Nazarov cyclization, a conrotatory 4π electrocyclization reaction of a divinyl ketone through the formation of the pentadienyl cation intermediate, is promoted by a Lewis acid to provide cyclopentenone. Our group has reported an enhancement of a reaction rate based on a microwave specific effect. An activation of an equilibrium of conformers was hypothesized and the microwave irradiation could formally increase a frequency of a formation of the suitable conformer, resulting in the acceleration of the reaction rate. In this work, our strategy for the acceleration of the ring-closure in Nazarov cyclization is the microwave-assisted activation of the equilibrium between conformers of the pentadienyl cation intermediate. When a copper-catalyzed Nazarov cyclization using aryl vinyl ketone derivative containing 1,3-dicarbonyl moiety was carried out under microwave irradiation conditions, the Nazarov cyclization was dramatically accelerated and the kinetic rate by the microwave irradiation was 5.8 times higher than that by a conventional heating conditions. We discussed about the microwave specific effect in terms of a reaction dynamics and the hypothesis of the conformational equilibrium activation by the microwave specific effect was theoretically supported.
