On the fate of biliverdin-IIIα-dimethyl ester formed by scrambling during syntheses of biliverdin-IXα-dimethyl ester from bilirubin

In preparations of biliverdin-IXα-dimethyl ester (2 b) from bilirubin-IXα (1) the ratio of the XIIIα- and IIIα-isomers3 b and4 b, formed via intermolecular scrambling, should be unity. However, irrespective of the synthetic variant considered, the amount of4 b obtained usually is exceptionally low. This is partly ascribed to a consecutive reaction of4 b in acidic methanol affording the chiral diastereomeric bridged biliverdins5 (a undb) and6 (a andb), respectively.