The structure of dihydro-4H-cyclobuta [c]isochromene 21b was confirmed by single-crystal X-ray diffraction (Figure ). The X-ray structure clearly confirms that the cis-product is formed, i.e., featuring the proton and phenyl group at the quaternary carbon atoms within the 4-membered ring in a mutual cis-configuration. [c]oxocins to dihydro-4H-cyclobuta [c]isochromenes is a powerful methodology to construct strained, fused 4,6-ring structures, which widely exist in many bioactive structures. Moreover, the 1H-2-benzo [c]oxocins also can be efficiently transformed to dihydro-1Hepidioxybenzo [c]oxocines under irradiation, in high yields via a [4+2] cycloaddition with photochemically generated 1 O2 using meso-tetraphenylporphyrin (TPP) as the photosensitizer (Table ). Thus, the efficient conversion of dihydro-1H-epidioxybenzo [c]oxocines also provides efficient synthetic protocols to construct medium-sized rings with strained endoperoxide substituents, which are found in many natural pharmaceuticals with great bioactivity. The cycloisomerizations to dihydro-4H-cyclobuta [c]isochromenes provide a promising, new photoswitchable system that utilized visible light for 1H-2-benzo [c]oxocins featuring aryl substituents at the enol ether R1-position. Intrigued by the above results, we continued to investigate the reverse process entailing the ring-opening of the strained, fused 4,6-membered dihydro-4Hcyclobuta [c]isochromene ring compounds. Initial efforts targeting the photochemical ring-opening of 1b quickly proved impossible as the intramolecular [2+2] cyclization breaks the conjugated structure of the 1H-2benzo [c]oxocins.