The [4+2]-ene pathways were not considered in previous calculations by Ess et al. For the concerted [4+2]-cycloadditions and [6+4]-ene reactions, each of these pathways can generate two stereo isomers as a result of different relative orientations of the two cyclohexadiene rings in the TS structures. These isomers are termed endo and exo for the [4+2] cycloaddition adducts and threo (also could be called rac or dl) and erythro (also could be called meso) for the [6+4] ene-adducts.