Ga(OTf)3 catalyzed synthesis of 1,4-dihydropyrimidin-2(1H)-ones

Article abstract of DOI:10.3987/COM-14-13118

Gallium(III) triflate was used to catalyze Biginelli-like reactions under solvent-free condition to obtain dihydropyrimidinone derivatives in good to excellent yields and short reaction time.

Full text of DOI:10.3987/COM-14-13118

HETEROCYCLES, Vol. 91, No. 1, 2015
105
HETEROCYCLES, Vol. 91, No. 1, 2015, pp. 105 - 112. © 2015 The Japan Institute of Heterocyclic Chemistry
Received, 3rd November, 2014, Accepted, 3rd December, 2014, Published online, 25th December, 2014
DOI: 10.3987/COM-14-13118
Ga(OTf)3 CATALYZED SYNTHESIS OF 1,4-DIHYDROPYRIMIDIN-
2(1H)-ONES
Jingjing Xia* and Kehua Zhang
Key Laboratory of Functional Molecule Design and Interface Process, Anhui
Jianzhu University, Heifei, Anhui, P.R.China, 230601; Email: xiajj@ahjzu.edu.cn
Abstract – Gallium(III) triflate was used to catalyze Biginelli-like reactions under
solvent-free condition to obtain dihydropyrimidinone derivatives in good to
excellent yields and short reaction time.
Dihydropyrimidinones (DHPMs) has attracted increasing attentions in organic and medicinal chemistry
due to their versatile pharmacological and therapeutic properties, such as calcium channel modulators, D1a
adrenoaceptor-selective antagonists,1-3 anticancer drugs to inhibit kinesin motor proteins,4,5 Rho-kinase
inhibitors,6 anti-HIV activity7 and so on.
The first synthesis of DHPMs was reported by an Italian chemist, Pietro Biginelli, in 1893 through a simple
one-pot, three-component condensation reaction of urea, ethyl acetoacetate and an aromatic aldehyde.8 The
Biginelli reaction was not widely applied until the early 1990s as the increasing requirement of biological
active compounds, which made this multicomponent reaction attractive. In the past years, this reaction has
gained much more attention and several modified procedures have been reported in order to improve its
efficiency. Lewis acids catalyzed DHPMs synthesis is one of the most widely used methods, such as
9
.
10
BF3 OEt2, FeCl3 6H2O, LaCl3,11 Yb(OTf)3,12 Cu(OTf)2,13 InBr3,14 Zr(NO3)3,15 BiCl3,16 Mn(OAc)3,17
CaCl2,18 I2,19 ZrOCl2,20 montmorillonite KSF,21 LiBr22 etc. Although extensive investigations have devoted
to improve the efficiency of Biginelli reaction, we, here, provide a relatively simple operation and
environmental friendly reaction condition to carry out Biginelli reaction.
.
As a water-compatible strong Lewis acid, Ga(OTf)3 has been widely used to catalyze organic reactions,23
such as Beckmann rearrangements,24 Friedel–Crafts reactions,25,26 aldoxime dehydration,27,28
regioselective rearrangements of 2-substituted vinylepoxides,29 asymmetric Mukaiyama aldol reactions,30
constructions of fused-bicyclolactones,31 and so on. Herein, we report a new utility of gallium(III) triflate
catalyzed neat Biginelli reaction. Compared to other methods reported in the literature,
gallium(III)-promoted reactions are straightforward, giving good to excellent yields. Moreover, this

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HETEROCYCLES, Vol. 91, No. 1, 2015
reaction condition can also expand to cycloketones, giving a novel transformation for the synthesis of
dihydropyrimidinones.
Initially, the reaction conditions were screened by exploring a typical Biginelli reaction of ethyl
acetoacetate, benzaldehyde and urea with catalyzed Ga(OTf)3 in various solvents, as collected in Table 1.
10 mol% Ga(OTf)3 was employed to catalyze this reaction under various solvents, such as THF, DCM,
acetonitrile, ethanol and toluene. THF and DCM gave trace amount of the desired product after refluxing
for 12 hours, which might be attributed to the low boiling points of the solvents (Table 1, entries 1 and 3).
Low yields were also obtained in toluene and acetonitrile solutions (Table 1, entries 2 and 4). Ethanol can
give a relatively better yield of 88% (Table 1, entry 5). However, when this reaction was carried out without
solvent at 90 oC, 98% yield product can be obtained within 0.5 h (Table 1, entry 6). The yield was a little bit
lower (90%) when the catalyst amount was reduced to 5 mol% (Table 1, entry 7).
Table 1. Reaction condition optimization on the Ga(OTf)3 catalyzed Biginelli reactiona
Entry
Solvent
THF
Ga(OTf)3/(mol%)
Reflux time/h
Yield/%
trace
30
1
2
10
10
10
10
10
10
5
12
12
12
12
12
0.5
12
toluene
3
DCM
trace
50
4
MeCN
5
EtOH
88
6b
7b
solvent-free
solvent-free
98
90
a: The reactant condition: benzaldehyde (1.0 mmol), ethyl acetoacetate (1.0 mmol), urea (1.2 mmol) in 10
mL solvent;
b: Reaction temperature: 90 oC.
With the best reaction condition (Table 1, entry 6) in hand; the reaction scope was explored to different
aldehydes, alkyl acetoacetates and urea or thiourea. The collected results were presented in Table 2. First of
all, different benzaldehydes with both electron donating (–OMe) and electron withdrawing (–NO2 and –F)
groups were investigated, giving high yield (> 90%) within less than one hour (Table 2, entries 1–5 and

HETEROCYCLES, Vol. 91, No. 1, 2015
107
9–11). Alkyl aldehydes were also explored (Table 2, entries 7 and 8), but with relatively low yields of 84%
and 82%, respectively. Thioureas can also be applied in this reaction condition (Table 2, entries 12 and 13)
with good yields.
Table 2. Ga(OTf)3 catalyzed Biginelli reaction of aldehydes, alkyl acetoacetates and urea under
solvent-free conditiona
Entry
R1
R2
X
O
O
O
O
O
Time (h)
0.5
Product
4a
Yield (%)
1
2
3
4
5
C6H5
OEt
OEt
OEt
OEt
OEt
98
93
95
94
96
4-MeO-C6H4
4-Cl-C6H4
4-F-C6H4
4-NO2-C6H4
0.5
4b
0.5
4c
0.25
0.25
4d
4e
6
OEt
O
0.75
90
4f
7
8
Me b
n-C6H13
C6H5
OEt
OEt
O
O
O
O
O
S
0.5
1.0
0.5
0.5
0.5
0.5
0.5
84
82
94
90
92
91
85
4g
4h
4i
9
OMe
OMe
OMe
OEt
10
11
12
13
4-MeO-C6H4
4-Cl-C6H4
C6H5
4j
4k
4l
C6H5
OMe
S
4m
a: The reactant condition: aldehyde (1.0 mmol), alkyl acetoacetate (1.0 mmol) and urea (1.2 mmol);
b: From 40% acetaldehyde solution.
The successful scope investigation mentioned above (Table 2) prompted us to further expand the versatility
of Biginelli reaction by replacing alkyl acetoacetate to cycloketones. Although Pan group has reported the
one-pot multicomponent Biginelli reactions between cycloalkanones, urea or thiourea, and aldehydes
catalyzed by TMSCl,32 a combination solvent of DMF/MeCN was used. Liu group mentioned a solvent and
catalyst free reaction condition, but the aldehydes were only restricted for aromatic aldehydes and no
cycloketones were investigated.33 In this reaction, Ga(OTf)3 catalyzed cycloketones, urea and aldehydes

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were also succeeded under solvent-free conditions with moderate to high yields (Table 3). Benzaldehydes
with both electron donating (–Me and –OMe) and electron withdrawing (–NO2) groups were tested with
good yields (Table 3, entries 1–5 and 7). A seven-membered ring ketone, cycloheptanone, was also tried
with a 83% yield (Table 3, entry 6).
Table 3. Ga(OTf)3 catalyzed Biginelli reaction of cycloketone, aldehyde, and urea under solvent-free
conditiona
Entry
R1
n
1
1
1
1
1
3
Time (h)
0.5
Product
Yield (%)a
1
2
3
4
5
6
C6H5
94
86
88
89
88
83
6a
6b
6c
6d
6e
6f
4-NO2-C6H4
4-Cl-C6H4
4-MeO-C6H4
4-Me-C6H4
C6H5
0.25
0.75
0.5
0.75
1.0
7
1
1.0
82
6g
a: The reactant condition: benzaldehyde (2.0 mmol), cycloketone (1.0 mmol) and urea (1.2 mmol).
In conclusion, Ga(OTf)3 can effectively catalyze Biginelli reaction of alkyl acetoacetates (or cycloketones),
aldehydes and urea or thiourea under solvent-free conditions. The functional group diversity,
environmental friendly condition, high yield and mild reactions conditions will promote this reaction with
wide applications.
EXPERIMENTAL
All chemicals (AR grade) were obtained from commercial resources and used without further purification.
1H NMR (400 MHz) spectra were recorded on a Varian Mercury MHz spectrometer in CDCl3 with TMS as
the internal standard. All products are known and they were also identified by melting point, which were
determined using XT-4 apparatus and are not corrected. High-resolution mass spectra (HRMS) were
obtained using a GCT-TOF instrument.

HETEROCYCLES, Vol. 91, No. 1, 2015
109
General procedure for the synthesis of DHPMs
To a mixture of alkyl acetoacetate (or cycloketone) (1.0 mmol), aldehyde (1.0 mmol, for cycloketones:
2.0 mmol) and urea (or thiourea) (1.2 mmol), 0.1 equivalent Ga(OTf)3 was added. The mixture was
stirred at 90 oC for appropriate time. After reaction completed by TLC analysis, water was added and the
product was extracted with EtOAc. The organic layer was dried over anhydrous Na2SO4 and evaporated.
The residue was recrystallized from methanol or ethanol to obtain pure products.
NMR and HR-MS for part of the compounds:
Ethyl 6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4a). mp 202–203 oC (Lit.9 202–204 oC).
1H NMR (CDCl3, 400 MHz) δ 8.04 (br, 1H, NH), 7.33–7.25 (m, 5H, ArH), 5.74 (br, 1H, NH), 5.05 (s, 1H, CH), 4.08 (q,
J = 7.2 Hz, 2H, CH2O), 2.36 (s, 3H, CH3), 1.18 (t, J = 7.2 Hz, 3H, CH3).
o
Ethyl 4-(4-methoxyphenyl)-6-methyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4b). mp 201–202 C (Lit.9
201–203 oC). 1H NMR (DMSO-d6, 400 MHz) δ 9.14 (br, 1H, NH), 7.70 (br, 1H, NH), 7.15 (d, J = 8.6 Hz, 2H), 6.88 (d, J
= 8.6 Hz, 2H), 5.10 (d, J = 3.4 Hz, 1H), 3.98 (q, J = 7.2 Hz, 2H, CH2O), 3.72 (s, 3H, CH3), 2.24 (s, 3H, CH3), 1.11 (t, J =
7.2 Hz, 3H, CH3). HR-MS(EI): calcd for C15H18N2O4: 290.1267, found: 290.1271.
o
Ethyl 4-(4-chlorophenyl)-6-methyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4c). mp 214–216 C (Lit.9
213–215 oC). 1H NMR (DMSO-d6, 400 MHz) δ 9.25 (br, 1H, NH), 7.78 (br, 1H, NH), 7.39 (d, J = 8.4 Hz, 2H), 7.24 (d, J
= 8.4 Hz, 2H), 5.14 (d, J = 3.0 Hz, 1H), 3.98 (q, J = 7.1 Hz, 2H, CH2O), 2.26 (s, 3H), 1.10 (t, J = 7.1 Hz, 3H, CH3).
o
Ethyl 4-(4-fluorophenyl)-6-methyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4d). mp 176–178 C (Lit.34
175–177 oC). 1H NMR (CDCl3, 400 MHz) δ 8.13 (br, 1H, NH), 7.26 (d, J = 8.4 Hz, 2H), 7.01 (d, J = 8.4 Hz, 2H), 5.40 (d,
J = 3.2 Hz, 1H), 4.08 (q, J = 7.2 Hz, 2H, CH2O), 2.33 (s, 3H), 1.18 (t, J = 7.2 Hz, 3H, CH3).
o
Ethyl 4-(4-nitrophenyl)-6-methyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4e). mp 207–211 C (Lit.9
o
1
208–211 C). H NMR (DMSO-d6, 400 MHz) δ 9.38 (br, 1H, NH), 8.20 (d, J = 8.6 Hz, 2H), 7.50 (d, J = 8.6 Hz, 2H),
5.26 (d, J = 2.0 Hz, 1H), 3.97 (q, J = 7.2 Hz, 2H, CH2O), 2.26 (s, 3H), 1.10 (t, J = 7.2 Hz, 3H, CH3).
Ethyl 4-(2-furanyl)-6-methyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4f). mp 209–212 oC (Lit.34
207–209 oC). 1H NMR (CDCl3, 400 MHz) δ 7.86 (br, 1H, NH), 7.32 (d, J = 6.0 Hz, 1H), 6.29–6.24 (m, 2H), 6.13 (d, J =
6.2 Hz, 1H), 5.47 (s, 1H, CH), 4.15 (q, J = 7.2 Hz, 2H, CH2O), 2.35 (s, 3H), 1.20 (t, J = 7.2 Hz, 3H, CH3).
o
o
1
Ethyl 4,6-dimethyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4g). mp 194–196 C (Lit.14 194–195 C). H
NMR (CDCl3 , 400 MHz): δ 8.11 (br, 1H, NH), 5.70 (br, 1H, NH), 4.43–4.32 (m, 1H, CH), 4.20 (q, J = 7.2 Hz, 2H,
CH2O), 2.28 (s, 3H, CH3), 1.29–1.26 (m, 6H, 2×CH3). HR-MS(EI): calcd for C9H14N2O3: 198.1004, found: 198.1003.
Ethyl 4-hexyl-6-methyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4h). mp 208–210 oC (Lit.12 208–210 oC).
1H NMR (CDCl3, 400 MHz): δ 8.15 (br, 1H, NH), 5.87 (br, 1H, NH), 4.35–4.25 (m, 1H, CH), 4.19 (q, J = 7.2 Hz, 2H,
CH2O), 2.30 (s, 3H, CH3), 1.55–1.52 (m, 2H, CH2), 1.35–1.10 (m, 11H, CH3 + 4×CH2), 0.88 (t, J = 7.2 Hz, 3H, CH3).
o
Methyl 6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4i). mp 210–212 C (Lit.9 209–212
oC). 1H NMR (DMSO-d6, 400 MHz) δ 8.10 (br, 1H, NH), 7.33–7.28 (m, 5H, ArH), 5.38 (d, J = 2.6 Hz, 1H), 3.61 (s, 3H),
2.35 (s, 3H).
Methyl 4-(4-methoxyphenyl)-6-methyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4j). mp 193–194 oC

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HETEROCYCLES, Vol. 91, No. 1, 2015
o
1
(Lit.9 192–194 C). H NMR (DMSO-d6, 400 MHz) δ 9.16 (br, 1H, NH), 7.71 (br, 1H, NH), 7.20(d, J = 8.4 Hz, 2H),
6.83 (d, J = 8.4 Hz, 2H), 5.30 (d, J = 2.2 Hz, 1H), 3.76 (s, 3H), 3.62 (s, 3H), 2.31 (s, 3H).
Methyl 4-(4-chlorophenyl)-6-methyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4k). mp 205–207 oC (Lit.12
204–207 oC). 1H NMR (DMSO-d6, 400 MHz) δ 9.21 (br, 1H, NH), 7.71 (br, 1H, NH), 7.15 (d, J = 8.4 Hz, 2H), 6.88 (d, J
= 8.4 Hz, 2H), 5.10 (d, J = 2.8 Hz, 1H), 3.54 (s, 3H), 2.27 (s, 3H).
o
Ethyl 6-methyl-4-phenyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4l). mp 231–234 C (Lit.14 232–234
oC). 1H NMR (CDCl3, 400 MHz): δ 7.88 (br, 1H, NH), 7.28–7.26 (m, 5H, ArH), 5.39 (d, J = 3.4 Hz, 1H, CH), 4.05 (q, J
= 7.2 Hz, 2H, CH2O), 2.35 (s, 3H, CH3), 1.16 (t, J = 7.2 Hz, 3H, CH3). HR-MS(EI): calcd for C14H16N2O2S: 276.0932,
found: 276.0940.
Methyl 6-methyl-4-phenyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (4m). mp 220–222 oC (Lit.35
221–222 oC). 1H NMR (CDCl3, 400 MHz): δ 7.90 (br, 1H, NH), 7.30–7.25 (m, 5H, ArH), 5.25 (d, J = 3.4 Hz, 1H, CH),
3.60 (s, 3H, CH3), 2.27 (s, 3H, CH3).
7-Benzylidene-4-phenyl-3,4,6,7-tetrahydro-1H-cyclopenta[d]pyrimidin-2(5H)-one (6a). mp 237–239 oC (Lit.32
o
1
236–239 C). H NMR (DMSO-d6, 400 MHz): δ 8.78 (s, 1H, NH), 7.41–7.23 (m, 11H, ArH, =CH), 6.64 (s, 1H, NH),
5.16 (s, 1H, CH), 2.84–2.80 (m, 2H, CH2), 2.36–2.35 (m, 1H, CH), 2.04–1.98 (m, 1H, CH). HR-MS (EI): calcd for
C20H18N2O: 302.1419, found: 302.1418.
7-(4-Nitrobenzylidene)-4-(4-nitrophenyl)-3,4,6,7-tetrahydro-1H-cyclopenta[d]pyrimidin-2(5H)-one
(6b).
mp
o
o
1
280–282 C (Lit.34 281–282 C). H NMR (DMSO-d6, 400 MHz): δ 8.88 (s, 1H, NH), 8.36–8.18 (m, 4H, ArH),
7.66–7.38 (m, 4H, ArH), 6.90–6.79 (m, 2H, NH+CH), 5.40 (s, 1H, CH), 2.93–2.89 (m, 2H, CH2), 2.27–2.19 (m, 1H,
CH), 2.04–2.02 (m, 1H, CH).
7-(4-Chlorobenzylidene)-4-(4-chlorophenyl)-3,4,6,7-tetrahydro-1H-cyclopenta[d]pyrimidin-2(5H)-one (6c). mp
o
o
1
227–228 C (Lit.32 226–228 C). H NMR (DMSO-d6, 400 MHz): δ 8.84 (s, 1H, NH), 7.47–7.28 (m, 9H, ArH, =CH),
6.63 (s, 1H, NH), 5.18 (s, 1H, CH), 2.83–2.78 (m, 2H, CH2), 2.44–2.38 (m, 1H, CH), 2.03–1.97 (m, 1H, CH).
7-(4-Methoxybenzylidene)-4-(4-methoxyphenyl)-3,4,6,7-tetrahydro-1H-cyclopenta[d]pyrimidin-2(5H)-one
(6d).
mp 250–252 oC (Lit.32 250–252 oC). 1H NMR (DMSO-d6, 400 MHz): δ 8.65 (s, 1H, NH), 7.27 (d, 2H, J = 8.4Hz, ArH),
7.16 (d, 2H, J = 8.4Hz, ArH), 7.05 (s, 1H, ArH), 6.94 (d, 4H, J = 7.5Hz, ArH, =CH), 6.58 (s, 1H, NH), 5.10 (s, 1H, CH),
3.76 (s, 6H, 2×OCH3), 2.79 (s, 2H, CH2), 2.40–2.36 (m, 1H, CH), 2.01–1.94 (m, 1H, CH); HR-MS (EI): calcd for
C22H22N2O3: 362.1630; found: 362.1626.
o
7-(4-Methylbenzylidene)-4-(p-tolyl)-3,4,6,7-tetrahydro-1H-cyclopenta[d]pyrimidin-2(5H)-one (6e). mp 239–241 C
o
1
(Lit.32 238–241 C). H NMR (DMSO-d6, 400 MHz): δ 8.99 (s, 1H, NH), 7.40–7.25 (m, 9H, ArH, =CH), 6.94 (s, 1H,
NH), 5.20 (s, 1H, CH), 2.85–2.83 (m, 2H, CH2), 2.44–2.36 (m, 1H, CH), 2.10 (s, 7H, 2 ×CH3+CH). HR-MS (EI): calcd
for C22H22N2O: 330.1732; found: 330.1729.
o
9-Benzylidene-4-phenyl-3,4,6,7,8,9-hexahydro-1H-cyclohepta[d]pyrimidin-2(5H)-one (6f). mp 287–288 C (Lit.34
o
1
286–288 C). H NMR (DMSO-d6, 400 MHz): δ 9.02 (s, 1H, NH), 7.43–7.22 (m, 11H, ArH, =CH), 6.93 (s, 1H, NH),
5.24 (s, 1H, CH), 2.85–2.83 (m, 2H, CH2), 2.44–2.43 (m, 1H, CH), 2.09 (s, 5H, CH, 2CH2).
4-(Naphthalen-1-yl)-7-(naphthalen-1-ylmethylene)-3,4,6,7-tetrahydro-1H-cyclopenta[d]pyrimidin-2(5H)-one (6g).
mp 262–264 oC (Lit.34 260–264 oC). 1H NMR (DMSO-d6, 400 MHz): δ 9.20 (s, 1H, NH), 8.39–6.76 (m, 14H, ArH), 7.05
(s, 1H, =CH), 6.03 (s, 1H, NH), 5.29 (s, 1H, CH), 2.80–2.70 (m, 2H, CH2), 2.45–2.41 (m, 1H, CH), 1.83–1.75 (m, 1H,

HETEROCYCLES, Vol. 91, No. 1, 2015
111
CH). HR-MS (EI): calcd for C28H22N2O: 402.1732; found: 402.1725.
ACKNOWLEDGEMENTS
This work is financially supported by the Anhui Provincial Natural Science Foundation (No.
1208085QB24).
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