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Diversity Oriented Synthesis of Peptide Derivatives as Antibiotics

Final Report Summary - ANTIBIOTICS DOS (Diversity oriented synthesis of peptide derivatives as antibiotics)

The screening of libraries of small organic molecules to identify useful modulators of biological systems is fundamental in the drug-discovery process and chemical biology studies in general. It has been argued that the more structurally diverse the molecules of the library are, the more likely will be to find a bioactive hit. To this end, diversity-oriented synthesis (DOS) is an excellent tool for the efficient generation of libraries of small molecules with a high degree of structural, and thus functional, diversity that interrogate large areas of chemical space.

We have developed a strategy for the DOS of a library of structurally unique and diverse macrocyclic peptidomimetics from simple, readily available amino acid starting materials. In a proof-of-concept study, a library of 14 such compounds displaying both scaffold and stereochemical diversity was generated [1]. The molecules prepared contain motifs present in many biologically active compounds (macrocycles, peptidomimetics and diketopiperazines).

Our strategy of synthesis was based around a build / couple / pair (B/C/P) approach (scheme 1). The build step involves the preparation of two types of chiral building blocks: the first contains an azide and a free amine, whereas the second contains an alkyne and a carboxylic acid. Coupling of three of these building blocks via amide bond formation furnished a range of tripeptide derivatives providing the basis for stereochemical diversity. The subsequent pair phase provided the basis for scaffold diversity and comprised of two cyclisation steps: a macrocyclisation via 1,3-dipolar cycloaddition and an intramolecular cyclisation to introduce the diketopiperazine (DKP) motif into the macrocyclic framework (scheme 2).

Computational analyses demonstrated that the library accesses both chemical space not explored by molecules from 'traditional' medicinal chemistry collections and also more populated areas, showing the versatility of the developed synthetic methodology. In preliminary biological assays a number of library compounds showed interesting biological properties; in particular, compound 14 showed significant antibacterial activity against Staphylococcus aureus. Because of the modular nature of our synthetic strategy, we anticipate that it holds significant potential for the DOS of libraries of macrocyclic peptidomimetics with greater levels of diversity.

A wide variety of natural and unnatural amino acids could be used in the build step of the synthesis, which would allow the generation of considerable stereochemical and scaffold diversity. In addition, the final library compounds should prove amenable to further modification and diversification. The generation of the proof-of-concept library required the development of two previously undescribed pieces of synthetic methodology.

First, we have described a protocol for the macrocyclic ring closure of chiral alpha-azido acids via regioselective and epimerization free 1,5-disubstituted triazole formation. The macrocycle products were obtained in superior yields to those described in the literature. Second, we have reported the development of a unique method for the DKP synthesis using solid-supported N-methylmorpholine in combination with microwave heating. The procedure allowed for the synthesis of structurally complex DKPs in excellent yields. Crucially, this method may represent a previously undescribed general strategy for the synthesis of DKP-based structures that could be difficult to isolate by other means. Owing to their robust and flexible natures, and given the presence of macrocyclic peptidomimetics and DKPs in many molecules of biological interest, it is envisaged that both previously undescribed methodologies will prove to be useful in a wider synthetic context.

[1] Isidro-Llobet, A.; Murillo, T.; Bello, P.; Cilibrizzi, A.; Hodgkinson, J.; Galloway, W.; Bender, A.; Welch, M.; Spring, D. PNAS. 2011, 108, 6793-6798.