A novel and convenient synthesis of 4-hydroxy-6-methyl-3-(1-(phenylimino) ethyl)-2H-pyran-2-one derivatives under ultrasound irradiation

Article abstract of DOI:10.1016/j.ultsonch.2011.01.006

A facile, efficient and environment-friendly protocol for the synthesis of 4-hydroxy-6-methyl-3-(1-(phenylimino)ethyl)-2H-pyran-2-one derivatives has been developed by the convenient ultrasound-mediated condensation of amine with dehydroacetic acid. This method provides several advantages over current reaction methodologies including a simple work-up procedure, shorter reaction times and higher yields.

Full text of DOI:10.1016/j.ultsonch.2011.01.006

Ultrasonics Sonochemistry 18 (2011) 1048–1051
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
A novel and convenient synthesis of 4-hydroxy-6-methyl-3-(1-(phenylimino)
ethyl)-2H-pyran-2-one derivatives under ultrasound irradiation
Huiyan Wang a,b, Yi Zou a,c, Xuan Zhao a, Daqing Shi a,
⇑
a Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
b Department of Chemical Engineering, Huaihai Institute of Technology, Lianyungang 222005, PR China
c School of Chemistry and Chemical Engineering, Xuzhou Normal University, Xuzhou 221116, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 April 2010
Received in revised form 11 January 2011
Accepted 14 January 2011
Available online 20 January 2011
A facile, efficient and environment-friendly protocol for the synthesis of 4-hydroxy-6-methyl-3-(1-
(phenylimino)ethyl)-2H-pyran-2-one derivatives has been developed by the convenient ultrasound-mediated
condensation of amine with dehydroacetic acid. This method provides several advantages over current
reaction methodologies including a simple work-up procedure, shorter reaction times and higher yields.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Schiff base
Azomethine
Dehydroacetic acid
Ultrasound irradiation
Synthesis
1. Introduction
ethyl)-2H-pyran-2-one derivatives 3 via an efficient condensation
of amine 1 and dehydroacetic acid 2 in ethanol in the presence
The Schiff base (or azomethine), named after Hugo Schiff, has a
wide range of applications. For example, it can be used as a cata-
lyst, nonlinear optical materials, electroluminescent materials
and so on. In recent years, scientists have found that the Schiff base
plays a very important role in catalytic and electronic transfer pro-
cesses of organisms [1]. The synthesis of novel Schiff bases and the
research of their physiological activities has become important due
to their anticancer and antimicrobial activities [2–4]. As a part of
drug development programs, numerous methods for the synthesis
of various kinds of Schiff base have been reported [5–9].
In recent years, ultrasound has become a highly useful method
for performing a wide range of chemical reactions and processes,
including chemical synthesis, materials production and water
treatment [10–12]. Most of the effects arise from cavitation
[13,14]. Furthermore, the application of ultrasound irradiation in
organic reactions is also rapidly increasing. A large number of or-
ganic reactions can be carried out in a higher yield, shorter reaction
time and milder conditions under ultrasonication [15–22].
of p-toluenesulfonic acid (p-TSA) under ultrasonic irradiation
(Scheme 1).
2. Methods
2.1. Apparatus and analysis
All reagents were purchased from commercial sources and used
without further purification. Ultrasonication was performed in a
KQ-250E medical ultrasound cleaner with a frequency of 40 kHz
and an output power of 250 W. Melting points are uncorrected.
IR spectra were recorded on a Varian F-1000 spectrometer in KBr
with absorptions in cmÀ1 1H NMR were determined on a Varian
.
Invoa-300/400 MHz spectrometer in CDCl3 solution. J values are
in Hz. Chemical shifts are expressed in ppm downfield from inter-
nal standard TMS. HRMS analyses were carried out using TOF-MS.
X-ray diffractions were recorded on a Siemens P4 diffractometer.
The reaction flask was located at the maximum energy area in
the cleaner, and the surface of the reactants was placed slightly
lower than the level of the water. Observation of the surface of
the reaction solution during vertical adjustment of vessel depth
will show the optimum position by the point at which maximum
surface disturbance occurs. The reaction temperature was con-
trolled by addition or removal of water from ultrasonic bath.
In order to expand the application of ultrasound in the synthesis
of organic compounds, we wish to report a novel and efficient
method for the synthesis of 4-hydroxy-6-methyl-3-(1-(phenylimino)
⇑
Corresponding author.
E-mail address: dqshi@suda.edu.cn (D. Shi).
1350-4177/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2011.01.006

H. Wang et al. / Ultrasonics Sonochemistry 18 (2011) 1048–1051
1049
3H, CH3), 2.38 (s, 3H, CH3), 2.56 (s, 3H, CH3), 5.72 (s, 1H, CH),
6.95 (d, J = 8.0 Hz, 1H, ArH), 7.16 (s, 1H, ArH), 7.27 (d, J = 8.0 Hz,
1H, ArH), 15.77 (s, 1H, OH); HRMS calculated for C15H1435Cl NO3
[M+]: 291.0662, found: 291.0660.
2.4.5. Compound 3e
4-hydroxy-6-methyl-3-(1-((4-nitrophenyl)imino)ethyl)-2H-
pyran-2-one. Gray solid; m.p. 198–200 °C; IR (KBr, cmÀ1): 3429,
3128, 2927, 1726, 1650, 1567, 1469, 1339, 1062, 945, 857,725;
1H NMR (400 MHz, CDCl3): d (ppm) 2.19 (s, 3H, CH3), 2.66 (s, 3H,
CH3), 5.80 (s, 1H, CH), 7.37 (d, J = 8.4 Hz, 2H, ArH), 8.34 (d,
J = 8.0 Hz, 2H, ArH), 16.39 (s, 1H, OH); HRMS calculated for
Scheme 1. Synthesis of 4-hydroxy-6-methyl-3-(1-(phenylimino)ethyl)-2H-pyran-
2-one derivatives.
C
14H12N2O5 [M+]: 288.0746, found: 288.0743.
2.2. Classical procedure for the synthesis of 4-hydroxy-6-methyl-3-(1-
(phenylimino)ethyl)-2H-pyran-2-one derivatives (3)
2.4.6. Compound 3f
A 100 mL flask was charged with amine 1 (10 mmol), dehydro-
acetic acid 2 (10 mmol) and p-TSA (0.5 mmol) in ethanol (30 mL).
The mixture was stirred at reflux. After the completion of the reac-
tion (monitored by TLC), the reaction was allowed to cool, and the
ethanol was evaporated under reduced pressure. The solid residue
was washed with ethanol to afford the pure product as solid.
3-(1-((2-chlorophenyl)imino)ethyl)-4-hydroxy-6-methyl-2H-
pyran-2-one. White solid; m.p. 158–160 °C (lit.[23d] m.p. 156 °C);
IR (KBr, cmÀ1): 3435, 3081, 2927, 1697, 1642, 1563, 1460, 1365,
1062, 946, 855,757; 1H NMR (400 MHz, CDCl3): d (ppm) 2.17 (s,
3H, CH3), 2.53 (s, 3H, CH3), 5.78 (s, 1H, CH), 7.24 (t, J = 7.6 Hz, 1H,
ArH), 7.32–7.38 (m, 2H, ArH), 7.53 (dd, J1 = 1.6 Hz, J2 = 7.2 Hz, 1H,
ArH), 15.91 (s, 1H, OH).
2.3. Ultrasound-promoted synthesis of 4-hydroxy-6-methyl-3-(1-
(phenylimino)ethyl)-2H-pyran-2- one derivatives (3)
2.4.7. Compound 3g
3-(1-((3-chlorophenyl)imino)ethyl)-4-hydroxy-6-methyl-2H-
pyran-2-one. White solid; m.p. 116–117 °C (lit.[23a] m.p. 118 °C);
IR (KBr, cmÀ1): 3429, 3063, 2647, 1706, 1652, 1568, 1467, 1385,
1070, 997, 839, 796; 1H NMR (400 MHz, CDCl3): d (ppm) 2.14 (s,
3H, CH3), 2.58 (s, 3H, CH3), 5.73 (s, 1H, CH), 7.07 (d, J = 7.6 Hz,
1H, ArH), 7.19 (s, 1H, ArH), 7.33–7.40 (m, 2H, ArH), 15.91 (s, 1H,
OH).
A 100 mL flask was charged with amine 1 (10 mmol), dehydro-
acetic acid 2 (10 mmol) and p-TSA (0.5 mmol) in ethanol (10 mL).
The mixture was sonicated in the water bath of an ultrasonic clea-
ner at 30 °C. After the completion of the reaction (monitored by
TLC), the reaction was allowed to cool, the ethanol was evaporated
under reduced pressure. The solid residue was washed with etha-
nol to afford the pure product as solid.
2.4.8. Compound 3h
2.4. Data spectra of products
3-(1-(benzylimino)ethyl)-4-hydroxy-6-methyl-2H-pyran-2-
one. Light yellow solid; m.p. 79–80 °C (lit.[23c] m.p. 79–81 °C); IR
(KBr, cmÀ1): 3442, 3057, 2917, 1700, 1643, 1571, 1472, 1387, 1059,
995, 874, 728; 1H NMR (400 MHz, CDCl3): d (ppm) 2.12 (s, 3H, CH3),
2.67 (s, 3H, CH3), 4.69 (d, J = 5.6 Hz, CH2), 5.68 (s, 1H, CH), 7.26–
7.31 (m, 2H, ArH), 7.34 (d, J = 6.8 Hz, 1H, ArH), 7.39 (t, J = 7.6 Hz,
2H, ArH), 14.54 (s, 1H, OH); HRMS calculated for C15H15NO3
[M+]: 257.1052, found: 257.1052.
2.4.1. Compound 3a
4-hydroxy-6-methyl-3-(1-(p-tolylimino)ethyl)-2H-pyran-2-
one. White solid; m.p. 154–156 °C (lit.[23a] m.p. 155 °C); IR (KBr,
cmÀ1): 3398, 3036, 2929, 1701, 1650, 1565, 1471, 1383, 1067,
946, 837, 764; 1H NMR (300 MHz, CDCl3): d (ppm) 2.16 (s, 3H,
CH3), 2.39 (s, 3H, CH3), 2.58 (s, 3H, CH3), 5.75 (s, 1H, CH), 7.05 (d,
J = 6.3 Hz, 2H, ArH), 7.24 (d, J = 6.0 Hz, 2H, ArH), 15.66 (s, 1H, OH).
2.4.9. Compound 3i
2.4.2. Compound 3b
3-(1-((4-fluorophenyl)imino)ethyl)-4-hydroxy-6-methyl-2H-
pyran-2-one. White solid; m.p. 147–148 °C; IR (KBr, cmÀ1): 3422,
3079, 2927, 1725, 1661, 1575, 1477, 1329, 1063, 998, 831, 771;
1H NMR (400 MHz, CDCl3): d (ppm) 2.16 (s, 3H, CH3), 2.57 (s, 3H,
CH3), 5.75 (s, 1H, CH), 7.15 (d, J = 6.4 Hz, 4H, ArH), 15.75 (s, 1H,
OH); HRMS calculated for C14H12F NO3 [M+]: 261.0801, found:
261.0799.
3-(1-((4-chlorophenyl)imino)ethyl)-4-hydroxy-6-methyl-2H-
pyran-2-one. White solid; m.p. 138–140 °C (lit.[23b] m.p. 140 °C);
IR (KBr, cmÀ1): 3404, 3079, 2725, 1718, 1651, 1568, 1466, 1327,
1091, 949, 845, 719; 1H NMR (300 MHz, CDCl3): d (ppm) 2.10 (s,
3H, CH3), 2.47 (s, 3H, CH3), 5.80 (s, 1H, CH), 7.39 (d, J = 8.7 Hz,
2H, ArH), 7.54 (d, J = 8.7 Hz, 2H, ArH), 15.72 (s, 1H, OH); HRMS cal-
culated for C14H1235Cl NO3 [M+]: 277.0505, found: 277.0503.
2.4.10. Compound 3j
2.4.3. Compound 3c
3-(1-(benzo[d][1,3]dioxol-5-ylimino)ethyl)-4-hydroxy-6-
methyl-2H-pyran-2-one. Gray solid; m.p. 184–185 °C; IR (KBr,
cmÀ1): 3441, 3088, 2917, 1693, 1650, 1566, 1471, 1364, 1037,
931, 869, 779; 1H NMR (400 MHz, CDCl3): d (ppm) 2.15 (s, 3H,
CH3), 2.57 (s, 3H, CH3), 5.74 (s, 1H, CH), 6.03 (s, 2H, OCH2O), 6.62
(d, J = 8.8 Hz, 2H, ArH), 6.83 (d, J = 7.6 Hz, 1H, ArH), 15.58 (s, 1H,
ArH); HRMS calculated for C15H13NO5 [M+]: 287.0794, found:
287.0796.
3-(1-((4-methoxyphenyl)imino)ethyl)-4-hydroxy-6-methyl-
2H-pyran-2-one. Yellow solid; m.p. 174–175 °C (lit.[23c] m.p. 180–
182 °C); IR (KBr, cmÀ1): 3439, 3089, 2929, 1695, 1619, 1563, 1472,
1331, 1069, 945, 833, 715; 1H NMR (400 MHz, CDCl3): d (ppm) 2.16
(s, 3H, CH3), 2.58 (s, 3H, CH3), 3.84 (s, 3H, CH3), 5.75 (s, 1H, CH),
6.96 (d, J = 8.8 Hz, 2H, ArH), 7.10 (d, J = 8.8 Hz, 2H, ArH), 15.59 (s,
1H, OH).
2.4.4. Compound 3d
2.4.11. Compound 3k
3-(1-((3-chloro-4-methylphenyl)imino)ethyl)-4-hydroxy-6-
methyl-2H-pyran-2-one. Light yellow solid; m.p. 167–169 °C; IR
(KBr, cmÀ1): 3440, 3081, 2984, 1707, 1656, 1563, 1471, 1359,
1067, 954, 830,765; 1H NMR (400 MHz, CDCl3): d (ppm) 2.13 (s,
4-hydroxy-6-methyl-3-(1-(quinolin-6-ylimino)ethyl)-2H-pyran-
2-one. White solid; m.p. 196–198 °C; IR (KBr, cmÀ1): 3427, 3044,
2925, 1717, 1650, 1574, 1479, 1365, 1063, 941, 836, 793; 1H
NMR (400 MHz, CDCl3): d (ppm) 2.19 (s, 3H, CH3), 2.68 (s, 3H,

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H. Wang et al. / Ultrasonics Sonochemistry 18 (2011) 1048–1051
Table 1
3. Results and discussion
The model reaction catalyzed by p-TSA in different solvents under ultrasound
irradiationa.
To achieve suitable conditions for the synthesis of 4-hydroxy-6-
methyl-3-(1-(phenylimino) ethyl)-2H-pyran-2-one derivatives 3,
various reaction conditions have been investigated in the reaction
of p-toluidine 1a and dehydroacetic acid 2 as a model reaction.
Initially, we examined the effect of the solvent through some
experiments. To search for the optimal solvent, the ultrasonic-
assisted reaction of the aromatic amine and dehydroacetic acid
was examined using ethanol, methanol, toluene, acetonitrile, tetra-
hydrofuran (THF), dioxane and water as solvent. The results were
listed in Table 1. Methanol and water afforded moderate yields
of the desired products (Table 1, entries 2 and 7). Poor results were
observed when the reactions were carried out in toluene, acetoni-
trile, THF and dioxane. The reactions took a long time and the
yields were low (Table 1, entries 3–6). Nevertheless, the reaction
using ethanol as the solvent gave the best result (Table 1, entry
1). Thus, ethanol was chosen as the solvent for all further reactions.
Next, we attempted to further improve the yield by performed
experiments in 30, 40 and 50 °C under ultrasonic irradiation
(Table 1). The effect, however, was not remarkable. Therefore, the
temperature was not the main factor affecting the reaction.
In order to apply this reaction to a library synthesis, we have ex-
tended the reaction of aryl amine with aliphatic and heterocyclic
amine under similar conditions (ethanol/30 °C/ultrasound/p-TSA),
furnishing the respective 4-hydroxy-6-methyl-3-(1-(phenylimino)
ethyl)-2H-pyran-2-one in good yields (Scheme 1). The optimized
results are summarized in Table 2. We found that the results were
excellent in terms of yields using aryl amine carrying electron-
donating substituents. Under the same conditions, however, the
reactions with aryl amine carrying electron-withdrawing groups
required a longer time and the yields of some products notably
decreased (Table 2, entries 2,5,7,9). It was also found that the
ortho-substituted aromatic amines generally gave very low yields,
even trace of the products, probably due to the large steric effect
of 2,6-dichloroaniline (Table 2, entry 6,12). That is to say, steric
factors played a key role in affecting the rates of reaction and the
reactions required a longer time.
Entry
Solvent
Temperature (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
Ethanol
Methanol
Toluene
Acetonitrile
THF
Dioxane
Water
Ethanol
Ethanol
30
30
30
30
30
30
30
40
50
4
5.5
24
24
24
24
7
86
76
40
38
32
35
65
85
87
4
4
a
Reaction conditions: p-toluidine (10 mmol), dehydroacetic acid (10 mmol),
p-TSA (0.5 mmol), solvent (10 mL) and the ultrasonic power 250 W, irradiation
frequency 40 kHz.
b
Yields of isolated products.
Table 2
Synthesis of 4-hydroxy-6-methyl-3-(1-(phenylimino)ethyl)-2H-pyran-2-one deriva-
tives 3 under ultrasonic irradiationa.
Entry
R
Product MP (°C)
Traditional
method
Sonochemical
method
Time
(h)
Yieldb Time
(%)
Yieldb
(%)
(h)
1
2
3
4-CH3C6H4
4-ClC6H4
4-
CH3OC6H4
4-CH3-3-
ClC6H3
4-NO2C6H4
2-ClC6H4
3-ClC6H4
C6H5CH2
4-FC6H4
3,4-
OCH2OC6H3
quinoline-
6-yl
3a
3b
3c
154–156
138–140
174–175
4
9
3.5
69
67
50
4
7.5
3.5
86
78
80
4
3d
167–169
5
88
4
95
5
6
7
8
9
3e
3f
3g
3h
3i
198–200 40
158–160 10
126–127 10
41
52
74
77
69
74
20
69
79
85
86
88
88
8
9
1
7
5
79–80
1.5
147–148 12
10
3j
184–185
196–199 20
36
6
11
12
3k
3l
50
9
62
To find the specific effect of ultrasound on this reaction, all pre-
viously mentioned reactions were carried out under the same con-
ditions in the absence of ultrasound irradiation (Table 2). It was
observed that the reaction times increased considerably and the
yields of the products decreased under conventional reflux condi-
tions. Thus, ultrasonic irradiation was found to have a beneficial ef-
fect on the synthesis of 4-hydroxy-6-methyl-3-(1-(phenylimino)
ethyl)-2H-pyran-2-one derivatives which was superior to the tradi-
tional method with respect to yields, reaction times, simplicity and
safety.
2,6-Cl2C6H3
–
Trace
20
Trace
a
Reaction conditions: amine (10 mmol), dehydroacetic acid (10 mmol), p-TSA
(0.5 mmol), ethanol (10 mL) and the ultrasonic power 250 W, irradiation frequency
40 kHz.
b
Yields of isolated products.
CH3), 5.18 (s, 1H, CH), 7.48–7.54 (m, 2H, ArH), 7.85 (s, 1H, ArH),
8.20 (t, J = 9.6 Hz, 2H, ArH), 8.98 (s, 1H, ArH), 16.12 (s, 1H, OH);
HRMS calculated for C17H14N2O3 [M+]: 294.1004, found: 294.1005.
Fig. 1. X-ray crystal structure of 3h.

H. Wang et al. / Ultrasonics Sonochemistry 18 (2011) 1048–1051
1051
To the best of our knowledge, this new procedure provides the
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The molecular structure of the product 3h is shown in Fig. 1.
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[24] Crystallographic data for 3h have been deposited at the Cambridge
Crystallographic Data Centre with the deposition number CCDC 806998.
Copies of these data can be obtained free of charge via www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge, CB2 1EZ, UK; Fax: +44 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk).
We acknowledge the financial support from the Foundation of
the Natural Science Foundation of China (No. 21072144), the Major
Basic Research Project of the Natural Science Foundation of the
Jiangsu Higher Education Institutions (No. 10KJA150049) and Key
Laboratory of Organic Synthesis of Jiangsu Province (No. KJS0812).
We are grateful to the Analytical and Testing Center of Soochow
University for supports in IR, 1H NMR and HRMS analyses.