C3-H Formylation of 2-Arylquinolines via Intermolecular Cyclizative Rearrangements Coupled with Selective C-C Bond Cleavages.

The direct, C3-selective formylation of azaarenes via site-specific C-H functionalization represents a step-economical strategy to upgrade simple feedstocks into structurally diverse N-heterocyclic building blocks. Despite the synthetic significance of this transformation, regioselective meta-formylation protocols for quinolines have remained largely unexplored to date. Herein, we report a practical strategy coupling intermolecular cyclizative rearrangement and selective C-C bond cleavage for the highly C3-selective formylation of 2-arylquinolines with excellent chemoselectivity. Specifically, quinoline N-oxides and dimethyl acetylenedicarboxylate (DMAD) undergo a sequential cascade process involving [3+2] heteroannulation, [3,5]-sigmatropic rearrangement, and three successive selective C-C bond cleavage events, thus affording the corresponding 3-carboxaldehyde azaarenes with high efficiency.