Iron(III)-Catalyzed Cascade Reaction between Nitroolefins and 2-Aminopyridines: Synthesis of Imidazo[1,2-a]pyridines and Easy Access towards Zolimidine

Imidazopyridine Family: Versatile and Promising Heterocyclic Skeletons for Different Applications

In recent years, there has been increasing attention focused on various products belonging to the imidazopyridine family; this class of heterocyclic compounds shows unique chemical structure, versatile optical properties, and diverse biological attributes. The broad family of imidazopyridines encompasses different heterocycles, each with its own specific properties and distinct characteristics, making all of them promising for various application fields. In general, this useful category of aromatic heterocycles holds significant promise across various research domains, spanning from material science to pharmaceuticals. The various cores belonging to the imidazopyridine family exhibit unique properties, such as serving as emitters in imaging, ligands for transition metals, showing reversible electrochemical properties, and demonstrating biological activity. Recently, numerous noteworthy advancements have emerged in different technological fields, including optoelectronic devices, sensors, energy conversion, medical applications, and shining emitters for imaging and microscopy. This review intends to provide a state-of-the-art overview of this framework from 1955 to the present day, unveiling different aspects of various applications. This extensive literature survey may guide chemists and researchers in the quest for novel imidazopyridine compounds with enhanced properties and efficiency in different uses.

Copper-Catalyzed Annulation of Electrophilic Benzannulated Heterocycles with 2-Aminopyridine and 2-Aminoquinoline: Direct Access toward Polyring-Fused Imidazo[1,2-a]pyridines

We have developed a direct method for the synthesis of polyring-fused imidazo[1,2-a]pyridines via a copper-catalyzed annulation of electrophilic benzannulated heterocycles with 2-aminopyridine and 2-aminoquinoline. From 3-nitroindoles and 2-aminopyridine, we could synthesize tetracenes, viz., indole-fused imidazo[1,2-a]pyridines, and by starting from 2-aminoquinoline, we could generate pentacenes, viz., indolo-imidazo[1,2-a]quinolines. In addition, we could also extend the methodology toward the synthesis of benzothieno-imidazo[1,2-a]pyridines starting from 3-nitrobenzothiophene. Furthermore, the basic photophysical properties of these synthesized heteroacenes were evaluated.

Visible-light-induced cascade reaction: a sustainable approach towards molecular complexity

Photoredox catalysis has demonstrated rapid evolution in the field of synthetic organic chemistry. On the other hand, the splendour of cascade reactions in providing complex molecular architectures renders them a cutting-edge research area. Therefore, the merging of photocatalysis with cascade synthesis brings out a synthetic paradigm with immense potential. The development of photocascade catalysis for a target molecule with a particular molecular skeleton and stereochemical framework presents certain challenges but provides a robust and environmentally benign synthetic alternative. This comprehensive review assembles all the accomplishments and highlights of visible-light-induced cascade reactions with literature coverage up to October 2022.

Synthetic Imidazopyridine-Based Derivatives as Potential Inhibitors against Multi-Drug Resistant Bacterial Infections: A Review

Fused pyridines are reported to display various pharmacological activities, such as antipyretic, analgesic, antiprotozoal, antibacterial, antitumor, antifungal, anti-inflammatory, and antiapoptotic. They are widely used in the field of medicinal chemistry. Imidazopyridines (IZPs) are crucial classes of fused heterocycles that are expansively reported on in the literature. Evidence suggests that IZPs, as fused scaffolds, possess more diverse profiles than individual imidazole and pyridine moieties. Bacterial infections and antibacterial resistance are ever-growing risks in the 21st century. Only one IZP, i.e., rifaximin, is available on the market as an antibiotic. In this review, the authors highlight strategies for preparing other IZPs. A particular focus is on the antibacterial profile and structure-activity relationship (SAR) of various synthesized IZP derivatives. This research provides a foundation for the tuning of available compounds to create novel, potent antibacterial agents with fewer side effects.

Iodine-Catalyzed Multicomponent Synthesis of Highly Fluorescent Pyrimidine-Linked Imidazopyridines

Herein, we report a metal-free one-pot three-component reaction of aryl methyl ketones, 2-aminopyridines, and barbituric acids for the synthesis of pyrimidine-linked imidazopyridines using a catalytic amount of molecular iodine in DMSO medium. This process involves a one-pot C-H oxidation, followed by the formation of one C-C and two C-N bonds. A wide variety of aryl methyl ketones and 2-aminopyridines were found to be suitable for this methodology. The UV and fluorescence properties of the synthesized products were studied in water and DMSO media. Most of the synthesized products exhibited very good to excellent fluorescence quantum yield. Among all the products, compounds 4p and 4q showed the maximum fluorescence quantum yield (0.36) in water medium under basic conditions and compound 4c showed the maximum fluorescence quantum yield (0.75) in DMSO medium.

Eosin Y-Catalyzed Synthesis of 3-Aminoimidazo[1,2-a]Pyridines via the HAT Process under Visible Light through Formation of the C-N Bond

A comfortable, environment-friendly, and metal-free approach for synthesizing the biologically important moiety aminoimidazopyridine through the multicomponent reaction of benzylamine, 2-aminopyridine, and t-butyl isocyanide under visible light using eosin Y as a photocatalyst has been developed. Inexpensive, nontoxic, the effortless accessibility of starting materials, and nonparticipation of particular glassware and a photoreactor system are important qualities of the current approach. Strangely, the mild conditions, environment-friendly, and enumerating tolerance of an extensive range of both electron-donating and electron-withdrawing groups are additional features of the approach.

Fe(iii)-Catalyzed synthesis of steroidal imidazoheterocycles as potent antiproliferative agents

An efficient and practical method has been developed for the synthesis of steroidal imidazoheterocycles via cost-effective and environmentally benign FeCl₃-catalyzed oxidative amination. A library of steroidal imidazo[1,2-a]pyridines was directly synthesized from readily available 2-aminopyridines and steroidal ketones in aerobic conditions. The synthesized compounds were screened for activity on human microsomal cytochrome P450s CYP7, CYP17 and CYP21. Antiproliferative activity of two lead compounds 3ia and 3la was additionally evaluated against the human MCF-7 (breast cancer), SKOV3 (ovarian cancer), and 22Rv1 (prostate cancer) cell lines. Steroidal imidazo[1,2-a]pyridine 3la which is a substrate molecule for CYP17A1 with IC₅₀ = 1.7 μM (MCF-7), 3.0 (SKOV3), and 6.0 μM (22Rv1) has proved to be more active than reference drug cisplatin.

Series of Functionalized 5-(2-Arylimidazo[1,2-a]pyridin-3-yl)pyrimidine-2,4(1H,3H)-diones: A Water-Mediated Three-Component Catalyst-Free Protocol Revisited

A water-mediated and catalyst-free practical method for the synthesis of a new series of pharmaceutically interesting functionalized 5-(2-arylimidazo[1,2-a]pyridin-3-yl)pyrimidine-2,4(1H,3H)-diones has been accomplished based on a one-pot multicomponent reaction between arylglyoxal monohydrates, 2-aminopyridines/2-aminopyrimidine, and barbituric/N,N-dimethylbarbituric acids under reflux conditions. The salient features of this protocol are avoidance of any additive/catalyst and toxic organic solvents, use of water as reaction medium, clean reaction profiles, operational simplicity, ease of product isolation/purification without the aid of tedious column chromatography, good to excellent yields, and high atom-economy and low E-factor.

Iodine Promoted Efficient Synthesis of 2-Arylimidazo[1,2-a]pyridines in Aqueous Media: A Comparative Study between Micellar Catalysis and an "On-Water" Platform

In a new and environmentally sustainable approach, a series of 2-arylimidazo[1,2-a]pyridine derivatives were synthesized in aqueous media in the presence of iodine as a catalyst. The reaction proceeded by condensation of various aryl methyl ketones with 2-aminopyridines to afford 2-arylimidazo[1,2-a]pyridines in good overall yields. Although several of the reactions were efficiently performed "on water", the addition of a surfactant, namely, sodium dodecyl sulphate , was found effective in terms of substrate scope and yield enhancement. Both methods were successfully used for the gram-scale synthesis of a marketed drug, zolimidine. The simple experimental setup, water as "green" media, and inexpensive catalyst are some of the merits of this protocol.

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.

Palladium-catalyzed C3-selective C–H oxidative carbonylation of imidazo[1,2-a]pyridines with CO and alcohols: a way to access esters

A palladium-catalyzed C3-selective C–H oxidative carbonylation for the synthesis of various esters from imidazo[1,2-a]pyridines and CO with high efficiency and high atom economy has been developed.

A convenient approach for the preparation of imidazo[1,2-a]-fused bicyclic frameworks via IBX/NIS promoted oxidative annulation

An IBX/NIS-induced intramolecular oxidative annulation of Mannich-type substrates is reported. This metal-free approach involving iodination, NH-oxidation, intramolecular C-N bond formation, and retro-Claisen-Schmidt sequence provides the construction of imidazo[1,2-a]pyridine, imidazo[1,2-a]pyrimidine as well as imidazo[1,2-a]pyrazine frameworks with yields up to 93%. In addition, a sequential one-pot process is also presented.

Facile synthesis of 2-arylimidazo[1,2-a]pyridines catalysed by DBU in aqueous ethanol

A facile protocol for the synthesis of 2-arylimidazo[1,2-a]pyridines was developed using two-component cyclization of substituted 2-aminopyridines and substituted phenacyl bromides in 65–94% yield. The reaction was DBU catalysed using green solvent, aqueous ethanol (1 : 1 v/v) at room temperature. The atom economy of the products was calculated to be 66.25–73.41%. The developed protocol successfully exhibits a broad substrate scope, less reaction time, high to moderate yield and multigram scale synthesis.

Lewis Acid-Catalyzed Intermolecular Annulation: Three-Component Reaction toward Imidazo[1,2-a]pyridine Thiones

A Lewis acid-catalyzed three-component annulation reaction of 2-aminopyridines and ynals with elemental sulfur was established. A series of imidazo[1,2-a]pyridine thiones was obtained in moderate to excellent yields. The merits of this transformation include easily available starting materials, multiple C-heteroatom bond formation in one pot, good functional group tolerance, elemental sulfur as S source, operational simplicity, etc.

DBU-mediated annulation of 2-aryl-3-nitro-2H-chromenes with 1,3-cyclohexanediones for the synthesis of benzofuro[2,3-c]chromenone derivatives

The DBU-mediated annulations of 2-aryl-3-nitro-2H-chromenes with 1,3-cyclohexanediones have been developed. This reaction involves a highly efficient domino sequence consisting of regioselective intermolecular Michael addition, intramolecular nucleophilic addition and aromatization as key unit steps. The reaction appears to be general for a variety of 2-aryl-3-nitro-2H-chromenes and tolerates the presence of aromatic moieties with electron-withdrawing and electron-donating substituents. This transformation provides a straightforward synthetic protocol for constructing benzofuro[2,3-c]chromenone derivatives.

A convergent synthesis of vinyloxyimidazopyridine via Cu(i)-catalyzed three-component coupling

The synthesis of vinyloxyimidazopyridine with complete regio- and stereoselectivity has been achieved by the Cu(i)-catalyzed three-component coupling of 2-aminopyridine, 2-oxoaldehyde and alkyne. This protocol is operationally very simple and has much potential for the synthesis of heteroarylated vinyl ethers from basic chemicals. Steroidal imidazopyridinyl vinyl ether was obtained successfully from ethynylestradiol.

Ce(iii)-catalyzed highly efficient synthesis of pyridyl benzamides from aminopyridines and nitroolefins without external oxidants

An efficient synthesis of a variety of pyridyl benzamides from 2-aminopyridines and nitroolefins is described. This rare-earth-metal-catalyzed reaction provides the corresponding products with broad substrate scope in moderate to excellent yields, in the absence of additives and external oxidants. Water is used as the source of the carbonyl oxygen atom in pyridyl benzamides. Furthermore, 2-substituted oxazolo[4,5-b]pyridines are formed in good yields under the standard conditions when 2-aminopyridin-3-ols are used as the substrates.

Facile synthesis of substituted quinolines by iron(iii)-catalyzed cascade reaction between anilines, aldehydes and nitroalkanes

A library of substituted quinolines has been synthesized by the reaction of aldehydes, anilines and nitroalkanes using a catalytic amount of Fe(iii) chloride.

Alumina-Supported Gold Nanoparticles as a Bifunctional Catalyst for the Synthesis of 2-Amino-3-arylimidazo[1,2-a]pyridines

The bifunctional catalytic efficacy of alumina-supported gold nanoparticles (Au/Al₂O₃) was investigated for the synthesis of a series of 2-amino-3-aryl-imidazopyridines through the chemoselective reduction of the corresponding 2-nitro-3-aryl-imidazo[1,2-a]pyridines in high isolated yields. This highly efficient protocol was initially applied for the synthesis of 2-nitro-3-aryl imidazo[1,2-a]pyridines via the reaction between 2-aminopyridine and nitroalkenes catalyzed by the present catalytic system Au/Al₂O₃. Moreover, the heterogeneous surface γ-Al₂O₃ was also found to catalyze this pathway in a comparable manner. However, only Au/Al₂O₃ was further proved as the appropriate catalytic system for the selective transfer hydrogenation of the synthesized 2-nitro imidazopyridine derivatives into the corresponding 2-amino-3-aryl imidazo[1,2-a]pyridines using NaBH₄ as a hydrogen-donor molecule. In addition, the one-pot two-step reaction between nitroalkenes and aminopyridines in the presence of Au/Al₂O₃-NaBH₄ provided directly the fast and facile synthesis of 2-amino-3-aryl imidazopyridines, highlighting a useful synthetic application of the catalytic protocol.

Cyanide-Free Ce(III)-Catalyzed Highly Efficient Synthesis of α-Iminonitriles from 2-Aminopyridines and Nitroalkenes via Intermolecular Dehydration Reaction

A direct and rapid method for the synthesis of α-iminonitriles achieved good to excellent yields. A novel intermolecular dehydration reaction between 2-aminopyridines and nitroalkenes is reported via a rare-earth-metal catalyst. The merits of this transformation include cyanide-free protocol, short reaction time, simple operation, water as the only byproduct, commercially available reagents, good functional group tolerance, etc. Moreover, nitroalkenes are demonstrated as a new "CN" source in this transformation.

RhIII-Catalyzed Synthesis of Isoquinolones and 2-Pyridones via Annulation of N-Methoxyamides and Nitroalkenes

The Rh(III)-catalyzed synthesis of 4-substituted isoquinolones and 2-pyridones by the annulation of N-methoxyamides and nitroalkenes has been developed. Both aliphatic and aromatic nitroalkenes were effective inputs. Annulations also proceeded for aromatic, alkenyl, and heteroaromatic C-H bond starting materials. Moreover, benzoic acid provided a novel nitrodihydroisocoumarin. The structure and relative stereochemistry of this compound, which is an oil at room temperature, was determined unambiguously by single crystal X-ray diffraction of its inclusion complex with a hydrogen-bonded host framework.

Heterogeneous Gold(I)-Catalysed Annulation between 2-Aminopyridines and Propiolaldehydes Leading to 3-Acylimidazo[1,2-A]Pyridines

The heterogeneous annulation between 2-aminopyridines and propiolaldehydes was achieved in CH2Cl2 at 25 °C in the presence of 3 mol% of MCM-41-immobilised phosphine gold(I) complex (MCM-41-PPh3-AuCl) and AgSbF6 under air, yielding a variety of 3-acylimidazo[1,2-a] pyridines in good yields. This heterogeneous gold(I) catalyst can be easily prepared by a simple procedure, recovered by filtration of the reaction solution and recycled up to seven times without significant loss of activity.