Sequential Diels–Alder Reactions in a Conjugated Tetraene System

Abstract

A sequence of two Diels–Alder reactions is observed for the reaction of bis-diene ( E)-3-[2-(furan-2-yl)vinyl]-5,5-dimethylcyclohex-2-en-1-one with two equivalents of N-phenylmaleimide. This study shows that the electron-deficient furanyl site, which is primarily “inactive” towards a [4 + 2] reaction, easily undergoes cycloaddition with the second equivalent of N-phenylmaleimide to produce the final bis-adduct. This is because the first Diels–Alder reaction between N-phenylmaleimide and the more reactive cyclohex-2-en-1-one site of the diene converts the 2-furanyl ring to a more electron-rich component, since the electron-withdrawing double bond attached to the furan ring changes to an electron-releasing alkyl group after the first cycloaddition. Consequently, the resulting furanyl diene is then able to undergo the second cycloaddition.