Concise Synthesis of (±)-Clopidogrel via Carboxylation of Benzylamine with CO2

Article abstract of DOI:10.1080/00397911.2015.1061671

A concise and efficient synthesis of (±)-clopidogrel, an antithrombotic agent, is achieved by inserting CO₂ at the benzylic position as the key reaction without using any toxic transition metals. The overall yield of the synthetic process is 38% and the salient features include operationally simple process chemistry and fewer steps.

Full text of DOI:10.1080/00397911.2015.1061671

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Concise Synthesis of (±)-Clopidogrel via
Carboxylation of Benzylamine with CO2
V. Venkataramasubramaniana & Arumugam Sudalaia
a Chemical Engineering and Process Development Division, National
Chemical Laboratory, Pune, Maharashtra, India
Accepted author version posted online: 26 Jun 2015.Published
online: 26 Jun 2015.
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To cite this article: V. Venkataramasubramanian & Arumugam Sudalai (2015): Concise Synthesis
of (±)-Clopidogrel via Carboxylation of Benzylamine with CO2, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, DOI:
10.1080/00397911.2015.1061671
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Synthetic Communications1, 0: 1–7, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397911.2015.1061671
CONCISE SYNTHESIS OF ( )-CLOPIDOGREL VIA
CARBOXYLATION OF BENZYLAMINE WITH CO2
V. Venkataramasubramanian and Arumugam Sudalai
Chemical Engineering and Process Development Division, National Chemical
Laboratory, Pune, Maharashtra, India
GRAPHICAL ABSTRACT
Abstract A concise and efficient synthesis of (ꢀ)-clopidogrel, an antithrombotic agent, is
achieved by inserting CO2 at the benzylic position as the key reaction without using any
toxic transition metals. The overall yield of the synthetic process is 38% and the salient
features include operationally simple process chemistry and fewer steps.
Keywords Antithrombatic agent; clopidogrel; CO2 insertion; Plavix
INTRODUCTION
(S)-Methyl-2-(2-chlorophenyl)-2-(6,7-dihydrothieno[3,2-c]pyridin-5(4H)-yl)acet-
ate, also known as (S)-clopidogrel (1a), is marketed as hydrogen sulfate salt. It is a
potent antiaggregant and antithrombotic drug demonstrated in several experimental
models of thrombosis[1] and was the second top-selling drug worldwide in 2005
with the commercial trade name of Plavix. It was marketed and licensed by Sanofi
in 1986. The drug was launched on the market following a successful clinical
evaluation[2] and studies, which has shown that it tends to block the platelet
aggregation more than aspirin 2 and ticlopidine 3, even at much lower dosage.[3]
Clopidogrel (1a) has an absolute S configuration[4] and the corresponding R enantio-
mer is totally devoid of antiaggregating activity. Because of the high biological activity
of (S)-clopidogrel, many organic chemists are interested in synthesizing it via chiral or
resolution pathway.
There are mainly two kinds of strategies known for its synthesis, such as (i) sub-
stitution reaction of 2-chlorophenylglycine or derivatives of 2-chloromandelic acid
Received April 21, 2015.
Address correspondence to A. Sudalai, Chemical Engineering and Process Development Division,
National Chemical Laboratory, Dr. Homi Bhabha Road, Pune Maharashtra, 411008 India. E-mail:
a.sudalai@ncl.res.in
1

2
V. VENKATARAMASUBRAMANIAN AND A. SUDALAI
Scheme 1. Reagents and conditions: (i) thiophene-2-ethanolamine (1.1 equiv), MgSO4(2 equiv), CH2Cl2,
20 min, 25 °C; (ii) NaBH4(2.5 equiv), MeOH, 15 min, 25 °C, 92% for two steps; (iii) Boc2O (1.5 equiv),
Et3N (1.5 equiv), DMAP (40 mol%), CH3CN, 7 h, 25 °C, 85%; (iv) CO2(1–2 atm), n-BuLi (1 equiv),
THF, ꢁ78 °C, 3 h, then MeI (2 equiv), 25 °C, 3 h, 65%; (v) TFA (2.5 equiv), CH2Cl2, 0 °C, 4 h, and
paraformaldehyde (1.2 equiv), 24 h, one pot, 68% yield.
and (ii) attack of nitrogen nucleophile onto a-halogen substituted phenyl acetonitrile
or phenyl acetate, both involving SN2 substitution reaction. Many other recent meth-
ods are known for the synthesis of clopidogrel such as SN2 displacement,[5] stereose-
lective hydrogenation,[6] Mannich-type multicomponent reaction,[7] Cu(II)-catalyzed
oxidative coupling reaction,[8] and patented methods,[9] involving the resolution of
(ꢀ)-clopidogrel using 10-L-camphorsulfonic acid (L-CSA) or its precursor a-amino
ester using D- or L- tartaric acid. Some of these intermediates in turn were obtained
via simple SN2 displacement reactions and NaCN addition on imines.[9a] Some of
these reported methods do utilize highly poisonous cyanide as nucleophile and highly
expensive toxic transition metals as catalyst in existing methods of its synthesis.
Thus, there is a need for an efficient synthesis of (ꢀ)-clopidogrel (ꢀ)-1 with one
carbon homologation using insertion of CO2 at the benzylic position,[10] and this
strategy has not been reported previously.
RESULTS AND DISCUSSION
CO2 insertion for an alternative energy sources is a topic of current interest
because of the constant rise in global warming and atmospheric CO2 concentra-
tions.[11] Because it is an attractive one-carbon source for organic synthesis, due to
its low cost, low toxicity, and ease of handling, we were interested in its utility in
some of organic transformations to synthesize the various substituted cyclic carbo-
nates and propargylic acids.[12] Herein, we present a concise synthesis of (ꢀ)-clopido-
grel (ꢀ)-1 by insertion of CO2 at the benzylic position as the key reaction from
readily available and cheap starting materials.
Retrosynthetic analysis of (ꢀ)-clopidogrel (ꢀ)-1 reveals that a -amino ester 7
could be visualized as the key intermediate, which in turn can be obtained from
the insertion of CO2 at the benzylic position in Boc-protected benzylamine 8 derived
from 2-chlorobenzaldehyde (4).
Accordingly, the synthesis of (ꢀ)-clopidogrel (ꢀ)-1 was undertaken starting
from commercially available 2-chlorobenzaldehyde (4). The condensation of 2-chlor-
obenzaldehyde (4) with thiophene-2-ethanolamine in CH2Cl2 and anhydrous MgSO4

SYNTHESIS OF ( )-CLOPIDOGREL
3
Figure 1. Structure of (S)-clopidogrel (1a), aspirin (2), and ticlopidine (3).
at room temperature provided the corresponding imine. The subsequent reduction of
imine using NaBH4 in methanol at 0 °C gave the N-substituted-2-chlorobenzylamine
5, which was then protected as Boc-carbamate 6 by treating it with (Boc)2O in
1
CH3CN at room temperature. (The H NMR spectrum of 6 showed the presence
of rotameric proton signals at d 1.41 (brs, 4H), and 1.49 (brs, 5H) as two broad sing-
lets for nine Boc-methyl protons and at d 4.45 (brs, 1H) and 4.55 (brs, 1H) as two
broad singlets for two benzylic protons. Its 13C NMR spectrum also displayed the
presence of rotameric carbon signals at d 155.3, and 155.6 for carbonyl carbon
present in the Boc group.) Insertion of CO2 at benzylic position of carbamate 6
was achieved by using n-BuLi as a strong base with the bubbling of CO2 at ꢁ 78 °
C in dry tetrahydrofuran (THF) to provide the a -amino acid as its lithium salt,
as indicated by the formation of turbid pale yellow solution after 3 h. Subsequently,
a -amino acid salt was treated with NaHCO3 and MeI in dimethylformamide
(DMF), which gave the a -amino ester 7 in 65% yield. To make this process in an
asymmetric fashion, CO2 was bubbled to a solution containing carbamate 6,
(þ)-sparteine (1 equiv),[13] and n-BuLi at ꢁ 78 °C. However, we ended up with the
a -amino ester 7 in only 3% ee. The Boc group in a -amino ester 7 was deprotected
using trifluoroacetic acid in CH2Cl2 at 25 °C, followed by electrophilic aromatic
cyclization (Pictet–Spengler type) on the thiophene ring was achieved, on treatment
with paraformaldehyde in one pot at 10 °C. It was then quenched with saturated
NaHCO3 solution, which furnished the (ꢀ)-clopidogrel free base, (ꢀ)-1, in 68% yield
and whose spectral data are in complete agreement with those reported in the
literature.[5]
CONCLUSION
In conclusion, a concise synthesis of (ꢀ)-clopidogrel with an overall yield of
38% is described. The key reaction employed here is the insertion of CO2 at the
benzylic position, which has been unprecedented for its synthesis. The salient fea-
tures include fewer steps and avoiding the use of toxic transition metals.
EXPERIMENTAL
All reactions were carried out under a nitrogen atmosphere. Unless otherwise
stated, all the chemicals and reagents were obtained commercially. Dry solvents were
prepared by the standard procedures. Analytical thin-layer chromatography (TLC)
was done on precoated silica-gel plates (Kieselgel 60 F254, Merck). Column chroma-
tographic purifications were done with 230- to 400-mesh silica gel. NMR spectra
were recorded in CDCl3 on AV 200 and AV 400 MHz Bruker NMR spectrometers.

4
V. VENKATARAMASUBRAMANIAN AND A. SUDALAI
All chemical shifts are reported in parts per million (ppm) downfield from
tetramethylsilane (TMS), and peak multiplicities are referred to as singlet (s), doublet
(d), quartet (q), and multiplet (m). Elemental analyses were performed on an
Elmentar-Vario-EL (Heraeus Company Ltd., Germany). Infrared (IR) spectra
were recorded in CHCl3 using Shimadzu FTIR-8400 spectrophotometer. Liquid
chromatography–mass spectrometry (LC-MS) was carried out on a Thermo
Finnigan Surveyor MSQ LC-MS instrument.
N-(2-Chlorobenzyl)-2-(thiophen-2-yl)ethan-1-amine (5)
2-(Thiophen-2-yl)ethan-1-amine (3.3 g, 25.63 mmol) in CH2Cl2 (15 mL) and
anhydrous Na2SO4 were added to a stirred solution of 2-chlorobenzaldehyde (3 g,
21.36 mmol) in CH2Cl2 (60 mL) at 25 °C. The reaction mixture was allowed to stir
at the same temperature for 30 min. This was followed by the addition of MeOH
(30 mL) and NaBH4(2 g, 53.4 mmol) at 0 °C, and this mixture was allowed to stir
for 30 min. It was then quenched with saturated NH4Cl solution. The solvents were
evaporated in rotavapour and the residue was extracted with CH2Cl2 (3 × 60 mL).
The combined organic layers were washed with brine, dried over anhydrous Na2SO4,
and concentrated under reduced pressure to give the crude benzylamine. Silica-gel
column chromatographic purification of the crude product using petroleum ether =
EtOAc = Et3N (70:25:5) gave benzylamine 5 (4.9 g) as a yellow liquid.
Yield: 4.9 g, 92%; IR (CHCl3, cmꢁ1): nmax 3440, 3331, 2912, 2826, 1471, 1441,
1
1049, 1037, 752, 696; H NMR (200 MHz, CDCl3): d 1.61 (br s, 1H), 2.84–2.98 (m,
2H), 2.99–3.11 (m, 2H), 3.89 (s, 2H), 6.78–6.85 (m, 1H), 6.86–6.96 (m, 1H), 7.11 (dd,
J ¼ 5.1, 1.1 Hz, 1H), 7.15–7.26 (m, 2H), 7.28–7.40 (m, 2H); 13C NMR (50 MHz,
CDCl3): d 30.4, 50.3, 50.9, 123.4, 124.9, 126.7, 128.1, 129.4, 129.9, 133.6, 137.5,
142.3. Analysis: C13H14ClNS requires C, 62.02; H, 5.61; N, 5.56; S, 12.73. Found:
C, 61.89; H, 5.72; N, 5.71; S, 12.65%.
tert-Butyl-(N-2-chlorobenzyl)-(N0-2-(thiophen-2-yl)ethyl) carbamate (6)
(Boc)2O (3.9 g, 18 mmol) in dry CH3CN (10 mL) was added to a solution of the
benzyl amine 5, (2.3 g, 9 mmol), triethylamine (1.38 mL, 9.93 mmol), and DMAP
(0.6 g, 4.51 mmol) in dry CH3CN (20 mL) under nitrogen at 0 °C, and the mixture
was allowed to warm to room temperature and stirred for 7 h. The reaction mixture
was then quenched with water, the solvent was evaporated, and the crude product
was extracted with CH2Cl2 (3 × 30 mL). The combined organic extracts were washed
with water (2 × 50 mL) and brine, dried over anhydrous Na2SO4, and concentrated
under reduced pressure. Silica-gel column chromatographic purification of the crude
product using petroleum ether = EtOAc (95:5) gave carbamate 6 (2.7 g) as a colorless
liquid.
Yield: 2.7 g, 85%; IR (CHCl3, cmꢁ1): nmax 3019, 2981, 2839, 2400, 1698, 1612,
1
1585, 1368, 1216, 1035, 1167; H NMR (500 MHz, CDCl3): d 1.41 (brs, 4H), 1.49
(brs, 5H), 3.00 (brs, 1H), 3.07 (brs, 1H), 3.41 (brs, 1H), 3.49 (brs, 1H), 4.45 (brs,
1H), 4.55 (brs, 1H), 6.75 (brs, 1H), 6.89 (dd, J ¼ 5.0, 3.5 Hz, 1H), 7.10 (d, J ¼ 5.2 Hz,
1H), 7.16–7.29 (m, 3H), 7.33 (d, J ¼ 7.6 Hz, 1H); 13C NMR (125 MHz, CDCl3): d
28.4, 29.0, 47.6, 48.9, 49.1, 49.2, 76.7, 77.3, 80.0, 96.2, 123.7, 125.2, 126.9, 128.0,

SYNTHESIS OF ( )-CLOPIDOGREL
5
128.3, 128.4, 129.1, 129.5, 133.0, 133.4, 135.7, 135.9, 141.1, 155.3, 155.6. Analysis:
C18H22ClNO2S required C, 61.44; H, 6.30; N, 3.98; S, 9.11. Found: C, 61.26; H,
6.01; N, 3.79; S, 9.02%.
Methyl-2-((tert-butoxycarbonyl)(2-(thiophen-2-yl)ethyl)amino)-2-(2-
chlorophenyl)acetate (7)
n-BuLi (2.96 mL, 4.7 mmol) was added dropwise to a stirred solution of
carbamate 6 (1.5 g, 4.3 mmol) in dry THF, at ꢁ 78 °C under N2 atmosphere for
15 min. CO2(1 atm) gas was bubbled through the reaction mixture and allowed to stir
for 3 h until it formed a pale yellow turbid solution. This was followed by the
addition of NaHCO3 (540 mg, 6.39 mmol) and MeI (1.5 g, 10.65 mmol) in DMF
and stirred for further 3 h at 25 °C. The reaction mixture was quenched with satu-
rated NH4Cl solution, and the solvent was evaporated under reduced pressure.
The obtained crude mixture was extracted with EtOAc (3 × 15 mL) and washed with
brine (3 × 10 mL). The combined organic extracts were dried over anhydrous Na2SO4
and concentrated. Silica-gel column chromatographic purification of the crude pro-
duct using petroleum ether = EtOAc (90:10) as eluent gave a -amino ester 7 (1.13 g)
as a colorless liquid.
Yield: 1.13 g, 65%; IR (CHCl3, cmꢁ1): nmax 2977, 1751, 1696, 1398, 1367, 1304,
1
1251, 1214, 1169, 966, 756; H NMR (200 MHz, CDCl3): d 1.51 (s, 9H), 2.32–2.35
(m, 1H), 3.01–3.04 (m, 1H), 3.15–3.23 (m, 1H), 3.38–3.53 (m, 1H), 3.79 (s, 3H),
6.13 (brs, 1H), 6.52 (brs, 1H), 6.78–6.82 (m, 1H), 7.01 (d, J ¼ 4.9 Hz, 1H), 7.22–
7.35 (m, 3H), 7.43–7.48 (m, 1H); 13C NMR (50 MHz, CDCl3): d 28.3, 29.5, 47.6,
52.2, 60.0, 60.8, 80.5, 96.0, 123.2, 124.6, 126.6, 126.9, 130.0, 133.1, 135.4, 141.1,
154.4, 155.3, 170.8. LCMS (ESI, m=z) calculated for C20H24ClNO4SNa (MþNa)þ
432.1; found 432.13. Analysis: C20H24ClNO4S required C, 55.49; H, 5.59; N, 3.24;
S, 7.41. Found: C, 55.38; H, 5.46; N, 3.15; S, 7.37%.
Methyl 2-(2-Chlorophenyl)-2-(6,7-dihydrothieno[3,2-c]pyridin-5(4H)-
yl)acetate ( )-Clopidogrel (1)
Trifluoroacetic acid (0.25 mL, 3.2 mmol) was added to a solution of a -amino
ester 7 (435 mg, 1.06 mmol) in dry CH2Cl2 and kept under stirring for 4 h at room
temperature. Subsequently, paraformaldehyde (38 mg, 1.3 mmol) was added to the
reaction mixture and allowed to stir for 24 h at room temperature. It was then
quenched with saturated NaHCO3 solution at 0 °C and further stirred for 10 min.
The organic layer was separated and the aqueous layer was extracted with CH2Cl2.
The combined organic extracts were washed with water (2 × 10 mL) and brine, dried
over anhydrous Na2SO4, and concentrated under reduced pressure. Silica-gel column
chromatographic purification of the crude product using petroleum ether=EtOAc
gave (ꢀ)-clopidogrel 1 (0.24 g) as a pale yellowish liquid.
Yield: 0.24 g, 70%; IR (CHCl3, cmꢁ1): nmax 2977, 1741, 1654, 1434, 1203, 1167,
1042, 755; 1H NMR (200 MHz, CDCl3): d 2.88 (brs, 4H), 3.53–3.78 (m, 5H), 4.89 (s,
1H), 6.65 (d, J ¼ 5.2 Hz, 1H), 7.04 (d, J ¼ 5.2 Hz, 1H), 7.22–7.25 (m, 1H), 7.28–7.32
(m, 1H), 7.35–7.44 (m, 1H), 7.56–7.80 (m, 1H); 13C NMR (50 MHz, CDCl3): d 25.5,
48.3, 50.7, 52.1, 67.7, 76.4, 77.6, 96.2, 122.8, 125.2, 127.2, 129.4, 129.8, 130.0, 133.1,

6
V. VENKATARAMASUBRAMANIAN AND A. SUDALAI
133.8, 134.7, 171.0. Analysis: C16H16ClNO2S required C, 59.72; H, 5.01; N, 4.35; S,
9.96. Found: C, 59.61; H, 4.95; N, 4.15; S, 9.76%.
ACKNOWLEDGMENT
The authors are thankful to V. V. Ranade, chair, Chemical Engineering and
Process Development Division, for his constant encouragement and support.
FUNDING
V. V. thanks the Council for Scientific and Industrial Research, New Delhi, for
the award of a senior research fellowship.
SUPPLEMENTAL MATERIAL
Supplemental data for this article can be accessed on the publisher’s website.
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