A facile approach for the synthesis of 3,4-dihydropyrimidin-2-(1H)-ones using a microwave promoted Biginelli protocol in ionic liquid

Article abstract of DOI:10.1007/s12039-012-0278-5

Carboxy functionalized ionic liquid [cmmim][BF₄] has been demonstrated to be an efficient and green catalyst for the one pot synthesis of 3,4-dihydropyrimidin-2-(1H)-ones heterocycles under microwave irradiation. The ionic liquid-microwave stra

Full text of DOI:10.1007/s12039-012-0278-5

c
J. Chem. Sci. Vol. 124, No. 4, July 2012, pp. 921–926. ꢀ Indian Academy of Sciences.
A facile approach for the synthesis of 3,4-dihydropyrimidin-2-(1H)-ones
using a microwave promoted Biginelli protocol in ionic liquid
ABHISHEK N DADHANIA∗, VAIBHAV K PATEL and DIPAK K RAVAL
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar 388 120, India
e-mail: abhishekdadhania@gmail.com
MS received 5 December 2011; revised 25 February 2012; accepted 6 March 2012
Abstract. Carboxy functionalized ionic liquid [cmmim][BF4] has been demonstrated to be an efficient and
green catalyst for the one pot synthesis of 3,4-dihydropyrimidin-2-(1H)-ones heterocycles under microwave
irradiation. The ionic liquid-microwave strategy represents an easy access to Biginelli compounds with high
yields and purity. The protocol was found to be compatible with different structurally diverse aldehydes. The
ionic liquid was recycled and reused in at least six subsequent reactions with consistent activity.
Keywords. Ionic liquid; Biginelli reaction; microwave irradiation; recyclability.
21
1. Introduction
and ZnI2 were employed for reaction along with
microwave irradiation (scheme 1). However, limita-
tions such as use of toxic solvents, expensive cata-
lyst, requirement of excess reagents/catalysts, laborious
work up procedures, etc. restrict the practical synthetic
utility of this chemistry.
Research interest in 3,4-dihydropyrimidin-2-(1H)-ones
(‘Biginelli compounds’, DHPMs), has surged rapidly,
owing to the pharmacological properties1–5 associ-
ated with many derivatives of this privileged hetero-
cyclic core. The biological activity1 of these Biginelli
compounds has been found in the form of antiviral,
antitumor, antibacterial and antiinflammatory proper-
ties. In addition to the medicinal applications, many
functionalized DHPMs have emerged as potent cal-
cium channel blockers,2 antihypertensive agents,3 α1a-
adrenergic antagonists4 and neuropeptide Y (NPY)
antagonists.5 Several recently isolated marine alka-
loids6 with interesting biological activities also con-
tain the dihydropyrimidinone-5-carboxylate core. The
best-known method for DHPMs is the classical Big-
inelli synthesis.7 Although it has been known for more
than a century, it is still the most useful method for the
preparation of such a class of compounds. Since 1893,
the synthesis of the DHPMs nucleus by the Biginelli
reaction procedure has been extensively explored.8
Many catalysts9 have been reported for the synthesis
of DHPMs, either by modification in classical Biginelli
The concept of speeding up synthetic transforma-
tions by microwave irradiation has been the focus of
considerable attention in recent years and is becoming
an increasingly popular technology.22 Recently, popu-
lar trend in this area is to use ionic liquids or mix-
ture of ionic liquids as reaction media in microwave
animated transformation.23 Likewise, the use of room-
temperature ionic liquids (ILs) as green reaction media
in organic transformation has gained considerable
importance due to their solvating ability, negligible
vapour pressure and easy recyclability.24 Moreover,
due to their high polarity and ionic nature, ionic liq-
uids couple very effectively with microwaves through
a dipolar polarization and ionic conduction mecha-
nism25 to push the reaction in forward direction very
quickly.
The literature survey reveals that the potentiality of
microwave/ionic liquid (MW/ILs) synergetic couple
reaction or by development of novel multistep strate- uniquely for the synthesis of Biginelli compounds
gies. In addition, many catalysts such as amberlyst- has not been explored much. Therefore, it appeared
15/AcOH,10 HClO4,11 H3PW12O40,12 TMSCl/MeCN,13 to us that a MW/ILs strategy would be ideally suited
PSSA/H2O,14 I2/Al2O3,15 sulphated ZrO2,16 SbCl3/ for the synthesis of this privileged heterocycle. Pro-
Al2O3,17 TsOH/AcOH,18 NBS/EtOH,19 In(OTf)3
ceeding in the same direction, we opted to evolve an
efficient and eco-friendly process for the preparation
of ionic liquid 1-carboxymethyl-3-methylimidazolium
tetrafluoroborate [cmmim][BF4] via microwave
irradiation.
20
∗For correspondence
921

922
Abhishek N Dadhania et al.
Scheme 1. Various catalysts employed for the synthesis of DHPMs under microwave irradiation.
1
We report here a microwave sponsored modified instrument. H NMR, 13C NMR and DEPT-135 spec-
preparation of [cmmim][BF4] and disclose the suc- tra were recorded on Bruker Avance 400 MHz spec-
cessful outcome of MW/ILs strategy to synthesize 3, trometer in CDCl3/DMSO-d6 with TMS as the internal
4-dihydropyrimidin-2-(1H)-ones heterocycle in ex- standard.
cellent isolated yields. The IL could be recycled and
successfully reused in at least six consequent runs.
The process holds well without the need of additional
catalyst.
2.1 Synthesis of 1-carboxymethyl-3-methylimidazolium
tetrafluoroborate [cmmim][BF4] under microwave
irradiation
Synthesis of [cmmim][BF4] was reported by Gonghua
Song et al. via conventional method.26 We have suc-
cessfully attempted the same under microwave irra-
2. Experimental
All chemicals were of research grade and were used as diation in comparatively short duration. Aliquots of
obtained from Sigma-Aldrich, Alfa-Aesar and SDFCL. chloroacetic acid (1 equiv.) was added over a period of
The melting points were determined in capillary tubes 60 min to 1-methyl-imidazole (1 equiv.) at room tem-
using heavy paraffin liquid in Thiele tube. Melting perature under stirring, and then this set-up was irradi-
points were uncorrected and were compared with the ated in the MW oven 4 times at the 20% power level
reported literature values. The reaction progress and (140 W) and 4 times at the 30% power level (210 W) for
purity of products were determined by TLC silica gel 60 s with 2 min intermittent cooling period after each
plates (Merck 60 F254). The reactions were performed in irradiation cycle. The solid thus obtained was washed
scientific microwave system (Catalyst system ‘CATA-R’, with acetonitrile, and dried under vacuum. The inter-
700 W) using 25 mL round-bottomed flask. IR Spec- mediate, 1-carboxymethyl-3-methylimidazolium chlo-
tra were recorded on a Shimadzu FT-IR 8400 spec- ride was obtained as a white solid (mp, 172–174◦C).
trometer. Mass spectra were recorded on AB APPLIED The [cmmim][Cl] (1 equiv.) and NaBF4 (1 equiv.) were
BIOSYSTEMS IMDS SCIEX, API-2000 LC/MS/MS dissolved in H2O and irradiated in the MW oven at

Microwave promoted Biginelli protocol in ionic liquid
923
60% power level (420 W) for 10 min with intermit- reaction mixture was poured onto crushed ice (∼20 gm)
tent cooling of 2 min after 5 min of irradiation. Water and stirred well. The solid separated was washed with
was removed from the reaction mixture by subjecting ice cold water and then recrystallized from hot ethanol
it to distillation for 4 h at 80◦C under reduced pres- or ethyl acetate to afford pure DHPMs, 4a–x. The com-
sure (10 mmHg) to give the product [cmmim][BF4] as bined aqueous filtrate was subjected to vacuum at 80◦C
a slight yellowish oil. Ionic liquid was characterized under reduced pressure (10 mmHg) for 4 h to leave
1
by H-NMR and 13C-NMR. All the data were in good behind the IL in near complete recovery, pure enough
agreement with the reported one.
to recycle. The recovered ionic liquid was found to be
equally effective for at least six cycles in the synthesis
of 4a.
2.2 General procedure for the preparation
of DHPMs
2.3 Spectral data of some selected compounds
A mixture of aldehyde 1 (10 mmol), ethyl acetoac-
etate 2 (EAA, 10 mmol), urea (or thiourea) 3 (11 mmol)
and [cmmim][BF4] (200 mg) was charged into a 25 mL
round bottom flask. The mixture was stirred gently
with a spatula for a few seconds and was subjected to
microwave irradiation at 40% power level (CATA-R,
700 W) for appropriate time as shown in table 1. After
completion of the reaction (as indicated by TLC) the
2.3a 5-Ethoxycarbonyl-6-methyl-4-phenyl-3,4-
dihydropyrimidin-2(1H)-one (4a): IR (KBr) 3233,
2954, 1730, 1690, 1206, 698 cm−1; 1H NMR δ 9.21 (s,
1H), 7.74 (s, 1H), 7.25 (m, 5H), 5.14 (s, 1H), 3.98 (q,
J = 7.1 Hz, 2H), 2.25 (s, 3H), 1.09 (t, J = 7.1 Hz,
3H); 13C NMR δ 13.9, 17.7, 53.9, 59.1, 99.2, 126.1,
127.1, 128.3, 144.7, 148.2, 152.0, 165.2. Calc. for
Table 1. Synthesis of DHPMs using various aldehydes with ethyl acetoacetate and urea/thiourea in [cmmim][BF4].
Reaction time (min)a
% Yieldb
Product
R-
X
MW
ꢀ
MW
ꢀ
Mp/◦C observed (lit.)
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
4s
Ph–
4- O2N–C6H4–
4-F–C6H4–
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
S
1.5
3.0
2.5
2.5
1.5
1.5
2.0
2.5
1.5
2.0
2.0
2.0
1.5
2.0
2.0
2.5
3.0
1.5
2.0
2.5
2.0
2.5
3.0
3.0
90
97
96
93
96
95
97
92
97
96
94
95
93
91
92
94
96
92
89
92
94
95
92
95
91
92
88
83
90
89
86
81
84
88
83
87
81
82
86
92
94
89
81
82
80
88
86
84
79
201–202 (202–204)28
206–208 (207–209)29
174–176 (175–176)31
192–194 (193–195)28
214–215 (215–216)29
200–202 (202–204)28
208–210 (209–211)28
221–222 (222–224)28
228–230 (230–232)28
178–180 (180–182)30
202–203 (204–205)30
214–216 (215–217)28
198–200 (200–202)30
206–208 (205–207)33
196–198 (196–197)33
212–214 (212)35
240
180
150
120
150
90
3-Cl–C6H4–
4-H3C–C6H4–
4-MeO–C6H4–
2-Furyl-
2-Cl–C6H4–
4-HO–C6H4–
3,4,5-(MeO)3-C6H2-
4-HO,3-MeO–C6H3-
2-Thienyl
2-HO–C6H4–
2-Br–C6H4–
3-Br–C6H4–
2,5-(MeO)2-C6H3-
3,4-(MeO)2-C6H3-
2-Pyridyl
3- O2N–C6H4–
2- O2N–C6H4–
4-Cl–C6H4–
Ph–
90
150
180
150
120
90
120
150
150
180
150
180
210
180
120
150
270
174–176 (175–177)30
212–214 (212–214)36
228–230 (230–232)34
208–210 (210)35
4t
4u
4v
4w
4x
210–212 (212-213)30
207–208 (208-210)28
150–151 (150–152)28
201–203 (202–204)32
4-MeO–C6H4–
3,4,5-(MeO)3-C6H2-
S
S
MW = Microwave irradiation (280 W); ꢀ = Conventional method (80◦C)
aAll the reactions were run till the completion as indicated by TLC
byield after crystallization

924
Abhishek N Dadhania et al.
Scheme 2. General reaction scheme for the synthesis of DHPMs.
C14H16N2O3: C, 64.60; H, 6.20; N, 10.76. Found: C,
64.61; H, 6.18; N, 10.74. MS: 261 (M+1).
In order to carry out such transformation by elim-
inating the use of molecular solvent, we examined
the influence of MW irradiation on a neat mix-
ture of ethyl acetoacetate, benzaldehyde, urea and
carboxy functionalized ionic liquid 1-carboxymethyl-
3-methylimidazolium tetrafluoroborate [cmmim][BF4].
In order to investigate the proportion of IL, irradia-
tion time and power level of MW set-up, we carried
out series of experiments. From these experiments, we
found an optimum set of conditions that afford DHPMs
in excellent isolated yields. Evidently, this optimum
condition employed aldehyde 1, ethyl acetoacetate 2
and urea (or thiourea) 3 in the ratio of 1.0:1.0:1.1 by
using 200 mg [cmmim][BF4] as reaction promoter at
power level 4 (280 W). Time taken for the comple-
tion of each conversion, aldehyde employed, isolated
yields and melting points of products are summarized
in table 1. The amount of IL does not appear to be crit-
ical as we have run successful experiments with 50–
200 mg of IL per mmol of aldehyde. However, the pres-
ence of about 200 mg of IL ensures the homogeneity of
the reaction mixture in particular when solid aldehydes
were employed. All the reactions were monitored by
TLC and taken to completion. All the compounds were
2.3b 5-Ethoxycarbonyl-6-methyl-4-(4-fluorophenyl)-
3,4-dihydropyrimidin-2(1H)-one (4c): IR (KBr)
1
3240, 2980, 1730, 1640, 1230, 1150, 790 cm−1; H
NMR δ 9.25 (s, 1H), 7.77 (s, 1H), 7.21 (m, 4H), 5.15
(s, 1H), 3.99 (q, J = 7.1 Hz, 2H), 2.26 (s, 3H), 1.09 (t,
J = 7.1 Hz, 3H); 13C NMR δ 13.9, 17.8, 53.6, 59.5,
123.4, 127.5, 145.2, 149.3, 151.6, 152.1, 164.8. Calc.
for C14H15FN2O3: C, 60.42; H, 5.43; N, 10.07. Found:
C, 60.44; H, 5.42; N, 10.10.
2.3c 5-Ethoxycarbonyl-6-methyl-4-(4-methoxyphenyl)-
3,4-dihydropyrimidin-2(1H)-one (4f): IR (KBr) 3206,
1
2956, 1740, 1679, 1245, 1180, 1040, 875 cm−1; H
NMR δ 9.14 (s,1H), 7.66 (s, 1H), 6.99 (m, 4H), 5.07
(s, 1H), 3.96 (q, J = 6.8 Hz, 2H), 3.70(s, 3H), 2.23 (s,
3H), 1.09 (t, J = 6.8 Hz, 3H); 13C NMR δ 14.5, 18.2,
53.78, 55.47, 59.60, 99.9, 114.1, 127.8, 136.4, 148.3,
158.8, 165.8. Calc. for C15H18N2O4: C, 62.06; H, 6.25;
N, 9.65. Found: C, 62.04; H, 6.28; N, 9.67. MS: 291
(M+1).
1
well-characterized by melting points, H NMR, 13C
NMR and DEPT-135 spectral data. Additional confor-
2.3d 5-Ethoxycarbonyl-6-methyl-4-(3,4,5-trimetho-
xyphenyl)-3,4-dihydropyrimidin-2(1H)-thione (4x):
IR (KBr) 3284, 2967, 1720, 1632, 1234, 1140,
1
780 cm−1; H NMR δ 10.21 (s,1H), 9.49 (s, 1H), 7.29
(m, 2H), 5.02 (s, 1H), 6.40 (s, 2H), 3.94 (q, J = 6.6 Hz,
2H), 3.57 (s, 9H) 2.13 (s, 3H), 1.04 (t, J = 7.0 Hz,
3H); 13C NMR δ 14.9, 17.7, 54.2, 56.1, 60.1, 60.4,
101.2, 103.5, 137.1, 139.4, 145.7, 153.3, 165.4, 174.5.
Calc. for C17H22N2O5S: C, 55.72; H, 6.05; N, 7.64.
Found: C, 55.69; H, 6.03; N, 7.62.
3. Results and discussion
A variety of aldehydes (1a–x) were chosen to be con-
densed with ethyl acetoacetate (2) and urea/thiourea
(3) as shown in the scheme 2.
Figure 1. Recyclability of ionic liquid.

Microwave promoted Biginelli protocol in ionic liquid
925
Scheme 3. Plausible mechanistic pathway.
mation of compounds was also achieved by IR and mass
IL promotes the reactions due to its inherent
spectral studies. All the data were in agreement with Brønsted acidity. Hydrogen bonding is formed between
the reported literature. All the aldehydes have reacted carboxylic proton of IL and carbonyl oxygen of alde-
in short reaction times under MW irradiations to afford hyde as well as ethyl acetoacetate during the reaction.
DHPMs in excellent isolated yields. MW/ILs induced The formation of hydrogen bond between IL and sub-
protocol showed the ability to tolerate variations in aro- strate leads them to the activation.27 Based on this,
matic aldehydes containing both electron donating and the plausible mechanistic pathway for this reaction is
electron withdrawing substituents.
suggested (scheme 3).
In a typical experiment, three reaction components
along with IL were simply mixed in a 25 mL round
bottom flask and irradiated under MW. After cool-
ing, the products were easily isolated by dilution with
water followed by filtration of precipitated DHPMs.
The DHPMs, thus isolated were pure enough for all
practical purposes. However, all the DHPMs were sub-
jected to crystallization and the yields reported after
purification step. The aqueous filtrate was subjected to
vacuum at 80◦C/10 mm Hg for 4 h to remove water and
leave behind [cmmim][BF4] pure enough for the next
run. The ionic liquid thus recycled was used at least six
times for the microwave-induced reaction of 4a with
prominent retention in its activity. The obtained results
are plotted in figure 1.
4. Conclusion
The combined effect of microwave-ionic liquid cou-
ple provides an easy and green route to synthesize
DHPMs via Biginelli reaction. The milder reaction con-
ditions, absence of additional catalyst, high reaction
rates, excellent yields, easy work up procedures and
MW-IL strategy make this procedure an attractive alter-
native to the conventional acid/base catalysed thermal
processes and is environment friendly with minimal or
no waste.
Acknowledgements
From the series of experiments, it was observed that
the reaction did not proceed even after several min-
utes of irradiation in molecular solvents such as ethanol,
ethyl acetate and acetonitrile instead of IL under similar
MW set-up conditions. No formation of the desired het-
erocycle was observed when reaction was carried out
under simple stirring at 30◦C. For the comparison pur-
pose, the reactions were also carried out in the presence
of the same amount of the [cmmim][BF4] in an oil bath
at 80◦C. The results are summarized in table 1. It is
clear that the reactions under microwave irradiation led
to relatively high yields in shorter reaction time. From
this experimental evidence, it can be stated that it is the
synergetic effect of MW/ILs couple that has promoted
this transformation in absence of any added catalyst.
Authors thank Head, Department of Chemistry, Sar-
dar Patel University for providing necessary research
facilities. DKR is grateful to authorities of Sardar Patel
University, Vallabh Vidyanagar for allocation of
research funding in the form of seed grant-2011. He
also thanks the University Grants Commission (UGC),
New Delhi for providing Meritorious Fellowship.
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