A concise route to a bicyclo[4.2.1] ring system, based on an intramolecular Diels-Alder reaction, has been developed. Preliminary results on the interplay between Lewis acids and high pressure to facilitate the reaction are described. Products with isomerized double bonds were obtained with 1 equivalent of Lewis acid at 0 degrees C. The stereochemistry of these compounds has been determined by X-ray crystallography. Double bond isomerization is suppressed by the use of excess Lewis acid at -78 degrees C or high pressure.