In connection with our continuing studies on bioactive substances from the Japanese marine organisms, we have isolated three new diterpenoids, stolonidiol (1), stolonidiol monoacetate (2), and the compound 3, from the soft coral (Clavularia sp.) as the major secondary metabolites. The compound 1 and 2 showed a strong cytotoxic activity against P388 leukemia cells in vitro (IC_<50> 0.015μg/ml for each compound). The plane structures of 1 and 2 were elucidated on the basis of chemical reactions and two-dimensional (2D) NMR spectra (^1H J-resolved spectrum, ^<13>C-^1H COSY, COLOC, and INADEQUATE). The relative stereochemistries were determined by measurement of high dilution IR spectrum and 2D NOESY, and the absolute stereochemistries were elucidated by CD measurement of the derivative 8. The X-ray crystallographic analysis of the derivative 10 confirmed the full structures of 1 and 2. The structure of 3 was also deduced on the basis of chemical reactions and spectroscopic data. The structures of 1 and 2 are characterized by new bicyclo-[9.3.0]tetradecane skeleton having the cis stereochemistry between the methyl group at C-1 and the alkyl group at C-12, which is different from that of the dolabellane-type bicyclic diterpenoids.
