Achieving Skeletal Diversity in Peptide Macrocycles through The Use of Heterocyclic Grafts

Unnatural Cyclopeptide Synthesis via Cu-Catalyzed 1,3-Dipolar Cycloaddition of Azomethine Ylides

Cyclic peptides are valued synthetic targets in organic and medicinal chemistry. Herein, we report an efficient strategy for the synthesis of unnatural cyclic peptides via the Cu-catalyzed 1,3-dipolar cycloaddition of azomethylene ylides. Linear precursors of different lengths and bearing diverse amino acids (26 examples) are shown to be compatible with this method, affording good yields and complete endo-diastereoselectivities. Density functional theory (DFT) calculations support a stepwise mechanism in which Cu plays a key role in the preorganization of the reactants.

Identification of "Structural Pin" Interactions and their Significance for the Conformational Control of Macrocyclic Scaffolds

While peptide macrocycles with rigidified conformations have proven to be useful in the design of chemical probes of protein targets, conformational flexibility and rapid interconversion can be equally vital for biological activity and favorable physicochemical properties. This study introduces the concept of "structural pin", which describes a hydrogen bond that is largely responsible for stabilizing the entire macrocycle backbone conformation. Structural analysis of macrocycles using nuclear magnetic resonance (NMR), molecular modelling and X-ray diffraction indicates that disruption of the structural pin can drastically influence the conformation of the entire ring, resulting in novel states with increased flexibility. This finding provides a new tool to interrogate dynamic behaviour of macrocycles. Identification of structural pins offers a useful conceptual framework to understand positions that can either be modified to give flexible structures or retained to maintain the rigidity of the scaffold.

General Installation of (4H)-Imidazolone cis-Amide Bioisosteres Along the Peptide Backbone

Imidazolones represent an important class of heterocycles present in a wide range of pharmaceuticals, metabolites, and bioactive natural products and serve as the active chromophore in green fluorescent protein. Recently, imidazolones have received attention for their ability to act as a nonaromatic amide bond bioisotere which improves pharmacological properties. Herein, we present a tandem amidine installation and cyclization with an adjacent ester to yield (4H)-imidazolone products. Using amino acid building blocks, we can access the first examples of α-chiral imidazolones that have been previously inaccessible. Additionally, our method is amenable to on-resin installation which can be seamlessly integrated into existing solid-phase peptide synthesis protocols. Finally, we show that peptide imidazolones are potent cis-amide bond surrogates that preorganize linear peptides for head-to-tail macrocyclization. This work represents the first general approach to the backbone and side-chain insertion of imidazolone bioisosteres at various positions in linear and cyclic peptides.

Thioimidates provide general access to thioamide, amidine, and imidazolone peptide-bond isosteres

Thioamides, amidines, and heterocycles are three classes of modifications that can act as peptide-bond isosteres to alter the peptide backbone. Thioimidate protecting groups can address many of the problematic synthetic issues surrounding installation of these groups. Historically, amidines have received little attention in peptides due to limitations in methods to access them. The first robust and general procedure for the introduction of amidines into peptide backbones exploits the utility of thioimidate protecting groups as a means to side-step reactivity that ultimately renders existing methods unsuitable for the installation of amidines along the main-chain of peptides. Further, amidines formed on-resin can be reacted to form (4H)-imidazolone heteorcycles which have recently been shown to act as cis-amide isosteres. General methods for heterocyclic installation capable of geometrically restricting peptide conformation are also under-developed. This work is significant because it describes a generally applicable and divergent approach to access unexplored peptide designs and architectures.

Synthesis of α-Aryl Secondary Amides via Nickel-Catalyzed Reductive Coupling of Redox-Active Esters

The transition-metal-catalyzed α-arylation of secondary amides remains a synthetic challenge due to the presence of a free N-H bond. We report a strategy to synthesize secondary α-aryl amides via a Ni-catalyzed reductive arylation of redox-active N-hydroxyphthalimide (NHP) esters of malonic acid half amides. This transformation proceeds under mild conditions and displays excellent chemoselectivity for amide α-arylation in the presence of other enolizable carbonyls. The NHP ester substrates are readily prepared from Meldrum's acid.

Divergent, C-C Bond Forming Macrocyclizations Using Modular Sulfonylhydrazone and Derived Substrates

A divergent approach to C-C bond forming macrocycle construction is described. Modular sulfonylhydrazone and derived pyridotriazole substrates with three key building blocks have been constructed and cyclized to afford diverse macrocyclic frameworks. Broad substrate scope and functional group tolerance have been demonstrated. In addition, site-selective postfunctionalization allowed for further diversification of macrocyclic cores.

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).

Diversity-Oriented A3-Macrocyclization for Studying Influences of Ring-Size and Shape of Cyclic Peptides: CD36 Receptor Modulators

Cyclic peptide diversity has been broadened by elaborating the A³-macrocyclization to include various di-amino carboxylate components with different N^ε-amine substituents. Triple-bond reduction provided new cyclic peptide macrocycles with Z-olefin and completely saturated structures. Moreover, cyclic azasulfurylpeptides were prepared by exchanging the propargylglycine (Pra) component for an amino sulfamide surrogate. Examination of such diversity-oriented methods on potent cyclic azapeptide modulators of the cluster of differentiation 36 receptor (CD36) identified the importance of the triple bond as well as the N^ε-allyl lysine and azaPra residues for high CD36 binding affinity. Cyclic azapeptides which engaged CD36 effectively reduced pro-inflammatory nitric oxide and downstream cytokine and chemokine production in macrophages stimulated with a Toll-like receptor-2 agonist. Studying the triple bond and amine components in the multiple-component A³-macrocyclization has given a diverse array of macrocycles and pertinent information to guide the development of ideal CD36 modulators with biomedical potential for curbing macrophage-driven inflammation.

One Reacts as Two: Applications of N-Isocyaniminotriphenylphosphorane in Diversity-Oriented Synthesis

N-Isocyaniminotriphenylphosphorane (NIITP) is a functionalized isonitrile that has been extensively applied in a variety of organic reactions during the last two decades. This Review summarizes the most important applications in organic synthesis of this versatile reactant, with the focus posed on mechanistic and methodological aspects allowing the generation of molecular diversity. NIITP combines the reactivity of isonitriles with that of phosphoranes to enable chemical transformations employed in the construction of compound libraries. Here, we cover from the initial applications of NIITP in the Nef isocyanide reaction to further derivations that render a variety of heterocyclic scaffolds. The presence of the isonitrile moiety in this singular compound makes possible the double addition of nucleophiles and electrophiles, which followed by inter(intra)molecular aza-Wittig type transformations enable several multicomponent and tandem processes. In particular, we stress the impact of NIITP in oxadiazole chemistry, from the early two-component transformations to recent examples of multicomponent reactions that take place in the presence of suitable electrophiles. In addition, we briefly describe the role of NIITP chemistry in generating skeletal and conformational diversity in cyclic peptides. The reaction of NIITP with alkynes is thoroughly revised, with particular emphasis on silver-catalyzed processes that have been developed in the last years. Biomedicinal applications of some reaction products are also mentioned along with a perspective of future applications of this reactant.

Strategies for Fine-Tuning the Conformations of Cyclic Peptides

Cyclic peptides are promising scaffolds for drug development, attributable in part to their increased conformational order compared to linear peptides. However, when optimizing the target-binding or pharmacokinetic properties of cyclic peptides, it is frequently necessary to "fine-tune" their conformations, e.g., by imposing greater rigidity, by subtly altering certain side chain vectors, or by adjusting the global shape of the macrocycle. This review systematically examines the various types of structural modifications that can be made to cyclic peptides in order to achieve such conformational control.

Diverse N-Substituted Azole-Containing Amino Acids as Building Blocks for Cyclopeptides

The preparation of 16 oxazole- or thiazole-containing amino esters bearing a wide array of N-substitution is reported. These were accessed in 40-92% yield via an AgClO4-promoted substitution reaction between a primary amine and a chloromethyl-functionalized thiazole or oxazole. These new synthetic building blocks will be useful for the preparation of new cyclopeptide analogues bearing heterocyclic backbone modifications. Four macrocyclic N-substituted oligoamides that include thiazole or oxazole heterocycles were obtained, following cyclooligomerization reactions of azole-modified N-substituted amino acids.

Synthetic Lugdunin Analogues Reveal Essential Structural Motifs for Antimicrobial Action and Proton Translocation Capability

Lugdunin, a novel thiazolidine cyclopeptide, exhibits micromolar activity against methicillin-resistant Staphylococcus aureus (MRSA). For structure-activity relationship (SAR) studies, synthetic analogues obtained from alanine and stereo scanning as well as peptides with modified thiazolidine rings were tested for antimicrobial activity. The thiazolidine ring and the alternating d- and l-amino acid backbone are essential. Notably, the non-natural enantiomer displays equal activity, thus indicating the absence of a chiral target. The antibacterial activity strongly correlates with dissipation of the membrane potential in S. aureus. Lugdunin equalizes pH gradients in artificial membrane vesicles, thereby maintaining membrane integrity, which demonstrates that proton translocation is the mode of action (MoA). The incorporation of extra tryptophan or propargyl moieties further expands the diversity of this class of thiazolidine cyclopeptides.

Heteroaryl Rings in Peptide Macrocycles

This Review is devoted to the chemistry of macrocyclic peptides having heterocyclic fragments in their structure. These motifs are present in many natural products and synthetic macrocycles designed against a particular biochemical target. Thiazole and oxazole are particularly common constituents of naturally occurring macrocyclic peptide molecules. This frequency of occurrence is because the thiazole and oxazole rings originate from cysteine, serine, and threonine residues. Whereas other heteroaryl groups are found less frequently, they offer many insightful lessons that range from conformational control to receptor/ligand interactions. Many options to develop new and improved technologies to prepare natural products have appeared in recent years, and the synthetic community has been pursuing synthetic macrocycles that have no precedent in nature. This Review attempts to summarize progress in this area.

Multicomponent Reaction Toolbox for Peptide Macrocyclization and Stapling

In the past decade, multicomponent reactions have experienced a renaissance as powerful peptide macrocyclization tools enabling the rapid creation of skeletal complexity and diversity with low synthetic cost. This review provides both a historical and modern overview of the development of the peptide multicomponent macrocyclization as a strategy capable to compete with the classic peptide cyclization methods in terms of chemical efficiency and synthetic scope. We prove that the utilization of multicomponent reactions for cyclizing peptides by either their termini or side chains provides a key advantage over those more established methods; that is, the possibility to explore the cyclic peptide chemotype space not only at the amino acid sequence but also at the ring-forming moiety. Owing to its multicomponent nature, this type of peptide cyclization process is well-suited to generate diversity at both the endo- and exo-cyclic fragments formed during the ring-closing step, which stands as a distinctive and useful characteristic for the creation and screening of cyclic peptide libraries. Examples of the novel multicomponent peptide stapling approach and heterocycle ring-forming macrocyclizations are included, along with multicomponent methods incorporating macrocyclization handles and the one-pot syntheses of macromulticyclic peptide cages. Interesting applications of this strategy in the field of drug discovery and chemical biology are provided.

Conformationally stable peptide macrocycles assembled using the Petasis borono-Mannich reaction

Synthesis and the structural analysis of conformationally stable peptide macrocycles assembled using the Petasis borono-Mannich reaction are reported.

Synthesis of macrocyclic peptidomimetics via the Ugi-click-strategy

The Ugi-click-strategy was employed for the synthesis of 12–28 membered 1,2,3-triazole derived macrocyclic peptidomimetics.