Beiträge zur Chemie der Pyrrolpigmente, 74. Mitt.

Abstract

10-(2-Pyridyl)-, 10-(2-pyrrolyl)-, and 10-(2-imidazolyl)-bilatrienes-abc were prepared and their configurations and conformations in several solvents were investigated using UV-VIS-spectroscopy and NMR-techniques. Whereas the pyridyl and the pyrrolyl derivatives adopt the common circular helical arrangement of the bilin chromophor in a variety of solvents like chloroform, methanol, and dimethylsulfoxid, the imidazolyl derivative exhibits a dramatic solvent dependence of its geometry. It is of circular helical form in methanol; in chloroform, however, it adopts a “stretched” arrangement at the 10-methine position with an intermediate exchange (NMR-timescale) between the two identical tautomers at room temperature. In dimethylsulfoxide the population of the two diastereomers is comparable and exchange at room temperature is slow enough to allow their characterisation by¹H-NMR. A variable temperature study allowed a determination of thermodynamic and kinetic parameters for this isomerising system.