The first 6′-O-sulfated phenylanthraquinones:: isolation from Bulbine frutescens, structural elucidation, enantiomeric purity, and partial synthesis

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 anthraquirione 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-deniethylknipholone. (c) 2005 Elsevier Ltd. All rights reserved.