Self-catalyzed syntheses, structural characterization, DPPH radical scavenging-, cytotoxicity-, and DFT studies of phenoxyaliphatic acids of 1,8-dioxo-octahydroxanthene derivatives

Article abstract of DOI:10.1016/j.molstruc.2013.11.016

One-pot, in-water syntheses of phenoxyaliphatic acids of 1,8-dioxo-octahydroxanthene derived from dimedone and formylphenoxyaliphatic acids are reported. Geometries of compounds 2b, 2c, and 5a have been examined crystallographically. The synthesized compo

Full text of DOI:10.1016/j.molstruc.2013.11.016

Journal of Molecular Structure 1059 (2014) 51–60
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Self-catalyzed syntheses, structural characterization, DPPH radical
scavenging-, cytotoxicity-, and DFT studies of phenoxyaliphatic
acids of 1,8-dioxo-octahydroxanthene derivatives
G.S. Suresh Kumar a, A. Antony Muthu Prabhu b, P.G. Seethalashmi c, N. Bhuvanesh d, S. Kumaresan e,
⇑
a Faculty of Chemistry, Department of Science and Humanities, Dr. Mahalingam College of Engineering and Technology (Dr. MCET), Udumalai Road, Pollachi 642 003, Tamil Nadu, India
b Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli 627 012, Tamil Nadu, India
c Department of Chemistry, A.P.C. Mahalaxmi College for Women, Thoothukudi 628 002, Tamil Nadu, India
d Department of Chemistry, Texas A & M University, College Station, TX 77842, USA
e Department of Chemistry, Noorul Islam University, Kumaracoil, Thuckalai 629 180, Tamil Nadu, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
Phenoxyaliphatic acids act as self-
catalysts for the syntheses of
xanthenes.
ꢀ
ꢀ
ꢀ
Xanthene 2a has better radical
scavenging- and anticancer activities.
DFT studies confirm the stabilization
through intermolecular H-bonding.
Compound 2c has the highest energy
gap, highest hardness, and lowest
softness.
2c
ꢀ
Self-catalyzed formation of 2a is
proved by experimental and DFT
studies.
SCXRD of 2c
DFT of 2c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 August 2013
Received in revised form 6 November 2013
Accepted 6 November 2013
Available online 16 November 2013
One-pot, in-water syntheses of phenoxyaliphatic acids of 1,8-dioxo-octahydroxanthene derived from
dimedone and formylphenoxyaliphatic acids are reported. Geometries of compounds 2b, 2c, and 5a have
been examined crystallographically. The synthesized compounds showed better DPPH radical scavenging
activity and cytotoxicity against A431 cancer cell line. The molecular properties of all synthesized xanth-
enes have been investigated using single crystal XRD and DFT method. Self-catalyzed Bronsted–Lowry
acid catalytic behavior was also investigated by both experimental and theoretical methods.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Xanthenes
Formylphenoxyaliphatic acids
DPPH radical scavenging
DFT studies
1. Introduction
of molecular properties with DFT provides a close relationship be-
tween theory and experiment. This usually leads to important
Density Functional Theory (DFT) finds increasing use in applica-
tions related to biological systems. The calculation of a wide range
clues about the geometric, electronic, and spectroscopic properties
of the systems concerned. Advancements in methodology and
implementations have reached a point where predicted properties
of reasonable to high quality can be obtained [1]. B3LYP/3-21G(d)
method has been successfully used to predict the geometry and
electronic structure of a molecule due to its more advantages [2].
⇑
Corresponding author. Tel.: +91 94431 82502.
E-mail address: skumarmsu@yahoo.com (S. Kumaresan).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.11.016

52
G.S. Suresh Kumar et al. / Journal of Molecular Structure 1059 (2014) 51–60
Xanthenes have received more attention due to their bioactivi-
(s, 2H), 5.20 (s, 1H), 6.63 (d, J = 8.7 Hz, 1H), 6.89 (t, J = 7.5 Hz,
1H), 6.96 (d, J = 7.5 Hz, 1H), 7.11 (t, J = 7.8 Hz, 1H); 13C NMR
(75 MHz, d ppm): 18.7, 21.7, 23.6, 26.7, 35.3, 45.1, 59.9, 105.1,
110.1, 116.6, 122.5, 123.5, 127.6, 147.7, 157.9, 164.5, 191.9; IR
ties against human pathogens [3]. They exhibit antifungal-,
antiviral-, antibacterial-, anti-inflammatory, etc. activity [4].
Furthermore, xanthenes are used in dyes, laser technologies, and
fluorescent materials for visualization of biomolecules [5]. Due to
the versatile application possibilities of xanthenes, there is a
continuous exploration for the formulation of newer synthetic
entities via efficient methods. Phenoxyaliphatic acid derivatives
possess a wide range of diverse bioactivities such as anti-inflam-
matory, antioxidant, antibacterial, analgesic, antisickling, antipae-
mic, antiplatelet, DPPH radical scavenging, DNA protection,
non-prostanoid prostacyclin mimetic, diuretic, and growth
regulatory [6].
Water finds an important role in organic syntheses as solvent
because of its abundance and less toxicity. Based on the solubility
of starting materials in water and reaction conditions, water-med-
iated reactions are classified either as ‘in-water’ process or ‘on-
water’ process [7,8].
Syntheses of xanthenes from dimedone and aromatic alde-
hyde(s) using Bronsted–Lowry acid catalysts have been reported
earlier [9]. To the best of our knowledge, we have not encountered
any report regarding the use of aromatic aldehydes bearing carbox-
ylic acid group in the side chain for the formation of 1,8-dioxo-
octahydroxanthenes. This mooted us to carry the one-pot synthe-
ses of 1,8-dioxo-octahydroxanthenes from dimedone and formyl-
phenoxyacetic acid(s) at elevated temperature in water without
any external acid catalyst. In the present study, we have prepared
different xanthene derivatives making use of the inherent self cat-
alyzing capacity of the carboxylic acid group present in the phen-
oxyaliphatic acids. DPPH radical scavenging activity, cytotoxicity,
and molecular properties of the newly synthesized compounds
were studied by single crystal XRD and DFT methods.
(KBr, m
, cmÀ1): 1756, 1664; ESI MS: m/z (M + H) 425.0. Anal.Calcd.
for C25H28O6: C, 70.74; H, 6.65%. Found: C, 70.72; H, 6.68%.
2.2.2. 2-(2-methoxy-6-(3,3,6,6-tetramethyl-1,8-dioxo-2,3,4,5,6,7,8,9-
octahydro-1H-xanthen-9-yl)phenoxy)acetic acid (2b)
Pale yellow solid, 84%, m.p. 221–223 °C; 1H NMR (300 MHz, d
ppm): 0.97 (s, 6H), 1.08 (s, 6H), 2.14 (dd, 4H), 2.49 (s, 4H), 3.78
(s, 3H), 4.67 (s, 2H), 4.99 (s, 1H), 6.64 (d, J = 7.3 Hz, 1H), 6.74 (d,
J = 7.8 Hz, 1H), 6.88 (t, J = 7.8 Hz, 1H); ESI MS: m/z (M-H) 453.4;
Anal.Calcd. for C26H30O7: C, 68.71; H, 6.65%. Found: C, 68.76; H,
6.69%.
2.2.3. 2-[4-(3,3,6,6-Tetramethyl-1,8-dioxo-2,3,4,5,6,7,8,9-octahydro-
1H-xanthen-9-yl)phenoxy]acetic acid (2c)
Pale yellow solid, 80%, m.p. 210–212 °C; 1H NMR (300 MHz, d
ppm): 0.99 (s, 6H), 1.11 (s, 6H), 2.00 (s, 4H), 2.46 (s, 4H), 4.75 (s,
2H), 5.30 (s, 1H), 7.24 (m, 4H); IR (KBr, cmÀ1): 1758, 1662; ESI
MS: m/z (M + H) 425.0; Anal.Calcd. for C25H28O6: C, 70.74; H,
6.65%. Found: C, 70.76; H, 6.69%.
2.2.4. 3,3,6,6-Tetramethyl-9-phenyl-3,4,5,6,7,9-hexahydro-1H-
xanthene-1,8(2H)-dione (5a)
Pale yellow solid, m.p. 188–190 °C, 86%, 1H NMR (300 MHz, d
ppm): 0.99 (s, 6H), 1.10 (s, 6H), 2.20 (4H), 2.67 (s, 4H), 4.75 (s,
1H), 7.12 (d, J = 7.5 Hz, 1H), 7.21 (t, J = 7.2 Hz, 2H), 7.27 (d,
J = 8.1 Hz, 2H); Anal.Calcd. for C23H26O3: C, 78.83; H, 7.48%; Found:
C, 78.80; H, 7.44.
2.2.5. Ethyl 2-[2-(3,3,6,6-tetramethyl-1,8-dioxo-2,3,4,5,6,7,8,9-
octahydro-1H-xanthen-9-yl)phenoxy]acetate (5b)
2. Experimental
Yellow solid, 82%, m.p. 178–180 °C; 1H NMR (300 MHz, d ppm):
0.97 (s, 6H), 1.08 (s, 6H), 1.29 (t, J = 7.0 Hz, 3H), 2.17 (s, 2H), 2.53 (s,
2H), 4.25 (q, J = 6.9 Hz, 2H), 4.54 (s, 2H), 4.87 (s, 1H), 6.63 (d,
J = 8.1 Hz, 1H), 6.91 (t, J = 7.5 Hz, 1H), 7.09 (t, J = 7.5 Hz, 1H), 7.68
(d, J = 8.1 Hz, 1H); 13C NMR (75 MHz, d ppm): 14.0, 27.1, 29.0,
29.9, 31.8, 40.7, 50.6, 60.8, 65.9, 111.5, 113.3, 121.2, 127.6, 130.7,
132.6, 156.0, 162.7, 168.4, 196.4; IR (KBr, cmÀ1): 1762, 1662; ESI
MS: m/z (M + H) 453.0; Anal.Calcd. for C27H32O6: C, 71.66; H,
7.13%; Found: C, 71.62; H, 7.16%.
2.1. General
Phenoxyaliphatic acids [10] and their corresponding esters [11]
were synthesized as per the reported methods. Melting points
were measured in open capillary tubes and are uncorrected. Infra-
red spectra were recorded on a JASCO FT-IR Model 410 spectropho-
tometer (KBr disc). Band positions are reported in reciprocal
centimetres (cmÀ1). The 1H NMR and 13C NMR spectra were re-
corded on a Bruker (Avance) 300 MHz NMR instrument using
TMS as internal standard and CDCl3 as solvent. The electrospray
mass spectra were recorded on a THERMO Finnigan LCQ Advantage
2.3. Single crystal XRD
The crystal structures were determined using a BRUKER APEX 2
X-ray (three-circle) diffractometer. The data reduction was done
with the program APEX2 [12]. The absorption correction was em-
ployed using the program SADABS [13]. The structure solution
was obtained using SHELXTL (XS) [14] and refined on F2 to conver-
gence [14,15]. Absence of additional symmetry was verified using
PLATON (ADDSYM) [16]. Powder X-ray diffraction data were re-
corded at room temperature using XPERT-PRO diffractometer sys-
max ion trap mass spectrometer. Samples (10 lL) (dissolved in sol-
vent such as methanol/acetonitrile/water) were introduced into
the ESI source through Finnigan surveyor autosampler. Elemental
analyses were performed on a Perkin Elmer 2400 Series II Elemen-
tal CHNS analyzer. Silica gel-G plates (Merck) were used for TLC
analysis with a mixture of hexanes and ethyl acetate as eluent.
2.2. General synthesis of phenoxyaliphatic acids of xanthene
tem with CuK 1, CuK 2, and CuKb with radiation of wavelength
a
a
1.54060, 1.54443, and 1.39225 Å respectively.
Formylphenoxyaliphatic acid (1 equi.) was dissolved in water
(10 mL) at 80 °C and dimedone (2 equi.) was added with vigorous
stirring. After 10 min, the reaction mixture was cooled to room
temperature, the solid was filtered, washed with water, dried well,
and recrystallized from hot ethanol.
2.4. DPPH radical scavenging assay
The radical scavenging activity of the compounds was evaluated
as per the method of Blois [17] with a slight modification [18]. The
compounds (2a–c, 5b) and BHA at different concentrations (25, 50,
75, and 100 ppm in 1 mL) were taken in different test tubes. Four
milliliters of 0.1 mM methanolic solution of DPPH was added to
these tubes and shaken vigorously. The tubes were allowed to
stand at 27 °C for 20 min. The control was prepared as above in
2.2.1. 2-[2-(3,3,6,6-Tetramethyl-1,8-dioxo-2,3,4,5,6,7,8,9-octahydro-
1H-xanthen-9-yl)phenoxy]acetic acid (2a)
Pale yellow solid, 82%, m.p. 206–208 °C; 1H NMR (300 MHz, d
ppm): 0.99 (s, 6H), 1.11 (s, 6H), 2.21 (s, 4H), 2.49 (s, 4H), 4.85

G.S. Suresh Kumar et al. / Journal of Molecular Structure 1059 (2014) 51–60
53
the absence of any compound and methanol was used for the base-
line correction. Optical density (OD) of the samples was measured
at 517 nm. Radical scavenging activity was expressed as the inhibi-
tion percentage and was calculated using the following formula:
% Radical scavenging activity = (Control OD – Sample OD/Con-
trol OD) Â100.
energy of the compound derived from electron-transfer (radical
cation) and the respective neutral compound; IPE = Ecation À En;
IPo = ÀEHOMO, the electron affinity is computed as the energy differ-
ences between the neutral molecule and the anion molecule:
EA = En À Eanion; EAo = ÀELUMO, respectively. From these calcula-
tions the other parameters such as electronegativity (
v), electro-
chemical potential
(
l), hardness
(
g), softness
(r), and
2.5. In vitro anticancer activity
electrophilicity index (
x
) have been calculated [22–25].
The human skin epithelial cell carcinoma cell line (A431), hu-
man stomach adenocarcinoma cancer cell line (AGS), and glioblas-
toma cell line (U373MG) were obtained from National Centre for
Cell Science (NCCS), Pune and grown in Dulbecco’s Modified Eagle’s
Medium (DMEM) containing 10% fetal bovine serum (FBS). All cells
were maintained at 37 °C, 5% CO2, 95% air, and 100% relative
humidity. Maintenance cultures were passaged weekly and the
culture medium was changed twice a week [19].
3. Results and discussion
3.1. Syntheses of phenoxyaliphatic acids of xanthenes
We studied the model reaction for the synthesis of 2-[2-{(3,4,6,
7-tetrahydro-3,3,6,6-tetramethylxanthen-1,8-dione-2-yl)methyl}
phenoxy]acetic acid (2a) derived from dimedone (5 mmole) and 2-
formylphenoxyacetic acid (2.5 mmole).
Experiments were carried out in various solvents like hexanes,
benzene, toluene, acetonitrile, DMF, ethanol, aqueous ethanol
(1:1), and water. The reactions did not proceed in hexanes, ben-
zene, and toluene, but scarcely proceeded in acetonitrile, DMF,
2.5.1. Cell treatment procedure
The monolayer cells were detached with trypsin-ethylenedi-
aminetetraacetic acid (EDTA) to make single cell suspensions and
the viable cells were counted using a hemocytometer and diluted
with a medium containing 5% FBS to a give final density of
1 Â 105 cells/mL. One hundred microlitres per well of cell suspen-
sion were seeded into 96-well plates at a plating density of 10,000
cells/well and incubated to allow for cell attachment at 37 °C, 5%
CO2, 95% air, and 100% relative humidity. After 24 h, the cells were
treated with serial concentrations of the extracts and fractions.
They were initially dissolved in DMSO and further diluted in serum
free medium to produce five concentrations. One hundred microli-
tres per well of each concentration was added to plates to obtain
final concentrations of 300, 150, 75, 37.5, and 18.75
volume in each well was 200 L and the plates were incubated
at 37 °C, 5% CO2, 95% air and 100% relative humidity for 48 h. The
medium without samples served as control. Triplicate was main-
tained for all concentrations.
and
ethanol
to
give
2-[2-{bis(4,4-dimethyl-2,6-diox-
ocyclohexyl)methyl}phenoxy]acetic acid (4c) (vide Supporting
information). In the case of aqueous ethanol as solvent, both 2a
(15%) and 4c (42%) were observed. In aqueous medium, the reac-
tion afforded 2a (78%). Subsequently, further optimization of the
reaction conditions, including reaction temperature, reaction time,
and volume of water was investigated. Satisfyingly, after a series of
experiments, water was proved to be the best solvent and the yield
of 2a reached 82% when the reaction was performed in water
(10 mL for 5 mmole) at 80 °C for 10 min (Scheme 1). Both starting
materials were miscible at 80 °C. After 10 min, the product 2a was
thrown out from the clear solution. Thus, this reaction seems to oc-
cur via ‘in-water process’ [8].
The pH of the reaction mixture was monitored at 5.84. This
manifests the formation of hydronium ion from 2-formylphenoxy-
acetic acid (pKa 3.04) in water during the reaction (Fig. 1). Cyclode-
hydration was ensued in a single step (Fig. 1). Earlier reports
confirmed the need for an acid or a base as catalyst for the cyclode-
hydration [9,26].
Carboxylic ester aldehyde (3b) and sodium salt of acid aldehyde
(3c) were used to prove the essentiality of the carboxylic acid
group for the reaction to go (vide Supporting information for the
evidence for self-catalyzed in-water reaction). Both ethyl 2-(2-
formylphenoxy)acetate (3b) and sodium salt of 2-formylphenoxy-
acetic acid (3c) behaved similar to benzaldehyde (3a) [26].
From one-pot syntheses of xanthenes (2a, 5a, and 5b) (vide Sup-
porting information, Scheme s1), it is believed that the carboxylic
acid group in 2-formylphenoxyacetic acid behaves as a Bronsted–
Lowry acid catalyst in water [27] affording 2a (Fig. 1). Based on
this, we proposed a reasonable reaction mechanism for the synthe-
sis of 2a through ‘in-water’ process (Fig. 1). We extended the same
lM. The final
l
2.5.2. MTT assay
MTT is a yellow water soluble tetrazolium salt. A mitochondrial
enzyme in living cells, succinate-dehydrogenase, cleaves the tetra-
zolium ring, converting the MTT to an insoluble purple formazan.
The amount of formazan produced is directly proportional to the
number of viable cells. After 48 h of incubation, 15 lL of MTT
(5 mg/mL) in phosphate buffered saline (PBS) was added to each
well and incubated at 37 °C for 4 h. The medium with MTT was
then flicked off and the formazan crystals formed were solubilized
in 100 lL of DMSO and the absorbance was measured at 570 nm
using a microplate reader. The % cell inhibition was determined
using the following formula.
% Cell inhibition = 100 – Abs (sample)/Abs (control) Â100.
Nonlinear regression graph was plotted between % cell inhibi-
tion and log10 concentration and IC50 was determined using Graph-
Pad Prism software.
2.6. Computational methodology
The theoretical energy calculation of xanthenes in gas phase
was performed using the analytical gradient method of DFT with
Becke’s three parameters (B3) exchange functional together with
the Lee–Yang–Parr (LYP) non-local correlation functional, symbol-
ized B3LYP [20] by means of 3-21G (d) basis set implemented in
Gaussian 09 W software package [21]. The electronic properties
were calculated from both the Koopmans’ theorem (orbital energy
consideration represented by subscript O) and the total energies
denoted with subscript E of the species as follows: the ionization
potential is calculated as the energy differences between the
Scheme 1. Synthesis of 2 from dimedone and formylphenoxyacetic acids.

54
G.S. Suresh Kumar et al. / Journal of Molecular Structure 1059 (2014) 51–60
Fig. 1. Synthesis of 2a through ‘in-water’ process.
condition for the syntheses of xanthenes 2b and 2c from 2-(2-for-
myl-6-methoxyphenoxy)acetic acid (1b) and 4-formylphenoxy-
acetic acid (1c) with dimedone. The newly synthesized
compounds 2a, 2b, 2c, and 5b were characterized by single crystal
XRD, IR, NMR, and mass analysis.
[28] designator C11(13) forming a one dimensional zig-zag chain
(Figs. 4 and 5). The O6–H6Á Á ÁO2 bond distance is 2.623 Å which
is nearly linear with an angle of 169.99°.
In compound 5b, O6–C26–C27 group is found to be disordered
and successfully modeled in two positions. Only one position of the
ORTEP diagram is shown for clarity [Fig. 2(c)]. Compound 5b is sta-
bilized by short contacts without any H-bonding. Compounds 2c
3.2. Single crystal XRD studies
and 5b have C–HÁ Á Á
p
interactions. The crystal structures of 2b,
stacking force.
2c, and 5b do not exhibit any
p–p
Compounds 2b and 2c were crystallized using aqueous ethanol
(1:1 v/v) and compound 5b was crystallized using ethanol under
slow evaporation. These compounds have been characterized by
single crystal X-ray diffraction studies. The ORTEP views of the
compounds 2b, 2c, and 5b are shown in Fig. 2. The crystallographic
data and structural refinement details for the compounds 2b, 2c,
and 5b are given in Table 1.
Systematic reflection conditions and statistical tests of the data
suggested that the compound 2b was crystallized as 2bÁH2O. It has
a centrosymmetric space group P21/c. In the crystal lattice, two
molecules of 2b are joined together through two water molecules
by intermolecular hydrogen bonding. Hydrogen bonding is formed
between H-atom of carboxyl group and O-atom of water molecule.
The C–O bond lengths [C26–O6: 1.212 Å, C26–O7: 1.321 Å] indi-
cate that the acid moiety is present as –COOH. The different types
of hydrogen bonding present in this crystal are O7–H7Á Á ÁO20(w),
O20(w)–H20A(w)Á Á ÁO2, and O20(w)–H20B(w)Á Á ÁO2. Carbonyl oxy-
gen forms bifurcated hydrogen bonding with two water molecules.
Two molecules of the compound 2b are connected together
through two water molecules by three types of hydrogen bondings
with Etter’s graph set [28] notation R22(13). In the formation of the
dimer, a square ring with graph set designation R24(8) is produced
(Fig. 3).
3.3. DPPH radical scavenging activity
The role of antioxidants is their interaction with oxidative free
radicals. The free radical scavenging activity of the compounds
2a–c and 5b was tested using DPPH model system [17,18] and
the results are presented in Table 2. It reveals that at 25 ppm con-
centration, 2a has better scavenging activity (72.46 3.84) fol-
lowed by 2c. Ester 5b has lower scavenging activity than its
corresponding acid 2a.
3.4. In vitro anticancer activity
We screened the growth inhibition or antiproliferative effects of
2a–c and 5b on the cancer cell lines A431, AGS, and U373MG fol-
lowing MTT assay [19]. These xanthenes have IC50 more than
300 lM on AGS and U373MG cell line. Table 3 reveals that 2a
has lower IC50 (128.2) than other derivatives on A431 cell line (vide
Supporting information for the growth inhibition effect of 2a on
the cancer cell line A431).
The crystal structure of 2c shows that the molecules are self-
associated. An intermolecular hydrogen bonding motif O6–
H6Á Á ÁO2 is observed between the carboxyl group of one molecule
and the carbonyl oxygen of another molecule. The C–O bond
lengths [C25–O6: 1.328 Å, C25–O5: 1.197 Å] indicate that the acid
moiety is present as –COOH. These molecules are connected
through O6–H6Á Á ÁO2 hydrogen bonding with Etter’s graph set
3.5. Theoretical studies using DFT method
Gas phase geometry optimizations were performed starting
from the crystal structure 2bÁH2O, 2c, and 5b at the B3LYP/3-
21G(d) level of theory [21]. The calculated and experimental values
of the selected geometrical parameters of 5b are given in Table 4.
The calculated geometrical parameter represents good correlation

G.S. Suresh Kumar et al. / Journal of Molecular Structure 1059 (2014) 51–60
55
Fig. 2. ORTEP view of (a) 2bÁH2O, (b) 2c, and (c) 5b.
Table 1
Crystal data and structure refinement of 2bÁH2O, 2c, and 5b.
2bÁH2O
2c
5b
CCDC no.
882629
26H32O8
472.52
Colourless block
0.40 Â 0.32 Â 0.16
0.71073 Å
875572
C25H28O6
424.47
Colourless thin plate
0.10 Â 0.07 Â 0.01
1.54178 Å
Orthorhombic
Pna2(1)
875479
C27H32O6
452.53
Colourless thin plate
0.20 Â 0.16 Â 0.14
0.71073 Å
Monoclinic
P2(1)/c
Empirical formula
Formula weight
Appearance
C
Crystal size (mm3)
Wavelength (Å)
Crystal system
Space group
Monoclinic
P21/c
Unit cell dimensions
a = 10.2999(18) Å
b = 17.749(3) Å
c = 13.256(2) Å
a = 19.5079(17) Å
b = 5.9111(6) Å
c = 18.7710(17) Å
a = 9.6070(11) Å
b = 23.486(3) Å
c = 11.1112(13) Å
a
=
c
= 90°
a
= b =
c
= 90°
a = c = 90°
b = 100.370(2)°
2383.8(7) Å3
4
b = 109.8120(10)°
2358.6(5) Å3
4
Volume
Z
2164.5(3) Å3
4
Density (calculated)
Absorption coefficient
F(000)
1.317 Mg/m3
1.303 Mg/m3
1.274 Mg/m3
0.097 mmÀ1
0.756 mmÀ1
0.089 mmÀ1
1008
904
968
Theta range for data collection
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
Largest diff. peak and hole
2.29–27.49°
1.044
R1 = 0.0457, wR2 = 0.1164
R1 = 0.0743, wR2 = 0.1343
0.259 and À0.265 e ÅÀ3
4.53–60.00°
1.039
R1 = 0.0549, wR2 = 0.1430
R1 = 0.0648, wR2 = 0.1503
0.244 and À0.289 e.ÅÀ3
2.25–27.46°
1.052
R1 = 0.0406, wR2 = 0.0996
R1 = 0.0536, wR2 = 0.1095
0.445 and À0.366 e.ÅÀ3

56
G.S. Suresh Kumar et al. / Journal of Molecular Structure 1059 (2014) 51–60
Fig. 3. H-bonding pattern for 2b.
Fig. 4. H-bonding pattern for 2c in 1D zig-zag chain.
Fig. 5. 2D view for 2c along b axis.
Table 2
Table 3
DPPH radical scavenging activity of 2a–c and 5b.
Calculation of IC50 using % growth inhibition against log10 concentration (lM) for
A431 cell line.
Samples
% Radical scavenging activity at different concentrations (ppm)
25
50
75
100
Compounds
2a
2b
2c
5b
2a
2b
2c
5b
72.46 3.84
60.49 2.26
61.84 0.83
42.88 4.20
93.81 0.01
73.76 8.27
61.04 5.13
62.23 4.41
44.76 8.68
90.85 0.89
77.73 1.32
63.98 3.62
68.87 1.48
46.07 8.42
96.58 0.19
79.23 1.53
61.95 2.54
67.04 1.66
49.91 6.93
95.04 0.13
IC50
(
lM)
128.2
258.4
288.4
>300
BHA
optimized structures of 2a, 2b, 2c, and 5b are shown in Fig. 6 (vide
Supporting information, 2aÁH2O and 2bÁH2O).
Table 5 reveals the total energies, frontier molecular orbital
energies, and dipole moments of the compounds in gas phase.
The obtained total energy values are À886730.54 kcal/mol for 2a,
À934439.46 kcal/mol for 2aÁH2O, À958204.43 kcal/mol for 2b,
À1005909.46 kcal/mol for 2bÁH2O, À886727.59 kcal/mol for 2c
and À935805.04 kcal/mol for 5b. The total energy difference of
the hydrated xanthenes (2aÁH2O and 2bÁH2O) and their parent
compounds (2a and 2b) is 47707 kcal/mol. The geometry of O2
and H7Á Á ÁO7 groups in 2bÁH2O helps accommodating a water mol-
with experimental values which can germ into the calculation of
the other parameters for the compound 5b.
Similarly the ground state geometrical optimization was done
on 2bÁH2O and 2c. The theoretical parameters were found to be
nearly the same as the experimental parameters (Table 4). Our pre-
vious work disclosed that the ortho- substituted phenoxyaliphatic
acid derivatives were stabilized through water molecule [6b,18b].
So we used the same DFT method for the geometrical optimization
[29] of the molecule 2a and its monohydrate (2aÁH2O). The

G.S. Suresh Kumar et al. / Journal of Molecular Structure 1059 (2014) 51–60
57
Table 4
Selected bond lengths and bond angles of 2bÁH2O, 2b, 2c, and 5b.
2bÁH2O
2b
2c
5b
Bond
XRD data
DFT data
DFT Data
Bond
XRD data
DFT data
Bond
XRD data
DFT Data
Bond length (Å)
C1–C2
C4–C5
C2–C3
C3–C4
C3–H3
C9–O2
C12–O3
C26–O6
C26–O7
O7–H7
1.344(2)
1.331(2)
1.513(2)
1.511(2)
1.000
1.230(18)
1.222 (19)
1.212(2)
1.321(2)
0.840
1.344
1.341
1.517
1.515
1.092
1.243
1.240
1.222
1.363
1.007
1.344
1.341
1.514
1.513
1.091
1.246
1.242
1.236
1.334
1.063
C1–C6
1.355(5)
1.347(6)
1.503(5)
1.509(5)
1.000
1.244(5)
1.232(5)
1.197(6)
1.329(5)
0.850
1.343
1.342
1.513
1.518
1.094
1.243
1.240
1.216
1.385
0.997
C1–C6
C10–C15
C6–C9
C9–C10
C9–H9A
C5–O2
C11–O3
C25–O5
C25–O6A
C25–O6B
1.340(17)
1.343
1.341
1.515
1.515
1.094
1.242
1.243
1.226
1.494
–
C10–C15
C9–C10
C6–C9
C9–H9A
C5–O2
C11–O3
C25–O5
C25–O6
O6–H6
1.340(18)
1.511(17)
1.511(17)
1.000
1.225(15)
1.222(16)
1.188(19)
1.413(2)
1.392(3)
Bond angle (°)
C6–C7–C8
C13–C14–C15
C1–O1–C5
C2–C3–C4
C2–C9–C8
C4–C12–C13
C18–C3–H3
O6–C26–O7
C26–O7–H7
C20–O4–C24
C19–O5–C25
107.33(13)
108.16(13)
118.11(12)
108.96(13)
118.39(13)
117.64(14)
109.20
121.06(16)
109.50
116.06(13)
114.86(12)
108.23
108.04
118.16
110.18
116.03
116.03
106.79
123.70
105.78
118.78
116.20
108.18
108.32
118.36
110.53
116.43
115.94
107.12
123.17
117.18
120.86
118.18
C2–C3–C4
107.9(3)
107.9(3)
118.2(3)
108.6(3)
118.5(3)
118.3(3)
109.0
125.3(4)
113.3
119.5(3)
108.3
108.4
117.8
109.8
116.2
116.1
107.8
123.9
108.6
118.1
C2–C3–C4
108.93(10)
111.77(11)
118.22(10)
109.02(10)
118.70(11)
118.22(11)
107.3
108.20
108.16
117.97
109.93
116.16
116.07
106.59
124.25
C12–C13–C14
C1–O1–C15
C6–C9–C10
C6–C5–C4
C10–C11–C12
C18–C9–H9A
O5–C25–O6
C25–O6–H6
C21–O4–C24
C12–C13–C14
C1–O1–C15
C6–C9–C10
C6–C5–C4
C10–C11–C12
C18–C9–H9A
O5–C25–O6A
O5–C25–O6B
123.53(17)
120.77(17)
ecule through H-bonding for further stabilization. This is sup-
ported by DFT studies (Table 4).
by different colours [30]. In the case of 2bÁH2O (Fig. 7), the neg-
ative region (red) is localized on the carbonyl oxygen (O2),
phenoxy oxygen (O5) and oxygen from water (O20) with a min-
imum value of À0.09031 a.u. However, the positive region (light
blue) is localized on the carboxylic acid hydrogen (H7) and the
H atoms H20A and H20B of water with a maximum value of
0.06061 a.u. The green region represents segments of zero
potential.
3.5.1. Molecular Electrostatic Potential (MEP)
Molecular Electrostatic Potential (MEP) of 2aÁH2O, 2bÁH2O,
2c, and 5b was determined using the same DFT method (vide
Supporting information, 2aÁH2O, 2c, and 5b). The different val-
ues of the electrostatic potential at the surface are represented
Fig. 6. Optimized structures of (a) 2a, (b) 2b, (c) 2c, and (d) 5b.

58
G.S. Suresh Kumar et al. / Journal of Molecular Structure 1059 (2014) 51–60
Table 5
Some molecular properties of xanthenes calculated using DFT at the B3LYP/3-21G(d) basis set in gaseous phase.
Entry
Total energy (kcal/mol) EHOMO (eV) ELUMO (eV)
D
E (eV) Dipole moment (Debye) I (eV)
A (eV)
v
(eV)
l
(eV)
g
(eV)
r
(eV)
x (eV)
2a
À886730.54
À934439.46
À958204.43
À6.1271
À6.5356
À5.9535
À5.5620
À6.3756
À5.8964
À1.4364
À2.0554
À1.7585
À1.9420
À1.5602
À1.4214
4.6907
4.4802
4.1950
3.6200
4.8154
4.4750
4.9442
12.9432
10.3698
11.9313
4.1523
6.1271 1.4364 3.7817 À3.7817 2.3453 0.4264 3.0489
6.5356 2.0554 4.2955 À4.2955 2.2401 0.4464 4.1184
5.9535 1.7585 3.8560 À3.8560 2.0975 0.4767 3.5444
5.5620 1.9420 3.7520 À3.7520 1.8100 0.5525 3.8888
6.3756 1.5602 3.9679 À3.9679 2.4077 0.4153 3.2695
5.8964 1.4214 3.6589 À3.6589 2.2375 0.4469 2.9916
2aÁH2O
2b
2bÁH2O À1005909.46
2c
5b
À886727.59
À935805.04
4.6925
I – ionization potential; A – electron affinity;
v
– electronegativity;
l
– electrochemical potential; g – absolute hardness; r – softness; x – nucleophilicity.
Fig. 7. Molecular Electrostatic Potential (MEP) of 2bÁH2O.
Thus, Fig. 7 confirms the existence of an intermolecular O–
HÁ Á ÁO interaction. This was confirmed from the single crystal
XRD studies (Fig. 3). The Molecular Electrostatic Potential map of
2b (vide Supporting information) is nearly the same as that of
2bÁH2O (Fig. 7). The maximum and minimum potential values
are different for 2b (À0.08276 to +0.08276 a.u) and 2bÁH2O
(À0.09301 to +0.09301 a.u). This further confirms that the pres-
ence of a water molecule increases the stability of 2bÁH2O.
In the MEP of 2c (vide Supporting information), the negative re-
gion (red) is located on the carbonyl oxygen atoms (O2, O3, and
O5) with a minimum value of À0.07064 a.u, and the maximum po-
sitive region (blue) on carboxylic acid hydrogen atom (H6O) with a
maximum value of 0.07064 a.u. This confirms the existence of an
intermolecular O–HÁ Á ÁO interaction. Fig. 4 reveals that the inter-
molecular H-bonding was observed in single crystal XRD. But in es-
ter 5b, the MEP (vide Supporting information) revealed the
negative region (red) on carbonyl oxygen atoms (O2, O3, and O5)
with a maximum value of À0.07047 a.u but no positive region
(blue) was observed. This showed that there is no intermolecular
dipole–dipole interaction and the molecule may be stabilized by
short contacts. Single crystal XRD of 5b also confirmed the pres-
ence of short contacts. Using the above data, the intermolecular
interaction of 2aÁH2O (no single crystal) was solved using MEP.
Fig. 7 showed that 2aÁH2O has similar type of MEP as that of 2bÁH2-
O, excepting that the Molecular Electrostatic Potential energy is
varied (À0.09163 to +0.09163 a.u).
3.5.2. Frontier molecular orbital analysis
We have investigated the electronic structures of the com-
pounds 2a–c and 5b using DFT method [31]. The electron density
of HOMO is the same for 2a, 2c and 5b (Fig. 8). The electron density
is mainly located on the phenyl ring with a little amount on the
middle ring and the carbonyl group of other two rings of xanthene.
But the electron density for 2b is mainly located on the phenyl ring.
The electron density of LUMO for 2a, 2c, and 5b is located with the
major portion on the middle ring and the carbonyl group of xan-
thene and a very little amount in phenyl ring whereas for 2b that
is present at the middle ring and the carbonyl group of xanthene
part. Among the non-hydrated compounds 2a–c and 5b, com-
pound 2b has minimum HOMO–LUMO energy gap (4.19 eV). These
results suggest that 2b enjoys more stability than the other
molecules.
The proposed reaction mechanism is also supported by energy
minimization studies [32]. The HOMO–LUMO energy gap for the
intermediates I, II, III, IV, and V (Fig. 1) are 3.0265, 6.2548,
5.7677, 6.3296, and 5.6877 eV respectively (vide Supporting infor-
mation). These values indicate that when the reaction takes place,
the
DE increases for the formation of intermediate I to V as pro-
posed in Fig. 1. The change in energy (4.6907 eV) is much signifi-
cant in V which results in the formation of the final product, 2a.
The optimized structures of the intermediates III, IV, and V show
the proton transfer from ÀCOOH group to the corresponding
nucleophilic site. This is also observed in the para- isomer (vide

G.S. Suresh Kumar et al. / Journal of Molecular Structure 1059 (2014) 51–60
59
bonding with water molecules have the lowest values. Single crys-
tal XRD and DFT studies confirmed that the hydrated ortho isomers
are more stable. Experimental and theoretical results confirmed
the proposed mechanistic aspects of the self-catalyzed syntheses
HOMO
LUMO
of
phenoxyaliphatic
acid
derivatives
of
1,8-dioxo-
octahydroxanthene.
Acknowledgements
Authors thank Dr. K. Ramalakshmi, Plantation Products Spices
and Flavour Technology Department, Central Food Technological
Research Institute, Council of Scientific and Industrial Research
(CSIR), Mysore, India. One of the authors (G.S. Suresh Kumar) grate-
fully thanks Madurai Kamaraj University, Madurai, Tamilnadu, In-
dia and CDRI, Lucknow, India for spectral studies.
(a)
(b)
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.201
3.11.016.
References
[1] (a) M. Orio, D.A. Pantazis, F. Neese, Photosynth Res. 102 (2009) 443;
(b) F.D. Proft, P. Geerlings, Chem. Rev. 101 (2001) 1451.
[2] M.E. Zandler, F. D’Souza, C.R. Chimie 9 (2006) 960.
[3] (a) J. Jeyakanthan, D. Velmurugan, Cryst. Res. Technol. 34 (1999) 1334;
(b) A. Domling, I. Ugi, Angew. Chem., Int. Ed. Engl. 39 (2000) 3168.
[4] (a) S.B. Krasnoff, D. Faloon, E. Williams, D.M. Gibson, Biocont. Sci. Tech. 9
(1999) 215;
(b) R.W. Lambert, J.A. Martin, J.H. Merrett, K.E.B. Parkes, G.J. Thomas, PCT Int.
Appl. WO 9706178, Chem. Abstr. 126 (1997) 212377y;
(c) T. Hideo, Jpn. Tokkyo Koho JP 56005480, Chem. Abstr. 95 (1981) 80922b;
(d) J.P. Poupelin, G. Saint-Rut, O. Foussard-Blanpin, G. Narcisse, G. Uchida-
Ernouf, R. Lacroix, Eur. J. Med. Chem. 13 (1978) 67.
(c)
(d)
[5] S.M. Menchen, S.C. Benson, J.Y.L. Lam, W. Zhen, D. Sun, B.B. Rosenblum, S.H.
Khan, M. Taing, Chem. Abstr. 139 (2003) p54287f. U. S. Patent, US6583168;
(b) O. Sirkeeioglu, N. Talinli, A. Akar, J. Chem. Res. Synop. (1995) 502;
(c) M. Ahmad, T.A. King, B.H. Cha, J. Lee, J. Phys. D: Appl. Phys. 35 (2002) 1473.
[6] (a) V. Bala, Y.S. Chhonker, S.R. Hashim, Asian J. Chem. 22 (5) (2010) 3447;
(b) G.S. Suresh Kumar, P.G. Seethalakshmi, N. Bhuvanesh, S. Kumaresan, J. Mol.
Str. 1034 (2013) 302;
(c) G.S. Suresh Kumar, P.G. Seethalakshmi, D. Sumathi, N. Bhuvanesh, S.
Kumaresan, J. Mol. Str. 1035 (2013) 476;
(d) G.S. Suresh Kumar, P.G. Seethalakshmi, N. Bhuvanesh, S. Kumaresan, J. Mol.
Str. 1050 (2013) 88.
[7] S. Marc-Olivier, L. Chao-Jun, Chem. Soc. Rev. 41 (2012) 1415.
[8] R.N. Butler, A.G. Coyne, Chem. Rev. 110 (2010) 6302.
[9] (a) H. Guang-Fan, C. Bin, C. Li-Zhuang, H. Xiao-Lei, J. Heterocyclic Chem. 49
(2012) 195;
Fig. 8. HOMO–LUMO energy structures of (a) 2a, (b) 2b, (c) 2c, and (d) 5b.
Supporting information). This confirms the self catalyzed Bron-
sted–Lowry acid catalytic process.
(b) P.P. Salvi, A.M. Mandhare, A.S. Sartape, D.K. Pawar, S.H. Han, S.S. Kolekar,
Comptes Rendus Cheimie 14 (2011) 883;
(c) F. Dong, Y. Jin-Ming, L. Zu-Liang, J. Heterocyclic Chem. 48 (2011) 468;
(d) W. Rui-Hua, C. Bin, Z. Wen-Tao, H. Guang-Fan, Synth. Comm. 40 (2010)
1867.
The results from Table 5 show that 2c has the highest energy
gap, highest hardness and lowest softness; and 2bÁH2O has the
lowest energy gap, lowest hardness, and highest softness. Com-
pound 5b has the lowest nucleophilicity/electron accepting charac-
ter. This is confirmed from DPPH radical scavenging activity
(Table 2).
[10] B.S. Furniss, A.J. Hannaford, P.W.G. Smith, R. Austin, Vogel’s Text book of
Practical Organic Chemistry, fifth ed., Longman Scientific & Technical, 1989.
[11] (a) G.S. Suresh Kumar, S. Kumaresan, J. Chem. Sci. 124 (4) (2012) 857;
(b) G.S. Suresh Kumar, S. Kumaresan, Int. J. Curr. Chem. 3 (3 & 4) (2012) 91;
(c) G.S. Suresh Kumar, S. Kumaresan, C. Shiju, Bijoy Mohan Das, G. Anantharaj,
S. Subramanian, Int. J. Basic Appl. Chem. Sci. 2 (2) (2012) 1.
[12] APEX2, Program for Data Collection on Area Detectors, BRUKER AXS Inc., 5465
East Cheryl Parkway, Madison, WI 53711–5373 USA.
4. Conclusion
[13] G.M. Sheldrick, SADABS, Program for Absorption Correction of Areadetector
Frames, BRUKER AXS Inc., 5465 East Cheryl Parkway, Madison, WI 53711–
5373, USA.
In summary, one-pot, in-water syntheses of phenoxyaliphatic
acids of 1,8-dioxo-octahydroxanthene derived from dimedone
and formylphenoxyaliphatic acids have been reported. We investi-
gated the geometry and bonding nature of xanthene derivatives by
single crystal X-ray technique. Xanthene 2b is stabilized through
intermolecular H-bonding with water molecules. Compound 2a
has better scavenging activity and antiproliferative effects on
A431 cell line than other derivatives. Total energy, Molecular Elec-
trostatic Potential map, and HOMO–LUMO results indicated that
compounds, 2a and 2b stabilized by intermolecular hydrogen
[14] (a) G.M. Sheldrick, Acta Cryst. A64 (2008) 112;
(b) XS, BRUKER AXS Inc., 5465 East Cheryl Parkway, Madison, WI USA, pp.
53711–5373.
[15] O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H. Puschmann, J. Appl.
Cryst. 42 (2009) 339.
[16] A.L. Spek, J. Appl. Cryst. 36 (2003) 7.
[17] M.S. Blois, Nature 4617 (1958) 1199.
[18] (a) K. Ramalakshmi, G. Hithamani, K.R. Asha, L.J. Mohan Rao, Int. J. Food Sci.
Technol. 46 (2011) 109;
(b) G.S. Suresh Kumar, S. Kumaresan, A. Antony Muthu Prabhu, P.G.
Seethalakshmi, Spectrochim. Acta A 101 (2013) 254.

60
G.S. Suresh Kumar et al. / Journal of Molecular Structure 1059 (2014) 51–60
[19] (a) T. Mosmann, J. Immunol. Methods 65 (1983) 55;
[23] A. Antony Muthu Prabhu, V.K. Subramanian, N. Rajendiran, Spectrochim. Acta
96A (2012) 95.
[24] (a) T. Sivasankar, A. Antony Muthu Prabhu, M. Karthick, N. Rajendiran, J. Mol.
Stuct. 1028 (2012) 57;
(b) A. Monks, D. Scudiero, P. Skehan, R. Shoemaker, K. Paull, D. Vistica, C. Hose,
J. Lonley, P. Cronise, A. Vaigro-Wolff, M. Gray-Goodrich, H. Cmpbell, J. Mayo, M.
Boyd, J. Natl. Cancer Inst. 83 (1991) 757.
[20] (a) C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1993) 5612;
(b) A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
(b) M. Jude Jenita, A. Antony Muthu Prabhu, N. Rajendiran, Ind. J. Chem. 51A
(2012) 1686.
[21] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y.
Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro,
M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M.
Cossi, N. Rega, J.M. Millam, M. Klene, J. E. Knox, J.B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C.
Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth,
P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman,
J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian Inc., Wallingford CT, 2009.
[22] (a) A. Antony Muthu Prabhu, N. Rajendiran, J. Fluore. 22 (2012) 1461;
(b) G.S. Suresh Kumar, A. Antony Muthu Prabhu, S. Jegan Jenniefer, N.
Bhuvanesh, P. Thomas Muthiah, S. Kumaresan, J. Mol. Str. 1047 (2013)
109.
[25] M. Linder, T. Brinck, J. Org. Chem. 77 (2012) 6563.
[26] M. Bayat, H. Imanieh, S.H. Hossieni, Chinese Chem. Lett. 20 (2009) 656.
[27] (a) A. Corma, H. Garcia, Chem. Rev. 103 (2003) 4307;
(b) T. Akiyama, Chem. Rev. 107 (2007) 5744.
[28] (a) M.C. Etter, J.C. MacDonald, J. Bernstein, Acta Cryst. B46 (1990) 256;
(b) M.C. Etter, Acc. Chem. Res. 23 (1990) 120;
(c) J. Bernstein, R.E. Davis, L. Shimoni, N.L. Chang, Angew. Chem. Int. Ed. Engl.
34 (1995) 1555;
(d) M.C. Etter, J. Phys. Chem. 95 (1991) 4601.
[29] I. Coric, D. Milic, D.M. Calogovic, L. Tomaskovic, Struct. Chem. 20 (2009) 73.
_
_
[30] E. Inkaya, M. Dinçer, E. Sßahan, I. Yıldırım, Spectrochim. Acta A 114 (2013) 92.
[31] A.G. Al-Sehemi, A. Irfan, A.M. Fouda, Spectrochim. Acta A 111 (2013) 223.
[32] (a) I. Abulkalam Azath, P. Puthiaraj, K. Pitchumani, ACS Sustainable Chem. Eng.
1 (1) (2013) 174;
(b) R.S. Bertoa, F. Saliu, M. Bruschi, B. Rindone, Tetrahedron 68 (2012) 8267;
(c) G. Gayatri, G. Narahari Sastry, J. Chem. Sci. 117 (5) (2005) 573.