A Novel Push-Pull Diels-Alder Diene : Reactions of 4-Alkoxy- or 4-Phenylsulfenyl-5-chalcogene-substituted 1-Phenylpenta-2,4-dien-1-one with Electron-Deficient Dienophiles

5-(Phenylselenenyl)- and 5-(phenylsulfenyl)-4-ethoxy-1-phenyl-2,4-pentadien-1-ones (2) and (3) underwent [4+2] cycloaddition with N-methyl and N-phenylmaleimides and successive isomerization to give the 7-benzoyl-3a,4,5,7a-tetrahydro-1H-isoindole-1,3(2H)-diones 5, 8 and 9 in good yields. The 4-ethoxy group on the 2,4-pentadien-1-one was found to be effective to facilitate the cycloaddition with dienophiles. We also performed other [4+2] cycloadditions of 2,4-pentadien-1-ones with DMAD or naphthoquinone.