TiO2–SiO2 Catalyzed Eco-friendly Synthesis and Antioxidant Activity of Benzopyrano[2,3-d]pyrimidine Derivatives

Article abstract of DOI:10.1002/jhet.2856

A cost-effective and eco-friendly synthesis of benzopyrano[2,3-d]pyrimidine derivatives has been developed via three component one-pot tandem approach by condensing different salicylaldehydes and secondary amines with malononitrile in the presence of TiO₂–SiO₂ catalyst at 80°C under solvent-free conditions. Mild experimental conditions, reusability of the catalyst, and cost effectiveness are the merits of this procedure. Compounds 4g, 4h, and 4i bearing 2-OMe group on the hydroxyphenyl group linked to the central carbon present in between the two nitrogen atoms of the pyrimidine ring were found to exhibit good antioxidant activity while other compounds have moderate antioxidant activity.

Full text of DOI:10.1002/jhet.2856

Month 2017
TiO2–SiO2 Catalyzed Eco-friendly Synthesis and Antioxidant Activity of
Benzopyrano[2,3-d]pyrimidine Derivatives
Shaik Ahammed Kabeer, Gajjala Raghavendra Reddy, Poola Sreelakshmi, Darla Mark Manidhar, and
*
Cirandur Suresh Reddy
Department of Chemistry, Sri Venkateswara University, Tirupati 517 502, India
*
E-mail: csrsvu@gmail.com
Received July 11, 2016
DOI 10.1002/jhet.2856
Published online 00 Month 2017 in Wiley Online Library (wileyonlinelibrary.com).
A cost-effective and eco-friendly synthesis of benzopyrano[2,3-d]pyrimidine derivatives has been devel-
oped via three component one-pot tandem approach by condensing different salicylaldehydes and secondary
amines with malononitrile in the presence of TiO2–SiO2 catalyst at 80°C under solvent-free conditions. Mild
experimental conditions, reusability of the catalyst, and cost effectiveness are the merits of this procedure.
Compounds 4g, 4h, and 4i bearing 2-OMe group on the hydroxyphenyl group linked to the central carbon
present in between the two nitrogen atoms of the pyrimidine ring were found to exhibit good antioxidant ac-
tivity while other compounds have moderate antioxidant activity.
J. Heterocyclic Chem., 00, 00 (2017).
INTRODUCTION
benzopyrano[2,3-d]pyrimidines were tested for cytotoxic
activity against a panel of cancer cell lines, and a number
of them showed to cause significant perturbation in cell
cycle kinetics [9]. In view of their pharmacological
importance, few synthetic approaches have been developed
for them using different catalysts such as LiClO4 [10],
Na2MoO4·2H2O [9], [Bmim]BF4 [11], Fe3O4 and SBA-15
[12], Fe(II)–benzoylthiourea complex bound silica
nanoparticles [Fe(II)–BTU–SNPs] [13], Brønsted acidic
ionic liquids [14], and dibutylamine (DBA) [15]. However,
these synthetic methods suffer from one or more
drawbacks, such as long reaction times, use of expensive
organic solvents, and homogeneous catalyst. Therefore,
The multicomponent reactions (MCRs) are highly
important and popular for organic chemical synthetic
research due to their wide range of advantages such as
increased selectivity, low energy consumption, and
minimum wastage [1]. These reactions are also important
in pharmaceutical chemistry for rapid generation of several
components with structural diversity which are useful for
drug discovery [2]. The new MCRs with green synthetic
methods in organic chemistry are evoking increased
interest due to their eco-friendly nature [3]. Totally, the
current organic chemistry research field is well renowned
with the application of MCRs, and they serve as a potential
method for the synthesis of new molecules with atom
economy and time economy [4]. The access of MCRs to
design and synthesize the bio-active compounds with
diversity orientation has strongly attracted the synthetic
community in both industry and academia [5]. Its
application for the synthesis of new heterocycles [6]
specifically for the pyrimidine derivatives [7] has been
turned as an innovative method during the last decade.
Benzopyrano[2,3-d]pyrimidines are attracting much
attention due to their wide range of pharmacological and
biological activities such as antifungal, antiinflammatory,
analgesic, and antibacterial activities [8]. Several
there arises
a need to develop more convenient,
environmental friendly, and practically feasible method for
the preparation of benzopyrano[4,3-d]pyrimidines.
In recent years, heterogeneous catalysts have gained much
importance because of their ease of handling, high catalytic
activities, reusability, and environmental benefits [16].
Among them, titania–silica (TiO2–SiO2) is found to be an
efficient heterogeneous catalyst in synthetic chemistry. It
received considerable attention because of its nontoxicity,
cost effectiveness, recyclability, ease of handling,
experimental simplicity, and commercial availability of
required chemicals at low cost and remarkable ability to
suppress side reactions with acid sensitive substrates [17].
© 2017 Wiley Periodicals, Inc.

S. A. Kabeer, G. R. Reddy, P. Sreelakshmi, D. M. Manidhar, and C. S. Reddy
Vol 000
These factors encouraged us to study the synthesis of
benzopyrano[4,3-d]pyrimidines using TiO2–SiO2.
conditions. The reaction was slow at room temperature and
required a long reaction time (Entry 16, Table 1) for its
sustenance, but it was completed in 15 min at 80°C
temperature (Entry 14, Table 1). We investigated the same
reaction using different catalysts at 80°C, under solvent-free
conditions, and the results were listed in Table 1. We found
that TiO2–SiO2 afforded maximum product yields. In the
catalyst optimization studies with 2, 5, and 10 mol% of
TiO2–SiO2, we obtained the yields 73, 92, and 94,
respectively (Entries 13–15, Table 1). Therefore, 5 mol% of
TiO2–SiO2 was sufficient for completion of the reaction,
and excess amount of catalyst did not increase the yields
significantly (Entry 14, Table 1).
Reactive oxygen species (ROS) such as superoxide,
hydroxyl, and nitric oxide (NO) are generated in the
human body and cause damage to lipids, proteins, and
DNA leading to carcinogenesis, drug-associated toxicity,
and inflammation. Furthermore, ROS play a significant
role in the inhibition of life-limiting chronic metabolic
disorders in the living organisms [18]. Antioxidants are
molecules, natural or synthetic, capable of interacting
with free radicals and inhibiting their chain reactions
ultimately before the damaging of essential vital
molecules in vivo [19]. Thus, we have studied the
antioxidant properties of the title compounds having
phenolic groups on them, which easily generates the free
radicals and arrests the damage of the vital molecules.
As part of our ongoing research program, we developed
some environment benign methodologies for the synthesis
of new organic molecules by using silica supported TiO2 as
a catalyst and successfully extended its application again
for the synthesis of benzopyranopyrimidine derivatives
[20–24]. Finally, to the best of our knowledge, this is the
first report on the synthesis of these compounds under
solvent-free conditions.
Table 1
Influence of various catalysts on the synthesis of compound 4a at
80°C.
Entry
Catalyst
None
p-TSA
CSA
ZnCl2
NiCl2·6H2O
AlCl3
FeCl3
CuBr2
Catalyst (mol%)
Time
Yield (%)a
1
2
3
4
5
6
7
8
—
5
5
5
5
5
5
5
5
5
5
5
2
5
10
5
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
12 h
35
48
60
58
49
29
72
55
60
72
68
55
73
9
Cu(OTf)2
InCl3
RESULTS AND DISCUSSION
10
11
12
13
b14
15
c16
BF3–SiO2
FeCl3–SiO2
TiO2–SiO2
TiO2–SiO2
TiO2–SiO2
TiO2–SiO2
Synthesis. We report a novel synthesis of benzopyrano
[4,3-d]pyrimidine derivatives (4a–l) via pseudofour
component condensation of salicylaldehydes, malononitrile,
and secondary amines catalyzed by 5% TiO2–SiO2 at 80°C,
under solvent-free conditions (Scheme 1).
As a model reaction, we attempted the condensation of
salicyladehyde, malononitrile, and piperdine in the presence
of 5 mol% of TiO2–SiO2 catalyst under solvent-free
92, 90, 85
94
69
aIsolated yields.
bYields when catalyst was reused three times.
cReaction was carried out at room temperature.
The bold emphasis is used for easy identification of the optimized catalyst
concentration (5mol%).
Scheme 1. Synthesis of benzopyrano[4,3-d]pyrimidine derivatives.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2017
TiO2–SiO2 Catalyzed Eco-friendly Synthesis and Antioxidant Activity of
Benzopyrano[2,3-d]pyrimidine Derivatives
Examination of the effect of various solvents such as
nitromethane, acetone, acetonitrile, 1,4-dioxane, ethanol,
toluene, DMF, and THF, in the presence of 5 mol% of
TiO2–SiO2 at 80°C revealed that presence of solvent retards
the reaction (8 h) and results in poor product yields. But the
solvent-free reaction was completed within 15 min with
92% yield (Entry 9, Table 2).
After optimizing the reaction conditions, we reacted
different salicylaldehydes (1a–d) with malononitrile (2)
and different secondary amines (3a–d), and obtained a
variety of benzopyrano[2,3-d]pyrimidines (4a–l). In all
the cases, with either electron-donating or electron-
withdrawing groups on salicylaldehydes, reaction proceeded
smoothly with malononitrile, and different secondary
amines in the presence of 5% TiO2–SiO2 at 80°C and
formed the corresponding benzopyrano[2,3-d]pyrimidines
(4a–l) in good to excellent yields.
recovered by simple filtration from the reaction mixture.
In our experiment, the successive yields were obtained in
92, 90, and 85% without loss of the effective catalyst
activity (Entry 14, Table 1).
The compounds reported in the literature were
characterized by comparing their melting points and
NMR, and presently synthesized new compounds were
1
characterized by comparing their melting points, H, 13C
NMR, and HRMS spectral studies.
A plausible mechanism is presented in the formation of
benzopyrano[4,3-d] pyrimidines catalyzed by TiO2–SiO2
(Scheme 2). The Knoevenagel condensation of
salicylaldehyde (1) and malononitrile (2) forms (a) and
then on dehydration forms (b), which on Pinner reaction
ultimately affords (c). The secondary amine (3) attacks
the cyano group of (c) to produce intermediate (d), which
reacts with another molecule of salicylaldehyde (1) to
form (e). A simple proton transfer in (e) affords the
benzopyrano[4,3-d]pyrimidine (4).
The TiO2–SiO2 catalyst was reused for three subsequent
runs as it is insoluble in dichloromethane and was easily
Antioxidant activity. The compounds 4a–l were tested
for in vitro antioxidant activity by NO, 2,2-diphenyl-1-
picrylhydrazyl (DPPH), ferric reducing-antioxidant power
(FRAP), and H2O2 methods. The observed data on the
antioxidant activity are given in Table 3. Majority of the
compounds showed good activities for NO scavenging,
DPPH reduction, and H2O2 inhibition as shown in
Table 3. Among these the compounds, 4g, 4h, and 4i
Table 2
Effect of various solvents on the synthesis of compound 4a.
Entry
Solvent (2 mL)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
9
CH3NO2
CH3COCH3
CH3CN
1,4-Dioxane
CH3CH2OH
Toluene
DMF
THF
Solvent free
8
8
8
8
8
8
8
8
25
30
73
67
80
59
62
73
92
exhibited these properties to much extent.
Nitric oxide free radical scavenging activity. The NO free
radical scavenging activity of 4a–l reveals that the
compounds 4h, 4g, 4i, and 4c showed the highest NO
scavenging activity which is comparable to the reference
standard. The remaining compounds exhibited moderate
15 (min)
aIsolated yields.
Scheme 2. Plausible reaction mechanism for the formation of benzopyranopyrimidines.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

S. A. Kabeer, G. R. Reddy, P. Sreelakshmi, D. M. Manidhar, and C. S. Reddy
Vol 000
Table 3
Antioxidant activity studies of 4a–l with various methods.
,
Percentage of inhibition (IC50 values)a b
S. no.
Nitric oxide
DPPH
H2O2
FRAP
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
69.53 0.48
68.24 0.35
83.45 0.50
75.63 0.37
66.38 0.29
70.56 0.56
89.44 0.25
91.46 0.57
89.33 0.87
64.27 0.48
67.43 0.59
66.38 0.32
95.57 0.29
89.67 1.14
90.84 1.38
84.55 1.52
87.38 1.14
90.45 1.21
73.47 1.44
80.33 0.72
78.56 1.65
79.56 0.98
94.56 1.39
84.52 1.11
82.55 1.37
97.46 1.25
71.58 0.36
70.46 0.49
73.68 0.37
76.34 0.23
67.85 0.42
71.56 0.61
84.37 0.69
84.58 0.33
86.74 0.67
67.56 0.24
64.77 0.56
61.76 0.74
88.25 0.43
1.145 0.016
1.086 0.036
1.546 0.024
1.323 0.049
1.183 0.026
1.165 0.028
2.124 0.045
2.193 0.036
2.094 0.031
0.923 0.022
1.812 0.023
1.158 0.031
2.362 0.042
Ascorbic acid
aIC50 values are expressed as mean SD of three experiments.
bConcentration in 100 μg/mL.
scavenging activity. The results were expressed as a
percent of scavenged NO in Table 3.
2,2-Diphenyl-1-picrylhydrazyl free radical scavenging
for 13C NMR in DMSO, and chemical shifts were
referenced to TMS and reported in δ scale. Mass
spectra were recorded on a Jeol SX 102 DA/600 mass
spectrometer, and elemental analysis was performed on a
Thermo Finnigan Instrument.
assay.
Among the title compounds, 4a–l tested for
antioxidant activity by DPPH method, the compounds
4j, 4b, 4e, and 4a showed remarkably high antioxidant
activity, while the other compounds showed moderate
activity when compared with the standard ascorbic acid,
and the results were presented in Table 3.
Chemistry. General procedure for the preparation of
TiO2–SiO2 catalyst. TiO2–SiO2 catalyst was prepared by
adding titanium isopropoxide [Ti(O-i-C3H7)4]/isopropanol
solution (10 mmol) in dropwise manner to the solution
of tetraethylorthosilicate [Si(O―C2H5)4]/isopropanol
(20 mmol). The obtained mixture was vigorously stirred
for 1 h at 90°C under basic pH conditions, aged for a day,
and then the formed precipitate was dispersed by means
of ultrasonication over 30 min and then filtered and then
dried over muffle furnace at 500°C to get fine TiO2–SiO2
catalyst [25,26].
Hydrogen peroxide scavenging assay.
The scavenging
ability of hydrogen peroxide by compounds 4i, 4h, 4g,
and 4d was found to be comparable with the reference
compound ascorbic acid (Table 3). This may be attributed
to the phenolics, which can donate electrons to hydrogen
peroxide thereby neutralizing it to water. This scavenging
activity of hydrogen peroxide is concentration dependent.
Ferric reducing-antioxidant power (FRAP) assay. Ferric
General procedure for the synthesis of benzopyrano[4,3-d]
reducing-antioxidant power assay was done on the target
compounds 4a–l. Compounds 4h, 4g, 4i, and 4k showed
the highest antioxidant activity as determined by FRAP
assay (Table 3). Their capacity for reducing ferric ion
was comparable to that of ascorbic acid.
pyrimidines (4a–l).
To a mixture of salicylaldehyde 1
(2 mmol), malononitrile 2 (1 mmol), and secondary
amine 3 (1 mmol) was added TiO2–SiO2 (5 mol%). The
mixture was kept stirred at 80°C for the required time
(Table 3). The progress of the reactions was monitored
by thin layer chromatography (TLC) on 250-μm silica
plates using 8:2 n-hexane and ethyl acetate mixture as
an eluent. After completion of the reaction as indicated
by TLC, methylene dichloride (5 mL) was added. The
reaction mixture was filtered to collect the catalyst. It
was washed with methylene dichloride (2 × 3 mL) and
dried before use. The filtrate was washed with water
(8 mL), dried over anhydrous Na2SO4, and filtered.
The collected filtrate was evaporated under reduced
pressure, and the obtained residue was recrystallized
from ethanol to get pure benzopyrano[4,3-d]pyrimidines
(4a–l).
EXPERIMENTAL
General.
All reagents were obtained from Sigma-
Aldrich and Alfa Aesar and were used directly without
further purification. Melting points were recorded on
Guna Digital Melting Point apparatus. All the IR spectra
of the title compounds were recorded on Bruker Alpha-
Eco ATR-FTIR interferometer with a single reflection
sampling module equipped with ZnSe crystal. NMR
spectra were recorded on Bruker AMX 500-MHz NMR
1
spectrometer operating at 500 MHz for H and 125 MHz
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2017
TiO2–SiO2 Catalyzed Eco-friendly Synthesis and Antioxidant Activity of
Benzopyrano[2,3-d]pyrimidine Derivatives
2-(4-(Piperidin-1-yl)-5H-chromeno[2,3-d]pyrimidin-2-yl)phe
(d, J = 8.8 Hz, 1H), 7.07 (d, J = 8.4 Hz, 1H), 7.17–7.25
(m, 3H), 8.24 (d, J = 3.0 Hz, 1H) ppm. 13C NMR
(125 MHz, DMSO-d6): δ 25.4, 48.5, 66.6, 97.2, 118.3,
119.1, 119.2, 120.4, 123.7, 128.2, 128.3, 128.4, 129.5,
132.7, 148.7, 158.9, 161.0, 163.7, 164.7 ppm. MS (EI) m/z:
429 (M+). Anal. Calcd for C21H17Cl2N3O3 (%): C, 58.62;
nol (4a). White powder, yield 92%, mp 168–170°C. IR
(ZnSe): ν 3252 (―OH) cmꢀ1 1H NMR (500 MHz,
.
DMSO-d6): δ 1.70–1.82 (m, 6H), 3.43 (t, J = 4.9 Hz,
4H), 3.91 (s, 2H), 5.86 (s, 1H), 6.91 (t, J = 8.0 Hz, 1H),
6.97 (d, J = 8.0 Hz, 1H), 7.09 (t, J = 7.3 Hz, 1H),
7.16–7.27 (m, 3H), 7.34 (t, J = 7.3 Hz, 1H), 8.41 (d,
J = 8.0 Hz, 1H) ppm. 13C NMR (125 MHz, DMSO-d6):
δ 24.2, 25.5, 25.8, 49.4, 97.3, 116.9, 117.4, 118.5, 118.7,
119.4, 124.2, 128.0, 128.4, 129.1, 132.6, 150.5, 160.3,
161.8, 164.2, 165.0 ppm. MS (EI) m/z: 360 (M + H)+.
Anal. Calcd for C22H21N3O2 (%): C, 73.52; H, 5.89; N,
H, 3.98; N, 9.77. Found: C, 58.54; H, 3.94; N, 9.70.
5-Chloro-2-(8-chloro-4-thiomorpholino-5H-chromeno[2,3-d]
pyrimidin-2-yl)phenol (4f). Yellow powder, yield 90%, mp
1
235–237°C. IR (ZnSe): ν 3296 (―OH) cmꢀ1. H NMR
(500 MHz, DMSO-d6): δ 2.85 (t, J = 4.9 Hz, 4H), 3.76
(t, J = 4.9 Hz, 4H), 3.87 (s, 2H), 5.88 (s, 1H), 6.91
(d, J = 8.8 Hz, 1H), 7.12 (d, J = 8.4 Hz, 1H), 7.20–7.31
(m, 3H), 8.32 (d, J = 3.0 Hz, 1H) ppm. 13C NMR
(125 MHz, DMSO-d6): δ 25.4, 27.3, 50.8, 97.5, 118.4,
119.2, 119.3, 120.6, 123.8, 128.3, 128.4, 129.6, 132.8,
148.8, 158.9, 161.0, 165.4 ppm. MS (EI) m/z: 445 (M+).
Anal. Calcd for C21H17Cl2N3O2S (%): C, 56.51; H, 3.84;
11.69. Found: C, 73.44; H, 5.89; N, 11.63.
2-(4-Morpholino-5H-chromeno[2,3-d]pyrimidin-2-yl)phenol
(4b).
White powder, yield 90%, mp 199–201°C. IR
(ZnSe): ν 3267 (―OH) cmꢀ1 1H NMR (500 MHz,
.
DMSO-d6): δ 3.49 (t, 4H, J = 4.5 Hz), 3.91 (t, 4H,
J = 4.5 Hz), 3.92 (s, 2H), 5.89 (s, 1H), 6.92 (t, 1H,
J = 8.0 Hz), 6.98 (d, 1H, J = 8.0 Hz), 7.11 (t, 1H,
J = 7.3 Hz), 7.17–7.29 (m, 3H), 7.36 (t, J = 7.3 Hz, 1H),
8.40 (d, J = 8.0 Hz, 1H) ppm. 13C NMR (125 MHz,
DMSO-d6): δ 25.4, 48.5, 66.6, 97.6, 116.9, 117.5, 118.3,
118.8, 118.9, 124.4, 128.2, 128.5, 129.1, 132.8, 150.2,
160.2, 161.9, 164.0, 164.6 ppm. MS (EI) m/z: 361 (M+).
Anal. Calcd for C21H19N3O3 (%): C, 69.79; H, 5.30; N,
N, 9.41. Found: C, 56.42; H, 3.80; N, 9.36.
2-Methoxy-6-(9-methoxy-4-(piperidin-1-yl)-5H-chromeno[2,3-
d]pyrimidin-2-yl)phenol (4g). White powder, yield 90%, mp
1
178–180°C. IR (ZnSe): ν 3284 (―OH) cmꢀ1. H NMR
(500 MHz, DMSO-d6): δ 1.66–1.80 (m, 6H), 3.48
(t, J = 4.9 Hz, 4H), 3.82 (s, 3H), 3.89 (s, 3H), 3.95 (s, 2H),
5.91 (s, 1H), 6.75 (d, J = 7.7 Hz, 1H), 6.78–6.86 (m, 2H),
6.95 (d, J = 7.7 Hz, 1H), 7.01 (t, J = 8.0 Hz, 1H), 7.91
(d, J = 8.0 Hz, 1H) ppm. 13C NMR (125 MHz, DMSO-d6):
δ 24.1, 25.4, 25.7, 49.3, 55.8, 55.9, 97.3, 110.2, 113.6,
117.5, 118.5, 119.6, 120.2, 120.9, 123.9, 139.9, 147.9,
148.5, 150.6, 161.9, 164.3, 164.7 ppm; MS (EI) m/z:
419(M+). Anal. Calcd for C24H25N3O4 (%): C, 68.72; H,
11.63. Found: C, 69.74; H, 5.22; N, 11.60.
2-(4-Thiomorpholino-5H-chromeno[2,3-d]pyrimidin-2-yl)phe
nol (4c). Yellow powder, yield 93%, mp 239–241°C. IR
(ZnSe): ν 3276 (―OH) cmꢀ1 1H NMR (500 MHz,
.
DMSO-d6): δ 2.86 (t, J = 4.9 Hz, 4H), 3.76 (t, J = 4.9 Hz,
4H), 3.91 (s, 2H), 6.92 (t, J = 8.1 Hz, 1H), 6.98
(d, J = 8.1 Hz, 1H), 7.12 (t, J = 7.3 Hz, 1H), 7.17–7.28
(m, 3H), 7.36 (t, J = 7.3 Hz, 1H), 8.39 (d, J = 8.0 Hz, 1H)
ppm. 13C NMR (125 MHz, DMSO-d6): δ 25.5, 27.3, 50.7,
97.9, 117.1, 117.6, 118.4, 118.8, 119.1, 124.5, 128.3,
128.5, 129.1, 133.0, 150.4, 160.3, 162.1, 165.4 ppm. MS
(EI) m/z: 377 (M+). Anal. Calcd for C21H19N3O2S (%): C,
6.01; N, 10.02. Found: C, 68.70; H, 5.99; N, 15.21.
2-Methoxy-6-(9-methoxy-4-morpholino-5H-chromeno[2,3-d]
pyrimidin-2-yl)phenol (4h). White powder, yield 89%, mp
1
210–220°C. IR (ZnSe): ν 3279 (―OH) cmꢀ1. H NMR
(500 MHz, DMSO-d6): δ 3.46 (t, J = 4.4 Hz, 4H), 3.88
(t, J = 4.4 Hz, 4H), 3.85 (s, 3H), 3.87 (s, 3H), 3.91 (s,
2H), 5.93 (s, 1H), 6.70 (d, J = 7.7 Hz, 1H), 6.77–6.87
(m, 2H), 6.94 (d, J = 7.7 Hz, 1H), 6.99 (t, J =8.0 Hz,
1H), 8.04 (d, J = 8.0 Hz, 1H) ppm. 13C NMR (125 MHz,
DMSO-d6): δ: 25.4, 48.5, 55.90, 55.96, 66.5, 97.6, 110.5,
113.9, 117.7, 118.4, 119.6, 119.7, 120.9, 124.1, 139.7,
147.9, 148.5, 150.5, 162.1, 164.2 ppm. MS (EI) m/z:
421(M+). Anal. Calcd for C23H23N3O5 (%): C, 65.55; H,
66.82; H, 5.07; N, 11.13. Found: C, 66.72; H, 5.03; N, 11.05.
5-Chloro-2-(8-chloro-4-(piperidin-1-yl)-5H-chromeno[2,3-d]
pyrimidin-2-yl)phenol (4d). White powder, yield 89%, mp
1
255–257°C. IR (ZnSe): ν 3246 (―OH) cmꢀ1. H NMR
(500 MHz, DMSO-d6): δ 1.73–1.81 (m, 6H), 3.43
(t, J = 4.7 Hz, 4H), 3.87 (s, 2H), 5.91 (s, 1H), 6.89
(d, J = 8.8 Hz, 1H), 7.11 (d, J = 8.4 Hz, 1H), 7.19–7.28
(m, 3H), 8.35 (d, J = 3.0, Hz, 1H) ppm. 13C NMR
(125 MHz, DMSO-d6): δ 24.2, 25.5, 25.9, 49.5, 97.0,
118.3, 119.0, 119.4, 121.0, 123.7, 128.2, 128.32, 128.35,
129.4, 132.6, 149.0, 159.0, 161.0, 165.1 ppm. MS (EI)
m/z: 427 (M+). Anal. Calcd for C22H19Cl2N3O2 (%): C,
61.69; H, 4.47; N, 9.81. Found: C, 61.55; H, 4.41; N, 9.75.
5-Chloro-2-(8-chloro-4-morpholino-5H-chromeno[2,3-d]pyri
5.50; N, 9.97. Found: C, 65.43; H, 5.44; N, 9.95.
2-Methoxy-6-(9-methoxy-4-thiomorpholino-5H-chromeno[2,3-
d]pyrimidin-2-yl)phenol (4i). Yellow powder, yield 92%,
mp 220–222°C. IR (ZnSe): ν 3294 (―OH) cmꢀ1 1H
.
NMR (500 MHz, DMSO-d6): δ 2.85 (t, J = 4.7 Hz,
4H), 3.75 (t, J = 4.7 Hz, 4H), 3.89 (s, 2H), 3.93 (d,
J = 2.2 Hz, 6H), 5.85 (s, 1H), 6.77 (d, J = 7.7 Hz,
1H), 6.82–6.88 (m, 2H), 6.97 (d, J = 7.7 Hz, 1H),
6.05 (t, J =8.0 Hz, 1H), 8.08 (d, J = 8.0 Hz, 1H)
ppm. 13C NMR (125 MHz, DMSO-d6): δ 25.6, 27.4,
50.8, 56.06, 56.09, 97.9, 110.7, 114.1, 117.9, 118.5,
midin-2-yl)phenol (4e).
White powder, yield 92%, mp
1
230–232°C. IR (ZnSe): ν 3305 (―OH) cmꢀ1. H NMR
(500 MHz, DMSO-d6): δ 3.48 (t, J = 4.5 Hz, 4H), 3.85
(s, 2H), 3.90 (t, J = 4.5 Hz, 4H), 5.90 (s, 1H), 6.85
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

S. A. Kabeer, G. R. Reddy, P. Sreelakshmi, D. M. Manidhar, and C. S. Reddy
Vol 000
119.7, 120.0, 121.0, 124.3, 148.2, 148.7, 150.6, 162.3,
164.6, 165.0 ppm. MS (EI) m/z: 437 (M+). Anal.
Calcd for C23H23N3O4S (%):C, 63.14; H, 5.30; N,
9.60. Found: C, 63.02; H, 5.22; N, 9.55.
for the spontaneous generation of NO from it. In the
evaluation, initially, 3 mL of the analytical mixture was
prepared by mixing
2 mL of 10 mM sodium
nitroprusside, 0.5-mL saline phosphate buffer, and
0.5 mL of title compounds and made up to 100 μg/mL
concentration and incubated at 25°C for 3 h. After 3 h of
incubation, 0.5 mL of the mixture was mixed with 1.5-
mL Griess reagent [1.0% sulphanilamide, 2.5% H3PO4
and 0.1% N-(1-naphtyl)ethylenediaminedihydrochloride]
and subjected for the measurement of absorbance at
546 nm by using UV–visible spectrophotometer [27].
Similarly, the control experiments were also conducted
under same conditions and with the same procedure.
Then, the scavenging activity was evaluated by using the
following equation by taking the difference between the
absorbance of the test and the control experiments read at
λ 546 nm and expressed the NO percent scavenging
activity. The increased absorbance of the compounds
with concentration indicates its increased reducing power
by this method.
5-Methoxy-2-(8-methoxy-4-(piperidin-1-yl)-5H-chromeno[2,3-
d]pyrimidin-2-yl)phenol (4j). White powder, yield 94%, mp
1
170–172°C. IR (ZnSe): ν 3298 (―OH) cmꢀ1. H NMR
(500 MHz, DMSO-d6): δ 1.67–1.81 (m, 6H), 3.40 (t,
J = 4.9 Hz, 4H), 3.79 (s, 3H), 3.81 (s, 2H), 3.82 (s,
3H), 4.69 (brs, 1H), 6.45–6.50 (m, 2H), 6.65 (dd,
J = 2.5, 8.4 Hz, 1H), 6.72 (d, J = 2.5 Hz, 1H), 7.07 (d,
J = 8.4 Hz, 1H), 8.30 (d, J = 8.4 Hz, 1H) ppm. 13C
NMR (125 MHz, DMSO-d6): δ 24.2, 24.7, 25.8, 49.3,
55.2, 55.3, 96.8, 101.1, 101.8, 106.3, 110.8, 111.3,
111.8, 128.9, 130.3, 151.0, 159.3, 161.6, 162.0, 163.3,
164.9 ppm. MS (EI) m/z: 419 (M+); Anal. Calcd for
C24H25N3O4 (%): C, 68.72; H, 6.01; N, 10.02. Found: C,
68.60; H, 5.97; N, 9.94.
5-Methoxy-2-(8-methoxy-4-morpholino-5H-chromeno[2,3-d]
pyrimidin-2-yl)phenol (4k). White powder, yield 90%, mp
1
227–229°C. IR (ZnSe): ν 3277 (―OH) cmꢀ1. H NMR
NO Radical scavenging ð%Þ
(500 MHz, DMSO-d6): δ 3.46 (t, J = 4.4 Hz, 4H), 3.80
(s, 3H), 3.83 (s, 5H), 3.89 (t, J = 4.4 Hz, 4H), 4.76
(brs, 1H), 6.46–6.51 (m, 2H), 6.67 (dd, J = 2.5, 8.4 Hz,
1H), 6.72 (d, J = 2.5 Hz, 1H), 7.08 (d, J = 8.4 Hz, 1H),
8.28 (d, J = 8.4 Hz, 1H) ppm. 13C NMR (125 MHz,
DMSO-d6): δ 24.8, 48.5, 55.2, 55.4, 66.6, 97.1, 101.1,
101.9, 106.6, 110.8, 111.1, 111.6, 128.9, 130.4, 151.0,
159.5, 161.8, 162.0, 163.5, 164.6 ppm. MS (EI) m/z: 421
(M+). Anal. Calcd for C23H23N3O5 (%): C, 65.55; H,
5.50; N, 9.97. Found: C, 65.42; H, 5.46; N, 9.93.
½ðAbsorbance of control-Absorbance of test sampleÞꢁ
¼
ꢂ100
ðAbsorbance of controlÞ
2,2-Diphenyl-1-picrylhydrazyl
(DPPH)
free
radical
scavenging assay. 2,2-Diphenyl-1-picrylhydrazyl (DPPH),
a nitrogen-centered stable free radical has been used to
characterize the antioxidant activity of the title compounds;
1.0 mL of 0.1 mM methanolic DPPH solution was added
to 3.0 mL of methanolic test solutions of 100 μg/mL
concentrations. Then, the absorbance was measured at
517 nm for all the test compounds by spectrophotometric
studies. In the analysis, the reduced DPPH free radical
gives a strong absorption maximum at λ 517 nm. A radical
scavenging antioxidant reacts with DPPH stable free
radical and converts into 1,1-diphenyl-2-picrylhydrazine. A
blank was prepared without adding compound and tested
for the absorbance similarly, and change in the absorbance
has been used to measure antioxidant properties. Lower
absorbance value of the analytical sample indicates its
higher free radical scavenging activity. The antioxidant
activity of title compounds was expressed as IC50 and
compared with standard [28]. The DPPH radical scavenging
capacity was calculated using the following equation:
5-Methoxy-2-(8-methoxy-4-thiomorpholino-5H-chromeno[2,
3-d]pyrimidin-2-yl)phenol (4l). Yellow powder, yield 89%,
mp 214–216°C. IR (ZnSe): ν 3258 (―OH) cmꢀ1. 1H NMR
(500 MHz, DMSO-d6): δ 2.84 (t, J = 4.9 Hz, 4H), 3.73
(t, J = 4.9 Hz, 4H), 3.79 (s, 2H), 3.80 (s, 3H), 3.83
(s, 3H), 5.86 (s, 1H), 6.45–6.51 (m, 2H), 6.67 (dd, J = 2.5,
8.4 Hz, 1H), 6.72 (d, J = 2.5 Hz, 1H), 7.07 (d, J = 8.4 Hz,
1H), 8.27 (d, J = 8.4 Hz, 1H) ppm. 13C NMR (125 MHz,
DMSO-d6): δ 24.7, 27.3, 50.6, 55.2, 55.4, 97.2, 101.1,
101.9, 106.5, 110.8, 111.0, 111.6, 128.9, 130.3, 150.9,
159.4, 161.5, 161.7, 162.0, 163.5 ppm. MS (EI) m/z: 437
(M+). Anal. Calcd for C23H23N3O4S (%): C, 63.14; H,
5.30; N, 9.60. Found: C, 63.02; H, 5.25; N, 9.55.
Antioxidant activity.
All the in vitro antioxidant
% DPPH radical scavenging
experiments were performed in triplicate, and the data was
expressed with standard deviation, and ascorbic acid was
used as the standard in all the four methods. Each method
has been performed at four different concentrations and
viz., 25, 50, 75, and 100 μg/mL, and the title compounds
were observed to enough potent at 100 μg/mL
concentration.
½ðAbsorbance of control-Absorbance of test sampleÞꢁ
¼
ꢂ100
ðAbsorbance of controlÞ
Hydrogen peroxide (H2O2) scavenging activity. The effect
of hydrogen peroxide scavenge ability of the title
compounds was determined based on the standard
procedure [29]. A solution of 100 mM H2O2 was
prepared, and to this 2 mL of 100 μg/mL title compound
Nitric oxide free radical scavenging assay.
We have
chosen the sodium nitroprusside buffer solution method
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2017
TiO2–SiO2 Catalyzed Eco-friendly Synthesis and Antioxidant Activity of
Benzopyrano[2,3-d]pyrimidine Derivatives
solution was added in phosphate buffer saline of pH
7.4. Absorbance of H2O2 at 230 nm was determined
spectrophotometrically after 10 min against a blank
solution containing phosphate buffer without H2O2.
The increased absorbance of the compounds with
concentration indicates increased reducing power.
reactive free radicals and facilitates to pair up with the
other free radicals generated in vivo.
Acknowledgments. We thank the Science and Engineering
Research Board (SERB), India for providing financial assistance
through a research project grant F.No.: SB/S1/OC-96/2013, Dt:
05-11-2014.
H2O2 radical scavenging ð%Þ
½ðAbsorbance of control-Absorbance of test sampleÞꢁ
¼
ꢂ100
ðAbsorbance of controlÞ
REFERENCES AND NOTES
Ferric reducing-antioxidant power (FRAP) assay.
The
capacity of compounds to reduce Fe+3 complex [2,4,6-
Tris(2-pyridyl)-s-triazine (TPTZ)] to the Fe+2 form was
considered as an equivalent for the corresponding FRAP
value. The reducing power of Fe+3 was determined
by measuring the change in absorption at 593 nm
spectrophotometrically [30]. The reagent was prepared by
mixing a 10.0 mL of TPTZ solution, with 10 mL of
FeCl3·6H2O solution and 100 mL of acetate buffer at pH
3.6. Then, 3 mL of 100-μL FRAP reagent is mixed and
incubated for 30 min at 37°C, and change in absorbance at
593 nm was recorded and compared to the absorbance of
pure reagent. The results were expressed as ascorbic
equivalent (mmol/100g compound). In this method, the
colorless [Fe(III)–TPTZ] complex was reduced to deep
blue colored [Fe(II)–TPTZ] complex by the compounds.
The increased absorbance of the compounds with
concentration indicates increased reducing power. Ascorbic
acid was used as reference, and FRAP value was measured
by using the equation:
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CONCLUSION
We have developed an efficient and environmentally
benign new method for the synthesis of benzopyrano
[4,3-d]pyrimidines by using 5% TiO2–SiO2 as a recyclable
heterogeneous catalyst, under solvent-free conditions. The
major advantages of this methodology are experimental
simplicity, simple work-up, reusable catalyst, short reaction
times, and high product yields. Interestingly, some
compounds 4g, 4h, and 4i are proved to be better
antioxidants as evidenced from all the four methods and
are comparable to standard ascorbic acid. Analysis of the
structure–activity relationship for the structure of the title
compounds 4a–l with the obtained in vitro antioxidant
activity results reveals that the ―OCH3 substituted
compounds have exhibited remarkably high activity when
compared to other substituted analogues. It is due to the
fact that ―OCH3 group stimulates the formation of
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet