Preparation of pyran derivatives promoted by FeCl3· 6H2O in ionic liquid

Article abstract of DOI:10.3184/030823408X324715

The condensation reaction of chaicone and 1,3-cyclohexanedione was smoothly carried out in [bmim][BF₄] with catalysis by FeCI ₃·6H₂O. A series of substituted pyrans were obtained by this reaction, in high yields under mild conditions through a simple operational procedure. In addition, the ionic liquid together with the catalyst used can be conveniently recovered and efficiently reused.

Full text of DOI:10.3184/030823408X324715

JOURNAL OF CHEMICAL RESEARCH 2008
JUNE, 357–360
RESEARCH PAPER 357
Preparation of pyran derivatives promoted by FeCl ·6H O in ionic liquid
3
2
Xinying Zhang , Quanjian Lv , Xuesen Fan and Guirong Qua
a
a,b
a*
a
School of Chemical and Environmental Sciences, Henan Key Laboratory for Environmental Pollution Control, Henan Normal
University, Xinxiang, Henan 453007, P.R. China
bZhengzhou College of Animal Husbandry Engineering, Zhengzhou, Henan 450011, P.R. China
Theꢀcondensationꢀreactionꢀofꢀchalconeꢀandꢀ1,3-cyclohexanedioneꢀwasꢀsmoothlyꢀcarriedꢀoutꢀinꢀ[bmim][BF ]ꢀwithꢀ
4
catalysisꢀbyꢀFeCl ·6H O.ꢀAꢀseriesꢀofꢀsubstitutedꢀpyransꢀwereꢀobtainedꢀbyꢀthisꢀreaction,ꢀinꢀhighꢀyieldsꢀunderꢀmildꢀ
3
2
conditionsꢀthroughꢀaꢀsimpleꢀoperationalꢀprocedure.ꢀInꢀaddition,ꢀtheꢀionicꢀliquidꢀtogetherꢀwithꢀtheꢀcatalystꢀusedꢀcanꢀ
beꢀconvenientlyꢀrecoveredꢀandꢀefficientlyꢀreused.
Keywords: pyran derivatives, ionic liquid, FeCl ·6H O, green synthesis
3
2
Pyran derivatives are of considerable interest because of their
wide range of biological properties, such as spasmolytic,
diuretic, anti-coagulant, anti-cancer, anti-anaphylactic
activity. In addition, they can be used as cognitive enhancers
for the treatment of neurodegenerative disease, including
Alzheimer’s disease, amylotrophic lateral sclerosis, and
Parkinson’s disease.1 In particular, the 4H-benzopyran
moiety is a constituent of the structures of a series of natural
products with interesting biological and pharmacological
liquids, such as good solvating capability, wide liquid range,
negligible vapour pressure, and ease of recycling, offer an
environmentally acceptable organic synthesis. Many classical
organic processes have successfully been carried out in ionic
liquids. Recently, we have developed novel methods for the
preparation of various biologically important heterocyclic
compounds, such as 1,4-DHPs, xanthenedione and quinolines
-5
in multi-component reactions by using ionic liquids as novel
10-13
reaction medium and/or promoter.
Herein we report our
6
,7
activities. Consequently the development of novel methods
for the synthesis of these heterocyclic compounds has been
of considerable interest in both organic and medicinal fields.
The condensation of α, β-unsaturated ketones with 1,3-
cyclohexanedione promoted by various catalysts has appeared
study of a novel preparation of substituted pyran derivatives
from reaction of chalcones (1) with 1,3-cyclohexanedione
(
2) in the presence of FeCl ·6H O in an ionic liquid, 1-
3 2
butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF ])
4
(Scheme 1).
8
as a novel alternative. In this regard, anhydrous zinc chloride
Our study was initiated by the treatment of 3-(4-nitrophenyl)-
was recently utilised to catalyse this reaction to afford
substituted benzopyran derivatives. However, this method
needs to be further improved because of the low yields. It has
also been reported that a combination of glacial acetic acid
and phosphoric anhydride can promote the condensation of
α,β-unsaturated ketones with 1,3-cyclohexanedione, but the
use of strongly acidic compounds conflicts with the principles
of green chemistry and may exclude some acid sensitive
1
-phenylprop-2-en-1-one (1a), previously prepared from 4-
nitrobenzaldehyde and acetophenone, with 5,5-dimethyl-1,3-
cyclohexanedione (2) in [bmim][BF ] at room temperature
4
without added catalyst. TLC analysis showed that no reaction
took place after the mixture was stirred at room temperature
for several hours. The reaction was then attempted at elevated
temperatures and it turned out that when it was run at 80°C,
it gradually gave a new substance. However, the reaction was
still incomplete after the mixture had been stirred at 80°C for
2 days. At this point, the reaction was interrupted and the new
9
substrates. As a result, the development of efficient, milder
and greener synthetic methods for the preparation of 4H-
benzopyran derivatives is still required.
Green chemistry technologies based on ionic liquid which
circumvent the use of volatile organic compounds are
attractive in organic synthesis. The unique properties of ionic
1
substance was separated and identified. H NMR spectroscopy
together with mass spectrometry result showed it is the desired
product 3a with a yield of 35% (Scheme 2).
O
Ar1
O
O
O
FeCl 6H O
3
2
Ar1
Ar2
+
8
0°C, [bmim]BF4
Ar2
O
1
2
3
Scheme 1
NO2
O
O
O
+
[bmim]BF4
O
O N
2
O
3a
1
a
2
Scheme 2
*
Correspondent. E-mail: xuesen.fan@yahoo.com

3
58 JOURNAL OF CHEMICAL RESEARCH 2008
To improve the above reaction with regard to reaction time
and yield, several mild Lewis acid type catalysts including
products and the reaction time. For example, for substrates
bearing electron-withdrawing groups on the aromatic rings,
shorter reaction time was needed and higher yields were
obtained (for 3a, 3b, 3f and 3j). On the other hand, a longer
reaction time was needed and lower yields were usually
obtained with substrates bearing electron-donating groups on
the aromatic rings (for 3i).
It is well documented that, compared with classical organic
solvents, reactions carried out in ionic liquids possess the
advantages not only of green chemistry, but are often simpler
processes and show enhanced reactivity and better yields.
This was demonstrated by comparing [bmim]BF4 with
several classical organic solvents including ethanol, toluene
and THF. The following is the results (shown in Table 3) of
investigations by using 1a and 2 as model substrates for the
preparation of 3a (Scheme 2).
InCl ·4H O, FeCl ·6H O, CeCl ·7H O, Fe (SO ) ∙xH O and
3
2
3
2
3
2
2
4 3
2
ZnCl were then tried by using 1a and 2 as model substrates.
2
The results were listed in Table 1.
From the results listed in Table 1, it can be concluded that
the above reaction was significantly improved by the addition
of Lewis acid catalyst. Of the catalysts examined, FeCl ·6H O
3
2
turned out to be the most effective one. For example, in the
presence of 20 mol% FeCl ·6H O, reaction of 1a and 2 gave
3
2
3
a exclusively after the mixture was stirred at 80°C for 6 h
with a yield of 80%. In the presence of other catalysts, the
yields were somewhat lower and the reaction needed a longer
time to complete. As for the loading of FeCl ·6H O, the results
3
2
listed in Table 1 showed that 20 mol % of FeCl ·6H O gave
3
2
a good result and increasing this amount to 50 mol % did not
make any obvious difference in terms of yield and reaction
time. Therefore, 20 mol % was chosen as the optimum loading
of FeCl ·6H O.
It can be seen that of the four solvents studied, [bmim][BF4]
gave the highest yields for 3a (80%, Table 3, entry 1) at 80°C.
3
2
Moreover, using [bmim][BF ] as the reaction medium also
Encouraged by the results obtained with 1a and 2, a variety
of substituted chalcones were tested to investigate the scope
and generality of this novel method. The results are listed
in Table 2. It showed that a wide range of chalcones reacted
4
made the separation process much easier. For example, in
[
bmim][BF ], 3a is in solid state and could be conveniently
4
isolated with high purity by filtration. On the other hand,
when ethanol, toluene or THF were used, a complicated
mixture was formed and 3a could only be obtained by column
chromatography.
smoothly with 2 in [bmim][BF ] and gave 3 in good yields. It
4
is worth noting that the electronic properties of the aromatic
ring of the chalcones has some effects on the yield of the
Table 1ꢀ Reactionꢀofꢀ1aꢀandꢀ2ꢀinꢀ[bmim][BF ]ꢀwithꢀvariousꢀcatalysts
4
Entryꢀ
Catalyst/mol%ꢀ
Time/hꢀ
Temperature/°Cꢀ
Yield/%
1
2
3
4
5
6
7
8
9
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
–ꢀ
24ꢀ
8ꢀ
80ꢀ
80ꢀ
80ꢀ
80ꢀ
80ꢀ
80ꢀ
80ꢀ
80ꢀ
80ꢀ
28
68
80
60
70
58
82
60
36
InCl ·4H Oꢀ(20)ꢀ
3
2
FeCl ·6H Oꢀ(20)ꢀ
6ꢀ
3
2
CeCl ·7H Oꢀ(20)ꢀ
12ꢀ
10ꢀ
15ꢀ
6ꢀ
6ꢀ
6ꢀ
3
2
Fe (SO ) ·xH Oꢀ(20)ꢀ
2
4 3
2
ZnCl ꢀ(20)ꢀ
2
FeCl ·6H Oꢀ(50)ꢀ
3
2
FeCl ·6H Oꢀ(10)ꢀ
3
2
FeCl ·6H Oꢀ(5)ꢀ
3
2
Table 2ꢀ Preparationꢀofꢀ3ꢀpromotedꢀbyꢀFeCl ·6H Oꢀ(20%ꢀmol)ꢀinꢀionicꢀliquid
3
2
Productsꢀ
Ar1ꢀ
Ar2ꢀ
Time/hꢀ
6ꢀ
Temperature/°Cꢀ
80ꢀ
Yield/%
80
3
a
O N
2
ꢀ
ꢀ
3
b
ꢀ
ꢀ
6ꢀ
80ꢀ
81
O N
2
3
3
c
Cl
ꢀ
ꢀ
ꢀ
ꢀ
8ꢀ
8ꢀ
80ꢀ
80ꢀ
75
70
d
3
3
e
f
H
3
C
ꢀ
ꢀ
ꢀ
ꢀ
10ꢀ
6ꢀ
80ꢀ
80ꢀ
70
85
O N
2
3
g
ꢀ
ꢀ
8ꢀ
80ꢀ
76
Br
Cl
3
3
3
h
i
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
8ꢀ
10ꢀ
6ꢀ
80ꢀ
80ꢀ
80ꢀ
75
62
82
H CO
3
j
Cl
O N
2

JOURNAL OF CHEMICAL RESEARCH 2008 359
Table 3ꢀPreparationꢀofꢀ3aꢀinꢀdifferentꢀsolvents
a
Entryꢀ
Productꢀ
Solventꢀ
Temperature/°Cꢀ
Time/hꢀ
Yield /%
1
2
3
4
a
ꢀ
ꢀ
ꢀ
ꢀ
3a
3a
3aꢀ
3a
[bmim][BF4]ꢀ
Ethanolꢀ
Tolueneꢀ
THFꢀ
80
6ꢀ
6ꢀ
6ꢀ
6ꢀ
80
28
40
46
Refluxꢀ
80ꢀ
Refluxꢀ
InꢀtheꢀpresenceꢀofꢀFeCl ·6H Oꢀ(20ꢀmol%).
3
2
Table 4ꢀ Reusabilityꢀofꢀ[bmim][BF ]ꢀandꢀcatalystꢀforꢀtheꢀpreparationꢀofꢀ3a
4
Entryꢀ
Roundꢀ
ILꢀrecovered/%ꢀ
Time/hꢀ
Temperature/°Cꢀ
Yield/%
1
2
3
4
a
ꢀ
ꢀ
ꢀ
ꢀ
1ꢀ
2ꢀ
3ꢀ
4ꢀ
96ꢀ
95ꢀ
92ꢀ
90ꢀ
6ꢀ
6ꢀ
6ꢀ
6ꢀ
80ꢀ
80ꢀ
80ꢀ
80ꢀ
80
82
78
75
InꢀtheꢀpresenceꢀofꢀFeCl ·6H Oꢀ(20ꢀmol%).
3
2
The possibility of the recovery and reutilisation of the catalyst
and solvent was then studied. Upon completion, the product
was in fact in a solid state and could be simply collected
CH), 5.68(d, 1H, J = 4.8 Hz, =CH), 7.37–7.49 (m, 4H, ArH), 7.62
d, 2H, J = 7.6 Hz, ArH), 7.72(d, 1H, J = 7.6 Hz, ArH), 8.07 (d,
(
+
1
H, J = 8.4 Hz, ArH), 8.19 (s, 1H, ArH); MS: m/z 398 [M + Na] .
Anal. Calcd for C H NO : C 73.6, H 5.6, N 3.7; found: C 73.8,
2
3
21
4
by suction. The filtrate containing [bmim][BF ] together
4
H 5.6, N 3.7%.
c: M.p. 110–111°C; IR (KBr) (cm ): 3080, 3050, 2950, 2871,
with the immobilised Fe (III) was then dried at 100°C.
Investigations by using 1a and 2 as model substrates again
showed that successive reuse of the recovered ionic liquid
and catalyst in the same reaction gave 3a with a yield of 82%
-1
3
1
1
679, 1658, 1627, 1500; H NMR (CDCl ): 1.08 (s, 3H, CH ),
3
3
1.16 (s, 3H, CH ), 2.22–2.58 (m, 4H, 2 × CH ), 4.51 (d, 1H, J = 4.8
3
2
Hz, CH), 5.70 (d, 1H, J = 4.8 Hz, =CH), 7.26–7.31 (m, 4H, ArH),
(Table 4, Entry 2). It was also observed that in the fourth
7.35–7.42 (m, 3H, ArH), 7.61(d, 2H, J = 7.6 Hz, ArH); MS: m/z 387
+
[
M + Na] . Anal. Calcd for C H ClO : C 75.7, H 5.8; found: C
round, 3a could be obtained with good yield by using the
ionic liquid and the catalyst recovered from the third round
23 21
2
7
5.5, H 6.0%.
d: M.p. 140–141°C (lit. 140°C); IR (KBr)(cm ): 3068, 3030,
8
-1
3
(Table 4, entry 4).
1
2
946, 2872, 1679, 1658, 1627, 1488; H NMR (CDCl ): 1.06 (s, 3H,
3
In conclusion, we have demonstrated that the condensation
reaction between chalcone and 1,3-cyclohexanedione could
CH ), 1.17 (s, 3H, CH ), 2.23–2.59 (m, 4H, 2 × CH ), 4.49 (d, 1H,
3
3
2
J = 4.8 Hz, CH), 5.70 (d, 1H, J = 4.8 Hz, =CH), 7.14–7.18 (m, 1H,
ArH), 7.25–7.38 (m, 7H, ArH), 7.57–7.59 (m, 2H, ArH).
be carried out smoothly in [bmim][BF ] with catalysis
4
-1
by FeCl ·6H O. In this reaction, substituted pyrans were
3e: M.p. 84–86°C; IR (KBr) (cm ): 3050, 3026, 2955, 2871, 1680,
3
2
1
1
656, 1628, 1512; H NMR (CDCl ): 1.06 (s, 3H, CH ), 1.13 (s, 3H,
obtained efficiently. It should be noted that this procedure
offers several advantages including the use of low-loading of
a green catalyst, green solvent, improved yields and simple
experimental procedures. In addition, easy workup has been
realised compared with the use of conventional organic
solvent in that the products can be obtained by filtration upon
completion of the reaction.
3
3
CH ), 2.18–2.58 (m, 7H, CH , 2 × CH ), 4.56 (d, 1H, J = 4.8 Hz,
3
3
2
CH), 5.69 (d, 1H, J = 4.8 Hz, =CH), 7.08 (d, 2H, J = 7.6 Hz, ArH),
.21 (d, 2H, J = 7.6 Hz, ArH), 7.30–7.37 (m, 3H, ArH), 7.58 (d, 2H,
7
+
J = 8.0 Hz, ArH). MS: m/z 367 [M + Na] . Anal. Calcd for C H O :
2
4
24
2
C 83.7, H 7.0; found: C 83.5, H 6.9%.
3f: M.p. 148–150°C; IR (KBr) (cm-1): 3068, 3025, 2958, 2866,
1
1658, 1625, 1594, 1516; H NMR (CDCl ): 1.12 (s, 3H, CH ), 1.18
3
3
(
s, 3H, CH ), 2.24–2.67 (m, 4H, 2 × CH ), 4.57 (d, 1H, J = 4.8 Hz,
3
2
CH), 5.94 (d, 1H, J = 4.8 Hz, =CH), 7.22–7.33 (m, 5H, ArH), 7.77
Experimental
(
d, 2H, J = 8.4 Hz, ArH), 8.24 (d, 2H, J = 8.4 Hz, ArH); MS: m/z 398
Melting points were measured by a Kofler micromelting point
apparatus and were uncorrected. H NMR spectra were determined
on a Bruker AC 400 spectrometer as CDCl or DMSO-d solutions.
Chemical shifts (d) were expressed in ppm downfield from the
internal standard tetramethylsilane and coupling constants J were
given in Hz. Mass spectra were obtained in ESI mode using a Bruker
Esquire 3000 mass spectrometer. Elemental analyses were performed
on an EA-1110 instrument.
+
[M + Na] . Anal. Calcd for C H NO : C 73.6, H 5.6, N 3.7; found:
2
3
21
4
1
3
6
-1
1
3
3
(
s, 3H, CH ), 2.22–2.62 (m, 4H, 2 × CH ), 4.51 (d, 1H, J = 4.8 Hz,
3
2
CH), 5.73 (d, 1H, J = 4.8 Hz, =CH), 7.19–7.22(m, 1H, ArH), 7.29–
7
+
.34 (m, 4H, ArH), 7.46–7.53 (m, 4H, ArH); MS: m/z 431 [M + Na] .
Anal. Calcd for C H BrO : C 67.5, H 5.2; found: C 67.2, H 5.4%.
2
3
21
2
8
-1
3
h: M.p. 172–174°C (lit. 175–176°C); IR (KBr) (cm ): 3079,
046, 2958, 2870, 1688, 1655, 1626, 1490; H NMR (CDCl3):
General procedure for the preparation of substituted pyran
derivatives (3)
1
3
1
4
7
.09 (s, 3H, CH ), 1.16 (s, 3H, CH ), 2.22–2.63 (m, 4H, 2 × CH ),
.51 (d, 1H, J = 4.8 Hz, CH), 5.74 (d, 1H, J = 4.8 Hz, =CH),
.20–7.34 (m, 5H, ArH), 7.36 (d, 2H, J = 8.4 Hz, ArH), 7.53(d, 2H,
3
3
2
To [bmim][BF ] 1 ml were added chalcone 1 mmol, 1, 3-cyclo-
4
hexanedione 1 mmol and FeCl ·6H O (20 mol%). The reaction
mixture was stirred at 80°C until the reaction was complete (monitored
by TLC). Then, the mixture was cooled to room temperature and
3
2
J = 8.4 Hz, ArH).
i: M.p. 133–134°C; IR (KBr) (cm ): 3066, 3018, 2971, 2870,
-1
3
5
0% ethanol water 1 ml was added. The solids were collected by
1
1
1
4
686, 1658, 1620, 1460; H NMR (DMSO-d ): 1.06 (s, 3H, CH ),
.13 (s, 3H, CH ), 2.18–2.58 (m, 4H, 2 × CH ), 3.81 (s, 3H, OCH ),
.47 (d, 1H, J = 4.8 Hz, CH), 5.57 (d, 1H, J = 4.8 Hz, =CH), 6.89 (d,
suction and rinsed with water and ethanol, and then dried to give 3
high purity. The ionic liquid layer was dried at 100°C under reduced
pressure to recover the ionic liquid together with the catalyst.
6
3
3
2
3
-
1
2H, J = 9.6 Hz, ArH), 7.14–7.18 (m, 1H, ArH), 7.25–7.33 (m, 4H,
3
a: M.p. 131–132°C; IR (KBr) (cm ): 3090, 3051, 2960, 2874,
+
1
ArH), 7.51 (d, 2H, J = 9.6 Hz, ArH); MS: m/z 383 [M + Na] . Anal.
1
680, 1652, 1590, 1516; H NMR (CDCl ): 1.09 (s, 3H, CH ), 1.18
3
3
Calcd for C H O : C 80.0, H 6.7; found: C 80.2, H 6.7%.
(
s, 3H, CH ), 2.22–2.61 (m, 4H, 2 × CH ), 4.65 (d, 1H, J = 4.8 Hz,
24 24
3
3
2
-1
CH), 5.67 (d, 1H, J = 4.8 Hz, =CH), 7.40–7.42 (m, 3H, ArH), 7.52 (d,
H, J = 8.0 Hz, ArH), 7.61–7.62(m, 2H, ArH), 8.17 (d, 2H, J = 8.0 Hz,
3j: M.p. 202–204°C; IR (KBr) (cm ): 3065, 3012, 2966, 2868,
1670, 1656, 1627, 1512; H NMR (CDCl3): 1.10 (s, 3H, CH3), 1.18
1
2
+
ArH); MS: m/z 398 [M + Na] . Anal. Calcd for C H NO : C 73.6,
H 5.6, N 3.7; found: C 73.8, H 5.4, N 3.6%.
(s, 3H, CH3), 2.23–2.60 (m, 4H, 2 × CH2), 4.54 (d, 1H, J = 4.8 Hz,
CH), 5.89 (d, 1H, J = 4.8 Hz, =CH), 7.25–7.30 (m, 4H, ArH), 7.76 (d,
2H, J = 8.8 Hz, ArH), 8.25(d, 2H, J = 8.8 Hz, ArH); MS: m/z 432 [M
2
3
21
4
-
1
3
b: M.p. 139–141°C; IR (KBr) (cm ): 3075, 3066, 2960, 2868,
1
+
1
681, 1660, 1631,1529; H NMR (CDCl ): 1.11 (s, 3H, CH ), 1.18
+ Na] . Anal. Calcd for C23H20ClNO4: C 67.4, H 4.9, N 3.4; found:
3
3
(
s, 3H, CH ), 2.23–2.67 (m, 4H, 2 × CH ), 4.66 (d, 1H, J = 4.8 Hz,
C 67.4, H 4.7, N 3.6%.
3
2

3
60 JOURNAL OF CHEMICAL RESEARCH 2008
This work was financially supported by the National Natural
Science Foundation of China (No. 20772025) and Program
for Science & Technology Innovation Talents in Universities
of Henan Province (No. 2008HASTIT006) and Department of
Education of Henan Province (No. 2008A150013).
4
5
6
7
8
9
0
Y.L. Zhang, B.Z. Chen, K.Q. Zheng, M.L. Xu and X.H. Lei, Yao Xue Xue
Bao, 1982, 17, 17.
L. Bonsignore, G. Loy, D. Secci and A. Calignano, Eur. J. Med. Chem.,
1
993, 28, 517.
S. Hatokeyama, N. Ochi, H. Numata and S. Takano, Chem. Commun.,
998, 17, 1202.
1
R. Gonzalez, N. Martin, C. Seoane, J.L. Marco, A. Albert and F.H. Cano,
Tetrahedron Lett., 1992, 33, 3809.
M.G. Ahmed, S.A. Ahmed, U.K.R. Romman, M.A. Hena and S. Kiyooka,
Indian J. Chem., 2002, 41B, 368.
Received 24 March 2008; accepted 10 May 2008
Paper 08/5177 doi: 10.3184/030823408X324715
V.K. Ahluwalia, S. Kiran, R. Aggarwal and K.K. Arora, Indian J. Chem.,
1
991, 30B, 1095.
References
1
X.S. Fan, X.Y. Hu, X.Y. Zhang and J.J. Wang, Aust. J. Chem., 2004, 57,
1
2
3
A. Shaabani, A.H. Rezayan, A. Sarvary, A. Rahmati and H.R. Khavasi,
Catal. Commun., 2008, 9, 1082.
L. Fotouhi, M.M. Heravi, A. Fatehi and K. Bakhtiari, Tetrahedron Lett.,
1067.
11 X.S. Fan, X.Y. Hu, X.Y. Zhang and J.J. Wang, Can. J. Chem., 2005, 83, 16.
12 X.S. Fan, Y.Z, Li, X.Y. Zhang, X.Y. Hu and J.J. Wang, Can. J. Chem.,
2006, 84, 794.
2
007, 48, 5379.
K. Singh, J. Singh and H. Singh, Tetrahedron, 1996, 52, 14273.
13 X.Y. Zhang, X.S. Fan, H.Y. Niu and J.J. Wang, Green Chem., 2003, 5, 267.