Solvent Induced Circular Dichroism in Conformational Analysis: Bile Pigments

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Abstract
The applicability of solvent induced circular dichroism (SICD) for the conformational analysis of bile pigments was investigated. The S-(-)-ethyl lactate induced rotational strengths for octaethylbilindion (4) and its dihydroderivative 5 are remarkably high. Related compounds, e.g., the isomeric purpurines 1 and 2 and formyltripyrrinon 3 exhibit an optical activity which is smaller by > 1 order of magnitude. 1-3 are essentially free from steric strain so that a flat arrangement of the chromophore is most likely. The closed conformations of 4 and 5 experience considerable steric repulsion of their terminal rings, so that a helical topology is energetically favored. This distinction is reflected in the magnitude of the SICD observed and demonstrates its applicability for the conformational analysis of bile pigments.