Modern Review,peptide cyclization

The synthesis of cyclic peptides has emerged as a pivotal area in medicinal chemistry, offering enhanced stability, improved membrane permeability, and the potential to overcome the limitations of linear peptides as drug candidates. A cornerstone in achieving these desirable properties is the effective implementation of an on-resin peptide cyclisation strategy. This approach leverages the advantages of solid-phase synthesis, allowing for efficient and controlled macrocyclization directly on the resin. This article delves into the intricacies of on-resin cyclisation of peptides, exploring various methodologies, key considerations, and the underlying principles that drive successful peptide cyclisation.

The Rationale Behind On-Resin Cyclisation

Cyclisation of peptides is an approach borrowed from nature, serving to constrain the peptide's conformation and thereby enhance its affinity and specificity for biological targets. While solution-phase cyclization is possible, on-resin macrocyclization offers several distinct advantages. The primary benefit is the ability to perform the cyclization reaction at a higher effective concentration, which can be crucial for driving the reaction to completion without significant side-product formation, often referred to as pseudo-dilute conditions. Furthermore, the solid support facilitates easy separation of the desired cyclic product from excess reagents and byproducts through simple washing steps. This simplifies purification, and in some cases, cyclic peptides are released from the solid phase in the pure form, negating the need for extensive downstream processing. The use of automated peptide synthesizers using solid phase supports further streamlines the entire process, from linear peptide assembly to the final cyclization step.

Key Strategies for On-Resin Peptide Cyclisation

A variety of strategies have been developed to achieve on-resin peptide cyclisation, each with its own strengths and applicability. These can broadly be categorized based on the type of bond formed during cyclization:

* Head-to-Tail Cyclisation: This is perhaps the most common and biologically relevant form of peptide cyclisation, involving the formation of a new peptide bond between the N-terminus of one end of the peptide and the C-terminus of the other. Various coupling reagents commonly employed in solid-phase peptide synthesis can be utilized for this purpose. A crucial aspect of this strategy is the careful selection of the resin. For instance, using a hyper acid-labile resin, such as 2-CTC, can facilitate the removal of the peptide from the resin without compromising the newly formed cyclic structure. On-resin head-to-tail cyclization of cyclotetrapeptides serves as a prime example of this methodology. Researchers have also explored methods to improve the efficiency of on-resin head-to-tail cyclisation of peptides, investigating factors like resin loading which can significantly impact purity and overall success.

* Side-Chain to Terminus Cyclisation: This approach involves forming a bond between a side chain of an amino acid and either the N-terminus or C-terminus of the peptide. This can lead to macrocycles with different structural constraints and biological properties.

* Side-Chain to Side-Chain Cyclisation: This strategy offers even greater conformational rigidity by forming a bond between two amino acid side chains. This often requires specific amino acid side chain modifications or the incorporation of unnatural amino acids.

Advanced Methodologies and Chemistries

Beyond the fundamental approaches, innovative chemistries have been developed to enhance the efficiency and versatility of on-resin peptide cyclisation.

* Click Chemistry Approaches: The advent of click chemistry has revolutionized peptide synthesis, and on-resin cyclisation is no exception. On-resin peptide macrocyclization using thiol-ene click chemistry has emerged as a powerful and rapid synthetic strategy. This method offers high yields and excellent functional group tolerance. Similarly, a robust and facile strategy employing thiol-Michael click chemistry via an N-methyl vinyl sulfonamide has been successfully developed for on-resin macrocyclization of peptides. These click chemistry approaches often involve pre-functionalizing the peptide with appropriate reactive groups on the resin, allowing for efficient coupling upon activation.

* Disulfide Bond Formation: For peptides containing cysteine residues, the formation of an intramolecular disulfide bridge is a common and effective method for cyclization. Solid phase peptide synthesis: Disulphide intramolecular cyclisation protocols often involve the use of oxidizing agents like iodine in DMF to facilitate the formation of the S-S bond while the peptide is still attached to the resin.

* Lactamisation: Three different strategies to obtain cyclic peptides via lactamization have been described, which involve the formation of an amide bond. This can be achieved through various coupling methods, often requiring activation of the carboxyl group or amine.

* Enzymatic Cyclisation: Emerging enzymatic methods are also being explored for on-resin peptide cyclisation. A one-pot strategy combining sortase A-mediated on-resin peptide cleavage and in situ cyclisation has been developed for the synthesis of cyclic peptides.

Critical Considerations for Successful On-Resin Cyclisation

Several factors are paramount to achieving successful **