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.

Palladium-Catalyzed Regioselective C3-Allylic Alkylation of 2-Aryl Imidazopyridines with MBH Carbonates

Imidazopyridine is an important framework that constitutes several pharmaceutical drugs and biologically active molecules. Herein, we present the palladium-catalyzed regioselective C3-allylic alkylation of 2-aryl imidazopyridines with MBH carbonates. This strategy furnishes a broad spectrum of C3-allylated imidazopyridines, and their structures have been unequivocally established using X-ray analysis. Besides, the reaction can be easily scaled up on a gram scale, and the ensuing product can be smoothly manipulated into synthetically useful entities.

Recent Advances in Microwave-Assisted Copper-Catalyzed Cross-Coupling Reactions

Cross-coupling reactions furnishing carbon–carbon (C–C) and carbon–heteroatom (C–X) bond is one of the most challenging tasks in organic syntheses. The early developed reaction protocols by Ullmann, Ullman–Goldberg, Cadiot–Chodkiewicz, Castro–Stephens, and Corey–House, utilizing elemental copper or its salts as catalyst have, for decades, attracted and inspired scientists. However, these reactions were suffering from the range of functional groups tolerated as well as severely restricted by the harsh reaction conditions often required high temperatures (150–200 °C) for extended reaction time. Enormous efforts have been paid to develop and achieve more sustainable reaction conditions by applying the microwave irradiation. The use of controlled microwave heating dramatically reduces the time required and therefore resulting in increase in the yield as well as the efficiency of the reaction. This review is mainly focuses on the recent advances and applications of copper catalyzed cross-coupling generation of carbon–carbon and carbon–heteroatom bond under microwave technology.

On water catalyst-free synthesis of benzo[d]imidazo[2,1-b] thiazoles and novel N-alkylated 2-aminobenzo[d]oxazoles under microwave irradiation

A highly efficient unprecedented catalyst-free microwave-assisted procedure for synthesizing benzo[d]imidazo[2,1-b]thiazoles and N-alkylated 2-aminobenzo[d]oxazol in green media was developed. The transformation provided rapid access to functionalized benzo[d]imidazo[2,1-b]thiazoles from 2-aminobenzothiazole and N-alkylated 2-aminobenzo[d]oxazole from 2-aminobenzoxazole scaffolds under mild transition-metal-free conditions. This synthetic manipulation is expected to greatly expand the repertoire of reaction types in heterocyclic chemistry and pave the way for new syntheses of bioactive compounds.

A highly efficient unprecedented catalyst-free microwave-assisted procedure for synthesizing benzo[d]imidazo[2,1-b]thiazoles and N-alkylated 2-aminobenzo[d]oxazol in green media was developed.

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.

Transition metal-catalyzed C-H bond functionalization in multicomponent reactions: a tool toward molecular diversity

Transition metal-catalyzed C-H bond functionalization has numerous applications in organic synthesis as a powerful type of bond transformation. In particular, the combination of C-H functionalization with other types of chemical transformations in the manner of multicomponent reactions is an even more beneficial tool in the synthesis of small organic molecules because such reactions provide a platform for the rapid generation of high molecular diversity and complexity by making use of the advantages of both latent C-H bond transformation and the multicomponent reaction. Herein, we provide a review highlighting the research advances in the multicomponent reactions built upon transition metal-catalyzed C-H bond functionalization. The content spans from the reactions featuring the functionalization of C(sp³)-H, C(sp²)-H and C(sp)-H bonds over the last decade.

1,3-Dipolar Cycloaddition Reactions for the Synthesis of Novel Oxindole Derivatives and Their Cytotoxic Properties

The multicomponent reaction between isatin, amino acid, but-2-ynedioates, and phenacyl bromide has been developed using microwave irradiation under catalyst and base-free conditions in aqueous medium. This synthetic protocol is useful for the synthesis of various functionalized spirooxindole derivatives. This MCR exhibits a broad substrate scope with excellent yields and shorter reaction time. Additionally the synthesized spirooxindole derivatives were evaluated for their anticancer activity against three human cancer cell lines: MCF-7 (breast), A549 (lung), and HeLa cervical. Most of the compounds showed moderate to potent cytotoxic activity against the tested cell lines.

H2O2/DMSO-Promoted Regioselective Synthesis of 3,3'-Bisimidazopyridinylmethanes via Intermolecular Oxidative C(sp(2))-H Bond Activation of Imidazoheterocycles

In the past decade, metal-free approaches for C-C bond formation have attracted a great deal of attention due to their ease of use and low cost. This report represents a novel and metal-free synthesis of 3,3'-bisimidazopyridinylmethanes via intermolecular oxidative C(sp(2))-H bond functionalization of imidazo[1,2-a]pyridines with dimethyl sulfoxide as the carbon synthon (CH2) using H2O2 as a mild oxidant under air. A library of 3,3'-bis(2-arylimidazo[1,2-a]pyridin-3-yl)methanes has been achieved in good to excellent yields. The present methodology has been successfully applied to imidazo[2,1-b]thiazoles and imidazo[2,1-b]benzothiazoles. Furthermore, the current approach was also extended for the synthesis of unsymmetrical 3,3'-bisimidazopyridinylmethanes under optimized reaction conditions. A mechanistic pathway is proposed on the basis of experiments with radical scavengers and DMSO-d6 and ESI-MS observations.

A convenient and rapid microwave-assisted synthesis of spirooxindoles in aqueous medium and their antimicrobial activities

An efficient, regioselective and diastereoselective synthesis of diversely functionalized spirooxindoles under microwave irradiation in aqueous medium; most of the compounds show significant antimicrobial activities.