Highly enantioselective biomimetic intramolecular dehydration: Kinetic resolution of β-hydroxy ketones catalyzed by β-turn tetrapeptides

Article abstract of DOI:10.1016/j.tetlet.2013.03.087

Racemic β-hydroxy ketones were kinetically resoluted into the enantiopure isomers and (E)-α,β-unsaturated ketones using catalytic asymmetric intramolecular dehydration for the first time. Synthetic tetrapeptides were used to imitate fatty acid dehydratase

Full text of DOI:10.1016/j.tetlet.2013.03.087

Tetrahedron Letters 54 (2013) 2828–2832
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Highly enantioselective biomimetic intramolecular dehydration: kinetic
resolution of b-hydroxy ketones catalyzed by b-turn tetrapeptides
Zhi-Xue Du a, Li-Yuan Zhang a, Xin-Yuan Fan a, Feng-Chun Wu a, Chao-Shan Da a,b,c,
⇑
a Institute of Biochemistry and Molecular Biology, School of Life Sciences, Lanzhou University, Lanzhou 730000, China
b State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
c Key Lab of Preclinical Study for New Drugs of Gansu Province, Lanzhou University, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Racemic b-hydroxy ketones were kinetically resoluted into the enantiopure isomers and (E)-a,b-unsatu-
Received 15 December 2012
Revised 11 March 2013
Accepted 19 March 2013
Available online 27 March 2013
rated ketones using catalytic asymmetric intramolecular dehydration for the first time. Synthetic tetra-
peptides were used to imitate fatty acid dehydratases to efficiently discriminate racemic b-hydroxy
ketones, enantioselectively catalyze the intramolecular dehydration, and result in highly enantioenriched
b-hydroxy and (E)-a,b-unsaturated ketones in the environmentally benign process. Mechanistically, the
high discrimination of the racemic substrates and successive enantioselective dehydration are highly
dependent on the cooperative catalysis of the NH2 and COOH groups of the peptide.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Asymmetric catalysis
Biomimetic course
Dehydration
Peptide
Kinetic resolution
It is very interesting to imitate enzymes in cells with elaborate-
designed synthetic peptides as catalysts.1 And actually varieties of
synthetic peptides have been developed to effectively discriminate
two optically opposite isomers and kinetically resolute them to get
useful enantiomers with very good results in the cases of alcohols
and amines.2 The dehydratase only catalyzes intramolecular dehy-
tion of racemic b-hydroxy ketones. This protocol results in (R)-b-
hydroxy ketones with the highest 97% ee and 33 s values.
We have successfully shown that, in an analogy to aldases,7 N-
terminal primary amine based tetrapeptides with stable b-turn
secondary structure (Figure 1) are efficient organocatalysts for
the asymmetric aldol reactions. We found that the aldols could un-
dration of the
rated carbonyls and leave
the synthesis of fatty acids, whereas the hydratase catalyze the
hydration of (E)- ,b-unsaturated carbonyls to generate -b-hydro-
L
-b-hydroxy carbonyls to produce (E)-
a
,b-unsatu-
dergo dehydration to afford a,b-unsaturated ketones in the pro-
D
-b-hydroxy carbonyls untouched in
cess.8 This phenomenon denoted the possibility of the chiral
discrimination of the peptides in the catalytic dehydration of the
racemic aldols. In this work we intended to explore their chiral dis-
crimination and catalytic asymmetric intramolecular dehydration
in the biomimetic process. Dehydration of racemic 1 (Eq. 1) was
used to be the model reaction for condition optimization.
a
D
xy carbonyls in the degradation of fatty acids.3 We are greatly
interested in imitating the dehydratase to catalyze the racemic b-
hydroxy carbonyls to intramolecularly dehydrate to produce their
enantioenriched isomers. Such works will be considerably helpful
for people to understand the mechanism of the dehydration from
the enzymes and to utilize them to produce enantioenriched b-hy-
droxy carbonyls. To our knowledge, such a biomimetic catalytic
course has not been disclosed as yet. With racemic b-hydroxy ke-
tones in hands, the reported methods to access enantiomers com-
prised the routine kinetic resolution4 with asymmetric acylation of
the alcohols5 and the retro-Aldol reaction of the substrates with
antibody.6 Herein we report our preliminary results on this biomi-
metic protocol with tetrapeptide-catalyzed asymmetric dehydra-
We first screened the tetrapeptides to select the optimistic
organocatalyst out of the peptides listed in Figure 1 in view of s
as well as ee value. As can be seen from Table 1, PEP-1 shows
the highest s value with 56% ee. The other three tetrapeptides all
O
H
H
N
PEP-1: Val-D-Pro-Gly-Leu-OH (R1=i-Pr, R2=i-Bu)
PEP-2: Plg-D-Pro-Gly-Leu-OH (R1=Ph, R2=i-Bu)
PEP-3: Chg-D-Pro-Gly-Leu-OH (R1=c-Hex, R2=i-Bu)
PEP-4: Plg-D-Pro-Gly-Phe-OH (R1=Ph, R2=Bn)
N
H
H N
O
R1
O
R2
+
−O
NH3
O
⇑
Corresponding author. Tel./fax: +86 931 891 5208.
E-mail address: dachaoshan@lzu.edu.cn (C.-S. Da).
Figure 1. Tetrapeptides for enantioselective dehydration.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.087

Z.-X. Du et al. / Tetrahedron Letters 54 (2013) 2828–2832
2829
Table 1
Selection of the optimal tetrapeptidea
O
OH
O
OH
O
tetrapeptide (20 mmol%)
oC, MeOH
+
Eq. 1
0
+
( )-1
1b
−
1a
Entry
Peptide
Cb (%)
eec (%)
sd
1
2
3
4
PEP-1
PEP-2
PEP-3
PEP-4
46
53
60
54
56
59
69
55
8.4
5.7
5.3
4.7
a
20% Benzoic acid was used as an additive, and the reaction was lasted for about 24 h.
The conversion was determined by GC analysis of the crude reaction mixture.
b
c
Determined with chiral HPLC. The configuration was determined in comparison of the retention time of the major isomer on chiral HPLC with the reported result.
d
s means selectivity factor, s = [ln(1 À C)(1 À ee)]/[ln(1 À C)(1 + ee)].4
show low s value. Therefore, we selected PEP-1 to be the ideal
organocatalyst to imitate the fatty acid dehydratase.
strates with the acetonic moiety result in P90% ee. The highest
enantioselectivity is up to 97%. Either with electron-withdrawing
or electron-donating functional groups in the phenyl ring, racemic
b-hydroxy ketones can be selectively dehydrated and result in sat-
isfied ee (P90%). Even the substrate with a remote phenyl group
(entry 15) or a heterocyclic group (entry 16), ee is still very high
(97% and 95%, respectively). Normally in the peptide-catalytic
asymmetric aldol reaction, however, aryl aldehydes with elec-
tron-donating groups and alkyl aldehydes are difficult substrates
from acetone with lower ee and much poorer yield.9 This biomi-
metic process provides another usable protocol to highly enantio-
pure b-hydroxy ketones from acetone, especially to those which
are not easy to be obtained from the organocatalytic asymmetric
aldol reaction. To the most substrates, the s values are higher than
10 except for four cases (entries 8, 9, 12, and 16). Three cases give s
values more than 21 (entries 3, 4, and 10). The highest s value, up
to 33, is achieved with 3a. To the cyclic substrates (entries 17 and
19), the ee and s values are both only moderate. The substrate of 17
with two diastereoisomers showed a little decrease in the syn/anti
from 1/1 to 1/1.7 after 72 h (entry 23). This shows that PEP-1 has
high specificity of substrates from acetone; the other substrates
derived from other ketones are very difficult to be dehydrated (en-
We then started to optimize the other reaction conditions (Ta-
ble 2). The achiral additives were firstly observed and benzoic acid
was assured to be the optimal additive (entries 1–3). Secondly, the
reaction temperature was investigated and 0 °C was determined to
be optimal to ee and s values (entries 1 and 4–5). Thirdly, the sol-
vents were extensively screened, but the reaction hardly proceeds
in most of them (entries 1 and 6–12). To our gratified, EtOH, as a
green solvent, is the second solvent in which the reaction can flu-
ently proceed. It results in the highest 91% ee and s (12.3) value,
much more optimal than MeOH. The result makes the reaction pro-
ceed under a green condition. The fact that the peptides are hard to
dissolve in EtOH encouraged us to successfully reduce the load of
PEP-1 to 5% (entries 12–15). Finally we tried to decrease the load
of benzoic acid, but the experimental results did not support this
effort (entries 15–19).
With the ideal reaction conditions, a series of racemic sub-
strates were experimented. As shown in Table 3, all of the sub-
Table 2
Optimization of reaction conditionsa
tries 21 and 22). The
a,b-unsaturated ketones are all E conforma-
Entry
PEP-1
Solvent
Additive (%)
Cb (%)
eec (%)
sd
tion. It is notable that (E)-a
,b-unsaturated ketones as the
products of the dehydration are not merely useless side-products,
they are usable synthetic intermediates for numerous organic syn-
thesis and they are actually the purposed products in the original
biochemical course.3 The great difference in the polarity between
the two products makes them easily separated and obtained using
column chromatography with silica gel. This result remarkably re-
duces the reaction waste and thus further enhances the green of
the process.
To explore the mechanism of this process, we synthesized N-
Boc-PEP-1 and PEP-1-OMe to catalyze the dehydration (Eq. 1)
with no addition of benzoic acid due to avoidance of the complex-
ity. The results (Table 4) show that N-Boc-PEP-1 and PEP-1-OMe
are both unable to catalyze the dehydration under the optimized
reaction conditions at all, but PEP-1 promotes the reaction with
moderate ee and s values on the same conditions. This result
clearly indicates that the chiral discrimination and dehydration
are seriously dependent upon the cooperative catalysis of the
bifunctional groups of the tetrapeptide.
1
2
20%
20%
20%
20%
20%
20%
20%
20%
20%
20%
20%
20%
3%
MeOH
MeOH
MeOH
MeOH
MeOH
DMF
DMSO
DCE
MeCN
H2O
Brine
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
PhCOOH (20)
p-Cl-PhCOOH (20)
1-NapCOOH (20)
PhCOOH (20)
PhCOOH (20)
PhCOOH (20)
PhCOOH (20)
PhCOOH (20)
PhCOOH (20)
PhCOOH (20)
PhCOOH (20)
PhCOOH (20)
PhCOOH (20)
PhCOOH (20)
PhCOOH (20)
PhCOOH (0)
46
58
69
58
55
—
—
—
—
—
56
60
70
44
69
—
—
—
—
—
8.4
4.5
3.7
4.0
7.2
—
—
—
—
—
3
4e
5f
6
7
8
9
10
11
12
13
14
15
16
17
18
19
—
—
—
60
66
61
67
47
51
43
45
91
91
90
92
53
61
48
52
12.3
8.1
10.8
8.0
6.6
7.0
7.2
7.4
5%
20%
5%
5%
5%
PhCOOH (5)
PhCOOH (8)
PhCOOH (10)
5%
a
The model reaction is Eq. 1. The load of PEP-1 and the additive is mmol %
equivalent to the substrate.
The conversion was determined by GC (entries 1–5, 12) or 1H NMR (entries 13–
b
Zeitler and co-workers observed a direct aldol condensation in
aid of in situ NMR technology.10 They found that aldol condensa-
tion was more pronounced than aldol addition while using a high
loading catalyst. This report would denote the possibility that race-
mic aldols underwent an enantioselective retro-aldol reaction
rather than the intramolecular dehydration to allow accessing ki-
19) of the crude products.
c
Determined by chiral HPLC. The configuration was determined in comparison of
the retention time of the major isomer on chiral HPLC with the reported result.
s = [ln(1 À C)(1 À ee)]/[ln(1 À C)(1 + ee)].4
d
e
The reaction temperature is rt.
f
The reaction temperature is À20 °C.

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Z.-X. Du et al. / Tetrahedron Letters 54 (2013) 2828–2832
Table 3
The biomimetic stereoselective dehydration of racemic b-hydroxy ketones
OH
O
OH
O
O
5 mmol% PEP-1, 20 mmol% PhCOOH
EtOH, 0 oC
Eq. 2
+
R1
R1
R1
R2 R3
1 ~ 23
R2 R3
1a ~ 23a
R2 R3
1b ~ 23b
Entry
R1
R2, R3
Time (h)
Ca (%)
Yieldb (a, %)
Yieldb (b, %)
eec (%)
sd
1
2
3
4
5
6
7
8
Ph
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
H,H
–(CH2)2–
–(CH2)2–
–(CH2)2–
–(CH2)2–
H,iPr
Me,H
–(CH2)2–
60
141
160
100
80
72
60
100
90
60
59
57
52
55
60
58
60
66
71
55
62
68
63
58
68
69
49
89
65
90
—
41
43
48
45
40
42
40
34
29
45
38
32
37
42
32
31
51
11
35
10
—
50
51
22
49
51
50
53
59
63
48
55
60
55
50
56
54
38
65
53
60
—
94
92
90
92
96
93
91
91
93
91
93
93
94
91
97
95
52
16.2
17.6
33
22.7
17
4-NO2–C6H4
2-NO2–C6H4
3-NO2–C6H4
4-F–C6H4
4-Br–C6H4
4-Cl–C6H4
1-Napth
16.8
12.3
8.1
6.7
21.2
11.7
7.9
11.5
14.9
10.1
8.3
5.5
3
9
2-Napth
10
11
12
13
14
15
16
17e
18e
19e
20e
21
22f
23g
2-MeO–C6H4
3-MeO–C6H4
4-Me–C6H4
2-F3C–C6H4
4-F3C–C6H4
C6H4C2H4
2-Furan
2-Napth
2-Napth
3-NO2-C6H4
3-NO2–C6H4
4-NO2–C6H4
2-NO2–C6H4
2-Napth
72
60
160
110
110
48
168
300
168
300
154
154
72
92
35
73
—
2
2.1
—
—
1.2
—
21
—
79
—
7
—
12(1:1.7)
a
b
C
The conversion was calculated by (100%—yield of 1a).
Isolated yield.
ee of the chiral alcohol was determined with chiral HPLC, and the configuration was determined in comparison of the retention time of the major isomer with the
reported result on chiral HPLC.
d
s = [ln(1 À C)(1 À ee)]/[ln(1 À C)(1 + ee)].4
e
The syn isomer was used as the substrate. The reaction was performed at room temperature.
f
The anti-isomer was used as the substrate.
The racemic substrate of syn/anti = 1/1. The displayed values are data of syn isomer. The value in the parentheses is syn/anti, and ee of the anti isomer is 3%. The reaction
g
was performed at room temperature.
Table 4
Dehydration with the protected PEP-1a
O
COO−
O
Cb (%)
eec (%)
sd
N
NH
Entry
Peptide (mmol %)
H
O+H2
Ph
O
H
1
2
3
PEP-1 (5)
N-Boc-PEP-1 (20)
PEP-1-OMe (20)
47
—
—
53
—
—
6.6
—
—
Ph
I
II
a
O
OH
Ph
The reaction was allowed to last for 48 h before the result was analysed. Ethanol
was used as the solvent, and no additive was added. The reaction temperature was
H2O
O
+
0 °C. N-Boc-PEP-1 is Boc-Val-D-Pro-Gly-Leu-OH, and PEP-1-OMe is H2N-Val-D-Pro-
(R)-1a
Gly-Leu-OMe.
b
The conversion was determined by 1H NMR of the crude reaction mixture.
Determined with chiral HPLC.
H2O
OH
*
c
d
s = [ln(1 À C)(1 À ee)]/[ ln(1 À C)(1 + ee)].4
Ph
1
COO−
Ph
netic resolution of the racemic aldols,4 succeeded by aldol conden-
sation procedure to afford ,b-unsaturated ketones. Given the low
N+H
COOH
a
NH2
O
loading of the tetrapeptide and the slow reaction of our tetrapep-
tide-catalyzed Aldol addition reaction with excessive acetone,8
we suggested this possibility impossible in this work. This sugges-
tion was further supported by two experiments, in which 1 equiv
4-methoxybenzaldehyde or 4-nitrobenzaldehyde to 1 was addi-
tionally introduced in the general catalytic dehydration procedure,
and no signal was observed with ESI-MS spectroscopy of the reac-
tion of acetone, possibly in situ generated from the retro-Aldol
reaction, with stoichiometric 4-methoxy-benzaldehyde or 4-nitro-
benzaldehyde (Supplementary data, pages S80 and S81).
H2O
PEP-1
Ph
III
1b
Figure 2. The biomimetic dehydration process.
the amine group of PEP-1 intermolecularly dehydrates and forms
an imine intermediate I. The b-turn tetrapeptide PEP-1 effectively
discriminates the racemic 1 and forms the stable intermediate I
with (S)-1a but leaves (R)-1a untouched. The hydrogen bonds be-
tween COOH and OH make the facial choice of 1, and the two selec-
tive points’ interactions of four groups from PEP-1 and 1 make the
Based on these observations, we envisioned the reaction pro-
cess as illustrated in Figure 2. Firstly, the carbonyl group of 1 and

Z.-X. Du et al. / Tetrahedron Letters 54 (2013) 2828–2832
2831
Figure 3. ESI-MS spectrum of reaction Eq. 1.
key intermediate I stable and contribute to the effective chiral
discrimination.
Program for Changjiang Scholars and Innovative Research Team
in University (IRT1137).
Then we tried to find the intermediates of the envisioned pro-
cess with ESI-MS (Figure 3, also see Supplementary data, page
S82). The intermediate III was fortunately recorded although I
and II were not found anyway. This might be that III is much more
stable than I and II because of its relatively stable conjugated struc-
ture. The intermediate III was also reconfirmed by HR-MS in ESI
method (calcd. for [C28H40N4O5+H]+ 513.3071, found 513.3071,
see Supplementary data, page S83).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
03.087.
References and notes
In conclusion, we have successfully developed a biomimetic
process with dehydratase-like b-turn tetrapeptides for the first
time. The peptides efficiently discriminate the racemic b-hydroxy
ketones and enantioselectively catalyze the intramolecular dehy-
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Acknowledgments
We are greatly thankful to the financial support from the Na-
tional Natural Science Foundation of China (No. 21072087) and

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Z.-X. Du et al. / Tetrahedron Letters 54 (2013) 2828–2832
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