Introduction
Melia azedarach L. (Meliaceae) is a deciduous tree which is native to Persia, India, and China, and distributed widely in China, Japan, and Southeast Asia. The dried stem bark, root bark, and fruit of this plant have been used as anthelmintic, antifeedant, and for the treatment of leprosy, eczema, asthma, malaria, fever, cholelithiasis, acariasis, and pain (Tang and Eisenbrand, 2011; Jung and Shin, 1990). Previous phytochemical studies on this plant have afforded a wide range of compounds including limonoid, triterpenoid, steroid, alkaloid, and flavonoid glycosides (Zhao et al., 2010). M. azedarach and limonoids have attracted considerable interest because of their biological activivities, including antioxidant, anticancer, antimicrobial, antifeedant, insecticidal, antifungal, anti-inflammatory, and analgesic effects (Tan and Luo, 2011; Marimuthu et al., 2013; Kim et al., 1994; Akihisa et al., 2013; Liu et al., 2011; Carpinella et al., 2003a, 2003b; Xie et al., 2008).
We investigated the chemical constituents of the fruits of this plant and found a new apotirucallane-type triterpenoid along with five known compounds. The structure of the new compound was elucidated by means of spectroscopic methods including 2D NMR techniques and mass spectrometry to be named as 3α-tigloylsapelin D (1).
Experimental
General Experimental Procedures − Optical rotations were determined on JASCO DIP-370 polarimeter at 25 ℃. NMR spectra were obtained using a Bruker AMX-500 MHz NMR spectrometer. HR-FAB-MS and ESI-MS spectra were obtained on JMS 700 (JEOL, Tokyo, Japan) and VG Autospec Ultima (Micromass, Manchester, UK) mass spectrometers, respectively. Open column chromatography was performed using a silica gel 60 (Kiesel gel 60, 700 - 230 and 230 - 400 mesh, Merck). Preparative HPLC was carried out on a Waters system (two 515 pumps and a 2996 photodiode array detector) and a YMC J’sphere ODS-H80 column (4 μm, 150 × 20 mm), using the mixed solvent system CH3CN/H2O at a flow rate of 6.0 mL/min. Thin layer chromatography (TLC) was performed on precoated silica gel 60 F254 (0.25 mm, Merck). All other chemicals and reagents were analytical grade.
Plant Material − The dried fruits of M. azedarach were purchased from Kyung-dong market, Seoul, Korea, in October 2010. The voucher specimen (CBNU 2010-003) was identified by Emeritus Professor Kyong Soon Lee and deposited at the Herbarium of College of Pharmacy, Chungbuk National University.
Extraction and Isolation − The dried fruits (1.0 kg) were extracted three times with methanol (3 × 2 L) at room temperature. The solvent was removed at reduced pressure to give a brown residue (179 g). The extract was suspended in H2O (1 L), and separated with n-hexane (3 × 1 L), CH2Cl2 (3 × 1 L), and EtOAc (3 × 1 L), respectively. The CH2Cl2 extract (12 g) was then subjected to column chromatography over silica gel (column: 20 cm × 7.0 cm) with CH2Cl2 - MeOH gradient (100 : 0 - 0 : 100) to yield 7 fractions (MA1 - MA7). Following silica gel column chromatography of MA4 fraction with a solvent gradient of EtOAc in n-hexane yielded 6 subfractions (MA4-1 - MA4-6). Among them, MA4-1 fraction yielded compound 1 (2.6 mg) by an additional purification step on the RPHPLC (MeCN : H2O gradient from 85 : 15 to 100 : 0). And then, compound 2 (3.0 mg) was isolated by RPHPLC (MeCN : H2O = 75 : 25, isocratic) from AM4-4 fraction. Following silica gel column chromatography of MA3 fraction with a solvent gradient of ethyl acetate in nhexane yielded 6 subfractions (MA3-1-MA3-6). Compounds 3 (4.5 mg) and 4 (2.5 mg) were isolated from MA3-2-1 fraction by RP-HPLC (MeCN : H2O = 80 : 20, isocratic). The MA3-3-5 fraction was further separated on RPHPLC (MeCN : H2O gradient from 60 : 40 to 80 : 20) to give compounds 5 (4.0 mg) and 6 (2.0 mg).
3α-Tigloylsapelin D (1) − Amorphous powder; αD25 : −49 (c 0.05, CHCl3); 1H-NMR (500 MHz, CDCl3) and 13C-NMR (125 MHz, CDCl3); see Table 1; HR-FBA-MS m/z 595.3977 [M+ Na]+ (calcd for C35H56O6Na, 595.3975).
Table 1.a) Assignments were confirmed by HMQC, HMBC and NOESY spectra.
Meliasenin B (2) − White solid; αD25 : −28 (c 0.1, MeOH); 1H-NMR (500 MHz, CD3OD): δ 5.69 (1H, br d, J = 2.5 Hz, H-7), 5.17 (1H, t, J = 7.0 Hz, H-24), 4.31 (1H, td, J = 10.5, 7.5 Hz, H-16), 3.21 (1H, t, J = 8.0 Hz, H-3), 3.10 (1H, td, J = 10.0, 3.5 Hz, H-9), 1.72 (3H, s, H-27), 1.66 (3H, s, H-26), 1.33 (3H, s, H-29), 1.29 (3H, s, H-30), 1.09 (3H, s, H-18), 1.05 (3H, s, H-29), 0.90 (3H, s, 19); 13C-NMR (125 MHz, CD3OD): δ 200.8 (6), 181.0 (C-21), 169.4 (C-8), 132.1 (C-25), 123.7 (C-7), 123.3 (C-24), 81.7 (C-16), 78.0 (C-3), 65.5 (C-5), 57.4 (C-17), 55.9 (C-14), 49.7 (C-9), 44.9 (C-20), 44.3 (C-10), 39.2 (C-13), 37.8 (C-4), 36.3 (C-1), 34.4 (C-15), 28.8 (C-22), 28.8 (C-12), 28.8 (C-30), 27.5 (C-28), 26.0 (C-2), 25.6 (C-23), 24.4 (C-26), 20.1 (C-18), 16.5 (C-27), 16.1 (C-11), 14.1 (C-29), 12.9 (C-19). ESI-MS m/z 491.43 [M + Na]+.
Sendanolactone (3) − White solid; αD25 : −35 (c 0.1, MeOH); 1H-NMR (500 MHz, CD3OD): δ 5.78 (1H, br d, J = 2.5 Hz, H-7), 5.17 (1H, t, J = 7.5 Hz, H-24), 4.30 (1H, td, J = 11.5, 8.0 Hz, H-16 Hz), 1.72 (3H, s, H-27), 1.66 (3H, s, H-26), 1.37, 1.36, 1.34, 1.15, 1.05 (15H, s, H-18, 19, 28, 29, 30 each); 13C-NMR (125 MHz, CD3OD): δ 215.5 (C-3), 199.1 (C-6), 180.8 (C-21), 169.8 (C-8), 132.1 (C-25), 123.5 (C-7), 123.3 (C-24), 81.6 (C-16), 64.9 (C-5), 57.3 (C-17), 55.9 (C-14), 48.6 (C-9), 48.2 (C-4), 46.7 (C-20), 44.8 (C-10), 39.1 (C-13), 36.4 (C-1), 34.3 (C-15), 33.5 (C-2), 28.8 (C-30), 28.7 (C-22), 28.6 (C-12), 25.6 (C-23), 24.4 (C-27), 24.3 (C-28), 20.7 (C-29), 19.9 (C-18), 16.5 (C-26), 16.1 (C-11), 12.4 (C-19). ESI-MS m/z 489.50 [M+ Na]+.
(−)-12β-Hydroxykulactone (4) − Colorless oil; αD25 : −24 (c 0.1, MeOH); 1H-NMR (500 MHz, CD3OD): δ 5.41 (1H, br s, H-7), 5.14 (1H, t, J = 7.0 Hz, H-24), 4.26 (1H, dd, J = 18.0, 10.0 Hz, H-16α), 3.98 (1H, dd, J = 9.5, 5.0 Hz, H-12α), 2.85 (1H, td, J = 14.5, 5.0 Hz, H-9), 1.69 (3H, s, H-27), 1.65 (3H, s, H-26), 1.41 (3H, s, H-30), 1.15 (3H, s, 28), 1.08 (3H, s, H-29), 1.04 (3H, s, H-19), 0.87 (3H, s, H-18); 13C-NMR (125 MHz, CD3OD): δ 217.7 (C-3), 182.0 (C-21), 143.6 (C-8), 131.8 (C-25), 123.9 (C-24), 118.5 (C-7), 82.5 (C-16), 70.8 (C-12), 54.7 (C-13), 53.1 (C-17), 52.4 (C-5), 48.4 (C-4), 48.3 (C-9), 45.3 (C-20), 44.0 (C-14), 38.1 (C-1), 36.0 (C-15), 35.1 (C-10), 34.4 (C-2), 33.3 (C-30), 30.1 (C-11), 28.4 (C-22), 25.4 (C-23), 24.7 (C-26), 24.1 (C-6), 23.7 (C-29), 20.7 (C-28), 19.3 (C-18), 16.9 (C-27), 11.7 (C-19). ESI-MS m/z 491.25 [M + Na]+.
Cinamodiol (5) −White solid; αD25 : −34 (c 0.125, MeOH); 1H-NMR (500 MHz, CD3OD): δ 5.37(1H, m, H-7), 5.16 (1H, br td, J = 6.8 Hz, H-24), 4.28 (1H, dd, J = 17.5, 9.5 Hz, H-16α), 3.96 (1H, dd, J = 9.5, 5.0 Hz, H-12α), 3.20 (1H, dd, J = 10.0, 6.0 Hz, H-3α), 1.71 (3H, s, H-27), 1.66 (3H, s, H-26), 1.37 (3H, s, H-30), 0.97 (3H, s, H-29), 0.88 (3H, s, H-19), 0.87 (3H, s, H-28), 0.82 (3H, s, H-18), 13C-NMR (125 MHz, CD3OD): δ 181.9 (C-21), 143.4 (C-8), 131.6 (C-25), 124.0 (C-24), 118.6 (C-7), 82.4 (C-16), 78.3 (C-3), 70.9 (C-12), 54.6 (C-14), 53.0 (C-17), 50.8 (C-5), 48.5 (C-9), 45.2 (C-20), 44.0 (C-13), 38.6 (C-4), 36.9 (C-1), 35.8 (C-15), 34.9 (C-10), 33.0 (C-30), 30.1 (C-22), 28.4 (C-11), 26.9 (C-2), 26.7 (C-28), 25.3 (C-27), 24.5 (C-23), 24.9 (C-6), 19.0 (C-19), 16.6 (C-26), 13.8 (C-29), 12.0 (C-18). ESI-MS m/z 493.33 [M+ Na]+.
3α-Hydroxytirucalla-7,24(25)-dien-6-oxo-21,16-olide (6) − White solid; αD25 : −42 (c 0.1, MeOH); 1H-NMR (500 MHz, CD3OD): δ 5.69 (1H, br d, J = 2.5 Hz, H-7), 5.17 (1H, br dt, J = 6.0 Hz, H-24), 4.31 (1H, dt, J = 10.5, 7.5 Hz, H-16α), 3.29 (1H, br s, H-3α), 3.20 (1H, dt, J = 8.0, 2.0 Hz, H-9), 1.72 (3H, s, H-27), 1.67 (3H, s, H-26), 1.34 (3H, s, H-28), 1.23 (3H, s, H-28), 1.14 (3H, s, H- 29), 1.07 (3H, s, H-18), 0.92 (3H, s, H-19); 13C-NMR (125 MHz, CD3OD): δ 202.2 (C-6), 181.1 (C-21), 169.5 (C-8), 132.1 (C-25), 123.7 (C-7), 123.3 (C-24), 81.7 (C-16), 76.0 (C-3), 60.5 (C-5), 57.4 (C-17), 55.9 (C-14), 49.7 (C-9), 44.8 (C-20), 44.1 (C-10), 39.1 (C-13), 36.4 (C-4), 34.4 (C-15), 30.6 (C-1), 28.8 (C-22), 28.8 (C-12), 28.8 (C-30), 27.3 (C-28), 25.6 (C-23), 24.4 (C-27), 24.2 (C-2), 20.7 (C-29), 20.1 (C-18), 16.5 (C-26), 16.0 (C-11), 13.1 (C-19). ESI-MS m/z 491.33 [M + Na]+.
Results and Discussion
The CH2Cl2-soluble fraction of the dried fruits of M. azedarach was subjected to silica gel column chromatography and preparative RP-HPLC which led to the isolation of six compounds including a new apotirucallane-type triterpenoid, 3α-tigloylsapelin D (1) (Fig. 1). Among these compounds, five known compounds were identified as meliasenin B (2) (Zhang et al., 2010), sendanolactone (3) (Faizi et al., 2002; Ochi et al., 1977), (-)-12β-hydroxykulactone (4) (Cantrell et al., 1999), cinamodiol (5) (Kelecom et al., 1996), and 3α-hydroxytirucalla-7,24(25)-dien-6-oxo-21,16-olide (6) (Tan et al., 2010) by comparing the 1H-HMR, 13C-NMR and MS spectroscopic data with those in the literature.
Fig. 1.The structures of compounds 1 - 6 isolated from M. azedarach.
Compound 1 was isolated as white amorphous powder. The HR-FAB-MS showed the quasi-molecular ion peak at m/z 595.3977 [M + Na]+ (calcd for C35H56O6Na, 595.3975), consistent with the molecular formula of C35H56O6. The 1H-NMR spectrum (Table 1) showed seven tertiary methyl groups (δH 0.89, 0.93, 0.94, 1.02, 1.11, each 3H, s), one olefinic proton (δH 5.52), and five oxygenated protons (δH 3.46, 3.90, 3.93, 4.01, 4.71). The 13C-NMR spectra (Table 1) showed the presences of seven methyls, eight methylenes (one oxygenated), nine methines (three oxygenated, one olefinic), and six quaternary carbons (one olefinic and one oxygenated). The remaining 1H-NMR and 13C-NMR data (Table 1) indicated the presence of a tigloyl group [δH 6.88 (1H, dd, 7.0, 1.5 Hz, H-3'), 1.87 (3H, t, 1.0 Hz, H-4'), and 1.81 (3H, dd, 7.0, 1.0 Hz, H-5'); δC 167.6, 136.7, 129.2, 14.4, 12.1]. The aforementioned data suggested that compound 1 was a triterpenoid having an apotirucallane skeleton with a tigloyl ester group in the molecule. Moreover, these data closely resembled to those of sapelin D and 3α-senecioyl-21,24R-epoxyapotirucall-14-ene-7α,23R,25-triol, apotirucallane-type triterpenoid from Entandrophragma cylindricurn and Cedrela sinensis, respectively (Lyons and Taylor, 1976; Mitsui et al., 2005, 2007). The location of tigloyl group was elucidated as C-3 by the observed HMBC correlation between H-3 (δH 4.71) and C-1' (δC 167.6) (Fig. 2). In the HMBC spectrum of compound 1, the correlations between the methylene protons [δH 3.46 (1H, dd, 11.0, 2.5 Hz, H-21α) and δH 4.01 (1H, d, 11.0 Hz, H-21β)] and the methine carbon [δC 86.5 (C-24)] suggested that C-21 was linked to C-24 via an oxygen bridge to form a cyclic ether (Fig. 2). The relative configuration of the tetracyclic core in compound 1 was established on the basis of NOESY experiments (Fig. 3). The NOESY correlations of H-3β/CH3-29, CH3-19/CH3-29, CH3-19/CH3-30, and CH3-30/H-7 indicated the tigloyl ester side chain at C-3 and hydroxyl group at C-7 were both in α-orientation. The large coupling constant (J = 9.0 Hz) between H-23 and H-24 indicated that the tetrahydropyran ring of the side chain at C-17 was in a chair conformation (Mitsui et al., 2005; Zhang et al., 2012). Further NOE correlations between H-17/H-21β, H-17/H-23, CH3-18/H-20, H-20/H-21α, H-21α/H-22α, H-21α/H-24, and H-22α/H-24 revealed that the configuration at C-23 and that at C-24 were both R. Therefore, the structure of 1 was determined as 3α-tigloyl-21,24Repoxyapotirucall-14-ene-7α,23R,25-triol, and named 3α-tigloylsapelin D (1).
Fig. 2.Key HMBC (→) correlations of compound 1.
Fig. 3.Key NOESY (↔) correlations of compound 1.
The genus Melia is well-known as a rich and valuable source of highly functionalized nortriterpenoid such as limonoid derivatives (Tan and Luo, 2011). Apotirucallanetype triterpenoids, which have undergone the apo-euphol rearrangement to form a C = C bond at C-14 with the methyl group shifted to C-8, were isolated from the genus Melia (Zhao et al., 2010; Mitsui et al., 2005; Nakanishi et al., 1986; Zeng et al., 1995a; Rogers et al., 1998; Fukuyama et al., 2000).
However, apotirucallane-type triterpenoids having a substituted tetrahydropyran ring side chain have been rarely found (Zeng et al., 1995b, 1995c). Therefore, the isolation of 3α-tigloylsapelin D (1) in this study provides further example of the rare apotirucallae-type triterpenoid with tetrahydropyran ring system in the molecule.
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