Hydrogen bonding and π-stacking in dipyrrinone acid dimers of xanthobilirubic acid and chiral analogs

Abstract

Xanthobilirubic acid and its analogs self-associate strongly through intermolecular hydrogen bonding between their carboxylic acid and dipyrrinone components, forming π-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: βS-methylxanthobilirubic acid (1) has [α]D20 = −314° and Δ∈434max = −10.9, Δ∈388max = +5.7 (CHCl3). In contrast, methyl esters show weaker rotations and vanishingly small CD: the methyl ester of (1) has [α]D20 +62° and Δ∈435 ⪡ 0.5, Δ∈ ⪡ 0.5.