Zirconocene dichloride catalyzed multi-component synthesis of 1-amidoalkyl-2-naphthols at ambient temperature

Article abstract of DOI:10.1007/s11164-015-2136-9

Zirconocene dichloride (Cp₂ZrCl₂) was found to be highly efficient catalyst for the synthesis of structurally diverse 1-amidoalkyl-2-naphthols by one-pot multi-component reaction (MCR) of 2-naphthol with a variety of aryl aldehydes a

Full text of DOI:10.1007/s11164-015-2136-9

Res Chem Intermed
DOI 10.1007/s11164-015-2136-9
Zirconocene dichloride catalyzed multi-component
synthesis of 1-amidoalkyl-2-naphthols at ambient
temperature
Sharanabasappa Khanapure1 Megha Jagadale1
•
•
Rajashri Salunkhe1 Gajanan Rashinkar1
•
Received: 13 March 2015 / Accepted: 11 June 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Zirconocene dichloride (Cp2ZrCl2) was found to be highly efficient catalyst
for the synthesis of structurally diverse 1-amidoalkyl-2-naphthols by one-pot multi-
component reaction (MCR) of 2-naphthol with a variety of aryl aldehydes and amides
under ambient temperature. The activation of both carbonyl groups of aryl aldehydes and
the o-quinine methide intermediate accounts for the high reactivity of Cp2ZrCl2.
Graphical Abstract
GRAPHICAL ABSTRACT
OH
O
NH2
C H O
NH
OH
O
H2N
NH2
Keywords Multi-component reaction Á Zirconocene dichloride Á 1-Amidoalkyl-2-
naphthols Á o-Quinine methide
&
Gajanan Rashinkar
gsr_chem@unishivaji.ac.in
1
Department of Chemistry, Shivaji University, Kolhapur 416004, M.S., India
123

S. Khanapure et al.
Introduction
Organozirconocenes represent a very powerful answer to the quest for new-
generation, metallocene-based catalysts in organic synthesis. The synthetic appli-
cations of organozirconocenes have received a decisive impetus since the
pioneering work of Schwartz [1–5] and Negeshi [6]. Among them, cationic
zirconocene has been advanced in recent years as a versatile catalyst for mediating
synthetically important transformations [7–9]. Zirconocene dichloride (Cp2ZrCl2)
and its derivatives constitute an important class of cationic zirconocene complexes
that are particularly promising for synthetic applications due to their stability,
commercial availability and non-hazardous nature [10]. Cp2ZrCl2 has been
extensively used as a catalyst in polymerization reactions [11–13]. The catalytic
activity of Cp2ZrCl2 is attributed to its weak acidity. However, despite the
impressive catalytic potential of Cp2ZrCl2, its utility in organic synthesis is only
sporadically demonstrated. The catalytic processes initiated by Cp2ZrCl2 include
synthesis of multi-substituted vinyl silanes [14], bis(indolyl) methanes [15],
cyclobutenylphosphaonates [16] and quinozolin-4(3H)-ones [17]. Cp2ZrCl2 has
also been used as a catalyst in acetylation of phenols/alcohols/amines [18], coupling
of terminal alkynes as well as intramolecular coupling of amines [19, 20], and in the
Refortmastky and Barbier reaction [21]. To tap the barely exploited potential of
Cp2ZrCl2 in organic synthesis, we sought to explore its catalytic activity in synthesis
of scaffolds with a high therapeutic value.
Multi-component reactions (MCRs) are arguably one of the most stimulating,
dynamic and synthetically powerful areas in contemporary organic synthesis for
generating high levels of molecular diversity from three or more reactants with a
minimal number of operations [22–24]. The importance of molecular diversity has
been clearly recognized to identify the lead compounds in drug discovery and
development programs. Indeed, MCRs are intrinsically connected to sustainable
chemistry as they comply with the principles of green chemistry in terms of
economy of steps as well as many of the stringent criteria of an ideal organic
synthesis. The MCRs are of particularly great utility when they lead to the formation
of privileged medicinal scaffolds. 1-Amidoalkyl-2-naphthol scaffolds are of
significant medicinal relevance since they constitute an important class of
intermediates that can be converted into hypertensive and bradycardiac-active
1-aminoalkyl-2-naphthols by amine hydrolysis reactions [25]. Recently, Abou-
Elmagd et al. [26] reported promising cytotoxicity and antiviral activity of
1-amidoalkyl-2-naphthols. The intriguing structural features and promising biolog-
ical activities of 1-amidoalkyl-2-naphthols have rendered them the status of a novel
pharmacophore in the context of drug design. Consequently, much effort has been
made towards the development of efficient methods for their synthesis. 1-Ami-
doalkyl-2-naphthols are prepared by one-pot MCR of 2-naphthol with aryl
aldehydes and amides. A large number catalysts such as piper-betle-shaped nano-
S [27], SiCl4 ? ZnCl2 [28], graphite-supported perchloric acid [29], RuCl2(PPh3)3
[30], [Cu(CH3CN)4PF6] [31], silica gel-supported SO3H [32], zinc benzene
sulphonate [33], [NMP]HSO4 [34], [MeC(OH)2]ClO4 [35], sulphonic acid-
123

Zirconocene dichloride catalyzed multi-component synthesis…
O
NH2
OH
CHO
NH
Cp2ZrCl2
rt, solvent
OH
O
+
+
H2N
NH2
Scheme 1 Cp2ZrCl2 catalyzed multi-component synthesis of 1-amidoalkyl-2-naphthols
functionalized imidazolium salts [36], silica gel-supported dual acidic ionic liquid
[37], trityl chloride [38], H3Mo12O40P [39], montmorillonite K10 [40], SSA [41],
P-TSA [42] etc. have been reported for the synthesis of 1-amidoalkyl-2-naphthols.
However, despite significant progress, there is still room for improvement,
especially towards developing a facile protocol working at ambient temperature.
In our continued interest in the development of highly expedient methodologies
for the synthesis of bioactive scaffolds [43, 44], we report herein the Cp2ZrCl2-
catalyzed synthesis of 1-amidoalkyl-2-naphthols by MCR of 2-naphthol with a
variety of aryl aldehydes and amides at ambient temperature (Scheme 1).
Experimental
Melting points were determined in an open capillary and are uncorrected. All
reactions were carried out under air atmosphere in dried glassware. Infrared spectra
were measured with a Perkin-Elmer Spectrum One FTIR spectrometer. The samples
1
were examined as KBr discs *5 % w/w. H NMR and 13C NMR spectra were
1
recorded on a Brucker AC (300 MHz for H NMR and 75 MHz for 13C NMR)
spectrometer using deuterated dimethylsulfoxide (DMSO-d6 ) as the solvent and
tetramethylsilane (TMS) as an internal standard. Chemical shifts (d) are expressed
in parts per million (ppm) values with TMS as the internal reference, and coupling
constants are expressed in hertz (Hz). Mass spectra were recorded on a Shimadzu
QP2010 gas chromatograph mass spectrometer. All the chemicals were obtained
from local suppliers and used as received.
General procedure for the synthesis of 1-amidoalkyl-2-naphthols
A mixture of 2-naphthol (1 mmol), aldehyde (1 mmol), acetamide or benzamide or
urea (1.2 mmol) and zirconocene dichloride (20 mol%) in ethylene dichloride
(EDC; 5 mL) was stirred at room temperature for a specified time. After completion
of the reaction, as indicated by thin layer chromatography (TLC), the reaction
mixture was quenched in cold water. The obtained crude solid was filtered and
purified by column chromatography on silica gel (Merck, 60–120 mesh, ethyl
acetate: hexane) to afford the pure product which was then characterized by
123

S. Khanapure et al.
spectroscopic methods such as infrared (IR), 1H NMR, 13C NMR and mass
spectroscopy.
Spectral data of representative compound
N-[(4-Methylphenyl)-(2-hydroxynapthalen-1-yl)-methyl]acetamide (Table 2, entry
1
4p) Mp: 233 °C, H NMR (300 MHz, DMSO-d6): d 1.92 (s, 3H), 2.23(s, 3H),
7.04–7.01 (m, 5H), 7.21 (d, J = 8.8 Hz, 1H), 7.20 (t, J = 7.1 Hz, 1H), 7.31 (m,
1H), 7.71 (d, J = 8.8 Hz, 1H), 7.75 (d, J = 7.9 Hz, 1H) 7.81(brd, 1H), 8.31 (d,
J = 8.1 Hz, 1H), 9.89 (s, 1H) ppm; 13C NMR (75 MHz, DMSO-d6): 20.1, 22.4,
47.3, 118.1, 118.7, 121.9, 122.4, 125.1, 126.4, 128.8, 128.1, 128.6, 132.5, 134.4,
139.2, 143.1, 152.5, 169.2 ppm; IR (KBr): t = 3417, 3317, 3074, 1625, 1599, 1567,
1517, 1464, 1395, 12,838, 1209, 1148, 1059, 941, 889, 788, 741, 718 cm-1; MS
(EI): m/z 305 (M?).
Results and discussion
We initiated our studies by examining the effectiveness of different solvents on the
model reaction of 2-naphthol, benzaldehyde and urea using a catalytic amount of the
Cp2ZrCl2 (20 mol%). The polar protic solvents, such as methanol and ethanol, gave
a poor yield of corresponding product (Table 1, entry 2 and 3). No improvement
was detected when the same reaction was carried out using polar aprotic solvents
such as DMF and acetonitrile (Table 1, entries 1, 6). To our delight, the desired
Table 1 Optimization of the reaction conditions using different solvents
O
NH2
OH
CHO
NH
O
OH
+
Cp2ZrCl2
rt, EDC
+
H2N
NH2
Sr. no.
Solvent
Reaction time (h)
Yield (%)a
1.
2.
3.
4.
5.
6.
DMF
12
15
18
6
65
40
45
91
40
40
Methanol
Ethanol
Ethylene dichloride
Dichloromethane
Acetonitrile
24
24
Reaction conditions: 2-Naphthol (1 mmol), benzaldehyde (1 mmol), urea (1.2 mmol) and
Cp2ZrCl2(20 mol %) in solvent (5 mL) were stirred at room temperature
a
Isolated yields after chromatography
123

Zirconocene dichloride catalyzed multi-component synthesis…
Table 2 Optimization study for the amount of Cp2ZrCl2
O
NH2
OH
CHO
NH
O
OH
+
Cp2ZrCl2
rt, EDC
+
H2N
NH2
Sr. no.
Amount of catalyst
(mol%)
Reaction
time (h)
Yield (%)a
1.
2.
3.
4.
5.
6.
7.
00
05
10
15
20
25
30
24
24
19
14
6
No reaction
45
54
58
91
91
92
8
8
Reaction conditions: 2-Naphthol (1 mmol), benzaldehyde (1 mmol), urea (1.2 mmol) and Cp2ZrCl2 in
EDC (5 mL) were stirred at room temperature
a
Isolated yields of purified products
product was obtained in good to excellent yield in a polar, aprotic solvent such as
ethylene dichloride (Table 1, entry 4).
The optimum quantity of catalyst was assessed by performing the model reaction
in the presence of various amounts of Cp2ZrCl2 (Table 2). The best results were
obtained when 20 mol% of catalyst was used (Table 2, entry 5). The use of higher
catalyst loading ([20 mol%) did not improve the yield significantly (Table 2, entry
6 and 7). On the other hand, for loading of catalyst below 20 mol %, Cp2ZrCl2 did
not promote the reaction to a synthetically useful degree (Table 2, entries 2–4). It is
worthy of note that when the model reaction was carried out in the absence of
Cp2ZrCl2, no product was formed either at ambient temperature or under reflux
conditions for 24 h (Table 2, entry 1).
With optimized conditions in hand, we evaluated the scope of the protocol by
reacting 2-naphthol and different amides such as urea/acetamide/benzamide with
electronically and structurally diverse aryl aldehydes. The results are summarized in
Table 3. The nature of substituents on the aromatic ring had a profound effect on the
yield of the product. In all cases, the corresponding 1-amidoalkyl-2-naphthols were
the sole products and no anomalies were observed. It was observed that substituents
in the aromatic ring of aldehydes have a delicate effect on the reaction process.
Aromatic aldehydes with electron-withdrawing groups reacted faster than those
with electron-donating groups. To further expand the scope of the present method,
the replacement of aryl aldehydes with heterocyclic and aliphatic aldehydes was
examined. To our delight, furfuraldehyde, thiophene-2-aldehyde (Table 3, entries f
and g) as well as formaldehyde (Table 3, entry h) reacted, giving satisfactory
123

S. Khanapure et al.
Table 3 Cp2ZrCl2 -catalyzed reaction of 2-naphthol, aldehydes, and urea or acetamide or benzamide
O
R
R1
NH
OH
O
Cp 2ZrCl2
rt, EDC
R1CHO
+
+
OH
H2
N
R
1
2
3
4
Aryl
Aldehyde
(2)
Amide
(3)
Product
(4)
Time
(h)
Yieldb
(%)
Physical Const.c
(°C)
Entry
O
NH2
CHO
O
NH
OH
a
7
5
91
94
170
H2N
NH2
(168-170)[55]
N O2
CHO
O
N H2
O H
O
N H
b
c
179
(179-180)[56]
NO2
H2N
NH2
CHO
NH2
O H
Br
O
NH
O
7
5
89
93
171
(170-172)[56]
H2N
H2N
NH2
NH2
Br
CHO
O
NH2
OH
Cl
O
167
(168)[32]
NH
d
Cl
O
NH2
NH
OH
H3C
O
CHO
8
7
62
45
117
e
(118-120)[53]
H2N
NH2
CH3
.
O
NH2
NH
O
f
163
(162.3-163.7)[27]
O
CHO
O
OH
H2N
H2N
NH2
NH2
O
NH2
O
NH
OH
g
h
7
42
159
S
(159-161)[27]
CHO
S
O
NH2
OH
O
H
NH
H
CHO
5
9
51
63
175
H2N
H2N
NH2
(174)[32]
NH2
CHO
H
3CO
O
O
NH
186
(182-184)[27]
NH2
i
OH
OCH3
CHO
OCH3
O
NH2
NH
OH
H
3CO
O
--------
J
10
Trace
H3CO
OCH3
H3CO
H2N
NH2
OCH3
123

Zirconocene dichloride catalyzed multi-component synthesis…
Table 3 continued
Aryl
Aldehyde
(2)
Amide
(3)
Product
(4)
Time
(h)
Yieldb
(%)
Physical Const.c
(°C)
Entry
O
CH3
CHO
O
NH
k
8
8
89
90
245
H2N
CH3
OH
(245-246)[55]
NO2
CHO
O
O
CH3
OH
l
NH
213
H2N
CH3
(212-215)[40]
NO2
CHO
O2N
O
CH3
O
O
NH
OH
m
7
7
8
91
87
85
247
H2
N
N
CH3
CH3
CH3
(249)[32]
NO2
Cl
O
CH3
OH
CHO
NH
n
225
H2
(226)[32]
Cl
B r
O
C H3
O
N H
CHO
251
H2
N
O H
o
p
q
(250-252)[56]
Br
CHO
CH3
H3
C
O
CH3
OH
O
O
NH
9
7
66
52
223
H2N
H2N
CH3
CH3
(222-223)[55]
O
CH3
NH
H
244
H
CHO
(242)[32]
OH
H3CO
O
CH3
CHO
O
NH
OH
r
8
9
79
70
187
H2N
CH3
(182-183) [48]
OCH3
CHO
OCH3
O
H3CO
H3CO
CH3
NH
OH
O
191
s
t
OCH3
H3CO
H2N
CH
3
(190-193) [53]
OCH3
CHO
O
O
NH
9
86
235
NH2
(234-236)[51]
OH
123

S. Khanapure et al.
Table 3 continued
Aryl
Aldehyde
(2)
Amide
(3)
Product
(4)
Time
(h)
Yieldb
(%)
Physical Const.c
(°C)
Entry
CHO
O
O2N
O
NH
OH
u
6
90
227
NH2
(228)[32]
NO2
CHO
O
Cl
O
NH
OH
v
NH2
7
88
68
179
(177-178)[51]
Cl
CHO
O
H3
C
O
NH
OH
w
10
175
NH2
(175-177)[53]
CH3
O
O
CHO
Br
Br
HN
NH2
x
y
7
8
86
76
228
(228-230)[51]
OH
O
H3CO
O
CHO
NH
OH
196
NH2
(197-199)[48]
OCH3
CHO
O
OCH3
O
NH2
H3CO
H3CO
z
9
67
233
(234-236)[53]
OCH3
NH
OH
H3CO
OCH3
Reaction conditions: 2-Naphthol (1 mmol), aldehyde (1 mmol), Urea/benzamide/acetamide (1.2 mmol)
and Cp2ZrCl2 (20 mol%,) in EDC (5 mL) were stirred at rt
a
Isolated yields of purified products
product yields. The molecular structure of products 4a2z were elucidated from
their IR and 1H and 13C NMR spectra, as described for 4p. The mass spectrum of 4p
displayed the molecular ion peak at m/z = 305 (M?), which is in agreement with
the proposed structure. The IR spectrum of 4p shows broad absorption bands at
3417 for O–H stretching and a sharp band at 3317 for N–H stretching of amide.
Also, three absorption bands at 1625, 1599 and 1567 cm-1, which are related to
C=O, C=N and C=C stretching, respectively, clearly indicated the most significant
1
functional groups of the product. The H NMR spectra of 4p exhibited two sharp
singlet signals at d 1.92 and 2.23 ppm, readily recognized as a methyl group, as well
as broad singlets at 8.31 and 9.89 ppm, indicating N–H and O–H, respectively.
Eleven signals in the aromatic region gave rise to characteristic peaks for an
123

Zirconocene dichloride catalyzed multi-component synthesis…
aromatic, and one signal for active methylene proton. Observation of 18 distinct
1
signals in the H-decoupled 13C NMR spectrum of 4p is in agreement with the
proposed structure. In all other cases, spectral and physical data are in harmony with
the proposed structures and are in good agreement with the reported values [55].
A mechanistic account simulating the probable sequence of events in the
Cp2ZrCl2 -catalyzed synthesis of 1-amidoalkyl-2-naphthols is given in Scheme 2.
Formation of amidoalkylnaphthols in the reaction of 2-naphthol, aldehyde, and
amides has been reported, possibly through ortho-quinonemethides (o-QMs) [56].
Initially, Cp2ZrCl2 coordinates with aryl aldehyde causing electrophilic activation
of a carbonyl moiety which triggered a nucleophilic attack upon 2-naphthol to
C l
Zr
H
O
O
O
C l
O
-H 2 O
Ar
H
Ar
H
Ar
o-QM
N H
O
O H
R
Ar
O
H
R
N H
2
C l
Ar
N H
O
Zr
C l
O
Ar
Scheme 2 Proposed mechanism for the synthesis of 1-amidoalkyl-2-naphthols using Cp2ZrCl2
Table 4 Comparison of different catalysts for the reaction of benzaldehyde, acetamide and 2-naphthol
Sr. no. Catalyst used Amount of Temp Time Yield References
catalyst
(°C)
(h)
(%)
1.
Fe(HSO4)3/solvent-free
5 mol%
0.1 g/mol
1 mol%
100 mg
85
1.08
1.5
2
83
89
90
82
89
96
85
90
94
81
93
72
89
[45]
2.
Montmorillonite K10
125
125
125
125
65
[40]
3.
K5CoW12O403H2O/solvent-free
HClO4_SiO2/solvent-free
p-TSA/solvent-free
[46]
4.
6.50
6
[47]
5.
10 mol%
[42]
6.
Polymer-supported sulphonic acid 170 mg
5
[57]
7.
Iodine
5 mol%
Room temp. 10
[49]
8.
Sr(OTf)2/CHCl3
Indion
10 mol%
65
10
10
[50]
9.
0.25 g/mol 110
10 mol%
0.75 g/mol 125
[51]
10.
11.
12.
13.
Sulphamic acid
Al(H2PO4)
Ce(SO4)2
Room temp. 12
21
Under reflux 36
Room temp.
[52]
[53]
–
[54]
Cp2ZrCl2
20 mol%
8
This work
123

S. Khanapure et al.
generate ortho-quinonemethide (o-QM). Further, Cp2ZrCl2 facilitates 1,4-nucle-
ophilic addition of amides on o-QMs, affording the desired 1-amidoalkyl-2-
naphthol.
In order to show the merit of Cp2ZrCl2 in comparison with the other catalysts
used for the similar reaction, we have summarized several results for the preparation
of N-[(phenyl)-(2-hydroxy-naphthalen-1-yl)-methyl] acetamide from benzaldehyde,
2-naphthol and acetamide in Table 4. It is evident from these results that Cp2ZrCl2is
an highly effective catalyst for the synthesis of 1-amidoalkyl-2-naphthols at ambient
temperature.
Conclusion
In conclusion, we have developed an efficient methodology for multi-component
synthesis of 1-amidoalkyl-2-naphthols at ambient temperature by using zirconocene
dichloride as a congruous catalyst. In addition, this method offers several significant
advantages such as a mild reaction condition, a clean reaction profile, an easy work
up and a high yield.
Compliance with Ethical Standards
Ethical standards The manuscript represents valid work; neither this manuscript nor one with sub-
stantially similar content under my authorship has been published or is being considered for publication
elsewhere (except as described in the manuscript submission); and copies of any closely related manu-
scripts are enclosed in the manuscript submission. I confirm that this work is an accurate representation of
the trial results.
Conflict of interest The authors declare that they have no conflicts of interest.
References
1. J. Schwartz, J.A. Labinger, Angew. Chem. 88, 402 (1976)
2. J. Schwartz, J.A. Labinger, Angew. Chem. Int. Ed. Engl. 15, 333 (1976)
3. C.A. Bertelo, J. Schwartz, J. Am. Chem. Soc. 98, 262 (1976)
4. D.W. Hart, T.F. Blackburn, J. Schwartz, J. Am. Chem. Soc. 97, 679 (1975)
5. D.W. Hart, J. Schwartz, J. Am. Chem. Soc. 96, 8115 (1974)
6. E. Negishi, Takahashi, T. Acc. Chem. Res. 27, 124 (1994) (and references cited therein)
7. K. Suzuki, L. Hintermann, S.Yamanoi, in Titanium and Zirconium in Organic Synthesis, ed. I. Marek
(Wiley-VCH, Gmb H&Co. KGaA, 2002)
8. B.G. Harvey, C.L. Mayne, A.M. Arif, R. Tomaszewski, R.D. Ernst, J. Am. Chem. Soc. 128, 1770
(2006)
9. R.A. Stockland Jr, S.R. Foley, R.F. Jordan, J. Am. Chem. Soc. 125, 796 (2003)
10. A. G. Davies, in Lewis Acids in Organic Synthesis, ed. H. Yamamoto (Wiley-VCH, Weinheim, 2000)
11. M.H. Milani, M.O. De Souza, R.F. De Souza, Cat. Commun. 11, 1094 (2010)
12. K.T. Li, C.L. Dai, C.Y. Li, Polym. Bull. 64, 749 (2010)
13. K.T. Li, C.L. Dai, C.W. Kuo, Cat. Commun. 8, 1209 (2007)
14. Y. Nishihara, D. Saito, K. Tanemura, S. Noyori, K. Takagi, Org. Lett. 11, 3546 (2009)
15. M.L. Kantam, K. Aziz, P.R. Likhar, Cat. Lett. 98, 117 (2004)
16. Y. Sinelnikove, A. Rubinstein, M. Srebnik, A.A.A. Al Quntar, Tetrahedron Lett. 50, 867 (2009)
17. J. Jadhav, S. Khanapure, R. Salunkhe, G. Rashinkar, Appl. Organomet. Chem. 27, 486 (2013)
123

Zirconocene dichloride catalyzed multi-component synthesis…
18. M.L. Kantam, K. Aziz, P.R. Likhar, Cat. Commun. 7, 484 (2006)
19. M. Makabe, Y. Sato, M. Mori, Synthesis 9, 1369 (2004)
20. O.P. Pandey, S.K. Sengupta, C.M. Tripathi, Molecules 10, 653 (2005)
21. M. Chouhan, R. Sharma, V.A. Nair, Appl. Organomet. Chem. 25, 470 (2011)
22. J.K. Padia, M. Field, J. Hinton, K. Meecham, J. Pablo, R. Pinnock, B.D. Roth, L. Singh, N.S.
Chauhan, B.K. Trivedi, L.J. Webdale, Med. Chem. 41, 1042 (1998)
23. M.A. Khilil, R. Soliman, A.M. Farghaly, A.A. Bekhit, Arch. Pharm. 327, 27 (1994)
24. J.F. Wolf, T.L. Rathman, M.C. Sleevi, J.A. Campbell, T.D. Greenwood, J. Med. Chem. 33, 161
(1990)
25. M. Schleiss, J. Eickhoff, S. Auerochs, M. Leis, S. Abele, S. Rechter, Y. Choi, J. Anderson, G. Scott,
W. Rawlinson, D. Michel, S. Ensminger, B. Klebl, T. Stamminger, M. Marschall, Antivir. Res. 79, 49
(2008)
26. W.S.I. Abou-Elmagd, A.I. Hashem, Med. Chem. Res. 22, 2005 (2013)
27. V.K. Das, M. Borah, A.J. Thakur, J. Org. Chem. 78, 3361 (2013)
28. T.A. Salama, Synlett 24, 713 (2013)
29. Z.K. Lei, L. Xiao, X.Q. Lu, Molecules 18, 1653 (2013)
30. X. Zhu, Y.R. Lee, S.H. Kim, Bull. Korean Chem. Soc. 33, 2799 (2012)
31. E. Soleimani, M. Zainali, Synth. Commun. 42, 1885 (2012)
32. D.A. Kotadia, S.S. Soni, J. Mol. Catal. A: Chem. 44, 353 (2012)
33. M. Wang, Z.G. Song, Y. Liang, Synth. Commun. 42, 582 (2012)
34. K.M. Deshmukh, Z.S. Qureshi, Y.P. Patil, B.M. Bhanage, Synth. Commun. 42, 93 (2012)
35. F. Tamaddon, J.M. Bistgani, Synlett 20, 2947 (2011)
36. M.A. Zolfigol, A. Khazaei, A.R. Moosavi-Zare, A. Zare, V. Khakyzadeh, Appl. Catal. A 400, 70
(2011)
37. Q. Zhang, J. Luo, Y. Wei, Green Chem. 12, 2246 (2010)
38. A. Khazaei, M.A. Zolfigol, A.R. Moosavi-Zare, A. Zare, A. Parhami, A. Khalafi-Nezhad, Appl.
Catal. A 386, 179 (2010)
39. P. Gawand, H. Deokar, B. Langi, A. Yadav, A. Chaskar, Synth. Commun. 39, 4171 (2009)
40. S. Kantevari, S.V.N. Vuppalapati, L. Nagarapu, Catal. Commun. 8, 1857 (2007)
41. G. Srihari, M. Nagaraju, M.M. Murthy, Helv. Chim. Acta 90, 1497 (2007)
42. M.M. Khodaei, A.R. Khosropour, H. Moghanian, Synlett 06, 916 (2006)
43. R. Kurane, J. Jadhav, S. Khanapure, R. Salunkhe, G. Rashinkar, Green Chem. 15, 1849 (2013)
44. J. Jadhav, V. Gaikwad, R. Kurane, R. Salunkhe, G. Rashinkar, Tetrahedron 69, 2920 (2013)
45. H.R. Shaterian, H. Yarahmadi, M. Ghashang, Bioorg. Med. Chem. Lett. 18, 788 (2008)
46. L. Nagarapu, M. Baseeruddin, S. Apuri, S. Kantevari, Catal. Commun. 8, 1729 (2007)
47. B. Das, D.N. Kumar, K. Laxminarayana, B. Ravikanth, Helv. Chim. Acta 90, 1330 (2007)
48. J.W. Qing, A.N. Li-Tao, Z.J. Ping, Chin. J. Chem. 26, 1697 (2008)
49. B. Das, K. Laxminarayana, B. Ravikanth, R. Rao, J. Mol. Catal. A: Chem. 261, 180 (2007)
50. W.K. Su, W. Y. Tang, J.J. Li, J. Chem. Res. 123, 128 (2008)
51. S.B. Patil, P.R. Singh, M.P. Surpur, S.D. Samant, Synth. Commun. 37, 1659 (2007)
52. R.R. Nagawade, D.B. Shinde, Chin. J. Chem. 25, 1710 (2007)
53. H.R. Shaterian, A. Amirzadeh, F. Khorami, M. Ghashang, Synth. Commun. 38, 2983 (2008)
54. N.P. Selvam, P.T. Perumal, Tetrahedron Lett. 47, 7481 (2006)
55. H.R. Shaterian, A. Hosseinian, M. Ghashang, Synth. Commun. 38, 3375 (2008)
56. P. Zhang, Z.H. Zhang, Monatsh. Chem. 140, 199 (2009)
57. A.N. Li-Tao, L.U.X. Hua, D.F. Qing, J.W. Qin, Z.J. Ping, Chin. J. Chem. 26, 2117 (2008)
123