Intramolecular hydrogen bonding between remote termini

[10]-Semirubin (1) and [20]-semirubin (2), with dipyrrinones covalently lin ked to C-10 and C-20 fatty acid chains, were synthesized following Friedel- Crafts acylation of the parent 9-H dipyrrinone, (4Z)-2,3,7,8-tetramethyl-(1 0H)-dipyrrin-1-one, with the diacid chlorides of decanedioic acid and eicos anedioic acid, respectively. Both of these bright yellow pigments are monom eric 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 (V PO) measurements. Ln contrast, their methyl esters are dimeric in CHCl3. H- 1{H-1}-Nuclear Overhauser effects are found between the lactam NH and carbo xylic acid OH of 1 and 2, consistent with the type of intramolecular hydrog en bonding found in bilirubin (the yellow pigment of jaundice) and its anal ogs. (C) 2001 Elsevier Science Ltd. All rights reserved.