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Determination of the Absolute Configurations of Natural Products via Density Functional Theory Calculations of Vibrational Circular Dichroism, Electronic Circular Dichroism, and Optical Rotation: The Iridoids Plumericin and Isoplumericin
journal contribution
posted on 2007-04-27, 00:00 authored by P. J. Stephens, J. J. Pan, F. J. Devlin, K. Krohn, T. KurtánThe absolute configurations (ACs) of the iridoid natural products, plumericin (1) and isoplumericin (2),
have been re-investigated using vibrational circular dichroism (VCD) spectroscopy, electronic circular
dichroism (ECD) spectroscopy, and optical rotatory dispersion (ORD). Comparison of DFT calculations
of the VCD spectra of 1 and 2 to the experimental VCD spectra of the natural products, (+)-1 and (+)-2,
leads unambiguously to the AC (1R,5S,8S,9S,10S)-(+) for both 1 and 2. In contrast, comparison of time-dependent DFT (TDDFT) calculations of the ECD spectra of 1 and 2 to the experimental spectra of
(+)-1 and (+)-2 does not permit definitive assignment of their ACs. On the other hand, TDDFT calculations
of the ORD of (1R,5S,8S,9S,10S)-1 and -2 over the range of 365−589 nm are in excellent agreement
with the experimental data of (+)-1 and (+)-2, confirming the ACs derived from the VCD spectra.
Thus, the ACs initially proposed by Albers-Schönberg and Schmid are shown to be correct, and the
opposite ACs recently derived from the ECD spectra of 1 and 2 by Elsässer et al. are shown to be
incorrect. As a result, the ACs of other iridoid natural products obtained by chemical correlation with 1
and 2 are not in need of revision.