Isoheleproline: a new amino acid-sesquiterpene adduct from Inula helenium.

A new amino acid-sesquiterpene adduct, isoheleproline (1), was isolated from the roots of Inula helenium (elecampane), together with four known sesquiterpene lactones (2-5). The planar configuration of 1 was elucidated on the basis of spectroscopic data analysis, and the relative configuration of 1 was determined by performing a detailed analysis of NOESY correlations and comparing its physicochemical data with the D- and L-proline adducts of 2 obtained by Michael addition. This is the first report of a new amino acid-sesquiterpene adduct from Inula plants.