Chemistry of the azaindoles (review)

Computational Study of the Electron Spectra of Vapor-Phase Indole and Four Azaindoles

After geometry optimization, the electron spectra of indole and four azaindoles are calculated by density functional theory. Available experimental photoemission and excitation data for indole and 7-azaindole are used to compare with the theoretical values. The results for the other azaindoles are presented as predictions to help the interpretation of experimental spectra when they become available.

Recent developments in the synthesis of azaindoles from pyridine and pyrrole building blocks

The azaindole framework is ubiquitous in bioactive natural products and pharmaceuticals. This review highlights the synthetic approaches to azaindoles with advantages and limitations, mechanistic pathways and biological importance.

Azaindole therapeutic agents

Azaindole structural framework is an integral part of several biologically active natural and synthetic organic molecules; and several FDA approved drugs for various diseases. In the last decade, quite a number of literature reports appeared describing the pharmacology, biological activity and therapeutic applications of a variety of azaindole molecules. This prompted the organic and medicinal chemistry community to develop novel synthetic methods for various azaindoles and test them for a bioactivity against a variety of biological targets. Herein, we have summarized the biological activity of therapeutically advanced clinical candidates and several preclinical candidate drugs that contain azaindole structural moiety.

Recent advances in the global ring functionalization of 7-azaindoles

The 7-azaindole building block has attracted considerable interest in the field of drug discovery in the current portfolio. Because of their powerful medicinal properties, the development of synthetic, elegant techniques for the functionalization of 7-azaindoles continues to be an active area of research. Advances in metal-catalyzed chemistry have recently supported the successful development of a number of novel and effective methods for functionalization of the 7-azaindole template. This review reports state-of-the-art functionalization chemistry of 7-azaindoles with an aspiration to highlight the global ring functionalization of 7-azaindoles that are potential as pharmacophores for various therapeutic targets. Other relevant reviews focused on 7-azaindole synthesis, properties and applications have also been reported. However, none of these reviews have been dedicated to the results achieved in the field of metal-catalyzed cross-coupling/C-H bond functionalized reactions. So we wish to discuss and summarize the advances made since 2011 in this field toward 7-azaindole functionalization.

Regioselective C-H sulfenylation of N-sulfonyl protected 7-azaindoles promoted by TBAI: a rapid synthesis of 3-thio-7-azaindoles

This paper describes the regioselective C-3 sulfenylation of N-sulfonyl protected 7-azaindoles with sulfonyl chlorides. In this transformation, dual roles of TBAI serving as both promoter and desulfonylation reagent have been demonstrated. The reaction proceeded smoothly under simple conditions to afford 3-thio-7-azaindoles in moderate to good yields with broad substrate scopes. This protocol refrains from using transition-metal catalysts, strong oxidants or bases, and shows its practical synthetic value in organic synthesis.

A novel, practical and highly regioselective TBAI promoted C-3 sulfenylation reaction of N-sulfonyl protected 7-azaindoles with sulfonyl chlorides is presented here.

Synthesis of Highly Substituted Pyridines via [4 + 2] Cycloadditions of Vinylallenes and Sulfonyl Cyanides

A convergent strategy for the synthesis of multisubstituted pyridines is described. Vinylallenes combine with commercially available arylsulfonyl cyanides in Diels-Alder cycloadditions to generate isopyridine cycloadducts that are converted to pyridines upon further heating or addition of a base. The 2-sulfonylpyridine products undergo nucleophilic aromatic substitution reactions with oxygen and carbon nucleophiles to provide access to a variety of highly substituted pyridines.

Azaindole-BODIPYs: Synthesis, fluorescent recognition of hydrogen sulfate anion and biological evaluation

The synthesized and sensing capability of two novel azaindole substituted mono and distyryl BODIPY dyes against bisulfate anion were reported. Structural characterizations of the targeted compounds were conducted by using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, ¹H and ¹³C NMR spectroscopies. Photophysical properties of the azaindole substituted BODIPY compounds were investigated employing absorption and fluorescence spectroscopies in acetonitrile solution. It was found that the final compounds 3 and 4 exhibited exclusively selective and sensitive turn-off sensor behavior on HSO₄^- anion. Additionally, the stoichiometry ratio of the targeted compounds to bisulfate anion was measured 0.5 by Job's method. Also, density function theory was performed to the optical response of the sensor for targeted compounds. Furthermore, the cytotoxicity of Azaindole-BODIPYs was examined against living human leukemia K562 cell lines.

Spectroscopic study of the ground and excited state prototropic equilibria of 4-azaindole

The ground and singlet excited state prototropism of 4-azaindole, 4AI, in acid and basic aqueous solutions, inside and outside the pH range, has been systematically studied by using absorption and fluorescence spectroscopic techniques. These studies have thrown light on some interesting aspects on the nature and the photophysics of the 4-AI prototropic species. Thus, the changes of the 4AI absorption spectra reveal the existence of four ground state species; the pyridinic protonated cation, C (pK(a)(C)=7.5±0.1), the neutral molecule, N (pK(a)(N)=15.5±0.5), the pyrrolic deprotonated anion, A, and a previously unnoticed dication, DC (pK(a)(DC)=-4.6±0.4). Besides the emissions of these species, a new fluorescence profile appears in alkaline solutions at around 500nm. This extra band has been ascribed to the neutral phototautomer, NT. What is more relevant to this study is the fact that the position and the intensity of the emission band assigned to the monoprotonated cation are very different from those observed for the normal cation of the 7-azaindole, 7-AI. This together with the fact that for the formation of the DC species a cationic precursor with a quinoid structure must be invoked, have prompted us to assign this cationic emission to the isomeric CI cations. Finally, the excited-state pK(a)s of the prototropic species of 4AI have been theoretically estimated by using the Förster-Weller cycle.

Palladium-catalyzed amination of unprotected halo-7-azaindoles

Simple and efficient procedures for the Pd-catalyzed cross-coupling of primary and secondary amines with halo-7-azaindoles(pyrrolo[2,3-b]pyridine) are presented. Previously, no general method was available to ensure the highly selective reaction of the heteroaryl halide in the presence of the unprotected azaindole N-H. Using palladium precatalysts recently reported by our group, such reactions are easily accomplished under mild conditions that can be applied to cross-coupling reactions with a wide array of aliphatic and aromatic amines.