Revisiting empirical rules for the determination of the absolute configuration of cascarosides and other (ox-)anthrones.

The introduction of the C10 -stereocenter of (ox-)anthrones by plant organisms is not stereospecific. Consequently, often, both (10S)- and (10R)-diastereomers can be found in the same plant. Motivated by the importance of a correct assignment of the configuration at C10 , this study revisits the nuclear magnetic resonance and electronic circular dichroism-based empirical rules for the determination of the absolute configuration by molecular dynamic simulations and electronic circular dichroism spectrum calculations. Furthermore, a vibrational circular dichroism spectroscopic characterization of these large and conformationally very flexible molecules reveals spectral signatures, which can be used to specifically distinguish the C10 stereochemistry. A detailed analysis of the underlying vibrational modes suggests that the observed spectral pattern of the investigated cascarosides may be generally characteristic for the C10 -stereocenter of (ox-)anthrones and that they can be used for empirical spectra-structure correlations.