E. Kolvari et al. / Tetrahedron 70 (2014) 1383e1386
1385
Table 6
proceeded cleanly. The results obtained by the two different tech-
UVevis data of the some of 3,4-dihydropyrimidin-2-(1H)-ones
niques: conventional heating (method A) and MW irradiation
(method B), were compared. As shown in Table 4, the microwave-
assisted nano-g-Fe2O3eSO3H catalyzed reactions were superior to
nm
Compound (4)
Aryl
l
max
Acetone
Ethanol
Acetonitrile
those using conventional heating. MW-assisted chemistry was
used due to the efficiency of the interaction of MWs with the polar
nano-catalyst,16 as it allows rapid heating of the reaction mixture to
required temperatures and the precise control of the reaction
temperature. With two methods, the aryl group substitution with
different groups and with the same groups located at different
positions of the aromatic ring has been shown not to have much
effect on the formation of the final product and afford the expected
products 4 and 5 in good to high yields. The products were char-
acterized by IR, 1H NMR, and 13C NMR spectroscopy, and also by
comparison with authentic samples.
Ph (4a)
p-MeC6H4 (4b)
p-ClC6H4 (4c)
p-FC6H4 (4f)
o-ClC6H4 (4i)
p-O2NC6H4 (4e)
3,4-MeOeC6H4 (4h)
4-HOe3-EtO-C6H3 (4j)
208
210
210
210
210
d
279
278
279
273
278
278
268
280
279
275
280
251
281
d
210
210
275
275
3. Results and discussion
Importantly, note that the ferromagnetic property of nano-g-
Fe2O3eSO3H made the isolation and reuse of this catalyst very
easy. After completion of the reaction, the mixture was triturated
with ethyl acetate. Within a few seconds after stirring was
stopped, the reaction mixture turned clear and catalyst was de-
posited on the magnetic bar, which was easily removed using an
external magnet. After being washed with acetone and dried in air,
In conclusion, an efficient, sustainable and green procedure for
synthesis of 3,4-dihydropyrimidin-2-(1H)-ones has been de-
veloped using a magnetically separable and easily recyclable nano-
g-Fe2O3eSO3H catalyst in solvent-free medium under thermal or
microwave conditions. Easy magnetic separation of the catalyst
eliminates the requirement of catalyst filtration after completion of
the reaction, which is an additional greener attribute of this
reaction.
the nano-
activation even after five rounds of synthesis of product 4a
(Table 5). The characterization of the nano- -Fe2O3eSO3H before
g-Fe2O3eSO3H can be directly reused without any de-
g
and after reuse five times showed the same particle size by
transmission electron microscopy (TEM; Fig. 1b) and the same
crystal structure by XRD.
4. Experimental section
4.1. Instrumentation, analysis, and starting materials
All chemicals were purchased from Merck, Fluka or Acros
companies and used without any further purification. Nano-g-
Table 5
Caption reuse of nano-
g
-Fe2O3eSO3H in the synthesis of 3,4-dihydropyrimidin-2-
Fe2O3eSO3H was prepared with the reported method.14 Microwave
LG oven MG 555f model was used. NMR spectra were recorded with
a Bruker Avance 300 spectrometer (1H NMR 300 MHz and 13C NMR
75 MHz) in pure deuterated chloroform with tetramethylsilane
(TMS) as the internal standard. Presented UVevis spectra were
obtained as ethanol solutions (10ꢁ5 M) on a Shimadzu UV-1650PC
spectrophotometer.
(1H)-onesa
Run
Yield (%)a
Method A
Method B
1
2
3
4
5
95
95
95
93
93
97
97
96
96
94
4.2. General procedure for the synthesis of 3,4-
dihydropyrimidin-2-(1H)-ones under conventional heating
method (method A)
a
Isolated yield.
A
mixture of aldehyde (1.0 mmol), ethyl acetoacetate
(1.0 mmol), urea (1.5 mmol) and nano-g-Fe2O3eSO3H (0.1 g) was
heated at 60 ꢀC. After completion of the reaction (monitored by
TLC), the mixture was cooled to room temperature and triturated
with ethyl acetate (5 mL). In the presence of a magnetic stirrer bar,
nano-g-Fe2O3eSO3H moved onto the stirrer bar steadily and the
reaction mixture turned clear within 10 s. The catalyst can be iso-
lated by simple decantation. After evaporation of the solvent, the
crude product was recrystallized from EtOH/H2O to give a pure
product.
Fig. 1. TEM pictures of nano-g-Fe2O3eSO3H before use (a) and after reuse five times (b).
Because of extended conjugation with inbuilt 3,4-
dihydropyrimidin-2-(1H)-one ring, these compounds are ex-
pected to show optical behavior. Therefore the UVevis absorption
spectra were recorded in different solvents (Table 6).
4.3. General procedure for the synthesis of 3,4-
dihydropyrimidin-2-(1H)-ones under microwave irradiation
method (method B)
In acetonitrile UVevis absorption spectra appeared around
251e281 nm range, in ethanol UVevis absorption spectra appeared
around 268e280 nm and in acetone absorption bands appeared
around 208e210 nm range. Comparatively in acetonitrile and
ethanol solutions 3,4-dihydropyrimidin-2-(1H)-ones exhibited
relatively higher absorption than the acetone solution, also the
results couldn’t be classified according to the substituents on the
phenyl ring.
A
mixture of aldehyde (1.0 mmol), ethyl acetoacetate
(1.0 mmol), urea (1.5 mmol), and nano- -Fe2O3eSO3H was placed
g
in a microwave reaction vial. The LG microwave oven MG 555f was
programmed to 250 W at 60 ꢀC. The progress of the reaction was
monitored by TLC. After completion of the reaction, ethyl acetate
(5 mL) was added and the catalyst was separated by applying an
external magnet. The solvent was evaporated and the residue was
Kolvari, Eskandar
