Syntheses of Bile Pigments. Part 17. Synthesis of a non‐racemizable urobilin derivative

The optically active urobilin model compound 7 was synthesized, in which Me groups instead of H‐atoms are bound to the asymmetric centers, thus preventing loss of chirality by tautomerization. The key intermediate of the eleven‐step synthesis of 7 is the 1,4,5,10‐tetrahydro‐10‐hydroxy‐1‐oxo‐11H‐dipyrrin‐9‐carboxylate rac‐2, which could be resolved into enantiomers by fractional crystallization of the corresponding methyl N‐[1‐(naphth‐1‐yl)ethyl]carbamates 3 and 4. The absolute configuration of enantiomerically pure (−)‐2 was determined by X‐ray diffraction analysis of its camphor‐10‐sulfonate 5. As the CD spectrum of the urobilin analogue 7 obtained from (−)‐(R)‐2 displays a positive Cotton effect, the present results prove, in connection with previous work, that substitution of Me groups for the H‐atoms bound to the asymmetric centers of a chiral urobilin chromophore do not influence the relationship between absolute configuration of the latter and its helicity.