Multicomponent Reaction Toolbox for Peptide Macrocyclization and Stapling

Ligation, Macrocyclization, and Simultaneous Functionalization of Peptides by Multicomponent Reactions (MCR)

Multicomponent reactions (MCRs) are recently expanding the plethora of solid-phase protocols for the synthesis and derivatization of peptides. Herein, we describe a solid-phase-compatible strategy based on MCRs as a powerful strategy for peptide cyclization and ligation . We illustrate, using Gramicidin S as a model peptide, how the execution of on-resin Ugi reactions enables the simultaneous backbone N-functionalization and cyclization, which are important types of derivatizations in peptide-based drug development or for incorporation of conjugation handles, or labels.

Peptide Macrocyclization Through Palladium-Catalyzed Late-Stage C-H Activation

Cyclic peptides are an important class of bioactive compounds for the chemical biology and pharmaceutical industry. Chemical synthesis of highly constrained cyclic peptides is often challenging. Here we describe the synthetic strategy of peptide macrocyclization through late-stage palladium-catalyzed C-H activation. These methods utilize endogenous backbone amides in the peptide sequence as directing groups and are efficient in the preparation of small-to-middle size peptide macrocycles.

Ru-MACHO-Catalyzed Direct Inter/Intramolecular Macrocyclization of Alcohols and Ketones

Herein we describe a new approach for end-to-end cyclization to construct macrocycles through the inter/intramolecular dehydrogenative coupling of alcohols and ketones in the presence of a Ru-MACHO catalyst. This method is highly atom economical and sustainable and can be used for many substrates. Additionally, this method results in the generation of only water as the byproduct. Moreover, in this approach, high dilution of the reactants is crucial for cyclization and high-yield macrocycle synthesis.

Iodine-Mediated Tryptathionine Formation Facilitates the Synthesis of Amanitins

Synthetic methods on the macrocyclization of peptides are of high interest since they facilitate the synthesis of various types of potentially bioactive compounds, e.g. addressing targets like protein-protein-interactions. Herein, we report on an efficient method to construct tryptathionine-cross-links in peptides between the amino acids Trp and Cys. This reaction not only is the basis for the total synthesis of the death cap toxin α-amanitin but also provides rapid access to various new amanitin analogues. This study for the first time presents a systematic compilation of structure-activity relations (SAR) of amatoxins with regard to RNA polymerase II inhibition and cytotoxicity with one amanitin derivative of superior RNAP II inhibition. The present approach paves the way for the synthesis of structurally diverse amatoxins as future payloads for antibody-toxin conjugates in cancer therapy.

Solid-Phase Peptide Macrocyclization and Multifunctionalization via Dipyrrin Construction

We introduce a new and highly efficient synthetic protocol towards multifunctional fluorescent cyclopeptides by solid-phase peptide macrocyclization via dipyrrin construction, with full scope of proteinogenic amino acids and different ring sizes. Various bicyclic peptides can be created by dipyrrin-based crosslinking and double dipyrrin-ring formation. The embedded dipyrrin can be either transformed to fluorescent BODIPY and then utilized as cancer-selective targeted protein imaging probe in vitro, or directly employed as a selective metal sensor in aqueous media. This work provides a valuable addition to the peptide macrocyclization toolbox, and a blueprint for the development of multifunctional dipyrrin linkers in cyclopeptides for a wide range of potential bioapplications.

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.

Peptide Cyclization Mediated by Metal-Free S-Arylation: S-Protected Cysteine Sulfoxide as an Umpolung of the Cysteine Nucleophile

Covalent linking of side chains provides a method to produce cyclic or stapled peptides that are important in developing peptide-based drugs. A variety of crosslinking formats contribute to fixing the active conformer and prolonging its biological activity under physiological conditions. One format uses the cysteine thiol to participate in crosslinking through nucleophilic thiolate anions or thiyl radicals to form thioether and disulfide bonds. Removal of the S-protection from an S-protected Cys derivative generates the thiol, which functions as a nucleophile. S-Oxidation of a protected Cys allows the formation of a sulfoxide that operates as an umpolung electrophile. Herein, the applicability of S-p-methoxybenzyl Cys sulfoxide (Cys(MBzl)(O)) to the formation of a thioether linkage between tryptophan and Cys has been investigated. The reaction of peptides containing Cys(MBzl)(O) and Trp with trifluoromethanesulfonic acid (TFMSA) or methanesulfonic acid (MSA) in TFA in the presence of guanidine hydrochloride (Gn ⋅ HCl) proceeded to give cyclic or stapled peptides possessing the Cys-Trp thioether linkage. In this reaction, strong acids such as TFMSA or MSA are necessary to activate the sulfoxide. Additionally, Gn ⋅ HCl plays a critical role in producing an electrophilic Cys derivative that combines with the indole by aromatic electrophilic substitution. The findings led us to conclude that the less-electrophilic Cys(MBzl)(O) serves as an acid-activated umpolung of a Cys nucleophile and is useful for S-arylation-mediated peptide cyclization.

Diversity-orientated synthesis of macrocyclic heterocycles using a double SNAr approach

An efficient macrocyclisation approach based on the double aromatic nucleophilic substitution (SNACK) was developed. This methodology allows a facile incorporation of heterocyclic motifs into macrocyclic rings and rapid synthesis of a significant number of structurally diverse macrocycles. SNACK macrocyclisation enables preparation of stable diastereoisomers of conformationally restricted macrocycles (atropisomers). Practical application of SNACK macrocyclisation in a drug discovery project was exemplified by the identification of high affinity macrocyclic binders of B-cell lymphoma 6 (BCL6).

Triethylammonium Hydrogen Sulfate [Et3NH][HSO4]-Catalyzed Rapid and Efficient Multicomponent Synthesis of Pyrido[2,3-d]pyrimidine and Pyrazolo[3,4-b]pyridine Hybrids

An operationally simple, one-pot multicomponent reaction has been developed for the assembly of pyrido[2,3-d]pyrimidine and pyrazolo[3,4-b]pyridine derivatives (4a-4am) in excellent yields (92-94%) with high purity. The reactions were easy to perform simply by mixing of electron-rich amino heterocycles (including aminouracils and aminopyrazoles), aldehyde, and acyl acetonitrile in the presence of [Et3NH][HSO4] under solvent-free conditions. The remarkable feature of the present approach is that the ionic liquid possesses dual solvent-catalytic engineering capability. Results of this study revealed that 1 mmol of the ionic liquid catalyst under solvent-free conditions at 60 °C is the best reaction parameter for the construction of fused pyridine and pyrimidine derivatives in excellent yields. The present methodology showed good results under gram-scale conditions, thereby indicating its applicability in industrial as well as academic settings in the near future.

3-Substituted 2-isocyanopyridines as versatile convertible isocyanides for peptidomimetic design

We report the use of 3-substituted 2-isocyanopyridines as convertible isocyanides in Ugi four-component reactions. The N-(3-substituted pyridin-2-yl)amide Ugi products can be cleaved by amines, alcohols, and water with Zn(OAc)2 as a catalyst. In addition, the applicability of the method was demonstrated in constrained di-/tripeptides bearing acid and base sensitive protective groups obtained via Ugi-4CR post-condensation modifications.

Amide bond formation: beyond the dilemma between activation and racemisation

The development of methods for amide bond formation without recourse to typical condensation reagents has become an emerging research area and has been actively explored in the past quarter century. Inspired by the structure of vitamin B12, we have developed a metal-templated macrolactamisation that generates a new wave towards classical macrolactam synthesis. Further, distinct from the extensively used methods with condensation reagents or catalysts based on catalyst/reagent control our metal-catalysed methods based on substrate control can effectively address long-standing challenges such as racemisation in the field of peptide chemistry. In addition, the substrate-controlled strategy demonstrates the feasibility of "remote" peptide bond-forming reaction catalysed by a metal-ligand complex. Moreover, an originally designed hydrosilane/aminosilane system can avoid not only racemisation but also unnecessary waste production. This feature article documents our discovery and application of our original approaches in amide bond formation.

A comprehensive survey upon diverse and prolific applications of chitosan-based catalytic systems in one-pot multi-component synthesis of heterocyclic rings

Heterocyclic compounds are among the most prestigious and valuable chemical molecules with diverse and magnificent applications in various sciences. Due to the remarkable and numerous properties of the heterocyclic frameworks, the development of efficient and convenient synthetic methods for the preparation of such outstanding compounds is of great importance. Undoubtedly, catalysis has a conspicuous role in modern chemical synthesis and green chemistry. Therefore, when designing a chemical reaction, choosing and or preparing powerful and environmentally benign simple catalysts or complicated catalytic systems for an acceleration of the chemical reaction is a pivotal part of work for synthetic chemists. Chitosan, as a biocompatible and biodegradable pseudo-natural polysaccharide is one of the excellent choices for the preparation of suitable catalytic systems due to its unique properties. In this review paper, every effort has been made to cover all research articles in the field of one-pot synthesis of heterocyclic frameworks in the presence of chitosan-based catalytic systems, which were published roughly by the first quarter of 2020. It is hoped that this review paper can be a little help to synthetic scientists, methodologists, and catalyst designers, both on the laboratory and industrial scales.

Towards a streamlined synthesis of peptides containing α,β-dehydroamino acids

Investigation of a strategy to streamline the synthesis of peptides containing α,β-dehydroamino acids (ΔAAs) is reported. The key step involves generating the alkene moiety via elimination of a suitable precursor after it has been inserted into a peptide chain. This process obviates the need to prepare ΔAA-containing azlactone dipeptides to facilitate coupling of these residues. Z-dehydroaminobutyric acid (Z-ΔAbu) could be constructed most efficiently via EDC/CuCl-mediated dehydration of Thr. Formation of Z-ΔPhe by this or other dehydration methods was unsuccessful. Production of the bulky ΔVal residue could be accomplished by DAST-promoted dehydrations of β-OHVal or by DBU-triggered eliminations of sulfonium ions derived from penicillamine derivatives. However, competitive formation of an oxazoline byproduct remains problematic.

2-Cyanoisonicotinamide Conjugation: A Facile Approach to Generate Potent Peptide Inhibitors of the Zika Virus Protease

The rapid generation and modification of macrocyclic peptides in medicinal chemistry is an ever-growing area that can present various synthetic challenges. The reaction between N-terminal cysteine and 2-cyanoisonicotinamide is a new biocompatible click reaction that allows rapid access to macrocyclic peptides. Importantly, 2-cyanoisonicotinamide can be attached to different linkers directly during solid-phase peptide synthesis. The synthesis involves only commercially available precursors, allowing for a fully automated process. We demonstrate the approach for four cyclic peptide ligands of the Zika virus protease NS2B-NS3. Although all peptides display the substrate recognition motif, the activity strongly depends on the linker length, with the shortest cyclization linker corresponding to highest activity (K i = 0.64 μM). The most active cyclic peptide displays affinity 78 times higher than that of its linear analogue. We solved a crystal structure of the proteolytically cleaved ligand and synthesized it by applying the presented chemistry to peptide ligation.

On the scope of the double Ugi multicomponent stapling to produce helical peptides

The stabilization of helical structures by peptide stapling approaches is now a mature technology capable to provide a variety of biomedical applications. Recently, it was shown that multicomponent macrocyclization is not only an effective way to introduce conformational constraints but it also allows to incorporate additional functionalities to the staple moiety in a one-pot process. This work investigates the scope of the double Ugi multicomponent stapling approach in its capacity to produce helical peptides from unstructured sequences. For this, three different stapling combinations were implemented and the CD spectra of the cyclic peptides were measured to determine the effect of the multicomponent macrocyclization on the resulting secondary structure. A new insight into some structural factors influencing the helicity type and content is provided, along with new prospects on the utilization of this methodology to diversify the molecular tethers linking the amino acid side chains.

Palladium-catalyzed Ugi-type reaction of 2-iodoanilines with isocyanides and carboxylic acids affording N-acyl anthranilamides

The first palladium-catalyzed Ugi-type multicomponent reaction for the synthesis of N-acyl anthranilamides from isocyanides, 2-iodoanilines and carboxylic acids has been developed. This method provides expeditious and highly efficient access to structurally diverse N-acyl anthranilamides from readily available starting materials with good functional group compatibility. The utility of this method has been demonstrated by the late stage functionalization of two commercial drugs: Flurbiprofen and Loxoprofen.

Dual Functional Pd-Catalyzed Multicomponent Reaction by Umpolung Chemistry of the Oxygen Atom in Electrophiles

A Pd-catalyzed multicomponent reaction was developed by trapping oxomium ylide with nitrosobenzene via Pd-promoted umpolung chemistry. The Pd catalyst plays two important roles: diazo compound decomposed catalyst and Lewis acid for the activation of nitrosobenzene. This strategy provides some insight into a new way for discovery of multicomponent methodology to construct complex molecules. The developed method also provides rapid access to a series of O-(2-oxy) hydroxylamine derivatives, which exhibit good anticancer activity in osteosarcoma cells.

Cys-Cys and Cys-Lys Stapling of Unprotected Peptides Enabled by Hypervalent Iodine Reagents

Easy access to a wide range of structurally diverse stapled peptides is crucial for the development of inhibitors of protein-protein interactions. Herein, we report bis-functional hypervalent iodine reagents for two-component cysteine-cysteine and cysteine-lysine stapling yielding structurally diverse thioalkyne linkers. This stapling method works with unprotected natural amino acid residues and does not require pre-functionalization or metal catalysis. The products are stable to purification and isolation. Post-stapling modification can be accessed via amidation of an activated ester, or via cycloaddition onto the formed thioalkyne group. Increased helicity and binding affinity to MDM2 was obtained for a i,i+7 stapled peptide.

Peptide-based inhibitors of protein-protein interactions: biophysical, structural and cellular consequences of introducing a constraint

Protein-protein interactions (PPIs) are implicated in the majority of cellular processes by enabling and regulating the function of individual proteins. Thus, PPIs represent high-value, but challenging targets for therapeutic intervention. The development of constrained peptides represents an emerging strategy to generate peptide-based PPI inhibitors, typically mediated by α-helices. The approach can confer significant benefits including enhanced affinity, stability and cellular penetration and is ingrained in the premise that pre-organization simultaneously pays the entropic cost of binding, prevents a peptide from adopting a protease compliant β-strand conformation and shields the hydrophilic amides from the hydrophobic membrane. This conceptual blueprint for the empirical design of peptide-based PPI inhibitors is an exciting and potentially lucrative way to effect successful PPI inhibitor drug-discovery. However, a plethora of more subtle effects may arise from the introduction of a constraint that include changes to binding dynamics, the mode of recognition and molecular properties. In this review, we summarise the influence of inserting constraints on biophysical, conformational, structural and cellular behaviour across a range of constraining chemistries and targets, to highlight the tremendous success that has been achieved with constrained peptides alongside emerging design opportunities and challenges.

Multicomponent synthesis and anti-proliferative screening of biaryl triazole-containing cyclophanes

We report a practical two-step approach involving a Ugi 4-CR/ azide-alkyne cycloaddition for the synthesis of biaryl-containing cyclophanes. The series represents an extension of our previously reported macrocycles as an effort to enhance the anti-proliferative activity of this scaffold. In this variant, we incorporate a biphenyl moiety in the framework, thus enhancing the macrocycle size, lipophilicity, and structural diversity. Macrocycles were tested against different cell lines, being more cytotoxic against prostate (PC-3 and DU-145) and breast (MCF-7) tumor cells. Gratifyingly, the most active compound showed a significative enhancement of PC-3 growth inhibition with respect to our previous series, reaffirming the potential anti-proliferative activity of this kind of cyclophanes.

Ring-Opening Metathesis Polymerization of Norbornene-Based Monomers Obtained via the Passerini Three Component Reaction

Ring-opening metathesis polymerization is a robust method to synthesize a variety of polymers by using ring-strained molecules as monomers, e.g., norbornenes. However, the synthesis of monomers with multiple functional groups remains a challenge, albeit peptide functional norbornenes have previously been used. Here, the Passerini three component reaction is exploited to synthesize norbornenes with two variable functional groups varying in bulkiness and distance from the polymerizable alkene. The results indicate that the functional groups do not affect the kinetics of the polymerization, whereas the length of the linker has a minor effect. Furthermore, a diblock-type copolymer is synthesized in a one-pot fashion, also indicating good control of the polymerization process. The thermal properties of all polymers are evaluated, highlighting the effect of monomer composition. This synthetic approach can be transferred to a variety of compounds, thus promising highly diverse polymers with complex compositions and architectures.

Marine furanocembranoids-inspired macrocycles enabled by Pd-catalyzed unactivated C(sp3)-H olefination mediated by donor/donor carbenes

Biomimetic modularization and function-oriented synthesis of structurally diversified natural product-like macrocycles in a step-economical fashion is highly desirable. Inspired by marine furanocembranoids, herein, we synthesize diverse alkenes substituted furan-embedded macrolactams via a modular biomimetic assembly strategy. The success of this assembly is the development of crucial Pd-catalyzed carbene coupling between ene-yne-ketones as donor/donor carbene precursors and unactivated Csp³‒H bonds which represents a great challenge in organic synthesis. Notably, this method not only obviates the use of unstable, explosive, and toxic diazo compounds, but also can be amenable to allenyl ketones carbene precursors. DFT calculations demonstrate that a formal 1,4-Pd shift could be involved in the mechanism. Moreover, the collected furanocembranoids-like macrolactams show significant anti-inflammatory activities against TNF-α, IL-6, and IL-1β and the cytotoxicity is comparable to Dexamethasone.

Furanocembranoid-like natural products with the alkene-substituted furan scaffold display a range of biological activities, but are difficult to access. Here, the authors report a modular biomimetic strategy to synthesise diverse alkene-substituted furan-containing macrolactams via palladium-catalysed unactivated Csp³-H olefination.

Synthetic enzyme-catalyzed multicomponent reaction for Isoxazol-5(4H)-one Syntheses, their properties and biological application; why should one study mechanisms?

In this work, we describe the application of a synthetic enzyme (synzyme) as the catalyst to promote the multicomponent synthesis of isoxazol-5(4H)-one derivatives. The catalytic system could be used up to 15 times without any notable loss of its activity. Some derivatives showed fluorescence and their photophysical data were evaluated. The mechanism of the reaction was, for the first time, investigated and, among the three reaction pathway possibilities, only one was operating under the developed conditions. ESI-MS(/MS) allowed for both the simultaneous monitoring of the multicomponent reaction (MCR) and the proposition of a kinetic model to explain the transformation. The kinetic model pointed firmly to only one reaction pathway and helped to discard the other two possibilities. The antimicrobial abilities of all synthesized derivatives against Gram-positive and Gram-negative strains were also evaluated. The abilities of functional chromophores (fluorescent compounds) as live cell-imaging probes were verified and one of the multicomponent adducts could stain early endosomes selectively in bioimaging experiments.

How and Why to Investigate Multicomponent Reactions Mechanisms? A Critical Review

We review the most innovative efforts and greatest challenges faced when elucidating multicomponent reactions (MCRs) mechanisms. When compared to traditional reactions, the often two or more concurrent reactions pathways and the greater number of possible intermediates in MCRs turn their mechanistic investigation both a harder and trickier task. The common approaches used to investigate reaction mechanisms are often unable to clarify MCRs mechanisms; hence few but clever approaches are currently used to determine these mechanisms and to depict their key transformations. Their complexity has required most innovative approaches and the use of a number of unique techniques that have shed light over the favored pathway selected from the myriad of alternatives theoretically available for MCRs. This review focuses on the most successful efforts applied by a few leading groups to perform these puzzlingly investigations.

Small and Simple, yet Sturdy: Conformationally Constrained Peptides with Remarkable Properties

The sheer size and vast chemical space (i.e., diverse repertoire and spatial distribution of functional groups) underlie peptides’ ability to engage in specific interactions with targets of various structures. However, the inherent flexibility of the peptide chain negatively affects binding affinity and metabolic stability, thereby severely limiting the use of peptides as medicines. Imposing conformational constraints to the peptide chain offers to solve these problems but typically requires laborious structure optimization. Alternatively, libraries of constrained peptides with randomized modules can be screened for specific functions. Here, we present the properties of conformationally constrained peptides and review rigidification chemistries/strategies, as well as synthetic and enzymatic methods of producing macrocyclic peptides. Furthermore, we discuss the in vitro molecular evolution methods for the development of constrained peptides with pre-defined functions. Finally, we briefly present applications of selected constrained peptides to illustrate their exceptional properties as drug candidates, molecular recognition probes, and minimalist catalysts.

Polymer Beads Decorated with Dendritic Systems as Supports for A3 Coupling Catalysts

Abstract

The gel type microscopic polymer beads bearing epoxy functionalities were modified using the two-stage procedures in order to decorate their surface with the moieties of the zeroth order PAMAM type dendrimer and different heterocyclic aldehydes (2-pyridinecarboxaldehyde, 2-pyrrolidinecarboxaldehyde, furfural or 2-thiophenecarboxaldehyde). The polymeric supports provided in this manner were then used for the immobilization of copper(II) ions. The resulting materials were characterized using different instrumental techniques (optical microscopy, SEM, FTIR microscopy, DR UV–Vis, ICP-OES, and thermal analysis). They were also used as catalysts in the model A3 coupling reaction of benzaldehyde, morpholine and phenylacetylene. The best catalytic activity was found for the polymeric catalyst bearing 2-pyridinecarboxaldehyde moieties. It turned out to be effective in the A3 coupling reactions included different benzaldehyde, alkyne, and secondary amine derivatives, as well. It could also be recycled several times without a significant decrease in its activity in the model A3 coupling reaction.

Graphic Abstract

On-resin multicomponent protocols for biopolymer assembly and derivatization

Solid-phase synthesis represents the methodological showcase for technological advances such as split-and-pool combinatorial chemistry and the automated synthesis of peptides, nucleic acids and polysaccharides. These strategies involve iterative coupling cycles that do not generate functional diversity besides that incorporated by the amino acids, nucleosides and monosaccharide building blocks. In sharp contrast, multicomponent reactions (MCRs) are traditionally used to generate both skeletal and appendage diversity in short, batchwise procedures. On-resin MCRs have traditionally been employed for the construction of heterocycle and peptidomimetic libraries, but that scenario has changed recently, and today the focus is more on the solid-phase derivatization of peptides and oligonucleotides. This review presents relevant experimental details and addresses the synthetic scope of such on-resin multicomponent protocols employed to accomplish specific biopolymer covalent modifications that are practically inviable by traditional solution-phase methodologies. Recommendations are provided to facilitate the implementation of solid-supported protocols and avoid possible pitfalls associated with the selection of the polymeric resin, the solvent and the order and amount of the reagents employed. We describe procedures comprising the multicomponent lipidation, biotinylation and labeling of both termini and the side chains, as well as the use of MCRs in the traceless on-resin synthesis of ligated and cyclic peptides. Solid-phase protocols for the assembly of α-helical and parallel β-sheet peptides as well as hybrid peptide-peptoid and peptide-peptide nucleic acid architectures are described. Finally, the solid-supported multicomponent derivatization of DNA oligonucleotides is illustrated as part of the DNA-encoded library technology relying on MCR-derived heterocyclic compounds.

Strategies to expand peptide functionality through hybridisation with a small molecule component

Combining different compound classes gives molecular hybrids that can offer access to novel chemical space and unique properties. Peptides provide ideal starting points for such molecular hybrids, which can be easily modified with a variety of molecular entities. The addition of small molecules can improve the potency, stability and cell permeability of therapeutically relevant peptides. Furthermore, they are often applied to create peptide-based tools in chemical biology. In this review, we discuss general methods that allow the discovery of this compound class and highlight key examples of peptide-small molecule hybrids categorised by the application and function of the small molecule entity.

Modulating Protein-Protein Interactions by Cyclic and Macrocyclic Peptides. Prominent Strategies and Examples

Cyclic and macrocyclic peptides constitute advanced molecules for modulating protein-protein interactions (PPIs). Although still peptide derivatives, they are metabolically more stable than linear counterparts, and should have a lower degree of flexibility, with more defined secondary structure conformations that can be adapted to imitate protein interfaces. In this review, we analyze recent progress on the main methods to access cyclic/macrocyclic peptide derivatives, with emphasis in a few selected examples designed to interfere within PPIs. These types of peptides can be from natural origin, or prepared by biochemical or synthetic methodologies, and their design could be aided by computational approaches. Some advances to facilitate the permeability of these quite big molecules by conjugation with cell penetrating peptides, and the incorporation of β-amino acid and peptoid structures to improve metabolic stability, are also commented. It is predicted that this field of research could have an important future mission, running in parallel to the discovery of new, relevant PPIs involved in pathological processes.

Photochemical Methods for Peptide Macrocyclisation

Photochemical reactions have been the subject of renewed interest over the last two decades, leading to the development of many new, diverse and powerful chemical transformations. More recently, these developments have been expanded to enable the photochemical macrocyclisation of peptides and small proteins. These constructs benefit from increased stability, structural rigidity and biological potency over their linear counterparts, providing opportunities for improved therapeutic agents. In this review, an overview of both the established and emerging methods for photochemical peptide macrocyclisation is presented, highlighting both the limitations and opportunities for further innovation in the field.

Structure-Acid Lability Relationship of N-Alkylated α,α-Dialkylglycine Obtained via a Ugi Multicomponent Reaction

Using the classical Ugi four-component reaction to fuse an amine, ketone, carboxylic acid, and isocyanide, here we prepared a short library of N-alkylated α,α-dialkylglycine derivatives. Due to the polyfunctionality of the dipeptidic scaffold, this highly steric hindered system shows an interesting acidolytic cleavage of the C-terminal amide. In this regard, we studied the structure-acid lability relationship of the C-terminal amide bond (cyclohexylamide) of N-alkylated α,α-dialkylglycine amides 1a–n in acidic media and, afterward, it was established that the most important structural features related to its cleavage. Then, it was demonstrated that electron-donating effects in the aromatic amines, flexible acyl chains (Gly) at the N-terminal and the introduction of cyclic compounds into dipeptide scaffolds, increased the rate of acidolysis. All these effects are related to the ease with which the oxazolonium ion intermediate forms and they promote the proximity of the central carbonyl group to the C-terminal amide, resulting in C-terminal amide cleavage. Consequently, these findings could be applied for the design of new protecting groups, handles for solid-phase synthesis, and linkers for conjugation, due to its easily modulable and the fact that it allows to fine tune its acid-lability.

Synthesis of trisubstituted hydroxylamines by a visible light-promoted multicomponent reaction

A green and efficient route for the synthesis of trisubstituted hydroxylamines from β-keto ester, 2-nitrosopyridine and aryldiazoacetates has been reported. This multicomponent reaction occurred under mild conditions without catalysts or additives.

Highly diastereoselective synthesis of vicinal diamines via a Rh-catalyzed three-component reaction of diazo compounds with diarylmethanimines and ketimines

A Rh-catalyzed selective three-component reaction of diazo compounds with diarylmethanimines and ketimines is reported that offers an efficient and convenient access to vicinal diamine derivatives with two tertiary stereocenters in high yields.

Rh-Catalyzed C-H alkylation enabling modular synthesis of CF3-substituted benzannulated macrocyclic inhibitors of B cell responses

Inspired by aspirin and chalcone, herein, we describe a modular biomimetic strategy to achieve a new class of CF3-bearing benzannulated macrolactams. The key to the success of macrolactams was the utilization of a highly chemoselective Rh(iii)-catalyzed native carboxylic acid-directed C-H alkylation. Moreover, the unique CF3-containing benzannulated macrocycles showed decent immunosuppressive effects on B cells in vitro, including proliferation, activation, and antibody production upon specific stimulation implicating TLR and BCR signaling.

Benzene-1,3-diol derivatives as the inhibitors of butyrylcholinesterase: An emergent target of Alzheimer’s disease

Molecular docking is a powerful and significant approach for the identification of lead molecules on the basis of virtual screening. With it a large number of compounds can be tested and based on the scoring function and ranking, the conclusion can be made about how the selected compounds can inhibit the targeted protein/receptor. Considering the importance of selective inhibitors of cholinesterase in the treatment of Alzheimer disease, this research is focused on the determination of the mechanism of binding interactions of few benzene-1,3-diol derivatives within the active site of both acetyl-choline-sterase (AChE) and butyrylcholinesterase (BChE). All the selective ligands were found to have a greater binding affinity with the BChE when compared to that of AChE, by an average value of ~?28.4 and ~?12.5 kJ/mol, respectively. The results suggested that the identified inhibitors can be used as the lead com-pounds for the development of novel inhibitors of the targeted enzymes against some specific diseases, thus opening the possibility of new therapeutic strategies.

Heck macrocyclization in natural product total synthesis

Covering: 1981-2020 Heck macrocyclization is a logical extension of the award-winning Mizoroki-Heck reaction. Through covalent linking of two otherwise discrete coupling partners, the resultant chimeric substrate is transformed into a large ring with enhanced rigidity and unique functional group disposition. Pioneered in the early 1980s, this methodology has evolved into a competent option for creating diverse macrocycles. Despite its growing influence, hitherto no systematic survey has ever appeared in the literature. The present review delineates the state-of-the-art of Heck macrocyclization in the context of natural product synthesis. Sixteen selected cases, each examined from a different perspective, coalesce into the view that the title reaction is a viable tool for synthesis-enabled macrocycle research.

Cyanobacteria and Eukaryotic Microalgae as Emerging Sources of Antibacterial Peptides

Cyanobacteria and microalgae are oxygen-producing photosynthetic unicellular organisms encompassing a great diversity of species, which are able to grow under all types of extreme environments and exposed to a wide variety of predators and microbial pathogens. The antibacterial compounds described for these organisms include alkaloids, fatty acids, indoles, macrolides, peptides, phenols, pigments and terpenes, among others. This review presents an overview of antibacterial peptides isolated from cyanobacteria and microalgae, as well as their synergism and mechanisms of action described so far. Antibacterial cyanopeptides belong to different orders, but mainly from Oscillatoriales and Nostocales. Cyanopeptides have different structures but are mainly cyclic peptides. This vast peptide repertoire includes ribosomal and abundant non-ribosomal peptides, evaluated by standard conventional methodologies against pathogenic Gram-negative and Gram-positive bacteria. The antibacterial activity described for microalgal peptides is considerably scarcer, and limited to protein hydrolysates from two Chlorella species, and few peptides from Tetraselmis suecica. Despite the promising applications of antibacterial peptides and the importance of searching for new natural sources of antibiotics, limitations still persist for their pharmaceutical applications.

Peptide macrocyclization by transition metal catalysis

Peptide macrocyclization has traditionally relied on lactam, lactone and disulfide bond-forming reactions that aim at introducing conformational constraints into small peptide sequences. With the advent of ruthenium-catalyzed ring-closing metathesis and copper-catalyzed alkyne-azide cycloaddition, peptide chemists embraced transition metal catalysis as a powerful macrocyclization tool with relevant applications in chemical biological and peptide drug discovery. This article provides a comprehensive overview of the reactivity and methodological diversification of metal-catalyzed peptide macrocyclization as a special class of late-stage peptide derivatization method. We report the evolution from classic palladium-catalyzed cross-coupling approaches to more modern oxidative versions based on C-H activation, heteroatom alkylation/arylation and annulation processes, in which aspects such as chemoselectivity and diversity generation at the ring-closing moiety became dominant over the last years. The transit from early cycloadditions and alkyne couplings as ring-closing steps to very recent 3d metal-catalyzed macrocyclization methods is highlighted. Similarly, the new trends in decarboxylative radical macrocyclizations and the interplay between photoredox and transition metal catalysis are included. This review charts future perspectives in the field hoping to encourage further progress and applications, while bringing attention to the countless possibilities available by diversifying not only the metal, but also the reactivity modes and tactics to bring peptide functional groups together and produce structurally diverse macrocycles.