Chemoenzymatic synthesis of the chiral herbicide: (S)-metolachlor

Article abstract of DOI:10.1139/V06-129

A chemoenzymatic approach for the production of (5)-metolachlor, one of the most widely used herbicides, has been developed. The starting material (S)-N-(2-ethyl-6-methylphenyl)alanine was obtained by the use of lipase-catalyzed hydrolytic kinetic resolution. Under the optimal conditions, the good activity and excellent enantioselectivity of lipase B from Candida antarctica (CAL-B, E > 100) are achieved in diethyl ether - water (15% v/v), which is about 9.7-fold more enantioselective than that in a pure buffered aqueous solution (E = 12.1). After a simple extraction procedure is used to separate the acid product from the remaining ester, the remaining ester is racemized, providing the basis for the continuous resolution process. Then (S)-metolachlor is synthesized by a simple chemical method using the enantiomerically pure (S)-acid.

Full text of DOI:10.1139/V06-129

1058
Chemoenzymatic synthesis of the chiral herbicide:
(S)-metolachlor
Liangyu Zheng, Suoqin Zhang, Fang Wang, Gui Gao, and Shugui Cao
Abstract: A chemoenzymatic approach for the production of (S)-metolachlor, one of the most widely used herbicides,
has been developed. The starting material (S)-N-(2-ethyl-6-methylphenyl)alanine was obtained by the use of lipase-
catalyzed hydrolytic kinetic resolution. Under the optimal conditions, the good activity and excellent enantioselectivity
of lipase B from Candida antarctica (CAL-B, E > 100) are achieved in diethyl ether – water (15% v/v), which is about
9.7-fold more enantioselective than that in a pure buffered aqueous solution (E = 12.1). After a simple extraction pro-
cedure is used to separate the acid product from the remaining ester, the remaining ester is racemized, providing the
basis for the continuous resolution process. Then (S)-metolachlor is synthesized by a simple chemical method using the
enantiomerically pure (S)-acid.
Key words: (S)-metolachlor, herbicide, CAL-B, (S)-N-(2-ethyl-6-methylphenyl)alanine, resolution.
Résumé : On a mis au point une approche chimioenzymatique à la production du (S)-métolachlore, un des herbicides
les plus utilisés. Le produit de départ, la (S)-N-(2-éthyl-6-méthylphényl)alanine, a été préparé en faisant appel à une ré-
solution par hydrolyse cinétique catalysée par une lipase. Dans des conditions optimisées, la meilleure activité et
l’excellente énantiosélectivité de la lipase B provenant de la Candida antartica (CAL-B, E > 100) sont obtenues en
opérant dans un mélange d’éther éthylique et d’eau (15% v/v), une énantiosélectivité qui est environ 9,7 fois plus
élevée que dans une solution aqueuse pure (E = 12,1). Après avoir utilisé une simple procédure d’extraction pour éli-
miner le produit acide et récupérer de l’ester résiduel, ce dernier peut être racémisé et servir dans un processus
continue de résolution. La synthèse du (S)-métolachlore est ensuite complétée par une méthode chimique simple en uti-
lisant l’acide (S) énantiomériquement pur.
Mots clés : (S)-métolachlore, herbicide, CAL-B, (S)-N-(2-éthyl-6-méthylphényl)alanine, dédoublement.
[Traduit par la Rédaction] Zheng et al. 1063
Introduction
stereoisomers differ in their ability to inhibit the growth of
weeds, it is reported that the herbicidal activity is mainly in-
fluenced by the chiral center, the (S)-enantiomers (Fig. 1)
providing higher herbicidal activity than the (R)-enantiomers
(3). Consequently, it is recognized that enriching the iso-
meric ratio in favor of the (S)-enantiomer will increase the
biological activity of the herbicide. Thus, providing excel-
lent weed control at rates substantially lower than those for
racemic metolachlor and showing a reduced risk of environ-
mental contamination. Therefore, it is necessary to develop a
new catalyst system to allow the production of enantio-
mercially enriched (S)-metolachlor on a commercial scale.
(S)-N-(2-ethyl-6-methylphenyl)alanine ((S)-2) is an impor-
tant precursor in the synthesis of (S)-metolachlor, which is
currently produced by chemical synthesis in large quantities
(4). The chemical method presently used requires drastic re-
action conditions that may cause racemization, decomposi-
tion, or side reactions. The high energy consumption
required for a long reaction time at a high temperature is un-
favorable especially for industrial processes (5). As alterna-
tive to such processes, a highly active biocatalyst could be
used in a very efficient manner. Among various commercial
enzymes, lipases are attractive in terms of availability, the
fact that there is no need of cofactors, high stability, and ac-
tivity in organic media (6–8). The resolution of racemates by
enzyme is still the most important approach in the industrial
synthesis of optically pure compounds (9,10), as it is often
As optically pure single enantiomers are often more spe-
cific targets and have fewer side effects than their corre-
sponding racemates, there is a real driving force for the
production of new drugs and agrochemicals with high opti-
cal purity (1). The herbicide metolachlor (N-(1′-methyl-2′-
methoxyethyl)-N-chloroacetyl-2-ethyl-6-methylaniline) has
been widely used for over 20 years with a main strength in
the control of weeds following pre-emergence application.
In addition, weed resistance has not been developed, making
metolachlor an important option in weed-management strate-
gies (2). Metolachlor comprises four stereoisomers, the
isomerism of which is based on a combination of a chiral
center in the aliphatic side chain and a chiral axis between
the phenyl group and the nitrogen atom. Although the four
Received 11 June 2006. Accepted 2 August 2006. Published
on the NRC Research Press Web site at http://canjchem.nrc.ca
on 12 October 2006.
L. Zheng, G. Gao, and S. Cao. Key Laboratory for
Molecular Enzymology and Engineering of Ministry of
Education, Jilin University, Changchun, 130021, People’s
Republic of China.
S. Zhang1 and F. Wang. College of Chemistry, Jilin
University, Changchun, 130021, People’s Republic of China.
1Corresponding author (e-mail: zhangsq@jlu.edu.cn).
Can. J. Chem. 84: 1058–1063 (2006)
doi:10.1139/V06-129
© 2006 NRC Canada

Zheng et al.
1059
Fig. 1. Structure of (S)-metolachlor.
Fullerton, USA) with a 59 cm (49 cm to detector) × 50 µm
i.d. eCAPTM neutral capillary (Beckman, Fullerton, USA).
The conversion of the reaction was determined by using
100 mmol/L triethylamine – acetic acid buffer (TEAA, pH =
5.5) as the background electrolyte. The enantiomeric excess
of (S)-2 was successfully analyzed in the buffer by using
40 mmol/L 2,6-di-O-methyl-β-cyclodextrin (DM-β-CD, Beckman,
Fullerton, USA) as a buffer additive. The analysis was per-
formed with an applied voltage of –20 kV, and the
absorbance was recorded at 200 nm.
CH3
C2H5
N
CH2OCH3
COCH2Cl
CH3
Enantiomeric ratio (E) of the hydrolysis of racemic N-(2-
ethyl-6-methylphenyl) alanine methyl ester was calculated
from the conversion (c) and enantiomeric excess (eep) of (S)-
2, using the equation
the most economical and convenient way to prepare enantio-
merically pure compounds. The main disadvantage of a res-
olution process compared with an enantioselective synthesis
is that the maximum theoretical yield is 50%. Therefore,
racemization of the unwanted enantiomer is of critical im-
portance for economically and environmentally acceptable
resolutions. A combination of classical resolution processes
with racemization to give asymmetric transformations will
be necessary to keep up with the advances in asymmetric
synthesis (11).
E = ln[1 – c(1 + eep)]/ln[1 – c(1 – eep)]
where eep = (cS – cR)/(cS + cR), where cS and cR are concen-
trations of the (S)- and (R)-enantiomers, respectively (12).
The absolute configuration of the enantiomers was estab-
lished by comparison of the measured optical rotation with
the literature data (4).
Herein, we first report
a practical lipase-catalyzed
Preparation of (R,S)-N-(2-ethyl-6-methylphenyl) alanine
methyl ester: (R,S)-1
hydrolytic kinetic resolution of (S)-2, the key intermediate
for (S)-metolachlor. To obtain the higher enantiomercially
pure (S)-acid, the catalytic properties of different lipases are
compared, the conditions of lipase-catalyzed hydrolysis are
optimized, and the remaining ester is racemized. Then, (S)-
metolachlor is synthesized by a chemical method based on
(S)-2.
The reaction mixture of 2-ethyl-6-methylaniline (8.4 mL,
60 mmol), NaHCO3 (5.5 g, 65 mmol), and methyl 2-
bromopropionate (180 mmol) was stirred under nitrogen at-
mosphere and slowly heated to 120–125 °C in 1 h. Then, the
dark reaction mixture was continually kept at the same tem-
perature for 18 h with stirring. After cooling, the reaction
mixture was transferred into 30 mL of ice water and ex-
tracted with ethyl acetate. The ethyl acetate fractions were
dried over anhydrous Na2SO4 and concentrated in a rotary
evaporator at 40 °C. After normal work-up, the resulting es-
ter was purified by column chromatography on silica gel us-
ing ethyl acetate – petroleum ether (1:5) as the eluant to
furnish the corresponding ester. N-(2-ethyl-6-methylphenyl)
Experimental
Materials
Pseudomonas sp. lipase (PSL) and Candida cylindracea
A.Y. lipase (AYL) were purchased from Amano Pharmaceu-
tical Co., Ltd. ( Nagoya, Japan). Candida antarctica lipase B
(CAL-B) was kindly donated by Novo Nordisk Industries
(Guangzhou, China). Porcine pancreatic lipase (PPL) was
purchased from Shanghai Dongfeng biochemical reagent
Co., Ltd. (Shanghai, China). Candida lipolytic lipase (CLL)
was provided by Wuxi enzyme preparation plant (Wuxi,
China). Penicillium expansum lipase (PEL) was provided by
Nantong Pharmaceutical Co., Ltd. (Nantong, China). The
authenticity of compounds prepared during the study was
confirmed by spectroscopic analysis, including 300 MHz
NMR (Mercury-300B, VARIAN, Palo Alto, USA), and
GC–MS (Saturn 220, VARIAN, Palo Alto, USA). Reactions
were routinely monitored on silica gel plates (Qingdao
Haiyang Chemical Co., LTD., Qingdao, China) using UV
light for detection of the spots. Optical rotation was
measured with a WZZ-1S digital automatic polarimeter
(Shanghai, China). All the organic solvents were reagent
grade and used without further purification. Other reagents
were all analytical grade or better.
1
alanine methyl ester (8.9 g, 67.2% yield): H NMR (CDCl3)
δ: 7.02–6.96 (m, 2H, aromatic H), 6.88–6.83 (t, 1H, J =
7.2 Hz, aromatic H), 3.96–3.94 (q, 1H, J = 6.9 Hz, CHCH3),
3.81 (s, 1H, NH), 3.66 (s, 3H, OCH3), 2.69–2.66 (m, 2H,
CH2CH3), 2.30 (s, 3H, aromatic CH3), 1.38–1.35 (d, 3H, J =
6.9 Hz, CH2CH3), 1.26–1.21 (t, 3H,
J = 7.5 Hz,
CHCH3).GC–MS m/z (%): 221(M+, 25), 162 (100), 133
(30), 77 (11); GC: 98% area.
Biocatalytic hydrolysis of (R,S)-1 in aqueous buffer
with organic compound
CAL-B (200 mg) and (R,S)-1 (2.2 g, 10 mmol) were
added to the aqueous phosphate buffer (100 mmol/L,
pH 8.0) containing diethyl ether (15% v/v) 100 mL. The
mixture was stirred at 25 °C and the pH was maintained us-
ing 0.1 mol/L NaOH solution. When the hydrolysis reached
49% conversion, a saturated solution of NaHCO3 (100 mL)
was added to the reaction mixture. The mixture was then ex-
tracted with ether (3 × 100 mL) to remove the unchanged es-
ter. The aqueous mixture was then acidified to pH 5.5 with
0.1 mol/L HCl and was extracted again with ether (3 ×
100 mL) to remove the acid product. Acid extracts were
Determination of conversion and enantiomeric excess (eep)
The analysis of the reaction mixtures and the determina-
tion of enantiomeric excesses of (S)-2 were performed by
capillary zone electrophoresis (P/ACE MDQ, Beckman,
© 2006 NRC Canada

1060
Can. J. Chem. Vol. 84, 2006
Scheme 1. Lipase-catalyzed hydrolysis of (R,S)-1 and chemical racemization.
Racemization
CH3
H
C2H5
N
CH3
CH3
H
C2H5
N
C2H5
N
COOCH3
Lipase
COOCH3
COOH
*
+
H
CH3
CH3
CH3
(R)-1
(R,S)-1
(S)-2
dried over anhydrous Na2SO4 and evaporated to give (S)-2
(1.0 g, eep > 98%).
and filtered. The filtrate was washed with 10% HCl, 10%
Na2CO3, and saturated NaCl. The organic phase was dried
over anhydrous Na2SO4, and subsequent concentration under
reduced pressure afforded the yellow oil (S)-4 (2.1 g, 95%
yield): [α]20 +11.6°(c 1.1%, methanol), 96.6% eep based on
Racemization of (R)-N-(2-ethyl-6-methylphenyl) alanine
methyl ester: (R)-1
[α]20 +12°D(c 1.166%, methanol) reported in literature (4).
D
The solution of (R)-1 (1.0 g, 4.5 mmol) in toluene (5 mL)
was added to a mixture of n-butyraldehyde (0.32 g,
4.5 mmol) and benzoic acid (0.22 g, 1.8 mmol) under nitro-
gen atmosphere. After refluxing for 3 h, the mixture was
cooled to room temperature and washed with 5% Na2CO3
(3×5 mL) and H2O (5 mL). The organic layer was separated,
dried over anhydrous Na2SO4, and concentrated under re-
duced pressure. The resulting products were purified by sil-
ica gel chromatography using ethyl acetate – petroleum ether
(1:5) as the eluant to produce the corresponding (R, S)-1
(0.9 g, 90% yield, R:S = 50:50).
GC–MS m/z (%): 269.2 (M+, 12), 162.2 (100), 133.1 (21);
GC: 92% area.
Preparation of (S)-N-(1′-methyl-2′-methoxyethyl)-N-
chloroacetyl-2-ethyl-6-methylaniline: (S)-5
The reaction mixture of (S)-4 (2.0 g, 7.4 mmol), 2,2-
dimethoxypropane (1.69 g, 14.8 mmol), and p-toluene-
sulfonic acid (0.26 g, 1.1 mmol) dissolved in methanol
(5 mL) was heated under reflux for 36 h. After cooling the
reaction mixture to room temperature, methanol was evapo-
rated under reduced pressure. The residue was taken up in
ethyl acetate (10 mL), and the solution was washed with
10% Na2CO3 and water. The organic layer was dried over
anhydrous MgSO4 and concentrated in vacuo. The residual
oil was purified by column chromatography on silica gel us-
ing ethyl acetate – petroleum ester (1:6) as the eluant to fur-
nish the corresponding (S)-metolachlor, (S)-5 (1.4 g, 66%
Preparation of (S)-N-(1′-methyl-2′-hydroxyethyl)-2-ethyl-
6-methylaniline: (S)-3
To a stirred solution of NaBH4 (0.9 g, 23 mmol) in anhy-
drous THF (20 mL) under nitrogen atmosphere, a solution of
(S)-2 (eep > 98%, 1.9 g, 9 mmol) in anhydrous THF (10 mL)
was added dropwise. Then, the concentrated H2SO4
(0.7 mL) was added slowly below 20 °C. The reaction mix-
ture was stirred at room temperature overnight, and the reac-
tion was quenched by slow addition of methanol (20 mL).
After concentrating the mixture in vacuo, 5 mol/L sodium
hydroxide (10 mL) was added, and the reaction mixture was
heated under reflux for 3 h. The crude mixture was filtered
and extracted with CHCl3 (3 × 20 mL). The organic layer
was dried over anhydrous Na2SO4 and concentrated under
reduced pressure to give (S)-3 (1.6 g, 92% yield): [α]20
+6.8° (c 2.0%, methanol), 97.1% eep based on [α]20 +7D°
(c 1.941%, methanol) reported in literature (4). GC–MDS m/z
(%): 193.1 (M+, 13.2), 162.2 (100); GC: 90% area.
yield): [α]20 –8.2 1° (c 2.1%, n-hexane), 91.1% eep based
D
on [α]20 –9.0 1° (c 2.073%, n-hexane) reported in litera-
D
1
ture (4). H NMR (CDCl3), δ: 7.24–7.13 (m, 3H, aromatic
H), 4.22 (m, 1H, CHCH3), 3.94 (dd, J = 4.2 and 9.4 Hz) and
3.75 (dd, J= 4.2 and 9.4 Hz) for one diastereomeric proton
of CHCH2OCH3), 3.62–3.61 (s, 2H, CH2Cl), 3.51 (dd, J =
4.2 and 9.4 Hz) and 3.49 (dd, J = 4.2 and 6.5 Hz) for one
diastereomeric proton of CHCH2OCH3, 3.27–3.26 (d, 3H,
OCH3), 2.62–2.54 (m, 2H, CH2CH3), 2.26 (s, 1H, aromatic
CH3), 2.24 (s, 2H, aromatic CH3), 1.28–1.23 (t, 3H, J =
7.5 Hz, CH2CH3), 1.17–1.14 (q, 3H, J = 7.0 Hz, CHCH3).
GC–MS m/z (%): 284 (M+, 60), 252 (38), 250 (100), 218
(47), 178 (25), 162 (37), 73 (60); GC: 98% area.
Preparation of (S)-N-(1′-methyl-2′-hydroxyethyl)-N-
chloroacetyl-2-ethyl-6-methylaniline: (S)-4
To a solution of (S)-3 (1.6 g, 8.3 mmol) and Na2CO3
(0.88 g, 8.3 mmol) dissolved in benzene (10 mL) under ni-
trogen atmosphere, chloroacetyl chloride (0.94 g, 8.3 mmol)
was slowly added at 15–20 °C with efficient stirring. The re-
action mixture was then stirred for 3 h at room temperature
Results and discussion
In a general experimental procedure, readily accessible
racemic methyl ester 1 was used as a substrate in the identi-
fication of a suitable lipase for the enantioselective hydroly-
sis (Scheme 1), and the screening results were listed in
Table 1. The lipase CAL-B was very active, and 48.2% of
© 2006 NRC Canada

Zheng et al.
1061
Table 1. Lipase-catalyzed hydrolysis of (R,S)-1.
Lipase
Time (h)
Conversion (%)
eep (%)
E value
Major enantiomer
2
60
72
72
72
72
10.6
>100
>100
99.2
8.6
CAL-B
PSL
PPL
CLL
AYL
PEL
48.2
48.2
1.7
0.24
2.0
69.4
99.0
99.0
98.0
78.8
58.5
S-(–)
R-(+)
R-(+)
S-(–)
S-(–)
S-(–)
0.12
3.8
Note: Reaction conditions: buffer: 100 mmol/L phosphate buffer (pH = 8.0, 5.0 mL); substrate: 0.5 mmol;
enzyme: 105 U; temperature: 37 °C.
Table 2. CAL-B-catalyzed hydrolysis of (R,S)-1 in different aqueous-organic media.
Organic solvent
Log P
Time (h)
Conversion (%)
eep (%)
E value
None
—
2
2
2
2
2
2
2
2
2
2
32.5
29.7
31.7
28.2
19.6
28.3
17.4
27.1
25.1
23.1
78.7
83.7
82.9
82.8
87.8
84.8
84.3
82.1
87.6
86.5
12.1
15.9
15.5
14.6
19.0
16.8
13.9
13.7
20.1
17.9
Acetonitrile
Acetone
THF
Diethyl ether
DIPE
–0.33
–0.23
0.49
0.85
1.90
2.50
3.20
3.50
4.00
Toluene
Cyclohexane
n-Hexane
n-Heptane
Note: Reaction conditions: substrate: 0.5 mmol; CAL-B: 10 mg; organic compound/phosphate buffer
(100 mmol/L, pH= 8.0, 5.0 mL) 1:9; temperature: 25 °C.
(R,S)-1 was hydrolyzed only after 2 h at 37 °C; however, it
displayed poor enantioselectivity (E = 10.6) towards the (S)-
enantiomer of the acid. The E value of >100 and
enantiomeric excess (eep) of >98% were obtained by lipase
PSL and PPL, but they preferably produced the (R)-acid and
needed longer reaction time. Other enzymes investigated in
this work gave both low conversion and enantioselectivity,
making them practically useless. As a result of its higher ac-
tivity and stereospecificity towards the (S)-enantiomer, the
lipase CAL-B was chosen from the enzymes screened for
further study.
Initially we investigated the influence of the micro-
environment on the CAL-B-catalyzed resolution and found
that the enantioselectivity of CAL-B was still too low for a
large-scale preparation (data not shown). This may be as-
cribed to the ability of CAL-B to catalyze the hydrolysis of
both (R)- and (S)-enantiomers of substrate, thus leading to
the low enantioselectivity of enzyme. According to this as-
sumption, if we take measures to enantioselectively inhibit
the reaction rate of CAL-B-catalyzed hydrolysis, and as a re-
sult, the (S)-enantiomer becomes overwhelmingly the faster
reacting enantiomer, the enantioselectivity of CAL-B may be
enhanced.
pure buffered medium, despite a decrease in enzyme activ-
ity.
According to the previously mentioned results, it is wor-
thy to note that the use of diethyl ether and n-hexane as ad-
ditives is preferred (E = 19.0 and 20.1, respectively). Herein,
the effects of diethyl ether and n-hexane amounts on the ac-
tivity and enantioselectivity of CAL-B were studied. The ad-
ditive amount of the organic compound is an important
parameter worthy of careful optimization because it may
have a strong influence on enzyme activity and enantio-
selectivity. When diethyl ether was added to the reaction me-
dium, the maximum E value of CAL-B was obtained at 15%
(v/v) (E > 100, Fig 2), which was approximately 9.7-fold
more enantioselective than that in the pure buffered medium.
For n-hexane, the results differed and the maximum
enantioselectivity of CAL-B was stabilized at the additive
amount ranging from 10 to 15% (v/v) (E = 20.1, Fig 3). The
conversion ratios of reactions for various amounts of organic
compounds were also measured and it was found that the ra-
tio decreased with an increase of additive amount of organic
compound. Based on these results, we presume that diethyl
ether is a stronger and more (R)-selective inhibitor than n-
hexane.
The strategy of enantioselective inhibition by adding an
organic compound to the reaction medium to enhance the se-
lectivity of lipase had previously been reported by our group
(13). So, in this study, organic compounds with different
hydrophobicities (log P) (14, 15) were added to the reaction
system to observe their effect on CAL-B-catalyzed hydroly-
sis (Table 2). In all experiments, the biocatalytic system
shows the same stereopreference for (R,S)-1 giving (S)-acid.
Also, the addition of organic compounds generally enhanced
the enantioselectivity of CAL-B compared with that in a
After CAL-B-catalyzed hydrolysis finished under the opti-
mized conditions, the enantiomercially pure (S)-2 (>98%
eep) and the remaining ester (R)-1 were extracted with ether.
The two compounds can be separated easily by partitioning
in ether or water. This simple extraction–partition procedure
is especially convenient and suitable for a large-scale opera-
tion. The remaining ester can be recycled by chemical
racemization, which is achieved through treatment with a
mixture of n-butyraldehyde and benzoic acid dissolved in to-
luene (Scheme 1). The aldehyde-catalyzed racemization of
© 2006 NRC Canada

1062
Can. J. Chem. Vol. 84, 2006
Fig. 2. Effect of different volume fractions of diethyl ether on
the conversion and enantioselectivity of CAL-B-catalyzed hydro-
lysis of (R,S)-1.
Fig. 3. Effect of different volume fractions of n-hexane on the
conversion and enantioselectivity of CAL-B-catalyzed hydrolysis
of (R,S)-1.
25
20
15
10
5
60
50
40
30
20
10
0
140
120
100
80
60
50
40
30
20
10
0
E
Conv.
60
40
E
Conv.
20
0
0
0
5
10
15
20
25
0
5
10
15
20
25
n-Hexane (% v/v)
Diethyl Ether (% v/v)
Scheme 2. Synthesis of (S)-metolachlor from enzymatically prepared (S)-2.
CH3
CH3
C2H5
N
C2H5
N
CH2OH
NaBH4/H2SO4
COOH
H
H
CH3
CH3
(S)-2
(S)-3
ClCOCH2Cl
CH3
C2H5
N
CH3
C2H5
CH2OH
CH2OCH3
N
COCH2Cl
COCH2Cl
CH3
CH3
(S)-4
(S)-Metolachlor, (S)-5
amino acids and derivatives is the most prominent example
of how to derive a functional group connected to a chiral
center and affect optical stability. Herein, we also investi-
gated the effects of other aldehydes including formaldehyde,
acetaldehyde, propionaldehyde, n-valeraldehyde, and benz-
aldehyde on the racemization, and found that the experimen-
tal results were similar. However, considering the economic
factor and the boiling point of the aldehyde, we selected n-
butyraldehyde as the racemic reagent. Very similar results
were also reported by Park et al. (6).
The chemical synthesis of (S)-metolachlor was then car-
ried out using enantiomercially pure (S)-2 as a substrate via
a number of pathways, which included reduction, chloro-
acetylation, and etherification (Scheme 2). The reduction of
(S)-2 to (S)-3 is conveniently carried out in an inert solvent.
Suitable reducing agents are lithium aluminium hydride and
© 2006 NRC Canada

Zheng et al.
1063
sodium borohydride, but the selectivity of lithium aluminium
hydride towards the functional group of the compound is
poor and it is rather expensive. Thus, it is preferable to use
sodium borohydride in the form of a complex, especially as
a sodium borohydride – tetrahydrofuran complex. The con-
version of (S)-3 into (S)-4 is carried out in an inert solvent
by the reaction with a chloroacetylating agent, such as
chloroacetyl chloride, in the presence of a base. Suitable
bases may be alkali metal carbonates, triethylamine, and
pyridine. Suitable inert solvents may be hydrocarbons, such
as hexane, cyclohexane, benzene, toluene, and chloroben-
zene. In this work, Na2CO3 and benzene were selected as the
base and inert solvent, respectively. The reaction was carried
out at a low or moderately elevated temperature. The etheri-
fication of the 2-hydroxy group of (S)-4 was carried out by
heating the reaction mixture, dissolved in methanol, in the
presence of a strong acid at a reflux temperature. A strong
acid suitable for the reaction is p-toluenesulfonic acid. Then
(S)-metolachlorwas obtained with the optical purity of
91.1% eep. Compared with racemic metolachor, (S)-metola-
chlor has a markedly superior herbicidal action against
weeds without increasing phytotoxicity towards cultivated
plants.
the process, is being done in our laboratory and will be re-
ported soon.
Acknowledgements
The authors are grateful for the financial support from
the National Natural Science Foundation of China
(No.20432010) and the Foundation of Research Program of
Jilin University, China.
References
1. J. Liu, Y. Zhang, L. Qiu, F. Yang, L. Ye, and Y. Xia. J. Ind.
Microbiol. Biotechnol. 31, 495 (2004).
2. P.J. O’Connell, C.T. Harms, and J.R.F. Allen. Crop Protection,
17, 207 (1998).
3. B.T. Cho and Y. S.Chun. Tetrahedron: Asymmetry, 3, 337
(1992).
4. H. Moser and C.Vogel. US Patent 5-002-606, 1991.
5. I. Osprian, M.H. Fechter, and H. Griengl. J. Mol. Catal. B, 24–
25, 89 (2003).
6. O.J. Park, S.H. Lee, T.Y. Park, S.W. Lee, and K.H. Cho. Tetra-
hedron: Asymmetry, 16, 1221 (2005).
7. K.E. Jaeger and T. Eggert. Curr. Opin. Biotechnol. 13, 390
(2002).
8. H. Stamatis, A. Xenakis, and F.N. Kolisis. Biotechnol. Adv.
17, 293 (1999).
Conclusion
9. Y. Chikusa, Y. Hirayama, M. Ikunaka, T. Inoue, S. Kamiyama,
M. Moriwaki, Y. Nishimoto, F. Nomoto, K. Ogawa, T. Ohno,
K. Otsuka, A.K. Sakota, N. Shirasaka, A. Uzura, and K.
Uzura. Org. Process Res. Dev. 7, 289 (2003).
10. M. Kinoshita and A.Ohno. Tetrahedron, 52, 5397 (1996).
11. E.J. Ebbers, G.J.A. Ariaans, J.P.M. Houbiers, A. Bruggink,
and B. Zwanenburg. Tetrahedron, 53, 9417 (1997).
12. C.S. Chen, Y. Fujimoto, G. Girdaukas, and C.J. Sih. J. Am.
Chem. Soc. 104, 7294 (1982).
13. L.Y. Zheng, S.Q. Zhang, Y. Feng, S.G. Cao, J.S. Ma, L.F.
Zhao, and G. Gao. J. Mol. Catal. B, 31, 117 (2004).
14. A. Cipiciani, M. Cittadini, and F. Fringuelli. Tetrahedron, 54,
7883 (1998).
We successfully developed a chemoenzymatic procedure
for the synthesis of enantiomerically pure (S)-metolachlor.
We took advantage of the higher activity and excellent
enantioselectivity of lipase CAL-B, chemical racemization
of the remaining ester, and chemical reactions including re-
duction, chloroacetylation, and etherification. The method is
easy to perform with standard equipment and can be widely
applied. At the same time, we found that organic compounds
strongly influenced the selectivity of CAL-B. Diethyl ether,
in particular, was found to achieve the highest enantio-
selectivity of CAL-B (E > 100) in the hydrolysis of (R,S)-1.
Further work in this area, including the repeated use of en-
zymes to decrease the enzymatic cost and scale-up results of
15. T.W. Chiou, C.C. Chang, C.T. Lai, and D.F. Tai. Bioorg. Med.
Chem. Lett. 7, 433 (1997).
© 2006 NRC Canada