The absolute configuration of natural (-)-stercobilin and other urobilinoid compounds

Chromic acid degradation of natural (-)-stercobilin (1) yields 2(R)-methyl-3(R)-ethylsuccinimide (+2), whereby the absolute configuration of 1 at the chiral centers C-1, C-2, C-7, and C-8 is established. The substituted oxo-tetrahydrodipyrromethane precursor, 5, for the total synthesis of (-)-stercobilins 3 and 4, in which the relative configuration between the asymmetric centers is known, yields 2(S)-methyl-3(S)-ethylsuccinimide (-2) under the same conditions of degradation. Nuclear magnetic resonance studies of 1 and 3 show that in 1 the hydrogen atoms at C-2 and C-2', as well as those at C-7 and C-7', are trans relative to one another. Accordingly, natural (-)-stercobilin possesses the 2'(S), 7'(S) configuration, and has the configuration formula 6(1 (R), 2(R), 2'(S), 7'(S), 7(R), 8(R)). These results, coupled with those of earlier studies, also establish the absolute configuration of the (+)-urobilin 7 and of the phycobilin 8 at C-7'.