Thermal cyclization of N-[2-(2-propenyl)-1-naphthyl] ketenimines: intramolecular Diels–Alder reaction versus [1,5] hydrogen migration. Synthesis of dibenz[b,h]acridines and benzo[h]quinolines

The thermal treatment of N-(2-propenyl)-1-naphthylamines provided the expected aza-Claisen rearranged products, 2-(2-propenyl)-1-naphthylamines and benz[g]indoles, these last derived from an intramolecular hydroamination reaction on those primary products. The 2-(2-propenyl)-1-naphthylamines were converted into their triphenylphosphazene derivatives, which by aza-Wittig reaction with disubstituted ketenes yielded N-[2-(2-propenyl)-1-naphthyl] ketenimines. The heating of these ketenimines in boiling toluene induced their cyclization either via an intramolecular Diels–Alder reaction, to afford dibenz[b,h]acridines, or via [1,5] hydrogen migration from the sp3 carbon atom of the propenyl substituent to the central carbon atom of the ketenimine fragment, followed by a 6π electrocyclic ring closure, to give benzo[h]quinolines.