Synthesis of dihydropyridine and piperidine derivatives via an unexpected reaction of pyridine with acetyl chloride

Acetyl chloride reacts with pyridine to give a mixture of N-acetyl-1,4- and 1,2-dihydropyridyl acetic acid (1a,b) after water quenching. The reactioninvolves the formation of a zwitterionic ketene enolate intermediate which results from deprotonation of the acetyl moiety of the in situ formed N-acetyl pyridinium ion.The effect on the reaction outcome of different parameters as temperature, pyridine/acetyl chloride molar ratio, and as Lewis acids and triflate counterion presence has been studied in detail, and a reaction mechanism has been proposed.