AbstractThe partial synthesis of 10,22‐dihydro‐4,5‐dioxo‐4,5‐secopheophorbide a (1) from pheophorbide a methyl ester (2) is described. A regioselective, photooxygenolytic reaction of (pheophorbidato a methyl ester)cadmium(II)(3) provides the entry to the crucial 4,5‐secoporphinoid structure in form of the (10,22‐dihydro‐4,5‐dioxo‐4,5‐seco‐pheophorbidato a methyl ester)cadmium(II) (4). The hydride reduction of this 4,5‐dioxo‐4,5‐secophytoporphyrin ester occurs selectively at the ‘eastern’ meso‐position to lead (after demetallation) to 10,22‐dihydro‐4,5‐dioxo‐4,5‐secopheophorbide a methyl ester (5). This oxobilin‐carbaldehyde has the structure assigned earlier to an ester of an isolation form of the red pigment(s) from Chlorella protothecoides. Hydrolysis of the propanoate ester function of 5, selectively catalyzed by pig liver esterase, then yields the title compound 1. The red tetrapyrrole 1 may represent an intermediary chlorophyll catabolite in degreening plants.