Synthesis of a fused N-bridged [3.3.1]nonadiquinoline multicyclic skeleton via a metal-free formal [4 + 2] cycloaddition/Mannich/dearomatization domino reaction

We present a novel green synthetic protocol for the construction of a broad range of substituted fused azabicyclo[3.3.1]nonadiquinoline scaffolds through a formic acid catalyzed formal [4 + 2] annulation/Mannich/dearomatization domino reaction. This unified strategy shows a broad substrate scope with high functional group tolerance, resulting in good to high yields under environmentally benign, transition-metal-free, low-cost, and operationally simple conditions. The current method outperforms existing methods in terms of green and sustainable conditions. The reaction mechanism was confirmed based on DFT calculations.