Synthetically useful β,γ-unsaturated carbonyl compounds having a quaternary carbon at the α-position were prepared with high stereoselectivity by the reaction of a dienolate anion derived from α,β-unsaturated imide having a chiral auxiliary and electrophiles (ethylacetate and allyl iodide as the C2 and C3 unit, respectively). This method was applied to a short asymmetric synthesis of (+)-ethosuximide