Polyethylene Glycol (PEG-400): An Efficient and Recyclable Reaction Medium for the Synthesis of Pyrazolo[3,4-b]pyridin- 6(7H)-one Derivatives

Article abstract of DOI:10.3390/molecules181113139

A mild and efficient synthesis of pyrazolo[3,4-b]pyridine-6(7H)-one derivatives via a three-component reaction of an aldehyde, Meldrum's acid and 3-methyl-1H-pyrazol-5-amine using recyclable polyethylene glycol (PEG)-400 as a reaction medium is described.

Full text of DOI:10.3390/molecules181113139

Molecules 2013, 18, 13139-13147; doi:10.3390/molecules181113139
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Polyethylene Glycol (PEG-400): An Efficient and Recyclable
Reaction Medium for the Synthesis of Pyrazolo[3,4-b]pyridin-
6(7H)-one Derivatives
Xiaoxing Zhong, Guolan Dou * and Deming Wang
School of Safety Engineering, China University of Mining &Technology, Xuzhou 221116, Jiangsu, China
* Author to whom correspondence should be addressed; E-Mail: gldoucumt@163.com;
Tel.: +86-187-9628-7182; Fax: +86-516-6588-0089
Received: 27 September 2013; in revised form: 16 October 2013 / Accepted: 16 October 2013 /
Published: 24 October 2013
Abstract: A mild and efficient synthesis of pyrazolo[3,4-b]pyridine-6(7H)-one derivatives via
a three-component reaction of an aldehyde, Meldrum’s acid and 3-methyl-1H-pyrazol-5-amine
using recyclable polyethylene glycol (PEG)-400 as a reaction medium is described. This
method has the advantages of accessible starting materials, good yields, mild reaction
conditions and begin environmentally friendly.
Keywords: pyrazolo[3,4-b]pyridine-6(7H)-one; recyclable polyethylene glycol; aqueous
media; synthesis
1. Introduction
Pyrazolo[3,4-b]pyridine-6-ones are a promising class of heterocyclic compounds that have been
shown to have potential in the treatment of several diseases, including bipolar disorder, diabetes, dementia,
Alzheimer’s disease, schizophrenia, depression and cancer [1,2]. Pyrazolo[3,4-b]pyridine-6-ones were first
synthesized by condensing ethylidenemalonic acid diethyl ester and 1-ethyl-5-amino methylpyzole in
refluxing dimethylformamide and water for 94 h and resulted in a 53% yield of the target product [3].
Quiroga et al. [4]. reported the synthesis of dihydropyrazolo[3,4-b]pyridine-6-ones by refluxing
equimolar amounts of aminopyrazole and the appropriate Meldrum’s acid benzylidene derivatives in
nitrobenzene for 30 min, resulting in yields ranging from 42% to 72%. Martinez-Teipel et al. [5].
developed a new synthetic route to this class of compound in 37%–92% yield involving an
intermolecular cyclization of 2-methoxy-6-oxo-1,4,5,6-tetrahydropyridine-3-carbonitriles with

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hydrazines. Dress et al. [1]. also reported the synthesis of these compounds in 30%–75% yield by using
condensation reactions that involved refluxing aminopyrazole with the appropriate aldehyde and dimedone
derivatives in ethanol for 6–8 h. Recently, an efficient synthsis of pyrazolo[3,4-b]pyridine-6-one
derivatives using intermolecular cyclization reactions under solvent-free microwave conditions and
ultrasound irradiation conditions was reported, resulting in yields of 40%–60% and 60%–95%,
respectively [6]. Shi et al. [7] reported the synthesis of 3-methyl-1,4-disubstituted-4,5-dihydro-1H-
pyrazolo[3,4-b]pyridine-6(7H)-ones via the three-component L-proline-catalyzed reaction of an
aldehyde, 3-methyl-1-phenyl-1H-pyrazol-5-amine, and Meldrum's acid. However, although these
methods have successfully led to a large library synthesis of pyrazolo[3,4-b]pyridine-6-ones, many of
them still suffer from drawbacks such as requiring the use of harmful organic reagents, unsatisfactory
yields and long reaction times. Therefore, a method with higher yield and environmentally-friendly
manipulation needs to be developed.
Multi-component reactions (MCRs), in which multiple reactions are combined into a synthetic
operation have been used extensively in synthetic chemistry to form carbon-carbon bonds [8–15]. Such
reactions offer a wide range of possibilities for the efficient construction of highly complex molecules
in a single procedural step, thus avoiding the complicated purification operations and allowing savings
of both solvents and reagents. In the past decade, there has been tremendous development in the area of
three- and four-component reactions, and great efforts continue to be made to develop new MCRs [16–24].
Recently, Mamaghani et al. reported an one-pot three-component reaction for the synthesis of novel
derivatives of pyrazolo[3,4-b]pyridine-6(7H)-ones in 3–4 min with excellent yields (87%–95%) from
5-amino-3-methyl-1H-pyrazole, Meldrum’s acid and aryl aldehydes under ultrasonic irradiation [25].
Nevertheless, this method needs specific experimental facilities, which limits its use in mass production.
Over the years, there has been a growing recognition that water has become an attractive medium for
many organic reactions, such as Diels-Alder reactions [26], Claisen rearrangement reactions [27,28],
Reformatsky reactions [29,30] and pinacol-coupling reactions [31], not only for the advantages
concerning the avoidance of expensive drying reactions, catalysts and solvents, but also for some
unique reactivity and selectivity [32–35]. On the other hand, organic reactions in water without using
harmful organic solvents is one of the current focuses today, especially in our current environmentally
conscious society, because water is abundant, nontoxic and environmentally-friendly when compared
with the traditionally used organic solvents. As we all know, poly(ethylene glycol) (PEG) is a
thermally stable, inexpensive, recoverable, and non-toxic hydrophilic polymer. Meanwhile, the high
solubility of PEGs in water and several organic solvents including alcohol and acetone [36] instead of
their insolubility in less polar solvents such as hexane makes them easy to recover and high
performance solvents for organic reactions [37–39]. Therefore, the use of an obviously benign and
inexpensive solvent like water and PEG could yield significant green chemistry benefits. Herein we
envisaged a simple and efficient one-pot three-component protocol for the synthesis of
pyrazolo[3,4-b]pyridine-6(7H)-ones in moderate to high yields, using environmentally-friendly
polyethylene glycol (PEG) as a recyclable reaction medium.

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2. Results and Discussion
When the three components, an aldehyde 1, Meldrum’s acid (2) and 3-methyl-1H-pyrazol-5-amine (3)
were treated in water in the presence of PEG 400 at 90 °C for about 15 min, the desired products―
4-substituted-3-methyl-4,5-dihydro-2H-pyrazolo[3,4-b]pyridin-6(7H)-ones 4 were obtained (Scheme 1).
Scheme 1. The synthesis of pyrazolo[3,4-b]pyridine-6(7H)-one derivatives in aqueous media.
R
O
PEG 400, H2O
O
NH
+
RCHO
+
90 oC
N
N
NH2
O
N
N
O
O
H
H
1
2
3
4
A range of novel valuable structures 4 were thus synthesized in good to excellent yields by this
simple four-component reaction in aqueous media. The results are summarized in Table 1.
Table 1. The synthesis of pyrazolo[3,4-b]pyridine-6(7H)-one derivatives in aqueous media.
Entry
Product
4a
R
Isolated Yield (%)
1
2
3
4
5
6
7
8
9
4-CH3C6H4
3-CH3C6H4
4-CH3OC6H4
3-CH3OC6H4
4-BrC6H4
3-BrC6H4
2-CH3C6H4
3-ClC6H4
88
93
89
84
88
92
89
90
86
4b
4c
4d
4e
4f
4g
4h
4i
n-propyl
As shown in Table 1, we were pleased to find that the method was applicable to a broad substrate
scope of substituted aldehydes. Aldehydes containing various electron-donating and electron-withdrawing
substituents were reacted under the experimental conditions, and the corresponding products were
obtained in good yields. Therefore, no remarkable electronic effects were observed in the reaction. Good
yields was also obtained when the alkyl alhedyde butyraldehyde was reacted (Table 1, entry 9).
With regard to sustainable chemistry issues, reagent recyclability is an important question. The
separation of the products and the reaction medium were explored for the synthesis of product 4a in
PEG-400-H2O. We were pleased to find that the entire reaction medium could be successfully recycled
for up to five runs with limited loss of activity (the yield decreased from 93% to 70% after
5 runs, Table 2).

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Table 2. Recycling and reuse of PEG-400-H2O.
Run
Yield (%)
1
2
3
93
89
82
4
5
76
70
All the products were characterized by 1H-NMR, IR and HRMS spectra. The structure of compound 4c
was further confirmed by X-ray diffraction analysis [40]. The molecular structure of 4c is shown in Figure 1.
Figure 1. Molecular structure of 4c.
Although the mechanism of the reaction has not yet been established, a possible explanation is
proposed in Scheme 2. The reaction might thus proceed via sequential condensation, addition,
cyclization, and elimination. First, a Knoevenagel condensation between aldehydes 1 with Meldrum’s
acid (2) affords intermediate A. The Michael addition of A with 3-methyl-1H-pyrazol-5-amine (3)
would then furnish the intermediate product B, which subsequently undergoes an intramolecular
cyclization and then releases acetone and carbon dioxide to give product 4.
Scheme 2. Possible mechanism for the formation of product 4.
O
O
O
N
O
( 3 )
NH2
O
N
C R
H
RCHO
+
H
O
O
O
A
1
2
O
R
R
O
- CH3COCH3
- CO2
N
NH
NH
O
N
O
N
H2N
O
H
B
4

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3. Experimental
3.1. General Information
Commercial solvents and reagents were used as received. IR spectra were obtained on a Nicolet
6700 spectrophotometer. 1H-NMR spectra were recorded using a Bruker DPX-400 MHz instrument, at
293 K unless otherwise noted, with the residual peaks of the solvent DMSO-d6 (δ = 2.50) used for
reference. HRMS were obtained on a micromass GCT-TOF instrument. X-Ray crystallographic
analysis was performed with a Rigaku Mercury diffractometer.
3.2. General Procedure for the Synthesis of Pyrazolo[3,4-b]pyridine-6(7H)-ones 4 in Aqueous Media
To a stirred solution of polyethylene glycol (PEG)-400 (5 mL) in water (10 mL), aldehyde 1
(2 mmol), Meldrum’s acid (2, 2 mmol) and 3-methyl-1H-pyrazol-5-amine (3, 2 mmol) were added and
the mixture stirred at 90 °C, until the reaction was complete as indicated by TLC (about 15 min). After
completion of the reaction, the crystalline powder formed was collected by filtration, washed with
water and recrystallized from ethanol to give pure 4. The recovered PEG with water was reused for
further cycles.
3-Methyl-4-p-tolyl-4,5-dihydro-2H-pyrazolo[3,4-b]pyridin-6(7H)-one (4a). M.p. >300 °C; IR (KBr):
1
3203, 3160 cm−1 (NH), 1650 cm−1 (C=O). H-NMR (DMSO-d6) δ: 1.87 (s, 3H, CH3), 2.26 (s, 3H,
CH3), 2.54 (d, J = 6.0 Hz, 1H, CH), 2.74–2.78 (m, 1H, CH), 4.09 (t, J = 6.4 Hz, 1H, CH), 7.04–7.12
(m, 4H, ArH), 10.27 (s, 1H, NH), 11.79 (s, 1H, NH); HRMS: m/z calcd. for C14H16N3O: 242.12879
(M+H); found 242.12880.
3-Methyl-4-m-tolyl-4,5-dihydro-2H-pyrazolo[3,4-b]pyridin-6(7H)-one (4b). M.p. >300 °C; IR (KBr):
1
3200, 3150 cm−1 (NH), 1640 cm−1 (C=O). H-NMR (DMSO-d6) δ: 1.83 (s, 3H, CH3), 2.26 (s, 3H,
CH3), 2.54 (d, J = 6.0 Hz, 1H, CH), 2.74–2.79 (m, 1H, CH), 4.09 (t, J = 6.4 Hz, 1H, CH), 7.04–7.12
(m, 4H, ArH), 10.27 (s, 1H, NH), 11.79 (s, 1H, NH); HRMS: m/z calcd. for C14H16N3O: 242.12879
(M+H); found 242.12878.
4-(4-Methoxyphenyl)-3-methyl-4,5-dihydro-2H-pyrazolo[3,4-b]pyridin-6(7H)-one (4c). M.p. >300 °C;
IR (KBr): 3180, 3155 cm−1 (NH), 1640 cm−1 (C=O). 1H-NMR (DMSO-d6) δ: 1.83 (s, 3H, CH3), 2.72–2.78
(m, 2H, CH2), 3.73 (s, 3H, OCH3), 4.09 (t, J = 6.4 Hz, 1H, CH), 6.86–6.88 (m, 2H, ArH), 7.07–7.09
(m, 2H, ArH),10.25 (s, 1H, NH), 11.78 (s, 1H, NH); HRMS: m/z calcd. for C14H16N3O2: 258.12370
(M+H); found 258.12369.
4-(3-Methoxyphenyl)-3-methyl-4,5-dihydro-2H-pyrazolo[3,4-b]pyridin-6(7H)-one (4d). M.p. >300 °C; IR
1
(KBr): 3180, 3150 cm−1 (NH), 1640 cm−1 (C=O). H-NMR (DMSO-d6) δ: 1.85 (s, 3H, CH3), 2.56 (d,
J = 5.6 Hz, 1H, CH), 2.78–2.84 (m, 1H, CH), 3.72 (s, 3H, OCH3), 4.17 (t, J = 6.4 Hz, 1H, CH), 6.73–6.81
(m, 3H, ArH), 7.23 (t, J = 8.0 Hz, 1H, ArH), 10.28 (s, 1H, NH), 11.81 (s, 1H, NH); HRMS: m/z calcd.
for C14H16N3O2: 258.12370 (M+H); found 258.12372.
4-(4-Bromophenyl)-3-methyl-4,5-dihydro-2H-pyrazolo[3,4-b]pyridin-6(7H)-one (4e). M.p. >300 °C;
IR (KBr): 3190, 3160 cm−1 (NH), 1643 cm−1 (C=O). 1H-NMR (DMSO-d6) δ: 1.85 (s, 3H, CH3), 2.56 (d,

Molecules 2013, 18
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J = 5.6 Hz, 1H, CH), 2.78–2.84 (m, 1H, CH), 4.17 (t, J = 6.4 Hz, 1H, CH), 7.13–7.15 (m, 2H, ArH),
7.50–7.52 (m, 2H, ArH), 10.32 (s, 1H, NH), 11.85 (s, 1H, NH); HRMS: m/z calcd. for C13H13BrN3O:
306.02365 (M+H); found 306.02362.
4-(3-Bromophenyl)-3-methyl-4,5-dihydro-2H-pyrazolo[3,4-b]pyridin-6(7H)-one (4f). M.p. >300 °C;
IR (KBr): 3190, 3160 cm−1 (NH), 1640 cm−1 (C=O). 1H-NMR (DMSO-d6) δ: 1.86 (s, 3H, CH3), 2.50–2.58
(m, 1H, CH), 2.79–2.83 (m, 1H, CH), 4.17 (t, J = 6.0 Hz, 1H, CH), 7.16–7.41 (m, 4H, ArH), 10.33
(s, 1H, NH), 11.86 (s, 1H, NH); HRMS: m/z calcd. for C13H13BrN3O: 306.02365 (M+H); found
306.02371.
3-Methyl-4-o-tolyl-4,5-dihydro-2H-pyrazolo[3,4-b]pyridin-6(7H)-one (4g). M.p. >300 °C; IR (KBr):
1
3180, 3150 cm−1 (NH), 1645 cm−1 (C=O). H-NMR (DMSO-d6) δ: 1.73 (s, 3H, CH3), 2.37 (s, 3H,
CH3), 2.39–2.47 (m, 1H, CH), 2.73–2.78 (m, 1H, CH), 4.34 (t, J = 6.4 Hz, 1H, CH), 6.88–7.20 (m, 4H,
ArH), 10.30 (s, 1H, NH), 11.81 (s, 1H, NH); HRMS: m/z calcd. for C14H16N3O: 242.12879 (M+H);
found 242.12871.
4-(3-Chlorophenyl)-3-methyl-4,5-dihydro-2H-pyrazolo[3,4-b]pyridin-6(7H)-one (4h) M.p. >300 °C;
IR (KBr): 3190, 3155 cm−1 (NH), 1645 cm−1 (C=O). 1H-NMR (DMSO-d6) δ: 1.86 (s, 3H, CH3), 2.77 (dd,
J1 = 5.6 Hz, J2 = 16 Hz, 1H, CH), 2.79–2.84 (m, 1H, CH), 4.20 (t, J = 6.0 Hz, 1H, CH), 7.14–7.35 (m,
4H, ArH), 10.34 (s, 1H, NH), 11.87 (s, 1H, NH); HRMS: m/z calcd. for C13H13ClN3O: 262.07417
(M+H); found 262.07416.
3-methyl-4-propyl-4,5-dihydro-2H-pyrazolo[3,4-b]pyridin-6(7H)-one (4i) M.p. 259–260 °C; IR (KBr):
3175, 3140 cm−1 (NH), 1645 cm−1 (C=O). 1H-NMR (DMSO-d6) δ: 0.85–0.86 (m, 3H, CH3), 1.17–1.93
(m, 4H, 2CH2), 2.18 (s, 3H, CH3), 2.24–2.26 (m, 1H, CH), 2.77–2.78 (m, 1H, CH), 3.51
(t, J = 7.2 Hz, 1H, CH), 10.13 (s, 1H, NH), 11.66 (s, 1H, NH); HRMS: m/z calcd. for C10H16N3O:
194.12879 (M+H); found 194.12874.
4. Conclusions
In summary, we have demonstrated a mild and highly efficient protocol for the synthesis of
4-substituted-3-methyl-4,5-dihydro-2H-pyrazolo[3,4-b]pyridin-6(7H)-ones in excellent yields by using
recyclable polyethylene glycol (PEG)-400 as a reaction medium. Environmental acceptablility, high
yields, easy work-up, cleaner reaction profiles, environmentally friendly solvent, and recyclability of
PEG are the notable features of this protocol.
Acknowledgments
We are grateful to The Joint Funds of the National Natural Science Foundation of China and
Shenhua Group Corporation Limited (No. 51134020), the National Natural Science Foundation of
China Youth Science Foundation (Grant No. 51204171 and 51004105), and A Project Funded by the
Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Conflicts of Interest
The authors declare no conflict of interest.
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40. Crystallographic data for the structure of 4c (CCDC-967181) have been deposited at the
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charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: +44
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Sample Availability: Samples of the compounds 4 are available from the authors.
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