Synthesis of benzo[4,5]imidazo[1,2-: A] pyrimidines and 2,3-dihydroquinazolin-4(1 H)-ones under metal-free and solvent-free conditions for minimizing waste generation

Article abstract of DOI:10.1039/c8ra07256f

Br?nsted acidic ionic liquid was found to be an efficient and recyclable catalyst for the synthesis of benzo[4,5]imidazo[1,2-a]pyrimidines and 2,3-dihydroquinazolin-4(1H)-ones. The reactions proceeded smoothly with a broad scope of substrates providing the expected products in good to excellent yields under an atom-economical pathway. The low-cost recyclable catalyst, metal- and solvent-free conditions, and the ease of product isolation are the highlighted advantages in solving the issue of trace metal contamination in synthesized pharmaceuticals.

Full text of DOI:10.1039/c8ra07256f

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Synthesis of benzo[4,5]imidazo[1,2-a]pyrimidines
and 2,3-dihydroquinazolin-4(1H)-ones under
metal-free and solvent-free conditions for
minimizing waste generation†
Cite this: RSC Adv., 2018, 8, 36392
*
Phuong Hoang Tran,
Thanh-Phuong Thi Bui, Xuan-Quynh Bach Lam
and Xuan-Trang Thi Nguyen
Brønsted acidic ionic liquid was found to be an efficient and recyclable catalyst for the synthesis of benzo
[4,5]imidazo[1,2-a]pyrimidines and 2,3-dihydroquinazolin-4(1H)-ones. The reactions proceeded smoothly
with a broad scope of substrates providing the expected products in good to excellent yields under an
atom-economical pathway. The low-cost recyclable catalyst, metal- and solvent-free conditions, and
the ease of product isolation are the highlighted advantages in solving the issue of trace metal
contamination in synthesized pharmaceuticals.
Received 31st August 2018
Accepted 12th October 2018
DOI: 10.1039/c8ra07256f
rsc.li/rsc-advances
Results and discussion
Synthesis of benzo[4,5]imidazo[1,2-a]pyrimidines
Introduction
The invention of new catalysts for the synthesis of biologically
active compounds is essential to the advancement of green
chemistry.1–3 Classical aqueous inorganic acids including
H2SO4, HF, HCl, HNO3, and H3PO4 have been used for various
organic transformations. Nevertheless, these homogeneous
Brønsted acids are not suitable for widespread application due
to separation and environmental issues.4 Ionic liquids have
attracted considerable attention in many elds due to their
structural diversity, low cost, and low toxicity.5–7 The use of ionic
liquids as green catalysts for chemical reactions has presented
many advantages in a sustainable synthesis.8–13 These homo-
geneous catalysts are thermally stable, highly active, and easily
recyclable.14 The most popular Brønsted acidic ionic liquids
have been easily prepared by the reaction of 1,4-butane sultone
or 1,3-propane sultone with N-methylimidazole.15 Thus, these
Brønsted acidic ionic liquids have been widely used as green
catalysts for various organic transformations.16–24 Herein, we
report the synthesis of benzo[4,5]imidazo[1,2-a]pyrimidines
and 2,3-dihydroquinazolin-4(1H)-ones catalyzed by a Brønsted
acidic ionic liquid under solvent-free conditions. The presented
method is considered an essential solution for the environ-
mental problems in industrial processes. The current synthesis
scheme contributes to the sustainable and low-cost pathway for
the preparation of benzo[4,5]imidazo[1,2-a]pyrimidines and
2,3-dihydroquinazolin-4(1H)-ones.
Pyrimidine derivatives are backbones of various pharmaceutical
drugs with various applications including anticancer,25–29 anti-
inammatory,30–32 antibacterial,31,33–35 and antifungal activi-
ties.35–37 Over the past decade, many approaches have been
achieved for the preparation of pyrimidine derivatives.38–42
However, multi-step or low-yield procedures and harmful
conditions could limit the widespread use of these approaches.
Recently, multicomponent reactions have provided new pros-
pects for the advancement in organic synthesis to eliminate
these drawbacks.43,44 Herein, the Brønsted acidic ionic liquid
was used for the rst time to catalyze the synthesis of benzo[4,5]
imidazo[1,2-a]pyrimidines via a one-pot multicomponent reac-
tion. Initially, [(4-SO3H)BMIM]HSO4 ionic liquid was prepared
as reported in the literature.22 The structure of [(4-SO3H)BMIM]
HSO4 was conrmed by NMR, FT-IR, TGA, and HR-MS (ESI,
Section S2†). The as-synthesized [(4-SO3H)BMIM]HSO4 was ob-
tained in high yield aer a simple purication step. Thermog-
ravimetric analysis shows that [(4-SO3H)BMIM]HSO4 is
thermally stable (up to 250 ꢀC). Next, we investigated the model
reaction between 2-aminobenzimidazole, ethyl acetoacetate,
and benzaldehyde in the presence of different catalysts
(10 mol%) at 100 ꢀC for 120 min (Table 1). When the model
reaction was performed using metal chlorides such as AlCl3,
ꢀ
FeCl3, FeCl2, and ZnCl2 as the catalysts at 100 C for 120 min,
the expected product was obtained in 32% to 60% yield (Table 1,
entries 1–4). These results encouraged us to test a series of
Lewis acidic deep eutectic solvents with zinc chloride. However,
the yields of the desired product drastically decreased (Table 1,
entries 5–7). Other deep eutectic solvents were investigated by
Department of Organic Chemistry, Faculty of Chemistry, University of Science, Viet
Nam National University, Ho Chi Minh City 721337, Viet Nam. E-mail: thphuong@
hcmus.edu.vn; Tel: +84 903706762
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra07256f
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Table 1 Yields for reaction of 2-aminobenzimidazole, ethyl acetoa-
cetate, and benzaldehyde with different catalystsa
attain the expected products in high yields (88–92%), and the
substituent position did not signicantly affect the reaction
yield. In the interest of extending the scope of reaction, we
examined acetonylacetone under the current method. To our
delight, the reactions proceed smoothly to provide the desired
products in high yields (88–93%) and shorter reaction times
(60–90 min). Moreover, the method avoids the use of volatile
organic solvents, harmful conditions, and additives making it
greener than the previous reports. In addition, the pure prod-
ucts were obtained easily by recrystallization; thus, this method
shows promise for application in industry.
The comparison of the present method with previously re-
ported literature is shown in Table 2. The multicomponent
reaction of 2-aminobenzimidazole, ethyl acetoacetate, and
benzaldehyde in the presence of [(4-SO3H)BMIM]HSO4 afforded
the expected product in good yield within 2 h under metal-free
and solvent-free conditions (Table 2, entry 6). The previous
studies reported that the same reaction also achieved the target
products in moderate to good yields, but some methods
required longer reaction time and/or high temperature (Table 2,
entries, 1–5). More importantly, the [(4-SO3H)BMIM]HSO4 could
be recovered and reused without any considerable loss in
catalytic activity in a test of six cycles of reuse.
To get more information about the plausible mechanism, we
performed three control experiments under the optimized
conditions. 2-Benzylidene-3-oxobutanoate intermediate was
detected as the major product (by GC-MS) when the model
reaction was performed for 20 min. Thus, the reaction mainly
followed Knoevenagel condensation between benzaldehyde and
ethyl acetoacetate in the rst step, where the [(4-SO3H)BMIM]
HSO4 acted as a Brønsted acidic catalyst. The control experi-
ments with only two components, i.e., the reaction of benzal-
dehyde with ethyl acetoacetate, also provided the desired
product (2-benzylidene-3-oxobutanoate) in 70% yield, which
was higher than the reaction yield of 2-aminobenzimidazole
and benzaldehyde. The intermediate could completely convert
into the desired product when 2-aminobenzimidazole was
added. Based on the above control experiments and previous
literature, the plausible mechanism is proposed, as presented
in Scheme 2. The reaction rst followed Knoevenagel conden-
sation between benzaldehyde and ethyl acetoacetate to produce
2-benzylidene-3-oxobutanoate as intermediate, which could
further react with 2-aminobenzimidazole through Michael
reaction. Finally, the desired product was obtained through
cyclization and dehydration. The [(4-SO3H)BMIM]HSO4 played
a vital role in four steps of the reaction.
Entry
Catalyst
Yieldb (%)
1
2
3
4
5
6
7
8
AlCl3
FeCl3
FeCl2
ZnCl2
57
60
32
60
0
22
12
59
48
52
67
69
89
[CholineCl][ZnCl]
[CholineCl][ZnCl]2
[CholineCl][ZnCl]3
[CholineCl][ethylene glycol]2
[CholineCl][glucose]
[CholineCl][oxalic acid]2
[EMIM]Cl
9
10
11
12
13
[BMIM]BF4
[(4-SO3H)BMIM]HSO4
a
Reaction condition: 2-aminobenzimidazole (1.0 mmol), ethyl
acetoacetate (1.0 mmol), benzaldehyde (1.0 mmol), and catalyst
(10 mol%) at 100 ꢀC for 120 min. b Isolated yield.
replacing ZnCl2 with ethylene glycol, oxalic acid or glucose
(Table 1, entries 8–10), but the yields of the product were not
satisfactory. Thus, we focused on ionic liquids. Three types of
ionic liquids, namely, [EMIM]Cl, [BMIM]BF4, and [(4-SO3H)
BMIM]HSO4, were tested for the model reaction. When the
reactions were performed in ionic liquid media, the yields of the
desired product drastically increased (Table 1, entries 11–13).
Among these ionic liquids, [(4-SO3H)BMIM]HSO4 showed the
best efficiency, allowing the reaction to afford 89% yield (Table
1, entry 13). As shown in Table 1, metal chlorides could be
employed in the model reaction. However, these metal chlo-
rides are moisture sensitive and easily decompose in the pres-
ence of a small amount of water. Furthermore, these metal
chlorides could not be recovered and reused aer the aqueous
workup. The prominent feature of [(4-SO3H)BMIM]HSO4 is
stability in water and in almost all cases, catalytic use, recovery,
and reuse of [(4-SO3H)BMIM]HSO4 are possible. In our study,
we tested different ratios of [(4-SO3H)BMIM]HSO4 with the aim
of obtaining the desired product in high yield. The best yield
was obtained in the presence of 10 mol% [(4-SO3H)BMIM]HSO4.
When the amount of catalyst was increased to 15 mol%, the
yield slightly improved. All attempts to reduce the amount of
[(4-SO3H)BMIM]HSO4 loading led to diminished yields.
With the optimized conditions in hand, we focused on using
a variety of aromatic aldehydes. The results are presented in
Scheme 1. A series of p-substituted benzaldehyde smoothly
reacted with 2-aminobenzimidazole and ethyl acetoacetate to
provide benzo[4,5]imidazo[1,2-a]pyrimidines in excellent yields.
Electron-donating group on benzaldehyde, such as methox-
ybenzaldehyde, gave the expected products in excellent yield
(90%), but longer reaction time was required (150 min).
Electron-withdrawing groups on benzaldehyde, such as 2-
nitrobenzaldehyde and 4-nitrobenzaldehyde, were all effectively
reactive and provided the corresponding products in excellent
yields within shorter reaction times (75 min). Benzaldehyde
bearing halogen substituents could also react smoothly to
Synthesis of 2,3-dihydroquinazolin-4(1H)-ones
Quinazolinones are valuable bioactive compounds possessing
antitumor,45–47 antiviral,48,49 antibacterial,50,51 and anti-
inammatory activities.52,53 Traditionally, both homogeneous
and heterogeneous acidic catalysts can catalyze the synthesis of
2,3-dihydroquinazolin-4(1H)-ones.54–56 Recently, various proto-
cols have been reported for the preparation of 2,3-dihy-
droquinazolin-4(1H)-ones with the aid of improving the
reaction efficiency.57–61 Moreover, 2,3-dihydroquinazolin-4(1H)-
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Scheme 1 [(4-SO3H)BMIM]HSO4-catalyzed synthesis of benzo[4,5]imidazo[1,2-a]pyrimidines.
ones have been successfully synthesized using ionic liquids SO3H)BMIM]HSO4 acidic ionic liquid could be used as an active
including [BMIM]BF4,62 poly(4-vinylpyridine) supported acidic catalyst for the preparation of 2,3-dihydroquinazolin-4(1H)-ones
ionic liquids,63 and imidazolium triate.64 However, methods presumably due to strong effect of Brønsted acidic groups on
that involved these ionic liquids presented some drawbacks, the ionic liquid. Interestingly, various aromatic aldehydes
including high temperature, prolonged reaction time, and the reacted successfully under the optimized conditions, and the
use of volatile organic solvents. In this study, we report that [(4- expected product was obtained in excellent yields in the
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Table 2 Comparison of the present method with previous literature for the one-pot multicomponent synthesis of benzo[4,5]imidazo[1,2-a]
pyrimidine
Entry
Catalytic system
Reaction conditions
Yield (%)
1
2
3
4
5
6
1,1,3,3-N,N,N0,N0-Tetramethylguanidinium triuoroacetate (30 mol%)
[bmim][BF4] (3 mL)
L-Proline (20 mol%), H2O
Thiamine hydrochloride (5 mol%), H2O
a-Zirconium sulfophenylphosphonate (12 mol%)
Current work: [(4-SO3H)BMIM]HSO4 (10 mol%), solvent-free
100 ꢀC, 5 h
90 ꢀC, 7 h
Reux, 3 h
Reux, 3 h
80 ꢀC, 20 h
100 ꢀC, 2 h
73 (ref. 26)
86 (ref. 36)
91 (ref. 30)
90 (ref. 41)
73 (ref. 42)
92
presence of [(4-SO3H)BMIM]HSO4 (Scheme 3). The substituent method avoids the use of volatile organic solvents, harmful
on the benzene did not affect the reaction rate. In addition, pure conditions, and metal catalysts, thus showing promise for
products were obtained easily by recrystallization. Remarkably, application in industrial processes.
the condensation between anthranilamide and benzaldehyde
The comparison of the presented method with previously
was successfully performed on a 50 mmol scale, and the ex- reported literature is shown in Table 3. The reaction of
pected yield was obtained equally with 1 mmol scale. This anthranilamide and benzaldehyde in the presence of [(4-SO3H)
Scheme 2 Proposed mechanism for the preparation of benzo[4,5]imidazo[1,2-a]pyrimidines.
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Scheme 3 Reaction scope with aldehydes.
BMIM]HSO4 afforded the expected product in excellent yield at acidic ionic liquid and the desired product could be obtained
room temperature for 30 min under metal-free and solvent-free from the intermediate by the intramolecular nucleophilic
conditions (Table 3, entry 6). The previous studies reported that attack of the nitrogen (–NH2) on the amide group to imine
the same reaction also achieved the target products in good to carbon.
excellent yields, but some methods required the use of
Recyclability of the catalyst is a necessary feature for the-
a solvent, high temperature and/or longer reaction time (Table application in industrial processes. The recyclability of
3, entries, 1–5). Additionally, the [(4-SO3H)BMIM]HSO4 could be [(4-SO3H)BMIM]HSO4 in this study was investigated in the
recovered and reused without any considerable loss of catalytic model reactions with benzaldehyde under optimal condi-
activity in a test of six cycles.
tions (Fig. 1). Aer completion of the reaction, the reaction
On the basis of our experiments and the literature, we mixture was diluted with ethanol, and the desired product
proposed the reaction mechanism for the cyclocondensation was obtained by ltration. The catalyst was recovered, dried
between anthranilamide and aromatic aldehydes (Scheme under vacuum for 6 h and reused for next cycles. The yields of
4).56,69 The rst step could be the formation of imine as the product slightly decreased aer the sixth cycle. The FT-IR
a critical intermediate under the presence of [(4-SO3H)BMIM] spectra conrmed that there was no change in functional
HSO4. Next, the imine would be activated by the Brønsted groups of [(4-SO3H)BMIM]HSO4 (Fig. 2).
Table 3 Comparison of the presented method with previous literature for the synthesis of 2-phenyl-2,3-dihydroquinazolin-4(1H)-one
Entry
Catalytic system
Reaction conditions
Yield (%)
1
2
3
4
5
6
[BMIM]PF6 (3.0 mL)
Cp2TiCl2 (1 mol%), ethanol (0.5 mL)
Fe3O4@SiO2@GMSI-VB1 (10 mg), H2O (1.0 mL)
MNPs/DETA-SA (15 mg), H2O (2.0 mL)
p-Sulfonic acid calix[4]arene(1.0 mol%), H2O (1.0 mL)
Current work: [(4-SO3H)BMIM]HSO4 (10 mol%), solvent-free
75 ꢀC, 35 min
30 ꢀC, 8 min
80 ꢀC, 80 min
90 ꢀC, 40 min
r.t., 18 min
89 (ref. 62)
96 (ref. 65)
95 (ref. 66)
94 (ref. 67)
94 (ref. 68)
98
r.t., 30 min
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Scheme 4 A plausible mechanism for the synthesis of 2,3-dihydroquinazolin-4(1H)-one.
Conclusions
We have developed a facile, efficient, and atom-economic
method for preparing benzo[4,5]imidazo[1,2-a]pyrimidines
and 2,3-dihydroquinazolin-4(1H)-ones under metal- and
solvent-free conditions. The presented method provides
a straightforward and green approach for the preparation of
biologically nitrogen-heterocyclic compounds from the readily
available starting materials in good to excellent yields within
short reaction time. Remarkably, [(4-SO3H)BMIM]HSO4
Brønsted acidic ionic liquid showed to be particularly suitable
for these transformations as highlighted by its environmental
friendliness, low-cost, recyclability, and simplicity of operation.
Experimental
Materials and instrumentation
Fig. 1 Recycling test.
All the reagents were purchased from Merck, Acros, and Sigma-
Aldrich and used without further purication. Ultrasonic irra-
diation was performed in Elma sonic S30H. FT-IR spectra were
analyzed using a Bruker Vertex 70 (KBr pellets). Thermogravi-
metric analysis was performed on Analyzer TGA Q5000. 1H and
13C NMR spectra were performed on a Bruker Advance 500.
High-resolution electrospray ionization mass spectrometry
(HRMS-ESI) was performed on a Bruker micrOTOF-QII MS at
80 eV.
Synthesis of [(4-SO3H)BMIM]HSO4 under solvent-free
sonication
[(4-SO3H)BMIM]HSO4 was synthesized via a one-pot two-step
procedure under solvent-free sonication according to our
previous literature report.22
General procedure for the synthesis of benzo[4,5]imidazo[1,2-
a]pyrimidines
A mixture of benzaldehyde (106 mg, 1 mmol), ethyl acetoacetate
(130 mg, 1 mmol), 2-aminobenzimidazole (133 mg, 1 mmol)
and [(4-SO3H)BMIM]HSO4 (31.6 mg, 0.1 mmol) was heated
100 ꢀC and the progress of the reaction was monitored by TLC.
Fig. 2 FT-IR spectra of the fresh and the six-times reused catalyst.
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Aer completion of the conversion, the reaction mixture was 16 A. C. Cole, J. L. Jensen, I. Ntai, K. L. T. Tran, K. J. Weaver,
quenched with cold ethanol (10 mL). The crude product was
ltered and washed with petroleum ether (10 mL), and then
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18 J. Yang, H. Zhou, X. Lu and Y. Yuan, Catal. Commun., 2010,
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General procedure for the synthesis 2,3-dihydroquinazolin-
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20 A. Wang, X. Zheng, Z. Zhao, C. Li, Y. Cui, X. Zheng, J. Yin and
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23 Q. T. Nguyen, A.-H. Thi Hang, T.-L. Ho Nguyen, D.-K. Nguyen
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4(1H)-one
A mixture of anthranilamide (136 mg, 1 mmol), benzaldehyde
(106 mg, 1 mmol), and [(4-SO3H)BMIM]HSO4 (31.6 mg, 0.1
mmol) was sonicated for 30 min at room temperature and the
progress of the reaction was monitored by TLC or GC-MS. Aer
completion of the conversion, the reaction mixture was
quenched with ethanol (10 mL). The crude product was ltered
and washed with petroleum ether (2 ꢁ 5 mL), and then puried
by recrystallization from ethanol to afford the desired pure
product.
25 T. Raj, H. Sharma, Mayank, A. Singh, T. Aree, N. Kaur,
N. Singh and D. O. Jang, ACS Sustainable Chem. Eng., 2017,
5, 1468–1475.
Conflicts of interest
There are no conicts to declare.
26 A. Shaabani, A. Rahmati and S. Naderi, Bioorg. Med. Chem.
Lett., 2005, 15, 5553–5557.
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C. Gambacorti-Passerini, L. Scapozza, D. Gueyrard and
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Acknowledgements
This research was funded by Viet Nam National University, Ho
Chi Minh City (VNU-HCM) under grant number 562-2018-18-03.
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