Synthesis of Covalently Linked dimeric Derivatives of Chlorophyll a, Pyrochlorophyll a, Chlorophyll b, and Bacteriochlorophyll a

Article abstract of DOI:10.1021/jo01298a043

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 80percent aqueous trifluoroacetic acid to yield the corresponding pheorbides.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 75percent+ 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 50percent for the former method and 70percent 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 b and bacterial series macrocycles was carried out with a mixture of IMgBHT and lithium 2,2,6,6-tetramethylpiperidie in thiophene, all at ambient temperature.Both mono- and dimethalated derivatives were isolated and characterized in each case.