Chemical constituents of Dendrobium gratiosissimum and their cytotoxic activities

Two new bibenzyl derivatives, named dengraols A 1 and B 2, are isolated from stems of Dendrobium gratiosissimum Rchb. (Orchidaceae), together with seven known compounds: 3,5,4'-trihydroxybibenzyl 3, 3,4'-dihydroxy- 5-methoxybibenzyl 4, 3, 4 -dihydroxy-5, 4' - dimethoxy bibenzyl 5, 3, 4' - dihydroxy - 4, 5, 3'-trimethoxybibenzyl (moscatilin) 6 and 5, 4'- dihydroxy -3, 3'-dimethoxybibenzyl (gigantol) 7, 5, 3', 4'-trihydroxy - 3- methoxy-bibenzyl (tristin) 8 and 5, 3'- dihydroxy- 3-methoxybibenzyl (batatasin III) 9. Among the isolated compounds, dengraols A 1 and B 2, moscatilin 6 and gigantol 7 showed inhibitory activity of proliferation on HL-60 cells with IC 50 values at 2.1, 6.4, 0.082 and 10.6 (μM, respectively.