New and efficient synthetic approaches for the regioisomeric and iminium impurities of clopidogrel bisulfate

Article abstract of DOI:10.1021/op300110m

New and concise synthetic routes have been devised for the regioisomeric and iminium impurities of clopidogrel bisulfate. The synthesis features utilization of commercially available starting materials and simple reactions.

Full text of DOI:10.1021/op300110m

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New and Efficient Synthetic Approaches for the Regioisomeric and
Iminium Impurities of Clopidogrel Bisulfate†
Sampath Aalla,‡ Goverdhan Gilla,*,‡ Raghupathi Reddy Anumula,‡ Kavitha Charagondla,‡
Prabhakar Reddy Vummenthala,§ and Pratap Reddy Padi⊥
‡Research and Development, Integrated Product Development, Dr. Reddy’s Laboratories Ltd., Innovation Plaza, Survey Nos. 42, 45,
46, and 54, Bachupally, Qutubullapur, Ranga Reddy-500 072, Andhra Pradesh, India
§Department of Chemistry, University College of Science, Osmania University, Hyderabad-500 007, Andhra Pradesh, India
⊥Research and Development, Macleods Pharmaceuticals Ltd., G-2, Mahakali Caves Road, Shanthi Nagar, Andheri (E), Mumbai-400
093, Maharashtra, India
ABSTRACT: New and concise synthetic routes have been devised for the regioisomeric and iminium impurities of clopidogrel
bisulfate. The synthesis features utilization of commercially available starting materials and simple reactions.
INTRODUCTION
Clopidogrel bisulfate (Figure 1) is a potent oral antiplatelet agent
often used in the treatment of coronary artery disease and
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Figure 2. Structures of clopidogrel regioisomer and clopidogrel iminium.
Extensive literature search revealed that two different synthetic
routes have been reported for regioisomer (Scheme 1).7−10 The
Figure 1. Structure of clopidogrel bisulfate.
first synthetic route (Scheme 1a) involves reaction of vinyl ketone
1 with glycine derivative 2 to provide intermediate 3, which upon
reaction with ethyl cyanoacetate (4) furnished compound 5.
Finally, diazotization followed by reduction using H3PO2 resulted in
clopidogrel regioisomer with an overall yield of 36%. Another route
(Scheme 1b) involves bromination of acid derivative 6 followed
by esterification of bromo derivative 7. Then, condensation of
compound 8 and thieno-piperidine 9 furnished the clopidogrel
regioisomer. Thieno-piperidine 9 is not commercially available
and was synthesized as shown in the synthetic Scheme 2. How-
ever, these synthetic approaches have some disadvantages, such
as lower overall yield (36%), usage of expensive and com-
mercially unavailable starting material 1, a higher number of steps
(seven steps, including the preparation of 9), usage of corrosive
reagent bromine, pyrophoric reagent LAH, and expensive
reagents NaBH4 and SnCl2. The iminium impurity was isolated
from the peroxide degradation by preparative HPLC. Isolation of
impurities by preparative HPLC is a laborious process. To the
best of our knowledge, there is no commercial source nor a
reported synthesis for iminium impurity. In view of the dis-
advantages associated with the synthesis of clopidogrel regio-
isomer and clopidogrel iminium, we intended to devise simple
and short synthetic approaches. Herein we report a facile three-
step synthesis for the regioisomer with an overall yield of 59%
and a single-step synthesis for iminium impurity with 70% yield
from the commercially available starting materials.
peripheral vascular and cerebrovascular diseases. Clopidogrel
works by helping to prevent harmful blood clots.1 It is marketed
by Bristol-Myers Squibb and Sanofi-Aventis under the trade
name Plavix, which is the world’s second-highest-selling
pharmaceutical with sales of $5.9 billion. The mechanism of
action of clopidogrel is irreversible blockade of the adenosine
diphosphate (ADP) receptor P2Y12, which is important in
platelet aggregation. Studies have shown that clopidogrel is more
effective in blocking platelet aggregation than aspirin and
ticlopidine even at a much lower dosage.2
A literature survey on clopidogrel bisulfate revealed the
presence of various pharmacoepia-listed impurities, such as
related compound A, related compound B (regioisomer), related
compound C, and related compound D.3,4 Mohan et al. reported
a new impurity, clopidogrel iminium, which is the principle
oxidative impurity of clopidogrel bisulfate.5
In view of stringent quality requirements,6 quantification of an
impurity present in the active pharmaceutical ingredient (API) is
receiving significant importance from the regulatory authorities
as well as pharmaceutical companies. Hence, there is a con-
siderable demand for a substantial quantity of impurity standards.
Often, the pharmacoepial impurity standards are available, but
with unaffordable prices. Therefore, easily accessible synthetic
routes with the lowest cost are sought after. In the present study,
we focused on the synthesis of the regioisomer and the iminium
impurity of clopidogrel bisulfate (Figure 2).
Received: May 2, 2012
†Dr. Reddy’s communication no. IPDO-IPM-00331
© XXXX American Chemical Society
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Scheme 1. Reported synthetic schemes for the clopidogrel regioisomer
Scheme 2. Reported synthetic schemes for thieno-piperidine 9
Figure 3. Retro-synthetic pathway of regioisomer.
into the hydrochloride salt of the regioisomer with 99.5% purity
in 90% yield under Pictet−Spengler reaction conditions
(Scheme 3). This newly designed synthetic scheme involves
Scheme 3. New synthetic scheme for the clopidogrel
regioisomer
RESULTS AND DISCUSSION
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The retro synthetic analysis indicated that the regioisomer can be
synthesized from the commercially available starting materials
(see Figure 3).
On the basis of the retrosynthetic pathway, methyl 2-amino-
(2-chlorophenyl)acetate (2) and 2-(thiophene-3-yl)ethanol
(19) were identified as starting materials. Compound 19 was
reacted with the p-toluenesulfonyl chloride in the presence of
triethylamine in toluene followed by a typical workup that
afforded the compound 20 with 99.2% purity in 93% yield. The
thus-obtained tosyl derivative 20 was subjected to reaction with
compound 2 in the presence of dipotassium hydrogen phosphate
in water medium to afford N-alkylated compound 21 in 71%
yield with 99.0% purity. Finally, compound 21 was converted
utilization of simple reactions and cheaply available reagents with
an overall yield of 59%; thus, it is advantageous over the reported
synthetic routes for the regioisomer.
Having developed a new and efficient synthetic route for
regioisomer, we focused on moving towards the clopidogrel
iminium impurity. In the reported literature clopidogrel iminium
impurity was isolated from the peroxide degradation, which
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Scheme 4. Synthetic scheme for clopidogrel iminium impurity
involves treatment of clopidogrel bisulfate with hydrogen
peroxide at 80 °C for 3 h followed by isolation using preparative
liquid chromatography (LC).5 To have a straightforward
synthesis, we embarked upon designing a synthetic route for
the iminium impurity. On the basis of the structure of the
clopidogrel iminium impurity, it was envisioned that bromina-
tion of clopidogrel and subsequent elimination of bromine would
result in the bromide salt of the clopidogrel iminium impurity.
Therefore, clopidogrel was subjected to bromination in the
presence of N-bromosuccinimide (NBS) to provide a reaction
intermediate, the bromo derivative 22 that upon warming
converted into the bromide salt of clopidogrel iminium impurity
in 70% yield with 99.5% purity (Scheme 4). Synthesis of clopidogrel
iminium impurity following the newly developed synthetic route is
simple and more cost-effective than the reported isolation method
by preparative HPLC.
Methyl 2-(2-Chlorophenyl)-2-(2-(thiophen-3-yl)ethyl-
amino)acetate Hydrochloride (21). A mixture of compound
2 (25 g, 0.1252 mol), compound 20 (42.4 g, 0.1501 mol),
dipotassium hydrogen phosphate (43.5 g, 0.2497 mol), and
water (12.5 mL) was stirred at 100 °C for 12 h. Then the reaction
mixture was cooled to room temperature, ethyl acetate (125 mL)
and water (125 mL) were charged, and the mixture was stirred
for 30 min. The aqueous layer was separated and extracted with
ethyl acetate (25 mL). The combined organic layer was washed
with water (50 mL), and aqueous HCl was slowly added (35%,
13.5 mL, 0.1294 mol) and stirred for 30 min at 10−15 °C. The
resultant reaction mixture was heated to 60 °C, stirred for 15 min,
and then cooled to 10−15 °C and stirred for 45 min. The
precipitated solid was filtered and washed with ethyl acetate (25
mL). The wet solid was dried at 65 °C under reduced pressure to
provide 31 g (71%) of a white solid with 99.0% purity by HPLC.
Mp: 169 °C; IR (KBr, cm−1): 3444, 3068, 2921, 1741, 1583, 1436,
1
CONCLUSION
1325, 1269, 1219, 1190, 1039, 786, 755; H NMR (400 MHz,
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DMSO-d6): δ 7.71 (d, J = 8.0 Hz, 1H), 7.63 (d, J = 7.6 Hz, 1H),
7.55−7.48 (m, 3H), 7.27 (s, 1H), 6.99 (d, J = 4.8 Hz, 1H), 5.58 (s,
1H), 3.74 (s, 3H), 3.17−3.03 (m, 4H); 13C NMR (100 MHz,
CDCl3): δ 167.3, 136.6, 134.8, 131.7, 130.5, 130.4, 128.5, 127.9,
127.3, 126.0, 122.2, 58.6, 53.7, 45.8, 26.3; HRMS (ESI): Calcd for
C15H16ClNO2S (M+ + H) 310.0669, Found 310.0656.
In conclusion, we have developed new and efficient synthetic
approaches for the regioisomer and iminium impurity of
clopidogrel bisulfate. These synthetic routes are concise, and
start from the commercially available materials, and provide easy
access to the synthesis of the regioisomer and the iminium
impurity of chlopidogrel bisulfate.
Methyl 2-(2-Chlorophenyl)-2-(4,7-dihydrothieno[2,3-
c]pyridin-6(5H)-yl)acetate Hydrochloride (Clopidogrel
Regioisomer). The mixture of compound 21 (15 g, 0.0433
mol) and aq formaldehyde solution (37%, 120 mL, 1.3875 mol)
was stirred for 7 h at 25−35 °C. The precipitated solid was
filtered, washed with acetone (15 mL), and dried at 65 °C under
reduced pressure to afford 14 g (90%) of an off-white solid with
99.5% purity by HPLC. Mp: 148 °C; IR (KBr, cm−1): 2953,
1757, 1591, 1434, 1325, 1243, 1220, 1159, 1063; 1H NMR (400
MHz, DMSO-d6 + CDCl3): δ 7.84 (d, J = 8.4 Hz, 1H), 7.44 (d,
J = 7.6 Hz, 1H), 7.37−7.30 (m, 2H), 7.14 (d, J = 4.8 Hz, 1H),
6.80 (d, J = 5.6 Hz, 1H), 5.10 (s, 1H), 3.97 (d, J = 8.0 Hz, 2H),
3.75 (s, 3H), 3.06 (d, J = 4.4 Hz, 2H), 2.82 (d, J = 5.2 Hz, 2H);
13C NMR (100 MHz, DMSO-d6 + CDCl3): δ 169.5, 134.0, 132.4,
129.7, 129.6, 129.4, 127.0, 126.3, 122.7, 65.8, 52.0, 48.9, 47.5,
23.9; HRMS (ESI): Calcd for C16H17ClNO2S (M+ + H)
322.0669, Found 322.0670.
5-[(2-Chlorophenyl)methoxycarbonylmethyl]-6,7-
dihydrothieno[3,2-c]pyridin-5-ium Bromide (Clopidog-
rel Iminium). To a solution of clopidogrel (25 g, 0.0777 mol) in
dichloromethane (500 mL) was added N-bromosuccinimide (14 g,
0.0786 mol) at 0−5 °C over a period of 15 min. The resultant
reaction mixture was stirred for 8 h at 0−5 °C, warmed to room
temperature, and stirred for 36 h. The reaction mixture was
concentrated under reduced pressure below 55 °C. Acetone
(50 mL) was charged and stirred for 1 h at room temperature.
The precipitated solid was filtered and washed with acetone (5 mL);
the wet solid was dried at 55 °C under reduced pressure to furnish
21.8 g (70%) of pale-yellow solid with 99.5% purity by HPLC. Mp:
157 °C; IR (KBr, cm−1): 3039, 2903, 1738, 1621, 1512, 1225, 754;
1H NMR (400 MHz, CDCl3): δ 10.2 (s, 1H), 7.92 (d, J = 8.8 Hz,
EXPERIMENTAL SECTION
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The 1H NMR and 13C NMR spectra were measured in CDCl3,
DMSO-d6, and a mixture of CDCl3 and DMSO-d6 on a Varian
Gemini 400 MHz, FT NMR spectrometer. Chemical shifts are
reported in δ (ppm) relative to TMS (δ 0.0), DMSO-d6 (δ 39.50)
and CDCl3 (δ 77.0). FT IR spectra were recorded on the solid
state as KBr dispersion using Perkin-Elmer 1650 FT IR
spectrophotometer. The mass spectra (70 eV) were recorded
on an HP-5989A LC−MS spectrometer. Melting points were
determined by using the capillary method on POLMON (model
MP-96) melting point apparatus. A liquid chromatograph
equipped with a variable-wavelength UV detector and integrator
was used in recording HPLC data.
2-(Thiophen-3-yl)ethyl Tosylate (20). To a solution of
p-toulenesulfonyl chloride (40.8 g, 0.2140 mol) in toluene (100
mL) was added 2-(thiophene-3-yl)ethanol (19, 25 g, 0.1950
mol) and triethylamine (32.5 g, 0.3212 mol) at 0−5 °C. Then
temperature was raised to 25−35 °C and stirred for 12 h. The
unwanted solid was filtered and washed with toluene (2 ×
25 mL). Total filtrates were combined, washed with water (2 ×
50 mL) and concentrated below 70 °C under reduced pressure to
furnish 51.4 g (93%) of brown colored oily compound with
99.2% purity by HPLC. IR (KBr, cm−1): 3091, 1610, 1519, 1316,
1257, 1196, 1153, 824, 789; 1H NMR (400 MHz, CDCl3): δ 7.72
(d, J = 6.4 Hz, 2H), 7.30 (d, J = 8.2 Hz, 2H), 7.22 (d, J = 8.2 Hz,
1H), 6.96 (s, 1H), 6.86 (d, J = 5.2 Hz,1H), 4.20 (t, J = 7.0 Hz,
2H), 2.99 (t, J = 7.0 Hz, 2H), 2.44 (s, 3H); 13C NMR (100 MHz,
CDCl3): δ 144.8, 136.4, 133.0, 129.9, 128.3, 128.1, 125.9, 122.2,
70.0, 29.8, 21.7; Anal. Calcd for C13H14O3S2: C, 55.29; H, 5.0; S,
22.71; Found: C, 55.32; H, 5.04; S, 22.70.
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1H), 7.59 (d, J = 4.8 Hz, 1H), 7.44−7.37 (m, 4H), 7.30 (d, J = 5.2
Hz, 1H), 4.45−4.38 (m, 1H), 3.84 (s, 3H), 3.82−3.74 (m, 1H),
3.57−3.48 (m, 1H), 3.33−3.25 (m, 1H); 13C NMR (100 MHz,
CDCl3): δ 167.3, 162.0, 152.9, 134.9, 133.6, 132.4, 130.9, 128.9,
128.8, 128.5, 127.7, 127.3, 71.2, 54.1, 47.6, 23.6; HRMS (ESI):
Calcd for C16H15ClNO2S (M+) 320.0512, Found 320.0523.
AUTHOR INFORMATION
Corresponding Author
*To whom correspondence should be addressed. E-mail:
goverdhang@drreddys.com.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the management of Dr. Reddy’s Laboratories Ltd.,
IPDO, Bachupally, for supporting this work.
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