The thermal reaction between (E) or (Z)-pyrazolones (1) and 2,3-dimethylbutadiene (2) gave 1,2-dimethyl-5-arylcyclohexen-4-spiro-4' (1'-phenyl-3'-substituted-5'-pyrazolones) (3, 4), from a Diels-Alder reaction, and 4-aryl-2-isopropenyl-2-methyl-5-substituted-7-phenyldihydropyran [3,2-d] pyrazoles (5, 6), from a heterodiene reaction. The effect of the configuration of the starting pyrazolone, as well