Bioassay-guided fractionation of an EtOAc-soluble extract of Casearia membranacea has resulted in the isolation of six new clerodane diterpenes, caseamembrins A-F (1-6), and a known compound, rel-(2S,5R,6R,8S,9S,10R,18S,19R)-diacetoxy-18,19-epoxy-6-hydroxy-2-(2-methylbutanoyloxy)cleroda-3,13(16),14-triene (7). The structures of 1-6 were established on the basis of extensive 1D and 2D NMR spectroscopic analysis. In addition, the new derivatives, 8 and 9, were prepared by acylation of 7 and 3, respectively. The isolated diterpenoids and their derivatives were tested against human prostate (PC-3) and hepatoma (Hep3B) cancer cells. Compounds 1, 3-5, and 7 exhibited cytotoxicity against both tumor cells, with IC(50) values below 3 micromicro, while compounds 2, 6, 8, and 9 were less effective.

Clerodanes are a group of novel diterpenoids with interesting biological effects such as antitumor-related activities. [1][2][3][4][5] Casearia membranacea HANCE (Flacourtiaceae) is the only species that belongs to the genus Casearia in Taiwan. 6) During the course of searching for novel antitumor agents from the terrestrial plants in Taiwan,[7][8][9] an ethanolic extract of the leaves and twigs of C. membranacea collected in Ken-ting showed significant cytotoxicity against the human prostate tumor cells (PC-3). To explore the origin of biological activity, bioassay-guided fractionation of the extract of the leaves and twigs of this species has resulted in the isolation of two new clerodane diterpenes, designated as caseamenbrols A (1) and B (2), together with the known casearlucin A (3). Herein, we wish to report the isolation, structural elucidation and cytotoxicity of these novel compounds. Results and DiscussionThe ethanolic extract of C. membranacea was fractionated by repeated column (Si gel and Sephadex LH-20) chromatography and preparative TLC to furnish caseamenbrols A (1, 0.012%) and B (2, 0.024%) in addition to casearlucin A (0.0023%).10)The FAB-MS and 13 C-NMR spectra of 1 revealed its molecular weight as 518 and molecular formula C 29 H 42 O 8 . The UV spectrum showed absorption due to conjugated diene at l 225 nm. The IR spectrum displayed absorption bands specific for a hydroxyl group (3445 cm -6). Taking into account three olefinic bonds and three ester carbonyls, the presence of three rings is justified to satisfy nine degrees of unsaturation. The spectral data of 1 are in conformity with the basic skeleton of clerodane diterpenes previously isolated from Casearia. 11,12) The presence of a six-carbon diene side chain at C-9 was confirmed by the long range correlations between H-12/C-9; H-14/C-12, C-13, C-16; H-15/C-13/C-14, C-16. In addition to casearlucin A (3), two new clerodane diterpenes, caseamembrols A (1) and B (2) have been isolated from the leaves and twigs of Casearia membranacea by bioassay-guided fractionation. The structures of the new compounds were established on the basis of extensive 1D-and 2D-NMR spectroscopic analysis. Compounds 1-3 exhibited significant cytotoxicity against human prostate (PC-3) cancer cells.

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