Absolute Stereostructure of the Labdane-type Diterpene Alcohol from Alpinia tonkinensis Gagnep

The labdane-type diterpene 15,16-epoxy-8(17),13(16),14-labdatrien-12ξ-ol (1) was isolated together with 2'-hydroxy-4',6'-dimethoxychalcone and 2,3-dihydro-4',7-di-O-methylkaempferol from the rhizomes of the endemic Vietnamese medicinal plant Alpinia tonkinensis Gagnep. (Zingiberaceae). Although both diastereomers of the furanolabdane alcohol 1 provided by the chiral center at the 12-position have already been reported as semisynthetic intermediates, the absolute configuration at the 12-position, which affects the chemical shifts of the 17-exomethylene protons, remains to be clarified. In this study, the natural furanolabdane 1, which possessed the same 1Hand 13C-NMR spectroscopic data as those of one of the semisynthetic diastereoisomeric pair, was determined to be in the (12S)-form from spectroscopic data and by applying the modified Mosher's method. Author