The didehydrocorrinoid derivative of vitamin B12, ‘pyrocobester’1(hexamethylCoα, Cob̃‐dicyano‐7‐de (carboxymethyl)‐7,8‐didehydrocobyrinate), is oxygenated in the presence of visible light and molecular oxygen to give the previously unknown ‘5,6‐dioxosecopyrocobester’3(hexamethylCoα, Cob̃‐dicyano‐5,6‐dioxo‐7‐de(carboxymethyl)‐7,8‐didehydro‐5,6‐secocobyrinate) under regioselective cleavage of the macrocycle at the 5,6‐position. Efficiency and yield of this reaction involving ‘singlet oxygen’ depend on the so