Apart from a direct pathway involving the thermal ringopening of dihydro-4H-cyclobuta [c]isochromenes b to dihydronapthalenes d via o-QDM3, these species can of course also form by thermal ring-contraction of 1H-2benzo [c]oxocins a, also proceeding (according to DFT) via ortho-quinodimethide intermediate o-QDM3 (Scheme 2). One clear observation for the aryl-substituted 1H-2benzo [c]oxocins (e.g., R = Ph in Scheme 1) is that the total overall highest-energy barrier from a to d via TS4 is +28.5 kcal mol -1 , which is higher than the total highest barrier for thermal switching of b to a pathway (i.e., the barrier to generate TS1 in Scheme 1; +25.2 kcal mol -1 for R = Ph). The DFT results are therefore in agreement with the experimental thermal switching results, and explain how the anti-Woodward-Hoffmann product a can be formedin favor of dihydronapthalene formation-upon mild heating, while applying harsher reaction conditions (i.e., higher temperatures with longer times) leads to formation of the thermodynamically favored dihydronapthalene product d.