Tricyclic compounds, containing a cyclopentanone ring and an alkylidene group α to the carbonylic function, have been obtained by reaction of allylic halides with bicyclo[2.2.1]hept-2-ene or bicyclo[2.2.1]hepta-2,5-diene and carbon monoxide in the presence of a triarylphosphine palladium as complex catalyst. Retro Diels-Alder type reactions, which in part occur during the carbonylation, lead to alkyliden- cyclopentenones.