Enzymatic resolution of the chiral auxiliary 2-methoxy-2-phenylethanol

Article abstract of DOI:10.1016/S0957-4166(02)00246-X

Several lipase-catalyzed processes are evaluated for the resolution of 2-methoxy-2-phenylethanol, a chiral auxiliary in the synthesis of optically active 1,4-dihydropyridines. Candida antarctica lipase B (CAL B) catalyzes the enzymatic acylation of the pr

Full text of DOI:10.1016/S0957-4166(02)00246-X

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 13 (2002) 1091–1096
Enzymatic resolution of the chiral auxiliary
2-methoxy-2-phenylethanol
Mar´ıa I. Monterde,a Rosario Brieva,a V´ıctor M. Sa´nchez,b Miguel Bayodb and Vicente Gotora,*
aDepartamento de Qu´ımica Orga´nica e Inorga´nica, Facultad de Qu´ımica, Universidad de Oviedo, 33071 Oviedo, Spain
bAstur-Pharma S. A. Pol´ıgono Industrial de Silvota, parcela 23, 33192 Llanera (Asturias), Spain
Received 26 April 2002; accepted 20 May 2002
Abstract—Several lipase-catalyzed processes are evaluated for the resolution of 2-methoxy-2-phenylethanol, a chiral auxiliary in
the synthesis of optically active 1,4-dihydropyridines. Candida antarctica lipase B (CAL B) catalyzes the enzymatic acylation of
the primary alcohol with high enantioselectivity. © 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Results and discussion
Enantiomerically pure 2-methoxy-2-phenylethanol, 1,
has been described as a useful chiral auxiliary in the
synthesis of 1,4-dihydropyridines.1 Increasing interest
in the preparation of these compounds from the
pharmaceutical industry means that there is a demand
for the development of new synthetic methods for 1
which are amenable to large-scale production and the
use of enzymes as catalysts is very suitable for this
purpose.2 While lipase-catalyzed processes in organic
solvents have been widely used for the preparation of
enantiomerically pure alcohols and the enzymatic res-
olution of secondary alcohols is now well estab-
lished,3 the resolution of primary alcohols has been
less studied. Nevertheless, lipases have been success-
fully used for the preparation of enantiopure primary
alcohols.4 Recently, Kanerva and co-workers have
described the resolution of 1-phenylethan-1,2-diol
through sequential methanolysis of the mono- or
diacetate derivatives.5 Herein, we describe the applica-
tion of lipases for the resolution of 2-methoxy-2-
phenylethanol.
Our initial experiments were designed to find the most
suitable lipase for catalyzing the transesterification of
2-methoxy-2-phenylethanol, ( )-1. In a first set of
experiments, vinyl acetate was chosen as the acyl donor
and also as the reaction solvent. These enzymatic pro-
cesses were carried out at 30°C (Scheme 1, Table 1).
Lipase B from C. antarctica (CAL B) showed the
highest enantioselectivity (E=47). All of the tested
lipases but C. rugosa lipase showed the same stereo-
chemical preference, with the (R)-enantiomer being
preferentially acylated. In view of these promising
results, we decided to study the effect of the reaction
parameters on the enantioselectivity of the acylation of
alcohol ( )-1. Taking into account that the temperature
can have a marked effect on the enantioselectivity,7 we
lowered to 15°C. Nevertheless, the enantioselectivities
were not significantly enhanced (Table 2).
Next, we studied the influence of the organic solvent in
the CAL B-catalysed acylation. Table 3 summarizes the
O
R
OMe
OMe
OMe
Lipase
+
OH
OAc
OH
+
O
Ph
Ph
Ph
(±)-1
(R)-2
(S)-1
R = H, Me
Scheme 1.
* Corresponding author. E-mail: vgs@sauron.quimica.uniovi.es
0957-4166/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(02)00246-X

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M. I. Monterde et al. / Tetrahedron: Asymmetry 13 (2002) 1091–1096
Table 1. Lipase-catalyzed acetylation of (9)-1 with vinyl acetate, at 30°C. R=H
Lipase (mg/mmol substrate)
Time (h)
ca (%)
Remaining alcohol 1
Product 2
Ec
b
Conf.
e.e.sb (%)
Conf.
e.e.p (%)
CAL B (150)
CAL B-L2 (10)
CAL-A-L5 (100)
CRL-L3A (10)
CRL-L3 (200)
PPL (150)
PPL-L7 (200)
PSL (150)
PSL-L6 (50)
0.5
0.5
0.2
7
1.5
11
7
22
3
3
49
59
77
36
74
36
27
43
67
49
76
12
(S)
(S)
(S)
(R)
(R)
(S)
(S)
(S)
Nd
(S)
(S)
(S)
89
\99d
94
15
61
39
28
36
(R)
(R)
(R)
(S)
(S)
(R)
(R)
(R)
Nd
(R)
(R)
(R)
86
70
27
27
28
68
77
46
Nd
28
32
22
47
40
5
2
3
8
10
4
Nd
2
11
2
Nd
28
MML-L9 (10)
ASL-L10 (100)
TLL-L8 (200)
1.5
7
\99d
2
a Conversion, c=e.e.s/(e.e.s+e.e.p).
b Determined by gas chromatography.
c Enantiomeric ratio, E=ln[(1−c)(1+e.e.p)]/ln[(1−c)(1−e.e.p)].6
d Only the (S)-enantiomer is detected by gas chromatography, the E value has been calculated taking e.e.s=99.9.
Table 2. Lipase-catalyzed acetylation of (9)-1 with vinyl acetate, at 15°C. R=H
Lipase
Time (h)
c (%)
Remaining alcohol 1
Product 2
E
e.e.s (%)
e.e.p (%)
CAL B
CAL B-L2
CAL-A-L5
0.6
0.6
0.2
54
56
46
96
99
52
85
76
60
48
39
7
Table 3. CAL B-catalyzed acetylation of (9)-1 in organic solvents
Solvent
Ra
T (°C)
Time (h)
c (%)
Remaining alcohol 1
Product 2
E
e.e.s (%)
e.e.p (%)
Hexane
Hexane
Hexane
Hexane
Dioxane
Dioxane
iPr2O
Me
Me
H
50
30
50
30
50
30
50
30
30
30
1.3
23
1
23
46
46
24
26
0.75
0.75
48
47
59
55
13
9
38
9
75
54
40
51
88
73
Nd
Nd
1
Nd
99
96
56
59
62
60
Nd
Nd
2
Nd
33
82
5
6
12
8
Nd
Nd
Nd
Nd
11
39
H
Me
Me
Me
Me
H
iPr2O
Ethyl acetate
Acetonitrile
H
a 2 equiv. of the acyl donor.
results of the experiments at 30 and 50°C. The reactions
were carried out using 2 equiv. of vinyl acetate or
isopropenyl acetate. Under these conditions, a strong
influence of the organic solvent upon the enantioselec-
tivity of the process was observed. It is apparent that
acetonitrile was the best solvent for the CAL B
catalysed reaction: after only 45 min, 54% conversion
was achieved, with an E value of 39. Nevertheless, the
use of the acyl donor vinyl acetate also as the reaction
solvent is preferable to other organic solvents (see
Table 1). Very low reaction rates and enantioselectivi-
ties were observed in other tested solvents.
Another common approach for the resolution of
racemic alcohols is enzymatic hydrolysis. Taking into
account that the best enzyme for the acylation is the
CAL B lipase, we examined the hydrolysis of the acetyl
derivative ( )-2 catalyzed by this lipase under different
conditions (Scheme 2).
CAL B induces a high reaction rate in the hydrolysis of
the substrate in aqueous media (phosphate buffer, pH
7) and 41% conversion was obtained after only 30 min.
However, the enantioselectivity of this process is rela-
tively low (E=10). When the reaction was carried out

M. I. Monterde et al. / Tetrahedron: Asymmetry 13 (2002) 1091–1096
1093
OMe
OMe
OMe
OMe
Lipase
Ac2O
OAc
OH
OAc
OH
+
Ph
Ph
Ph
Ph
CH2Cl2,py
(±)-2
(S)-2
(R)-1
(±)-1
Scheme 2.
of this process was lower than in the absence of the
additive. Finally, the best biocatalytic conditions found
for the alkoxycarbonylation with diallyl carbonate were
applied to the process with different carbonates (diben-
zyl carbonate, vinylacetoxime carbonate and phenyl
p-nitrophenyl carbonate), but in all cases the reaction
rates and enantiomeric ratios were very low.
in organic solvents with a small amount of water, only
the reaction in water–saturated hexane shows a moder-
ate reaction rate, 41% conversion was obtained after 7
h, but again the enantioselectivity of the process is low
(E=9). When other organic solvents were used as
reaction media for the enzymatic hydrolysis, very low
conversions were obtained.
As in the case of the acylation, we examined the CAL
B-catalyzed hydrolysis of the carbonate derivative ( )-3
(Scheme 4). The experiments were carried out in differ-
ent organic solvents, and the results obtained are sum-
marized in Table 5. The lipase induces a low to
moderate reaction rate depending on the organic sol-
vent used. Unfortunately, the enantioselectivity of these
hydrolytic processes was low in all cases.
Enzymatic alkoxycarbonylation or the enzymatic
hydrolysis of carbonates are also useful procedures for
the resolution of racemic alcohols.8 We first studied the
influence of the organic solvent in the alkoxycarbonyla-
tion of 2-methoxy-2-phenylethanol, ( )-1 with diallyl
carbonate in the presence of CAL B (Scheme 3).
Table 4 summarizes the results of these experiments.
The best enantiomeric ratio was obtained in THF (E=
30), with a moderate reaction rate (20% conversion
after 32 h). In order to improve the enantioselectivity
and the reaction rate we carried out the process at
different temperatures. At 15°C the reaction rate
decreased considerably, but we did not observe an
improvement in enantioselectivity. At 60°C the process
is faster, but the enantiomeric ratio decreases (E=10).
We also studied the effect of a small amount of triethyl
amine in the reaction media, but the enantioselectivity
3. Conclusions
We have described the resolution of 2-methoxy-2-
phenylethanol, ( )-1 via a CAL B-catalyzed acylation
and alkoxycarbonylation. High enantioselectivities can
be achieved by an appropriate selection of the reaction
parameters. In general, the best conditions for CAL B
acylation involved the use of vinyl acetate as an acyl
donor and also as the solvent. On the other hand,
O
OMe
OMe
OMe
CAL-B
+
OH
O
O
OH
+
O
O
Ph
organic solvent
Ph
Ph
O
(±)-1
(S)-1
(R)-3
Scheme 3.
Table 4. CAL B-catalyzed alkoxycarbonylation of (9)-1 in organic solventsa
Solvent
T (°C)
Time (h)
c (%)
Remaining alcohol 1
Product 3
E
c
e.e.sb (%)
e.e.p (%)
1,4-Dioxane
tBuOMe
Toluene
iPr2O
30
30
30
30
30
30
30
15
30
60
30
24
45
34
48
96
96
96
96
32
27
96
32
41
30
67
47
42
25
28
20
28
35
38
7
30
Nd
9
50
29
35
23
30
47
83
10
69
Nd
10
68
86
90
93
77
86
15
1
7
Nd
1
9
17
26
30
10
21
Et2O
Acetonitrile
CH2CCl2
THF
THF
THF
THFd
a 2 equiv. diallyl carbonate.
b Determined by HPLC.
c Determined by 1H NMR in the presence of Eu(hfc)3.
d 0.1 equiv. of Et3N were added to the reaction media.

1094
M. I. Monterde et al. / Tetrahedron: Asymmetry 13 (2002) 1091–1096
O
OMe
OMe
CH2Cl2, py
0ºC
+
O
O
OH
Cl
O
Ph
Ph
O
(±)-3
(±)-1
OMe
OMe
CAL-B
organic solvent/ H2O
O
O
OH
+
Ph
Ph
O
(R)-1
(S)-3
Scheme 4.
Table 5. CAL B-catalyzed hydrolysis of carbonate (9)-3 in water–organic solvents, at 30°Ca
Solvent
Time (days)
c (%)
Remaining carbonate 3
Product 1
E
e.e.s (%)
e.e.p (%)
Dioxane
Acetonitrile
THF
4
7
9
3
34
13
14
39
41
13
13
47
81
85
77
71
14
15
9
Hexane
10
a 10 equiv. H2O.
alkoxycarbonylation catalyzed by CAL B afforded
moderated enantioselectivities using diallyl carbonate as
the alkoxycarbonylating agent in THF, at 30°C. The
enzymatic acylation is preferable to the alkoxycarbonyl-
ation. Taking into account the simplicity and ease of
scale-up in lipase-catalyzed reactions, the method has
great potential applicability within the pharmaceutical
industry.
reagents were purchased from Aldrich Chemie. Solvents
were distilled over an adequate desiccant and stored
under nitrogen. Flash chromatography was performed
using Merck silica gel 60 (230–400 mesh).
4.2. Lipase-catalyzed acetylation of ( )-2-methoxy-2-
phenylethanol with vinyl acetate. General procedure
The reaction mixture contained alcohol ( )-1 (0.985
mmol), vinyl acetate (1.971 mmol) and the lipase
(amount indicated in Table 1) in the corresponding
organic solvent (7 mL) (when vinyl acetate itself was
also the solvent of the reaction, 49.25 mmol of this
reactant were used). The mixture was shaken at 250
rpm in a rotary shaker. The progress of the reaction
was monitored by TLC (hexane/ethyl acetate, 7:3). The
enzyme was removed by filtration and washed with
ethyl acetate. The solvent was evaporated under
reduced pressure and the crude residue was purified by
flash chromatography on silica gel (hexane/ethyl ace-
tate, 4:1) to afford compound (R)-(−)-2 and the corre-
sponding enantiomer of the remaining substrate
(S)-(+)-1. The purification procedure affords both com-
pounds quantitatively, and the yields obtained are in
accordance with the corresponding conversion shown
in Tables 1–3.
4. Experimental
4.1. General
Enzymatic reactions were carried out in a Gallenkamp
incubatory orbital shaker. Immobilized C. anctarctica
lipase B (CAL B), Novozym 435, was a gift from Novo
Nordisk. Pseudomonas cepacia lipase lyophilized (PSL)
was purchased from Amano Pharmaceutical Co.
Porcine pancreas lipase (PPL) is a product of Sigma. C.
antarctica B (CALB-L2), Candida rugosa (CRL, L3 y
L3A –purified–, Candida antarctica A (CALA-L5),
Pseudomonas sp. (PSL-L6), porcine pancreas (PPL-L7),
Thermomyces lanuginosa (TLL-L8), and Mucor Miehei
(MML-L9) lipases were obtained from Roche
Diagnostics.
Melting points were taken using a Gallenkamp appara-
tus and were uncorrected. Optical rotations were mea-
sured using a Perkin–Elmer 241 polarimeter and are
4.2.1. (R)-(−)-2-Methoxy-2-phenylethyl acetate, (R)-2.
1
Colorless oil; [h]2D5 −82 (c=1.0, CHCl3), e.e. 95%; H
NMR (CDCl3) l (ppm): 2.08 (s, 3H, CH3), 3.29 (s, 3H,
CH3), 4.16–4.2 (m, 2H, CH2), 4.40–4.44 (m, 1H, CH),
7.34–7.35 (m, 5H, CH); 13C NMR (CDCl3) l (ppm):
20.9 (CH3), 56.9 (CH3), 67.8 (CH2), 81.46 (CH), 126.8
(CH), 128.2 (CH), 129.5 (CH), 137.7 (C), 170.8 (CꢀO);
MS (EI) m/z 194 (M+, <1), 77 (18), 91 (17), 105 (6), 121
(100), 122 (25), 134 (11). HRMS calcd for C9H10NO
[M+−(OCOCH3)−H] 134.07309, found 134.0731.
1
quoted in units of 10−1 deg cm2 g−1. H and 13C NMR
were obtained with TMS (tetramethylsilane) as internal
standard using a Bruker AC-300 (1H–300 MHz and
13C–75.5 MHz) spectrometer. Mass spectra were
recorded on a Hewlett–Packard 5987A and Finigan
MAT/95 spectrometers. Microanalyses were performed
on a Perkin–Elmer 240B elemental analyzer. All

M. I. Monterde et al. / Tetrahedron: Asymmetry 13 (2002) 1091–1096
1095
4.3. Synthesis of ( )-2-methoxy-2-phenylethyl acetate,
( )-2
yields obtained are in accordance with the corre-
sponding conversion showed in Table 4.
Acetic anhydride (11.82 mmol) was added under
nitrogen to a 0°C solution of alcohol ( )-1 (9.85
mmol) in 25 mL of CH2Cl2, pyridine (11.82 mmol)
and a catalytic amount of dimethylaminopyridine.
The resulting solution was stirred for 4 h. The result-
ing mixture was extracted with HCl (1N) and
CH2Cl2. The organic layers were dried over Na2SO4,
filtered and evaporated under reduced pressure. The
crude residue was purified by flash chromatography
on silica gel with hexane/ethyl acetate (4:1) to afford
compound ( )-2 in quantitative yield.
4.5.1. (R)-(−)-2-Methoxy-2-phenylethyl allyl carbonate,
(R)-3. Colorless oil; [h]2D5 −63 (c=1.0, CHCl3), e.e.
1
93%; H NMR (CDCl3) l (ppm), J (Hz): 3.29 (s, 3H,
CH3), 4.16–4.32 (m, 2H, CH2), 4.45–4.49 (m, 1H,
3
3
CH), 4.62–4.65 (dd, 2H, CH2, JHH=5.6, JHH=1.3),
5.25–5.39 (m, 2H, CH+CH), 5.87–6.00 (m, 1H, CH),
7.38–7.39 (m, 5H, CH); 13C NMR (CDCl3) l (ppm):
56.9 (CH3), 68.4 (CH2), 70.9 (CH2), 81.3 (CH), 118.7
(CH), 126.8 (CH), 128.3 (CH), 128.5 (CH), 131.4
(CH), 137.4 (C), 154.8 (CꢀO); MS (ESI+) m/z (%):
259.1 [(M+Na)+, 25].
4.4. Lipase-catalyzed hydrolysis of ( )-2-methoxy-2-
phenylethyl acetate
4.6. Synthesis of ( )-2-methoxy-2-phenylethyl allyl
carbonate, ( )-3
Two procedures could be used.
Allyl chloroformate (19.7 mmol) was slowly added to
a solution of the alcohol ( )-1 (9.85 mmol) and pyri-
dine (19.7 mmol) in CH2Cl2 under nitrogen at 0°C.
The solution was stirred for 5 h and extracted with
1N HCl and CH2Cl2. The organic fraction was dried
over Na2SO4, filtered and evaporated under reduced
pressure. The residue was subjected to flash chro-
matography on silica using hexane/AcOEt (4:1) to
afford the racemic carbonate ( )-3 (95% yield).
Procedure 1:
A
mixture of ( )-2-methoxy-2-
phenylethyl acetate (0.77 mmol) and CAL B lipase
(150 mg) suspended in phosphate buffer at pH 7
(0.05 M, 4 mL), was shaken at 250 rpm. The enzyme
was removed by filtration and washed with water and
CH2Cl2. The mixture was extracted with CH2Cl2 and
the combined organic fractions were dried over
Na2SO4, filtered and evaporated under reduced pres-
sure. The residue was purified by flash chromatogra-
phy on silica gel with hexane/AcOEt (4:1).
4.7. Lipase-catalyzed hydrolysis of ( )-2-methoxy-2-
phenylethyl allyl carbonate, ( )-3
Procedure 2: The reaction mixture contained ( )-2-
methoxy-2-phenylethyl acetate (0.77 mmol), CAL B
lipase (150 mg) in the corresponding water–saturated
organic solvent (7 mL). The mixture was shaken at
30°C and 250 rpm in a rotary shaker. The progress
of the reaction was monitored by TLC (hexane/ethyl
acetate, 4:1). After removal of the enzyme by filtra-
tion, the filtrate was washed with CH2Cl2 and the
solvents were evaporated under reduced pressure. The
crude residue was purified by flash chromatography
on silica gel (hexane/ethyl acetate, 4:1) to afford (R)-
(−)-1 and the corresponding enantiomer of the
remaining substrate (S)-(+)-2.
The reaction mixture contained the carbonate ( )-3
(0.42 mmol), water (4.2 mmol) and the lipase (150
mg) in the corresponding organic solvent (7 mL). The
mixture was shaken at 30°C and 250 rpm in a rotary
shaker. The progress of the reaction was monitored
by TLC (hexane/ethyl acetate, 9:1). After removal of
the enzyme by filtration, the filtrate was washed with
CH2Cl2 and the solvents evaporated under reduced
pressure. The crude residue was purified by flash
chromatography on silica gel (hexane/ethyl acetate,
9.5:0.5) to afford (R)-(−)-1 and the corresponding
enantiomer of the remaining substrate (S)-(+)-3.
4.8. Determination of the enantiomeric excess of (R)-1,
(S)-1, (R)-2 and (S)-2
4.5. Lipase-catalyzed alkoxycarbonylation of ( )-2-
methoxy-2-phenylethanol. General procedure
The enantiomeric excesses of (R)-1, (S)-1, (R)-2 and
(S)-2 were determined by direct analysis on a chiral
GC RtbDEXse column (30 m×0.25 mm; Restek).
Column temperature: 95°C. Sample concentration 0.5
mg/mL. Two peaks (tR 49.4 and 51.1 min) for (R)-1
and (S)-1 and two peaks (69.2 and 71 min) for (R)-2
and (S)-2, respectively, were resolved.
The reaction mixture contained alcohol ( )-1 (0.985
mmol), the corresponding carbonate (1.971 mmol)
and the lipase in the chosen organic solvent (7 mL)
(Table 4). The mixture was shaken at 250 rpm in a
rotary shaker. The progress of the reaction was moni-
tored by TLC using the solvent system hexane/ethyl
acetate (7:3). The enzyme was removed by filtration
and washed with ethyl acetate. The solvent was evap-
orated under reduced pressure and the crude residue
was purified by flash chromatography on silica gel
(hexane/ethyl acetate, 9.5:0.5) to afford compound
(R)-(−)-3 and the corresponding enantiomer of the
remaining substrate (S)-(+)-1. The purification proce-
dure affords both compounds quantitatively and the
4.9. Determination of the enantiomeric excess of (R)-3
and (S)-3
The enantiomeric excesses of (R)-3 and (S)-3 were
determined by 1H NMR analysis in the presence of
the chiral shift reagent Eu(hfc)3.

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M. I. Monterde et al. / Tetrahedron: Asymmetry 13 (2002) 1091–1096
bolts, M.; Witholt, B. Nature 2001, 409, 258–268.
Acknowledgements
3. Kazlauskas, R. J.; Weissfloch, A. N. E.; Rappaport, A. T.;
Cuccia, L. A. J. Org. Chem. 1991, 56, 2656–2665.
4. Gonzalo, G.; Brieva, R.; Sa´nchez, V. M.; Bayod, M.;
Gotor, V. J. Org. Chem. 2001, 66, 8947–8953.
This work was supported by the Ministerio de Ciencia
y Tecnolog´ıa (FEDER project 1FD97-1255-C02-01).
We express our appreciation to Novo Nordisk Co. for
the generous gift of the lipase CAL B.
5. Virsu, P.; Liljeblad, A.; Kanerva, A.; Kanerva, L. T.
Tetrahedron: Asymmetry 2001, 12, 2447–2455.
6. Chen, C. S.; Fujimoto, Y.; Girdaukas, G.; Sih, C. J. J.
Am. Chem. Soc. 1982, 104, 7294–7299.
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