Bi-bicyclic and bi-tricyclic compounds from Dendrobium thyrsiflorum

Coumarin–benzofuran, dimeric phenanthrene and phenanthrene–phenanthraquinone derivatives, along with seven known ones, were isolated from Dendrobium thyrsiflorum (Orchidaceae). Four of the compounds showed significant cytotoxic activities against Hela, K-562 and MCF-7 cell lines. One bi-bicyclic and two bi-tricyclic derivatives of coumarin–benzofuran, phenanthrene–phenanthrene and phenanthrene–phenanthraquinone, along with seven known compounds, were isolated from stems of Dendrobium thyrsiflorum Rchb.f. (Orchidaceae). On the basis of chemical, NMR ( 1H, 13C, HMQC, HMBC and NOESY) and mass spectrometry data, their structures were elucidated as denthyrsin [3-(5′,6′-dimethoxybenzofuran-2′-yl)-6,7-dimethoxy-2H-chromen-2-one; 1], denthyrsinol (4,5′-dimethoxy-[1,1′]biphenanthrenyl-2,5,4′,7′-tetraol; 2), and denthyrsinone (7,4′,7′-trihydroxy-2,2′,8′-trimethoxy-[5,1′]biphenanthrenyl-1,4-dione; 3). Compounds 1– 3 and denthyrsinin (1,5,7-trimethoxyphenanthrene-2,6-diol; 4) showed significant cytotoxic activities against Hela (13.5, 9.3, 9.9 and 2.7 μM, respectively), K-562 (0.45, 1.6, 6.0 and 2.3 μM, respectively) and MCF-7 (18.1, not tested, 3.5 and 4.8 μM, respectively) cell lines.