A novel process for antimalarial drug pyronaridine tetraphosphate

Article abstract of DOI:10.1021/op400357f

A novel process for preparation of pyronaridine tetraphosphate, an antimalarial drug substance, is reported. The overall yields are 54% and >99.8% (including five chemical steps). Formation and control of possible impurities are also described.

Full text of DOI:10.1021/op400357f

Technical Note
pubs.acs.org/OPRD
A Novel Process for Antimalarial Drug Pyronaridine Tetraphosphate
Yu Liu,*,†,‡ Zixue Zhang,‡ Anfei Wu,† Xiaoli Yang,† Yong Zhu,† and Nan Zhao†
†API Research Centre, Shanghai Desano Pharmaceutical Company, Shanghai 20103, P.R. of China
‡Novel Technology Center of Pharmaceutical Chemistry, Shanghai Institute of Pharmaceutical Industry, Shanghai Engineering,
Research Center of Pharmaceutical Process, 1111 North Zhongshan No.1 Road, Shanghai 200437, P.R. of China
S
* Supporting Information
ABSTRACT: A novel process for preparation of pyronaridine tetraphosphate, an antimalarial drug substance, is reported. The
overall yields are 54% and >99.8% (including five chemical steps). Formation and control of possible impurities are also
described.
INTRODUCTION
Pyronaridine tetraphosphate (Figure 1) was first synthesized in
the 1970s and has been extensively used in China as an
RESULTS AND DISCUSSION
■
■
An improved process for preparation of pyronaridine
tetraphosphate, which is free from associated impurities and
does not make use of column chromatography or other tedious
purification methods, is reported.
A convergent synthesis route was designed as shown in
Scheme 2, wherein the two key building blocks, 4-amino-2,6-
bis(pyrrolidin-1-ylmethyl) phenol 12 and 7,10-dichloro-2-
methoxybenzo[b][1,5]naphthyridine 4, are combined in a
one-step reaction.5
Preparation of 7,10-Dichloro-2-methoxybenzo[b]-
[1,5]naphthyridine (4). The Ullman reaction mediated by
CuO proceeded smoothly with very high regioselectivity.
However, an impurity was produced during the steps of
cyclization and chlorination (Scheme 3).
Figure 1. Structure of pyronaridine tetraphosphate 1.
Impurity IV could further react with 12, and the resulting
impurity in the next step cannot be removed effectively due to
the close Rf values and the relative retention time of 0.95.
Different conditions were tried to eliminate the impurity but
failed. We concluded that the impurity was produced via an
intermediate (as shown in Scheme 4) under acidic conditions.
Thus, a base had to be used to scavenge the HCl formed in the
POCl3 and in the reaction, and Et3N was chosen as the most
suitable base in the process. Qualified compound 4 was
obtained after the reaction was worked up, filtered, and purified
in toluene.
Preparation of 4-Amino-2,6-bis(pyrrolidin-1-
ylmethyl)phenol (12). The temperature and sequence of
the start of material addition are very important in Mannich
reactions. Primarily, the reaction was refluxed in ethanol, where
compound 10 remained, and a single substituted impurity was
produced in the reaction. Additional amounts of pyrrolidine
and paraformaldehyde did not resolve the problem.
antimalarial monotherapy, showing good activity and toler-
ability. Owing to the drug’s high antimalarial activity, safety,
tolerability, stability, and excellent water solubility, the World
Health Organization plans to complete preclinical and clinical
trials with the aim of replacing chloroquine with pyronaridine
as the first line of treatment for malaria, particularly in Africa.1
The reported synthesis route is a linear synthetic strategy (as
shown in Scheme 1).2 The synthesis starts from the reaction of
two compounds, 2,4-dichlorobenzoic acid (1) and 6-methox-
ypyridin-3-amine (2), coupled under the mediation of CuO to
obtain 4-chloro-2-(6-methoxypyridin-3-ylamino)benzoic acid
(3).3 Compound 3 undergoes cyclization and chlorination by
refluxing in POCl3 to produce 7,10-dichloro-2-methoxybenzo-
[b][1,5]naphthyridine (4), followed by SNAr reaction with
aminophenol, Mannich reaction, and salt with orthophosphoric
acid to obtain the final product.4 However, no data on the
purity of the final product have been revealed in the literature.
Further duplication of these experiments at our laboratory
resulted in the formation of impurity I (Figure 2) due to the
overreaction of the Mannich reaction, which is very difficult to
eliminate from the final product without affecting the yield.
Thus, developing a process for the preparation of
pyronaridine tetraphosphate that can control the formation of
impurities and does not require tedious purification techniques
in obtaining the desired purity is necessary.
In the next step, the reaction proceeded in pyrrolidine
without the solvent because p-nitrophenol is very soluble in
pyrrolidine. The synthesis progressed completely, albeit with an
intense uncontrolled exothermic reaction, which is deemed
unsafe during scale up. Other solvents were screened, such as
Received: December 13, 2013
Published: January 28, 2014
© 2014 American Chemical Society
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dx.doi.org/10.1021/op400357f | Org. Process Res. Dev. 2014, 18, 349−353

Organic Process Research & Development
Technical Note
Scheme 1. Reported synthesis route
methanol and isopropanol. The reaction works smoothly in
isopropanol without sudden heat production, and the starting
material p-nitrophenol was totally consumed. Sequence of raw
material was also considered. The addition of pyrrolidine to
paraformaldehyde and nitrophenol suspension in isopropanol
was regarded as the safest addition sequence.
Although the purity of the reaction was greatly improved by
reaction condition optimization, there was a multisubstituted
impurity (Figure 3) in the reaction, which was further
converted to impurity I. Removal of impurity I could not be
achieved after its formation, so this impurity must be removed
at this step in order to obtain the qualified API. Because
Figure 2. Formation impurity in the final product.
Scheme 2. Convergent synthesis route
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Organic Process Research & Development
Technical Note
Scheme 3
Scheme 4
Table 1. Screening of Solvent and Reaction Temperature of the Mannich Reaction
a
a
a
a
entry
solvent
MeOH
temp (°C)
impurity II (%)
impurity III (%)
product (%)
p-nitrophenol (%)
1
2
3
4
5
6
65
75
85
75
85
95
38.64
8.00
3.98
2.53
2.92
3.01
0.50
0.67
0.11
1.21
0.10
0.10
47.74
81.76
62.40
75.15
90.15
89.25
4.26
0.41
ND
EtOH
pyrrolidine
i-PrOH
i-PrOH
i-PrOH
1.25
ND
ND
a
Purity measured by HPLC.
Scheme 5
to obtain a qualified active pharmaceutical ingredient (API)
with purity of >99.8% and single impurity of less than 0.1%.
On the basis of the optimized conditions, the batch was
carried out on 50 kg scale, and the results were reproduced at
the plant level.
Figure 3. Formation impurities in the synthesis of compound II.
compound 11 is oil, we have to solidify it for the convenience
of recrystallization. Several salts, such as phosphoric acid,
hydrochloric acid, oxalic acid, and sulfuric acid, were screened
in order to get a better solid. Finally we found that the HCl salt
of compound 11 was a light-yellow solid, the purity of the HCl
salt of compound 11 could be improved to >99.8%, and a single
impurity could be reduced to <0.1% after recrystallization.
After high-quality compound 11 was obtained, we examined
the catalytic hydrogenation conditions. 2,6-Substituted amino-
phenol 12 is sensitive to air, but its HCl salt is much more
stable than its isolated base. Thus, an equimolar concentration
of HCl aqua solution was added to the solution to substantially
improve reaction rates. No impurity was produced in this step.
Preparation of Pyronaridine Tetraphosphate (1). The
last step is SNAr reaction. Generally, this kind of reaction works
under basic conditions. Interestingly, this reaction proceeds
under acidic conditions. HCl or H2SO4 aqua solution addition
to the solvent would produce an impurity (impurity V)
(Scheme 5). We speculated that the HCl salt of compound 12
could promote the reaction work without additional acid, as we
expected that the reaction worked well with a molecular ratio of
1:1 of the two starting materials. After the reaction was
completed, the HCl salt of compound 9 was filtered and
isolated with 20% NaOH aqua solution and then mixed with
orthophosphoric acid to obtain pyronaridine tetraphosphate.
The phosphoric acid salt was slurried in refluxed 75% ethanol
CONCLUSION
■
We have successfully developed a novel, commercially viable
manufacturing process for pyronaridine tetraphosphate with
high purity and satisfactory yield. The impurities produced in
this process were well controlled and removed.
EXPERIMENTAL DETAILS
■
General Methods. All reagents were purchased from
Sinopharm, and the purities of these reagents are greater than
99.0%. NMR spectra of the intermediates were recorded on a
Bruker 300 NMR and 400 NMR spectrometer using TMS as an
internal standard. EI−MS spectra were obtained on a Finnigan
MAT 95 mass spectrometer, and ESI−MS spectra were
obtained on a Kratos MS 80 mass spectrometer. Purity analysis
was carried out on an HPLC (Agilent 1200 Series RRLC,
Agilent technologies) using an optima PAK C18 (5 mm;
4.6150 mm) column. HPLC solvents consisted of potassium
phosphate buffer:acetonitrile (9:1) (solvent A) and potassium
phosphate buffer:acetonitrile (2:8) (solvent B). One liter of the
buffer (pH 2) consisted of KH2PO4 (1.36 g) and 1-
octanesulfonic acid sodium salt hydrate (2.163 g). The flow
rate was maintained at 1 mL/min, and the column temperature
was maintained at 40 °C.6
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Organic Process Research & Development
Technical Note
4-Chloro-2-(6-methoxypyridin-3-ylamino)benzoic
Acid (3). The suspension of 2,4-dichlorobenzoic acid (7.16 kg,
37.5 mol) 1, 6-methoxypyridin-3-amine (5.0 kg, 40.3 mol) 2,
potassium carbonate (2.65 kg, 19.0 mol), and CuO (30.0 g,
0.38 mol) in 20 L isopentanol was refluxed at 128−130 °C,
with the production of CO2 gas. The reaction was cooled to
100 °C after 10 h and quenched with 7 L water. Then, the pH
of the solvent was adjusted to 11 with 3 L of 10% NaOH aqua
solution at the same temperature. The obtained solution was
cooled to 40−50 °C and filtered to remove the copper salt. The
filtrate was adjusted to pH 3 with 6 N HCl to obtain a grey
precipitate. The precipitate was slurried with 30 L hexane; then
the product was obtained by filtration and dried to a water
content of less than 0.3% to afford a grey solid 3 (8.62 kg,
2HCl: C, 50.80; H, 6.66; Cl, 18.74; N, 11.11. Found: C, 50.83;
H, 6.72; N, 11.09. MS (ESI, m/z) 306 ([M + H]+, 100%).
4-Amino-2,6-bis(pyrrolidin-1-ylmethyl)phenol (12).
Up to 10% Pd/C (495.0 g, 70% water) was added to a
solution of compound 11 (5.0 kg, 13.0 mol) and 250 mL
concentrated HCl aqua solution in 25 L methanol. The mixture
was stirred under H2 atmosphere under 3 bar pressure at 25 °C.
The reaction was terminated after 5.5 h. The reaction mixture
was filtered, and the filtrate was concentrated to produce clear
oil. Subsequently, the oil was solidified in the mixture solution
(18.2 L of 95% ethanol and 60.7 L of ethyl acetate), followed
by filtration and drying at 60 °C for 4 h to afford a white solid
(4.98 kg, 97.9% yield, 98.68% purity by HPLC), mp 190−191
1
°C; H NMR (400 MHz, DMSO) δ 10.57 (s, 2H, br), 7.62(s,
1
1H), 7.60(s, 1H), 4.49 (s, 4H), 3.45 (m, 8H), 1.95 (m, 8H).
Anal. Calcd for C16H25N3O. 3HCl: C, 49.94; H, 7.33; N, 10.92.
Found: C, 49.90; H, 7.41; N, 10.89. MS (ESI, m/z) 276 ([M +
H]+, 100%).
82.5% yield, 97.45% purity), mp 198−200 °C; H NMR (300
MHz, DMSO) δ 13.11 (s, 1H, br), 9.48 (s, 1H), 8.13 (d, J = 2.3
Hz, 1H), 7.88 (d, J = 8.5 Hz, 1H), 7.70 (dd, J = 8.7, 2.7 Hz,
1H), 6.90 (d, J = 8.7 Hz, 1H), 6.76 (dd, J = 8.5, 2.0 Hz, 1H),
6.69 (d, J = 2.0 Hz, 1H), 3.88 (s, 3H). Anal. Calcd for
C13H11ClN2O3·HCl: C, 49.54; H, 3.84; N, 8.89; Found: C,
49.60; H, 3.80; N, 8.95. MS (ESI, m/z) 279 ([M + H]+, 100%),
Pyronaridine Tetraphosphate (1). The solution of 7,10-
dichloro-2-methoxybenzo[b][1,5]naphthyridine (4.0 kg, 14.3
mol) 4 and 4-amino-2,6-bis(pyrrolidin-1-ylmethyl)phenol 12
(5.5 kg, 14.3 mol) in 45.8 L ethanol was stirred at 50 °C for 2.5
h. The solvent was removed under reduced pressure, and the
obtained yellow solid was dissolved in 50 L water. Afterwards,
the solvent was adjusted to pH 10−12 with 5 L of 20% NaOH
water solution. The solid was collected by filtration and washed
with water until neutral. The solid was suspended in 20 L water
without drying; orthophosphoric acid was then added to the
suspension to adjust the pH to 2−3 and stirred for 1 h.
Isopropanol (40 L) was added to the solvent, after which the
yellow solid was precipitated and collected by filtration. The
solid was purified by refluxing the slurry containing 15 L of 75%
ethanol to obtain the qualified product 1 (11.2 kg, 86% yield,
+
235 ([M − 44] , 15%).
7,10-Dichloro-2-methoxybenzo[b][1,5]naphthyridine
(4). POCl3 (8.9 kg, 58.0 mol) was slowly added to the
suspension of 4-chloro-2-(6-methoxypyridin-3-ylamino)benzoic
acid 3 (4.0 kg, 14.3 mol) and Et3N (6.4 kg, 63.4 mol) in 20 L
toluene under N2 atmosphere at 8−10 °C. Then, the reaction
temperature was heated to reflux for 4 h. The solvent was
cooled to 10 °C and slowly transferred to another vessel with
10 L ice water, while keeping the temperature below 10 °C.
The pH of the solvent was adjusted to 8−9 with 25% NaOH
aqua solution at 10 °C. Precipitates were collected and slurried
in 5 L toluene at 70 °C; then after the solvent was cooled to 0−
10 °C, the solid was filtered and dried at 65 °C for 4 h as a grey
solid 4 (3.8 kg, 95% yield, 99.10% purity), mp 185−186 °C; 1H
NMR (400 MHz, DMSO) δ 8.43 (d, J = 9.3 Hz, 2H), 7.83 (dd,
J = 9.2, 2.0 Hz, 1H), 7.51 (d, J = 9.2 Hz, 1H), 4.15 (s, 3H).
Anal. Calcd for C13H8Cl2N2O: C, 55.94; H, 2.89; N, 10.04.
Found: C, 55.90; H, 2.84; N, 10.10. MS (ESI, m/z) 279 ([M +
H]+, 100%).
1
and 99.8% purity), mp 180−182 °C; H NMR (300 MHz,
DMSO) δ 8.25 (d, J = 9.2 Hz, 1H), 8.00 (s, 1H), 7.90 (d, J =
9.3 Hz, 1H), 7.32 (t, J = 9.7 Hz, 2H), 7.13 (s, 2H), 4.02 (s,
3H), 3.87 (s, 2H), 2.91 (m, 6H), 1.85 (m, 6H), 1.25 (m, 6H).
Anal. Calcd for C29H32ClN5O2·4H3PO4: C, 38.27; H, 4.87; N,
7.70. Found: C, 38.32; H, 4.80; N, 7.73. MS (ESI, m/z) 518
([M + H]+, 100%).
4-Nitro-2,6-bis(pyrrolidin-1-ylmethyl)phenol (11). Pyr-
rolidine 7 (10.0 kg, 148.0 mol) was added to a suspension of p-
nitrophenol 10 (3.95 kg, 28.4 mol) and paraformaldehyde 8
(4.56 kg, 152.0 mol) in isopropanol (15 L) at 20−30 °C over 1
h. Then, the reaction temperature was raised to 50 °C until the
solvent became clear, after which the temperature was raised to
85−90 °C for 3 h. The solvent was evaporated to dryness under
reduced pressure, and the obtained yellow oil was dissolved in
10 L DCM. The organic solvent was washed with water (2 × 5
L), and the water phase was combined and extracted with
DCM 5 L. The organic phases were combined and removed in
vacuo to obtain a yellow oil. The yellow oil was dissolved in
isopropanol (16 L) and cooled to 5 °C. A 2 L 23% HCl/
MeOH solution was added to the isopropanol solution and
stirred for 3 h until a light-yellow precipitate was produced
from the solvent. The solid was filtered and washed with
isopropanol (1 L) to obtain a light-yellow solid weighing 11.6
kg. The solid was recrystallized in 30 L methanol to furnish the
compound as a light-yellow solid 11 (8.8 kg, 82.0% yield,
ASSOCIATED CONTENT
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: liuyu_tianjin@126.com. Phone: +86 21 55514600-
271. Fax: +86 21 65169893.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the staff of Analytical Department of Desano
Pharmaceutical Company for the purity and assay determi-
nation.
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