Abstract
The structure of the dimeric eudesmanolide hydroxy-bis-dihydrofarinosin (1) from Encelia farinosa followed after contrasting their 1 H and 13 C NMR spectra with those of encelin (6), hydroxy-bis-dihydroencelin (3), and farinosin (4). Structure1 was verified by single-crystal X-ray diffraction analysis, which further provided the stereochemistry of the hydroxy group at C-4. Comparison of the experimental vibrational circular dichroism spectrum of its derived diacetate 2 with that calculated by density functional theory provided the absolute configuration, which resulted the same as that of its biogenetic precursor 4. Evaluation of several chemical shift differences between the two eudesmanolide fragments of 1 and 3 allows also ascertaining the yet not reported absolute configuration of the C-4 stereogenic center of 3. Copyright © 2016 John Wiley & Sons, Ltd.
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