Catalyst-Controlled Regiodivergent Oxidative Annulation of 2-Arylimidazo[1,2-a]pyridines with Cinnamaldehyde Derivatives for Construction of Fused N-Heterocyclic Frameworks

Access to pyrido-pyrimidinone and imidazopyridine via Fe(III)-mediated denitrogenative annulation of tetrazolopyridine with β-keto ester

An iron(III)-mediated denitrogenative annulation of tetrazolopyridine with β-keto esters was developed to produce two classes of nitrogen-containing heterocycles; pyrido-pyrimidinone and imidazopyridine, and the selectivity of product formation was controlled by alkali metal halide salts present in the reaction medium. Mechanistic studies suggested that these reactions likely proceed via a radical mechanism. This single-step reaction yielded a diverse range of these N-heterocycles linked to different natural product scaffolds. A gram-scale synthesis was carried out to confirm the scalability of this method.

Ru(II)-catalyzed oxidative (4+3) C-H/C-H annulation of 2-aryl-4H-pyrido[1,2-a]pyrimidin-4-ones with allyl alcohol

A ruthenium(II)-catalyzed direct oxidative (4+3) C-H/C-H annulation of 2-aryl-4H-pyrido[1,2-a]pyrimidin-4-ones with allyl alcohol has been described. The developed method yielded the corresponding annulated products in moderate to good yields. In addition, based on the control experimental results, a possible reaction mechanism has been proposed for the (4+3) C-H/C-H annulation reaction.

Rhodaelectro-Catalyzed Synthesis of Pyrano[3,4-b]indol-1(9H)-ones via the Double Dehydrogenative Heck Reaction between Indole-2-carboxylic Acids and Alkenes

A rhodaelectro-catalyzed double dehydrogenative Heck reaction of indole-2-carboxylic acids with alkenes has been developed for the synthesis of pyrano[3,4-b]indol-1(9H)-ones. The weakly coordinating carboxyl group is utilized twice as a directing group to activate the C-H bonds throughout the reaction. This reaction precedes an acceptorless dehydrogenation under exogenous oxidant-free conditions in an undivided cell with a constant current.

Photoredox-Cobaloxime Catalysis for Selective Oxidative Dehydrogenative [4+2] Annulation of Imidazo-Fused Heterocycles with Alkenes

A selective oxidative [4+2] annulation of alkenes with imidazo-fused heterocycles has been developed by using the synergistic combination of photoredox and cobaloxime catalysts. It allows facile access to various imidazole-fused polyaromatic scaffolds accompanied by H2 evolution. This protocol features high regioselectivity as well as a broad substrate scope. Detailed mechanistic studies indicate that twice the electron/H transfer processes facilitated by this catalytic system achieve the annulation π-extension of imidazo-fused heterocycles with alkenes.

Visible-Light-Promoted and EDA Complex-Driven [4 + 2] Annulation for the Construction of Naphtho[1',2':4,5]imidazo[1,2-a]pyridines

A green visible-light-promoted and electron donor-acceptor (EDA) complex-driven synthetic strategy for the construction of value-added naphtho[1',2':4,5]imidazo[1,2-a]pyridines from 2-arylimidazo[1,2-a]pyridines with Z-α-bromocinnamaldehydes has been accomplished under photocatalyst- and transition-metal-free conditions. This efficient annulation approach provides a new and straightforward pathway for the annulative π-extension of imidazo[1,2-a]pyridine-based aromatics. Moreover, the sustainable methodology exhibits simple operation, a wide range of substrates, benign conditions, and good functional group compatibility.