Add time:09/05/2019 Source:sciencedirect.com
xanthobilirubic acid (cas 15770-19-1) 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.
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