Synthesis of all-L cyclic tetrapeptides using pseudoprolines as removable turn inducers

Production of constrained L-cyclo-tetrapeptides by epimerization-resistant direct aminolysis

The synthesis of constrained 12-membered rings is notably difficult. The main challenges result from constraints during the linear peptide cyclization. Attempts to overcome constraints through excessive activation frequently cause peptidyl epimerization, while insufficient activation of the C-terminus hampers cyclization and promotes intermolecular oligomer formation. We present a β-thiolactone framework that enables the synthesis of cyclo-tetrapeptides via direct aminolysis. This tactic utilizes a mechanism that restricts C-terminal carbonyl rotation while maintaining high reactivity, thereby enabling efficient head-to-tail amidation, reducing oligomerization, and preventing epimerization. A broad range of challenging cyclo-tetrapeptides ( > 20 examples) are synthesized in buffer and exhibits excellent tolerance toward nearly all proteinogenic amino acids. Previously unattainable macrocycles, such as cyclo-L-(Pro-Tyr-Pro-Val), have been produced and identified as μ-opioid receptor (MOR) agonists, with an EC50 value of 2.5 nM. Non-epimerizable direct aminolysis offers a practical solution for constrained peptide cyclization, and the discovery of MOR agonist activity highlights the importance of overcoming synthetic challenges for therapeutic development.

Synthesis and Functionalization of Azetidine-Containing Small Macrocyclic Peptides

Cyclic peptides are increasingly important structures in drugs but their development can be impeded by difficulties associated with their synthesis. Here, we introduce the 3-aminoazetidine (3-AAz) subunit as a new turn-inducing element for the efficient synthesis of small head-to-tail cyclic peptides. Greatly improved cyclizations of tetra-, penta- and hexapeptides (28 examples) under standard reaction conditions are achieved by introduction of this element within the linear peptide precursor. Post-cyclization deprotection of the amino acid side chains with strong acid is realized without degradation of the strained four-membered azetidine. A special feature of this chemistry is that further late-stage modification of the resultant macrocyclic peptides can be achieved via the 3-AAz unit. This is done by: (i) chemoselective deprotection and substitution at the azetidine nitrogen, or by (ii) a click-based approach employing a 2-propynyl carbamate on the azetidine nitrogen. In this way, a range of dye and biotin tagged macrocycles are readily produced. Structural insights gained by XRD analysis of a cyclic tetrapeptide indicate that the azetidine ring encourages access to the less stable, all-trans conformation. Moreover, introduction of a 3-AAz into a representative cyclohexapeptide improves stability towards proteases compared to the homodetic macrocycle.

The Role of Attractive Non-Covalent Interactions in Peptide Macrocyclization

The efficiency of macrocyclization reactions relies on the appropriate conformational preorganization of a linear precursor, ensuring that reactive ends are in spatial proximity prior to ring closure. Traditional peptide cyclization approaches that reduce the extent of terminal ion pairing often disfavor cyclization-conducive conformations and can lead to undesired cyclodimerization or oligomerization side reactions, particularly when they are performed without high dilution. To address this challenge, synthetic strategies that leverage attractive noncovalent interactions, such as zwitterionic attraction between chain termini during macrocyclization, offer a potential solution by reducing the entropic penalty associated with linear peptides adopting precyclization conformations. In this study, we investigate the role of (N-isocyanoimino)triphenylphosphorane (Pinc) in facilitating the cyclization of linear peptides into conformationally rigid macrocycles. The observed moderate diastereoselectivity is consistent with the preferential Si-facial addition of Pinc, where the isocyanide adds to the E-iminium ion on the same face as the l-proline amide group. The resulting peptide chain reveals that the activated phosphonium ylide of Pinc brings the reactive ends close together, promoting cyclization by enclosing the carboxylate within the interior of the pentapeptide and preventing the formation of byproducts. For shorter peptides with modified peptide backbones, the cyclization mechanism and outcome are redirected, as nucleophilic motifs such as thiazole and imidazole can covalently trap nitrilium intermediates. The isolation of the intermediate in the unproductive macrocyclization pathway, along with nuclear magnetic resonance and density functional theory studies, provides insights into heterocycle-dependent selectivity. The Pinc-driven macrocyclization process has generated diverse collections of cyclic molecules, and our models offer a comprehensive understanding of observed trends, facilitating the development of other heterocycle-forming macrocyclization reactions.

Diastereoselective synthesis of cyclic tetrapeptide pseudoxylallemycin A illuminates the impact of base during macrolactamization

Therapeutic agents with unique molecular structures and new mechanisms of action are needed to confront the phenomenon of multidrug resistance among bacteria. Pseudoxylallemycins, cyclic tetrapeptide (CTP) natural products, have exhibited modest antibiotic activity, but their synthesis has proven challenging. Inherent ring strain in CTPs decreases the rate of cyclization in lieu of polymerization and racemization pathways, which has resulted in previous syntheses describing mixtures of diastereomers containing predominantly an undesired epimer. We have optimized the cyclization step of pseudoxylallemycin A to favor production of the natural diastereomer; notably, variation of the base, temperature, and solvent with peptide coupling reagent propylphosphonic anhydride (T3P) afforded exquisite selectivity for the natural product in as high as 97 : 3 DR, and our conditions can provide the natural product in up to 32% overall yield through 8 steps. Employing weaker bases than those typically used in peptide coupling reactions led to the greatest improvement in diastereoselectivity, and these studies demonstrated that the identity of the amine base has enormous impact on the rate of C-terminal epimerization when T3P is used, a variable usually considered of lesser consequence when combined with typical amide coupling reagents. Toward fully characterizing pseudoxylallemycin stereoisomers, variable temperature NMR was described as a tool to more clearly analyze CTPs that exhibit multiple conformational states. These synthetic and spectroscopic insights were applied toward synthesizing several natural product analogues, and their antibacterial activity was examined using microdilution assays.

Synthesis of L-cyclic tetrapeptides by backbone amide activation CyClick strategy

Cyclic tetrapeptides exhibit high cellular permeability and a wide range of biological properties and thus have gained great interest in the field of medicinal chemistry. We synthesized highly strained 12-membered head to tail cyclic peptides with varying reactive amino acids, without oligomerization using the exclusively intramolecular CyClick chemistry. This occurs by a two-step process involving the low-energy formation of a 15 atom-containing cyclic imine, followed by a chemoselective ring contraction of the peptide backbone generating a highly strained 12 atom-containing cyclic tetrapeptide. This reaction exhibited high substrate scope and generated head to tail cyclic tetrapeptides with varying amino acids at the N-terminus, showing chemoselectivity without the need for side group protection.

Highly Efficient Cyclization Approach of Propargylated Peptides via Gold(I)-Mediated Sequential C-N, C-O, and C-C Bond Formation

A rapid and efficient cyclization of unprotected N-propargylated peptides using the Au(I) organometallic complex is reported. The method relies on the activation of the propargyl functionality using gold(I) to produce a new linkage with the N-terminus amine at the cyclization site. The presented method features a fast reaction rate (within 20 min), mild conditions, chemoselectivity, wide sequence scope, and high yields (up to 87%). The strategy was successfully tested on a wide variety of 30 unprotected peptides having various sequences and lengths, thus providing access to structurally distinct cyclic peptides. The practical usefulness of this method was demonstrated in producing peptides that bind efficiently to Lys48-linked di- and tetra-ubiquitin chains. The new cyclic peptide modulators exhibited high permeability to living cells and promoted apoptosis via binding with the endogenous Lys48-linked ubiquitin chains.

Macrocyclic Tetramers-Structural Investigation of Peptide-Peptoid Hybrids

Outstanding affinity and specificity are the main characteristics of peptides, rendering them interesting compounds for basic and medicinal research. However, their biological applicability is limited due to fast proteolytic degradation. The use of mimetic peptoids overcomes this disadvantage, though they lack stereochemical information at the α-carbon. Hybrids composed of amino acids and peptoid monomers combine the unique properties of both parent classes. Rigidification of the backbone increases the affinity towards various targets. However, only little is known about the spatial structure of such constrained hybrids. The determination of the three-dimensional structure is a key step for the identification of new targets as well as the rational design of bioactive compounds. Herein, we report the synthesis and the structural elucidation of novel tetrameric macrocycles. Measurements were taken in solid and solution states with the help of X-ray scattering and NMR spectroscopy. The investigations made will help to find diverse applications for this new, promising compound class.

Peptide Cyclization at High Concentration

The emergence of cyclic peptides as pharmaceuticals has led to an eruption of new methodologies for macrocyclization. However, the cyclization of peptides at high concentrations presents a challenge due to the production of side products like dimers and oligomers. This factor is more pronounced with the cyclization of peptides composed of fewer than seven amino acids, thus has created a need for a new synthetic strategy. Herein, we will elucidate a new chemoselective method termed ‘CyClick’ that works in an exclusively intramolecular fashion preventing the formation of commonly occurring side products such as dimers and oligomers, even at relatively high concentration.

1 Introduction

2 Known Methodologies

3 Novel CyClick Chemistry

4 Conclusion and Outlook

Synthetic strategies to access staphylococcus auto-inducing peptides as quorum sensing modulators

The accessory gene regulator (agr) quorum-sensing system is arguably the most important regulator of staphylococcus virulence and has been the focus of tremendous interest in the development of effective therapies for pathogenic bacterial infections. With regards to chemotherapeutic based strategies, the significant proportion of currently reported agr-system modulating molecules are mimics of the native ArgC substrate, which is a thioester-based macrocyclic peptide know as the auto-inducing peptide. Over the past two decades, more than two-hundred synthetic analogues have been reported. This review traces the development of the synthetic strategies employed to synthesise these analogues with a particular focus on macrocyclisation. At present these synthetic approaches can be clustered into five broad categories (1) solution-phase cyclisation, (2) immobilised carbodiimide assisted cyclisation, (3) concomitant on-resin cleavage and macrocyclisation, (4) Boc-compatible chemoselective thioesterification, and (5) Fmoc-compatible chemoselective thioesterification. The advantages and limitation provided by each of the approaches are compared and contrasted with a view towards potential reaction scale-up.

Investigations of the key macrolactamisation step in the synthesis of cyclic tetrapeptide pseudoxylallemycin A

The total synthesis and structural confirmation of naturally occurring all l-cyclic tetrapeptide pseudoxylallemycin A is reported. X-ray crystallography revealed that the linear precursor adopted an all-trans (ttt) extended linear conformation, while its cyclic derivative adopts a trans,cis,trans,cis (tctc) conformation. Two kinetically favoured cyclic conformers prone to hydrolysis initially formed rapidly during cyclisation, with subsequent conversion to the thermodynamically stable tctc macrocycle taking place slowly. We postulate the initial unstable cyclic product undergoes an unprecedented nucleophilic ring opening with either the T3P or PyAOP by-products to give the linear ttt structure as a reactivated species and through a series of equilibria is slowly consumed by cyclisation to the thermodynamic product pseudoxylallemycin A. Consumption of the reactivated species by formation of pseudoxylallemycin A requires a trans-cis isomerism to occur and necessitates moderately increased reaction temperatures. Cyclisation with T3P was found to provide the greatest stereoretention. Synthesis and X-ray crystallography of the C-terminal epimer demonstrated its cyclisation to be kinetically favoured and to proceed without epimerisation despite also bearing an all-trans backbone.

A Chemical Biology Approach to Understanding Molecular Recognition of Lipid II by Nisin(1-12): Synthesis and NMR Ensemble Analysis of Nisin(1-12) and Analogues

Natural products that target lipid II, such as the lantibiotic nisin, are strategically important in the development of new antibacterial agents to combat the rise of antimicrobial resistance. Understanding the structural factors that govern the highly selective molecular recognition of lipid II by the N-terminal region of nisin, nisin(1-12), is a crucial step in exploiting the potential of such compounds. In order to elucidate the relationships between amino acid sequence and conformation of this bicyclic peptide fragment, we have used solid-phase peptide synthesis to prepare two novel analogues of nisin(1-12) in which the dehydro residues have been replaced. We have carried out an NMR ensemble analysis of one of these analogues and of the wild-type nisin(1-12) peptide in order to compare the conformations of these two bicyclic peptides. Our analysis has shown the effects of residue mutation on ring conformation. We have also demonstrated that the individual rings of nisin(1-12) are pre-organised to an extent for binding to the pyrophosphate group of lipid II, with a high degree of flexibility exhibited in the central amide bond joining the two rings.

A gold mine for drug discovery: Strategies to develop cyclic peptides into therapies

As a versatile therapeutic modality, peptides attract much attention because of their great binding affinity, low toxicity, and the capability of targeting traditionally "undruggable" protein surfaces. However, the deficiency of cell permeability and metabolic stability always limits the success of in vitro bioactive peptides as drug candidates. Peptide macrocyclization is one of the most established strategies to overcome these limitations. Over the past decades, more than 40 cyclic peptide drugs have been clinically approved, the vast majority of which are derived from natural products. The de novo discovered cyclic peptides on the basis of rational design and in vitro evolution, have also enabled the binding with targets for which nature provides no solutions. The current review summarizes different classes of cyclic peptides with diverse biological activities, and presents an overview of various approaches to develop cyclic peptide-based drug candidates, drawing upon series of examples to illustrate each strategy.

Cyclic Tetrapeptides from Nature and Design: A Review of Synthetic Methodologies, Structure, and Function

Small cyclic peptides possess a wide range of biological properties and unique structures that make them attractive to scientists working in a range of areas from medicinal to materials chemistry. However, cyclic tetrapeptides (CTPs), which are important members of this family, are notoriously difficult to synthesize. Various synthetic methodologies have been developed that enable access to natural product CTPs and their rationally designed synthetic analogues having novel molecular structures. These methodologies include the use of reversible protecting groups such as pseudoprolines that restrict conformational freedom, ring contraction strategies, on-resin cyclization approaches, and optimization of coupling reagents and reaction conditions such as temperature and dilution factors. Several fundamental studies have documented the impacts of amino acid configurations, N-alkylation, and steric bulk on both synthetic success and ensuing conformations. Carefully executed retrosynthetic ring dissection and the unique structural features of the linear precursor sequences that result from the ring dissection are crucial for the success of the cyclization step. Other factors that influence the outcome of the cyclization step include reaction temperature, solvent, reagents used as well as dilution levels. The purpose of this review is to highlight the current state of affairs on naturally occurring and rationally designed cyclic tetrapeptides, including strategies investigated for their syntheses in the literature, the conformations adopted by these molecules, and specific examples of their function. Using selected examples from the literature, an in-depth discussion of the synthetic techniques and reaction parameters applied for the successful syntheses of 12-, 13-, and 14-membered natural product CTPs and their novel analogues are presented, with particular focus on the cyclization step. Selected examples of the three-dimensional structures of cyclic tetrapeptides studied by NMR, and X-ray crystallography are also included.

Macrocyclisation of small peptides enabled by oxetane incorporation

Cyclic peptides are an important source of new drugs but are challenging to produce synthetically. We show that head-to-tail peptide macrocyclisations are greatly improved, as measured by isolated yields, reaction rates and product distribution, by substitution of one of the backbone amide C[double bond, length as m-dash]O bonds with an oxetane ring. The cyclisation precursors are easily made by standard solution- or solid-phase peptide synthesis techniques. Macrocyclisations across a range of challenging ring sizes (tetra-, penta- and hexapeptides) are enabled by incorporation of this turn-inducing element. Oxetane incorporation is shown to be superior to other established amino acid modifications such as N-methylation. The positional dependence of the modification on cyclisation efficiency is mapped using a cyclic peptide of sequence LAGAY. We provide the first direct experimental evidence that oxetane modification induces a turn in linear peptide backbones, through the observation of d _NN (i, i + 2) and d _αN (i, i + 2) NOEs, which offers an explanation for these improvements. For cyclic peptide, cLAGAY, a combination of NMR derived distance restraints and molecular dynamics simulations are used to show that this modification alters the backbone conformation in proximity to the oxetane, with the flexibility of the ring reduced and a new intramolecular H-bond established. Finally, we incorporated an oxetane into a cyclic pentapeptide inhibitor of Aminopeptidase N, a transmembrane metalloprotease overexpressed on the surface of cancer cells. The inhibitor, cCNGRC, displayed similar IC₅₀ values in the presence or absence of an oxetane at the glycine residue, indicating that bioactivity is fully retained upon amide C[double bond, length as m-dash]O bond replacement.

Synthesis of chiral nine and twelve-membered cyclic polyamines from natural building blocks

A rational strategy for the facile and efficient cyclization of amino acid-based linear precursors forming nine and twelve-membered cyclic peptidomimetics is reported.

The Pseudoproline Approach to Peptide Cyclization

The development of efficient methods for the synthesis of cyclic peptides is of interest because of the many potential applications of this class of molecule. Pseudoprolines are derived from serine, threonine, and cysteine and can be used as traceless turn-inducers to facilitate the cyclization of a wide range of linear peptide precursors. The incorporation of a pseudoproline into the peptide to be cyclized generally results in a cyclization reaction that proceeds more quickly and with higher yield than that of an analogous sequence without the pseudoproline. Installation of a pseudoproline at the C-terminal position of a linear peptide sequence has also been shown to eliminate any epimerization of this residue during the reaction. Following pseudoproline-mediated cyclization, these turn-inducers can be removed on treatment with acid in a similar manner to other protecting groups to provide the native peptide sequence, and in the case of cysteine-derived pseudoprolines, the resulting cysteine can be readily converted into alanine through desulfurization. These traceless turn-inducers have been successfully used in the synthesis of cyclic peptides containing either serine, threonine, cysteine or alanine residues.

A new member of the fusaricidin family - structure elucidation and synthesis of fusaricidin E

Two hitherto unknown fusaricidins were obtained from fermentation broths of three Paenibacillus strains. After structure elucidation based on tandem mass spectrometry and NMR spectroscopy, fusaricidin E was synthesized to confirm the structure and the suggested stereochemistry. The synthesis was based on a new strategy which includes an efficient access to the 15-guanidino-3-hydroxypentadecanoyl (GHPD) side chain from erucamide.

Peptide Macrocyclization Assisted by Traceless Turn Inducers Derived from Ugi Peptide Ligation with Cleavable and Resin-Linked Amines

A multicomponent approach enabling the installation of turn-inducing moieties that facilitate the macrocyclization of short and medium-size oligopeptides is described. The strategy comprises the Ugi ligation of peptide carboxylic acids and isocyanopeptides in the presence of aldehydes and acid or photolabile amines followed by cyclization and cleavage of the backbone N-substituents to render canonical cyclopeptides. Implementing the approach on solid phase with the use of Rink amide resins led to a new class of backbone amide linker strategy.

The road to the synthesis of "difficult peptides"

The last decade has witnessed a renaissance of peptides as drugs. This progress, together with advances in the structural behavior of peptides, has attracted the interest of the pharmaceutical industry in these molecules as potential APIs. In the past, major peptide-based drugs were inspired by sequences extracted from natural structures of low molecular weight. In contrast, nowadays, the peptides being studied by academic and industrial groups comprise more sophisticated sequences. For instance, they consist of long amino acid chains and show a high tendency to form aggregates. Some researchers have claimed that preparing medium-sized proteins is now feasible with chemical ligation techniques, in contrast to medium-sized peptide syntheses. The complexity associated with the synthesis of certain peptides is exemplified by the so-called "difficult peptides", a concept introduced in the 80's. This refers to sequences that show inter- or intra-molecular β-sheet interactions significant enough to form aggregates during peptide synthesis. These structural associations are stabilized and mediated by non-covalent hydrogen bonds that arise on the backbone of the peptide and-depending on the sequence-are favored. The tendency of peptide chains to aggregate is translated into a list of common behavioral features attributed to "difficult peptides" which hinder their synthesis. In this regard, this manuscript summarizes the strategies used to overcome the inherent difficulties associated with the synthesis of known "difficult peptides". Here we evaluate several external factors, as well as methods to incorporate chemical modifications into sequences, in order to describe the strategies that are effective for the synthesis of "difficult peptides". These approaches have been classified and ordered to provide an extensive guide for achieving the synthesis of peptides with the aforementioned features.

An update on new methods to synthesize cyclotetrapeptides

Cyclotetrapeptides are important bioactive lead drug molecules that display a wide spectrum of pharmacological activities. However, the synthesis of cyclotetrapeptides from their linear precursors is challenging due to the highly constrained conformation required for cyclisation, thus hampering their progress to a clinical setting. This review provides an account of the reported methods used for the synthesis of cyclotetrapeptides.

Syntheses of some α-cyclic tripeptides as potential inhibitors for HMG-CoA Reductase

α-Cyclic tripeptides (CtPs) are the most rigid members of the cyclic peptide family. However, due to their synthetic difficulty, biological activity has remained undisclosed. The incorporation of side-chain-protected natural amino acids into functional CtPs was performed to explore the potential biological functions. Several novel CtPs that consist of protected serine (S(Bn)) and/or glutamate (E(OBn)) were prepared from corresponding linear tripeptides by chemical synthesis. There is a strong possibility for CtPs that contain 3 phenyl groups to correlate with atorvastatin structure. The binding effects in human HMG-CoA reductase (hHMGR) activities were first evaluated by molecular docking. High docking scores were received with these CtPs for enzyme. Therefore, enzymatic assays were carried out and the compound cyclo(S(Bn))3 was indeed able to moderately inhibit hHMGR (IC50 = 110 μM).

Cysteine pseudoprolines for thiol protection and peptide macrocyclization enhancement in Fmoc-based solid-phase peptide synthesis

Contrary to other studies, here we describe cysteine (Cys) pseudoproline-containing peptides with short deprotection times in TFA. The deprotection times fell in the same range as other protecting groups commonly used in SPPS (e.g., 1–3 h). Moreover, when using Cys pseudoprolines as peptide macrocyclization-enhancing moieties a considerable reduction in reaction time was observed compared to a peptide containing trityl protected Cys.

A macrolactonization approach to the total synthesis of the antimicrobial cyclic depsipeptide LI-F04a and diastereoisomeric analogues

The cyclic peptide core of the antifungal and antibiotic cyclic depsipeptide LI-F04a was synthesised by using a modified Yamaguchi macrolactonization approach. Alternative methods of macrolactonization (e.g., Corey-Nicolaou) resulted in significant epimerization of the C-terminal amino acid during the cyclization reaction. The D-stereochemistry of the alanine residue in the naturally occurring cyclic peptide may be required for the antifungal activity of this natural product.

A versatile scaffold for site-specific modification of cyclic tetrapeptides

A novel scaffold that can be used to prepare conformationally homogeneous cyclic tetrapeptides equipped with a β-amino acid residue is disclosed. It is shown that regioselective structural modification can be accomplished using thiols and azide nucleophiles, commonly associated with rich downstream chemistry. The method should find application in efforts to constrain privileged tripeptide sequences in rigid molecular scaffolds.

Synthesis of sansalvamide A peptidomimetics: triazole, oxazole, thiazole, and pseudoproline containing compounds

Peptidomimetic-based macrocycles typically have improved pharmacokinetic properties over those observed with peptide analogs. Described are the syntheses of 13 peptidomimetic derivatives that are based on active Sansalvamide A structures, where these analogs incorporate heterocycles (triazoles, oxazoles, thiazoles, or pseudoprolines) along the macrocyclic backbone. The syntheses of these derivatives employ several approaches that can be applied to convert a macrocyclic peptide into its peptidomimetic counterpart. These approaches include peptide modifications to generate the alkyne and azide for click chemistry, a serine conversion into an oxazole, a Hantzsch reaction to generate the thiazole, and protected threonine to generate the pseudoproline derivatives. Furthermore, we show that two different peptidomimetic moieties, triazoles and thiazoles, can be incorporated into the macrocyclic backbone without reducing cytotoxicity: triazole and thiazole.

Improving the Fmoc Solid Phase Synthesis of the Cyclic Hexapeptide Complement C5a Antagonist, PMX205

The anti-inflammatory drug, PMX205, is an antagonist of the C5a complement receptor and has been shown to be effective in rodent models of amyotrophic lateral sclerosis and Alzheimer's disease. This cyclic hexapeptide (c[Arg-Trp-D-Cha-Pro-Orn]-Hca) has been reported to produce relatively low yields for both the linear peptide assembly and the cyclization reaction in solution and solid phase syntheses. During attempts to reproduce the solid phase methodology, a catastrophic loss of substitution was encountered which could be avoided or reduced by the use of 2-chlorotrityl resin. Likewise, the cyclization reaction could be significantly improved by the use of FDPP (pentafluorophenyl diphenylphosphinate) at high dilution (up to 80% purified yield). Both improvements are accomplished with commercially available products.

Contemporary strategies for peptide macrocyclization

Peptide macrocycles have found applications that range from drug discovery to nanomaterials. These ring-shaped molecules have shown remarkable capacity for functional fine-tuning. Such capacity is enabled by the possibility of adjusting the peptide conformation using the techniques of chemical synthesis. Cyclic peptides have been difficult, and often impossible, to prepare using traditional synthetic methods. For macrocyclization to occur, the activated peptide must adopt an entropically disfavoured pre-cyclization conformation before forming the desired product. Here, we review recent solutions to some of the major challenges in this important area of contemporary synthesis.