3-Fluorofuran-2(5H)-one (1) and three 3,4-difluorofuran-2(5H)-ones 2-4, α,β-unsaturated lactones possessing fluorinated double bonds, were applied as dienophiles in DielsAlder reactions with normal electron demand using diphenylisobenzofuran or cyclopentadiene as dienes. In the same reactions, furan or 2,3-dimethylbuta-1,3-diene were completely unreactive. Three structural factors of furan-2(5H)-ones appeared to have an effect on the reactivity, regioselectivity and diastereoselectivity of the [4+2] cycloadditions: the number of fluorine atoms attached to the double bond and the number and the bulkiness of alkyl substituents at the 5-position of the furan-2(5H)-one system. The monofluorinated furan-2(5H)-one 1 was generally more reactive than the difluorinated furan-2(5H)-ones 2-4. While the reactions of the furan-2(5H)-ones 2-4 with isobenzofuran exclu-