A revisit to the Hantzsch reaction: Unexpected products beyond 1,4-dihydropyridines

A novel green and efficient one-pot three-component synthesis of 2-aryl-pyridines in good to excellent yields has been reported. The methodology initially involved the formation of 1,2-dihydropyridine intermediates via reaction of a variety of aromatic aldehydes with ethyl (methyl) acetoacetate and ammonium acetate, which were the same starting materials as the Hantzsch reaction, under solvent-, catalyst- and heat-free (at room temperature) conditions, followed by air oxidation for 72 hours. In this paper, we also systematically reinvestigated the classic Hantzsch reaction under different reaction conditions, analyzed the main products as well as byproducts, corrected some mistakes in the literature and elucidated the reaction mechanism.