Isolation and structure of a 20,21-epoxybufenolide series from "Ch'an Su"

Steroid bufenolides resulting from epoxidation of the 17beta-2-pyrone ring of bufadienolides are rare. Five 20,21-epoxybufenolides, namely, 20S,21-epoxyresibufogenin (1), 20R,21-epoxyresibufogenin (2), 3-O-formyl-20S,21-epoxyresibufogenin (3), 3-O-formyl-20R,21-epoxyresibufogenin (4), and 3-oxo-20S,21-epoxyresibufogenin (5), were isolated from the Chinese toad skin extract drug Ch'an Su. The structures were elucidated by spectroscopic and chemical methods. The configuration at C-20 was assigned by the analysis of difference NOE spectra. The cancer cell (KB and MH-60) growth inhibition by the new 20,21-epoxybufenolides was examined, and 20,21-epoxides 1, 2, and 5 were found to significantly inhibit the leukemia MH-60 cell line.