Intramolecular Diels–Alder reaction of cyclopenta-1, 3-diene derivatives 16 generated in situ from the corresponding 4-(pent-4-enyl) cyclopent-2-enone ethylene ketals 15 is investigated. 4-[(E)-5-(Ethoxycarbonyl) pent-4-enyl] cyclopent-2-enone ethylene ketal 15a and 4-(5, 5-dicyanopent-4-enyl) cyclopent-2-enone ethylene ketal 15b were prepared in 5 steps from cyclopent-2-enone ethylene ketal 5. Upon heating at 120 C, compound 15a ...