The Witkop–Winterfeldt oxidation converts tetrahydropyridoindoles into pyrroloquinolones and cinnolines by an unprecedented scaffold rearrangement

The Fischer indole reaction between phenylhydrazines and tosyl-4-piperidone furnishes tetrahydropyrido[4,3-b]indoles. In a Witkop–Winterfeldt-oxidation using ozone such indole derivatives are converted into medium-sized dicarbonyl ring systems, which cyclize to pyrroloquinolones. A detailed study of the reaction intermediates and the characterization of a cinnoline betaine side product formed by an unprecedented ring closure mechanism are reported.