The annulation of a methylenecyclopropane with acyl cyanoalkenes using DABCO or quinuclidine as a catalyst was developed to give 2,3-dihydofurans. The stoichiometric amount of the Lewis bases promoted the isomerization of 2,3-dihydrofurans to furans. 1H NMR spectroscopy of the reaction in situ revealed that the methylenecyclopropane is opened by the Lewis base to form a reaction intermediate that is added to the cyanoalkenes.