Copper-Mediated Aerobic Oxidative Synthesis of 3-Bromo-imidazo[1,2-a]pyridines with Pyridines and Enamides

Electrochemical Activation of the C-X Bond on Demand: Access to the Atom Economic Group Transfer Reaction Triggered by Noncovalent Interaction

An atom economic method demonstrates the involvement of noncovalent interaction via hydrogen or halogen bonding interaction in triggering paired electrolysis for the group transfer reactions. Specifically, this method demonstrated the bromination of several aromatic and heteroaromatic compounds through the activation of the C(sp³)-Br bond of organic-bromo derivatives on demand. This electrochemical protocol is mild, and mostly no additional electrolyte is needed, which makes the workup process straightforward. Unlike the existing regioselective monobromination methods, this work utilizes a relatively small amount (1.2 equiv) of bromine surrogates that releases bromine on demand under the electrochemical condition and after completion of the reaction generates acetophenone as a useful byproduct. Green metrics indicate this protocol has a very good atom efficiency with an E-factor of 26.86 kg of waste/1 kg of product. In addition to the scale-up process, this strategy could be extended to the transfer of chlorine and thioaryl units. An extensive mechanistic study is accomplished to validate the hypothesis of noncovalent interaction using computational, spectroscopic, and cyclic voltammetry studies. Finally, the applicability of this newly developed nonbonding interaction to trigger paired electrolysis was extended to the chemoselective debromination of several dihalo organic compounds.

Ultrasound-Assisted Iodination of Imidazo[1,2-α]pyridines Via C-H Functionalization Mediated by tert-Butyl Hydroperoxide

A novel metal catalyst-free and environmentally friendly method for the regioselective iodination of imidazo[1,2-α]pyridines at their C3 position is disclosed, which has a wide substrate scope and could be sustainable. This reaction proceeds through ultrasound acceleration in the presence of a green alcohol solvent. Compared with a conventional heating system, the reaction efficiency and the rate are significantly improved and the iodine atom economy is maximized using ultrasound techniques.

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.

Perfluorocarboxylic Anhydrides Triggered Cyclization: Access to 4-Perfluoroalkylpyridines

A double nucleophilic addition-cyclization-elimination cascade is developed, that allows various 2,6-diaryl-4-perfluoroalkylpyridines to be synthesized in one step from easily available enamides and perfluorocarboxylic anhydrides. The procedure is also operationally simple and scalable and provides access to the facial construction of 4-fluoroalkylpyridines, which are of great interest in medicinal chemistry.

Recent advances on the transition-metal-catalyzed synthesis of imidazopyridines: an updated coverage

A comprehensive account of recent advances in the synthesis of imidazopyridines, assisted through transition-metal-catalyzed multicomponent reactions, C-H activation/functionalization and coupling reactions are highlighted in this review article. The basic illustration of this review comprises of schemes with concise account of explanatory text. The schemes depict the reaction conditions along with a quick look into the mechanism involved to render a deep understanding of the catalytic role. At some instances optimizations of certain features have been illustrated through tables, i.e., selectivity of catalyst, loading of the catalyst and percentage yield with different substrates. Each of the reported examples has been rigorously analyzed for reacting substrates, reaction conditions and transition metals used as the catalyst. This review will be helpful to the chemists in understanding the challenges associated with the reported methods as well as the future possibilities, both in the choice of substrates and catalysts. This review would be quite appealing to a wider range of organic chemists in academia and industrial R&D sectors working in the field of heterocyclic syntheses. In a nutshell, this review will be a guiding torch to envisage: (i) the role of various transition metals in the domain dedicated towards method development and (ii) for the modifications needed thereof in the R&D sector.

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.

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.

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.

C-H Functionalization of Azines

Azines, which are six-membered aromatic compounds containing one or more nitrogen atoms, serve as ubiquitous structural cores of aromatic species with important applications in biological and materials sciences. Among a variety of synthetic approaches toward azines, C-H functionalization represents the most rapid and atom-economical transformation, and it is advantageous for the late-stage functionalization of azine-containing functional molecules. Since azines have several C-H bonds with different reactivities, the development of new reactions that allow for the functionalization of azines in a regioselective fashion has comprised a central issue. This review describes recent advances in the C-H functionalization of azines categorized as follows: (1) S_NAr reactions, (2) radical reactions, (3) deprotonation/functionalization, and (4) metal-catalyzed reactions.

Insights into selenylation of imidazo[1,2-a]pyridine: synthesis, structural and antimicrobial evaluation

An efficient methodology was developed to synthesize novel organoselenium derivatives of formyl-substituted imidazo[1,2-a]pyridine by nucleophilic aromatic substitution.

Regiocontrolled functionalization of 2,3-dihalogenoimidazo[1,2-a]pyridines by Suzuki–Miyaura and Sonogashira cross-coupling reactions

An efficient method for regiocontrolled functionalization of 2,3-dihalogenoimidazo[1,2-a]pyridine was developed.

Synthesis of Disubstituted 3-Phenylimidazo[1,2-a]pyridines via a 2-Aminopyridine/CBrCl3 α-Bromination Shuttle

A versatile protocol for the synthesis of disubstituted 3-phenylimidazo[1,2-a]pyridines by coupling 2-aminopyridine with phenylacetophenones, phenylacetones, or β-tetralone has been developed. Isolated yields of up to 97% were obtained at 80 °C within 5 h. The 2-aminopyridine/CBrCl₃ system acts as an α-bromination shuttle by transferring Br from CBrCl₃ to the α-carbon of the carbonyl moiety. This triggers a series of steps with double C-N/C-N bond formation to the final product. The distinct advantages of this protocol include the use of commercially available inexpensive substrates, simplicity of a metal-free one-pot synthesis, and ease of scale-up to multigram quantities.

Novel and efficient transformation of enamides into α-acyloxy ketones via an acyl intramolecular migration process

Hydrogen peroxide and anhydride mediated transformation of enamides to afford substituted α-acyloxy ketones is described.

A facile and efficient method for the synthesis of 1,2,4-trisubstituted imidazoles with enamides and benzylamines

A novel and practical method for the construction of 1,2,4-trisubstituted imidazoles with enamides and benzylamines catalysed by CuBr and I₂ has been developed.