Four new limonoids from the seeds of Chukrasia tabularis A. Juss.

Phytochemistry Letters 19 (2017) 12–17

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Four new limonoids from the seeds of Chukrasia tabularis A. Juss. Li Yia,1, Hongjian Zhangb,1, Xiaomeng Tianb , Jie Luob , Jun Luob,* , Lingyi Kongb,* a

Testing & Analysis Center, Nanjing Normal University, Nanjing 210023, People’s Republic of China State Key Laboratory of Natural Medicines, Department of Natural Medicinal Chemistry, China Pharmaceutical University, 24 Tong Jia Xiang, Nanjing 210009, People’s Republic of China b

A R T I C L E I N F O

Article history: Received 12 September 2016 Received in revised form 31 October 2016 Accepted 7 November 2016 Available online xxx Keywords: Chukrasia tabularis Limonoids Phragmalin Mexicanolide ECD exciton chirality method

A B S T R A C T

Two new C-15 enolic acyl phragmalin-type limonoid orthoesters (1-2) which possessed a C-15-propionyl phragmalin skeleton and two new mexicanolide-type limonoids (3-4) were isolated from the ethanol extract of seeds of Chukrasia tabularis A. Juss. Their structures were established on the basis of spectroscopic analyses and electronic circular dichroism (ECD) exciton chirality method. Additionally, all of the compounds were screened against three human tumor cell lines MCF-7, SMMC-7721, and U2OS. ã 2016 Published by Elsevier Ltd on behalf of Phytochemical Society of Europe.

1. Introduction Limonoids are the characteristic metabolites of the Meliaceae plants and possess diverse structures and significant biological activities, have been studied extensively in the past (Fang et al., 2011; Tan and Luo, 2011). Among them, phragmalin-type limonoids, a class of structurally complex and biologically important compounds attracting a broad range of interests in both organic chemistry and agrochemistry (Yadav et al., 2012; Cui et al., 2005; Liu et al., 2013; Razafimahefa et al., 2014; Yang et al., 2012; Zhang et al., 2011), are the major secondary metabolites of the plants from Chukrasia genus (Meliaceae) (Fan et al., 2007; Hu et al., 2013; Liu et al., 2012; Luo et al., 2012, 2009a, 2011; Nakatani et al., 2004). Specially, C-15 enolic acyl phragmalin-type limonoids are a series of novel limonoids with a biosynthetically extended enolic acyl side chain at C-15. In 2009 (Luo et al., 2009b) and 2015 (Luo et al., 2015), we had reported some C-15 enolic acyl phragmalin-type limonoids with novel C-16/C-30 d-lactone ring from C. Tabularis. Further investigation led to the isolation of two new C-15 enolic acyl phragmalin-type limonoids, chukvelutilide Y (1) and chukvelutilide Z (2) with C-16/C-30 d-lactone ring (D ring), and two new mexicanolide-type limonoids ivorenoid G (3) and andirolide Q (4) from the seeds of C. tabularis A. Juss. Herein, the

* Corresponding authors. E-mail addresses: [email protected] (J. Luo), [email protected], [email protected] (L. Kong). 1 These authors contributed equally.

isolation, structural elucidation, and bioactive test of these three compounds were reported. 2. Results and discussion Chukvelutilide Y (1) was isolated as a white amorphous powder with the molecular formula of C43H52O20 as determined by the HRESIMS at m/z 911.2940 [M + Na]+ (Calcd for C43H52O20Na: 911.2944), indicating 18
of unsaturation. The characteristic UV absorption bands at 268 nm suggested nucleus of chukvelutilide Y was enolated. Further, the whole feature of 1H and 13C NMR data (Table 1), especially a characteristic enolic proton signal at dH 13.75 and a series of d-ketolactone carbon signals at dC 180.1 (C-10 ), 92.3 (C-15) and 169.9 (C-16), suggested that compound 1 was a C-15 enolic acyl phragmalin-type limonoid orthoester derivative (Luo et al., 2009b). Extensive analysis of the 2D NMR spectra (HSQC and HMBC) of compound 1, particularly HMBC correlations (Fig. 3) from the enolic proton signal 10 -OH (dH 13.75) and H-14 (dH 3.44) to d-ketolactone carbon signals (C-10 and C-15) and from proton signals of an ethyl group [dH 2.54 (m), 2.41 (m), each 1H and 1.23 (t, J = 7.5 Hz), 3H] to the carbon signal at dC 180.1 (C-10 ) suggested that propionyl group in biosynthetically extended pathway was attached at C-15. Further, the HMBC correlations of H-3 (dH 5.52, s)/C-2 (dC 83.0), H-30 (dH 5.72, s)/C-2, H-3 (dH 5.52, s)/C-1000 (dC 168.0), H-17 (dH 5.89, s)/C-10000 (dC 168.6), H-19 (dH 4.14, 4.66, d, J = 11.5 Hz)/C-10000 (dC 169.2), revealed that the acetoxy groups occurred at C-2, C-3, C-17, and C-19. Three oxygenated quaternary carbons signals at dC 84.5 (C-1), dC 80.0 (C-8), and dC 82.8 (C-9)

http://dx.doi.org/10.1016/j.phytol.2016.11.004 1874-3900/ã 2016 Published by Elsevier Ltd on behalf of Phytochemical Society of Europe.

L. Yi et al. / Phytochemistry Letters 19 (2017) 12–17 Table 1 1 H NMR (500 MHz) and No.

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C NMR (125 MHz) Data of 1and 2 in CDCl3.

1

dH (J in Hz) 1 2 3 4 5 6a 6b 7 8 9 10 11 12 13 14 15 16 17 18 19a 19b 20 21 22 23 28 29a 29b 30 31 32 10 20 30 10 -OH 2-OAc

5.52 (s) 3.17 (d, 11.0) 2.96 (d, 17.5) 2.47 (m)

5.02 (d, 2.0) 4.55 (d, 2.0) 3.44 (s)

5.89 (s) 1.54 (3H, s) 4.14 (d, 11.5) 4.66 (d, 11.5) 7.54 (s) 6.41 (s) 7.29 (s) 1.01 (3H, s) 1.87 (2H,m)

2

dC

dH (J in Hz)

84.5 83.0 79.9 46.3 35.5 32.3

84.6 71.4 5.50 (s) 80.8 46.7 2.63 (d, 11.0) 34.4 2.91 (dd, 17.5, 4.5) 30.6 2.59 (m) 171.3 79.1 82.9 45.5 4.42 (s) 69.6 4.61 (d, 1.0) 72.3 44.7 3.39 (s) 43.4 91.8 170.0 5.75 (s) 69.6 1.62 (3H, s) 18.0 4.29 (d, 14.0) 67.8 4.78 (d, 14.0) 122.1 7.33 (s) 140.8 6.32 (s) 110.0 7.32 (t-like, 1.0) 143.0 1.10 (3H, s) 14.2 1.89 (m) 39.1 1.90 (m) 5.38 (s) 73.4 119.6 1.69 (3H, s) 21.0 181.0 2.41, 2.54 (2H, m) 25.5 1.24 (3H, t, 7.5) 10.9 13.8

172.6 80.0 82.8 48.1 69.9 71.7 44.5 43.2 92.3 169.9 70.2 17.9 66.3 122.3 141.0 109.9 142.4 14.4 39.9

3-OAc

73.4 119.7 1.68 (3H,s) 20.8 180.1 2.41, 2.54 (2H, m) 25.5 1.23 (3H, t,7.5) 11.0 13.8 2.10 (3H, s) 20.9; 169.6 2.37 (3H, s) 21.6; 16.8

12-OAc 17-OAc

2.04 (3H, s)

19-OAc

1.65 (3H, s)

12OCCH2Me

7-OCH3

5.72 (s)

20.7; 168.6 20.5; 169.2 172.5

2.33 (3H, s) 1.74 (3H, s) 1.91 (3H, s)

dC

20.7; 169.7 20.1; 169.3 20.5; 168.7

2.00 (m); 1.77 (m) 26.4 0.88 (7.5, t, 3H) 8.3 3.72 (3H, s) 51.8

showed a characteristic orthoacetate system. Comparing NMR data with chukvelutilide A (Luo et al., 2009a,b) indicated that compound 1 possessed the same C-15 enolic propionyl phragmalin skeleton with a 1,8,9-ortho-acetate and an C-16/C-30 d-lactone ring. Thus, the planar structure of 1 was established as shown in Fig. 1. The ROESY correlations of 1 from H-5 to H-30 and H-11, from H-17 to H-12 and H-30, and from H-30 to H-12 indicated b-orientations of these protons. Moreover, ROESY correlations of H-14/Me-18, H-29a/H-3, and H-29b/Me-19 revealed that these protons had a-orientation. Further, the absolute configuration of 1 was determined by the ECD exciton chirality method. The ECD spectrum of 1 exhibited negative chirality resulting from the exciton coupling between two different chromophores of the furan 0 ring at 225 nm (De +0.992) and the 1 ,15a,b-unsaturated ketone at 202 nm (De 8.688), respectively. The clockwise manner of the two chromophores in space thus defined the absolute configuration of 1 (Fig. 2) as 2R, 3S, 5S, 8R, 9R, 10S, 13R, 17R, 30R by ECD exciton chirality method, which was as same as chukvelutilide I

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(Luo et al., 2015). Therefore, the 2D structure of 1 (Chukvelutilide Y) was determined as depicted in Fig. 3. Chukvelutilide Z (2) was assigned a molecular formula of C37H42O17 as determined by HRESIMS displaying a m/z ion at 781.2316 [M+Na]+ (calcd for C37H42O17Na: 781.2314). The NMR data (Table 1) of 2 resembled those reported for chukvelutilide E with a C-7-O-C-19 lacton ring (Luo et al., 2009b), except for the numbers and location of acetyl groups, which was confirmed by HMBC correlation (see SI S11). The significant ROESY correlations (see SI S12) of H-12/H-17, H-17/H-30, H-30/H-5, and Me-18/H-14, as well as the NMR data, coupling patterns, and CD spectrum, suggested that 2 possessed the same relative configuration as chukvelutilide E (Luo et al., 2009b). Therefore, the structure of 2 was assigned as shown in Fig. 1. Ivorenoid G (3) showed a HRESIMS ion at m/z 655.2726 [M+Na]+ (calcd for C33H44O12Na: 655.2725), corresponding to the molecular formula C33H44O12 with 12
of unsaturation. The absence of UV absorption at 268 nm and proton signal at dH 13.75 show that 3 was not a typical phragmalin with C-15 alcohol. The proton signals of four tertiary methyls (dH 0.76, 1.06, 1.19, 1.36, each 3H, s), an acetyl (dH 2.10, 3H, s), an isopropyl (dH 2.64, 1H, m; 1.17, 1.17, each 3H, d, J = 7.0 Hz) and a methoxy (dH 3.72, 3H, s) were observed in the 1H NMR spectrum (Table 2). The 13C NMR with HSQC experiments revealed 33 carbon resonances including 8  CH3, 4  CH2, 10  CH (three oxygenated and three olefinic), and 11  C (four carbonyl, one olefinic and three oxygenated) groups. Extensive analysis of HMBC cross-peaks of H-28 (dH 0.76, 3H, s)/C-4 (dC 39.7), H-29 (dH 1.36, 3H, s)/C-4 (dC 39.7) revealed that compound 3 was a mexicanolide-type limonoid, which structure was closely related to that of ivorenoid D (Wu et al., 2014). The only difference was the presence of isobutyryloxy moiety (dH 1.17, 3H, d, J = 7.0 Hz; 1.17, 3H, d, J = 7.0 Hz; 2.64, 1H, m; dC 19.0, 18.9, 33.9, 177.2) in 3 replacing the 3-OAc of ivorenoid D (Wu et al., 2014), which was confirmed by the HMBC correlations from H-3 (dH 5.52, s) to C-10 (dC 176.0) (Fig. 3). The stereochemistry of H-3 was assigned to b-oriented by the ROESY correlation between H-3 and the proton of 1-OH (Fig. 4). Andirolide Q (4) was a white amorphous powder, had the molecular formula C31H40O11, as established by HRESIMS (m/z 589.2639, calcd for C31H41O11: 589.2643). The MS and data from 1D- and 2D-NMR studies (1H NMR, 13C NMR, HMBC, HSQC, and ROESY) indicated that 4 were similar to 3, except for the presence of a carbonyl at C-1 (dC 204.5) and a D14,15 double bond (dC 165.5, 117.0), and the absence of a 2-methylpropanoyl group. The structural assignment of 4 was further confirmed via the HMBC spectrum. Therefore, the structure of andirolide Q (4) was elucidated as shown. The compounds 1–4 were evaluated in vitro for cytotoxic activities against MCF-7 (human breast cancer cell line), SMMC7721 (human hepatic cancer cell line), and U2OS (human osteosarcoma cell line) using the MTT method. As a result, all compounds showed no obvious cytotoxic activities with IC50 values more than 50 mM compared to the positive control. Cisplatin was used as the positive control, and the experiments were conducted for three independent replicates. 3. Experimental section 3.1. General experimental procedures Optical rotations were measured using a JASCO P-1020 polarimeter, whereas CD spectra were obtained on a JASCO 810 spectropolarimeter, and UV spectra were recorded quantitatively on a Shimadzu UV-2501 PC spectrophotometer. IR (KBr-disks) spectra were recorded using a Bruker Tensor 27 spectrometer. NMR spectra were recorded on Bruker ACF-500 NMR instrument (1H: 500 MHz, 13C: 125 MHz) with TMS as the internal standard. Mass

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L. Yi et al. / Phytochemistry Letters 19 (2017) 12–17

Fig. 1. Structures of compounds 1–4.

spectra were obtained on a MS Agilent 1100 Series LC/MS Trap mass spectrometer (ESI–MS) and a Micro Q-TOF MS (HRESIMS). All of the solvents used were of analytical grade (Jiangsu Hanbang Sci. & Tech. Co. Ltd). Silica gel (200–300 mesh, Qingdao Haiyang Chemical Co. Ltd) and RP-C18 (40–63 mm, FuJi) were used for column chromatography (CC). Analytical HPLC was performed on an Agilent 1260 Series instrument with a DAD detector using a shim-pack VP-ODS column (250  4.6 mm, 5 mm). Preparative HPLC was performed using an Agilent 1100 Series with a Shim-park RP-C18 column (20  200 mm, 10 mm) and with a binary channel UV detector Shimadzu SPD-20A at 210 and 230 nm (Shimadzu, Tokyo, Japan). 3.2. Plant material Air-dried seeds of Chukrasia tabularis A. Juss. were purchased from ZhanJiang, Guangdong Province, China, and authenticated by Professor Mian Zhang of the Research Department of Pharmacognosy, China Pharmaceutical University. A voucher specimen (No. 2013-MLZ) has been deposited at the Department of Natural Medicinal Chemistry, China Pharmaceutical University.

3.3. Extraction and isolation Air-dried seeds (5.0 kg) of C. tabularis were extracted by refluxing with 95% ethanol (20 L  3). After filtration, the EtOH extract was concentrated under reduced pressure to yield a crude extract (720 g) and then extracted with CHCl3 to yield the CHCl3 extract (150 g). The oily CHCl3 extract was dissolved in MeOH/H2O (1:1, v/v, 1 L), and successively extracted with petroleum ether (PE). After removal of the fatty components, an extract (48.0 g) was obtained. This extract was subjected to silica gel CC (200–300 mesh, i.d. 60  5 cm) and eluted with gradient CHCl3/ MeOH (1:0 to 2:1, v/v) to afford five fractions (Fr. A-E) according to TLC monitoring and HPLC-DAD analysis. Fr. B (6.3 g) was chromatographed on a column of reversed-phase C18 silica gel (i.d. 40  5 cm) eluted with MeOH/H2O (3:7 to 7:3, v/v) to give five sub-fractions (Fr. B1-B5). Fr. B3 (1.8 g) was chromatographed on a column of silica gel CC (200–300 mesh, i.d. 30  3 cm) eluted successively with a gradient of DCM/MeOH (100:1 to 30:1, v/v) to yield three sub-fractions (Fr. B3a-B3c). Fr. B3b (0.9 g) was subjected to reversed-phase C18 silica gel CC (i.d. 30  2.5 cm) eluted with MeOH/H2O (1:1 to 3:1, v/v) to give three sub-fractions (Fr. B3b1-

L. Yi et al. / Phytochemistry Letters 19 (2017) 12–17

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Fig. 2. CD spectrum of 1; the bold lines denote the electric transition dipole of the chromophores for 1.

Fig. 3. Key HMBC (!) and ROESY ($) correlations of 1.

B3b3), then Fr. B3b1 (62.0 mg)was separated by preparative HPLC using MeOH/H2O (60:40, v/v; flow rate = 10 mL/min; UV = 210, 230 nm) as the mobile phase to give 3 (20.0 mg, tR 28.4 min). Fr. B3b3 (25.6 mg) was separated by preparative HPLC using CH3CN/ H2O (50:50, v/v; flow rate = 10 mL/min; UV = 210, 230 nm) as the mobile phase to give 2 (9.2 mg, tR 33.6 min). Fr. B4 (2.4 g) was subjected to reversed-phase C18 silica gel CC (i. d. 30  3 cm) eluted with MeOH/H2O (65:35, v/v) to give three subfractions (Fr. B4a-B4c), then Fr. B4b was separated by preparative HPLC using MeOH/H2O (65:35, v/v; flow rate = 10 mL/min; UV = 210, 230 nm) as the mobile phase to yield 4 (3.1 mg, tR 28.4 min). Fr. B5 (1.2 g) was applied to column of silica gel CC (200– 300 mesh, i.d. 30  3 cm) eluted successively with a gradient of

CHCl3/MeOH (100:1 to 50:1, v/v) to yield three sub-fractions (Fr. B5a-B5c). Fr. B5b (0.6 g) was subjected to reversed-phase C18 silica gel CC (i.d. 30  3 cm) eluted with MeOH/H2O (60:40, v/v) to give three sub-fractions (Fr. B5b1-B5b3), and then Fr. B5b2 (45.3 mg) was separated by preparative HPLC using CH3CN/H2O (60:40, v/v; flow rate = 10 mL/min; UV = 210, 230 nm) as the mobile phase to yield 1 (6.0 mg, tR 14.2 min). 3.3.1. Chukvelutilide Y (1) White amorphous powder; [a]25D 40.0 (c 0.100, CH3OH); UV (CH3OH) lmax (log e) 200 (0.626), 268 (0.468) nm; CD (CH3OH, De) 225 (+0.992), 269 (-6.716) nm; IR (KBr) nmax 3455, 2980, 1742, 1640, 1371, 1222 cm1; 1H NMR (CDCl3, 500 MHz) and 13C NMR (CDCl3,

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L. Yi et al. / Phytochemistry Letters 19 (2017) 12–17

Table 2 1 H NMR (500 MHz) and No.

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C NMR (125 MHz) Data of 3 and 4 in CDCl3.

3

dH (J in Hz) 1 2 3 4 5 6a 6b 7 8 9 10 11 12a 12b 13 14 15 16 17 18 19 20 21 22 23 28 29 30 10 20 30 40 1-OH 2-OAc 7-OCH3

5.52 (s) 2.78 (m) 3.01 (d,16.0) 2.35 (dd,16.0,9.5)

1.55 (m) 4.23 (m) 1.38 (m) 2.00 (dd,14.0,4.0) 2.17 (dd,7.5,1.5) 2.77 (m) 2.82 (d, 7.5) 5.22 (s) 1.06 (3H, s) 1.19 (3H, s) 7.53 (s) 6.44 (d,1.0) 7.46 (t,1.5) 0.76 (3H, s) 1.36 (3H, s) 1.92 (dd,14.5,1.5), 2.71 (m) 2.64 (m) 1.17 (d,7.0) 1.17 (d,7.0) 7.21(s) 2.10 (3H, s) 3.72 (3H, s)

4

dC 108.1 93.6 75.6 39.7 40.6 32.1 177.2 78.6 67.4 47.5 64.3 45.2 36.7 43.6 27.3 169.3 77.8 22.3 22.8 121.0 140.8 109.8 143.2 22.2 24.2 39.7 176.0 33.9 19.0 18.9 22.2; 174.5 52.1

dH (J in Hz)

5.02 (s) 2.61 (d,7.0) 2.39 (d,7.0) 2.36 (m)

2.17 (m) 5.53 (m) 1.45 (m) 1.83 (m)

6.28 (s)

dC 204.5 88.3 80.2 43.2 39.6 32.6 173.7 20.0 66.1 54.5 74.9 30.9 39.0 165.5 117.0

2.73 (m) 1.30 (m) 1.30 (m)

164.6 78.9 21.2 18.0 120.2 141.6 110.4 143.0 21.4 24.9 79.2 175.1 34.2 19.1 18.4

3.71 (3H, s)

52.1

5.37 (s) 1.28 (3H, s) 1.04 (3H, s) 7.46 (s) 6.48 (br s) 7.45 (s) 0.94 (3H, s) 0.86 (3H, s) 4.29 (s)

125 MHz), see Table 1; ESIMS m/z: 906.4 [M + NH4]+ (100); HRESIMS m/z: 911.2940 [M+Na]+ (calcd. for C43H52O20Na: 911.2944).

3.3.2. Chukvelutilide Z (2) White amorphous powder; [a]25D 17.2 (c 0.100, CH3OH); UV (CH3OH) lmax (log e) 205 (0.18), 269 (0.25) nm; CD (CH3OH, De) 239 (+0.903), 269 (-5.903) nm; IR (KBr) nmax 3464, 2939, 1747, 1641, 1372, 1236 cm1; 1H NMR (CDCl3, 500 MHz) and 13C NMR (CDCl3, 125 MHz), see Table 1; ESIMS m/z: 793.7 [M+Cl] (100); HRESIMS m/z: 781.2316 [M+Na]+ (calcd. for C37H42O17Na: 781.2314). 3.3.3. Ivorenoid G (3) White amorphous powder; [a]25D 55.8 (c 0.100, CH3OH); UV (CH3OH) lmax (log e) 209 (0.635), 245 (0.016) nm; CD (CH3OH, De) 225 (+2.078) nm; IR (KBr) nmax 3456, 2975, 174735, 1372, 1226 cm1; 1H NMR (CDCl3, 500 MHz) and 13C NMR (CDCl3, 125 MHz), see Table 2; ESIMS m/z: 650.4 [M+NH4]+ (100); HRESIMS m/z: 655.2726 [M+Na]+ (calcd. for C33H44O12Na: 655.2725). 3.3.4. Andirolide Q (4) White amorphous powder; [a]25D 32.6 (c 0.100, CH3OH); UV (CH3OH) lmax (log e) 209 (0.635), 245 (0.016) nm; CD (CH3OH, De) 217 (+19.262), 243 (-2.1372) nm; IR (KBr) nmax 3456, 2973, 1726, 1385, 1279 cm1; 1H NMR (CDCl3, 500 MHz) and 13C NMR (CDCl3, 125 MHz), see Table 2; ESIMS m/z: 589.3 [M+H]+ (100); HRESIMS m/z: 589.2639 [M+H]+ (calcd. for C31H41O11: 589.2643). 3.4. Cytotoxicity assay All new compounds were tested for cytotoxicity against MCF-7 (human breast cancer cell line), SMMC-7721 (human hepatic cancer cell line) and U2OS (human osteosarcoma cell line) using the MTT method (Zhou et al., 2013). Cells were plated in 96-well culture plates (5  103 cells per well). After incubation over night, the cells were treated with different concentrations of each compound for 48 h. DMSO (0.1%) was used as a vehicle. MTT (5 mg/ mL) was dissolved in PBS and filter sterilized, then 20 mL of the prepared solution was added to each well and cells were incubated until a purple precipitate was visible. The formed formazan crystals were dissolved in DMSO (150 mL/well) by constant shaking for 10 min. The absorbance was measured on an ELISA reader (SpectraMax Plus 384, Molecular Devices, Sunnyvale, CA) at a test wavelength of 570 nm and a reference wavelength of 630 nm. After treatment, cell viability was detected and IC50 values were

Fig. 4. Key HMBC (!) and ROESY ($) correlations of 3.

L. Yi et al. / Phytochemistry Letters 19 (2017) 12–17

calculated by the Reed and Muench method (Reed and Muench, 1938). Acknowledgments This research work was financially supported by the National Natural Sciences Foundation of China (81573550), the Youth Fund Project of Basic Research Program of Jiangsu Province (Natural Science Foundation, BK20160077), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Ph.D. Programs Foundation of Ministry of Education of China (20120096130002). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. phytol.2016.11.004. References Cui, J., Deng, Z., Li, J., Fu, H., Proksch, P., Lin, W., 2005. Phragmalin-type limonoids from the mangrove plant Xylocarpus granatum. Phytochemistry 66, 2334–2339. Fan, C.Q., Wang, X.N., Yin, S., Zhang, C.R., Wang, F.D., Yue, J.M., 2007. Tabularisins AD, phragmalin ortho esters with new skeleton isolated from the seeds of Chukrasia tabularis. Tetrahedron 63, 6741–6747. Fang, X., Di, Y.T., Hao, X.J., 2011. The advances in the limonoid chemistry of the Meliaceae family. Curr. Org. Chem. 15, 1363–1391. Hu, K., Liu, J.Q., Li, X.N., Chen, J.C., Zhang, W.M., Li, Y., Li, L.Q., Guo, L.L., Ma, W.G., Qiu, M.H., 2013. Chukfuransins A-D, four new phragmalin limonoids with b-Furan ring involved in skeleton reconstruction from chukrasia tabularis. Org. Lett. 15, 3902–3905. Liu, H.B., Zhang, H., Li, P., Wu, Y., Gao, Z.B., Yue, J.M., 2012. Kv1: 2 potassium channel inhibitors from Chukrasia tabularis. Org. Biomol. Chem. 10, 1448–1458.

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Liu, J.Q., Peng, X.R., Zhang, W.M., Shi, L., Li, X.Y., Chen, J.C., Qiu, M.H., 2013. Swietemahalactone, a rearranged phragmalin-type limonoid with antibacterial effect, from Swietenia mahagoni. RSC Adv. 3, 4890–4893. Luo, J., Wang, J.S., Luo, J.G., Wang, X.B., Kong, L.Y., 2009a. Chukvelutins A-C: 16norphragmalin limonoids with unprecedented skeletons from Chukrasia tabularis var. velutina. Org. Lett. 11, 2281–2284. Luo, J., Wang, J.S., Wang, X.B., Huang, X.F., Luo, J.G., Kong, L.Y., 2009b. Chukvelutilides A-F: phragmalin limonoids from the stem barks of Chukrasia tabularis var. velutina. Tetrahedron 65, 3425–3431. Luo, J., Wang, J.S., Luo, J.G., Wang, X.B., Kong, L.Y., 2011. Velutabularins A-J: phragmalin-type limonoids with novel cyclic moiety from Chukrasia tabularis var. velutina. Tetrahedron 67, 2942–2948. Luo, J., Li, Y., Wang, J.S., Lu, J., Kong, L.Y., 2012. Three new C-15-isobutyryl 16norphragmalin-type limonoids from Chukrasia tabularis var. velutina. Phytochem. Lett. 5, 249–252. Luo, J., Zhang, H.J., Quasie, O., Shan, S.M., Zhang, Y.M., Kong, L.Y., 2015. Further C-15acyl phragmalin derivatives from Chukrasia tabularis A. Juss. Phytochemistry 117, 410–416. Nakatani, M., Abdelgaleil, S.A., Saad, M.M., Huang, R.C., Doe, M., Iwagawa, T., 2004. Phragmalin limonoids from Chukrasia tabularis. Phytochemistry 65, 2833–2841. Razafimahefa, S., Mutulis, F., Mutule, I., Liepinsh, E., Dambrova, M., Cirule, H., Svalbe, B., Yahorava, S., Yahorau, A., Rasolondratovo, B., 2014. Libiguins A and B: novel phragmalin limonoids isolated from neobeguea mahafalensis causing profound enhancement of sexual activity. Planta Med. 80, 306–314. Reed, L.J., Muench, H., 1938. A simple method of estimating fifty per cent endpoints. Am. J. Epidemiol. 27, 493–497. Tan, Q.G., Luo, X.D., 2011. Meliaceous limonoids: chemistry and biological activities. Chem. Rev. 111, 7437–7522. Wu, W.B., Zhang, H., Liu, H.C., Dong, S.H., Wu, Y., Ding, J., Yue, J.M., 2014. Ivorenoids A-F: limonoids from Khaya ivorensis. Tetrahedron 70, 3570–3575. Yadav, P.A., Suresh, G., Prasad, K.R., Rao, M.S., Babu, K.S., 2012. New phragmalin-type limonoids from Soymida febrifuga. Tetrahedron Lett. 53, 773–777. Yang, W., Kong, L., Zhang, Y., Tang, G., Zhu, F., Li, S., Guo, L., Cheng, Y., Hao, X., He, H., 2012. Phragmalin-type limonoids from Heynea trijuga. Planta Med. 78, 1676–1682. Zhang, Q., Di, Y.T., He, H.P., Fang, X., Chen, D.L., Yan, X.H., Zhu, F., Yang, T.Q., Liu, L.L., Hao, X.J., 2011. Phragmalin- and mexicanolide-type limonoids from the leaves of Trichilia connaroides. J. Nat. Prod. 74, 152–157. Zhou, Z.B., Luo, J.G., Pan, K., Shan, S.M., Zhang, W., Kong, L.Y., 2013. Bioactive benzofuran neolignans from Aristolochia fordiana. Planta Med. 79, 1730–1735.