The treatment of the enolate of a delta-ethylenic malonate with an unsaturated iodide in the presence of a Palladium(O) complex leads stereospecifically to a cyclopentane with the simultaneous formation in a trans fashion of the two carbon-carbon bonds. All the results authorize to conclude that this cyclisation occurs by nucleophilic attack of the enolate onto the double bond activated by the sigma-palladic unsaturated species.