Stereochemistry and conformational analysis of hemirubin

Intramolecularly hydrogen-bonded analogs of the natural bile pigment, bilirubin, are very scarce. Nuclear Overhauser effect NMR studies of the newest analog, a hemirubin (1), confirms intramolecular hydrogen bonding and a ridge-tile-shaped conformation. H-1 NMR studies of 1 at sufficiently low temperatures detect conformational enantiomerization, for which an activation barrier of Delta G(double dagger)similar to 16 kcal/mol at 25 degrees C in CDCl3 has been estimated. Evidence for the presence of dimeric association at low temperatures or high concentrations of 1 is found by the appearance of new sets of resonances, data from which may be used to calculate: Delta G degrees(298) (K) + 3.4 kca/mol, Delta H degrees - 5.6 kcal/mol and Delta S degrees -30.3 cal/deg/mol. (C) 2000 Elsevier Science Ltd. All rights reserved.