BREAKDOWN OF CHLOROPHYLL - PARTIAL SYNTHESIS OF A PUTATIVE INTERMEDIARY CATABOLITE - PRELIMINARY COMMUNICATION

The partial synthesis of 10,22-dihydro-4,5-dioxo-4,5-secopheophorbide a (1) from pheophorbide a methyl ester (2) is described. A regioselect ive, photooxygenolytic reaction of (pheophorbidato a methyl ester)cadm ium(II) (3) provides the entry to the crucial 4,5-secoporphinoid struc ture in form of the (10,22-dihydro-4,5-dioxo-4,5-secopheophorbidato 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 th e structure assigned earlier to an ester of an isolation form of the r ed pigment(s) from Chlorella protothecoides. Hydrolysis of the propano ate 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.