One-flask synthesis of pyrazolo[3,4-d]pyrimidines from 5-aminopyrazoles and mechanistic study

Article abstract of DOI:10.3390/molecules22050820

A novel one-flask synthetic method was developed in which 5-aminopyrazoles were reacted with N,N-substituted amides in the presence of PBr₃. Hexamethyldisilazane was then added to perform heterocyclization to produce the corresponding pyrazolo[

Full text of DOI:10.3390/molecules22050820

molecules
Article
One-Flask Synthesis of Pyrazolo[3,4-d]pyrimidines
from 5-Aminopyrazoles and Mechanistic Study
Wan-Ping Yen 1,2, Shuo-En Tsai 1,2, Naoto Uramaru 3, Hiroyuki Takayama 4
and Fung Fuh Wong 1,
*
1
School of Pharmacy, China Medical University, No. 91 Hsueh-Shih Rd., Taichung 40402, Taiwan;
a5850478@yahoo.com.tw (W.-P.Y.); u100003044@cmu.edu.tw (S.-E.T.)
Program for Biotech Pharmaceutical Industry, China Medical University, No. 91 Hsueh-Shih Rd.,
Taichung 40402, Taiwan
Department of Environmental Science, Nihon Pharmaceutical University,
Komuro Inamachi Kita-adachi-gun, Saitama-ken 10281, Japan; uramaru@nichiyaku.ac.jp
Department of Medico Pharmaceutical Science, Nihon Pharmaceutical University, Komuro Inamachi
Kita-adachi-gun, Saitama-ken 10281, Japan; h.takayama@nichiyaku.ac.jp
Correspondence: wongfungfuh@yahoo.com.tw or ffwong@mail.cmu.edu.tw;
Tel.: +886-422-053-366 (ext. 5603); Fax: +886-422-078-083
2
3
4
*
Academic Editor: Philippe Belmont
Received: 18 March 2017; Accepted: 11 May 2017; Published: 16 May 2017
Abstract: A novel one-flask synthetic method was developed in which 5-aminopyrazoles were
reacted with N,N-substituted amides in the presence of PBr3. Hexamethyldisilazane was then
added to perform heterocyclization to produce the corresponding pyrazolo[3,4-d]pyrimidines in
suitable yields. These one-flask reactions thus involved Vilsmeier amidination, imination reactions,
and the sequential intermolecular heterocyclization. To study the reaction mechanism, a series
of 4-formyl-1,3-diphenyl-1H-pyrazol-5-yl-N,N-disubstituted formamidines, which were conceived
as the chemical equivalent of 4-(iminomethyl)-1,3-diphenyl-1H-pyrazol-5-yl-formamidine, were
prepared and successfully converted into pyrazolo[3,4-d]pyrimidines. The experiments demonstrated
that the reaction intermediates were the chemical equivalents of 4-(iminomethyl)-1,3-diphenyl-
1H-pyrazol-5-yl)formamidines. The rate of the reaction could be described as being proportional
to the reactivity of amine reactants during intermolecular heterocyclization, especially when
hexamethyldisilazane was used.
Keywords: pyrazolo[3,4-d]pyrimidines; pyrimidines; vilsmeier reaction; heterocyclization;
hexamethyldisilazane
1. Introduction
One-flask reactions possess significant advantages and have emerged as a powerful tool in
synthetic organic chemistry and reaction design approaches [
one-flask reactions in organic syntheses are their green chemistry nature and high atom economy
due to the lack of workup or the isolation of intermediates involved [ 12]. We previously reported
1–3]. The main advantages of using
4–
an efficient one-pot three-component synthesis of pyrazolo[3,4-d]pyrimidines that involved treatment
of 5-aminopyrazoles with formamide using PBr3 as the coupling agent (Scheme 1) [13–15].
Molecules 2017, 22, 820; doi:10.3390/molecules22050820
www.mdpi.com/journal/molecules

Molecules 2017, 22, 820
2 of 12
Scheme 1. Synthesis of pyrazolo[3,4-d]pyrimidines by the different synthetic strategy via the Vilsmeier
reaction and intramolecular or intermolecular heterocyclization.
Pyrazolopyrimidine derivatives are important structural moieties, found in pharmacologically
active compounds such as a novel series of glucokinase activators [16], antibacterial [17],
antifungal [18
anticancer compounds [28
cells [30 31]. The pyrazolo[3,4-d]pyrimidine core is also isomeric with the biologically significant purine
system [32 33]. Numerous synthetic methods were developed for preparing pyrazolopyrimidine
derivatives [34 37]. However, most of these methods are not straightforward and their purification
,19], antioxidant [20], antitumor [21
–24], herbicidal [25], antivirus [26,27],
,
29], and effective inhibitors of inflammatory mediators in intact
,
,
–
steps are troublesome. Therefore, new and convenient routes for the synthesis of pyrazolo[3,4-d]
pyrimidine systems have attracted considerable attention [13–15].
In this study, we extended our previous one-pot three-component approach for the
synthesis of a series of pyrazolo[3,4-d]pyrimidine derivatives to develop a novel one-flask
synthesis involving Vilsmeier amidination, imination reactions, and sequential intermolecular
heterocyclization. First, 5-aminopyrazoles were treated with various Vilsmeier agents, which were
generated from the corresponding amide solvents, including N,N-dimethylformamide (DMF),
N,N-diethylformamide (DEF), N,N-diisopropylformamide, N,N-di-n-butylformamide, piperidine-1-
carbaldehyde, and pyrrolidine-1-carbaldehyde in the presence of tribromophosphine PBr3, to produce
the corresponding 4-(iminomethyl)-1,3-diphenyl-1H-pyrazol-5-yl-N,N-disubstituted formamidine
intermediates (Scheme 1) [38,39]. Without isolating the intermediates, we sequentially evaluated
the intermolecular heterocyclization reactivity between 4-(iminomethyl)-1,3-diphenyl-1H-pyrazol-
5-yl-formamidines and amines such as hexamethyldisilazane, hexamethylenetetramine, lithium
bis(trimethylsilyl)amine, and sodium bis(trimethylsilyl)amine. These experimental results revealed
that commercially available N,N-dimethylformamide (DMF)/PBr3 and hexamethyldisilazane were
the optimal Vilsmeier agent and the promotor, respectively. Specifically, we successfully combined
the Vilsmeier amidination and imination reactions with intermolecular heterocyclization to design
a high-efficiency one-flask synthesis for the preparation of a series of pyrazolo[3,4-d]pyrimidines.
2. Results and Discussion
To optimize the one-flask process for the synthesis of pyrazolo[3,4-d]pyrimidine derivatives
3a
mechanism the study illustrated in Scheme 2 was performed. 5-Amino-1,3-diphenylpyrazole (1a) was
prepared by our previously developed method [38 39] and used as the model starting material to
–n via the sequential Vilsmeier reaction and intermolecular heterocyclization and explain the
,
improve the intermolecular heterocyclization reaction conditions. Following the reliable published
procedure for the Vilsmeier reaction 5-aminopyrazole 1a was treated with 3.0 equivalent of PBr3 in

Molecules 2017, 22, 820
3 of 12
N,N-dimethylformamide (DMF) solution at 60 ◦C for 1.0–2.0 h. The corresponding 4-(iminomethyl)-
1,3-diphenyl-1H-pyrazol-5-yl-formamidine 2a was thus obtained in excellent yield (>90%).
Scheme 2. The newly developed one-flask for synthesis of pyrazolo[3,4-d]pyrimidine derivatives via
Vilsmeier reaction and the sequential heterocyclization.
Without isolation of intermediate 2a, various amines, including hexamethyldisilazane
(NH(SiMe3)2), hexamethylenetetramine, lithium bis(trimethylsilyl)amine (LiN(SiMe3)2), and sodium
bis(trimethylsilyl)amine (NaN(SiMe3)2) were added into the reaction mixture and the solution
was heated at reflux for 3–5 h to establish the best heterocyclization conditions (see Table 1).
Without the amine agent, only 4-formyl-1,3-diphenyl-1H-pyrazol-5-yl-formamidine 7a, which is the
chemical equivalent of 4-(iminomethyl)-1,3-diphenyl-1H-pyrazol-5-yl-formamidine intermediate 2a
,
was isolated after work-up and purification (see entry 1 in Table 1). Among the amines, the corresponding
pyrazolo[3,4-d]pyrimidine product 3a can be produced and isolated in yields ranging from 26% to
91%. Based on the results, we found that commercially available hexamethyldisilazane (NH(SiMe3)2)
provided the best result (91% yield) and the reactivity tendency of the amines was NH(SiMe3)2 >
NaN(SiMe3)2 > LiN(SiMe3)2 > hexamethylenetetramine (see the Entries 2–5 in Table 1). We next tried
different amounts of NH(SiMe3)2, including 1.0, 2.0, 3.0, and 4.0 equivalents. The corresponding
pyrazolo[3,4-d]pyrimidine product 1a was obtained in 56–91% yield, with the best yield (91%)
corresponding to 3 equivalents of (NH(SiMe3)2) (see the Entries 5 and 6–8 in Table 1). Consequently,
we believe that 3.0 equivalent of NH(SiMe3)2 is the optimum amount for our reaction conditions.
Table 1. The study of amine agents in the one-flask for synthesis of pyrazolo[3,4-d]pyrimidines.
Entry
Amine Agents
Equiv.
Yields (%) of Compound 3a
a
1
2
3
Without base
Hexamethylenetetramine
Lithium bis(trimethylsilyl)amine (LiN(SiMe3)2)
Sodium bis(trimethylsilyl)amine
(NaN(SiMe3)2)
-
3
3
-
33
67
4
3
81
5
6
7
8
Hexamethyldisilazane (NH(SiMe3)2)
Hexamethyldisilazane (NH(SiMe3)2)
Hexamethyldisilazane (NH(SiMe3)2)
Hexamethyldisilazane (NH(SiMe3)2)
a
3
1
2
4
91
56
63
75
1H-pyrazol-5-yl-N,N-disubstituted formamidine 2a was isolated.
To determine the reactivity of the different Vilsmeier agents (HC(O)NR1R2 + PBr3), we used
different amide solvents, including N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF),
N,N-diisopropylformamide, N,N-di-n-butylformamide, piperidine-1-carbaldehyde, and pyrrolidine-1-
carbaldehyde in the presence of 3.0 equivalent of PBr3 to prepare the corresponding types of Vilsmeier
reagent. Compound 1a was allowed to react sequentially with these different Vilsmeier reagents at
◦
60 C for 1.0–2.0 h. When the starting material 1a was fully consumed, 3.0 equivalents of NH (SiMe3)2
were added to the reaction mixture which was heated at reflux for 3.0–5.0 h. After the work-up
and purification, the corresponding pyrazolo[3,4-d]pyrimidine 3a was obtained in 56–91% yields
(see Table 2). Based on the study, commercially available DMF was the best solvent for the preparation

Molecules 2017, 22, 820
4 of 12
of the Vilsmeier reagent in this new one-flask procedure. Based on our optimized experimental results,
we believe the most reliable procedure for the one-flask synthesis of pyrazole[3,4-d]pyrimidines
involves the treatment of 5-aminopyrazole 1a with 3.0 equivalent of PBr3 in DMF solution at 60 ◦C
for 1.0–2.0 h. When the Vilsmeier reaction was comple◦ted, the ◦resulting mixture was added with
3.0 equivalents of NH(SiMe3)2 then heated at reflux at 70 C to 80 C for 3.0–5.0 h (monitored by TLC).
After work-up and purification by chromatography, the corresponding pyrazolo[3,4-d]pyrimidine 3a
was obtained in excellent yield (91%, see Table 2).
Table 2. The study of the reactivity of the different Vilsmeier agents in the one-flask synthesis of
pyrazole[3,4-d]pyrimidines.
H
O
NR1R2
N
N
NH2
N
H
2. NH(SiMe3)2
1.
H
NR1R2
PBr3
Ph
N
Ph
N
Ph N
NR1R2
N
N
N
Ph
Ph
Ph
2a
1a
3a
Entry
Amide Solvents
Yields (%) of Compound 3a
1
2
3
4
5
6
N,N-dimethylformamide (DMF)
N,N-diethylformamide (DEF)
N,N-diisopropylformamide
N,N-di-n-butylformamide
piperidine-1-carbaldehyde
pyrrolidine-1-carbaldehyde
91
86
83
81
69
56
Application of the optimized one-flask inter-heterocyclization procedure to 5-amino-1,
3-disubstituted pyrazoles 1b bearing various N1 substituents, including o-Me-Ph, o-Cl-Ph, m-Me-Ph,
–
i
m-Cl-Ph, m-NO2-Ph, p-Me-Ph, p-Cl-Ph, and p-Br-Ph, also proceeded smoothly to give the corresponding
pyrazolo[3,4-d]pyrimidines 3a–i in 78–91% yields (see Table 3).
Table 3. The results of the one-flask synthesis of pyrazolo[3,4-d]pyrimidines from 5-aminopyrazoles,
DMF/PBr3 and NH(SiMe3)2.
H
O
N
N
NH2
N
NMe2
H
2. NH(SiMe3)2
1.
H
NMe2
PBr3
X
N
X
N
X N
NMe2
N
N
N
W
W
W
1a-n
2a-n
3a-n
Yields of 3a–n (%)
Substrates
X
W
No.
Intermolecular Reaction
Intramolecular Reaction a
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
Ph
o-Me-Ph
o-Cl-Ph
m-Me-Ph
m-Cl-Ph
m-NO2-Ph
p-Me-Ph
p-Cl-Ph
p-Br-Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3a
3b
3c
3d
3e
3f
3g
3h
3i
91
78
86
89
91
87
91
87
81
79
87
84
88
91
96
93
91
92
92
87
-
91
95
93
91
93
91
94
Me
3j
3k
3l
Ph
Ph
Ph
Ph
t-Bu
p-Me-Ph
p-Cl-Ph
3m
p-OMe-Ph 3n
a
the reported data has been published and cited in ref. [13–15].

Molecules 2017, 22, 820
5 of 12
For further investigation of the effect of the substituent on the C-3 of the pyrazole ring, the same
conditions were employed with 5-amino-1-phenyl-3-substituted pyrazoles 1j that contained methyl,
t-butyl, p-Me-Ph, p-Cl-Ph, or p-OMe-Ph groups at the C-3 position of the pyrazole ring. The reaction
also proceeded smoothly gave the corresponding products 3j in 79–91% yields (see Table 3).
All pyrazolo[3,4-d]pyrimidines 3a were fully characterized by spectroscopic methods and the physical
–
n
–
n
–
n
properties and spectroscopic characteristics of the pyrazolo[3,4-d]pyrimidines 3a–n were consistent
with our published data [13–15].
For further comparison of the reactivity between this new intermolecular Vilsmeier
heterocyclization and the previously published intramolecular heterocyclization method [13 15],
5-aminopyrazoles were treated with formamide/PBr3. Based on the results of Table 3, the corresponding
pyrazolopyrimidines 3a were obtained in 78–91% yields by the intermolecular heterocyclization
–
–
n
route and in 87–96% yields by intramolecular heterocyclization, respectively. The data suggests that
the intramolecular heterocyclization is more favorable as it provided the better isolated yields.
We propose a plausible mechanism for the newly developed one-flask cascade for synthesis
of pyrazolo [3,4-d]pyrimidines as shown in Scheme 3. Initiallly, N,N-dimethylformamide (DMF)
reacted with the coupling agent PBr3 to form the Vilsmeier reactive species
Sequentially, 5-amino-1,3-disubstituted pyrazoles 1a reacted with the reactive species
the amidination and imination reaction to give the 1H-pyrazol-5-yl-N,N-disubstituted formamidine
intermediates 2a (see Scheme 3). When the Vilsmeier reaction was complete (by monitoring TLC),
NH(SiMe3)2 was directly added into the reaction mixture to perform the substitution reaction with
the imino group to generate intermediate . A sequential intermolecular heterocyclization reaction
then took place to produce intermediate . After the desilylation reaction occurred caused by bromide
4
in situ [40
–44].
–n
4
to undergo
–n
5
6
anion and water, the corresponding pyrazolo[3,4-d]pyrimidines 3a–n were obtained in good yields.
Scheme 3. A plausible mechanism for the newly developed one-flask procedure for the synthesis of
pyrazolo[3,4-d]pyrimidines.
To further study the mechanism, 4-formyl-1,3-diphenyl-1H-pyrazol-5-yl-N,N-dimethyl
formamidine 7a was synthesized [20] and reacted with various amines including NH(SiMe3)2,
hexamethylenetetramine, LiN(SiMe3)2, and NaN(SiMe3)2, to carry out the intermolecular
heterocyclization. The heterocyclization was successfully and smoothly underwent to give pyrazole

Molecules 2017, 22, 820
6 of 12
[3,4-d]pyrimidine product 3a in 37–91% yields. Particularly, NH(SiMe3)2 was most efficient
base for heterocyclization to afford the desired product in 91% yield (see Entry 4 in Table 4).
The similar reactivity tendency of heterocyclization was observed in this study: NH(SiMe3)2 >
NaN(SiMe3)2 > LiN(SiMe3)2 > hexamethylenetetramine (see Entries 1–4 in Table 4). 4-formyl-1,
3-diphenyl-1H-pyrazol-5-yl-N,N-disubtituted formamidines 7b–e with grafting the different
amino-substituent on amidinyl groups, such as NEt2, N(i-Pr)2, N(n-Bu)2, and piperidinyl, were
then allowed to reacted with NH(SiMe3)2 in DMF solution at reflux to give the corresponding
pyrazolo[3,4-d]pyrimidine 3a for the investigation of the reactivity of substrates. Based on
the experimental result, among of starting substrates 7b–e displayed the good to excellent
reactivity in heterocyclization, except for 7c possessing the bulky N (i-Pr)2 substituent moiety
on amidinyl groups (see Entries 1 and 5–8 in Table 4). Furthermore, 4-formyl-1,3-diphenyl-1H-
pyrazol-5-yl-N,N-dimethyl formamidine 7a with the NMe2 substituent on amidinyl groups
was the best suitable reactant in the intermolecular heterocyclization (91%, see Table 4).
The above results also gave more proof to our proposed mechanism, for example, the new one-flask
reaction would take place through 4-(iminomethyl)-1H-pyrazol-5-yl-formamidine intermediates 2a–n.
On the other hands, the commercial available N,N-dimethylformamide (DMF) in the presence of PBr3
and hexamethyldisilazane were the best Vilsmeier agent and the promoted cyclization base.
Table 4. The mechanistic study for the intermolecular heterocyclization from 4-formyl-1,3-disubstituted-
1H-pyrazol-5-yl-formamidines 7a–n with various amines.
NR1R2
N
N
N
H
H
O
Amines
DMF
Ph
N
Ph N
N
N
Ph
Ph
7a-f
3a
Yields of 3a (%)
Entry
Substrates
NR1R2
Amines
1
2
3
4
5
6
7
8
7a
7a
7a
7a
7b
7c
7d
7e
NMe2
NMe2
NMe2
NMe2
NEt2
Hexamethylenetetramine
LiN(SiMe3)2
3a
3a
3a
3a
3a
3a
3a
3a
37
51
84
91
89
61
81
86
NaN(SiMe3)2
NH(SiMe3)2
NH(SiMe3)2
NH(SiMe3)2
NH(SiMe3)2
N(i-Pr)2
N(n-Bu)2
Piperidinyl
NH(SiMe3)2
3. Experimental Section
3.1. General Information
All chemicals were reagent grade and used as purchased. All reactions were carried out under
nitrogen atmosphere and monitored by TLC analysis. Flash column chromatography purification of
compounds was carried out by gradient elution using hexanes in ethyl acetate (EA) unless otherwise
stated. Commercially available reagents were used without further purification unless otherwise
1
noted. H-NMR were recorded at 200, 400, or 500 MHz and 13C-NMR recorded at 50, 100, or 125 MHz,
respectively, in CDCl3, CH3OD, and DMSO-d6 as solvent (see supplementary materials). The standard
abbreviations s, d, t, q, and m refer to singlet, doublet, triplet, quartet, and multiplet, respectively.
Coupling constant (J), whenever discernible, have been reported in Hz. Infrared spectra (IR) were
recorded as neat solutions or solids; and mass spectra were recorded using electron impact or
electrospray ionization techniques. The wavenumbers reported are referenced to the polystyrene

Molecules 2017, 22, 820
7 of 12
1601 cm–1 absorption. High-resolution mass spectra were obtained by means of a JMS-HX110 mass
spectrometer (JEOL, Tokyo, Japan).
3.2. Standard Procedure for the Synthesis of Pyrazolo[3,4-d]pyrimidines 3a–n
The optimized procedure involved the treatment of 5-aminopyrazoles 1a–n (1.0 equiv)
with PBr3 (~3 equiv.) in various amide solutions including N,N-dimethylformamide (DMF),
N,N-diethylformamide (DEF), N,N-diisopropylformamide, N,N-di-n-butylformamide, piperidine-1-
◦
carbaldehyde, or pyrrolidine-1-carbaldehyde (2 mL) at 50–60 C for 1.0–2.0 h. When the reaction
was completed (as monitored by TLC), an amine such as hexamethyldisilazane (NH(SiMe3)2),
hexamethylenetetramine, lithium bis(trimethylsilyl)amine (LiN(SiMe3)2), or sodium bis(trimethylsilyl)
amine (NaN(SiMe3)2) was added into the reaction mixture which was stirred at reflux for 3–5 h. When
the intermolecular heterocyclization was complete, the resulting mixture was added to saturated
sodium bicarbonate (15 mL) and extracted with dichloromethane (15 mL
were dried over MgSO4, filtered, and concentrated under reduced pressure. The residue was purified
×
2). The organic extracts
by column chromatography on silica gel to give the corresponding pyrazolo[3,4-d]pyrimidines 3a–n in
69–91% yields.
1,3-Diphenyl-1H-pyrazolo[3,4-d]pyrimidine (3a) [13
(hexane–EtOAc). 1H-NMR (CDCl3, 400 MHz):
7.53–7.57 (4H, m, ArH), 8.06 (2H, d, J = 8.00 Hz, ArH), 8.31 (2H, d, J = 8.00 Hz, ArH), 9.12 (1H, s),
– ,
45]. Light-yellow solid; m.p. 158–159 ◦C
15
δ
7.34–7.38 (1H, m, ArH), 7.48–7.51 (1H, m, ArH),
9.51 (1H, s). 13C-NMR (CDCl3, 100 MHz,):
δ
114.24, 121.46 (2
×
C), 126.86, 127.39 (2
×
C), 129.24 (4
×
C),
129.64, 131.50, 138.50, 145.00, 152.82, 153.34, 155.61. IR (KBr): 1632, 1586, 1554, 1497, 1366, 1219 cm–1.
EIMS m/z: 272 (M+, 100), 273 (18), 271 (31), 142 (11), 77 (34), 69 (24), 51 (11).
1-(2-Methylphenyl)-3-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3b) [13
–
15]. Light-yellow solid; m.p.
1
140–141 ◦C (hexane–EtOAc). H-NMR (CDCl3, 400 MHz):
δ 2.48 (3H, s, CH3), 7.18 (1H, d, J = 7.60 Hz,
ArH), 7.42–7.45 (1H, m, ArH), 7.50 (1H, d, J = 8.00 Hz, ArH), 7.54–7.58 (2H, m, ArH), 8.05–8.10 (2H, m,
ArH), 9.12 (1H, s), 9.50 (1H, s). 13C-NMR (CDCl3, 100 MHz):
21.6 (CH3), 114.18, 118.77, 122.17, 127.42
(2 C), 127.77, 129.06, 129.23 (2 C), 129.61, 131.55, 138.38, 139.33, 144.92, 152.80, 153.32, 155.60.
IR (KBr): 1636, 1497, 1223, 1096 cm–1. EIMS m/z: 286 (M+, 100), 287 (20), 285 (19), 77 (10).
δ
×
×
1-(2-Chlorophenyl)-3-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3c) [13
15]. Yellow solid; m.p. 139–140 ◦C
(hexane–EtOAc). 1H-NMR (CDCl3, 400 MHz):
7.48–7.51 (3H, m, ArH), 7.53–7.57 (2H, m, ArH),
7.60–7.64 (2H, m, ArH), 8.05 (2H, d, J = 8.00 Hz, ArH), 9.08 (1H, s), 9.54 (1H, s). 13C-NMR (CDCl3,
–
δ
100 MHz):
δ
113.01, 125.54, 127.38 (2
×
C), 127.72, 128.38, 129.65 (2
×
C), 130.08, 130.82, 131.40, 132.19,
134.74, 145.70, 152.87, 154.50, 155.91. IR (KBr): 3012, 1636, 1582, 1497, 1362, 1223, 1084 cm–1. EIMS m/z:
306 (M+, 96), 308 (28), 307 (M+ + 1, 15), 272 (15), 271 (100), 195 (11), 77 (42), 75 (10), 51 (11).
◦
1-(3-Methylphenyl)-3-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3d) [13
(hexane–EtOAc). 1H-NMR (CDCl3, 400 MHz):
7.41–7.45 (1H, m, ArH), 7.49 (1H, d, J = 7.20 Hz, ArH), 7.53–7.57 (2H, m, ArH), 8.05–8.10 (4H, m, ArH),
9.16 (1H, s), 9.53 (1H, s). 13C-NMR (CDCl3, 100 MHz):
21.60, 114.15, 118.70, 122.11, 127.38 (2 C),
127.72, 129.02, 129.19 (2 C), 129.57, 131.52, 138.36, 139.28, 144.86, 152.75, 153.27, 155.55. IR (KBr):
1632, 1613, 1585, 1493, 1420, 1366, 1265 cm–1. EIMS m/z: 286 (M+, 100), 287 (22), 285 (21), 77 (9).
–
15]. Yellow solid; m.p. 80–81 C
δ
2.52 (3H, s, CH3), 7.18 (1H, d, J = 8.00 Hz, ArH),
δ
×
×
◦
1-(3-Chlorophenyl)-3-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3e) [13
(hexane–EtOAc). H-NMR (CDCl3, 400 MHz):
–
15]. White solid; m.p. 185–186 C
1
δ 7.31 (1H, d, J = 8.00 Hz, ArH), 7.44–7.50 (2H, m, ArH),
7.52–7.57 (2H, m, ArH), 8.32–8.42 (2H, m, ArH), 9.14 (1H, s), 9.49 (1H, s). 13C-NMR (CDCl3, 100 MHz):
δ
114.51, 118.90, 121.07, 126.61, 127.40 (2
×
C), 129.25 (2
×
C), 129.85, 130.21, 131.17, 134.95, 139.58,
145.62, 152.88, 153.61, 155.73. IR (KBr): 1585, 1555, 1489, 1404, 1366, 1312, 1215, 1088 cm–1. EIMS m/z:
306 (M+, 100), 308 (32), 307 (M+ + 1, 26), 305 (22), 77 (16).

Molecules 2017, 22, 820
8 of 12
◦
1-(3-Nitrophenyl)-3-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3f) [13
(hexane–EtOAc). H-NMR (CDCl3, 400 MHz):
–
15]. Yellow solid; m.p. 179–180 C
1
δ 7.54–7.61 (3H, m, ArH), 7.73 (1H, dd, J = 8.0, 16.4 Hz,
ArH), 8.08 (2H, d, J = 8.00 Hz, ArH), 8.19 (1H, d, J = 8.00 Hz, ArH), 8.87 (1H, d, J = 8.00 Hz, ArH),
9.20 (1H, s), 9.37 (1H, s), 9.55 (1H, s). 13C-NMR (CDCl3, 100 MHz):
114.74, 115.70, 120.87, 126.04,
127.51 (2 C), 129.36 (2 C), 130.16, 130.18, 130.88, 139.64, 146.08, 148.87, 153.14, 154.00, 156.05. IR
(KBr): 1636, 1528, 1489, 1346, 1003 cm–1. EIMS m/z: 317 (M+, 100), 318 (17), 77 (14).
δ
×
×
1-(4-Methylphenyl)-3-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3g) [13
15]. Brown solid; m.p. 133–134 ◦C
(hexane–EtOAc). 1H-NMR (CDCl3, 400 MHz):
2.39 (3H, s, CH3), 7.30 (2H, d, J = 8.00 Hz, ArH),
7.45 (1H, d, J = 8.00 Hz, ArH), 7.50 (2H, dd, J = 7.2, 14.8 Hz, ArH), 8.00 (2H, d, J = 8.00 Hz, ArH),
8.11 (2H, d, J = 8.00 Hz, ArH), 9.07 (1H, s), 9.43 (1H, s). 13C-NMR (CDCl3, 100 MHz):
20.98, 113.92,
C), 131.43, 135.94, 136.56, 144.47,
–
δ
δ
121.21 (2
×
C), 127.18 (2
×
C), 128.76 (2
×
C), 129.37, 129.61 (2
×
152.58, 152.95, 155.33. IR (KBr): 1636, 1589, 1516, 1386, 1219, 1088 cm–1. EIMS m/z: 286 (M+, 100),
287 (22), 285 (28), 77 (10). HRMS Calcd. for C18H14N4: 286.1218; Found: 286.1216.
1-(4-Chlorophenyl)-3-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3h) [13
(hexane–EtOAc). 1H-NMR (CDCl3, 400 MHz):
7.48–7.51 (3H, m, ArH), 7.53–7.57 (2H, m, ArH),
8.03 (2H, d, J = 8.00 Hz, ArH), 8.32 (2H, d, J = 8.00 Hz, ArH), 9.11 (1H, s), 9.49 (1H, s). 13C-NMR
–
15]. Yellow solid; m.p. 147–148 ◦C
δ
(CDCl3, 100 MHz):
δ
114.32, 122.24 (2
×
C), 127.37 (2
×
C), 129.23 (2
×
C), 129.28 (2
×
C), 129.78, 131.24,
132.11, 137.13, 145.23, 152.89, 153.36, 155.66. IR (KBr): 1632, 1555, 1497, 1215, 1054 cm–1. EIMS m/z: 306
(M+, 100), 308 (31), 307 (M+ + 1, 23), 305 (17), 77 (14).
◦
1-(4-Bromophenyl)-3-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3i) [13
(hexane–EtOAc). H-NMR (CDCl3, 400 MHz):
–
15]. Yellow solid; m.p. 180–181 C
1
δ 7.51–7.59 (3H, m, ArH), 7.66 (2H, d, J = 8.40 Hz, ArH),
8.05 (2H, d, J = 8.00 Hz, ArH), 8.29 (2H, d, J = 8.00 Hz, ArH), 9.13 (1H, s), 9.51 (1H, s). 13C-NMR (CDCl3,
100 MHz):
δ
114.37, 119.99, 122.53 (2
×
C), 127.37 (2
×
C), 129.23 (2
×
C), 129.79, 131.22, 132.24 (2
×
C),
137.63, 145.29, 152.88, 153.40, 155.67. IR (KBr): 1586, 1555. 1481, 1400, 1389, 1215, 1072 cm–1. EIMS m/z:
350 (M+, 100), 352 (M+ + 2, 99), 353 (15), 351 (27), 194 (14), 77 (30).
◦
3-Methyl-1-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3j) [13
–
15
,
46]. Brown solid; m.p. 77–78 C (hexane–EtOAc).
1H-NMR (CDCl3, 400 MHz): δ 2.70 (3H, s, CH3), 7.32 (1H, dd, J = 7.20, 14.80 Hz, ArH), 7.50–7.79 (2H,
dd, J = 7.60, 15.60 Hz, ArH), 8.19 (2H, d, J = 8.00 Hz, ArH), 9.07 (1H, s), 9.16 (1H, s). 13C-NMR (CDCl3,
100 MHz):
δ
12.59, 115.79, 121.07 (2
×
C), 126.49, 129.22 (2
×
C), 138.50, 143.35, 151.77, 152.77, 155.70.
IR (KBr): 3240, 1643, 1503, 1439, 1211 cm–1. EIMS m/z: 210 (M+, 100), 211 (16), 209 (27), 195 (13), 142
(15), 77 (37), 69 (11), 57 (16), 55 (13), 51 (13).
◦
3-tert-Butyl-1-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3k) [13
1H-NMR (CDCl3, 400 MHz):
ArH), 8.22 (2H, d, J = 8.00 Hz, ArH), 9.04 (1H, s), 9.32 (1H, s). 13C-NMR (CDCl3, 100 MHz):
–
15]. Yellow solid; m.p. 45–46 C (hexane–EtOAc).
δ
1.57 (9H, s, t-Bu), 7.28–7.32 (1H, m, ArH), 7.51 (2H, dd, J = 7.60, 15.60 Hz,
30.05
δ
(3
×
C), 34.51, 114.02, 121.17 (2
×
C), 126.33, 129.13 (2
×
C), 138.66, 152.84, 153.16, 154.88, 155.04. IR
(KBr): 3048, 2967, 2666, 1636, 1578, 1508, 1427, 1366, 1188, 1096 cm–1. EIMS m/z: 252 (M+, 43), 238 (18),
237 (100), 222 (12), 105(11), 77(17), 57(11).
◦
3-(4-Methylphenyl)-1-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3l) [13
(hexane–EtOAc). 1H-NMR (CDCl3, 400 MHz):
7.55 (2H, dd, J = 8.00, 16.00 Hz, ArH), 7.95 (2H, d, J = 8.00 Hz, ArH), 8.30 (2H, d, J = 8.00 Hz, ArH),
9.12 (1H, s), 9.49 (1H, s). 13C-NMR (CDCl3, 100 MHz):
21.39, 114.24, 121.41 (2 C), 126.74, 127.23
(2 C), 128.64, 129.18 (2 C), 129.87 (2 C), 138.51, 139.77, 145.06, 152.79, 153.27, 155.52. IR (KBr):
3117, 1582, 1501, 1223, 1092 cm–1. EIMS m/z: 286 (M+, 100), 287 (21), 285 (26).
–15]. White solid; m.p. 138–139 C
δ
2.44 (3H, s, CH3), 7.36 (3H, d, J = 6.80 Hz, ArH),
δ
×
×
×
×
3-(4-Chlorophenyl)-1-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3m) [13
–
15
,
45]. Light-yellow solid; m.p.
1
194–193 ◦C (hexane–EtOAc). H-NMR (CDCl3, 400 MHz):
δ 7.37 (1H, dd, J = 7.60, 15.20 Hz, ArH),
7.51–7.57 (m, 4 H, ArH), 8.00 (2H, d, J = 8.00 Hz, ArH), 8.28 (2H, d, J = 8.00 Hz, ArH), 9.13 (1H, s),
9.47 (1H, s). 13C-NMR (CDCl3, 100 MHz):
δ
114.03, 121.47 (2 C), 127.01, 128.51 (2 C), 129.27
×
×

Molecules 2017, 22, 820
9 of 12
(2
×
C), 129.48 (2
×
C), 129.98, 135.71, 138.43, 143.85, 152.60, 153.37, 155.69. IR (KBr): 1632, 1555, 1504,
1404, 1219, 1092 cm–1. EIMS m/z: 306 (M+, 100), 308 (33), 307 (M+ + 1, 26), 305 (22), 77 (10).
3-(4-Methoxylphenyl)-1-phenyl-1H-pyrazolo[3,4-d]pyrimidine (3n) [13
–
15]. Light-yellow solid; m.p.
1
169–170 ◦C (hexane–EtOAc). H-NMR (CDCl3, 400 MHz):
δ 3.86 (3H, s, OCH3), 7.04 (2H, d, J = Hz,
ArH), 7.33 (1H, dd, J = 7.60, 14.80 Hz, ArH), 7.52 (2H, dd, J = 7.60, 15.60 Hz, ArH), 7.96 (2H, d,
J = 8.00 Hz, ArH), 8.28 (2H, d, J = 8.00 Hz, ArH), 9.08 (1H, s), 9.43 (1H, s). 13C-NMR (CDCl3, 100 MHz):
δ
55.36 (OCH3), 114.13, 114.57 (2
×
C), 121.26 (2
×
C), 124.01, 126.61, 128.61 (2
×
C), 129.14 (2
×
C),
138.51, 144.74, 152.66, 153.18, 155.43, 160.72. IR (KBr): 3059, 1632, 1613, 1528, 1501, 1431, 1362, 1300,
1258, 1219, 1173, 1092 cm–1. EIMS m/z: 302 (M+, 100), 303 (22), 287 (23), 77 (15).
4. Conclusions
We have successfully developed the one-flask method to synthesize pyrazolo[3,4-d]pyrimidines
by treating 5-amino-pyrazoles, in presence of PBr3 coupling agent and then hexamethyldisilazane.
In this new one-flask reaction was contained Vilsmeier reaction and the sequential intermolecular
heterocyclization two steps. Based on the improved studies of the different type of Vilsmeier agents
and amines, we found the commercial available DMF/PBr3 and hexamethyldisilazane were the
best Vilsmeier agent and the efficient base for this newly developed one-flask synthesis. For the
mechanistic study, 4-(iminomethyl)-1,3-diphenyl-1H-pyrazol-5-yl-N,N-disubstituted formamidines
were demonstrated as the reaction intermediates by using a series of 4-formyl-1,3-diphenyl-1
H-pyrazol-5-yl-N,N-disubstituted formamidines successfully reacted with amines to give pyrazolo
[3,4-d]pyrimidines due to they were conceived as the chemical equivalent species. On the other hands,
the order of reactivity of amines in intermolecular heterocyclization was NH(SiMe3)2 > NaN(SiMe3)2
> LiN(SiMe3)2 > hexamethylenetetramine. Through the further comparison variation reactive study
between intramolecular and intermolecular Vilsmeier heterocyclization reaction, we found the
intramolecular heterocyclization be able to provide the better results.
Supplementary Materials: Supplementary materials are available online.
Acknowledgments: We are grateful to the Tsuzuki Institute for Traditional Medicine and the Ministry of Science
and Technology of the Republic of China (MOST 105-2113-M-039-001) for financial support.
Author Contributions: Fung Fuh Wong conceived and designed the experiments; Wan-Ping Yen and Shuo-En
Tsai performed the experiments; Naoto Uramaru, Hiroyuki Takayama, Shuo-En Tsai, and Wan-Ping Yen analyzed
the data; Fung Fuh Wong, Hiroyuki Takayama, and Naoto Uramaru contributed reagents/materials/analysis
tools; Fung Fuh Wong, Wan-Ping Yen, and Shuo-En Tsai wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
Ugi, I.; Dömling, A.; Werner, B. Since 1995 the new chemistry of multicomponent reactions and their libraries,
including their heterocyclic chemistry. J. Heterocycl. Chem. 2000, 37, 647–658. [CrossRef]
Posner, G.H. Multicomponent one-pot annulations forming 3 to 6 bonds. Chem. Rev. 1986, 86, 831–844.
[CrossRef]
Weber, L.; Illgen, K.; Almstetter, M. Discovery of new multi component reactions with combinatorial methods.
Synlett 1999, 366–374. [CrossRef]
Fogg, D.E.; dos Santos, E.N. Tandem catalysis: A taxonomy and illustrative review. Coord. Chem. Rev. 2004
248, 2365–2379. [CrossRef]
,
Shindoh, N.; Takemoto, Y.; Takasu, K. Auto-Tandem Catalysis: A Single Catalyst Activating Mechanistically
Distinct Reactions in a Single Reactor. Chem. Eur. J. 2009, 15, 12168–12179. [CrossRef] [PubMed]
Yin, G.D.; Zhou, B.H.; Meng, X.G.; Wu, A.X.; Pan, Y.J. Efficient C C Double-Bond Formation Reaction
−
via a New Synthetic Strategy: A Self-Sorting Tandem Reaction. Org. Lett. 2006, 8, 2245–2248. [CrossRef]
[PubMed]

Molecules 2017, 22, 820
10 of 12
7.
8.
Gao, M.; Yang, Y.; Wu, Y.D.; Cong, C.; Shu, W.M.; Zhang, D.X.; Cao, L.P.; She, N.F.; Wu, A.X. An efficient
synthesis of hydantoins via sustainable integration of coupled domino processes. Org. Lett. 2010, 12,
4026–4029. [CrossRef] [PubMed]
Zhu, Y.P.; Gao, Q.H.; Lian, M.; Yuan, J.J.; Liu, M.C.; Zhao, Q.; Yang, Y.; Wu, A.X. A sustainable byproduct
catalyzed domino strategy: Facile synthesis of a-formyloxy and acetoxy ketones via iodination/nucleophilic
substitution/hydrolyzation/oxidation sequences. Chem. Commun. 2011, 47, 12700–12702. [CrossRef]
[PubMed]
9.
Allenn, A.E.; MacMillan, D.W.C. Synergistic catalysis: A powerful synthetic strategy for new reaction
development. Chem. Sci. 2012, 3, 633–658. [CrossRef] [PubMed]
10. Harschneck, T.; Kirsch, S.F. One-pot synthesis of 1, 2-Dihydropyridines: Expanding the diverse reactivity of
propargyl vinyl ethers. J. Org. Chem. 2011, 76, 2145–2156. [CrossRef] [PubMed]
11. Zhu, Y.P.; Jia, F.C.; Liu, M.C.; Wu, A.X. A multipathway coupled domino strategy: Metal-free oxidative
cyclization for one-pot synthesis of 2-acylbenzothiazoles from multiform substrates. Org. Lett. 2012, 14,
4414–4417. [CrossRef] [PubMed]
12. Guilarte, V.; Fernández-Rodríguez, M.A.; Garcría-Garcría, P.; Hernando, E.; Sanz, R. A Practical,
One-Pot Synthesis of Highly Substituted Thiophenes and Benzo[b]thiophenes from Bromoenynes and
o-Alkynylbromobenzenes. Org. Lett. 2011, 13, 5100–5103. [CrossRef] [PubMed]
13. Chang, C.H.; Tsai, H.J.; Huang, Y.Y.; Lin, H.Y.; Wang, L.Y.; Wu, T.S.; Wong, F.F. Selective synthesis of
pyrazolo[3,4-d]pyrimidine, N-(1H-pyrazol-5-yl)formamide, or N-(1H-pyrazol-5-yl)formamidine derivatives
from N-1-substituted-5-aminopyrazoles with new Vilsmeier-type reagents. Tetrahedron 2013, 69, 1378–1386.
[CrossRef]
14. Huang, Y.Y.; Wang, L.Y.; Chang, C.H.; Kuo, Y.H.; Kaneko, K.; Takayama, H.; Kimura, M.; Juang, S.H.;
Wong, F.F. One-pot synthesis and antiproliferative evaluation of pyrazolo[3,4-d]pyrimidine derivatives.
Tetrahedron 2012, 68, 9658–9664. [CrossRef]
15. Ismail, N.S.M.; Ali, E.M.H.; Ibrahim, D.A.; Serya, R.A.T.; El Ella, D.A.A. Pyrazolo[3,4-d]pyrimidine based
scaffold derivatives targeting kinases as anticancer agents. Future J. Pharm. Sci. 2016, 2, 20–30. [CrossRef]
16. Peter Bonn, P.; Mikael Brink, D.; Fagerhag, J.; Jurva, U.; Robb, G.R.; Schnecke, V.; Henriksson, A.S.;
Waring, M.J.; Westerlund, C. The discovery of a novel series of glucokinase activators based on
a pyrazolopyrimidine scaffold. Bioorg. Med. Chem. Lett. 2012, 22, 7302–7305. [CrossRef] [PubMed]
17. Bakavoli, M.; Bagherzadeh, G.; Vaseghifar, M.; Shiri, A.; Mehdi Pordel, M.; Mashreghi, M.; Pordeli, P.;
Araghi, M. Molecular iodine promoted synthesis of new pyrazolo[3,4-d]pyrimidine derivatives as potential
antibacterial agents. Eur. J. Med. Chem. 2010, 45, 647–650. [CrossRef] [PubMed]
18. Holla, B.S.; Mahalinga, M.; Karthikeyan, M.S.; Akberali, P.M.; Shetty, N.S. Synthesis of some novel
pyrazole[3,4-d]pyrimidine derivatives as potential antimicrobial agents. Bioorg. Med. Chem. 2006, 14,
2040–2047. [CrossRef] [PubMed]
19. Shamroukh, A.H.; Rashad, A.E.; Sayed, H.H. Synthesis of Some Pyrazolo[3,4-d]pyrimidine Derivatives for
Biological Evaluation. Phosphorous Sulfur Silicon Relat. Elem. 2005, 180, 2347–2360. [CrossRef]
20. Rigueiroa, F.; Teixeiraa, S.; Salaheldinb, A.M.; Oliveira-Camposb, A.M.F.; Rodriguesb, L.M.; Peixotoc, F.;
Oliveira, M.M. Evaluation of antioxidant properties of some pyrazolo[3,4-d]pyrimidines derivatives and
their effects on mitochondria bioenergetics. Biochim. Biophys. Acta 2008, 1777, S10.17. [CrossRef]
21. Kumar, A.; Ahmad, I.; Chhikara, B.S.; Tiwari, R.; Mandal, D.; Parang, K. Synthesis of
3-phenylpyrazolopyrimidine-1,2,3-triazole conjugates and evaluation of their Src kinase inhibitory and
anticancer activities. Bioorg. Med. Chem. Lett. 2011, 21, 1342–1346. [CrossRef] [PubMed]
22. Ghorab, M.M.; Ragab, F.A.; Alqasoumi, S.I.; Alafeefy, A.M.; Aboulmag, S.A. Synthesis of some new
pyrazolo[3,4-d]pyrimidine derivatives of expected anticancer and radioprotective activity. Eur. J. Med. Chem.
2010, 45, 171–178. [CrossRef] [PubMed]
23. Schenone, S.; Bruno, O.; Bondavalli, F.; Ranise, A.; Mosti, L.; Giulia Menozzi, G.; Paola Fossa, P.; Donnini, S.;
Santoro, A.; Ziche, M.; et al. Antiproliferative activity of new 1-aryl-4-amino-1H-pyrazolo[3,4-d]pyrimidine
derivatives toward the human epidermoid carcinoma A431 cell line. Eur. J. Med. Chem. 2004, 39, 939–946.
[CrossRef] [PubMed]
24. EI-Bendary, E.R.; Badria, F.A. Synthesis, DNA-binding, and Antiviral Activity of Certain Pyrazolo[3,4-d]
pyrimidine Derivatives. Arch. Pharm. 2000, 333, 99–103. [CrossRef]

Molecules 2017, 22, 820
11 of 12
25. Liu, H.; Wang, H.Q.; Liu, Z.J. Synthesis and herbicidal activity of novel pyrazolo[3,4-d]pyrimidin-4-one
derivatives containing aryloxyphenoxypropionate moieties. Bioorg. Med. Chem. Lett. 2007, 17, 2203–2209.
[CrossRef] [PubMed]
26. Chern, J.H.; Shia, K.S.; Hsu, T.A.; Tai, C.L.; Lee, C.C.; Lee, Y.C.; Chang, C.S.; Tseng, S.N.; Shih, S.R.
Design, synthesis, and structure–activity relationships of pyrazolo [3,4-d] pyrimidines: A novel class of
potent enterovirus inhibitors. Bioorg. Med. Chem. Lett. 2004, 14, 2519–2525. [CrossRef] [PubMed]
27. Rashad, A.E.; Hegab, M.I.; Abdel-Megeid, R.E.; Fathalla, N.; Abdel-Megeid, F.M.E. Synthesis and anti-HSV-1
evaluation of some pyrazoles and fused pyrazolopyrimidines. Eur. J. Med. Chem. 2009, 44, 3285–3291.
[CrossRef] [PubMed]
28. Rahmoui, A.; Souiei, S.; Belkacem, M.A.; Romdhane, A.; Bouajila, J.; Jannet, H.B. Synthesis and
biological evaluation of novel pyrazolopyrimidines derivatives as anticancer and anti-5-lipoxygenase agents.
Bioorg. Chem. 2016, 66, 160–168. [CrossRef] [PubMed]
29. El-Kalyoubi, S.; Agili, F. A Novel Synthesis of Fused Uracils: Indenopyrimidopyridazines, Pyrimidopyridazines,
and Pyrazolopyrimidines for Antimicrobial and Antitumor Evalution. Molecules 2016, 21, 1714. [CrossRef]
[PubMed]
30. Quintela, J.M.; Peinador, C.; Gonzalez, L.; Devesa, I.; Ferrandiz, M.L.; Alcaraz, M.J.; Riguera, R.
6-Dimethylamino 1H-pyrazolo[3,4-d]pyrimidine derivatives as new inhibitors of inflammatory mediators in
intact cell. Bioorg. Med. Chem. 2003, 11, 863–868. [CrossRef]
31. Abdelazeem, A.H.; Abdelatef, S.A.; El-Saadi, M.T.; Omar, H.A.; Khan, S.I.; McCurdy, C.R.; El-Moghazy, S.M.
Novel pyrazolopyrimidine derivatives targeting COXs and iNOS enzymes; design, synthesis and biological
evaluation as potential anti-inflammatory agents. Eur. J. Pharm. Sci. 2014, 62, 197–221. [CrossRef] [PubMed]
32. Avasthi, K.; Farooq, S.M.; Raghunandan, R.; Ma, P.R. Design and synthesis of pyrazolo[3,4-d]pyrimidine
1
core based dissymmetrical ‘Leonard linker’ compounds: H-NMR and crystallographic evidence for folded
conformation due to arene interaction. J. Mol. Struct. 2006, 785, 106–113. [CrossRef]
33. Venkatesan, G.; Paira, P.; Cheong, L.C.; Vamsikrishna, K.; Federico, S.; Klotz, K.N.; Spalluto, G.;
Pastorin, G. Discovery of simplified N2-substituted pyrazolo[3,4-d]pyrimidine derivatives as novel adenosine
receptor antagonists: Efficient synthetic approaches, biological evaluations and molecular docking studies.
Bioorg. Med. Chem. 2014, 22, 1751–1765. [CrossRef] [PubMed]
34. Molina, P.; Arques, A.; Vinader, M.V. Tandem aza-wittig reaction/electrocyclic ring-closure a facile entry
to the synthesis of fused pyrimidines: Preparation of pyrazolo[3,4-d] and 1,2,3-triazolo[4,5-d]pyrimidine
derivatives. Tetrahedron Lett. 1987, 28, 4451–4454. [CrossRef]
35. Sureja, D.K.; Dholakia, S.P.; Vadalia, K.R. Synthesis of some novel pyrazolo[3,4-d] pyrimidin-4(5H)-one
derivatives as potential antimicrobial agents. J. Pharm. Bioallied. Sci. 2016, 8, 321–326. [CrossRef] [PubMed]
36. Slavish, P.J.; Price, J.E.; Hanumesh, P.; Webb, T.R. Efficient synthesis of pyrazolopyrimidine libraries. J. Comb.
Chem. 2010, 12, 807–809. [CrossRef] [PubMed]
37. Gilbert, A.M.; Caltabiano, S.; Koehn, F.E.; Chen, Z.J.; Francisco, G.D.; Ellingboe, J.W.; Kharode, Y.;
Mangine, A.; Francis, R.; TrailSmith, M.; et al. Pyrazolopyrimidine-2,4-dione sulfonamides: Novel and
selective calcitonin inducers. J. Med. Chem. 2002, 45, 2342–2345. [CrossRef] [PubMed]
38. Wen, K.S.; Lin, H.Y.; Huang, Y.Y.; Kaneko, K.; Takayama, H.; Kimura, M.; Juang, S.H.;
Wong, F.F. Chemoselective synthesis, antiproliferative activities, and SAR study of 1H-pyrazol-5-yl-N,
N-dimethyl-formamidines and pyrazolyl-2-azadienes. Med. Chem. Res. 2012, 21, 3920–3928. [CrossRef]
39. Cheng, K.M.; Huang, Y.Y.; Huang, J.J.; Kaneko, K.; Kimura, M.; Takayama, H.; Juang, S.H.;
Wong, F.F. Synthesis and antiproliferative evaluation of N,N-disubstituted-N0-[1-aryl-1H-pyrazol-5-yl]-
methnimidamides. Bioorg. Med. Chem. Lett. 2010, 20, 6781–6784. [CrossRef] [PubMed]
40. Srivastava, V.; Negi, A.S.; Kumar, J.K.; Gupta, M.M. A simple, convenient and chemoselective formylation of
sterols by Vilsmeier reagent. Steroids 2006, 71, 632–638. [CrossRef] [PubMed]
41. El-Shishtawy, R.M.; Almeida, P. A new Vilsmeier-type reaction for one-pot synthesis of pH sensitive
fluorescent cyanine dyes. Tetrahedron 2006, 62, 7793–7798. [CrossRef]
42. Pan, W.; Dong, D.; Wang, K.; Zhang, J.; Wu, R.; Xiang, D.; Liu, Q. Efficient One-Pot Synthesis of Highly
Substituted Pyridin-2(1H)-ones via the Vilsmeier
Org. Lett. 2007, 9, 2421. [CrossRef] [PubMed]
−
Haack Reaction of 1-Acetyl,1-Carbamoyl Cyclopropanes.
43. Huang, Y.Y.; Kaneko, K.; Takayama, H.; Kimura, M.; Wong, F.F. New investigation of Vilsmeier-type reaction
using pyrazolones with various amides. Tetrahedron Lett. 2011, 52, 3786–3792. [CrossRef]

Molecules 2017, 22, 820
12 of 12
44. Wong, F.F.; Huang, Y.Y. Novel Vilsmeier-type methylenation for synthesis of dipyrazolylmethane derivatives
using formamide or N-methylformamide. Tetrahedron 2011, 67, 3863–3867. [CrossRef]
45. Simay, A.; Takacs, K.; Horvath, K.; Dvortsak, P. Vilsmeier-Haack reaction of 5-amino- and 5-acylaminopyrazoles.
Acta Chim. Acad. Sci. Hung. 1980, 105, 127–139.
46. Jachak, M.N.; Avhale, A.B.; Medhane, V.J.; Toche, R.B. A convenient route for the synthesis of pyrazolo
[3,4-d]pyrimidine, pyrazolo[3,4-b][1,6]naphthyridine and pyrazolo[3,4-b]quinoline derivatives. J. Heterocycl.
Chem. 2006, 43, 1169–1175. [CrossRef]
Sample Availability: Samples of the compounds are available from the authors.
©
2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).