Syntheses of bile pigments. Part 15. First unequivocal assignment of the absolute configuration of an urobilinoid bile pigment by X‐ray diffraction analysis of its synthetic precursor

Abstract
(−)‐(4S,16S)‐8, 12‐bis[de(2‐carboxyethyl)]mesourobilin‐IIIα hydrochloride (8) has been synthesized from the enantiomerically pure 1,4,5,10‐tetrahydro‐1‐oxodipyrrin‐9‐carboxylic‐acid precursor 6a whose absolute configuration was determined by X‐ray diffraction analysis of the N‐[(S)‐1‐(1‐naphthyl)ethyl] carboxamide 7b. The present results prove unequivocally that an (S,S)‐configurated urobilin chromophore displays a negative Cotton effect in the VIS absorption range. However, the helicity of the inherently dissymmetric chromophore remains undetermined.