[4+2]Cycloaddition Reaction of Substituted 2-Pyrones with Maleimides and Stability of the Adducts

Abstract

For the purpose of clarification of the factors influencing the Diels-Alder (DA) reactivity of 2-pyrones and the stability of the DA adducts, the DA reactions of four 2-pyrones with N-substituted maleimides were investigated. Wher as the reactions of methyl 2-pyrone-5carboxylate (MP) with N-substituted maleimides only gave endo-DA adducts [1], those of 2-pyrone [1] and ethyl 4, 6-dimethyl-2-pyrone-5-carboxylate (EP) with the maleimides gave ( I) and 1: 2 endo-endo-D A. adducts [2] which were formed through decarboxylation of the corresponding [4+2] adducts, followed by, addition of another molecule of the maleimides. On the other hand, the reactions of 4, 6, dimethy1-2-pyrone (DMP) afforded, only [2] even under mild conditions.
The reactiv ity of 2-pyrones with maleimide diminished in order; DMP>P>EP>MP, showing that electron-rich 2-pyrones are more reactive than the electron-defficient ones. However, the acceleration effect of 4, 6-dimethyl groups seems to be smaller than the deceleration effect of 5-ethoxycarbonyl group. The reactivity of MP with N-substituted maleimides diminished by more electron-withdrawing N-substituents, suggesting that a 5-niethoxycarbonyl group on 2-pyrone makes the 2-pyrone an inverse electron-demand manner even in the reaction with maleimides.
The thermostability of [1] was found to be enhanced by an electron-withdrawing substituent of 2-pyrone at 5-position and to be decreased by 4, 6-dimethyl groups.