Studies on the chemoenzymatic synthesis of (R)- and (S)-methyl 3-aryl-3-hydroxypropionates: The influence of toluene-pretreatment of lipase preparations on enantioselective transesterifications

Article abstract of DOI:10.1016/j.tetasy.2013.06.004

Two series (para- and meta-substituted) of racemic methyl esters of 3-aryl-3-hydroxypropionic acid were prepared after which the enantiomers were separated by an enzyme-catalyzed transesterification. Several lipases were investigated as the catalyst. The influence of the enzyme pretreatment, as well as substrate concentration, reaction temperature, stirring manner, and substrate conversion on the stereochemical outcome of the biotransformation process were investigated in detail. The best results were achieved by using solvent-pretreated lipase from Pseudomonas fluorescens or Burkholderia cepacia suspended in toluene, and vinyl acetate as the acetyl group donor.

Full text of DOI:10.1016/j.tetasy.2013.06.004

Tetrahedron: Asymmetry xxx (2013) xxx–xxx
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Studies on the chemoenzymatic synthesis of (R)- and (S)-methyl
3-aryl-3-hydroxypropionates: the influence of toluene-pretreatment
of lipase preparations on enantioselective transesterifications
⇑
Paweł Borowiecki , Maria Bretner
Warsaw University of Technology, Faculty of Chemistry, Noakowskiego St. 3, 00-664 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Two series (para- and meta-substituted) of racemic methyl esters of 3-aryl-3-hydroxypropionic acid were
prepared after which the enantiomers were separated by an enzyme-catalyzed transesterification. Sev-
eral lipases were investigated as the catalyst. The influence of the enzyme pretreatment, as well as sub-
strate concentration, reaction temperature, stirring manner, and substrate conversion on the
stereochemical outcome of the biotransformation process were investigated in detail. The best results
were achieved by using solvent-pretreated lipase from Pseudomonas fluorescens or Burkholderia cepacia
suspended in toluene, and vinyl acetate as the acetyl group donor.
Received 15 May 2013
Accepted 24 June 2013
Available online xxxx
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
VI originally designed for the treatment of parasitic worm
infections but currently used as an immunomodulatory chemo-
The use of enantiomerically enriched intermediates in drug syn-
thesis is particularly desirable in the pharmaceutical industry.1 The
stereochemistry of biologically active compounds receives special
attention due to the implications of different physiological effects
resulting from interactions with drug-targets (receptors, enzymes,
and transporters) in living systems.2 Thus, it is obvious that the
safety profile of a drug based on enantiopure pharmaceutically ac-
tive ingredients is more reliable and provides an important thera-
peutic strategy for new drug development.
Therefore, simple and inexpensive methods for the preparation
of both enantiomeric forms of methyl 3-aryl-3-hydroxypropio-
nates are vital since they are important building blocks in the syn-
thesis of several therapeutically active compounds. The most
prominent examples are the selective serotonin-reuptake inhibi-
tors (SSRIs), such as fluoxetine3 I and dapoxetine4 II used in the
treatment of major depression (panic and obsessive-compulsive
disorder, bulimia nervosa) and male sexual dysfunctions (prema-
ture ejaculation), respectively (see Fig. 1). Other examples are: nor-
epinephrine re-uptake inhibitors (NRI), such as atomoxetine5 III
approved for attention-deficit hyperactivity disorder (ADHD),
nisoxetine6 IV widely used as a biological activity standard or tol-
terodine7 V antimuscarinic drug used for the treatment of urinary
incontinence. Based on the retrosynthetic strategy, enantiomeri-
cally pure 3-hydroxy-3-phenylpropionates have been also consid-
ered as a potential core structure for the synthesis of levamisole8
therapeutic in cancer adjuvant therapy, and the therapeutically
important antidepressant paroxetine9 VII.
Various methodological approaches were developed toward
obtaining optically active 3-aryl-3-hydroxypropionates.10 One of
the most common enzymatic route is the bioreduction of prochiral
b-keto esters, which can be catalyzed by numerous native micro-
bial,11 genetically engineered12 or plant13 dehydrogenases (ADHs).
Unfortunately, the asymmetric bioreduction of the ketone group
via an isolated alcohol dehydrogenase requires the addition of hy-
dride donating cofactors (mainly NADH or NADPH), which are pro-
hibitively expensive on a multi-gram scale. In turn, the living cells
of microorganisms can handle this without the need for external
regeneration of cofactors due to cellular metabolic machinery.
However, microbial cells usually contain several different ADHs,
which can display different or opposite enantioselectivities toward
a given substrate (sometimes the addition of selective inhibitors is
required in order to prevent hydrolysis and/or decarboxylation of
b-keto esters).14 Lipase-catalyzed hydrolysis of b-hydroxy esters
is another frequently performed procedure.15 Such an approach
usually leads to optically active products with good to excellent
enantioselectivity. Although often highly efficient, the above
mentioned methodologies suffers from one major limitation: they
involve employing an aqueous reaction media, which requires te-
dious product isolation and purification, therefore leading to mod-
erate yields. Moreover, the reaction work-up may give an emulsion
that settles slowly, so the scaling-up of the process requires the
product extraction from large volumes of broth by huge volumes
of organic solvent, canceling the benefits of biotransformation as
an environmentally friendly process.
⇑
Corresponding author. Tel.: +48 22 660 53 42; fax: +48 22 628 27 41.
E-mail addresses: pawel_borowiecki@onet.eu, pborowiecki@ch.pw.edu.pl
(P. Borowiecki).
0957-4166/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2013.06.004
Please cite this article in press as: Borowiecki, P.; Bretner, M. Tetrahedron: Asymmetry (2013), http://dx.doi.org/10.1016/j.tetasy.2013.06.004

2
P. Borowiecki, M. Bretner / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
N
*
HCl
O
*
HCl
O
N
H
dapoxetine (Kutub, TM Priligy)
II
atomoxetine (Strattera, TM Eli Lilly)
III
H
N
O
*
O
HCl
CF3
O
*
OH
O
HCl
fluoxetine (Prozac, TM Eli Lilly)
N
H
I
OMe
X
IV nisoxetine (94939, TM Eli Lilly)
X = H or F
O
HCl
O
HN
O
*
HO
HCl
N
*
*
F
paroxetine (Paxil, TM GlaxoSmithKline)
VII
tolterodine (Detrol, TM Pfizer)
V
HCl
N
N
S
*
levamisole (Ergamisol, TM Janssen Pharmaceutica)
Figure 1. Pharmacologically active compounds possessing a 3-aryl-3-hydroxypropionate unit.
VI
In sharp contrast, the advantages of procedures carried out in
ference in size the better selectivity factor (E) of the reaction. This
statement prompted us to focus our efforts directly on the kinetic
resolution of methyl esters of 3-aryl-3-hydroxypropionic acids
since these were less bulky substituents in comparison with those
described in the previous literature. Herein we report a detailed
survey of the lipase-catalyzed KR of the titled compounds, consid-
ering mainly the effects of: (i) the donor and acceptor type substit-
uents and their position in the phenyl ring; (ii) the ratio of the
substrate : biocatalyst : acetyl group donor; (iii) the type of solvent;
(iv) the temperature; (v) the manner of stirring; (vi) the conversion
rate; and finally (vii) the influence of lipase pretreatment on the
enzyme activity and enantioselectivity.
organic solvents include: good solubility of the vast majority of
nonpolar reactants as well as the insolubility of enzymes, which al-
lows their easy separation from a product solution, and recycling
without a significant loss of activity or enantioselectivity. In non-
polar organic solvents, biocatalyst stability toward heat and autol-
ysis is enhanced, background reactions are reduced, and overall
work-up is improved providing better integration into tandem che-
moenzymatic processes.16
Herein we report a new approach for kinetic resolutions (KR) of
para- and meta-substituted 3-aryl-3-hydroxy propionate enantio-
mers based on lipase-catalyzed O-acetylation. With the aim of find-
ing conditions that can be widely used, we concentrated our efforts
toward the development of a convenient chemoenzymatic route,
which could provide new opportunities to industrial syntheses of
pharmaceuticals based on a b-keto ester scaffold. Furthermore,
we have found that only a few papers have been devoted to the
enantioselective acylation of 3-aryl-3-hydroxypropionates,17 and
to the best of our knowledge, none of them applies to their methyl
esters as we have described herein. According to the empirical rule
formulated by Kazlauskas,18 lipase-catalyzed enantiomer discrimi-
nation of a secondary alcohol is based on the difference in size of
substituents attached to the asymmetric center. The greater the dif-
2. Results and discussion
2.1. Synthesis of the racemic starting materials
Despite the existence of well-known, straightforward (e.g., aldol
condensation,19 Reformatsky reaction20) or multistep (Claisen
condensation,21 Baylis–Hillman22 or Blaise reaction23) preparation
procedures of racemic 3-aryl-3-hydroxypropionates, we decided to
adopt a different strategy shown in Scheme 1. The reason for this
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P. Borowiecki, M. Bretner / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
3
O
O
O
OH
O
OAc
O
i)
ii)
iii)
OMe
OMe
OMe
X
X
X
X
Y
Y
Y
Y
1a-e; 2b-e
3a-e; 4b-e
5a-e; 6b-e
7a-e; 8b-e
for 1,3,5,7,9 and 11:
a
b
c
d
e
: X = H; Y = H
: X = Br; Y = H
: X = Cl; Y = H
: X = OMe; Y = H
: X = F; Y = H
iv)
OH
O
OAc
O
2 4 6 8 10
12
:
for , , , , and
b: X = H; Y = Br
c: X = H; Y = Cl
(S)
(R)
OMe
OMe
+
d: X = H; Y = OMe
e: X = H; Y = F
X
X
Y
9a e
Y
(R)-(+)-11a-e; (R)-(+)-12b-e
10b e
(S)-(-)- - ; (S)-(-)-
-
Scheme 1. Lipase-mediated kinetic resolution of methyl 3-aryl-3-hydroxypropionates. Reagents and conditions: (i) dimethyl carbonate (3 equiv), NaH (4 equiv), anhydrous
PhCH3, 105–110 °C; (ii) NaBH4 (0.5 equiv), MeOH, 0–5 °C; (iii) (CH3CO)2O (1 equiv), dry pyridine (1.2 equiv), anhydrous CH2Cl2, DMAP (15 mg), reflux or rt; (iv) vinyl acetate
(5–10 equiv), enzyme, TBME or PhCH3, 30–60 °C, 500 rpm (lab shaker or magnetic stirrer).
was our parallel work devoted to whole-cell PEDH-mediated enan-
tioselective reduction of prochiral ketones 3a–e; 4b–e, which will
be presented elsewhere.
Table 3
Synthesis of racemic b-acetoxy esters (7a–e; 8b–e)
Entry
Compound
X
Y
t (h)
Yieldb (%)
According to the method reported by Holub et al.24 condensa-
tion of the appropriate acetophenone enolate 1a–e; 2b–e and di-
methyl carbonate provided the b-keto esters 3a–e; 4b–e in
moderate to excellent yields (46–95%, Table 1).
1
2
3
4
5
6
7
8
9
7a
7b
7c
7d
7e
8b
8c
8d
8e
H
H
H
H
H
12
3a
85
66
60
36
63
90
97
90
95
Br
Cl
OCH3
F
H
H
3a
3a
H
3a
In the second step of the synthesis, the regioselective reduction
of the carbonyl group in the presence of ester group, by using 0.5-
fold molar equivalent of methanolic solution of NaBH4 was carried
out at low temperature (0–5 °C). In this manner b-hydroxy esters
5a–e; 6b–e were accomplished in high to nearly quantitative
yields (79–99%, Table 2). It is noteworthy that if the reaction
temperature increased over 10 °C or ethanol was used instead of
Br
Cl
OCH3
F
24
24
24
24
H
H
a
Reflux.
Isolated yield.
b
Table 1
methanol as the solvent, 1-arylpropane-1,3-diols or alcoholysis
by-products were formed, respectively.
Synthesis of b-keto esters 3a–e; 4b–e
Entry
Compound
X
Y
t (h)
Yielda (%)
b-Acetoxy esters 7a–e; 8b–e, used as the analytical standards
for the resolution products of racemic b-hydroxy esters 5a–e;
6b–e, were prepared by esterification of 5a–e and 6b–e with acetic
anhydride in dry dichloromethane in the presence of freshly dis-
tilled pyridine and a catalytic amount of 4-dimethylaminopyridine
(DMAP) at room temperature if not indicated otherwise. These
compounds were obtained with acceptable yields (36–97%,
Table 3).
1
2
3
4
5
6
7
8
9
3a
3b
3c
3d
3e
4b
4c
4d
4e
H
H
H
H
H
20
20
20
6
6
6
6
16
10
95
70
81
61
81
46
71
70
59
Br
Cl
OCH3
F
H
H
H
Br
Cl
OCH3
F
H
H
a
Isolated yield.
2.2. The kinetic resolution of methyl 3-hydroxy-3-
phenylpropanoate 5a
Table 2
Synthesis of racemic b-hydroxy esters 5a–e; 6b–e
One of the major requirements of a lipase-catalyzed kinetic res-
olution is the optimization of the reaction conditions. Hence in the
model reaction, methyl 3-hydroxy-3-phenylpropanoate 5a was
used as a pilot phenyl-based b-hydroxy ester. Vinyl acetate was
introduced as the acylating agent in order to shift a favorable ther-
modynamic equilibrium state in the direction of ester synthesis.
This selection was carried out intentionally since such a property
excludes the presence of high-boiling by-products, which often
hamper the separation procedure, when relatively expensive fatty
acid vinyl esters are employed (i.e., vinyl laurate). As a starting
point, the screening of 10 of the most frequently used commer-
cially available enzyme preparations as biocatalysts was carried
Entry
Compound
X
Y
ta (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
5a
5b
5c
5d
5e
6b
6c
6d
6e
H
H
H
H
H
0.5
1
1.5
1
1.5
1
1
79
99
99
82
88
98
91
90
96
Br
Cl
OCH3
F
H
H
H
Br
Cl
OCH3
F
H
H
1
1
a
Overall time of reaction.
Isolated yield.
b
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4
P. Borowiecki, M. Bretner / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
Table 4
Next, the dependence of the enantiomeric excess of (+)-(R)-11a
and (S)-(ꢀ)-9a (Fig. 2) on the degree of substrate 5a conversion
was investigated. The plots presented in Figure 2 clearly indicate
that the reaction time is an extremely important parameter espe-
cially for the enantiopurity of unreacted alcohol (S)-(ꢀ)-9a. During
optimization, we realized that even a 3 percent increase of the con-
version could change the ee-value dramatically from 90% up to
>99% (Fig. 2, blue curve j, conv. = 48% after 120 h vs conv. = 51%
after 168 h). The reverse situation occurs for the acetyl ester (R)-
(+)-11a, where the difference in ee is less significant, with barely
a 3% decrease of the enantiopurity from 98% to 95% being observed
(Fig. 2, red curve N after 45 and 168 h).
Lipase-catalyzed transesterification of methyl 3-hydroxy-3-phenylpropanoate 5a
after 139 h
Enzymea
Solvent
Conv.b (%)
ees (%)
eep (%)
Ed
c
c
Entry
1
2
3
4
5
6
Amano
AK
Amano
PS-IM
Novozym
435
TBME
PhCH3
TBME
PhCH3
TBME
PhCH3
42
46
51
44
46
34
70
84
98
77
81
49
96
97
96
98
95
96
103
175
226
232
98
80
a
Conditions: 5a 300 mg, lipase 50 mg, solvent 2 mL, vinyl acetate 716 mg
(5 equiv), 30 °C, 500 rpm (laboratory shaker).
b
Based on GC, for confirmation the
% conversion was calculated from the
When we applied more drastic conditions (T = 60 °C, magnetic
stirrer) toward resolution of 5a, the reaction time decreased, and
so we decided to investigate in more detail how this would affect
the selectivity of the reaction (Fig. 3). The progress of the four reac-
tions carried out under different conditions at elevated tempera-
tures was monitored by HPLC at regular 1 day intervals. During
the experiments, it unexpectedly turned out that the most active
and stable enzyme was native Amano AK in toluene. This lipase
preparation catalyzed the acetylation reaction with 50% conversion
and the same enantiomeric excess for (R)-(+)-11a (91% ee) and (S)-
(ꢀ)-9a (92% ee) as Amano PS-IM in TBME, but the reaction time
was approximately 104 h shorter. Exceeding the conversion rate
up to 56% in (Amano AK)-catalyzed reaction turned out to be ben-
eficial for obtaining alcohol (S)-(ꢀ)-9a with high enantiomeric ex-
cess (Fig. 3, 99% ee). The two reactions represented by the two
charts depicted on the right side of Figure 3 were sluggish (50%
conversion could not be achieved even after 1 week) with barely
a noticeable decrease of enantiomeric excess of (R)-(+)-11a.
enantiomeric excess of the unreacted alcohol (ees) and the product (eep) according
to the formula conv. = ees/(ees + eep).
c
Determined by HPLC analysis by using a Chiralcel OD-H column.
d
Calculated
according
to
Chen
et
al.,25
using
the
equation:
E ¼ fln ½ð1 ꢀ conv:Þð1 ꢀ eesÞꢁg=fln ½ð1 ꢀ conv:Þð1 þ eesÞꢁg.
out. We tested lipases isolated from Burkholderia cepacia (Amano
PS, Amano PS-IM, Chirazyme L-1, PS-Immobead 150—Sigma nr:
54327), Pseudomonas fluorescens (Amano AK), Candida antarctica
(Novozym 435, Chirazyme L-2), Thermomyces lanuginosus (Ther-
momyces lanuginosus-Immobead 150—Sigma nr: 76546), Mucor
miehei (Lipozyme RM IM), and Aspergillus niger (Amano A). Initially,
the influence of various co-solvents (toluene, n-hexane, TBME, 1,4-
dioxane, and 2-methyl-2-butanol) on the reaction of 5a was inves-
tigated. The results indicated that the most suitable solvents for
the enzymatic reactions were TBME and toluene, which offer the
best compromise between conversion and selectivity. Based on
these findings, transesterification reactions of 5a catalyzed by
three of the most active enzymes (see Table 4) in two different
(in terms of lipophilicity and polarity) media [toluene (Log P
2.73; 0.36 D) and TBME (Log P 1.3; 1.4 D)] were performed at
30 °C using a fivefold molar excess of vinyl acetate. Processes were
arbitrarily stopped according to GC indications. Experiments
showed that Amano PS-IM suspended in TBME appeared to cata-
lyze the formation of the acetate (R)-(+)-11a and alcohol (S)-(ꢀ)-
9a with very high enantiomeric excesses (96% ee and 98% ee,
respectively) (Table 4, entry 3). Furthermore, Amano PS-IM dis-
played very good enantioselectivity (E >200) and higher activity
than the other enzymes (as it catalyzed the reaction with 51% sub-
strate conversion after 139 h).
2.3. The kinetic resolution of methyl 3-(4-bromophenyl)-3-
hydroxypropanoate 5b and methyl 3-(4-chlorophenyl)-3-
hydroxypropanoate 5c
In our subsequent studies, optimization of the preparative-scale
enantiomer resolution of para-bromo 5b and para-chloro phenyl-
3-hydroxypropanoate 5c was performed under similar conditions
as described for 5a. However, for these compounds we were unable
to follow the reaction course by HPLC, because the enantiomers
could not be separated on a Chiralcel OD-H column when the crude
reaction mixture was analyzed. Therefore, the progress of the en-
zyme-catalyzed reaction was monitored by GC, the products were
separated chromatographically, and their enantiomeric excess was
determined with the use of HPLC. The same procedure was applied
to the remaining compounds 5d–e and 6b-f. Another difficulty at
this stage was the low reaction rate (at room temperature only
traces of the desired products were found for all of the enzymes
employed after 17 days). Primarily, we thought that increasing
the temperature up to a reasonable value (T = 60 °C) could speed
up the reaction as for 5a. Hence toluene was chosen as the reaction
medium for further studies. After incorporating some other
changes, such us doubling the molar excess of vinyl acetate up to
10 equiv, and replacing the laboratory shaker with a magnetic stir-
rer in order to improve mass transfer between the liquid reagents
and solid biocatalyst, we observed a significant acceleration of the
lipase-catalyzed transesterifications of alcohols 5b and 5c (Tables 5
and 6).
The results of the kinetic resolution of 5b (Table 5) demon-
strated that extending the reaction time provides sufficient enan-
tiomeric purity (98% ee) of the unreacted alcohol (S)-(ꢀ)-9b if
conversion of the substrate slightly exceed 53% (see Table 5, entries
4 and 6). The product (R)-(+)-11b in a highly enantiomerically en-
riched form (94% ee) was obtained if the conversion did not pass
over 41% (see Table 5, entry 3). The results clearly indicate that
Amano PS, Amano AK, and Amano PS-IM display moderate
Figure 2. Dependence of enantiomeric excess ee (%) of (R)-(+)-11a (red curve, N)
and (S)-(ꢀ)-9a (blue curve, j) on the conversion degree of 5a during the Amano PS-
IM-catalyzed acetylation with vinyl acetate in a TBME solution at 30 °C (lab shaker).
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P. Borowiecki, M. Bretner / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
5
Figure 3. Dependence of the enantiomeric excess ee (%) of (R)-(+)-11a (red curve, N) and (S)-(ꢀ)-9a (blue curve, j) on the conversion degree of 5a during lipase-catalyzed
acetylation with vinyl acetate in a TBME and toluene solutions at 60 °C (magnetic stirrer).
Table 5
Table 6
Preparative-scale lipase-catalyzed resolution of methyl 3-(4-bromophenyl)-3-
Preparative-scale lipase-catalyzed resolution of methyl 3-(4-chlorophenyl)-3-
hydroxypropanoate 5b
hydroxypropanoate 5c
Enzymea
t (d)
Conv.b (%)
ees (%)
eep (%)
Ed
Entry
Enzymea
t (d)
Conv.b (%)
ees (%)
eep (%)
Ed
c,f
c
c
c
Entry
Yielde (%)
Yielde (%)
Yielde (%)
Yielde (%)
1
2
3
4
5
6
Amano
PS
Amano
AK
Amano
PS-IM
10
14
10
14
10
14
34
37
41
53
39
53
46 (85)
54 (95)
65 (93)
98 (68)
57 (95)
98 (46)
89 (38)
92 (75)
94 (36)
86 (85)
90 (47)
85 (62)
27
41
63
60
34
60
1
2
3
4
5
6
Amano
PS
Amano
AK
Amano
PS-IM
10
14
4
14
13
14
31
33
34
51
40
55
42 (85)
44 (75)
47 (51)
98 (67)
59 (87)
98 (73)
87 (48)
98 (46)
91 (55)
95 (77)
88 (52)
81 (55)
22
153
34
180
28
43
a
a
Conditions: 5b 600 mg, lipase 200 mg, toluene 6 mL, vinyl acetate 1.99 g
(10 equiv), 60 °C, 500 rpm (magnetic stirrer).
Conditions: 5c 600 mg, lipase 200 mg, toluene 6 mL, vinyl acetate 2.4 g
(10 equiv), 60 °C, 500 rpm (magnetic stirrer).
b
b
Based on GC, for confirmation the
%
conversion was calculated from the
Based on GC, for confirmation the % conversion was calculated from the
enantiomeric excess of the unreacted alcohol (ees) and the product (eep) according
to the formula conv. = ees/(ees + eep).
enantiomeric excess of the unreacted alcohol (ees) and the product (eep) according
to the formula conv. = ees/(ees + eep).
c
c
Determined by HPLC analysis by using a Chiralcel OD-H column.
Determined by HPLC analysis by using a Chiralcel OD-H column.
d
d
Calculated
according
to
Chen
et
al.,25
using
the
equation:
Calculated
according
to
Chen
et
al.,25
using
the
equation:
E ¼ fln ½ð1 ꢀ conv:Þð1 ꢀ eesÞꢁg=fln ½ð1 ꢀ conv:Þð1 þ eesÞꢁg.
E ¼ fln ½ð1 ꢀ conv:Þð1 ꢀ eesÞꢁg=fln ½ð1 ꢀ conv:Þð1 þ eesÞꢁg.
e
e
Isolated yield calculated on the basis of the theoretical number of moles arising
from conversion rate, relative to the theoretical amount, that is, when 50% con-
version is reached, up to half of the acetate could be obtained.
Isolated yield calculated on the basis of the theoretical number of moles arising
from conversion rate, relative to the theoretical amount, that is, when 50% con-
version is reached, up to half of the acetate could be obtained.
f
Determined by HPLC analysis after transforming alcohol into ester since alcohol
was indivisible on a Chiralcel OD-H column.
enantioselectivity (E = 27–63). This is good enough as an industri-
ally acceptable E-value should be larger than 20.
In a similar manner, the resolution of racemic 3-(4-chloro-
phenyl)-3-hydroxypropanoate 5c gave enantiomerically enriched
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6
P. Borowiecki, M. Bretner / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
Table 7
Table 8
Solvent-pretreated (Amano AK)-catalyzed KR of 5b and 5c after 12 days
Preparative-scale lipase-catalyzed resolution of methyl 3-(4-methoxyphenyl)-3-
hydroxypropanoate 5d
c
c
Entry
Substratea
Conv.b (%)
ees (%)
eep (%)
Ed
Yielde (%)
Yielde (%)
Entry
Enzymea
t (d)
Conv.b (%)
ees (%)
eep (%)
Ed
c
c
Yielde (%)
Yielde (%)
1
2
5b
5c
51
54
98f (67)
98 (56)
93 (53)
83 (62)
127
49
1
2
3
4
5
Amano PS
Amano AK
Amano PS-
IM
11
3
3
4
4
32
47
48
50
39
40 (70)
79 (76)
87 (35)
93 (60)
48 (92)
85 (48)
89 (72)
96 (64)
94 (68)
74 (40)
18
41
140
110
11
a
Conditions: 5b or 5c 600 mg, lipase 200 mg (pretreated), toluene 6 mL, 10 equiv
of vinyl acetate, 60 °C, 500 rpm (magnetic stirrer).
Based on GC, for confirmation the
enantiomeric excess of the unreacted alcohol (ees) and the product (eep) according
to the formula conv. = ees/(ees + eep).
b
% conversion was calculated from the
Novozym 435
a
Conditions: 5d 600 mg, lipase 200 mg, toluene 6 mL, vinyl acetate 2.46 g
(10 equiv), 60 °C, 500 rpm (magnetic stirrer).
c
Determined by HPLC analysis by using a Chiralcel OD-H column.
Calculated
d
b
according
to
Chen
et
al.,25
using
the
equation:
Based on GC, for confirmation the % conversion was calculated from the
E ¼ fln ½ð1 ꢀ conv:Þð1 ꢀ eesÞꢁg=fln ½ð1 ꢀ conv:Þð1 þ eesÞꢁg.
enantiomeric excess of the unreacted alcohol (ees) and the product (eep) according
to the formula conv. = ees/(ees + eep).
e
Isolated yield calculated on the basis of the theoretical number of moles arising
from conversion rate, relative to the theoretical amount, that is, when 50% con-
version is reached, up to half of the acetate could be obtained.
c
Determined by HPLC analysis by using a Chiralcel OD-H column.
d
Calculated
according
to
Chen
et
al.,25
using
the
equation:
f
Determined by HPLC analysis after transforming alcohol into ester since alcohol
E ¼ fln ½ð1 ꢀ conv:Þð1 ꢀ eesÞꢁg=fln ½ð1 ꢀ conv:Þð1 þ eesÞꢁg.
e
was indivisible on a Chiralcel OD-H column.
Isolated yield calculated on the basis of the theoretical number of moles arising
from conversion rate, relative to the theoretical amount, that is, when 50% con-
version is reached, up to half of the acetate could be obtained.
ester (R)-(+)-11c (98% ee) and alcohol (S)-(ꢀ)-9c (98% ee) (Table 6,
entries 2, 4, and 6) if the proper conversion degrees were retained.
After many trials, the unsatisfactory low rate of the investigated
transesterification reactions prompted us to search for a procedure,
which could meet economical demands. Recent studies on lipase-
catalyzed reactions in ionic liquids26 have revealed the beneficial
enhancement of enzyme activity. Moreover, it was also suggested
that the addition of bases27 (e.g., triethylamine, DMAP, pyridine,
imidazole, and 2,6-lutidine), macrocyclic tetraamines,28 thiacrown
ethers,29 or silver oxide30 as well as microwave-assisting31 of en-
zyme-catalyzed reactions can increase the reaction rate and enan-
tiomeric selectivity. Encouraged by these suggestions, we decided
to incorporate one of the proposed methodologies toward our
investigated compounds. For this purpose, the known and often
applied ‘enzyme conditioning’32 method was investigated. It usu-
ally involves leaving the biocatalyst over a specified time period
in the reaction solvent with occasional agitation. Such a simple
and cost-effective pretreatment led to an increase in the activity
of the selected lipase (Amano AK) (Table 7). This was observed
by improvement of the reaction rate. For comparable conversions,
reactions lasted 2 days less (Table 5 entry 4 vs Table 7 entry 1 and
Table 6 entry 4 vs Table 7 entry 2) probably due to the elimination
of a ‘time-lag’. Examination of enzyme-pretreatment showed that
the enantioselectivity of the reaction remains acceptably high (E
>49) for both reactions studied (Table 7, entries 1 and 2), yielding
enantiomers with high enantiomeric purity (83–98% ee). Despite
the fact that the enzymatic reactions took as long as 12 days, the
results in terms of enantiomeric purities compared well with pro-
cedures previously reported in the literature.
Table 9
Preparative-scale lipase-catalyzed resolution of methyl 3-(4-fluorophenyl)-3-
hydroxypropanoate 5e
Enzymea
t (d)
Conv.b (%)
ees (%)
eep (%)
Ed
c,f
c
Entry
Yielde (%)
Yielde (%)
1
2
3
4
Amano PS
Amano AK
Amano PS-IM
Novozym 435
11
4
3
47
52
52
49
72 (84)
99 (76)
96 (89)
91 (98)
81 (97)
92 (64)
89 (83)
95 (38)
20
126
67
3
124
a
Conditions: 5e 600 mg, lipase 200 mg, toluene 6 mL, vinyl acetate 2.6 g
(10 equiv), 60 °C, 500 rpm (magnetic stirrer).
b
Based on GC, for confirmation the
% conversion was calculated from the
enantiomeric excess of the unreacted alcohol (ees) and the product (eep) according
to the formula conv. = ees/(ees + eep).
c
Determined by HPLC analysis by using a Chiralcel OD-H column.
Calculated
d
according
to
Chen
et
al.,25
using
the
equation:
E ¼ fln ½ð1 ꢀ conv:Þð1 ꢀ eesÞꢁg=fln ½ð1 ꢀ conv:Þð1 þ eesÞꢁg.
e
Isolated yield calculated on the basis of the theoretical number of moles arising
from conversion rate, relative to the theoretical amount, that is, when 50% con-
version is reached, up to half of the acetate could be obtained.
f
Determined by HPLC analysis after transforming alcohol into ester since it was
indivisible on a Chiralcel OD-H column.
out to be a selective catalyst. The enzyme preparations examined
gave excellent reaction enantioselectivities (Table 8, entries 3
and 4; Table 9, entries 2 and 4, E >110), but the enantiomeric ex-
cesses obtained were strongly dependent upon the applied exper-
imental conditions. For example, the results of (Amano PS-IM)-
catalyzed KR ofpara-methoxyphenyl derivative 5d showed an in-
crease in the ee of the remaining alcohol (S)-(ꢀ)-9d when carried
out until 50% conversion (Table 8, entry 4, 93% ee). In turn, acetate
(R)-(+)-11d with the highest enantiopurity (96% ee) was obtained
when the reaction was arrested close to 48% conversion (Table 8,
entry 3).
2.4. The kinetic resolution of methyl 3-(4-methoxyphenyl)-3-
hydroxypropanoate 5d and methyl 3-(4-fluorophenyl)-3-
hydroxypropanoate 5e
A similar procedure was used in the kinetic resolution of two
other alcohols 5d and 5e. Once again HPLC analysis failed to sepa-
rate the enantiomers from the crude reaction mixture, so we had to
include a product purification step as well. Since the lipase-cata-
lyzed transesterification of these compounds proceeded faster than
the corresponding para-bromo 5b and para-chloro 5c derivatives,
there was no need for pretreatment, and the enantioselectivities
of several lipase preparations were examined. The results are pre-
sented in Tables 8 and 9.
In our investigations, we found that the kinetic resolution (KR)
of 5e catalyzed by Amano AK resulted in almost enantiomerically
pure (99% ee) para-fluoro alcohol (S)-(ꢀ)-9e (Table 9, entry 2),
while an acetate of the opposite stereochemistry (R)-(+)-11e could
be obtained with high enantiomeric excess (95% ee) if Novozym
435 was used as the biocatalyst and the conversion did not exceed
49% (Table 9, entry 4).
2.5. The kinetic resolution of meta-substituted methyl 3-
Detailed examination of the results revealed that the stereose-
lective acetylation of 5d and 5e proceeded particularly efficiently
if Amano PS-IM or Amano AK was used. However, in the resolution
of para-fluoro 5e enantiomers, Novozym 435 unexpectedly turned
phenyl-3-hydroxypropanoates 6b–e
For the rest of the synthesized substrates 6b–e, the employed
procedure was identical to the one described for methyl 3-phe-
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7
Table 10
Effect of enzyme-pretreatment on lipase-catalyzed resolution of meta-substituted b-hydroxy esters 6b–e
d
d
Entry
Substrate
Enzyme
t (d)
Conv.c (%)
ees (%)
eep (%)
Ee
1
2
3
4
5
6
7
8
6b
Amano AK
6a
5b
7a
1b
6a
5b
7a
1b
5a
4b
7a
3b
5a
3b
7a
2b
46
46
49
35
49
43
47
41
48
39
39
49
51
49
52
50
84
76
93
51
92
66
87
67
88
59
62
91
98
88
>99
93
97
91
95
94
95
88
97
96
94
94
97
94
93
92
90
93
175
49
134
54
129
31
188
99
95
59
124
103
127
70
Amano PS-IM
Amano AK
6c
6d
6e
Amano PS-IM
Amano AK
9
10
11
12
13
14
15
16
Amano PS-IM
Amano AK
Amano PS-IM
99
94
a
b
c
Conditions: 6b–e 600 mg, lipase 200 mg, toluene 6 mL, 10 equiv of vinyl acetate, 60 °C, 500 rpm (magnetic stirrer).
Conditions: 6b–e 600 mg, lipase 200 mg (pretreated), toluene 6 mL, 10 equiv of vinyl acetate, 60 °C, 500 rpm (magnetic stirrer).
Based on GC, for confirmation the% conversion was calculated from the enantiomeric excess of the unreacted alcohol (ees) and the product (eep) according to the formula
conv. = ees/(ees + eep).
d
Determined by HPLC analysis by using a Chiralcel OD-H column.
e
Calculated according to Chen et al.,25 using the equation: E ¼ fln ½ð1 ꢀ conv:Þð1 ꢀ eesÞꢁg=fln ½ð1 ꢀ conv:Þð1 þ eesÞꢁg.
nyl-3-hydroxypropanoates 5b–e. Reactions were conducted in a
toluene solution containing 10-molar excess of vinyl acetate as
an acyl transfer reagent with a substrate/enzyme weight ratio of
3:1. Since the results described in the previous paragraph indicated
the superiority of Amano AK and Amano PS-IM, we limited our
experiments to these enzyme preparations. At this stage, taking
into account the moderate conversions observed for all of the
meta-substituted 6b–e derivatives (t >5 days), we incorporated
the pretreatment methodology. In general, the results of the per-
formed experiments (Table 10), show that in the lipase-catalyzed
transesterification of meta-substituted derivatives 6b–e, the high-
est enantioselectivity toward 6b (E = 175) and 6e (E = 127) was ob-
tained with Amano AK. In turn, the best results with regard to
enantioselectivity toward 6c (E = 188) and 6d (E = 124) were
achieved by using Amano PS-IM.
As demonstrated in Table 10, the enantiomeric excesses of the
remaining alcohols (S)-(ꢀ)-10b–e and the formed acetates (R)-
(+)-12b–e were sufficiently high with the most promising results
as follows: 91–100% ee for the slower reacting and 93–97% ee for
the faster reacting enantiomers. It is worth noting that the influ-
ence of conversion on the enantiomeric excess of the product dra-
matically changes the outcome of the stereochemical course,
especially with regard to the ee-values of the slower reacting alco-
hols (Table 10, entries 13 vs 14 and 15 vs 16).
As indicated in Table 10, solvent pretreatment of the enzymes is
important. After the enzyme-conditioning activity of Amano AK li-
pase increased, we considerably shortened the reaction time to at
least 1–2 days to establish appropriate conversion (Table 10, en-
tries 1 vs 2 or 13 vs 14). In order to verify if the above-described
phenomenon exists for other lipase, Amano PS-IM was also exam-
ined. The difference between reaction courses in this case was even
more evident. Remarkable enhancement resulted in time-saving,
which increased up to 5 days (Table 10, entries 15 vs 16). Unfortu-
nately, reducing the reaction time led to a significant decrease in
enantioselectivity (E = 175 down to E = 49 and E = 127 down to
E = 70) for comparable conversion observed (Table 10, entries 1
vs 2 or 13 vs 14). An interesting observation must be noted herein.
When comparing conventional KR trials with the use of both inves-
tigated enzymes, the reaction lasted less in the case of Amano AK
(Table 10, entries 5 vs 7). The reverse situation occurs while taking
into consideration enzyme pretreatment methodology, and Amano
PS-IM exhibited higher activity (Table 10, entries 14 vs 16). This
tendency cannot be explained by the simple experiment presented
herein and remains a challenge.
2.6. Determination of the stereochemistry of optically active
alcohols (S)-(ꢀ)-9a–e, (S)-(ꢀ)-10b–e
The absolute configurations of the obtained (S)-(ꢀ)-b-hydroxy
esters (ꢀ)-9a–e, (ꢀ)-10b–e were established by comparison of
their specific rotation signs with the literature data. Some of the
remaining optically active alcohols (ꢀ)-10b, (ꢀ)-10c, (ꢀ)-10e are
not reported, and thus their absolute configurations could be only
deduced on the basis of signs of the specific rotations of ethyl ester
derivatives with the assumption that these two classes of com-
pounds belong to the same homologous series, and chemical corre-
lation can be applied. Additionally, according to the method
evaluated by Hoff et al.33for the prediction of the absolute config-
uration of secondary alcohols, the similarity of the elution order on
an HPLC chiral column of all the compounds obtained in the same
enzyme-catalyzed kinetic resolution manner suggests the validity
of our assumptions as the rest of the b-hydroxy esters have the
same (3S)-stereochemistry. This assignment is also in agreement
with the rule formulated by Kazlauskas et al.34 (S)-(ꢀ)-9a:
½
a 2D4
ꢁ
¼ ꢀ52:4 (c 1.0, CHCl3, 98% ee), lit.: {½aꢁD ¼ ꢀ17 (c 1.0,
MeOH)}35 and {½a 2D5
ꢁ
¼ ꢀ49:9 (c 1.0, CHCl3)}.11d (S)-(ꢀ)-9b:
½
a 2D4
ꢁ
¼ ꢀ19:7 (c 1.3, CHCl3, 54% ee), lit.: {½aꢁD ¼ ꢀ30:3 (c 2.43,
CHCl3)}.35 (S)-(ꢀ)-9c: ½a 2D4
¼ ꢀ19:5 (c 1.2, CHCl3, 44% ee), lit.:
ꢁ
{½a D
ꢁ
¼ ꢀ12:9 (c 1.21, MeOH)}35 and {½a D
ꢁ
¼ ꢀ18 (c 1.0, EtOH)}.36
(S)-(ꢀ)-9d: ½a 2D4
¼ ꢀ20 (c 2.15, CHCl3, 79% ee), lit.: {½aꢁD ¼ ꢀ29 (c
ꢁ
1.0, CHCl3, 89% ee)}36 and {½a D20
ꢁ
¼ ꢀ27:7 (c 0.5, CHCl3)}.37 (S)-(ꢀ)-
9e: ½a 2D4
¼ ꢀ8 (c 2.1, CHCl3, 91% ee), lit.: only for (R)-enantiomer
ꢁ
{½a 2D0
ꢁ
¼ þ32:7 (c 1.0, CHCl3)}.38 (S)-(ꢀ)-10b: ½a 2D4
¼ ꢀ33:5 (c 1.74,
ꢁ
CHCl3, 93% ee), lit.: only ethyl ester was reported {½a D20
¼ ꢀ38:6
ꢁ
(c 1.0, CHCl3)}39 (S)-(ꢀ)-10c: ½a 2D4
¼ ꢀ40:6 (c 1.22, CHCl3, 92% ee),
ꢁ
lit.: not reported. (S)-(ꢀ)-10d: ½a D24
¼ ꢀ41:2 (c 1.21, CHCl3, 91%
ꢁ
ee), lit.: {½a 2D4
ꢁ
¼ ꢀ28:7 (c 1.0, CHCl3)}.37 (S)-(ꢀ)-10e: ½a 2D4
¼ ꢀ46
ꢁ
(c 1.31, CHCl3, >99% ee), lit.: not reported.
3. Conclusion
The simple procedure described herein offers a convenient and
easy method for the stereoselective chemoenzymatic synthesis of
enantiopure aryl b-hydroxy esters. We determined the conditions
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8
P. Borowiecki, M. Bretner / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
for the enzymatic kinetic resolution of variety of substituted 3-
phenyl-3-hydroxy methyl esters demonstrating its applicability
in preparative-scale synthesis, providing both enantiomeric forms
of the target compounds with excellent enantioselectivities and in
good to high isolated yields. We have demonstrated the usefulness
of this procedure in giving access to enantiomerically enriched b-
hydroxy esters without the need for special equipment. The out-
come of lipase-catalyzed transesterification reactions revealed
Amano AK and Amano PS-IM as the most efficient enzymes. The
experimental data collected clearly indicate that enantioselectivi-
ties were the best for the simple phenyl ring 5a (E >200), lower
for the bromo- and chloro-substituted compounds 5b-c, 6b-c
(E = 22–188), and slightly worse for methoxy- and fluoro-deriva-
tives 5d–e, 6d–e (E = 11–140). A slight influence of the nature of
the substituent was observed especially with regard to enantio-
meric excess results. It turned out that both isomers form with
excellent enantiomeric purities (>99% ee) could be achieved only
in the case of a non-substituted phenyl ring 5a as well as for unre-
acted para- and meta-fluoro-substituted alcohols 5e, 6e, respec-
tively. We postulate that the replacement of the hydrogen atom
by its bioisostere (fluorine atom) is the key reason for this effect.
Since we discovered that the conversions of conventional KR ap-
proaches were low for para-bromo- and -chloro derivatives 5b,
5c, we focused our attention on factors which affect the activity
of lipases in a positive manner. Many techniques for increasing en-
zyme-catalyzed reaction rates have been established, but in this
case we examined the influence of a simple solvent-pretreated en-
zyme methodology. The elaboration of a new enzymatic protocol
led us to the following findings: (i) the solvent pretreatment meth-
od can effectively increase a lipase activity (Amano PS-IM is partic-
ularly vulnerable), thus diminishing in some cases the time of the
resolution process by up to six orders-of-magnitude, but unfortu-
nately (ii) simultaneously the chosen pretreatment strategy may
deteriorate the enantioselectivity factor. However, we believe that
this phenomenon has a strong practical meaning, and more de-
tailed investigations will enable better optimization of lipase-cata-
lyzed processes.
and Chiralcel OD-H (Diacel) chiral column using mixtures of n-hex-
ane/iso-propyl alcohol as the mobile phase in appropriate ratios gi-
ven in the Experimental; the HPLC analyses were carried out in an
isocratic manner; flow (f) is given in mL/min; racemic alcohols and
esters were used as standards. Optical rotations were measured
with a P20 polarimeter (Belligham & Stanley Ltd, line D spectrum
of sodium) in a 2 dm long cuvette. UV spectra were measured with
Cary 3 spectrometer. 1H and 13C NMR spectra were measured with
a Varian Mercury 400BB spectrometer operating at 400 MHz for 1H
and 100 MHz for 13C nuclei, chemical shifts (d) are given in parts
per million (ppm) relative to tetramethylsilane (TMS) as internal
standard; signal multiplicity assignment: s, singlet; d, doublet; t,
triplet; q, quartet; m, multiplet; coupling constant (J) are given in
hertz (Hz).
4.2. Synthesis of b-keto esters 3a–d and 4b–e
In a three-necked round bottomed flask equipped with a reflux
condenser and a magnetic stirrer, NaH (4 equiv, 60% mineral oil
dispersion), was placed under nitrogen and washed with toluene
(3 ꢂ 10 mL). Next the solution of dimethyl carbonate (3 equiv) in
toluene (50 mL) was added. The appropriate derivative of aceto-
phenone (10 g) in toluene (50 mL) was added dropwise over 1 h
to the resultant suspension, with stirring at rt. The reaction mix-
ture was subsequently diluted with toluene (250 mL) and stirred
vigorously at 105–110 °C for the time given in Table 1. The result-
ing mixture was cooled to rt, and glacial acetic acid (8 mL) was
added dropwise until a heavy, pasty solid appeared. This was trea-
ted with a solution of conc. HCl (30 mL) in ice water (200 mL). The
separated aqueous solution was extracted with AcOEt
(3 ꢂ 200 mL). The organic layers were neutralized by extraction
with saturated NaHCO3 (2 ꢂ 200 mL) and dried over anhydrous
Na2SO4. The volatiles were then removed under reduced pressure
and the resultant crude oil was purified by vacuum distillation to
give the appropriate b-keto ester as a keto-enol tautomer.
4.2.1. Methyl 3-oxo-3-phenylpropanoate 3a
Colorless oil; yield 95%; bp 80–85 °C, p = 0.05 mmHg; Rf = 0.72
[PhCH3/AcOEt (3:1)]; 1H NMR (CDCl3, 400 MHz) d: 3.70 (s, 3H),
3.75 (s, 1H, enol form), 3.97 (s, 2H), 5.64 (s, 0.3H, enol form),
7.34–7.46 (m, 3H), 7.52–7.57 (m, 0.9H, enol form), 7.71–7.75 (m,
0.6H, enol form), 7.88–7.93 (m, 2H), 12.50 (s, 0.3H, enol form);
13C NMR (CDCl3, 100 MHz) d: 45.49, 51.24 (enol form), 52.25,
86.82 (enol form), 125.76, 128.20 (enol form), 128.25 (enol form),
128.50, 131.01 (enol form), 133.48, 135.61, 167.59, 171.08 (enol
form), 173.13 (enol form), 191.99; GC [100–260 (10 °C/min)]:
tR = 7.32.
4. Experimental
4.1. General
All commercially available reagents (Aldrich, Fluka and POCH)
were used without further purification. Novozym SP 435 (lipase
from Candida antarctica B immobilized on a macroporous acrylic
resin, >10000 U/g), Amano PS (native lipase from Burkholderia
cepacia ealier Pseudomonas cepacia, >23.000 U/g), Amano PS-IM (li-
pase from Burkholderia cepacia immobilized on diatomite, 500 U/g),
and Amano AK (native lipase fromPseudomonas fluorescens,
>20.000 U/g) were purchased from Novo Nordisk Co. and Amano
Pharmaceutical Co. respectively. Melting points were obtained
with an MPA100 Optimelt SRS apparatus. Thin-layer chromatogra-
phy was carried on TLC aluminum plates with silica gel Kieselgel
60 F254 (Merck) (0.2 mm thickness film) using UV light as a visual-
izing agent. The chromatographic analyses (GLC) were performed
with an HP Series II 5890 instrument equipped with a flame ioni-
zation detector (FID) and fitted with HP-50+ (30 m) semipolar col-
umn. Helium (2 mL/min) was used as the carrier gas; retention
times (tR) are given in minutes under these conditions. Column
chromatography was performed using Silica gel 60 (Merck) of
4.2.2. Methyl 3-(4-bromophenyl)-3-oxopropanoate 3b
White solid; mp 44–45 °C; yield 70%; bp 124–130 °C,
p = 0.1 mmHg; Rf = 0.8 [PhCH3/AcOEt (5:1)]; 1H NMR (CDCl3,
400 MHz) d: 3.72 (s, 3H), 3.77 (s, 1H, enol form), 3.95 (s, 2H),
5.62 (s, 0.3H, enol form), 7.48–7.54 (m, 1 H, enol form), 7.56–
7.62 (m, 2.3H, partially enol form), 7.75–7.82 (m, 2H), 12.46 (s,
0.3H, enol form); 13C NMR (CDCl3, 100 MHz) d: 45.40, 51.39 (enol
form), 52.39, 87.20 (enol form), 125.64 (enol form), 127.40,
128.94 (enol form), 129.84, 131.61 (enol form), 131.95, 132.02
(enol form), 134.46, 167.46, 170.00 (enol form), 173.16 (enol form),
191.24; GC [100–260 (10 °C/min)]: tR = 5.59.
4.2.3. Methyl 3-(4-chlorophenyl)-3-oxopropanoate 3c
40–63 lm. A mixture of toluene/ethyl acetate was used as eluent
White solid; mp 45–47 °C; yield = 81%; bp 102–106 °C,
p = 0.1 mmHg; Rf = 0.82 [PhCH3/AcOEt (5:1)]; 1H NMR (CDCl3,
400 MHz) d: 3.73 (s, 3H), 3.78 (s, 1H, enol form), 3.96 (s, 2H),
5.62 (s, 0.3H, enol form), 7.34–7.40 (m, 0.6H, enol form), 7.41–
(2:1, 3:1, 5:1, 9:1 or 10:1 v/v depending on substance purified).
The enantiomeric excesses of the resulting esters and alcohols
were determined by HPLC analysis performed on Shimadzu CTO-
10ASV chromatograph equipped with an STD-20A UV detector
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P. Borowiecki, M. Bretner / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
9
7.45 (m, 2H), 7.66–7.70 (m, 0.6H, enol form), 7.84–7.89 (m, 2H),
12.48 (s, 0.3H, enol form); 13C NMR (CDCl3, 100 MHz) d: 45.52,
51.43 (enol form), 52.47, 87.21 (enol form), 127.29 (enol form),
128.72 (enol form), 129.05, 129.84, 130.86 (enol form), 134.15,
137.27 (enol form), 140.26, 167.56, 170.07 (enol form), 173.27
(enol form), 191.08; GC [150–260 (10 °C/min)]: tR = 1.81.
3H), 3.93 (s, 2H), 5.59 (s, 0.3H, enol form), 6.90–6.95 (m, 0.6H, enol
form), 7.04–7.07 (m, 0.6H, enol form), 7.21–7.33 (m, 2H), 7.38–7.44
(m, 2H), 12.46 (s, 0.3H, enol form); 13C NMR (CDCl3, 100 MHz) d:
45.43, 51.14 (enol form), 52.11, 55.00 (enol form), 55.10, 86.99
(enol form), 110.97 (enol form), 112.31, 116.89 (enol form),
118.12 (enol form), 119.94, 120.85, 129.29 (enol form), 129.51,
134.40 (enol form), 136.96, 159.43 (enol form), 159.62, 167.67,
170.95 (enol form), 173.19 (enol form), 191.99; GC [100–260
(10 °C/min)]: tR = 5.34.
4.2.4. Methyl 3-(4-methoxyphenyl)-3-oxopropanoate 3d
Yellowish oil; yield 61%; bp 137 °C, p = 0.05 mmHg; Rf = 0.66
[PhCH3/AcOEt (3:1)]; 1H NMR (CDCl3, 400 MHz) d: 3.68 (s, 3H),
3.72 (s, 0.23H, enol form), 3.77 (s, 0.23H, enol form), 3.80 (s, 3H),
3.90 (s, 2H), 5.53 (s, 0.07H, enol form), 6.83–6.91 (m, 2H), 7.63–
7.69 (m, 0.28H, enol form), 7.82–7.89 (m, 2H), 12.52 (s, 0.07H, enol
form); 13C NMR (CDCl3, 100 MHz) d: 45.23, 51.07 (enol form),
52.16, 55.15 (enol form), 55.31, 85.11 (enol form), 113.66 (enol
form), 113.74, 125.35 (enol form), 127.56 (enol form), 128.75,
130.67, 161.96 (enol form), 163.81, 167.99, 171.19 (enol
form), 173.45 (enol form), 190.68; GC [150–260 (10 °C/min)]:
tR = 2.63.
4.2.9. Methyl 3-(3-fluororophenyl)-3-oxopropanoate 4e
Colorless oil; yield 59%; bp 75–78 °C, p = 0.05 mmHg; Rf = 0.84
[PhCH3/AcOEt (3:1)]; 1H NMR (CDCl3, 400 MHz) d: 3.70 (s, 3H),
3.75 (s, 1H, enol form), 3.95 (s, 2H), 5.61 (s, 0.3H, enol form),
7.06–7.13 (m, 0.3H, enol form), 7.21–7.28 (m, 0.6H, enol form),
7.28–7.34 (m, 1H), 7.39–7.44 (m, 0.3H, enol form), 7.48–7.50 (m,
1H), 7.56–7.59 (m, 1H), 7.65–7.68 (m, 1H), 12.44 (s, 0.3H, enol
form); 13C NMR (CDCl3, 100 MHz) d: 45.49, 51.35 (enol form),
52.32, 87.70 (enol form), 112.89 (d, J = 22.71 Hz, enol form),
114.93 (d, J = 22.95 Hz), 117.93 (d, J = 21.44 Hz, enol form),
120.65 (d, J = 21.17 Hz), 121.50 (d, J = 3.10 Hz, enol form), 124.17
(d, J = 3.05 Hz), 129.99 (d, J = 8.37 Hz, enol form), 130.37 (d,
J = 8.35 Hz), 135.39 (d, J = 7.61 Hz, enol form), 137.78 (d, J = 6.06
Hz), 162.60 (d, J = 246 Hz, enol form), 162.66 (d, J = 249.1 Hz),
167.43 (enol form), 169.61 (enol form), 173.13, 191.09; GC
[100–260 (10 °C/min)]: tR = 6.57.
4.2.5. Methyl 3-(4-fluorophenyl)-3-oxopropanoate 3e
Yellowish oil; yield 81%; bp 110 °C, p = 0.1 mmHg; Rf = 0.72
[PhCH3/AcOEt (5:1)]; 1H NMR (CDCl3, 400 MHz) d: 3.67 (s, 3H),
3.71 (s, 1H, enol form), 3.93 (s, 2H), 5.54 (s, 0.3H, enol form),
7.00–7.06 (m, 0.6H, enol form), 7.06–7.10 (m, 2H), 7.67–7.71 (m,
0.6H, enol form), 7.87–7.94 (m, 2H), 12.48 (s, 0.3H, enol form);
13C NMR (CDCl3, 100 MHz) d: 45.28, 51.16 (enol form), 52.16,
86.53 (enol form), 115.35 (d, J = 21.60 Hz, enol form), 115.67 (d,
J = 21.71 Hz), 128.01 (d, J = 9.21 Hz, enol form), 131.01 (d, J = 9.93
Hz), 132.14 (d, J = 3.02 Hz, enol form), 164.76 (d, J = 251.8 Hz, enol
form), 165.79 (d, J = 255.3 Hz), 167.56, 170.06 (enol form), 173.17
(enol form), 190.66; GC [100–260 (10 °C/min)]: tR = 6.86.
4.3. Synthesis of racemic b-hydroxy esters 5a–d and 6b–e
In a two-necked round bottomed flask an appropriate b-keto es-
ter (7.5 g) was dissolved in methanol (220 mL) under a nitrogen
atmosphere at ice-water temperature. Next, sodium borohydride
(0.5 equiv) was added portionwise with stirring at a rate sufficient
enough to maintain the reaction temperature at 0–5 °C. The reac-
tion mixture was stirred for the time given in Table 2. Next, brine
(200 mL) was added and the resultant suspension was stirred at
room temperature for 10 min. The reaction mixture was diluted
by the addition of AcOEt (100 mL) and distilled water (100 mL),
and the layers were separated. The aqueous layer was extracted
with AcOEt (3 ꢂ 200 mL). The combined organic layers were
washed with brine (200 mL) and dried over MgSO4. Next, the sol-
vent was evaporated to dryness leaving a crude product, which
was chromatographed on silica gel eluting with the appropriate
mixture of PhCH3/AcOEt (various ratios, depending on the sub-
strate) to afford a b-hydroxy ester.
4.2.6. Methyl 3-(3-bromophenyl)-3-oxopropanoate 4b
Yellowish oil; yield 46%; bp 118–120 °C, p = 0.1 mmHg;
Rf = 0.78 [PhCH3/AcOEt (3:1)]; 1H NMR (CDCl3, 400 MHz) d: 3.70
(s, 3H), 3.75 (s, 1H, enol form), 3.94 (s, 2H), 5.60 (s, 0.3H, enol
form), 7.21–7.25 (m, 0.3H, enol form), 7.28–7.32 (m, 1H), 7.51–
7.35 (m, 0.3H, enol form), 7.61–7.63 (m, 1H), 7.64–7.66 (m, 0.3H,
enol form), 7.78–7.81 (m, 0.3H, enol form), 7.83–7.84 (m, 1H),
8.00–8.01 (m, 1H), 12.42 (s, 0.3H, enol form); 13C NMR (CDCl3,
100 MHz) d: 45.23, 51.32 (enol form), 52.25, 87.65 (enol form),
122.43 (enol form), 122.82, 124.30 (enol form), 126.81, 128.75
(enol form), 129.82, 130.11, 131.09 (enol form), 133.80 (enol form),
134.98 (enol form), 136.28, 137.28, 167.27, 169.22 (enol form),
172.92 (enol form), 190.86; GC [100–260 (10 °C/min)]: tR = 5.54.
4.3.1. Methyl 3-hydroxy-3-phenylpropanoate 5a
4.2.7. Methyl 3-(3-chlorophenyl)-3-oxopropanoate 4c
Colorless oil; yield 79%; Rf = 0.53 [PhCH3/AcOEt (3:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.64–2.80 (m, 2H), 3.47 (d, J = 3.84 Hz, 1H),
3.69 (s, 3H), 5.11 (dt, J = 9.15, 3.67 Hz, 1H), 7.23–7.40 (m, 5H);
13C NMR (CDCl3, 100 MHz) d: 43.13, 51.72, 70.14, 125.52, 127.64,
128.38, 142.50, 172.55; UV/VIS: = kmax = 254 nm; GC [170–260
(10 °C/min)]: tR = 2.16; HPLC [hexane–i-PrOH (90:10); f = 0.7]:
tR = 13.819, 20.258 or HPLC [hexane–i-PrOH (95:5); f = 0.8]:
tR = 18.772, 29.743.
Colorless oil; yield 71%; bp 130–131 °C, p = 0.05 mmHg; Rf = 0.8
[PhCH3/AcOEt (3:1)]; 1H NMR (CDCl3, 400 MHz) d: 3.70 (s, 3H),
3.75 (s, 1H, enol form), 3.95 (s, 2H), 5.60 (s, 0.3H, enol form),
7.26–7.31 (m, 0.3H, enol form), 7.34–7.40 (m, 1H), 7.48–7.49 (m,
0.3H, enol form), 7.50–7.51 (m, 1H), 7.56–7.59 (m, 0.3H, enol
form), 7.68–7.69 (m, 0.3H, enol form), 7.74–7.77 (m, 1H), 7.84–
7.85 (m, 1H), 12.43 (s, 0.3H, enol form); 13C NMR (CDCl3,
100 MHz) d: 45.27, 51.29 (enol form), 52.23, 87.65 (enol form),
123.86 (enol form), 125.87, 126.40 (partially enol form), 128.16,
129.58 (enol form), 129.89, 130.88 (enol form), 133.39 (enol form),
134.38 (enol form), 134.81, 137.14, 167.31, 169.38 (enol form),
172.98 (enol form), 190.97; GC [100–260 (10 °C/min)]: tR = 4.25.
4.3.2. Methyl 3-(4-bromophenyl)-3-hydroxypropanoate 5b
White solid fusible at room temperature; yield 99%; Rf = 0.57
[PhCH3/AcOEt (5:1)]; 1H NMR (CDCl3, 400 MHz) d: 2.66–2.71 (m,
2H), 3.26 (br s, 1H), 3.70 (s, 3H), 5.07 (dd, J = 8.33, 4.52 Hz, 1H),
7.22–7.23 (m, 2H), 7.43–7.48 (m, 2H); 13C NMR (CDCl3, 100 MHz)
d: 42.92, 51.92, 69.56, 121.53, 127.33, 131.56, 141.44, 172.49;
UV/VIS: kmax = 225 nm; GC [180–260 (10 °C/min)]: tR = 3.69; HPLC:
compound was indivisible on available Chiralcel OD-H column
(alcohol was acetylated and only then ee was determined).
4.2.8. Methyl 3-(3-methoxyphenyl)-3-oxopropanoate 4d
Colorless oil; yield 70%; bp 112–115 °C, p = 0.05 mmHg;
Rf = 0.69 [PhCH3/AcOEt (3:1)]; 1H NMR (CDCl3, 400 MHz) d: 3.67
(s, 3H), 3.71 (s, 1H, enol form), 3.74 (s, 1H, enol form), 3.75 (s,
Please cite this article in press as: Borowiecki, P.; Bretner, M. Tetrahedron: Asymmetry (2013), http://dx.doi.org/10.1016/j.tetasy.2013.06.004

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P. Borowiecki, M. Bretner / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
4.3.3. Methyl 3-(4-chlorophenyl)-3-hydroxypropanoate 5c
White solid; mp 39–41 °C; yield 99%; Rf = 0.55 [PhCH3/AcOEt
(5:1)]; 1H NMR (CDCl3, 400 MHz) d: 2.61–2.74 (m, 2H), 3.37 (br s,
1H), 3.69 (s, 3H), 5.07 (dd, J = 8.69, 4.17 Hz, 1H), 7.25–7.31 (m,
4H); 13C NMR (CDCl3, 100 MHz) d: 43.00, 51.85, 69.49, 126.96,
128.56, 133.35, 140.98, 172.43; UV/VIS: kmax = 225 nm; GC [150–
260 (10 °C/min)]: tR = 4.75; HPLC [hexane–i-PrOH (98:2); f = 0.5]:
tR = 47.844, 49.977.
51.96, 69.56, 112.56 (d, J = 22.09 Hz), 114.47 (d, J = 21.17 Hz),
121.04 (d, J = 2.79 Hz), 129.92 (d, J = 8.28 Hz), 144.99 (d, J = 7.37
Hz), 162.72 (d, J = 245.3 Hz), 172.36. UV/VIS: kmax = 254 nm; GC
[150–160 (2 °C/min) ꢃ 160–260 (10 °C/min)]: tR = 3.92; HPLC
[hexane–i-PrOH (90:10); f = 0.8]: tR = 9.936, 16.259.
4.4. Preparation of racemic b-acetoxy esters 7a–d; 8b–e
A mixture of the appropriate b-hydroxy ester (300 mg) in dry
CH2Cl2 (25 mL), acetic anhydride (1 equiv), freshly distilled pyri-
dine (1.2 equiv), and a catalytic amount of DMAP (15 mg) was
placed in a round-bottomed flask. The reagent mixture was then
stirred under the conditions given in Table 3. Reaction progress
was followed by thin layer chromatography (TLC). The reaction
was carried until full consumption of the substrate was achieved.
Next, iced water (25 mL) was poured into the reaction mixture
and resultant biphasic solution was acidified with 10% HCl aq
(5 mL). The organic layer was separated and the aqueous phase
was extracted with CH2Cl2 (3 ꢂ 25 mL). The combined organic lay-
ers were washed with saturated solution of NaHCO3 (2 ꢂ 75 mL),
water (75 mL), and dried over anhydrous Na2SO4. The drying agent
was filtered off and the volatiles were evaporated in vacuo. The
resultant crude product was purified by column chromatography
using the appropriate mixture of PhCH3/AcOEt (different ratios,
depending on the substrate) as an eluent.
4.3.4. Methyl 3-(4-methoxyphenyl)-3-hydroxypropanoate 5d
White solid; mp 28–30 °C; yield 82%; Rf = 0.52 [PhCH3/AcOEt
(3:1)]; 1H NMR (CDCl3, 400 MHz) d: 2.62–2.79 (m, 2H), 3.26 (br s,
1H), 3.69 (s, 3H), 3.77 (s, 3H), 5.06 (dd, J = 9.16, 3.93 Hz, 1H),
6.85–6.87 (m, 2H), 7.26–7.28 (m, 2H); 13C NMR (CDCl3, 100 MHz)
d: 43.10, 51.70, 55.12, 69.81, 113.75, 126.82, 134.68, 159.04,
172.59; UV/VIS: kmax = 225 nm; GC [180–260 (10 °C/min)]:
tR = 3.51; HPLC [hexane–i-PrOH (95:5); f = 0.5]: tR = 35.972, 38.461.
4.3.5. Methyl 3-(4-fluorophenyl)-3-hydroxypropanoate 5e
Colorless oil; yield 88%; Rf = 0.52 [PhCH3/AcOEt (5:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.62–2.75 (m, 2H), 3.36 (br s, 1H), 3.69 (s, 3H),
5.08 (dd, J = 9.03, 4.07 Hz, 1H), 6.97–7.06 (m, 2H), 7.28–7.35 (m,
2H); 13C NMR (CDCl3, 100 MHz) d: 43.13, 51.83, 69.55, 115.25 (d,
J = 21.37 Hz), 127.28 (d, J = 7.60 Hz), 138.27 (d, J = 3.09 Hz),
162.16 (d, J = 245.2 Hz), 172.53. UV/VIS: kmax = 225 nm; GC [150–
260 (10 °C/min)]: tR = 2.94; HPLC: compound was indivisible on
available Chiralcel OD-H column (alcohol was acetylated and only
then ee was determined).
4.4.1. Methyl 3-(acetyloxy)-3-phenylpropanoate 7a
Colorless oil; yield 85%; Rf = 0.83 [PhCH3/AcOEt (2:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.05 (s, 3H), 2.76 (dd, J = 15.81, 5.19 Hz, 1H),
2.98 (dd, J = 15.81, 9.03 Hz, 1H), 3.67 (s, 3H), 6.17 (dd, J = 8.92,
5.08 Hz, 1H), 7.22–7.43 (m, 5H); 13C NMR (CDCl3, 100 MHz) d:
20.96, 41.12, 51.77, 71.98, 126.36, 128.29, 128.53, 139.15, 169.68,
170.11; UV/VIS: kmax = 254 nm; GC [170–260 (10 °C/min)]:
tR = 2.74; HPLC [hexane–i-PrOH (90:10); f = 0.7]: tR = 9.989,
13.191 or HPLC [hexane–i-PrOH (95:5); f = 0.8]: tR = 10.843,
15.172.
4.3.6. Methyl 3-(3-bromophenyl)-3-hydroxypropanoate 6b
Colorless oil; yield 98%; Rf = 0.66 [PhCH3/AcOEt (3:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.63–2.76 (m, 2H), 3.52 (d, J = 3.81 Hz, 1H),
3.71 (s, 3H), 5.08 (dt, J = 8.03, 4.20 Hz, 1H), 7.16–7.31 (m, 2H),
7.36–7.43 (m,
1
H), 7.52–7.55 (m, 1H); 13C NMR (CDCl3,
100 MHz) d: 43.02, 51.98, 69.49, 122.49, 124.11, 128.65, 129.96,
130.67, 144.62, 172.29; UV/VIS: kmax = 225 nm; GC [150–190
(10 °C/min) ꢃ 190–200
(2 °C/min) ꢃ 200–260
(10 °C/min)]:
tR = 6.34; HPLC [hexane–i-PrOH (90:10); f = 0.8]: tR = 10.542,
4.4.2. Methyl 3-(acetyloxy)-3-(4-bromophenyl)propanoate 7b
White crystals fusible at room temperature; yield 66%; Rf = 0.65
[PhCH3/AcOEt (9:1)]; 1H NMR (CDCl3, 400 MHz) d: 2.05 (s, 3H),
2.73 (dd, J = 15.81, 5.65 Hz, 1H), 2.95 (dd, J = 15.81, 8.58 Hz, 1H),
3.66 (s, 3H), 6.10 (dd, J = 8.69, 5.53 Hz, 1H), 7.21–7.28 (m, 2H),
7.45–7.51 (m, 2H); 13C NMR (CDCl3, 100 MHz) d: 20.96, 40.88,
51.89, 71.38, 122.33, 128.21, 131.74, 138.19, 169.64, 169.87; UV/
VIS: kmax = 225 nm; GC [180–260 (10 °C/min)]: tR = 4.19; HPLC
[hexane–i-PrOH (95:5); f = 0.4]: tR = 16.770, 18.577.
20.322.
4.3.7. Methyl 3-(3-chlorophenyl)-3-hydroxypropanoate 6c
Colorless oil; yield 91%; Rf = 0.61 [PhCH3/AcOEt (3:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.64–2.75 (m, 2H), 3.54 (d, J = 3.84 Hz, 1H),
3.71 (s, 3H), 5.08 (dt, J = 8.13, 4.29 Hz, 1H), 7.19–7.29 (m, 3H),
7.36–7.38 (m, 1H); 13C NMR (CDCl3, 100 MHz) d: 43.01, 51.98,
69.56, 123.63, 125.74, 127.74, 129.66, 134.26, 144.39, 172.30;
UV/VIS: kmax = 225 nm; GC [150–190 (10 °C/min) ꢃ 190–200
(2 °C/min) ꢃ 200–260 (10 °C/min)]: tR = 4.84; HPLC [hexane–i-
PrOH (90:10); f = 0.8]: tR = 10.192, 19.480.
4.4.3. Methyl 3-(acetyloxy)-3-(4-chlorophenyl)propanoate 7c
White crystals fusible at room temperature; yield 60%; Rf = 0.63
[PhCH3/AcOEt (10:1)]; 1H NMR (CDCl3, 400 MHz) d: 2.04 (s, 3H),
2.73 (dd, J = 15.92, 5.53 Hz, 1H), 2.95 (dd, J = 15.92, 8.69 Hz, 1H),
3.66 (s, 3H), 6.11 (dd, J = 8.58, 5.42 Hz, 1H), 7.26–7.36 (m, 4H);
13C NMR (CDCl3, 100 MHz) d: 20.92, 40.91, 51.85, 71.32, 127.89,
128.76, 134.14, 137.68, 169.62, 169.86; UV/VIS: kmax = 225 nm;
GC [150–260 (10 °C/min)]: tR = 5.50; HPLC [hexane–i-PrOH
(95:5); f = 0.5]: tR = 15.612, 17.139.
4.3.8. Methyl 3-(3-methoxyphenyl)-3-hydroxypropanoate 6d
Colorless oil; yield 90%; Rf = 0.48 [PhCH3/AcOEt (3:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.66–2.79 (m, 2H), 3.35 (d, J = 3.57 Hz, 1H),
3.71 (s, 3H), 3.79 (s, 3H), 5.07–5.12 (m, 1H), 6.80–6.84 (m, 1H),
6.90–6.95 (m, 2H), 7.22–7.28 (m, 1H); 13C NMR (CDCl3, 100 MHz)
d: 43.19, 51.87, 55.17, 70.14, 110.94, 113.20, 117.72, 129.41,
144.05, 159.52, 172.48; UV/VIS: kmax = 225 nm; GC [190–200
(10 °C/min) ꢃ 200–210
(2 °C/min) ꢃ 210–260
(10 °C/min)]:
4.4.4. Methyl 3-(acetyloxy)-3-(4-methoxyphenyl)propanoate 7d
Colorless oil; yield 36%; Rf = 0.68 [PhCH3/AcOEt (3:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.02 (s, 3H), 2.74 (dd, J = 15.76, 5.50 Hz, 1H),
2.97 (dd, J = 15.58, 8.80 Hz, 1H), 3.65 (s, 3H), 3.78 (s, 3H), 6.12
(dd, J = 8.80, 5.32 Hz, 1H), 6.84–6.89 (m, 2H), 7.25–7.33 (m, 2H);
13C NMR (CDCl3, 100 MHz) d: 21.02, 40.96, 51.75, 55.15, 71.68,
113.87, 127.90, 131.18, 159.49, 169.76, 170.19; UV/VIS:
kmax = 225 nm; GC [180–260 (10 °C/min)]: tR = 4.15; HPLC [hex-
ane–i-PrOH (95:5); f = 0.5]: tR = 21.969, 24.756.
tR = 3.13; HPLC [hexane–i-PrOH (90:10); f = 0.8]: tR = 14.944,
18.442.
4.3.9. Methyl 3-(3-fluorophenyl)-3-hydroxypropanoate 6e
Colorless oil; yield 96%; Rf = 0.71 [PhCH3/AcOEt (3:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.66–2.77 (m, 2H), 3.50 (d, J = 3.81 Hz, 1H),
3.71 (s, 3H), 5.05–5.18 (m, 1H), 6.90–7.03 (m, 1H), 7.08–7.20 (m,
2H), 7.22–7.40 (m, 1H); 13C NMR (CDCl3, 100 MHz) d: 43.02,
Please cite this article in press as: Borowiecki, P.; Bretner, M. Tetrahedron: Asymmetry (2013), http://dx.doi.org/10.1016/j.tetasy.2013.06.004

P. Borowiecki, M. Bretner / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
11
4.4.5. Methyl 3-(acetyloxy)-3-(4-fluorophenyl)propanoate 7e
Colorless oil; yield 63%; Rf = 0.63 [PhCH3/AcOEt (5:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.08 (s, 3H), 2.78 (dd, J = 15.76, 5.50 Hz, 1H),
3.01 (dd, J = 15.86, 8.71 Hz, 1H), 3.70 (s, 3H), 6.19 (dd, J = 8.61,
5.50 Hz, 1H), 7.04–7.09 (m, 2H), 7.36–7.41 (m, 2H); 13C NMR
(CDCl3, 100 MHz) d: 21.02, 41.10, 51.89, 71.44, 115.57 (d,
J = 22.14 Hz), 128.46 (d, J = 8.37 Hz), 135.10 (d, J = 2.97 Hz),
137.86, 162.6 (d, J = 247.5 Hz), 169.73, 170.04. UV/VIS: kmax = 262 -
nm; GC [150–260 (10 °C/min)]: tR = 3.63; HPLC [hexane–i-PrOH
(95:5); f = 0.5]: tR = 15.101, 16.190.
gas chromatography. For HPLC analysis, the samples taken from
the reaction mixture (20 L) were condensed in vacuo, diluted
with hexane: iso-PrOH (2:1) (500 lL) and centrifuged before
l
injection.
4.6. Enzyme solvent-pretreatment
The biocatalyst (200 mg) used in the transesterification studies
was kept in toluene (4 mL) overnight with gentle stirring (50 rpm)
at ambient temperature.
4.4.6. Methyl 3-(acetyloxy)-3-(3-bromophenyl)propanoate 8b
Colorless oil; yield 90%; Rf = 0.71 [PhCH3/AcOEt (5:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.06 (s, 3H), 2.73 (dd, J = 15.95, 5.24 Hz, 1H),
2.94 (dd, J = 15.95, 8.81 Hz, 1H), 3.67 (s, 3H), 6.10 (dd, J = 8.81,
5.24 Hz, 1H), 7.18–7.24 (m, 1H), 7.26–7.31 (m, 1H), 7.41–7.45 (m,
1H), 7.49–7.51 (m, 1H); 13C NMR (CDCl3, 100 MHz) d: 21.08,
41.08, 51.98, 71.24, 122.54, 125.07, 129.30, 130.09, 131.35,
141.34, 169.45, 169.67; UV/VIS: kmax = 225 nm; GC [150–190
4.7. General procedure for the preparative scale lipase-catalyzed
kinetic resolution of racemic b-hydroxyesters
Racemic b-hydroxy ester (600 mg) dissolved in toluene (2 mL)
and vinyl acetate (10 equiv) were added into a round-bottomed
flask containing the appropriate enzyme (200 mg) suspended in
toluene (4 mL). The reaction mixture was heated to the desired
temperature and stirred with a magnetic stirrer (500 rpm, IKA
RCT basic). Aliquots of the sample were periodically withdrawn
at fixed intervals, diluted with ethyl acetate, and analyzed by gas
chromatography. When the reaction reached the desired conver-
sion, the enzyme was filtered off and washed carefully with a mix-
ture of PhCH3/AcOEt (1:1) (10 mL). The permeate was condensed
in vacuo and subjected to silica gel column chromatography using
PhCH3/AcOEt (different ratios, depending on the substrate) as an
eluent to give an enantiomerically enriched product (acetyl ester)
and the corresponding alcohol, respectively. The detailed condi-
tions, yields, and enantiomeric excess data for the resolved prod-
ucts are listed in Tables 5–10. The specific rotations for the
enantioenriched alcohols are mentioned in Section 2.6, and for es-
(10 °C/min) ꢃ 190–200
(2 °C/min) ꢃ 200–260
(10 °C/min)]:
tR = 7.27; HPLC [hexane–i-PrOH (98:2); f = 0.8]: tR = 14.649, 15.778.
4.4.7. Methyl 3-(acetyloxy)-3-(3-chlorophenyl)propanoate 8c
Colorless oil; yield 97%; Rf = 0.72 [PhCH3/AcOEt (5:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.06 (s, 3H), 2.73 (dd, J = 15.95, 5.24 Hz, 1H),
2.94 (dd, J = 15.95, 8.81 Hz, 1H), 3.67 (s, 3H), 6.11 (dd, J = 8.81,
5.24 Hz, 1H), 7.20–7.29 (m, 3H), 7.32–7.36 (m, 1H); 13C NMR
(CDCl3, 100 MHz) d: 21.04, 41.05, 51.95, 71.29, 124.56, 126.40,
128.41, 129.80, 134.35, 141.11, 169.44, 169.67; UV/VIS:
kmax = 225 nm; GC [150–190 (10 °C/min) ꢃ 190–200 (2 °C/
min) ꢃ 200–260 (10 °C/min)]: tR = 5.64; HPLC [hexane–i-PrOH
(98:2); f = 0.8]: tR = 13.428, 14.499.
ters are as follows: (R)-(+)-11a: [
ee); (R)-(+)-11b: [ ]D24 = +46.4 (c 1.12, CHCl3, 92% ee); (R)-(+)-
11c: ]D24 = +42 (c 1.04, CHCl3, 98% ee); (R)-(+)-11d:
]D24 = +22 (c 1.9, CHCl3, 89% ee); (R)-(+)-11e: [ ]D24 = +11 (c
0.95, CHCl3, 95% ee); (R)-(+)-12b: [ ]D24 = +36.2 (c 1.34, CHCl3,
97% ee); (R)-(+)-12c: [ ]D24 = +41.1 (c 1.4, CHCl3, 97% ee); (R)-
(+)-12d: ]D24 = +46.3 (c 1.2, CHCl3, 97% ee); (R)-(+)-12e:
]D24 = +39.7 (c 1.56, CHCl3, 93% ee).
a]D24 = +50.1 (c 1.0, CHCl3, 96%
a
4.4.8. Methyl 3-(acetyloxy)-3-(3-methoxyphenyl)propanoate 8d
Colorless oil; yield 90%; Rf = 0.71 [PhCH3/AcOEt (5:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.05 (s, 3H), 2.74 (dd, J = 15.81, 4.97 Hz, 1H),
2.94 (dd, J = 15.92, 9.15 Hz, 1H), 3.66 (s, 3H), 3.78 (s, 3H), 6.13
(dd, J = 9.26, 4.97 Hz, 1H), 6.80–6.85 (m, 1H), 6.87–7.00 (m, 2H),
7.21–7.35 (m, 1H); 13C NMR (CDCl3, 100 MHz) d: 21.00, 41.17,
51.77, 55.10, 71.82, 111.90, 113.44, 118.36, 129.50, 140.61,
159.43, 169.45, 169.90; UV/VIS: kmax = 225 nm; GC [190–200
[a
[a
a
a
a
[
a
[a
Acknowledgments
(10 °C/min) ꢃ 200–210
(2 °C/min) ꢃ 210–260
(10 °C/min)]:
tR = 3.84; HPLC [hexane–i-PrOH (98:2); f = 0.8]: tR = 20.522, 30.868.
The project was co-financed by The European Regional Devel-
opment Fund under The Innovative Economy Operational Pro-
gramme 2007–2013: ‘Biotransformations for Pharmaceutical and
Cosmetic Industry’ POIG.01.03.01-00-158/09. These studies were
partially supported by Warsaw University of Technology, Faculty
of Chemistry. In addition, we are thankful to Eng. Sc. students: Pat-
4.4.9. Methyl 3-(acetyloxy)-3-(3-fluorophenyl)propanoate 8e
Colorless oil; yield 95 %; Rf = 0.71 [PhCH3–AcOEt (5:1)]; 1H NMR
(CDCl3, 400 MHz) d: 2.05 (s, 3H), 2.73 (dd, J = 15.95, 5.00 Hz, 1H),
2.93 (dd, J = 15.95, 8.81 Hz, 1H), 3.66 (s, 3H), 6.12 (dd, J = 8.81,
5.24 Hz, 1H), 6.93–7.00 (m, 1H), 7.02–7.07 (m, 1H), 7.10–7.14 (m,
1H), 7.25–7.33 (m, 1H); 13C NMR (CDCl3, 100 MHz) : 20.98,
41.04, 51.88, 71.27 (d, J = 1.84 Hz), 113.19 (d, J = 22.09 Hz),
115.13 (d, J = 20.26 Hz), 121.92 (d, J = 3.70 Hz), 130.07 (d,
J = 8.28 Hz), 141.58 (d, J = 6.44 Hz), 162.55 (d, J = 246.71 Hz),
169.43, 169.68; UV/VIS: kmax = 225 nm; GC [150–160 (2 °C/
min) >> 160–260 (10 °C/min)]: tR = 5.43; HPLC [hexane-i-PrOH
(98:2); f = 0.8]: tR = 13.087, 14.414.
´
´
rycja Niemynska, Katarzyna Kordowska and Marta Trzebinska for
partial participation in the enzymatic studies. We express our spe-
cial gratitude to Professor Jan Plenkiewicz for helpful discussions
and collaborative support while writing this work.
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