Diterpenoids and Limonoids from the Leaves and Twigs of Swietenia mahagoni

Three new compounds, including two diterpenoids, nemoralisins H and I (1 and 2), and a limonoid, 2-methoxy khayseneganin E (3), along with four known constituents (4–7), were isolated from the leaves and twigs of Swietenia mahagoni. Their chemical structures were elucidated by means of spectroscopic analysis. The cytotoxities of these isolated constituents were assayed. Electronic supplementary material The online version of this article (doi:10.1007/s13659-014-0006-6) contains supplementary material, which is available to authorized users.

With the aim of searching for structurally unique and bioactive chemical constituents, we isolated the leaves and twigs of S. mahagoni and gained two new diterpenoids (1 and 2), one new limonoid (3) and four known compounds (4-7) (Fig. 1). In this paper, we reported the isolation and structural elucidation of new compounds, as well as cytotoxicities of all the compounds against five human cancer cell lines.

Results and Discussion
The air-dried powder of leaves and twigs of S. mahagoni was extracted with 70 % aqueous acetone at room temperature three times to give the residue, which was then partitioned between EtOAc and water to get the EtOAcsoluble fraction. Then, three new constituents together with four known compounds were acquired by a series of chromatographic methods. Herein, we described the isolation and structural elucidation of these new compounds.
Compound 1, colorless oil, was assigned a molecular formula C 20   exhibited high resemblance with nemoralisin A isolated from Aphanamixis grandifolia (Meliaceae) [13], and they showed the same molecular formula and the similar skeletal structure (a,b-unsaturated ketone was connected with one a,b-unsaturated d-lactone by an aliphatic chain). The only difference between 1 and nemoralisin A was that the hydroxyl group located at C-4 in 1 but at C-8 in nemoralisin A.  The 1D NMR data (Table 2) of 3 indicated that it was a phragmalin-type limonoid and similar with khayseneganin E [15][16][17]. However, a methoxy in 3 replaced the hydroxyl group at C-2 in khayseneganin E, which was supported by  HMBC correlation between OMe (d H 3.68) and the ketal carbon (d C 106.9) (Fig. 2). The relative configuration of 3 was determined by analysis of the ROESY spectrum, in which correlations of H-5/H-6, H-5/H-12b, H-12b/H-17, and H-6/Me-28 suggested that these groups are all b-oriented. In addition, Me-19, Me-18, H-9, and H-3 were assigned as a-oriented on the basis of the ROESY correlations between Me-19/H-9 and between H 2 -29/H-3, Me-18/H-3, suggesting that OH-3 is b-oriented. Consequently, the structure of 3 was determined as shown, named 2methoxy khayseneganin E. Four known constituents: nemoralisin C (4) [13], khayanolide B (5) [18], khayseneganin G (6) [17], and deacetylkhayanolide E (7) [19], were identified by the comparison of their spectroscopic data with those reported in the literature.
Compounds 1-7 were tested for in vitro inhibitory activities against HL-60, SMMC-7721, A549, MCF-7 and SW480 human tumor cell lines. All the tested samples showed no activities against the mentioned cell lines with IC 50 [ 40 lM. Much to our delight, Liu J. et al. [20] reported that aphadilactones A-D, which can be considered as the dimers of diterpenoid compounds (1, 2 and 4) showed potent and selective inhibition against the diacylglycerol o-acyltransferase-1 (DGAT-1) enzyme, and are the strongest natural DGAT-1 inhibitors discovered to date. So emphasis can be laid on this class of compounds in our future search of potential DGAT-1 inhibitors.

General Experimental Procedures
Optical rotations were obtained with a Jasco P-1020 polarimeter. UV (in MeOH) and IR (in CHCl 3 ) spectra were measured on Shimadzu UV-2401 PC spectrophotometer and Bruker Tensor-27 infrared spectrophotometer, respectively. ESIMS spectra were recorded on an API QSTAR Pulsar spectrometer. EIMS and HREIMS were performed on a Waters Autospec Premier P776. 1D and 2D NMR spectra were recorded on Bruker DRX-500 and Bruker Avance III-600 MHz spectrometers. Chemical shifts (d) were expressed in ppm with reference to the TMS resonance. Semi-preparative HPLC studies were carried out on an Agilent 1100 liquid chromatograph with a Zorbax SB-C18 (9.4 mm 9 25 cm) column. Column chromatography was performed using Silica gel [(200-300)  and Sephadex LH-20 [(20-150) lm, Pharmacia] were also used for column chromatography.

Plant Material
The leaves and twigs of S. mahagoni were collected from Xishuangbanna, Yunnan Province, China. A voucher sample has been deposited in the State Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of Sciences.