Vibrational Circular Dichroism Similarity-Guided Absolute Configuration Determination of 11-Acyloxytremetones

Abstract

Although tremetone [5-acetyl-2-(1-methylvinyl)-2,3-dihydrobenzofuran] has only one stereogenic center, the absolute configuration (AC) determination of its naturally occurring 11-acyloxy derivatives 1 and 2 by vibrational circular dichroism (VCD) turned out to be difficult. Similarity-based comparison of the experimental VCD spectrum of 11-coumaryloxytremetone (1), isolated from Parastrephia quadrangularis, with spectra calculated using popular density functional theory (DFT) levels of theory, provided poor enantiomeric similarity indices ( ESI), even when the p-coumaroyl ester group of 1 was replaced by the acetyl group in 2. In search for a better understanding of these results, IR-guided individual scaling factors (ISFs), recently introduced as part of the Vibrational Spectra Similarity and Analysis Tool (VISSAT) software, were used to correct DFT frequencies, while a VCD-guided conformational analysis was developed to explore conformational preferences. These studies showed that for both molecules 72% of the individual conformations gave ESI values in favor of the ( R) enantiomer. Likewise, when conformer abundances were optimized to produce the best possible similarity for each enantiomer, the obtained ESI values were always larger for the ( R) isomer than for the ( S) isomer. These results point toward the ( R) AC in both compounds and highlight the incorrect conformer abundance prediction by DFT calculations as the potential source of the initial difficulties. In addition, the AC of 1 was independently verified using the Flack and Hooft parameters gained after a single-crystal x-ray diffraction (XRD) study.

Individual scale factor approach for the vibrational circular dichroism similarity‐guided spectral and conformational analysis of perezone and dihydroperezone

“…As recently shown for the highly flexible 11‐acyloxytremetones, 17 a likely culprit to this problem is the incapacity of the commonly used DFT levels of theory to consistently predict accurate relative free energies and, therefore, correct conformational abundances. It has been proposed 31 that this lack of consistency is related to the fortuitous error balances that are known to occur, particularly with the B3LYP functional.…”

“…The worst cases of these VCD rebel molecules 16,17 are the recently studied tremetones, 17 which could not be solved using widely used VCD calculation methodologies and could finally be cleared-up using the recently developed 16 VISSAT software.…”