Aza-1,2,3-triazole-3-alanine Synthesis via Copper-Catalyzed 1,3-Dipolar Cycloaddition on Aza-progargylglycine

Solid phase submonomer azapeptide synthesis

Azapeptides contain at least one aza-amino acid, where the α-carbon has been replaced by a nitrogen atom, and have found broad applicability in fields ranging from medicinal chemistry to biomaterials. In this chapter, we provide a step-by-step protocol for the solid phase submonomer synthesis of azapeptides, which includes three steps: (1) hydrazone activation and coupling onto a resin-bound peptide, (2) chemoselective semicarbazone functionalization for installation of the aza-amino acid side chain, and (3) orthogonal deprotection of the semicarbazone to complete the monomer addition cycle. We focus on semicarbazone functionalization by N-alkylation with primary alkyl halides, and describe conditions for coupling onto aza-amino acids. Such divergent methods accelerate the synthesis of peptidomimetics and allow the rapid introduction of a wide variety of natural and unnatural side chains directly on solid support using easily accessible submonomers.

Synthesis and Biomedical Potential of Azapeptide Modulators of the Cluster of Differentiation 36 Receptor (CD36)

The innovative development of azapeptide analogues of growth hormone releasing peptide-6 (GHRP-6) has produced selective modulators of the cluster of differentiation 36 receptor (CD36). The azapeptide CD36 modulators curb macrophage-driven inflammation and mitigate atherosclerotic and angiogenic pathology. In macrophages activated with Toll-like receptor-2 heterodimer agonist, they reduced nitric oxide production and proinflammatory cytokine release. In a mouse choroidal explant microvascular sprouting model, they inhibited neovascularization. In murine models of cardiovascular injury, CD36-selective azapeptide modulators exhibited cardioprotective and anti-atherosclerotic effects. In subretinal inflammation models, they altered activated mononuclear phagocyte metabolism and decreased immune responses to alleviate subsequent inflammation-dependent neuronal injury associated with retinitis pigmentosa, diabetic retinopathy and age-related macular degeneration. The translation of GHRP-6 to potent and selective linear and cyclic azapeptide modulators of CD36 is outlined in this review which highlights the relevance of turn geometry for activity and the biomedical potential of prototypes for the beneficial treatment of a wide range of cardiovascular, metabolic and immunological disorders.

Tandem Catalysis: Synthesis of Nitrogen-Containing Heterocycles

In this Review, we consider all the publications since the beginning of the century that describe tandem reactions resulting in the formation of five-membered aromatic nitrogen heterocycles (thiazole, imidazole, indole, tetrazole, triazole, and isoxazole). The contents of this review are organized by taxonomy and type of tandem catalysis. It covers orthogonal, auto-, and assisted tandem catalysis, providing an overview of tandem reactions applied tonitrogen heterocycles reported in the literature up to March 2020. We believe that this compilation of data will provide a necessary starting reference to developthe applications of tandem catalysis in medicinal chemistry.

Azo-triazolide bis-cyclometalated Ir(iii) complexes via cyclization of 3-cyanodiarylformazanate ligands

In this work we describe the synthesis of sterically encumbered 1,5-diaryl-3-cyanoformazanate bis-cyclometalated iridium(iii) complexes, two of which undergo redox-neutral cyclization during the reaction to produce carbon-bound 2-aryl-4-arylazo-2H-1,2,3-triazolide ligands. This transformation offers a method for accessing 2-aryl-4-arylazo-2H-1,2,3-triazolide ligands, a heretofore unreported class of chelating ligands. One formazanate complex and both triazolide complexes are structurally characterized by single-crystal X-ray diffraction, with infrared spectroscopy being the primary bulk technique to distinguish the formazanate and triazolide structures. All complexes are further characterized by UV-Vis absorption spectroscopy and cyclic voltammetry, with the triazolide compounds having similar frontier orbital energies to the formazanate complexes but much less visible absorption.

Azapeptide Synthesis Methods for Expanding Side-Chain Diversity for Biomedical Applications

Mimicry of bioactive conformations is critical for peptide-based medicinal chemistry because such peptidomimetics may augment stability, enhance affinity, and increase specificity. Azapeptides are peptidomimetics in which the α-carbon(s) of one or more amino acid residues are substituted by nitrogen. The resulting semicarbazide analogues have been shown to reinforce β-turn conformation through the combination of lone pair-lone pair repulsion of the adjacent hydrazine nitrogen and urea planarity. Substitution of a semicarbazide for an amino amide residue in a peptide may retain biological activity and add benefits such as improved metabolic stability. The applications of azapeptides include receptor ligands, enzyme inhibitors, prodrugs, probes, and imaging agents. Moreover, azapeptides have proven therapeutic utility. For example, the aza-glycinamide analogue of the luteinizing hormone-releasing hormone analogue Zoladex is a potent long-acting agonist currently used in the clinic for the treatment of prostate and breast cancer. However, the use of azapeptides was hampered by tedious solution-phase synthetic routes for selective hydrazine functionalization. A remarkable stride to overcome this bottleneck was made in 2009 through the introduction of the submonomer procedure for azapeptide synthesis, which enabled addition of diverse side chains onto a common semicarbazone intermediate, providing a means to construct azapeptide libraries by solution- and solid-phase chemistry. In brief, aza residues are introduced into the peptide chain using the submonomer strategy by semicarbazone incorporation, deprotonation, N-alkylation, and orthogonal deprotection. Amino acylation of the resulting semicarbazide and elongation gives the desired azapeptide. Since the initial report, a number of chemical transformations have taken advantage of the orthogonal chemistry of semicarbazone residues (e.g., Michael additions and N-arylations). In addition, libraries have been synthesized from libraries by diversification of aza-propargylglycine (e.g., A³ coupling reactions, [1,3]-dipolar cycloadditions, and 5-exo-dig cyclizations) and aza-chloroalkylglycine residues. In addition, oxidation of aza-glycine residues has afforded azopeptides that react in pericyclic reactions (e.g., Diels-Alder and Alder-ene chemistry). The bulk of these transformations of aza-glycine residues have been developed by the Lubell laboratory, which has applied such chemistry in the synthesis of ligands with promising biological activity for treating diseases such as cancer and age-related macular degeneration. Azapeptide analogues of growth hormone-releasing peptide-6 (His-d-Trp-Ala-Trp-d-Phe-Lys-NH₂, GHRP-6) have for example been pursued as ligands of the cluster of differentiation 36 receptor (CD36) and show promising activity for the development of treatments for angiogenesis-related diseases, such as age-related macular degeneration, as well as for atherosclerosis. Azapeptides have also been employed to make a series of conformationally constrained second mitochondria-derived activator of caspase (Smac) mimetics that exhibit promising apoptosis-inducing activity in cancer cells. The synthesis of cyclic azapeptide derivatives was used to make an aza scan to study the conformation-activity relationships of the anticancer agent cilengitide, cyclo(RGDf-N(Me)V), and its parent counterpart cyclo(RGDfV), which exhibit potency against human tumor metastasis and tumor-induced angiogenesis. Innovations in the synthesis and application of azapeptides will be presented in this Account, focusing on the creation and use of side-chain diversity in medicinal chemistry.

Triazolyl-donor-acceptor chromophore-decorated unnatural amino acids and peptides: FRET events in a β-turn conformation

The β-turn conformation and FRET process were established in the designed tripeptide containing fluorescent triazolyl donor and acceptor-decorated unnatural amino acids separated by a natural alanine.

Peptide scanning for studying structure-activity relationships in drug discovery

Peptide-based therapeutics have grown in importance over the last few decades. Furthermore, peptides have been extensively used as lead compounds in the drug discovery process to investigate the nature of chemical space required for molecular recognition and activity at a variety of targets. This critical commentary reviews scanning techniques, which employ natural and non-proteinogenic amino acids to facilitate understanding of structural requirements for peptide biological activity. The value of sequence analysis by such methods is highlighted by examples, in which the elements for peptide affinity and activity have been elucidated and employed to prepare peptidomimetic leads for drug development.

N-Amino-imidazolin-2-one peptide mimic synthesis and conformational analysis

Base-promoted 5-exo-dig cyclizations of aza-propargylglycinamides provided N-amino-imidazolin-2-one peptide mimics, which exhibited turn geometry in X-ray crystallographic and NMR spectroscopic analyses. Sonogashira coupling prior to cyclization afforded N-amino-imidazolin-2-ones with diverse 4-position aromatic substituents with potential to serve as Phe and Trp mimics.

Azapeptides and their therapeutic potential

Azapeptides are peptide analogs in which one or more of the amino residues is replaced by a semicarbazide. This substitution of a nitrogen for the α-carbon center results in conformational restrictions, which bend the peptide about the aza-amino acid residue away from a linear geometry. The resulting azapeptide turn conformations have been observed by x-ray crystallography and spectroscopy, as well as predicted based on computational models. In biologically active peptide analogs, the aza-substitution has led to enhanced activity and selectivity as well as improved properties, such as prolonged duration of action and metabolic stability. In light of these characteristics, azapeptides have found important uses as receptor ligands, enzyme inhibitors, drugs, pro-drugs, probes and imaging agents. Recent improvements in synthetic methods for their procurement have ushered in a new era of azapeptide chemistry. This review aims to provide a historical look at the development of azapeptide science along with a focus on recent developments and perspectives on the future of this useful tool for medicinal chemistry.