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

Insights into the mechanism of 3d transition-metal-catalyzed directed C(sp3)-H bond functionalization reactions

The growing interest in the catalytic activity of earth-abundant 3d transition-metals has led to the development of new and more sustainable methods for C-H bond functionalization reactions. However, this is an emerging field which involves considerable mechanistic complexity as the mode of action of 3d transition metals differs markedly from the well-studied mechanisms of precious metals. In this review, we present an overview of the research efforts in Ni-, Cu-, Fe- and Co-catalyzed directed C(sp3)-H bond functionalization reactions, covering design principles and mechanistic discussions, along with potential applications and limitations. To conclude, the unresolved challenges and future viewpoints are highlighted. We aspire for this review to serve as a relevant and valuable reference for researchers in this swiftly progressing field, helping to inspire the development of more original and innovative strategies.

Thiourea compounds as multifaceted bioactive agents in medicinal chemistry

Microbial resistance (MR) and cancer are global healthcare pitfalls that have caused millions of deaths and pose a significant pharmaceutical challenge, with clinical cases increasing. Thioureas are preferred structures in medicinal chemistry, chemosensors, and organic synthesis platforms. In fact, thiourea (TU) moieties serve as a common framework for several medications and bioactive substances, demonstrating a wide range of therapeutic and pharmacological accomplishments. The integration of the thiourea moiety into a diverse range of organic molecules has resulted in very flexible compounds with widespread uses in medicinal chemistry. Moreover, for over a century, TU and its metal complexes have been characterized for their biological activity. Finally, we provide an assessment and future outlook of different organo-thiourea derivatives, from the very beginning to the most recent discoveries in medicinal activity.

Iron-Catalyzed Site-Selective Bromination of Benzylic C(sp3)-H Bonds

An iron-catalyzed chemo- and site-selective benzylic C-H bromination has been described. The practical approach uses the C-H substrate as the limiting reagent and commercially available iron(II) bromide at a loading of 1 mol % as the catalyst without the involvement of any extrinsic ligand. The simple and mild reaction can be readily scaled up to gram quantity with good functional group tolerance, offering a convenient route for the late-stage diversification of complex bioactive natural products and pharmaceutical molecules through sequential benzylic C-H bromination.

Recent advances in photoinitiated Pd-catalyzed desaturation and Heck-type reactions via 1,5-hydrogen atom transfer

Photoredox-catalyzed 1,5-hydrogen atom transfer has rapidly developed, which has made remarkable achievements in the activation of remote C(sp3)-H bonds and successfully addressed the selectivity issues faced by transition metal catalysis. In recent years, the discovery of hybrid Pd radical intermediates and their application in photoredox-catalyzed 1,5-hydrogen atom transfer has gained considerable importance, which obviated the necessity of an additional photosensitizer. In light of the significant role of alkenes in organic synthesis and hybrid Pd radical intermediates, this review aims to summarize efforts on photoinduced Pd-catalyzed radical relay Heck and desaturation reactions via 1,5-hydrogen atom transfer, briefly discussing the design, selectivity, reaction mechanisms and future research prospects.

Late-Stage Pd(II)-Catalyzed C(sp3)-H Functionalization of Peptides Directed by a Removable, Backbone-Inserted Amidoxime Ether

Palladium(II)-catalyzed C-H functionalization has attracted considerable attention as a pathway to late-stage modification of peptides. Herein, we report the Pd-catalyzed C(sp3)-H arylation of peptides directed by an amidoxime ether, which can be easily incorporated into peptides at any amide bond. Site- and stereoselective arylation of peptides has been achieved, including unprecedented functionalization of internal residues of native peptides. Removal of the amidoxime ether was achieved to generate the parent amide and facilitate a traceless C-H functionalization process.

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.

Palladium(II)-Catalyzed Site-Selective C(sp3)-H Alkenylation of Oligopeptides

An innovative palladium-catalyzed alkenylation of peptides and vinyl iodides has been developed. This method does not require the introduction of a directing group and uses carboxylic acid groups as endogenous directing groups. It is noteworthy that two key building blocks for the ilamycins and CXCR7 modulators were prepared using our methodology. In addition, the free carboxylic acid residue can be linked to a variety of other compounds, providing a novel approach to the synthesis of peptide drugs in the future.

Ag(I)-Mediated Site-Selective C(sp2)-H Chalcogenation of Tryptophan-Peptides with Dichalcogenides at Room Temperature

This report presents a silver-mediated site-selective chalcogenation of tryptophan-containing peptides with various dichalcogenides (disulfides/diselenides) at room temperature in good to excellent yields. The significant features include broad substrate scope, functional group diversity, late-stage modification of drug molecules (Dopamine and Levodopa), and various valuable postsynthetic transformations under mild conditions.

RuII-Catalyzed C-H Activated Diverse Cyclization with Transformation of Substrate-DG to Functional Groups: Synthesis of Functionalized Indoles and Indenones

We present an elegant and efficient method for Ru(II)-catalyzed C-H activation, followed by a diverse range of intermolecular cross-dehydrogenative coupling reactions. This process is facilitated by an intrinsic directing group (DG) and includes the in situ transformation of the DG into common and useful functional groups. Notably, this method avoids the installation and deinstallation of the directing group. Our approach enables the selective functionalization of benzimidate, coupled with the cyclization of o-alkynyl-aniline, resulting in the high-yield synthesis of diverse compounds such as indoles, and indenones. The sequential formation of C-N, C-C, and C-O bonds, followed by hydrolysis, underscores the versatile in situ transformation of the directing group. This work not only broadens the synthetic toolbox for constructing complex heterocyclic structures but also highlights the potential for sustainable and selective synthesis of valuable compounds.

Discovery and development of tyrosine-click (Y-click) reaction for the site-selective labelling of proteins

With the versatile utility of bio-conjugated peptides and proteins in the fields of agriculture, food, cosmetics and pharmaceutical industry, the design of smart protocols to conjugate and modulate biomolecules becomes highly desirable. During this process, the most important consideration for biochemists is the retention of configurational integrity of the biomolecules. Moreover, this type of bioconjugation of peptide and protein becomes frivolous if the reaction is not performed with precise amino acid residues. Hence, chemo-selective, as well as site-selective reactions, that are biocompatible and possess an appropriate level of reactivity are necessary. Based on click chemistry, there are so many tyrosine (Y) conjugation strategies, such as sulfur-fluoride exchange (SuFEx), sulfur-triazole exchange (SuTEx), coupling with π-allyl palladium complexes, diazonium salts, diazodicarboxyamide-based reagents etc. Among these techniques, diazodicarboxyamide-based Y-conjugation, which is commonly known as the "tyrosine-click (Y-click) reaction", has met the expectations of synthetic and biochemists for the tyrosine-specific functionalization of biomolecules. Over the past one and a half decades, significant progress has been made in the classical organic synthesis approach, as well as its biochemical, photochemical, and electrochemical variants. Despite such progress and increasing importance, the Y-click reaction has not been reviewed to document variations in its methodology, applications, and broad utility. The present article aims to provide a summary of the approaches for the modulation of biomolecules at the hotspot of tyrosine residue by employing the Y-click reaction. The article also highlights its application for the mapping of proteins, imaging of living cells, and in the fields of analytical and medicinal chemistry.

Iridium-catalyzed diacylmethylation of tyrosine and its peptides with sulfoxonium ylides

Pyridyloxy-directed Ir(III)-catalyzed diacylmethylation of protected tyrosines was achieved with alkyl and (hetero)aryl sulfoxonium ylides, furnishing tyrosine-based unnatural amino acids in good yields. Furthermore, the late stage exemplification of the strategy was successfully accomplished in tyrosine-containing dipeptides, tripeptides and tetrapeptides in moderate yields. This methodology is distinguished by its site-selectivity, tolerance of sensitive functional groups, scalability, and retention of the chiral configuration for tyrosine motifs.

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.

Cu(II)-Mediated Sulfonylation of (Hetero)arenes with TosMIC Using Monodentate Directing Groups

Monodentate chelation-assisted direct ortho-C-H sulfonylation of (hetero)arenes using TosMIC as the novel sulfonylating reagent has been developed. A broad range of substrates, including indolines, indoles, 2-phenylpyridines, and others were well tolerated to afford the corresponding products in moderate to good yields. Mechanistic studies revealed that the sulfonyl radical might be involved. Inspired by the above discovery, preliminary para-C-H sulfonylation of naphthalene substrate was also successfully realized. The current protocol featured with cheap metal catalysis, good functional group compatibility, and operational convenience.

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

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

Late-stage modification of bioactive compounds: Improving druggability through efficient molecular editing

Synthetic chemistry plays an indispensable role in drug discovery, contributing to hit compounds identification, lead compounds optimization, candidate drugs preparation, and so on. As Nobel Prize laureate James Black emphasized, "the most fruitful basis for the discovery of a new drug is to start with an old drug"1. Late-stage modification or functionalization of drugs, natural products and bioactive compounds have garnered significant interest due to its ability to introduce diverse elements into bioactive compounds promptly. Such modifications alter the chemical space and physiochemical properties of these compounds, ultimately influencing their potency and druggability. To enrich a toolbox of chemical modification methods for drug discovery, this review focuses on the incorporation of halogen, oxygen, and nitrogen-the ubiquitous elements in pharmacophore components of the marketed drugs-through late-stage modification in recent two decades, and discusses the state and challenges faced in these fields. We also emphasize that increasing cooperation between chemists and pharmacists may be conducive to the rapid discovery of new activities of the functionalized molecules. Ultimately, we hope this review would serve as a valuable resource, facilitating the application of late-stage modification in the construction of novel molecules and inspiring innovative concepts for designing and building new drugs.

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.

Palladium-Catalyzed Regiodivergent C-H Olefination of Imidazo[1,2a]pyridine Carboxamide and Unactivated Alkenes

Despite remarkable successes in linear and branched vinyl (hetero) arene synthesis, regiodivergent C-H olefination with a single catalytic system has remained underdeveloped. Overcoming this limitation, a Pd/MPAA-catalyzed regiodivergent C-H olefination of imidazo[1,2a] pyridine carboxamides with unactivated terminal alkenes to generate branched and linear olefinated products depending upon the electronic nature of alkenes is reported herein. Moreover, this protocol can be applied for C-H deuteriation of the corresponding heteroarenes with D2 O as deuterium source. Preliminary experimental studies combined with computational investigations (DFT studies) suggest that regiodivergent olefination can be controlled by olefin insertion and β-hydride elimination steps.

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.

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.

Decatungstate-Catalyzed C(sp3)-H Alkylation of a Val Residue Proximal to the N-Terminus Controlled by an Electrostatic Interaction

The decatungstate photocatalyst [W₁₀O₃₂]^4- efficiently promoted the C(sp³)-H alkylation of the trifluoroacetic acid salt of valine methyl ester (H-Val-OMe·TFA) with electron-deficient alkenes under UV irradiation. The electrostatic interaction between the cationic ammonium group (⁺NH₃) of the main chain and anionic [W₁₀O₃₂]^4- played an important role in this reaction. The influence of various protected amino acids in the C(sp³)-H alkylation was investigated as the model reaction for the alkylation of Val-containing peptides. The introduction of an alkyne moiety into Val through this alkylation was successful, and successive copper-catalyzed azide-alkyne cycloaddition (CuAAC) was demonstrated. The C(sp³)-H bond of a Val residue located at the second from the N-terminus was also successfully converted. C(sp³)-H alkylation of oligopeptides containing two Val residues selectively proceeded proximally to the N-terminus.

Unlocking the Nucleophilicity of Strong Alkyl C-H Bonds via Cu/Cr Catalysis

Direct functionalization of inert C-H bonds is one of the most attractive yet challenging strategies for constructing molecules in organic chemistry. Herein, we disclose an unprecedented and Earth abundant Cu/Cr catalytic system in which unreactive alkyl C-H bonds are transformed into nucleophilic alkyl-Cr(III) species at room temperature, enabling carbonyl addition reactions with strong alkyl C-H bonds. Various aryl alkyl alcohols are furnished under mild reaction conditions even on a gram scale. Moreover, this new radical-to-polar crossover approach is further applied to the 1,1-difunctionalization of aldehydes with alkanes and different nucleophiles. Mechanistic investigations reveal that the aldehyde not only acts as a reactant but also serves as a photosensitizer to recycle the Cu and Cr catalysts.

Application of functional peptides in the electrochemical and optical biosensing of cancer biomarkers

Early screening and diagnosis are the most effective ways to prevent the occurrence and progression of cancers, thus many biosensing strategies have been developed to achieve economic, rapid, and effective detection of various cancer biomarkers. Recently, functional peptides have been gaining increasing attention in cancer-related biosensing due to their advantageous features of a simple structure, ease of synthesis and modification, high stability, and good biorecognition, self-assembly and antifouling capabilities. Functional peptides can not only act as recognition ligands or enzyme substrates for the selective identification of different cancer biomarkers but also function as interfacial materials or self-assembly units to improve the biosensing performances. In this review, we summarize the recent advances in functional peptide-based biosensing of cancer biomarkers according to the used techniques and the roles of peptides. Particular attention is focused on the use of electrochemical and optical techniques, both of which are the most commonly used techniques in the field of biosensing. The challenges and promising prospects of functional peptide-based biosensors in clinical diagnosis are also discussed.

Native Amino Group Directed Site-Selective ε-C(sp2)-H Iodination of Primary Amines

Selective remote C-H activating amines using unmodified NH₂ as a native directing group demonstrate compelling synthetic utilities. The 3-arylpropan-1-amine moiety is present in many drugs and candidates in clinical trials. Selective iodination of 3-arylpropan-1-amines on remote aryl rings gives valuable intermediates for modifying bioactive molecules and synthesizing quinolones. Here we report the first palladium-catalyzed selective ε-C(sp²)-H iodination of free 3-arylpropan-1-amines via a seven-membered palladacycle.

Tunable Electrochemical Peptide Modifications: Unlocking New Levels of Orthogonality for Side-Chain Functionalization

Electrochemical transformations provide enticing opportunities for programmable, residue-specific peptide modifications. Herein, we harness the potential of amidic side-chains as underutilized handles for late-stage modification through the development of an electroauxiliary-assisted oxidation of glutamine residues within unprotected peptides. Glutamine building blocks bearing electroactive side-chain N,S-acetals are incorporated into peptides using standard Fmoc-SPPS. Anodic oxidation of the electroauxiliary in the presence of diverse alcohol nucleophiles enables the installation of high-value N,O-acetal functionalities. Proof-of-principle for an electrochemical peptide stapling protocol, as well as the functionalization of dynorphin B, an endogenous opioid peptide, demonstrates the applicability of the method to intricate peptide systems. Finally, the site-selective and tunable electrochemical modification of a peptide bearing two discretely oxidizable sites is achieved.

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.

Bioinspired Palladium-Catalyzed Intramolecular C(sp3 )-H Activation for the Collective Synthesis of Proline Natural Products

Bioinspired palladium-catalyzed intramolecular cyclization of amino acid derivatives containing a vinyl iodide moiety by C-H activation enabled rapid access to a wide range of functionalized proline derivatives with an exocyclic olefin. To demonstrate the practicality of this methodology, the functionalized prolines were used as intermediates for the synthesis of several natural products: lucentamycin A, oxotomaymycin, oxoprothracarcin, and barmumycin.

N-Terminal Selective C-H Azidation of Proline-Containing Peptides: a Platform for Late-Stage Diversification

A methodology for the C-H azidation of N-terminal proline-containing peptides was developed employing only commercially available reagents. Peptides bearing a broad range of functionalities and containing up to 6 amino acids were selectively azidated at the carbamate-protected N-terminal residue in presence of the numerous other functional groups present on the molecules. Post-functionalizations of the obtained aminal compounds were achieved: cycloaddition reactions or C-C bond formations via a sequence of imine formation/nucleophilic addition were performed, offering an easy access to diversified peptides.

Chemo- and Site-Selective Replacement of N-Terminal Carbamates in Peptides

In peptide synthesis, it is important to distinguish the terminal amino group and carry out the selective transformation of only the N-terminal protecting group. We describe herein a reaction for the chemo- and site-selective replacement of carbamates with various other carbamates only at the N-terminus of peptides. We demonstrate the scope of carbamates and peptides and the introduction of fluorine into a peptide. This strategy is applicable to the late stage of peptide synthesis.

Umpolung strategies for the functionalization of peptides and proteins

Umpolung strategies, defined as synthetic approaches which reverse commonly accepted reactivity patterns, are broadly recognized as enabling tools for small molecule synthesis and catalysis. However, methods which exploit this logic for peptide and protein functionalizations are comparatively rare, with the overwhelming majority of existing bioconjugation approaches relying on the well-established reactivity profiles of a handful of amino acids. This perspective serves to highlight a small but growing body of recent work that masterfully capitalizes on the concept of polarity reversal for the selective modification of proteinogenic functionalities. Current applications of umpolung chemistry in organic synthesis and chemical biology as well as the vast potential for further innovations in peptide and protein modification will be discussed.

Pd-catalysed C-H functionalisation of free carboxylic acids

Pd-catalysed C-H functionalisation of free carboxylic acids has drawn significant attention over the last few years due to the predominance of carboxylic acid moieties in pharmaceuticals and agrochemicals. But their coordinating ability was overlooked and masked by exogenous directing groups for a long time. Even other crucial roles of carboxylic acids as additives and steric inducers that directly influence the mode of a reaction have been widely neglected. This review aims to embrace all of the diverse aspects of carboxylic acids except additive and steric effects by concisely and systematically describing their versatile role in Pd-catalysed proximal and distal C-H activation reactions that could be implemented in the pharmaceutical and agrochemical industries. In addition, the mechanistic perspectives along with several recent strategies developed in the last few years discussed here will serve as educational resources for future research.

Synthetic Thiol and Selenol Derived Amino Acids for Expanding the Scope of Chemical Protein Synthesis

Cells employ post-translational modifications (PTMs) as key mechanisms to expand proteome diversity beyond the inherent limitations of a concise genome. The ability to incorporate post-translationally modified amino acids into protein targets via chemical ligation of peptide fragments has enabled the access to homogeneous proteins bearing discrete PTM patterns and empowered functional elucidation of individual modification sites. Native chemical ligation (NCL) represents a powerful and robust means for convergent assembly of two homogeneous, unprotected peptides bearing an N-terminal cysteine residue and a C-terminal thioester, respectively. The subsequent discovery that protein cysteine residues can be chemoselectively desulfurized to alanine has ignited tremendous interest in preparing unnatural thiol-derived variants of proteogenic amino acids for chemical protein synthesis following the ligation-desulfurization logic. Recently, the 21st amino acid selenocysteine, together with other selenyl derivatives of amino acids, have been shown to facilitate ultrafast ligation with peptidyl selenoesters, while the advancement in deselenization chemistry has provided reliable bio-orthogonality to PTMs and other amino acids. The combination of these ligation techniques and desulfurization/deselenization chemistries has led to streamlined synthesis of multiple structurally-complex, post-translationally modified proteins. In this review, we aim to summarize the latest chemical synthesis of thiolated and selenylated amino-acid building blocks and exemplify their important roles in conquering challenging protein targets with distinct PTM patterns.

Hypervalent Iodine-Mediated Late-Stage Peptide and Protein Functionalization

Hypervalent iodine compounds are powerful reagents for the development of novel transformations. As they exhibit low toxicity, high functional group tolerance, and stability in biocompatible media, they have been used for the functionalization of biomolecules. Herein, we report recent advances up to June 2021 in peptide and protein modification using hypervalent iodine reagents. Their use as group transfer or oxidizing reagents is discussed in this Minireview, including methods targeting polar, aromatic, or aliphatic amino acids and peptide termini.

Overcoming the Challenges toward Selective C(6)-H Functionalization of 2-Pyridone with Maleimide through Mn(I)-Catalyst: Easy Access to All-Carbon Quaternary Center

An earth-abundant and inexpensive Mn(I)-catalyzed alkylation of 2-pyridone with maleimide has been reported for the first time, in contrast to previously reported Diels-Alder products. Notably, an unexpected rearrangement has been discovered in the presence of acetic acid, which also provides a unique class of compounds bearing three different N-heterocycles with an all-carbon quaternary center. Furthermore, single crystal X-ray and HRMS revealed a five-membered manganacycle intermediate. This methodology tolerates a wide variety of functional groups delivering the alkylated products in moderate to excellent yields.

Electrochemistry for the Chemoselective Modification of Peptides and Proteins

Although electrochemical strategies for small-molecule synthesis are flourishing, this technology has yet to be fully exploited for the mild and chemoselective modification of peptides and proteins. With the growing number of diverse peptide natural products being identified and the emergence of modified proteins as therapeutic and diagnostic agents, methods for electrochemical modification stand as alluring prospects for harnessing the reactivity of polypeptides to build molecular complexity. As a mild and inherently tunable reaction platform, electrochemistry is arguably well-suited to overcome the chemo- and regioselectivity issues which limit existing bioconjugation strategies. This Perspective will showcase recently developed electrochemical approaches to peptide and protein modification. The article also highlights the wealth of untapped opportunities for the production of homogeneously modified biomolecules, with an eye toward realizing the enormous potential of electrochemistry for chemoselective bioconjugation chemistry.

Theoretical Investigate of the α-Substitution Effect on γ-C(sp3)−H Arylation of Amines: Structure-Reactivity Relationship (SRR) Studies

Structure-reactivity relationship (SRR) studies to understand the α-substitution effect toward Pd-catalyzed γ-C(sp3)−H arylation enabled by catalytic transient directing groups have carefully been performed by DFT calculations. Selected transient directing groups...

Specific assembly of dihydrobenzofuran frameworks via Rh(iii)-catalysed C–H coupling of N-phenoxyacetamides with 2-alkenylphenols

The synergistic dual directing group-enabled and Rh(iii)-catalysed redox-neutral C–H functionalization/[3+2] annulation has been realized for the synthesis of dihydrobenzofurans.

Amine-Directed Mizoroki-Heck Arylation of Free Allylamines

The transition metal-catalyzed Mizoroki−Heck reaction is a powerful method to synthesize C–C bonds, allowing access to several important pharmaceuticals. Traditionally free amines have not been compatible with these approaches due...

Direct Access to Bridged Polycyclic Skeletons by Merging Oxidative C-H Annulation and Cascade [4 + 2] Cycloaddition

We report a step-economic strategy for the direct synthesis of bridged polycyclic skeletons by merging oxidative C-H annulation and cascade cycloaddition. In the protocol, spiro[cyclopentane-1,3'-indoline]-2,4-dien-2'-ones were first synthesized by oxidative C-H annulation of ethylideneoxindoles with alkynes. Subsequent cascade [4 + 2] cycloaddition with dienophiles gave the bridged bicyclo[2.2.1]quinolin-2(1H)-ones and enabled the one-pot construction of two quaternary carbon centers and three C-C bonds. Mechanistic investigations of the latter suggest a cascade ring-opening, 1,5-sigmatropic rearrangement, and [4 + 2] cycloaddition process.

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

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