Catalytic application of task specific ionic liquid on the synthesis of benzoquinazolinone derivatives by a multicomponent reaction

Article abstract of DOI:10.1016/j.tetlet.2013.11.011

Benzoquinazolin-2-one derivatives were synthesized by using a catalytic amount of task specific ionic liquid, [1-methyl-3-(4-sulfobutyl)imidazolium-4- methylbenzenesulfonate] through a one-pot multicomponent Biginelli reaction of α-tetralone, aldehyde, and urea/thiourea in excellent yields within a short reaction time. Mechanism studies suggest that the reaction proceeds through iminium intermediate and C2-H of the TSIL plays a major role on its catalytic activity. The catalyst has been reused four times without any significant loss in catalytic activity. Large scale reaction by using this TSIL suggests the applicability of this methodology for bulk synthesis of quinazolinone derivatives.

Full text of DOI:10.1016/j.tetlet.2013.11.011

Tetrahedron Letters 55 (2014) 235–239
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Catalytic application of task specific ionic liquid on the synthesis
of benzoquinazolinone derivatives by a multicomponent reaction
⇑
⇑
Matiur Rahman, Anirban Sarkar, Monoranjan Ghosh, Adinath Majee , Alakananda Hajra
Department of Chemistry, Visva-Bharati (A Central University), Santiniketan 731235, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Benzoquinazolin-2-one derivatives were synthesized by using a catalytic amount of task specific ionic
liquid, [1-methyl-3-(4-sulfobutyl)imidazolium-4-methylbenzenesulfonate] through a one-pot multicom-
Received 19 August 2013
Revised 31 October 2013
Accepted 2 November 2013
Available online 14 November 2013
ponent Biginelli reaction of
a-tetralone, aldehyde, and urea/thiourea in excellent yields within a short
reaction time. Mechanism studies suggest that the reaction proceeds through iminium intermediate
and C2-H of the TSIL plays a major role on its catalytic activity. The catalyst has been reused four times
without any significant loss in catalytic activity. Large scale reaction by using this TSIL suggests the appli-
cability of this methodology for bulk synthesis of quinazolinone derivatives.
Keywords:
Ionic liquid
Benzoquinazolin-2-one
Biginelli reaction
Hydrogen bond
Ó 2013 Elsevier Ltd. All rights reserved.
Multicomponent reactions (MCRs) have been proved to be a
new tool in organic and medicinal chemistry as it provides rapid
building of complex structure in a convergent and atom economi-
cal way.1
generation of waste, avoid the use of auxiliary substances, and
minimize the energy requirement.5 The aspect of hazardness of
the use of volatile organic solvents as auxiliary substances can be
addressed by the use of ionic liquids6 and the generation of waste
and the consumption of energy can be minimized by the accelera-
tion of reaction rates through the use of catalysts. However, there
are some unresolved environmental, health, and safety issues in
the use of ILs as solvents in large quantities.7–9 Thus, its use in
small quantities become an alternate choice to exploit the benefits
of ILs.10 The application of Brønsted acidic task-specific ionic liq-
uids (TSILs) as catalytic materials is growing continuously in the
field of catalysis.11 Combining the useful characteristics of solid
acids and mineral acids, TSILs have been applied to replace tradi-
tional mineral liquid acids, such as hydrochloric acid and sulfuric
acid in chemical reactions. After the report by Forbes and Davis
for the synthesis of a new class of phosphonium- and imidazolium
ion-based ionic liquids equipped with a pendant acidic sulfonic
acid moiety, Brønsted acidic ionic liquids (BAILs) have drawn con-
siderable attention.12 BAILs are more preferable over the tradi-
tional ionic liquids as these combine strong acidity with the use
of nonvolatile ionic liquids. As a result many transformations have
been carried out using this type of acidic IL.11
The quinazolinone moiety has a resourceful pharmacophoric
feature and is present in many naturally occurring compounds.
Thus it is of great importance to chemists and biologists. Com-
pounds with this structural framework are also clinically useful
and possess a wide range of biological activities such as antiviral,
antimalarial, anticonvulsant, antibacterial, diuretic, hypnotic,
hypoglycemic, antitumoral, and antihypertensive.2 The Biginelli
reaction, which was discovered more than a century ago is one
of the most important reactions for the synthesis of dihydropyri-
midinones by a three-component coupling of 1,3-dicarbonyl com-
pounds, aldehydes, and urea.3 Several improved procedures have
been developed mainly based on 1,3-dicarbonyl compounds.3c–f
However, only a few protocols have been reported using cyclic aryl
ketones for the Biginelli condensation which produced quinazoline
derivatives.4 But these methodologies suffer from some stern
drawbacks like use of TMSCl,4a,e HCl,4b,d HClO4,4g FeCl3,4f and other
limitations such as narrow substrate scope and4c long reaction
time.4f Thus, the quest remains for a practical and convenient
method.
In continuation of our efforts toward the development of green
methodologies using task specific ionic liquids (TSILs),10b–f we
present in this study the use of 1-methyl-3-(4-sulfobutyl)imidazo-
lium-4-methylbenzenesulfonate as a catalyst for the synthesis of
benzoquinazolin-2-one and -2-thione in a one-pot, three-compo-
A new class of reaction medium has emerged–ionic liquids (ILs)
that have been projected as green solvents. The tight legislation to
maintain greenness in synthetic processes directs to prevent the
nent reaction of
(Scheme 1).
a-tetralone, an aldehyde, and urea or thiourea
⇑
Corresponding authors. Tel./fax: +91 3463 261526.
E-mail addresses: adinath.majee@visva-bharati.ac.in (A. Majee), alakananda.hajra@
visva-bharati.ac.in (A. Hajra).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.11.011

236
M. Rahman et al. / Tetrahedron Letters 55 (2014) 235–239
that the use of lower amount of catalyst decreased the yield, but
R
O
CHO
X
use of higher quantity of catalyst did not improve the yield. Neither
higher temperature (100 °C) nor longer reaction time (120 min)
improved the yield of reaction (Table 1, entries 8 and 9). Moderate
yield was obtained at lower temperature (Table 1, entry 10). More-
over, p-toluenesulfonic acid is not so effective like TSIL (A). Finally,
the optimized condition for this reaction was chosen as use of
1 mol % catalyst (TSIL A) at 80 °C for 90 min (Table 1, entry 2).
To explore the generality and scope, several structurally
TSIL (1 mol%)
H2N
NH2
R
°
NH
90 min., 80 C, heat
n
n
n = 1,2
N
H
X
O
TSIL:
O
S
Me
X= O, S
N
N
SO3H
Me
O
Scheme 1. Synthesis of quinazolinones catalyzed by TSIL.
diverse aldehydes were treated with
a-tetralone (1 mmol) and
urea/thiourea (1 mmol) under optimized reaction conditions
and the results are summarized in Table 2. Both aromatic alde-
hydes substituted with electron donating group or electron with-
drawing group underwent clean conversion to the desired
products. The catalyst was compatible with various functional
groups such as –Br (Table 2, entry 2) and –NO2 (Table 2, entries
3 and 4). These reactions suggest the mildness of the catalyst.
Acid sensitive aldehyde (piperonal) was also unaffected under
To find out a suitable TSIL, a model reaction was chosen by
using benzaldehyde (1 mmol), -tetralone (1 mmol), and urea
(1 mmol) in the presence of different TSILs and the results are sum-
a
marized in Table 1.
From Table 1 it is evident that TSIL A (Table 1, entry 2) was the
most efficient catalyst which provided 85% yield within 90 min. It
was also observed that catalytic efficiency depends on electronic
environment and physiochemical properties of TSIL. With decreas-
ing chain length of TSIL B catalytic properties decreased (Table 1,
entry 3).13 The poor catalytic effect was observed when C2-H
was replaced by C2-Me (TSIL C) and afforded 56% yield in 90 min
(Table 1, entry 4). It suggests that C2-H plays a crucial role to cat-
alyze the reaction.10g Reaction without catalyst afforded very poor
yield (<10%). It is worthy to mention that the Biginelli reaction for
the synthesis of dihydropyrimidinones proceeded very well under
catalyst and solvent-free conditions.3f It was also clear from Table 1
these reaction conditions (Table 2, entries
7 and 8). The
acid-sensitive heterocyclic substrate such as 2-thiophenecarbox-
aldehyde efficiently afforded the desired product without accom-
panying self-condensation (Table 2, entry 9). In general all the
reactions were clean. No formation of undesired product was
found. It is notable to mention that the pure product was iso-
lated from the reaction mixture without use of any chromato-
graphic method. Only ethanol was used for recrystallization to
get the analytically pure products.
Table 1
Optimization of reaction conditionsa
Ph
CHO
O
O
NH
TSIL (mol%)
H2N
NH2
Temp., time
heat
N
H
O
Entry
1c
Catalyst
Mol (%)
0.5
Temp (°C)
Time (min)
90
Yieldb (%)
49
OTs
OTs
(A)
80
N
N
SO3H
SO3H
Me
(A)
2
3
1
1
80
80
90
90
85
68
N
N
N
Me
OTs
OTs
(B)
SO3H
N
Me
N
N
(C)
SO3H
4
1
80
90
56
Me
Me
(D)
N
N
N
N
5
6
1
2
80
80
90
90
52
86
SO3
Me
Me
OTs
OTs
(A)
(A)
(A)
SO3H
7
8
9
5
1
1
80
100
80
90
90
86
86
85
N
N
N
N
SO3H
SO3H
Me
Me
OTs
OTs
(A)
(A)
120
N
N
N
N
SO3H
SO3H
Me
OTs
10
1
60
120
52
Me
11
12
—
—
1
80
80
90
90
<10
24
p-Toluenesulfonic acid
a
b
c
Reaction conditions: benzaldehyde (1 mmol),
Isolated yield.
Reaction was carried out on 5 mmol.
a-tetralone (1 mmol), and urea (1 mmol) in neat conditions.

M. Rahman et al. / Tetrahedron Letters 55 (2014) 235–239
237
Table 2
Table 3
TSIL catalyzed synthesis of tetrahydro-1H-benzo[h]quinazolin-2-ones/2-thiones
Reusability of TSIL Aa
Use/
Scalec
(mmol)
Amount
Amount
Recoveredf
(%)
Yieldg
(%)
cycleb
usedd (g)
recoverede (g)
R
CHO
O
Fresh
1st
2nd
3rd
25
22
20
17
15
0.098
0.086
0.076
0.066
0.058
0.090
0.078
0.071
0.057
0.050
91
90
88
86
84
84
83
83
82
82
X
°
80 C, heat
+
NH
R
+
H2N
NH2
TSIL A (1 mol%)
90 min.
N
H
X
1 mmol
X = O, S
1 mmol
1 mmol
4th
a
Benzaldehyde and a-tetralone (1 equiv) were treated with urea (1 equiv) in the
presence of TSIL A (1 mol %) at 80 °C for 90 min.
b
Indicates the status of the IL use.
The amount of benzaldehyde, a-tetralone, and urea used in reaction.
The amount of TSIL A used.
The amount of TSIL A recovered.
Indicates to the recovered TSIL A.
Yield of the 4-phenyl-3,4,5,6-tetrahydro-1H-benzo[h]quinazolin-2-one urea
Entry
1
Substrate
CHO
Urea/thiourea
Yielda (%)
85
c
d
e
f
O
H2N
H2N
H2N
H2N
NH2
NH2
NH2
NH2
g
O
O
O
Br
CHO
2
3
82
68
(5 mmol) using 1 mol % catalyst.
O2N
O2N
CHO
yields of the 4-phenyl-3,4,5,6-tetrahydro-1H-benzo[h]quinazolin-
2-one (Table 3).
4
70
CHO
Then we performed large scale preparation of 4-phenyl-3,4,5,6-
tetrahydro-1H-benzo[h]quinazolin-2-one by the reaction of benz-
O
S
Me
Me
CHO
CHO
5
6
80
80
H2N
H2N
H2N
NH2
NH2
NH2
aldehyde (50 mmol),
a-tetralone (50 mmol), and urea (50 mmol)
in presence of 1 mol% TSIL A which afforded 84% yield in 90 min.
Finally we have tried to understand the mechanistic course of
the reaction under the catalytic influence of TSIL. The three-com-
O
O
O
O
O
CHO
ponent reaction among
been proposed to progress by three characteristic pathways
(Scheme 3): (a) Knovenagel adduct formation of aldehyde and
a-tetralone, benzaldehyde, and urea have
7
8
9
78
82
82
S
O
S
a-
CHO
H2N
H2N
H2N
NH2
NH2
NH2
tetralone followed by aza-Michael addition of urea and after that
intramolecular cyclodehydration (Path A), (b) intermediate forma-
tion of iminium moiety by condensation between benzaldehyde
CHO
S
and urea, followed by attack through enol form of
the iminium moiety (Path B), (c) enaminone formation by conden-
sation of urea with -tetralone followed by subsequent aldol type
a-tetralone to
CHO
a
10
80
reaction of enaminone with that of aldehyde by cyclodehydration
to give the expected product (Path C).15 To understand the role
of ionic liquid in the three-component reaction, three sets of reac-
tions have been performed: equimolar mixture of benzaldehyde
HO
a
Isolated yield.
and
a-tetralone (path A), benzaldehyde and urea (path B), and
a-tetralone and urea (path C) were heated at 80 °C in the presence
and absence of TSIL A. In the path A and path C no reaction took
place. The starting materials remain unchanged (TLC) even after
two hours in path A and C. This result eliminates the Knovenagel
(path A) and enaminone (path C) routes. Interestingly in the ab-
sence or in the presence of TSIL A a white solid was obtained even
after five minutes (path B) which is shown in Scheme 4. This sug-
gests the three-component reaction has been taking place through
iminium mechanism.16 The obtained white solid (imine) product
We have successfully synthesized another biologically active
compound 4-benzo[1,3]dioxol-5-yl-1,3,4,5-tetrahydro-indeno[1,2-d]
pyrimidin-2-one14 using piperonal, 1-indanone, and urea giving
moderate yields in 90 min (Scheme 2).
The reusability of the catalyst is an important benefit especially
for commercial applications. We planned to evaluate the possibil-
ity of recovery and reuse of TSIL A. After completion of the reaction
the TSIL A was recovered and four consecutive reaction cycles were
performed without any significant loss in the catalytic activity or
product yields. The recovery efficiency was 91–84% with 84–82%
has been treated with
a-tetralone in the absence and in the
presence of TSIL A (Scheme 4). In the absence of TSIL very low
amount (<10%) of desired product was obtained but in the pres-
ence of TSIL A the required product 4-phenyl-3,4,5,6-tetrahydro-
1H-benzo[h]quinazolin-2-one was obtained in 84% yield. Therefore
TSIL A shows its divergent and essential task during the reaction of
O
O
active methylene carbon of
with that of imine and followed by cyclodehydration step. How-
ever the imine (white solid) was treated with -tetralone in the
a-tetralone (through its enol form)
CHO
O
O
°
80 C, heat, 90 min.
a
NH
NH
+
+
H2N
NH2
O
presence of TSIL A (Table 1, entry 4), devoid of C2-H, moderate
yield of product (56%) was obtained. This suggests that C2-H plays
a crucial role in the course of the reaction probably activating the
aldehydic oxygen through hydrogen bond formation.10g
In conclusion, we have demonstrated the remarkable catalytic
activity of a TSIL for the practical and convenient synthesis of
benzoquinazolinone derivatives using only 1 mol % catalyst. Our
O
TSIL A (1 mol%)
O
1 mmol
1 mmol
1 mmol
76 %
4-benzo[1,3]dioxol-5-yl-1,3,4,5-tetrahydro-indeno
Scheme
2. Synthesis
of
[1,2-d]pyrimidin-2-one.

238
M. Rahman et al. / Tetrahedron Letters 55 (2014) 235–239
O
CHO
CHO
O
Path A
O
H2N
O
NH2
Knovenagel Mechanism
O
O
Path B
NH
N
NH2
H2N
NH2
N
H
O
CHO
Iminium Mechanism
O
N
NH2
O
Path C
NH2
O
H2N
Enamine Mechanism
Scheme 3. Probable reaction pathway.
O
CHO
O
O
5 min. 80 0C
no catalyst
90 min. 80
no catalyst
C
°
N
NH2
H2N
NH2
<10%
1 mmol 1 mmol
white solid
O
CHO
O
O
N
5 min. 80 0
C
NH
NH2
H2N
NH2
°
90 min. 80
C
no catalyst
N
H
O
TSIL (A), 1 mol%
1 mmol 1 mmol
white solid
84%
O
CHO
O
O
N
5 min. 80 0
C
NH
NH2
H2N
NH2
°
90 min. 80
C
no catalyst
N
H
O
TSIL (C), 1 mol%
1 mmol 1 mmol
white solid
52%
Scheme 4. The role of TSIL in three-component reaction.
observation suggests that the reaction proceeds through iminium
intermediate and C2-H plays a major role on its catalytic activities.
This protocol has the advantages of operational simplicity, reus-
ability of the catalyst, no need of column chromatographic separa-
tion, and shorter reaction time. It is also applicable for large scale
synthesis and expected to be attractive for academic and industrial
use due to its environmentally benign reaction conditions.
References and notes
1. For reviews please see: (a) Dömling, A. Chem. Rev. 2006, 106, 17; (b) Zhu, J.;
Bienaymé, H. Multicomponent Reactions; Wiley-VCH: Weinheim, 2005; (c)
Wipf, P.; Stephenson, C. R. J.; Okumura, K. J. Am. Chem. Soc. 2003, 125, 14694.
2. (a) Stevens, G. Diuretics; Academic Press Inc.: New York, 1963; p 12; (b) Hess, H.
J.; Cronin, T. H.; Sciabine, A. J. Med. Chem. 1968, 11, 130; (c) Gupta, C. M.;
Hussain, S. T.; Bhaduri, A. P.; Khana, N. M.; Mukherjee, S. K. Nature 1969, 223,
524.
3. (a) Biginelli, P. Gazz Chem. Ital. 1893, 23, 360; (b) Biginelli, P. Ber 1891, 24, 2962;
(c) Kappe, C. O. Acc. Chem. Res. 2000, 33, 879; (d) Suresh; Sandhu, J. S. Arkivoc
2012, i, 66; (e) Ranu, B. C.; Hajra, A.; Jana, U. J. Org. Chem. 2000, 65, 6270; (f)
Ranu, B. C.; Hajra, A.; Dey, S. S. Org. Process. Res. Dev. 2002, 6, 817.
4. (a) Mirza-Aghayan, M.; Moradi, A.; Bolourtchain, M.; Boukherroub, R. Synth.
Commun. 2010, 40, 8; (b) Kaur, B.; Kaur, R. Arkivoc 2007, xv, 315; (c) Azizian, J.;
Mohammadi, A. M.; Karimi, A. R.; Mohammadizadeh, M. R. App. Catal. A: Gen.
2006, 300, 85; (d) Abelman, M. M.; Smith, S. C.; James, D. R. Tetrahedron Lett.
2003, 44, 4559; (e) Sabitha, G.; Reddy, K. B.; Srinivas, R.; Yadav, J. S. Helv. Chim.
Acta 2005, 88, 2996; (f) Wang, Z. T.; Xe, L. W.; Xia, C. G.; Wang, H. Q. Tetrahedron
Lett. 2004, 45, 7951; (g) Narahari, S. R.; Reguri, B. R.; Gudaparthi, O.; Mukkanti,
K. Tetrahedron Lett. 2012, 53, 1543.
Acknowledgments
A.H. and A.M. acknowledge the financial support from the
Department of Science and Technology, Govt. of India (Grant No.
SR/S5/GC-05/2010). We are thankful to DST-FIST and UGC-SAP.
A.S. and M.G. thank CSIR and UGC for their fellowships.
Supplementary data
5. (a) Clark, J. H. Green Chem. 1999, 1, 1; (b) Tundo, P.; Anastas, P.; Black, D. S.;
Breen, J.; Collins, T.; Memoli, S.; Miyamoto, J.; Polyakoff, M.; Tumas, W. Pure
Appl. Chem. 2000, 72, 1207; (c) Welton, T. Chem. Rev. 1999, 99, 2071.
6. (a) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 139; (b) Jain, N.; Kumar,
A.; Chauhan, S.; Chauhan, S. M. S. Tetrahedron 2005, 61, 1015.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
11.011.

M. Rahman et al. / Tetrahedron Letters 55 (2014) 235–239
239
7. (a) Clark, J. H. Pure Appl. Chem. 2001, 73, 103; (b) Anastas, P. A.; Bartlett, L. B.;
Kirchhoff, M. M.; Williamson, T. C. Catal. Today 2000, 55, 11; (c) Smiglak, M.;
Reichert, W. M.; Holbrey, J. D.; Wilkes, J. S.; Sun, L.; Thrasher, J. S.; Kirichenko,
K.; Singh, S. A.; Katritzky, R.; Rogers, R. D. Chem. Commun. 2006, 2554.
8. Costello, D. M.; Brown, L. M.; Lamberti, G. A. Green Chem. 2009, 11, 548.
9. Coleman, D.; Gathergood, N. Chem. Soc. Rev. 2010, 39, 600.
10. (a) Sawanta, A. D.; Rauta, D. G.; Darvatkara, N. B.; Salunkhe, M. M. Green Chem.
Lett. Rev. 2011, 4, 41; (b) Rahman, M.; Majee, A.; Hajra, A. J. Heterocycl. Chem.
2010, 47, 1230; (c) Das, S.; Majee, A.; Hajra, A. Green Chem. Lett. Rev. 2011, 4,
349; (d) Kundu, D.; Majee, A.; Hajra, A. Green Chem. Lett. Rev. 2011, 4, 205; (e)
Santra, S.; Majee, A.; Hajra, A. Tetrahedron Lett. 2011, 52, 3825; (f) Das, S.;
Rahman, M.; Kundu, D.; Majee, A.; Hajra, A. Can. J. Chem. 2010, 88, 150; (g)
Chakraborti, A. K.; Raha Roy, S. J. Am. Chem. Soc. 2009, 131, 6902; (h)
Chakraborti, A. K.; Raha Roy, S.; Kumar, D.; Chopra, P. Green Chem. 2008, 10,
1111; (i) Sarkar, A.; Raha Roy, S.; Parikh, N.; Chakraborti, A. K. J. Org. Chem.
2011, 76, 7132; (j) Ranu, B. C.; Banerjee, S. Org. Lett. 2005, 7, 3049; (k) Ranu, B.
C.; Banerjee, S. J. Org. Chem. 2005, 70, 4517; (l) Liu, W.; Gao, S.; Feng, C.; Zang,
X.; Zhou, X.; Ma, J.; Wang, C. Chin. J. Org. Chem. 2012, 32, 962.
11. (a) Kotadia, D. A.; Soni, S. S. J. Mol. Catal. A: Chem. 2012, 353, 44; (b) Fang, D.;
Zhou, X.-L.; Ye, Z.-W.; Liu, Z.-L. Ind. Eng. Chem. Res. 2006, 45, 7982; (c) Leng, Y.;
Wang, J.; Zhu, D.; Ren, X.; Ge, H.; Shen, L. Angew. Chem., Int. Ed. 2009, 48, 168.
12. (a) Cole, A. C.; Jensen, J. L.; Ntai, I.; Tran, K. L. T.; Weaver, K. J.; Forbes, D. C.;
Davis, J. H. J. Am. Chem. Soc. 2002, 124, 5962; (b) Myles, L.; Gore, R. G.;
Gathergood, N.; Connon, S. J. Green Chem. 2013, 15, 2740; (c) Ressmann, A. K.;
Gaertner, P.; Bica, K. Green Chem. 2011, 13, 1442; (d) Liu, X.; Ma, H.; Wu, Y.;
Wang, C.; Yang, M.; Yana, P.; Welz-Biermann, U. Green Chem. 2011, 13, 697.
13. (a) Asano, K.; Matsubara, S. Org. Lett. 2009, 11, 1757; (b) Sarkar, A.; Raha Roy,
S.; Kumar, D.; Madaan, C.; Rudrawar, S.; Chakraborti, A. K. Org. Biomol. Chem.
2012, 10, 281.
14. Maddry, J. A.; Severson, W.; Jonsson, C. B.; Noah, J. W.; Noah, D. L.; Chen, X. U.S.
Patent WO2010078229 A1, 2010.
15. De Souza, R. O. M. A.; da Penha, E. T.; Milage, H. M. S.; Garden, S. J.; Esteves, P.
M.; Eberlin, M. N.; Antunes, O. A. C. Chem. Eur. J. 2009, 15, 9799.
16. (a) Kappe, C. O. J. Org. Chem. 1997, 62, 7201; (b) Zhu, Y.-L.; Huang, S.-L.;
Pan, Y.-J. Eur. J. Org. Chem. 2005, 2354.