ΔH 298K in kcal/mol point calculations give opposite results that the threo [6+4]-ene pathway is the most favored pathway, with TS-threo-4 being 1 kcal/mol lower than TS-endo-2. The CCSD(T) computed activation enthalpies agree excellently with the experimental activation energies (Table ). Calculations show that the entropy of TS-threo-4 is 5 J/mol•K smaller than TS-endo-2; this entropic difference corresponds to a difference of 1. formed C-C bond (~180 degrees, see Figure ) is necessary in order to reach a geometry for the second bond to form to give syn-3. This might be the reason that syn-3 has a lower product ratio than exo-2 in experiment.