Solvent-free synthesis of polyfunctional tetrahydropyrimidines promoted by recyclable ionic liquid

Article abstract of DOI:10.1007/s13738-012-0202-4

The synthesis of nitrogen containing heterocyclic compounds from diethyl ester, formaldehyde, and various aromatic/aliphatic amines via a simple and efficient one-pot multicomponent reaction in ionic liquid is described. The catalyst can be recycled at least three times without obvious loss in catalytic activity. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s13738-012-0202-4

J IRAN CHEM SOC (2013) 10:695–699
DOI 10.1007/s13738-012-0202-4
ORIGINAL PAPER
Solvent-free synthesis of polyfunctional tetrahydropyrimidines
promoted by recyclable ionic liquid
•
Mazaahir Kidwai Neeraj Kumar Mishra
•
Saurav Bhardwaj
Received: 26 March 2012 / Accepted: 30 November 2012 / Published online: 9 January 2013
Ó Iranian Chemical Society 2012
Abstract The synthesis of nitrogen containing heterocy-
clic compounds from diethyl ester, formaldehyde, and
various aromatic/aliphatic amines via a simple and efficient
one-pot multicomponent reaction in ionic liquid is descri-
bed. The catalyst can be recycled at least three times
without obvious loss in catalytic activity.
Furthermore, the discovery of novel multicomponent
reactions [9–12] can be considered as an interesting topic
for academic research. Therefore, considerable effort has
been dedicated to the development of synthesis and
mechanism of the multicomponent reaction.
Among nitrogen containing heterocycles [13–17],
pyrimidine and their derivatives such as pyrazolopyrimi-
dines, isooxazolopyrimidines, pyrimidopyrimidines, and
tetrahydropyrimidines have generated a widespread interest
owing to the vast range of biological activities [18, 19] that
these compounds possess. They have been used as pre-
ventative and prophylactic drugs in the treatment of male
erectile dysfunction [20]. Specifically, tetrahydropyrimi-
dines containing an amino acid unit have attracted much
attention due to their potential biological activities [21]
such as M1 muscarinic receptor agonists for treatment of
Alzheimer’s disease [22, 23], human immunodeficiency
virus (HIV) protease inhibitors [24], and other inhibitors
against mycobacterium tuberculosis [25]. Literature meth-
ods for the synthesis of fused pyrimidines [26–29] gener-
ally involve either cyclization of substituted pyrimidines or
condensation of specific moieties with pyrimidine [30–33].
The synthesis of tetrahydropyrimidines relies on three as
well as four-component system and both route provided
excellent yields of products. In our knowledge only few
versions for the synthesis of multifunctional tetrahydro-
pyrimidines have been presently developed [33, 34].
Although, entirely these methods have not been satisfac-
tory and are associated with drawbacks such as toxic
reagents/solvents, stoichiometric amounts of catalysts, long
reaction times, unsatisfactory yields, complex procedure,
and mostly there is no recovery of the catalyst.
Keywords Multicomponent reaction Á
Tetrahydropyrimidines Á Heterocyclic compounds Á Ionic
liquid Á Recyclability
Introduction
The search for novel compounds with beneficial thera-
peutic effects is at the forefront of medicinal chemistry
research. With this in mind, developing procedures for the
efficient generation of small heterocyclic organic mole-
cules is of paramount importance in identifying the next
generation of drug candidates. In this context, multicom-
ponent reactions have emerged as powerful and bond-
forming efficient tools that allow the preparation of
polycyclic targets by connecting several components in a
one-pot sequential and efficient manner [1–5]. Due to their
high versatility, small heterocycles [6–8] were particularly
attractive in multicomponent reaction for pharmaceutical
development.
M. Kidwai Á N. K. Mishra Á S. Bhardwaj
Green Chemistry Research Laboratory, Department
of Chemistry, University of Delhi, Delhi 110007, India
Present Address:
During the past few years, ionic liquids [35–39] have
been demonstrated as efficient and practical alternatives of
green reaction medium for many important organic
M. Kidwai (&)
Jiwaji University, Gwalior, MP, India
e-mail: kidwai.chemistry@gmail.com
123

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J IRAN CHEM SOC (2013) 10:695–699
transformations [6–8, 40–42]. In recent times, ionic liquids
have been marched for beyond showing its significant role
in controlling the reaction as catalyst [43, 44]. A number of
ionic liquid with unique properties [45–48] have been
successfully applied in many types of reactions such as
Friedel–Craft acylations [49], alkylations [50], and Man-
nich-type reaction [51, 52]. Considering these elegant
discoveries of addition activity of ionic liquid, we envisage
that other types of multicomponent reaction could also be
catalyzed by ionic liquid. Thus keeping in mind a more
efficient and economical methodology for the synthesis of
bioactive tetrahydropyrimidines, we report herein an
environmentally benign route for the synthesis of poly-
functional tetrahydropyrimidine derivatives using ionic
liquid as a recyclable catalyst.
aniline (2.0 mmol), 2 mL ionic liquid, and formaldehyde
(3 mmol) was heated at 80–90 °C for 60 min. The progress
of reaction mixture was monitored by TLC. After comple-
tion of the reaction, the mixture was cooled to room tem-
perature. The product was extracted with diethyl ether
(3 9 10 mL) (ionic liquid being insoluble in ether). Ether
layer was decanted, dried over anhydrous MgSO4, and
concentrated under reduced pressure. After the solvent was
removed vacuo, the crude product was purified by pre-
parative TLC with petether/ethylacetate (90:10) as the
eluent to afford the desired product in 89 % yield. The rest
of viscous ionic liquid was thoroughly washed with ethyl
acetate and recycled in subsequent reactions.
General procedure for the synthesis of polysubstituted
tetrahydropyrimidine derivatives via four-component
reaction
Experimental
Method (B) In a 50 ml round bottom flask, but-2-yn-
edioic acid diethyl ester 1 (1 mmol) and aniline 2 (R1NH2)
(1 mmol) in 2 mL ionic liquid were mixed very slowly and
stirred at room temperature for 5–10 min. To this, other
different amines (R2NH2)/benzylamine 4 (1.1 mmol) and
formaldehyde 3 (3 mmol) was added with stirring at
80–90 °C for 40 min. The progress of reaction mixture was
monitored by TLC. After completion of the reaction, the
mixture was cooled to room temperature. The product was
extracted with diethyl ether (3 9 10 mL) (ionic liquid
being insoluble in ether). Ether layer was decanted, dried
over anhydrous MgSO4, and concentrated under reduced
pressure. After the solvent was removed vacuo, the crude
product was purified by preparative TLC with petether/
ethylacetate (90:10) as the eluent to afford the desired
product in 93 % yield. The rest of viscous ionic liquid was
thoroughly washed with ethyl acetate and recycled in
subsequent reactions.
Chemicals and apparatus
General
The materials were purchased from Sigma-Aldrich and
Merk, and used without any purification. The ionic liquids
were prepared by reported procedures [53]. All reactions
and purity of tetrahydropyrimidines were monitored by
thin layer chromatography (TLC) using aluminum plates
coated with silica gel (merk) using hexane-ethylacetate
(90:10) as an eluent. The isolated products were further
purified by preparative TLC using silica gel G (particle size
10–40 microns, 300 mesh) purchased from Spectrochem
Pvt. Ltd. Mumbai, India. 1H NMR and 13C NMR (300 and
75 MHz, respectively) spectra were recorded on Bruker
Avance Spectrospin 300 (300 MHz). All NMR samples
were run in CDCl3 and chemical shifts are expressed as d
relative to internal Me4Si. IR spectra were obtained on
Perkin Elmer FT-IR spectrometer spectrum-2000 using
potassium bromide pellets or as liquid films between two
sodium chloride pellets. ESI–MS mass spectra were
recorded on a Waters LCT Micromass. The temperature of
the reaction mixture was measured through a non-contact
infrared thermometer (AZ, Mini Gun Type, Model 8868).
All of the obtained products are known compounds.
Spectral data for the selected compounds: 1,3-diphenyl-
1,2,3,6-tetrahydropyrimidine-4,5-dicarboxylic acid diethyl
ester (5a): Yellowish oil15; IR (Film): mmax = 3,248, 2,984,
1,752, 1,696, 1,544, 1,486, 1,370, 1,243, 1,188, 1,107,
1
1,035, 745 cm-1; HNMR (300 MHz, CDCl3): d = 7.32
(m, 10H), 4.86 (s, 2H), 4.28 (s, 2H), 4.17 (q, J = 7.2 Hz,
2H), 3.96 (q, J = 7.2 Hz, 2H), 1.24 (t, J = 6.9 Hz, 3H),
0.99 (t, J = 7.6 Hz, 3H); 13CNMR (75 MHz, CDCl3):
d = 165.3, 164.2, 148.6, 146.1, 143.5, 129.8, 129.3, 128.7,
126.7, 125.6, 122.8, 118.3, 117.4, 101.5, 69.3, 62.5, 60.0,
47.3, 14.3, 13.8, 13.1 m/z (ESI–MS) 380.01 (M ? 1,
requires 380.17).
General procedure for the synthesis
of tetrahydropyrimidines
General procedure for the synthesis of polysubstituted
tetrahydropyrimidine derivatives via three-component
reaction
1,3-Dibutyl-1,2,3,6-tetrahydropyrimidine-4,5-dicarbox-
ylic acid diethyl ester (5b): Yellowish oil15; IR (Film):
mmax = 3,436, 2,981, 1,728, 1,685, 1,572, 1,490, 1,332,
1,265, 1,228, 1,140, 1,035 cm-1 1HNMR (300 MHz,
;
Method (A) In a 50 ml round bottom flask a stirred mix-
ture containing but-2-ynedioic acid diethyl ester (1 mmol),
CDCl3): 4.35 (q, J = 7.2 Hz, 2H), 4.26 (q, J = 7.2 Hz,
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J IRAN CHEM SOC (2013) 10:695–699
697
Scheme 1 One-pot synthesis of
polysubstituted
tetrahydropyrimidines via
MCRs
R2
COOEt
EtOOC
Ionic liquid
N
R1NH
+
+ HCHO R2NH2
+
2
80-90 oC
EtOOC
N
40 min-5h
COOEt
1
R1
5a-5l
2
3
4
2H), 3.69 (s, 2H), 3.38 (s, 2H), 2.77 (t, J = 7.4 Hz, 2H),
2.26 (t, J = 7.4 Hz, 2H) 1.63 (m, 4H), 1.30 (m, 4H), 1.11
(m, 6H), 0.93 (m, 6H); 13CNMR (75 MHz, CDCl3):
d = 166.0, 164.1, 147.6, 91.5, 67.4, 59.3, 51.8, 50.1, 48.4,
31.6, 30.2, 20.9, 19.7, 14.3, 13.8, 13.4 m/z (ESI–MS)
340.09 (M ? 1, requires 340.45).
Table 1 Optimization of the reaction conditions for teytrahydro-
pyrimidines in different ionic liquids
Entry
Ionic liquid
Time (min)
Yield (%)a
1
2
3
4
[bmim]PF6
[bmim]Cl
[bmim]BF4
[bmim]Br
60
60
60
60
30
32
89
80
3-Benzyl-1-phenyl-1,2,3,6-tetrahydropyrimidine-4,5-
dicarboxylic acid diethyl ester (5g): Yellowish oil15; IR
(Film): mmax = 3,440, 2,974, 1,722, 1,672, 1,558, 1,465,
Reaction conditions: 1.0 equivalent of but-2-ynedioic acid diethyl
ester, 2.0 equivalent of aniline, 3.0 equivalent of formaldehyde; dif-
ferent ionic liquids (2 mL); temperature 80–90 °C
1,388, 1,261, 1,206, 1,105, 1,028, 752 cm-1 1HNMR
;
(300 MHz, CDCl3): d = 7.46–7.27 (m, 7H), 6.92–6.87 (m,
3H), 4.41 (s, 2H), 4.28 (q, J = 7.6 Hz, 2H), 4.22 (s, 2H),
4.18 (q, J = 7.5 Hz, 2H), 4.05 (s, 2H), 1.06 (m, 6H);
13CNMR (75 MHz, CDCl3): d = 165.3, 164.4, 148.0,
137.2, 130.5, 128.3, 127.6, 120.8, 117.5, 95.1, 65.2, 62.3,
59.8, 54.6, 45.8, 14.7, 13.2 m/z (ESI–MS) 393.68 (M ? 1,
requires 394.18).
a
Isolated yields
Table 2 Recycling of ionic liquid
No. of cyclesa
Fresh
Run 1
Run 2
Run 3
Yield (%)b
Time (min)
a
89
60
89
60
88
60
86
60
Reaction conditions: 1.0 equivalent of but-2-ynedioic acid diethyl
ester, 2.0 equivalent of aniline, 3.0 equivalent of formaldehyde; ionic
liquid [bmim]Br; temperature 80–90 °C
Results and discussion
On the basis of this consideration, we assumed that the
MCRs [51, 52] would proceed smoothly in the presence of
ionic liquid as well. Recently, as a part of our ongoing
research program for the synthesis of nitrogenous hetero-
cyclic compounds, here we became interested to develop a
more efficient and recyclable catalytic method by the use
of ionic liquid ([bmim]BF4) for the synthesis of potentially
biologically active polysubstituted 1,2,3,6-tetrahydropyr-
imidines via a three- and four-component condensation
reaction in order to conduct a new architecture of hetero-
cyclic compounds containing a- and b-amino acid unit.
Initially, we first allowed but-2-ynedioic acid diethyl
ester (1 mmol), aniline (2 mmol), and formaldehyde
(3 mmol) in the presence of ionic liquid ([bmim]BF4)
(2 mL) at 80–90 °C. The reaction completed almost
instantly (determined by TLC) and afforded the corre-
sponding product 1,3-diphenyl-1,2,3,6-tetrahydropyrimi-
dine-4,5-dicarboxylic acid diethyl ester with 89 % yield
(Scheme 1).
b
Isolated yields
Table 3 Ionic liquid ([bmim]BF4) mediated one-pot synthesis of
teytrahydropyrimidines via MCRs
Entry R1NH2
R2NH2
Product Time Yield
(min) (%)a
1
PhNH2
PhNH2
5a
5b
5c
5d
5e
5f
60
40
89
95
93
86
93
91
86
84
80
82
62
70
2
n-BuNH2
BnNH2
n-BuNH2
BnNH2
3
40
4
4-MeC6H4NH2 4-MeC6H4NH2
60
5
PhNH2
PhNH2
BnNH2
PhNH2
PhNH2
PhNH2
PhNH2
PhNH2
BnNH2
40
6
n-BuNH2
50
7
PhNH2
5g
5h
5i
40
8
4-MeC6H4NH2
4-ClC6H4NH2
3-MeC6H4NH2
2-MeC6H4NH2
120
120
120
360
360
9
10
11
12
5j
5k
We then turned our attention to investigate the effect of
different ionic liquids on the rate of reaction and the yield
of products. During this process, [bmim]BF4 was found to
be more appropriate ionic liquid (Table 1, entry 3). But the
reaction was found to be slow and a complex mixture of
4-NO2C6H4NH2 5l
Reaction conditions: 1.0 equivalent of but-2-ynedioic acid diethyl
ester, 2.0 equivalent of aniline, 3.0 equivalent of formaldehyde; ionic
liquid [bmim]Br (2 mL); temperature 80–90 °C
a
Isolated yields
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J IRAN CHEM SOC (2013) 10:695–699
compound was observed when the reaction was carried out
in the presence of [bmim]Br, [bmim]Cl, and [bmim]PF6.
Therefore, we chose [bmim]BF4 as a catalyst for the syn-
thesis of tetrahydropyrimidines.
the products obtained are interesting nitrogen containing
heterocyclic molecules containing a- and b-amino acid
blocks.
Acknowledgments Authors Neeraj Kumar Mishra and Saurav
Bhardwaj thank to Council of Scientific and Industrial Research
(CSIR), New Delhi, India for the grant of Research Associateship and
Senior Research Fellowship.
The ionic liquid could be recycled and reused for three
times without obvious loss in activity. Since ionic liquid
([bmim]BF4) is immiscible in ether, the desired product
may be extracted with it and the retained ionic liquid phase
may be reused. The second and third runs using recovered
ionic liquid afforded almost similar yields to those obtained
in the first run (Table 2).
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