A new synthesis of fully phosphorylated flavones as potent pancreatic cholesterol esterase inhibitors

Article abstract of DOI:10.1039/c0ob00640h

Five flavones possessing one to four phenolic groups were fully phosphorylated efficiently and the obtained compounds showed excellent pancreatic cholesterol esterase (CEase) inhibitory activities with IC ₅₀ in the nanomolar range, which were m

Full text of DOI:10.1039/c0ob00640h

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Organic &
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PAPER
A new synthesis of fully phosphorylated flavones as potent pancreatic
cholesterol esterase inhibitors†
Guoping Peng,a Yidan Du,b Yingling Wei,b Jingming Tang,b Ai-Yun Peng*b and Liqun Rao*a
Received 29th August 2010, Accepted 28th January 2011
DOI: 10.1039/c0ob00640h
Five flavones possessing one to four phenolic groups were fully phosphorylated efficiently and the
obtained compounds showed excellent pancreatic cholesterol esterase (CEase) inhibitory activities with
IC50 in the nanomolar range, which were much more potent than their parent compounds. The
inhibition mechanism and kinetic characterization studies indicate that they are irreversible competitive
inhibitors.
Introduction
Pancreatic cholesterol esterase (CEase; EC 3.1.1.13) is a nonspe-
cific a/b hydrolase that can catalyze the hydrolysis of cholesterol
esters, triacylglycerides, phospholipids and lipid-soluble vitamin
esters prior to their absorption. Since CEase plays important
roles in the dietary cholesterol absorption, inhibition of CEase
has aroused much interest as a new potential approach to treat
hypercholesterolemia.1,2
Many efforts have been made in the development of potent
inhibitors of CEase. Several classes of CEase inhibitors have been
reported, including substituted 6-chloro-2-pyrones,3,4 1,3-oxazin-
Fig. 1 Structures and inhibitory activities of some potent CEase
inhibitors.
4-ones,5,6 carbamates,7–9 phosphonates,10,11 high molecular weight
sulfated polysaccharides12 and 3-alkoxychloroisocoumarins.13 In
our previous work, we have shown that some phosphaiso-
coumarins are reversible competitive inhibitors of CEase.14 How-
ever, most of the above inhibitors (Fig. 1) are not potent and safe
enough for practical use. Therefore, it is still desirable to develop
novel CEase inhibitors with more potent activities and lower side
effects.
In the present study we designed and synthesized a series of
phosphorylated flavones as potential inhibitors of CEase on the
basis of the following considerations. First, there exists some simi-
larity in the skeleton structure between cholesterol and flavones
(Fig. 2), and so phosphorylated flavones may act as alternate
Fig. 2 Structures of cholesterol and flavones.
substrate-based inhibitors of CEase. Second, the target phosphates
have tetrahedral structures that may mimic the transition states of
the hydrolysis of cholesterol esters and thereby inhibit the activity
of CEase. Third, phosphorylated flavones may act as prodrugs to
inhibit CEase. Flavones are a class of polyphenolic compounds
that are widely distributed in plant foods and many studies have
shown that flavones have the potential to become drugs especially
in cancer and cardiovascular disease prevention and therapy.15–17
However, most of the flavones show poor solubility not only in
water but also in lipids, which reduces their absorption in vivo and
limits their further practical applications. Phosphates are often
used as prodrugs to improve the bioavailabilities of the parent
agents and several phosphorylated flavones have been reported as
anticancer agents,18 but the phosphorylated flavones containing
aCollege of Bioscience and Biotechnology, Hunan Agricultural University,
Furong District, Changsha, 410428, China. E-mail: raoliqun@163.com;
Fax: 86 0731 84612870; Tel: 86 0731 84673601
bSchool of Chemistry & Chemical Engineering, Sun Yat-sen Univer-
sity, 135 Xingangxi Lu, Guangzhou, 510275, China. E-mail: cespay@
mail.sysu.edu.cn; Fax: 86 20 84112245; Tel: 86 020 84110918
† Electronic supplementary information (ESI) available: NMR spectra
for 2a–2e; procedure for enzyme assays and graphs for determination of
inhibitors IC50 values. See DOI: 10.1039/c0ob00640h
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Table 1 Optimization of reaction conditions toward fully phosphory-
Table 2 DMAP-catalyzed synthesis of fully phosphorylated flavonesa
lated flavonesa
Entry Substrate
Product
Isolated yield (%)
Phosphorylated
reagent (equiv.)
Isolated
yield (%)
1
2
3
4
5
1a (5,7,3¢,4¢-OH) 2a (5,7,3¢,4¢-OP(O)(OEt)2) 91
Entry Catalyst (equiv.) Solvent
1b (7,3¢,4¢-OH)
1c (5,7-OH)
1d (7-OH)
2b (7,3¢,4¢-OP(O)(OEt)2)
2c (5,7-OP(O)(OEt)2)
2d (7-OP(O)(OEt)2)
2e (6-OP(O)(OEt)2)
83
86
80
74
b
1
2
3
4
5
—
—
—
—
CCl4
HP(O)(OEt)2 (12)
CCl4 + DMF HP(O)(OEt)2 (20)
CCl4 + THF HP(O)(OEt)2 (20)
\
b
\
1e (6-OH)
b
\
a All reactions were carried out at 2.5 mmol scale in THF (30 mL) with
DMAP (2 equiv. per OH), Et3N (2 equiv. per OH), ClP(O)(OEt)2 (3 equiv.
per OH) at room temperature for 24 h.
b
THF
THF
ClP(O)(OEt)2 (12)
ClP(O)(OEt)2 (12) 2a (91)
\
DMAP (8)
a All reactions were carried out at room temperature for 24 h with the
addition of 8 equiv. of Et3N. b The reaction led to a complicated mixture.
Table 3 Inhibitory effects on CEase of flavones 1a–1e and 2a–2e
more than two phenolic groups and their inhibitory activities on
CEase have not been reported so far.
Compound
IC50/nM Compound
IC50/nM
2a
2b
2c
2d
2e
3.89
26.1
3.76
2.44
390
1a
1b
1c
1d
1e
28 100
35 700
nia
ni
ni
Results and discussion
Chemistry
Phosphaisocoumarin Ab 1900
Phosphaisocoumarin Bb 4800
We initiated this study with the phosphorylation of luteolin (1a)
possessing four phenolic groups and the results are summarized
in Table 1. The Atherton–Todd reaction has proved to be a useful
method to phosphorylate flavonoids with one or two phenolic
groups.19,20 However, under typical Atherton–Todd reaction con-
ditions, the reaction of 1a with HP(O)(OEt)2 and Et3N in CCl4 only
led to a complicated mixture (entry 1). Taking into account that
this result is probably caused by the incomplete phosphorylation
and the weak solubility of luteolin in CCl4, we tried to optimize
the reaction by increasing the amount of HP(O)(OEt)2 and adding
a polar solvent (e.g. DMF), but all attempts lead to unfavorable
results (entries 2, 3). Since it has been reported that DMAP (4-
dimethylaminopyridine) can catalyze the full sulfation of flavones,
then we turned our attention to the use of DMAP as a catalyst.
Gratifyingly, we finally found that when 1a was treated with 3.0
equiv of ClP(O)(OEt)2, 2.0 equiv of Et3N, 2.0 equiv of DMAP per
hydroxy in THF, the starting material 1a was consumed completely
and a single tetraphosphorylated product 2a was isolated in 91%
yield.
a ni, no inhibition at 100 mM. b In line with data in ref. 14.
Biology
Structure–activity correlations. The inhibitory activity of 2a–
2e and 1a–1e against CEase was investigated according to Hosie
et al.21 with some modifications and the results are summarized in
Table 3. Compared with phosphaisocoumarins A, B (Table 3) and
other known CEase inhibitors (Fig. 1), phosphorylated flavones
showed excellent inhibitory activity toward CEase. According to
the data in Table 3, the following structure–activity relationships
could be deduced.
With these optimized conditions in hand, the reactions of five
flavones 1a–1e with excess ClP(O)(OEt)2, Et3N and DMAP were
carried out in THF at room temperature and the results are
summarized in Table 2. This reaction is very efficient and all
flavones tested could be converted to the desired products 2a–
2e in good to excellent yields. It is also worthy to note that the
purification is very simple and convenient since only a single most
non-polar fully phosphorylated flavone was formed in the end for
each reaction. Among these products, compounds 2a, 2b and 2e
are new compounds; compounds 2c and 2d have been prepared
using the Atherton–Todd reaction by Zhao’s group.19,20 However,
no yield was given for compound 2c in the literature,19 although
2d was obtained in 92% yield.20a
(1) Phosphorylation of flavones can significantly enhance the
inhibition of CEase. The parent flavones (1a–1e) showed weak
(IC50 > 25 mM) or no inhibition activities against CEase, while
their phosphorylated derivatives (2a–2e) displayed excellent CEase
inhibitory activities with IC50 in the nanomolar range. For
example, tetra-phosphorylated luteolin 2a has an IC50 value of
3.89 nM, and was 7224-fold more potent than luteolin 1a (IC50
=
28.1 mM).
(2) The position of the phosphate group has a large effect on the
inhibitory activity. For example, compound 2d with a phosphoryl
group at positon 7 (IC50 = 2.44 nM) was nearly 160-fold more active
than 2e with a phosphoryl group at positon 6 (IC50 = 390 nM). All
results showed that introduction of a phosphoryl group at positon
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7 is a very important beneficial factor. In addition, phosphoryl
groups at position 4¢ and/or 3¢ are unnecessary or even adverse
since both 2a and 2c showed nearly the same activity and 2b
exhibited less activity.
Mechanism and kinetic characterization. To determine
whether compounds 2 are irreversible or reversible inhibitors of
CEase, we measured residual activity of CEase after incubation
with the potent inhibitors 2a and 2d according to the literature
protocol.22 The results showed that 2a and 2d (both at nanomolar
concentrations) inhibit CEase in a progressive manner; the enzyme
activity decreases as the incubation time increases (Fig. 3). Both
compounds were characterized as irreversible inhibitors of CEase
since they meet some of the criteria proposed by Abeles and
Maycock.23 For example, the inhibition was time-dependent and
followed the first-order kinetics over the observed time period
(Fig. 3).
Fig. 4 Lineweaver–Burk plots for inhibition of CEase by 2a and 2d
with p-nitrophenylbutyrate (pNPB) as the substrate. (A) and (B) represent
kinetics of compounds 2a and 2d, respectively. Ki = [I]/[Km(app)/Km - 1].
Km(app) determined with inhibitor and pNPB and Km with pNPB alone.
Fig. 3 Time-dependent inhibition of CEase by compounds 2a and 2d.
The kinetic behavior of compounds 2a and 2d on the CEase
using p-nitrophenylbutyrate as the substrate was then determined
from Lineweaver–Burk double reciprocal plots (Fig. 4).4 The
results indicated that on a short time scale they were competitive
inhibitors of CEase, suggesting that these inhibitors and the
substrate might competitively bind with the same site of the free
enzyme. The Ki and Km values for 2a (calculated from the plots) are
3.73 nM and 0.11 mM, and the Ki and Km values for 2d are 1.17 nM
and 0.19 mM. It is apparent that the Km value is significantly
greater than the Ki value for both compounds, indicating that the
affinity between the enzyme and the inhibitor is far higher than the
affinity between the enzyme and the substrate. These results may
explain why phosphorylated flavones 2 show such good inhibitory
activities against CEase.
tivities than the parent flavones. Compounds 2a, 2c and 2d
were found to be the most potent inhibitors with IC50 value
of 3.89 nM, 3.76 nM and 2.44 nM, respectively, indicating that
introduction of a phosphoryl group at position 7 of flavone was
beneficial to the inhibition of CEase. This work suggested that
phosphorylated flavones might act as interesting lead compounds
for developing new effective and safe CEase inhibitors which may
have good prospects for practical application in the treatment of
hypercholesterolemias.
Experimental
General
Anhydrous THF was dried according to standard procedures.
Unless otherwise noted, all other reagents were obtained from
commercial sources and used without further purification. NMR
spectra were recorded on a Varian Mercury-Plus 300 (1H 300 MHz;
13C 75.4 MHz; 31P 121 MHz). ESI-mass spectra were recorded on
a LCMS-2010A Liquid Chromatograph mass spectrometer. IR
spectra were recorded as KBr pellets on a Bruker Equinox 55
Conclusion
In conclusion, the present investigation not only developed a new
effective and convenient way to synthesize fully phosphorylated
flavones, but also provided a new class of potent irreversible
competitive CEase inhibitors. The results showed that all the
synthesized compounds had much more potent inhibitory ac-
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FT/IR spectrometer. Melting points were not corrected. Column
chromatography was performed on 200–300 mesh silica gel. Thin-
layer chromatography was conducted on Kieselgel 60 F254.
Diethyl flavone-◦6-yl phosphate (2e). A pale yellow solid. Yield:
1
74%. Mp: 67–68 C. H NMR (300 MHz, CDCl3): d 7.88–7.97
(m, 3H), 7.49–7.65 (m, 5H), 6.81 (s, 1H), 4.21–4.31 (m, 4H), 1.39
(t, J = 7.2 Hz, 6H); 13C NMR (75.4 MHz, CDCl3): d 177.4, 163.4,
153.0, 147.7 (d, J = 5.1 Hz), 131.6, 131.4, 128.9 (d, J = 3.0 Hz),
126.3, 126.1, 124.7, 119.6, 115.7 (d, J = 3.3 Hz), 106.9, 64.9 (d,
J = 6.3 Hz), 16.2 (d, J = 6.0 Hz); 31P NMR (121 MHz, CDCl3):
d -5.24; MS (ESI): m/z (%): 375 [(M+1)+, 100]. Anal. Calcd for
C19H19O6P: C, 60.96; H, 5.12. Found C, 60.96; H, 5.40. IR (KBr,
cm-1): 2988, 1643, 1498, 1393, 1261, 1235, 1181, 1026.
General procedure for the synthesis of 2a–2e. To a stirring
solution of 1 (2.5 mmol), DMAP (2.0 mmol per –OH group),
Et3N (2.0 mmol per –OH group) in anhydrous THF (30.0 mL),
a solution of ClP(O)(OEt)2 (DEPC, 30 mmol) in anhydrous THF
(20 mL) was added dropwise in an ice-water bath over 30 min.
After stirring at room temperature for 24 h under nitrogen, the
reaction mixture was diluted with EtOAc and washed with 0.5 M
HCl, 5% (w/v) K2CO3, brine and water, and then dried over
anhydrous Na2SO4. After removal of the solvent in vacuo, the
residue was purified by column chromatography on silica gel
with petroleum ether/EtOAc (4 : 1–2 : 1) as eluent to give the
corresponding product 2.
Assay procedure and determination of inhibitor IC50
CEase (porcine) was from Worthington, p-nitrophenyl butyrate
(pNPB) was from Sigma. The CEase inhibition was assayed
according to Hosie et al.21 with some modifications. CEase activity
was measured by following the hydrolysis of the colorimetric
substrate pNPB. The temperature was maintained at 25.0 0.2
◦C. All compounds and pNPB were dissolved in acetonitrile. The
final concentration of acetonitrile in the test solution was 1.5%.
First, 3.25 U of CEase (2.35 mg mL-1) were preincubated with
different concentrations of inhibitor in 0.1 M sodium phosphate
buffer (pH 7.04, containing 0.1 M NaCl) for 15 min. Then,
the pNPB (0.07 mM) was added to the reaction mixture and
the enzyme reaction was monitored for 1 min by measuring the
change in absorbance at 405 nm. The measurement was performed
in triplicate for each concentration and averaged before further
calculation. The computer program used for the analysis of data
was Origin 7.5.
Diethyl 4-oxo-2-phenyl-4H-chromen-3¢,4¢,5,7-yl phosphate (2a).
Oil. Yield: 91%. 1H NMR (300 MHz, CDCl3) d 7.85 (s, 1H), 7.53–
7.65 (m, 2H), 7.33 (s, 1H), 7.24 (s, 1H), 6.59 (s, 1H), 4.26–4.40 (m,
16H), 1.34–1.40 (m, 24H); 13C NMR (75.4 MHz, CDCl3) d 175.7,
159.9, 157.8, 153.8 (d, J = 3.4 Hz), 150.3 (d, J = 6.6 Hz), 144.2,
141.8, 128.2, 123.4, 121.7, 119.4, 113.7 (d, J = 5.5 Hz), 110.2 (d,
J = 6.3 Hz), 109.0, 105.7 (d, J = 5.4 Hz), 65.3, 65.2, 65.1, 65.0,
16.1 (m, 4C); 31P NMR(121 MHz, CDCl3) d: -6.65, -6.47, -5.88,
-5.43. MS: m/z 853(M+, 100); Anal. Calcd. for C31H46O18P4: C,
44.83; H, 5.58. Found C, 44.93; H, 5.74; IR (film, cm-1): 2919,
1654, 1419, 1330, 1273, 1105, 1029.
Hexaethyl tri-phosphoric ester of apigenin (2b). Oil. Yield:
83%. H NMR (300 MHz, CDCl3): d 7.75–7.79 (m, 2H), 7.15–
1
Mechanism of CEase inhibition
7.30 (m, 4H), 6.54 (s, 1H), 4.12–4.36 (m, 12H), 1.20–1.35 (m,
2a and 2d were assayed in 0.1 M sodium phosphate buffer
(pH 7.04, containing 0.1 M NaCl) using p-nitrophenyl butyrate
(pNPB) as the substrate to determine whether they are reversible
or irreversible inhibitors. The temperature was maintained by a
temperature controllable water bath kept at 25 0.2 ◦C. The
inhibitors and the enzyme were preincubated in the absence of
substrate for different time periods. Then, the pNPB (0.10 mM)
was added to the reaction mixture and the enzyme reaction was
monitored for 1 min by measuring the change in absorbance at
405 nm. The measurement was performed in triplicate for each
time interval and averaged before further calculation.
18H); 13C NMR (75.4 MHz, CDCl3 : d 175.7, 160.7, 157.7, 153.6
)
(d, J = 5.1 Hz), 153.1 (d, J = 6.2 Hz), 150.1 (d, J = 8.7 Hz), 127.6,
127.3, 120.3, 113.6 (d, J = 6.3 Hz), 109.9, 108.3, 105.5, 65.1, 65.0,
64.8, 64.7, 16.1 (m, 4C); MS (ESI): m/z (%): 678 [(M+1)+, 100].
Anal. Calcd for C27H37O14P3: C, 47.80; H, 5.50. Found C, 47.54;
H, 5.70. IR (film, cm-1): 2936, 1656, 1479, 1383, 1286, 1030, 988.
Tetraethyl bis-phosphoric es◦ter of chrysin (2c). A pale yellow
solid. Yield: 86%; Mp: 79–81 C (lit.19 83–84 ◦C). 1H NMR (300
MHz, CDCl3): d 7.77–7.86 (m, 2H), 7.41–7.53 (m, 3H), 7.31–7.40
(m, 1H), 7.17–7.28 (m, 1H), 6.64 (s, 1H), 4.24–4.40 (m, 8H), 1.31–
1.40 (m, 12H); 13C NMR (75.4 MHz, CDCl3): d 175.9, 161.7,
157.8, 153.7 (d, J = 8.4 Hz), 150.2 (d, J = 7.1 Hz), 131.5, 130.7,
128.8, 125.9, 113.7 (d, J = 8.6 Hz), 109.9, 108.6, 105.6 (d, J = 3.7
Hz), 65.2, 65.1, 16.1, 16.0; 31P NMR (121 MHz, CDCl3): d -6.28,
-6.52. MS (ESI): m/z (%): 526 [M+, 100]. IR (film, cm-1): 2986,
1651, 1438, 1380, 1278, 1156, 1031, 979.
Kinetic characterization of CEase inhibition
The general procedure of the kinetic characterization of CEase is
also similar to the above inhibition assays. First, 3.25 U of CEase
(2.35 mg mL-1) were preincubated with the inhibitor in sodium
phosphate buffer for 15 min. Then, different concentrations
of pNPB were added to the reaction mixture and the enzyme
reaction was monitored for 1 min by measuring the change in
absorbance at 405 nm. Triplicate sets of data were collected for
each inhibitor concentration. Kinetics of CEase inhibition was
generally analyzed by Lineweaver–Burk plots.4
Diethyl flavone-7-yl phosphate (2d). A pale yellow solid. Yield:
80%; Mp: 61–63 ◦C (lit.20 60–61 ◦C). 1H NMR (300 MHz, CDCl3):
d 8.20 (d, J = 8.7 Hz, 1H), 7.91–7.94 (m, 2H), 7.53–7.57 (m, 4H),
7.25–7.29 (m, 1H), 6.84 (s, 1H), 4.24–4.34 (m, 4H), 1.41 (dt, J1 =
7.2 Hz, J1 = 1.5 Hz, 6H); 13C NMR (75.4 MHz, CDCl3): d 177.3,
163.5, 156.8, 154.6 (d, J = 6.9 Hz), 131.6, 131.4, 128.9, 127.4, 126.1,
121.0, 117.8 (d, J = 7.5 Hz), 108.9 (d, J = 4.3 Hz), 107.5, 65.1 (d,
J = 7.6 Hz), 16.1 (d, J = 7.5 Hz); 31P NMR (121 MHz, CDCl3):
d -5.87. MS (ESI): m/z (%): 374 [M+, 100]. IR (film, cm-1): 2985,
1642, 1444, 1374, 1280, 1158, 1040, 976.
Acknowledgements
This work was supported by research grants from the National
Natural Science Foundation of China (Grant No. 20602043) and
the Fundamental Research Funds for the Central Universities.
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12 US Pat., 7071174, 2006.
Notes and references
13 J. J. Heynekamp, L. A. Hunsaker, T. A. Vander Jagt, R. E. Royer, L. M.
Deck and D. L. Vander Jagt, Bioorg. Med. Chem., 2008, 16, 5285–5294.
14 B. Li, B. Zhou, H. Lu, L. Ma and A.-Y. Peng, Eur. J. Med. Chem., 2010,
45, 1955–1963.
1 WO Pat., 2010014541, 2010.
2 D. Y. Hui and P. N. Howles, J. Lipid Res., 2002, 43, 2017–2030.
3 M. Stoddard Hatch, W. M. Brown, J. A. Deck, L. A. Hunsaker, L.
M. Deck and D. L. Vander Jagt, Biochim. Biophys. Acta, 2002, 1596,
381–391.
15 H. L. Liu, W. B. Jiang and M. X. Xie, Recent Pat Anti-Canc., 2010, 5,
152–164.
4 L. M. Deck, M. L. Baca, S. L. Salas, L. A. Hunsaker and D. L. Vander
Jagt, J. Med. Chem., 1999, 42, 4250–4256.
5 M. Pietsch and M. Gu¨tschow, J. Med. Chem., 2005, 48, 8270–
8288.
6 M. Pietsch and M. Gu¨tschow, J. Biol. Chem., 2002, 277, 24006–24013.
7 G. Lin, W.-C. Liao and S.-Y. Chiou, Bioorg. Med. Chem., 2000, 8,
2601–2607.
8 G. Lin, C.-T. Shieh, H.-C. Ho, J.-Y. Chouhwang, W.-Y. Lin and C.-P.
Lu, Biochemistry, 1999, 38, 9971–9981.
16 L. H. Cazarolli, L. Zanatta, E. H. Alberton, M. S. R. B. Figueiredo,
P. Folador, R. G. Damazio, M. G. Pizzolatti and F. R. M. B. Silva,
Mini-Rev. Med. Chem., 2008, 8, 1429–1440.
17 H. Harnafi and S. Amrani, Parmaco. Rev., 2007, 1, 193–202.
18 T. Zhang, X. Chen, L. Qu, J. Wu, R. Cui and Y. Zhao, Bioorg. Med.
Chem., 2004, 12, 6097–6105.
19 X.-L. Chen, L.-B. Qu, T. Zhang, H.-X. Liu, F. Yu, X. Cheng and Y.-F.
Zhao, Anal. Chem., 2004, 71, 211–217.
20 (a) J. Wu, X. Chen, L. Qu, J. Lu and Y. Zhao, Fenxi Ceshi Xuebao,
2006, 25, 22–26; (b) X. Chen, T. Zhang, H. X. Liu, L.-B. Qu, Y. Z. Yu
and Y. Zhao, Chin. Chem. Lett., 2004, 15, 343–346.
9 S. R. Feaster, K. Lee, N. Baker, D. Y. Hui and D. M. Quinn,
Biochemistry, 1996, 35, 16723–16734.
10 H. Schmidinger, R. Birner-Gruenberger, G. Riesenhuber, R. Saf, H.
Susani-Etzerodt and A. Hermetter, ChemBioChem, 2005, 6, 1776–1781.
11 G. B. Quistad, S. N. Liang, K. J. Fisher, D. K. Nomura and J. E. Casida,
Toxicol. Sci., 2006, 91, 166–172.
21 L. Hosie, L. D. Sutton and D. M. Quinn, J. Biol. Chem., 1987, 262,
260–264.
22 N Zhang and J. E. Casida, Bioorg. Med. Chem., 2002, 10, 1281–1290.
23 R. H. Abeles and A. L. Maycock, Acc. Chem. Res., 1976, 9, 313–319.
2534 | Org. Biomol. Chem., 2011, 9, 2530–2534
This journal is
The Royal Society of Chemistry 2011
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