title: A C-H...O=C hydrogen bond? Intramolecular hydrogen bonding in a novel semirubin.
abstract: (4Z)-8-(5-Carboxypentyl)-9-butyl-2,3-diethyl-dipyrrin-1-one (1), a new analogue of xanthobilirubic acid, (4Z)-8-(carboxyethyl)-2,7,9-dimethyl-3-ethyl-dipyrrin-1-one, was synthesized in four steps from the known 2,3-diethyl-dipyrrin-1-one. Whereas xanthobilirubic acid (which is a model for one-half of bilirubin, the yellow pigment of jaundice) and its homologues with hexanoic and longer acid chains at C-8 engage only in intermolecular hydrogen bonding, 1 is found to engage in intramolecular hydrogen bonding. In CDCl(3) solution, dipyrrinone 1 adopts an anti-Z conformation, and its hexanoic acid COOH is hydrogen-bonded to the lactam H-N-C=O and to the pyrrole C(7)-H but not to the pyrrole NH. The latter constitutes an example of a hydrogen bond of the type C-H...O=C, weak and detected typically in crystals. Dipyrrinone 1 is found by vapor pressure osmometry to be monomeric in CHCl(3), but its methyl ester (2) tends toward being dimeric, like that of methyl xanthobilirubinate, which is dimeric.
authors.affiliation: Department of Chemistry, University of Nevada, Reno, Nevada 89557-0020, USA.
authors.forename: M T
authors.initials: MT
authors.lastname: Huggins
authors.forename: D A
authors.initials: DA
authors.lastname: Lightner
mesh_terms: D001663:Bilirubin / Q000737:chemistry; D002208:Caproates / Q000737:chemistry; D002725:Chloroform; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D011758:Pyrroles / Q000737:chemistry*; D012997:Solvents; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet; D014835:Volatilization
chemical_list: D002208:Caproates; D011758:Pyrroles; D012997:Solvents; C415191:semirubin; C037652:hexanoic acid; D002725:Chloroform; D001663:Bilirubin