Synthesis of Repaglinide Congeners

Article abstract of DOI:10.1080/00397911.2015.1064139

This report describes a synthesis of two potent impurities of repaglinide, benzyl repaglinide 1 and repaglinide isomer 2, from commercially available raw materials: 2-fluoro benzonitrile, (S)-3-methyl-1-[2-(piperidin-1-yl)phenyl]butylamine (5), and 3-ethoxy-[4-(ethoxycarbonyl)phenyl]acetic acid (7). These impurities are the crucial components in determining the quality of the drug substance, repaglinide, during its manufacturing.

Full text of DOI:10.1080/00397911.2015.1064139

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Synthesis of Repaglinide Congeners
Dhanraj T. S. S. Sundaramab, Jayati Mitraa, C. H. Rajesha, Aminul Islama, Koilpillai Joseph
Prabahara, Battula Venkateswara Raob & Sanasi Paul Douglasb
a Chemical Research and Development, APL Research Center-II, Indrakaran (V), Andhra
Pradesh, India
b Department of Engineering chemistry, Andhra University College of Engineering (A), Andhra
University, Visakhapatnam, Andhra Pradesh, India
Accepted author version posted online: 25 Jun 2015.
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To cite this article: Dhanraj T. S. S. Sundaram, Jayati Mitra, C. H. Rajesh, Aminul Islam, Koilpillai Joseph Prabahar, Battula
Venkateswara Rao & Sanasi Paul Douglas (2015): Synthesis of Repaglinide Congeners, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, DOI: 10.1080/00397911.2015.1064139
To link to this article: http://dx.doi.org/10.1080/00397911.2015.1064139
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Synthesis of Repaglinide Congeners
Dhanraj T. S. S. Sundaram1,2,, Jayati Mitra1, C. H. Rajesh1, Aminul Islam1, Koilpillai
Joseph Prabahar1, Battula Venkateswara Rao2, Sanasi Paul Douglas2
1Chemical Research and Development, APL Research Center-II, Indrakaran (V), Andhra
Pradesh, India
2Department of Engineering chemistry, Andhra University College of Engineering (A),
Andhra University, Visakhapatnam, Andhra Pradesh, India
Address corresponds to Dhanraj T.S.S. Sundaram, Chemical Research and Development,
APL Research Center-II, Aurobindo Pharma Ltd., Survey No. 71 & 72, Indrakaran (V),
Medak district- 502329, Andhra Pradesh, India. E-Mail: drajchem2002@rediffmail.com
Abstract
This report describes a synthesis of two potent impurities of repaglinide, benzyl
repaglinide 1 and repaglinide isomer 2 from commercially available raw materials 2-
fluoro benzonitrile, (S)-3-methyl-1-[2-(piperidin-1-yl)phenyl]butylamine (5) and 3-
ethoxy-[4-(ethoxycarbonyl)phenyl]acetic acid (7). These impurities are the crucial
components in determining the quality of the drug substance, repaglinide during its
manufacturing.
1

KEYWORDS: Hypoglycemic agent, N-acetyl-L-glutamic acid, impurity, repaglinide
isomer, benzyl repaglinide
INTRODUCTION
Repaglinide 3 is an antidiabetic drug and chemically known as (S)-(+)-2-ethoxy-4-(2-{3-
methyl-1-[2-(piperidin-1-yl)phenyl]butylamino}-2-oxoethyl)benzoic acid, which lowers
the blood glucose levels by stimulating the release of insulin from the pancreas (Fig.
1).[1,2] It is an oral hypoglycemic agent used for treating type-2 non-insulin dependent
diabetes mellitus.[3,4]
Synthesis of 3 as depicted above, involves the condensation of two key raw materials
(S)-3-methyl-1-[2-(piperidin-1-yl)phenyl]butylamine (5) and 3-ethoxy-[4-
(ethoxycarbonyl)phenyl]acetic acid (7) followed by ester hydrolysis in basic medium.[5]
Most of the API’s (active pharmaceutical ingredients) are prepared by the multistep
chemical synthesis and during its synthesis some of the process related impurities, also
called as related substances may form in trace levels and retain in the drug substance.
These impurities may have a significant impact on the quality and safety of API based on
their toxic behavior. Therefore, it is always recommended by ICH guidelines to study the
origin and control of such impurities which are present in the API at a level ≥ 0.05%
w/w.[6] Furthermore, it is equally important to identify, synthesize and characterize such
impurities in a pure form to check the analytical parameters such as specificity, linearity,
accuracy, limit of detection (LOD), limit of quantification (LOQ) and relative retention
factor.[7]
2

During the process development of 3 in our laboratory, we have anticipated the plausible
formation of the potent impurities namely, (S)-2-ethoxy-4-[2-({2-phenyl-1-[2-(piperidin-
1-yl)phenyl]ethyl}amino)-2-oxoethyl]benzoic acid (1, benzyl repaglinide) and 2-(4-{(S)-
3-methyl-1-[2-(piperidin-1-yl)phenyl]butylcarbamoyl}-3-ethoxyphenyl)acetic acid (2,
repaglinide isomer). These impurities were the expected contaminants from the key raw
materials 5 and 7. Based on their importance, 1 is described as a qualified impurity in the
pharmacopieal monograph with a limit of not more than 0.1% w/w in the drug substance
3.[8] To the best of our knowledge, the synthesis of these impurities were not reported so
far. Due to the stringent impurity profiling of API from the drug regulatory authorities
and their importance, we herein report the synthesis and characterization of these
impurities (1 and 2).
RESULT AND DISCUSSION
The addition of Grignard reagent, prepared by the method of Leigh (reaction medium
containing toluene with one equivalent of THF), one could expect the formation of
benzyl counterpart along with the regular product.[9,10] It is evident from the literature that
on continuous heating of refluxing toluene in Grignard reaction condition, benzyl
substituted product gets formed in minor level (Scheme 1). Similarly, addition of
isobutylmagnesium bromide on nitrile 4 in toluene/THF mixture at higher temperature
results 5 as a major product with a plausible formation of 6. The introduction of benzyl
moiety is attributed from toluene which is used as a solvent during the insertion of
isobutyl group in nitrile 4. (S)-Benzylamine 6 which reacts in a similar way as 5 is
considered as a route cause for the contamination of benzyl repaglinide 1 in 3.
3

Benzyl repaglinide 1 was prepared as shown in Scheme 2, starting with nitrile 4. The
addition of Grignard reagent (benzylmagnesium chloride) over nitrile 4, prepared in
toluene/THF medium afforded imine intermediate 9. Reduction of 9 using NaBH4 gave a
racemic benzylamine 10. Since, benzyl repaglinide 1 is a chiral impurity, we have
screened racemic benzylamine 10 with different optically active reagents to obtain
chirally pure 1. Finally, promising results were obtained with N-acetyl-L-glutamic acid.
(S,S)-Glutamate salt 11 on basification yielded (S)-2-phenyl-1-[2-(piperidin-1-
yl)phenyl]ethylamine (6). Coupling of 6 with the acid chloride derivative of phenylacetic
acid 7 in presence of triethylamine gave amide derivative 12 which on saponification
afforded 1 in good yield. The chromatographic and enantiomeric purity of 1 was found to
be above 99% by HPLC. The chemical structure of 1 was confirmed by spectral analysis
and in comparison with 3.
The origin of impurity 2 is again due to the contamination of phenylacetic acid 7
containing benzoic acid 8. The literature reports the preparation of phenylacetic acid 7
starting with O-alkylation of 2- hydroxy-4-methylbenzoic acid.[11] While preparing 7
from its penultimate under basic conditions afforded 7 as a major product along with 2-
ethoxy-[4-(carboxymethyl)]benzoic acid (13) and 2-ethoxy-[4-
(ethoxycarbonyl)methyl]benzoic acid (8), as an expected contaminants. Di-acid 13 is
mainly responsible for the formation of repaglinide dimer, a major impurity of 3.[12]
The benzoic acid 8 was prepared, starting with 7 as shown in Scheme 3. Hydrolysis of 7
under basic condition leads to the formation of 13 as a major product.[13]
4

Di-acid 13 on esterification using ethanol in toluene containing catalytic amount of
sulfuric acid, afforded 8 as a major product along with di-ester 14 in a good yield. The
compound 8 was distinguished from 7 by the chemical shift values of proton at C6
position in the aromatic ring of 8 (δ 8.13 ppm) against the proton at C5 position of 7 (δ
7.74 ppm). This difference in the chemical shift values were due to the shielding effect
created by the ester group in 7. Benzoic acid 8 was converted into its acid chloride
derivative, using thionyl chloride and coupled with 5 to obtain amide intermediate 15,
which on hydrolysis under basic condition, resulted repaglinide isomer 2 in a good yield
with purity more than 94% by HPLC (Scheme 4).
CONCLUSION
We have successfully demonstrated the synthesis and characterization of 1 and 2, the
potent impurities of repaglinide. This study not only helped us to establish the impurity
profile of 3, but also allowed us to understand the cause for its generation and control.
EXPERIMENTAL
Melting points were determined with a Reichert Thermopan apparatus.1H and 13C NMR
spectra were recorded with a Bruker Avance 300 MHz and Varian 400 and 500 MHz
spectrometer using TMS as the internal standard in DMSO-d6 or CDCl3. IR spectra were
recorded using a Perkin-Elmer spectrum one Fourier Transform (FT) IR
spectrophotometer. High-resolution mass spectral analyses were performed using the
electrospray ionization (ESI) method on Xevo G2 QTOf mass spectrometer. HPLC
measurements were run on a Zorbax SB-Aq (150 mm x 4.6mm, 5µm; make: Agilent).The
5

flow rate was maintained at 1.5 mL / min with a column temperature of 45 oC. UV
detection occurred at λ = 240 nm. All reagents were used as purchased unless otherwise
stated. Reactions were performed under nitrogen atmosphere and the work-up were done
in a well-ventilated fuming hood.
Preparation Of (S)-2-Ethoxy-4-[2-({2-Phenyl-1-[2-(Piperidin-1-
Yl)Phenyl]Ethyl}Amino)-2-Oxoethyl]Benzoic Acid (1)
Sodium hydroxide (0.6 g, 15.0 mmol) was added to a stirred solution of 12 (5.0 g, 9.7
mmol) in a mixture of methanol (50 mL) and water (25 mL) at 25-30 °C. The reaction
mixture was stirred for 4 h at reflux. Thereafter, it was concentrated under vacuum to
remove methanol and diluted with water (50 mL) at 25-30 °C. Added CH2Cl2 (20 mL)
and stirred for 10 min at 25-30 °C. The organic layer was discarded and the aqueous layer
was extracted with CH2Cl2 (2 x 50 mL) at pH 4.5-5.0. The solvent was evaporated under
vacuum and the residue was dissolved in 2-propanol (15 mL) at reflux. The solution was
cooled to 5-10 °C and stirred for 2 h. The product was filtered, washed with cold 2-
propanol (5 mL) and dried under reduced pressure at 45 °C to obtain 1 as white solid.
Yield 3.9 g (82.9%); Purity 99.94%; Chiral purity 99.99%; Mp132-134 °C; [α]20 +25.6°
D
(c1, methanol ); 1H NMR(500 MHz, DMSO-d6) δ 1.31 (t, 3H, J = 7.0 Hz), 1.60 (m, 6H),
2.46 (br s, 2H), 2.77 (dd, 1H, J1 = 10.0 Hz, J2 = 14.0 Hz), 2.88 (dd, 1H, J1 = 5.0 Hz, J2 =
13.5 Hz), 3.03 (br s, 2H), 3.41 (s, 2H), 3.97 (q, 2H, J = 6.5 Hz), 5.52 (m, 1H), 6.68 (dd,
1H, J1 = 1.5 Hz, J2 = 7.5 Hz), 6.87 (d, 1H, J = 1.5 Hz), 7.09 (m, 2H), 7.21 (m, 6H), 7.42
(dd, 1H, J1 = 1.0 Hz, J2 = 7.5 Hz), 7.48 (d, 1H, J = 8.0 Hz), 8.57 (d, 1H, J = 8.5 Hz,
exchangeable with D2O), 12.41 (br s, 1H, exchangeable with D2O); 13C NMR (100 MHz,
6

CDCl3) δ 14.5, 24.0, 26.9, 26.9, 43.1, 43.8, 52.8, 54.7, 65.8, 113.2, 116.3, 122.7, 122.7,
124.9, 126.2, 127.6, 128.0, 128.1, 129.1, 133.8, 137.2, 137.9, 143.0, 152.3, 157.4, 165.3,
168.4; HRMS (ESI, QTOF) for C30H34N2O4 [M+H]+: m/z calcd: 487.2598; found:
487.2593.
Preparation Of 2-(4-{(S)-3-Methyl-1-[2-(Piperidin-1-Yl)Phenyl]Butylcarbamoyl}-3-
Ethoxyphenyl)Acetic Acid (2)
To a stirred suspension of 8 (2.0 g, 7.9 mmol) in toluene (20 mL), thionyl chloride (1.2 g,
10.1 mmol) was added at 25-30 °C. The mixture was stirred for 2 h at 25-30 °C. This
acid chloride solution was added over 5 (1.6 g, 6.8 mmol) in toluene (20 mL), contains
triethylamine (0.8 g, 7.9 mmol) at 25-30 °C. The reaction mixture was stirred for an hour
and diluted with water (20 mL). The mixture was stirred for 15 min and the organic layer
was separated. The solvent was removed under reduced pressure to afford amide
intermediate 15. The residue was dissolved in aqueous ethanol (20 mL, 50% v/v) and
treated with sodium hydroxide (0.6 g, 15.0 mmol) for 3 h at 60 °C. The reaction mixture
was concentrated under vacuum and diluted with water (20 mL). To the suspension
CH2Cl2 (40 mL) was added and stirred for 10 min at 25-30 °C. The organic layer was
discarded and the aqueous layer was extracted with CH2Cl2 (2 x 50 mL) at pH 4.5-5.0.
The solvent was evaporated under vacuum and the residue was purified by column
chromatography (CH2Cl2/ hexane) to yield 2 as a pale yellow solid. Yield 1.720 g
(58.3%); Mp 106-108°C; [α]20 +77.8° (c 0.5, methanol ); Purity 95.44%; 1H NMR (500
D
MHz, DMSO-d6) δ 0.95 (dd, 6H, J1 = 6.5 Hz, J2 = 6.0 Hz), 1.42 (m, 4H), 1.63 (m, 9H),
2.62 (br s, 2H), 3.14 (br s, 2H), 3.60 (s, 2H), 4.16 (m, 2H), 5.58 (m, 1H), 6.90 (dd, 1H, J1
7

= 1.0 Hz, J2 = 8.0 Hz), 7.05 (m, 2H), 7.18 (m, 2H), 7.32 (dd, 1H, J1 = 2.0 Hz, J2 = 8.0
Hz), 7.62 ( d, 1H, J = 7.5 Hz), 8.37 (d, 1H, J = 8.5 Hz); 13C NMR (100 MHz, CDCl3) δ
14.2, 22.2, 23.1, 24.4, 25.4, 26.7, 41.2, 47.1, 47.8, 54.6, 64.7, 113.1, 120.5, 121.1, 122.2,
124.2, 125.8, 127.5, 132.5, 138.6, 139.6, 152.3, 156.9, 164.5, 175.2; HRMS (ESI, QTOF)
for C27H36 N2O4[M+H]+: m/z calcd: 453.2754; found: 453.2750.
SUPPLEMENTAL MATERIAL
Full experimental detail, 1H and 13C NMR spectra, HPLC traces for this article can be
accessed on the publisher’s website.
ACKNOWLEDGMENTS
Authors thank the management of Aurobindo Pharma Ltd., for the permission to publish
this work. Authors are also grateful to the Chemical and Analytical Research
Departments, Aurobindo Pharma Ltd. for their support to this work.
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9

Figure 1. Repaglinide (3) along with its related substances, benzyl repaglinide 1 and
repaglinide isomer 2.
10

Scheme 1. Expected formation of benzylmagnesium bromide.
11

Scheme 2. Synthesis of benzyl repaglinide 1.
12

Scheme 3. Synthesis of benzoic acid 8.
13

Scheme 4. Synthesis of repaglinide isomer 2.
14