Thermolysis of 1-alkenylbenzocyclobutenyl-1-carboxylic acid produced 4-alkylideneisochroman-3-ones in good yields via an unprecedented tandem electrocyclic-[1,5]sigmatropic process of o-quinodimethane. Alternatively, thermolysis of the corresponding methyl ester afforded the dihydronaphthalenes via E-transition state in excellent yields.