For concerted [4+2]-ene reactions, two pathways are possible. In each of them, there exist configurational isomers for the TS, for example, TS-5 and TS'-5. However, both of these two TS structures lead to the same dimer (i.e., 5). The configurational isomerism in the TS disappears in the dimer because the bottom ring in the adduct becomes 1,4cyclohexadiene that does not contain any stereocenter.