Heterodimeric ent-kauranoids from Isodon tenuifolius.

Thirteen new heterodimeric ent-kauranoids, bistenuifolins A-M (1-13), were isolated from the aerial parts of Isodon tenuifolius. The constituent units of compounds 1-6 were linked by a six-membered dihydropyran ring, while those of compounds 7-11 were joined by a rare single carbon-carbon bond (C-16→C-17'). The constituent units of 12 and 13 were linked via an unusual cyclobutane moiety. The structures of these metabolites were established via spectrometric analyses, and the absolute configurations of 1 and 4 were defined by single-crystal X-ray diffraction. Selected compounds were evaluated for their cytotoxicity against a small panel of human tumor cell lines; bistenuifolin B (2) exhibited weak inhibitory effects.