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Six diterpenes, including one new natural product, were isolated from a CHCl 3 extract of the stem bark of Thuja standishii. The new compound has been characterized as 15-oxolabda-8(17),13 Z-dien-19-oic acid. The known compounds were identified as ferruginol (2), sugiol (3), isocupressic acid (4), sandaracopimaric acid (5) and 15-oxolabda-8(17),13 E-dien-19-oic acid (6). Compounds 2-5 and the derivatives 4a and 4b were tested for their inhibitory effects on Epstein-Barr virus early antigen (EBV-EA) activation induced by 12-O-tetradecanoylphorbol 13-acetate (TPA). Compounds 2, 3, 4 and 5 showed strong inhibitory effect on EBV-EA induction (100 % inhibition at 1000 mol ratio/TPA).

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The absolute stereostructure of a novel skeletal diterpene, standishinal (1), from the bark of Thuja standishii was confirmed by X-ray crystallographic analyses of 1 and its p-bromobenzoate derivative. Aromatase inhibitory activities of standishinal, eight known diterpenes, totarol, 12-methoxyabieta-8,11,13-trien-11-ol, 12-hydroxy-6,7-seco-abieta-8,11,13-triene-6,7-dial, trans-communic acid, labda-8(17),13-dien-12 R,15-olid-19-oic acid, 12 S-hydroxylabda-8(17),13(16),14-trien-19-oic acid, 13-oxo-15,16-dinorlabda-8(17),11 E-dien-19-oic acid and 14-oxo-15-norlabda-8 (17),12 E-dien-19-oic acid from the plant, and four synthetic analogs were evaluated using a recombinant human aromatase. Among them, standishinal and its diacetate derivative had significant inhibitory activities.

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