We report density functional theory (M06-2X) studies of a series of dehydro-Diels-Alder (DDA) reactions. For these and the parent reaction, the stepwise mechanisms have similar barriers, whereas the barriers of the concerted mechanisms differ significantly. The reactivity of DDA reactions is controlled by distortion energy. The concerted and stepwise mechanisms of the hexadehydro-Diels-Alder (HDDA) reaction are competitive with activation barriers of similar to 36 kcal/mol. This is because a large distortion energy (similar to 43 kcal/mol) is required to achieve the concerted transition state geometry. MD simulations reveal that productive concerted trajectories display a strong angle bending oscillation (similar to 25 degrees oscillation amplitude), while the stepwise trajectories show only a chaotic pattern and less pronounced bending vibrations.