Previously unknown [4+2]-cycloaddition of 5-arylideneimidazol-4-ones with dienes leading to 1,3-diazaspiro[4.5]deca-1,7-dien-4-ones is proceeding under either Lewis acid catalysis or upon thermal initiation. Arylideneimidazol-4-ones with electron-donating groups display low reactivity, which could be overcome by introduction of trifluoromethyl group in the 2-position of imidazol-4-one.