CBr4 Mediated Oxidative C–N Bond Formation: Applied in the Synthesis of Imidazo[1,2-α]pyridines and Imidazo[1,2-α]pyrimidines

Zn(II)-Stabilized Radical Ligand Enabled Radical-Type C(sp3)-H Activation-Cascade Cyclization to Imidazopyridines

A Zn(II)-stabilized radical-ligand enabled tandem cyclization via radical-type C(sp3)-H functionalization of N-benzylpyridin-2-amines with terminal alkynes producing straightforward access to a wide variety of imidazo[1,2-a]pyridines in moderate to good yields is reported. In the presence of KOtBu the Zn(II)-catalyst [ZnIILaCl2] (1 a) (La=2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline) undergoes one-electron reduction to the active catalyst [ZnII(La)•-Cl2] [1 a]- bearing a ligand-centered radical. Upon coordination of N-benzylpyridin-2-amine to [1 a]-, the radical-ligand abstracts a hydrogen atom from the benzylic position, forming a benzylic radical intermediate which, through radical addition with the alkyne generates a vinyl radical intermediate. Subsequent cyclization via intramolecular nucleophilic attack by the pyridine nitrogen produces imidazo[1,2-a]pyridines. Control experiments and spectroscopic investigation confirm the radical-ligand assisted tandem radical-type C(sp3)-H activation, addition, and cyclization steps.

Coumarin-imidazopyridine hybrids and their first-in-class ZnII metal complexes as potent dual entry and replication inhibitors of Zika viral infection

In this study, we synthesized and characterized a series of coumarin-imidazopyridine hybrid ligands (HL1-HL4) and their corresponding Zn(II) complexes (C1-C4). The ligands were synthesized via a two-step process in 56-90 % yields. The resulting ligands, were utilized to form Zn(II) complexes, characterized by conductivity measurements, HRMS, IR, 1H NMR spectroscopy and X-ray diffractions. Biological evaluations revealed that these compounds exhibited potent antiviral activity against Zika virus (ZIKV), with EC50 values ranging from 0.55 to 4.8 µM and SI of up to 1490. Notably, the complexes (the first-in-class Zn(II) anti-ZIKV complexes) generally displayed enhanced activity compared to their respective ligands, with some compounds outperforming the reference antiviral, ribavirin. The Time of Addition assay suggested that while some compounds interfere with both viral entry (with a virucidal component) and replication phases, other only acted in replication phases. These results together with molecular modeling studies on ZIKV Envelope protein and ZIKV NS2B-NS3 offered insights for their mode of actions and further optimizations.

Investigating catalytic pathways: a comparative review of homogeneous and heterogeneous catalysis for 3-aroylimidazo[1,2-a]pyridine synthesis

3-aroylimidazo[1,2-a]pyridines represent a class of derivatives in the imidazo[1,2-a]pyridine family known for their important biological and pharmaceutical activities. Consequently, various methodologies have been designed to simplify the synthesis of this structure, with an emphasis on the use of cost-effective starting materials and environmentally friendly protocols. All the methods developed in recent years (from 2016 to 2023) rely on homogeneous or heterogeneous catalysts. Therefore, we aim to perform a comparative analysis between these two approaches, elucidating their respective advantages and limitations. The first part of this work focuses on techniques employing homogeneous catalysts, followed by the next section devoted to heterogeneous catalysts. This comprehensive review should be of substantial interest to researchers in the fields of organic and medicinal chemistry, as it provides a valuable resource for their research.

HPW-Catalyzed environmentally benign approach to imidazo[1,2-a]pyridines

The imidazo[1,2-a]pyridine moiety is present in drugs with several biological activities. The most direct way of obtaining this nucleus is the Groebke-Blackburn-Bienaymé three-component reaction (GBB-3CR) between aminopyridines, aldehydes, and isocyanides under both Lewis and Brønsted acid catalysis. However, several catalysts for this reaction have major drawbacks such as being expensive, extremely dangerous, strong oxidizing, and even explosive. In this scenario, heteropolyacids emerge as greener and safer alternatives due to their very strong Brønsted acidity. In particular, phosphotungstic acid (HPW) is an economical and green attractive catalyst for being cheap, non-toxic, and is known for its chemical and thermal stability. Herein, we report a straightforward approach to the GBB-3CR using HPW as catalyst in ethanol under microwave (μw) heating. This convenient environmentally benign methodology is broad in scope, provides the heterobicyclic products in high yields (up to 99%), with a low catalyst loading (2 mol %) in only 30 minutes, and allows the successful use of aliphatic aldehydes, substrates not so frequently explored with most usual catalysts for this reaction. Furthermore, the aforementioned advantages make this methodology very attractive and superior to the existing ones.

Pyrimidine-fused Dinitrogenous Penta-heterocycles as a Privileged Scaffold for Anti-Cancer Drug Discovery

Pyrimidine-fused derivatives that are the inextricable part of DNA and RNA play a key role in the normal life cycle of cells. Pyrimidine-fused dinitrogenous penta-heterocycles including pyrazolopyrimidines and imidazopyrimidines is a special class of pyrimidine-fused compounds contributing to an important portion in anti-cancer drug discovery, which have been discovered as core structure for promising anti-cancer agents used in clinic or clinical evaluations. Pyrimidine-fused dinitrogenous penta-heterocycles have become one privileged scaffold for anti-cancer drug discovery. This review consists of the recent progress of pyrimidine-fused dinitrogenous penta-heterocycles as anti-cancer agents and their synthetic strategies. In addition, this review also summarizes some key structure-activity relationships (SARs) of pyrimidine-fused dinitrogenous penta-heterocycle derivatives as anti-cancer agents.

I2-Mediated [3 + 2] annulation of methyl-azaarenes with alkyl 2-isocyanoacetates or amino acid ester hydrochlorides: selective synthesis of iodine-functionalized and non-iodine-functionalized fused imidazoles

An I2-mediated [3 + 2] annulation of methyl-azaarenes with alkyl 2-isocyanoacetates or amino acid ester hydrochlorides has been demonstrated. This strategy involves the C≡N cleavage of isocyanides and can selectively...

Structural Determinants for the Mode of Action of Imidazopyridine DS2 at δ-Containing γ-Aminobutyric Acid Type A Receptors

Despite the therapeutic relevance of δ-containing γ-aminobutyric acid type A receptors (GABA_ARs) and the need for δ-selective compounds, the structural determinants for the mode and molecular site of action of δ-selective positive allosteric modulator imidazo[1,2-a]pyridine DS2 remain elusive. To guide the quest for insight, we synthesized a series of DS2 analogues guided by a structural receptor model. Using a fluorescence-based fluorometric imaging plate reader membrane potential assay, we found that the δ-selectivity and the pharmacological profile are severely affected by substituents in the 5-position of the imidazopyridine core scaffold. Interestingly, the 5-methyl, 5-bromo, and 5-chloro DS2 analogues, 30, 35, and 36, were shown to be superior to DS2 at α4β1δ as mid-high nanomolar potency δ-selective allosteric modulators, displaying 6-16 times higher potency than DS2. Of these, 30 also displayed at least 60-fold selectivity for α4β1δ over α4β1γ2 receptor subtypes representing a potential tool for the selective characterization of δ-containing GABA_ARs in general.

Annulation of Conjugated Azine-Imine with a Sulfoxonium Ylide in a Noncarbenoid Route to Synthesize Multisubstituted Imidazole-Fused Heterocycles

A new [4+1]-annulation of in situ generated heterocyclic azine-aldimines with β-keto sulfoxonium ylides has been developed. The reaction constructs N-fused imidazole rings. In the reaction, the ylides play a dual-functional role of a nucleophilic 1,1-dipolar one-carbon synthon and a source of an internal oxidant, dimethyl sulfoxide, that promotes in situ dehydrogenation to product scaffolds. The method enables access to imidazo-pyridine, pyrazine, and pyrimidine heteroaromatics.

Palladium-catalyzed regioselective direct C–H bond alkoxycarbonylation of 2-arylimidazo[1,2-a]pyridines

A palladium catalyzed method for regioselective alkoxycarbonylation of imidazo[1,2-a]pyridines using W(CO)₆, as the CO source, and alcohols has been reported.

Recent advances in the application of tetrabromomethane in organic synthesis

This review article covers the use of tetrabromomethane as mediator, catalyst and reagents for organic synthesis for the period from 2007 to 2020.

Synthesis of imidazo[1,2-a][1,3,5]triazines by NBS-mediated coupling of 2-amino-1,3,5-triazines with 1,3-dicarbonyl compounds

An efficient NBS-promoted synthesis of substituted imidazo[1,2-a][1,3,5]triazines under mild conditions has been developed.

Direct intramolecular double cross-dehydrogentive-coupling (CDC) cyclization of N-(2-pyridyl)amidines under metal-free conditions

A facile transition-metal-free protocol to form 2-iminoimidazo[1, 2-a]-pyridines bearing a -CHBr₂ group and an aza-quaternary carbon center at the 3 position from N-(2-pyridyl)amidines substrates, in which the new heterocyclic skeletons constructed from amidines via radical reactions or nucleophilic substitution reactions are promoted only by CBr₄ under mild conditions, is demonstrated. The reactions were realized by intramolecular CDC reaction involving C-N and C-C bond formation via the sequential C(sp³)-H bifunctionalization mode on the same carbon atom under mild conditions. Moreover, this work also provides an excellent and representative example for CBr₄ as an efficient reagent to initiate radical reactions under initiator-free conditions or to give rise to nucleophilic substitution reactions only by base.

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.

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.

KI-catalyzed C-S bond formation via an oxidation relay strategy: efficient access to various α-thio-β-dicarbonyl compounds

An efficient and practical methodology to obtain α-thio-β-dicarbonyl compounds was presented under alkaline conditions via potassium iodide (KI) catalysis; various symmetrical/unsymmetrical 1,3-dicarbonyl compounds were obtained under an aerobic atmosphere in moderate to excellent yields, with good functional group tolerance. Notably, a widely used anti-inflammatory drug butazodine could be modified with our protocol, even on a gram scale.

Photocatalytic regeneration of brominating agent in the visible light-mediated synthesis of imidazo[1,2-a]pyridines

Regenerating the brominating agent by erythrosine B closes a catalytic cycle for the construction of the imidazo[1,2-a]pyridine framework.

Chemodivergent synthesis of N-(pyridin-2-yl)amides and 3-bromoimidazo[1,2-a]pyridines from α-bromoketones and 2-aminopyridines

N-(Pyridin-2-yl)amides and 3-bromoimidazo[1,2-a]pyridines were synthesized respectively from α-bromoketones and 2-aminopyridine under different reaction conditions.

An efficient synthesis of benzothiazole using tetrabromomethane as a halogen bond donor catalyst

A CBr₄ catalyzed simple, mild, and efficient protocol has been developed for the synthesis of 2-substituted benzothiazole from 2-aminothiophenols and N-methylthioamides under solvent free conditions.

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

Multicomponent reactions (MCRs) are robust tools for the rapid synthesis of complex, small molecule libraries for use in drug discovery and development. By utilizing MCR chemistry, we developed a protocol to functionalize the C-3 position of imidazo[1,2-a]pyridine through a three component, decarboxylation reaction involving imidazo[1,2-a]pyridine, glyoxalic acid, and boronic acid.

Tandem radical cyclization of N-methacryloyl benzamides with CBr4 to construct brominated isoquinolinediones

A simple cumene (isopropylbenzene, IPB) promoted auto-oxidation involved tandem radical cyclization of N-methacryloyl benzamides using stable and easy-to-handle CBr₄ as the bromine source is described. This strategy provides an efficient and practical approach for the synthesis of bromine containing isoquinolinediones. This method also presents a new way to generate bromine radicals using a mild auto-oxidation pathway.

Iodine–NH4OAc mediated regioselective synthesis of 2-aroyl-3-arylimidazo[1,2-a]pyridines from 1,3-diaryl-prop-2-en-1-ones

An efficient, metal-free regioselective synthesis of 2-aroyl-3-arylimidazo[1,2-a]pyridines from 1,3-diaryl-prop-2-en-1-ones has been developed by their reaction with 2-aminopyridine in the presence of I₂/NH₄OAc.

Metal- and base-free synthesis of imidazo[1,2-a]pyridines through elemental sulfur-initiated oxidative annulation of 2-aminopyridines and aldehydes

An elemental sulfur-based metal- and base-free system enables oxidative imidazo[1,2-a]pyridine annulations.

CBr4-mediated cross-coupling reactions of α-amino carbonyl compounds with alcohols and thiols to build C–O and C–S bonds, respectively

The direct C(sp³)–H bond functionalization of α-amino carbonyl compounds with CBr₄ to construct C–O and C–S bonds.

Synthesis of Imidazopyridines via Copper-Catalyzed, Formal Aza-[3 + 2] Cycloaddition Reaction of Pyridine Derivatives with α-Diazo Oxime Ethers

The Cu-catalyzed, formal aza-[3 + 2] cycloaddition reaction of pyridine derivatives with α-diazo oxime ethers in trifluoroethanol was used to synthesize imidazopyridines via the release of molecular nitrogen and elimination of alcohol. These methods enabled modular synthesis of a wide range of N-heterobicyclic compounds such as imidazopyridazines, imidazopyrimidines, and imidazopyrazines with an α-imino Cu-carbenoid generated from the α-diazo oxime ethers and copper.