Determination of the Absolute Configurations of Natural Products Using TDDFT Optical Rotation Calculations: The Iridoid Oruwacin

We report the determination of the absolute configuration (AC) of the iridoid natural product oruwacin by comparison of the optical rotations, [alpha] D, of its two enantiomers, calculated using time-dependent density functional theory (TDDFT), to the experimental [alpha] D value, +193. Conformational analysis of oruwacin using density functional theory (DFT) identifies eight conformations which are significantly populated at room temperature. [alpha] D values of these eight conformations are calculated using TDDFT at the B3LYP/aug-cc-pVDZ//B3LYP/6-31G* level, leading to the conformationally averaged [alpha] D values of -193 for the (1 R,5 S,8 S,9 S,10 S)-enantiomer and +193 for the (1 S,5 R,8 R,9 R,10 R)-enantiomer. Comparison of the calculated [alpha] D values to the value of the natural product proves that naturally occurring oruwacin has the AC 1 S,5 R,8 R,9 R,10 R. This AC is opposite to that assigned by Adesogan by comparison of the [alpha] D of oruwacin to that of the iridoid plumericin. Our results show that the assignment of the AC of a natural product by comparison of its [alpha] D to that of a chemically related molecule can be unreliable and should not be assumed to be definitive.