![]() ![]() Under brief reaction, bilirubin, its dimethyl ester and biliverdin dimethyl ester afforded only low yields of vinyl-neoxanthobilirubinic acid ( Scheme 1) or its methyl ester ? all with an endo vinyl group. In the mid-1920s Hans Fischer renewed his investigations of the constitutional structure of bilirubin and learned subsequently that bilirubins and biliverdins are cleaved to 9-H dipyrrinones in boiling resorcinol. Linear representations of (A) bilirubin, (B) biliverdin and (C) mesobilirubin and their 9-H dipyrrinone products isolated from molten resorcinol. 9-Methyldipyrrinones are likewise synthesized from monopyrroles, but direct conversion of these synthetically more accessible pigments to synthetically useful 9-H dipyrrinones has not been achieved. 1 The 9-H dipyrrinone precursors, as well as 9-formyldipyrrinones have been prepared by synthesis, typically from monopyrroles. Although these pigments are not biosynthesized in nature by conjoining two dipyrrinones, bilirubin and biliverdin analogs have been prepared synthetically by coupling two 9-H dipyrrinones with formaldehyde or its equivalent, or by coupling a 9-formyldipyrrinone with a 9-H dipyrrinone ? or even by oxidative coupling of 9-methyldipyrrinones. 2 In both bilirubin and biliverdin, the dipyrrinone units are connected by a single carbon, C(10). Dipyrrinones 1 are the chromophores of bilirubin ( Scheme 1), the yellow-orange pigment of mammalian bile and of jaundice, and they also constitute the two halves of biliverdin ( Scheme 1), the blue-green biological precursor of bilirubin and the pigment of non-mammalian bile. ![]()
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