Heterocycles 30: Lipase catalyzed kinetic resolution of racemic 1-(2-aryl-4-methyl-thiazol-5-yl)ethanols

Article abstract of DOI:10.1016/j.tetasy.2011.12.005

The lipase mediated selective O-acylation and alcoholysis of the corresponding alcohols/butanoates have been investigated for the production of both (R)- and (S)-1-(2-aryl-thiazol-5-yl)ethanols. In terms of stereoselectivity and activity, CaL-B mediated selective acylation gave similar good results (c = 50%, E 200) in comparison with CaL-A mediated alcoholysis (c = 50%, E 200). Using the sequential kinetic resolution: first a CaL-B catalyzed (R)-selective O-acylation of racemic alcohols rac-2a-d followed by a CaL-A mediated methanolysis of the previously obtained enantiomerically enriched (R)-butanoates (R)-3a-d yielded (with 39-46% yields) both enantiopure forms of the 1-(2-aryl-thiazol-5-yl)ethanols.

Full text of DOI:10.1016/j.tetasy.2011.12.005

Tetrahedron: Asymmetry 22 (2011) 2165–2171
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Tetrahedron: Asymmetry
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Heterocycles 30: Lipase catalyzed kinetic resolution of racemic
1-(2-aryl-4-methyl-thiazol-5-yl)ethanols
Denisa Hapa˘u a, Jürgen Brem b, Valentin Zaharia a,
⇑
a Department of Organic Chemistry, ‘Iuliu Haßtieganu’ University of Medicine and Pharmacy, Ro-400012 Cluj-Napoca, Victor Babeßs 41, Romania
b Department of Biochemistry and Biochemical Engineering, Babeßs-Bolyai University of Cluj-Napoca, Ro-400028 Cluj-Napoca, Arany János 11, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
The lipase mediated selective O-acylation and alcoholysis of the corresponding alcohols/butanoates have
been investigated for the production of both (R)- and (S)-1-(2-aryl-thiazol-5-yl)ethanols. In terms of ste-
reoselectivity and activity, CaL-B mediated selective acylation gave similar good results (c = 50%, E »200)
in comparison with CaL-A mediated alcoholysis (c = 50%, E »200). Using the sequential kinetic resolution:
first a CaL-B catalyzed (R)-selective O-acylation of racemic alcohols rac-2a–d followed by a CaL-A med-
iated methanolysis of the previously obtained enantiomerically enriched (R)-butanoates (R)-3a–d yielded
(with 39–46% yields) both enantiopure forms of the 1-(2-aryl-thiazol-5-yl)ethanols.
Received 21 November 2011
Accepted 13 December 2011
Available online 12 January 2012
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
center in the structure of a bioactive compound can cause a higher
selectivity during interaction with protein receptors and provides
Thiazole and its derivatives are important in biological systems
and are important building-blocks for the synthesis of various
antitumor, anti-inflammatory, antiviral, and antimicrobial1–6
drugs. Some examples of thiazole based pharmaceutical products
include Ritonavir (HIV protease inhibitor), Bleomycine (anticancer-
ous glycopeptidic antibiotic), Pramipexole (dopamine agonist
anti-Parkinsonian), Meloxicam, and Fanetizole (non-steroidal
anti-inflammatory drugs). In particular, the 2-phenylthiazole ring
system, can be found in the structure of some anti-inflammatory
products (Fentiazac), and has provided a valuable template for
new anti-inflammatory,2 antimicrobial3, and antiflaviviral4 agents.
Some 2-phenylthiazole-4-carboxamides have shown significant
cytostatic activity by inducing the apoptosis of three human cancer
cell lines including T47D (breast cancer), HT29 (colon cancer), and
Caco-2 (colorectal cancer).5 Other phenylthiazole bearing com-
pounds have been reported as selective peroxisome proliferator
specific action to the medication.
Optically active secondary alcohols have found multiple appli-
cations as chiral building blocks for the synthesis of various enan-
tiopure pharmaceuticals. Thus, the development of efficient
strategies toward such highly enantiomerically enriched com-
pounds is a concern of both pharmaceutical and fine chemical
industries.7
Due to their ability to accept a wide range of substrates and
maintain their stability, chemo- and regio- enantioselectivities
even in non-conventional environments, lipases have been widely
used in enzymatic kinetic resolutions.8 Baker’s yeast-mediated
bioreduction of prochiral ketones has proven to be another
convenient method to obtain enantiomerically pure secondary
alcohols.9
As a continuation of research devoted to the development of
enzymatic procedures for optically active heterocyclic compounds,
we decided to investigate the biocatalytic synthesis of new enan-
tiopure 2-aryl-thiazolyl-ethanols.
activated receptor (PPARa) agonists and are good candidates for
the treatment of lipid imbalance and atherosclerosis.6
All these biological activities are the result of a complex interac-
tion between chiral receptors and drug molecules. One of the cur-
rent concerns in the drug-design area is to provide a targeted
therapy, by improving the effectiveness of action for biologically
active compounds. Stereochemistry is an essential property in
pharmacology that can be exploited. The presence of a stereogenic
The recently described asymmetric bioreduction of heteroaryl-
ethanone derivatives mediated by Saccharomyces cerevisiae,10
which provided the enantiopure (S)-alcohols with good conver-
sions and high enantioselectivity, led us to first investigate the
cellular reduction with Baker’s yeast of 1-(2-phenyl-4-methyl-
thiazol-5-yl)ethanone 1a.
Since the two enantiomers of a new chiral compound can show
different toxicity or therapeutic properties, both need to be synthe-
sized. However, in most cases, the efficient conversion of the (S)-
alcohol into the corresponding (R)-enantiomer, requires additional
steps.
⇑
Corresponding author. Tel.: +40 264 594 148; fax: +40 264 597 257.
E-mail address: vzaharia@umfcluj.ro (V. Zaharia).
0957-4166/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2011.12.005

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D. Hapau et al. / Tetrahedron: Asymmetry 22 (2011) 2165–2171
Table 1
The lipase-catalyzed kinetic resolution proved to be a more effi-
Lipase-catalyzed acetylation of rac-2a with vinyl acetate
cient procedure for the synthesis of both enantiomers of 1-(2-aryl-
4-methyl-thiazol-5-yl)ethanols rac-2a–d.
Entry
Enzyme
cB (%)
ee(S)-2a (%)
ee(R)-3a (%)
E
The absolute configuration of the enantiopure compounds ob-
tained was determined by a detailed 1H NMR study of the Mosher’s
derivatives of (S)-2b.
1
2
3
4
5
6
7
8
CaL-B
CaL-A
Lipase FA
PPL
AK
CRL
41
48
11
25
35
10
14
1
63
63
3a
1
50
9
92
69
24a
3
91
79
6
43
10
2
1
34
9
2. Results and discussion
2.1. Chemical synthesis
MML
BCL
1
1
1
4
56
A
B
Inverse selectivity.
After 14 h.
As previously described in the literature,11 the 1-(2-aryl-4-
methyl-thiazol-5-yl)ethanones 1a–d were synthesized by using
the Hantzsch condensation of various thioamides with 3-
chloroacetylacetone.
As illustrated in Scheme 1, racemic 1-(2-aryl-4-methyl-thiazol-
5-yl)ethanols rac-2a–d were prepared by reduction with sodium
borohydride of the corresponding ketones 1a–d.
Table 2
Lipase-catalyzed acetylation of rac-2a with CaL-B in various solvents
Entry
Solvent
ca (%)
ee(S)-2a (%)
ee(R)-3a (%)
E
1
2
3
4
5
6
7
8
9
n-Hexane
n-Octane
Cyclohexane
Chloroform
Toluene
DIPE
THF
Acetonitrile
MTBE
53
53
52
24
50
45
26
39
49
97
97
98
30
94
78
34
61
88
87
86
92
97
93
94
97
96
93
58
54
110
81
92
83
92
92
77
2.2. Enzymatic synthesis
To investigate the stereoselectivity of the reactions involving
chiral 1-(2-phenyl-4-methyl-thiazol-5-yl)ethanols rac-2a–d and
their esters rac-3a–d, the chromatographic separation of the enan-
tiomers was first established (Table 6). The base-line separation of
all enantiomers of rac-2,3a–d was performed using various HPLC
chiral columns.
a
After 36 h.
of rac-2a in the presence of several enzymes in pure vinyl acetate
was tested (Scheme 1, Table 1). CaL-B was found to be the most
selective enzyme for the kinetic resolution of rac-2a.
It is known that the properties of the solvent and the nucleo-
phile can significantly influence the activity and selectivity of
enantioselective enzymatic acylation.12 The CaL-B mediated acyla-
tion of rac-2a with vinyl acetate in several solvents was tested
(Table 2) and cyclohexane was found to be the most appropriate
solvent for the CaL-B catalyzed acetylation of rac-2a.
The CaL-B mediated acylation of rac-2a in cyclohexane was also
carried out with vinyl propanoate and vinyl butanoate (Table 3,
entries 2 and 3) and it was found that the highest selectivity and
reactivity for the CaL-B catalyzed acylation of rac-2a was obtained
using vinyl butanoate as the acyl donor (E >200). Applying the
optimal conditions found for the acylation of rac-2a for the kinetic
resolution of rac-2b–d gave good to excellent ee values for all of
the resolution products (Table 3, entries 4–6).
2.2.1. Analytical scale biotransformations
2.2.1.1. Baker’s yeast mediated bioreduction of 1-(2-phenyl-4-
methyl-thiazol-5-yl)ethanone 1a. In order to investigate the Ba-
ker’s yeast-mediated reduction of ketone 1a, analytical scale reac-
tions were performed under fermenting and non-fermenting
conditions. Due to the unsatisfactory ee values for the isolated
(S)-2a (ee 70%), further optimization of this procedure was also
investigated by the addition of various additives (n-hexane, allyl
alcohol, MgSO4, ethyl bromoacetate, and so on). In some cases,
the enantiomeric purity of the bioreduction product increased
slightly (from the initial 70% to 81%), but the conversion of the
reaction considerably decreased.
2.2.1.2. Analytical scale enzymatic acylation of rac-2a–d. Since
the Baker’s yeast-mediated bioreduction gave unsatisfactory result
further the analytical scale enantiomer selective lipase-catalyzed
acylations of racemic 1-(2-aryl-4-methyl-thiazol-5-yl)ethanols
rac-2a–d were studied. We used rac-2a as the model compound
for optimization of the enzymatic acylation. First the acetylation
It is important to note that the rate of the enzymatic reaction
was influenced by the nature of the substituent on the phenyl ring.
1-3
a
Ar
N
S
N
S
N
S
N
S
i
iv
Ar
Ar
Ar
Ar
+
O
HO
rac-2a-d
O
HO
C3H7
(R)-3a-d
(S)-2a-d
1a-d
b
O
v
iii
ii
N
S
N
S
N
S
N
c
d
v
Ar
Ar
Ar
Ar
+
S
HO
O
HO
O
C3H7
C3H7
Br
(S)-2a-d
(S)-2a
rac-3a-d
(R)-2a-d
O
O
Scheme 1. Synthesis and biotransformation of the 1-(2-phenylthiazol-5-yl)ethanol derivatives and their esters. Reagents and conditions: (i) NaBH4/MeOH; (ii) Saccharomyces
cerevisiae; (iii) (CH3–CH2–CH2–CO)2O, Et3N, DMAP/CH2Cl2, rt; (iv) CaL-B, vinyl butanoate/cyclohexane; (v) CaL-A, MeOH/DIPE.

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D. Hapau et al. / Tetrahedron: Asymmetry 22 (2011) 2165–2171
Table 3
CaL-B mediated acylation with various acyl donors in cyclohexane
Entry
Acylating agent
Compound
Time (h)
c (%)
ee(S)-2a-d (%)
ee(R)-3a-d (%)
E
1
2
3
4
5
6
Vinyl acetate
2a
2a
2a
2b
2c
2d
36
36
36
45
19
69
52
50
50
50
49
50
98
94
98
97
97
92
96
98
97
>99.5
98
110
Vinyl propanoate
Vinyl butanoate
Vinyl butanoate
Vinyl butanoate
Vinyl butanoate
174
»200
»200
»200
»200
>99.5
Table 4
Enantioselective methanolysis of rac-3a-d, with different types of immobilized CaL-A, in diisopropyl ether as solvent
Entry
Compd.
Enzyme
c (%)
Time (h)
ee(S)-2a (%)
ee(R)-3a (%)
E
1
2
3
4
5
6
3a
3a
3a
3b
3c
3d
CaL-A CLEA
IMMCal-A
CaL-A on Celite
CaL-A CLEA
CaL-A CLEA
CaL-A CLEA
50
52
45
50
50
50
36
48
48
48
56
56
>99.5
10
40
>99.5
>99.5
>99.5
>99.5
11
33
>99.5
>99.5
>99.5
»200
1.3
3.1
»200
»200
»200
Table 5
Yields and ee values for the preparative scale synthesis of (S)- and (R)-2a–d and (R)-3a–d.
Producta
Yieldc (%)
ee (%)
[
a
]
D
Producta
Yieldc (%)
ee (%)
[
a
]
D
Productb
Yieldc (%)
ee (%)
[a]
D
d
d
d
Entry
1
2
3
4
(S)-2a
(S)-2b
(S)-2c
(S)-2d
45
45
46
44
98.5
98.5
92
À53.4
À56
(R)-3a
(R)-3b
(R)-3c
(R)-3d
44
44
45
43
98
97
>99.5
96.2
+152
+144
+167.6
+99.2
(R)-2a
(R)-2b
(R)-2c
(R)-2d
41
40
42
39
>99.5
>99.5
>99.5
>99.5
+52.8
+56.8
+57.6
+44.5
À54
91.5
À40
a
b
c
From the enzymatic acylation of rac-2a–d.
From the enzymatic methanolysis of (R)-3a–d.
Isolated yields based on racemic starting material.
c 1.0, CHCl3, t = 25 °C.
d
Thus, when brominated rac-2d was used as the substrate, the acyl-
ation proceeded with the lowest velocity (Table 3, entry 6).
which is able to produce conformational changes. In this way the
available space in the active site became wider (essentially
infinite8). In other words, while rac-2a as a low volume substrate
fits perfectly in the active site of CaL-B, rac-3a with a higher
volume fits perfectly in the catalytic site of CaL-A.
2.2.1.3. Analytical scale enzymatic alcoholysis of rac-3a–d and
(R)-3a–d. It is known that lipases usually retain their enantiopre-
ference during hydrolysis or alcoholysis. Consequently, such reac-
tions should result in the opposite enantiomeric forms of the
reaction counterparts.
Several lipases (such as CaL-B, AK, CRL, and BCL, etc.) in various
solvents and in the presence of various amounts (2–10 equiv) of
nucleophiles (methanol, ethanol, propanol, and butanol) showed
poor activity in the alcoholysis process of rac-3a–d. Only CaL-A
proved to be an efficient biocatalyst for the enzymatic alcoholysis
of rac-3a–d. While IMMCal-A T2–150 (covalently immobilized
CaL-A) and CaL-A immobilized on Celite were inadequate biocata-
lysts (E <5, Table 4, entries 2 and 3), it was found that CaL-A CLEA
(Candida antarctica lipase A reticulated with glutaraldehyde), in
diisopropyl ether, in the presence of methanol (10 equiv) as a
nucleophile was the most appropriate biocatalyst for the resolu-
tion of rac-3a–d (Table 4, entries 1 and 4–6).
2.2.2. Preparative scale synthesis of both (R)- and (S)-2a–d
Using the optimal conditions found for the analytical scale
enzymatic acylation of 1-(2-phenyl-4-methyl-thiazol-5-yl)etha-
nols rac-2a–d, followed by the CaL-A catalyzed methanolysis of
the previously formed (R)-3a–d, the preparative scale enzymatic
synthesis of both (S)- and (R)-2a–d was performed. During pre-
parative scale transformations, no significant differences were
found for the reaction rates and enantiopurity of the resolution
products compared with those found for the analytical scale
reactions (Table 5).
2.3. The absolute configuration of the kinetic resolution
products
By comparing the behavior of the two lipases from Candida ant-
arctica it can be concluded that both CaL-A and CaL-B are highly ac-
tive biocatalysts for the acylation of rac-2a. However, while CaL-B
acted as a highly selective enzyme, CaL-A showed poor selectivity
(Table 1, entries 1 and 2). In contrast with these results, while CaL-
B was inactive for the alcoholysis of the corresponding racemic
butanoate rac-3a, CaL-A proved to be the most appropriate enzyme
for this reaction. These results can be explained by comparing the
restricted active site of CaL-B8 with the special properties of CaL-A,
The (S)-absolute configuration of the novel enantiopure second-
ary alcohols was confirmed by a detailed 1H NMR study of the
Mosher’s derivatives of (À)-2b.13 Thus, the enantiomerically pure
alcohol (À)-2b remaining untransformed during the CaL-B-medi-
ated acylation was esterified with both (R)-MTPA and (S)-MTPA.
The resulting diastereomers were differentiated by their 1H NMR
spectra (Fig. 1).
In the case of (S)-MTPA-(À)-2b, the strong diamagnetic effect of
the phenyl group caused the proximal CH3 protons to be more

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D. Hapau et al. / Tetrahedron: Asymmetry 22 (2011) 2165–2171
CH3
S
N
(S)
H3C
H
CH3
O
H3CO
Ph
(R)
O
CF3
(R)-MTPA-(S)-2b
CH3
S
N
(S)
H3C
H
CH3
OCH3
O
Ph
(S)
O
CF3
(S)-MTPA-(S)-2b
3.50
3.00
2.50
2.00
Figure 1. Signals of the CH3 protons and OCH3 protons from the 1H NMR spectra of (R)-MTPA-(S)-2b and of (S)-MTPA-(S)-2b.
shielded, d = 1.66 ppm, when compared with their analogues in
(R)-MTPA-(À)-2b (d = 1.73 ppm) where the distance between the
phenyl and the methyl group is higher (Fig. 1). Also, the methoxy
protons of the (S)-MTPA-(À)-2b were found to be more shielded,
d = 3.48 ppm, in comparison with the methoxy protons of the
(R)-MTPA-(À)-2b (d = 3.55 ppm), due to the diamagnetic effect of
the 2-phenylthiazole ring system.
These results confirmed that the stereochemical outcome of the li-
pase-catalyzed kinetic resolutions were in accordance with the
Kazlauskas empirical rule.14
4. Experimental
The CH3 protonsof the (R)-MTPA-(À)-2b located at the 4-position
of the thiazole ring were found to be more shielded (d = 2.50 ppm)
when compared with the (S)-MTPA-(À)-2b (d = 2.53 ppm). Based
on these observations, the (S)-absolute configuration can be
assigned for (À)-2b.
4.1. Analytical methods
The 1H NMR and 13C NMR spectra were recorded in CDCl3 solu-
tion on a Brucker Avance DPX-300 spectrometer operating at 300
and 75 MHz, respectively. Chemical shifts on the scale are ex-
pressed in ppm values from TMS as the internal standard. EI mass
spectra were taken on a VG 7070E mass spectrometer operating at
70 eV and are characterized by m/z (relative intensity %) values.
High performance liquid chromatography analyses were carried
out on Agilent 1200 instrument. Enantiomeric separation was per-
formed using a Chiralpak IB column (4.6 Â 250 mm) for rac-2,3a,b,
respectively, Chiralpak IC column (4.6 Â 250 mm) for rac-2,3c,d. In
all cases, a gradient separation procedure was first established,
using a mixture of n-hexane and 2-propanol as eluent, all at
1 mL/min flow rate. The gradient separation procedure and the
retention times are shown in Table 6.
The configurations of all the unreacted enantiomers in the
CaL-B catalyzed acylations were assigned as (S) on the basis of
the same sign of their specific rotation.
3. Conclusions
Two kind of enzymatic methods were investigated for the pro-
duction of new enantiomerically pure 1-(2-aryl-4-methyl-thiazol-
5-yl)ethanols. Compared to bioreduction, the lipase-catalyzed
kinetic resolution proved to be more efficient, yielding highly
enantiopure products.
Thin layer chromatography (TLC) was carried out on Merck Kie-
selgel 60 F254 sheets. Spots were visualized by treatment with 5%
ethanolic phosphomolybdic acid solution and heating of the dried
plates. Preparative chromatographic separations were performed
using column chromatography on Merck Kieselgel 60 (63–
Both procedures, the enantioselective CaL-B mediated acylation
with vinyl butanoate of 1-(2-aryl-4-methyl-thiazol-5-yl)ethanols
(S)-2a–d in cyclohexane, and the CaL-A-CLEA catalyzed methanol-
ysis of the corresponding racemic esters rac-3a–d in DIPE have
been proven to be highly stereoselective procedures. These results
were a valuable prerequisite for the preparative scale production of
(R)-, (S)-2a–d, and (R)-3a–d.
200 lm). The specific rotation was determined on a Bellingham-
Stanley ADP 220 polarimeter.
Melting points were determined on open glass capillaries using
an Electrothermal IA 9000 digital melting point apparatus and are
uncorrected.
The absolute configuration of (À)-2b isolated as the unreacted
stereoisomer during the enzymatic acylation of rac-2b was as-
signed to be (S) by a 1H NMR study of its Mosher’s derivatives.

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D. Hapau et al. / Tetrahedron: Asymmetry 22 (2011) 2165–2171
Table 6
The retention times of the enantiomers of rac-2,3a–d
Compound
tR (min)
Compound
tR (min)
HPLC column
Gradient separation
Time (min) % iPrOH
(R)-2a
(S)-2a
(R)-2b
(S)-2b
(R)-2c
(S)-2c
(R)-2d
(S)-2d
12.88
11.15
14.29
12.17
24.19
18.59
13.76
13.33
(R)-3a
(S)-3a
(R)-3b
(S)-3b
(R)-3c
(S)-3c
(R)-3d
(S)-3d
4.75
5.28
4.76
IB
IB
IC
IC
0–5
5.1–12
0–6
6.5–17
0–11
11.5–20
0–8
8.5–15
3
15
3
15
2
15
2
20
5.68
10.19
10.96
6.86
7.51
4.2. Reagents and solvents
4.3.1.3. 1-(4-Methyl-2-p-tolylthiazol-5-yl)ethanol rac-2c. Yield:
93%; white solid; mp: 112 °C; 1H NMR: 300 MHz, CDCl3): d = 1.54
(d, J = 6.38 Hz, 3H); 2.35 (s, 3H); 2.36 (s, 3H); 5.13 (q, J = 6.23 Hz,
1H); 7.19 (d, J = 8.1 Hz, 2H); 7.74 (d, J = 8.1 Hz, 2H) 13C NMR:
(75 MHz, CDCl3) d = 15.2; 21.3; 25.4; 63.6; 126.1; 129.5; 130.9;
136.9; 139.9; 148.0; 165.5; HRMS: M+ found (M+ calculated for
The commercial chemicals and solvents were products of
Aldrich or Fluka. All solvents were purified and dried by standard
methods as required.
Baker’s yeast was produced by Budafok Ltd. Hungary and was
purchased from a local store. Lipase B from Candida antarctica
(CaL-B, Novozym 435) was purchased from Novozymes, Denmark.
Lipases from Candida rugosa (CrL), Mucor miehei (MML), and hog
pancreas (PPL) were purchased from Fluka. Lipases from Pseudo-
monas fluorescens (AK), Burkholderia cepacia (BcL) and Rhyzopus
oryzae (Lipase F) were from Amano, Europe, England. Lipase A from
C. antarctica immobilized on Celite (CaL-A) was a generous gift
from the University of Turku (Finland), lipase A from C. antarctica
reticulated with glutaraldehyde (CLEA) or covalently immobilized
(IMMCal-A T2–150) were purchased from Fluka and Chiralvision,
respectively.
C
13H15NOS): 233.0876 (233.0874); ESI+-MS: m/z (%) = 234 (M+1,
8); 233 (M, 54); 219 (13); 218 (100); 216 (8); 190 (16); 118 (15);
109 (5); 71 (5); 45 (7).
4.3.1.4. 1-(2-(4-Bromophenyl)-4-methylthiazol-5-yl)ethanol rac-
2d. Yield: 93%; white solid; mp: 121 °C; 1H NMR: (300 MHz,
CDCl3): d = 1.55 (d, J = 6.4 Hz, 3H); 2.38 (s, 3H); 5.16 (q, J = 3.2 Hz,
1H); 7.52 (d, J = 8.5, 2H); 7.72 (d, J = 8.5 Hz, 2H) 13C NMR:
(75 MHz, CDCl3): d = 15.2; 25.5; 63.7; 124.0; 127.6; 132.0; 132.5;
137.8; 148.5; 164.0; HRMS: M+ found (M+ calculated for
C
12H12BrNOS): 296.9819 (296.9823); ESI+-MS: m/z (%) = 300 (M+1,
81Br, 9); 299 (M, 81Br, 65); 298 (M+1, 79Br, 10); 297 (M, 79Br, 62);
256 (17); 254 (18); 184 (9); 182 (9); 89 (8); 73 (9); 71 (9); 45
(16); 43 (13).
4.3. Chemical synthesis of racemic compounds rac-2,3a–d
4.3.1. Synthesis of racemic 1-(2-aryl-4-methyl-thiazol-5-
yl)ethanols rac-2a–d
4.3.2. Synthesis of racemic 1-(2-aryl-4-methyl-thiazol-5-yl)ethyl
butanoates rac-3a–d by chemical acylation of the
corresponding racemic 1-(2-aryl-4-methyl-thiazol-5-
yl)ethanols rac-2a–d
To a solution of racemic 1-(2-aryl-4-methyl-thiazol-5-yl)etha-
nol rac-2a–d (0.561 mmol) in dry CH2Cl2 (4 mL), Et3N (0.616 mmol,
To a solution of 1-(2-aryl-4-methyl-thiazol-5-yl)ethanone 1a–d
(3 mmol) dissolved in a minimum amount of methanol, sodium
borohydride (6 mmol) was added slowly. The mixture was stirred
at room temperature for 1 h. After quenching the reaction mixture
by the addition of water (10 ml), methanol was removed by rota-
tory evaporation. The aqueous mixture was neutralized using a
solution of 3 M HCl, and then extracted with dichloromethane.
After the organic layer was dried over anhydrous Na2SO4, the sol-
vent was removed by rotatory evaporation to afford compounds
rac-2a–d.
62.4 mg, 86
lL), n-butanoic anhydride (0.617 mmol, 97.6 mg,
100 L), and DMAP (0.016 mmol, 2 mg) were added. The mixture
l
was stirred at room temperature overnight and then quenched
with water (10 mL). The isolated organic layer was dried over
anhydrous Na2SO4 and the solvent was distilled off by rotatory
evaporation. The crude product was purified by column chroma-
tography on silica gel (eluent: dichloromethane/acetone = 25:1
(v/v)) to give rac-3a–d as a liquid.
4.3.1.1. 1-(4-Methyl-2-phenylthiazol-5-yl)ethanol rac-2a. Yield:
92%; semisolid; 1H NMR: (300 MHz, acetone-d6): d = 1.52 (d,
J = 6.2 Hz, 3H); 2.44 (s, 3H); 5.26 (q, J = 6.4 Hz, 1H); 7.45-7.54 (m,
3H); 7.89-8.02 (m, 2H); 13C NMR: (75 MHz, acetone d = 15.3;
25.7; 63.6; 127.2; 129.9; 130.1; 131.7; 145.5; 150.7; 165.9; HRMS:
M+ found (M+ calculated for C12H13NOS): 219.0712 (219.0717);
ESI+-MS: m/z (%): 221 (M+2, 18); 220 (M+1, 100); 219 (M, 63);
205 (12); 204 (62); 203 (5); 176 (8); 104 (4); 71 (8); 45 (7);
4.3.2.1. 1-(4-Methyl-2-phenylthiazol-5-yl)ethyl butanoate rac-
3a. Yield: 92%; liquid; 1H NMR: (300 MHz, CDCl3): d = 0.92 (t,
J = 7.3 Hz, 3H); 1.54–1.67 (m, 5H); 2.28 (t, J = 7.5 Hz, 2H); 2.49 (s,
3H); 6.19 (q, J = 6.5 Hz, 1H); 7.38–7.44 (m, 3H); 7.85–7.92 (m,
2H); 13C NMR: (75 MHz, CDCl3): d = 13.5; 15.2; 18.3; 22.8; 36.2;
65.2; 126.3; 128.8; 130.0; 132.6; 133.2; 149.9; 166.0; 172.6;
HRMS: M+ found (M+ calculated for C16H19NO2S): 289.1132
(289.1136); ESI+-MS: m/z (%) = 291 (M+2, 20); 290 (M+1, 100);
289 (M, 20); 219 (21); 201 (47); 104 (7); 71 (9); 65 (11); 43 (18).
4.3.1.2. 1-(4-Methyl-2-m-tolylthiazol-5-yl)ethanol (rac-2b).
Yield: 95%, white solid, mp: 93.5 °C; 1H NMR: 300 MHz, CDCl3):
d = 1.60 (d, J = 6.35 Hz, 3H); 2.42 (s, 3H); 2.43 (s, 3H); 5.20 (q,
J = 5.8 Hz, 1H); 7.25 (d, J = 7.39 Hz, 1H); 7.33 (t, J = 7.39 Hz, 1H);
7.69 (d, J = 7.54 Hz, 1H); 7.76 (s, 1H); 13C NMR: (75 MHz, CDCl3):
d = 15.2; 21.3; 25.4; 63.7; 123.5; 126.7; 128.7; 130.6; 133.5;
137.2; 138.6; 148.2; 165.6; HRMS: M+ found (M+ calculated for
4.3.2.2. 1-(4-Methyl-2-m-tolylthiazol-5-yl)ethyl butanoate rac-
3b. Yield: 92%; liquid; 1H NMR: 300 MHz, CDCl3 d = 0.92 (t,
J = 7.4 Hz, 3H); 1.6–1.7 (m, 5H); 2.29 (t, J = 7.4 Hz, 2H); 2.40 (s,
3H); 2.50 (s, 3H); 6.2 (q, J = 6.6 Hz, 1H); 7.20 (d, J = 7.5 Hz, 1H);
7.29 (t, J = 7.6 Hz, 1H); 7.66 (d, J = 7.6 Hz, 1H); 7.73 (s, 1H); 13C
NMR: 75 MHz, CDCl3 d = 13.6; 15.4; 18.4; 21.3; 22.9; 36.2; 65.4;
C
13H15NOS): 233.0871 (233.0874); ESI+-MS: m/z (%) = 234 (M+1,
9); 233 (M, 53); 219 (15); 218 (100); 216 (7); 190 (18); 118 (17);
109 (6); 71 (3); 45 (5);

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D. Hapau et al. / Tetrahedron: Asymmetry 22 (2011) 2165–2171
2170
123.6; 126.8; 128.7; 130.7; 132.4; 133.5; 138.6; 150.1; 166.1;
172.6; HRMS: M+ found (M+ calculated for C17H21NO2S):
303.1296 (303.1293); ESI+-MS: m/z (%) = 304 (M+1, 6); 303 (M,
25); 218 (13); 217 (17); 216 (100); 215 (37); 190 (7); 118 (6); 99
(5); 98 (7); 71 (7); 65 (5); 59 (9); 43 (7).
4.5.2. Enzymatic kinetic resolution of racemic 1-(2-phenyl-
4-methyl-thiazol-5-yl)ethanol rac-2a with vinyl acetate in
different solvents with CaL-B
To
yl)ethanol rac-2a (22.7
vinyl acetate (217 mol, 20
a
solution of racemic 1-(2-phenyl-4-methyl-thiazol-5-
mol, 5 mg) in different solvents (200 L),
L) and lipase CaL-B (2.5 mg) were
l
l
l
l
added. The reaction mixtures were shaken at 1300 rpm at room
temperature. Samples were analyzed by HPLC as described in Sec-
tion 4.5.1. Data on conversion and enantiomeric composition of the
products for the solvents tested are presented in Table 2.
4.3.2.3. 1-(4-Methyl-2-p-tolylthiazol-5-yl)ethyl butanoate rac-
3c. Yield: 93%; liquid; 1H NMR: 300 MHz, CDCl3 d = 0.92 (t,
J = 7.4 Hz, 3H); 1.59-1.69 (m, 5H); 2.28 (t, J = 7.4 Hz, 2H); 2.36 (s,
3H); 2.47 (s, 3H); 6.19 (q, J = 6.6 Hz, 1H); 7.2 (d, J = 8.0 Hz, 2H);
7.77 (d, J = 8.2 Hz, 2H); 13C NMR: 75 MHz, CDCl3 d = 13.5; 15.3;
18.3; 21.3; 22.9; 36.2; 65.3; 126.2; 129.5; 130.9; 132.0; 140.1;
150.0; 166.1; 172.6; HRMS: M+ found (M+ calculated for
4.5.3. CaL-B mediated enzymatic kinetic resolution with
various acyl donors of racemic 1-(2-phenyl-4-methyl-
thiazol-5-yl)ethanol rac-2a in cyclohexane
C
17H21NO2S): 303.1297 (303.1293); ESI+-MS: m/z (%) = 304 (M+1,
To
yl)ethanol rac-2a (22.7
ester (20 L) and lipase CaL-B (1 mg) were added. The reaction
a
solution of racemic 1-(2-phenyl-4-methyl-thiazol-5-
6); 303 (M, 29); 218 (12); 217 (20); 216 (100); 215 (25); 190 (6);
118 (6); 99(5); 98 (6); 71 (5); 59 (8); 43 (7).
l
mol, 5 mg) in cyclohexane (200 L), vinyl
l
l
mixtures were shaken at 1300 rpm at room temperature. Samples
taken periodically were analyzed by HPLC similarly as described in
Section 4.5.1. Data on conversion and enantiomeric composition of
the products for the acyl donors tested are presented in Table 3.
4.3.2.4. 1-(2-(4-Bromophenyl)-4-methylthiazol-5-yl)ethyl but-
anoate rac-3d. Yield: 91%; liquid; 1H NMR: 300 MHz, CDCl3
d = 0.90 (t, J = 7.4 Hz, 3H); 1.56-1.66 (m, 5H); 2.27 (t, J = 7.4 Hz,
2H); 2.46 (s, 3H); 6.17 (q, J = 6.6 Hz, 1H); 7.51 (d, J = 8.5 Hz, 2H);
7.74 (d, J = 8.5 Hz, 2H); 13C NMR 75 MHz, CDCl3 d = 13.5; 15.3;
18.3; 22.8; 36.1; 65.2; 124.1; 127.6; 131.9; 132.4; 133.0; 150.4;
164.4; 172.5; HRMS: M+ found (M+ calculated for C16H18BrNO2S):
367.0246 (367.0241); ESI+-MS: m/z (%) = 370 (M+1, 81Br, 7); 369
(M, 81Br, 40); 368 (M+1, 79Br, 7); 367 (M, 79Br, 40); 256 (6); 254
(6); 89 (32); 77 (20); 71 (12); 59 (12); 43 (15).
4.5.4. Enzymatic alcoholysis of racemic ester rac-3a and
(R)-3a–d
To a solution of rac-3a (17.3 lmol, 5 mg) in DIPE (200 lL), CaL-A
(10 mg) and alcohol (10 equiv) were added. The reaction mixture
was shaken at 1300 rpm at room temperature. Samples were ana-
lyzed by HPLC as described in Section 4.5.1. The enzymatic alcohol-
ysis of the enantiopure esters (R)-3a-d was performed in a similar
manner. Data on conversion and enantiomeric composition of the
products are presented in Table 4.
4.4. Asymmetric bioreduction of 1-(2-phenyl-4-methyl-thiazol-
5-yl)ethanone 1a
4.4.1. Analytical scale non-fermenting bioreduction of 1a
Baker’s yeast (2.5 g) was suspended in water (25 mL). After stir-
ring for 15 min, 1-(2-phenyl-4-methyl-thiazol-5-yl)-ethanone 1a
(10 mg) dissolved in ethanol (0.5 mL) was added into the resulting
cell suspension. Samples (3 mL) were taken periodically every 24 h
over 8 days and extracted with ethyl acetate (9 mL). The organic
layer was dried over anhydrous Na2SO4 and used for HPLC analysis
without further purification.
4.6. Preparative scale enzyme mediated biotransformation
of 1-(2-aryl-4-methyl-thiazol-5-yl)ethanols rac-2a–d and
their enantiopure butanoates (R)-3a–d
4.6.1. Preparative scale enzymatic acylation of racemic alcohols
rac-2a–d
Into a mixture of racemic 1-(2-aryl-4-methyl-thiazol-5-yl)etha-
nol (rac-2a-d, 100 mg) in cyclohexane (4 mL), vinyl butanoate
(7 equiv) and lipase CaL-B (20 mg) were added. The reaction mix-
ture was shaken at 1300 rpm at room temperature. The reactions
were monitored by HPLC and were stopped at approx. 50% conver-
sion. The enzyme was removed by filtration. Solvents were re-
moved by rotatory evaporation and the crude product was
purified by column chromatography on silica gel using a mixture
of dichloromethane/acetone 25:1 v/v as eluent, resulting in enanti-
omerically pure (S)-1-(2-aryl-4-methyl-thiazol-5-yl)ethanols (S)-
2a–d and (R)-1-(2-aryl-4-methyl-thiazol-5-yl)ethyl butanoates
(R)-3a–d.
4.4.2. Analytical scale fermenting bioreduction of 1a
Baker’s yeast (2.5 g) and sucrose (1 g) were added to water
(25 mL). After stirring for 15 min, 1-(2-phenyl-4-methyl-thiazol-
5-yl)ethanone 1a (10 mg) dissolved in ethanol (0.5 mL) was added
into the resulting cell suspension. Further experiments were per-
formed as described in the previous section.
4.5. Analytical scale enzyme mediated biotransformation of
1-(2-aryl-4-methyl-thiazol-5-yl)ethanols rac-2a–d and their
esters rac-3a–d
4.6.2. Preparative scale enzymatic methanolysis of (R)-
butanoates (R)-3a–d
4.5.1. Enzymatic kinetic resolution of racemic 1-(2-phenyl-
4-methyl-thiazol-5-yl)ethanol rac-2a with vinyl acetate and
different lipases
To a solution of (R)-3a–d (100 mg) in diisopropyl ether (4 mL),
CaL-A CLEA (200 mg) and methanol (10 equiv) were added. The
reaction mixture was shaken at 1300 rpm at room temperature.
When (R)-3a–d were completely transformed into the correspond-
ing alcohols (R)-2a–d (HPLC analysis), the enzyme was removed by
filtration. Solvents were removed by rotatory evaporation and the
crude product was purified by column chromatography on silicagel
using a mixture of dichloromethane/acetone 25:1 v/v as eluent,
resulting in enantiomerically pure (R)-1-(2-aryl-4-methyl-thia-
zol-5-yl)ethanols (R)-2a–d.
To
a
solution of racemic 1-(2-phenyl-4-methyl-thiazol-5-
mol, 5 mg) in vinyl acetate (200 L) lipase
yl)ethanol rac-2a (22.7
l
l
(10 mg) was added. The reaction mixture was shaken at 1300 rpm
at room temperature. For HPLC analysis, samples were taken from
the reaction mixture (20 lL), diluted to 2000 lL with a mixture of
n-hexane and 2-propanol (1:1 v/v) and filtered before injection.
Data on conversion and enantiomeric composition of the products
for the lipases tested are presented in Table 1.

˘
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D. Hapau et al. / Tetrahedron: Asymmetry 22 (2011) 2165–2171
5. Aliabadi, A.; Shamsa, F.; Ostad, S. N.; Emami, S.; Shafiee, A.; Davoodi, J.;
Foroumadi, A. Eur. J. Med. Chem. 2010, 45, 5384–5389.
MS and NMR spectra of the optically active compounds were
indistinguishable from those of their racemates.
6. Sierra, M. L.; Beneton, V.; Boullay, A.-B.; Boyer, T.; Brewster, A. G.; Donche, F.;
Forest, M.-C.; Fouchet, M.-H.; Gellibert, F. J.; Grillot, D. A.; Lambert, M. H.;
Laroze, A.; Le Grumelec, C.; Linget, J.-M.; Montana, V. G.; Nguyen, V.-L.;
Nicodeme, E.; Patel, V.; Penfornis, A.; Pineau, O.; Pohin, D.; Potvain, F.; Poulain,
G.; Ruault, C.; Saunders, M.; Toum, J.; Xu, H. E.; Xu, R. X.; Pianetti, P. M. J. Med.
Chem. 2007, 50, 685–695.
4.7. Preparation of the Mosher’s esters
A
solution of (R)-MTPA (0.064 mmol, 15 mg) and DCC
7. Breuer, M.; Ditrich, K.; Habicher, T.; Hauer, B.; Keßeler, M.; Stürmer, R.;
Zelinski, T. Angew. Chem., Int. Ed. 2004, 43, 788–824.
(0.077 mmol, 15.7 mg) in anhydrous dichloromethane (2 mL) was
stirred for 30 min followed by addition of (S)-2b (0.128 mmol,
29.87 mg) and DMAP (1.5 mg). The reaction mixture was stirred
for 17 h at room temperature. The solvent was further removed
by rotatory evaporation, and the product was purified by column
chromatography on silicagel, using a mixture of dichlorometh-
ane/acetone 25:1 (v/v) as eluent. The (S)-MTPA-(S)-2b diastereo-
mer was prepared by a similar procedure.
8. Bornscheuer, U. T.; Kazlauskas, R. J. Hydrolases in Organic Synthesis: Regio- and
Stereoselective Biotransformations; Wiley-VCH: Weinheim-New York, 2006;
Bencze, L. C.; Paizs, C.; Toßsa, M. I.; Irimie, F. D. Tetrahedron: Asymmetry 2011, 22,
675–683; Brem, J.; Liljeblad, A.; Paizs, C.; Tosßa, M. I.; Irimie, F. D.; Kanerva, L. T.
Tetrahedron: Asymmetry 2011, 22, 315–322; Toßsa, M. I.; Pilbák, S.; Moldovan, P.;
Paizs, C.; Szatzker, G.; Szakács, G.; Novák, L.; Irimie, F. D.; Poppe, L. Tetrahedron:
Asymmetry 2008, 19, 1844–1852; Paizs, C.; Tosßa, M. I.; Bódai, V.; Szakács, G.;
Kmecz, I.; Simándi, B.; Majdik, C.; Novák, L.; Irimie, F. D.; Poppe, L. Tetrahedron:
Asymmetry 2003, 14, 1943–1949; Brem, J.; Paizs, C.; Toßsa, M. I.; Vass, E.; Irimie,
F. D. Tetrahedron: Asymmetry 2009, 20, 489–496.
9. Podea, P. V.; Paizs, C.; Tosßa, M. I.; Irimie, F. D. Tetrahedron: Asymmetry 2008, 19,
1959–1964; Paizs, C.; Toßsa, M. I.; Majdik, C.; Moldovan, P.; Novák, L.; Kolonits,
P.; Marcovici, A.; Irimie, F. D.; Poppe, L. Tetrahedron: Asymmetry 2003, 14, 1495–
1501.
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