Thermal [3+2] Cycloaddition Reaction of N-Acyliminophenanthridinium Betaine with Allenoates: Facile Access to Phenanthridine-fused Tetracyclic Heterocycles

The thermal [3+2] cycloaddition reaction of N-acyliminophenanthridinium betaines with various allenoates has been investigated. The reaction was operationally simple and proceeded smoothly under mild reaction conditions, providing a variety of aromatic tetracyclic heterocycles in moderate to excellent yields.