Double retro-Diels-Alder cleavage by flash thermolysis gave the new, unstable polyunsaturated ketones and hydrocarbons 1–5 for which NMR, IR and UV spectra at low temperature are reported. The Diels—Alder adducts of compounds 1–3 with various dienes, the 1, 6 additions of several nucleophilic agents with 4, and the isomerisation of 5 in p- ethynylstyrene support the proposed structures.