Separation and identification of an impurity from the istradefylline intermediate

Article abstract of DOI:10.1039/c9ra09074f

Istradefylline is a selective adenosine antagonist for the A_2a receptor, and it is used to treat the Parkinson's disease and improve dyskinesia in the early stage of the Parkinson's disease. An impurity in the istradefylline intermediate A₁ (6-amino-1,3-diethyl-2,4-(1H,3H)-pyrimidinedione) was identified by high performance liquid chromatography (HPLC); it was separated by preparative HPLC and further characterized by UV, IR, MS, NMR, 2D NMR and single-crystal XRD analyses. The impurity was identified as (E)-N-ethyl-2-cyano-3-ethylamino-2-butenamide, which originated from the synthetic process of the intermediate A₁. The structure of this impurity might affect the efficiency and safety of istradefylline; therefore, the research and control of this impurity are necessary for ensuring the quality of istradefylline.

Full text of DOI:10.1039/c9ra09074f

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Separation and identification of an impurity from
the istradefylline intermediate†
Cite this: RSC Adv., 2020, 10, 14493
a
Haojie Xu,ab Yiyun Wang,ab Hongyi Wang,ab Zhonghui Zheng,b Zihui Meng,
*
Min Xuea and Zhibin Xua
Istradefylline is a selective adenosine antagonist for the A2a receptor, and it is used to treat the Parkinson's
disease and improve dyskinesia in the early stage of the Parkinson's disease. An impurity in the istradefylline
intermediate A1 (6-amino-1,3-diethyl-2,4-(1H,3H)-pyrimidinedione) was identified by high performance
liquid chromatography (HPLC); it was separated by preparative HPLC and further characterized by UV, IR,
MS, NMR, 2D NMR and single-crystal XRD analyses. The impurity was identified as (E)-N-ethyl-2-cyano-
3-ethylamino-2-butenamide, which originated from the synthetic process of the intermediate A1. The
structure of this impurity might affect the efficiency and safety of istradefylline; therefore, the research
and control of this impurity are necessary for ensuring the quality of istradefylline.
Received 3rd November 2019
Accepted 17th March 2020
DOI: 10.1039/c9ra09074f
rsc.li/rsc-advances
istradefylline was used for patients whose symptoms were not
well controlled, and it showed good safety and efficiency.15,16
1. Introduction
Currently, istradefylline is synthesized by the amidation,
cyclization and methylation of intermediates A and B.17 Inter-
mediate A was synthesized from 1,3-diethylurea via cyclization
with cyanoacetic acid, nitrosation and reduction. Intermediate
B was prepared from veratraldehyde by condensation with
malonic acid and acyl chlorination (Fig. 2).
Impurities including organics, inorganics and solvent
residuals can be usually found in the nal drugs, and they
might originate from the starting materials, intermediates,
production processes and storage.18 Organics are the most
common impurities in pharmaceutical products, especially
unreacted intermediates, by-products or degradation products
that occur during storage. B. W reported the HPLC analysis of
istradefylline by isocratic elution using acetonitrile-phosphate
buffer as the mobile phase.19 However, no other studies
related to the analysis of istradefylline have been reported.
By-products usually have similar physical and chemical
properties to those of drugs due to their similar chemical
Parkinson's disease is a common progressive neurological
disease, where the lesions are mainly located in substantia
nigra, striatum and globus pallidus.1,2 Ageing is the most
important known risk, and the incidence of Parkinson's disease
rises dramatically with age. The incidence rates for people aged
50 to 59 and 70 to 79 are 17.4 and 93.1 per 100 thousand people,
respectively.3,4 There are individual differences in the Parkin-
son's disease and patients with hyperactivity as the main
symptom might be diagnosed in the early stage.5 The initial
symptoms in sequence are tremors, stiffness or slow movement,
gait disorder, myalgia and spasm, mental disorders such as
depression and stress, language disorder and myasthenia, and
drooling and mask-like face. Currently, the main therapeutic
medicines can relieve the clinical symptoms of PD.6–9 However,
a single treatment method may not be completely effective, and
the entire treatment process should be targeted according to the
different stages and severity of the disease.10
Istradefylline (KW-6002, (E)-8-(3,4-dimethoxystyryl)-1,3-
diethyl-7-methyl-3,7-dihydro-1H-purine-2,6-dione), a selective
adenosine A2a receptor antagonist (Fig. 1), was developed by
Kyowa Hakko Kirin, Japan.11 It is used to treat Parkinson's
disease; it can improve the motor function of PD patients by
altering the activity of neurons and can also improve early
dyskinesia.12–14 During phase I and phase II clinical trials,
adjunct with L-dopa and other anti-Parkinson drugs,
aSchool of Chemistry and Chemical Engineering, Beijing Institute of Technology,
102488, Beijing, China. E-mail: mengzh@bit.edu.cn
bShandong Xinhua Pharmaceutical Co., Ltd., No.1 Lutai Avenue, 255086, Zibo,
Shandong, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra09074f
Fig. 1 Chemical structure of Istradefylline.
RSC Adv., 2020, 10, 14493–14499 | 14493
This journal is © The Royal Society of Chemistry 2020

View Article Online
RSC Advances
Paper
Fig. 2 Reagents and conditions: (a) AC2O, 90–95 ꢂC, 1 h; 40% NaOH, 90–95 ꢂC, 15 min; (b) NaNO2, HAC, 60 ꢂC, 0.5 h; (c) H2, Ni, 0.5 Mpa, 1 h; (d)
Py, reflux, 6 h; (e) SOCl2, DCM, DMF, rt, 2 h; (f) Py, DCM, rt, 16 h; (g) NaOH, C2H5OH, reflux, 15 min; (CH3O)2CO, DMF, K2CO3, 140 ꢂC, 1.5 h.
structures. In order to ensure that the drugs do not have adverse
effects, it is extremely important to control the impurities,
which might affect the efficiency and safety of the drugs.20
The istradefylline intermediate A1 (Fig. 2) is the active frag-
ment in the intermediate A, and its impurities are directly
related to the impurities of istradefylline. The purpose of this
study is to separate the impurities of the istradefylline inter-
mediates by prep-HPLC and conrm their structure by UV, IR,
NMR, MS and single-crystal X-ray diffraction analyses.
2.3. Separation of impurity
A prep-HPLC method was developed for the separation of the
impurity present in the istradefylline intermediate A1. Inter-
mediate A1 (0.4 g), water (8 mL) and acetonitrile (8 mL) were
dissolved in an ultrasonic water bath. Preparative performance
liquid chromatography was carried out on KNAUER-AZURA
(Knauer, Germany). The impurity was separated and puried
by prep-HPLC on Ceres B equipped with a preparation column
(250 ꢀ 30 mm, 7 mm) using acetonitrile/water (30/70, v/v) as the
mobile phase at a ow rate of 30 mL minꢁ1 and detected at a UV
wavelength of 268 nm.
2. Experiment and methods
2.1. Materials
2.4. Characterization and analysis of the impurity
Istradefylline intermediate A1 was obtained from Shandong
Xinhua Pharmaceutical Co., Ltd. (Zibo, China). Acetonitrile and
puried water were used for sample preparation and mobile
phase. HPLC grade acetonitrile was purchased from Merck
(Darmstadt, Germany). Water was puried through a Milli-Q
water purication system (Millipore, USA).
The separated impurity was characterized by UV, IR, MS, NMR,
2D NMR and single-crystal X-ray diffraction. Ultraviolet
absorption spectra were recorded on UV-2550 (SHIMADZU,
Kyoto, Japan). Infrared absorption spectra were recorded on
Vertex-70 FT-IR (Bruker, Germany) over the range of 4000–
400 cmꢁ1 by the pressed tablet method using KBr.
For NMR analysis, we used deuterated dimethylsulfoxide
(DMSO-d6) as the solvent and tetramethylsilane (TMS) as the
2.2. HPLC analysis of the impurity
HPLC analyses were performed on an LC-20AD system (SHI- internal standard. 2D NMR correlation spectra (Correlated
MADZU, Kyoto, Japan) equipped with an SPD-10 Avp detector. Spectroscopy (COSY)), 1H Detected Heteronuclear Single
The impurity was prepared by dilution into the mobile phase Quantum Coherence (HSQC), 1H Detected Heteronuclear
and then was ltered through a 0.45 mm organic membrane. Multiple Bond Correlation (HMBC) and Distortionless
The impurity was analyzed by HPLC on Agilent ZORBOX Enhancement by Polarization Transfer Spectroscopy (DEPT)
equipped with a C18 chromatographic column (150 ꢀ 4.6 mm, were recorded on INOVA-600 (Varian, USA) for the assignment
5 mm). Mobile phase A (acetonitrile) and mobile phase B of the related chemical groups.
(puried water) were used for HPLC. The separation was
The high-resolution mass spectrum (HRMS) was detected on
accomplished by a gradient elution according to the program 1200RRLC-6520 Accurate-Mass Q-TOF (Agilent, USA) with ESI in
(Tmin/A : B): T0/20 : 80, T15/60 : 40 and T20/90 : 10 to T50 at the positive ion mode.
ow rate of 1.0 mL minꢁ1. The column temperature was set at
Single crystals were picked and analysed for X-ray structural
35 ꢂC and the detector wavelength operated at 268 nm (Fig. S1†). analysis on a Bruker apex2 X-ray diffractometer with Mo-Ka
14494 | RSC Adv., 2020, 10, 14493–14499
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Fig. 3 HPLC chromatogram of (a) gradient elution, (b) acetonitrile/water (35 : 65, v/v), and (c) acetonitrile/water (30 : 70, v/v).
ꢀ
radiation (l ¼ 0.71073 A) at 296 K. The parameters of crystals protocol including the elution method, mobile phase and
are summarized in Table 3. The single crystal X-ray diffraction injection volume was optimized and the results are shown in
spectrum is shown in Fig. S11.†
Fig. 3.
According to Fig. 3, good separation can be achieved by
gradient elution, and the peak of the intermediate A1 is far away
from the impurity peak (a ¼ 4.1); however, the column balance
required a long time with a large solvent demand in the
preparative mode. Therefore, isocratic elution was applied.
When acetonitrile/water (35 : 65, v/v) was applied (a ¼ 2.47),
3. Results and discussion
3.1. Separation of the impurity by prep-HPLC
To obtain a sufficient amount of the impurity for characteriza-
tions, prep-HPLC was used to obtain the impurity. The HPLC
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 14493–14499 | 14495

View Article Online
RSC Advances
Paper
Table 1 Comparative 1H and 13C NMR data of impurity (DMSO-d6, 25
is the K-band transition peak of the carbonyl group. The K band
is characteristic of a p–p* transition, and it can be deduced that
the impurity contains the structure of a conjugated group.
The IR spectrum of the impuriꢁty1 is shown in Fig. S3† and tꢁh1e
ꢂC)a
Impurity chemical shis (ppm)
13C
1H
No. of H
Multiplicity
DEPT
absorption bands at 3361 cm , 2963 cmꢁ1, 2929 cm
,
Position
2187 cmꢁ1, 1699 cmꢁ1, 1624 cmꢁ1, 1453 cmꢁ1 and 1376 cmꢁ1
correspond to the functional groups, namely, NH, CH3, CH2,
CO, CONH and CN. It can be deduced that the impurity has
similar functional groups to that of intermediate A1.
1
2
3
4
5
6
7
8
9
167.68
70.13
167.78
17.34
120.81
37.97
14.90
33.49
15.18
—
—
—
2.17
—
3.32
1.14
3.10
1.00
—
—
—
3H
—
—
—
—
*
*
*
b
*
a
b
a
b
1
The H NMR, 13C NMR and DEPT data of the impurity are
—
2H
3H
2H
3H
Overlap
J ¼ 7.2 Hz
J ¼ 7.1, 5.8 Hz
J ¼ 7.1 Hz
shown in Table 1 and the proton correlation data are shown in
Table 2. In the 1H NMR spectrum of the impurity (Fig. S4†),
there are seven groups of hydrogen with the integral ratio of
1 : 1 : 2 : 2 : 3 : 3 : 3 from low eld to high eld. It can be
deduced that the impurity contains three methyl groups, two
methylene groups and two hydrogens. In addition to the two
methylene groups and two methyl groups, the impurity has one
more methyl group than the intermediate A1. In the 13C NMR
spectrum of the impurity (Fig. S5†), the molecular backbone of
the impurity has nine carbons, which is inconsistent with that
of A1, which has eight carbons. According to the chemical shi
and DEPT data, the impurity contains three primary carbons,
two secondary carbons and four quaternary carbons. It can be
deduced that the structure might contain two methylene
groups, three methyl groups, one secondary amine, a cyano-
bonded ketone and a carbon–carbon double bond.
The MS spectrum of the impurity shows a protonated
molecule [M + H]+ at m/z 182.1 in Fig. S6,† which is signicantly
different from that for the intermediate A1 (m/z 183.1008).
Therefore, it can be deduced according to the above-
mentioned characterization that the impurity is a by-product
of intermediate A1 and is identied as (E)-N-ethyl-2-cyano-3-
ethylamino-2-butenamide, as shown in Fig. 4.
a
Highlighted: a position part in the molecule, a-negative peaks in
DEPT, b-positive peaks in DEPT, * four quaternary carbons.
there was a small impurity peak in the front of the target
impurity peak, which might be included by the impurity peak
when a large amount of the sample was injected. Consequently,
acetonitrile/water (30/70, v/v) was further investigated; the
impurity and intermediate A1 could be separated well (a ¼ 2.89),
and the small impurity peak before the target impurity could
also be separated well. Then, a larger injection volume (4 mL)
was tested further, and the small impurity peak before that of
the intermediate A1 could still be completely separated. Thus,
the prep-HPLC conditions were set as follows: acetonitrile/water
(30 : 70, v/v), isocratic elution, and 4 mL injection volume.
Under these conditions, the separation was satisfactory, and
economical solvent demand and short run time were achieved.
The nal pale yellow impurity product was extracted with
dichloromethane and concentrated to dryness under a reduced
pressure.
In order to further conrm the deduced structure of the
impurity, the DEPT spectrum (Fig. S7†), 1H–1H correlation
spectrum (COSY) (Fig. S8†), 13C–1H correlation spectrum
(HMQC) (Fig. S9†) and remote 13C–1H correlation spectrum
(HMBC) (Fig. S10†) were obtained to characterize hydrogen and
carbon. The structure was conrmed again as (E)-N-ethyl-2-
cyano-3-ethylamino-2-butenamide.
3.2. Characterization of the obtained impurity
The impurity obtained from prep-HPLC was characterized by
UV, IR, MS, NMR, 2D NMR and proton correlation spectroscopy.
The UV absorption spectra of the obtained impurity in H2O, HCl
(0.1 mol Lꢁ1), NaOH (0.1 mol Lꢁ1) and methanol solutions are
shown in Fig. S2.† There is an absorption peak at 285 nm, which
Table 2 H–H, H–C correlation of the impurity by COSY, HSQC and HMBC
Impurity
No.
dC/dH
COSY
HSQC
HMBC
C
Structure
1
2
3
4
5
6
7
8
167.68
70.13
167.78
17.34/2.17
120.81
37.97/3.32
14.90/1.14
33.49/3.10
15.18/1.00
7.09
—
—
—
—
—
—
—
—
H-8,4
H-4
H-6,4
—
H-4
H-7
H-6
H-9
H-8
—
4ꢂ
4ꢂ
4ꢂ
1ꢂ
4ꢂ
2ꢂ
1ꢂ
2ꢂ
1ꢂ
—
—
—
—
H-7
H-6
H-9
H-8
H-8
H-6
H-6
H-7
H-8
H-9
—
9
8-NH
6-NH
10.69
—
—
14496 | RSC Adv., 2020, 10, 14493–14499
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Table 3 Crystal data and structure refinement for the impurity
is related to H-9 (d1.00) and remotely related to H-8 (d3.10),
which is conrmed to be methyl C-9. The primary carbon peak
(d17.34) is related to H-4 (d2.17) and not remotely related to the
HSQC spectrum, which is assigned to C-4. Two sets of secondary
carbon peaks can be observed in the DEPT spectrum. The
secondary carbon peak (d37.97) is related to H-6 (d3.32) and is
remotely related to H-4 (d2.17), which is conrmed to be
methylene C-6. The secondary carbon peak (d33.49) is related to
H-8 (d3.10) and remotely related to H-9 (d1.00), which is
conrmed to be methylene C-8. Four sets of quaternary carbons
peaks can be conrmed from the DEPT spectrum. The quater-
nary carbon peak (d167.68) is related to H-8 (d3.10) and H-4
(d2.17) in the HMBC spectrum, which is assigned to quater-
nary C-1. The HMBC spectrum shows that C (d70.13) is remotely
related to H-4 (d2.17), which is conrmed to be quaternary C-2.
C (d167.78) is remotely related to H-4 (d2.17) and H-6 (d3.32),
which is conrmed to be quaternary C-3. C (d120.81) is remotely
related to H-4 (d2.17), which is conrmed to be quaternary C-5
(Table 2).
Parameter
Content
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
C9H15N3O
181.24
150
Monoclinic
P21/c
8.28(4)
12.60(6)
9.92(5)
ꢀ
a/A
ꢀ
b/A
ꢀ
c/A
a/ꢂ
b/ꢂ
g/ꢂ
90
105.94(10)
90
995.63(8)
ꢀ3
Volume/A
Z
4
1.21
11 037
1.06
rcalc g cmꢁ3
Reections collected
Goodness-of-t on F2
R indexes
R1 ¼ 0.0883, wR2 ¼ 0.1261
According to the COSY spectrum, combined with the HSQC
and DEPT spectra, H (d1.00) is a set of three peaks with three
protons related to H-8 (d3.10) and correlated to C-9 (d15.18),
which is conrmed to be H-9. H (d1.14) is a set of three peaks
with three protons related to H-6 (d3.32) and correlated to C-7
(d14.90), which is conrmed to be H-7. H (d2.17) is a set of
single peaks with three protons correlated to C-4 (d17.34) and
remotely related to C-1(167.68), C-2(70.13), C-3(167.78) and C-
5(120.81), which is conrmed to be H-4. H (d3.10) is related to
H-9 (d1.00) and correlated to C-8 (d33.49), which is conrmed to
be H-8. H (d3.32) is related to H-7 (d1.14) and correlated to C-6
(d37.97), which is conrmed to be H-6.
3.3. Formation mechanism of the impurity
Although the formation mechanism of an imidazole diketone
from the condensation of cyanoacetic acid with diethyl urea has
not been reported, Baccolini reported that the condensation
reaction of 2-carbonyl propionaldehyde and 2,3-diol propio-
naldehyde with urea can be carried out to obtain 5-methyl-
imidazolidine-2,4-dione.21 Based on the structure of 5-
methylimidazolidine-2,4-dione, we deduce that the formation
mechanism of (E)-N-ethyl-2-cyano-3-ethylamino-2-butenamide
might be as follows (Fig. 5): when condensation reaction is
carried out, cyanoacetic acid reacts with diethyl urea and acetic
anhydride to obtain intermediate A1. Simultaneously, the
intermediate A1 will undergo a decarboxylation reaction at
a high temperature to generate (E)-N-ethyl-2-cyano-3-ethyl-
amino-2-butenamide.
The DEPT spectrum shows the presence of three sets of
primary carbon peaks. The primary carbon peak (d14.90) is
related to H-7 (d1.14) and remotely related to H-6 (d3.32), which
is conrmed to be methyl C-7. The primary carbon peak (d15.18)
Fig. 4 Structure of (E)-N-ethyl-2-cyano-3-ethylamino-2-butenamide.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 14493–14499 | 14497

View Article Online
RSC Advances
Paper
Fig. 5 Reaction mechanism.
(E)-N-Ethyl-2-cyano-3-ethylamino-2-butenamide as a byprod-
uct of intermediate A1 reacts in the subsequent synthetic
process. It might eventually affect the safety and efficiency of
istradefylline, but its pharmacology and toxicity are unknown
yet. The presence of the impurity in the intermediate A1 poses
a signicant risk to the safety and efficiency of istradefylline.
The strict control of the reaction temperature and the reduction
in acidity during the synthetic process, especially during the
distillation of acetic acid, may reduce the formation of this
byproduct. Through research using analytical methods, we can
reduce the quality risk of the drug by using HPLC to detect and
control this impurity.
References
1 W. Chen, H. Wang, H. Wei, S. Gu and H. Wei, J. Neurol. Sci.,
2013, 324, 21–28.
2 J. Lu, J. Cui, X. Li, X. Wang, Y. Zhou, W. Yang, M. Chen,
J. Zhao and G. Pei, PLoS One, 2016, 11, e0166415.
3 H. Nagayama, O. Kano, H. Murakami, K. Ono, M. Hamada,
T. Toda, R. Sengoku, Y. Shimo and N. Hattori, J. Neurol.
Sci., 2019, 396, 78–83.
4 A. Dalpiaz, B. Cacciari, C. B. Vicentini, F. Bortolotti,
G. Spalluto, S. Federico, B. Pavan, F. Vincenzi, P. A. Borea
and K. Varani, Mol. Pharm., 2012, 9, 591–604.
5 H. H. Fernandez, D. R. Greeley, R. M. Zweig, J. Wojcieszek,
A. Mori, N. M. Sussman and U. S. S. Group, Park. Relat.
Disord., 2010, 16, 16–20.
4. Conclusions
6 R. A. Hauser, L. M. Shulman, J. M. Trugman, J. W. Roberts,
A. Mori, R. Ballerini, N. M. Sussman and U. S. S. G.
Istradefylline, Mov. Disord., 2008, 23, 2177–2185.
7 K. Ishibashi, Y. Miura, K. Wagatsuma, J. Toyohara,
K. Ishiwata and K. Ishii, Neuropharmacology, 2018, 143,
106–112.
8 S. Uchida, K. Soshiroda, E. Okita, M. Kawai-Uchida, A. Mori,
P. Jenner and T. Kanda, Eur. J. Pharmacol., 2015, 747, 160–
165.
9 K. Yamada, M. Kobayashi, A. Mori, P. Jenner and T. Kanda,
Pharmacol., Biochem. Behav., 2013, 114–115, 23–30.
10 H. Kataoka and K. Sugie, J. Neurol. Sci., 2018, 388, 233–234.
11 Y. Mizuno, T. Kondo and G. Japanese Istradefylline Study,
Mov. Disord., 2013, 28, 1138–1141.
12 W. K. D. Ko, S. M. Camus, Q. Li, J. Yang, S. McGuire,
E. Y. Pioli and E. Bezard, Neuropharmacology, 2016, 110,
48–58.
In this study, an impurity from the istradefylline intermediate
A1 was separated by prep-HPLC. Its structure was identied as
(E)-N-ethyl-2-cyano-3-ethylamino-2-butenamide by means of
UV, IR, MS, NMR, 2D NMR and single-crystal X-ray diffraction
analyses. The impurity is actually a by-product of the interme-
diate A1, and it is formed due to the high temperature and acidic
conditions employed during the synthesis of intermediate A1.
The structure of this impurity has a potential impact on the
safety and efficiency of istradefylline. In order to ensure the
quality of istradefylline, HPLC is needed to detect and control
the content of this impurity.
Conflicts of interest
The authors declare that there are no conicts of interest.
13 P. A. LeWitt, M. Guttman, J. W. Tetrud, P. J. Tuite, A. Mori,
P. Chaikin, N. M. Sussman and U. S. S. Group, Ann.
Neurol., 2008, 63, 295–302.
14 K. Matsuura, H. Kajikawa, K.-i. Tabei, M. Satoh, H. Kida,
N. Nakamura and H. Tomimoto, Neurosci. Lett., 2018, 662,
158–161.
Acknowledgements
The authors wish to thank the Shandong Analytical Testing
Center for their 2D NMR services, Prof. Li Qingshan from
Nankai University and Prof. Guo Yong from the Lanzhou
institute of Chemical & Physics for their contribution in iden-
tifying the structure of the impurity.
14498 | RSC Adv., 2020, 10, 14493–14499
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
15 S. Uchida, K. Soshiroda, E. Okita, M. Kawai-Uchida, A. Mori, 19 B. Wu and H. Cui, CN Pat., 105891377A, 2016.
P. Jenner and T. Kanda, Eur. J. Pharmacol., 2015, 766, 25–30. 20 Q. Xu, A. Khan, D. Gao, K. M. Adams, F. Tadjimukhamedov,
16 S. Uchida, T. Tashiro, M. Kawai-Uchida, A. Mori, P. Jenner
and T. Kanda, J. Pharmacol. Sci., 2014, 124, 480–485.
17 L. Fan and H. Xingpu, Chin. J. Pharm., 2010, 41, 241–243.
18 N. Kumar, S. R. Devineni, P. R. Gajjala, S. K. Dubey and
P. Kumar, J. Pharm. Anal., 2017, 7, 394–400.
S. Tan and J. T. Simpson, J. Pharm. Anal., 2018, 8, 96–102.
21 G. Baccolini and C. Boga, Tetrahedron Lett., 2011, 52, 1713–
1717.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 14493–14499 | 14499