Semirubin. A novel dipyrrinone strapped by intramolecular hydrogen bonds.
(4Z)-9-(5-Carboxypentyl)-2,3,7,8-tetramethyl-(10H)-dipyrrin- 1-one (1, semirubin), a new dipyrrinone model for one-half of bilirubin, the yellow-orange neurotoxic pigment of jaundice, was synthesized following Friedel-Crafts acylation of 2,3,7, 8-tetramethyl-(10H)-dipyrrin-1-one (5) with the half-ester acid chloride of adipic acid. Unlike other dipyrrinone models for bilirubin, such as the xanthobilirubic acids, which engage only in intermolecular hydrogen bonding, 1 is unique in having been designed and found to engage in intramolecular hydrogen bonding, between the carboxylic acid and the dipyrrinone lactam and pyrrole. This important conformation-determining structural characteristic, shared by 1 and bilirubin, renders them less polar than their methyl esters and leaves them monomeric in nonpolar solvents, where their esters are dimeric. The corresponding 10-oxo analogue (3) of 1 serves as a model for 10-oxo-bilirubin, a presumed bilirubin metabolite in alternate pathways for bilirubin excretion. Like 1, 3 is found to engage in intramolecular hydrogen bonding. Unlike the methyl ester of 1, the ethyl ester of 3 is not intermolecularly hydrogen bonded in nonpolar solvents.
Huggins
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MT
Department of Chemistry, University of Nevada, Reno, Nevada 89557-0020, USA.
Lightner
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DA
D000215:Acylation; D001663:Bilirubin / Q000737:chemistry*; D002855:Chromatography, Thin Layer; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D011758:Pyrroles / Q000138:chemical synthesis / Q000737:chemistry*; D012996:Solutions; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet
D011758:Pyrroles; D012996:Solutions; C415191:semirubin; D001663:Bilirubin