The leaves of Viburnum awabuki (Caplifoliaceae) are known to have been used as traditional fish stupefying agent and fish poison for purpose of catching fish in Okinawa islands. Kawazu reported the isolation of a piscicidal activating agent, vibsanine A, a plant growth inhibitor, vibsanine B (1), and vibsanine C (4), which belong to unprecedented 11-membered and 7-membered ring diterpenes, respectively. Hence, we have proposed the new term "Vibsanine" for this type of diterpenes. Although occurrence of intriguing diterpenoids is anticipated, further chemical study on V. awabuki has not been done since then. From viewpoint of structural interest and biological activity of vibsanine-type diterpenes we have reinvestigated chemical constituents of V. odoratissimum and V. awabuki collected in Ishigaki island and Tokushima, resulting in the isolation of seven new vibsanine-type diterpenes 2, 3, 5-9, together with three unprecedented diterpenes 10, 11 and 12 named neovibsanines A, B and D. As first of all, the stereochemistries of vibsanines B (1) and C (4) which have remained unclear have to be defined. HPLC and ^1H NMR spectrum of vibsanine B indicated that 1 is a mixture of two conformational isomers as ratio of 99: 1. The relative configurations for both conformers were elucidated on the basis of NOESY data and then were clarified to be consistent with two most stable conformations obtained by MM2 calculation. Next, upon refluxing 1 in toluene an intermolecular Cope rearrangement took place to give 7-membered ring vibsanines 4 (85.9 %), 4a (11.2 %), 4b (1.6 %) and 4c (0.2 %). This result implies that 7-membered ring vibsanines may be biosynthesized via Cope rearrangement from 11-membered ring vibsanines. The absolute configurations of vibsanines B (1) and C (4) have been unambiguously determined based on the result of X-ray crystallographic analysis of 4e. The structures of new 7-membered ring vibsanines G (5), H (6), K (7), L (8), and L hydroperoxide (9) were elucidated by spectral data and comparison of ^<13>C NMR data with those of congeners. On the other hand, the plane structures of unprecedented neovibsanines A (1), B (2) and D (3) were determined by extensive analysis of ^1H-^1H COSY, HMQC, TOCSY and HMBC and their stereochemistries were elucidated by NOESY. It is worthy of note that the neovibsanine-type compound 13 was derived via a consecutive bond cleavage and formation upon irradiation of vibsanine B (1) by high pressure Hg lamp. Finally, cytotoxicity of vibsanine-type diterpenoids were examined, in particular, to know the effect of a hydroperoxy group presented in the molecule.
