1H-2-benzo [c]oxocins functionalized with biologically relevant substituents could also be isomerized to the desired products in high yields (24b-27b, Figure ). As the control experiments in Table demonstrate that [Co(TPP)] does not interfere with the ring-closing process, Figure . Substrate scope of isomerization of 1H-2-benzo [c]oxocins to dihydro-4H-cyclobuta [c]isochromenes. Standard reaction conditions: 1H-2-benzo [c]oxocins (5 mg) dissolved in CD2Cl2 (0.55 mL); reactions performed in NMR tubes located 10 cm from light source at room temperature and irradiated for 7 h. Isolated yields. For 13b-19b, 22b and 23b, UV light (365 nm) was used instead of white light. The yields of 14b, 17b and 19b irradiating with white light are shown between parentheses. a one-pot synthesis of dihydro-4Hcyclobuta [c]isochromenes was also found to proceed in excellent yield (Figure ). Irradiation of aldehyde substrate 1 with UV light yielded 1H-2-benzo [c]oxocin 1a in situ, which is immediately photoisomerized to the desired dihydro-4H-cyclobuta [c]-isochromene 1b utilizing a onepot approach.