Configurational Variation of a Natural Compound within Its Source Species. The Unprecedented Case of Areolal in
Héctor M Arreaga-González1, Gabriela Rodríguez-García1, Rosa E Del Río1
1Instituto de Investigaciones Químico-Biológicas, Universidad Michoacana de San Nicolás de Hidalgo , Ciudad Universitaria , Morelia , Michoacán 58030 , Mexico.
Abstract:
The exceptional case of a natural compound that shows drastic absolute configuration variations within the same species was examined. Sequential samples of areolal (1) isolated from Piptothrix areolare showed dextrorotatory (ee 32%), almost racemic (ee 4%), levorotatory (ee 82%), and again dextrorotatory (ee 10%) values. Enantiomeric compositions of this epoxythymol derivative were determined from individual plant specimens collected from the same geographical location over a 46-day period, which were processed using the same extraction and isolation methods. Detection of this unusual phenomenon was possible by analysis of NMR data recorded in the presence of BINOL as a chiral solvating agent. The absolute configuration of (-)-(8S)-areolal followed from vibrational circular dichroism data of an enantiomerically enriched sample, while single-crystal X-ray diffraction and supramolecular analyses revealed interactions that diminish the crystal entropy in rac-1. These results might be related with environmental factors and biochemical processes, suggesting the need of strict evaluations of enantiomeric composition of natural products that could be considered for human applications.
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