Efficient atom-economic one-pot multicomponent synthesis of benzylpyrazolyl coumarins and novel pyrano[2,3-c]pyrazoles catalysed by 2-aminoethanesulfonic acid (taurine) as a bio-organic catalyst

Article abstract of DOI:10.1080/00397911.2019.1619772

The present work describes eco-friendly multicomponent protocol for the synthesis in excellent yields of structurally diverse benzylpyrazolyl coumarin 5 (a–s) involving the reaction of 4-hydroxycoumarin, ethyl acetoacetate, hydrazine hydrate/phenyl hydraz

Full text of DOI:10.1080/00397911.2019.1619772

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
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Efficient atom-economic one-pot multicomponent
synthesis of benzylpyrazolyl coumarins and
novel pyrano[2,3-c]pyrazoles catalysed by 2-
aminoethanesulfonic acid (taurine) as a bio-
organic catalyst
Asha V. Chate, Badrodin Ayyub Shaikh, Giribala M. Bondle & Sunil M. Sangle
To cite this article: Asha V. Chate, Badrodin Ayyub Shaikh, Giribala M. Bondle & Sunil M. Sangle
(2019): Efficient atom-economic one-pot multicomponent synthesis of benzylpyrazolyl coumarins
and novel pyrano[2,3-c]pyrazoles catalysed by 2-aminoethanesulfonic acid (taurine) as a bio-
organic catalyst, Synthetic Communications, DOI: 10.1080/00397911.2019.1619772
To link to this article: https://doi.org/10.1080/00397911.2019.1619772
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https://doi.org/10.1080/00397911.2019.1619772
Efficient atom-economic one-pot multicomponent synthesis
of benzylpyrazolyl coumarins and novel
pyrano[2,3-c]pyrazoles catalysed by 2-aminoethanesulfonic
acid (taurine) as a bio-organic catalyst
Asha V. Chatea, Badrodin Ayyub Shaikha, Giribala M. Bondlea, and Sunil M. Sangleb
aDepartment of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, India;
bRajaram College, Kolhapur, India
ABSTRACT
ARTICLE HISTORY
Received 27 March 2019
The present work describes eco-friendly multicomponent protocol
for the synthesis in excellent yields of structurally diverse benzylpyra-
zolyl coumarin 5 (a–s) involving the reaction of 4-hydroxycoumarin,
ethyl acetoacetate, hydrazine hydrate/phenyl hydrazine hydrate and
aldehydes, also novel pyrano[2,3-c]pyrazole derivatives 8 (a–k) inte-
grated by isonicotinic acid hydrazide from reaction of aldehyde, ethyl
acetoacetate, malononitrile with isoniazid, employing water as a
reaction medium and 2-aminoethanesulfonic acid (taurine) as the
catalyst. This new methodology endowed the advantages such as
short reaction time, recovery of catalysts after catalytic reaction and
reusing them without losing their activity and alleviate of operation.
KEYWORDS
2-Aminoethanesulfonic acid;
benzylpyrazolyl coumarin;
green reaction;
multicomponent reaction;
pyrano[2,3-c]pyrazoles
GRAPHICAL ABSTRACT
Introduction
The challenging task in chemistry is to develop practical methods, reaction media, con-
ditions, and the use of materials based on the principles of green chemistry.
The concept of ‘‘Green Chemistry’’ has emerged as one of the guiding principles of
CONTACT Asha V. Chate
author-chateav@gmail.com
Department of Chemistry, Dr. Babasaheb Ambedkar
Marathwada University, Aurangabad 431 004, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc
ß 2019 Taylor & Francis Group, LLC

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A. V. CHATE ET AL.
environmentally benign synthesis.[1–3] The use of bio-organic catalyst in organic synthe-
sis has become a subject of intense investigation, in particular, green catalyst which offer
advantages in clean and sustainable chemistry as they can be nontoxic, readily access-
ible, biodegradable and retrievable which successfully used for the synthesis of greener
reaction. Thus, there are still some challenges imbedded in this class of cyclizations,
such as performing the reaction under green catalysis, and green reaction media.
Organocatalysts have emerged as biomimetic catalysts, displaying advantages of being
both greener as well as biocompatible.[4–11] 2-Aminoethanesulfonic acid (Taurine) is a
b-amino acid, It is a kind of sulfur-containing amino acid which does not participate in
the biosynthesis of protein,[12] but it is considered as a conditionally semi essential
amino acid for mammals,[13] as it plays an important role in brain development,[14] also
recognized to have the effect on the antitumor activity,[15,16] antioxidant activity,[17]
antihypertensive activity[18] and membrane fluidity.[19] At present, the greater demand
of taurine from food additives market[20–22] calls for its chemical synthesis. In addition
to this, they improve water solubility and are capable to form H-bonds while their
strong inductive effect can be utilized to tune pKa values of adjacent or remote amino
groups.[23] On the other hand the structural and electronic properties might mimic
transition states to tetrahedral intermediates.[24]
In recent times, 2-aminoethanesulfonic acid (taurine) was used for the preparation of
bio-active barbituric and thiobarbituric acid derivatives,[25] Knoevenagel reaction between
aldehydes and malononitrile,[26] and silica gel supported taurine in the oxidation of sul-
fides to their corresponding disulfides.[27] Moreover, to the best of our knowledge, there
are no other reports of the catalytic activity of this b-amino acid for organic transforma-
tions. Currently, conventional step-by-step synthetic methods cannot meet the demands
of high-throughput screening to identify potential therapeutic agents in drug research and
development. To meet these demands, cascade reactions/domino reaction and multicom-
ponent reaction have been developed, MCRs are atom economical because of accelerating
chemical reactions, converting three or more components incorporated into the final
product via a simple one-pot route.[28] They fulfill the requirements of green chemistry
and for producing vast libraries of desired medicinal scaffolds in a benign way.[29]
Oxygen-containing heterocyclic frameworks exist in many natural products and syn-
thetic pharmaceutics, so the construction of this class of scaffolds has attracted great
attention from the organic community,[30] in this perspective, benzylcoumarins skeletal
structure exits in a wide range of biologically active drugs and natural products. These
biologically active compounds include the warfarin, phenprocumene, and coumatetralyl
(Figure 1), showed antibacterial, anti-HIV, antiviral, and anticoagulant activities.[31]
Similarly, pyrazolones are another important heterocyclic compounds (Figure 2) with a
broad spectrum of biological activities. To list a few phenazone, propyphenazone, and
ampyrone (Figure 2) are well known antipyretic and antianalgesic drugs. Also, pyrazo-
lones are generally known for anti-fungal, antimycobacterial, antibacterial, anti-inflam-
matory, antitumor, antidepressant, and anti-tubercular activities.[32] It is anticipated that
the integration of the 3-benzyl coumarin and pyrazolone moieties leading to benzylpyra-
zolyl coumarin scaffolds could be fascinating and beneficial from the biological point of
view. Till date there are few reports available on the synthesis of this benzyl
pyrazolylcoumarins.[33–40]

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Figure 1. Some biologically active 3-substituted coumarin.
Figure 2. Bioactive pyrazolone moieties.
Figure 3. Bioactive pyrano[2,3-c]pyrazoles moieties.
In recent times, various substituted pyrazole and 4H-pyran-annulated heterocyclic
scaffolds have been reported to demonstrate diverse biological activities (Figure 3)
including anticancer,[41] antimicrobial,[42] antimalarial,[43] anti-HIV,[44] and anti-inflam-
matory activities.[45] Particularly, 2-amino-3-cyano-4H-pyran containing heterocycles
showed encouraging anticancer and antibacterial activities.[46] Hence, it would be an
attractive idea to incorporate isoniazid (INH) with pyran and pyrazolo moieties, because
it might lead to a new dimension of structural diversity to the molecules. The fictional-
ization of the INH is possible via its reactive hydrazide group which can easily react
with carbonyl compounds to form hydrazones or undergo cyclization reactions. Such a
molecular modification can possibly results in enhancement of the activity and some-
times may also help in reducing the toxicity of the molecule. It is surprising that there
is no compound possessing such a molecular skeleton reported in the literature to date.
Taking this into account, Herein, we report the synthesis of structurally diverse ben-
zylpyrazolyl coumarins 5 (a–s) and novel pyrano[2,3-c]pyrazole derivatives 8 (a–k) inte-
grated by isonicotinohydrazide by the use of tauine organocatalyst, that shows enhanced

4
A. V. CHATE ET AL.
Scheme 1. General scheme for the synthesis of benzylpyrazolyl coumarin 5 (a–s) and novel 6-amino-
1-isonicotinoyl-3-methyl-4-phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile derivatives 8 (a–k).
Scheme 2. Standard model reaction.
activity and improved performance in the selected benchmark transformation when
compared to typically used other catalysts (Scheme 1).
Result and discussion
During the course of our initial investigation reaction of 4-hydroxycoumarin, ethyl ace-
toacetate, hydrazine hydrate and benzaldehyde was used as the standard model reaction
in a one-pot manner to investigate experimental conditions including catalysts and sol-
vents (Scheme 2). Initially standard model reaction was carried out in the absence of
catalyst in water at room temp. to 70 ꢀC for 24 h. It was observed that no remarkable
target product was obtained (Table 1, entry 1). In the next step, the model reaction was
conducted in the presence of different catalysts, such as PTSA, acetic acid, Ca(OTf)2,
b-CD, and taurine (Table 1, entries 2–6), the obtained yields of product was low. When
catalytic amounts of taurine was added to the same reaction mixture at 70 ꢀC, the
desired product (5a) was isolated with moderate yield (66%) after 60 min. After achiev-
ing this encouraging result, the reaction conditions were then screened with the aim of
optimizing the yield of 5a. We were delighted to find that taurine (15 mol%) was

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Table 1. Effect of different catalyst on 5a.a
Entry
Catalyst
Solvent (10 mL)
Time (min/h)
Temp (ꢀC)
Yieldb (%)
c
1.
2.
3.
4.
5.
6.
–
PTSA
CH3-COOH
Ca(OTf)2
b-CD
H2O
H2O
H2O
H2O
H2O
H2O
24 h
24 h
1 h
3.5 h
60 min
20 min
70
70
70
70
70
70
–
30
72
80
76
92
Taurine
aAll reactions were carried out using 4-hydroxy coumarin (1 mole), ethyl acetoacetate (1 mole), hydrazine hydrate/phenyl
hydrazine hydrate (1 mole), aldehyde (1 mole) and taurine (15 mol%) using water as a solvent.
bIsolated yield.
cReaction failed to provide any product.
Table 2. Effect of solvent on the synthesis of 5a.a
Entry
Catalyst (mol %)
Solvent (10 mL)
Temp (ꢀC)
Time (min)
Yieldb (%)
1.
2.
3.
4.
5.
15
15
15
15
15
Ethanol
Methanol
CCl4
CH3CN
Toluene
70
70
70
70
120
180
90
210
180
300
84
79
67
14
61
aAll reactions were carried out using 4-hydroxy coumarin (1 mole), ethyl acetoacetate (1 mole), hydrazine hydrate/phenyl
hydrazine hydrate (1 mole), aldehyde (1 mole) and taurine (1 mole) using water as a solvent.
bIsolated yield.
superior for the reaction, and the yield of product 5a was 92% in water for 20 min
(Table 1, entry 6).
Next, we screened protic and aprotic solvents on the reaction. Obviously, protic sol-
vents were better for the reaction, because they stabilize the carbocation intermediate, a
polar protic solvent, such as ethanol and methanol, have a permanent dipole which
means that the delta negative (partial negative charge) on the molecule will have dipole-
dipole interactions with the carbocation, stabilizing it, along with this protic solvent can
interact electrostatically with the nucleophile thereby stabilizing it. This reduces the
reactivity of the nucleophile which favors a reaction. Encouraged by this result, we car-
ried on with examining the effect of solvents using different aprotic solvents also such
as toluene, CHCl3, CCl4 and CH3CN using same catalyst equivalents with respect to the
substrate on this reaction at heating temperature and results are incorporated in
(Table 2, entries 1–5). It was clear that the highest yield of product 5a was achieved
when the reaction was performed in water (Table 1, entry 6). The progress of the reac-
tion at room temperature in water was slow, and became even slower as it proceeded;
however, the rate was greatly increased at increasing temperature, so the choice was
made to pursue these conditions instead. Furthermore, the influence of the amount of
taurine catalyst was examined to increase the efficiency of the reaction (Table 3).
With the best reaction conditions in hand, we tested the generality of this method-
ology with different aldehydes and phenyl hydrazine. Results listed in Table 4, demon-
strate that most of the amines tested underwent smooth transformation to afford the
corresponding products in satisfied yields. The electron-donating and electron-with-
drawing groups were successfully converted to the corresponding products in excellent
yields (80–94%) within very short time periods (20–30 min); moreover, the electronic
effects of the substituent were not observed. The same MCR also proceeds with phenyl
hydrazine and also with isoniazid in place of hydrazine hydrate, which indicate that

6
A. V. CHATE ET AL.
Table 3. Effect of catalyst (mole %) on 5a.a
Entry
Catalyst (mole %)
Solvent (10 mL)
Temp (ꢀC)
Time (min)
Yieldb (%)
1.
2.
3.
4.
5
10
15
20
H2O
H2O
H2O
H2O
70
70
70
70
60
56
20
20
66
82
92
92
aAll reactions were carried out using 4-hydroxy coumarin (1 mole), ethyl acetoacetate (1 mole), hydrazine hydrate/phenyl
hydrazine hydrate (1 mole), aldehyde (1 mole), using water as a solvent.
bIsolated yield.
reaction was also smoothly preceded with the hydrazide (Table 4, entry 20). Relatively
higher yields of the products were observed in case of phenyl hydrazine. This could be
due to the delocalization of the lone pair of one nitrogen atom onto the phenyl ring.
Encouraged by the participation of diversely substituted aryl aldehydes, we extended
our methodology to check the participation of substituted heterocyclic aldehydes and
aliphatic aldehydes, and found all these were equally participating to yield the products
in good to excellent yields (entries 5j, 5k, 5m, Table 4). No distinct substitution effect
was observed for this reaction. The isolated products were extremely pure without the
need for any costly purification steps, which can be expensive in terms of time, materi-
als and overall yield.
Inspired by these tempting results obtained for cyclocondensation of benzylpyrazolyl
coumarin to be performed to show the ability of taurine in the promotion of these
kinds of reactions. we extended the same protocol for synthesis of novel 6-amino-1-iso-
nicotinoyl-3-methyl-4-phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile derivatives
reacting various substituted aldehyde (1 mole) with malononitrile (1 mole), ethyl acetoa-
cetate (1 mole), isoniazid (1 mole) (Scheme 3). Surprisingly, we were delighted to find
that the taurine furnished the desired product within short time period with respect to
excellent yields of products under similar reaction conditions (Table 5, entries 1–11),
suggesting that the taurine is essential to promote the reaction efficiency.
Recycling of the catalyst
The recyclability of taurine-catalyst was studied for synthesis of 4-((4-hydroxy-2-oxo-
2H-chromen-3-yl)(phenyl)methyl)-5-methyl-1H-pyrazol-3(2H)-one (5a) under the opti-
mized conditions. After completion of the reaction, the catalyst was separated from
ꢀ
reaction mixture by simple filtration and filtrate was cooled at 5 C, the white shiny
taurine was reappeared and then filtrates directly, dried and after that applied for
repeated reactions for the same transformation under the same reaction conditions
(Table 4). To our delight, the catalyst can be reused as a minimum three times with lit-
tle deactivation an essential aspect of green chemistry (Figure 4).
Reaction mechanism
As shown in (Scheme 4), the possible mechanism for the construction of benzylpyra-
zolyl coumarin, first ethylacetoacetate and phenylhydrazine will react to give the pyrazo-
lone (I), which will undergo Knoevenagel condensation with aldehydes to give the
adduct Knoevenagel adduct (II). In the next step pyrazolone (I) and Knoevenagel adduct
(II) will undergo taurine-catalyzed Michael addition to form (III) and the final product (5a).

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Table 4. Taurine-catalyzed synthesis of benzylpyrazolyl coumarin derivativesa 5 (a–t).
M. P. (ꢀC)
Sr. No.
Entry
Product
Time (min.)
20
Yieldb (%)
92
Found
Report. [33–40]
232–234
1.
5a
230–233
2.
3.
5b
5c
20
25
83
88
221–223
227–229
–
–
4.
5.
6.
5d
5e
5f
25
25
25
82
83
88
200–202
225–227
237–239
201–203
–
–
7.
5g
5h
5i
20
25
20
20
86
85
87
80
217–219
223–225
231–234
204–206
–
8.
–
9.
233–235
207–209
10.
5j
11.
5k
25
86
218–220
–
(continued)

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A. V. CHATE ET AL.
Table 4. Continued.
M. P. (ꢀC)
Report. [33–40]
Sr. No.
Entry
Product
Time (min.)
20
Yieldb (%)
88
Found
12.
5l
215–217
–
13.
14.
15.
16.
5m
5n
5o
5p
20
25
25
20
76
85
81
78
207–209
219–221
222–224
241–243
–
223–225
225–227
244–247
17.
18.
5q
5r
20
20
83
91
233–235
220–222
–
–
19.
20.
5s
5t
20
20
89
85
216–218
187–190
–
–
aAll reactions were carried out using 4-hydroxy coumarin (1 mole), ethyl acetoacetate (1 mole), hydrazine hydrate/phenyl
hydrazine hydrate (1 mole), aldehyde (1 mole) and using taurine (15 mole%) in water as a solvent (10 mL).
bIsolated yield.

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Scheme 3. Synthesis of pyrano[2,3-c]pyrazole-5-carbonitrile.
Table 5. Taurine-catalyzed synthesis of 6-amino-1-isonicotinoyl-3-methyl-4-phenyl-1,4-dihydropyr-
ano[2,3-c]pyrazole-5-carbonitrile derivativesa 8 (a–k).
Sr. No.
Entry
Product
Time (min.)
35
Yieldb (%)
M. P. (ꢀC)
1.
8a
90
99–101
2.
3.
4.
5.
8b
8c
8d
8e
35
30
30
35
89
88
80
89
121–123
161–164
118–120
126–128
(continued)

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A. V. CHATE ET AL.
Table 5. Continued.
Sr. No.
Entry
Product
Time (min.)
30
Yieldb (%)
86
M. P. (ꢀC)
6.
8f
120–123
7.
8g
35
81
133–135
8.
9.
8h
8i
35
30
84
81
98–100
154–158
10.
11.
8j
35
35
84
89
110–112
177–180
8k
aAll reactions were carried out using ethyl acetoacetate (1 mole), aldehyde (1 mole), malononitrile (1 mole), isoniazid
(1 mole), and using taurine (15 mole%) in water as a solvent (10 mL).
bIsolated yield.
The possible formation mechanism of densely functionalized novel pyrano[2,3-c]pyra-
zole is suggested in (Scheme 5). During the reaction, the aldehydes are activated, and
the Knoevenagel products (II) are formed via reaction of anion (I) with activated alde-
hydes and hydrazine of isoniazid attacked the carbonyl group of the activated ethyl ace-
toacetate. Then, loss of H2O, and intramolecular nucleophilic attack by another NH

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11
95
90
85
80
92
91
90
90
90
Catalyst
recovery
88
88
84
Yield
Fresh Run Run Run
1
2
3
Figure 4. Recycle and recovery of 2-Aminoethanesulfonic acid (Taurine) and its effect on yield.
Scheme 4. A possible mechanism for the formation of benzylpyrazolyl coumarin.
group of hydrazine of isonizid to the next carbonyl group of ethyl acetoacetate afforded
(III) after removing one molecule of EtOH. In the next step, the Michael addition
occurred between (II) and (III) in the presence of taurine, which gave intermediate
(IV). After intramolecular cyclization and oxidation, final products (8a) were formed.
Conclusion
In this work, a novel, convenient, and efficient approach to the synthesis of potentially
active benzylpyrazolyl coumarin 5 (a–s) and novel pyrano[2,3-c]pyrazole 8 (a–k) has
been reported based on the multicomponent reaction in good to excellent yield in a
short period of time. Additionally the use of readily available and inexpensive catalyst
taurine make this system more atom economical, minimally available starting materials,
which makes the overall synthesis, is applicable in the quick access to relevant pharma-
ceutical molecules with pyrazole and isoniazide core heterocycle

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A. V. CHATE ET AL.
Scheme 5. A possible mechanism for the formation of 6-amino-1-isonicotinoyl-3-methyl-4-phenyl-1,4-
dihydropyrano[2,3-c]pyrazole-5-carbonitrile.
Acknowledgements
The authors are thankful to Professor Charansingh H. Gill for his invaluable discussions
and guidance.
Funding
The authors are thankful to Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada
University, Aurangabad and this work was supported by Special Assistance Programme SAP,
University Grants Commission, New Delhi, India, intended to encourage the pursuit of excellence
and teamwork in advanced teaching and research to accelerate the realization of international
standards in specific fields.
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