Diterpenoids from Leonurus japonicus

Leonurus japonics Houtt. (Lamiaceae) is an annual or biennial herbaceous plant widely distributed and cultivated in China. The dried herb is used in TCM for the treatment of various diseases, especially menstrual disturbances, dysmenorrhea, and amenorrhea. Recently, phytochemical studies on this plant have been reported. Our previous investigation on two plants of the family Lamiaceae resulted in the isolation of a number of new labdane-type diterpenes. In our continuing research on secondary metabolites from L. japonicus, five diterpenoids including two new and three known ones were isolated and identified. The new compounds (Fig. 1) were elucidated as 1-((2S)-2-(3-(furan-3yl)propanoyl)-2,6,6-trimethylcyclohexyl)-2-hydroxybutan1-one (1), and 1-((1S,3R,3aS)-3-(2-(furan-3-yl)ethyl)-3methoxy-3a,7,7-trimethyloctahydroisobenzo furan-1-yl)propan-1-one (2). The known compounds were identified as seco-labdane (3), 6b-hydroxy-15,16-epoxylabda-8,13(16), 14-trien-7-one (4), and leojaponin (5). The ethanol extract of the powdered herb of L. japonicus was first separated by macroporious resin (D101) and then repeatedly separated by silica gel column chromatography, Sephadex LH-20, and preparative HPLC to give compounds 1–5. Compound 1 was obtained as a colorless oil, [α] D + 36.3 (c 0.1, MeOH), and was established to have a molecular formula of C20H30O4 by HRESIMS ([M + H] + m/z 335.2178; calcd. 335.2222). The H, C NMR and DEPT data (Table 1) exhibited resonances for three tertiary methyl groups (δH 0.76, 0.87, 0.90; δC 29.3, 29.5, 25.6, respectively), two carbonyl groups (δC 215.7, 213.2), a monosubstituted furan (δH 7.33, 7.23, 6.25; δC 110.9, 123.8, 139.2, 142.8), five methylenes (δC 37.2, 34.2, 31.4, 26.7, 19.5, 19.1), two methines including one oxygenated (δH 3.58 s, 4.03 d J = 8.0 Hz; δC 53.8, 80.6), and two quaternary carbons (δC 34.5, 49.9). These spectroscopic data indicated 1 to be an 8,9-seco-labdane diterpenoid with a hydroxyl group (δH 3.44 s). The H NMR data of 1 were similar to those of the lagopsin F and 8,9-secohispanolone except for the absence of the resonances of an acetoxy group and a replacement of carbonyl with a hydroxyl group in lagopsin F. However, carefully analysis of the H-H COSY spectra, the cross peak between the two protons of methines (H-5 and H-7) was not observed, and correlation from H-7 to H2-8 (δH 1.30 m) was obviously detected. It suggested that the hydroxyl group was connected to C-7, rather than C-6 as lagopsin F. The proposed structure of 1 was further confirmed by the HMBC correlations between H3-20 and C-1/C-5/C-9/C-10; H3-18 and C-19/C4/C-5; H3-17 and C-7/C-8; H-16 and C-13/C-14/C-15; H15 and C-13/C-14/C-16; H-14 and C-13/C-15/C-16; H2-12 and C-9/C-11/C-13/C-14/C-16; H2-11 and C-9/C-12/C-13; H-8 and C-7/C-17; H-5 and C-1/C-3/C-4/ C-6/C-7/C-9/C10, and H-7 and C-5/C-6/C-8/C-17 (Fig. 2). However, the C-7 relative configuration could not be assigned. Therefore, the structure of 1 was identified as 1-((2S)-2-(3-(furan-3yl)propanoyl)-2,6,6-trimethylcyclo-hexyl)-2-hydroxybutan -1-one, and named secoleojaponol. Compound 1 has not been previously reported. Compound 2, a colorless oil, [α] D 3.6 (c 0.1, MeOH) was shown to have a formula of C21H32O4 by HRESIMS ([M + H] m/z 349.2020; calcd. 349.2015). The NMR data (Table 1) showed that compound 2 has three tertiary methyl groups (δH 0.75, 1.04, 1.12; δC 33.6, 22.2, 17.0, respectively) and a methoxy group (δH 3.35, s; δC 48.8) a propanoyl group [δH 1.08 (3H, t, J = 8.0 Hz), 2.65 (2H, q, J = 8.0 Hz ); δC 7.6,