Stereostructures of New Labdane-Type Diterpenes, Excoecarins F, G1, and G2 from the Wood of Excoecaria agallocha

Excoecaria agallocha LINN (Euphorbiaceae) is distributed on seashores and edge-mangroves throughout tropical Africa, Asia, and northwest Australia. The Excoecaria genus is well known to contain skin irritants, generally called irritant Excoecaria factors. The bark and wood of this tree have been used in traditional medicines for flatulence in Thailand. The resinous wood including the latex of E. agallocha has been used as a substitute for agarwood (Jinko) incense in Okinawa, Japan. The piscicidal constituent excoecariatoxin characterizing the daphnane diterpene ester and some related compounds has been obtained from the twigs, bark, and latex of E. agallocha in Japan and Thailand, respectively. Daphnaneand tigliane-type diterpene esters are known to be skin irritants and tumor promoters. A novel phorbol ester acting as an anti-HIV agent was isolated from the leaves and stems of E. agallocha collected in northwest Australia. We reported the isolation and structure elucidation of some diterpenes from the resinous wood of this plant, and also reported their inhibitory effect on Epsetin-Barr virus early antigen (EBV-EA) activation induced by 12-O-tetra-decanoyl phorbol-13-acetate (TPA) in Raji cells. As a continuing study, we isolated three new diterpenes, excoecarin F (1), excoecarin G1 (2), and G2 (3) from the resinous wood of E. agallocha. In this paper, we describe the stereochemistry of the new compounds. The ether extract of the resinous wood was purified by repeated ordinary and reversed-phase silicagel column chromatography and finally preparative HPLC to give excoecarins F (1, 0.0011%), G1 (2, 0.0008%), and G2 (3, 0.0007%). The positive detection of excoecarin F (1), [a]D 220.8° (c51.0, MeOH), for 2,6-dichlorophenol-indophenol sodium salt on TLC revealed the presence of a carboxylic acid group. The molecular formula of colorless syrup dimethyl ester 1a, [a]D 221.7° (c51.0, CHCl3), was determined by HR-FABMS measurement to be C22H37O6Cl. The IR spectrum of 1a showed hydroxyl (3358 cm), carbonyl (1740, 1726 cm), ether (1109, 1074, 987 cm), and alkyl halide (1259, 740 cm) groups. The H-NMR spectrum of 1a showed signals for methine and methylene protons on each carbon bearing a hydroxyl group and/or chloride group (d 3.44, 3.60, 3.71) (Table 1). The proton and carbon signals in the Hand CNMR spectra of 1a were very similar to those of ent-13-epi8,13-epoxy-2,3-secolabd-14-en-2,3-oic acid dimethyl ester (4), except for some signals due to the side chain [1a: dC 77.4, dH 3.71 (1H, dd, J52.3, 9.0 Hz, 14-H), 46.4, 3.44 (1H, dd, J59.0, 11.0 Hz, 15-H), 46.4, 3.60 (1H, dd, J52.3, 11.0 Hz, 15-H)] (Tables 1 and 2). These data and detailed Cand 456 Communications to the Editor Chem. Pharm. Bull. 47(3) 456—458 (1999) Vol. 47, No. 3