Four new 6-nor-5(6→7)abeo-abietane type diterpenes and antitumoral cytotoxic diterpene constituents from the bark of Taiwania cryptomerioides

Four new 6-nor-5(6→7)abeo-abietane type diterpenes designated as taiwaniaquinone G, taiwaniaquinone H, taiwaniaquinol E and taiwaniaquinol F and eight known diterpenes, taiwaniaquinones A (5), D (6), E, F (8), and taiwaniaquinols A (9), B, C (11), D (12) were isolated from the bark of Taiwania cryptomerioides. Their structures were elucidated on the basis of spectroscopic studies. These twelve diterpenes were evaluated for in vitro anti-tumoral cytotoxic activity. The result demonstrated that compounds 5, 6, 8, 9, 11, and 12 bearing an aldehyde group possessed potent cytotoxic activity against KB epidermoid carcinoma cancer cell lines with IC₅₀ values of 6.9, 7.2, 4.4, 8.3, 8.1, and 3.5 μM, respectively.