Activation energies, Ea (kcal/mol) Enthalpy change, ΔH (kcal/mol)   Endo Exo Endo Exo 1,3-butadiene +7.45 +9.69 -56.15 -56.15 cyclopentadiene +4.69 +6.33 -37.85 -39.87

Table 2: Energetics of the Diels-Alder reactions of cyclobutenone with 1,3-butadiene and cyclopentadiene (parent reactions).