Glycine catalyzed convenient synthesis of 2-amino-4H-chromenes in aqueous medium under sonic condition

Article abstract of DOI:10.1016/j.ultsonch.2012.01.006

A one-pot three-component condensation of an aldehyde, malononitrile, and resorcinol has been achieved by ultrasound method. The reaction has been catalyzed by glycine in aqueous medium. This protocol afforded corresponding 2-amino-4H-chromenes in shorter

Full text of DOI:10.1016/j.ultsonch.2012.01.006

Ultrasonics Sonochemistry 19 (2012) 725–728
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Short Communication
Glycine catalyzed convenient synthesis of 2-amino-4H-chromenes in aqueous
medium under sonic condition
⇑
Bandita Datta, M.A. Pasha
Department of Studies in Chemistry, Bangalore University, Central College Campus, Palace Road, Bengaluru 560001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A one-pot three-component condensation of an aldehyde, malononitrile, and resorcinol has been
achieved by ultrasound method. The reaction has been catalyzed by glycine in aqueous medium. This
protocol afforded corresponding 2-amino-4H-chromenes in shorter reaction times, high yields and sim-
ple work-up procedures with the green aspects by avoiding toxic reagents.
Received 24 November 2011
Received in revised form 20 December 2011
Accepted 5 January 2012
Available online 21 January 2012
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Aldehydes
Resorcinol
Malononitrile
Glycine
Water and ultrasound
1. Introduction
profound research activity in the development of organic reactions
in aqueous media which offers key advantages such as rate
Considering the increasing environmental pollution and its
drastic impact on living systems, emerging areas of green
chemistry demand eco-friendly organic chemical processes [1].
Development of alternative energy sources have become a major
consequence due to the consumption of energy for heating and
cooling which causes a major adverse effect on the environment.
In this regard, a number of organic chemists are engaged in the
development of green chemistry protocols i.e. utilization of non-
classical energy resources like ultrasound irradiation technique
and microwave energy [2]. Use of ultrasound has proved to be
one of the stepping stone towards the green syntheses. Ultrasound
enhances the reactivity of the chemical reactions via the process of
acoustic cavitation [3]. The assistance of ultrasonic irradiation
efficiently shortens the reaction times. Simple experimental proce-
dure, very high yields, increased selectivities, and clean reaction of
many ultrasound-induced organic transformations offer additional
convenience in the field of synthetic organic chemistry [4].
One of the major factors for a green chemical process in solution
involves the choice of inexpensive, safe and non-toxic solvents.
Water being abundant in nature is the first choice. In addition to
satisfying the above criteria, water has also special effects on reac-
tion arising from intra- and inter-molecular non-covalent interac-
tions leading to assembly processes [5]. Since the pioneering
studies by Breslow on Diels–Alder reactions [6], there has been
enhancement and insolubility of the final products, and facilitates
their isolation by simple filtration. The chemical effects resulting
from the irradiation of aqueous solutions with ultrasound were
for the first time introduced by Loomis and co-workers [7].
Chromenes are important oxygenated heterocyclic compounds
endowed with activities such as antidepressant, antihypertensive,
anti-tubulin, antiviral, antioxidative activity [8,9]. Chromenes are
known to activate potassium channels and inhibit phosphodiester-
ase IV and dihydrofolate reductases [10–17]. Chromenes are also
used as cosmetics, pigments [18], and potential biodegradable
agrochemicals [19]. A few procedures have been reported for the
preparation of substituted 2-amino-4H-chromenes from resor-
cinol, aldehydes and malononitrile using piperazine [20], DBU
[21], NEt3 [22] and by an electrochemical synthesis in propanal
[23].
In view of the inherent properties of glycine such as environ-
mental compatibility, non-corrosives, ready availability and cost
effectiveness; this catalyst has started evoking interest in the
organic chemists to use it as an organo base in organic synthesis.
In addition, molecules like cinchona alkaloids and other amino
acids have also been used as organocatalysts in various organic
syntheses, and have been proved to be highly efficient catalysts
in multi-component reactions [24,25]. In continuation of our inter-
est towards the exploration of use of glycine as an organocatalyst
in organic synthesis [26a] and in the development of new synthetic
methodologies [26], herein, we wish to report a one-pot multi-
component synthesis of 2-amino-4H-chromenes from araldehydes
⇑
Corresponding author.
E-mail address: m_af_pasha@ymail.com (M.A. Pasha).
1350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2012.01.006

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B. Datta, M.A. Pasha / Ultrasonics Sonochemistry 19 (2012) 725–728
1
2.2.3. Spectral data of 4h
1H NMR (DMSO-d6, 400 MHz): 4.90 (s, 1H, CH); 6.51 (s, 2H,
R
NH2); 6.82–6.84 (d, J = 8.1 Hz, 1H, ArH); 6.99 (s, 2H, ArH); 7.60–
7.66 (m, 2H, ArH); 8.01 (s, 1H, ArH); 8.07–8.09 (m, 1H, ArH);
9.75 (s, 1H, OH) ppm; 13C NMR (DMSO-d6, 100 MHz): 34.08,
53.54, 102.73, 105.93, 110.70, 111.46, 112.65, 120.83, 129.96,
142.78, 149.53, 157.30, 157.78, 161.33; Mass (m/z): 298 e/z.
R
N
H
O
Glycine
CN
NH2
+
)))), H2O
OH
HO
O
HO
N
2.2.4. Spectral data of 4j
2
1H NMR (DMSO-d6, 400 MHz): 3.29 (s, 1H, CH); 4.652 (s, 2H,
NH2); 6.39 (s, 1H, ArH); 6.46–6.49 (m, 1H, ArH); 6.76–6.89 (m,
2H, ArH); 7.16–7.18 (m, 2H, ArH); 7.34–7.36 (m, 1H, ArH); 9.68
(s, 1H, OH) ppm.
4
3
Scheme 1. Synthesis of 2-amino-4H-chromenes under sonic condition.
(1), malononitrile (2) and resorcinol (3) at 28–30 °C in water as a
solvent under sonic condition in good to excellent yield within
9–45 min (Scheme 1).
2.2.5. Spectral data of 4m
1H NMR (DMSO-d6, 400 MHz): 4.75 (s, 1H, CH); 6.13 (s, 2H,
NH2); 6.33–6.36 (d, J = 12 Hz, 1H, ArH); 6.40 (s, 1H, ArH); 6.52–
6.54 (m, 1H, ArH); 6.93–6.96 (d, J = 12 Hz, 2H); 7.50 (s, 1H, ArH);
9.79 (s, 1H, OH) ppm.
2. Methods
2.2.6. Spectral data of 4n
2.1. Materials and instruments
1H NMR (DMSO-d6, 400 MHz): 4.86 (s, 1H, CH); 6.38 (s, 2H,
NH2); 6.39–6.47 (m, 1H, ArH); 6.75–6.77(d, J = 8.0 Hz, 1H, ArH);
6.88 (s, 1H, ArH); 7.09–7.16 (m, 3H, ArH); 7.21–7.26 (m, 2H,
ArH); 9.75 (s, 1H, OH) ppm.
All starting materials were commercial products, and were used
without further purification except liquid aldehydes, which were
distilled before use. Yields refer to yield of the isolated products.
Melting points were measured on a Raaga, Chennai, Indian make
melting point apparatus. Nuclear magnetic resonance spectra were
obtained on a 400 MHz Bruker AMX instrument in DMSO-d6 using
TMS as a standard. LC–Mass spectra were performed on an Agilent
Technologies 1200 series instrument. HRMS analyses were carried
out using ESI-Q TOF instrument. Infrared spectra were recorded
using Shimadzu FT-IR-8400s spectrophotometer as KBr pellets.
All the reactions were studied using SIDILU, Indian make sonic
bath working at 35 kHz (constant frequency, 120 W) maintained
at 28–30 °C without mechanical stirring.
3. Results and discussion
In search of the best experimental reaction condition, the reac-
tion of 4-methoxy benzaldehyde, malononitrile, and resorcinol in
the presence of glycine in aqueous medium was considered as a
standard and a model reaction. When the reaction was carried
out in the absence of any catalyst the product was not detected
(Table 1, entry 1). In the presence of glycine the reaction was pos-
sible, and in order to determine the appropriate concentration of
the catalyst used, we investigated the model reaction at different
concentrations of glycine such as 5, 10, 15, and 20 mol%. The prod-
uct was formed in 67%, 78%, 96%, and 94% yield, respectively (Table
1). This indicates that 15 mol% of glycine is sufficient to carry out
the reaction smoothly.
2.2. General procedure for the synthesis of 2-amino-4H-chromenes
A
mixture of aromatic aldehyde (5 mmol), malononitrile
(5 mmol), resorcinol (5 mmol) and glycine (15 mol%) in water
(5 mL) was sonicated in a sonic bath working at 35 kHz (constant
frequency) maintained at 28–30 °C (by circulating water). After
completion of the reaction (monitored on TLC), product was taken
into ethyl acetate (10 mL) and separated. The organic layer was
washed successively with water (5 mL), sat. NaHCO3 (5 mL), water
(5 mL) and then dried over anhydrous Na2SO4 to get the crude
product in almost pure form. The analytical grade sample was ob-
tained by silica gel column chromatography (EtOAc: Hexane:: 1:9).
The spectral data of some of the representative products:
In order to evaluate the effect of solvent, various solvents such
as hexane, DCM, DMF, ethanol and water were used for the model
reaction in the presence of glycine under sonic condition. Use of
hexane and DCM yielded the Knoevenagel condensation adduct
as major product and the desired product was formed only in
14% and 35%, respectively (LC–MS). Reaction in DMF and ethanol
resulted in 86% and 82% yields, respectively. As Imax (maximum
cavitation intensity) and TImax (the temperature at which Imax is
reached) of any solvent has a profound effect in sonochemical reac-
tivity (for water Imax is 100), Imax of water is responsible for the in-
crease in the reaction rate compared to the other solvents for
which Imax is less [27]. Further, when the transition state or the
product is more polar than the reactants then polar solvents stabi-
2.2.1. Spectral data of 4a
1H NMR (DMSO-d6, 400 MHz): 5.54 (s, 1H, CH); 5.95 (s, 2H,
NH2); 6.23 (s, 1H, ArH); 6.34–6.35 (d, J = 4.0 Hz, 1H, ArH); 6.74–
6.75 (d, J = 4.0 Hz, 1H, ArH); 6.78–6.06 (m, 5H, ArH); 8.53 (s, 1H,
OH) ppm; Mass (m/z): 264 e/z.
Table 1
Evaluation of catalytic activity of glycine in the synthesis of 2-amino-4H-chromene.a
Entry
Amount of glycine (mol%)
Time (min)
Yield (%)b
2.2.2. Spectral data of 4d (Novel)
1
2
3
4
5
0
5
10
15
20
36
18
18
9
ND
67
78
96
94
1H NMR (DMSO-d6, 400 MHz): 3.70 (s, 3H, CH3); 4.54 (s, 1H,
CH); 6.38 (s, 2H, NH2); 6.44–6.47 (m, 2H, ArH); 6.75–6.77 (d,
J = 8.0 Hz, 3H, ArH); 6.83–6.85 (t, J = 8.0 Hz, 1H, ArH); 7.05 (s, 1H,
OH); 9.62 (s, 1H, OH) ppm; 13C NMR (DMSO-d6, 100 MHz): 34.08,
53.54, 59.53, 102.73, 105.93, 111.46, 112.65, 120.83, 129.96,
142.78, 149.53, 157.30, 157.78, 161.33 pm; HRMS (m/z):
334.0093 e/z.
9
a
Reaction condition: resorcinol (2 mmol), 4-methoxybenzaldehyde (2 mmol),
malononitrile (2 mmol) and glycine under sonication.
b
Isolated yields.

B. Datta, M.A. Pasha / Ultrasonics Sonochemistry 19 (2012) 725–728
727
Table 2
+
_
+
Effect of solvent on the rate of the reaction and yield of the product.a
NH3 COO
NH3
COOH
Yield (%)b
O
Entry
Solvent
Time (min)
O
CN
CN
H
H
1
2
3
4
5
6
Solvent-free
Hexane
DCM
DMF
Ethanol
Water
54
36
36
18
18
18
5
H
CN
H
_
+
14
35
86
82
95
H
CN
a
Reaction condition: aromatic aldehyde (2 mmol), malononitrile (2 mmol), res-
O
H
orcinol (2 mmol) and glycine (15 mol%) and water (5 mL).
CN
CN
b
H
Isolated yield.
CN
CN
H
Table 3
Synthesis of 2-amino-4H-chromenes from aromatic aldehydes, resorcinol, malono-
nitrile and catalytic glycine under aqueous sonic condition.
H
Entry
Aldehyde
Producta
Time (min)
Yieldb (%)
CN
1
2
3
4
5
6
7
8
H
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
9
94
91
92
92c
90
95
88
92
92
91
92
89
90
96
93
NC
HO
CN
4-OMe
3,4-(OMe)2
3-OMe,4-OH
4-Me
4-OH
3,4-O–CH2–O–
4-Cl
2-Cl
4-F
2-F
3-NO2
18
27
30
24
20
36
16
27
20
18
36
32
36
45
O
HO
N
H
O
9
H
10
11
12
13
14
15
CN
NH
CN
O
HO
2-NO2
2-Furfuryl
2-Thiophene
O
NH2
HO
Scheme 2. A plausible mechanism of formation of 2-amino-4H-chromenes.
a
Reaction condition: resorcinol (2 mmol), aromatic aldehyde (2 mmol), malon-
onitrile (2 mmol) and glycine (15 mol%) under sonication.
furan-2-carbaldehyde were equally amenable to these conditions
(Table 3, entries 14 and 15). All the results are summarized in Table
3. Formation of the product was confirmed with the help of IR, 1H
NMR, and mass spectral analysis or comparison of the melting
points with the products prepared by reported methods [20,21].
The reproducibility of the reaction has also been checked for
three times each for benzaldehyde and 4-hydroxybenzaldehyde.
It was found that, every time the optimised reaction condition
holds good for the synthesis of aminochromenes. These results
have been presented in Table 4.
b
Isolated yields.
Novel compound.
c
Table 4
Reproducibility of the optimized reaction.a
Entry
Aldehyde
Product
Time (min)
Yieldb (%)
1
2
3
4
5
6
H
H
H
4-OH
4-OH
4-OH
4a
4a
4a
4f
4f
4f
9
94
93
94
95
95
93
18
27
30
24
24
The reaction is expected to proceed via the Knoevenagel con-
densation of an aromatic aldehyde and malononitrile, followed
by the cyclo-condensation reaction with resorcinol in water to
form the desired product as shown in Scheme 2.
a
Reaction conditions: resorcinol (2 mmol), aromatic aldehyde (2 mmol), mal-
ononitrile (2 mmol) and glycine (15 mol%) under sonication.
Further experiments were designed for investigating the
influence of ultrasound on the synthesis of 2-amino-4H-chromenes
(Table 5). Representative reactions were carried out with benzalde-
hyde and aromatic aldehydes containing functional groups like
NO2, OH, OMe; separately in water (5 mL) using glycine
(15 mol%) as the catalyst at 28–30 °C with magnetic stirring in
the presence and absence of ultrasound and found that 25%
chromene was obtained for the condensation of benzaldehyde,
resorcinol and malononitrile in the absence of ultrasonic irradia-
tion; the Knoevenagel condensation product-arylmethylidenemal-
ononitrile and the unreacted resorcinol were isolated. Whereas
94% yield of aminochromene was obtained under the influence of
ultrasound, indicating that this one-pot condensation reaction
can be greatly accelerated under sonic condition.
Upon irradiation with ultrasound, the formation, growth, and
implosive collapse of bubbles can create extreme physical and
chemical conditions in liquids [27a], leading to short-lived local-
ized hot-spots which produce relatively high temperature for this
condensation reaction to occur, and the rate of the reaction got
well accelerated under sonic condition. In the present reaction,
b
Isolated yields.
lize the transition state. The more polar the solvent the faster the
reaction will be. Hence, compared to non-polar solvents polar
solvents like DMF and ethanol enhanced the rate of the reaction
and gave products in good yields.
The reaction was also studied under solvent-free condition and
only trace amount of the desired product was obtained, which
clearly indicates that, water is essential for shifting the reaction
to the product side. In the presence of water, the reaction not only
went to completion efficiently but also furnished the product in
excellent yield (95%, Table 2).
For assessing the generality of optimized reaction condition, a
wide range of substituted aldehydes were allowed to undergo this
three-component condensation. Aromatic aldehydes with
electron-withdrawing and electron-donating functionalities such
as Cl, F, OH, Me, OMe, and NO2 (Table 3, entries 2–13) were found
to be compatible under the optimized reaction condition.
Heteroaromatic aldehydes such as thiophene-2-carbaldehyde and

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B. Datta, M.A. Pasha / Ultrasonics Sonochemistry 19 (2012) 725–728
Table 5
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Entry
R1
Product (4)
Time/yield
Time/yield
(min/(%))c
(
, min/(%))b
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1
2
3
4
5
H
4a
4b
4f
4l
4m
9/25
9/94
4-OMe
4-OH
3-NO2
2-NO2
20/40
30/47
20/35
40/45
18/91
20/95
36/89
32/90
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Reaction condition: aromatic aldehyde (2 mmol), malononitrile (2 mmol), res-
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Sonication.
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got well accelerated under sonic condition. In the present reaction,
the reactants are partially dissolved initially, and the reaction be-
gins only under the influence of ultrasound which can be explained
as follows: when cavitation occurs near the solid surface (of solid
substrates), cavity collapse is non-spherical and as a result of this
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facial boundary layers. At a liquid–liquid interface (heteroge-
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also leads to the mutual injection of droplets of one liquid into
the other one, producing emulsions as in the present reaction.
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reactant contact surface gets increased. The driving force for the
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An ultrasound-assisted method for the synthesis of 2-amino-
4H-chromene derivatives is studied in details. This one-pot
condensation reaction, leading to a series of polysubstituted
chromenes can be carried out efficiently using glycine as an organ-
ocatalyst in water as solvent. Ultrasound accelerates greatly the
reaction rate and enhances the yields. The method described in this
paper is mild and energy efficient and provides a very reliable pro-
cedure for the synthesis of 2-amino-4H-chromene derivatives.
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