The resulting dihydro-4Hcyclobuta [c]isochromenes exhibit absorption in the UV-C region (i.e., colourless, as shown in Figure ) that also promote the ring-closure of 1a to 1b. However, we noticed that the dihydro-4H-cyclobuta [c]isochromene 1b could thermally reverse back to the original 1H-2-benzo [c]oxocin 1a (Figure , see SI for more details). This result was quite surprising to us, as-according to the Woodward-Hoffmann rules-thermal ring-opening should proceed in a conrotatory manner to produce the twisted 8-membered ring isomer 1a' as the expected product (Figure ). However, interrogation of the thermal ring-opening of 1b in the dark revealed the formation of the twisted structure 1a' was not observed at all. In a few rare cases, formation of anti-Woodward-Hoffmann products resulting from of ring strain have been reported, with the harsh reaction conditions required to overcome steric barriers typically leading to formation of only trace amounts of the anti-Woodward-Hoffmann ring-opened products. In our case, the anti-Woodward-Hoffmann product 1a is regenerated in high yield upon mild (> 60°C) heating. We surmise that the twisted 8-membered ring 1a' is strained (uphill by +9.9 kcal mol -1 with respect to 1b according to DFT, vide infra), hampering its direct formation via thermal ring-opening from 1b. At the same time, direct thermal ring-opening of 1b to the non-twisted starting material 1a violates the Woodward-Hoffmann rules, as such attempts always led to the hypothetical twisted product 1a'. The twisted product 1a' was not observed experimentally, congruent with our inability to find a transition state for such a process with DFT calculations.