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

Abstract 7,10-Epoxy -1,5-guaia-3, 11-dien-8, 12-olide has been isolated from dried leaves of Hedyosmum arborescens Swartz. The structure, vibrational frequencies, infrared and VCD intensities, NMR 1 H and 13 C spectra have been calculated by the density functional theory (DFT) method at the B3LYP/6-31+G(d,p) levels for four stereoisomers of this natural guaianolide. This study shows that the comparison of the experimental and calculated 1 H and 13 C 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 7 S , 10 R -epoxy-1 R ,5 R -guaia-3,11-dien-8 S ,12-olide stereoisomer.