Late‐Stage Peptide Macrocyclization by Palladium‐Catalyzed Site‐Selective C−H Olefination of Tryptophan

Transition‐metal‐catalyzed C?H activation has shown potential in the functionalization of peptides with expanded structural diversity. Herein, the development of late‐stage peptide macrocyclization methods by palladium‐catalyzed site‐selective C(sp²)?H olefination of tryptophan residues at the C2 and C4 positions is reported. This strategy utilizes the peptide backbone as endogenous directing groups and provides access to peptide macrocycles with unique Trp–alkene crosslinks.     

Internal Peptide Late‐Stage Diversification: Peptide‐Isosteric Triazoles for Primary and Secondary C(sp3)−H Activation

Secondary C(sp³)?H arylations were accomplished by palladium catalysis with triazoles as peptide bond isosteres. The unique power of this approach is highlighted by the possibility of achieving secondary C(sp³)?H functionalizations on terminal peptides as well as the unprecedented positional‐selective C(sp³)?H functionalization of internal peptide positions, setting the stage for modular peptide late‐stage diversification.     

Stapled Peptides by Late‐Stage C(sp3)−H Activation

Despite the importance of stapled peptides for drug discovery, only few practical processes to prepare cross‐linked peptides have been described; thus the structural diversity of available staple motifs is currently limited. At the same time, C−H activation has emerged as an efficient approach to functionalize complex molecules. Although there are many reports on the C−H functionalization of amino acids, examples of post‐synthetic peptide C−H modification are rare and comprise almost only C(sp²)−H activation. Herein, we report the development of a palladium‐catalyzed late‐stage C(sp³)−H activation method for peptide stapling, affording an unprecedented hydrocarbon cross‐link. This method was first employed to prepare a library of stapled peptides in solution. The compatibility with various amino acids as well as the influence of the size (i ,i +3 and i ,i +4) and length of the staple were investigated. Finally, a simple solid‐phase procedure was also established.     

Site‐Selective δ‐C(sp3)−H Alkylation of Amino Acids and Peptides with Maleimides via a Six‐Membered Palladacycle

The site‐selective functionalization of unactivated C(sp³)?H bonds remains one of the greatest challenges in organic synthesis. Herein, we report on the site‐selective δ‐C(sp³)?H alkylation of amino acids and peptides with maleimides via a kinetically less favored six‐membered palladacycle in the presence of more accessible γ‐C(sp³)?H bonds. Experimental studies revealed that C?H bond cleavage occurs reversibly and preferentially at γ‐methyl over δ‐methyl C?H bonds while the subsequent alkylation proceeds exclusively at the six‐membered palladacycle that is generated by δ‐C?H activation. The selectivity can be explained by the Curtin–Hammett principle. The exceptional compatibility of this alkylation with various oligopeptides renders this procedure valuable for late‐stage peptide modifications. Notably, this process is also the first palladium(II)‐catalyzed Michael‐type alkylation reaction that proceeds through C(sp³)?H activation.     

Late-Stage Peptide Macrocyclization by Palladium-Catalyzed Site-Selective C−H Olefination of Tryptophan

Transition-metal-catalyzed C−H activation has shown potential in the functionalization of peptides with expanded structural diversity. Herein, the development of late-stage peptide macrocyclization methods by palladium-catalyzed site-selective C(sp²)−H olefination of tryptophan residues at the C2 and C4 positions is reported. This strategy utilizes the peptide backbone as endogenous directing groups and provides access to peptide macrocycles with unique Trp–alkene crosslinks.     

Disulfide bond cleavage in TEMPO-free radical initiated peptide sequencing mass spectrometry

The gas‐phase free radical initiated peptide sequencing (FRIPS) fragmentation behavior of o‐TEMPO‐Bz‐conjugated peptides with an intra‐ and intermolecular disulfide bond was investigated using MSⁿ tandem mass spectrometry experiments. Investigated peptides included four peptides with an intramolecular cyclic disulfide bond, Bactenecin (RLC RIVVIRVC R), TGF‐α (C HSGYVGVRC ), MCH (DFDMLRC MLGRVFRPC WQY) and Adrenomedullin (16–31) (C RFGTC TVQKLAHQIY), and two peptides with an intermolecular disulfide bond. Collisional activation of the benzyl radical conjugated peptide cation, which was generated through the release of a TEMPO radical from o‐TEMPO‐Bz‐conjugated peptides upon initial collisional activation, produced a large number of peptide backbone fragments in which the S? S or C? S bond was readily cleaved. The observed peptide backbone fragments included a‐, c‐, x‐ or z‐types, which indicates that the radical‐driven peptide fragmentation mechanism plays an important role in TEMPO‐FRIPS mass spectrometry. FRIPS application of the linearly linked disulfide peptides further showed that the S? S or C? S bond was selectively and preferentially cleaved, followed by peptide backbone dissociations. In the FRIPS mass spectra, the loss of ?SH or ?SSH was also abundantly found. On the basis of these findings, FRIPS fragmentation pathways for peptides with a disulfide bond are proposed. For the cleavage of the S? S bond, the abstraction of a hydrogen atom at C_β by the benzyl radical is proposed to be the initial radical abstraction/transfer reaction. On the other hand, H‐abstraction at C_α is suggested to lead to C? S bond cleavage, which yields [ion ± S] fragments or the loss of ?SH or ?SSH. Copyright © 2011 John Wiley & Sons, Ltd.     

Water Soluble Antioxidant Dextran–Quercetin Conjugate with Potential Anticancer Properties

Quercetin, a naturally occurring potent antioxidant, is limited in therapeutic use, owing to its poor water solubility and stability. Herein, a method of conjugating quercetin to an aldehyde functionalized dextran via an HCl catalyzed condensation reaction to yield a water soluble quercetin functionalized polymer is reported. The prepared conjugate is characterized by ¹H and ¹H‐¹³C heteronuclear single quantum correlation (HSQC) NMR, which demonstrate that conjugation occurs via both the A‐ and B‐rings of quercetin. The degree of quercetin functionalization can be tuned by varying the reaction temperature and/or the concentration of the HCl catalyst. However, as temperatures and HCl concentrations are increased above 40 °C and 2 m , respectively, the increase in functionalization is accompanied by an increase in the oxidation of the conjugated quercetin and a decrease in polymer yield. The prepared conjugate is shown to have improved stability compared with native quercetin while maintaining substantial free‐radical scavenging activity. Anticancer activity is evaluated in vitro in a neuroblastoma cell line. The dextran–aldehyde–quercetin conjugate prepared at 40 °C and 2 m HCl is shown to be cytotoxic to neuroblastoma cells (SH‐SY5Y–IC₅₀ = 123 µg mL⁻¹ and BE(2)‐C–IC₅₀ = 380 µg mL⁻¹) but shows no activity against nonmalignant MRC‐5 cells at concentrations up to 400 µg mL⁻¹.     

Late‐Stage Peptide Diversification through Cobalt‐Catalyzed C−H Activation: Sequential Multicatalysis for Stapled Peptides

Bioorthogonal late‐stage diversification of structurally complex peptides has enormous potential for drug discovery and molecular imaging. In recent years, transition‐metal‐catalyzed C?H activation has emerged as an increasingly viable tool for peptide modification. Despite major accomplishments, these strategies largely rely on expensive palladium catalysts. We herein report an unprecedented cobalt(III)‐catalyzed peptide C?H activation, which enables the direct C?H functionalization of structurally complex peptides, and sets the stage for a multicatalytic C?H activation/alkene metathesis/hydrogenation strategy for the assembly of novel cyclic peptides.     

C（sp3）-H bond functionalization of oximes derivatives via 1,5-hydrogen atom transfer induced by iminyl radical

Oximes derivatives have been vastly used in organic synthesis. In this review, C（sp³）-H bond functionalization of oximes derivatives via iminyl radical induced 1,5-hydrogen atom transfer was discussed. According to the different type of products, this review was divided into three parts:（1） C（sp³）-H bond functionalization for C-C bond formation.（2） C（sp³）-H bond functionalization for C-N bond formation.（3） C（sp³）-H bond functionalization for C-S, C-F b...     

Silver‐Mediated Intermolecular 1,2‐Alkylarylation of Styrenes with α‐Carbonyl Alkyl Bromides and Indoles

A new iron‐facilitated silver‐mediated radical 1,2‐alkylarylation of styrenes with α‐carbonyl alkyl bromides and indoles is described, and two new C?C bonds were generated in a single step through a sequence of intermolecular C(sp³)?Br functionalization and C(sp²)?H functionalization across the alkenes. This method provides an efficient access to alkylated indoles with broad substrate scope and excellent selectivity.     

Conjugation of arginine–glycine–aspartic acid peptides to thermoreversible N‐isopropylacrylamide polymers

Thermoreversible polymeric biomaterials are finding increased acceptance in tissue engineering applications. One drawback of the polymers is their synthetic nature, which does not allow direct interaction of mammalian cells with the polymers. This limitation may be alleviated by grafting arginine–glycine–aspartic acid (RGD) containing peptides onto the polymer backbone to facilitate interactions with cell‐surface integrins. Toward this goal, N‐isopropylacrylamide (NiPAM)‐based thermoreversible polymers containing amine‐reactive N‐acryloxysuccinimide (NASI) groups were synthesized. Conjugation of RGD‐containing peptides to polymers was demonstrated with ¹H NMR spectroscopy and reverse‐phase high‐pressure liquid chromatography. The conjugation reaction was optimal at 4 °C and pH of 8.0, and increased with the increasing NASI content of polymers. With a peptide grafting ratio of 0.25 mol %, there was no significant change in the lower critical solution temperature of the polymers. Finally, the NASI‐containing polymers, cast as films, on tissue culture polystyrene, were shown to conjugate to RGD‐containing peptides and support C2C12 cell attachment. We conclude that NASI‐containing thermoreversible polymers are amenable for grafting biomimetic peptides to impart cell adhesiveness to the polymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3989–4000, 2003     

Enantioselective Copper(I)/Chiral Phosphoric Acid Catalyzed Intramolecular Amination of Allylic and Benzylic C−H Bonds

Radical‐involved enantioselective oxidative C?H bond functionalization by a hydrogen‐atom transfer (HAT) process has emerged as a promising method for accessing functionally diverse enantioenriched products, while asymmetric C(sp³)?H bond amination remains a formidable challenge. To address this problem, described herein is a dual Cu^I/chiral phosphoric acid (CPA) catalytic system for radical‐involved enantioselective intramolecular C(sp³)?H amination of not only allylic positions but also benzylic positions with broad substrate scope. The use of 4‐methoxy‐NHPI (NHPI=N‐hydroxyphthalimide) as a stable and chemoselective HAT mediator precursor is crucial for the fulfillment of this transformation. Preliminary mechanistic studies indicate that a crucial allylic or benzylic radical intermediate resulting from a HAT process is involved.     

Photoinduced Remote Functionalization of Amides and Amines Using Electrophilic Nitrogen Radicals

The selective functionalization of C(sp³)?H bonds at distal positions to functional groups is a challenging task in synthetic chemistry. Reported here is a photoinduced radical cascade strategy for the divergent functionalization of amides and protected amines. The process is based on the oxidative generation of electrophilic amidyl radicals and their subsequent transposition by 1,5‐H‐atom transfer, resulting in remote fluorination, chlorination and, for the first time, thioetherification, cyanation, and alkynylation. The process is tolerant of most common functional groups and delivers useful building blocks that can be further elaborated. The utility of this strategy is demonstrated through the late‐stage functionalization of amino acids and a dipeptide.     

Monofluoroalkenylation of Dimethylamino Compounds through Radical–Radical Cross‐Coupling

An unprecedented and challenging radical–radical cross‐coupling of α‐aminoalkyl radicals with monofluoroalkenyl radicals derived from gem‐difluoroalkenes was achieved. This first example of tandem C(sp³)?H and C(sp²)?F bond functionalization through visible‐light photoredox catalysis offers a facile and flexible access to privileged tetrasubstituted monofluoroalkenes under very mild reaction conditions. The striking features of this redox‐neutral method in terms of scope, functional‐group tolerance, and regioselectivity are illustrated by the late‐stage fluoroalkenylation of complex molecular architectures such as bioactive (+)‐diltiazem, rosiglitazone, dihydroartemisinin, oleanic acid, and androsterone derivatives, which represent important new α‐amino C?H monofluoroalkenylations.     

Dual C(sp3)−H Bond Functionalization of N‐Heterocycles through Sequential Visible‐Light Photocatalyzed Dehydrogenation/[2+2] Cycloaddition Reactions

Herein we describe a mild method for the dual C(sp³)?H bond functionalization of saturated nitrogen‐containing heterocycles through a sequential visible‐light photocatalyzed dehydrogenation/[2+2] cycloaddition procedure. As a complementary approach to the well‐established use of iminium ion and α‐amino radical intermediates, the elusive cyclic enamine intermediates were effectively generated by photoredox catalysis under mild conditions and efficiently captured by acetylene esters to form a wide array of bicyclic amino acid derivatives, thus enabling the simultaneous functionalization of two vicinal C(sp³)?H bonds.     

Functionalization of C‐H Bonds by Photoredox Catalysis

Visible‐light photoredox catalysis has been successfully used in the functionalization of inert C?H bonds including C(sp²)‐H bonds of arenes and C(sp³)‐H bonds of aliphatic compounds over the past decade. These transformations are typically promoted by the process of single‐electron‐transfer (SET) between substrates and photo‐excited photocatalyst upon visible light irradiation (household bulbs or LEDs). Compared with other synthetic strategies, such as the transition‐metal catalysis and traditional radical reactions, visible‐light photoredox approach has distinct advantages in terms of operational simplicity and practicability. Versatile direct functionalization of inert C(sp²)‐H and C(sp³)‐H bonds including alkylation, trifluoromethylation, arylation and amidation, has been achieved using this practical strategy.     

A comparison of the fragmentation pathways of [Cu(II)(Ma)(Mb)]•2+ complexes where Ma and Mb are peptides containing either a tryptophan or a tyrosine residue

[Cu^II(M_a)(M_b)]^?2+ complexes, where M_a and M_b are dipeptides or tripeptides each containing either a tryptophan (W) or tyrosine (Y) residue, have been examined by means of electrospray tandem mass spectrometry. Collision‐induced dissociations (CIDs) of complexes containing identical peptides having a tryptophan residue generated abundant radical cations of the peptides; by contrast, for complexes containing peptides having a tyrosine residue, the main fragmentation channel is dissociative proton transfer to give [M_a + H]⁺ and [Cu^II(M_b – H)]^?+. When there are two different peptides in the complex, each containing a tryptophan residue, radical cations are again the major products, with their relative abundances depending on the locations of the tryptophan residue in the peptides. In the CIDs of mixed complexes, where one peptide contains a tryptophan residue and the other a tyrosine residue, the main fragmentation channel is formation of the radical cation of the tryptophan‐containing peptide and not proton transfer from the tyrosine‐containing peptide to give a protonated peptide. Copyright © 2010 John Wiley & Sons, Ltd.     

α‐Aminoxy‐Acid‐Auxiliary‐Enabled Intermolecular Radical γ‐C(sp3)−H Functionalization of Ketones

A method for site‐specific intermolecular γ‐C(sp³)?H functionalization of ketones has been developed using an α‐aminoxy acid auxiliary applying photoredox catalysis. Regioselective activation of an inert C?H bond is achieved by 1,5‐hydrogen atom abstraction by an oxidatively generated iminyl radical. Tertiary and secondary C‐radicals thus formed at the γ‐position of the imine functionality undergo radical conjugate addition to various Michael acceptors to provide, after reduction and imine hydrolysis, the corresponding γ‐functionalized ketones.     

Sulfamate Esters Guide Selective Radical‐Mediated Chlorination of Aliphatic C−H Bonds

Masked alcohols are particularly appealing as directing groups because of the ubiquity of hydroxy groups in organic small molecules. Herein, we disclose a general strategy for aliphatic γ‐C(sp³)?H functionalization guided by a masked alcohol. Specifically, we determine that sulfamate ester derived nitrogen‐centered radicals mediate 1,6‐hydrogen‐atom transfer (HAT) processes to guide γ‐C(sp³)?H chlorination. This reaction proceeds through a light‐initiated radical chain‐propagation process and is capable of installing chlorine atoms at primary, secondary, and tertiary centers.     

A Genetically Encoded,Phage‐Displayed Cyclic‐Peptide Library

Superior to linear peptides in biological activities, cyclic peptides are considered to have great potential as therapeutic agents. To identify cyclic‐peptide ligands for therapeutic targets, phage‐displayed peptide libraries in which cyclization is achieved by the covalent conjugation of cysteines have been widely used. To resolve drawbacks related to cysteine conjugation, we have invented a phage‐display technique in which its displayed peptides are cyclized through a proximity‐driven Michael addition reaction between a cysteine and an amber‐codon‐encoded N^?‐acryloyl‐lysine (AcrK). Using a randomized 6‐mer library in which peptides were cyclized at two ends through a cysteine–AcrK linker, we demonstrated the successful selection of potent ligands for TEV protease and HDAC8. All selected cyclic peptide ligands showed 4‐ to 6‐fold stronger affinity to their protein targets than their linear counterparts. We believe this approach will find broad applications in drug discovery.