Ring closure reactions of γ-cycloheptatrienyl-substituted α, β-unsaturated carbonyl compounds by acid were examined. Treatment of 2-methyl-4- (2, 4, 6-cycloheptatrien-4-yl) -2-pentenal (II) with hydrobromic acid at room temperature in acetic acid afforded 1, 3-dimethyl- (III) and 2, 4-dimethyl-5H-benzocycloheptene (IV) accompanied by the production of 1, 3, 8-trimethylnaphthalene (V). On the ring closure of 5- (2, 4, 6-cycloheptatrien-4-yl) -3-hexen-2-one (VIII) by a similar procedure, 1, 4-dimethyl-5H-benzocycloheptene (IX), 1, 4, 5-trimethylnaphthalene (X), and 1, 4, 6-trimethylnaphthalene (XI) were isolated.