Synthesis of 6-Amino-2,4-dihydropyrano[2,3-c ]pyrazol-5-carbonitriles Catalyzed by Silica-Supported Tetramethylguanidine under Solvent-Free Conditions

Article abstract of DOI:10.1080/00397911.2014.913634

An efficient, high-yielding, and rapid protocol has been developed for the synthesis of diversity-oriented 6-amino-2,4-dihydropyrano[2,3-c]pyrazol-5- carbonitriles derivatives via a four-component, one-pot cyclocondensation reaction of ethyl acetoacetate, hydrazine hydrate, aldehydes, and malononitrile using silica-supported tetramethylguanidine as a heterogeneous catalyst for the first time. The protocol proves to be efficient and environmentally benign in terms of very easy workup, good yields, and ease of recovery of catalyst. In addition, the present method is superior in terms of green media, the amount of catalyst, and reaction time.

Full text of DOI:10.1080/00397911.2014.913634

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Synthesis of 6-Amino-2,4-
dihydropyrano[2,3-c]pyrazol-5-
carbonitriles Catalyzed by Silica-
Supported Tetramethylguanidine Under
Solvent-Free Conditions
Amol B. Atar a , Jong Tae Kim a , Kwon Taek Lim a & Yeon Tae Jeong a
a Department of Image Science and Engineering , Pukyong National
University , Busan , Korea
Accepted author version posted online: 19 May 2014.Published
online: 02 Jul 2014.
To cite this article: Amol B. Atar , Jong Tae Kim , Kwon Taek Lim & Yeon Tae Jeong (2014)
Synthesis of 6-Amino-2,4-dihydropyrano[2,3-c]pyrazol-5-carbonitriles Catalyzed by Silica-
Supported Tetramethylguanidine Under Solvent-Free Conditions, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 44:18, 2679-2691,
DOI: 10.1080/00397911.2014.913634
To link to this article: http://dx.doi.org/10.1080/00397911.2014.913634
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Synthetic Communications1, 44: 2679–2691, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.913634
SYNTHESIS OF 6-AMINO-2,4-DIHYDROPYRANO-
[2,3-c]PYRAZOL-5-CARBONITRILES CATALYZED BY
SILICA-SUPPORTED TETRAMETHYLGUANIDINE
UNDER SOLVENT-FREE CONDITIONS
Amol B. Atar, Jong Tae Kim, Kwon Taek Lim, and
Yeon Tae Jeong
Department of Image Science and Engineering, Pukyong National University,
Busan, Korea
GRAPHICAL ABSTRACT
Abstract An efficient, high-yielding, and rapid protocol has been developed for the synthesis
of diversity-oriented 6-amino-2,4-dihydropyrano[2,3-c]pyrazol-5-carbonitriles derivatives
via a four-component, one-pot cyclocondensation reaction of ethyl acetoacetate, hydrazine
hydrate, aldehydes, and malononitrile using silica-supported tetramethylguanidine as a
heterogeneous catalyst for the first time. The protocol proves to be efficient and environ-
mentally benign in terms of very easy workup, good yields, and ease of recovery of catalyst.
In addition, the present method is superior in terms of green media, the amount of catalyst,
and reaction time.
Keywords Four-component reaction; heterogeneous catalyst; pyrano[2,3-c]pyrazole;
silica-supported tetramethylguanidine
Received February 5, 2014.
Address correspondence to Yeon Tae Jeong, Department of Image Science and Engineering,
Pukyong National University, Busan 608-737, Korea. E-mail: ytjeong@pknu.ac.kr
Color versions of one or more of the figures in the article can be found online at www.tandfonline.
com/lsyc.
2679

2680
A. B. ATAR ET AL.
INTRODUCTION
Pyrano[2,3-c]pyrazole derivatives are building blocks with a broad range of
biological activities such as antimicrobial,[1] insecticidal,[2] and anti-inflammatory[3]
activites. Furthermore, compounds containing pyrano[2,3-c]pyrazole moiety have
been reported to exhibit enzyme inhibitiory,[4] anticancer,[5] and antifungal[6] activity
apart from being significant intermediates for the construction of complex hetero-
cycles.[7] The pharmaceutical importance of pyrano[2,3-c]pyrazole and their utiliza-
tion in organic transformations has directed great research activity toward the
construction of the skeletons of different kinds of heterocycles.[8] As a result of
their biological and synthetic importance, a number of synthetic methods for the
preparation of 6-amino-2,4-dihydropyrano[2,3-c]pyrazol-5-carbonitriles have been
developed for the reaction of ethylacetoacetate, hydrazine hydrate, aldehyde, and
malononitrile in the presence of different catalysts.[9–22] However, some of these pro-
cedures have certain limitations, such as prolonged reaction time, the use of toxic
solvents, or poor yields. The recovery and reusability of the catalyst is also a
problem. Therefore, it is still desirable to seek a green and ecofriendly protocol that
uses a highly efficient and reusable catalyst for the preparation of 6-amino-2,4-
dihydropyrano[2,3-c]pyrazol-5-carbonitriles.
Recently, heterogeneous catalysts have been highly acknowledged for the
sustainable development of any catalytic process because of their easy recovery,
recyclability, minimization of undesired toxic wastes, and E factor.[23] The usage
of heterogeneous metal Lewis acid catalyst instead of traditional homogeneous metal
Lewis and Brønsted acid catalysts could be a more environmentally friendly alterna-
tive. The problems usually connected with the homogeneous catalysts can be
easily overcome by heterogenization of a homogeneous counterpart onto insoluble
inorganic or organic solid support in which active components are strongly
immobilized.[24] Solid catalysts provide numerous opportunities for recovering and
recycling catalysts from reaction environments. Among the various heterogeneous
catalysts, particularly, silica-supported tetramethylguanidine has the advantages of
low cost, ease of preparation, environmental friendliness, high efficiency, and
catalyst recyclability (Fig. 1).
It is evident from the previous literature that silica-supported tetramethyl-
guanidine has generated enormous interest as a green and a potential solid catalyst
to construct carbon–carbon and carbon–heteroatom bonds in various organic
transformations.[25,26] Considering these subjects and based on our previous endea-
vors in exploring novel and practical multicomponent reactions to synthesize useful
Figure 1. Silica-supported tetramethylguanidine.

SILICA-SUPPORTED TETRAMETHYLGUANIDINE
2681
Scheme 1. Synthesis of 6-amino-4-alkyl=aryl-3-methyl-2,4-dihydropyrano[2,3-c]pyrazole-carbonitriles
catalyzed by SiO2TMG under neat conditions.
heterocyclic compounds,[27–30] we decided to investigate efficiency of silica-supported
tetramethylguanidine catalyst for the synthesis of 6-amino-2,4-dihydropyrano[2,3-
c]pyrazol-5-carbonitriles. Herein we report a tandem synthesis of 6-amino-2,4-
dihydropyrano[2,3-c]pyrazol-5-carbonitriles derivatives from ethyl acetoacetate,
hydrazine hydrate, aldehydes, and malononitrile using silica-supported tetramethyl-
guanidine as expeditious reusable catalyst in an excellent yield (Scheme 1).
RESULTS AND DISCUSSION
The studies were initiated to optimize the reaction conditions for a model reac-
tion of hydrazine hydrate 1, ethyl acetoacetate 2, benzaldehyde 3a, and malononitrile
4 in the presence of different catalysts and solvents (Table 1). To establish the real
effectiveness of the catalyst for the synthesis of 6-amino-3-methyl-4-(3-phenyl)-
2,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile, a test reaction was performed with-
out catalyst using hydrazine hydrate 1, ethyl acetoacetate 2, benzaldehyde 3a, and
malononitrile 4 in ethanol at reflux. It was found that only 10% yield of product
was obtained in the absence of catalyst even after 6 h (Table 1, entry 1). To develop
a viable approach, the model reaction was investigated by employing different het-
erogeneous catalysts such as SiO2, BF3 ꢀ SiO2, Zn(OTf)2 ꢀ SiO2, ZnCl2 ꢀ SiO2, TiO2 ꢀ
SiO2, SiO2TMG, and FeCl3 ꢀ SiO2. Among all screened catalysts SiO2TMG gave
the best result in view of yield and reaction time (Table 1, entry 9). In contrast,
SiO2, BF3 ꢀ SiO2, Zn(OTf)2 ꢀ SiO2, ZnCl2 ꢀ SiO2, TiO2 ꢀ SiO2, and FeCl3 ꢀ SiO2 did
not afford the desired product in good yields (Table 1, entries 2–6 and 8).
To see the effect of solvent, we screened different solvents such as EtOH, acet-
onitrile, methanol, and water at reflux. It was observed that under solvent conditions
the reactions required longer times (2–4 h) to afford comparable yields (Table 1,
entries 7 and 10–12). When the reaction was performed under solvent-free con-
ditions, good yield of target product was obtained (Table 1, entry 9). Moreover,
we found that the yields were obviously affected by the amount of SiO2TMG loaded.
When 5, 10, 15, and 20 mol% of SiO2TMG were used, the yields were 81%, 96%,
95%, and 92%, respectively (Table 1, entries 9 and 13–15). Therefore, 10 mol% of
SiO2TMG was sufficient and optimal quantity for the completion of the reaction.

2682
A. B. ATAR ET AL.
Table 1. Optimizationa of promoters, solvents, and temperature in the synthesis of 5a
No.
Catalyst (10 mol%)
Solvent
Condition
Time
Yieldb (%)
1
2
—
SiO2
Ethanol
Neat
Reflux
100 ꢁC
Reflux
100 ꢁC
100 ꢁC
80 ꢁC
6 h
4 h
10
38
45
51
56
48
85
60
96
85
78
82
81
95
92
3
BF3 ꢀ SiO2
Ethanol
Water
Neat
5 h
6 h
4
Zn(OTf)2 ꢀ SiO2
ZnCl2.SiO2
5
8 h
6
TiO2 ꢀ SiO2
ACN
10 h
2 h
7
SiO2TMG
Ethanol
Neat
Neat
Reflux
80 ꢁC
8
FeCl3 ꢀ SiO2
SiO2TMG
SiO2TMG
4 h
9
100 ꢁC
100 ꢁC
Reflux
Reflux
100 ꢁC
100 ꢁC
100 ꢁC
30 min
2 h
3 h
10
11
12
13
14
15
Water
ACN
SiO2TMG
SiO2TMG
Methanol
Neat
Neat
4 h
1 h
SiO2TMG (5 mol%)
SiO2TMG (15 mol%)
SiO2TMG (20 mol%)
30 min
30 min
Neat
aReaction conditions: hydrazine hydrate (1.5 mmol), ethyl acetoacetate (1 mmol), benzaldehyde
(1 mmol), malononitrile (1 mmol), and catalyst (10 mol%, except entries 13, 14, and 15).
bIsolated yield.
Encouraged by the general interest of this convergent and experimentally
simple transformation, we then evaluated its scope and efficiency in the generation
of 6-amino-2,4-dihydropyrano[2,3-c]pyrazol-5-carbonitriles arrays by recourse to
a diverse range of several substituted aryl=aliphatic aldehydes. It was gratifying
to observe that most of the tested substrates exhibited satisfactory reactivity profiles,
in all cases leading to a smooth heterocyclization sequence that readily afforded
the target structures (Table 2). Compared with aromatic aldehydes, aliphatic
aldehyde afforded relatively lower yields of the corresponding 6-Amino-2,4-
dihydropyrano[2,3-c]pyrazol-5-carbonitriles. A possible mechanism of this one-pot
reaction is expected on the basis of reported literature[15] (Scheme 2).
The probable mechanism for the formation of 6-amino-2,4-dihydropyrano-
[2,3-c]pyrazol-5-carbonitriles using SiO2TMG as a catalyst is outlined in Scheme 2.
=
The condensation of C O and NH2 on the ethyl acetoacetate and hydrazine
hydrate and the subsequent loss of ethanol occurred via intramolecular nucleophilic

SILICA-SUPPORTED TETRAMETHYLGUANIDINE
2683
Table 2. Synthesisa of 6-amino-4-alkyl=aryl-3-methyl-2,4-dihydropyrano[2,3-c]pyrazole-carbonitriles
No.
Aldehyde
Product
Time (min)
Yieldb (%)
Mp (ꢁC) [Ref.]
5a
30
96
245–246[13]
5b
5c
5d
5e
5f
20
30
20
30
20
98
95
96
95
95
233–234[13]
147–148[13]
244–245[19]
260–262[19]
179–180[13]
(Continued )

2684
A. B. ATAR ET AL.
Table 2. Continued
No.
Aldehyde
Product
Time (min)
Yieldb (%)
Mp (ꢁC) [Ref.]
5g
40
92
211–212[13]
5h
35
91
247–248[21]
5i
45
89
182–183[15]
5j
20
98
250–251[13]
5k
55
90
189–190[–]
(Continued )

SILICA-SUPPORTED TETRAMETHYLGUANIDINE
2685
Table 2. Continued
No.
Aldehyde
Product
Time (min)
Yieldb (%)
Mp (ꢁC) [Ref.]
5l
25
93
221–222[13]
5m
40
90
168–169[21]
5n
35
88
250–252[19]
5o
15
98
248–249[22]
5p
45
90
210–211[13]
(Continued )

2686
A. B. ATAR ET AL.
Table 2. Continued
No.
Aldehyde
Product
Time (min)
Yieldb (%)
Mp (ꢁC) [Ref.]
5q
60
79
143–144[12]
5r
35
92
259–260[21]
5s
30
93
225–226[13]
5t
55
85
248–250[–]
5u
40
85
190–191[21]
5v
35
87
175–176[13]
(Continued )

SILICA-SUPPORTED TETRAMETHYLGUANIDINE
2687
Table 2. Continued
No.
Aldehyde
Product
Time (min)
Yieldb (%)
Mp (ꢁC) [Ref.]
5w
25
95
244–245[19]
5x
40
90
209–211[15]
aReaction conditions: hydrazine hydrate (1.5 mmol), ethyl acetoacetate (1 mmol), aldehydes (1 mmol),
malononitrile (1 mmol), and SiO2TMG catalyst (10 mol%).
bIsolated yield.
substitution to deliver intermediate pyrazole-5-one. Knoevenagel condensation
between an malononitrile and aldehyde produced a species that underwent
Michael addition with the pyrazole-5-one. Further rearrangement via the subsequent
intramolecular nucleophilic attack of OH to CN may have generated the expected
target product. The SiO2TMG catalyst may have facilitated both the conden-
sation reaction and the nucleophilic attack.
Scheme 2. Probable mechanism for the formation of 6-amino-2,4-dihydropyrano[2,3-c]pyrazol-5-
carbonitriles using SiO2TMG as a catalyst.

2688
A. B. ATAR ET AL.
Table 3. Comparative studya of the efficacy of silica-supported tetramethylguanidine using phenyl
hydrazine and hydrazine hydrate
Time (min)
Yieldb (%)
No.
Aldehyde
H2N-NH2
Ph-NH-NH2
H2N-NH2
Ph-NH-NH2
1
2
3
4
5
p-Bromobenzaldehyde
p-Methoxybenzaldehyde
Benzaldehyde
20
40
30
35
35
45
60
55
65
60
95
92
96
92
87
92
88
93
90
88
m-Hydroxybenzaldehyde
p-Methylbenzaldehyde
aReaction conditions: Phenyl hydrazine=hydrazine hydrate (1.5 mmol), ethyl acetoacetate (1 mmol),
aldehydes (1 mmol), malononitrile (1 mmol), and SiO2TMG catalyst (10 mol%).
Table 4. Recycling and reuse of SiO2TMG
Entry
Reaction cycle
Yielda (%)
1
2
3
4
1 (fresh run)
96
96
95
95
2
3
4
aIsolated yield.
We also carried out the comparative study of the efficacy of silica-supported
tetramethylguanidine between phenyl hydrazine and hydrazine hydrate as shown
in Table 3. As compared to hydrazine hydate, phenyl hydrazine take more time
for the completion of reaction with slightly lower yield.
The reusability of the SiO2TMG catalyst is one of its most important benefits
and makes it useful for commercial applications as well. Thus the recovery
and reusability of the catalyst were investigated. The recyclability of the catalyst
was checked with model reaction (Table 4, entries 1–4). The catalyst was recovered
after completion of the first fresh run, the reaction mixture was cooled to room
temperature, and then the residue was treated with boiling ethanol, filtered through
a sintered funnel, and washed thoroughly with the same solvent. The catalyst was
separated from the reaction mixture. The recovered SiO2TMG was dried at
90–100 ꢁC for 12 h and tested in up to three more reaction cycles. The same catalyst
was reused for subsequent reactions (three runs) with fresh substrates under the
same conditions. The catalyst showed excellent recyclability in all these reactions
(Table 4), as the reaction times and yield remained almost the same without loss
of catalytic activity.
EXPERIMENTAL
Chemicals were purchased from Aldrich, Merck, and Alfa Aesar chemical
companies. The NMR spectra were recorded in dimethylsulfoxide (DMSO-d6)
on a Jeol JNM ECP 400 NMR instrument using Tetramethylsilane (TMS) as an

SILICA-SUPPORTED TETRAMETHYLGUANIDINE
2689
internal standard. High-resolution mass spectra (HRMS) were recorded on a Jeol
JMS-700 mass spectrometer. Melting points were taken in open capillaries on an
Electrothermal-9100 instrument (Japan).
Preparation of Silica-Supported Tetramethylguanidine (SiO2TMG)
Catalyst
Silica-supported tetramethylguanidine was prepared by adopting the literature
precedent.[31] 1,1,3,3-Tetramethylguanidine (TMG, 0.2 mol) and mixed xylenes were
mixed together and heated to 120 ꢁC. Chloropropyl trimethoxysilane (0.081 mol) was
added dropwise over 2 h, and the solution allowed to react for 2 h. It treated with
activated silica in presence of triethyl amine, and the solution was allowed to react
for 1 h. On cooling, a solid precipitate was filtered and dried at 85–90 ꢁC to yield
the desired product.
General Procedure for the Synthesis of 6-Amino-2,4-
dihydropyrano[2,3-c]pyrazol-5-carbonitriles
To a prestirred mixture of ethyl acetoacetate (1 mmol), hydrazine hydrate
(1 mmol), aldehydes (1 mmol), and malononitrile (1 mmol) were added, followed by
SiO2TMG (10 mol%). The reaction mixture was then heated at 100 ꢁC. The reaction
was monitored by thin-layer chromatography (TLC). After completion of the reac-
tion, the reaction mixture was cooled to room temperature. The residue was treated
with boiling ethanol, filtered through a sintered funnel, and washed thoroughly with
the same solvent. The combined filtrate was evaporated in vacuo to afford the crude
product, which was recrystallized from ethanol to afford pure compound 5. The spec-
tral and analytical data of the model representative compound are given here.
6-Amino-3-methyl-4-phenyl-2,4-dihydropyrano[2,3-c]pyrazole-5-
carbonitrile (5a)
1
Yield 96%; white powder; mp 245–246 ꢁC. H NMR (400 MHz, DMSO-d6): d
12.1 (s, 1H), 7.31 (t, J ¼ 12 Hz, 2H), 7.22 (t, J ¼ 8 Hz, 1H), 7.16 (d, J ¼ 8 Hz, 2H),
6.88 (s, 2H), 4.59 (s, 1H), 1.77 (s, 3H). 13C NMR (100 MHz, DMSO-d6): d 160.9,
154.7, 144.4, 135.6, 128.4, 126.7, 120.8, 97.6, 57.2, 36.2, 9.7. HRMS (ESI, m=z):
calcd. for C14H12N4O (m=z) 252.1011; found 252.1011.
CONCLUSION
In summary, we have described an efficient and environmentally benign proto-
col for the synthesis of fully substituted 6-amino-2,4-dihydropyrano[2,3-c]pyrazol-5-
carbonitriles via reaction of ethyl acetoacetate, hydrazine hydrate, and malononitrile
with diverse aldehydes using silica-supported tetramethylguanidine as expeditious
reusable catalyst in excellent yields. The main advantages of this present method-
ology are the simple workup, easy recovery of catalyst, and lack of need for anhy-
drous conditions, base, or any additional activators.

2690
A. B. ATAR ET AL.
FUNDING
This work was supported by a research grant from Pukyong National
University (2014).
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
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