Heterodimeric <i>ent</i>-Kauranoids from <i>Isodon tenuifolius</i>

Thirteen new heterodimeric <i>ent</i>-kauranoids, bistenuifolins A–M (<b>1</b>–<b>13</b>), were isolated from the aerial parts of <i>Isodon tenuifolius</i>. The constituent units of compounds <b>1</b>–<b>6</b> were linked by a six-membered dihydropyran ring, while those of compounds <b>7</b>–<b>11</b> were joined by a rare single carbon–carbon bond (C-16→C-17′). The constituent units of <b>12</b> and <b>13</b> were linked via an unusual cyclobutane moiety. The structures of these metabolites were established via spectrometric analyses, and the absolute configurations of <b>1</b> and <b>4</b> 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 (<b>2</b>) exhibited weak inhibitory effects