Acid catalyzed Knoevenagel condensation of thiobarbituric acid and aldehyde at room temperature

Article abstract of DOI:10.1080/00397911.2020.1751203

Knoevenagel reactions have been performed by the action of various unsymmetrical thiobarbituric acids containing activated methylene carbon and electron deficient center of aromatic aldehydes using small amount of acetic acid as initiator in ethanolic med

Full text of DOI:10.1080/00397911.2020.1751203

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Acid catalyzed Knoevenagel condensation
of thiobarbituric acid and aldehyde at room
temperature
Vinod D. Deotale & Madhukar G. Dhonde
To cite this article: Vinod D. Deotale & Madhukar G. Dhonde (2020): Acid catalyzed Knoevenagel
condensation of thiobarbituric acid and aldehyde at room temperature, Synthetic Communications
To link to this article: https://doi.org/10.1080/00397911.2020.1751203
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2020.1751203
Acid catalyzed Knoevenagel condensation of thiobarbituric
acid and aldehyde at room temperature
Vinod D. Deotale and Madhukar G. Dhonde
Department of Chemistry, Shri Mathuradas Mohota College of Science, Nagpur, India
ABSTRACT
ARTICLE HISTORY
Received 16 January 2020
Knoevenagel reactions have been performed by the action of various
unsymmetrical thiobarbituric acids containing activated methylene car-
bon and electron deficient center of aromatic aldehydes using small
amount of acetic acid as initiator in ethanolic medium. The present
protocol proceeded smoothly on room temperature stirring using ethyl
alcohol as solvent with the help of initiator. The work-up procedure is
very simple and products have been purified by simple recrystalliza-
tion. Thus rendering the methodology is good and all synthesized mol-
KEYWORDS
Acetic acid catalyst;
aldehydes; Knoevenagel
products; TBAs
1
ecules were characterized by H, 13C NMR, and MS spectra.
GRAPHICAL ABSTRACT
Ar
N
O
O
Ar
N
O
O
Ar''
H
Ar''
H
CH3CO2H:EtOH
rt, 0.9-1.4 h
O
S
S
-25 Examples
- without column
- 76-85% yields
N
N
Ar'
Ar'
Introduction
The Knoevenagel reaction is a condensation between activated methylene carbon and
carbonyl compounds in acidic or basic medium. The condensation is catalyzed by a
weak base such as an amine and have powerful tool for carbon–carbon double bond
formation strategy.[1] In the past few decades, several Knoevenagel condensation reac-
tions have been promoted under distinct catalysts.[2,3] However few reports are most
prominent on activated methylene carbon in water medium,[4] or with organo-
catalyst,[5] and In(III) catalyzed using acetic anhydride as promoter.[6] In fact, to
increased electrophilicity or leaving-group ability of aldehydes in the presence of the
acid additive could accelerate the Knoevenagel condensation which is further increased
polar character due to carbon–carbon double bond network.
The Knoevenagel reaction is the most common synthetic strategy to produce electron
deficient carbon–carbon double bond center. It has been widely employed in the prep-
aration of benzylidene derivatives and important intermediates which is used in varied
organic transformations. Therefore, alkylated and benzylidene derivatives of thiobarbitu-
ric acid have attracted attention of researchers toward medicinal chemistry,[7–11]
CONTACT Madhukar G. Dhonde
madhudash2001@yahoo.co.in
Department of Chemistry, Shri Mathuradas
Mohota College of Science, Nagpur, 444024 MS, India.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
V. D. DEOTALE AND M. G. DHONDE
therapeutic drugs, pharmacological action[12–15] and some of them are more significant
due to their diverse biological activity.[16] An insertion of an aryl, amino, or a methyl
moiety at 5-position of thiobarbituric acid enhances the antidepressant activities[17] has
been reported. Arylidene-pyrimidine-2,4,6-trione, arylidene-2-thioxo-dihydro-pyrimi-
dine-4,6-dione and its derivatives were found to have hypotensive, transiquilizer and
good anti-bacterial agents.[18,19] The barbituric acid and 2-thiobarbituric acid undergoes
Knoevenagel condensations with aldehydes to give 5-substituted derivatives.[20,21] In
addition, synthesis of benzylidenes, useful intermediate products, and also gives struc-
tural versatility in different nucleophiles, such as diketones, ketothioesters, 1,3-ketoest-
ers[22,23] malonates, malononitriles[24] keto amides, and cyclic esters and with different
aromatic[25] or aliphatic aldehydes.[26] Benzylidene thiobarbituric acids react with car-
bon nucleophiles[27,28] because of the fact that the active double bond in benzylidene
carbon can easily be reduced[29,30] these compounds can be used for the synthesis of
unsymmetrical disulfides[31,32] and for the mild oxidation of alcohols.[33,34] Kinetics of
electrophilic alkylations of barbiturate and thiobarbiturate anions[35] and electro-
philicity of 5-Benzylidene-1,3-dimethylbarbituric and -thiobarbituric acids[36] had been
proved the strongly polarized exocyclic double bond.
TBAs have activated methylene carbon act as key precursor which is used in
Knoevenagel condensation.[37] Recently we have reported solvent-free synthesis of
Thiobarbituric acids as a precursor using Green Catalyst[38] and HPLC purification
technique.[39] Effort to extend the continuation of our earlier work, here we wish to
report the synthesis of benzylidene thiobarbituric acid derivatives by the reaction of thi-
obarbituric acid and aromatic aldehyde using catalytic amount of weak acid i.e., acetic
acid in ethanolic medium at room temperature. The simple isolation and purification
route used to make benzylidene derivatives is depicted in Scheme 1.
Results and discussion
We initially carried out the optimization by the reaction of 3-phenyl-2-thioxo-1-(4-
methylphenyl)-dihydropyrimidine-4,6(1H,5H)-dione 1b and 4-methoxybenzaldehyde 2
on room temperature stirring as model substrate and progress of the reaction has been
monitored by TLC (Table 1). We examine the first reaction by loading 10 mol% of
CH3CO2Na as a catalyst in solvent free condition, only 45% of 3b was obtained at 4 h
(entry 1). On the basis of first result, solvent may be important for the completion of
reaction. Then we attempted the reaction with 10 mol% of CH3CO2Na in aqueous
medium which offered 40% of 3b (entry 2) because aromatic substituent is insoluble in
water hence organic solvent is crucial for completion of reaction.
We planned to optimize the reaction condition by taking the mixture of
CH3CO2Na:H2O in 1:1, 1:2 and CH3CO2Na:H2O:EtOH, 1:1:2 respectively, 48–58% of
3b was obtained and 1b remains unreacted (entries 3–5). Aqueous medium does not
support completion of reaction hence by switching the aqueous medium to organic.
Further optimization carried out by using mixture of 1:1 and 1:2 proportions of
CH3CO2Na:EtOH, 66–73% of 3b was obtained (entries 6 and 7). We have taken lot of
effort to developing clean methodology using CH3CO2Na in varying medium but did
not give notable impact.

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SYNTHETIC COMMUNICATIONSV
3
Ar
N
Ar
O
O
O
N
Ar''
H
CH3CO2H, EtOH,
rt, 0.9-1.4 h
Ar''
S
S
O
N
N
O
Ar'
Ar'
2
1a-x
3a-x
3
1-Ar
3-Ar’
Ar”
3
1-Ar
3-Ar’
Phenyl
Ar”
a
Phenyl
Phenyl
Phenyl
Phenyl
4-Methoxyphenyl
4-Methoxyphenyl
4-Methoxyphenyl
4-Methoxyphenyl
4-Methoxyphenyl
4-Methoxyphenyl
4-Methoxyphenyl
m
4-Methoxyphenyl
3-Chlorophenyl
4-Methylphenyl
3-Methylphenyl
3-Chlorolphenyl
3-Chlorolphenyl
4-Chlorophenyl
4-Chlorophenyl
4-Methoxyphenyl
4-Chlorophenyl
3-Methoxyphenyl
4-Methylphenyl
4-Chlorophenyl
4-Methylphenyl
b
c
d
e
f
n
o
Phenyl
4-Chlorophenyl
4-Chlorophenyl
4-Chlorophenyl
4-Chlorophenyl
4-Chlorophenyl
4-Chlorophenyl
4-Chlorophenyl
4-Chlorophenyl
4-Chlorophenyl
4-Chlorophenyl
4-Chlorophenyl
4-Methoxylphenyl
4-Methylphenyl
4-Methylphenyl
4-Methylphenyl
p
q
r
2-Methylphenyl
3-Methylphenyl
4-Methylphenyl
3-Methoxyphenyl
2-Methylphenyl
3-Methylphenyl
4-Methylphenyl
3-Methoxyphenyl
4-Methoxyphenyl
4-Methoxyphenyl
s
g
h
i
t
4-Methoxylphenyl 4-Methoxylphenyl 4-Methoxyphenyl
Ethyl
u
v
w
3-Chlorophenyl
4-Chlorophenyl
Phenyl
3-Chlorophenyl
4-Chlorophenyl
Phenyl
4-Methoxyphenyl
4-Methoxyphenyl
4-Chlorophenyl
4-Chlorophenyl
4-Methyl phenyl
4-Methylphenyl
4-Methylphenyl
Ethyl
j
k
l
4-Methylphenyl
Phenyl
x
Scheme 1. The reaction is given above.
No significant improvement was observed in the present optimization, therefore, we
changed the strategy by loading the catalytic amount of weak acid i.e., acetic acid to
afford 49% of 3b in absence of solvent (entry 8). When the reactions were carried out
in different proportions of CH3CO2H:H2O & CH3CO2H:H2O:EtOH) did not give
appreciable results (entries 9–12).
Therefore we confirmed that the aromatic substituent present in 1b is more easily
soluble in organic solvent hence, ethanol medium is good option to carried out the fur-
ther reaction protocol. It was interesting to expand the scope for completion of reaction
we carried out the same reaction by catalytic amount of 1:1 proportion of
CH3CO2H:EtOH, 76% and at 1:2 proportion, 84% of 3b were isolated (entries 13–14).
Encouraged this result (entry 14), we compared the ability by reducing the mol % of
CH3CO2H & EtOH, low to moderate yields were obtained (entries 15–17). In all the
cases studied, CH3CO2H:EtOH (1:2) was found to be the best choice for the rest reac-
tions to give corresponding 5-arylbenzylidene-1,3-diaryl-2-thioxodihydropyrimidine-4,6-
(1H,5H)-diones (3a-x) in excellent yields. The results are summarized in Table 2.
Material and methods
Experimental method
Synthesis of 5-(4-methoxybenzylidene)-1-(4-methylphenyl)-3-phenyl-2-thioxodihydro-
pyri-midine-4,6-(1H,5H)-dione (3b)
5-(4-Methoxybenzylidene)-1-(4-methylphenyl)-3-phenyl-2-thioxodihydropyrimidine-4,6-
(1H,5H)-dione (3b) was synthesized by the interaction of 1-(4-methylphenyl)-3-phenyl-

4
V. D. DEOTALE AND M. G. DHONDE
Table 1. Optimization of reaction conditions of 1-(4-methylphenyl)-3-phenyl-2-thioxo-dihydropyrimi-
dine-4,6(1H,5H)-dione (1b) and 4-methoxybenzaldehyded (2).
CH3
CH3
O
O
O
O
O
H
N
N
N
N
CH3CO2H, EtOH,
rt, 1.25 h
S
S
OCH3
OCH3
3b
2
1b
Loading
Time
Yielde
Entry
Catalyst
(mol%)
(h)
(%)
1
2
3
4
5
6
7
8
CH3CO2Na
10
10
10
10
10
10
10
10
10
10
10
10
10
10
7.5
5.0
2.5
4.0
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
1.25
3.5
3.5
3.5
35f
30f
40f
42f
50f
58
[CH3CO2Na : H2O]a
[CH3CO2Na : H2O : EtOH]a
[CH3CO2Na : H2O : EtOH]b
[CH3CO2Na : H2O : EtOH]c
[CH3CO2Na : EtOH]a
[CH3CO2Na : EtOH]b
CH3CO2H
65
43f
40f
46f
49f
51f
65
9
[CH3CO2H : H2O]a
10
11
12
13
14
15
16
17
[CH3CO2H : H2O : EtOH]a
[CH3CO2H : H2O : EtOH]b
[CH3CO2H : H2O : EtOH]c
[CH3CO2H : EtOH]a
[CH3CO2H : EtOH]b
[CH3CO2H : EtOH]b
84
63
[CH3CO2H : EtOH]b
55
[CH3CO2H : EtOH]b
40f
Mole proportion of catalyst and solvent (1:1)a; (1:2)b;(1:1:2)c; General reaction conditionsd: 1-(4-methylphenyl)-3-phenyl-
2-thioxo-dihydropyrimidine-4,6(1H,5H)-dione (10 mmol), 4-methoxybenzaldehyde (10 mmol); epresent yield, f1b not
consumed completely.
Optimized reaction condition has been found as significant, hence rest of the products have been prepared using
same condition.
2-thioxo-dihydropyrimidine-4,6(1H,5H)-dione, (1b, 310 mg, 1 mmol) and p-methoxy
benzaldehyde (2, 136 mg, 1 mmol) was loaded with 10 mol % of acetic acid (0.57 mL)
and ethanol (1.04 mL) in 1:2 proportion at room temperature stirring over 1 h. The pro-
gress of reaction was monitored by TLC. After completion of reaction, mixture was dis-
solved in ethyl acetate then washed with brine solution followed by water. Solvent was
evaporated under vacuum; the obtained residue was dried and finally purified by recrys-
tallization from ethanol (305 mg, 84%). Yellow solid, mp 261–263 ꢀC. 1H NMR
400 MHz, (CDCl3) ppm: d 8.63 (s, 1H, ¼C–H), 8.44–8.41 (m, 2H C2,C6-Ar00), 7.52–7.50
(m, 2H, C2,C6,-Ar), 7.47–7.45 (m, 3H, C3,C4,C5-Ar), 7.33–7.26 (m, 2H, C2,C6-Ar0),
7.19–7.15 (m, 2H, C3,C5-Ar0), 6.95–6.93 (m, 2H, C3,C5-Ar00), 3.91 (s, 3H, Ar00OCH3),
2.42 (s, 3H, Ar0CH3). 13C NMR (100 MHz, CDCl3) ppm: d 21.60, 55.93, 114.49, 114.76,
125.99, 128.50, 128.81, 128.95, 129.66, 130.49, 137.40, 138.94, 139.60, 140.20, 161.46,
162.51, 165.43, 180.81. HRMS (m/z) 429.1272 [M þ H]þ, Calcd. 429.1267 [M þ H]þ.

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SYNTHETIC COMMUNICATIONSV
5
Table 2. Structure of different functionalized products.
CH3
OCH3
O
O
O
O
O
H3C
N
N
N
N
N
N
N
S
S
S
S
N
H3C
O
O
O
OCH3
OCH3
OCH3
OCH3
3a, 1.1h, 81%
H3C
3b, 1h, 84%
CH3
3d, 1h, 79%
OCH3
3c, 1.2h, 76%
H3CO
O
O
O
O
N
N
N
N
N
S
S
S
S
N
N
N
O
O
O
OCH3
OCH3
OCH3
O
OCH3
H3C
Cl
H3CO
CH3
3f, 1.1h, 78%
OCH3
3h, 1.2h, 78%
CH3
3e, 1h, 80%
3g, 0.9h, 77%
Cl
O
O
O
O
N
N
N
N
N
N
S
S
S
S
N
N
O
OCH3
O
O
OCH3
Cl
O
3k, 1h, 80%
CH3
Cl
Cl
3l, 1h, 81%
3j, 1h, 76%
3i, 1h, 78%
OCH3
Cl
CH3
Cl
O
O
O
O
N
N
N
N
N
S
S
S
S
N
N
N
O
Cl
O
Cl
O
O
Cl
Cl
H3C
CH3
3m, 1h, 79%
CH3
3p, 0.9h, 84%
3o, 1h, 82%
3n, 1h, 77%
OCH3
OCH3
O
O
O
O
N
N
N
N
S
S
S
S
N
N
N
N
O
O
Cl
O
O
3t, 1.1h, 85%
Cl
Cl
Cl
Cl
Cl
Cl
Cl
3s, 1h, 79%
3r, 1h, 82%
Cl
3q, 1h, 80%
OCH3
CH3
O
O
O
O
N
N
N
N
N
S
S
S
S
N
N
N
O
O
O
Cl
Cl
Cl
O
Cl
CH3
CH3
CH3
H3CO
3w, 1.3h, 82%
3x, 1.1h, 83%
3u, 1.3h, 83%
3v, 1.4h, 78%

6
V. D. DEOTALE AND M. G. DHONDE
Conclusion
We have developed a simple efficient approach for the synthesis of a series of 5-substi-
tuted arylbenzylidene-1,3-diaryl-2-thioxodihydropyrimidine-4,6-(1H,5H)-diones under
simple room temperature stirring. This present protocol is considered as eco-friendly,
inexpensive, offer the advantage of easier operational simplicity, simple reaction workup
and purified by crystallization without column chromatography. All synthesized prod-
ucts (3a-x) were characterized and confirmed by 1H, 13C NMR, HRMS and
MS techniques.
1H, 13C NMR and MS spectra of all synthesized compounds is provided in supple-
mentary file.
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