Ring-closing metathesis in peptides

Peptide macrocyclisation via intramolecular interception of visible-light-mediated desulfurisation

Synthetic methods that enable the macrocyclisation of peptides facilitate the development of effective therapeutic and diagnostic tools. Herein we report a peptide cyclisation strategy based on intramolecular interception of visible-light-mediated cysteine desulfurisation. This method allows cyclisation of unprotected peptides in an aqueous solution via the installation of a hydrocarbon linkage. We explore the limits of this chemistry using a range of model peptides of increasing length and complexity, including peptides of biological/therapeutic relevance. The method is applied to replace the native disulfide of the peptide hormone, oxytocin, with a proteolytically/redox-stable hydrocarbon, and internal macrocyclisation of an MCL-1-binding peptide.

Peptide Carbocycles: From -SS- to -CC- via a Late-Stage "Snip-and-Stitch"

One way to improve the therapeutic potential of peptides is through cyclization. This is commonly done using a disulfide bond between two cysteine residues in the peptide. However, disulfide bonds are susceptible to reductive cleavage, and this can deactivate the peptide and endanger endogenous proteins through covalent modification. Substituting disulfide bonds with more chemically robust carbon-based linkers has proven to be an effective strategy to better develop cyclic peptides as drugs, but finding the optimal carbon replacement is synthetically laborious. We report a new late-stage platform wherein a single disulfide bond in a cyclic peptide can serve as the progenitor for any number of new carbon-rich groups, derived from organodiiodides, using a Zn:Cu couple and a hydrosilane. We show that this platform can furnish entirely new carbocyclic scaffolds with enhanced permeability and structural integrity and that the stereochemistry of the new cycles can be biased by a judicious choice in silane.

A study on selective transformation of norbornadiene into fluorinated cyclopentane-fused isoxazolines

This work presents an examination of the selective functionalization of norbornadiene through nitrile oxide 1,3-dipolar cycloaddition/ring-opening metathesis (ROM)/cross-metathesis (CM) protocols. Functionalization of commercially available norbornadiene provided novel bicyclic scaffolds with multiple stereogenic centers. The synthesis involved selective cycloadditions, with subsequent ROM of the formed cycloalkene-fused isoxazoline scaffolds and selective CM by chemodifferentiation of the olefin bonds of the resulting alkenylated derivatives. Various experimental conditions were applied for the CM transformations with the goal of exploring substrate and steric effects, catalyst influence and chemodifferentiation of the olefin bonds furnishing the corresponding functionalized, fluorine-containing isoxazoline derivatives.

Macrocyclization strategies for cyclic peptides and peptidomimetics

Peptides are a growing therapeutic class due to their unique spatial characteristics that can target traditionally "undruggable" protein-protein interactions and surfaces. Despite their advantages, peptides must overcome several key shortcomings to be considered as drug leads, including their high conformational flexibility and susceptibility to proteolytic cleavage. As a general approach for overcoming these challenges, macrocyclization of a linear peptide can usually improve these characteristics. Their synthetic accessibility makes peptide macrocycles very attractive, though traditional synthetic methods for macrocyclization can be challenging for peptides, especially for head-to-tail cyclization. This review provides an updated summary of the available macrocyclization chemistries, such as traditional lactam formation, azide-alkyne cycloadditions, ring-closing metathesis as well as unconventional cyclization reactions, and it is structured according to the obtained functional groups. Keeping peptide chemistry and screening in mind, the focus is given to reactions applicable in solution, on solid supports, and compatible with contemporary screening methods.

Discovery and characterization of novel peptide inhibitors of the NRF2/MAFG/DNA ternary complex for the treatment of cancer

Pathway activating mutations of the transcription factor NRF2 and its negative regulator KEAP1 are strongly correlative with poor clinical outcome with pemetrexed/carbo(cis)platin/pembrolizumab (PCP) chemo-immunotherapy in lung cancer. Despite the strong genetic support and therapeutic potential for a NRF2 transcriptional inhibitor, currently there are no known direct inhibitors of the NRF2 protein or its complexes with MAF and/or DNA. Herein we describe the design of a novel and high-confidence homology model to guide a medicinal chemistry effort that resulted in the discovery of a series of peptides that demonstrate high affinity, selective binding to the Antioxidant Response Element (ARE) DNA and thereby displace NRF2-MAFG from its promoter, which is an inhibitory mechanism that to our knowledge has not been previously described. In addition to their activity in electrophoretic mobility shift (EMSA) and TR-FRET-based assays, we show significant dose-dependent ternary complex disruption of NRF2-MAFG binding to DNA by SPR, as well as cellular target engagement by thermal destabilization of HiBiT-tagged NRF2 in the NCI-H1944 NSCLC cell line upon digitonin permeabilization, and SAR studies leading to improved cellular stability. We report the characterization and unique profile of lead peptide 18, which we believe to be a useful in vitro tool to probe NRF2 biology in cancer cell lines and models, while also serving as an excellent starting point for additional in vivo optimization toward inhibition of NRF2-driven transcription to address a significant unmet medical need in non-small cell lung cancer (NSCLC).

Alkyne-Bridged α-Conotoxin Vc1.1 Potently Reverses Mechanical Allodynia in Neuropathic Pain Models

Several Conus-derived venom peptides are promising lead compounds for the management of neuropathic pain, with α-conotoxins being of particular interest. Modification of the interlocked disulfide framework of α-conotoxin Vc1.1 has been achieved using on-resin alkyne metathesis. Although introduction of a metabolically stable alkyne motif significantly disrupts backbone topography, the structural modification generates a potent and selective GABAB receptor agonist that inhibits Cav2.2 channels and exhibits dose-dependent reversal of mechanical allodynia in a behavioral rat model of neuropathic pain. The findings herein support the hypothesis that analgesia can be achieved via activation of GABABRs expressed in dorsal root ganglion (DRG) sensory neurons.

α-Conotoxin Peptidomimetics: Probing the Minimal Binding Motif for Effective Analgesia

Several analgesic α-conotoxins have been isolated from marine cone snails. Structural modification of native peptides has provided potent and selective analogues for two of its known biological targets-nicotinic acetylcholine and γ-aminobutyric acid (GABA) G protein-coupled (GABAB) receptors. Both of these molecular targets are implicated in pain pathways. Despite their small size, an incomplete understanding of the structure-activity relationship of α-conotoxins at each of these targets has hampered the development of therapeutic leads. This review scrutinises the N-terminal domain of the α-conotoxin family of peptides, a region defined by an invariant disulfide bridge, a turn-inducing proline residue and multiple polar sidechain residues, and focusses on structural features that provide analgesia through inhibition of high-voltage-activated Ca2+ channels. Elucidating the bioactive conformation of this region of these peptides may hold the key to discovering potent drugs for the unmet management of debilitating chronic pain associated with a wide range of medical conditions.

Catalytic Asymmetric Access to Noncanonical Chiral α-Amino Acids from Cyclic Iminoglyoxylates and Enamides

Here we describe an enantioselective Mannich reaction of cyclic iminoglyoxylates with enamides by virtue of chiral phosphoric acid catalysis in a one-pot manner. The wide substrate scope, mild reaction conditions, and constantly excellent enantioselectivities (>95% ee in most cases) render this protocol highly practical for the rapid construction of valuable noncanonical chiral α-amino-acid building blocks.

Modification of Proteins Using Olefin Metathesis

Olefin metathesis has revolutionized synthetic approaches to carbon-carbon bond formation. With a rich history beginning in industrial settings through its advancement in academic laboratories leading to new and highly active metathesis catalysts, olefin metathesis has found use in the generation of complex natural products, the cyclization of bioactive materials, and in the polymerization of new and unique polymer architectures. Throughout this review, we will trace the deployment of olefin metathesis-based strategies for the modification of proteins, a process which has been facilitated by the extensive development of stable, isolable, and highly active transition-metal-based metathesis catalysts. We first begin by summarizing early works which detail peptide modification strategies that played a vital role in identifying stable metathesis catalysts. We then delve into protein modification using cross metathesis and finish with recent work on the generation of protein-polymer conjugates through ring-opening metathesis polymerization.

Solid-phase synthesis of biaryl bicyclic peptides containing a 3-aryltyrosine or a 4-arylphenylalanine moiety

A methodology for the solid-phase synthesis of biaryl bicyclic peptides containing a Phe-Phe, a Phe-Tyr or a Tyr-Tyr motif has been devised. This approach comprises two key steps. The first one involves the cyclization of a linear peptidyl resin containing the corresponding halo- and boronoamino acids via a microwave-assisted Suzuki–Miyaura cross coupling. This step is followed by the macrolactamization of the resulting biaryl monocyclic peptidyl resin leading to the formation of the expected biaryl bicyclic peptide. This study provides the first solid-phase synthesis of this type of bicyclic compounds being amenable to prepare a diversity of synthetic or natural biaryl bicyclic peptides.

Ring-closing metathesis of unprotected peptides in water

Ring-closing metathesis (RCM) is an attractive reaction for the preparation of artificially designed peptides. Until now, RCM has been used for fully or partially protected peptides. Herein, the first RCM of unprotected peptides in water was achieved using a water-soluble Ru catalyst.

Total synthesis and cyclization strategy of samoamide A, a cytotoxic cyclic octapeptide rich in proline and phenlalanine isolated from marine cyanobacterium

Samoamide A is a cyclic octapeptide rich in proline and phenylalanine residues isolated from an American Samoa marine cyanobacterium, which exhibits potent activity against H460 human non-small-cell lung cancer cells (IC₅₀ of 1.1 μM). The first total synthesis of samoamide A was achieved by employing a strategy of a solid-phase peptide synthesis (SPPS) and a head-to-tail cyclization selecting free steric-hinrance connection sites. Then the final product was purified and identified. This strategy not only provides a basis in producing potent cytotoxic agents for drug discovery, but also provides a reference to the total synthesis of proline-rich peptides.

Mechanism and chemoselectivity origins of bioconjugation of cysteine with Au(iii)-aryl reagents

A detailed computational study is presented on the reaction mechanism of selective cysteine S-arylation by cationic Au(iii)-aryl reagents. The chemoselectivity origins have been elucidated through comparison with potential N- and O-arylation, showing that the acidity and nucleophilicity of the residue are two inherent controlling factors.

Ring closing metathesis of unprotected peptides

An efficient and expedient route to the synthesis of dicarba peptides from protecting group-free sequences is reported using Ru-alkylidene catalysed olefin metathesis. A range of cyclic peptides was prepared from linear peptides containing two Z-crotyl glycine residues. Free amine groups were masked as salts with Brønsted acids preventing in situ catalyst decomposition. Excellent RCM conversion was obtained in both DMF and methanol.

Natural products and ring-closing metathesis: synthesis of sterically congested olefins

This review highlights RCM reactions towards the synthesis of sterically congested natural products throughout the recent evolution of catalysts.

Recent advances in the synthesis of cyclic pseudopeptides

Constrained peptides pose tremendous value in drug discovery. For example, owing to their large surface areas, they offer novel ways at inhibiting protein-protein interactions. As this field has grown, it has become desirable to introduce non-peptidic functionality into these rings to enable differentiated structure activity relationships and improved pharmacokinetic properties. Recent advances in the synthesis of cyclic pseudopeptides include macrocyclization through cysteine alkylation, multicomponent reactions, decarboxylative couplings, and novel stapling chemistry.

Organometallic chemical biology: an organometallic approach to bioconjugation

This review summarizes the history and recent developments of the field of organometallic chemical biology with a particular emphasis on the development of novel bioconjugation approaches. Over the years, numerous transformations have emerged for biomolecule modification with the use of organometallic reagents; these include [3+2] cycloadditions, C–C, C–S, C–N, and C–O bond forming processes, as well as metal-mediated deprotection (“decaging”) reactions. These conceptually new additions to the chemical biology toolkit highlight the potential of organometallic chemistry to make a significant impact in the field of chemical biology by providing further opportunities for the development of chemoselective, site-specific and spatially resolved methods for biomolecule structure and function manipulation. Examples of these transformations, as well as existing challenges and future prospects of this rapidly developing field are highlighted in this review.

The Role of Disulfide Bond Replacements in Analogues of the Tarantula Toxin ProTx-II and Their Effects on Inhibition of the Voltage-Gated Sodium Ion Channel Nav1.7

Spider venom toxins, such as Protoxin-II (ProTx-II), have recently received much attention as selective Na_v1.7 channel blockers, with potential to be developed as leads for the treatment of chronic nocioceptive pain. ProTx-II is a 30-amino acid peptide with three disulfide bonds that has been reported to adopt a well-defined inhibitory cystine knot (ICK) scaffold structure. Potential drawbacks with such peptides include poor pharmacodynamics and potential scrambling of the disulfide bonds in vivo. In order to address these issues, in the present study we report the solid-phase synthesis of lanthionine-bridged analogues of ProTx-II, in which one of the three disulfide bridges is replaced with a thioether linkage, and evaluate the biological properties of these analogues. We have also investigated the folding and disulfide bridging patterns arising from different methods of oxidation of the linear peptide precursor. Finally, we report the X-ray crystal structure of ProTx-II to atomic resolution; to our knowledge this is the first crystal structure of an ICK spider venom peptide not bound to a substrate.

Ring Rearrangement Metathesis in 7-Oxabicyclo[2.2.1]heptene (7-Oxanorbornene) Derivatives. Some Applications in Natural Product Chemistry

Metathesis reactions is firmly established as a valuable synthetic tool in organic chemistry, clearly comparable with the venerable Diels-Alder and Wittig reactions and, more recently, with the metal-catalyzed cross-coupling reactions. Metathesis reactions can be considered as a fascinating synthetic methodology, allowing different variants regarding substrate (alkene and alkyne metathesis) and type of metathetical reactions. On the other hand, tandem metathesis reactions such Ring Rearrangement Metathesis (RRM) and the coupling of metathesis reaction with other reactions of alkenes such as Diels-Alder or Heck reactions, makes metathesis one of the most powerful and reliable synthetic procedure.

In particular, Ring-Rearrangement Metathesis (RRM) refers to the combination of several metathesis transformations into a domino process such as ring-opening metathesis (ROM)/ring-closing metathesis (RCM) and ROM-cross metathesis (CM) in a one-pot operation. RRM delivers complex frameworks that are difficult to assemble by conventional methods constitutingan atom economic process. RRM is applicable to mono- and polycyclic systems of varying ring sizes such as cyclopropene, cyclobutene, cyclopentene, cyclohexene, pyran systems, bicyclo[2.2.1]heptene derivatives, bicyclo[2.2.2]octene derivatives, bicyclo[3.2.1]octene derivatives and bicyclo[3.2.1]octene derivatives.

In this review our attention has focused on the RRM reactions in 7-oxabicyclo[2.2.1]heptene derivatives and on their application in the synthesis of natural products or significant subunits of them.