Bufadienolides. IX. Isolation and Structure of Resibufagin

A new bufadienolide, resibufagin (I), has been isolated from both"thin-plate"Ch'an Su and"disk-like"one. Based on the chemical and spectral evidence, the structure of resibufagin (I) was assigned to be 3β-hydroxy-19-oxo-14β, 15β-epoxy-5β-bufa-20, 22-dienolide. Treatment of I with sodium borohydride afforded an alcohol, named resibufaginol (III). Oxidation of an acetate (II) obtained from I with chromium trioxide gave a corresponding 10-carboxylic acid (V), which was methylated to afford a 10-methyl carboxylate (VI). Treatment of I or II with hydroxylamine hydrochloride and sodium acetate gave the corresponding oxime, VII or VIII, respectively. Acetylation of VII or VIII with acetic anhydride and pyridine afforded the same acetyloxime (IX). Furthermore, together I, there were obtained a bufogenin, which was identified to be marinobufagin (X), first isolation from Ch'an Su. Resibufagin (I) isolated from Ch'an Su is the third bufadienolide having 10-formyl group. Also resibufaginol (III) obtained from I is the third bufadienolide having 10-alcohol group, and may be obtained from toad venoms on further examination.