Bioassay-guided fractionation of the root tissue of Reevesia formosana led to isolation of 13 cardenolide glycosides, reevesiosides A–I and epi-reevesiosides F–I. Their structures were determined by means of spectroscopic analysis and single-crystal X-ray diffraction was performed using reevesioside A. Reevesioside A, reevesioside F, and epi-reevesioside F displayed especially potent cytotoxicity against the MCF-7 and NCI-H460 cancer cell lines, with IC50 values of 63 ± 2 and 19 ± 1, 72 ± 8 and 20 ± 0, and 34 ± 6 and 10 ± 1 nM, respectively. Identification of the sugar constituents and unusual 18,20-epoxide cardenolide glycosides are described herein. Cardiac glycosides were previously unknown in the Sterculiaceae family.
