I2/DMSO-catalyzed one-pot approach for the synthesis of 1,3,4-selenadiazoles

Article abstract of DOI:10.1039/d0ra10576g

A three-component cascade reaction for the synthesis of 1,3,4-selenadiazoles and their derivatives from arylaldehydes, hydrazine, and elemental selenium by using molecular iodine is reported. This strategy is operationally simple, well-suited to a wide range of functional groups, and provides the desired products in moderate to excellent yields. The proposed mechanism predicts that the reaction tolerated a radical process.

Full text of DOI:10.1039/d0ra10576g

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I2/DMSO-catalyzed one-pot approach for the
synthesis of 1,3,4-selenadiazoles†
Cite this: RSC Adv., 2021, 11, 5724
*
Suresh Kuarm Bowroju and Rajitha Bavanthula
A three-component cascade reaction for the synthesis of 1,3,4-selenadiazoles and their derivatives from
arylaldehydes, hydrazine, and elemental selenium by using molecular iodine is reported. This strategy is
operationally simple, well-suited to a wide range of functional groups, and provides the desired products
in moderate to excellent yields. The proposed mechanism predicts that the reaction tolerated a radical
process.
Received 16th December 2020
Accepted 23rd January 2021
DOI: 10.1039/d0ra10576g
rsc.li/rsc-advances
antitumor activity.11 However, little is known about the 2,5-
disubstituted 1,3,4-selenadiazoles.12
Introduction
Several methods have been reported for their preparation,
which include a ring-closure reaction of selenobenzamides with
hydrazine hydrate,13 reacting dimethylformamide azine with
hydrogen selenide,14 treatment of 1,2-diacetylhydrazine with
phosphorus pentaselenide,15 reaction of isoselenocyanates with
selenosemicarbazides16 or a carboxylic acid with selenosemi-
carbazide and phosphoryl chloride (Fig. 1).17 However, the
examples of these compounds have some shortcomings in the
literature are limited due to either lack of starting materials or
very low yield. Therefore, developing an effective and low-cost
method for synthesizing two-substituted 1,3,4-selenathiazoles
is still highly warranted. In recent times, the iodine/DMSO
combination has received considerable attention, in synthetic
organic chemistry, as an effective and eco-friendly oxidative
system as it has affected numerous organic trans-
formations.18–22 As our continuous work on the development of
a new method for the synthesis of various heterocyclic
Selenium, is an element which plays a crucial role in biology
and technology. It belongs to the chalcogen family, and is an
essential element for living organisms as a component of sele-
nomethionine, an amino acid occurring in the active centres of
some enzymes, for example, glutathione peroxidase. Due to
these signicant properties, selenium-containing compounds
such as biologically active molecules have been attractive to
researchers. Organic and inorganic compounds of selenium
also show unique electronic properties and are used as semi-
and superconductors in electro-optic devices and sensors. The
rst organoselenium compound diethyl selenide was synthe-
sized in 1836.1 1,3,4-Selenadiazoles and their derivatives are
closely related to the 1,3,4-thiadiazole compounds but their
properties are quite different from the 1,3,4-thiadiazole
compounds. 1,3,4-Selenadiazoles and their derivatives are
applied widely in pharmaceutical, agricultural, and materials
chemistry. 1,3,4-Selenadiazoles are important heterocycles with
an “N–C–Se” linkage which can work as the active center,
chelate certain metal ions in vivo, and show good tissue
permeability. The lower toxicity and in vivo stability of the
selenadiazole nucleus is attributed to its aromaticity. In
particular, the 1,3,4-selenadiazoles have displayed a broad
spectrum of biological activities including antibacterial, anal-
gesic, antitumor, anticonvulsant, and anti-inammatory drugs,
pesticides and fungicides.2–4 Furthermore, some of them have
been used as thermotropic liquid crystals, corrosion and
oxidation inhibitors, or as dyes or metal ion complexation
reagents.5–10 1,3,4-Selenadiazole-containing kidney-type gluta-
minase inhibitors showed improved cellular uptake and
Department of Chemistry, National Institute of Technology, Warangal, TS, India.
E-mail: rajithabhargavi@ymail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra10576g
Fig. 1 Methods for the synthesis of 1,3,4-selenadiazoles.
5724 | RSC Adv., 2021, 11, 5724–5728
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biologically active molecules,23–43 we herein report iodine/DMSO
catalysed a three-component reaction for the preparation of
1,3,4-selenathiazoles from the easily available, aryl aldehyde,
hydrazine and selenium powder.
Results and discussion
Initially, our studies commenced using benzaldehyde 1a,
hydrazine 2 and elemental selenium as a model substrate and
the results are presented in Table 1. Firstly, 1a, 2 and selenium
were taken in DMSO and the mixture was heated at 150 ꢀC for 4
hours, however, the reaction failed to proceed and no product
was detected (Table 1, entry 1). Due to the failure of this reac-
tion, conrming the importance of catalyst in this multicom-
ponent reaction. Therefore, we focused on sequential additions
and accordingly, we attempted a reaction with 1a, 2, selenium
Scheme 1 Synthesis of 2,5-diaryl-1,3,4-selenadiazoles.
to increase the temperature to 170 ꢀC, however, lowered the
yield of the desired product to 76% (Table 1, entry 10). To
conclude, the conditions described in entry 7 were found to be
optimal, allowing for maximum conversion to the desired
product 3a.
With the optimized reaction conditions in hand (Table 1,
entry 7), we subsequently investigated the substrate scope of
this transformation (Schemes 1 and 2). Benzaldehyde bearing
neutral, electron-donating, and electron-withdrawing substitu-
ents at the o/m/p-positions of the aromatic ring reacted
smoothly under the optimized reaction conditions, affording
the corresponding product in high yields (73–92%). Besides
different benzaldehydes, other benzothiophene, benzofuran,
indole aldehydes also yields 65–74%.
Moreover, the reaction also tolerated several functional
groups, such as nitrile, methoxy, iodo and amino. All reactions
proceeded smoothly to give the desired products in moderate to
good yields. The reactivities of phenyl rings with strongly
electron-withdrawing substituents (–CF3, –NO2) were higher
than those of phenyl rings with electron-donating substituents.
This suggests that electron-withdrawing groups enhance the
reaction efficiency (3e and 3g). Substrates with substituents at
the ortho or meta positions were also compatible with. Notably,
the position of the substituent did not affect the reaction
productivities and yields (3c, 3d and 3b, 3f). Aer replacing the
phenyl group with heteroaryl, such as benzothiophene, benzo-
furan, indole aldehydes. All reactions proceeded smoothly to
give the desired products in moderate to good yields. Compared
ꢀ
and TBAI in DMSO, and heated at 150 C for 4 hours. Encour-
agingly, the expected product 3a was obtained in an isolated
yield of 51% (Table 1, entry 2). Accordingly, we focused our
studies on to improve the yield, the reaction was attempted with
KI and NH4I affording 3a in 42% and 35% yield respectively
(Table 1, entries 3 and 4). Finally, I2 was shown to be the best
performing catalyst as the yield signicantly increased to 88%
(Table 1, entry 7). Many synthetic procedures use varying
amounts of I2 and reaction temperatures depending on the
reaction conditions employed to produce the desired product in
maximum yield. Thus, to complete the study, attempts were
made to decrease the amount of iodine to 5% and 10%,
however, lowered the yield of the desired product to 71% and
82% respectively (Table 1, entries 5 and 6). Furthermore,
attempt was made to increase the amount of iodine to 30%,
however, lowered the yield of the desired product to 79% (Table
1, entry 8). Further, the attempt was made to decrease the
temperature to 120 ꢀC, however, lowered the yield of the desired
product to 76% (Table 1, entry 9). Finally, the attempt was made
Table 1 Optimization of the reaction conditionsa
Temp. (0
Entry
Catalyst (%)
Solvent
ꢀC)
Yield (%)
1
2
3
4
5
6
7
8
9
—
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
150
150
150
150
150
150
150
150
120
170
Trace
51
35
42
71
82
88
79
76
TBAI (20)
NH4I (20)
KI (20)
I2 (5)
I2 (10)
I2 (20)
I2 (30)
I2 (20)
I2 (20)
10
71
a
1a (0.5 mmol), 2 (0.25 mmol), Se (0.25 mmol), catalyst (20 mol%), and
solvent (3 mL) under air at 150 ꢀC for 4 h.
Scheme 2 Synthesis of 2,5-diheteroaryl-1,3,4-selenadiazoles.
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and hydrazine 2. Iodine radicals produced upon heating the
molecular iodine. Then, iodine radicals get a proton from A to
obtain the radical intermediate B and HI. HI reacts with DMSO
to generate iodine, and the cycle continues.44,45 Later, radical
intermediate B reacts with selenium powder to afford the sele-
nium free-radical intermediate C.46 Again iodine radicals get
a proton from C to obtain the radical intermediate D and HI.
Finally, intermediate D undergoes cyclization to furnish the
desired product 3a.
Conclusion
We have successfully developed a practical way for the synthesis
of 1,3,4-selenadiazole through I2/DMSO promoted cascade
cyclization process. We reported a mild method by the multi-
component reaction of different aldehydes, elemental sele-
nium, and hydrazine. The present methodology has the
advantages of being free of metal, the use of simple operation.
Experimental section
General procedure for the synthesis of 2,5-diaryl-1,3,4-
selenadiazoles
Aldehyde (0.5 mmol), elemental selenium (0.25 mmol), hydra-
zine (0.25 mmol), iodine (20%), and DMSO (3 mL) were added to
the reaction vial. The mixture was stirred at 150 ꢀC for 4 h. Aer
completion of reaction (conrmed by TLC), water was added
and extracted with ethyl acetate. The combined organic layers
were dried over magnesium sulfate. Evaporation of the solvent
under reduced pressure provided the crude product, which was
puried by column chromatography on silica gel (1–3% MeOH
in dichloromethane as eluent) to give the corresponding target
product.
Scheme 3 Controlled experiments.
with indole aldehydes, benzothiophene, benzofuran showed
a lower reactivity in the reaction.
We have performed the several control experiments to
interpret the reaction mechanism (Scheme 3). The rst control
experiment was carried out with selenium powder and synthetic
diimine, it could be converted into the corresponding desired
product 3a in 92% yield. This reaction completed in 1 h. This is
because the process is one-step less than the standard reaction.
Later, when we replaced Se with KSeCN, desired product 3a was
obtained in moderate yield. Finally, radical inhibitors
tetramethyl-piperidin-1-oxyl (TEMPO) and butylated hydrox-
ytoluene (BHT) were used resulted in the inhibition of the
reaction. These results suggested that this reaction proceeded
through a radical pathway. We performed the scale up reactions
5 mmol and 10 mmol and obtained the desired product in 90%
and 85% yields, respectively.
2,5-Diphenyl-1,3,4-selenadiazole (3a)
White solid; Yield ¼ 88%; mp 115–117 ꢀC (lit. mp 116–118 ꢀC);47
1H NMR (400 MHz, DMSO-d6): d 7.85 (d, J ¼ 7.8 Hz, 4H), 7.21–
7.22 (m, 6H); 13C NMR (400 MHz, DMSO-d6): d 170.75, 140.41,
129.51, 129.44, 129.14, 127.47, 109.55; HRMS (ESI) m/z calcd for
C
14H10N2Se (M + H)+ 287.0269, found 287.0300; elemental
analysis: C, 58.96; H, 3.53; N, 9.82; Se, 27.69, found: C, 58.86; H,
3.62; N, 9.87; Se, 27.65.
From the above results, a proposed mechanism is presented
in Scheme 4. Initially, di-imine generated from benzaldehyde 1a
6,60-(1,3,4-Selenadiazole-2,5-diyl)bis(3-methylaniline) (3b)
1
ꢀ
Brown solid; yield ¼ 73%; mp 165–167 C; H NMR (400 MHz,
CDCl3): d 7.79 (d, J ¼ 8.0 Hz, 2H), 6.47 (s, 2H), 6.44 (brs, 2H),
2.25 (s, 6H); 13C NMR (400 MHz, CDCl3): 173.58, 151.03, 145.91,
132.03, 118.04, 116.86, 107.49, 21.67; HRMS (ESI) m/z calcd for
C
16H16N4Se (M + H)+ 345.0581, found 345.0565; elemental
analysis: C, 55.98; H, 4.70; N, 16.32; Se, 23.00, found C, 55.91; H,
4.72; N, 16.4; Se, 22.97.
2,5-Bis(3,4-dimethoxyphenyl)-1,3,4-selenadiazole (3c)
1
ꢀ
White solid; yield ¼ 84%; mp 210–212 C; H NMR (400 MHz,
CDCl3): d 7.74 (d, J ¼ 8.0 Hz, 2H), 7.74 (s, 2H), 6.9 (d, J ¼ 8.0 Hz,
2H), 3.93 (s, 6H), 3.92 (s, 6H); 13C NMR (400 MHz, CDCl3):
Scheme 4 Proposed mechanism.
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172.04, 153.71, 148.64, 124.58, 121.67, 112.26, 110.28, 56.05, (t, J ¼ 6.4 Hz, 4H), 1.69–1.65 (m, 4H), 1.35–1.22 (m, 16H), 0.83 (t,
55.95; HRMS (ESI) m/z calcd for C18H18N2O4Se (M + H)+ J ¼ 6.8 Hz, 6H); 13C NMR (400 MHz, DMSO-d6): 167.43, 162.72,
407.0155, found 407.0141; elemental analysis: C, 53.34; H, 131.78, 123.19, 114.62, 68.17, 31.65, 28.96, 28.83, 25.83, 22.47,
4.48; N, 6.91; O, 15.79; Se, 19.48, found C, 53.31; H, 4.50; N, 6.92; 14.36; HRMS (ESI) m/z calcd for C28H38N2O2Se (M + H)+
O, 15.68; Se, 19.59.
515.2109, found 515.2095; elemental analysis: C, 65.48; H,
7.46; N, 5.45; O, 6.23; Se, 15.37, found: C, 65.54; H, 7.44; N, 5.41;
O, 6.19; Se, 15.42.
2,5-Bis(2,6-dimethoxyphenyl)-1,3,4-selenadiazole (3d)
1
ꢀ
White solid; yield ¼ 85%; mp 223–225 C; H NMR (400 MHz,
CDCl3): d 7.33 (d, J ¼ 8.0 Hz, 2H), 6.55 (d, J ¼ 8.0 Hz, 4H), 3.85 (s,
12H); 13C NMR (400 MHz, CDCl3): 170.69, 157.81, 131.82,
111.59, 104.1, 56.16, 56.14; HRMS (ESI) m/z calcd for
2,5-Di(benzofuran-2-yl)-1,3,4-selenadiazole (3j)
1
ꢀ
White solid; yield ¼ 70%; mp 270–272 C; H NMR (400 MHz,
DMSO-d6): d 7.75 (d, J ¼ 8.0 Hz, 2H), 7.66 (d, J ¼ 8.0 Hz, 2H), 7.62
(s, 2H), 7.47 (t, J ¼ 8.4 Hz, 2H), 7.32 (t, J ¼ 8.0 Hz, 2H); 13C NMR
(400 MHz, DMSO-d6): 160.53, 155.4, 146.61, 127.98, 127.28,
124.24, 123.52, 113.95, 113.88, 112.49; HRMS (ESI) m/z calcd for
C
18H18N2O4Se (M + H)+ 407.0155, found 407.0141; elemental
analysis: C, 53.34; H, 4.48; N, 6.91; O, 15.79; Se, 19.48, found C,
53.29; H, 4.51; N, 6.89; O, 15.81; Se, 19.50.
C
18H10N2O2Se (M + H)+ 366.9939, found 366.9916; elemental
2,5-Bis(2-nitrophenyl)-1,3,4-selenadiazole (3e)
analysis: C, 59.19; H, 2.76; N, 7.67; O, 8.76; Se, 21.62, found: C,
59.15; H, 2.82; N, 7.65; O, 8.71; Se, 21.67.
1
ꢀ
Pale yellow solid; yield ¼ 90%; mp 265–267 C; H NMR (400
MHz, CDCl3): d 8.06–8.00 (m, 4H), 7.45 (t, J ¼ 8.0 Hz, 2H), 7.21
(d, J ¼ 7.6 Hz, 2H); 13C NMR (400 MHz, CDCl3): 171.31, 141.98,
133.57, 133.09, 132.1, 128.02, 94.71; HRMS (ESI) m/z calcd for
2,5-Bis(benzo[b]thiophen-2-yl)-1,3,4-selenadiazole (3k)
1
ꢀ
White solid; yield ¼ 65%; mp 265–268 C; H NMR (400 MHz,
DMSO-d6): d 8.07 (s, 2H), 8.0–7.95 (m, 4H), 7.46–7.41 (m, 4H);
13C NMR (400 MHz, DMSO-d6): 163.96, 141.74, 139.15, 135.15,
130.7, 130.65, 127.44, 126.13, 125.49, 123.38; HRMS (ESI) m/z
calcd for C18H10N2S2Se (M + H)+ 399.9454, found 399.9444;
elemental analysis: C, 54.41; H, 2.54; N, 7.05; S, 16.14; Se, 19.87,
found: C, 54.39; H, 2.51; N, 7.11; S, 16.10; Se, 19.89.
C
14H8N4O4Se (M + H)+ 376.9842, found 376.9870; elemental
analysis: C, 44.82; H, 2.15; N, 14.93; O, 17.06; Se, 21.04, found C,
44.75; H, 2.18; N, 14.89; O, 17.10; Se, 21.08.
6,60-(1,3,4-Selenadiazole-2,5-diyl)bis(2-methylaniline) (3f)
1
ꢀ
Light brown solid; yield ¼ 78%; mp 188–190 C; H NMR (400
MHz, DMSO-d6): d 7.63 (d, J ¼ 8.0 Hz, 2H), 7.16 (d, J ¼ 7.2 Hz,
2H), 6.48 (t, J ¼ 8.0 Hz, 2H), 2.09 (s, 6H); 13C NMR (400 MHz,
DMSO-d6): 172.69, 152.34, 137.04, 131.66, 125.6, 116.94, 112.01,
20.13; HRMS (ESI) m/z calcd for C16H16N4Se (M + H)+ 345.0481,
found 345.0490; elemental analysis: C, 55.98; H, 4.70; N, 16.32;
Se, 23.00, found C, 55.92; H, 4.71; N, 16.4; Se, 22.97.
2,5-Di(1H-indol-2-yl)-1,3,4-selenadiazole (3l)
1
ꢀ
White solid; yield ¼ 73%; mp 289–290 C; H NMR (400 MHz,
DMSO-d6): d 11.71 (s, 2H), 7.61 (d, J ¼ 8.4 Hz, 2H), 7.41 (d, J ¼
8.4 Hz, 2H), 7.21 (t, J ¼ 8.4 Hz, 2H), 7.06 (s, 2H), 7.05 (t, J ¼
8.0 Hz, 2H); 13C NMR (400 MHz, DMSO-d6): d 163.25, 137.66,
128.83, 127.28, 124.7, 122.34, 120.38, 112.91, 107.72; HRMS
(ESI) m/z calcd for C18H12N4Se (M + H)+ 365.0279, found
365.0296; elemental analysis: C, 59.51; H, 3.33; N, 15.42; Se,
21.74, found: C, 59.47; H, 3.32; N, 15.39; Se, 21.82.
2,5-Bis(3-(triuoromethyl)phenyl)-1,3,4-selenadiazole (3g)
1
ꢀ
White solid; yield ¼ 92%; mp 155–157 C; H NMR (400 MHz,
DMSO-d6): d 8.23 (s, 2H), 8.17 (d, J ¼ 7.6 Hz, 2H), 8.03 (d, J ¼
7.6 Hz, 2H), 7.67 (t, J ¼ 8.0 Hz, 2H); 13C NMR (400 MHz, DMSO-
d6): 166.10, 136.62, 134.16, 133.17, 132.48, 130.43, 118.48,
112.34; HRMS (ESI) m/z calcd for C16H8F6N2Se (M + H)+
422.9569, found 422.9550; elemental analysis: C, 45.62; H, 1.91;
F, 27.06; N, 6.65; Se, 18.75, found C, 45.58; H, 1.88; F, 27.04; N,
6.71; Se, 18.79.
2,5-Bis(5-methoxy-1H-indol-2-yl)-1,3,4-selenadiazole (3m)
1
ꢀ
White solid; yield ¼ 74%; mp 289–290 C; H NMR (400 MHz,
DMSO-d6): d 11.57 (s, 2H), 7.3 (d, J ¼ 8.8 Hz, 2H), 7.05 (s, 2H),
6.96 (s, 2H), 6.87 (dd, J ¼ 8.8 Hz, J ¼ 2.4 Hz, 2H), 3.71 (s, 6H),;
13C NMR (400 MHz, DMSO-d6): 161.17, 154.24, 132.99, 129.03,
127.57, 116.21, 113.78, 107.36, 102.38, 55.66; HRMS (ESI) m/z
calcd for C20H16N4O2Se (M + H)+ 425.0442, found 425.0458;
elemental analysis: C, 56.74; H, 3.81; N, 13.23; O, 7.56; Se, 18.65,
found: C, 56.69; H, 3.82; N, 13.27; O, 7.63; Se, 18.59.
2,20-(1,3,4-Selenadiazole-2,5-diyl)dibenzonitrile (3h)
Off-white solid; yield ¼ 80%; mp 159–160 ꢀC; 1H NMR (400
MHz, DMSO-d6): d 8.05 (d, J ¼ 8.0 Hz, 2H), 8.02 (d, J ¼ 6.8 Hz,
2H), 7.76–7.72 (m, 4H); 13C NMR (400 MHz, DMSO-d6): 165.68,
134.42, 133.76, 133.56, 133.32, 131.33, 118.16, 112.07; HRMS
(ESI) m/z calcd for C16H8N4Se (M + H)+ 336.9953, found 2,5-Bis(5-uoro-1H-indol-2-yl)-1,3,4-selenadiazole (3n)
336.9972; elemental analysis: C, 57.33; H, 2.41; N, 16.71; Se,
23.55, found: C, 57.30; H, 2.44; N, 16.76; Se, 23.50.
1
ꢀ
White solid; yield ¼ 71%; mp 268–270 C; H NMR (400 MHz,
DMSO-d6): d 11.83 (s, 2H), 7.42–7.34 (m, 4H), 7.08–7.03 (m, 2H),
7.03 (s, 2H); 13C NMR (400 MHz, DMSO-d6): 162.94, 158.74,
156.42, 134.39, 130.54, 127.36, 127.25, 114.21, 113.70, 113.44,
2,5-Bis(4-(heptyloxy)phenyl)-1,3,4-selenadiazole (3i)
White solid; yield ¼ 81%; mp 95–98 ꢀC; 1H NMR (400 MHz, 107.65, 106.51, 106.43, 106.29, 106.2; HRMS (ESI) m/z calcd for
DMSO-d6): d 7.84 (d, J ¼ 8.8 Hz, 4H), 6.96 (d, J ¼ 8.8 Hz, 4H), 3.99
C
18H10F2N4Se (M + H)+ 401.0111, found 401.0097; elemental
© 2021 The Author(s). Published by the Royal Society of Chemistry
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analysis: C, 54.15; H, 2.52; F, 9.52; N, 14.03; Se, 19.78, found: C, 22 W. Ge, X. Zhu and Y. Wei, RSC Adv., 2013, 3, 10817–10822.
54.21; H, 2.48; F, 9.53; N, 14.06; Se, 19.72.
23 B. S. Kuarm, Y. T. Reddy, J. V. Madhav, P. A. Crooks and
B. Rajitha, Bioorg. Med. Chem. Lett., 2011, 21, 524.
24 S. V. Laxmi, Y. T. Reddy, B. S. Kuarm, P. N. Reddy,
P. A. Crooks and B. Rajitha, Bioorg. Med. Chem. Lett., 2011,
21, 4329.
Conflicts of interest
The authors declare no conicts of interest.
25 B. K. Suresh, J. Venu Madhav, S. L. Laxmi, B. Rajitha,
Y. Thirupathi Reddy, P. Narsimha Reddy and P. A. Crooks,
Synth. Commun., 2010, 40, 3358.
26 B. S. Kuarm, J. Venu Madhav, S. L. Laxmi, B. Rajitha,
Y. Thirupathi Reddy, P. Narsimha Reddy and P. A. Crooks,
Synth. Commun., 2011, 41, 1719.
Acknowledgements
We are grateful to the Council of Scientic and Industrial
Research (CSIR), New Delhi, India.
27 B. K. Suresh and B. Rajitha, Synth. Commun., 2012, 42, 2382.
28 B. S. Kuarm, J. Venu Madhav and B. Rajitha, Lett. Org. Chem.,
2011, 8, 549.
29 B. K. Suresh, B. Janardan and B. Rajitha, Chin. J. Chem.,
2012, 30, 947.
30 J. V. Madhav, Y. Thirupathi Reddy and P. A. Crooks, Synth.
Commun., 2008, 38, 3215.
31 B. Rajitha, Arkivoc, 2008, Xiii, 145.
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