New cytotoxic bis-labdanic diterpenoids from Alpinia calcarata.

Two novel bis-labdanic diterpenoids named calcaratarin D and calcaratarin E were isolated from the rhizomes of Alpinia calcarata. Their structures were elucidated as a pair of stereoisomers on the basis of the spectral evidence. 2D NMR techniques including (1)H- (1)H COSY, HMQC, HMBC, NOESY spectra and HRFABMS were extensively applied to establish the structures. The two bis-labdanic diterpenoids showed cytotoxic activity against human KB cells in vitro.