Cyclopentenones of formula ##STR1## wherein each of symbols R.sup.1 and R.sup.2, when taken separately, represents an alkyl radical of C.sub.1 to C.sub.6, or a hydrogen atom, or, when taken together, they represent a polymethylene, or one of them designates a hydrogen atom and the other an alkyl radical of C.sub.1 to C.sub.6, are prepared starting from compounds having formula ##STR2## wherein R stands for a C.sub.1 to C.sub.6 alkyl radical or a phenyl group, via a catalytic reaction promoted by a metallo-organic compound of formulaMeX.sub.2 (R.sup.3 CN).sub.2 (III)wherein Me represents palladium or platinum, R.sup.3 represents a C.sub.1 to C.sub.3 alkyl radical or a phenyl group and X defines a halogen atom.