Abstract
Novel bilirubin and biliverdin congeners with propionic acids replaced by o-carboxyphenyl exhibit diastereomerism due to axial chirality about the carbon–carbon single bond linking the o-carboxyphenyl group to a pyrrole ring. Evidence for atropisomerism was found even in the monopyrrole precursor, ethyl 3,5-dimethyl-4-( o-carboxyphenyl)pyrrole-2-carboxylate. Like bilirubin, o-carboxyphenyl rubin 1a adopts an intramolecularly hydrogen-bonded ridge-tile conformation in nonpolar solvents. In solutions containing optically active amines or human serum albumin 1a exhibits intense bisignate exciton coupling-type induced circular dichroism for its long wavelength absorption near 400 nm.
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