Recent advances in the global ring functionalization of 7-azaindoles

Iodine-Catalyzed Regioselective C-3 Chalcogenation of 7-Azaindoles: Access to Benzothiophene-Fused 7-Azaindole Analogs

An iodine-catalyzed method has been reported for efficient regioselective C-3 sulfenylation, selenylation, thiocyanation, and selenocyanation of NH-free 7-azaindoles using thiophenols, diselenides, potassium thiocyanates, and selenocyanates, respectively. This approach showcases high efficiency and remarkable versatility, facilitating the synthesis of diverse chalcogenated 7-azaindoles. Additionally, the sulfenylated derivatives have been further diversified to generate a new array of benzothiophene-fused 7-azaindole cores of pharmaceutical interest. The synthetic flexibility of this protocol has been highlighted through the gram-scale synthesis of sulfonylated 7-azaindole-based bioactive 5-HT6 receptor agonists.

Azaindole: A Candidate Anchor for Regulating Charge Polarity and Inducing Resonance Transmission at the Fermi Level via Dehydrogenation

Tuning the polarity of charge carriers is essential for designing molecular logic devices in molecular electronics. In this study, the electrical transport properties of a family of azaindole-anchored single-molecule junctions have been investigated using density functional theory combined with the nonequilibrium Green's function method. The obtained results reveal that dehydrogenation is an effective method for reversing the polarity of charge carriers. The molecular junctions based on the entire azaindole unit are n-type and contain electrons as the principal charge carriers, whereas the dehydrogenated junctions are p-type and contain holes as the main carriers. Furthermore, the azaindole anchors undergo a transition from an electron-rich to an electron-deficient state due to dehydrogenation, which is the original cause of the charge carrier polarity conversion. Dehydrogenated molecular junctions also exhibit the Fermi pinning effect and a sharp highest occupied molecular orbital (HOMO) resonance peak at the Fermi level. In addition, using Pt electrodes instead of Au electrodes is a means of producing a HOMO resonance peak a for azaindole-based molecular junctions. This work demonstrates the enormous potential of utilizing azaindole-anchored molecular junctions for the implementation of molecular logic and multifunctional molecular devices.

Recent synthetic strategies for the functionalization of fused bicyclic heteroaromatics using organo-Li, -Mg and -Zn reagents

This review highlights the use of functionalized organo-Li, -Mg and -Zn reagents for the construction and selective functionalization of 5- and 6-membered fused bicyclic heteroaromatics. Special attention is given to the discussion of advanced syntheses for the preparation of highly functionalized heteroaromatic scaffolds, including quinolines, naphthyridines, indoles, benzofurans, benzothiophenes, benzoxazoles, benzothiazoles, benzopyrimidines, anthranils, thienothiophenes, purine coumarins, chromones, quinolones and phthalazines and their fused heterocyclic derivatives. The organometallic reagents used for the desired functionalizations of these scaffolds are generally prepared in situ using the following methods: (i) through directed selective metalation reactions (DoM), (ii) by means of halogen/metal exchange reactions, (iii) through oxidative metal insertions (Li, Mg, Zn), and (iv) by transmetalation reactions (organo-Li and Mg transmetalations with ZnCl2 or ZnO(Piv)2). The resulting reactive organometallic reagents allow a wide range of C-C, C-N and C-X cross-coupling reactions with different electrophiles, employing in particular Kumada or Negishi protocols among other transition metal (Pd, Ni, Co, Cu, Cr, Fe, etc.)-catalyzed processes. In addition, key developments concerning selective metalation techniques will be presented, which rely on the use of RLi, LDA and TMP metal bases. These methods are now widely employed in organic synthetic chemistry and have proven to be particularly valuable for drug development programs in the pharmaceutical industry. New and improved protocols have resulted in many Li, Mg and Zn organyls now being compatible with functionalized aryl, heteroaryl, alkenyl, alkynyl and alkyl compounds even in the presence of labile functional groups, making these reagents well-suited for C(sp2)-C(sp2), C(sp2)-C(sp) and C(sp2)-C(sp3) cross-coupling reactions with fused heteroaryl halides. In addition, the use of some transition metal-catalyzed processes occasionally allows a reversed role of the reactants in cross-coupling reactions, providing alternative synthetic routes for the preparation of fused heteroaromatic-based bioactive drugs and natural products. In line with this, this article points to novel methods for the functionalization of bicyclic heteroaromatic scaffolds by organometallic reagents that have been published in the period 2010-2023.

Design and Synthesis of Potential Multi-Target Antidepressants: Exploration of 1-(4-(7-Azaindole)-3,6-dihydropyridin-1-yl)alkyl-3-(1H-indol-3-yl)pyrrolidine-2,5-dione Derivatives with Affinity for the Serotonin Transporter

We describe the design, synthesis and structure-activity relationship of a novel series of 1-(4-(7-azaindole)-3,6-dihydropyridin-1-yl)alkyl-3-(1H-indol-3-yl)pyrrolidine-2,5-dione derivatives with combined effects on the serotonin (5-HT1A) and dopamine (D2) receptors and the serotonin (5-HT), noradrenaline (NA), and dopamine (DA) transporters as multi-target directed ligands for the treatment of depression. All of the tested compounds demonstrated good affinity for the serotonin transporter (SERT). Among them, compounds 11 and 4 emerged as the lead candidates because of their promising pharmacological profile based on in vitro studies. Compound 11 displayed a high affinity for the 5-HT1A (Ki = 128.0 nM) and D2 (Ki = 51.0 nM) receptors, and the SERT (Ki = 9.2 nM) and DAT (Ki = 288.0 nM) transporters, whereas compound 4 exhibited the most desirable binding profile to SERT/NET/DAT among the series: Ki = 47.0 nM/167.0 nM/43% inhibition at 1 µM. These results suggest that compounds 4 and 11 represent templates for the future development of multi-target antidepressant drugs.

Nickel binding enables isolation and reactivity of previously inaccessible 7-aza-2,3-indolynes

N-Heteroaromatics are key elements of pharmaceuticals, agrochemicals, and materials. N-Heteroarynes provide a scaffold to build these essential molecules but are underused because five-membered N-heteroarynes have been largely inaccessible on account of the strain of a triple bond in that small of a ring. On the basis of principles of metal-ligand interactions that are foundational to organometallic chemistry, in this work we report the stabilization of five-membered N-heteroarynes in the nickel coordination sphere. A series of 1,2-bis(dicyclohexylphosphino)ethane nickel 7-azaindol-2,3-yne complexes were synthesized and characterized crystallographically and spectroscopically. Ambiphilic reactivity of the nickel 7-azaindol-2,3-yne complexes was observed with multiple nucleophilic, electrophilic, and enophilic coupling partners.

Efficient and metal-free synthesis of 2-aroyl 7-azaindoles via thermally induced denitrogenative intramolecular annulation of 1,2,3,4-tetrazolopyridines

A facile and metal-free intramolecular denitrogenative annulation strategy for the preparation of novel 2-aroyl 7-azaindoles has been developed from 3-(tetrazolo[1,5-a]pyridin-8-yl)prop-2-en-1-one in the presence of the deep eutectic solvent Dowtherm A. The valuable features of the protocol include a short reaction time, absence of any metal catalyst, utilization of a eutectic solvent, easy product isolation, and very good yields of novel 2-aroyl 7-azaindoles.

CuI/DMAP-Catalyzed Oxidative Alkynylation of 7-Azaindoles: Synthetic Scope and Mechanistic Studies

An efficient and practical method for the N-alkynylation of 7-azaindoles has been established by using CuI/DMAP catalytic system at room temperature and in open air. This simple protocol has been successfully employed in the synthesis of a wide range of N-alkynylated 7-azaindoles with good yields. Also, this approach is well-suited for large-scale N-alkynylation reactions. The designed N-alkynylated 7-azaindoles were further subjected to Cu-/Ir-catalyzed alkyne-azide cycloaddition (CuAAC/IrAAC) or "click" reaction for the rapid synthesis of 1,4-/1,5 disubstituted 1,2,3-triazole decorated 7-azaindoles. A mechanistic study based on density functional theory (DFT) calculations and ultraviolet-visible (UV) spectroscopic studies revealed that the CuI and DMAP combination formed a [CuII (DMAP)2 I2 ] species, which acts as an active catalyst. The DFT method was used to assess the energetic viability of an organometallic in the C-N bond formation pathway originating from the [CuII (DMAP)2 I2 ] complex. We expect that the newly designed Cu/DMAP/alkyne system will offer valuable insights into the field of Cu-catalyzed transformations.

Unlocking New Prenylation Modes: Azaindoles as a New Substrate Class for Indole Prenyltransferases

Aza-substitution, the replacement of aromatic CH groups with nitrogen atoms, is an established medicinal chemistry strategy for increasing solubility, but current methods of accessing functionalized azaindoles are limited. In this work, indole-alkylating aromatic prenyltransferases (PTs) were explored as a strategy to directly functionalize azaindole-substituted analogs of natural products. For this, a series of aza-l-tryptophans (Aza-Trp) featuring N-substitution of every aromatic CH position of the indole ring and their corresponding cyclic Aza-l-Trp-l-proline dipeptides (Aza-CyWP), were synthesized as substrate mimetics for the indole-alkylating PTs FgaPT2, CdpNPT, and FtmPT1. We then demonstrated most of these substrate analogs were accepted by a PT, and the regioselectivity of each prenylation was heavily influenced by the position of the N-substitution. Remarkably, FgaPT2 was found to produce cationic N-prenylpyridinium products, representing not only a new substrate class for indole PTs but also a previously unobserved prenylation mode. The discovery that nitrogenous indole bioisosteres can be accepted by PTs thus provides access to previously unavailable chemical space in the search for bioactive indolediketopiperazine analogs.

Design, synthesis, anticonvulsant activity and structure-activity relationships of novel 7-Azaindole derivatives

In search of new-structure compounds with good anticonvulsant activity and low neurotoxicity, a series of 3-(1,2,3,6-tetrahydropyridine)-7-azaindole derivatives was designed and synthesized. Their anticonvulsant activities were evaluated by maximal electroshock (MES) and pentylenetetrazole (PTZ) test, and neurotoxicity was determined by the rotary rod method. In the PTZ-induced epilepsy model, compounds 4i, 4p and 5 k showed significant anticonvulsant activities with ED50 values at 30.55 mg/kg, 19.72 mg/kg and 25.46 mg/kg, respectively. However, these compounds did not show any anticonvulsant activity in the MES model. More importantly, these compounds have lower neurotoxicity with protective index (PI = TD50/ED50) values at 8.58, 10.29 and 7.41, respectively. In order to obtain a clearer structure-activity relationship, more compounds were designed rationally based on 4i, 4p and 5 k and their anticonvulsant activities were evaluated on PTZ models. The results demonstrated that the N-atom at the 7-position of the 7-azaindole and the double-bond in the 1,2,3,6-tetrahydropyridine skeleton was essential for antiepileptic activities.

Exploiting Coordination Behavior of 7-Azaindole for Mechanistic Investigation of Chan-Lam Coupling and Application to 7-Azaindole Based Pharmacophores

Multiple spectroscopic techniques, along with single-crystal X-ray analysis, have been used to reveal the detailed structural and electronic information on reaction intermediates of a new copper(II)-DBU catalytic system for the N-arylation of 7-Azaindole. The reaction mixture of Chan-Lam cross-coupling yields two dimeric copper(II)-7-azaindole complexes, including one attached with DBU, prior to adding arylboronic acid and are confirmed structurally and spectroscopically. A suitable mechanism has been proposed using the dimeric copper(II) complex as a catalyst for the coupling reactions. The role of DBU as a base and also as an auxiliary ligand in the course of the reaction has been established. The transmetalated monomeric aryl-copper(II) species generated from the dimeric unit is oxidized by another equivalent of copper(II) to yield an aryl-copper(III) intermediate for facile N-arylation, which has been authenticated with UV-vis spectroscopy. The regeneration of the copper(II)-catalyst by aerial oxidation of colorless copper(I) species (generated via reductive elimination and disproportionation step) is confirmed by mass and absorption spectroscopy. Detailed DFT and TD-DFT calculations help to rationalize the proposed reaction intermediates and their corresponding electronic transitions. Moreover, the confirmation of copper(I)-7-azaindole intermediate via HRMS reaffirmed the involvement of Cu(II)/Cu(III)/Cu(I) species in the Chan-Lam type of coupling. A medicinally-important 7-azaindole-based SHP2 inhibitor has been synthesized via sequential arylation.

CuF2/DMAP-Catalyzed N-Vinylation: Scope and Mechanistic Study

CuF₂/DMAP has been established as an excellent catalytic system for vinylsilane-promoted N-vinylation of amides and azoles at room temperature without an external fluoride source. A mechanism has been proposed on the basis of the isolation of reactive intermediate [Cu(DMAP)₄Cl₂], fluoride ion-assisted transmetalation, and ultraviolet-visible spectroscopic studies. The catalytic efficiency of the synthesized and structurally characterized [Cu(DMAP)₄Cl₂] complex has been demonstrated. The valuable monomers for water-soluble polymers, viz., NVP, NVC, and NVIBA, were synthesized on a gram scale.

Current Fragment-to-lead Approaches Starting from the 7-azaindole: The Pharmacological Versatility of a Privileged Molecular Fragment

Fragment-based drug discovery is one of the most powerful paradigms in the recent context of medicinal chemistry and is being widely practiced by academic and industrial researchers. Currently, azaindoles are among the most exploited molecular fragments in pharmaceutical innovation projects inspired by fragment-to-lead strategies. The 7-azaindole is the most prominent representative within this remarkable family of pyrrolopyridine fragments, as it is present in the chemical structure of several approved antitumor drugs and also of numerous therapeutic candidates. In this paper, a brief overview on existing proofs of concept in the literature will be presented, as well as some recent works that corroborate 7-azaindole as a privileged and pharmacologically versatile molecular fragment.

N-Aroyloxycarbamates as switchable nitrogen and oxygen precursor: Ir/Cu controlled divergent C–H functionalization of heteroarenes

Chemodivergent control in the functionalization of nitrogen-directed aromatic C–H bonds has been achieved by a switchable catalyst.

Oxidatively induced reactivity in Rh(iii)-catalyzed 7-azaindole synthesis: insights into the role of the silver additive

The Ag+ cations oxidize and promote the activity of Rh-intermediates in the coupling of 2-aminopyridine and alkyne to form 7-azaindole.

Pd and photoredox dual catalysis assisted decarboxylative ortho-benzoylation of N-phenyl-7-azaindoles

Directing group assisted decarboxylative ortho-benzoylation of N-aryl-7-azaindoles with α-keto acids has been achieved by synergistic visible light promoted photoredox and palladium catalysis. The approach tenders rapid entry to aryl ketone...

Synthesis of Unsymmetrical Biheteroarenes via Dehydrogenative and Decarboxylative Coupling: a Decade Update

The design and development of robust and efficient methods for installing one heterocycle with another is endowed as a ubiquitous and powerful synthetic strategy to access complex organic biheterocycles in recent days due to their pervasive applications in medicinal as well as material chemistry. This perspective presents an overview on the recent findings and developments for the synthesis of unsymmetrical biheteroarenes via dehydrogenative and decarboxylative couplings with literature coverage mainly extending from 2011 to 2021. For simplification of the readers, the article has been subcategorized based on the catalysts used in the reactions.

Two-pot synthesis and photophysical studies of 1,6-disubstituted 5-aza-indoles from succinaldehyde and N-aryl propargylic-imines

A two-pot synthesis of 5-aza-indoles has been developed from aqueous succinaldehyde and N-aryl propargylic-imines. This overall protocol involves: (i) the metal-free [3 + 2] annulation of aqueous succinaldehyde and N-aryl propargylic-imines to access 2-alkynyl-pyrrole-3-aldehydes and (ii) Ag-catalyzed 6-endo-dig-cyclization to obtain substituted 5-aza-indoles in the second pot. The 5-aza-indoles showed engaging photophysical activities, and the practicality of this pot-economic gram-scale synthesis has been demonstrated.

Deprotometalation-Iodolysis and Direct Iodination of 1-Arylated 7-Azaindoles: Reactivity Studies and Molecule Properties

Five protocols were first compared for the copper-catalyzed C-N bond formation between 7-azaindole and aryl/heteroaryl iodides/bromides. The 1-arylated 7-azaindoles thus obtained were subjected to deprotometalation-iodolysis sequences using lithium 2,2,6,6-tetramethylpiperidide as the base and the corresponding zinc diamide as an in situ trap. The reactivity of the substrate was discussed in light of the calculated atomic charges and the pKa values. The behavior of the 1-arylated 7-azaindoles in direct iodination was then studied, and the results explained by considering the HOMO orbital coefficients and the atomic charges. Finally, some of the iodides generated, generally original, were involved in the N-arylation of indole. While crystallographic data were collected for fifteen of the synthesized compounds, biological properties (antimicrobial, antifungal and antioxidant activity) were evaluated for others.

C-H activation reactions of nitroarenes: current status and outlook

Ring substitution reactions of nitroarenes remain an under-developed area of organic synthesis, confined to the narrow domains of S_NAr and S_NArH reactions. While searching for alternative methodologies, we took stock of the C-H activation reactions of nitroarenes which unearthed a variety of examples of nitro directed regioselective C-H functionalization reactions such as ortho-arylation, -benzylation/alkylation, and -allylation, oxidative Heck and C-H arylation reactions on (hetero)aromatic rings. A collective account of these reactions is presented in this review to showcase the existing landscape of C-H activation reactions of nitroarenes, to create interest in this field for further development and propagate this strategy as a superior alternative for ring substitution reactions of nitroarenes. The prospect of merging the C-H activation of nitroarenes with C-NO₂ activation, thereby harnessing NO₂ as a transformable multitasking directing group, is also illustrated.

Rhodium(III)-catalyzed oxidative alkylation of N-aryl-7-azaindoles with cyclopropanols

An efficient Rh(iii)-catalyzed C-H oxidative alkylation of N-aryl-7-azaindoles with cyclopropanols by merging tandem C-H and C-C cleavage was developed. This transformation features mild reaction conditions, high regioselectivity, and excellent functional group compatibility. The resulting β-aryl ketone derivatives can be readily transformed into 7-azaindole-containing π-extended polycyclic heteroarenes.