Characterization and phase transition study of a versatile molecular gel from a glucose-triazole-hydrogenated cardanol conjugate

Article abstract of DOI:10.1039/c3ra47540a

The synthesis and gelation properties of a structurally simple, renewable-resource-based glucose-triazole-hydrogenated cardanol conjugate (GTHCC) are reported. The conjugation of hydrogenated cardanol and glucose was done using a triazole linker employing copper(i) mediated acetylene azide cycloaddition 'click' chemistry in the key step. The GTHCC was found to form four different classes of gels; namely, (i) hydrogel from aqueous-protic solvents, (ii) organogel from non-polar solvents, (iii) gel from vegetable oil and (iv) gel from petroleum oil. In a water-methanol (1:1) mixture, the conjugate acted as a stable thermoreversible supergelator, even at a very low gelator concentration of 0.03% w/v. Unlike glycoside-based gels derived from hydrogenated cardanol and glucose, the GTHCC is stable under acidic as well as basic conditions, owing to the covalent linkages in the tether. The intrinsic fluorescence of the hydrogel was found to be sensitive towards a gel-sol transition.

Full text of DOI:10.1039/c3ra47540a

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Characterization and phase transition study of a
versatile molecular gel from a glucose-triazole-
hydrogenated cardanol conjugate†
Cite this: RSC Adv., 2014, 4, 12175
a
a
b
b
*
*
H. Surya Prakash Rao, M. Kamalraj, Jitendriya Swain and Ashok K. Mishra
The synthesis and gelation properties of a structurally simple, renewable-resource-based glucose-triazole-
hydrogenated cardanol conjugate (GTHCC) are reported. The conjugation of hydrogenated cardanol and
glucose was done using a triazole linker employing copper(I) mediated acetylene azide cycloaddition
‘click’ chemistry in the key step. The GTHCC was found to form four different classes of gels; namely, (i)
hydrogel from aqueous-protic solvents, (ii) organogel from non-polar solvents, (iii) gel from vegetable oil
and (iv) gel from petroleum oil. In a water–methanol (1 : 1) mixture, the conjugate acted as a stable
thermoreversible supergelator, even at a very low gelator concentration of 0.03% w/v. Unlike glycoside-
based gels derived from hydrogenated cardanol and glucose, the GTHCC is stable under acidic as well
as basic conditions, owing to the covalent linkages in the tether. The intrinsic fluorescence of the
hydrogel was found to be sensitive towards a gel–sol transition.
Received 11th December 2013
Accepted 9th January 2014
DOI: 10.1039/c3ra47540a
www.rsc.org/advances
a sugar (polar head) and a hydrogenated cardanol (3-pentade-
cylphenol; a non-polar tail) could display gelation properties.13
Introduction
Cardanol is a low-cost raw material that is generated in indus-
trial quantities from cashew nut shell liquid (CNSL), which is a
by-product of the cashew industry.14 Hydrogenated cardanol
serves as a raw material for technologically relevant surface
active products.3,14 Previously, John and co-workers studied the
applications of the cardanyl glucoside (1-o-30-n-(pentadecyl)
phenyl-a-D-glucopyranoside) and its siblings, and their gelation
properties.14,15 However, glucoside-based gels are susceptible to
hydrolysis under acidic conditions which limits their applica-
tions. We have now designed a glucose–triazole-hydrogenated
cardanol conjugate (GTHCC) 1 which is robust, stable under
acidic or basic conditions and can be prepared readily by
applying a copper(I) mediated acetylene azide (CuAAC) click
reaction. The GTHCC designed by us is a new class of
compound, and it contains a polar glucose head connected
through a triazole incorporated linker to the benzene ring
with a 15C nonpolar tail (Fig. 1). The conjugate 1 is robust,
as the hydrogenated cardanol and the glucose units are
linked covalently through the tether, which incorporates the
Gels are abundant and widely studied so materials with
various applications in pharmacology, material science,
cosmetics, lubrication, pollution control, electronics, sensing,
chromatography and surface engineering.1–8 Different gels can
be classied on the basis of the medium which they hold.7 The
formation of a molecular gel in different media mainly depends
upon non-covalent forces like p–p stacking, hydrogen bonding,
hydrophobic interactions and van der Waals forces.1,7–10
Molecular hydrogels could be formed by the assembly of low
molecular weight organic molecules into brous networks,
which entrap solvent molecules. The development of new
molecular gels has been a continuing and fascinating area of
applied organic chemistry research.1 A major challenge in this
eld is unravelling the preparation of structurally simple
molecular gels, generated from inexpensive and renewable raw-
materials and using convenient synthetic routes, for high
volume applications.
Molecular gels from sugar-based compounds are highly
attractive for development owing to their intrinsic chirality,
biocompatibility and natural abundance.11 Such gelators have
applications as advanced materials, drug delivery systems, for
separating biomolecules or for the formation of supramolecular
architecture.12 It occurred to us that the conjugates derived from
aDepartment of Chemistry, Pondicherry University, Pondicherry, 605014, India
bDepartment of Chemistry, Indian Institute of Technology Madras, Chennai, 600036,
India. E-mail: mishra@iitm.ac.in
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra47540a
Fig.
1 Structure of the glucose-triazole-hydrogenated cardanol
conjugate (GTHCC) 1.
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1,4-disubstituted triazole unit. Gels which show thermorever- Scanning Calorimetric measurements were performed using a
sible sol–gel transitions are an important class of gels and have TA-DSC Q-200 instrument. Fluorescence emission and uores-
been studied widely.7 Thermoreversible hydrogels exhibit a sol– cence anisotropy measurements were performed using a Fluo-
gel transition at a certain temperature, which occurs as a romax-4
uorescence
spectrophotometer.
Rheological
consequence of a sudden change in the solvation state.7,8 We measurements of the storage (G0) and loss (G00) modules were
report here the synthesis and thermosensitive gelation proper- taken for the GTHCC gel in (A) water–methanol (1 : 1), and (B)
ties of a glucose–triazole-hydrogenated cardanol conjugate cyclohexane solvents at 20 ꢀC, using a Physica MCR301
(GTHCC) 1 (Fig. 1) that satises the above criteria.
rheometer in the cone plate conguration (25 mM diameter,
1.002ꢀ angle and gap size 52 mM), which was equipped with a
thermocouple unit. Measurements were conducted with the
strain rate held at 1.00%. Temperature dependence was studied
Experimental section
Materials and general methods
for the GTHCC in water–methanol (1 : 1) using temperature
–1
ꢀ
ꢀ
For the synthesis and characterisation of the glucose-triazole- intervals of 1 C min , over the range 15–55 C.
hydrogenated cardanol conjugate, all the reagents and solvents
were purchased from Sigma-Aldrich, E-Merck or SRL, India.
Melting points were uncorrected and were determined using
open-ended capillary tubes on a VEEGO VMP-DS instrument.
Thin layer chromatography (TLC) was performed with silica gel-
G (SRL, India) or silica gel GF-254 (E-Merck) using hexanes–
ethyl acetate as the eluent, and the spots were visualized with a
short exposure to iodine vapor or UV light. Column chroma-
tography was carried out on silica gel (100–200 mesh, Acme
Synthetic Chemicals, India). Hexanes used in column chroma-
Gelation test
The gelation ability of the GTHCC in different fractions of
water–alcohol mixtures was determined by weighing 5–50 g Lꢁ1
of the compound and adding them to different solvents and
oils. Then the mixtures were heated until the solutions became
transparent, and were allowed to cool at 23–20 C. Gel forma-
tion was checked by the tube inversion method.18
ꢀ
Synthetic scheme for materials
ꢀ
tography refer to the 60–80 C boiling mixture. Infra Red (IR)
The synthesis of the GTHCC is straightforward and goes
through a CuAAC reaction between glucose diacetonide prop-
argyl ether 4 and hydrogenated cardanol based azide 3 as the
pivotal step (Scheme 1). The azide 3 was prepared from the
corresponding bromide 2. The acetonide protecting groups in
the GTHCC diacetonide 5 could be stripped sequentially or
selectively to provide mono-acetonide 6 or GTHCC 1. A small
fraction (<3%) of the b-anomer of 1 is preferentially crystallized
from column fractions during the column chromatographic
purication of a 1 : 1 mixture of a and b-anomers. The 1H NMR
spectrum of GTHCC 1 in CDCl3 displayed broad peaks for the
hydrogens on the glucose portion, indicating intermolecular
hydrogen bonding and aggregation. GTHCC was peracetylated
to 7 with acetic anhydride in pyridine. Both the mono-acetonide
6 and peracetylated product 7 did not show any inter-molecular
associations in CDCl3. Full descriptions of the synthetic
procedures and the molecular characterisation data are given in
the ESI.†
spectra were recorded as KBr pellets on a Bomem MB104
spectrometer or a Nicolet-6700 spectrometer. Specic rotations
were recorded in CHCl3 on a Rudolph Research Analytical
Automatic Polarimeter. The 1H nuclear magnetic resonance
(NMR) (400 MHz), 13C NMR (100 MHz) and Distortionless
Enhancement by Polarization Transfer (DEPT) spectra were
recorded as dilute solutions in a mixture of CDCl3 and CCl4
(1 : 1) or DMSO-d6 using a Bruker 400 MHz NMR spectrometer
with tetramethyl silane (TMS) (0 ppm) as the internal standard;
1
J values are given in Hz. H NMR spectral data are reported as
follows: chemical shi, multiplicity (s ¼ singlet, d ¼ doublet, t
¼ triplet, q ¼ quartet and m ¼ multiplet), coupling constant
and integration. High resolution mass spectra were recorded on
a Water Q-TOF micro mass spectrometer using the electron
spray ionization mode. 1-(2-bromoethoxy)-3-pentadecylbenzene
2 (ref. 16) and the propargyl glucose diacetonide ether 4 (ref. 17)
were prepared according to literature procedures. Cardanol was
purchased from Sabari Industries, Sedurapet, Pondicherry and
hydrogenated in a Paar Hydrogenation Apparatus. All other
starting materials or reagents were purchased from SRL India
and used as received.
Results and discussion
Gelation properties
All the solvents used for the gel preparation and gel char-
acterisation experiments were of spectroscopic grade. Triple- GTHCC 1 was tested for its gelation properties in different
distilled water was used for the experiment, which was prepared aqueous-protic solvent mixtures, vegetable oil, petroleum oil
using alkaline permanganate solution. Dynamic light scattering and non-polar organic solvents; the results are given in Table 1
(DLS) analysis was carried out with a Malvern Zetasizer nano (ESI Table 1†). It was found that when using GTHCC as a low
series, with a path length of 1 cm. The wavelength of the laser molecular weight gelator, four different classes of gel could be
used was 632.8 nm and the scattering angle was kept at 90ꢀ. found; that is, (i) hydrogel from aqueous-protic solvent
Viscosity measurements were performed using an SV-10 Vibro mixtures, (ii) organogel from non-polar solvents, (iii) gel from
Viscometer. FT-IR spectra were recorded using a Perkin Elmer vegetable oil and (iv) gel from petroleum oil. The GTHCC
Spectrum FT-IR instrument. The scanning electron microscope formed a hydrogel in a mixture of water and methanol, or water
images were taken using an F E I Quanta FEG 200 – High and ethanol, at the initial concentration of 5 g Lꢁ1. The critical
Resolution Scanning Electron Microscope. Differential gelation concentration (CGC) of GTHCC was determined for the
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Scheme 1
different fractions of the water–methanol and water–ethanol group of low molecular weight compounds which have a CGC of
solvent mixtures (ESI Table 1†). The best gelation ability was less than 1% (w/v).7,19,20
observed in the water–methanol (1 : 1) mixture, where CGC was
The GTHCC hydrogel formed from the water–methanol
reached at 0.25 mg mLꢁ1. The CGC of GTHCC hydrogel is less mixture (1 : 1) was stable for several days at room temperature.
than 0.03% (w/v). Therefore, GTHCC in a water–methanol (1 : 1) When the hydrogel was heated above 53 ꢀC it formed a clear
mixture can be regarded as a supergelator. Thus 1 belongs to a transparent solution, which indicates the complete conversion
Table 1 Results for the gelation test of GTHCC (at 5–50 g Lꢁ1) at room temperaturea
Aqueous–protic solvent mixture
State
Tgs
Water–methanol (1 : 1)
Water–ethanol (1 : 1)
Water–n-propanol (1 : 1)
Water–n-butanol (1 : 1)
Water
Methanol
Ethanol
n-Propanol
Butanol
Gel (CGC ¼ 0.025%, 0.25 mg mLꢁ1
)
42 ꢀ
33 ꢀ
C
C
Gel (CGC ¼ 0.08%, 0.8 mg mLꢁ1
)
Solution
Solution
Precipitate
Solution
Solution
Solution
Solution
Non-polar organic solvent
Cyclohexane
Toluene
Gel (CGC ¼ 2%, 20 mg mLꢁ1
)
Gel
Vegetable oil
Coconut oil
Mustard oil
Soybean oil
Gel
Gel
Gel
Petroleum oil
Petrol
Diesel
Gel (CGC ¼ 3.5%, 35 mg mLꢁ1
)
Gel
a
Tgs ¼ gel–sol transition temperature; CGC ¼ Critical Gelation Concentration at 23–20 ꢀC.
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of the gel into a sol state (Fig. 2). Upon gradual cooling the micellar aggregates, which show a diameter distribution in the
hydrogel could be reproduced, which shows that the molecule is range 3–5 mm (ESI Fig. 1(B)†). Fig. 7(B) shows the plot for the
a thermoreversible supergelator. The GTHCC formed stable changes in mean diameter of the hydrogel with the variation of
hydrogels even with 0.1 M aqueous HCl and methanol (1 : 1), temperature. An abrupt increase in micellar size has been
ꢀ
and 0.1 M aqueous NaOH and methanol (1 : 1), solutions. This observed at 49 C, which is due to the intermicellar interaction
indicates that the hydrogel is stable in highly acidic as well as leading to the formation of larger micellar aggregates. Aer 42 ꢀC
basic conditions.
the mean diameter increases sharply due to larger aggregates,
which are assembled together to form large ber networks or
gels. DLS studies of the GTHCC hydrogel indicate that the gela-
tion process involves different modes of aggregation. The
differential scanning calorimetric (DSC) analysis of the hydrogel
in the water–methanol (1 : 1) mixture provides further insight
into the changes observed in the physical state of the sample. The
endothermic peaks in the DSC plot represent the gel–sol transi-
tion temperature (Tgs) which provides information regarding the
interaction of the gelator molecules. Multiple endothermic
transition peaks have been observed in the DSC plot of the
hydrogel (Fig. 7(A)), which is due to the gelation process of
GTHCC in the water–methanol (1 : 1) mixture involving various
modes of aggregation. There is a sharp endothermic transition at
42 ꢀC (Tgs), which happens due to the melting of the gel to higher
micellar aggregates, whereas the small endothermic transition at
Rheology and uorescence studies of the gel
The formation of a true gel and the sol–gel transition temper-
ature are reliably inferred from the measurement of the rheo-
logical parameters of the sample.21–24 Fig. 3 shows the frequency
dependence of the storage modulus (G0) and loss modulus (G00)
of GTHCC gels in (A) water–methanol (1 : 1) and (B) cyclohexane
solvents. The results show linear regions of G0 and G00 with a
frequency sweep (1–100 rad sꢁ1). There is no change and
crossover observed in the storage modulus and loss modulus of
either gel over the frequency range. The storage modulus (G0) is
the dominant variable in both cases, which is the most impor-
tant characteristic of a molecular gel.21,22 Moreover, both the
storage and loss moduli are almost independent of the
frequency, indicating a gel-like behaviour. The frequency
measurements indicate that the gels did not show any change in
rheological properties over the frequency range.
ꢀ
49 C is due to the dissolution of larger micellar aggregates to
micellar structures, which is conrmed by DLS analysis. The
viscosity studies of the GTHCC hydrogel in the water–methanol
(1 : 1) mixture provide valuable information regarding the
behaviour of the gel with the variation of temperature. The
viscosity of the hydrogel decreases with an increase in tempera-
ture (Fig. 7(C)). A sharp decrease in viscosity was observed aer
Fig. 4 shows the changes in the storage modulus (G0) and the
loss modulus (G00) at an angular frequency of 10 rad sꢁ1, as a
function of temperature for the GTHCC gel in the water–
methanol (1 : 1) solvent mixture. The crossover point of the G0
and G00 above 49 C gives an indication of the transition from
ꢀ
ꢀ
ꢀ
49 C and aer 54 C it remains constant due to its complete
conversion from a gel state to a sol state.
the solid gel state to the uid sol state.22,23 This result suggests
ꢀ
that above 49 C the GTHCC gel formed in the aqueous meth-
The intrinsic uorescence emission and uorescence
anisotropy (Fig. 7(D) and ESI Fig. 4†) of GTHCC provides more
useful information regarding the gelation process in the water–
methanol (1 : 1) mixture. The GTHCC hydrogel in the water–
methanol (1 : 1) mixture gives an emission at 300 nm. The
hydrogel gives a higher emission intensity in the gel state at low
temperatures, whereas with an increase in temperature the
emission intensity decreases (Fig. 7(D) and ESI Fig. 4†). A sharp
decrease in uorescence intensity is observed at 50 ꢀC and aer
54 ꢀC it remains constant due to a complete conversion to a sol
state. The decrease in uorescence intensity with increase in
temperature is observed due to an increase in the non-radiative
decay process with the conversion of the gel to a sol state. The
uorescence anisotropy measurement of the GTHCC hydrogel
shows a similar trend in uorescence emission. A higher uo-
rescence anisotropy is observed in the gel state, indicating a
highly rigid and compact structure formed by the GTHCC
hydrogel, which restricts its rotational motion. With an increase
in temperature the uorescence anisotropy decreases, due to
the melting of the hydrogel to a sol state, which increases the
molecular rotation of the molecules.
anolic medium is converted to a sol state.
The gelation process of GTHCC in the water–methanol
mixture (1 : 1) is also studied by dynamic light scattering (DLS)
analysis. The DLS analysis has shown that the gelation process
involves a change of hydrodynamic diameters, with a mean
diameter from 22–23 nm to 5–7 mm (Fig. 5, 6, 7(B) and ESI
Fig. 1†). In the sol state (57 ꢀC), the DLS histogram plot (ESI
Fig. 1(A)†) indicates the formation of a micellar structure which
has a hydrodynamic diameter distribution from 20–50 nm.
Generally the diameters for micellar structures are in the range
of 1–300 nm.25,26 In the gel state the distribution of the hydro-
dynamic diameters increases due to the assembly of larger
From the above study of four conceptually different investi-
gations: DLS, DSC, viscosity and uorescence parameters, it is
concluded that the gelation process of the GTHCC supergelator
in a water–methanol (1 : 1) medium involves the formation of
three different states of GTHCC aggregation at different
Fig. 2 Demonstration of the thermoreversible supergelator charac-
teristics of a GTHCC hydrogel formed from a water–methanol mixture
(1 : 1).
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Fig. 3 Frequency dependence of the storage modulus (G0) and the loss modulus (G00) of GTHCC gel at 20 ꢀC in (A) water–methanol (1 : 1) and (B)
cyclohexane (strain rate 1.00%).
Fig. 6 DLS plot for distribution of the hydrodynamic diameter of
Fig. 4 Temperature dependence of the storage modulus (G0) and the
GTHCC with the variation of temperature in water–methanol (1 : 1)
solvent ([GTHCC] ¼ 2 mg mLꢁ1).
loss modulus (G00) of GTHCC gel in water–methanol (1 : 1) at a
frequency of 10 rad sꢁ1. [GTHCC] ¼ 2 mg mLꢁ1
.
Morphology of gel
HR-SEM images (Fig. 8(A) and ESI Fig. 3(A)†) of a dried gold-
coated hydrogel of GTHCC in a water–methanol (1 : 1) mixture
showed the formation of a spongy sheet -like structure which
has cavities of mm scale inside it. From the HR-SEM images it
can be concluded that the molecules of GTHCC assemble over
one another to form a sheet-like structure, and the cavities
formed by these assemblies entrap solvent molecules through
capillary forces and surface tension.14 This observation suggests
that the van der Waals hydrophobic interactions between the
nonpolar hydrogenated cardanol tails and the triazole linker
could be the main driving force for the gelation properties of the
compound. The FT-IR data (ESI Fig. 2†) of the hydrogel in the
deuterated water–methanol (1 : 1) mixture show a broad peak at
Fig. 5 Dynamic Light scattering (DLS) plot for the variation of mean
diameter of hydrogel with the variation of temperature in water–
methanol (1 : 1), ([GTHCC] ¼ 2 mg mLꢁ1). (Error ¼ ꢂ5%).
3450 cmꢁ1
, indicating intermolecular hydrogen bonding
between the –OH groups of the GTHCC conjugate.27 Neither
mono-acetonide 6 nor peracetylated GTHCC 7 formed a gel at
concentrations of up to 5 mg mLꢁ1 in the mixture of water and
methanol (1 : 1), indicating that intermolecular hydrogen
bonding is also crucial for gelation. Thus, along with van der
Waals hydrophobic interactions, intermolecular hydrogen
bonding plays an important role in formation of a supergelator
with an exceptionally high gelation capacity.
temperature ranges: at higher temperatures, above 49 ꢀC, the
transparent sol state causes the compound to exist in a micellar
organization; on decreasing the temperature, to between 38 ꢀC
to 44 ꢀC, higher micellar aggregates form; and the gel phase
ꢀ
forms below 27 C.
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Fig. 8 HR-SEM images of the GTHCC gel in (A) water–methanol
(1 : 1), (B) cyclohexane and (C) petrol.
The HR-SEM image of the GTHCC gel in petrol shows that the
compound aggregates to form entangled bre networks
(Fig. 8(C)). The CGC of GTHCC in petrol is 3.5%. In a physical
mixture of water and petrol, the addition of GTHCC forms a
phase-selective gel with the petrol, which separates from the
water and forms a layer that oats. This observation could have
implications in oil-slick management. The studied morphology
of GTHCC gel in different solvents displays signicant varia-
tions in the morphology of self-assembled systems.
Conclusion
In conclusion, we present a glucose-triazole-hydrogenated carda-
nol conjugate (GTHCC) as a new type of molecular gel, which is
synthesized using hydrogenated cardanol and glucose which are
sufficiently cheap and are from renewable natural resources.
GTHCC forms four different classes of gels in aqueous-protic
solvent mixtures, non-polar organic solvents, vegetable oils and
petroleum oils, which are well supported by rheological measure-
ments. In a water–methanol (1 : 1) mixture it shows highly stable
thermoreversible supergelation ability. The DLS, DSC, viscosity
measurements, intrinsic uorescence emission, uorescence
anisotropy measurements and SEM analysis shows the formation
of micellar structures (20–50 nm diameters) in the sol state.
Intermicellar aggregation leads to the formation of larger micellar
structures and the interaction of larger aggregates to form sheet-
like spongy gel networks which entrap the solvent molecule by
surface tension effects in the water–methanol (1 : 1). Owing to the
long non-polar tail and polar glucose head, GTHCC induces gela-
tion even in non-polar aprotic solvents like cyclohexane and petrol.
Fig.
7 (A) Endothermic plot of differential scanning calorimetric
analysis; (B) Dynamic Light scattering (DLS) plot for the variation of
mean diameter of hydrogel with the variation of temperature; (C) plot
for the viscosity of the hydrogel with the variation of temperature; (D)
plot for fluorescence intensity and fluorescence anisotropy of the
GTHCC hydrogel (lex ¼ 274 nm and lem ¼ 300 nm), in water–meth-
anol (1 : 1) solvent, ([GTHCC] ¼ 2 mg mLꢁ1). (Error ¼ ꢂ5%).
Signicantly, the GTHCC 1 was found to induce gelation in
non-polar aprotic solvents like cyclohexane and petrol. The HR-
SEM image of the GTHCC gel in cyclohexane shown in Fig. 8(B)
is comprised of twisted and interconnected sheets. At 500 nm
magnication (ESI Fig. 3(B)†) the GTHCC gel displayed the
brous nature of the interconnected sheets. The HR-SEM
images suggest that the hydrophobic interactions between the
nonpolar hydrogenated cardanol tails and the triazole linker are
the main driving force for the gelation of the compound in
Acknowledgements
cyclohexane. A similar observation was reported by the John The authors thank Professor Abhijit Deshpande, Department of
research group for cardanyl glucoside gelation in cyclohexane.14 Chemical Engineering, IIT Madras, and Mr K. Jagadeeshwar for
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their kind help in carrying out rheological measurements and 14 V. S. Balachandran, S. R. Jadhav, P. K. Vemula and G. John,
useful discussions. One of the authors, AKM, thanks the Chem. Soc. Rev., 2013, 42, 427.
Department of Science and Technology (DST), Government of 15 G. John, J. H. Jung, M. Masuda and T. Shimizu, Langmuir,
India, for nancial support. HSP thanks the Central Instru- 2004, 20, 2060.
mentation Facility, Pondicherry University for spectra. M. K. 16 O. A. Attanasi, G. Mele, P. Filippone, S. E. Mazzetto and
thanks Council for Scientic Industrial Research (CSIR) for
fellowship. J. S. thanks IIT Madras for research fellowships.
G. Vasapollo, ARKIVOC, 2009, (viii), 69–84.
17 (a) V. Baliah, A. Ekambaram and T. S. Govindarajan, Curr.
Sci., 1954, 23, 264; (b) A. Roy, S. K. Sahabuddin, B. Achari
and S. B. Mandal, Tetrahedron, 2005, 61, 365–
371.
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