Conformational analysis of perezone and dihydroperezone using vibrational circular dichroism

• Published in: Phytochemistry (Oxford) 2012-02, Vol.74, p.190-195
• Main Authors: Burgueño-Tapia, Eleuterio, Cerda-García-Rojas, Carlos M., Joseph-Nathan, Pedro
• Format: Article
• Language: English
• Subjects: Absolute configuration; Circular Dichroism; Compositae; Dihydroperezone; Models, Molecular; Molecular Conformation; Molecular Structure; Monte Carlo Method; Perezia adnata var. alamani; Perezone; Plant Extracts - chemistry; Sesquiterpenes - chemistry; Stereoisomerism; Vibration; Vibrational circular dichroism
• ISSN: 0031-9422; 1873-3700
• DOI: 10.1016/j.phytochem.2011.10.005

VCD spectra similarity of perezone (a) and dihydroperezone (b) suggests equivalent conformational behavior, which excludes published argumentation that π–π interactions between the side-chain double bond and the quinone ring controls the molecular conformation of a. [Display omitted] ► Experimental VCD spectra of perezone (1) and dihydroperezone (2) are quite similar. ► These spectra, when compared to calculated spectra, also show close resemblance. ► There is no evidence for published folded conformations of 1 owing to π–π interactions. The vibrational circular dichroism (VCD) spectra of perezone and dihydroperezone measured from CDCl3 solutions were quite similar, suggesting analogous conformations for both molecules. Their absolute configurations were confirmed by comparison of the experimental VCD spectrum of each compound with curves generated from theoretical calculations using density functional theory (DFT) at the B3LYP/DGDZVP level of theory taking into account their conformational mobility. Conformational analysis of the 8-(R) enantiomer showed 19 low energy conformers in a 2.4kcal/mol energy range, while for 8-(R), with the saturated side alkyl chain, 34 conformers were considered in the first 2kcal/mol. Initial analyses were carried out using a Monte Carlo searching with the MMFF94 molecular mechanics force field, all MMFF94 conformers were geometrically optimized using DFT at the B3LYP/6-31G(d) level of theory, followed by reoptimization and calculations of their vibrational frequencies at the B3LYP/DGDZVP level. Good agreement between the theoretical 8-(R) enantiomers and experimental VCD curves were observed for both.