Abstract: Cyclization of 1,1-dichloro-4, 8-dimethylnona-1, 3, 7-triene (Ⅳ) under the catalysis of boron trifluoride-etherate at room temperature gave a-isomer (Ⅴ) as the unique product in 90% yield. It was suggested that the cause leading to above result may be much more steric strain due to intramolecular crowding with the bulky dichlorovinyl group and adjacent three methyl groups in β-isomer (Ⅵ) than in a-isomer (Ⅴ),