N-Sulfonic acid modified poly(styrene-co-maleic anhydride): An efficient and recyclable solid acid catalyst for the synthesis of a wide range of spiropyrans

Article abstract of DOI:10.1007/s13738-014-0523-6

N-Sulfonic acid modified poly(styrene-co-maleic anhydride) as a recyclable solid acid catalyst efficiently catalyzed the one-pot three-component synthesis of spiropyran derivatives through a simple, convenient, and cost-effective approach in good yields a

Full text of DOI:10.1007/s13738-014-0523-6

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0523-6
ORIgINAl PAPER
N‑Sulfonic acid modified poly(styrene‑co‑maleic anhydride):
an efficient and recyclable solid acid catalyst for the synthesis
of a wide range of spiropyrans
Majid M. Heravi · Elaheh Hashemi · Fereshteh Azimian
Received: 7 June 2014 / Accepted: 5 August 2014
© Iranian Chemical Society 2014
Abstract N-Sulfonic acid modified poly(styrene-co-
maleic anhydride) as a recyclable solid acid catalyst effi-
ciently catalyzed the one-pot three-component synthesis of
spiropyran derivatives through a simple, convenient, and
cost-effective approach in good yields and selectivity.
nuclei enhance the biological activity to a significant
extent. Powerful cytostatic alkaloids like spirotrypros-
tatins and pteropodines are some distinguished examples
of spiroindoles [3]. Condensed spirooxindole compounds
with 4H chromenes provided pharmacologically active
systems with immense importance due to their diuretic,
spasmolitic, anticoagulant, anticancer, and antinaphylac-
tic activities [4]. Due to importance and value mentioned
above, the facile, green and heterogeneously catalyzed
synthesis of these moieties especially the nitrile substi-
tuted 4H chromenes which are effective drugs in human
neurodegenerative disorders [5] via multicomponent con-
densation is desirable.
Keywords Heterogeneous catalyst · Multicomponent
reactions · Poly(styrene-co-maleic anhydride) · Polymer-
supported catalyst · Recyclable solid acid catalyst
Introduction
In modern organic chemistry, considerable effort is being
made on the design of strategies and protocols leading to
provide structurally diverse and complex molecules, which
are preferably, biologically active. Multicomponent reac-
tions (MCR) have become popular for constructing com-
plex molecules because of using, protocols involving less
environmental impact, maintaining higher atom-economy
and if possible the recyclability and reusability of the
appropriate catalysts [1].
Spiro compounds as a well-known class of biologi-
cally active molecules are present in many naturally
occurring substances. It has found out; when indoles
which are present in many natural products, pharma-
ceuticals, and agrochemicals [2] involve in these spiro
A variety of catalysts such as piperidine [6], triethanola-
mine [7], l-proline [8], ethylenediaminediacetic acid [9],
triethylamine [10], porcine pancreas lipase [5], 1-N-butyl-
3-methyl-imidazolium tetrafluoroborate [11], sodium
octadecanoate [12], InCl3 [13], nanocrystalline MgO [14]
were applied for the synthesis of spiro[4H-pyran-oxin-
dole] derivatives, which each shows at least one drawback,
such as harsh reaction conditions, long reaction times, low
yields, complicated, tedious work up procedure, and tech-
nical complexity. Therefore, further development on the
environmental and economical impact of the reaction is of
interest and still in much demand.
Scientific interest for the development of heterogeneous
catalysts for fine chemical synthesis has become a major
area of research because of their simple recovery and reus-
ability, as well as their potential for incorporation in MCRs.
Heterogeneous solid acid catalysts as efficient, easy recy-
clable and recoverable, non-corrosive and environmental
friendly catalysts in organic reactions are extensively used
in academia and applicable in industry. Nowadays, more
than 100 transformations catalyzed by over 103 solid acids
are doable in large scale levels [15–21].
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-014-0523-6) contains supplementary
material, which is available to authorized users.
M. M. Heravi (*) · E. Hashemi · F. Azimian
Department of Chemistry, School of Science, Alzahra University,
Vanak, Tehran, Iran
e-mail: mmh1331@yahoo.com
1 3

J IRAN CHEM SOC
H2N
X
R3
O
O
R3
N
N
R1
R1
O
N
SMI-SO3H
N
O
+
NC
X
+
O
EtOH, reflux
N
R2
N
78-97%
3a, 3b
R2
2a
X= CN
1a-1d
R3= H, Ph
R1= H,Br, Cl
R2= H, Benzyl
4a:
R
R
1 = H, R2 = H, X = CN, R3 = H
1 = Cl, R2 = H, X = CN, R3 = H
4b:
4c: R1 = Br, R2 = H, X = CN, R3 = H
4d: R1 = H, R2 = Me, X = CN, R3 = Ph
4e: R1 = H, R2 = Benzyl, X = CN, R3 = H
CH CH2
O
n
O
N
N
Cl
SMI-SO3H
SO3H
Scheme 1 Synthesis of spirooxindole derivatives 4a–4e
In the course of our studies on the modification and use
of heterogeneous solid acid catalyst in organic transforma-
tions [22–27], and in continuation of our interest in the syn-
thesis of biologically heterocyclic systems via MCRs [28–
31] herein we wish to report a simple and efficient method
for the syntheses of a series of spirooxindoles with fused
tetrahydro chromene derivatives via the condensation reac-
tion of isatin, with different nitrilo active methylene compo-
nents, and various reagents including α-methylencarbonyl
in the presence of N-sulfamic acid modified poly(styrene-
co-maleic anhydride) catalyst (SMI-SO3H) as an efficient
and recyclable solid acid catalyst which has been recently
synthesized in our laboratory [32].
frequencies are reported in wave numbers (cm−1). Band
intensities are assigned as weak (w), medium (m), and
strong (s). All yields refer to isolated products.
Catalyst synthesis
SMI-SO3H was synthesized according to our previous
work [32] and applied to the aforementioned MCRs as the
catalyst.
Synthesis of spirooxindoles (4a–4e): general procedure
For the preparation of spiropyran derivatives (4a–4e), a
mixture of isatin derivatives 1a–1d (1 mmol), malononi-
trile (1 mmol), pyrazole 3a and 3b (1 mmol) and 0.03 g of
catalyst SMI-SO3H in ethanol (3 ml) was refluxed for an
indicated time (see Table 4). After completion of the con-
version, as monitored by TlC (n-hexane:EtOAc = 1:1), the
solid heterogeneous catalyst was easily filtrated. The recov-
ered catalyst was washed with acetone and dried under
reduced pressure in 70 °C for 3 h and stored for another
consecutive reaction to run. It can be used at least 6 times
in all the reaction, above with no appreciable loss of activ-
ity. The filtrate was concentrated to solidify and the crude
products were purified by recrystallization from aque-
ous ethanol to afford the title compounds and their physi-
cal data were compared with those of authentic samples
(Scheme 1).
Experimental
general
N,N-Dimethylformamide (DMF) and triethylamine (TEA)
were distilled and kept over 4 Å molecular sieve before
use. The other reagents were purchased from Aldrich and
Merck in high-grade quality and used as received (except
SMA). SMA used in this study is KARABOND SAM
and its general formula is [(C8H8)0.6 (C4H2O3)0.4]n with
anhydride/imide content = 40 %, Mn = 86,666 (g/mol),
Mw = 182,000 and Mw/Mn = 2.1.
1
The H NMR and 13C NMR spectra were recorded,
using Bruker Ultrashield 400 and 100 MHz, respectively,
Advance instrument, with DMSO-d6 used as solvent. Pro-
ton resonances are designated as singlet (s), doublet (d),
triplet (t) and multiplet (m). FTIR spectra were recorded
using KBr disks on FT-IR Bruker Tensor 27 instrument
in the 500–4,000 cm−1 region. The vibrational transition
Synthesis of spirooxindole derivatives 7a–7d and 8: general
procedure
Isatin derivatives 1a–1d (1 mmol), different nitrilo active
methylene components 2a and 2b (1 mmol), and 4-hydroxy
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J IRAN CHEM SOC
Scheme 2 Synthesis of spirooxindole derivatives 7a–7d and 8
coumarin (1 mmol) or dimedone (1 mmol) were reacted
under similar optimized reactions, in presence of 0.03 g of
catalyst and 3 ml of solvent at reflux condition, to afford
spirooxindole derivatives 7a–7d and 8 and their physical
data were determined (Scheme 2).
different MCRs, requiring acidic conditions [32]. In con-
tinuation of our ongoing research program on the develop-
ment of new catalysts and technologies for the synthesis of
biologically active heterocycles [22–27], herein we wish to
extend the applicability of SMI-SO3H catalyst in the pro-
motion of the facile and clean synthesis of spirooxindoles
with fused tetrahydro chromene. For the optimization of the
reaction condition, initially, the condensation of isatin (1a),
malononitrile (2a), and 3-methyl-1H-pyrazol-5-one (3a)
which could be catalyzed by SMI-SO3H was selected as a
model reaction. At first, the effect of different solvents on
the reaction times and yields of the product 4a was exam-
ined (Table 1) and ethanol was selected as the solvent of
choice. The optimization of the amount of the catalyst and
reaction temperature indicated that the best result is achiev-
able in the presence of 0.03 g of catalyst at reflux condition
(Table 2). We investigated the effectiveness of the catalyst
by comparing the reaction in 0.03 g of different appropriate
catalysts. From the result in Table 3, the superior activity of
our catalyst was vividly observed. It was found, more effi-
cient even than silica perchloric acid as an excellent acidic
catalyst [33] and commercially available Amberlyst-15
catalyst which has many applications in organic transfor-
mations [34]. To evaluate the scope and limitations of this
methodology, we extended this optimized conditions for
the preparation of spirooxindoles 4a–4e by condensation
reaction of isatin derivatives (1a–1d), malononitrile (2a)
and pyrazols 3a and 3b (Scheme 1). The selected deriva-
tives (4a–4d) were known and their melting point data
were compared with those of authentic samples and found
to be identical (Table 4) [35, 36].
Synthesis of spiropyrans derivatives 11a, 11b, 12a
and 12b: general procedure
Similar three-component reaction, in the presence of
0.03 g of catalyst and 3 ml of solvent at reflux condition,
acenaphthenequinone (1 mmol) or ninhydrin (1 mmol)
reacted with nitrilo active methylene compounds 2a and 2b
and (1 mmol) pyrazolone 3a and 3b (1 mmol) to provide
the spiropyrans derivatives 11a, 11b, 12a and 12b and their
physical data were determined (Scheme 3). The physical
1
and spectral (melting point (Mp), IR, and H NMR) data
for the derivative 12b as a new compound is as follows:
6′-amino-3′-methyl-1,3-dioxo-1′-phenyl-1,3
1′H-spiro[indene-2,4′-pyrano [2,3c]
dihydro-
pyrazole]-5′-
carbonitrile (12b): Yellow powder, Mp: 264–266 °C, IR
(KBr, ν, cm−1): 3,559, 3,366, 3,319, 3,188, 2,197, 1,712,
1
1,657, HNMR (400 MHz, DMSO-d6): δ (ppm) = 1.1 (s,
3H, CH3), 7.2–7.4 (m, 4H, Ar–H), 7.5 (s, 2H, NH2), 7.9–
8.1 (m, 5H, Ar–H).
Results and discussion
In our previous research, SMI-SO3H catalyst was success-
fully synthesized and applied as an efficient catalyst in
1 3

J IRAN CHEM SOC
R
H2N
O
N
O
N
NC
O
O
O
78-85%
R
N
O
O
O
10
N
12a: R = H
12b: R = Ph
SMI-SO3H
NC
X
+
Or
+
EtOH, reflux
Or
O
2a, and 2b
X= CN, COOEt
3a, and 3b
H2N
X
O
R
R = H, Ph
O
N
N
CH CH2
O
n
9
O
83-87%
N
N
11a: X = CN
11b: X = CO2Et
Cl
SO3H
SMI-SO3H
Scheme 3 Synthesis of spiropyrans derivatives 11a, 11b, 12a and 12b
Table 1 Synthesis of 4a in presence of different solvents
Table 3 Synthesis of 4a in presence of different catalysts at refluxed
condition
Entry
Solvent
Temperature (°C)
Time (h)
Yielda (%)
Entry
Catalyst
Time (h)
Yielda (%)
1
2
3
4
5
6
H2O
Reflux
Reflux
Reflux
Reflux
Reflux
100
1.1
0.3
1.5
1
73
88
31
42
27
69
1
2
3
4
5
6
Silica perchloric acid
AlCl3-HSO4
SMI-SO3H
Dawson
0.3
4
70
43
88
25
55
–
CH3CH2OH
CH3CN
DMF
0.3
6
CH2Cl2
–
3.3
2
Amberlyst-15
–
0.4
24
1 mmol (1a), 1 mmol (2a) and 1 mmol (3a) in presence of 0.03 g of
catalyst and 3 ml of solvent
1 mmol (1a), 1 mmol (2a) and 1 mmol (3a) in presence of 0.03 g of
catalyst and 3 ml of EtOH
a
Refers to the isolated yield
a
Refers to the isolated yield
Table 4 Synthesis of spirooxindole derivatives 4a–4e under opti-
mized conditions
Table 2 Synthesis of 4a in presence of different amount of catalyst
and temperature
Entry Product Time (min) Yielda (%) Mp (°C)/lit. Mp
Entry
Catalyst (g)
Temperature (°C)
Time (h)
Yielda (%)
[references]
1
2
3
4
5
6
0.01
0.02
0.03
0.04
0.03
0.03
Reflux
Reflux
Reflux
Reflux
R.T.
1.1
1
42
58
88
89
30
42
1
2
3
4
5
4a
4b
4c
4d
4e
20
25
25
30
45
88
97
92
87
78
281–283/279–280 [35]
240–241/239–240 [35]
282–283/282–283 [35]
220–222/219–220 [35]
213–214/210 [36]
0.3
0.3
3
50
1.5
1 mmol (1a–1d), 1 mmol (2a), 1 mmol (3a, 3b) in the presence of
0.03 g catalyst and 3 ml EtOH at reflux conditions
1 mmol (1a), 1 mmol (2a) and 1 mmol (3a)
a
a
Refers to the isolated yield
Refers to the isolated yield
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J IRAN CHEM SOC
Table 5 Synthesis of spirooxindole derivatives 7a–7d and 8 under
optimized conditions
dimedone (6) under the obtained optimized reaction condi-
tions (Scheme 2). We found out that using SMI-SO3H acid
catalyst makes this method extremely simple and suitable
to provide a variety of spiropyrans (7a–7d and 8) in a sin-
gle-step process. The selected synthesized derivatives (7a–
7d and 8) were known and their melting point data were
compared with those of authentic compounds and found to
be identical (Table 5) [37–39].
To further explore the potentiality of this process in
the heterocyclic synthesis, we used this approach for
the synthesis of spiropyrans. Acenaphthenequinone (9)
and ninhydrin (10) were reacted with pyrazolone deriva-
tives 3a and 3b (Scheme 3). The reaction proceeded
under similar conditions and afforded the corresponding
spiro-acenaphthylene derivatives in excellent yields and
selectivity (Table 6). Thus, using this catalyst provides
an efficient route to access spiropyrans. The synthesized
derivatives (11a, 11b, 12a) were known and their melt-
ing point data were compared with those of the authentic
samples and found to be identical (Table 6) [40, 41]. In
addition, spectra data of new compound 12b conformed
its structure.
Entry Product Time (min) Yielda (%) Mp (°C)/lit. Mp
[references]
1
2
3
4
5
7a
7b
7c
7d
8
30
30
40
30
40
88
85
96
87
84
252–253/250 [37]
206–208/210 [37]
285–287/285–286 [38]
241–242/244 [37]
256–257/258–260 [39]
1 mmol (1a–1d), 1 mmol (2a, 2b), 1 mmol (5 or 6) in the presence of
0.03 g catalyst and 3 ml EtOH at reflux conditions
a
Refers to the isolated yield
Table 6 Synthesis of spiropyrans derivatives 11a, 11b, 12a and 12b
under optimized conditions
Entry Product Time (min) Yielda (%) Mp (°C)/lit. Mp
[references]
1
2
3
4
11a
11b
12a
12b
90
83
87
78
85
229–231/227–230 [40]
210/210–211 [40]
251–253/250–252 [41]
264–266
115
45
50
The plausible mechanism for the formation of
spirooxindole 7a–7d and 8 involves the activation
of isatin by SMI-SO3H as an acid catalyst to generate
α,β-unsaturated dicyano adduct which is subsequently
subjected to the 1,4-addition of 1,3-dione. This adduct
undergoes intramolecular cyclization through [1, 3] -sig-
matropic proton shift of the iminopyrans which leads
to the formation of a 2-amino-4H-pyran ring system.
Because of the higher electrophilicity and lower steric
1 mmol (1a–1e), 1 mmol (2a, 2b), 1 mmol (5 or 6) in the presence of
0.03 g catalyst and 3 ml EtOH at reflux conditions
a
Refers to the isolated yield
Encouraged by these results, we turned our attention
to study the reactivity of an equimolar quantity of isatin
derivatives (1a–1e), different nitrolo active methylene
components (2a and 2b), and 4-hydroxy coumarin (5) or
Scheme 4 The possible mechanism for the preparation of spirooxindole 7a–7d and 8 with SMI-SO3H
1 3

J IRAN CHEM SOC
References
100
80
60
40
20
0
1. J. Zhu, H. Bienayme, Multicomponent Reactions (Wiley-VCH,
Weinheim, 2005)
2. M. Shiri, M.A. Zolfigol, H.g. Kruger, Z. Tanbakouchian, Chem.
Rev. 110, 2250 (2010)
3. T.H. Kang, K. Matsumoto, Y. Murakami, H. Takayama, M. Kita-
jima, N. Aimi, H. Watanabe, Eur. J. Pharmacol. 444, 39 (2002)
4. J. Skommer, D. Wlodkowic, M. Matto, M. Eray, J. Pelkonen,
leuk. Res. 30, 322 (2006)
4a
7a
11a
5. S.J. Chai, Y.F. lai, J.C. Zu, H. Zheng, Q. Zhu, P.F. Zhang, Adv.
1
2
3
4
5
6
Synth. Catal. 353, 371 (2011)
Run number
6. H. Kimio, O. Hirotaka, Chem. Pharm. Bull. 28, 648 (1980)
7. l.A. Shemchuk, V.P. Chernykh, R.g. Redkin, Russ. J. Org.
Chem. 144, 1789 (2008)
8. Y. li, H. Chen, C. Shi, D. Shi, S. Ji, J. Comb. Chem. 12, 231
(2010)
Fig. 1 The recyclability of the SMI-SO3H in the preparation of 4a,
7a, and 11a
9. g.S. Hari, Y.R. lee, Synthesis 3, 453 (2010)
10. V.Y. Mortikov, Y.M. litvinov, A.A. Shestopalov, l.A. Rodinovs-
kaya, A.M. Shestopalov, Russ. Chem. Bull. Int. Ed. 57, 2373
(2008)
11. R.M. Kurosh, Y.M. leila, Tetrahedron 67, 5693 (2011)
12. l.M. Wang, N. Jiao, J. Qiu, J.J. Yu, J.Q. liu, F.l. guo, Y. liu,
Tetrahedron 66, 339 (2010)
13. S.l. Zhu, S.J. Ji, Y. Zhang, Tetrahedron 63, 9365 (2007)
14. B. Karmakar, A. Nayak, J. Banerji, Tetrahedron lett. 53, 5004
(2012)
15. Y.M. Sania, A.W. Dauda, A.R.A. Aziz, Appl. Catal. A gen. 470,
140 (2014)
16. g.A. Benson, W.J. Spillane, Chem. Rev. 80, 151 (1980)
17. X. li, W. Eli, J. Mol. Catal. A Chem. 279, 159 (2008)
18. g. Chehardoli, M.A. Zolfigol, T. Faal-Rastgar, S. Mallakpour, A.
ghorbani-Choghamarani, J. Chem. Sci. 121, 441 (2009)
19. K. Pirkanniemi, M. Sillanpää, Chemosphere 48, 1047 (2002)
20. R. Dalpozzo, g. Bartoli, l. Sambri, P. Melchiorre, Chem. Rev.
110, 3501 (2010)
hindrance of the nitrile group toward nucleophilic addi-
tion, the nucleophilic addition of enolic oxygen occurs
preferably to nitrile rather than to the ester group, when
both nitrile and ester groups are present, providing the
desired 2-amino-4H-pyran with high regioselectivity
(Scheme 4).
Because of green chemistry viewpoint, the recyclabil-
ity and reusability of the catalyst was carried out for the
synthesis of 4a, 7a and 11a compounds in each selected
multi-component reaction. Upon completion of the reac-
tion, the catalyst was simply recovered by simple filtra-
tion, washing with acetone and drying at 70 °C for 3 h
and reused for another fresh reaction run. The results
clearly showed that the catalyst could be efficiently recov-
ered and recycled even after 6 cycles without suffering
any significant drop in its catalytic activity or the yield of
reaction (Fig. 1).
21. A.K. Chakraborti, B. Singh, S.V. Chankeshwara, A.R. Patel, J.
Org. Chem. 74, 5967 (2009)
22. M.M. Heravi, M. Tajbakhsh, A.N. Ahmadi, B. Mohajerani,
Monatsh. Chem. 137, 175 (2006)
23. M.M. Heravi, K. Bakhtiari, Z. Daroogheha, F.F. Bamoharram,
Catal. Commun. 8, 1991 (2007)
24. F.F. Bamoharram, M.M. Heravi, M. Roshani, M. Jahangir, A.
gharib, Appl. Catal. A gen. 302, 42 (2006)
Conclusions
25. M.M. Heravi, M. Khorasani, F. Derikvand, H.A. Oskoole, F.F.
In conclusion, we have successfully developed an interest-
ing mild three-component one-pot process with sequen-
tial Knoevenagel/Michael/cyclization reactions, catalyzed
by SMI-SO3H as a solid acid for the efficient synthesis of
diverse spiropyrans in good yields and high regioselectivity.
The catalyst not only plays a crucial role in the progress of
the reactions regarding the rate and yields of the products,
but with it, the common problems in the recovery and reuse
of the catalysts is also circumvented. This catalyst system
could be easily separated from the reaction mixture almost,
quantitatively, in high purity and being directly reused after
simple extraction, for several times with no noticeable loss
of activity, concluding from the comparison of the yield of
products for each run.
Bamoharram, Catal. Commun. 8, 1886 (2007)
26. M.M. Heravi, g. Rajabzadeh, F.F. Bamoharram, N. Seifi, J. Mol.
Catal. A-Chem. 256, 238 (2006)
27. M.M. Heravi, F.F. Bamoharram, g. Rajabzadeh, N. Seifi, M.
Khatami, J. Mol. Catal A Chem. 259, 213 (2006)
28. M.M. Heravi, K. Bakhtiari, Z. Daroogheha, F.F. Bamoharram, J.
Mol. Catal. A-Chem. 273, 99 (2007)
29. M.M. Heravi, B. Baghernejad, H.A. Oskooie, Tetrahedron lett.
50, 767 (2009)
30. M.M. Heravi, Z. Faghihi, J. Iran. Chem. Soc. 11, 209 (2014)
31. M.M. Heravi, B. Baghernejad, H.A. Oskooie, R. Hekmatshoar,
Tetrahedron lett. 49, 6101 (2008)
32. M.M. Heravi, E. Hashemi, Y.S. Beheshtiha, K. Kamjou, M. Tool-
abi, N. Hosseintash, J. Mol. Catal. A Chem. 392, 173 (2014)
33. R. Kumar, D. Kumar, A. K. Chakraborti, Synthesis, 299 (2007)
34. R. Pal, T. Sarkar, S. Khasnobis, ARKIVOC 570 (2012)
35. Y. liu, Z. Ren, W. Cao, J. Chen, H. Deng, M. Shao, Synth. Com-
mun. 41, 3620 (2011)
36. R. Redkin, l.A. Shemchuk, V.P. Chernykh, O.V. Shishkin, S.V.
Acknowledgments M. M. H is thankful to Alzahra University
Research Council for partial financial assistance.
Shishkin, Tetrahedron 63, 11444 (2007)
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J IRAN CHEM SOC
37. g. Shanthi, g. Subbulakshmi, P.T. Perumal, Tetrahedron 63, 2507
(2007)
40. M. Saeedi, M.M. Heravi, Y.S. Beheshtiha, H.A. Oskooie, Tetra-
hedron 66, 5345 (2010)
38. N. lashgari, g. Mohammadi Ziarani, A. Badiei, M. Zarezadeh-
Mehrizi, J. Heterocycl. Chem. (2014). doi:10.1002/jhet.1746
39. B. Maheshwar Rao, g. Niranjan Reddy, T. Vijaikumar Reddy,
B.l.A. Prabhavathi Devi, R.B.N. Prasad, J.S. Yadav, B.V. Subba
Reddy, Tetrahedron lett. 54, 2466 (2013)
41. R.M. Shaker, A.F. Mahmoud, F.F. Abdel-latif, J. Chin. Chem.
Soc. 52, 563 (2005)
1 3