Reactions of protoanemonin, 1, and 3-bromo- and 3-methoxy-5-methylene-2(5H)-furanones, 2 and 3, with dienes have been performed under both thermal and photochemical activation conditions. Photochemical cycloadditions of butadiene and hans-piperylene to this kind of furanones have afforded 1,2-adducts, 10-13, as stereoisomeric mixtures, in 40-60% yields, and have shown that furanones 1-3 react specifically at the exocyclic double bond as they do in thermal conditions. Furthermore, thermal cycloadditions have led to the production of the corresponding 1,4-adducts, 4-7, in good yields, but 10-13 have also been detected at the initial stages of these reactions. Compounds 10-13 have been proved to be intermediates in the production of adducts 4-7, isomerization taking place under heating and involving the formation of biradical species as intermediates. Moreover, cycloreversion from 1,4-adducts has not been observed, and only very small proportions (1-8%) of the corresponding furanones have been found when 1,2-adducts have been heated. in good agreement with these results, CASSCF and DFT nb initio theoretical calculations, performed on the reaction of 1 and butadiene to give 4, have shown the nonconcerted mechanism to be more favorable than the concerted process, and have allowed the location of all intermediates and transition states involved in the overall process. Since, in addition, the presence of radicals in the isomerization of 1,2-adducts to 1,4-adducts has been confirmed with ESR spin trapping techniques, we can conclude that the resulting Diels-Alder adducts are mainly produced via a biradical mechanism. To our knowledge, this is the first time that a biradical mechanism operating in a Diels-Alder cycloaddition and accounting for the production of 1,4-adducts has been evidenced and rationalized.