Design, synthesis, pericyclic chemistry and biomedical applications of azopeptides

The azapeptides are peptide mimics in which the alpha carbon of one or more amino acids has been replaced with a nitrogen atom. The primary goal of this doctorate study was to develop a new method for the synthesis of azapeptides by the application of azopeptides, which are azodicarbonyl analogs that possess an imino urea component. Oxidation of aza-glycine residues proved effective for making azopeptides, which were employed in pericyclic chemistry and examined by X-ray crystallography. Diels−Alder cyclization and Alder−ene reactions on azopeptides enabled respectively access to constrained aza-pipecolyl and azaallylglycinyl residues. X-ray analysis of an azopeptide in the solid state provided insight into imino urea configuration (Chapter 2). Employing the products from azopeptide chemistry as constrained valine analogs, mimics of the Ala-Val-Pro-Ile sequence from the second mitochondria derived activator of caspases (Smac) protein were synthesized and demonstrated ability to induce apoptosis in breast cancer cells (Chapter 3). Following the development of a method to synthesize azopeptides in solution, a solidphase approach was conceived to prepare azopeptides on Rink amide resin and may be amenable to combinatorial chemistry for library generation. This study was targeted on the synthesis of aza-analogs of the opioid peptides morphiceptin and endomorphins as well as the Cluster of Differentiation 36 receptor (CD36) ligand Growth Hormone Releasing Peptide-6 (GHRP-6, His-D-Trp-Ala-Trp-D-Phe-Lys-NH2). The former were examined due to their importance as opioid receptor subtype selective agonists with potential for developing novel analgesics. The latter was targeted in pursuit of selective CD36 modulators with therapeutic potential for treating diseases featuring macrophage-driven inflammation including age-related macular degeneration and atherosclerosis. Twelve aza-opioids were synthesized by replacing proline at the two position of the respective peptide ligands with different aza-pipecolate residues. Similarly, five aza-pipecolyl GHRP-6 analogs were synthesized using the solid-phase method to replace respectively the Ala3 and Trp4 residues (Chapter 4). Examination of the aza-opioids for inhibitory potency on electrically induced contractions of the guinea pig ileum and mouse vas deferens, and the aza-GHRP-6 analogs for capacity to diminish CD36-mediated overproduction of nitric oxide in macrophage cells iv after treatment with the Toll-like receptor-2-agonist fibroblast-stimulating lipopeptide, both demonstrated the utility of the aza-pipecolate methodology for studying the influence of conformation on peptide activity and selectivity. The novel methods for the synthesis of azopeptides in solution and on solid support described in this thesis are designed to enable their use in studies of structure-activity relationships with different biologically active peptides. In this respect, azopeptides have been applied in this research to make ligands of the melanocyte-inhibiting factor-1 (MIF-1), Smac, opioid and CD36 receptors. Considering the effectiveness of the synthetic methods and the potential applications of azopeptides, the findings of this thesis offer strong potential for the advancement of peptide science in the context of medicinal chemistry and chemical biology.

Keyphrases: Azapeptides, Tour β, Semicarbazone, Aza-valine, Semicarbazide, Azopeptide, Aza-Diels Alder, GHRP-6, Peptide mimicry, β-turn, Pericyclic