The Diels-AIder reaction of 5, 5, 5-trichloro-3-penten-2-one (1a) with cyclopentadiene afforded exo-2-acetyl-endo-3-trichloromethylbicyclo [2.2. 1] hept-5-ene (2a) stereoselectively. The stereochemistry of the reactions of cyclopentadiene with several γ- chlorinated α, β-unsaturated ketones was examined. The endo-directive abilities decreased in the following order: CCl 3> CCl 2 CH 3, CHCl 2, CCl (CH 3) 2> CH (CH 3) 2> CH 3. ...