Metal- and additive-free direct C3-aminomethylation of imidazo[1,2-a]pyridines with 1,3,5-triazinanes

Functionalization of Indoles with 1,3,5-Triazinanes: Chemistry of Aminomethylation vs the Hofmann-Martius-Type Rearrangement

We have developed efficacious routes toward the selective synthesis of two classes of compounds such as C-3 amino-methylated indoles and 4-indol-3-yl-methylanilines from the same precursors, namely, indoles and 1,3,5-triazinanes. It is reported that the controlled cleavage of 1,3,5-triazinanes can be effected by heat for the generation of aryl imine motifs, and we observed that the presence of Lewis acid influences the course of these transformations toward different products. The reaction toward indol-3-yl-methylanilines proceeds via a nucleophilic attack of indole to the aryl imine generated from the 1,3,5-triazinanes to form an amino-methylated product which undergoes a Lewis acid mediated Hofmann-Martius-type rearrangement. In the absence of a Lewis acid, the reaction between indoles and 1,3,5-triazinane afforded C-3 amino-methylated indoles. Experimentally, we could prove that the amino-methylated product was the intermediate formed during the Lewis acid catalyzed synthesis of 4-indol-3-yl-methylanilines and that the process proceeds in an intermolecular fashion. The selective synthesis of both classes of compounds was found to be general, and a library of molecules was generated.

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.

Catalyst-free inverse-electron-demand aza-Diels-Alder reaction of 4,4-dicyano-2-methylenebut-3-enoates and 1,3,5-triazinanes: access to polysubstituted tetrahydropyridines

An inverse-electron-demand aza-Diels-Alder reaction between 4,4-dicyano-2-methylenebut-3-enoates and 1,3,5-triazinanes under catalyst-free and additive-free conditions was developed, which provided a highly convenient and straightforward method to construct a series of polyfunctionalized tetrahydropyridines in high yields. This strategy features numerous advantages, including high efficiency, good functional group tolerance, broad substrate scope, and environmentally friendly conditions.

Reaction of imidazo[1,2-a]pyridines with coumarin-3-carboxylic acids: a domino Michael addition/decarboxylation/oxidation/annulation

A palladium-catalyzed decarboxylative domino reaction of imidazo[1,2-a]pyridines and coumarin-3-carboxylic acids has been developed, which provides access to dibenzoisochromenoimidazo[1,2-a]pyridin-6-ones possessing six fused rings.