During our previous studies, we observed that some of the prepared 1H-2-benzo [c]oxocins appeared to be unstable, with some of them slowly converting to other products. We initially assigned this instability to their intrinsic thermal instability at room temperature, but later we discovered that these conversions are actually triggered upon exposure to sunlight. Therefore, we sought to explore this reactivity in more detail. Since 1H-2-benzo [c]oxocins have a conjugated diene structure, we anticipated a lightinduced ring-contraction was occurring. Initial irradiation of a 1H-2-benzo [c]oxocin 1a solution (DCM, 365 nm UV light, aerobic conditions) revealed the formation of two new products: dihydro-4H-cyclobuta [c]isochromene (1b, generated by direct light-induced intramolecular [2+2] cyclization) and trace amounts of dihydro-1H-epidioxybenzo [c]oxocine (1c, from a [4+2] cycloaddition reaction with singlet oxygen ( 1 O2), experimental details in SI). Several reactions were performed to obtain more information about these transformations, differing conditions, varying solvent, light sources, and the inclusion of additives (Table ). According to these experimental results, the formation of both products requires the input of light. Furthermore, UV light is not needed as full conversion can be achieved by using white light (i.e., no conversion for entry 1, 5, 8 and 11 without light, with full conversion observed with white light).