Norsesquiterpenoids from the leaves of Croton tiglium

Short Communication DOI 10.1007/s13659-011-0035-3 Nat. Prod. Bioprospect. 2011, 1, 134–137 Norsesquiterpenoids from the leaves of Croton tiglium Wei BU,a,c,† Yan-Ni SHI,a,b,† Yong-Ming YAN,a Qing LU,a Guang-Ming LIU,c Yan LI,a,* and Yong-Xian CHENGa,* a State Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming 650201, China b Graduate University of Chinese Academy of Sciences, Beijing 100049, China c Faculty of Pharmacy, Dali University, Dali 671000, China † These authors contributed equally to this work. Received 21 November 2011; Accepted 14 December 2011 © The Author(s) 2011. This article is published with open access at Springerlink.com Abstract: Two new compounds, badounoids A (1) and B (2), together with 13 known norsesquiterpenes, were isolated from the leaves of Croton tiglium L. The structures of the new compounds were established by means of spectroscopic methods. The absolute configuration of badounoid B was determined by single-crystal X-ray diffraction analysis. All the known compounds were isolated from Croton plants for the first time which added a new chemical facet for this genus. The selected compounds were evaluated for their cytostatic activity against several cancer cell lines. None of them was found to be active. Keywords: Croton tiglium, badounoid, norsesquiterpenoid, cytostatic activity Introduction The intriguing structures of Euphorbiaceae plants and their diverse biological activities have attracted great interest in the recent years.1 Croton tiglium L. is a plant belonging to the family Euphorbiaceae, its seeds, a well-known traditional Chinese medicine have been extensively investigated. So far, diterpenoids, alkaloids, flavonoids, and steroids have been characterized from the seeds, they were found to have antitumor, antiinflammatory, analgesic, and lipid lowering effects.2 The leaves of C. tiglium have been used to treat diarrhea, tinea, pain, and hurt,3 however, little is known for its chemical profiling. During our search for active compounds from the leaves, fifteen norsesquiterpenes including two new ones were isolated and structurally identified. This paper describes their isolation and structural identification. Results and Discussion Badounoid A (1), isolated as colorless gums, had the molecular formula C14H20O3 derived from its positive HRESIMS at m/z 259.1318 [M + Na]+ (calcd. 259.1310), indicating five degrees of unsaturation. The IR spectrum showed the absorption bands for hydroxy (3431 cm−1) and ,-unsaturated carbonyl (1654 cm−1) groups. The 13C NMR and DEPT spectra revealed 14 carbon resonances, which are four methyl, one oxygenated methylene, four methine, and *To whom correspondence should be addressed. E-mail: five quaternary carbons (including one oxygenated carbon and one carbonyl), indicating that 1 is an analogue of 5. The 1H-1H COSY correlation of H-2 ( 6.99)/H-3 ( 6.18), and HMBC correlations of H-2, H-3, Me-14/C-4 ( 188.9), Me-14/C-5 ( 131.3), C-6 ( 161.9), and Me-12/C-1 ( 41.0), C-6 (Figure 1) suggested the west part of 1 as shown. The side chain of 1 was identified as a substituted isoprenyl group according to the following evidence: (i) 1H-1H COSY correlation of H-7 ( 6.43)/H-8 ( 5.91), (ii) HMBC correlations of H-8, H-10 ( 3.49), Me-11 ( 1.34)/C-9 ( 74.6). Further, HMBC correlations of H-7, H-8/C-6 established the linkage of the side chain with the ring. The JH-7,H-8 value of 16.3 Hz indicated a trans double W. BU and Y. N. SHI et al. Nat. Prod. Bioprospect. 2011, 1, 134–137 135 Table 1. NMR data for compounds 1 and 2.a (methanol-d4 for 1 and CDCl3 for 2, J in Hz, δ in ppm) 1 position 1 2 3 4 5 6 7 8 9 10 11 12 13 14 δC 41.0, C 160.0, CH 126.2, CH 188.9, C 131.3, C 161.9, C 125.0, CH 144.2, CH 74.6, C 70.7, CH2 24.7, CH3 27.0, CH3 26.9, CH3 13.3, CH3 2 δH δC 37.4, C 42.2, CH2 67.3, CH 36.4, CH2 29.1, CH 77.9, C 133.5, CH 133.7, CH 68.6, CH 23.8, CH3 27.1, CH3 25.6, CH3 15.8, CH3 6.99 (d, 9.9) 6.18 (d, 9.9) 6.43 (d, 16.3) 5.91 (d, 16.3) 3.49 (s) 1.34 (s) 1.27 (s) 1.27 (s) 1.94 (s) δH 1.38 (m); 1.82 (dd, 14.6, 3.4) 4.11 (m) 1.55 (m); 1.69 (m) 2.26 (m) 5.67 (d, 15.8) 5.74 (dd, 15.8, 5.1) 4.38 (m) 1.29 (d, 6.4) 1.15 (s) 0.82 (s) 0.75 (d, 6.8) a Data were recorded at 500 MHz for 1H NMR and 125 MHz for 13C NMR. bond. The configuration at C-9 still remained unresolved, since the stereochemistry determination at the chiral center of the conformationally flexible chain is always challengeable. Thus, the structure of 1 was deduced as shown, with a trivial name badounoid A. Figure 1. Selected HMBC (H→C) and COSY(－) correlations of 1. Badounoid B (2) was isolated as colorless crystals. The molecular formula of 2 was determined to be C13H24O3 from its HRESIMS at m/z 227.1652 [M  H] (calcd. 227.1647), requiring two degrees of unsaturation. The IR spectrum displayed the existence of OH (3430 cm−1) and C=C (1638 cm−1) functionalities. The NMR data of 2 resembled those of 6. Interpretation of 1H-1H COSY, HSQC and HMBC spectra of 2 disclosed that compounds 2 and 6 have the same planar structure. The ROESY correlation of H-5/H-7 suggested that Me-13 and OH-6 were spacially vicinal. Whereas, the scarcity of diagnostic ROESY signals made it difficult to assign the relative configuration at C-3. Thus, the configurations at C-3 and C-9 of flexible side chain were clarified by X-ray diffraction using Cu-irradiation (Figure 2), which also allowed the assignment of absolute configuration in 2 as 3R, 5R, 6S, and 9R. Therefore, the structure of 2 was determined as shown and given a name badounoid B. The known compounds were identified as 3β-hydroxy5α,6α-epoxy-7-megastigmen-9-one (3),4 4,5-dihydroblumenol A (4),5 (3S,4R)-3,4-dihydroxy-β-ionone (5),6 (3S,5R,6S,7E,9R)3,6-dihydroxy-5,6-dihydro--ionol (6),7 blumenol A (7),5 (3R,6R,7E)-3-hydroxy-4,7grasshopper ketone (8),8 megastigmadien-9-one (9),9 (+)-3-hydroxy-β-ionone (10),10 (6R,7E)-4,7-megastigmadien-3,9-dione (11),10 (S)-(+)-dehydrovomifoliol (12),11 (3S,5R,6S,7E)-3,5,6-trihydroxy-7megastigmen-9-one (13),12 corchoionol C (14),13 and (+)boscialin (15),14 respectively, by comparison with literature data. All these compounds were isolated from this genus for the first time. Figure 2. X-ray crystallographic structure of 2 showing the absolute configuration. Megastigmane norsesquiterpenoids have been widely found in the plants. However, their real role in the plants or in drug discovery is poorly known. It was reported that this type of norsesquiterpene possesses antiinflammatory activity. Whether the present isolates being also responsible for the traditional uses of the leaves in infectious diseases needs further investigation. In this study, the selected compounds (1, 3, 4, 1214) were evaluated for their cytostatic activity against HL60, SMMC-7721, A-549, MCF-7, and SW480 human cancer cells, however, all these compounds showed no activity in this assay. Among these mis (...truncated)