Divergent reactions of indoles with aminobenzaldehydes: indole ring-opening vs. annulation and facile synthesis of neocryptolepine

Indoles display a diverse pattern of reactivity upon reaction with different classes of aminobenzaldehydes. Whereas the acid-promoted reaction with secondary aminobenzaldehyde leads to indole annulation followed by spontaneous oxidation to neocryptolepine and analogues, the reaction with primary aminobenzaldehydes results in the formation of synthetically useful quinolines via a remarkably facile indole ring-opening.