Hydrogen bonding: HOC=O···H–N vs. HOC=O···H–C

A chloroform-soluble dipyrrinone, 8,9-bis-(5-carboxypentyl)-2,3-bis-(2-methoxyethoxy)-10H-dipyrrin-1-one, with solubilizing 2-methoxyethoxy β-substituents on the lactam ring and two hexanoic acid groups (one at C(9), the other at C(8) of the pyrrole ring) was synthesized for its ability to form intramolecular hydrogen bonds to the lactam unit from either carboxylic acid, whether in the syn-(Z) or anti-(Z) conformation. In the syn-(Z) conformation, such intramolecular hydrogen bonds can also include the pyrrole N-H. In the anti-(Z), intramolecular hydrogen bonds can include the pyrrole C(7)-H. Evidence for both monomer conformations in equilibrium is provided by 1H NMR analyses, which indicate that the syn-(Z) is favored over the anti-(Z) and predict an interconversion barrier of approximately 40 kJ mol−1.