Copper-catalyzed four-component synthesis of imidazo[1,2-a]pyridines via sequential reductive amination, condensation, and cyclization

First-row transition metal for isocyanide-involving multicomponent reactions (IMCR)

First-row transition metal catalyzed transformations that are able to construct complex molecules from simple, readily obtainable feedstocks have become a keystone of modern synthetic organic chemistry. Particularly, the multicomponent reaction (MCR) involving carbon-carbon (C-C) as well as carbon-heteroatom (C-X) bond formation plays an essential role in many chemical conversions, and insurgencies in these reactions powerfully improve the overall synthetic efficiency. Recently, MCRs emerges rapidly because of its greener sides like eco-friendly nature, swift and straightforward execution, high atom/step economy, and construction of aimed product with lowest or no by-product, usually in quantitative yield. Curiously, the exceptional divalent carbon atoms of isocyanides make them predominantly useful components in multicomponent reactions. As a result of widespread research over the past few decades, numerous well-designed and effective procedures for the first-row TM-catalyzed MCR to afford the various entities have been reported. These aspects are summarized in this review article. A particular focus on comparative discussion of various first-row transition-metal catalyzed isocyanide-based multicomponent reactions through mechanistic details included in the review article.

Synthesis of Imidazo[1,2-a]pyridine-Fused 1,3-Benzodiazepine Derivatives with Anticancer Activity via a One-Pot Cascade GBB-3CR/Pd(II)-Catalyzed Azide-Isocyanide Coupling/Cyclization Process

A new one-pot synthesis of imidazo[1,2-a]pyridine-fused 1,3-benzodiazepine derivatives via a sequential GBB-3CR/Pd(II)-catalyzed azide-isocyanide coupling/cyclization process was developed. The Groebke-Blackburn-Bienaymé three-component reactions (GBB-3CR) of 2-aminopyridine, 2-azidobenzaldehydes, and isocyanides in the presence of a catalytic amount of p-toluenesulfonic acid gave azide intermediates without separation. The reaction was followed by using another molecule of isocyanides to produce imidazo[1,2-a]pyridine-fused 1,3-benzodiazepine derivatives in good yields by the Pd(II)-catalyzed azide-isocyanide coupling/cyclization reaction. The synthetic approach produces novel nitrogen-fused polycyclic heterocycles under mild reaction conditions. The preliminary biological evaluation demonstrated that compound 6a inhibited glioma cells efficiently, suggesting potentially broad applications of the approach for synthesis and medicinal chemistry.

Hydromagnesite sheets impregnated with cobalt-ferrite magnetic nanoparticles as heterogeneous catalytic system for the synthesis of imidazo[1,2-a]pyridine scaffolds

This paper manifests an A³-coupling strategy assisted by novel hydromagnesite sheets impregnated with cobalt ferrite (CoFe₂O₄-HMS) magnetic nanoparticles (MNPs) as an environmentally benign nanocomposite to synthesize imidazo[1,2-a]pyridine scaffolds under ultrasonication. The synthesis of these biologically active derivatives was achieved through A³-coupling employing 2-aminopyridines derivatives, pertinent aryl aldehydes, and phenylacetylene in the presence of polyethylene glycol 400 (PEG 400) as a green solvent under aerobic conditions. Based on its high product yield (up to 94%) in a short reaction time, with a modest catalyst loading, excellent catalyst, and solvent recyclability without substantial loss of operation (up to five synthetic cycles), as demonstrated by the high ecological compatibility and sustainability factors, this strategy follows the principles of green chemistry. The synthesized nanocomposite was characterized via several spectroanalytical techniques, including PXRD, FE-SEM, HR-TEM, EDAX, ICP-AES, FT-IR, Raman spectroscopy, CO₂-TPD, TGA-DTA-DTG analyses, magnetic studies, and nitrogen porosimetry. Furthermore, the structures of synthesized compounds were confirmed based on FT-IR, ¹H NMR, ¹³C NMR, mass spectroscopy, and elemental analysis data.

Sustainable synthesis of imidazo[1,2-a]pyridine scaffolds assisted by hydromagnesite sheets impregnated with cobalt–ferrite (CoFe₂O₄-HMS) magnetic nanoparticles (MNPs).

The status of isocyanide-based multi-component reactions in Iran (2010-2018)

Isocyanides as key intermediates and magic reactants have been widely applied in organic reactions for direct access to a broad spectrum of remarkable organic compounds. Although the history of these magical compounds dates back more than 100 years, it still has been drawing widespread attention of chemists who confirmed their versatility and effectiveness. Because of their wide spectrum of pharmacological, industrial and synthetic applications, many reactions with the utilization of isocyanides are reported in the literature. In this context, Iranian scientist played a significant role in the growth of isocyanides chemistry. The present review article covers literature from the period starting from 2010 onward and encompasses new synthetic routes and organic transformation involving isocyanides by Iranian researchers. During this period, a diverse range of isocyanide-based multi-component reactions (I-MCRs) has been reported such as a new modification of Ugi, post-Ugi, Passerini and Groebke-Blackburn-Bienayme condensation reactions, isocyanide-based [1 + 4] cycloaddition reactions, isocyanide-acetylene-based MCRs, isocyanide and Meldrum's acid-based MCRs, several unexpected reactions besides green mediums and novel catalytic systems for the synthesis of diverse kinds of pharmaceutically and industrially remarkable heterocyclic and linear organic compounds. This review also emphasizes the neoteric applications of I-MCR for the synthesis of valuable peptide and pseudopeptide scaffolds, enzyme immobilization and functionalization of materials with tailorable properties that can play important roles in the plethora of applications.

Electro-organic synthesis of tetrahydroimidazo[1,2-a]pyridin-5(1H)-one via a multicomponent reaction

Electro-synthesis through a one-pot three-component condensation of corresponding aldehydes, Meldrum's acid, and 2-(nitromethylene)imidazolidine resulted in a series of novel tetrahydroimidazo[1,2-a]pyridine-5(1H)-one derivatives containing an electronegative pharmacophore (=CNO₂). The process was carried out in propanol medium with sodium bromide presented as electrolyte, inside an undivided cell with good to excellent yields. As a powerful entry into fused polycyclic structures related to bioactive heterocycles, this green protocol shows great potential.

The Groebke-Blackburn-Bienaymé Reaction

Imidazo[1,2-a]pyridine is a well-known scaffold in many marketed drugs, such as Zolpidem, Minodronic acid, Miroprofen and DS-1 and it also serves as a broadly applied pharmacophore in drug discovery. The scaffold revoked a wave of interest when Groebke, Blackburn and Bienaymé reported independently a new three component reaction resulting in compounds with the imidazo[1,2-a]-heterocycles as a core structure. During the course of two decades the Groebke Blackburn Bienaymé (GBB-3CR) reaction has emerged as a very important multicomponent reaction (MCR), resulting in over a hundred patents and a great number of publications in various fields of interest. Now two compounds derived from GBB-3CR chemistry received FDA approval. To celebrate the first 20 years of GBB-chemistry, we present an overview of the chemistry of the GBB-3CR, including an analysis of each of the three starting material classes, solvents and catalysts. Additionally, a list of patents and their applications and a more in-depth summary of the biological targets that were addressed, including structural biology analysis, is given.

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.

The use of magnetic starch as a support for an ionic liquid-β-cyclodextrin based catalyst for the synthesis of imidazothiadiazolamine derivatives

In this paper, a novel catalyst is introduced based on the introduction of an ionic liquid onto the β-cyclodextrin. The ionic liquid-β-cyclodextrin was anchored to magnetic starch (denoted βCD-IL@M-Starch) and fully characterized by several methods including TEM, TGA, VSM and FT-IR. The catalyst was used for the synthesis of imidazo[2,1-b][1,3,4]thiadiazol-5-amine and imidazo[1,2-a]pyridin-3-amine derivatives. βCD-IL@M-Starch catalyst showed very good activity in the synthesis of diphenylimidazo[2,1-b][1,3,4]thiadiazol-5-amine derivatives from the corresponding benzaldehyde, semicarbazide, benzaldehydes and isocyanides. The products were obtained in a mild reaction conditions in good isolated yields in the presence of βCD-IL@M-Starch as catalyst. The catalyst showed to be magnetically reusable, and gave very good results in 10 sequential reactions.

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.

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.