The transition state structures of the endo cycloaddition of E-1,3-pentadiene and maleic anhydride. This structure is only about 6.8 kcal/mol less stable than the precoordination complex. This activation barrier is very small and the Diels-Alder reaction can proceed quickly. Note that the endo activation barrier is 2.2 kcal/mol less than the barrier for the exo reaction. The higher stability of the endo precoordination complex together with the lower activation barrier of the endo reaction cause the preferential endo product formation.