Late-Stage Glycosylation of Peptides by Methionine-Directed β-C(sp3)-H Functionalization with 1-Iodoglycals

Using l-methionine (Met) as the endogenous directing group, we developed Pd-catalyzed β-C(sp3)-H glycosylation of peptides with 1-iodoglycals. A wide range of tri- to hexapeptides containing the Ala-Met motifs underwent Ala C-H glycosylation under the standard conditions to give the glycopeptides smoothly. 15 proteinogenic amino acids (with easily removable protecting groups) were well tolerated. Control experiments indicated that Met acted as a N,S-bidentate directing group and exhibited an effect superior to other amino acid residues such as l-aspartic acid (Asp), l-asparagine (Asn), and S-protected l-cysteine (Cys). In addition, further transformation by HFIP-promoted 1,4-elimination furnished another type of glycopeptide with the 1,3-diene motif, which provides a handle for further derivatization.

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.

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.

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.

Synthesis of Chlorinated Oligopeptides via γ- and δ-Selective Hydrogen Atom Transfer Enabled by the N-Chloropeptide Strategy

The introduction of a chlorine atom could potentially endow peptide derivatives with notable bioactivity and applicability. However, despite considerable recent progress in C(sp3)-H functionalization chemistry, a general method for the site-selective chlorination of inert aliphatic C-H bonds in peptides still remains elusive. Herein, we report a site-selective C(sp3)-H chlorination of oligopeptides based on an N-chloropeptide strategy. N-chloropeptides, which are easily prepared from the corresponding native oligopeptides, are smoothly degraded in the presence of an appropriate copper catalyst, and a subsequent 1,5-hydrogen atom transfer affords γ- or δ-chlorinated peptides in excellent yield. A wide variety of amino acid residues can thus be site-selectively chlorinated in a predictable manner. This method hence enables the efficient synthesis of otherwise less accessible, chlorine-containing peptide fragments of natural peptides. We moreover demonstrate here the successful estimation of the stereochemistry of the chlorinated carbon atom in aquimarin A. Furthermore, we reveal that side-chain-chlorinated peptides can serve as highly useful substructures with a fine balance between stability and reactivity, which renders them promising targets for synthetic and medicinal applications.

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.

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.

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.

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.

General Enantioselective and Stereochemically Divergent Four-Stage Approach to Fused Tetracyclic Terpenoid Systems

Tetracyclic terpenoid-derived natural products are a broad class of medically relevant agents that include well-known steroid hormones and related structures, as well as more synthetically challenging congeners such as limonoids, cardenolides, lanostanes, and cucurbitanes, among others. These structurally related compound classes present synthetically disparate challenges based, in part, on the position and stereochemistry of the numerous quaternary carbon centers that are common to their tetracyclic skeletons. While de novo syntheses of such targets have been a topic of great interest for over 50 years, semisynthesis is often how synthetic variants of these natural products are explored as biologically relevant materials and how such agents are further matured as therapeutics. Here, focus was directed at establishing an efficient, stereoselective, and molecularly flexible de novo synthetic approach that could offer what semisynthetic approaches do not. In short, a unified strategy to access common molecular features of these natural product families is described that proceeds in four stages: (1) conversion of epichlorohydrin to stereodefined enynes, (2) metallacycle-mediated annulative cross-coupling to generate highly substituted hydrindanes, (3) tetracycle formation by stereoselective forging of the C9-C10 bond, and (4) group-selective oxidative rearrangement that repositions a quaternary center from C9 to C10. These studies have defined the structural features required for highly stereoselective C9-C10 bond formation and document the generality of this four-stage synthetic strategy to access a range of unique stereodefined systems, many of which bear stereochemistry/substitution/functionality not readily accessible from semisynthesis.

Transition-Metal-Catalyzed Annulations Involving the Activation of C(sp3 )-H Bonds

The selective functionalization of C(sp3 )-H bonds using transition-metal catalysis is among the more attractive transformations of modern synthetic chemistry. In addition to its inherent atom economy, such reactions open unconventional retrosynthetic pathways that can streamline synthetic processes. However, the activation of intrinsically inert C(sp3 )-H bonds, and the selection among very similar C-H bonds, represent highly challenging goals. In recent years there has been notable progress tackling these issues, especially with regard to the development of intermolecular reactions entailing the formation of C-C and C-heteroatom bonds. Conversely, the assembly of cyclic products from simple acyclic precursors using metal-catalyzed C(sp3 )-H bond activations has been less explored. Only recently has the number of reports on such annulations started to grow. Herein we give an overview of some of the more relevant advances in this exciting topic.

Palladium-catalyzed enantioselective carbonylation reactions

Carbonylation, one of the most powerful approaches to the preparation of carbonylated compounds, has received significant attention from researchers active in various fields. Indeed, impressive progress has been made on this subject over the past few decades. Among the various types of carbonylation reactions, asymmetric carbonylation is a straightforward methodology for constructing chiral compounds. Although rhodium-catalyzed enantioselective hydroformylations have been discussed in several elegant reviews, a general review on palladium-catalyzed asymmetric carbonylations is still missing. In this review, we summarize and discuss recent achievements in palladium-catalyzed asymmetric carbonylation reactions. Notably, this review’s contents are categorized by reaction type.

Ligand Enabled Pd(II)-Catalyzed γ-C(sp3)-H Lactamization of Native Amides

γ-Lactams form important structural cores of a range of medicinally relevant natural products and clinical drugs, principal examples being the new generation of immunomodulatory imide drugs (IMiDs) and the brivaracetam family. Compared to conventional multistep synthesis, an intramolecular γ-C-H amination of aliphatic amides would allow for the direct construction of valuable γ-lactam motifs from abundant amino acid precursors. Herein we report a novel 2-pyridone ligand enabled Pd(II)-catalyzed γ-C(sp³)-H lactamization of amino acid derived native amides, providing the convenient synthesis of γ-lactams, isoindolinones, and 2-imidazolidinones. C6-Substitution of the 2-pyridone ligand is crucial for the lactam formation. This protocol features the use of N-acyl amino acids, which serve as both the directing group and cyclization partner, practical and environmentally benign tert-butyl hydrogen peroxide (TBHP) as the sole bystanding oxidant, and a broad substrate scope. The utility of this protocol was demonstrated through the two-step syntheses of a lenalidomide analog and brivaracetam from readily available carboxylic acids and amino acids.

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 bidentate auxiliary assisted remote C(sp3)-H functionalization

Pd-catalyzed C-H functionalisation affords effective synthetic tools to construct C-C and C-X bonds. Despite the challenges, the distal functionalization of C(sp³)-H bonds has witnessed significant developments and the use of bidentate auxiliaries has garnished this area by providing an opportunity to control reactivity as well as selectivity beyond proximal sites. This article covers the recent developments on the Pd-catalyzed bidentate auxiliary-assisted distal C(sp³)-H functionalization and is categorized based on the nature of functionalizations.

Merging C-H Activation and Strain-Release in Ruthenium-Catalyzed Isoindolinone Synthesis

The merger of strain-release of 1,2-oxazetidines with carboxylic acid directed C-H activation in catalytic synthesis of isoindolinones is reported for the first time. This reaction opens a new and sustainable avenue to prepare a range of structurally diverse isoindolinone skeletons from readily available benzoic acids. The success of late-stage functionalization of some bioactive acids, and concise synthesis of biologically important skeletons demonstrated its great synthetic potential in drug discovery. Mechanistic studies indicated a plausible C-H activation/β-carbon elimination/intramolecular cyclization cascade pathway.

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.

Synthesis of Cyclic Anhydrides via Ligand-Enabled C-H Carbonylation of Simple Aliphatic Acids

The development of C(sp³ )-H functionalizations of free carboxylic acids has provided a wide range of versatile C-C and C-Y (Y=heteroatom) bond-forming reactions. Additionally, C-H functionalizations have lent themselves to the one-step preparation of a number of valuable synthetic motifs that are often difficult to prepare through conventional methods. Herein, we report a β- or γ-C(sp³ )-H carbonylation of free carboxylic acids using Mo(CO)₆ as a convenient solid CO source and enabled by a bidentate ligand, leading to convenient syntheses of cyclic anhydrides. Among these, the succinic anhydride products are versatile stepping stones for the mono-selective introduction of various functional groups at the β position of the parent acids by decarboxylative functionalizations, thus providing a divergent strategy to synthesize a myriad of carboxylic acids inaccessible by previous β-C-H activation reactions. The enantioselective carbonylation of free cyclopropanecarboxylic acids has also been achieved using a chiral bidentate thioether ligand.

The Emergence of Palladium-Catalyzed C(sp3 )-H Functionalization of Free Carboxylic Acids

Palladium-catalyzed directing group assisted C-H bond activation has emerged as a powerful tool in synthetic organic chemistry. However, only recently, among various directing groups, widely available carboxylate moiety is recognized as a versatile candidate for the regioselective transformations. Notably, palladium-catalyzed carboxylate directed C(sp³ )-H bond activation and diverse functionalization is highly challenging and has gained huge attention for its versatile applications. Mono- and bidentate ligands have proven to be useful for accelerating the C(sp³ )-H bond activation step, which helps to control reactivity and selectivity (including enantioselectivity). In this Minireview, we discuss the recent progress made in palladium-catalyzed C(sp³ )-H bond functionalization reactions for the construction of C-C and C-Heteroatom bonds with the direction of free carboxylic acid.

General Access to Modified α-Amino Acids by Bioinspired Stereoselective γ-C-H Bond Lactonization

α-Amino acids represent a valuable class of natural products employed as building blocks in biological and chemical synthesis. Because of the limited number of natural amino acids available, and of their widespread application in proteomics, diagnosis, drug delivery and catalysis, there is an increasing demand for the development of procedures for the preparation of modified analogues. Herein, we show that the use of bioinspired manganese catalysts and H₂ O₂ under mild conditions, provides access to modified α-amino acids via γ-C-H bond lactonization. The system can efficiently target 1°, 2° and 3° γ-C-H bonds of α-substituted and achiral α,α-disubstituted α-amino acids with outstanding site-selectivity, good to excellent diastereoselectivity and (where applicable) enantioselectivity. This methodology may be considered alternative to well-established organometallic procedures.

Concerted vs Nonconcerted Metalation-Deprotonation in Orthogonal Direct C-H Arylation of Heterocycles with Halides: A Computational Study

A computational study on the base-assisted orthogonal C-H arylation of azoles with halides is reported. Although concerted metalation-deprotonation (CMD) is favored under acetate assistance at the C5 site that displays the best balance of nucleophilic and acidic character, the most acidic C2 site may be selectively targeted under carbonate assistance by taking advantage of a carbanionic-type (or non-concerted) metalation-deprotonation mechanism (nCMD). For the latter, several experimental probes including base, ligand, and solvent effects have been collected in favor of an outer-sphere deprotonation process after the formation of a [(L_n)(N1-heteroaryl)PdArX] complex. However, no computational analysis of this fundamental elementary step has been so far provided. We have carried out a series of density functional theory (DFT) calculations that delineate the structural and energetic aspects of the nCMD pathway. Starting with the oxa(thia)zole-4-carboxylates selected in our group to engineer the competitive C2 vs C5 arylation in azoles, we show that the energy barrier of the C2 anion generation is lying unexpectedly lower than the prior heterocycle coordination to Pd that is eventually identified as the rate-determining step. These calculations provide satisfactory explanations for the experimental observations of the divergence between nCMD and CMD reactivity, and notably a lower barrier at the C2 site for the nCMD process. On the other hand, the nCMD process is ineffective at the C5 site. Evaluation of various azoles reveals that the nCMD mechanism at C2 is viable from the most acidic (benzo)oxazoles to moderately acidic (benzo)thiazoles, as well as weakly acidic imidazoles. In all cases, in accordance with previously reported experimental data in orthogonal direct C-H arylation of azoles, the nCMD route is found energetically competitive to the CMD one at C5 for all azoles, except for imidazole which needs stronger basic conditions than simple carbonate assistance. Additionally, the acetate ligand, which is the base of choice for CMD, was found inefficient for nCMD and the comparative performance of acetate vs carbonate to assist CMD in the azole series reveals also considerable changes from electronically close but environmentally divergent C5-H vs C2-H bonds.

Site-selective and diastereoselective functionalization of α-amino acid and peptide derivatives via palladium-catalyzed sp3 C–H activation

This review introduces palladium-catalyzed C–H functionalization of amino acids and peptides.

Ruthenium-catalysed C–H/C–N bond activation: facile access to isoindolinones

A facile ruthenium-catalysed C–H/C–N bond activation and the subsequent annulation of readily available benzoic acids with in situ generated formaldimines are developed for the efficient synthesis of a wide range of biologically important isoindolinones.

Mechanistic understanding enables chemoselective sp3 over sp2 C–H activation in Pd-catalyzed carbonylative cyclization of amino acids

An understanding on the factors controlling C(sp²)–H vs, C(sp³)–H selectivity in Pd-catalyzed carbonylative cyclization of γ-arylated valine derivatives has allowed to reverse the remarkable selectivity of Pd for aryl C(sp²)–H over C(sp³)–H cleavage.

Palladium-catalyzed gaseous CO-free carbonylative C–C bond activation of cyclobutanones

A palladium-catalyzed carbonylative C–C bond activation reaction of cyclobutanones is reported, and it affords a variety of indanones bearing ester or amide groups using phenyl formate and benzene-1,3,5-triyl triformate as CO surrogates.

Site-selective modification of peptide backbones

Exciting developments in the site-selective modification of peptide backbones are allowing an outstanding fine-tuning of peptide conformation, folding ability, and physico-chemical and biological properties.

Diverse strategies for transition metal catalyzed distal C(sp3)-H functionalizations

Transition metal catalyzed C(sp3)-H functionalization is a rapidly growing field. Despite severe challenges, distal C-H functionalizations of aliphatic molecules by overriding proximal positions have witnessed tremendous progress. While usage of stoichiometric directing groups played a crucial role, reactions with catalytic transient directing groups or methods without any directing groups are gaining more attention due to their practicality. Various innovative strategies, slowly but steadily, circumvented issues related to remote functionalizations of aliphatic molecules. A systematic compilation has been presented here to provide insights into the recent developments and future challenges in the field. The Present perspective is expected to open up a new dimension and provide an avenue for deep insights into the distal C(sp3)-H functionalizations that could be applied routinely in various pharmaceutical and agrochemical industries.