Three New Ursane-Type Triterpenoids from the Stems of Saprosma merrillii
by
Dashuai Zhang
1,†, 
Wenhao Chen
1,†,
Wenxing Chen
2,
Xiaoping Song
1,
Changri Han
1,
Yan Wang
1 and
Guangying Chen
1,* 1
Key Laboratory of Tropical Medicinal Plant Chemistry of Ministry of Education, College of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158, Hainan, China
2
Jiangsu Key Laboratory for Pharmacology and Safety Evaluation of Chinese Materia Medica, Nanjing University of Chinese Medicine, Nanjing 210046, Jiangsu, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Molecules 2013, 18(12), 14496-14504; https://doi.org/10.3390/molecules181214496
Submission received: 25 September 2013
/
Revised: 13 November 2013
/
Accepted: 14 November 2013
/
Published: 25 November 2013
(This article belongs to the Section Natural Products Chemistry)
Abstract
:Three new ursane-type triterpenoids, 3α,6α,30-trihydroxy-ursan-28-oic acid (1), 3α,30-dihydroxy-6-oxo-ursan-28-oic acid (2) and 3α,6α,7α,30-tetrahydroxy-ursan-28-oic acid (3), together with one known triterpenoid, betulinic acid (4), one known anthraquinone, 1,7-dihydroxy-2-methylanthraquinone (5), four known phenols, 1,3,5-trimethoxybenzene (6), p-hydroxybenzoic acid (7), syringic acid (8), isovanillin (9), two steroids, sitosterol (10) and daucosterol (11), were isolated from the ethanol extract of the stems of S. merrillii. Their structures were elucidated on the basis of physical and spectral techniques, besides comparison with literature data. Compounds 1–3 showed inhibitory activities against the A549, HEPG2, and B16F10 cell lines.
1. Introduction
The genus Saprosma belongs to the family Rubiaceous which has about 50 species found around the world, and has been used as a traditional medicine for the treatment of fever and lumbocrural pain [1,2,3,4,5,6]. Iridoid glycosides [1,2,7], sulfur-containing iridoid glycosides [1,7,8,9], anthraquinones [3,7,9], and alkaloids [4] have been reported from this genus, and some of these compounds showed good anti-inflammatory, antibacterial and antitumor effect [1,2,3,4].
S. merrillii is an endemic plant in Hainan Island whose chemical constituents have not been investigated previously. As part of our continuing study on bioactive components from tropical medicinal plants from Hainan, the stems of S. merrillii were investigated. The EtOAc soluble fraction of the EtOH extract of S. merrillii, which showed cytotoxic activity against the A549 cell line, with an IC50 value of 65.66 μg/mL, was further purified by column chromatography to afford three new ursane-type triterpenoids 1–3 (Figure 1) and eight known compounds 4–11. Here, we wish to report on the isolation and structural elucidation of these compounds. Compounds 1–3 were evaluated for their cytotoxicity against the A549, MDA-MB-231, HEPG2 and B16F10 tumor cell lines.
Figure 1.
Structures of compounds 1–11.
2. Results and Discussion
The air-dried stems of S. merrillii were extracted three times with 80% EtOH at room temperature. The EtOH extract of the plant was suspended in water and then partitioned successively with petroleum ether and EtOAc. Column chromatography (CC) of the EtOAc-soluble fraction yielded three new compounds 1–3 and eight known compounds 4–11.
Compound 1 was isolated as a white amorphous solid. Its molecular formula was assigned to be C30H50O5 based on the HRESIMS (m/z: 513.3559 [M+Na]+, calcd. for C30H49O5Na, 513.3551) indicating six degrees of unsaturation. The IR spectrum showed absorption bands for hydroxyl (3,432 cm–1), carbonyl (1,704 cm–1) and C-O (1,040 cm–1) functions, respectively.
| NO. | 1 a | 2 b | 3 b | |||
|---|---|---|---|---|---|---|
| δC | δH | δC | δH | δC | δH | |
| 1 | 38.3 t | 1.54 (1H, m, H-1β), 1.63 (1H, m, H-1α) | 34.5 t | 1.47 (1H, m, H-1β), 1.65 (1H, m, H-1α) | 34.2 t | 1.64 (1H, m, H-1β), 1.83 (1H, m, H-1α) |
| 2 | 26.4 t | 1.51 (1H, m, H-1α), 2.05 (1H, m, H-1β) | 26.0 t | 1.56 (1H, m, H-1α), 1.92 (1H, m, H-1β) | 26.6 t | 1.50 (1H, m, H-1α), 2.04 (1H, m, H-1β) |
| 3 | 78.5 d | 3.26 (1H, t, 2.4) | 75.7 d | 3.19 (1H, br s) | 77.0 d | 3.28 (1H, t, 2.6) |
| 4 | 39.4 s | 37.2 s | 39.1 s | |||
| 5 | 50.2 d | 1.31 (1H, br.s) | 60.0 d | 2.77 (1H, s) | 48.7 d | 1.41 (1H, br.s) |
| 6 | 69.3 d | 4.34 (1H, m) | 213.7 s | 73.9 d | 4.11 (m) | |
| 7 | 42.9 t | 1.50 (1H, m, H-1α), 1.63 (1H, m, H-1β) | 52.8 t | 1.80 (1H, m, H-1α), 2.57 (1H, d, 11.6 H-1β) | 74.6 d | 3.62 (1H, dd, 4.4, 4.8) |
| 8 | 41.3 s | 48.2 s | 47.0 s | |||
| 9 | 49.8 d | 1.63 (1H, m) | 49.3 d | 1.73 (1H, m) | 49.2 d | 1.70 (1H, m) |
| 10 | 38.0 s | 44.3 s | 37.8 s | |||
| 11 | 22.1 t | 1.43 (1H, m, H-1α), 1.61 (1H, m, H-1β) | 22.1 t | 1.41 (1H, m, H-1α), 1.66 (1H, m, H-1β) | 21.7 t | 1.39 (1H, m, H-1α), 1.64 (1H, m, H-1β) |
| 12 | 33.2 t | 1.37 (1H, m, H-1β) 2.22 (1H, dt, 12.6, 2.8 H-1α) | 32.6 t | 1.48 (1H, m, H-1β), 2.24 (1H, m, H-1α) | 33.1 t | 1.35 (1H, m, H-1β), 2.20 (1H, dt, 12.8, 3.6 H-1α) |
| 13 | 38.8 d | 2.41 (1H, dt, 12.2, 3.6) | 39.1 d | 2.37 (1H, m) | 38.9 d | 2.41 (1H, dt, 12.8, 3.6) |
| 14 | 43.9 s | 43.8 s | 44.8 s | |||
| 15 | 31.0 t | 1.17 (1H, m, H-1β), 1.56 (1H, m, H-1α) | 30.4 t | 1.02 (1H, m, H-1β), 1.48 (1H, m, H-1α) | 36.6 t | 1.16 (1H, m, H-1β), 1.34 (1H, m, H-1α) |
| 16 | 28.6 t | 1.34 (1H, m, H-1α), 1.66 (1H, m, H-1β) | 27.6 t | 1.43 (1H, m, H-1α), 1.69 (1H, m, H-1β) | 28.5 t | 1.28 (1H, m, H-1α), 1.66 (1H, m, H-1β) |
| 17 | 57.9 s | 57.0 s | 57.0 s | |||
| 18 | 51.9 d | 1.54 (1H, m) | 51.0 d | 2.08 (1H, t, 4.0) | 51.6 d | 1.40 (1H, m) |
| 19 | 39.5 d | 1.83 (1H, m) | 38.8 d | 1.86 (1H, m) | 39.3 d | 1.87 (1H, m) |
| 20 | 44.6 d | 2.31 (1H, tt, 10.8, 3.0) | 44.2 d | 2.29 (1H, m) | 44.4 d | 2.30 (1H, m) |
| 21 | 24.7 t | 1.31 (1H, m, H-1α), 1.51 (1H, m, H-1β), | 24.4 t | 1.39 (1H, m, H-1α), 1.55 (1H, m, H-1β) | 24.4 t | 1.32 (1H, m, H-1α), 1.54 (1H, m, H-1β) |
| 22 | 36.8 t | 1.30 (1H, m, H-1α), 1.37 (1H, m, H-1β) | 37.7 t | 1.34 (1H, m, H-1α), 1.37 (1H, m, H-1β) | 37.9 t | 1.32 (1H, m, H-1α), 1.35 (1H, m, H-1β) |
| 23 | 28.9 q | 0.98 (3H, s) | 27.4 q | 0.94 (3H, s) | 28.7 q | 0.98 (3H, s) |
| 24 | 24.8 q | 1.20 (3H, s) | 22.6 q | 1.19 (3H, s) | 25.0 q | 1.22 (3H, s) |
| 25 | 15.4 q | 0.99 (3H, s) | 17.4 q | 0.86 (3H, s) | 15.6 q | 1.06 (3H, s) |
| 26 | 17.4 q | 1.28 (3H, s) | 16.5 q | 0.93 (3H, s) | 11.0 q | 1.21 (3H, s) |
| 27 | 18.0 q | 1.23 (3H, s) | 15.1 q | 1.14 (3H, s) | 17.8 q | 1.22 (3H, s) |
| 28 | 180.2 s | 177.7 s | 177.8 s | |||
| 29 | 18.8 q | 0.96 (3H, d, 6.8) | 18.8 q | 0.93 (3H, d, 7.0) | 18.8 q | 0.94 (3H, d, 6.8) |
| 30 | 64.2 t | 3.33 (1H, m, H-1β), 3.75 (1H, dd, 10.6, 4.4, H-1α) | 63.7 t | 3.37 (1H, dd, 10.0, 8.4, H-1β), 3.76 (1H, dd, 10.0, 4.6, H-1α) | 63.8 t | 3.34 (1H, m, H-1α), 3.76 (1H,10.2, 4.6, H-1β) |
The 1H-NMR spectrum of 1 showed the following signals: five tertiary methyl groups at δH 1.28, 1.23, 1.20, 0.99 and 0.98 (each 3H, s), a secondary methyl at δH 0.96 (3H, d, J = 6.8Hz, H-29), one oxygenated methylene protons δH 3.75 (1H, dd, J = 10.6, 4.4 Hz, H-30b), and 3.33 (1H, m, H-30a), two oxygenated methine protons δH 4.34 (1H, m, H-6), 3.26 (1H, t, J = 2.4 Hz, H-3) and a series of overlapped signals suggesting an ursane-type triterpenoid. The 13C-NMR spectrum revealed 30 carbon signals, which were further classified by DEPT and HSQC experiments as six methyls, 10 methylenes (one oxygenated), eight methines (two oxygenated), six quaternary carbons (including one carboxyl group) (Table 1). The aforementioned data implied that 1 was an ursane-28-oic-acid with three hydroxyls [10,11,12,13,14,15].
In the HMBC spectrum, the oxygenated methylene protons δH 3.75, 3.33 (H2-30) showed correlations to C-19 (δC 39.5), C-20 (δC 44.6) and C-21 (δC 24.7), indicating that one hydroxyl was linked at C-30. Moreover, two oxygenated methine protons δH 4.34 (H-6) and δH 3.26 (H-3), which were deduced by the HMBC correlations from the proton δH 4.34 to C-8 (δC 41.3), C-10 (δC 38.0) and from δH 3.26 to C-5 (δC 50.2), C-23 (δC 28.9) and C-24 (δC 24.8). These observations, together with the HMBC correlations between δH 1.34, 1.66 (H2-16) and 1.30, 1.37 (H2-22) and the carboxyl group δC 180.2 (C-28), were used to establish the molecular framework of 1 (Figure 2).
Figure 2.
Key 1H-1H COSY (bold bonds) and HMBC correlations (arrows) of 1–3.
The relative configuration of 1 was established by analysis of the key correlations displayed in the NOESY spectrum (Figure 3). The key correlation from δH 3.26 (H-3) to 1.20 (H3-24), proves hydrogen of C-3 is β-orientation. δH 4.34 (H-6) showed correlations to δH 0.99 (H3-25) and 1.28 (H3-26), proves hydrogen of C-6 is β-orientation. On the basis of the above discussion, the relative configuration of 1 was established as shown (Figure 1). Thus, compound 1 was determined to be 3α,6α,30-trihydroxyursan-28-oic acid.
Figure 3.
Key NOESY correlations of 1.
Compound 2 was isolated as a white amorphous solid. Its molecular formula was assigned to be C30H48O5 based on the HRESIMS, which showed a unit of two hydrogen atoms less than that of compound 1. Its NMR spectra (Table 1 and Experimental Section) were similar to those of 1, except for one oxygenated methine δH 4.34 (1H, m, H-6), δC 69.3 (C-6) to the ketone carbon at δC 213.7 (C-6) in 2. This was supported by the HMBC correlations between δH 2.77 (H-5), 1.80 and 2.57 (H2-7) and the carbonyl group (Figure 2). The relative configuration of compound 2 was obtained from the NOESY spectrum (Figure 4), the cross-peak between δH 3.19 (H-3) and 1.19 (H-24) indicated that the configuration of 2 was the same as that shown above for 1, in relevant parts of the molecule. Finally, the structure of 2 was determined to be 3α,30-dihydroxy-6-oxo-ursan-28-oic acid.
Figure 4.
Key NOESY correlations of 2.
Compound 3 was isolated as a white amorphous solid. Its molecular formula was assigned to be C30H50O6 based on the HRESIMS, which showed a unit of one oxygen atom more than that of compound 1. Its NMR spectra (Table 1 and Experimental Section) were similar to those of 1, except for one more oxygenated methine δH 3.62 (1H, t, J = 4.8 Hz, H-7) that showed correlation to δH 4.11 (H-6) in the 1H-1H COSY spectrum fixed its position. In the NOESY spectrum (Figure 5), the key correlations from δH 4.11 (H-6) to 1.06 (H3-25), δH 3.62 (H-7) to 1.21 (H3-26), proves hydrogens of C-6 and C-7 are both β-orientation. Thus, the structure of compound 3 was elucidated as 3α,6α,7α,30-tetrahydroxy-ursan-28-oic acid.
Figure 5.
Key NOESY correlations of compound 3.
Compounds 1–3 were evaluated for their anti-proliferative activities against the human lung cancer A549, human breast cancer MDA-MB-231, human hepatocellular carcinoma HEPG2 and mouse melanoma B16F10 cancer cell lines using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay method in vitro [16]. Compound 1 exhibited weak cytotoxicity against the B16F10 cell line, with an IC50 value of 72.72 μM, while 2 inhibited the proliferation of the A549 cell line with an IC50 value of 24.66 μM (Table 2).
| Compounds | IC50 (μM) | |||
|---|---|---|---|---|
| A549 | MDA-MB-231 | HEPG2 | B16F10 | |
| 1 | N | N | N | 72.72 |
| 2 | 24.66 | N | 127.80 | N |
| 3 | 129.22 | N | N | N |
N: means inactive with IC50 values above 150 μM.
3. Experimental
3.1. General
Optical rotations were measured with PolAAr 3005 polarimeter (Optical Activity Limited, Cambridgeshire, UK). IR spectra were recorded on a Thermo Nicolet 6700 (using KBr disks) spectrophotometer (Thermo Scientific, Madison, WI, USA). NMR spectra were acquired on a Bruker AV (400 MHz for 1H-NMR, ppm relative to TMS) spectrometer (Bruker, Hesse-Darmstadt, Germany). ESIMS spectra were recorded on an Agilent 1200 series HPLC (Agilent Technologies, Waldbroon, Germany) interfaced to an Bruker Esquire 6000 Ion Trap mass spectrometer equipped with an electrospray ionization source, and HRESIMS spectra were made on the Bruker Daltonics Apex-Ultra 7.0 T (Bruker Corporation, Billerica, MA, USA), respectively. Column chromatography of silica gel (200–300 mesh), all solvents for column chromatography were of analytical grade (Xilong Chemical Reagents Company, Ltd., Guangdong, China). Spots of compounds on TLC were visualized by spraying using 10% H2SO4 in EtOH (v/v) followed by heating.
3.2. Plant Material
S. merrillii was collected from Sanya, Hainan Province, China, in August 2009 and was identified by Professor Qiongxin Zhong of School of Life Science, Hainan Normal University. A voucher specimen (No. 200908QDRMS) was deposited at the Department of Key Laboratory of Tropical Medicinal Plant Chemistry of Ministry of Education, Hainan Normal University.
3.3. Extraction and Isolation
The stems of S. merrillii was air-dried (9.8 kg) and extracted three times with 80% EtOH (60 L × 3 d each) at room temperature. The EtOH extract (380 g) was suspended in water and then partitioned successively with petroleum ether and EtOAc. The EtOAc-soluble portion (60 g) was subject to Si gel CC (200–300 mesh), eluting with a gradient of mixtures of petroleum ether/EtOAc/MeOH (from 1:0:0 to 0:0:1) to give 10 major fractions (Frl–Fr10) according to TLC analysis. Frl (1.1 g) was recrystallized from EtOAc to afford 10 (0.5 g). Fr2 (0.2 g) was chromatographed on a Si gel column eluted with petroleum ether/EtOAc (from 1:0 to 1:1) to afford 4 (1.5 mg). Fr3 (0.4 g) was chromatographed on a Si gel column eluted with petroleum ether/EtOAc (from 1:0 to 1:2) to afford 5 (2.5 mg) and 6 (2.0 mg). Fr4 (0.5 g) was chromatographed on a silica gel column eluted with petroleum ether/EtOAc (from 1:0 to 1:2) to afford eight major subfractions (Fr4a–Fr4h). Fr4a (0.3 g) was further purified by Si gel column chromatography (petroleum ether/EtOAc, from 1:0 to 1:2) to obtain 7 (3.0 mg). Fr5 (0.6 g) was chromatographed on Si gel column eluted with petroleum ether/acetone (from 2:1 to 0:1) to afford 8 (3.5 mg). Fr6 (0.5 g) was chromatographed on Si gel column eluted with petroleum ether/CHC13/EtOAc (4:2:1) to afford six major subfractions, Fr6a–Fr6f. Fr6b (0.2 g) was further purified by Si gel (petroleum ether/CHC13/EtOAc, 2:4:1) to obtain 9 (2.0 mg). Fr7 (1.2 g) was chromatographed on Si gel column eluted with petroleum ether/EtOAc (from 1:0 to 0:1), recrystallized from MeOH to afford 1 (53 mg). Fr8 (1.2 g) was chromatographed on Si gel column eluted with petroleum ether/EtOAc (from 1:0 to 0:1), recrystallized from acetone to afford 2 (15 mg). Fr9 (1.2 g) was chromatographed on Si gel column eluted with petroleum ether/EtOAc (from 1:0 to 0:1), recrystallized from acetone to afford 3 (10.0 mg). Fr10 (5.0 g) was recrystallized from MeOH to afford 11 (1.1 g).
3.4. Characterization of Compounds 1–3
Compound 1: white amorphous solid; ![Molecules 18 14496 i001]()
−108.33 (c 0.0024, MeOH); IR (KBr) υmax 3432, 2949, 2873, 1704, 1641, 1459, 1383, 1278, 1181, 1040, 990 cm–1; 1H- and 13C-NMR data see Table 1; ESIMS m/z 489.4 [M−H]−, 513.3 [M+Na]+; HRESIMS m/z: 513.3559 [M+Na]+ (calcd for C30H49O5Na, 513.3551).
−108.33 (c 0.0024, MeOH); IR (KBr) υmax 3432, 2949, 2873, 1704, 1641, 1459, 1383, 1278, 1181, 1040, 990 cm–1; 1H- and 13C-NMR data see Table 1; ESIMS m/z 489.4 [M−H]−, 513.3 [M+Na]+; HRESIMS m/z: 513.3559 [M+Na]+ (calcd for C30H49O5Na, 513.3551).Compound 2: white amorphous solid; ![Molecules 18 14496 i001]()
−20.73 (c 0.0019, MeOH); IR (KBr) υmax 3440, 2948, 2873, 1704, 1636, 1458, 1387, 1174, 1070, 1032, 986, 928 cm–1; 1H- and 13C-NMR data see Table 1; ESIMS m/z: 487.6 [M−H]−, 511.5 [M+Na]+; HRESIMS m/z: 511.3389 [M+Na]+ (calcd for C30H48O5Na, 511.3394).
−20.73 (c 0.0019, MeOH); IR (KBr) υmax 3440, 2948, 2873, 1704, 1636, 1458, 1387, 1174, 1070, 1032, 986, 928 cm–1; 1H- and 13C-NMR data see Table 1; ESIMS m/z: 487.6 [M−H]−, 511.5 [M+Na]+; HRESIMS m/z: 511.3389 [M+Na]+ (calcd for C30H48O5Na, 511.3394).Compound 3: white amorphous solid; ![Molecules 18 14496 i001]()
–37.84 (c 0.0037, MeOH); IR (KBr) υmax 3442, 2951, 2875, 1704, 1647, 1458, 1384, 1229, 1176, 1110, 1035, 920 cm–1; 1H- and 13C-NMR data see Table 1; ESIMS m/z: 505.7 [M−H]−, 529.5 [M+Na]+; HRESIMS m/z: 529.3501 [M+Na]+ (calcd for C30H50O6Na, 529.3499).
–37.84 (c 0.0037, MeOH); IR (KBr) υmax 3442, 2951, 2875, 1704, 1647, 1458, 1384, 1229, 1176, 1110, 1035, 920 cm–1; 1H- and 13C-NMR data see Table 1; ESIMS m/z: 505.7 [M−H]−, 529.5 [M+Na]+; HRESIMS m/z: 529.3501 [M+Na]+ (calcd for C30H50O6Na, 529.3499).3.5. Cytotoxicity Assays
Test compounds 1–3 were dissolved in DMSO (final concentration, 0.1%). The cytotoxicity of compounds 1–3 against A549, MDA-MB-231, HEPG2 and B16F10 was determined by standard MTS assay [16]. Cells dispersed evenly in medium and were seeded in a 96-well plate with a density of 1 × 104 cells/well. Next day, cells were treated with various concentrations of samples (0–100 μM) for 48 h or treated with indicated concentrations for 48 h with six replicates of each treatment. After incubation, each well was added 20 μL of MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-car-boxymethoxy-phenyl)-2-(4-sulfophenyl)-2H-tetrazolium] reagent and incubated for 3 h. Cell viability was determined by measuring the optical density at 490 nm using a Biotek microplate reader (Biotek, Winooski, VT, USA). Untreated cells in medium were used as control. Corresponding groups without cells were used as blanks. All experiments were carried out with four replicates.
4. Conclusions
In conclusion, our phytochemical investigation of the stems of S. merrillii afforded three new triterpenoids: 3α,6α,30-trihydroxy-ursan-28-oic acid (1), 3α,30-dihydroxy-6-oxo-ursan-28-oic acid (2), 3α,6α,7α,30-tetrahydroxy-ursan-28-oic acid (3), together with one known triterpenoid, betulinic acid (4) [17], one known anthraquinone, 1,7-dihydroxy-2-methylanthraquinone (5) [18], four known phenols, 1,3,5-trimethoxybenzen (6) [19], p-hydroxybenzoic acid (7) [20], syringic acid (8) [21], isovanillin (9) [22], two steroids, sitosterol (10) and daucosterol (11). Compound 1 exhibited weak cytotoxicity against the B16F10 cell line with an IC50 value of 72.72 μM, while 2 inhibited the proliferation of the A549 cell line with an IC50 value of 24.66 μM (Table 2).
Acknowledgments
This work was supported financially by the National Natural Science Foundation of China (No. 21162009, 31360069) and 973 Program, Ministry of Science and Technology of China (Grant No. 2011CB512010).
Conflicts of Interest
The authors declare no conflict of interest.
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MDPI and ACS Style
Zhang, D.; Chen, W.; Chen, W.; Song, X.; Han, C.; Wang, Y.; Chen, G. Three New Ursane-Type Triterpenoids from the Stems of Saprosma merrillii. Molecules 2013, 18, 14496-14504. https://doi.org/10.3390/molecules181214496
AMA Style
Zhang D, Chen W, Chen W, Song X, Han C, Wang Y, Chen G. Three New Ursane-Type Triterpenoids from the Stems of Saprosma merrillii. Molecules. 2013; 18(12):14496-14504. https://doi.org/10.3390/molecules181214496
Chicago/Turabian StyleZhang, Dashuai, Wenhao Chen, Wenxing Chen, Xiaoping Song, Changri Han, Yan Wang, and Guangying Chen. 2013. "Three New Ursane-Type Triterpenoids from the Stems of Saprosma merrillii" Molecules 18, no. 12: 14496-14504. https://doi.org/10.3390/molecules181214496
APA StyleZhang, D., Chen, W., Chen, W., Song, X., Han, C., Wang, Y., & Chen, G. (2013). Three New Ursane-Type Triterpenoids from the Stems of Saprosma merrillii. Molecules, 18(12), 14496-14504. https://doi.org/10.3390/molecules181214496
