Polymer-supported zinc chloride: A highly active and reusable heterogeneous catalyst for one-pot synthesis of 2,4,5-trisubstituted imidazoles

Article abstract of DOI:10.1007/s00706-008-0086-2

A polymer-supported Lewis acid catalyst was prepared and found to be extremely efficient for the preparation of imidazole derivatives via three-component condensation of benzil, aldehydes, and ammonium acetate under mild conditions. The catalyst is stable

Full text of DOI:10.1007/s00706-008-0086-2

Monatsh Chem (2009) 140:541–546
DOI 10.1007/s00706-008-0086-2
ORIGINAL PAPER
Polymer-supported zinc chloride: a highly active and reusable
heterogeneous catalyst for one-pot synthesis of 2,4,5-trisubstituted
imidazoles
Liang Wang Æ Chun Cai
Received: 21 June 2008 / Accepted: 7 October 2008 / Published online: 15 November 2008
Ó Springer-Verlag 2008
Abstract A polymer-supported Lewis acid catalyst was
prepared and found to be extremely efficient for the prep-
aration of imidazole derivatives via three-component
condensation of benzil, aldehydes, and ammonium acetate
under mild conditions. The catalyst is stable and can be
easily recovered and reused at least four times without any
loss of activity.
N-(2-oxo)amides with ammonium trifluoroacetate [9] and
1,2-aminoalcohols in the presence of PCl5 [10], and many
more. Among these methods, the one-pot condensation of
diketones, aldehydes, and ammonium acetate catalyzed by
protic acids [11–13] and Lewis acids [14–16] is a typical
procedure which has been extensively investigated.
Recently, methods utilizing microwave irradiation [17–19],
ionic liquids [20], solid acid [21], and supported catalysts
[22–24] have also been described. However, some of the
reported methods suffer from drawbacks, for example non-
recyclability, harsh reaction conditions, long reaction times,
tedious work-up, expensive catalysts and apparatus, and
environmental pollution. Therefore, a need still exists for
versatile, simple, and environmentally friendly processes
which produce imidazoles under milder and practical
conditions.
Keywords Functional polymer ꢀ Heterogeneous catalyst ꢀ
Polymer-supported ꢀ 2,4,5-trisubstituted imidazoles
Introduction
The imidazole moiety is present in a wide range of natu-
rally occurring molecules [1] and pharmaceutically active
compounds [2]. Among these, 2,4,5-trisubstituted imida-
zoles are of great interest because of their chemical and
biological activity, making them common structures in
numerous synthetic compounds, for example fungicides
[3], herbicides [3], plant-growth regulators [4], and thera-
peutic agents [5].
In recent years, great efforts in catalysis research have
been devoted to the introduction and application of effec-
tive and safe heterogeneous catalysts [25]. The use of
polymer-supported catalysts in organic transformations has
been receiving extraordinary attention [26], and the design
of functionalized polymers carrying metal species has
generated substantial interest [27]. The utilization of
polymer-supported catalysts has several advantages in
preparative procedures. Such catalysts are as active as their
homogeneous counterparts while having the distinguishing
characteristics of being easily separable from the reaction
media, recyclability, greater selectivity, enhanced stability,
easier handling, non-toxicity, and low cost [27].
The original synthetic route to imidazoles utilized gly-
oxal, formaldehyde, and ammonia, as reported by Debus
[6]. Numerous classical methods for the synthesis of
multisubstituted imidazoles have subsequently been devel-
oped, for example the hetero-Cope rearrangement [7],
four-component condensation of arylglyoxals, primary
amines, carboxylic acids, and isocyanides [8], reaction of
Although many polymer supports were modified on the
basis of Merrifield resin, the chloromethyl methyl ether
used during its synthesis is carcinogenic. Furthermore,
problems such as poly-substitution and secondary cross-
linking lead to undefined structures. For this reason, Liu’s
method for preparation of a chloroacetylated polystyrene
L. Wang ꢀ C. Cai (&)
Chemical Engineering College,
Nanjing University of Science and Technology,
210094 Nanjing, People’s Republic of China
e-mail: c.cai@mail.njust.edu.cn
123

542
L. Wang, C. Cai
Ph
Ph
at different stages of synthesis. As seen from Fig. 1, the
C–Cl peaks at 1,285 and 700 cm-1 in the starting polymer
were practically absent or weakened after introduction of
DEA. A strong and broad band at 3,450 cm-1 in curve B
and in curve C was assigned to the –OH vibration, and the
sharp peaks at 1,100–1,200 cm-1 were attributed to the C–
N vibration. The sharp peaks at approximately 1,650–
1,700 cm-1 in the three curves were attributed to the C=O
vibration. Furthermore, there were some differences
between curve B and curve C and some special peaks
shifted to lower or higher wavenumbers to some extent.
Although it was difficult to tell whether the peak of –OH
changed after introducing zinc chloride, because of its
broad peak, the peak of the C–O vibration shifted to higher
wavenumber (*44 cm-1) in the spectra of the supported
catalyst, indicating there was a coordination between –OH
and Zn. Comparison of the IR spectra of the modified resin
and the supported catalyst revealed that peak of the C=O
N
Ph
Ph
cat
O
O
2NH4OAc
+
R
+
RCHO
N
H
Ethanol, reflux
HO
cat.
HO
Zn2+
N
-
O
. 2Cl
PS
Scheme 1
resin by acylation [28] offers the advantages of convenient
and safe preparation and increased chelating properties.
As a part of our program aiming at developing novel
functionalized polymer-supported catalysts, and in contin-
uation of our interest in heterogeneously catalyzed organic
reactions, we would like to report our investigation con-
cerning the preparation of a new functionalized polymer-
supported catalyst and the direct synthesis of 2,4,5-triary-
limidazoles (Scheme 1).
vibration had shifted towards lower frequency (*24 cm-1
)
from 1,676 to 1,652 cm-1, suggesting the coordination of
C=O and Zn. There was also a slight shift (*7 cm-1) to
lower wavenumbers for the C–N absorption peak, which
can be explained by coordination of N and the metal. The
IR spectra data are summarized in Table 1.
Results and discussion
The polymer-supported zinc catalyst was prepared starting
from a chloracetylated polystyrene resin which was first
reacted with diethanolamine (DEA) and later loaded with
ZnCl2.
Quantitative analysis of resin 2 (see ‘‘Experimental’’
section) and supported catalyst 3 was also conducted uti-
lizing elemental analysis and ICP analysis. Results showed
that the weight percentage of nitrogen, coming from the
DEA ligand, was 5.53%. The Zn content anchored on the
polymer support was determined by ICP and the loading
capacity was 1.47 mmol g-1. All these results suggested
that the DEA ligand and zinc chloride were successfully
anchored on to the polymer matrix.
Characterization of the functionalized resin and its
zinc complex
IR spectral analysis and elemental analysis
In order to ascertain the attachment of DEA and the metal
to the polymer support, IR spectra were recorded separately
XPS analysis
XPS analysis of resin 2 and supported catalyst 3 was per-
formed in order to confirm the formation of the complex.
The data are shown in Table 2.
C
As shown in Table 2, the Zn2p3/2 binding energy of the
metal complex was decreased by 1.1 eV compared with
ZnCl2; the change of Zn2p3/2 binding energy reflects an
increase of its electron density. The N1s binding energy of
the ligand and complex was 399.1 and 399.8 eV, which
means there is a coordination effect between nitrogen and
zinc. XPS analysis of the oxygen atoms of the complex
furnished three peaks, which may correspond to C=O,
–OH, and other types of oxygen. The binding energies of
O11s and O21s of the complex were both increased by
0.4 eV compared with those of the ligand while the binding
energy of O31s, which belongs to the uncoordinated oxy-
gen, remained the same. All these results indicated that the
polymer-supported zinc chloride was formed.
B
A
3900 3600 3300 3000 2700 2400 2100 1800 1500 1200 900 600
-1
Wavenumber/cm
Fig. 1 IR spectra of chloroacetylated polystyrene (a), diethanolamine
functionalized resin (b), and supported catalyst (c)
123

Polymer-supported zinc chloride
543
Table 1 Infrared spectral data of resins 1 and 2, and catalyst 3
Compound
m
OH/cm-1
m
CH2/cm-1
m
C=O/cm-1
m
C–N/cm-1
m
C–O/cm-1
m
C–Cl/cm-1
Resin 1
–
2,923
2,931
2,927
1,680
1,676
1,652
–
1285, 700
Weak
Resin 2
3,450
3,450
1,195
1,188
1,228
1,272
Catalyst 3
Weak
100
80
60
40
20
0
Table 2 XPS data of ZnCl2, functionalized resin 2, and catalyst 3
XPS peaks
Binding energy/eV
DEb/eV
ZnCl2
Resin 2
Catalyst 3
Zn2p3/2
N1s
1023.1
–
1022.0
399.8
533.7
532.2
528.4
-1.1
?0.7
?0.4
?0.4
–
–
–
–
–
399.1
533.3
531.8
528.4
O11s
O21S
O31S
The binding energy is referred to C1s = 284.8 eV
Thermal and pH effects on the stability of the supported
catalyst
0
2
4
6
8
10
12
14
The stability of supported catalyst is very important and is
responsible for the catalytic effect and for catalyst recycling.
It was found there was no weight loss of the functional resin
before 150 °C whereas for the supported catalyst the value
was 180 °C, indicating the catalyst was more stable than the
support. The pH of the solution also had an important effect
on the stability of the catalyst. Experiments were carried out
in buffer solutions of different pH to test the retention of zinc
ion on the support. As is apparent from Fig. 2, retention of
the zinc ion increased rapidly with increasing pH from 1 to
6. The retention achieved was highest at pH 7 and it
remained steady until pH 11. However, retention decreased
under strongly basic conditions. All these results indicated
the catalyst was very stable under neutral and weakly basic
conditions whereas the Zn ion was released from the support
rather easily under acidic conditions.
pH
Fig. 2 The relationship between pH and retention of zinc by the
polymer
occurred. An excellent yield was obtained when using the
DEA–ZnCl2 complex, indicating that the DEA ligand was
very effective (Table 3, entry 6). Supported catalyst 3
containing 250, 125, and 50 mg zinc chloride per g resin 2
was then evaluated in the model reaction. For the higher
catalyst loadings (entries 7 and 8) good yields were
obtained, whereas lower conversion was observed with
lower catalyst loading (entry 9). Next, we examined the
effect of the amount of catalyst (catalyst loading
250 mg g-1) on the reaction. No reaction was observed in
the absence of catalyst but on adding 5 mol% catalyst 3 to
the system under similar conditions the rate of the reaction
was obviously accelerated, but the yield was unsatisfac-
tory. Increasing the amount to 20 mol% not only increased
the yield but also shortened the reaction time, and results
clearly indicated that 15 mol% catalyst was sufficient.
To confirm the activity of catalyst 3, a comparison
experiment was carried out by using resin 2 alone under the
same experimental conditions; no product was isolated
(entry 11). This indicated that the resin itself did not pro-
mote the reaction. To discover whether the reaction
occurred in the solid matrix of the supported catalyst or
whether zinc chloride simply released into the ethanol was
responsible for the reaction, catalyst 3 was heated under
reflux in ethanol for 10 min and then isolated by filtration.
When reactants were added to the filtrate and heated under
One-pot synthesis of 2,4,5-trisubstituted imidazoles 4
Effect of different Lewis acid catalysts on the reaction
First, the condensation of benzil, benzaldehyde, and
ammonium acetate was chosen as model reaction to detect
whether the supported catalyst and other Lewis acids were
efficient. The results are summarized in Table 3.
It was found that conventional Lewis acids, for example
AlCl3, FeCl3ꢀ6H2O, and NiCl2ꢀ6H2O gave low to moderate
yields. Enlarging the amount of these catalysts did not
promote the reaction. When using zinc chloride as catalyst,
the reaction proceeded smoothly to afford the corre-
sponding product in good yield, and no side-reaction
123

544
L. Wang, C. Cai
C2H5OH were much better than non-polar solvents. No
reaction occurred when H2O was used as solvent, maybe
because aggregation of the catalyst, caused by its hydro-
phobic nature, led to inadequate access of the substrates to
the active sites of the catalyst. Although HOAc was most
effective, a low yield was obtained when the catalyst was
reused (Table 4, entry 5). These results indicated the metal
was readily released from the support by the acidic con-
ditions and HOAc itself could catalyze the condensation to
some extent. We therefore selected ethanol as solvent.
Table 3 Effect of the catalyst on the model reaction
Entry Catalyst
Amount of
catalyst/mol%
Time/h Yield/%a
1
–
–
20
20
20
20
3
NR
52
47
71
83
97
96
80
61
79
87
96
96
NR
NR
2
AlCl3
3
3
FeCl3ꢀ6H2O
NiCl2ꢀ6H2O
ZnCl2
3
4
3
5
3
6
Diethanolamine–ZnCl2 20
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
3
7b
8c
9d
10
11
12
13
14
15e
3
–
–
3
Effect of temperature on the reaction
3
–
3
5
The reaction temperature has a great influence on the
model reaction. The reactions were carried out in ethanol at
different temperatures ranging from 30 to 78 °C. The
results are presented in Table 5. It is clear that at lower
temperatures, even if the time was prolonged to 12 h, only
low yields were observed. Consequently, we chose 78 °C
as the optimal temperature for our reaction.
3
10
15
20
20
10
3
3
Resin 2
3
1.5
All reactions were carried out in ethanol at 78 °C
NR means no reaction
Recycling of the catalyst
a
Isolated yields
0.1 g catalyst 3 was used (catalyst loading was 250 mg g-1
b
)
A very important advantage of polymer-supported catalysts
is the possibility of catalyst recycling. In order to verify if
the catalyst could be reused, condensation of benzil,
benzaldehyde, and ammonium acetate was chosen as
model reaction (Scheme 1). It was found that the catalyst
was quantitatively recovered by simple washing and fil-
tration, and proved to be reusable four times without any
loss of catalytic activity, furnishing the corresponding
product with 96, 96, 95, and 94% isolated yields. Along
these lines, there were no significant differences between
the IR spectra of the initial and recovered catalyst, which
may indicate there was no degradation of the supported
catalyst. The extent of zinc ion leaching from the catalyst
was very low (\10 ppm) as judged by ICP analysis. From
the first run to the fourth run the zinc content of the
recovered catalyst remained almost the same. All these
results demonstrated that our catalyst was stable and has
practical utility.
c
d
e
0.1 g catalyst 3 was used (catalyst loading was 125 mg g-1
)
0.1 g catalyst 3 was used (catalyst loading was 50 mg g-1
Catalyst 3 was removed by filtration after 10 min
)
reflux for 1.5 h, no reaction took place (Table 3, entry 12).
These results indicated the reaction takes place in the solid
matrix and that release of ZnCl2 is negligible.
Effect of solvent on the reaction
Investigation of reaction medium for the process revealed
that solvents played a significant role in the reaction under
investigation. The results are summarized in Table 4. It
was found that polar solvents such as HOAc, CH3CN, and
Table 4 Effect of the solvent on the model reaction
Entry
Solvent
Time/h
Yield/%a
1
2
3
4
5
CH2Cl2
H2O
4
37
Table 5 Effect of the temperature on the model reaction
4
Trace
82
Entry
T/°C
Time/h
Yield/%a
CH3CN
C2H5OH
CH3COOH
2
1
2
3
4
30
45
60
78
12
12
2
21
53
80
96
1.5
1.5
96
98, 47b
All reactions were carried out under reflux conditions with 15 mol%
catalyst 3
1.5
a
Isolated yields
All reactions were carried out using 15 mol% catalyst 3 in ethanol
a
Isolated yields
b
Catalyst was reused
123

Polymer-supported zinc chloride
545
Synthesis of different 2,4,5-trisubstituted imidazole
derivatives 4
IR spectra were recorded in KBr disks with a Bomem
MB154S FT-IR spectrometer. Elemental analyses were
performed on a Yanagimoto MT3CHN recorder. Metal
content was measured by ICP analysis on a Varian AA240
analyzer after digesting the samples in conc. HNO3. XPS
analysis was performed with the PHI-5702/ESCA/SAM
equipped with an Mg Ka (1253.6 eV) X-ray source. The
binding energy (BE) of the C1s peak at 284.8 eV was taken
as internal standard. Thermogravimeric analyses were
carried out on a Shimadzu DT-30 instrument at a heating
Encouraged by the results described above, the one-pot
condensation of 2,4,5-trisubstituted imidazoles 4 catalyzed
by polymer-supported catalyst 3 was extended to other
aldehydes to explore the scope of this procedure. The
results are summarized in Table 6. Accordingly, a wide
range of aldehydes can be effectively used as substrates.
Aromatic aldehydes with either electron-donating or elec-
tron-withdrawing substituents afforded the corresponding
imidazoles in good to excellent yields (84–97%). However,
the reaction system was not suitable for aliphatic alde-
hydes. Even if the time was prolonged, only low yields
were obtained [15].
1
rate of 20 ° min-1 under an atmosphere of nitrogen. H
NMR spectra were recorded with a Bruker Advance
RX300 instrument. Mass spectra were recorded on a Saturn
2000GC/MS instrument or a Agilent 1100LC/MS instru-
ment. All products were known compounds and were
identified by comparison of their physical and spectral data
with those of authentic samples.
In conclusion, a novel DEA functionalized resin, 3, was
prepared from chloroacetylated polystyrene resin, and its
zinc chloride complex could easily be synthesized. The
catalyst exhibits high catalytic activity in the one-pot
synthesis of 2,4,5-trisubstituted imidazoles 4. The signifi-
cant advantages of this methodology are mild reaction
conditions, high yields, short reaction times, simple work-
up procedures, and low cost. Most importantly, the
immobilized catalyst was very stable and can be reused at
least four times without any loss of activity.
Typical procedure for synthesis of functionalized
polymer and its zinc complex
DEA functionalized polymer was synthesized by a slightly
modified literature method [28]. Pre-washed chloroacety-
lated styrene–divinylbenzene copolymer beads 1 (1.0 g)
were swollen in 30 cm3 dioxane for 2 h. DEA (1.05 g) was
added and the mixture was stirred at 110 °C for 24 h in the
presence of 0.79 g pyridine (10 mmol). The color of the
beads changed from brown to yellow. The beads were then
isolated by filtration, washed with water and methanol, and
dried at 80 °C under vacuum overnight. Yield, 1.756 g;
capacity, 4.1 mmol g-1. Elemental analysis of nitrogen
was 5.53%. Resin 2 (0.5 g) obtained above was added to a
solution of 0.476 g ZnCl2 in 20 cm3 MeOH and the mix-
ture was stirred at 65 °C for 6 h. The resulting beads, 3,
were isolated by filtration, washed with water and metha-
nol, and dried at 80 °C under vacuum overnight. Yield,
0.630 g; capacity, 1.52 mmol g-1 (1.47 mmol g-1 by ICP
analysis) (Scheme 2).
Experimental
Chloroacetylated polystyrene (7% divinylbenzene, 15.4%
chlorine, grain size range: 240–280 lm, surface area:
32 m2 g-1, pore diameter: 80–100 A, total pore volume:
˚
0.11 cm3 g-1) was obtained from Nanjing Microspheres
Hi-Efficiency Isolation Carrier; DEA, anhydrous zinc
chloride, and other reagents were obtained commercially
and used without further purification.
Table 6 Supported zinc chloride-catalyzed one-pot synthesis of
2,4,5-trisubstituted imidazoles
Stability of the supported catalyst under different
pH conditions
Entry
R
Product
Yield/%a
1
C6H5
4a
4b
4c
4d
4e
4f
96
94
85
97
84
87
89
23
31
The supported catalyst (0.05 g) was added to buffer solu-
tion of different pH and the mixture was stirred at 25 °C for
12 h. The catalyst was the removed by filtration and the
concentration of zinc ions in the solution was determined
by ICP. Retention of zinc on the polymer was calculated by
use of Eq. 1 [28]:
2
4-CH3–C6H4
4-OH–C6H4
4-CH3O-C6H4
4-NO2–C6H4
4-Cl–C6H4
4-Br–C6H4
n-C2H5
3
4
5
6
7
4g
4h
4i
C ꢁ V ꢁ 10ꢂ3
8b
9b
a
R% ¼
ꢁ 100%
ð1Þ
W ꢁ Q
n-C3H7
where Q is the metal amount adsorbed (mmol g-1), C is
the concentration of zinc ion in the solution (mmol dm-3),
Isolated yields
b
Reaction time was prolonged to 4 h
123

546
L. Wang, C. Cai
Scheme 2
OH
Zn2+
HO
O
O
OH
OH
O
-
ZnCl2
DEA
. 2Cl
N
PS
Cl
PS
PS
N
MeOH
dioxane
1
2
3
V is the volume (cm3), and W is the weight of supported
catalyst (g).
3. Maier T, Schmierer R, Bauer K, Bieringer H, Buerstell H, Sachse
B (1989) US Patent 4(820):335
4. Schmierer R, Mildenberger H, Buerstell H (1987) German Patent
3,614,364
General procedures for synthesis of 2,4,5-trisubstituted
imidazoles 4
5. Heeres J, Back LJJ, Mostmans JH, Vancutsem J (1979) J Med
Chem 22:1003
6. Debus H (1858) Liebigs Ann Chem 107:199
7. Lantos I, Zanng WY, Shiu Y, Eggleston DS (1993) J Org Chem
58:7092
8. Zhang C, Moran EJ, Woiwade TF, Short KM, Mjalli AMM
(1996) Tetrahedron Lett 37:751
9. Claiborne CF, Liverlon NJ, Nguyen KT (1998) Tetrahedron Lett
39:8939
10. Bleicher KM, Gerber F, Wuthrich Y, Alanine A, Caprella A
(2002) Tetrahedron Lett 43:7687
11. Liu FJ, Chen J, Zhao J, Zhao Y, Li L, Zhang H (2003) Synthesis
2661
12. Sarshar S, Siev D, Mjalli AMM (1996) Tetrahedron Lett 37:835
13. Weinmann H, Hahhe M, Koeing K, Mertin E, Tilstam U (2002)
Tetrahedron Lett 43:593
14. Kidwai M, Mothsra P, Bansal V, Somvanshi RK, Ethayathulla
AS, Dey S, Singh TP (2007) J Mol Catal A Chem 265:177
15. Wang LM, Wang YH, Tian H, Yao YF, Shao JH, Liu B (2006)
J Fluorine Chem 127:1570
16. Heravi MM, Bakhtiari K, Oskooie S, Taheri HA (2007) J Mol
Catal A Chem 263:279
17. Balalaie S, Hashemi MM, Akhbari M (2003) Tetrahedron Lett
44:1709
18. Sparks RB, Combs AP (2004) Org Lett 6:2473
19. Wolkenberg SE, Wisnoski DD, Leister WH, Wang Y, Zhao Z,
Lindsley CW (2004) Org Lett 6:1453
20. Xia M, Lu YD (2007) J Mol Catal A Chem 265:205
21. Shaabani A, Rahmati A, Farhangi E, Badri Z (2007) Catal
Commun 8:1149
A mixture of 1 mmol aldehyde, 0.210 g benzil (1 mmol),
0.148 g ammonium acetate (2.5 mmol), and 0.102 g 3
(0.15 mmol, based on zinc) in 2 cm3 ethanol was heated
under reflux for an appropriate time as indicated by TLC.
After completion of the reaction acetone was added to the
mixture and the catalyst was isolated by filtration and
washed with ether. The filtrate was concentrated on a
rotary evaporator under reduced pressure and the solid
product obtained was washed with water and recrystallized
from acetone–water 9:1 (v/v) to afford the pure products
4a–4i.
2,4,5-Triphenylimidazole (4a): Mp [ 260 °C (274–
278 °C [29]); 2-(4-methylphenyl)-4,5-diphenylimidazole
(4b): Mp 226–228 °C (227–228 °C [30]); 2-(4-hydroxy-
phenyl)-4,5-diphenylimidazole (4c): Mp 236–238 °C
(233 °C [31]); 2-(4-methoxyphenyl)-4,5-diphenylimida-
zole (4d): Mp 227–228 °C (227–228 °C; [15]); 2-(4-
nitrophenyl)-4,5-diphenylimidazole (4e): Mp 235–236 °C
(240 °C [30]; 2-(4-chlorophenyl)-4,5-diphenylimidazole
(4f): Mp 258–260 °C (261–263 °C [16]); 2-(4-bromophe-
nyl)-4,5-diphenylimidazole (4g): Mp 261–262 °C (261.5–
263.5 °C [32]); 2-ethyl-4,5-diphenylimidazole (4h): Mp
225–227 °C (223–224 °C [15]); 2-propyl-4,5-diphenyl-
imidazole (4i): 255–257 °C (256–258 °C [15]).
22. Usyatinsky AY, Khmelnitsky YL (2000) Tetrahedron Lett
41:5031
23. Xu Y, Wan LF, Salehi H, Deng W, Guo QX (2004) Heterocycles
63:1613
24. Shaabani A, Rahmati A (2006) J Mol Catal A Chem 249:246
25. Ley SV, Baxendale IR, Bream RN, Jackson PS, Leach AG,
Longbottom DA, Nesi M, Scott JS, Storer RI, Taylor SJ (2000)
J Chem Soc Perkin Trans 1(23):3815 and references cited therein
26. Bailey DC, Langer SH (1981) Chem Rev 81:109
27. Leadbeater NE, Macro M (2002) Chem Rev 102:3217
28. Wang YQ, Wei RQ, Liu XN, Liu Y (2007) Chin J Proc Eng 7:49
29. Alexander YU, Yuri LK (2000) Tetrahedron Lett 41:5031
30. Cook AH, Jones DG (1941) J Chem Soc 278
31. Shapi AS, Umesh CN, Sanjay SP, Thomas D, Rajgopal JL,
Kumar VS (2005) Tetrahedron 61:3539
Acknowledgments We thank Nanjing Microspheres Hi-Efficiency
Isolation Carrier Co. Ltd for providing chloroacetylated polystyrene
resin and the National Defense Committee of Science and Technol-
ogy of China (40406020301) for financial support.
References
1. Ho JZ, Hohareb RM, Ahn JH, Sim TB, Rapoport H (2003) J Org
Chem 68:109
2. Lombardino JG, Wiseman EH (1974) J Med Chem 17:1182
32. White DM, Sonnenberg J (1964) J Org Chem 29:1926
123