Intramolecular hydrogen bonding between remote termini

[10]-Semirubin (1) and [20]-semirubin (2), with dipyrrinones covalently linked to C10 and C20 fatty acid chains, were synthesized following Friedel–Crafts acylation of the parent 9-H dipyrrinone, (4Z)-2,3,7,8-tetramethyl-(10H)-dipyrrin-1-one, with the diacid chlorides of decanedioic acid and eicosanedioic acid, respectively. Both of these bright yellow pigments are monomeric in CHCl3 solvent: MWobs=394±20 for 1 (formula weight 386), and 533±20 for 2 (formula weight 526) as determined by vapor pressure osmometry (VPO) measurements. In contrast, their methyl esters are dimeric in CHCl3. 1H{1H}-Nuclear Overhauser effects are found between the lactam NH and carboxylic acid OH of 1 and 2, consistent with the type of intramolecular hydrogen bonding found in bilirubin (the yellow pigment of jaundice) and its analogs.