Chapter 48 â Diels-Alder Reaction 629 hat star by the Diels-Alder reaction? wO If the Diels-Alder reaction between dimethylmaleic anhydride and furan had worked, would cantharidin have been formed? Determine the heats of formation or the steric energies of the exo- and endo-anhydride adducts using a molecular mechanics program. What do these energies tell you about the mechanism of the reaction? Which molecule, norbornene dicarboxylic acid or compound X, would you predict to be more stable? Why?