Synthesis and characterization of potential impurities of Vigabatrin-An anti epileptic drug

Article abstract of DOI:10.1080/00397911.2018.1550200

The present work describes the synthesis and characterization of four potential impurities of Vigabatrin (1) namely 2-(2-aminobut-3-enyl)malonic acid (2) (Vigabatrin USP impurity-E), 2-(2-oxo-5-vinylpyrrolidin-1-yl)acetic acid (3) (USP Tablets impurity), 4-aminohexanoic acid (4) and 2,2′-oxo-5,5′-bispyrrolidinyl ether (5). Compound 4 is a possible process related impurity of 1 where as compound 5 is a process related impurity of 5-ethoxy-2-pyrrolinone (16). All these impurities have a significant impact on the quality of the drug product. This work is extremely useful for generic pharmaceutical industry.

Full text of DOI:10.1080/00397911.2018.1550200

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
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Synthesis and characterization of potential
impurities of Vigabatrin-An anti epileptic drug
Kishore Karumanchi, Senthil Kumar Natarajan, Sunil Gadde, Ramadas
Chavakula, Raghu Babu Korupolu & Kishore Babu Bonige
To cite this article: Kishore Karumanchi, Senthil Kumar Natarajan, Sunil Gadde, Ramadas
Chavakula, Raghu Babu Korupolu & Kishore Babu Bonige (2019): Synthesis and characterization
of potential impurities of Vigabatrin-An anti epileptic drug, Synthetic Communications, DOI:
10.1080/00397911.2018.1550200
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https://doi.org/10.1080/00397911.2018.1550200
Synthesis and characterization of potential impurities
of Vigabatrin-An anti epileptic drug
Kishore Karumanchia,b, Senthil Kumar Natarajana, Sunil Gaddea,
Ramadas Chavakulaa, Raghu Babu Korupolub, and Kishore Babu Bonigeb
aChemical Research and Development, APL Research Centre-II, Aurobindo Pharma Ltd., Indrakaran
Village, Kandi Mandal, Sangareddy, Telangana, India;bDepartment of Engineering Chemistry, A. U.
College of engineering (A), Andhra University, Visakhapatnam,
Andhra Pradesh, India.
ABSTRACT
ARTICLE HISTORY
Received 18 August 2018
The present work describes the synthesis and characterization of four
potential impurities of Vigabatrin (1) namely 2-(2-aminobut-3-enyl)-
malonic acid (2) (Vigabatrin USP impurity-E), 2-(2-oxo-5-vinylpyrroli-
din-1-yl)acetic acid (3) (USP Tablets impurity), 4-aminohexanoic acid
(4) and 2,20-oxo-5,50-bispyrrolidinyl ether (5). Compound 4 is a pos-
sible process related impurity of 1 where as compound 5 is a process
related impurity of 5-ethoxy-2-pyrrolinone (16). All these impurities
have a significant impact on the quality of the drug product. This
work is extremely useful for generic pharmaceutical industry.
KEYWORDS
Vigabatrin; USP impurity-E;
Tablets Impurity; Impurities;
Synthesis and characterization
GRAPHICAL ABSTRACT
Introduction
Vigabatrin [c-vinyl GABA or 4-amino-5-hexenoic acid] (1) is a synthetic analogue of
GABA (c-aminobutyric acid) is a highly selective enzyme-activated inhibitor of GABA-
T in mammalian brain. Inhibition of GABA-T by c-vinyl GABA, which replaces GABA
as a substrate for GABA-T, increases the level of GABA in the central nervous system
(CNS). Thus, Vigabatrin is useful for the treatment of disorders associated with deple-
tion of GABA levels in the CNS such as tardive dyskinesia, schizopherenia and epi-
R
lepsy.[1] Vigabatrin is marketed under the brand name SabrilV.
The major challenge for the organic chemist is to produce a quality, safe, reprodu-
cible synthetic schemes within the developmental space. Furthermore, it is extremely
challenging to identify the impurities or related substances which are important compo-
nents arise in smaller amounts during the synthesis/manufacturing of a drug substance.
Besides, the presence of impurities in an active pharmaceutical ingredient (API) has a
CONTACT Kishore Karumanchi
kkishore009@gmail.com
Chemical Research and Development, APL Research
Centre-II, Aurobindo Pharma Ltd., Survey No. 71 & 72, Indrakaran Village, Kandi Mandal, Sangareddy, Telangana, India.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

2
K. KARUMANCHI ET AL.
significant impact on the quality and safety of the drug. Therefore, assessment and con-
trol of the related substances is mandatory to obtain various regulatory approvals.
As per the monograph of US pharmacopeia (USP-40), the threshold limit of specified or
un-specified impurity of 1 should be <0.10%. ICH (International Conference on
Harmonization) recommended that any impurity at a level greater than the identification
threshold in any batch manufactured by the proposed commercial process should be iden-
tified, based on the fact that unidentified impurities may cause health hazards by making
the API (Active Pharmaceutical Ingredient) inferior in quality.[2] To have a better under-
standing of impurity profile, these impurities have to be identified, synthesized and con-
trolled in the manufacturing process. In addition, these impurities in a pure form have a
crucial importance in determining the analytical parameters such as accuracy, specificity,
linearity, limit of detection (LOD), limit of quantification (LOQ) and relative retention fac-
tor.[3] So it is necessary to synthesize and characterize all of the related substances or
impurities due to the crucial importance in the purity of a drug substance in the pharma-
ceutical industry. Based on the above facts and their importance, the synthesis and charac-
terization of these potential impurities (2, 3, 4 and 5) is reported.
Discussion
Various synthetic routes were available in literature for the preparation of 1. Gittos et al.[4]
describes the preparation of Vigabatrin involves (Scheme 1) the condensation of two com-
mercially available raw materials diethylmalonate (6) and 1,4-dichloro-2-butene (7) to
afford 1,1-bis-ethoxycarbonyl-2-vinylcyclopropane (8). Compound 8 is treated with
ammonia gas in formamide at 120 ꢀC to afford diethyl-2-vinylcyclopropane-1,1-dicarboxy-
late (9). Compound 9 is hydrolyzed with sodium ethoxide in ethanol and decarboxylated
O
O
O
O
NaOEt / Ethanol
Reflux, 3 h
Cl
H3C
O
O
CH3
Cl
+
CH3
H3C
O
O
CH2
6
7
8
Ammonia Gas / Formamide
120-130 °C, 16 h
O
O
H2N
Acetic acid
Reflux, 16 h
NaOEt / Ethanol
Reflux, 16 h
HO
CH2
CH2
CH2
O
O
O
N
H
N
N
H
H
9
10
11
COOH
H2C
NH2
1
Scheme 1. Reported preparation of Vigabatrin (1).

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3
O
COOH
NH2 COOH
2
HO
Aq. KOH
CH2
H2C
O
N
H
10
Scheme 2. Preparation of 2-(2-aminobut-3-enyl)malonic acid (2).
CH2
CH2
O
n-Butyllithium, HMPA THF
Br
CH3
O
O
N
N
H
+
O
12
COOCH3
13
11
NaOH, Aq. HCl, Methanol
CH2
O
N
3
COOH
Scheme 3. Preparation of 2-(2-oxo-5-vinylpyrrolidin-1-yl)acetic acid (3).
with acetic acid to afford 5-vinyl-2-pyrrolidinone (11). Compound 11 is further hydro-
lyzed in the presence of aq. HCl and purified to afford Vigabatrin (1).
2-(2-Aminobut-3-enyl)malonic acid (2) (Vigabatrin USP impurity-E) was extremely
crucial among all other related substances listed in US pharmacopeia (USP 40) for
Vigabatrin. Any unknown impurities were determined against 2 in the purity method
described in USP. Compound 2 may be originated by the ring cleavage of 10 during
the hydrolysis of 10 to form 11. Thus, compound 2 was prepared by the hydrolysis
(Scheme 2) of 10 in the presence of KOH and isolated from a mixture of aqueous
methanol after acidification with aqueous HCl. The biggest challenge was encountered
during the isolation of 2 because of the presence of large amount of inorganic salt
which was very difficult to eliminate completely. Furthermore, both the salts and com-
pound 2 were soluble in water. Due to the significance of the impurity, it was required
to isolate compound 2 in salt free form. To remove salts, ion exchange chromatography
was employed. But the attempt was unsuccessful due to degradation of the product.
Later, the residue was added minimum water and kept stationary for few days. The
product was precipitated but yield was less. Finally, compound 2 was isolated from a
mixture of water and methanol system.
2-(2-Oxo-5-vinylpyrrolidin-1-yl)acetic acid (3) was a USP tablets impurity, may be
originated during manufacturing of Vigabatrin tablet. It was believed that, the impurity
was formed due to degradation during the tablet preparation. Initially, an attempt to
prepare 3 using 2-bromoacetic acid instead of methyl-2-bromoacetate (12) was unsuc-
cessful. Thus, it was prepared by the condensation of 12 and 11 in the presence of n-
butyllithium to afford 2-(2-oxo-5-vinylpyrrolidin-1-yl)aceticacidmethylester (13) which

4
K. KARUMANCHI ET AL.
NaBH4 /
Ethanolic HCl /
Ethanolic HCl
C2H5
Ethanol
O
O
O
O
O
O
OH
N
H
N
H
N
H
15
16
14
EtMgBr /
VMgBr / THF
Aq. KOH /
CH2
COOH
Acetic acid / IPA
H2C
N
H
NH2
1
11
Scheme 4. Reported preparation of Vigabatrin 1.
O2N
COOH
Raney Ni, Hydrogenation
H3C
H3C
COOH
NH2
S
17
4
Scheme 5. Reported preparation of 4-aminohexanoic acid (4).
was further hydrolyzed insitu in the presence of a base at lower temperature to afford 3
(Scheme 3).
In another report for the synthesis of 1[5] (Scheme 4) involves by the reduction of
succinimide (14) in the presence of NaBH4 to form 5-hydroxy-2-pyrrolidinone (15) in
ethanol which upon treating with ethanolic HCl insitu to afford 5-ethoxy-2-pyrrolidi-
none (16). Compound 16 is treated in a mixture of EtMgBr (Ethyl Magnesium
Bromide) and VMgBr (Vinyl Magnesium Bromide) in THF to afford 11. Further, com-
pound 11 is hydrolyzed in the presence of KOH to afford 1.
4-Aminohexanoic acid (4) was a process related impurity, may be originated during
the preparation of 11 from 16 in a mixture of EtMgBr (Ethyl Magnesium Bromide) and
VMgBr (Vinyl Magnesium Bromide) in THF. It may be due to usage of excess moles of
EtMgBr which may lead to form 5-ethyl-2-pyrrolidinone (18). Further, compound 18
was carried forwarded and hydrolyzed to form 4 in the next step.
A single reported process of 4[6] was in the milligram quantities by Raney nickel cata-
lyzed reduction of 5-methyl-4-nitrothiophene-2-carboxylic acid (17) (Scheme 5).
However, there was no proper synthetic procedure and characterization described.
In our approach, compound 16 was treated with EtMgBr to afford 5-ethyl-2-pyrroli-
dinone (18) which was further hydrolyzed in the presence of KOH to afford 4. In
another approach, compound 4 was prepared by Pd/C catalyzed reduction of 1 in aque-
ous methanol (Scheme 6).
2,20-Oxo-5,50-bispyrrolidinyl ether (5) was observed as a key impurity during the
manufacturing of 5-ethoxy-2-pyrrolidinone (16) via 5-hydroxy-2-pyrrolidinone (15)
from succinimide (14). It is believed that, the impurity was originated by the self
condensation of compound 15 under thermal condition. The plausible mechanism for
compound 5 was stated in Scheme 7.

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5
COOH
Aq. KOH / Acetic acid / IPA
Method 1
EtMgBr / THF
C2H5
CH3
H3C
O
O
O
N
H
N
H
NH2
4
16
18
Pd/C, Aq. Methanol /
Hydrogenation
Method 2
COOH
H2C
NH2
1
Scheme 6. Preparation of 4-aminohexanoic acid (4).
OH
- H2O
+
OH
O
HO
O
O
O
5
O
O
N
H
15
N
H
N
H
N
H
N
H
Scheme 7. Plausible mechanism of 2,20-oxo-5,50-bispyrrolidinyl ether (5).
90-100 °C / 30 min
O
OH
O
5
O
O
N
H
N
H
N
H
neat, 77%
15
Scheme 8. Preparation of 2,20-oxo-5,50-bispyrrolidinyl ether (5).
The sole reported synthesis of 5[7] involves by treating compound 16 in large volume of
water under reflux condition to afford 5 with 54% yield where as in less volume of water
to afford 15. The above process was attempted to prepare 5 but obtained product was 15
instead of 5. The desired compound formation was not observed. Furthermore, it was
described that there was no molecular ion was observed for 5 in MASS analysis.[7]
However, it was required to prepare compound 5 in a pure form to meet the require-
ment. Hence, a simple and reproducible methodology was developed under solvent free
condition with 77% of yield and good quality. In our approach, compound 15[7] was
allowed for self-condensation at 90–100 ꢀC under neat conditions to afford 5 (Scheme 8).
Experimental Section
All melting points were uncorrected and were determined using a Reichert Thermopan
1
apparatus. H and 13C NMR spectra were recorded by a Bruker Advance 300 MHz and
Varian 500 MHz spectrometer using TMS as internal standard in D2O and DMSO-d6.
1
The H chemical shift values were reported in the d scale relative to TMS (d 0.00) and
the 13C chemical shift values were given relative to D2O, CDCl3 and DMSO-d6 as
internal standards. The IR spectra were recorded as KBr pellets using Perkin Elmer

6
K. KARUMANCHI ET AL.
Spectrum One Fourier Transform (FT) IR spectrophotometer. High-resolution mass
spectral (HRMS) analysis was performed using the electrospray ionization (ESI) method
and a Xevo G2 QTOF mass spectrometer. IR spectra were obtained from Perkin-Elmer
Spectrum ES version. All raw materials were purchased from commercial sources and
used without purification.
Preparation of (2-aminobut-3-enyl)malonic acid (Vigabatrin USP impurity-E) (2)
Compound 10 (8 g, 0.05 mol) was added to a solution of potassium hydroxide (3.2 g,
0.05 mol) in DM water (5 mL) at ambient temperature. The reaction mass was warmed
and stirred under reflux for 6 h. The reaction mass was acidified with Conc. HCl and
the water in the reaction mass was distilled out completely under reduced pressure to
obtain semisolid mass. Further, a mixture of DM water (4 mL) and methanol (40 mL)
was added. The reaction mass was stirred for 24 h at ambient temperature. The precipi-
tated product was collected by filtration, dried and characterized as 2 as an off-white
1
solid. H NMR (300 MHz, D2O): 2.13-2.22 (m, 1H), 2.28-2.38 (m, 1H), 3.32-3.36 (m,
1H), 3.81-3.89 (m, 1H), 5.45-5.50 (m, 2H), 5.77-5.89 (m, 1H); 13C NMR (300 MHz,
D2O): 31.73, 51.41, 52.99, 122.12, 132.07, 175.25, 175.45; HRMS (ESI) calculated for
C7H11NO4 (M þ H)þ 174.0766, found: 174.0768; Melting Point: Decomposed at 152 ꢀC;
IR (KBr) 3967, 3854, 3850, 3828, 3808, 3788, 3752, 3741, 2346, 2289, 2215,
1986, 1930 cmꢁ1
.
Preparation of 2-(2-oxo-5-vinylpyrrolidin-1-yl)acetic acid (3)
To a solution of 11 (10 g, 0.09 mol) in THF was added n-butyllithium (57 mL, 15% in
hexanes, d ¼ 0.68, 0.09 mol) at ꢁ65 to ꢁ70 ꢀC under nitrogen atmosphere. Compound
12 (13.8 g, 0.09 mol) was added portion wise through a dropping funnel at ꢁ65 to
ꢁ70 ꢀC. The reaction mass was warmed to 20–30 ꢀC and was added hexamethylphos-
phoramide (HMPA) (5 mL). The reaction mass was stirred for 5 h at the same tempera-
ture. Aqueous ammonium chloride was added cautiously and was stirred for 30 min at
same temperature. The layers were separated and the aqueous layer was extracted with
ethyl acetate. Combined organic extract was washed with brine solution. The organic
layer was separated and evaporated to afford 2-(2-oxo-5-vinylpyrrolidin-1-yl) aceticacid-
methylester (13) as pale brown colour oil (14 g). The above obtained residue was dis-
solved in methanol (140 mL) and sodium hydroxide (3.6 g, 0.09 mol) solution in
methanol (36 mL) was added portion wise for 30 min. The reaction mixture was stirred
for 16 h at 20–30 ꢀC and the methanol was distilled out completely under reduced pres-
sure. DM water (50 mL) was added to the obtained residue and was washed with DCM
(25 mL). The separated aqueous layer was acidified with conc. HCl and washed with
DCM (25 mL). The separated aqueous layer was saturated with NaCl and extracted with
DCM (100 mL). The separated organic layer distilled out completely under reduced
1
pressure to afford 3 (5 g, 33%). H NMR (300 MHz, DMSO-d6): 1.73–1.83 (m, 1H),
2.22–2.42 (m, 3H), 3.55 (d, 1H, J ¼ 8.7 Hz), 4.05–4.20 (m, 2H), 5.25–5.34 (m, 2H),
5.72–5.84 (m, 1H); 13C NMR (300 MHz, DMSO-d6): 24.98, 29.28, 61.09, 118.39, 138.05,

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7
þ
170.02, 174.343; HRMS (ESI) calculated for C8H11NO3 (M þ H)
170.0817, found:
170.0818; Melting Point: 112–115 ꢀC; IR (KBr) 3455, 3244, 2981, 2139, 1737, 1337 cmꢁ1
.
Preparation of 4-aminohexanoic acid (4) (Method 1)
Compound 18 (15 g, 0.13 mol) was added to a solution of potassium hydroxide (8.27 g,
0.13 mol) in DM water (15 mL) at ambient temperature. The reaction mass was warmed
to reflux and stirred for 2 h under reflux. The reaction mass was cooled to 25–30 ꢀC
and diluted with isopropanol (135 mL). Acetic acid (8 g, 0.13 mol) was further added
portion wise for 30 min. The slurry was stirred for 2 h at 20–30 ꢀC and further 2 h at
0–5 ꢀC. The precipitated product was collected by filtration and dried to afford 4 (13 g,
1
74%) as an off-white solid. H NMR (500 MHz, D2O): 0.97 (t, 3H, J ¼ 7 Hz), 1.63–1.76
(m, 2H), 1.83–1.95 (m, 2H), 2.31–2.34 (t, 2H, J ¼ 7.5 Hz), 3.19–3.24 (m, 1H); 13C NMR
(300 MHz, D2O): 8.60, 24.95, 28.18, 33.46, 53.14, 181.66; HRMS (ESI) calculated for
C6H13NO2 (M þ H)þ 132.1025, found: 132.1009; Melting Point: 181–183 ꢀC, lit.6
181–182 ꢀC; IR (KBr) 2966, 2941, 2820, 2672, 2626, 2205, 1663, 1453, 1348, 1296 cmꢁ1
.
Preparation of 4-Aminohexanoic acid (4) (Method 2)
Compound 1 (5 g), 10% Pd/C (0.2 g) was added to a mixture of DM water (10mL) and
methanol (50 mL) into an autoclave unit at ambient temperature. Hydrogen gas pressure of
5 Kg/cm2 was applied and the reaction mixture was stirred by maintaining at the same pres-
sure for 16h at 20–30ꢀC. The palladium catalyst was removed by hyflo bed filtration and
the solvent in the filtrate was distilled out completely under reduced pressure. Isopropanol
(15mL) was added to the obtained residue. The precipitated product was collected by filtra-
1
tion, dried and characterized as 4 (4.5 g, 90%) as an off-white power. The H NMR,
13C NMR and MASS spectral data were recorded and found identical (see above).
Preparation of 2,20-oxo-5,50-bispyrrolidinyl ether (5)
Compound 15 (10 g, 0.09 mol) was stirred at 90-100 ꢀC under neat conditions for 30 min.
The reaction mass was cooled to 30–35 ꢀC and DCM (100 mL) was added. The reaction
mass was stirred for 1 h at the same temperature. The reaction mass was further cooled to
20–30 ꢀC. The product was collected by filtration, dried and characterized as 5 as an off-
white solid (3.5 g, 77%). 1H NMR (300 MHz, DMSO-d6): 1.76–1.86 (m, 2H), 1.95–2.09 (m,
2H), 2.15–2.50 (m, 4H), 5.03 (d, 2H, J ¼ 3 Hz), 8.58 (s, 2H); HRMS (ESI) calculated for
þ
C8H12N2O3 (M þ Na) : 207.0746, found: 207.0762; Melting Point: 169–173 ꢀC; IR (KBr)
3182, 3108, 2991, 2968, 2929, 2773, 1447, 1422, 1404, 1373, 1183, 1160 cmꢁ1
.
Conclusion
A simple, efficient synthesis and characterization of 2, 3, 4 and 5, the potential impur-
ities of Vigabatrin have been successfully demonstrated. In addition, the possible path-
ways for the formation of these impurities are also described. The synthesis of these
impurities not only helps in obtaining good quality of the drug substance but also helps

8
K. KARUMANCHI ET AL.
in establishing the impurity profile of 1 by understanding the cause of its origin.
Keeping in view the regulatory importance of Vigabatrin impurities, our efforts to syn-
thesize and characterize them effectively have proved to be beneficial.
Acknowledgement
The authors gratefully acknowledge Aurobindo Pharma Limited for supporting the work. The
authors are also thankful to the Chemical Research Department and Analytical Research
Department for the support and co-operation.
Supporting Information
Full experiment detail, 1H NMR spectra, 13C NMR spectra and HRMS. This material can be
found via the “Supplementary Content” section of this article’s webpage.
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