The first 6′-O-sulfated phenylanthraquinones: isolation from Bulbine frutescens, structural elucidation, enantiomeric purity, and partial synthesis |
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Authors: | Joan Mutanyatta Merhatibeb Bezabih Michael Dreyer Nikolaus Kocher |
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Institution: | a Institute of Organic Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany b Department of Chemistry, University of Botswana, Private Bag 0022, Gaborone, Botswana c Swiss Tropical Institute, Socinstrasse 57, CH-4002 Basel, Switzerland d Institute of Inorganic Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany |
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Abstract: | From the roots of Bulbine frutescens, the first sulfated phenylanthraquinones were isolated, together with their known sulfate-free analogs. Their structures were elucidated by spectroscopic and chiroptical methods, by acid hydrolysis or by partial synthesis. The new compounds have the usual stereo-orientation at the biaryl axis (i.e., with the acetyl portion above the anthraquinone plane) except for sodium ent-knipholone 6′-O-sulfate (and thus, also its hydrolysis product, ent-knipholone), which exhibit an opposite axial configuration. We also describe the first stereoanalysis of natural phenylanthraquinones, some of which were found to be not enantiomerically pure, some even near-racemic. We furthermore, report on the first X-ray structure analysis of a phenylanthraquinone, viz. 4′-O-demethylknipholone. |
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Keywords: | Bulbine frutescens Sulfated phenylanthraquinones Axial chirality Enantiomeric resolution LC-CD coupling Crystal structure |
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