Kinetic resolution of mandelate esters via stereoselective acylation catalyzed by lipase PS-30

Article abstract of DOI:10.1016/j.tetlet.2014.02.095

By using lipase PS-30 as catalyst, the kinetic resolution of a series of racemic mandelate esters has been achieved via stereoselective acylation. The value of kinetic enantiomeric ratio (E) reached up to 197.5. Substituent effect is briefly discussed.

Full text of DOI:10.1016/j.tetlet.2014.02.095

Tetrahedron Letters 55 (2014) 2290–2294
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Tetrahedron Letters
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Kinetic resolution of mandelate esters via stereoselective acylation
catalyzed by lipase PS-30
a
Peiran Chen a,b, , Wenhong Yang
⇑
a Key Lab of Eco-Textile, The Ministry of Education, Donghua University, 2999 Renmin Road North, Songjiang District, Shanghai 201620, China
b Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute of Organic Chemistry, CAS, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
By using lipase PS-30 as catalyst, the kinetic resolution of a series of racemic mandelate esters has been
achieved via stereoselective acylation. The value of kinetic enantiomeric ratio (E) reached up to 197.5.
Substituent effect is briefly discussed.
Received 17 January 2014
Revised 17 February 2014
Accepted 24 February 2014
Available online 3 March 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Mandelate esters
Lipase PS-30
Kinetic resolution
Stereoselective acylation
Kinetic enantiomeric ratio (E)
Enantiopure mandelate esters are one kind of the most versatile
synthetic materials as they bear a hydroxyl group and a carbonyl
group on one chiral carbon.1,2 They provide a wide space for trans-
formation into a large number of chiral molecules, especially in a
wide range of pharmaceuticals and agrochemicals, such as oxybu-
tynin, clopidogrel, and cefadole.3–7 Mandelic acid is also used as a
chiral resolving agent.8–10 Formation of these chiral building blocks
or their analogues can roughly be divided into three major
categories: (1) kinetic resolution (KR) or dynamic kinetic resolu-
tion (DKR) of the racemic substrates promoted by enzymatic
and/or metallic catalysts (the best E value was up to 1892);11–29
(2) enantioselective reduction of the keto group of keto acid esters
by chiral boranes30–32 with the help of chiral auxiliaries,33–35 by
chiral metal catalysts, 36–49NADH50–52 or bio-catalysts;53–60 and
(3) Alpha-carbon oxidative hydroxylation of phenyl acetates.61–64
In the bio-catalyzed KR, a mandelate ester racemate is treated
by an acylating reagent in the presence of a lipase or an esterase
to stereoselectively obtain an acylated enantiomer and an
unreacted enantiomer,64–66 or alternatively, an acylated mandelate
ester racemate is stereoselectively hydrolyzed in the presence of a
lipase or an esterase to give a hydrolyzed enantiomer and an unre-
acted enantiomer.65,67–70 In ideal case, both of the enantiomers can
be obtained simultaneously in high enantiopurity with a yield
close to 50%, which is usually not good for final product but might
be good in the case where both of the resolved enantiomers are
useful as starting material in synthesis.
Following our previous investigation in the kinetic resolution of
cyanohydrins, allylic alcohol, and propargylic alcohols catalyzed by
lipases,71–74 in this Letter, we wish to report our study on kinetic
resolution of racemic mandelate esters.
O
NaO
N
O
OCH3
N
N N
Cl
O
N
N
O
OH
S
N
N
H
H
CH3
S
O
H
S
H
H
H
O
clopidogrel
hydrogensulfate
O
oxybutynin
cefadole
⇑
Corresponding author. Tel.: +86 21 67792594.
E-mail address: prchen@dhu.edu.cn (P. Chen).
http://dx.doi.org/10.1016/j.tetlet.2014.02.095
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

P. Chen, W. Yang / Tetrahedron Letters 55 (2014) 2290–2294
2291
Table 1
Stereoselective acetylation of methyl mandelates 1a–q with vinyl acetate catalyzed by lipase PS-30
lipase PS-30
OH
OH
OAc
vinyl acetate
OR'
OR'
OR'
+
R
R
R
O
O
Toluene, 25°C
O
Rac-1a~q
2a~q
a
b
Entry
1
Substrate
EeP
%
EeS
%
Ec
Cd
%
Ce HPLC %
Reaction time (day)
OH
OMe
OMe
90.1
93.4
90.0
59.0
41
50
6
6
O
1a
OH
2
89.8
89.9
42
49.0
O
H C
3
1b
CH3 OH
OMe
OMe
OMe
OMe
3
4
5
16.5
93.1
92.3
11.0
18.2
39.5
1.5
33.5
36.8
20
34
35
40.0
16.4
30.0
7
6
7
O
1c
Cl OH
O
1d
OH
Cl
O
1e
OH
6
7
8
9
93.8
94.5
96.1
/
99.6
65.8
70.8
/
192.5
70.1
106.9
/
47
40
42
/
51.5
41.0
42.4
/
6
5
5
/
O
Cl
Br
1f
OH
OMe
OMe
O
1g
OH
O
F
F
1h
F
OH
OMe
OMe
O
1i
OH
O
10
97.1
84.7
184.1
47
46.6
7
F
1j
(continued on next page)

2292
P. Chen, W. Yang / Tetrahedron Letters 55 (2014) 2290–2294
Table 1 (continued)
a
b
Entry
Substrate
EeP
%
EeS
%
Ec
Cd
%
Ce HPLC %
Reaction time (day)
F
OH
OMe
OMe
OMe
OMe
11
12
13
/
/
/
/
/
/
O
F
1k
OH
97.1
63.6
34.2
93.7
95.0
42
37
74.0
59.6
6
5
O
MeO
1l
OH
15.0
50.2
O
n-PrO
1m
OH
F C
3
O
14
96.0
2.5
39
2.5
5
CF
3
1n
O
OMe
OMe
15
16
93.8
96.4
39.2
94.3
57.8
40
45
70.5
49.4
4
5
OH
1o
OH
197.5
O
1p
OH
OEt
17
92.7
6.1
28.0
39
6.2
7
O
1q
a
ee(P) stands for enantiomeric excess of methyl mandelate of the fast reacting enantiomer of the methyl mandelate. Analysis was performed on Chiralcel OJ-H or OD-H
column with hexane/iPrOH in varying ratios to afford ee values.
b
ee(S) stands for enantiomeric excess of the slow reacting enantiomer of the methyl mandelate which was obtained after the methyl mandelate was converted into the
corresponding acetate then subjected to chiral HPLC analysis on Chiralcel OD-H column with hexane/i-PrOH in varying ratios.
c
E = ln[(1 À CHPLC)(1 À eeS)]/ln[(1 À CHPLC)(1 + eeS)], as defined in Ref. 75.
d
Determined by isolated yield of 2.
e
CHPLC = eeS/(eeP + eeS), as defined in Ref. 76.
In optimization of the reaction conditions, we chose LP-AK, LP-
AS, and PS-30 due to their high efficiency and good accessibility.
Screening using racemic ethyl mandelate as a model compound
showed that lipase PS-30 was the best one among the three, which
could give a value of kinetic enantiomeric ratio (E) up to 28.0 at
39% conversion. Therefore, lipase PS-30 was used throughout this
study.
For the acyl reagent, vinyl acetate and vinyl butyrate were
tested and vinyl acetate gave higher E value (up to 83.1 at 45% con-
version after 7 days), while using vinyl butyrate as the acyl donor,
the E value was only 1.7. Therefore, we used vinyl acetate as the
acylating reagent in the following study.
Various esters formed by mandelic acid and different alcohols
were tested. Higher enantioselectivity (E = 59) at 41% conversion
could be obtained with methyl mandelate as the substrate. The ki-
netic resolution of the mandelate esters with alkoxy groups larger
than ethoxy could not proceed.
Therefore, we could establish our reaction conditions: Race-
mic methyl mandelates were treated with vinyl acetate in the
presence of lipase PS-30 (the immobilized form of Burkholderia
cepacia lipase, purchased from Sigma–Aldrich) and 4A molecu-
lar sieves in toluene at 25 °C to afford O-acetyl (S)-methyl
mandelate and the unreacted (R)-methyl mandelate. The reac-
tion was stopped and worked up for enantiomeric excess (ee)
value evaluation. From the ee values, the values of kinetic
enantiomeric ratio (E), which is defined in Ref. 75,76, were
calculated.
Table 1 summarizes our results. Some substrates shown in
Table 1 (compounds shown in entries 4, 7, 10, 13, and 15) are for
the first time subjected to enzymatic KR. The E values of ten
substrates (entries 1, 2, 6–8, 10, 12, 15–17) are larger than 50.
According to Schneider et al.,67 value of E > 50 would be sufficient
for production of the desired enantiomer in good yield and
enantiomeric purity.

P. Chen, W. Yang / Tetrahedron Letters 55 (2014) 2290–2294
2293
Table 2
For discussion of data shown in Table 1, we take 1a (entry 1,
E = 59.0) as the parent compound for comparison with the others.
We can see the phenomena as follows:
Configuration assignment of 2a, 2h, 2o, 2q, 3b, 3d, and 3p by comparison with the
reported [ data
a
]
D
Compound
Measured values
Reported values
Ref.
[
a
]
ee (%)
90.1
[
a
]
ee (%)
D
D
R
3
R
R
COOMe
OH
2a
2h
2o
2q
3b
3d
3p
+100.04
(c 0.75, CHCl3)
+115.7
+124.6
(c 0.82, CHCl3)
+101.6
(c 1.09, CHCl3)
-7.5
(c 1.3, CHCl3)
+124.6
71 (S)
16
78
79
86
80
81
82
2
n
96.1
93.8
92.4
89.8
18.2
94.3
100 (S)
>99 (S)
71 (S)
90 (R)
62 (R)
99 (R)
1
(c 0.98, CHCl3)
À6.21
(c 0.5, CHCl3)
+110.4
(1) Compounds 1b, 1f–1h, 1j, and 1l (entries 2, 6–8, 10 and 12,
E = 70.1 to 192.5), which bear a substituent of moderate size
on meta- or para-position of the phenyl group (n = 0, R3 = H,
R1 or R2 with moderate size), gave higher E values, while
compounds 1m (entry 13, E = 15.0) and 1n (entry 14, no
reaction), which bear a substituent of larger size on para-
position (n = 0, R2 = R3 = H, R1 with large size), gave signifi-
cantly lower E values. This fact implies that size of substitu-
ent (i.e. the size of R group) has an influence on the
interaction of the substrate with the enzyme.
(2) When the size of substituents on the phenyl group is small
enough, such as F and Br (whose atomic radius is 42 and
94 pm, respectively, while that of H is 53 pm),77 steric hin-
drance would not be a key factor for the interaction of the
substrates and the enzyme, while electron-withdrawing
effect would make some positive contribution to the E val-
ues (entries 6–8, and 10, E = 70.1 to 192.5, while in entry
1, E = 59.0). However, no reaction occurred when the man-
delate substrates have a fluorine substituent on ortho-posi-
(c 0.97, CHCl3)
À93.1
(c 0.82, CHCl3)
À82.6
(c 0.74, CHCl3)
À57.2
(c 1.00, CHCl3)
À45
(c 0.59, CHCl3)
À23.6
(c 4.0, CHCl3)
À28.7
(c 1.07, CHCl3)
(c 1.61, CHCl3)
OH
OH
M
M
L
L
unfavored enentiomert
favored enantiomer
L= large size group; in this paper, L=aryl group
M= middle size groip; in this paper, M= alkynyl group
Figure 2. A schematic presentation of Kazlauskas’ rule.
tion (n = 0, R3 = F) (entries
attributed to that six-membered ring intramolecular
hydrogen-bond structure was formed between F and the
-hydroxyl group of the methyl mandelate (Fig. 1), which
9 and 11). This might be
a
a
inhibited the acylation of the hydroxyl group.
(3) Substitution on ortho-position of the phenyl group (n = 0,
R1 = R2 = H, R3 with large size) (entry 3, compound 1c,
E = 1.5) causes stereo-hindrance to the hydroxyl group, lead-
ing to dramatical decrease of E values.
Configuration of the other products (2c, 2e, 2f, 2g, 2j, 2l, 2m,
2n), where the aryl groups stand for the large size group and the
ester groups stand for the middle size group, was proposed to have
the same designation as the known ones based on Kazlauskas’ rule
(Fig. 2).83,84
In summary, we have achieved the KR of seventeen racemic
methyl mandelic esters via stereoselective acylation using lipase
PS-30 as the catalyst. Majority of the substrates gave values of E
close to or larger than 50, the best one achieves 197.5.
(4) In compounds 1p and 1q (n = 1 and 2, R1 = R2 = R3 = H, R1)
(entries 16 and 17, E = 57.8 and 197.5, respectively), the phe-
nyl group is relatively far away from the hydroxyl group,
leading to the decrease of stereo-hindrance around the
hydroxyl group, which is in favor of the interaction of the
substrate with the enzyme thus giving the best resolution.
(5) Ethyl mandelate (entry 18, compound 1r) gave lower E value
(28.0) than that of methyl mandelate (entry 1, E = 59.0). No
reaction occurred when the mandelic esters have alkoxyl
groups larger than ethoxy, therefore they could not be used
as the substrate of this enzymatic kinetic resolution reaction.
Acknowledgments
This work is financially supported by a grant of The National
Natural Science Foundation of China (NSFC, Grant No. 21272305),
The Natural Science Foundation of Shanghai Municipality (Grant
No. 12ZR1400200) and The Fundamental Research Funds for the
Central Universities.
Configuration of some acylated products (2a, 2h, 2o, and 2q)
and the unreacted alcohols (3b, 3d, and 3p) is further confirmed
by comparison of the observed optical rotations with those re-
ported in the literatures (Table 2). Some other products have been
reported in the references (2b–f, 2q in Ref. 85; 2l, 2p in Ref. 13,
Supplementary data
respectively), but their [
sponding references.
a
]
D
data could not be found in the corre-
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.02.
095.
H
O
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