A green and clean pathway: One pot, multicomponent, and visible light assisted synthesis of pyrano[2,3-: C] pyrazoles under catalyst-free and solvent-free conditions

Article abstract of DOI:10.1039/c7nj01688c

A mild, eco-efficient and metal-free synthetic protocol for the production of biologically significant scaffold dihydropyrano[2,3-c]pyrazoles via reaction between ethylacetoacetate (1), hydrazine hydrate (2), aromatic aldehydes (3) and malononitrile (4) a

Full text of DOI:10.1039/c7nj01688c

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A green and clean pathway: One pot, multicomponent, visible light assisted
synthesis of pyrano[2,3-c]pyrazoles under catalyst-free and solvent-free
conditions
Bhartendu Pati Tripathi,a# Anu Mishra,a# Pratibha Rai,a Yogesh Kumar Pandey,a Madhulika Srivastava,a
Snehlata Yadav,a Jaya Singhb and Jagdamba Singha*
aEnvironmentally Benign Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad-211002 (India); Tel:
+919415218507; E-mail: dr.jdsau@gmail.com
bDepartment of Chemistry, LRPG College, Sahibabad, Uttar Pradesh, India. E-mail: nimesh.singh@in.abb.com
# Equal contribution
Abstract:
A mild, eco-efficient and metal-free synthetic protocol for biologically significant scaffold
pyranopyrazole via reaction between ethylacetoacetate (1), hydrazine hydrate (2), aromatic
aldehydes (3) and malononitrile (4) as reactants and exploiting visible light as a reaction
promoter has been developed. Cost effectiveness, nontoxicity, being catalyst-free and solvent-
free, easy-availability of reactants and non-involvement of specific glasswares and photo reactor
system are some of the significant traits of the present protocol. The scope of current
methodology is further extended by using diethyl acetylene dicarboxylate (6) as a reactant in
place of ethylacetoacetate.
1

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Introduction
Increasing demand of the development of new techniques which necessitates green synthesis of
organic molecules leads towards new thoughts and new creations. The continual use of
hazardous reaction media and metal catalysts as reaction promoters in the laboratory has proven
to be ceaselessly harmful to our environment and to all living beings. Thus, organic chemists are
switching over to the maximum use of eco-friendly and sustainable resources.1 Consequently,
development of visible-light-initiated transformation has been considered as an impressive tool
for a large number of eco-efficient synthesis and constantly drags the attention of researchers
owing to the fact that employment of visible light provides a greener and more cost effective
alternative which is accessible to varied reactions.2-3 Generally, compact fluorescent lights
(CFLs) and light emitting diodes (LEDs) are used as a source of visible light for various
transformations.4 However there are a number of organic scaffolds which are unable to absorb
visible light.5 To conquer this impediment, chemists are shifting their attention towards photo
catalysts and photo sensitizers as transition metals, organic dyes and nanoparticles.6 Moreover, a
number of drawbacks are reported with transition metal photocatalysts like short durability,
adverse inherent malignancy and expensiveness.7 Similarly, in order to eliminate the use of toxic,
organic solvents which are responsible for a large scale generation of waste and pollution,
designation of a solvent-free synthesis is a big errand for the researchers and scientists. In this
way, a catalyst-free and solvent-free synthesis has emerged as an important probe.8 The
dihydropyrano[2,3-c]pyrazole nucleus and its derivatives represents a remarkable and attractive
scaffold in the field of medicinal chemistry and also acquire valuable biological as well as
pharmacological properties.9 They are documented to possess a range of biological properties
such as antimicrobial,10 insecticidal, molluscicidal,11 anticancer12 and anti inflammatory
activities.13 They are also potential inhibitors of human Chk1 kinase.14 They can also be used as
pharmaceutical elements as well as eco-friendly agrochemicals 15 (Figure 1).16
2

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OMe
F
F
F
F
F3C
N
F
CN
CH3
H2N
N
N
O
NH2
N
NC
O
H
Antibacterial agenta
Anticancer agentb
Ph
F
NC
H2N
N
R
N
O
N
O
R
O2N
OH
HO
CF3
NC
N
CH3
N
H2N
O
NC
N
N
H2N
O
H
Cl
In vitro cytotoxicity against cell
line U937 and B16 F10d,e
Human Chk 1kinase inhibitorc
Figure 1 Some biologically important derivatives of dihydropyrano[2,3-c]pyrazole
Literature survey reveals that numerous methods were developed for the synthesis of
Dihydropyrano[2,3-c]pyrazoles.17-24 Despite the fact that the reported methods have their own
importance but still some consequential drawbacks are associated with these methods as the use
of hazardous solvents, metal catalysts, inorganic bases, non recyclability of the catalyst and high
temperature. In continuation of our previous works in visible-light-promoted synthesis,25 we
envisioned the possibility for the synthesis of dihydropyrano[2,3-c]pyrazole initiating from
ethylacetoacetate (1), hydrazine hydrate (2), aromatic aldehydes (3) and malononitrile (4) under
solvent and catalyst-free condition in presence of CFL (22 W) irradiation (Scheme 1).
3

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Previous works
O
R1
R1
CHO
EtOOC
OR
Ref17-24
CN
O
(1)
O
+
NC
CN
R1
H N NH
H2O
2
2
N
HN
CN
(2)
N
H
N
OR
H2N
O
O
NH2
(3)
(4)
EtOOC
COOEt
(5)
(7)
(6)
Current work
O
R1
R1
CHO
22 W CFL
CN Solvent-free
EtOOC
OR
O
(1)
O
+
CN
NH2
NC
H2N NH2
H2O
R1
HN
N
Catalyst-free
CN
N
(2)
N
H
OR
O
rt
H2N
O
(3)
(4)
EtOOC
COOEt
(7)
(5)
(6)
Scheme 1 Synthesis of dihydropyrano[2,3-c]pyrazole
Results and Discussion
In the beginning, we have been investigated the synthesis of pyranopyrazoles using
ethylacetoacetate (1), hydrazine hydrate (2), 4-chlorobenzaldehyde (3a) and malononitrile (4) as
model substrates. Initially, the reaction was performed in EtOH using 1.0 mmol of each reactant
under 22 W compact florescent lamp (CFL) irradiation in catalyst-free condition. The desired
product was formed in 71% yield after 15 min (Table 1, entry 6). In our optimization studies, we
screened the effect of reaction media, CFL of different intensities and catalysts under different
reaction conditions to achieve the proper condition for the above transformation. First of all, we
investigated the effect of different organic solvents like DCM, DMSO, DMF, CH3CN, MeOH,
EtOH, EtOH:H2O as well as neat under catalyst-free condition. In each case different yields of
the desired product was obtained (Table 1, entries 1-10). In this instance, we observed that when
the model reactions were carried out under catalyst-free, neat condition the yield of the product
was maximum. Thus, we concluded that the neat condition is favorable for the present
transformation (Table 1, entry 10). In our next endeavor, we optimized different intensities of
CFL (Table 2, entries 1-4) and found that 22 W CFL is suitable for the present protocol (Table 2,
entry 3).
4

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Table 1 Optimization table of Solvent for the synthesis of compound (5a)a
Entry
Solvents
DCM
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
15
15
15
15
15
15
15
15
15
15
53
58
51
62
73
71
74
69
78
88
DMSO
DMF
CH3CN
MeOH
EtOH
EtOH:H2O (1:1)
EtOH:H2O (2:1)
EtOH:H2O (1:2)
Neat
aReaction conditions: 1 (1.0 mmol), 2 (1.0 mmol), 3a (1.0 mmol) and 4 (1.0 mmol) in solvent were irradiated under open air at
room temperature using CFL (22 W). bIsolated yield of the product (%).
Table 2 Optimization table of CFL for the synthesis of compound (5a)a
Entry
Reaction condition
CFL (18 W)
Time (min)
Yieldb (%)
1
2
3
4
30
30
30
30
74
81
88
88
CFL (20 W)
CFL (22 W)
CFL (23 W)
aReaction conditions: 1 (1.0 mmol), 2 (1.0 mmol), 3a (1.0 mmol) and 4 (1.0 mmol) in neat condition were irradiated under open
air at room temperature using CFL. bIsolated yield of the product (%).
5

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Further we optimized the reaction conditions by several control experiments including
effect of catalyst, benzoquinone and that of visible light (Scheme 2). In these attempts, we
observed that photocatalyst (Rose Bengal and Eosin Y) did not affect the yield of product
(Scheme 2, entries 1 and 2). So we decided to perform our reaction under catalyst-free condition
(Scheme 2, entry 2). When the reaction was carried out in presence of benzoquinone (2.0 mmol)
21% yield of product was obtained which clearly indicates that a radical intermediate must be
involved (Scheme 2, entry 3).25c, 26 A sequences of reactions to ensure the effect of day and dark
under catalyst- and solvent-free conditions were also investigated and it was further revealed that
in absence of visible light, the yield of product was significantly decreased (Scheme 2, entry 4),
this result was further confirmed by the work of R. H. Vekariya et al.21a
1
2
O
Photocatalyst-free
Photocatalyst
78%a / 85%b
(5a)
H2N NH2
88%
(5a)
H2O
Neat/CFL (22 W)
rt
Neat/CFL (22 W)
rt
O
(1)
O
(2)
+
CHO
CN
4
3
CN
Day/Dark
Neat/rt
39%c / 30%d
(5a)
Benzoquinone
(2.0 mmol)
Neat/CFL (22 W)
rt
21%
(5a)
(4)
Cl
(3a)
Reaction conditions: 1 (1.0 mmol), 2 (1.0 mmol), 3a (1.0 mmol) and 4 (1.0 mmol) in neat condition were irradiated under open
a
air at room temperature using CFL (22 W). Yield (%) of the product in presence of Rose Bengal. bYield (%) of the product in
presence of Eosin Y. cYield (%) of the product in presence of Day light. dYield (%) of the product in presence of dark.
Scheme 2 Control experimenta
With the optimized reaction conditions in hand, we further investigated the scope and
functional group potential of our present protocol with various substituted aromatic aldehydes (3)
(Table 3). The present protocol works well with varied substrates involving aromatic aldehydes
with electron donating group as well as electron withdrawing group (Table 3, entries 1-13).
6

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Table 3 Substrate scope for the synthesis of compound (5)a
Entry Aldehyde (3)
1
Product (5)
Time (min)
15
Yieldb (%) M.P.(OC )
88
230-23223
2
3
4
15
20
22
85
84
81
247-24922
258-26022
259-26122
7

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5
6
25
80
88
218-22120
15
192-
19418b
7
8
9
25
25
20
76
79
76
259-26122
247-24922
193-
19618a
10
25
79
207-21020
8

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11
12
13
15
87
76
56
222-
22518b
25
30
210-21320
222-22522
CHO
N
aReaction conditions: 1 (1.0 mmol), 2 (1.0 mmol), 3 (1.0 mmol) and 4 (1.0 mmol) in solvent- and catalyst-free condition were
irradiated under open air at room temperature using CFL (22 W). bIsolated yield of the product (%).
Further we extended our substrate scope by using diethyl acetylene dicarboxylate (6) in
place of ethylacetoacetate (Table 4, entries 1-8).
Table 4 Substrate scope for the synthesis of compound (7)a
Entry Aldehyde (3)
Product (7)
Time (min) Yieldb (%) M.P.
(OC)18d,24a,b
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15
86
75
239-241
1
2
20
209-212
20
20
68
86
219-221
205-208
3
4
15
22
90
70
212-215
187-191
5
6
10

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22
78
63
211-213
198-201
7
8
25
aReaction conditions: 6 (1.0 mmol), 2 (1.0 mmol), 3 (1.0 mmol) and 4 (1.0 mmol) in solvent- and catalyst-free condition were
irradiated under open air at room temperature using CFL (22 W). bIsolated yield of the product (%).
To set up the practicability of the developed protocol, we carried out the experiment on a
gram scale. Ethylacetoacetate (1, 10 mmol), hydrazine hydrate (2, 10 mmol), 4-
chlorobenzaldehyde (3a, 10 mmol, 1.40 g) and malononitrile (4, 10 mmol), was reacted to obtain
the dihydropyrano[2,3-c]pyrazole (5a) in 88% yield in about 15 minutes by using CFL and
common laboratory glasswares under catalyst- and solvent-free condition at room temperature
(Scheme 3).
Scheme 3 Practicability of the Current methodology
Mechanism
On the bedrock of literature,25b, 27, 28 a plausible mechanism for above photo-induced reaction is
given in Scheme 4. The reaction initiates by a general nucleophilic addition reaction between (1)
and (2) followed by elimination to form (A) which occurs in keto-enol tautomerism with (B).
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Now, in the presence of visible light, malononitrile (4) undergoes tautomerisation to give (C).
After this (C) and benzaldehyde derivatives (3) react to form Benzylidene malononitrile adduct
(D), which undergoes a photochemical activation to form a radical intermediate (E). Here after,
(E) abstracts a hydrogen from malononitrile to form intermediate (F) as well as malononitrile
radical. Now this radical abstracts a hydrogen from (B) to form intermediate (G). Afterward,
intermediate (F) and (G) coalesce to form (H) which further undergoes tautomerisation and
intramolecular cyclization to form the product (5).
R1
C N
C N
C
N
HN
N
H
C
C
R1
C N
R1
CN
(4)
H
O
NC
(C)
(D)
(E)
(3)
CN
O
O
O
CN
N NH
N NH
(1)
H
O
O
(B)
(A)
H2N NH2
H2O
CN
CN
(2)
C
NH
N
NH
R1
C N
O
(F)
(G)
H
O
H
H
N
N
NH
N
N
NH
N
O
NH
O
C
C
H
Tautomerisation
R1
Cyclization
NH2
R1
CN
(5)
C N
R1
C N
(H)
(I)
Scheme 4 Plausible mechanism for photo-induced synthesis of dihydropyrano[2,3-c]pyrazole (5)
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Conclusion
In summary, we have designed a highly efficient, environmently benign, multicomponent, one
pot photocatalytic pathway for the synthesis of biologically important scaffold pyranopyrazoles
via readily available starting materials ethylacetoacetate, hydrazine hydrate, aromatic aldehyde
and malononitrile. A visible light assisted, solvent-free, catalyst-free, metal free and easy workup
procedure are some of the prime advantages of our reaction. To the best of our knowledge, the
reported conditions are the first ever reported visible light mediated, catalyst free transformation
for the synthesis of highly substituted dihydropyrano [2,3-c] pyrazoles.
Acknowledgements
We sincerely thank SAIF, Punjab University, Chandigarh, for providing micro-analyses and
spectra. B. P. Tripathi and S. Yadav are thankful to UGC, New Delhi for Research fellowship.
The author A. Mishra and P. Rai are thankful to UGC, New Delhi for the award of Junior
Research Fellowship (JRF) andSenior Research Fellowship (SRF), respectively. Y. K. Pandey is
thankful to CSIR, New Delhi for the award of Junior Research Fellowship (JRF).
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