Four new selaginellin derivatives from Selaginella pulvinata: mechanism of racemization process in selaginellins with quinone methide

Nine selaginellin derivatives, including a novel isoquinoline-type selaginellin named selaginisoquinoline A (I), and three ethoxy selaginellins P-R (2-4), were isolated from the plant of Selaginella pulvinata. Their structures were determined by extensive spectroscopic analyses, and single-crystal X-ray diffraction, as well as CD calculations. Selaginisoquinoline A (1) is a racemate due to quinone methide-phenol tautomerism; while in the case of selaginellins P-R (2-4), whose tautomerism was blocked by alkylation of C10-OH, the interesting racemization process is induced by axial chirality and molecular rotation, thus existing as a pair of slowly interconverting enantiomers. Compounds 3, 5 and 9 showed moderate cytotoxicity against BGC-823 human gastric cancer cell line. All compounds, including four enantiomers, exhibited good antifungal activities against four pathogenic fungi cell lines with MIC80 values in the range of 4-32 mu g/mL. (C) 2015 Elsevier Ltd. All rights reserved.