Determination of the Preferred Stereoisomer of Natural Product Bisabolol in Chloroform Solution through Quantum Chemical Calculations of 1H NMR Chemical Shifts.

The elucidation of natural product structures and the differentiation of stereoisomers are important issues in organic chemistry. An example is the sesquiterpene (-)-α-bisabolol (αBis), having two stereogenic centers, αBis and a flexible side chain that generates high conformational freedom. Recently, new tools named DP4+ and ANN-PRA, which are probabilistic approaches, were used in combination to solve the relative configuration of αBis (α and epi-α diastereomers). Nuclear magnetic resonance (NMR) chemical shifts obtained from density functional theory (DFT) calculations in the vacuum were used by the DP4+ and ANN-PRA computational algorithms averaged by the Boltzmann population. Although such a procedure can provide an indication of the most probable enantiomer, no information on the spatial arrangement of the preferred molecular structure present in the NMR experiment (in CDCl3) could be obtained. In this work, we used the DFT methodology and the polarizable continuum model approach, with the inclusion of explicit CHCl3 solvent molecules, to calculate 1H NMR spectra for various distinct trial molecular structures of αBis, encompassing α and epimeric forms, varying relevant torsion angles to find plausible minimum energy structures on the potential energy surface, including solvent effects. Through comparison between the experimental and theoretical 1H NMR profiles in chloroform solution, we were able to unambiguously elucidate the predominant molecular structure (enantiomer α) that reproduced faithfully the experimental 1H NMR pattern. This could not be done in previous work employing the DP4+ and ANN-PRA tools; however, there is an agreement that the α stereoisomer should be predominant. The preferred α-Bis molecular structure reported here will most probably interact with biological targets.