It is clear that the transformation from 1a to dihydro-4Hcyclobuta [c]isochromene 1b is a non-catalyzed photoisomerization reaction (Table , entry 3 and 4). In the presence of air, transformation of 1a to either dihydro-4Hcyclobuta [c]isochromene 1b and dihydro-1Hepidioxybenzo [c]oxocine 1c is competitive, but in the absence of a photosensitizer, conversion to dihydro-4Hcyclobuta [c]isochromene 1b is predominant (Table , entry 2). Reducing the area of the air-solvent interface further inhibits the [4+2] cycloaddition with singlet oxygen leading to dihydro-1H-epidioxybenzo [c]oxocine 1c formation, instead affording the dihydro-4Hcyclobuta [c]isochromene 1b in a near quantitative yield (Table , entry 13). Obviously, the exclusion of oxygen by performing the photoisomerization under a protective N2 atmosphere leads to the fully-selective formation of 1b (Table , entries 3 and 4). In the presence of both air and meso-tetraphenylporphyrin (TPP) as photosensitizer (for in situ photochemical 1 O2 formation), the [4+2] cycloaddition reaction prevails, leading to selective formation of dihydro-1H-3,6-epidioxybenzo [c] -DCM white light fully converted 0 98 2 a Reaction conditions: substrate 1a (5 mg) and additive were mixed in DCM (1 mL), stirred at room temperature for 15 h; reactions preformed in 10 mL vials located 10 cm from light source. Conversion and the ratio of compounds were determined by integration of the 1 H-NMR signals in the presence of dimethyl sulfone as internal standard. c Reaction performed under a protective N2 atmosphere. No stirring. product ratio of 1b and 1c for reactions performed under air (Table and), but in the presence of [Co(TPP)] (i.e., the catalyst used to prepare 1a) fully suppresses the formation of 1c (Table and). Presumably, the paramagnetic [Co(TPP)] complex catalyses the relaxation of 1 O2 to 3 O2. However, the intense visible absorption of [Co(TPP)] also causes it to function as an internal light filter in solution, shielding the efficient irradiation of 1a by the white light source, requiring UV light with a higher power density for full conversion of 1a to 1b (Table and).