As the ring-closure process of conjugated diene [2+2] intramolecular cyclization induced by light has been well studied, we herein only focus on the thermal ringopening process. We explored pathways using two different type of substituents at the enol ether position (red line for R = Me, and blue line for R = Ph). The calculations were performed at the b3-lyp/def2-TZVP level of theory, using Grimme's D3 dispersion corrections ("zero" damping), taking the experimental observations into consideration. The computed mechanisms are shown in Scheme 1. As expected, direct thermal ring-opening of the 4-membered ring of the dihydro-4Hcyclobuta [c]isochromene b via TS1 proceeds in a conrotatory manner, following the Woodward-Hoffman rules to produce the twisted 1H-2-benzo [c]oxocin a'. This process is endergonic for both compounds (R= Me: +14.3 kcal mol -1 ; R = H +9.9 kcal mol -1 ) producing strained, thermodynamically unstable intermediates, featuring a high energy barrier in both cases (DFT R = Me: G ǂ 383K = +29.9 kcal mol -1 ; DFT R = Ph: G ǂ 383K = +25.2 kcal mol -1 ; The latter value is close to the experimental free energy barrier for R = Ph, as determined by Eyring analysis: G ǂ 383K = +30.5 kcal mol -1 , see SI Table ). However, the barrier is significantly lower for R = Ph than for R = Me, in good agreement with the experimental observations. The strained twisted intermediates a' are thermally unstable, and easily ring-open to produce o-QDM intermediate o-QDM-1, which is an exergonic step with a relatively low energy barrier (R = Me: +13.0 kcal mol -1 ; R = Ph: +12.4 kcal mol -1 ). A slight rotation around the single bond to convert o-QDM-1 to o-QDM-2 is then followed by a (nearly) barrierless 8-cyclization to produce the 1H-2benzo [c]oxocins a via TS3, thus completing the thermal switching process. This process is again exergonic, featuring a very low energy barrier (R = Me: +3.3 kcal mol - 1 ; R = Ph +2.1 kcal mol -1 ). Trace amounts of o-QDM-1 could also convert to o-QDM-3 to undergo 6-cyclisation producing the thermodynamically more stable dihydronaphthalene products d via TS4, which is also an exergonic process with a low energy barrier (R = Me: +13.6 kcal mol -1 ; R = Ph +13.4 kcal mol -1 ). This explains the occurrence of some fatigue upon thermal switching at high temperature. However, the energy barrier for conversion of o-QDM3 to dihydronaphthalene d (+13.4 kcal mol -1 ) is Scheme 2. Proposed mechanism for the thermal ring-contraction of 1H-2-benzo [c]oxocins (a) to dihydronaphthalenes (d), based on DFT calculations (b3-lyp, def2-TZVP, m4 grid, disp3). All Gibbs free energies (ΔG o 383K in kcal mol -1 ), including TS3 and TS4, are reported relative to the energy of intermediate a.