Synthesis of Covalently Linked Dimeric Derivatives of Chlorophyll A, Pyrochlorophyll A, Cchlorophyll B, and Bacteriochlorophyll A

Bis(chlorophyllide) ethylene glycol diesters were prepared for each of the title compounds. Pheophytins a and b isolated from alfalfa and bacteriochlorophyll a isolated from R. sphaeroides were treated with 80% aqueous trifluoroacetie acid to yield the corresponding pheophorbides. Pyropheophorbide was prepared by a literature procedure. Carbonic anhydride and benzotriazole-1-methanesulfonate activation methods were used in the esterification of the pheophorbides with ethylene glycol at ambient temperature. Each method yielded 75% + of the pheophorbide ethylene glycol monoester. These monoesters were treated with equimolar amounts of the corresponding pheophorbide by using benzotriazole-1-methanesulfonate/4-(dimethylamino)pyridine in CH ₂ Cl ₂ or dicyclohexylcarbodiimide/4-(dimethylamino)pyridine in CH ₂ Cl ₂ at ambient temperature. Yields of bis-(pheophorbide) ethylene glycol diesters averaged about 50% for the former method and 70% for the latter method. Insertion of the magnesium atoms into the a series macrocycles was accomplished with iodomagnesium 2, 6-di-tert-butyl-4-methylphenolate, IMgBHT, in CH ₂ Cl ₂ , while the metalation of the b and bacterial series macrocycles was carried out with a mixture of IMgBHT and lithium 2, 2, 6, 6-tetramethylpiperidide in thiophene, all at ambient temperature. Both mono- and dimetalated derivatives were isolated and characterized in each case.