Plant anticancer agents. XXX: Cucurbitacins from Ipomopsis aggregata (Polemoniaceae).

Isocucurbitacin B (I), 3-epi-isocucurbitacin B (II), and cucurbitacin B (III) were identified as the principal cytotoxic constituents of Ipomopsis aggregata (Pursh) V. Grant (Polemoniaceae). The structure of the new compound, II, was determined through analysis of its spectrometric characteristics.