Phytochemical study of the MeOH extracts prepared from Dendrobium capillipes Rchb.f. and Dendrobium secundum (Blume) Lindl. (Orchidaceae) led to the isolation of two new compounds, including a flavonol glycoside named quercetin-3-O- α -L-rhamnopyranosyl (1→2)- β -D-xylopyranoside from D. capillipes and a bibenzyl named 5-hydroxy-3,4,3',4',5'-pentamethoxybibenzyl from D. secundum, along with ten known compounds, including seven compounds from D. capillipes: chrysotobibenzyl, crepidatin, gigantol, chrysotoxine, moscatilin, kaempferol-3-O- α -L-rhamnopyranosyl (1→2)- β -D-xylopyranoside or lysimachiin and kaempferol-3-O- α -L-rhamnopyranosyl (1→2)- β -D-glucopyranoside; and three compounds from D. secundum: kaempferol-3,7-O-di- α -L-rhamnopyranoside, quercetin-3-O- α -L-rhamnopyranoside and kaempferol-3-O- α -L-rhamnopyranoside. Their structures were determined by analysis of their spectroscopic (MS, UV, IR and NMR) data and comparison with previously published values. In the cytoxicity study of the bibenzyls, moscatilin was found to be the most potent compound, inhibiting KB (oral human epidermal carcinoma) and NCI-H187 (human lung cancer) cells, but it had no cytotoxicity against MCF-7 (breast cancer) cells. Gigantol and 4,5,4'-trihydroxy-3,3'-dimethoxybibenzyl showed cytotoxicity against all cancer cell lines, but with weak activity. In addition, brittonin A, chrysotoxine and 4,5,4'-trihydroxy-3,3'-dimethoxybibenzyl showed no cytotoxicity against normal cells (Vero cells).
