substituent Activation energies, ΔEact (kcal/mol) Enthalpy change, ΔH (kcal/mol)
  Syn Anti Syn Anti
  endo exo endo exo endo exo endo exo
-CH3 +12.22 +12.03 +7.00 +9.22 -55.84 -55.94 -55.94 -55.84
-OH +8.53 +7.34 +7.39 +9.20 -54.89 -56.17 -56.98 -54.90
-CN +6.23 +7.28 +5.10 +7.91 -57.36 -58.00 -58.00 -57.36
-Cl +8.74 +8.30 +6.68 +8.77 -55.84 -56.33 -56.33 -55.84
Table 3: Energetics of the reaction of 4-monsubstituted cyclobutenones and 1,3-butadiene.