NOESY and DFT-GIAO Calculations Reveal Pervasive Errors in C20 Configurations of Taraxastane-3,20-diols: Proposals to Improve NMR Structure Determinations.

The current scientific consensus generally assigns incorrect C20 configurations to taraxastane-3β,20-diols and their oxo derivatives reported in 32 articles. Definitive configurations established here by NOESY, quantum mechanical NMR predictions, and C NMR γ-gauche effects warrant updating of the many erroneous structures. Suggestions to improve NMR structure determinations of such nonpolar polycyclic natural products include acquiring 2D spectra at higher resolution, suppressing zero-quantum coherence in NOESY, and judging proposed NOE correlations against H-H distances.