Synthesis of covalently linked dimeric derivatives of chlorophyll a, pyrochlorophyll a, chlorophyll 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 trifluoroacetic 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 CH₂Cl₂ or dicyclohexylcarbodiimide/4-(dimethylamino)pyridine in CH₂Cl₂ 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 CH₂Cl₂, while the metalation of the 6 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.