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 quinolinesvia a remarkably facile indole ring-opening.