Biocatalytic domino reaction: Synthesis of 2H-1-benzopyran-2-one derivatives using alkaline protease from Bacillus licheniformis

Article abstract of DOI:10.1039/c0gc00799d

A novel BLAP (alkaline protease from Bacillus licheniformis) catalyzed synthesis of 2H-1-benzopyran-2-one derivatives was achieved by domino Knoevenagel/intramolecular transesterification reaction. The control of enzymatic chemoselectivity between Knoevenagel/intramolecular transesterification and Knoevenagel/intramolecular hemiketalization could be realized by adjusting parameters including solvent, water content and temperature. The products were obtained in acceptable yields. This BLAP catalyzed selective domino reaction provided an alternative synthetic method for 2H-1-benzopyran-2-one derivatives.

Full text of DOI:10.1039/c0gc00799d

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PAPER
Biocatalytic domino reaction: synthesis of 2H-1-benzopyran-2-one
derivatives using alkaline protease from Bacillus licheniformis†
Chang-Heng Wang,‡ Zhi Guan‡ and Yan-Hong He*
Received 11th November 2010, Accepted 3rd May 2011
DOI: 10.1039/c0gc00799d
A novel BLAP (alkaline protease from Bacillus licheniformis) catalyzed synthesis of
2H-1-benzopyran-2-one derivatives was achieved by domino Knoevenagel/intramolecular
transesterification reaction. The control of enzymatic chemoselectivity between
Knoevenagel/intramolecular transesterification and Knoevenagel/intramolecular
hemiketalization could be realized by adjusting parameters including solvent, water content and
temperature. The products were obtained in acceptable yields. This BLAP catalyzed selective
domino reaction provided an alternative synthetic method for 2H-1-benzopyran-2-one derivatives.
1
. Introduction
The synthesis of 2H-1-benzopyran-2-one and its derivatives
has engrossed substantial attention from organic and medic-
inal chemists for many years as they belong to a class of
1
compounds with proven utility in medicinal chemistry. The
nucleus of 2H-1-benzopyran-2-one were called privileged struc-
tures in biological chemistry, and numerous pharmaceuticals
2
were based on development of this basic scaffold (Fig. 1).
Fig. 1 Biologically active 2H-1-benzopyran-2-one derivatives.
Of these, warfarin was the most well-known of a class of
been paid more and more attention. Not only do enzymes
work in anhydrous organic media, but they acquire remarkable
properties such as enhanced stability, altered substrate and
enantiomeric specificities, and the ability to catalyze unusual
2H-1-benzopyran-2-one derivatives used as anticoagulants. In
principle, 2H-1-benzopyran-2-one derivatives might provide
access to compounds like warfarin and also allowed the synthesis
of a wide variety of 3-alkylcoumarins for biological screening.
However the conventional methods for the preparation of 2H-1-
9
reactions which are impossible in aqueous media. Some ele-
3
gant works on enzyme catalyzed reactions in organic solvents
benzopyran-2-one derivatives suffered from major drawbacks
10
have been reported. While a multitude of single enzyme-
catalyzed single-step transformations was known, few examples
of the single enzyme catalyzed multistep conversions have
(
drastic conditions, stoichiometric amounts of Lewis or mineral
acids, multistep protocols, troublesome work-up procedures).
Some attempts have been reported to expand the synthetic
approach to functionalized 2H-1-benzopyran-2-one derivatives
11
been described. Therefore, the development of new enzymatic
catalysts in domino reaction to prepare 2H-1-benzopyran-2-
one derivatives is not only in great demand in enzymology
and synthetic methodologies, but also of practical importance.
Herein, we wish to report a novel discovery that the readily avail-
able alkaline protease BLAP (alkaline protease from Bacillus
licheniformis 2709, EC 3.4.21.14) promotes the domino Knoeve-
nagel/intramolecular transesterification reaction to form 2H-1-
benzopyran-2-one derivatives in organic solvents.
4
5
by using heteropolyacid-catalyzed procedures, ionic liquids,
6
microwave irradiation, and the calcined Mg–Al hydrotalcite as
solid base. Yet, to the best of our knowledge, enzyme-catalyzed
7
synthesis of 2H-1-benzopyran-2-one and its derivatives, which
could be performed under facile and mild reaction conditions,
has never been reported.
Since Klibanov (re)discovered that enzymes could maintain
8
their activities in organic solvents, enzymes as biocatalysts have
2
. Results and discussion
School of Chemistry and Chemical Engineering, Southwest University,
Chongqing, 400715, (China). E-mail: heyh@swu.edu.cn;
Fax: (+86)23-68254091
It has been reported recently that protease could catalyze the
transesterification reaction for the synthesis of N-substituted
imidazole derivatives containing a glucose branch in organic
†
1
Electronic supplementary information (ESI) available: See DOI:
0.1039/c0gc00799d
11
‡
Chang-Heng Wang and Zhi Guan contributed equally to this work
solvents. Furthermore, we found that alkaline protease could
2
048 | Green Chem., 2011, 13, 2048–2054
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catalyze Knoevenagel reaction in organic solvents which was
currently in progress in our laboratory. Based on this, we spec-
ulated that the Knoevenagel reaction between salicylaldehyde
and ethyl acetoacetate catalyzed by BLAP might take place to
generate intermediate A (Scheme 1), and 2H-1-benzopyran-2-
one might be fulfilled in succession under the same conditions.
Therefore, we decided to investigate the possibility of using
BLAP as a catalyst for the synthesis of 2H-1-benzopyran-2-one
derivatives by domino Knoevenagel/intramolecular transester-
ification reaction. Initially, the condensation of salicylaldehyde
and ethyl acetoacetate catalyzed by BLAP in DMSO at r.t. was
tested, and the desired product 2H-1-benzopyran-2-one 3f was
obtained successfully.
Fig. 2 The influence of organic solvents on chemoselectivity and
yield of BLAP-catalyzed domino reaction. Conditions: salicylalde-
-1
hyde (200 mg, 1.64 mmol), BLAP (40 mg ml ), ethyl acetoacetate
(
2
4.90 mmol), deionized water (0.5 ml) and organic solvent (5 ml) at
5 C.
◦
Scheme 1 BLAP-catalyzed reaction of salicylaldehyde with ethyl
acetoacetate.
In consideration of the various advantages of using organic
9
solvents in enzymatic reactions, a solvent screen was performed
using salicylaldehyde and ethyl acetoacetate as a model reaction.
Interestingly, when BLAP was used as a catalyst in different
organic solvents, the desired product 2H-1-benzopyran-2-one 3f
was obtained, but a by-product hemiketal 4 was also observed
which was formed by domino Knoevenagel/intramolecular
hemiketalization (Scheme 1). The results of various organic
solvents were given in Fig. 2. As can be seen from Fig. 2, 2H-
1
-benzopyran-2-one 3f could be obtained in the highest yield
with the best selectivity in DMSO, while hemiketal 4 could be
received with the highest yield in DMF. It was notable that the
chemoselectivity of BLAP could be adjusted, even reversed, by
changing solvents. In DMSO and ethanol, 3f was received as the
major product, while 4 was obtained as the major product in
such solvents as DMF, acetone, THF and so on. Thus, we chose
DMSO as the optimum solvent for the preparation of 3f in the
following studies.
Fig. 3 The influence of water content on BLAP-catalysed domino
-1
reaction. Conditions: BLAP (40 mg ml ), salicylaldehyde (200 mg,
.64 mmol), ethyl acetoacetate (4.90 mmol), water+DMSO (5 ml),
1
deionized water from 0 to 50% (water/[water+DMSO], v/v) at r.t. (25–
◦
3
0 C) for 86 h.
It was known that all enzymes need essentially bound water,
and the enzymatic activity in organic solvent depends on
desired product 2H-1-benzopyran-2-one 3f could be obtained
in the highest yield with the best selectivity in DMSO at 10%
(water/[water+DMSO], v/v) water content. However, the yield
of 3f decreased evidently once the water content surpassed 10%.
When the water content reached 30%, the by-product hemiketal
4 could be formed dominately. All the results indicated that water
was obviously essential in the biocatalytic domino reaction.
The chemoselectivity could be achieved by adjusting the water
12
water content. Moreover, the water content required to reach
the maximal activity is different in different organic solvents.
Thus, the experiments were performed to ascertain the catalytic
activity of BLAP with different amounts of water in the model
reaction system. From Fig. 3, it was found that the yield of
the enzymatic reaction could be enhanced by increasing the
concentration of water, and reached the highest yield. The
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content in organic solvent. Thus, the optimum water content for
the synthesis of 2H-1-benzopyran-2-one 3f was 10%.
Temperature also plays an important role in enzyme catalyzed
reaction, due to their effects on the enzyme stability and
Next, we investigated the effects of BLAP loading and molar
ratio of substrates on the enzymatic domino reaction (Table 1).
The results showed that 20 mg ml of BLAP loading and the
molar ratio 1 : 3 of salicylaldehyde to ethyl acetoacetate were the
optimum conditions.
-
1
reaction rate. Thus, a temperature screen was performed from
◦
3
0 to 80 C using the model reaction of salicylaldehyde with
To demonstrate the specific catalytic effect of BLAP on
the domino Knoevenagel/intramolecular transesterification se-
quence under optimized conditions, we performed some control
experiments (Table 2). The reaction between salicylaldehyde and
ethyl benzoylacetate catalyzed by BLAP under the optimized
conditions gave product 3d in satisfied yield of 73% after 48 h
(Table 2, entry 1), but no product was detected in the absence of
enzyme after 48 h (Table 2, entry 2). This clearly indicated that
BLAP had specific catalytic effect on the reaction. Furthermore,
ethyl acetoacetate (Fig. 4). It was found that the yield of the
by-product hemiketal 4 was slightly higher than the desired
◦
product 2H-1-benzopyran-2-one 3f at 35 C. However, once
◦
the temperature reached 55 C, 3f could be obtained in the
highest yield with the best selectivity. The results indicated that
the chemoselectivity also could be achieved by adjusting the
temperature. Thus, the optimum temperature for the synthesis
◦
of 3f was 55 C.
2
+
since BLAP is a Ca -dependent enzyme, EDTA (ethylene
diamine tetraacetic acid) was used to denature the enzyme.
We found that the EDTA-denatured BLAP completely lost its
catalytic activity for the domino reaction (Table 2, entry 3),
which demonstrated that the catalytic activity of BLAP was not
simply caused by the amino acids of the enzyme, and the native
tertiary structure of the enzyme might also be necessary for this
catalysis. In addition, to further prove that the catalytic activity
of BLAP for the domino reaction did not arise from unspecific
protein-derived activation of the reagents by, e.g., the surface
of the enzyme. The experiment catalyzed by inhibited enzyme
was conducted, and a complete inhibition of the catalytic
activity of BLAP was observed by using serine protease inhibitor
PMSF (phenylmethanesulfonyl fluoride) (Table 2, entry 4). This
indicated that the enzyme catalyzed domino reaction occurred
in the active site of BLAP or in close proximity similar to other
13
reported enzymes. On the other hand it could be assumed that
the protein surface of BLAP was predominantly catalytically
inactive in the process. Above experiments also excluded the
possibility that the catalysis was caused by the impurities of the
enzyme.
Furthermore, in order to verify that the second step (the
intramolecular transesterification) was also catalyzed by BLAP,
we attempted to synthesize the intermediate A (Scheme 1).
Unexpectedly, we could not obtain intermediate A although
Fig. 4 The influence of temperature on BLAP-catalyzed domino
-1
reaction. Conditions: BLAP (40 mg ml ), salicylaldehyde (200 mg,
.64 mmol), ethyl acetoacetate (4.90 mmol), deionized water (0.5 ml)
and DMSO (4.5 ml) at specified temperature for 45 h.
1
a
Table 1 Effects of BLAP loading and molar ratio of salicylaldehyde to ethyl acetoacetate on the yield of 3f in the domino reaction
-1
b
Entry
1a : 2a
BLAP (mg ml )
Time [h]
Yield 3f [%]
1
2
3
4
5
6
7
8
9
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 1
1 : 2
1 : 3
1 : 4
10
20
30
40
50
20
20
20
20
112
112
112
112
112
80
80
80
80
51
58
56
55
43
47
50
58
48
a
◦
b
All reactions were carried out using salicylaldehyde (200 mg, 1.64 mmol), deionized water (0.5 ml) and DMSO (4.5 ml) at 55 C. Yield of the
isolated product after chromatography on silica gel.
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050 | Green Chem., 2011, 13, 2048–2054
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a
Table 2 Control experiments for the reaction of salicylaldehyde and ethyl benzoylacetate
b
Entry
Catalyst
Time [h]
Yield [%]
1
2
3
4
BLAP
no enzyme
BLAP denatured with EDTA
BLAP inhibited with PMSF
48
48
48
48
73
ND
ND
ND
c
d
a
All reactions were carried out using salicylaldehyde (200 mg, 1.64 mmol), ethyl benzoylacetate (4.90 mmol), deionized water (0.5 ml), DMSO (4.5 ml)
-1
◦
b
c
◦
and catalyst (20 mg ml ) at 55 C. Yield of the isolated product after chromatography on silica gel. Pre-treated with EDTA at 100 C for 24 h.
d
◦
Pre-treated with PMSF at 25 C for 24 h.
a
Table 3 Control experiments of intramolecular transesterification
b
Entry
Catalyst
Time [h]
Yield [%]
1
2
3
4
BLAP
no enzyme
BLAP denatured with EDTA
BLAP inhibited with PMSF
48
48
48
48
75
52
53
49
c
d
a
-1
◦
b
All reactions were carried out using hemiketal 4 (38 mg), deionized water (0.05 ml), DMSO (0.45 ml) and catalyst (20 mg ml ) at 55 C. Yield of
c
◦
d
◦
the isolated product after chromatography on silica gel. Pre-treated with EDTA at 100 C for 24 h. Pre-treated with PMSF at 25 C for 24 h.
we have tried many different methods, but instead hemiketal
4
was received. It was apparent that the intermediate A is
unstable and so hemiketal 4 is easily formed. Because there
is a chemical equilibrium between the intermediate A and
hemiketal 4, we used 4 to conduct the same set of control
experiments presented in Table 2 to demonstrate the cat-
alytic effect of BLAP on the intramolecular transesterification
(
Table 3). The intramolecular transesterification catalyzed by
BLAP under the optimized conditions gave product 3f in
satisfied yield of 75% after 48 h (Table 3, entry 1). In the
absence of BLAP, 3f was only obtained in a low yield of
5
2% (Table 3, entry 2). The experiments using denatured and
inhibited BLAP gave the results similar to the blank experiment
Table 3, entries 3 and 4). It is obvious that BLAP promoted
(
the intramolecular transesterification. Based on the results of
control experiments, the BLAP catalyzed domino reaction was
validated.
Fig. 5 Concentration curves of 3f, 4 and salicylaldehyde in the process
We then detected the reaction process, and the time courses
were shown in Fig. 5. A quick increase of product 3f and
by-product 4 was observed during 0–10 h. After 10 h, the
concentration of 3f increased continually, but the concentration
of 4 decreased slightly. After 50 h, the product 3f reached the
equilibrium, and the concentration of 4 and salicylaldehyde
did not continue to decrease. The results showed that the
catalytic activity of the BLAP for Knoevenagel/intramolecular
transesterification is higher than the activity for Knoeve-
of BLAP catalyzed two-step domino reaction. Conditions: BLAP
-1
(
(
20 mg ml ), salicylaldehyde (200 mg, 1.64 mmol), ethyl acetoacetate
4.90 mmol), deionized water (0.5 ml) and DMSO (4.5 ml) at 55 C. 9
◦
reactions were set up at the same time, and each reaction was terminated
at a specified time. The concentrations were calculated by isolated
weights of 3f, 4 and salicylaldehyde.
nagel/hemiketalization under the optimal conditions. It was
also confirmed that the synthesis of 2H-1-benzopyran-2-one
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a
Table 4 Substrate scope of the domino Knoevenagel/intramolecular transesterification reaction
b
Entry
1
1
R
2
Product
Prod. No.
Time [h]
48
Yield [%]
Ph
3a
3b
3c
3d
3e
3f
75
68
60
73
69
58
45
56
30
48
25
2
3
4
5
6
7
8
9
OEt
163
117
48
CH
Ph
3
OEt
163
48
CH
Ph
3
3g
3h
3i
48
OEt
117
117
48
CH
Ph
3
1
1
0
1
3j
OEt
3k
117
2
052 | Green Chem., 2011, 13, 2048–2054
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Table 4 (Contd.)
b
Entry
1
R
2
Product
Prod. No.
Time [h]
192
Yield [%]
1
1
1
2
3
4
Ph
3l
18
10
48
OEt
3m
3n
192
288
CH
3
a
All reactions were carried out using salicylaldehyde derivative (1.64 mmol), b-keto ester (4.90 mmol), deionized water (0.5 ml), DMSO (4.5 ml) and
-1
◦
b
BLAP (20 mg ml ) at 55 C. Yield of the isolated product after chromatography on silica gel.
derivatives by domino Knoevenagel/intramolecular transester-
ification could be catalyzed by BLAP.
gave much lower yield than ethyl acetoacetate (Table 4, entries
12 and 14) due to the steric hindrance.
Finally, to investigate the generality and scope of this new en-
zyme activity, some other salicylaldehyde derivatives and b-keto
esters were used to expand upon this BLAP catalyzed domino
reaction under the optimized conditions (Table 4). It could be
seen that a wide range of substrates could be accepted by the
enzyme. Both electron-donating and electron-withdrawing func-
tionalities were compatible (Table 4, entries 1–11). It was clear
that the electronic factors of the substituents on the benzene ring
appeared to have great effects on the yields of the reaction. When
salicylaldehydes containing electron-donating substituents were
condensed with different b-keto esters by this procedure, the
products were obtained in good yields of 60–75% (Table 4,
entries 1–3). In contrast, the substituted salicylaldehydes bearing
electron-withdrawing substituents gave products in lower yields
of 25–56% (Table 4, entries 7–11). Moreover, compared with
other aromatic aldehydes, the yields of the reaction of 2-hydroxy-
3
. Conclusion
In conclusion, the synthesis of 2H-1-benzopyran-2-one deriva-
tives has been achieved using a BLAP catalyzed domino Kno-
evenagel/intramolecular transesterification reaction between
various salicylaldehydes and b-keto esters. Notably, we have
demonstrated an example of the combination of a domino
reaction with biocatalysis, in which the single enzyme displayed
an activity to catalyze two step conversions. Furthermore,
the control of enzymatic chemoselectivity between the Kno-
evenagel/intramolecular transesterification and the Knoeve-
nagel/hemiketalization could be realized by adjusting param-
eters including solvent, water content and temperature. This
BLAP catalyzed selective domino reaction provided an alterna-
tive synthetic method for 2H-1-benzopyran-2-one derivatives. It
is also a novel case of unnatural activities of existing enzymes.
1
-naphthaldehyde with b-keto esters were quite low probably
due to the steric hindrance effect of naphthyl (Table 4, entries
2–14). In addition, ethyl benzoylacetate generally offered
1
higher yields in shorter reaction times than ethyl acetoacetate
and diethyl malonate when reacting with salicylaldehyde and
substituted salicylaldehyde (Table 4, entries 1, 4, 7, 10). This
was probably because ethyl benzoylacetate more easily produces
stable carbanion with strong electron withdrawing character
than ethyl acetoacetate and diethyl malonate. However, when
reacting with 2-hydroxy-1-naphthaldehyde, ethyl benzoylacetate
Experimental
General information
Alkaline protease from Bacillus licheniformis 2709 (200 U/mg.
One unit of activity is the amount of enzyme that liberates
◦
1.0 mg of tyrosine from casein per minute at 40 C and pH
10.5) was purchased from Wuxi Xuemei Enzyme Co. Ltd.
This journal is © The Royal Society of Chemistry 2011
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(
WuXi, China). Unless otherwise noted, all reagents were
183, 163–168; (e) C. J. Palmer and J. L. Josephs, J. Chem. Soc.,
Perkin Trans. 1, 1995, 1, 3135–3152; (f) M. Taniguchi, Y. Q. Xiao,
X. H. Liu, A. Yabu, Y. Hada, L. Q. Guo, Y. Yamazoe and K. Baba,
Chem. Pharm. Bull., 1999, 47, 713–715; (g) D. A. Horton, G. T.
Bourne and M. L. Smythe, Chem. Rev., 2003, 103, 893–930; (h) M.
Noeldner, H. Hauer and S. S. Chatterjee, Drugs Future, 1996, 21, 779–
obtained from commercial suppliers and were used without
further purification. All reactions were monitored by thin-layer
chromatography (TLC) with Haiyang GF254 silica gel plates.
Flash column chromatography was carried out using100–200
mesh silica gel at increased pressure. For visualization, TLC
plates were either placed under ultraviolet light, or stained with
iodine vapor, or acidic vanillin. NMR spectra were recorded
on a 300 MHz spectrometer. Melting points were determined
on X-4 digital display micro melting point apparatus and were
uncorrected.
7
81.
(a) E. Knoevenagel and B. Dtsch, Chem. Ges., 1904, 37, 4461–4471;
b) E. C. Horning, Organic Syntheses, Coll. Vol. III, John Wiley &
3
4
(
Sons, New York, 1955, p. 281; (c) C. Wiener, C. H. Schoder and K. P.
Link, J. Am. Chem. Soc., 1957, 79, 5301–5303; (d) G. Jones, Organic
Reactions, Wiley & Sons, New York, 1967, Vol. 15, pp. 204; (e) F.
Bigi, L. Chesini, R. Maggi and G. Sartori, J. Org. Chem., 1999, 64,
1
2
033–1035; (f) A. Song, X. Wang and K. S. Lam, Tetrahedron Lett.,
003, 44, 1755–1758.
(a) R. Torviso, D. Mansilla, A. Beliza’n, E. Alesso, G. Moltrasio,
P. Va’zquez, L. Pizzio, M. Blanco and C. Ca’ceres, Appl. Catal., A,
General experimental procedure for the domino
Knoevenagel/intramolecular transesterification reaction
(
2
008, 339, 53–60; (b) G. P. Romanelli, D. Bennaedi, D. M. Ruiz, G.
Baronetti, H. J. Thomas and J. C. Autino, Tetrahedron Lett., 2004,
5, 8935–8939.
products 3a–n)
4
A 25 ml round-bottomed flask was charged with BLAP (20 mg
ml ), DMSO (4.5 ml) and deionized water (0.5 ml), to which
the salicylaldehyde derivative (1.64 mmol) and b-keto ester
5
6
(a) B. C. Ranu and R. Jana, Eur. J. Org. Chem., 2006, 3767–3770;
(b) A. Shaabani, R. Ghadari, A. Rahmati and A. H. Rezayan, J. Iran.
Chem. Soc., 2009, 6, 710–714.
-
1
H. Valizadeh, M. Mamaghani and A. Badrian, Synth. Commun.,
(
4.90 mmol) were introduced. The resulting mixture was stirred
2
005, 35, 785–790.
◦
for the specified amount of time at 55 C. The reaction was
terminated by filtering the enzyme. CH
filter paper to assure that products obtained were all dissolved
in the filtrate. 20 ml of water was then added to the filtrate,
and the filtrate was extracted three times with 20 ml of CH
The combined extracts were dried over anhydrous Na SO
the solvents were then removed under reduced pressure. The
crude products were purified by column chromatography with
petroleum ether/ethyl acetate as eluent.
7 A. Ramani, B. M. Chanda, S. Velu and S. Sivasanker, Green Chem.,
1999, 1, 163–165.
2
2
Cl was used to wash the
8
K. Griebenow and A. M. Klibanov, J. Am. Chem. Soc., 1996, 47,
1
1695–11700.
9
Enzymatic Reactions in Organic Media, (ed.: A. M. P. Koskinen, A.
M. Klibanov), Blackie-Pergamon: London, 1996.
2
Cl
2
.
1
0 (a) U. T. Bornscheuer and R. J. Kazlauskas, Angew. Chem., 2004, 116,
2
4
and
6
156–6165, (Angew. Chem., Int. Ed., 2004, 43, 6032–6040); (b) R.
Kourist, S. Bartch, L. Fransson, K. Hult and U. T. Bornscheuer,
ChemBioChem, 2008, 9, 67–69; (c) P. Carlqvist, M. Svedendahl, C.
Branneby, K. Hult, T. Brinck and P. Berglund, ChemBioChem, 2005,
6
1
, 331–336; (d) R. J. Kazlauskas, Curr. Opin. Chem. Biol., 2005, 9,
95–201; (e) P. Berglund and S. Park, Curr. Org. Chem., 2005, 9, 325–
Acknowledgements
336; (f) G. Hasnaoui-Dijoux, M. M. Elenkov, J. H. Lutje Spelberg, B.
Hauer and D. B. Janssen, ChemBioChem, 2008, 9, 1048–1051; (g) J.
M. Xu, F. Zhang, B. K. Liu, Q. Wu and X. F. Lin, Chem. Commun.,
Financial support from Natural Science Foundation Project of
CQ CSTC (2009BA5051) is gratefully acknowledged.
2
007, 20, 2078–2080; (h) F. W. Lou, B. K. Liu, Q. Wu, D. S. Lv and
X. F. Lin, Adv. Synth. Catal., 2008, 350, 1959–1962; (i) Q. Wu, J. M.
Xu, L. Xia, J. L. Wang and X. F. Lin, Adv. Synth. Catal., 2009, 351,
1833–1841; (j) R. C. Tang, Z. Guan, Y. H. He and W. Zhu, J. Mol.
Catal. B: Enzym., 2010, 63, 62–67.
References
1
S. Khode, V. Maddi, P. Aragade, M. Palkar, P. K. Ronad, S.
Mamledesai, A. H. M. Thippeswamy and D. Satyanarayana, Eur.
J. Med. Chem., 2009, 44, 1682–1688.
(a) R. S. Keri, K. M. Hosamani and H. R. Seetharama Reddy, Catal.
Lett., 2009, 131, 321–327; (b) R. Jana, R. Trivedi and J. A. Tunge,
Org. Lett., 2009, 11, 3434–3436; (c) K. Chun, S.-K. Park, H. M.
Kim, Y. Choi, M.-H. Kim, C.-H. Park, B.-Y. Joe, T. G. Chun, H.-
M. Choi, H.-Y. Lee, S. H. Hong, M. S. Kim, K.-Y. Nam and G.
Han, Bioorg. Med. Chem., 2008, 16, 530–535; (d) C. J. Wang, Y. J.
Hsieh, C. Y. Chu, Y. L. Lin and T. H. Tseng, Cancer Lett., 2002,
11 S. P. Yao, D. S. Lu, Q. Wu, Y. Cai, S. H. Xu and X. F. Lin, Chem.
Commun., 2004, 17, 2006–2007.
12 H. Fan, M. Kitagawa, T. Raku and Y. Tokiwa, Macromol. Biosci.,
2003, 3, 420–424.
13 (a) M. Svedendahl, K. Hult and P. Berglund, J. Am. Chem. Soc., 2005,
127, 17988–17989; (b) O. Torre, V. Gotor-Fernandez, I. Alfonso, L. F.
Garcia-Alles and V. Gotor, Adv. Synth. Catal., 2005, 347, 1007–1014;
(c) P. Carlqvist, M. Svedendahl, C. Branneby, K. Hult, T. Brinck and
P. Berglund, ChemBioChem, 2005, 6, 331–336; (d) O. Torre, I. Alfonso
and V. Gotor, Chem. Commun., 2004, 15, 1724–1725.
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