Dengratiols A-D, four new bibenzyl derivatives from Dendrobium gratiossimum.

Dengratiol A (1), an unprecedented bibenzyl derivative bearing a tropolone unit together with three pairs of bibenzyl enantiomers (±)-dengratiols B-D [(±)-2-(±)-4], were isolated from Dendrobium gratiossimum Rchb.f. The resolution of enantiomers was performed with chiral HPLC. Their structures were characterized by extensive spectroscopic data analysis and calculated electronic circular dichroism (ECD). A hypothetical biosynthetic pathway for 1 is proposed. Biological assay revealed that (-)-2 showed moderate antiviral effect against IAV with IC50 value of 6.3 μM, and (±)-2 displayed cytotoxic activities against five human tumor cell lines with IC50 values ranging from 15.5 to 42.5 μM.