Synthesis of 6-Substituted 6H-Indolo[2,3-b]quinolines from Isoindigos

A facile approach to 6-aryl/alkyl substituted 6H-indolo[2,3-b]quinolines from mono-N-substituted isoindigo derivatives in the presence of SnCl2·2H2O in acid media is described. Pyrrole and pyridine rings are synchronously constructed in one pot for these tetracyclic molecules. A plausible reduction/hydrolysis/decarboxylation/cyclization/aromatization domino reaction mechanism is proposed. Bis-N-substituted isoindigo only gives the corresponding reduction product, 3,3′-bioxindole.