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)-onesystem. 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 exclusively