A variety of alkylidenecyclopropanone acetals possessing a terminal diylophile were allowed to undergo an intramolecular [3 + 2]cycloaddition via a dipolar trimethylenemethane to obtain bicyclol[4.3.0] and [3.3.0] carbo- and heterocycles in good yields. The product selectivities were rigorously controlled by the concerted or stepwise nature of the cycloaddition reaction.