Synthesis of RP 48497, an impurity of eszopiclone

Article abstract of DOI:10.3390/molecules13081817

RP 48497 is a photodegradation product of eszopiclone, a non-benzodiazepine sedative-hypnotic used in the treatment of insomnia. We report herein the first synthesis of RP 48497 via reduction, chlorination, and recyclization of 6-(5-chloropyridin-2-yl)-7-hydroxy-6,7-dihydropyrrolo[3,4-b]pyrazin-5-one (3), a key intermediate in the synthesis of eszopiclone. The structure of RP 48497 was confirmed by its ¹H-NMR and MS data. The mechanism of the reduction step in the synthesis of RP 48497 was also studied and the key parameters were determined. These findings should be important for quality control purposes in the manufacture of eszopiclone.

Full text of DOI:10.3390/molecules13081817

Molecules 2008, 13, 1817-1821; DOI: 10.3390/molecules13081817
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.org/molecules
Communication
Synthesis of RP 48497, an Impurity of Eszopiclone
Yu Sha 1, Lei Zhang 1, Gui-Jie Du 2, Jian Ren 1 and Mao-Sheng Cheng 1,*
1
Key Lab of New Drug Design and Discovery of Liaoning Province, School of Pharmaceutical
Engineering, Shenyang Pharmaceutical University, Shenyang 110016, P. R. China;
E-mails: shayu@syphu.edu.cn; zl.719@163.com; renjian_1981@hotmail.com
2 Jinzhou Jiutai Pharmaceutical Co. Ltd, Jinzhou 121012, P.R. China; E-mail: duguijie@yahoo.com.cn
* Author to whom correspondence should be addressed: E-Mail: mscheng@syphu.edu.cn.
Received: 5 June 2008; in revised form: 8 August 2008 / Accepted: 12 August 2008 / Published: 22
August 2008
Abstract: RP 48497 is a photodegradation product of eszopiclone, a non-benzodiazepine
sedative-hypnotic used in the treatment of insomnia. We report herein the first synthesis of
RP 48497 via reduction, chlorination, and recyclization of 6-(5-chloropyridin-2-yl)-7-
hydroxy-6,7-dihydropyrrolo[3,4-b]pyrazin-5-one (3), a key intermediate in the synthesis of
1
eszopiclone. The structure of RP 48497 was confirmed by its H-NMR and MS data. The
mechanism of the reduction step in the synthesis of RP 48497 was also studied and the key
parameters were determined. These findings should be important for quality control
purposes in the manufacture of eszopiclone.
Keywords: RP 48497, eszopiclone, quality control
Introduction
Eszopiclone [(+)-6-(5-chloro-2-pyridinyl)-(7S)-(4-methylpiperazin-1-yl-carbonyloxy)-6,7-dihydro-
5H-pyrrolo[3,4-b]pyrazine-5-one, RP 27267, Estorra®, Lunesta®], the (S)-enantiomer of zopiclone, is
more active and less toxic than the racemic zopiclone. It is metabolized by three major pathways,
involving decarboxylation, oxidation and demethylation. The main urine metabolites are the less active

Molecules 2008, 13
1818
eszopiclone-N-oxide (RP 29753) and the inactive N-desmethyleszopiclone (RP 32273). RP 29753 can
also be considered as an impurity of eszopiclone and is formed by the oxidation of the latter. The
related substance RP 29307 is formed during the penultimate phase of the synthesis. It may pre-exist in
the starting material and it can also be formed by the hydrolysis of eszopiclone during storage. Finally,
RP 48497 is a non-chiral potential photodegradation product whose formation is caused by bright light
[1].
Figure 1. Structures of eszopiclone and its metabolites.
O
O
O
O
N
N
N
N
N
N
N
Cl
N
Cl
N
Cl
N
Cl
N
N
N
N
N
N
O
O
O
O
OH
O
O
O
RP 29307
N
N
O
N
N
N
N
N
N
H
N
Cl
N
RP 32273
Eszopiclone
RP 29753
RP 48497
As syntheses of the other impurities of eszopiclone, except RP 48497, have been previously
reported, the main aim of our work presented here was to synthesize this compound. This is significant
for the quality control of eszopiclone, whose chemical synthesis as shown in Scheme 1 has been
reported [2-5]:
Scheme 1. Synthetic route to eszopiclone.
O
O
O
N
N
N
N
N
N
a
b
HN
COOH
1
Cl
N
Cl
O
N
N
O
O
2
O
O
N
N
N
O
N
N
Cl
N
Cl
e
c
d
N
Cl
N
N
N
N
O
O
N
O
O
OH
N
N
3
Eszopiclone
N
N
(a) 5-Chloropyridin-2-ylamine, CH3CN, reflux; (b) Ac2O, reflux; (c) KBH4, 1,4-dioxane, H2O, 13
◦C; (d) 4-methylpiperazine-1-carbonyl chloride hydrochloride, Et3N, pyridine, CH2Cl2, reflux; (e)
tartaric acid or malic acid

Molecules 2008, 13
Results and Discussion
Synthesis
1819
Our synthetic route to RP 48497 using the same starting materials as that to eszopiclone is
illustrated in Scheme 2. Considering the instability of intermediate 3, its treatment with excess KBH4
or NaBH4 resulted in the cleavage of the carbon-nitrogen bond to yield compound 4. Finally,
compound 5, obtained by the chlorination of 4, was intramolecularly cyclized without purification in
the presence of NaH to form the target compound RP 48497 (6). The total yield was 40 %.
Scheme 2. Synthetic route to RP 48497.
O
O
O
O
N
N
N
N
N
N
N
N
a
b
c
O
N
Cl
N
Cl
HN
Cl
N
N
N
COOH
O
OH
O
3
1
2
O
O
O
N
N
N
N
h
g
HN
Cl
f
N
Cl
HN
Cl
Cl
N
N
N
N
N
OH
6
4
(RP 48497 )
5
(a) 5-Chloropyridin-2-yl-amine, CH3CN, reflux; (b) Ac2O, reflux; (c) NaBH4, 1,4-dioxane, H2O, 13
◦C; (d) 4-methylpiperazine-1-carbonyl chloride hydrochloride, Et3N, pyridine, CH2Cl2, reflux; (e)
tartaric acid or malic acid; (f) NaBH4, 1,4-dioxane, H2O; (g) CH2Cl2, SOCl2; (h) NaH, DMF
After studying in depth the reduction step, we found that the amount of reductant used played a vital
role in the transformation of the intermediate 2 to the intermediate 3. If more than 0.5 molecular
equivalents of reductant were present in the reaction mixtures, a certain amount of a byproduct whose
structure has been identified as compound 4 could be obtained, and the content of 4 would increase
when either the reaction temperature or the ratio of the reductant were increased. The polarity of 3 and
4, according to TLC, therefore, it is very difficult to separate them and purify of 3 from 4. The
mechanism of the transformation from 3 to 4 can be explained as shown in Scheme 3 [6]:
Scheme 3. Mechanism of formation of intermediate 4.
H
H B H
H
H B H
O
OH
HN
H
O
H
OH
N
H
N
N
N
N
-BH4
N
N
N
N
-BH4
H2O
Cl
Cl
N
Cl
N
Cl
N
N
N
N
O
O
O
O
4
3
For this reason, the reduction of intermediate 2 to 3 is thus a key step in the preparation of
eszopiclone. The content of compound 4 is a valid parameter for quality control of the intermediate 3.

Molecules 2008, 13
1820
Since this impurity 4 is mixed with the intermediate 3, it could also be further acylated with 4-
methylpiperazine-1-carbonyl chloride hydrochloride during the process to give compound 7 (Scheme
cheme 4), a ring-opening derivative of eszopiclone.
Scheme 4. Formation of compound 7.
O
O
N
N
HN
O
Cl
a
HN
OH
Cl
N
N
N
N
O
N
7
4
N
(a) 4-Methylpiperazine-1-carbonyl chloride hydrochloride, Et3N, pyridine, CH2Cl2, reflux
Considering that it is very hard to remove compound 7 as a consequence of the similar polarities of
eszopiclone and compound 7, it may be concluded that careful control on the reduction step to prevent
the formation of compound 4 is important in the quality control of eszopiclone.
Experimental
General
All commercially available reagents and solvents were used without further purification, unless
specified. Solvents were dried and re-distilled prior to use according to standard methods. Melting
1
points are uncorrected and were determined on a Büchi Melting Point B-540 apparatus. H-NMR
spectra were recorded on a Bruker ARX 300 MHz instrument, using DMSO-d6 as solvent and TMS as
the internal standard. Coupling constants (J) are expressed in Hz. Mass spectra were obtained on an
Agilent 1100 mass spectrometer. Column chromatography (CC) was performed on silica gel H and
analytical TLC on silica gel HF254.
3-Hydroxymethyl-pyrazine-2-carboxylic acid (5-chloro-pyridin-2-yl)-amide (4)
To a mixture of 1,4-dioxane (230 mL) and water (12 mL), was added 6-(5-chloropyridin-2-yl)-7-
hydroxy-6,7-dihydropyrrolo[3,4-b]pyrazin-5-one (3, 26.4 g, 0.10 mol).. After cooling to 10 ◦C, NaBH4
(2.7 g, 0.07 mol) was added to the above suspension. The mixture was stirred for 2 hours with the
◦
◦
temperature kept between 10 C to 15 C and then poured into ice/water (300 mL). Glacial acetic acid
was added to adjust the pH to 5-6 and the mixture was stirred for 30 minutes. The precipitated solid
◦
◦
1
was filtered and dried to give 20.0 g of a yellow product. Yield: 75.2 %; m.p.:176.0 C-177.3 C; H-
NMR δ: 10.89 (1H, s), 8.84 (1H, d, J = 2.31), 8.69 (1H, d, J = 2.25), 8.45 (1H, d, J = 2.43), 8.26 (1H,
d, J = 8.94), 8.03 (1H, dd, J = 2.61, 8.91), 5.34 (1H, t, J = 5.91), 4.93 (2H, d, J = 5.67).

Molecules 2008, 13
1821
3-Chloromethylpyrazine-2-carboxylic acid (5-chloro-pyridin-2-yl)-amide (5)
◦
To a 0 C suspension of 3-hydroxymethylpyrazine-2-carboxylic acid (5-chloropyridin-2-yl)-amide
(4, 30.0 g, 0.10 mol) in dichloromethane (120 mL), thionyl chloride (42 mL) was added dropwise
◦
while keeping the temperature below 10 C. After the addition, the mixture was stirred at room
temperature for 2 hours and then evaporated to dryness under reduced pressure. The off-white product
◦
was recrystallized from i-propane to give brown needles, 21.0 g. Yield: 65.4 %; m.p.:149.0 C-150.0
◦C.
6-(5-Chloropyridin-2-yl)-6,7-dihydropyrrolo[3,4-b]pyrazin-5-one (RP 48497, 6)
To a 0 ◦C suspension of NaH 4.3 g (70 %, 0.12 mol) in DMF (200 mL), 3-chloromethylpyrazine-2-
carboxylic acid (5-chloropyridin-2-yl)-amide (5, 17.5 g, 0.06 mol) was added slowly. After the
addition, the reaction mixture was stirred for 24 hours at room temperature. Additional NaH (70 %, 1.3
g, 0.04 mol) was then added and the reaction mixture was stirred for another 1 hour. The mixture was
poured into ice/water (200 mL). After the ice dissolved, the precipitated solid was filtered to give 11.8
g of a yellow powder. Yield: 77.2 %. The crude product can be further purified by CC (petroleum
ether-EtOAc, 10:1); m.p.: 222.0 ◦C-224.0 ◦C; 1H-NMR δ: 8.90 (2H, d, J = 2.60), 8.60 (1H, d, J = 9.00),
8.57 (1H, d, J = 2.58), 8.08 (1H, dd, J = 8.97, 2.64), 5.13 (2H, s); MS m/z: 47.2 [M+H]+, 269.1
[M+Na]+, 514.9 [2M+Na]+, 245.0 [M-H]-.
References
1. Piperak, S; Parissi-Poulou, M. Enantiomeric separation of zopiclone, its metabolites and products
of degradation on a cyclodextrin bonded phase. J. Chromatogr. A. 1996, 729, 19-28.
2. Cotrel, C.; Jeanmart, C.; Messer, M. N. Pyrrolo[3,4-b]pyrazine derivatives. U.S. Pat. 3,862,149,
1975.
3. Cotrel, C.; Jeanmart, C.; Messer, M. N.; Crisan, C. Heterocyclic compounds. U.S. Pat. 4,220,646,
1980.
4. Chen, HC; Liu ZZ; Zhang, CX. Synthesis of sedative hypnotic Zopiclone. J. Zheng Zhou Univ.
(zirankexueban). 1993, 12, 73-76.
5. Zhang, YF; Zuo, DS; Tian, KP; Shen, JM. Synthesis of the new sedative hypnotic Zopiclone.
Chin. J. Med. Chem. 1994, 4, 62-64.
6. Horii Z., Iwata C., Tamura Y. Reduction of phthalimides with sodium borohydride. J. Org. Chem.
1961, 26, 2273-2276.
Sample Availability: Samples are available from the authors.
© 2008 by the authors; licensee Molecular Diversity Preservation International, Basel, Switzerland.
This article is an open-access article distributed under the terms and conditions of the Creative
Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).