HYDROGEN-BONDING AND PI-STACKING IN DIPYRRINONE ACID DIMERS OF XANTHOBILIRUBIC ACID AND CHIRAL ANALOGS

Xanthobilirubic acid and its analogs self-associate strongly through intermolecular hydrogen bonding between their carboxylic acid and dipyrrinone components, forming rr-stacked dimers. In contrast, their methyl esters form planar dimers conjoined by dipyrrinone to dipyrrinone intermolecular hydrogen bonding. When a stereogenic center is present in the propionic side acid chain, unusually large optical rotations and exciton coupling circular dichroism may be observed for the optically active acids: beta S-methylxanthobilirubic acid (1) has [alpha](20)(D) = -314 degrees and Delta epsilon(434)(max) = -10.9, Delta epsilon(388)(max) = +5.7(CHCl3). In contrast, methyl esters show weaker rotations and vanishingly small CD: the methyl ester of (1) has [alpha](20)(D) and 62 degrees and Delta epsilon(435) much less than 0.5, Delta epsilon(390) much less than 0.5.