The treatment of 1-acylbicyclo[4.2.0]oct-3-ene with ethylaluminum dichloride gave 6-alkylbicyclo-[4.3.0]non-3-en-7-one in good yields. It was found that the stereoselectivity largely depended on the structure of acyl group and the acetyl derivatives gave cis-isomers with high stereoselectivity.