Installation of two alkoxy groups on a terminal methylene of trimethylenemethane (TMM) dramatically alters the electronic state and the reactivities of the TMM. Thermolysis of a 1,1-dialkoxy-2-methylenecyclopropane generates such a TMM, which exhibits a marked singlet dipolar character. It undergoes [3 + 2] cycloadditions to a variety of electron-deficient unsaturated compounds including alkenes, alkynes, oximes, carbonyl compounds, and fullerenes and reacts with active methelene compounds and organozinc reagents as a 1,3-dipolar synthon.