Asymmetric synthesis of (R)-(+)-etomoxir via enzymatic resolution

Article abstract of DOI:10.1016/S0957-4166(00)00304-9

An asymmetric synthesis of (R)-(+)-etomoxir 3, employing enzymatic resolution of ethyl 2-alkyl-2,3-dihydroxypropionate using Amano AK via transacylation is reported. Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00304-9

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 3395–3401
Asymmetric synthesis of (R)-(+)-etomoxir via enzymatic
resolution
Sang-sup Jew,* Eun-young Roh, Eun-young Baek, Labrouille´re Mireille, Hyung-ook Kim,
Byeong-seon Jeong, Mi-kyoung Park and Hyeung-geun Park*
College of Pharmacy, Seoul National University, Seoul 151-742, South Korea
Received 10 July 2000; accepted 4 August 2000
Abstract
An asymmetric synthesis of (R)-(+)-etomoxir 3, employing enzymatic resolution of ethyl 2-alkyl-2,3-
dihydroxypropionate using Amano AK via transacylation is reported. © 2000 Elsevier Science Ltd. All
rights reserved.
1. Introduction
Substituted oxirane-2-carboxylates such as palmoxirate,1 clomoxir,2 etomoxir,3 JG3814 have
been recognized as powerful hypoglycemic agents in animals including humans. Their hypo-
glycemic effect was associated with the irreversible inhibition of carnitine palmitoyl transferase
I (CPT I), which is essential for the transportation of palmitoyl CoA into the mitochondria
matrix side for its b-oxidation5.
* Corresponding authors. E-mail: ssjew@plaza.snu.ac.kr
0957-4166/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00304-9

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S. Jew et al. / Tetrahedron: Asymmetry 11 (2000) 3395–3401
The further mechanistic investigation revealed that only the (R)-enantiomer of etomoxir
shows biological activity by covalent bond formation between its CoA ester with CPT I.5c
Because of the stereospecificity for hypoglycemic activity, several asymmetric synthetic methods
of (R)-(+)-etomoxir have been reported using enzymatic resolution,6 Sharpless epoxidation7 and
asymmetric bromolactonization8 as key steps. Among them, Prasad’s enzymatic resolution
method is quite attractive for practical application. They reported the synthesis of 5 as an
intermediate for (R)-(+)-etomoxir (92% ee), but the known process7 for the conversion of 5 to
(R)-(+)-etomoxir is relatively impractical (four steps, 23%). Herein, we present the highly
enantioselective and practical synthesis of (R)-(+)-etomoxir via enzymatic resolution.
2. Results and discussion
First of all, ethyl 2-benzyl-2,3-dihydroxypropionate 69 was chosen as the substrate to
investigate the kinetic resolution with various lipases via transacylation10 using vinylacetate.
Transacylation was initiated by addition of various lipases and vinylacetate to 6 in methyl
tert-butylether (MTBE) at room temperature.11 After around 50% conversion of 6, 6a and 7
were isolated and the enantioselectivities were measured. The enantiomeric excess of 6 was
1
determined by H NMR analyses of the diasteromeric Mosher esters. In the case of 7, the same
method was employed after the hydrolysis of 7 under basic conditions. The absolute configura-
tions were assigned by comparison with literature data.9 As shown in Table 1, Amano AK gave
the best enantioselectivity of both diol 6a (R, 99% ee) and acetate 7 (S, 97% ee). Interestingly,
the stereo-orientation was the reverse with Amano AY.
Table 1
Model study of enzymatic resolution with ethyl 2-benzyl-2,3-dihydroxypropionate 6
No. Lipasea (90
mg/mmol of
substrate)
Temp (°C)
Solvent
Time (h)
Conv.b (%) 6a
7
E value
%eec (%y)
%eed (%y)
1
2
3
4
Amano PS
Amano A
Amano AK
Amano AY
20
25
20
20
MTBE
MTBE
MTBE
MTBE
4
24
2
53
38
51
36
96(R) (40)
40(R) (58)
99(R) (42)
43(S) (57)
85(S) (55)
64(S) (40)
97(S) (55)
76(R) (38)
48
7
210
11
24
a The enzymes, Pseudomonas cepacia lipase (Amano PS), Aspergillus niger lipase (Amano A), Pseudomonas
fluorescens lipase (Amano AK), and Candida rugosa lipase (Amano AY) were donated from Amano Co.
1
b The degree of conversion was determined by analysis of H NMR of the mixture of 6a and 7.
1
c The %ee was determined by H NMR spectrum with Mosher ester derivatives.
1
d The %ee was determined by H NMR spectrum with Mosher ester derivatives after conversion of 7 to enantiomer
of 6a by hydrolysis.

S. Jew et al. / Tetrahedron: Asymmetry 11 (2000) 3395–3401
3397
The highest E value (210) shown for Amano AK was chosen to be applied to the preparation
of (R)-etomoxir 3. The substrate 10 for enzymatic resolution was prepared from triethyl
phosphonoacetate in two steps (Scheme 1). The alkylation of 8 with 6-p-chlorophenoxyhexyl-
bromide, followed by Wittig reaction with formaldehyde gave 9. The dihydroxylation using
NMO and OsO4 provided diol 10. Enzymatic resolution of 10 with Amano AK was optimized
by using different solvents. As shown in Table 2, MTBE also showed the best selectivity
[(R)-10a, 99% ee:S-11, 96% ee)]. The treatment of methansulfonylchloride with 10a under basic
conditions gave mesylate 12, which was converted to (R)-etomoxir 3, [h]2D0 +8.55 (c 0.75, CHCl3),
98% ee (lit.7 8.56, c 1, CHCl3, >98% ee) by epoxidation under basic conditions.
Scheme 1. Reagents and conditions: (a) (i) p-ClPhOCH2(CH2)4CH2Br, NaH, DME, reflux, 12 h; (ii) NaH, (CH2O)n,
DME, rt, 12 h, 75%, (b) NMO, cat. OsO4. tert-BuOH:H2O (1:1), rt, 1 h, 96%, (c) lipase, MTBE, rt, 2 h, 44%, (d)
methanesulfonylchloride, triethylamine, CH2Cl2, (e) anhydrous K2CO3, abs. EtOH, rt, 6 h, 95%
Table 2
Enzymatic resolution of 10 with Amano AK
No. Lipase (90
mg/mmol of
substrate)
Temp (°C)
Solvent
Time (h)
Conv.a (%) 10a
11
E value
%eeb (%y)
%eec (%y)
1
2
3
4
Amano AK 20
Amano AK 20
Amano AK 20
Amano AK 20
DIPE
2
45
45
33
51
79 (R) (47)
80 (R) (42)
48 (R) (60)
98 (R) (44)
95 (S) (50) 138
\99 (S) (50) 220
\99 (S) (35) 125
96 (S) (54) 211
Benzene
Toluene
MTBE
6.5
7.5
2
1
a The degree of conversion was determined by analysis of H NMR of the mixture of 10a and 11.
b The %ee and absolute configuration were determined by comparison of specific rotation of 3 with literature
values.
c The %ee and absolute configuration were determined by specific rotation after conversion of 11 to the enantiomer
of 10a by hydrolysis.

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S. Jew et al. / Tetrahedron: Asymmetry 11 (2000) 3395–3401
3. Conclusion
In summary, highly enantioselective enzymatic resolution of ethyl 2-alkyl-2,3-dihydroxyprop-
ionate was developed by using Amano AK in MTBE. By this process, (R)-(+)-etomoxir
could be prepared in 30% yield and 98% ee over five steps from triethyl phosphonoacetate.
The high enantioselectivity could be applied to the synthesis of chiral 2-oxirane carboxylate
derivatives (1, 2, 4). Also, as the enantioselectivity of asymmetric dihydroxylation of ethyl
2-alkylacrylate is generally relatively poor ($70% ee),12 highly enantioselective chiral 2-alkyl-
2,3-dihydroxypropionate derivatives prepared from this new method should be useful as
chiral building blocks for asymmetric synthesis.
4. Experimental
4.1. General
Optical rotations were measured with a JASCO DIP-1000 digital polarimeter. Infrared
spectra were taken on a Perkin–Elmer 1710 FT-IR spectrometer. Mass spectra were obtained
on a VG Trio-2 GC-MS instrument; high resolution mass spectra were obtained on a HP
1
5890 Series II. H and 13C NMR spectra were measured with a JEOL JNM-LA 300, a JEOL
JNM-GCX 400 or a Bruker AMX-500 spectrometer using TMS as the internal standard. All
reactions were carried out under an argon atmosphere, using anhydrous solvents except for
those involving hydrolysis. Most reagents were obtained from commercial suppliers and used
without further purification unless noted. Tetrahydrofuran was distilled from Nao and ben-
zophenone.
4.2. Enzymatic resolution of ethyl 2,3-dihydroxy-2-benzyl propionate 6 with Amano AK via
transacylation
To 6 (100 mg, 0.45 mmol) and Amano AK (45 mg) in MTBE was added vinyl acetate
(116 mg, 1.35 mmol) and the reaction mixture was stirred at room temperature. The reaction
was stopped at around 50% conversion of 6 and filtered with Celite in vacuo. After removal
of solvent, the residue was dried in vacuo. After the determination of the degree of conver-
1
sion by H NMR, the residue was purified by column chromatography (ethyl acetate:n-hex-
ane=1:2) to give (R)-6a (42 mg, 42%) and (S)-7 (61 mg, 55%). Compound (R)-6a: [h]D20
−15.5 (c 0.85, CHCl3), 99% ee (lit.9 +10.43, c 0.86, CHCl3, 76% ee, (S)-6); IR (neat) 3445,
1
1733 cm−1; H NMR (CDCl3, 300 MHz) l 7.31–7.17 (m, 5 H), 4.26–4.15 (m, 2 H), 3.92 (t,
J=11.09 Hz, 1 H), 3.73–3.67 (m, 1 H), 3.37 (s, 1 H), 3.00 (d, J=13.65 Hz, 1 H), 2.90 (d,
J=13.65 Hz, 1 H), 2.19–2.14 (m, 1 H), 1.28 (t, J=7.14 Hz, 3 H); MS (EI) m/e 225 [M++1].
Compound (S)-7: [h]2D0 +15.0 (c 0.94, CHCl3); IR (neat) 3503, 1748 cm−1; H NMR (CDCl3,
1
300 MHz) l 7.60–7.19 (m, 5 H), 4.39–4.09 (m, 4 H), 3.00 (q, J=7.56 Hz, 2 H), 2.07 (s, 3
H), 1.26 (t, J=7.07 Hz, 3 H); MS (EI) m/e 267 [M+]. The hydrolysis of (S)-7 in the ethanol
solution of potassium carbonate gave (S)-6a: [h]2D0 +15.0 (c 0.65, CHCl3), 97% ee (lit.9
+10.43, c 0.86, CHCl3, 76% ee, (S)-6).

S. Jew et al. / Tetrahedron: Asymmetry 11 (2000) 3395–3401
3399
4.3. Ethyl 2-hydroxy-2-benzyl-3-[h-methoxy-h-(trifluoromethyl)phenylacetoxy]-propanoate
To a tetrahydrofuran solution of (R)-6a (20 mg, 0.09 mmol) and dimethylaminopyridine (22
mg, 0.18 mmol) was added (S)-(+)-a-methoxy-a-(trifluoromethyl)phenylacetic acid chloride
(MTPC) (46 mg, 0.18 mmol). After stirring the reaction solution for 30 min, tetrahydrofuran
was removed in vacuo. The residue was purified by column chromatography (ethyl acetate:
n-hexane=1:10) to give MTPC derivative of (R)-6a as a colorless oil (25 mg): 1H NMR (CDCl3,
300 MHz) l 7.58–7.19 (m, 10 H), 4.55 (s, 2 H), 4.40–4.36 (q, 2 H, minor), 4.17–4.09 (q, 2 H,
major), 3.52 (s, 3 H, minor), 3.50 (s, 3 H, major), 3.07 (q, 2 H), 1.21 (t, J=7.2 Hz, 3 H, major),
1.15 (t, J=7.1 Hz, 3 H, minor). The same procedure was employed for the preparation of
MTPC derivatives of (S)-6.
4.4. Ethyl 2-[6-(p-chlorophenoxy)hexyl]acrylate 97
To a 1,2-dimethoxyethane suspension of 95% NaH (1.84 g, 46.09 mmol) was added a
1,2-dimethoxyethane solution of triethylphosphonoacetate (9.14 mL, 46.09 mmol) at room
temperature. The reaction mixture was stirred for 1 h and then 6-(p-chlorophenoxy)-1-bromo-
hexane (14.10 g, 46.09 mmol) was added to the reaction mixture. The reaction mixture was
refluxed for 10 h. After the reaction mixture was cooled down to room temperature, 95% NaH
(1.84 g, 46.09 mmol) was added at 0°C. The reaction mixture was warmed to room temperature
and stirred for 1 h. The 1,2-dimethoxyethane solution of 95% paraformaldehyde (1.54 g, 51.17
mmol) was added at room temperature. The reaction mixture was stirred for 1 h. The excess
solvent was removed in vacuo and the residue was diluted with ethyl acetate. The ethyl acetate
solution was washed with water and brine, dried over anhydrous MgSO4, the residue was
purified by column chromatography (ethyl acetate:hexane=1:20) to give 97 as a colorless oil
1
(11.33g, 75%): IR (neat) 1717 cm−1; H NMR (CDCl3, 300 MHz) l 7.24–7.19 (m, 2 H),
6.84–6.78 (m, 2 H), 6.13 (s, 1 H), 5.51 (s, 1 H), 4.21 (q, J=7.15 Hz, 2 H), 3.91 (t, J=6.45 Hz,
2 H), 2.31 (t, J=6.95 Hz, 2 H), 1.85–1.33 (m, 8 H), 1.28 (t, J=6.83 Hz, 3 H); MS (EI) m/e 310
[M+].
4.5. Ethyl 2,3-dihydroxy-2-[6-(p-chlorophenoxy)hexyl]propanoate 10
To tert-butanol:H2O (1:1, 50 mL) a mixture of NMO (9 mmol) was added 0.08 M OsO4 in
toluene (2 mL, 0.4 mmol) and 9 (2.59 g, 8 mmol). The reaction solution was stirred for 1 h at
room temperature. The excess OsO4 was quenched by addition of NaHSO3 (1.7 g) and the
tert-butanol was removed in vacuo. The residue was diluted with water (20 mL) and extracted
with methylene chloride (20 mL×3). The combined methylene chloride was washed with water
(10 mL) and brine (10 mL), was removed in vacuo and the residue was purified by column
chromatography (methylene chloride:methanol=20:1) to give 10 (2.65 g, 96%) as colorless oil:
1
IR (neat) 3447, 1733 cm−1; H NMR (CDCl3, 300 MHz) l 7.22 (d, J=6.72 Hz, 2 H), 6.80 (d,
J=6.84 Hz, 2 H), 4.28 (m, 2 H), 3.90 (t, J=6.45 Hz, 2 H), 3.78 (t, J=10.47 Hz, 1 H), 3.62–3.57
(m, 1 H), 3.55 (s, 1 H), 2.17–2.13 (m, 1 H), 1.77–1.29 (m, 13 H); 13C NMR (CDCl3, 75 MHz)
l 175.21, 157.63, 129.22, 125.30, 115.70, 78.49, 68.11, 67.89, 62.28, 34.76, 29.32, 29.00, 25.76,
22.90, 14.19; MS (EI) m/e 344 [M+], HRMS (EI) calcd for C17H25O355Cl [M+] 344.1319, found
344.1305.

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S. Jew et al. / Tetrahedron: Asymmetry 11 (2000) 3395–3401
4.6. Enzymatic resolution of 10 with Amano AK via transacylation
To 10 (100 mg, 0.29 mmol) and Amano AK (30 mg/mmol of diol) in MTBE was added
vinyl acetate (75 mg, 0.87 mmol)) and the reaction mixture was stirred at room temperature.
The reaction was stopped at around 50% conversion of 10 and filtered with Celite in vacuo.
After removal of solvent, the residue was dried in vacuo. After the determination of the
1
degree of conversion by H NMR, the residue was purified by column chromatography (ethyl
acetate:n-hexane=1:2) to give (R)-10a (44%) and (S)-11 (54%). Compound (R)-10a: [h]D20
+6.05 (c 0.95, CHCl3). The other spectral data were exactly same as 10. Compound (S)-11:
[h]2D0 +4.4 (c 0.75, CHCl3); IR (neat) 1748 cm−1; H NMR (CDCl3, 300 MHz) l 7.26–7.20
1
(m, 2 H), 6.82–6.79 (m, 2 H), 4.31–4.13 (m, 4 H), 3.90 (t, J=6.45 Hz, 2 H), 3.41 (s, 1 H),
2.06 (s, 3 H), 1.80–1.15 (m, 13 H); 13C NMR (CDCl3, 75 MHz) l 174.13, 170.47, 157.65,
129.21, 125.29, 115.71, 76.35, 68.91, 68.09, 62.27, 34.95, 29.24, 28.98, 25.75, 22.80, 20.64,
14.16; MS (EI) m/e 386 [M+], HRMS (EI) calcd for C19H27O365Cl [M+] 386.1496, found
386.1426. The hydrolysis of (S)-11 in the ethanol solution of potassium carbonate gave
(S)-10a: [h]2D0 −5.95 (c 0.74, CHCl3).
4.7. (R)-(+)-Etomoxir 3
To an anhydrous tetrahydrofurane solution of 10a (33 mg, 0.10 mmol) and triethylamine
(15 mg, 0.15 mmol) was added methansulfonyl chloride (17 mg, 0.15 mmol) and the reaction
solution was stirred for 0.5 h at 0°C. After removal of the solvent in vacuo, the residue was
diluted with ethyl acetate (50 mL) and washed with water (10 mL) and brine (10 mL). The
organic solution was filtered, dried over anhydrous MgSO4 and evaporated to give almost
pure 12 (38 mg). The crude 12 was used without further purification. To the anhydrous
ethanol solution of 12 was added potassium carbonate (20 mg, 0.15 mmol) and the reaction
mixture was stirred for 6 h at room temperature. After the removal of ethanol, the residue
was purified by column chromatography (ethyl acetate:n-hexane=1:20) to give (R)-(+)-eto-
moxir 3 as a colorless oil (28 mg, 95% yield): [h]2D0 +8.55 (c 0.75, CHCl3), 98% ee (lit.7 +8.56,
1
c 1, CHCl3, >98% ee); IR (neat) 2960, 1740, 1500, 1250 cm−1; H NMR (CDCl3, 400 MHz)
l 7.21 (d, J=8.8 Hz, 2 H), 6.80 (d, J=8.8 Hz, 2 H), 4.21 (dq, J=7.2, 3.6 Hz, 2 H), 3.89 (t,
J=6.8 Hz, 2 H), 3.02 (d, J=5.86 Hz, 1 H), 2.78 (d, J=5.86 Hz, 1 H), 2.13–2.04 (m, 1 H),
1.78–1.73 (m, 2 H), 1.69–1.62 (m, 1H), 1.54–1.33 (m, 6 H), 1.28 (t, J =7.2 Hz, 3 H); 13C
NMR (CDCl3, 75 MHz) l 170.46, 157.67, 129.25, 125.29, 115.71, 68.13, 61.44, 57.02, 51.88,
31.16, 29.21, 29.01, 25.80, 24.70, 14.11; MS (EI) m/e 326 [M+], HRMS (EI) calcd for
C17H23O345Cl [M+] 326.1285, found 326.1282.
Acknowledgements
This work was supported by the grant of the Research Center of New Drug Development
of Seoul National University and the Research Institute of Pharmaceutical Sciences in Col-
lege of Pharmacy of Seoul National University. We also thank Amano Co. for the kind
donation of enzymes.

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3401
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11. The general reaction procedure for enzymatic resolution was as follows: to the mixture of the diol and enzyme
(90 mg/mmol of diol) in MTBE was added vinyl acetate (3.0 eq. of diol) and it was stirred at room temperature.
The reaction was stopped at around 50% conversion of diol and filtered with Celite in vacuo. After removal of
1
solvent, the residue was dried in vacuo. After the determination of the degree of conversion by H NMR, the
residue was purified by column chromatography (ethyl acetate:n-hexane=1:2) to give diol and acetate.
12. Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric Synthesis; Ojima, I., Ed. Catalytic asymmetric
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.