A new and useful synthesis of a title compounds is reported. They can be obtained easily by photocyclization of enedicarbonyl compounds 3 and 4. A different behaviour, ascribed to the geometry of a double bond, is observed; 3 are converted only to butenolides 5, while trans- isomers 4 are converted into alkyl-furyl-ketones 5 and 6. A possible mechanism is described.