Silica-supported heterogeneous catalysts-mediated synthesis of chalcones as potent urease inhibitors: in vitro and molecular docking studies

Article abstract of DOI:10.1007/s00706-019-02534-z

Abstract: We herein report a facile and high yielding protocol for silica-supported heterogeneous catalysts-mediated synthesis of chalcones. A comparison of results of our synthesis with conventional synthetic protocols is also being offered to assess the efficiency of the prepared catalysts. Biological evaluation of the newly synthesized compounds as urease inhibitors was performed. Most of the compounds were found to have potent urease inhibition activity. The chalcone 3-(3-hydroxyphenyl)-1-phenylpropenone was found to be the most potent with percentage inhibition 86.17 ± 0.89 and half maximal inhibitory concentration (IC₅₀) value 11.51 ± 0.03 μM. The molecular docking study emphasized that the same congeners 3-(furan-2-yl)-1-(4-hydroxyphenyl)propenone, 3-(4-hydroxyphenyl)-1-(4-methoxyphenyl)propanone, and 3-[4-(dimethylamino)phenyl]-1-(p-tolyl)propenone showed very good inhibitory potential against urease and show a higher docking scores 5718, 5940, 5596 and an ACE of ? 246.66, ? 244.79, and ? 243.06 kJ/mol, respectively than the control ligand. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s00706-019-02534-z

Monatshefte für Chemie - Chemical Monthly
https://doi.org/10.1007/s00706-019-02534-z
ORIGINAL PAPER
Silica‑supported heterogeneous catalysts‑mediated synthesis
of chalcones as potent urease inhibitors: in vitro and molecular
docking studies
Aeysha Sultan1 · Shanavas Shajahan2 · Tansir Ahamad3 · Saad M. Alshehri3 · Noreen Sajjad4 · Mehr‑un‑Nisa1 ·
Mian Habib Ur Rehman1 · Lokman Torun5 · Muhammad Khalid6 · Roberto Acevedo7
Received: 11 September 2019 / Accepted: 9 December 2019
© Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract
We herein report a facile and high yielding protocol for silica-supported heterogeneous catalysts-mediated synthesis of
chalcones. A comparison of results of our synthesis with conventional synthetic protocols is also being ofered to assess
the efciency of the prepared catalysts. Biological evaluation of the newly synthesized compounds as urease inhibitors was
performed. Most of the compounds were found to have potent urease inhibition activity. The chalcone 3-(3-hydroxyphenyl)-
1-phenylpropenone was found to be the most potent with percentage inhibition 86.17 0.89 and half maximal inhibitory
concentration (IC50) value 11.51 0.03 µM. The molecular docking study emphasized that the same congeners 3-(furan-2-yl)-
1-(4-hydroxyphenyl)propenone, 3-(4-hydroxyphenyl)-1-(4-methoxyphenyl)propanone, and 3-[4-(dimethylamino)phenyl]-
1-(p-tolyl)propenone showed very good inhibitory potential against urease and show a higher docking scores 5718, 5940,
5596 and an ACE of −246.66, −244.79, and −243.06 kJ/mol, respectively than the control ligand.
Graphic abstract
O
HO
OMe
Keywords Heterogeneous catalyst · Ligand · Docking · Urease · Chalcones
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s00706-019-02534-z) contains
supplementary material, which is available to authorized users.
5
* Roberto Acevedo
Department of Chemistry, Yilditz University of Engineering
roberto.acevedo.llanos@gmail.com
and Technology, Istanbul, Turkey
6
7
Department of Chemistry, Khawaja Ghulam Fareed
University of Engineering and Technology, Rahim Yar Khan,
Pakistan
1
Department of Chemistry, University of Education,
Faisalabad Campus, Faisalabad, Pakistan
2
Nano and Hybrid Materials Laboratory, Department
Facultad de Ingeniería y Tecnología, Universidad San
Sebastián, Bellavista 7, 420524 Santiago, Chile
of Physics, Periyar University, 636 011, Salem, India
3
Department of Chemistry, College of Science, King Saud
University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
4
Department of Chemistry, University of Lahore, Lahore,
Pakistan
Vol.:(0123456789)
1 3

A. Sultan et al.
butyrylcholinesterase, and lipoxygenase [20]. These sec-
ondary metabolites, along with their physiological impor-
tance, also serve as precursor/intermediates in organic
synthesis of compounds of medicinal importance. Some
examples of biologically active chalcones are shown in
Fig. 1 [21–24].
Introduction
Heterogeneous catalysts have been prepared by immo-
bilization of transition metal complexes on various sup-
ports and the nature of action and mechanism of catalysis
has been studied/developed [1]. The development of frst
mesoporous materials as heterogeneous catalysts in early
90 s led to extensive research which also triggered explo-
ration of silica-supported catalysts. Silica (SiO2) whether
found naturally or synthesized artifcially exists in variety
of forms i.e., gels, crystalline, and amorphous. Each form
exhibits diferent physicochemical properties. Because of
its greater surface area, synthetic silica is extensively used
as adsorbing material and as catalyst support. Silica has a
tetrahedral geometry with each silicon atom bonded to four
oxygen atoms and each oxygen atom being bound to two
silicon atoms. Two types of functional group: silanol groups
(Si–O–H) and siloxane groups (Si–O–Si) are present on the
silica surface. All the chemical processes and even the physi-
cal processes like adsorption takes place on the silanol sites;
the siloxane sites, which form the backbone of silica, remain
inert to most of the activities [2]. The innate combination of
inertness of siloxane group and activity of silanol groups in
silica ofers the advantages of chemical inertness and ease
of its modifcation in the presence/infuence of metals and
organic substances. These properties make silica an attrac-
tive support for heterogeneous catalysis [3].
Urease is a member of superfamily of enzymes ami-
dohydrolases and phosphotriesterases. These are high
molecular weight nickel-based metalloenzymes, which can
be found in bacteria, fungi, algae, plants, certain inverte-
brates, as well as in soil where these biological catalysts
mediate the hydrolysis of urea into CO2 and NH3. The
presence of ammonia in animals and human is considered
harmful, because of its toxic efect. Ureases, because of
their role in the production of ammonia, have been found
to be associated with the development of a number of
physiological disturbances such as pyelonephritis, urolithi-
asis, hepatic coma, hepatic encephalopathy, and urinary
catheter encrustation [25].
The use of metals in heterogenous catalysis has been
widely reported. The supported metal based heterogeneous
catalysts, owning to their selectivity and higher turnover
rates, hold special place in the feld of heterogenous cata-
lysts. The catalytic activity of these supported catalysts is
dependent upon sizes of metallic species, interaction of
the metallic species with supports, substrates, and solvents
[26–30]. In continuation with our previous studies toward
exploration of silica-supported heterogeneous catalysts
[31], the presented work is concerned with preparation of
various silica-supported transition metals based hetero-
geneous catalysts and evaluation of their efectiveness as
catalyst for synthesis of chalcones. Since urease inhibitory
activity of chalcones is still a neglected feld, the synthe-
sized compounds were evaluated for their in vivo and in
silico anti-urease potential.
Chalcones (1,3-diaryl-2-propenones) and their deriva-
tives have a wide range of biological activities, which
include (but not limited to) anti-inflammatory [4, 5],
antifungal [6], antioxidant [7, 8], antimalarial [9], anti-
tuberculosis [10], analgesic [11, 12], anti-HIV [13, 14],
anticancer [15-17], antitumor [18] activities. These also
act as inhibitors of α-amylase [19], acetylcholinesterase,
Fig. 1 Some examples of bio-
A
n
t
i
v
i
logically active chalcones
r
e
n
a
l
c
h
o
c
l
a
a
l
c
h
c
o
n
e
r
e
c
n
a
c
i
t
n
A
U
r
e
a
s
e
i
n
h
i
b
i
t
o
e
n
o
r
c
l
a
c
h
h
c
l
a
l
a
i
b
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i
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t
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e
n
A
1 3

Silica-supported heterogeneous catalysts-mediated synthesis of chalcones as potent urease…
metals), whereby these polarizes the C=O of aldehyde via
Results and discussion
an acid–base interaction.
Synthesis
Recycling test
The metal immobilized over silica was prepared by stir-
ring metal halides (NiCl2, FeCl3, ZnCl2, CuCl2) with silica
using dry ethanol as solvent at room temperature for 4 h fol-
lowed by evaporating the solvent to dryness under reduced
pressure and the dispersed solid solution was then heated
at 100 °C for 1 h under reduced pressure to aford active
silica-supported metal catalysts [32]. The efectiveness of
these silica-supported transition metal catalysts was evalu-
ated by carrying out a model reaction of condensation of
benzaldehyde and acetophenone under solvent-free condi-
tions. To confrm the role of catalyst in this condensation
reaction, the same model/control experiment was carried out
in the absence of any catalyst and using H2SO4 as a catalyst
as well. The fndings of model reaction are summarized in
Table 1, which clearly indicate that catalysts do have a role
in the reaction. Absence of any catalyst does not lead to
formation of product in signifcant yield.
We have studied the recycling efciency of the catalysts,
i.e. whether the catalysts can be reused further for several
cycles. To test the catalytic efciency for further cycles, a
control experiment was performed using benzaldehyde and
acetophenone. After each reaction cycle, the catalysts were
recovered by fltration, washed thoroughly with acetonitrile,
and then treated with 0.1 M HCl solution in ethanol at 338 K
for 8 h for regeneration and fnally dried at 373 K for 2 h.
The catalytic reactions have been carried out following the
same experimental procedure as that with the original cata-
lysts. The catalytic activity decreased very slightly in the
successive catalytic cycles until six control reactions. After
six cycles a very pronounced decreased in catalytic activity
was observed.
After successful model studies, the silica-supported het-
erogeneous catalysts were then evaluated for their efective-
ness toward diferent substituted acetophenones (PhAc 1)
and aldehydes (ArCHO 2). Signifcantly lower yields were
observed for H2SO4 mediated reaction of substituted PhAc
1 and ArCHO 2. Although SiO2-MClx mediated reactions
aforded products with signifcant yields; however, the yields
were quite lower for chalcones bearing nitro, hydroxy, and/
or amino groups. In case of silica-sulfuric acid, the yields
were quite high even for compounds with aforementioned
substituents (Table 2).
The conventional H2SO4 used in the condensation reac-
tion did not yield product in reasonable amount. The same
model reaction when tried with SiO2–H2SO4 yielded desired
chalcone 3 in excellent yield (94%). Metal based catalysts
also aforded better yields as compared with H2SO4; how-
ever, their yields were lower than that for SiO2–H2SO4. All
heterogeneous catalysts (SiO2–H2SO4 and metal based cata-
lysts) could be recycled by fltering them from the reaction
mixture and then washing the solid reagent by ethyl acetate
followed by drying in an oven at 100 °C for 30 min. We
believe that catalytic efect of SiO2–MClx catalysts (M=Fe,
Ni, Cu, Zn; x = 2, 3) is due to their ability to act as Brøn-
sted acid (due to presence of Si–OH groups), as well as
Lewis acid (due to presence of empty π orbital of transition
It is quite clear from Table 2 that SiO2–FeCl3 is the most
efcient and efective catalyst for synthesis of chalcones with
diferent substituent. SiO2–H2SO4 is quite efective catalyst,
but low yields are obtained with this catalyst when OH and
or NMe2 substituent are present. SiO2–NiCl2, SiO2–CuCl2,
and SiO2–ZnCl2 also gave reasonable yields, however, with
OH and NMe2 substituent, yields were signifcantly lower.
We suggest that lower yields with these substituents are due
to possible complexation of catalysts with O and N acting
as strong donors.
Table 1 Evaluation of efectiveness of catalysts for synthesis of chal-
cone 3
Entry Reagent/catalyst
Solvent
Time/min Product (yield/%)
Urease inhibition study
1
2
3
4
5
None
None
H2SO4
SiO2–H2SO4
–
210
3 (<10)
3 (15)
3 (45)
3 (92)
3 (96)
Dry EtOH 180
Dry EtOH 150
–
–
The biological activity of the chalcones 3a–3i toward urease
inhibition was determined using Berthelot assay [33]. Thiou-
rea was used as positive control. In the present study, a series
of synthesized chalcones 3a–3i have been tested for urease
inhibition activity (Table 3). The inhibition pattern of this
series found to be 3f>3e>3i>3 g>3b>3a>3h>3c>3
d and according to substituents 4′-OH–, 4-OCH3–>4′-OH–,
2-furyl– > 4′-CH3–, 4-(CH3)2NPh– > 4′-CH3–,
Ph–>3′-OH–, Ph–>H–, Ph–>4′-CH3–, 2-furyl–>3′-OH–,
2-furyl– > 4′-OH–, Ph–. The studies showed that 4′-OH–,
30
30
SiO2–FeCl3
(10 mol%)
6
7
8
SiO2–NiCl2
(10 mol%)
–
–
–
45
50
45
3 (88)
3 (72)
3 (78)
SiO2–CuCl2
(10 mol%)
SiO2–ZnCl2
(10 mol%)
1 3

A. Sultan et al.
Table 2 Comparison of efect of catalysts/reagents on yield of diferent substituted chalcones 3a–3i
Compound
R–
Ar–
Yield/%
H2SO4
#
^
^
^
^
SiO2–H2SO4
SiO2–FeCl3
SiO2–NiCl2
SiO2–CuCl2
SiO2–ZnCl2
3a
3b
3c
3d
3e
3f
3g
3h
3i
H–
Ph-
Ph-
2-furyl-
Ph-
2-furyl-
4-OMe-
Ph-
45
21
34
47
52
58
56
52
<10
92
90
79
86
84
90
86
97
84
96
92
94
93
97
95
98
96
88
88
80
72
75
82
86
91
95
82
72
56
46
48
43
52
68
72
34
78
59
44
52
57
61
72
76
45
3′-OH-
3′-OH-
4′-OH-
4′-OH-
4′-OH-
4′-Me-
4′-Me-
4′-Me-
2-furyl-
4-Me2NPh-
1.5 equivalent to acetophenone, 5 h refux in methanol; #SSA heating at 65 °C for 1.5 h under solvent-free conditions; ^rt, 70–110 min, solvent-
free conditions
4-OCH3– (3f, IC50, 11.51 0.03 μM) and 4′-OH–,
2-furyl– (3e, IC50, 14.62 0.01 μM) are the most potent
compounds against urease even better than the standard thi-
ourea. The four compounds 4′-CH3–, 4-(CH3)2NPh– (3i),
4′-CH3–, Ph– (3g), 3′-OH–, Ph– (3b) and H–, Ph– (3a)
have shown moderate enzyme inhibitions with IC50 val-
ues≤28 μM with the following order of decreasing activ-
ity as 4′-CH3–, 4-(CH3)2NPh– > 4′-CH3–, Ph– > 3′-OH–,
Ph–>H–, Ph–. However, compounds 4′-CH3–, 2-furyl– (3h),
3′-OH–, 2-furyl– (3c) and 4′-OH–, Ph– (3d) have exhibited
weak inhibitions.
urease (4H9M) via PatchDock [34]. PatchDock afords the
top 20 solutions and “solution 1” was picked as binding
pocket for docking analyses, as it is surrounded by most
signifcant residues designated in crystal structure of jack
bean urease receptor (4H9M) [35]. The docked structures
were inspected using Discovery Studio 4.5 Visualizer.
The binding afnities of the docked ligands were evalu-
ated as scores and ACE (atomic contact energy) of the
docked complexes. The hydrogen bonding and hydrophobic
interactions of each ligand were evaluated within binding
pocket of receptor protein. The conformation of the ligands
which illustrated the highest biological activities is showed
in Table 4, Figs. 2, 3, 4, 5 with their favorable contacts in
the binding pockets of enzyme. To get qualitative evaluation
and to recognize molecular basis of the calculated biological
activities (IC50), the docked complexes of ligands 3a–3i and
standard thiourea were investigated. Standard thiourea was
also docked with jack bean urease for comparison and has
illustrated score 1607 with an ACE value − 106.77 kJ/mol
(Fig. 2). Thiourea has showed hydrophobic contact poten-
It is important to note that mild electron donating groups
increase the urease inhibition. However, the size and the
position of the substituents also matter. In para substituted
ligands, the observed order of their activity has been found
as follows 4′-OH–, 4-OCH3–>4′-OH–, 2-furyl–>4′- CH3–,
4-(CH3)2NPh–>4′-CH3–, Ph–>4′-CH3–, 2-furyl–>4′-OH–,
Ph–. Similarly, among the meta substituted ligands, 3′-OH–,
Ph– (3b) has shown more inhibition of urease as compared
to 3′-OH–, 2-furyl– (3c). On the other hand, electron with-
drawing groups e.g., OH- and CH3-groups, especially, at
meta positions were found to reduce the urease inhibition.
Though, it is critical to establish structure–activity rela-
tionship of the studied chalcones 3a–3i possibly, owing
to the series of factors, i.e., size, shape, polarizability, and
electronegativity of a ligand playing essential function in
enzyme inhibition.
tial with pocket amino acids Val416, Tyr410, Met453, Ser456
,
Pro413, Leu460, Thr457, Phe464. In the beginning, assessment
of the docked complexes of jack bean urease with 3a–3i
disclosed that ligands 3e, 3f, and 3i exhibited important
interaction patterns. Visual scrutiny of these complexes
predicts major interactions between a binding conforma-
tion of ligand 3f with jack bean urease as compared to the
other eight ligands; all ligands showed superior interactions
than standard thiourea. Ligand 3f showed most potent inter-
action with jack bean urease with a score of 5940 and an
ACE of − 244.79 kJ/mol. The interacting residues of this
complex are Lys716, Lys709 (Fig. 5, Table 5). Ligand 3f has
shown a potential hydrogen bond between carbonyl directly
attached with 4-hydroxyphenyl group and amino of Lys716
(1.98 Å), while second hydrogen bond has formed between
4-methoxyphenyl group and amino of Lys709 (2.56 Å)
Molecular docking studies
The X-ray crystallographic structure of jack bean urease
protein was retrieved from RCSB Protein Data Bank (PDB
ID 4H9M). The preparation of protein structure was done
by deletion of water molecules, cofactor, and co-crystallized
ligands by utilizing Discovery Studio 4.5 Visualizer. Ligands
3a–3i and standard thiourea were docked with jack bean
1 3

Silica-supported heterogeneous catalysts-mediated synthesis of chalcones as potent urease…
Table 3 Determination of IC50
Conc. Inhibition
/mM /%
S. No.
1
Comp. Structure
IC50 /µMa
values of urease inhibition of
chalcones 3a–3i
O
3a
0.25
81.59 0.91 28.41 0.09
O
2
3
3b
0.25
81.85 0.86 27.11 0.03
OH
O
O
3c
0.25
61.29 0.72 58.81 0.11
OH
O
O
O
O
O
O
4
5
6
7
8
9
3d
0.25
0.25
0.25
0.25
0.25
58.75 0.36 74.35 0.19
83.42 0.92 14.62 0.01
86.17 0.89 11.51 0.03
84.61 0.51 25.78 0.11
56.58 0.41 35.44 0.11
HO
O
3e
HO
3f
HO
OMe
3g
Me
O
3h
Me
3i
0.25
0.25
81.81 0.81 23.52 0.01
98.21 0.18 21.25 0.15
Me
NMe2
Thiourea Std.
aResults are the mean of three independent experiments (n=3) S.D
indicating important interaction. Similarly, ligand 3f exhib-
ited hydrophobic contact potential with pocket amino acids
Glu718, Asp730, Pro717, Glu742, Val36, Thr33 and also depicted
π-stacking contact potential with Phe712 amino acid.
amino acids. The favorably causative structural elements
(atoms and torsions) on the whole binding energy are visu-
ally color in green, likewise the structural elements that are
not contributing favorably are colored in pink, and neutral
elements are in white (Fig. 3). The aromatic phenyl moieties
4-methoxyphenyl and 4-hydroxyphenyl ring are contribut-
ing favorably to the binding energy. The only unfavorable
structural element is the ethylenic scafold. This lead to
the supposition that if phenyl ring is substituted by some
other atoms, i.e., carbon or other heteroatoms, it may be a
reason for even better binding afnity and thereby showing
increased anti-urease activity.
To explain the structural elements responsible for the
indicative inhibitory efect against the binding site of jack
bean urease, the most active ligand 3f bound to protein
was subjected to binding afnity assessment using hydro-
gen bond donor–acceptor surface of Discovery Studio 4.5
Visualizer software. It permits diagrammatic estimation of
favorable and unfavorable supports as a result of the struc-
ture/bound conformation of inhibitor with the neighboring
1 3

A. Sultan et al.
Table 4 Docking results for the highest ranked biologically active complexes (urease inhibition)
Compd. Code IC50/µM
Score ACE/kJ mol−1 Amino acids show
Distance/Å
Amino acids show hydro-
phobic contacts
Amino acids show
π-stacking contacts
hydrogen bond
contacts
3a
3b
3c
28.41 0.09 4700
− 684.62
− 739.02
− 670.78
− 699.40
Lys716, Lys716
Lys716, Glu742
–
2.29, 2.49
Ala37, Thr33, Lys709, Glu718
Asp730, Glu742
,
Phe712
27.11 0.03 4656
58.81 0.11 4492
74.35 0.19 4708
2.93, 3.08, 3.03 Asp730, Glu718, Lys709, Tyr32, Phe712
Thr33
–
Thr33, Lys745, Val744, Glu742
Pro717, Glu718, Lys709
,
Phe712
3d
3e
Lys709
2.64
2.70, 2.48
Glu742, Thr33, Tyr32, Phe712
–
14.62 0.01 5718 − 1032.02
Lys716, Lys716
Lys709, Glu718, Asp730
,
Phe712
Glu742, Thr33
3f
11.51 0.03 5940 − 1024.20
Lys716, Lys709
–
1.98, 2.56
–
Glu718, Asp730, Pro717
Glu742, Val36, Thr33
,
Phe712
Phe712
3g
25.78 0.11 4702
35.44 0.11 4572
− 594.84
− 647.18
Glu718, Pro717, Asp730
,
Lys716, Glu742, Val36, Thr33,
Tyr32
3h
3i
–
–
–
–
Met746, Lys745, Tyr32, Glu742
,
–
–
Pro743, Glu718
23.52 0.01 5596 − 1016.96
Leu839, Thr33, Tyr32, Phe712
Lys745, Glu742, Ala16,
Pro743, Ala37, Leu13
,
Std
21.25 0.15 1600
− 433.92
–
–
Val416, Tyr410, Met453, Ser456
,
–
Thiourea
Pro413, Leu460, Thr457
Phe464
,
No. Specifc code assigned to compound, ACE Atomic contact energy determined by PatchDock (kJ/mol), IC50 experimental calculation of
inhibitory constant (µM), Distance hydrogen bond length analyzed from docked pose via ligand interaction tool of PatchDock
Fig. 2 Binding site interaction of standard thiourea 3D (left) and 2D (right)
Ligand 3e showed a score of 5718 with jack bean ure-
ase and an ACE of − 246.66 kJ/mol. The interacting resi-
dues of this complex are Lys716, Lys716 (Fig. 5, Table 5).
Ligand 3e has exposed a potential hydrogen bond between
carbonyl directly attached with 4-hydroxyphenyl group and
amino of Lys716 (2.48 Å), while second hydrogen bond has
formed between the same carbonyl directly attached with
4-hydroxyphenyl group and hydrogen attached with amino
group of Lys716 (2.70 Å). Ligand 3e demonstrated hydro-
phobic contact potential with pocket amino acids Lys709
,
Glu718, Asp730, Glu742, and Thr33. This complex also exhib-
ited π-stacking contact potential with Phe712 amino acid.
However, ligand 3i depicted score 5596 with an ACE
value − 243.06 kJ/mol. Ligand 3i exhibited no hydrogen
1 3

Silica-supported heterogeneous catalysts-mediated synthesis of chalcones as potent urease…
Fig. 3 Hydrogen bond donor–
acceptor surface of most active
inhibitor ligand 3f
with OH and NMe2 substituent, while same issue was not
observed in case of SiO2–FeCl3, which resulted in higher
product yields as with these substituents.
Biological assessment of the newly synthesized com-
pounds suggested that most of the compounds were found
to have potent urease inhibition activity. The ligand 3f was
found to be the most efective ligand with percentage inhi-
bition 86.17 0.89 and IC50 value 11.51 0.03 µM. The
molecular docking study highlighted that the same ligands
3e, 3f, and 3i illustrated high afnity to urease inhibition
with higher docking scores than the control ligand thiourea.
Ligands 3e, 3f, and 3i have proved very good inhibitory
potential against urease and showed higher docking scores
5718, 5940, 5596 and an ACE of − 246.66, − 244.79, and
− 243.06 kJ/mol, respectively, than control ligand thiourea
having score 1600 with an ACE value − 103.71 kJ/mol.
This is consistent with the urease inhibition values (in vitro)
of the ligands 3e, 3f, and 3i even better than standard drug
thiourea. In vitro and in silico results suggested that these
ligands are excellent urease inhibitors that make them efec-
tive anti-urease agents.
Fig. 4 Overlap of bound conformations of ligands 3e (pink), 3f
(golden), and 3i (purple) (color fgure online)
bonding with jack bean urease receptor. It revealed potential
for hydrophobic contact with pocket amino acids Leu839
,
Thr33, Tyr32, Phe712, Lys745, Glu742, Ala16, Pro743, Ala37,
Leu13. Hence, molecular docking studies of ligands 3a–3i
exhibited good urease inhibition activity and it is consistent
with the in vitro results.
Experimental
Conclusion
All chemicals used were purchase from Sigma, Pakistan.
All solid reactants were recrystallized prior use. Purity of
compounds was confrmed by thin layer chromatography
(TLC) using Al-backed pre-coated silica gel with fuorescent
indicator (0.25 mm thick layer). The spots were visualized
under UV lamp (λ=365 and 254 nm) of 8 W power as well
as using KMnO4 dip as universal locating agent. All syn-
thesized products were purifed either by column chroma-
tography using silica gel as stationary phase (0.6–0.2 mm,
60 Å mesh size, Merck) or by crystallization. Melting points
The current work dealt with a comparison of catalytic ef-
ciency of conventional protocols and silica-based heter-
ogenous catalysts for the synthesis of chalcones. A facile
and solvent-free synthesis of SiO2–H2SO4 and SiO2–MClx
catalysts was carried out and employed for preparation
of variety of substituted chalcones. The fndings suggest
SiO2–FeCl3 should be further explored for its catalytic activ-
ity for broader range of substituent. However, SiO2–H2SO4,
SiO2–NiCl2, and SiO2–ZnCl2 gave signifcant lower yields
1 3

A. Sultan et al.
Fig. 5 2D representation of the docked complexes 3e, 3f, and 3i
within jack bean urease receptor pocket showing their polar interac-
tions with the enzyme on right hand side and 3D poses of docked
ligands 3e, 3f, and 3i on left-hand side with PatchDock, showing
unfavorable bump (red), carbon hydrogen bond (light green), π-alkyl
(light pink), π-cation (brown), conventional hydrogen bond (green),
and amide π-stack (pink) interactions (color fgure online)
1 3

Silica-supported heterogeneous catalysts-mediated synthesis of chalcones as potent urease…
Table 5 Evaluation of
Entry
Catalyst
Cycle (yield/%)
Fresh
reversibility of silica-supported
catalysts
1
2
3
4
5
1
2
3
4
5
SiO2–H2SO4
SiO2–FeCl3
SiO2–NiCl2
SiO2–CuCl2
SiO2–ZnCl2
92
96
88
72
78
88
91
82
66
72
83
83
75
60
67
76
78
70
53
58
71
73
64
48
51
54
64
45
36
48
were determined using Gallenkamp melting point apparatus
(MF-8, Burladingen, Germany). The IR-spectra are recorded
on Prestige 21 spectrophotometer (Shimadzu, Japan) in KBr
discs. The LR-EIMS are carried out on a Fisons Autospec
Mass Spectrometer (VG, New Jersey, USA). The 1H NMR
(400 MHz) and 13C NMR (75 MHz) are recorded on Bruker,
Massachusetts, USA in CDCl3 using tetramethylsilane
(TMS) as internal standard.
white amorphous solid. Pure product was obtained as color-
less needles upon crystallization with dichloromethane.
Generalized protocol for SiO2–H2SO4 mediated
preparation of chalcones
To a well-stirred homogeneous solution of acetophe-
none (1 eq) and benzaldehyde (1.15 eq) was added 0.02 g
SiO2–H2SO4. The resulting suspension was stirred and
heated at 65 °C for 1.5 h. After completion of reaction, the
reaction mixture was cooled to ambient temperature and
fractioned between brine and CH2Cl2; the solid SSA was
fltered of. The SSA was washed with acetone to ensure
desorption of product from surface of catalyst. The organic
layer was dried over anhydrous Na2SO4, fltered, and con-
centrated under reduced pressure to aford the chalcone as
colorless solid [31].
Silica‑sulfuric acid
Sulfuric acid was added drop-wise to the well-stirred silica
gel (SiO2) and diethylether (Et2O) suspension. The resulting
reaction mixture was stirred for an hour followed by removal
of solvent in vacuo. The solid thus obtained was heated in
oven at 120 °C for 3 h to yield silica-sulfuric acid as a white
solid [31].
Generalized protocol for the SiO2–MClx mediated
preparation of chalcones
Silica‑supported metal catalysts
The silica-supported transition metal catalysts were pre-
pared by a modifed reported protocol [36]. In a 50 cm3
round-bottomed (RB) fask, 1.0 g silica and metal chloride
(1.0 mmol) were mixed in 5.0 cm3 of dry ethanol. The mix-
ture was stirred for 4 h under nitrogen atmosphere at room
temperature followed by removal of solvent under reduced
pressure at room temperature. The resulting solids were then
heated for 1 h at 100 °C to yield active metal chlorides dis-
persed over silica.
The heterogeneous mixture comprising of SiO2–MClx
(10 mol%), acetophenone (1 eq.), and benzaldehyde
(1.15 eq.) was stirred at room temperature for 7–110 min.
Progress of reaction was monitored by means of TLC. After
completion of reaction, the reaction mixture was fractioned
between brine and CH2Cl2 and catalyst (in solid state) was
fltered. To ensure desorption of product from the catalyst
surface, the SSA was washed with 25 cm3 acetonitrile. The
organic layer, upon drying over anhydrous Na2SO4 followed
by filtration and concentration under reduced pressure,
aforded chalcone as colorless solid. The characterization
data of substituted chalcones 3a–3i are given in Table 6.
Generalized protocol for H2SO4 mediated
preparation of chalcones under refuxing conditions
Determination of urease activity
Benzaldehyde (1.15 eq) was added to a stirred acidifed
methanolic solution (1.15 eq of H2SO4) of acetophenone
(1 eq.). The obtained reaction solution was heated under
refux for 3 h. Then, the solvent was removed under reduced
pressure and the reaction solution was sequentially neu-
tralized with NaHCO3:H2O (9:1) solution and fractioned
between H2O and EtOAc. The formed organic layers were
separated and dried over anhydrous Na2SO4, fltered, and
concentrated under reduced pressure to aford product as
Urease activity was determined using Berthelot assay.
The basic solution phenol and hypochlorite constitute the
Berthelot’s reagent, which develops blue complex with
ammonia. Ammonia is a product of urea hydrolysis [34].
The assay solution contained phosphate bufer (i.e. pH=7,
10, 50 mM), sample solution and enzyme solution (25 cm3,
0.015 unit) in each well of the 96-well plate. The solutions
1 3

A. Sultan et al.
Table 6 Physical
Compound
R–
Ar–
Rf*
M.p./°C
Lit. m.p./°C
Refer-
ences
characterization of diferent
substituted chalcones 3a–3i
3a
3b
3c
3d
3e
3f
3g
3h
3i
H
Ph
Ph-
2-furyl-
Ph-
2-furyl-
4-OMe-
Ph-
0.58
0.45
0.48
0.48
0.62
0.52
0.58
0.53
0.46
57
56–57
120.2–120.6
141
179–180
142
[37]
[31]
[38]
[39]
[39, 40]
[41]
[42]
[43]
[44]
3′-OH–
3′-OH–
4′-OH–
4′-OH–
4′-OH–
4′-Me–
4′-Me–
4′-Me–
120–121
140–142
180
142
158
108
64–67
107–111
158
108–110
62–64
110
2-furyl-
4-Me2NPh-
*Solvent system: EtOAc/n-hexane 1:3
were incubated at 37 °C for 10 min. Then, 40 cm3 of 20 mM
urea was incorporated to each well to reach the total volume
of 85 cm3. Contents were incubated at 37 °C for 10 min and
absorbance was measured at 625 nm using 96-well plate
reader Synergy HT, BioTek, USA. After that, 115 cm3 Berth-
elot’s reagent was added to each well and color intensity was
measured after 10 min. The following was employed for the
calculation of percentage inhibition:
9. Liu UM, Wilairat P, Croft SL, Tan AL, Go M (2003) Bioorg Med
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55:44
11. Singh HP, Chauhan CS, Pandeya SN, Sharma CS, Srivastava B,
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15. Vijay K (2010) Indian J Chem 49:1109
Percentage inhibition
[
16. Zuo Y (2012) Eur J Med Chem 50:393
]
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Gown AT, Rennison D (1998) Bioorg Med Chem 8:1051
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= 100 − (Abs. of test sample/Abs. of control) × 100 .
(1)
IC50 values of active compounds were determined from
EZ-Fit Enzyme kinetics software, Perrella International,
USA.
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Acknowledgements The authors are highly grateful to Higher Educa-
tion Commission (HEC) of Pakistan for funding Dr. Aeysha Sultan
under Start-up Research Grant Program (SRGP) for project No 1507,
as well as for fnancial assistance for EIMS and NMR analyses. We
are also thankful to Government College University Faisalabad for
assistance in FTIR and UV analyses. The authors thank the Research-
ers Supporting Project number (RSP-2019/6), King Saud University,
Riyadh, Saudi Arabia.
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