Vibsane‐Type Diterpenoids from Viburnum odoratissimum and Their Cytotoxic and HSP 90 Inhibitory Activities

Four new vibsane‐type diterpenoids, vibsanol I ( 1 ), 15‐hydroperoxyvibsanol A ( 2 ), 14‐hydroperoxyvibsanol B ( 3 ), 15‐ O ‐methylvibsanin U ( 4 ), and a new natural product, 5,6‐dihydrovibsanin B ( 5 ), as well as six known analogues, were isolated from the twigs and leaves of Viburnum odoratissimum . Their structures were elucidated by spectroscopic analyses and chemical derivatization method. All compounds showed different levels of cytotoxicity against five cell lines ( HL ‐60, A‐549, SMMC ‐7721, MCF ‐7, and SW 480). Remarkably, 14,18‐ O ‐diacetyl‐15‐ O ‐methylvibsanin U ( 4a ) showed significant cytotoxicity against HL ‐60, A‐549, SMMC ‐7721, MCF ‐7, and SW 480, with IC 50 values of 0.15 ± 0.01, 0.69 ± 0.01, 0.41 ± 0.02, 0.75 ± 0.03, and 0.48 ± 0.03   μ m , respectively. In addition, vibsanin K ( 10 ) was identified as a HSP 90 inhibitor with an IC 50 value of 19.16 μ m .