Catalyst free, C-3 functionalization of imidazo[1,2-a]pyridines to rapidly access new chemical space for drug discovery efforts

Micelle-Assisted C(sp2)-H Functionalization for C-Se and C-X Bond Formation in the Aqueous Medium

An environmentally sustainable, versatile, and cost-effective approach for C-Se and C-X (X = I, Br, and Cl) bond formation through C-H functionalization assisted by micellar catalysis in water is developed. The reaction utilizes a minimum amount of diorganyl diselenides and potassium halides for the respective functionalizations. The present protocol was suitable for scale-up synthesis, which directly provided the desired selenylated products without the need for chromatographic purification, in sufficient purity. The aqueous micellar catalysis system was reusable for up to 5 reaction cycles without compromising the reaction yield.

Unveiling the Significance of tert-Butoxides in Transition Metal-Free Cross-Coupling Reactions

The astounding reactivity of tert-butoxides in transition metal-free coupling reactions is driving the scientific community towards a new era of environmental friendly, as well as cost-effective, transformation strategies. Transition metal-catalyzed coupling reactions generate hazardous wastes and require harsh reaction conditions, mostly at elevated temperature, which increases not only costs but also environmental concerns regarding the methodology. Tert-butoxide-catalyzed/mediated coupling reactions have several advantages and potential applications. They can form carbon-carbon, carbon-heteroatom, and heteroatom-heteroatom bonds under mild reaction conditions. Mechanistic insights into these reactions include both ionic and radical pathways, with the fate of the intermediates depending on the reaction conditions and/or additives used in the reactions. Among all of the known tert-butoxides, potassium tert-butoxide has pronounced applications in transition metal-free coupling reactions as compared to other tert-butoxides, such as sodium and lithium tert-butoxides, because of the higher electropositivity of potassium compared to sodium and lithium. Moreover, potassium tert-butoxide can act as a source of base, nucleophile and single electron donors in various important transformations. In this review, we provide an extensive overview and complete compilation of transition metal-free cross-coupling reactions catalyzed/promoted by tert-butoxides during the past 10 years.

C3-Alkylation of Imidazo[1,2-a]pyridines via Three-Component Aza-Friedel-Crafts Reaction Catalyzed by Y(OTf)3

As an important class of nitrogen-containing fused heterocyclic compounds, imidazo[1,2-a]pyridines often exhibit significant biological activities, such as analgesic, anticancer, antiosteoporosis, anxiolytic, etc. Using Y(OTf)3 as a Lewis acid catalyst, a simple and efficient method has been developed for the synthesis of C3-alkylated imidazo[1,2-a]pyridines through the three-component aza-Friedel-Crafts reaction of imidazo[1,2-a]pyridines, aldehydes, and amines in the normal air atmosphere without the protection of inert gas and special requirements for anhydrous and anaerobic conditions. A series of imidazo[1,2-a]pyridine derivatives were obtained with moderate to good yields, and their structures were confirmed by 1H NMR, 13C NMR, and HRMS. Furthermore, this conversion has the advantages of simple operation, excellent functional group tolerance, high atomic economy, broad substrate scope, and can achieve gram-level reactions. Notably, this methodology may be conveniently applied to the further design and rapid synthesis of potential biologically active imidazo[1,2-a]pyridines with multifunctional groups.

Ketosulfonylmethylenation and sulfonylethyleneation of imidazoheterocycles with dimethylformamide as a methylene source

A hitherto unreported ketosulfonylmethylenation occurring at the C-3 position of imidazoheterocycles, with dimethylformamide as the methylene source was described. Using CoCl2·6H2O or Fe(acac)3 as efficient and inexpensive catalysts, some important biologically active methylenated compounds were prepared, with high efficacy, favorable functional group compatibilities, and a broad substrate scope.

Synthesis and site selective C-H functionalization of imidazo-[1,2-a]pyridines

Imidazo[1,2-a]pyridine has attracted much interest in drug development because of its potent medicinal properties, therefore the discovery of novel methods for its synthesis and functionalization continues to be an exciting area of research. Although transition metal catalysis has fuelled the most significant developments, extremely beneficial metal-free approaches have also been identified. Even though pertinent reviews focused on imidazo[1,2-a]pyridine synthesis, properties (physicochemical and medicinal), and functionalization at the C3 position have been published, none of these reviews has focused on the outcomes obtained in the field of global ring functionalization. We wish here to describe a brief synthesis and an overview of all the functionalization reactions at each carbon atom, viz, C2, C3, C5, C6, C7 and C8 of this scaffold, divided into sections based on site-selectivity and the type of functionalization methods used.

A multi pathway coupled domino strategy: I2/TBHP-promoted synthesis of imidazopyridines and thiazoles via sp3, sp2 and sp C-H functionalization

I2/TBHP-promoted, one-pot, multi pathway synthesis of imidazopyridines and thiazoles has been achieved through readily available ethylarenes, ethylenearenes and ethynearenes. I2/TBHP as an initiator and oxidant is used to realize the C-H functionalization of this domino reaction. Simple and available starting materials, wide range of functional group tolerance, high potential for drug activity of the products and application in production are the advantageous features of this method.

Synthesis and characterization of Fe3O4@Sal@Cu as a novel, efficient and heterogeneous catalyst and its application in the synthesis of 2-amino-4H-chromenes

This research paper presents the synthesis and characterization of the magnetic nanoparticle, Fe₃O₄@Sal@Cu, [Fe₃O₄@Si-CH₂-CH₂-CH₂-NH-NH-CO-N=CH-(2-HO-C₆H₄-)@Cu] as a green and retrievable catalyst. This catalyst was characterized by FTIR, XRD, EDX and TGA analyses. In addition, the catalytic activity of this new catalyst was investigated for the synthesis of 2-amino 4H-chromenes by producing good-to-excellent yields under mild reaction conditions. The other advantages of the developed nanocatalyst are its ecofriendliness, being easy to handle, high reusability and being magnetically separable. The synthesis of some new derivatives of 2-amino-4H-chromenes in the presence of this nanocatalyst is also reported.

Gold-catalyzed oxidation of terminal alkynes to glyoxals and their reactions with 2-phenylimidazo[1,2-a]pyridines: one-pot synthesis of 1,2-diones

A novel one-pot protocol for the convenient and efficient synthesis of (2-phenylimidazo[1,2-a]pyridin-3-yl)alkane-1,2-diones (3) in good yields (32-88%) from 2-phenylimidazo[1,2-a]pyridines (1) and terminal alkynes (2) has been established with a wide range of substrate scope. A tandem reaction sequence containing gold-catalyzed double oxidations of terminal alkynes to generate glyoxals, nucleophilic addition of 2-phenylimidazo[1,2-a]pyridines to glyoxals to yield α-hydroxyl ketones, and oxygenation of the α-hydroxyl ketones to afford the final products 3 under air atmosphere is involved in this method. Simple operation, mild reaction conditions, and widely available substrates make this strategy more affordable.

Discovery of novel class of histone deacetylase inhibitors as potential anticancer agents

Selective inhibition of histone deacetylases (HDACs) is an important strategy in the field of anticancer drug discovery. However, lack of inhibitors that possess high selectivity toward certain HDACs isozymes is associated with adverse side effects that limits their clinical applications. We have initiated a collaborative initiatives between multi-institutions aimed at the discovery of novel and selective HDACs inhibitors. To this end, a phenotypic screening of an in-house pilot library of about 70 small molecules against various HDAC isozymes led to the discovery of five compounds that displayed varying degrees of HDAC isozyme selectivity. The anticancer activities of these molecules were validated using various biological assays including transcriptomic studies. Compounds 15, 14, and 19 possessed selective inhibitory activity against HDAC5, while 28 displayed selective inhibition of HDAC1 and HDAC2. Compound 22 was found to be a selective inhibitor for HDAC3 and HDAC9. Importantly, we discovered a none-hydroxamate based HDAC inhibitor, compound 28, representing a distinct chemical probe of HDAC inhibitors. It contains a trifluoromethyloxadiazolyl moiety (TFMO) as a non-chelating metal-binding group. The new compounds showed potent anti-proliferative activity when tested against MCF7 breast cancer cell line, as well as increased acetylation of histones and induce cells apoptosis. The new compounds apoptotic effects were validated through the upregulation of proapoptotic proteins caspases3 and 7 and downregulation of the antiapoptotic biomarkers C-MYC, BCL2, BCL3 and NFĸB genes. Furthermore, the new compounds arrested cell cycle at different phases, which was confirmed through downregulation of the CDK1, 2, 4, 6, E2F1 and RB1 proteins. Taken together, our findings provide the foundation for the development of new chemical probes as potential lead drug candidates for the treatment of cancer.

BF3-etherate promoted facile access to vinyloxyimidazopyridines: a metal-free sustainable approach

A convenient and metal-free synthesis of vinyloxyimidazopyridine derivatives has been attained via BF₃.OEt₂ promoted one-pot multicomponent approach. This procedure involves a facile coupling of 2-aminopyridine derivatives with arylglyoxal and alkyne derivatives. BF₃.OEt₂ complexation has successfully catalyzed the reaction at room temperature. Utilization of transition metal-free catalyst, mild reaction conditions, easy handling and operational simplicity are key features of developed process.

Metal-free oxidative decarbonylative halogenation of fused imidazoles

An efficient strategy has been developed for the deformylative halogenation of carbaldehyde imidazo-fused heterocycles in the presence of TBHP controlled by temperature.

Visible-Light-Induced Dehydrogenative Imidoylation of Imidazo[1,2-a]pyridines with α-Amino Acid Derivatives and α-Amino Ketones

A new and efficient visible-light-promoted dehydrogenative cross-coupling reaction of imidazo[1,2-a]pyridines with α-amino carbonyl compounds toward imidoyl imidazo[1,2-a]pyridines is developed. A diverse range of imidazo[1,2-a]pyridines undergoes the dehydrogenative imidoylation smoothly with α-amino carbonyl compounds to access the corresponding products in satisfactory yields. We have also proposed the possible reaction mechanism based on preliminary mechanistic studies. The synthetic method has the advantages of wide substrate scope, good functional tolerance, and mild reaction conditions, which make this transformation more practical and sustainable.

Peroxide-mediated site-specific C–H methylation of imidazo[1,2-a]pyridines and quinoxalin-2(1H)-ones under metal-free conditions

An effective approach to realize the direct methylation of imidazo[1,2-a]pyridines and quinoxalin-2(1H)-ones with peroxides under metal-free conditions is described.

Synthesis of naphtho[1′,2′:4,5]imidazo[1,2-a]pyridines via Rh(iii)-catalyzed C–H functionalization of 2-arylimidazo[1,2-a]pyridines with cyclic 2-diazo-1,3-diketones featuring with a ring opening and reannulation

An unprecedented synthesis of functionalized naphtho[1′,2′:4,5]imidazo[1,2-a]pyridines via rhodium-catalyzed cascade reactions of 2-arylimidazo[1,2-a]pyridine-3-carbaldehydes with cyclic α-diazo-1,3-diketones is presented.

Catalyst-free direct cross-dehydrogenative coupling of imidazoheterocycles with glyoxal hydrates: an efficient approach to 1,2-diketones

An efficient and convenient methodology for catalyst-free cross-dehydrogenative coupling of imidazoheterocycles with glyoxal hydrates in good yields was developed. This methodology exhibits a broad substrate scope and excellent functional group tolerance and offers a straightforward means to produce different heterocycles such as imidazoheterocyclic quinoxaline, imidazoheterocyclic hydantoin and imidazoheterocyclic α-keto ketamine under relatively mild conditions. Biological evaluation showed that the most potent compound 3m possesses significant in vitro antiproliferative activities against human-derived lung cancer cell lines with an IC50 value of 14.8 μM.

One-Pot NBS-Promoted Synthesis of Imidazoles and Thiazoles from Ethylarenes in Water

A facile and eco-friendly method has been developed for the synthesis of imidazoles and thiazoles from ethylarenes in water. The reaction proceeds via in situ formation of α-bromoketone using NBS as a bromine source as well as an oxidant, followed by trapping with suitable nucleophiles to provide the corresponding products in good yields under metal-free conditions.

Intramolecular cyclization of imidazo[1,2-a]pyridines via a silver mediated/palladium catalyzed C–H activation strategy

Intramolecular cyclization of imidazo[1,2-a]pyridines has been developed via a C–H activation strategy to reach new chemical-space for drug discovery.

Photocatalyst-free decarboxylative aminoalkylation of imidazo[1,2-a]pyridines with N-aryl glycines enabled by visible light

A novel, green and efficient visible-light-promoted decarboxylative aminoalkylation reaction of imidazo[1,2-a]pyridines with N-aryl glycines has been described.