The reaction of 2,3-dimethyl-1,3-butadiene (1) with methyl acetoacetate in the presence of a catalyst composed of palladous chloride and p-phenyl-1-phospha-3-methyl-3-cyclopentene gave 3-carbomethoxy-5,6-dimethyl-5-hepten-2-one (2) (1:1 adduct). Alkali decomposition of 2 gave 5,6-dimethyl-5-hepten-2-one (3), which was a key intermediate for the preparation of α-irone. Similarly, isoprene gave a mixture of 5- and 6-methyl-5-hepten-2-one.