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The chemical and photochemical oxygenation of 2,3-dihydrooctaethyl-1,19(21,24H)-bilindione (1), as a model for the chromophores of both phytochrome Pr and phycocyanin, has been studied in neutral and alkaline solution, and in the presence of zinc ions. By comparison with octaethyl-1,19(21,24H)-bilindione (5), the influence of ring A hydrogenation on the reactivity of bilins has been assessed. In the dark, 1 is attacked selectively and rapidly at C-5 yielding "purpurins", while 5 reacts slowly, and is attacked predominantly at C-10. Photooxidation of 5 yields the tripyrrinic "purpurin" 7 only. Photoreactivity of 1 is considerably enhanced, yielding "purpurins" and "violins" rapidly. In spite of UV-vis  and mass spectroscopic similarities, the "purpurin" 7 differs from the "purpurins" 6a,b by the loss of ring A. The facile cleavage at the C-5 methine bridge and the spectroscopic properties of "purpurins" are discussed.

Studies on Plant Bile Pigments, II. Chemical and Photochemical Oxygenation of a Phytochrome P r Chromophore Model Pigment to Purpurins