Pericyclic Reaction Behavior of Cyclopentadienones toward Acyclic Conjugated Dienes. (3,3)-Sigmatropic Rearrangement and Double Diels-Alder Reactions of the endo(4+2).PI. Cycloadducts.

Cycloaddition of 2, 5-disubstituted-3, 4-diphenylcyclopentadienone with acyclic conjugated dienes gave the endo [4+2]π cycloadducts, which then underwent [3, 3]-sigmatropic rearrangement to give the corresponding endo [2+4]π cycloadduct on heating at 110°C. Pyrolysis of the endo [4+2]π cycloadducts at 170°C resulted in decarbonylation to give the double Diels-Alder adduct. The sequential pericyclic reaction behavior is discussed on the basis of X-ray diffraction analyses and molecular orbital calculation data.