Conformational analysis of three germacranolides by the PM3 semi-empirical method

The conformations of the C-6 lactonized germacranolides 1 3 were calculated by applying the PM3 semi-empirical method. The low-temperature 1H-NMR spectra of parthenolide (1) were also measured. The relations between the calculated and the experimentally determined geometries were established using a modified Karplus equation. The applied quantum-chemical calculations proved to be an efficient and easy-to-use tool for the elucidation and prediction of the properties of germacranolide-type sesquiterpene lactones.