Rapid microwave-assisted liquid-phase synthesis of 4-substituted-5- methoxycarbonyl-6-methyl-3,4-dihydropyridones on poly(ethylene glycol) support

Article abstract of DOI:10.1080/00397910701873144

4-Substituted-5-methoxycarbonyl-6-methyl-3,4-dihydropyridones have been efficiently prepared on the soluble polymer polyethylene glycol (PEG) 4000 by solvent-free microwave irradiation through a one-pot condensation of PEG-bound acetoacetate, Meldrum's acid, ammonium acetate, and the appropriate aldehyde in the presence of a catalytic amount of polyphosphoric acid. The polymer-supported synthesis provided the target compounds in excellent yield and purity with a facile workup procedure. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701873144

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Rapid Microwave‐Assisted
Liquid‐Phase Synthesis of
4‐Substituted‐5‐methoxycarbonyl‐6‐methyl‐3,4‐d
on Poly(ethylene Glycol) Support
Gui‐Yun Fu a , Xiao‐Lan Zhang a b , Shou‐Ri Sheng a , Mei‐Hong Wei a
Xiao‐Ling Liu a
&
a College of Chemistry and Chemical Engineering , Jiangxi Normal
University , Nanchang, China
b Department of Chemistry , Shangrao Teacher's College , Shangrao, China
Published online: 17 Apr 2008.
To cite this article: Gui‐Yun Fu , Xiao‐Lan Zhang , Shou‐Ri Sheng , Mei‐Hong Wei & Xiao‐Ling Liu (2008) Rapid
Microwave‐Assisted Liquid‐Phase Synthesis of 4‐Substituted‐5‐methoxycarbonyl‐6‐methyl‐3,4‐dihydropyridones
on Poly(ethylene Glycol) Support, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 38:8, 1249-1258, DOI: 10.1080/00397910701873144
To link to this article: http://dx.doi.org/10.1080/00397910701873144
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Synthetic Communicationsw, 38: 1249–1258, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701873144
Rapid Microwave-Assisted Liquid-Phase
Synthesis of 4-Substituted-5-
methoxycarbonyl-6-methyl-3,4-
dihydropyridones on Poly(ethylene Glycol)
Support
Gui-Yun Fu,1 Xiao-Lan Zhang,1,2 Shou-Ri Sheng,1
Mei-Hong Wei,1 and Xiao-Ling Liu1
1College of Chemistry and Chemical Engineering, Jiangxi Normal
University, Nanchang, China
2Department of Chemistry, Shangrao Teacher’s College, Shangrao,
China
Abstract: 4-Substituted-5-methoxycarbonyl-6-methyl-3,4-dihydropyridones have been
efficiently prepared on the soluble polymer polyethylene glycol (PEG) 4000 by solvent-
free microwave irradiation through a one-pot condensation of PEG-bound acetoacetate,
Meldrum’s acid, ammonium acetate, and the appropriate aldehyde in the presence of a
catalytic amount of polyphosphoric acid. The polymer-supported synthesis provided the
target compounds in excellent yield and purity with a facile workup procedure.
Keywords: Liquid-phase synthesis, microwave irradiation, PEG-bound acetoacetate,
4-substituted-5-methoxycarbonyl-6-methyl-3,4-dihydropyridones
Although already widely used as a powerful tool in the field of combinatorial
chemistry, solid-phase organic synthesis (SPOS) suffers from some inherent
disadvantages because of the heterogeneous reaction conditions, such as
high demands on the compatibility of the individual components, relatively
Received in Japan July 28, 2007
Address correspondence to Shou-Ri Sheng, College of Chemistry and Chemical
Engineering, Jiangxi Normal University, Nanchang 330027, China. E-mail:
shengsr@jxnu.edu.cn
1249

1250
G.-Y. Fu et al.
low reactivity and selectivity, harsh reactions, as well as the difficulty
characterizing the insoluble polymer-supported compounds.[1] This stimulated
the development of various approaches for construction of combinatorial
libraries in solution, among which liquid-phase organic synthesis (LPOS)
using soluble polymers provides an attractive strategy by incorporating the
positive aspects of classical solution chemistry.[2] It benefits from both the
advantageous features of homogeneous solution chemistry (high reactivity,
lack of diffusion phenomena, and ease of analysis without the cleavage-
and-check procedure) and of solid-phase methods (use of excessive regents
and easy isolation and purification of product). With these advantages, this
strategy has recently caught the attention of the scientific community as an
effective technique for the construction of combinatorial small organic
molecular libraries, especially heterocyclic compounds, of which many are
important structural motifs of pharmaceuticals or biologically active
compounds.[3] Among the various soluble polymers, polyethylene glycol
(PEG) is the most useful and promising.[4] 1,4-Dihydropyridines (1,4 DHP),
as one of the most important chemicals in the treatment of cardiovascular
diseases because of their calcium antagonist effects, have been widely
studied and used but still generate much current interest.[5] Classical
methods for the synthesis of 1,4-dihydropyridines are one-pot condensation
of aldehyde with ethyl acetoacetate and ammonia acetate either in acetic
acid or by refluxing in alcohols.[6] This method, however, involves long
reaction time, harsh reaction conditions, and the use of a large quantity of
volatile organic solvents and generally gives low yields. To the best of our
knowledge, there have been no reports about liquid-phase synthesis of
1,4-dihydropyridines, despite its well-documented use in solid-phase
synthetic procedure.[7] As part of an ongoing research program focused on
the use PEG-supported b-keto ester in liquid-phase organic synthesis,[8] we
report here on the liquid-phase synthesis of 4-substituted-5-methoxy-
carbonyl-6-methyl-3,4-dihydropyridones from readily available starting
materials. These 3,4-dihydro-2-oxopyridines are important key intermediates
for the preparation of o-chloroformyl 1,4-dihydropyridine, (DHP)s by the
Vilsmeier–Haack reaction, which can be further transformed in a wide
variety of pyridine-fused heterocyclic systems.[9]
The LPOS of 4-substituted-5-methoxycarbonyl-6-methyl-3,4-dihydro-
pyridones takes place from our previous preparation of immobilized PEG
4000–bound acetoacetate 1[8] as shown in Scheme 1. Simple acetoacetylation
Scheme 1.

PEG Support Synthesis of 3,4-Dihydropyridones
1251
could be performed by treatment of dihydroxyl PEG 4000 with 2,2,6-
trimethyl-4H-1,3-dioxin-4-one (toluene, 111 8C, 5 h). The quantitative
conversion of PEG to 1 was observed following the presence of the C55O
absorptions at 1743 and 1718 cm21, which correspond to ester and ketone
carbonyl groups, respectively. In this procedure, the condensation of PEG-
bound acetoacetate 1 with Meldrum’s acid, aldehydes, and ammonium
acetate would be the key step for the success of this protocol. As a model
study, the liquid-phase combinatorial synthesis of 4-phenyl-5-methoxycarbo-
nyl-6-methyl-3,4-dihydropyridone (4a) was first investigated using the
classical solution-phase methods. When PEG-bound acetoacetate 1 was
treated with benzaldehyde (2a), Meldrum’s acid, and ammonium acetate in
refluxing EtOH for 6 h[6b] or in refluxing AcOH for 14 h[6c] or even for
longer times, the condensation on PEG was not complete. Microscopy
fourier transform infrared (FT-IR)[10] study of the PEG-bound intermediate
3a showed a new, moderately strong absorption at 1690 cm21 corresponding
to carbonyl stretch group in the dihydropyridine ring but still exhibiting a rela-
tively intense band near 1718 cm21, which corresponds to the ketone carbonyl
group of PEG-bound acetoacetate 1. Correspondingly, the lower yields of the
target compound 4a cleaved from PEG using NaOMe were obtained using the
classic methods (entries 1 and 2). The application of microwave (MW)
irradiation in organic synthesis recently has been the focus of considerable
attention and is becoming an increasingly popular technology.[11] Rodrıguez
´
et al.[12] has reported that compounds 4 could be prepared from Meldrum’s
acid, methyl acetoacetate, and the appropriate aromatic aldehydes in the
presence of ammonium acetate under MW heating. Stimulated by this, a
better result was obtained in our attempts when employing the same
reaction conditions.[12] Interesting, after a series of experiments, the best
result was obtained when the a catalytic amount of nonvolatile, high-
boiling-point, and nonoxidant polyphosphoric acid (PPA) was added under
400 W of irradiation power for 5 min at 100 8C without solvent. This
reaction process, promoted by microwave, was also monitored by microscopy
FT-IR and showed complete disappearance of the ketone carbonyl stretch and
the appearance of a new carbonyl stretch group in the dihydropyridine ring
near 1690 cm21 of the PEG-bound intermediate 3a. This was further
confirmed by proton NMR spectroscopy of 3a, because the methyl proton
(CH3) of the PEG-bound acetoacetate 1 was shifted downfield from
d ¼ 2.27 to 2.42 ppm. When the irradiation was stopped, the solids were
treated with a small quantity of CH2Cl2; the PEG-bound intermediate 3a
was then precipitated by addition of diethyl ether and separated by filtration.
The target compound 4a was obtained in 88% yield and 95% (entry 3) high
performance liquid chromatography (HPLC) purity of crude product by
cleavage from the PEG support under the treatment of the PEG-bound
product 3a with NaOMe in MeOH with stirring overnight. Apparently, the
MW-assisted procedure leads to high yields, in the absence of solvent,
within short reaction times, and with simplified workup, as compared to

1252
G.-Y. Fu et al.
Table 1. Synthesis of 4-substituted-5-methoxycarbonyl-6-methyl-3,4-dihydropyri-
dones 4a–k
Entry
R (Aldehydes 2)
Product
Yield (%)a
Purity (%)b
c
1
2
C6H5 (2a)
C6H5 (2a)
C6H5 (2a)
4a
4a
4a
4b
4c
4d
4e
4f
30
60
88
85
84
86
90
91
92
91
89
86
85
—
—
c
3
95
94
93
94
96
96
97
96
95
93
93
4
2-NO2C6H4 (2b)
3-NO2C6H4 (2c)
4-NO2C6H4 (2d)
2,4-(NO2)2C6H3 (2e)
4-MeOC6H4 (2f)
2-ClC6H4 (2g)
4-ClC6H4 (2h)
4-CNC6H4 (2i)
4-CH3OCOC6H4 (2j)
n-C3H7 (2k)
5
6
7
8
9
4g
4h
4i
10
11
12
13
4j
4k
aIsolated yield based on loading of original HO-PEG-OH; The known products were
characterized from their spectra (1H NMR and IR) and comparison with authentic
samples.
bDetermined on HPLC analysis of crude products before purification.
cNot determined.
conventional heating methods. Following the synthesis of 4a on PEG, a set of
related compounds (4b–k) was prepared using an analogous synthetic
protocol. As shown in Table 1, the yields (84–92%) are excellent and the
purities are satisfactory (ꢀ93%). Additionally, it appeared that the present
method was effective for both aromatic (either with electron-withdrawing or
electron-donating groups) (entries 1–12) and aliphatic aldehydes (entry 13).
In some cases, a trace amount of the PEG residue might contaminate the
final products 4, which could be easily further purified by recrystallization
from hot ethanol or by passing the crude product through a silica-gel
column (10–15% ethyl acetate in hexane as the eluent).
On the other hand, the resin-bound intermediate 4 cleaved via another
process was further investigated as shown in Scheme 2. For example,
treatment of 3a with 50% trifluoroacetic acid (TFA) in CH2Cl2 at room
Scheme 2.

PEG Support Synthesis of 3,4-Dihydropyridones
1253
temperature for 1 h yielded 6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydropyri-
dine-5-carboxylic acid 5a in 90% yield and 95% HPLC purity.
In conclusion, an efficient liquid-phase synthetic technique for the con-
struction of 4-substituted-5-methoxycarbonyl-6-methyl-3,4-dihydropyri-
dones on soluble polymeric support has been developed, which utilized a
one-pot condensation of PEG-bound acetoacetate, Meldrum’s acid,
aldehydes, and ammonium acetate in the presence of a catalytic amount of
polyphosphoric acid (PPA) under solvent-free microwave promotion.
Compared with the classical solution-phase reactions, the yields could be con-
siderably improved and the reaction time could be shortened dramatically.
This versatile method has potential applications in combinatorial synthesis
of analogous heterocyclic compound libraries for chemical biological
screening and the drug discovery process.
EXPERIMENTAL
Melting points were determined on an X4 melting-point apparatus and are
uncorrected. 1H NMR (400-MHz) spectra were recorded on a Bruker
Avance (400-MHz) spectrometer. FT-IR spectra were taken on
a
Perkin-Elmer SP One FT-IR spectrophotometer. Microanalyses were
performed with a PE 2400 elemental analyzer. HPLC analysis was carried
out on an Agilent 1100 automated system with a PDA detector
(lmax ¼ 254 nm) using a gradient from 100% of the aqueous 0.1% TFA
(eluent A) to 60% eluent A–40% of 0.5% TFA in acetonitrile (eluent B)
over 35 min (0.8 mL/min) on a RP-18e column (100 Â 4.6 mm). MW exper-
iments were performed on a Galaz WP 800J-823 domestic microwave oven.
Meldrum’s acid, ammonium acetate, and different aldehydes were obtained
from commercial sources and used without further purification.
General Procedure for the Preparation of 4-Substituted-5-
methoxycarbonyl-6-methyl-3,4-dihydro-pyridones (4a–k)
At room temperature, PPA (2–3 drops) was added to the completely ground
powers of PEG 4000–bound acetoacetate 1[8] (1.0 g, 0.5 mmol), Meldrum’s
acid (1.0 mmol), ammonium acetate (1.2 mmol), and corresponding
aldehyde 2 (1.0 mmol). The resulting mixture was added into an open
vessel and stirred with a spatula for 30 s, and the vessel was placed inside a
large container filled with alumina at the center of the domestic microwave
oven. After irradiation at 400 W for 5–6 min at a temperature of 100 8C,
the mixture was cooled to room temperature and a small volume of dichloro-
methane (5 mL) was added it. After filtration, diethyl ether (30 mL) was
poured under stirring to precipitate the polymer 3. For completion of the pre-
cipitation, the suspension was left at 0 8C for another 30 min. The polymer 3

1254
G.-Y. Fu et al.
was collected and washed several times with diethyl ether (3 Â 10 mL). After
drying in vacuo, the solid was added to the NaOMe (1 N)/MeOH solution
(15 mL) to cleave the products at room temperature overnight (checked by
thin-layer chromatography TLC). The target compounds 4 were obtained by
extraction from the reaction mixture with EtOAc, dilution with H2O
(20 mL), and then removal of the solvent. The samples were further purified
by recrystallization from hot ethanol or passing the crude product through a
silica-gel column (10–15% ethyl acetate in hexane), if necessary.
Data
4-Phenyl-5-methoxycarbonyl-6-methyl-3,4-dihydropyridone (4a): Mp 197–
1
198 8C (lit.[6b] mp 197–198 8C); H NMR: d ¼ 7.75 (br s, 1H), 7.35–7.25
(m, 2H), 7.20–7.15 (m, 3H), 4.25 (dd, J ¼ 7.9, 1.5 Hz, 1H), 3.67 (s, 3H),
2.91 (dd, J ¼ 16.4, 7.9 Hz, 1H), 2.71 (dd, J ¼ 16.4, 1.5 Hz, 1H), 2.43
(s, 3H); IR (KBr): 3218 (NH), 1704 (CO, ester), 1690 (C55O) and 1620
(C55C) cm21
.
4-(2-Nitrophenyl)-5-methoxycarbonyl-6-methyl-3,4-dihydropyridone (4b): Mp
1
205–206 8C (lit.[6b] mp 205–207 8C); H NMR: d ¼ 7.98 (br s, 1H), 7.84
(d, J ¼ 8.2 Hz, 1H), 7.52–7.49 (m, 1H), 7.36–7.31 (m, 1H), 7.27
(d, J ¼ 7.8 Hz, 1H), 4.75 (dd, J ¼ 7.9, 1.5 Hz, 1H), 3.53 (s, 3H), 3.05 (dd,
J ¼ 16.3, 7.9 Hz, 1H), 2.82 (dd, J ¼ 16.3, 1.5 Hz, 1H), 2.45 (s, 3H); IR
(KBr): 3231 (NH), 1708 (CO, ester), 1688 (C55O), 1624 (C55C), 1351 and
1525 (NO2) cm21
.
4-(3-Nitrophenyl)-5-methoxycarbonyl-6-methyl-3,4-dihydropyridone (4c): Mp
1
206–207 8C (lit.[6b] mp 206–207 8C); H NMR: d ¼ 8.21 (br s, 1H), 8.01–
7.89 (m, 1H), 7.52–7.49 (m, 3H), 4.38 (dd, J ¼ 8.0, 1.5 Hz, 1H), 3.58
(s, 3H), 3.03 (dd, J ¼ 16.4, 8.0 Hz, 1H), 2.70 (dd, J ¼ 16.4, 1.5 Hz, 1H),
2.45 (s, 3H); IR (KBr): 3230 (NH), 1703 (CO, ester), 1687 (C55O), 1633
(C55C), 1352 and 1526 (NO2) cm21
.
4-(4-Nitrophenyl)-5-methoxycarbonyl-6-methyl-3,4-dihydropyridone (4d): Mp
1
224–225 8C (lit.[6b] mp 223–225 8C); H NMR: d ¼ 8.14 (d, J ¼ 8.8 Hz,
2H), 7.60 (br s, 1H), 7.18 (d, J ¼ 8.8 Hz, 2H), 4.30 (dd, J ¼ 8.0, 1.8 Hz, 1H),
3.62 (s, 3H), 3.01 (dd, J ¼ 16.5, 8.0 Hz, 1H), 2.50 (dd, J ¼ 16.5, 1.8 Hz,
1H), 2.46 (s, 3H); IR (KBr): 3230 (NH), 1705 (CO, ester), 1685 (C55O),
1637 (C55C), 1350 and 1518 (NO2) cm21
.
4-(2,4-Dinitrophenyl)-5-methoxycarbonyl-6-methyl-3,4-dihydropyridone (4e):
1
Mp 212–213 8C (lit.[12] mp 212–213 8C); H NMR: d ¼ 8.08 (s, 1H), 7.75
(br s, 1H), 7.25 ꢀ 7.15 (m, 2H), 4.38 (dd, J ¼ 8.2, 1.9 Hz, 1H), 3.65 (s, 3H),
3.11 (dd, J ¼ 16.5, 8.2 Hz, 1H), 2.75 (dd, J ¼ 16.5, 1.9 Hz, 1H), 2.52

PEG Support Synthesis of 3,4-Dihydropyridones
1255
(s, 3H); IR (KBr): 3235 (NH), 1710 (CO, ester), 1698 (C55O), 1641 (C55C),
1350 and 1525 (NO2) cm21
.
4-(4-Methoxyphenyl)-5-methoxycarbonyl-6-methyl-3,4-dihydropyridone (4f):
1
Mp 187–188 8C (lit.[6b] mp 188–190 8C); H NMR: d ¼ 8.78 (br s, 1H),
7.12 (d, J ¼ 8.4 Hz, 2H), 6.80 (d, J ¼ 8.4 Hz, 2H), 4.20 (dd, J ¼ 7.9,
1.5 Hz, 1H), 3.77 (s, 3H), 3.65 (s, 3H), 2.94 (dd, J ¼ 16.5, 7.9 Hz, 1H),
2.62 (dd, J ¼ 16.5, 1.5 Hz, 1H), 2.39 (s, 3H); IR (KBr): 3220 (NH), 1702
(CO, ester), 1690 (C55O) and 1618 (C55C) cm21
.
4-(2-Chlorophenyl)-5-methoxycarbonyl-6-methyl-3,4-dihydropyridone (4g): Mp
1
198–199 8C (lit.[9a] mp 198–200 8C); H NMR: d ¼ 8.80 (br s, 1H), 7.40–
7.04 (m, 4H), 4.67 (dd, J ¼ 8.2, 1.8 Hz, 1H), 3.60 (s, 3H), 2.91 (dd, J ¼ 16.4,
8.2 Hz, 1H), 2.71 (dd, J ¼ 16.4, 1.8 Hz, 1H), 2.45 (s, 3H); IR (KBr): 3216
(NH), 1706 (CO, ester), 1686 (C55O) and 1620 (C55C) cm21
.
4-(4-Chlorophenyl)-5-methoxycarbonyl-6-methyl-3,4-dihydropyridone (4h): Mp
1
202–203 8C (lit.[12] mp 202–203 8C); H NMR: d ¼ 8.80 (br s, 1H), 8.47
(d, J ¼ 1.7 Hz, 1H), 7.75 (d, J ¼ 8.7 Hz, 2H), 4.65 (dd, J ¼ 8.3, 1.9 Hz, 1H),
3.64 (s, 3H), 2.92 (dd, J ¼ 16.5, 8.3 Hz, 1H), 2.72 (dd, J ¼ 16.5, 1.9 Hz, 1H),
2.47 (s, 3H); IR (KBr): 3218 (NH), 1708 (CO, ester), 1685 (C55O), 1623
(C55C) cm21
.
4-(4-Cyanophenyl)-5-methoxycarbonyl-6-methyl-3,4-dihydropyridone (4i): Mp
1
258–259 8C (lit.[9a] mp 259–260 8C); H NMR: d ¼ 8.40 (br s, 1H), 7.64
(d, J ¼ 8.2 Hz, 2H), 7.30 (d, J ¼ 8.2 Hz, 2H), 4.32 (dd, J ¼ 8.1, 1.0 Hz, 1H),
3.68 (s, 3H), 3.02 (dd, J ¼ 16.6, 8.1 Hz, 1H), 2.70 (dd, J ¼ 16.6, 1.0 Hz,
1H), 2.46 (s, 3H); IR (KBr): 3224 (NH), 2222 (CN), 1700 (CO, ester), 1655
(C55O) and 1613 (C55C) cm21
.
4-(4-Methoxycarbonylphenyl)-5-methoxycarbonyl-6-methyl-3,4-dihydropyri-
done (4j): Mp 186–187 8C (lit.[9a] mp 186–188 8C); 1H NMR: d ¼ 8.90 (br s,
1H), 96 (d, J ¼ 8.3 Hz, 2H), 7.28 (d, J ¼ 8.3 Hz, 2H), 4.32 (dd, J ¼ 8.2,
1.1 Hz, 1H), 3.90 (s, 3H), 3.67 (s, 3H), 2.99 (dd, J ¼ 16.5, 8.2 Hz, 1H),
2.70 (dd, J ¼ 16.5, 1.1 Hz, 1H), 2.45 (s, 3H); IR (KBr): 3225 (NH), 1725
(CO, ester), 1650 (C55O) and 1612 (C55C) cm21
.
4-(n-Propyl)-5-methoxycarbonyl-6-methyl-3,4-dihydropyridone (4k): Mp 125–
1
126 8C; H NMR: d ¼ 7.85 (br s, 1H), 4.17 (m, 1H), 3.65 (s, 3H), 2.90 (dd,
J ¼ 16.2, 7.5 Hz, 1H), 2.86 (dd, J ¼ 16.2, 1.5 Hz, 1H), 2.42 (s, 3H), 1.10–
1.37 (m, 4H), 0.78 (t, J ¼ 7.2 Hz, 3H); 13C NMR (DMSO-d6, 100 MHz): d
¼170.4, 166.2, 149.3, 105.6, 51.6, 36.8, 34.1, 29.6, 24.5, 18.8, 13.8; IR (KBr):
3220 (NH), 1721 (CO, ester), 1658 (C55O) and 1625 (C55C) cm21. Anal.
calcd. for C11H17NO3: C, 62.54; H, 8.11; N, 6.63. Found: C, 62.58; H, 8.17;
N, 6.68.

1256
G.-Y. Fu et al.
Data for Compound 5a
1H NMR (DMSO-d6): d ¼ 9.97 (br s, 1H), 7.48–7.45 (m, 2H), 7.03–7.07
(m, 3H), 4.01 (dd, J ¼ 7.5, 1.0 Hz, 1H), 2.90 (dd, J ¼ 16.4, 7.5 Hz, 1H),
2.41 (dd, J ¼ 16.4, 1.0 Hz, 1H), 1.93 (s, 3H); 13C NMR (DMSO-d6):
d ¼ 171.5, 170.1, 145.3, 141.1, 128.3, 126.2, 123.5, 109.5, 40.1, 38.4, 18.8;
IR (neat): 3220 (NH), 1700 (CO, acid), 1692 (C55O) and 1624 (C55C) cm21
.
ACKNOWLEDGMENT
We gratefully acknowledge financial support from the National Natural
Science Foundation (NSF) of China (No. 20562005) and NSF of Jiangxi
Province (No. 0620021 and No. 2007GZW0185).
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