FORMATION OF OXONIACHLORINS ON PHOTOOXIDATION OF 20-TRIFLUOROACETOXY-CHLOROPHYLL AND 20-CHLORO-CHLOROPHYLL DERIVATIVES

Zinc complexes of both 20-trifluoroacetoxy- and 20-chloro-pheophorbide-alpha methyl ester yield the same isolable 20-oxonia-pheophorbide (2b) on irradiation with visible light in methylene chloride for 90 and 10 min, respectively. In the case of the 20-chloro derivative, the yield amounts to 40-50% along with 30% of starting material. Zinc complexes of 20-trifluoroacetyl- and 20-chloro-pyropheophorbide-alpha methyl ester behave likewise. In aqueous solution, the expected dihydro-biliverdin derivatives 2 are obtained among other products, some of them display a strong red fluorescence. In the presence of nucleophiles the oxoniachlorin macrocycle is readily cleaved, yielding the corresponding linear tetrapyrroles. Particularly, reaction with cyanide ions leads to biline derivatives, which could serve as intermediates for the synthesis of natural occurring luciferins from bioluminescent plankton. The possible role of some of the above mentioned reactions in connection with chlorophyll catabolism is discussed.