Superacidic Cyclization of Activated Anthranilonitriles into 2-Unsubstituted-4-aminoquinolines.

4-Aminoquinolines were prepared in a three-step synthesis starting from substituted anthranilonitriles. The condensation on 1,1,1-trichloro-4-ethoxybut-3-enone proceeded efficiently either neat or in refluxing EtOH. Cyclization in superacidic trifluoromethanesulfonic acid provided unstable intermediate, which upon treatment with NaOEt in ethanol, afforded the expected esters. Theoretical investigations pointed out a monoprotonated nitrilium as the reactive species during the cyclization process.