Light Promoted Synthesis of Quinoxalines and Imidazo[1,2‐ a ]pyridines via Oxybromination from Alkynes and Alkenes

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.

Halogen Bond-Activated Visible-Light-Mediated Regioselective C-H Arylation of 2-Phenylimidazo-[1,2-a]pyridines

An efficient method for transition metal-free halogen bond-assisted regioselective C-H arylation of 2-phenylimidazo-[1,2-a]pyridines under visible-light condition has been developed. The halogen bond between an aryl halide and base KO^tBu initiates an electron transfer process and generates an aryl radical, which catalyzes its coupling with 2-phenylimidazo-[1,2-a]pyridines to give arylated products in good yield. Several control experiments, density functional theory calculations, and ultraviolet-visible analysis indicate the presence of a halogen bond between an aryl halide and KO^tBu. This methodology has been successfully utilized to synthesize antileishmanial agents.

Iodine-Catalyzed Multicomponent Synthesis of Highly Fluorescent Pyrimidine-Linked Imidazopyridines

Herein, we report a metal-free one-pot three-component reaction of aryl methyl ketones, 2-aminopyridines, and barbituric acids for the synthesis of pyrimidine-linked imidazopyridines using a catalytic amount of molecular iodine in DMSO medium. This process involves a one-pot C-H oxidation, followed by the formation of one C-C and two C-N bonds. A wide variety of aryl methyl ketones and 2-aminopyridines were found to be suitable for this methodology. The UV and fluorescence properties of the synthesized products were studied in water and DMSO media. Most of the synthesized products exhibited very good to excellent fluorescence quantum yield. Among all the products, compounds 4p and 4q showed the maximum fluorescence quantum yield (0.36) in water medium under basic conditions and compound 4c showed the maximum fluorescence quantum yield (0.75) in DMSO medium.

Recent Developments in the Photochemical Synthesis of Functionalized Imidazopyridines

Imidazopyridines constitute one of the most important scaffolds in medicinal chemistry, as their skeleton could be found in a myriad of biologically active molecules. Although numerous strategies were elaborated for imidazopyridine preparation in the 2010s, novel eco-compatible synthetic approaches have emerged, conscious of climate change concerns. In this framework, photochemical methods have been promoted to conceive this heterocyclic motif over the last decade. This review covers the recently published works on synthesizing highly functionalized imidazopyridines by light induction.

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.

Recent Progress in Metal-Free Direct Synthesis of Imidazo[1,2-a]pyridines

This Mini-Review highlights the most effective protocols for metal-free direct synthesis of imidazo[1,2-a]pyridines, crucial target products and key intermediates, developed in the past decade. The emphases is given on the ecological impact of the methods and on the mechanistic aspects as well. The procedures efficiently applied in the preparation of important drugs and promising drug candidates are also underlined.