Two triterpenoid glycosides from the roots of Camellia oleifera and their cytotoxic activity

Article abstract of DOI:10.1080/10286020.2015.1004630

Two new triterpenoid saponins, oleiferoside N and oleiferoside O, were isolated from the EtOH extract from the roots of Camellia oleifera C. Abel. Their structures were elucidated as 16α-acetoxy-21β,22α-O-diangeloyloxy-23,28-dihydroxyolean-12-ene 3β-O-β-D-xylopyranosyl-(1 → 3)-α-L-arabinopyranosyl-(1 → 3)-β-D-glucuronopyranoside (1), 16α-acetoxy-21β-O-angeloyloxy-23,28-dihydroxy-22α-O-(2-methylbutanoyloxy)olean-12-ene 3β-O-β-D-xylopyranosyl-(1 → 3)-α-L-arabinopyranosyl-(1 → 3)-β-D-glucuronopyranoside (2), on the basis of spectroscopic analyses (IR, ESI-MS, HR-ESI-MS, 1D and 2D NMR) and acid hydrolysis. Both were characterized to be oleanane-type saponins with sugar moieties linked to C-3 of the aglycone. Cytotoxic activities of two saponins were evaluated against four human tumor cell lines (A549, B16, BEL-7402, and MCF-7) by using the MTT in vitro assay. Compounds 1 and 2 showed moderate cytotoxic activities toward the tested cell lines.

Full text of DOI:10.1080/10286020.2015.1004630

This article was downloaded by: [New York University]
On: 10 July 2015, At: 06:42
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: 5 Howick Place, London, SW1P 1WG
Journal of Asian Natural Products
Research
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/ganp20
Two triterpenoid glycosides from the
roots of Camellia oleifera and their
cytotoxic activity
Ping Yanga, Xia Lia, Yan-Li Liua, Qiong-Ming Xua, Yun-Qiu Lib & Shi-
Lin Yangab
a College of Pharmaceutical Science, Soochow University, Suzhou
215123, China
b College of Pharmaceutical Science, Guilin Medical University,
Guilin 541004, China
Published online: 28 Jan 2015.
Click for updates
To cite this article: Ping Yang, Xia Li, Yan-Li Liu, Qiong-Ming Xu, Yun-Qiu Li & Shi-Lin Yang (2015):
Two triterpenoid glycosides from the roots of Camellia oleifera and their cytotoxic activity, Journal
of Asian Natural Products Research, DOI: 10.1080/10286020.2015.1004630
To link to this article: http://dx.doi.org/10.1080/10286020.2015.1004630
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Journal of Asian Natural Products Research, 2015
http://dx.doi.org/10.1080/10286020.2015.1004630
Two triterpenoid glycosides from the roots of Camellia oleifera and
their cytotoxic activity
Ping Yanga, Xia Lia, Yan-Li Liua, Qiong-Ming Xua*, Yun-Qiu Lib* and Shi-Lin Yanga,b
b
aCollege of Pharmaceutical Science, Soochow University, Suzhou 215123, China; College of
Pharmaceutical Science, Guilin Medical University, Guilin 541004, China
(Received 12 October 2014; final version received 3 January 2015)
Two new triterpenoid saponins, oleiferoside N and oleiferoside O, were isolated from
the EtOH extract from the roots of Camellia oleifera C. Abel. Their structures were
elucidated as 16a-acetoxy-21b,22a-O-diangeloyloxy-23,28-dihydroxyolean-12-ene
3b-O-b-D-xylopyranosyl-(1 ! 3)-a-L-arabinopyranosyl-(1 ! 3)-b-D-glucuronopyra-
noside (1), 16a-acetoxy-21b-O-angeloyloxy-23,28-dihydroxy-22a-O-(2-methylbuta-
noyloxy)olean-12-ene 3b-O-b-D-xylopyranosyl-(1 ! 3)-a-L-arabinopyranosyl-
(1 ! 3)-b-D-glucuronopyranoside (2), on the basis of spectroscopic analyses (IR,
ESI-MS, HR-ESI-MS, 1D and 2D NMR) and acid hydrolysis. Both were characterized
to be oleanane-type saponins with sugar moieties linked to C-3 of the aglycone.
Cytotoxic activities of two saponins were evaluated against four human tumor cell lines
(A549, B16, BEL-7402, and MCF-7) by using the MTT in vitro assay. Compounds 1
and 2 showed moderate cytotoxic activities toward the tested cell lines.
Keywords: Camellia oleifera; Theaceae; triterpenoid saponins; oleiferosides NZO;
cytotoxic activities
1. Introduction
for the extract of the roots of C. oleifera in
our previous investigation [5]. Up to now,
22 triterpenoids have been identified from
the root extract, and many of them showed
potent cytotoxic activities [5,6]. Here,
we describe the isolation and structural
elucidation of two new triterpenoid sapo-
nins (1 and 2) (Figure 1). Their cytotoxic
activities against A549, B16, BEL-7402,
and MCF-7 human tumor cell lines were
evaluated by using 3-(4,5-dimethylthiazol-
2-yl)-2,5-diphenyltetrazolium bromide
(MTT) assay.
Camellia (Theaceae family) species are
widely distributed in China, Japan, India,
and many other South-East Asian
countries [1] and some are used as folk
medicines. For example, the roots of
Camellia oleifera are traditionally used
for the treatment of common cold,
bovillae, ardent fever, urinary tract infec-
tion, nephritis, edema, and threatened
abortion [2] due to their antimicrobial
[3], antioxidant and antitumor activities
[4]. According to the traditional Chinese
medicine theory, the roots, which are bitter
and cool-natured, can invigorate lung and
kidney, and can expel “wind” and “heat”.
Significant cytotoxic activities against
A549 human lung adenocarcinoma, B16
mouse melanoma, BEL-7402 human
hepatocarcinoma, and MCF-7 human
breast carcinoma cell lines were found
2. Results and discussion
Compound 1 was obtained as a white
amorphous powder. The negative-ion HR-
ESI-MS of 1 exhibited a quasimolecular
ion peak at m/z 1151.5568 [M–H]2,
indicating the molecular formula of
*Corresponding authors. Emails: xuqiongming@suda.edu.cn; leeyq88@126.com
q 2015 Taylor & Francis

2
P. Yang et al.
4'
3'
O
2'
1'
5'
29
20
30
18
O
4"
3"
19
H
H
21
22
O
H
12
1"
2"
5"
O
11
O
OH
17
26 13
14
6
3
25
OH
Ara
HO
4
28
GlcA
O
1
Xyl
4
4
15
16
5
2
9
6
5
O
O
HO
HO
HO
O
8
10
5
O
1
5
3
1
O
O
27
2
2
2
3
3
7
1
4
OH
OH
OH
2'"
1'"
O
24
H
CH2OH
23
4'
3'
O
2'
1'
5'
29
30
18
O
20
4"
3"
19
H
H
21
22
O
H
12
1"
2"
5"
O
11
25
O
OH
17
26 13
14
6
3
OH
Ara
HO
4
28
GlcA
O
1
Xyl
4
4
15
16
5
2
9
5
O
O
HO
HO
HMBC
HO
O
8
10
O
1
5
3
1
O
O
27
7
2
2
2
3
5
3
1
6
4
OH
OH
OH
2'"
1'"
O
NOESY
24
H
CH2OH
23
Figure 1. Key HMBC and NOESY correlations of compounds 1 and 2.
C58H88O23. The IR spectrum indicated the
presence of hydroxyl group (3456 cm21),
CvO group (1720 cm21) and a,b-unsatu-
rated ester group (1601, 1602 cm21). The
13C NMR (Table 1) spectrum showed the
resonances of 58 carbons, ascribable to 11
methyl, 11 methylene, 23 methine, and 13
quaternary carbons as revealed by the
HSQC experiment. The 1H NMR (Table 1)
spectrum showed six methyl proton
signals at d 0.87 (Me-26), 0.96 (Me-25),
1.00 (Me-24), 1.14 (Me-29), 1.37 (Me-30),
and 1.47 (Me-27), two isolated oxymethy-
lene proton signals at d 3.53 (1H, d,
J ¼ 10.0 Hz, H-28), 3.70 (1H, d,
J ¼ 10.0 Hz, H-28), as well as 3.75
(1H, d, J ¼ 11.0 Hz, H-23), 4.35 (1H, d,
J ¼ 11.0 Hz, H-23), four oxymethine pro-
ton signals at d 4.32 (1H, dd, J ¼ 11.0,
4.5 Hz, H-3), 5.59 (1H, br s, H-16), 6.06
(1H, d, J ¼ 10.5 Hz, H-21), and 6.36 (1H,
d, J ¼ 10.5 Hz, H-22), and one olefinic
proton signal at d 5.50 (1H, br s, H-12).
Furthermore, the proton signals of two
angeloyl (Ang) groups at d [6.05 (1H, d q,
J ¼ 7.5, 1.5 Hz, 21-O-Ang-30), 2.18 (3H,
d, J ¼ 7.5 Hz, 21-O-Ang-40), and 2.04
(3H, s, 21-O-Ang-50)]; d [6.01 (1H, d q,
J ¼ 7.5, 1.5 Hz, 22-O-Ang-300), 2.10 (3H,
d, J ¼ 7.5 Hz, 22-O-Ang-400), and 2.10
(3H, s, 22-O-Ang-500)] were observed. The
NMR spectroscopic data of compound 1
were similar to those reported for oleifero-
side G published in the literature [5],
except for the presence of signals at dC
107.0, 73.6, 74.5, 69.5, and 67.8, corre-
sponding to one xylopyranosyl group in
place of the rhamnopyranosyl group in
oleiferoside G. Besides, there was no
signal at dC 52.0 in compound 1, which

Journal of Asian Natural Products Research
3
d
d
d
d
d
d
d
d
d

4
P. Yang et al.
d
d
d
d
d
d
d
d

Journal of Asian Natural Products Research
5
was ascribed to the OCH3 group located
at C-6 of glucuronopyranosyl group in
oleiferoside G [5]. The anomeric proton
signals at d 5.20 (1H, d, J ¼ 7.0 Hz), 5.45
(1H, d, J ¼ 6.5 Hz), and 5.02 (1H, d,
J ¼ 7.5 Hz), which showed the HSQC
correlations to the carbons at d 105.0,
103.2, and 107.0, respectively, indicated
the presence of three sugar residues. The
acid hydrolysis and GC analysis confirmed
this fact. Acid hydrolysis of compound 1
and GC analysis revealed one unit of
D-glucuronic acid, one unit of L-arabinose,
and one unit of D-xylose. The HMBC
correlations between H-1 of glucuronic
acid at dH 5.20 and C-3 of aglycone at dC
81.9, between H-1 of arabinose at dH 5.45
and C-3 of glucuronic acid at dC 86.7,
between H-1 of xylose at dH 5.02 and C-3
of arabinose at dC 81.6, indicated the
glycosidic chain was located at C-3 of
aglycone and the sugar units were
connected as b-D-xylopyranosyl-(1 ! 3)-
a-L-arabinopyranosyl-(1 ! 3)-b-D-glu-
curonopyranosyl. The b-anomeric con-
figuration of the glucuronopyranosyl unit
and the xylopyranosyl unit was determined
group (1601 cm21). The NMR spectro-
scopic data of compound 2 were similar
to those of compound 1, except that the
olefinic carbons were replaced by the sp3
carbons, which suggested that an MB
group instead of an Ang group was
attached at C-22. The assignment was
supported by the HMBC correlations
between H-22 at dH 6.29 and C-100 of MB
at dC 176.1, between H-400 of MB at dH
0.98 and C-200 of MB at dC 41.9, C-300 of
MB at dC 26.9, between H-500 of MB at dH
1.26 and C-100 of MB at dC 176.1, C-200 of
MB at dC 41.9, C-300 of MB at dC 26.9.
Further analyses of the NMR data led to
the structure assignment of compound 2 as
16a-acetoxy-21b-O-angeloyloxy-23,28-
dihydroxy-22a-O-(2-methylbutanoyloxy)
olean-12-ene 3b-O-b-D-xylopyranosyl-
(1 ! 3)-a-L-arabinopyranosyl-(1 ! 3)-b-
D-glucuronopyranoside, and named as
oleiferoside O.
In conclusion, two new compounds
were isolated and identified from 70%
EtOH of C. oleifera. Both of them
possessed an olean-12-ene skeleton and
oligosaccharidic chains were linked to
C-3. Cytotoxic activities of these saponins
against four human tumor cell lines (A549,
B16, BEL-7402, and MCF-7) were eval-
uated by using the MTT in vitro assay.
It was found that both compounds had
moderate cytotoxic activity against A549
(human lung adenocarcinoma), B16
(mouse melanoma), BEL-7402 (human
hepatocarcinoma), and MCF-7 (human
breast carcinoma) (Table 2).
3
from the observation of the large JH-1,H-2
coupling constant, and the a-anomeric
configuration of the arabinopyranosyl unit
was assigned according to the NOESY
correlations between H-1 of arabinose at d
5.45 and H-3 of arabinose at d 4.32
(Figure 1). Thus, the structure of compound
1 was elucidated as 16a-acetoxy-21b,22a-
O-diangeloyloxy-23,28-dihydroxyolean-
12-ene 3b-O-b-D-xylopyranosyl-(1 ! 3)-
a-L-arabinopyranosyl-(1 ! 3)-b-D-glu-
curonopyranoside, and named as oleifero-
side N.
3. Experimental
3.1 General experimental procedures
Compound 2 was obtained as a white
amorphous powder. Its negative-ion HR-
ESI-MS showed a quasimolecular ion
peak at m/z 1153.5834 [M–H]2, which
inferred the molecular formula of
C58H90O23. The IR spectrum indicated
the presence of hydroxyl groups
(3456 cm21), CvO group (1720,
1724 cm21) and a,b-unsaturated ester
The specific rotation values were deter-
mined with a Perkin-Elmer model 241
polarimeter (PerkinElmer, Inc., Waltham,
MA, USA). UV spectra were recorded on a
UV2401PC spectrophotometer (Shimadzu
Corp., Kyoto, Japan). IR spectra were
recorded on a Perkin-Elmer 983 G
spectrometer (PerkinElmer, Inc.). 1H, 13C,

6
P. Yang et al.
Table 2. In vitro cytotoxic activities of compounds 1 and 2.
IC50 (mM)
Compound
A549
B16
7402
MCF-7
1
81.41 ^ 0.89
116.68 ^ 6.03
4.16 ^ 0.28
83.17 ^ 13.75
132.79 ^ 11.46
3.93 ^ 0.22
88.54 ^ 6.78
132.34 ^ 1.37
3.45 ^ 0.26
77.93 ^ 6.52
117.21 ^ 18.53
5.17 ^ 0.38
2
Norcantharidin
and 2D NMR spectra were recorded on a
Varian Inova 500 spectrometer (Varian,
Inc., Palo Alto, CA, USA) in C5D5N using
tetramethylsilane as the internal standard.
High-resolution electrospray ionization
mass spectra were determined on a Micro-
mass Q-TOF2 spectrometer (Waters Corp.,
London, UK). HPLC analysis and purifi-
cation were performed with an Agilent
Corp. SB-C18 semi-preparative HPLC
column (250 £ 9.4 mm i.d., 5 mm, Agilent
Corp., Palo Alto, CA, USA) on a Shimadzu
HPLC system composed of an LC-20AT
pump with an SPD-20A detector
(Shimadzu Corp.), and the wavelength for
detection was 203 nm. Medium pressure
liquid chromatography (MPLC) purifi-
cation was performed on a Bu¨chi Flash
Chromatography system composed of a C-
650 pump with a flash column (460 mm
£ 26 mm i.d., Bu¨chi Corp., Flawil, Switzer-
land). Silica gel (200–300 mesh) for
column chromatography and precoated
silica gel TLC plates were purchased from
Qingdao Marine Chemical Factory
(Qingdao, China). ODS for MPLC was
purchased from Merck KGaA (Darmstadt,
Germany). Sephadex LH-20 for column
chromatography was purchased from GE
Corp. (Piscataway, NJ, USA). GC was
conducted on a GC-14C (Shimadzu Corp.)
with a flame ionization detector (FID).
Compounds on the TLC were colored by
10% sulfuric acid alcohol solution.
by Professor Xiao-Ran Li at Soochow
University. A voucher sample (No.11-11-
06-01) was deposited in the herbarium of
the College of Pharmacy, Soochow
University.
3.3 Extraction and isolation
The air-dried roots (9.3 kg) of C. oleifera
were crushed with a grinder into fine
debris, and extracted twice with 70%
EtOH (2 £ 50 l) at 808C under reflux. The
solvent was subsequently removed under
reduced pressure to yield a residue
(0.45 kg), which was then dissolved in
distilled water and passed through a D101
macroporous resin column (i.d. 30 cm
£ 200 cm, Xi’an Sunresin New Materials
Co. Ltd, Xi’an, China). The column was
eluted in gradient with aqueous EtOH
(EtOH/H2O 0%, 30%, 60%, 80%, each
40 l). The 60% EtOH eluate (42 g) was
further vacuum chromatographed on a
silica gel (60–100 mesh) column (30
£ 18 cm), eluted with a gradient of CHCl3/
MeOH (100:0, 90:10, 80:20, 70:30, 60:40,
50:50, 40:60, 30:70, 0:100, each 6.0 l). The
CHCl3/MeOH (70:30) eluate (3.2 g) was
chromatographed by MPLC over an ODS
column, eluted with MeOH/H2O (50:50,
60:40, 70:30, 80:20, 90:10, 100:0, each
500 ml) at a flow rate of 20.0 ml/min to
afford six fractions. Fraction 4 (500–
1000 ml, 68 mg) was separated by semi-
preparative HPLC over an ODS column,
eluted with MeOH/H2O (65:35) to yield
compounds 2 (12 mg, tR 60.3 min) and 1
(10 mg, tR 64.7 min). The purities of the
isolated saponins were .95%, as deter-
mined by HPLC.
3.2 Plant material
The roots of C. oleifera C. Abel were
collected in Qichun, Hubei Province of
China in November 2011, and identified

Journal of Asian Natural Products Research
7
3.3.1 16a-Acetoxy-21b,22a-O-
diangeloyloxy-23,28-dihydroxyolean-12-
ene 3b-O-b-D-xylopyranosyl-(1 ! 3)-a-
L-arabinopyranosyl-(1 ! 3)-b-D-
glucuronopyranoside (1)
eliminated under N2, and 0.2 ml trimethyl-
silylimidazole (Tokyo Chemical Industry
Co., Ltd, 99%) was added. Then the
mixture was heated at 608C for another 2 h
and partitioned between n-hexane and
water. The organic layer was investigated
by GC under the following conditions:
capillary column, HP-5 (30 m £ 0.25 mm
£ 0.25 mm, Dikma Technologies, Beijing,
China); FID detector with a temperature of
2808C; injection temperature 2508C; initial
temperature 1608C, then raised to 2808C at
58C/min, final temperature maintained
for 10 min; carrier gas, N2. The standard
sugars experienced the same reaction and
GC conditions. The retention times of D-
glucuronic acid, D-xylose, and L-arabinose
were 19.770, 32.257, and 33.284 min,
respectively.
20
White amorphous powder; ½aꢀD þ 6.4
(c ¼ 0.12, MeOH); UV (MeOH) lmax
(log 1) 211 (4.18), 254 (4.34) nm; IR
n
max (KBr): 3456 (OH), 2965, 2918, 2849,
1720 (CvO), 1602 (CvCZCvO), 1601
(CvCZCvO), 1441, 1382, 1168,
1040 cm21
;
1H NMR (pyridine-d5,
500 MHz) and 13C NMR (pyridine-d5,
125 MHz) spectral data were given in
Table 1; HR-ESI-MS: m/z 1151.5568 [M–
H]2 (calcd for C58H87O23, 1151.5638).
3.3.2 16a-Acetoxy-21b,22a-O-
diangeloyloxy-23,28-dihydroxyolean-12-
ene 3b-O-b-D-xylopyranosyl-(1 ! 3)-a-
L-arabinopyranosyl-(1 ! 3)-b-D-
glucuronopyranoside (2)
3.5 Cytotoxicity assay
The MTT colorimetric assay was per-
formed to evaluate the cytotoxic activities
of the isolated saponins against A549,
B16, BEL-7402, and MCF-7 human tumor
cell lines (Cell Bank, Shanghai Institutes
for Biological Sciences, Chinese Academy
of Sciences). The amount of formazan was
determined by photometry at 570 nm.
Cells were plated into 96-well flat-
bottomed culture plates at a concentration
of 2 £ 105 cells per well in complete RPMI
(serum-free cell freezing medium) 1640
culture medium. After 24 h, the medium
containing fetal calf serum was removed
and the test solutions were applied to the
cells in different final concentrations at
0.625, 1.25, 2.5, 5, 10, 20, 650, 100, and
200 mM. After 24 h, MTT solution was
added to the wells and the plates were
incubated at 378C for 4 h. The positive
control group was treated with norcanthar-
idin (purity higher than 99.0% as deter-
mined by HPLC; Nanjing Zelang Medical
Technology Co. Ltd., Nanjing, China).
The amount of formazan was determined
by photometry at 570 nm. The results were
expressed as the percentage of absorbance
20
White amorphous powder; ½aꢀD þ 6.6
(c ¼ 0.12, MeOH); UV (MeOH) lmax
(log 1) 210 (4.18), 254 (4.32) nm; IR
max(KBr): 3456 (OH), 2963, 2912, 2843,
n
1720 (CvO), 1724 (CvO), 1601
(CvCZCvO), 1441, 1382, 1158,
1040 cm21
;
1H NMR (pyridine-d5,
500 MHz) and 13C NMR (pyridine-d5,
125 MHz) spectral data were given in
Table 1; HR-ESI-MS: m/z 1153.5834 [M–
H]2 (calcd for C58H89O23, 1153.5795).
3.4 Acid hydrolysis and sugar analysis
of compounds 1 and 2
Each saponin (2 mg) was dissolved in 2 N
HCl (2 ml) and stirred at 808C for 4 h. The
reaction mixture was extracted with
chloroform, and the aqueous layer was
evaporated to give a mixture of monosac-
charides. The residue was dissolved in
anhydrous pyridine (1 ml) followed by the
addition of 2 mg of L-cysteine methyl ester
hydrochloride (99%, Tokyo Chemical
Industry Co., Ltd, Tokyo, Japan). After
heating at 608C for 2 h, the solvent was

8
P. Yang et al.
Jiangsu Higher Education Institutions
(PAPD).
in the control cells in comparison to that in
the drug-treated cells. The IC50 values
(50% inhibitory concentration) of com-
pounds 1 and 2 are shown in Table 2. The
experiment was performed in triplicate for
each sample.
References
[1] P. Zhao, D.F. Gao, M. Xu, Z.G. Shi,
D. Wang, C.R. Yang, and Y.J. Zhang,
Chem. Biodivers. 8, 1931 (2011).
[2] T. Takemoto, T. Miyasi, and G. Kusano,
Phytochemistry 14, 2534 (1975).
[3] J.L. Hu, S.P. Nie, D.F. Huang, C. Li,
M.Y. Xie, and Y. Wan, J. Sci. Food. Agric.
92, 2443 (2012).
Acknowledgment
We are grateful to Dr Jon Parcher at the
University of Mississippi for helpful
discussions.
[4] X.C. Jin and Y. Ning, Int. J. Biol.
Macromol. 51, 364 (2012).
Disclosure statement
No potential conflict of interest was reported by
the authors.
[5] X. Li, J.Z. Zhao, C.P. Peng, Z. Chen,
Y.L. Liu, Q.M. Xu, I.A. Khan, and
S.L. Yang, Planta Med. 80, 590 (2014).
[6] S.L. Wang, Z. Chen, X.J. Tong, Y.L. Liu,
X. Li, Q.M. Xu, X.R. Li, and S.L. Yang,
Helv. Chim. Acta 96, 1126 (2013).
Funding
This work was financially supported by the
Priority Academic Program Development of