Green synthesis and characterization of 3-carboxycoumarin and ethylcoumarin-3-carboxylate via Knoevenagel condensation

Article abstract of DOI:10.14233/ajchem.2017.20142

Environmentally benign, simple, efficient and clean syntheses of 3-carboxycoumarins and ethyl coumarin-3-carboxylate have been synthesized via Knoevenagel condensation using green catalysts such as aqueous extract of Acacia concinna pods and amino acids. These syntheses were carried out in solvents as well as solvent-less media. The yields obtained (between 86 % to 96 %) for the reactions were equal or better than those previously reported. The products were isolated, purified and characterized through melting points, TLC, IR, GC Mass and ¹H NMR spectroscopy. The characterized molecules showed structural and compositional conformity with the predicted reactions. The green route used for these reactions has shown to be efficient in reducing the time of reaction and in avoiding the toxic nature of the catalysts employed in classical reaction.

Full text of DOI:10.14233/ajchem.2017.20142

Asian Journal of Chemistry; Vol. 29, No. 2 (2017), 261-266
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2017.20142
Green Synthesis and Characterization of 3-Carboxycoumarin and
Ethylcoumarin-3-carboxylate via Knoevenagel Condensation
1
1,*
2
SYED SALMAN SHAFQAT , AMIR AZAM KHAN , MISBAHUL AIN KHAN ,
3
1
4
SHANTI FARIDAH SALLEH , MOHD SYAHMI JAMALUDIN and PANG SUH CEM
1Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Kota Samarahan,
Sarawak, Malaysia
2Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur, Pakistan
3Department of Chemical Engineering and Energy Sustainability, Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Kota Samarahan,
Sarawak, Malaysia
4Department of Chemistry, Faculty of Resource Science &Technology, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia
*Corresponding author: E-mail: akamir@unimas.my
Received: 17 June 2016;
Accepted: 13 October 2016;
Published online: 30 November 2016;
AJC-18140
Environmentally benign, simple, efficient and clean syntheses of 3-carboxycoumarins and ethyl coumarin-3-carboxylate have been
synthesized via Knoevenagel condensation using green catalysts such as aqueous extract of Acacia concinna pods and amino acids. These
syntheses were carried out in solvents as well as solvent-less media. The yields obtained (between 86 % to 96 %) for the reactions were
equal or better than those previously reported. The products were isolated, purified and characterized through melting points, TLC, IR,
GC Mass and 1H NMR spectroscopy. The characterized molecules showed structural and compositional conformity with the predicted
reactions. The green route used for these reactions has shown to be efficient in reducing the time of reaction and in avoiding the toxic
nature of the catalysts employed in classical reaction.
Keywords: Green protocol, Acacia concinna, Amino acids, 3-Carboxycoumarins, Knoevenagel condensation.
Literature shows that there are few references for the use of
amino acids and their derivatives for catalyzing of almost all
types of chemical reactions [5-7] including Knoevenagel
condensation [8]. During the last two years, amino acids have
also been found as efficient catalyst in Biginelli synthesis [9],
triarylimidazoles [10] and pyrazoleacryloyl syntheses [11].
An aqueous extract of Acacia concinna pods has been
investigated as an environmentally benign catalyst for the
Knoevenagel condensation as well as acylation of amines [12].
The aqueous extract of these pods is acidic in nature (pH,
about 2.1) due to the presence of an acaciac acid which is a
trihydroxy-monocarboxylic acid (Fig. 1) [13].
These new approaches have been very useful in the deve-
lopment of more environmentally supportable products and
chemical processes [14].We had earlier explored the utility
amino acids for coumarin-3-carboxylic acid in only one reaction
[8]. With the benefit of foresight, we aimed to extend this work
for efficacy of amino acids and aqueous extract of Acacia
concinna pods as catalysts in other Knoevenagel reactions by
using variety of active methylene molecules.
INTRODUCTION
Knoevenagel condensation is a classic C-C bond forma-
tion condensation reaction between aldehydes or ketones and
active methylene compounds [1]. Usually ammonia and other
amines like pyridine and piperidine are used as a catalyst in
Knoevenagel reactions [2]. Pyridine and piperidine are health
hazards. Pyridine easily dissolves in water and harms both
animals and plants in aquatic systems. Piperidine can seriously
affect eyes, skin and respiratory system [3]. In recent times,
various modern homogeneous and heterogeneous catalysts
have also been introduced for this purpose, such as microwave-
promoted, ultrasound, clays, solid phase, ionic liquids and bio-
technology catalyzed as well as solvent-free Knoevenagel
reactions have been studied and reported [4]. But the use of
expensive reagents, drastic reaction conditions, undesired side
products, tedious workups, prolonged reaction times and poor
yields are still existed limitations of these methodologies. To
deal with these tangible obstacles, Knoevenagel reaction has
been studied within the green chemistry perspective.

262 Shafqat et al.
Asian J. Chem.
Perkins Elmer FTIR (spectrum RXI) spectrometer. Mass spectra
were recorded on mass (MAT 312 Model) and GC-MS Shimadzu
QP2010. 1H NMR spectra were recorded on (NMR; Jeol JNM-
ECA500).
OH
O
General procedure for the preparation of catalyst:
Powdered pods of Acacia concinna fruit (20 g) and water (100
mL) in a 250 mL conical flask were boiled for 15 min. The
material was allowed to stand overnight then filtered off and
the aqueous extract was employed as a catalyst for the follo-
wing syntheses. The major content of this extract was acacia
acid (pH = 3-4).
OH
OH
HO
Fig. 1. Chemical structure of acaciac acid
Knoevenagel condensation is one of the important protocols
for the synthesis of coumarins. Coumarin and its derivatives
have long been recognized to possess biological activities
including anticancer [15,16], antibacterial [17], antifungal [18],
anticoagulant [19], anti-inflammatory [20], antitumor [21],
anticonvulsant [22], anti-HIV activity [23] and cholinesterases
inhibitors [24,25]. In addition, these compounds are used as
additives in food and cosmetics, dispersed fluorescent bright-
ening agents and as dyes for tuning lasers [26]. Photoresponsive
coumarins when grafted to silica surface have been found as
drug carrier agents [27]. The coumarin part is dimerized by
the irradiation with UV light longer than 310 nm, the molecular
“hinged double doors” of the pore in MCM-41 are closed and
guest molecules are stored inside. On the other hand, when
these particles are exposed to a UV light of wavelength around
250 nm, cleaves the coumarin dimer to regenerate its monomer
and opens the “double doors” of the pore in MCM-41, resulting
in the release of guest molecules as shown in Scheme-I. Photo
controlled reversible release of phenanthrene as a guest mole-
cule from coumarin modified mesoporous silica is a typical
example of this process [27].
Synthesis of 3-carboxycoumarins
Method I: A mixture of salicylaldehyde (2 mmol) (1),
Meldrum’s acid (2 mmol) (2) and 0.05 g of amino acid and
5 mL ethanol were taken in a round-bottomed flask and heated
under reflux on water bath for 1 h. After completion of the
reaction, the reaction mixture was cooled to room temperature,
poured off in ice cold water and a solid product was filtered
off, washed well with water and recrystallized to obtain pure
product (Scheme-II).
O
O
O
Amino
acid
O
CH3
CH3
OH
H
+
O
O
O
OH
(1)
(2)
O
(3)
Scheme-II: Condensation of salicyldehyde and Meldrum’s acid in presence
of amino acid
Method II: A mixture of salicylaldehyde (1), malonic
acid (4) and 5 mL of 20 % aqueous extract of Acacia concinna
were heated under reflux for 2 h. The reaction mixture was
allowed to cool to room temperature then it was poured off in
ice cold water. Solid product was filtered off and the crude
product was recrystallized by water to obtain pure coumarin-
3-carboxylic acid (3) (Scheme-III).
O
O
O
O
O
O
310 nm
254 nm
O
O
O
O
H
OH
Acacia concina
HO
+
OH
O
O
extract
OH
O
(3)
O
(
4)
(1)
Scheme-I: Reversible photodimerization and photocleavage of coumarin
Scheme-III: Condensation of salicyldehyde and malonic acid in presence
of Acacia concina extract
derivatives
Due to extensive applications of coumarin and its deriva-
tives numerous synthetic routes to coumarins syntheses have
been the subject of extensive study over many decades[19,28-30].
In the present work, another green approach with efficient
yield is being utilized for the syntheses of coumarins. These
syntheses have been carried out by using five different amino
acids viz. glycine, L-proline, L-cysteine, tryptophan, β-alanine
as well as aqueous extract of Acacia concinna pods.
Synthesis of ethyl coumarin-3-carboxylate
Method-I: A mixture of salicylaldehyde (1), (2 mmol),
malonic ester (2 mmol) (5) and 0.05 g amino acid were taken
in a round-bottomed flask and heated along with stirring at
boiling water bath for 2 h. After completion of the reaction,
the reaction mixture was cooled to room temperature, poured
in ice cold water and a solid product was filtered off, washed well
with water and recrystallized to obtain pure product (Scheme-IV).
Method II: A mixture of salicylaldehyde (1), (2 mmol),
malonic ester (2 mmol) (5) and aqueous extract of Acacia
concinna were taken in a round-bottomed flask and heated
along with stirring at boiling water bath for 2 h. After comple-
tion of the reaction, the reaction mixture was cooled to room
EXPERIMENTAL
Most of the chemicals used during this research work were
of laboratory grade and used without further purification.
Commercial TLC plates and UV light were used as a spot
locating agent. Infrared spectra (IR) spectra were recorded on

Vol. 29, No. 2 (2017)
Green Synthesis of 3-Carboxycoumarin and Ethylcoumarin-3-carboxylate via Knoevenagel Condensation 263
O
O
Coumarin-3-carboxylic acid: In recent years, amino
O
O
acids have drawn much interest in different organic reactions
due to their experimental simplicity, ease of handling and high
reactivity.
Amino
acid
H
OEt
+
EtO
OEt
(5)
OH
O
O
(1)
(6)
Condensation of salicyldehyde (1) with Meldrum’s acid
(2) was carried out in the presence of glycine as catalyst as a
test reaction. This reaction gave a good yield of coumarin-3-
carboxylic acid (3). With a successful reaction it was then carried
out in the presence of some other amino acids (Table-1).Amino
acid catalyzed reactions also gave good (85 %) to excellent
(96 %) yields.
Scheme-IV: Condensation of salicyldehyde and malonic ester in presence
of amino acid
temperature, poured in ice cold water and a solid product was
filtered off, washed well with water and recrystallized to obtain
pure product (Scheme-V).
O
O
Amino acids have been recognized as catalysts due to their
existence in dipolar form known as zwitterions (ampholyte
ions) in aqueous media. Plausible mechanism for the glycine-
catalyzed (or any other amino acid-catalyzed) synthesis of
coumarin been proposed (Schemes VI and VII). The reaction
may proceed through two different pathways. PathA provides
carbanion formation that is produced by abstraction of active
proton from active methylene molecule. The carbanion inter-
mediate then condenses with the carbonyl carbons of salicyl-
dehyde (Ia) followed by dehydration to afford the intermediate
(IIIa), which then rearranges to the required coumarin.
Path B involves the activation of aldehydic oxygen by
acid part of glycine via intermolecular hydrogen bonding and
subsequent condensation with active methylene molecule
intermediate. The intermediate undergoes dehydration to
O
O
H
OEt
Acacia concinna
+
EtO
OEt
(5)
OH
O
O
(1)
(6)
Scheme-V: Condensation of salicyldehyde and malonic ester in presence
of aqueous extract of Acacia concina
RESULTS AND DISCUSSION
In continuation of our research interest concerning the
investigation of new natural catalyst and development of new
methodologies, a highly efficient method for Knoevenagel
synthesis of coumarin-3-carboxylic acid and ethyl coumarin-
3-carboxylate under mild conditions with excellent yields using
amino acid as well as aqueous extract of Acacia concinna pods
is reported in this paper.
O
O
O
H
H
N+
H
O
1
OR2
+
R O
OH
O
O
H
Glycinium ion
Carbanion
OR2
R1O
(IIa)
OH
(Ia)
R1=OH, OEt
R2=OH, OEt
Path A
O
H
N
H
OH
Glycine
O-
O
H2O
R1O
R2O
H
O
OH
O
R1O
O
H 2
O
OH
O
OH
(IIIa)
n
o
i
t
R1O
a
z
i
l
c
y
C
O
O
(IVa)
Scheme-VI: Plausible path A for synthesis of coumarin

264 Shafqat et al.
Asian J. Chem.
TABLE-1
PHYSICO-CHEMICAL PROPERTIES OF THE PRODUCTS OBTAINED BY
THE CONDENSATION OF SALICYLDEHYDE WITH MELDRUM’S ACID
Yield
(%)
90
85
88
93
86
95
m.p.
(°C)
187
188
187
189
m.p. (°C)
[Ref. 31]
Catalyst
Colour
White
Pale yellow
Yellow
White
Yellow
Mass (m/z)
1H NMR
IR (νmax, cm-1)
Glycine
190 (31 %) M+;
146 (100 %);
118 (67 %);
89 (60 %);
DMSO, 500 MHz
L-Proline
L-Cysteine
Tryptophan
β-Alanine
3317 (O-H), 2985
(arom C-H), 1725
(C=O)
OH: 12.21 (broad peak), 1H: 8.37
(s), 1H: 7.38 (d) overlapping Hz
7.64, 1H 7.72 (t), 1H: 7.41 (t)
overlapped, 1H 7.89 (d) 7.64
189-192
190
190
63 (64 %).
Acacia concinna Light brown
H
O
+
N
O
-
H
O
OH
O
O
H
Iminium ion
Path B
2
1
R O
OR
OH
(Ib)
(IIb)
H O
2
1
R =OH, OEt
2
O
H
N
R =OH, OEt
H
OH
-
Glycine
O
H
1
OR
+
N
H
2
OR
-
O
OH
O
OH
1
OR
O
2
O
OR
OH
(IIIb)
1
R O
O
O
(IVb)
Scheme-VII: Plausible path B for synthesis of coumarin
produce intermediate (IIIa). Cyclization and dehydration of
intermediate (IIIa) results in respective coumarin.
One of the major advantages of this protocol is the iso-
lation and purification of the desired product which have been
achieved by simple washing and crystallization of the crude
product.
Aqueous extract of Acacia concinna, another biodegra-
dable catalyst has been reported in this study.Aqueous extract
of Acacia concinna has been found as a catalyst in the acylation
of amines [13] and in Knoevenagel condensation [12] of
salicyldehyde (1) with malonic acid (4). Chavan and Bandgar
[12] optimized the concentration of aqueous extract of Acacia
concinna. They reported that 20 % solution gives the maximum
yield. They [12] also reported a comparison of yields of synthesis
of coumarin derivatives using various surfactants as catalysts
such as DBSA (68 %), SDS (60 %), Triton X-100 (52 %), CPB
(40 %), CTAB (45 %) and aqueous extract of Acacia concinna
(98 %). Keeping in view the previous studies, in this research
work the reactions were carried out on different concentrations
and verified that the optimum concentration is 20 %. So herein,
all the reported reactions were conducted in the presence of
20 % aqueous extract of Acacia concinna.
The rate enhancement in the aqueous extract of Acacia
concinna pods is mainly due to its surfactant property and
acidic pH. The extract can solubilize the reactant species
strongly by hydrogen bonding and helps to increase the number
of effective collisions between the reactant species. Further
encapsulation of the reactants in micellar cages may drive the
equilibrium toward the product side by expelling the water
molecule out of its hydrophobic interior that increases the speed

Vol. 29, No. 2 (2017)
Green Synthesis of 3-Carboxycoumarin and Ethylcoumarin-3-carboxylate via Knoevenagel Condensation 265
TABLE-2
PHYSICO-CHEMICAL PROPERTIES OF THE PRODUCTS OBTAINED BY
THE CONDENSATION OF SALICYLDEHYDE WITH MALONIC ACID
Yield
(%)
92
87
85
90
92
90
m.p.
(°C)
191
192
189
188
m.p. (°C)
[Ref. 31]
Catalyst
Colour
Mass (m/z)
1H NMR
IR (νmax, cm-1)
Glycine
White
Pale yellow
Yellow
White
Yellow
190 (31 %) M+;
146 (100 %);
118 (67 %);
89 (60 %);
DMSO, 500 MHz
L-Proline
L-Cysteine
Tryptophan
β-Alanine
3317 (O-H), 2985
(arom C-H), 1725
(C=O)
OH: 12.21 (broad peak), 1H: 8.37
(s), 1H: 7.38 (d) overlapping Hz
7.64, 1H 7.72 (t), 1H: 7.41 (t)
overlapped, 1H 7.89 (d) 7.64
189-192
190
190
63 (64 %).
Acacia concinna Light brown
TABLE-3
PHYSICO-CHEMICAL PROPERTIES OF THE PRODUCTS OBTAINED BY
THE CONDENSATION OF SALICYLDEHYDE WITH MALONIC ESTER
Yield
(%)
96
m.p.
(°C)
92
m.p. (°C)
[Ref. 31]
IR (νmax, cm-1)
Catalyst
Glycine
Colour
White
Mass (m/z)
1H NMR
L-Proline
Pale yellow
Yellow
White
90
86
92
90
95
90
92
93
93
92
DMSO, 500 MHz
3185 (arom C-H),
2986 (alkyl C-H)
1752 (C=O)
218 (31 %) M+;
173 (90 %);
146 (100 %)
L-Cysteine
Tryptophan
β-Alanine
1H 8.52 (s); 1H 7.63; 1H 7.35;
2H 4.41 (m); 4,6 1H; 3H`. 41 (t)
overlapped; 5,7 1H
92-95
Yellow
Acacia concinna Light brown
H2O, CH3COR2
O
O
OR1
OH
H
OR1
O
O
OH
OR2
O
O
Scheme-VIII: Micelle-promoted syntheses of coumarin
as well as the yields of products. This remarkable enhancement
in reaction rate prompted us to explore the potential of this
protocol for the synthesis of 3-carboxycoumarins and ethyl-
coumarin-3-carboxylate.The efficiency of this aqueous approach
was studied for the synthesis of 3-carboxycoumarin and
ethylcoumarin-3-carboxylate using mild reaction conditions
and the results are summarized in Tables 2 and 3.
A model reaction was performed by using salicyldehyde,
Meldrum’s acid and aqueous extract of Acacia concinna. The
product was isolated, purified and characterized. The results
verified the formation of coumarin-3-carboxylic acid.Afterwards
the syntheses were carried out by employing malonic acid.
The products (3) was characterized with the product obtained
with glycine (m.p., mixed m.p., TLC, IR). This remarkable
enhancement in reaction rate and efficiency of this approach
prompted us to explore the potential of this protocol for the
synthesis of ethyl coumarin-3-carboxylate.
Ethyl coumarin-3-carboxylate: In continuation of our
interest in the green catalysts condensation of salicyldehyde
(1) with ethyl acetoacetate (6) was carried out in the presence
of glycine. This reaction gave a yield of 92 %. The results
confirmed the presence the formation of ethyl coumarin-3-
carboxylate. So the catalytic activity of few other amino acids
and aqueous extract of Acacia concinna was attempted. All
these reactions gave excellent yields.
Conclusion
For the present work, only salicyldehyde and a few repre-
sentative active methylene compounds were used to check the
feasibility of amino acid as well as Acacia concinna catalysts
in these reactions. The yields in all these cases were comparable
and these catalysts can be extended using other salicyldehyde
derivatives and active methylene compounds. In conclusion,
an efficient and another eco-friendly method for the syntheses

266 Shafqat et al.
Asian J. Chem.
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time with high purity, mild reaction conditions and a simple
workup procedure makes this procedure attractive for large
scale preparation of coumarins.
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