Thermal rearrangements of 2-cyclopropylcycloalk-2-enones

Thermolysis of 2-(2,2-dimethylcyclopropyl)cyclopent-2-enone (2) and 2-cyclopropylcyclohex-2-enone (6) is shown to produce 2-(3-methylbut-2-enylidene)cyclopentanone (10) and 2-propylphenol (14), respectively, via homo[1,5]sigmatropic H-migration and isomerisation. Heating 2-cyclopropylcyclopent-2-enone (8) under a variety of conditions led only to decomposition; no products [e.g.(11)] resulting from vinylcyclopropane–cyclopentene rearrangement were detected.