Hetero-Diels-Alder (HDA) Strategy for the Preparation of 6-Aryl- and Heteroaryl-Substituted Piperidin-2-one Scaffolds: Experimental and Theoretical Studies

Preparation of piperidine-2-one scaffolds by the hetero-Diels-Alder (HAD) reaction, assisted by microwaves, is described. The versatility of this new approach has been demonstrated by the synthesis of racemic (+/-)-2-phenylpiperidine. Theoretical calculations have allowed us to clarify the factors that govern ring closure to form four-membered rings, arising from a Staudinger-type electrocyclization, and/or six-membered rings through a classical [4+2] HAD cyclization. This competition may be lowered by increasing the electronic demand of the dienophile, as anticipated by the computational studies.