IR, VCD, 1H and 13C NMR experimental and theoretical studies of a natural guaianolide: Unambiguous determination of its absolute configuration

This study shows that the comparison of the experimental and calculated 1H and 13C NMR spectra allows the determination of the most favorable diastereoisomers but is not sufficient to access to the absolute configuration of the 7,10-epoxy guaianolide since the two remaining enantiomers possess the same NMR spectra. The absolute configuration of this natural compound can be unambiguously established only by the comparison of the calculated and experimental VCD spectra. Indeed, a very good agreement between experimental and theoretical VCD spectra was obtained in the mid-infrared range for the 7S, 10R-epoxy-1R,5R-guaia-3,11-dien-8S,12-olide stereoisomer.