The meso-reactivity of porphyrins and related compounds. Part VIII. Substitution and addition reactions of octaethyl-21H,24H-bilin-1,19-dione, a model verdin system. X-Ray analyses of octaethyl-5-nitro-21H,24H-bilin-1,1 9-dione and of 4,5-diethoxy-octaethyl-4,5-dihydro-21H,24H-bilin-1,19-dione

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Abstract
Nitration (NaNO2–HOAc) and deuteriation (CF3CO2D) of octaethylbilindione (4), a model bile pigment, occur preferentially at the C(5) and C(15) positions; in contrast, nucleophilic attack (CN–, dimethylformamide) occurs at C(10). X-Ray analysis of the 5-nitrobilindione (6) reveals that the stereochemistry of the C(4)–C(5) double bond has changed with respect to that of the starting material: because of steric interactions involving the meso-nitro-group, ring A is twisted out of the plane of the other three rings. The resulting steric inhibition of conjugation is attended by a pronounced hypsochromic shift. Attempts to prepare halogenated bilindiones have been unsuccessful. However, bromine in ethanol (or methanol) produces the 4,5-dialkoxy-adducts [e.g. (8)]; these are believed to be formed via an oxidative route (attack of solvent on the radical cation). They are also formed by autoxidation of the cadmium complex of the bilindione in an alcoholic solvent. X-Ray analysis of the 4,5-diethoxy-adduct (8) confirms the general structure and reveals that the crystalline product obtained is the (4RS,5SR) diastereoisomer. The molecular structures and hydrogen-bonding systems of compounds (6) and (8) are compared and discussed. The bearing of these results on the molecular interpretation of the photoreversibility of phytochrome is considered.