Absolute configuration of esquelane derivatives from Adesmia boronioides by vibrational circular dichroism

The absolute configurations of natural esquelane derivatives 1 and 2, the main components of a commercially available fragrance, as well as their synthetic derivatives 3–7 were revised by using the vibrational circular dichroism (VCD) spectroscopy of 2 and 6 in combination with density functional theory calculations at the B3LYP/DGDZVP level of theory. The agreement between the experimental and calculated VCD spectra of these bisnorsesquiterpenes revealed that they are in fact (1S,4R,5S)-esquel-6-en-9-one 1 and (1S,4R,5R)-esquel-7-en-9-one 2 instead of the formerly reported enantiomers. The VCD method was also applied to cacalol acetate 9 in order to verify its absolute configuration, since cacalol 8 was previously used as a comparative motif to determine the absolute configuration of esquelane derivatives.