Catalyst-free and oxidant-free tandem aza-Mannich/cyclization/aromatization of C,N-cyclic azomethine imines with enamides: facile synthesis of 5,6-dihydropyrazolo[5,1-a]isoquinolines

A novel and efficient catalyst-free and oxidant-free tandem aza-Mannich/cyclization/aromatization reaction of C,N-cyclic azomethine imines with enamides has been developed. This practical one-step protocol enables a simple and environmentally friendly route toward the straightforward synthesis of highly substituted 5,6-dihydropyrazolo[5,1-a]isoquinolines. Different types of enamides and enamines, especially enamides derived from marketed drugs as well as bioactive molecules, are suitable substrates. CB1 cannabinoid receptor antagonist could be synthesized efficiently based on this methodology, illustrating that this would be a practical strategy to synthesize valuable structural motifs.