A magnetic solid sulfonic acid modified with hydrophobic regulators: An efficient recyclable heterogeneous catalyst for one-pot aza-Michael-type and Mannich-type reactions of aldehydes, ketones, and amines

Article abstract of DOI:10.1016/j.tetlet.2015.02.088

Two convenient green protocols for the synthesis of β-amino ketones have been developed which involve one-pot aza-Michael-type and Mannich-type reactions of a series of aldehydes, ketones, and amines in the presence of a catalytic amount of the magnetic solid sulfonic acid catalyst, Fe₃O₄@SiO₂@Me&Et-PhSO₃H, at room temperature. The catalyst can be reused four times without loss of activity.

Full text of DOI:10.1016/j.tetlet.2015.02.088

Accepted Manuscript
A magnetic solid sulfonic acid modified with hydrophobic regulators: An effi-
cient recyclable heterogeneous catalyst for one-pot aza-Michael-type and Man-
nich-type reactions of aldehydes, ketones and amines
Barahman Movassagh, Leili Tahershamsi, Akbar Mobaraki
PII:
S0040-4039(15)00369-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.02.088
TETL 45958
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
5 December 2014
27 January 2015
18 February 2015
Please cite this article as: Movassagh, B., Tahershamsi, L., Mobaraki, A., A magnetic solid sulfonic acid modified
with hydrophobic regulators: An efficient recyclable heterogeneous catalyst for one-pot aza-Michael-type and
Mannich-type reactions of aldehydes, ketones and amines, Tetrahedron Letters (2015), doi: http://dx.doi.org/
10.1016/j.tetlet.2015.02.088
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Graphical Abstract
Leave this area blank for abstract info.
A magnetic solid sulfonic acid modified with
hydrophobic regulators: An efficient recyclable
heterogeneous catalyst for one-pot aza-Michael-type
and Mannich-type reactions of aldehydes, ketones and amines
Barahman Movassagh,* Leili Tahershamsi, Akbar Mobaraki
O
O
NHR4
R3
R2
Cat.
R3CHO
R4NH2
R1
R1
+
+
EtOH, r.t.
R2
Cat. = Fe3O4@SiO2@Me&Et-PhSO3H

1
Tetrahedron Letters
journal homepage: www.els evier.com
A magnetic solid sulfonic acid modified with hydrophobic regulators: An
efficient recyclable heterogeneous catalyst for one-pot aza-Michael-type and
Mannich-type reactions of aldehydes, ketones and amines
Barahman Movassagh,* Leili Tahershamsi, Akbar Mobaraki
Department of Chemistry, K. N. Toosi University of Technology, P. O. Box 16315-1618, Tehran, Iran
ARTICLE INFO
ABSTRACT
Article history:
Received
Two convenient green protocols for the synthesis of β-amino ketones have been developed
which involve one-pot aza-Michael-type and Mannich-type reactions of a series of aldehydes,
ketones and amines in the presence of a catalytic amount of the magnetic solid sulfonic acid
catalyst, Fe3O4@SiO2@Me&Et-PhSO3H, at room temperature. The catalyst can be reused four
times without loss of activity.
Received in revised form
Accepted
Available online
Keywords:
2009 Elsevier Ltd. All rights reserved.
Core-shell magnetic solid acid
Heterogeneous catalyst
Aza-Michael reaction
Mannich-type reaction
One-pot reaction
of simple, recyclable, and environmentally friendly approaches
that can be performed at ambient temperature for both aza-
Michael and Mannich reactions are desirable.
The construction of carbon-nitrogen bonds, especially under
green and safe conditions, is an important transformation in
organic synthesis. The development of novel synthetic routes
leading to β-aminocarbonyl compounds has received
considerable interest due to their occurrence in a wide variety of
biologically active natural products;1 they also serve as key
intermediates for the synthesis of important nitrogen-containing
compounds such as β-lactams, β-amino acids, and β-amino
alcohols.2 One-pot Mannich reactions of aldehydes, ketones and
amines using a variety of Lewis acids,3 Brønsted acids,4 and
Lewis base5 catalysts have been reported. However, many of
these suffer from drawbacks such as long reaction times, toxicity,
and difficult separations after the reaction.
Another approach for preparing β-amino carbonyl compounds
is based on the addition of amines to α,β-unsaturated carbonyl
compounds (aza-Michael reaction). This method has a prominent
advantage over the Mannich reaction in that it covers a wide
range of nitrogen nucleophiles including amides, carbamates, and
sulfonamides, which can rarely be utilized using the conventional
Mannich condensation. A number of Lewis acid catalysts, solid
acids, and Brønsted acids,6 as well as basic catalysts7 have been
employed. Unfortunately, several drawbacks such as high costs,
harsh reaction conditions, long reaction times, the requirements
for large excesses of reagents or catalysts, and the use of toxic
solvents are associated with the above methods. Also, some of
the reported procedures were only applicable to aliphatic amines
and failed to work with aromatic amines.6h,7b Although recent
protocols have made this route more attractive, the development
Significant attention has been paid to solid acid catalysts due
to their potential applications for replacing liquid mineral acids in
industry;8 they exhibit advantages of easy separation of the
catalyst from the liquid reaction medium, minimal corrosion,
good recyclability, green chemical processes, and enhanced
product selectivity.9 In many reactions catalyzed by solid sulfonic
acids involving hydrophobic and hydrophilic substrates, the
water produced as a by-product is co-adsorbed by the catalyst
poisoning the surface and producing a more hydrophilic
environment, resulting in reduced catalytic activity.10 On the
other hand, the polarities of the reactants and products, the
acidity of the catalyst, and hydrophobic-hydrophilic balance on
the catalyst surface have significant effects on the progress of the
reaction. Further studies have addressed the design, synthesis and
catalytic applications of surface-modified SO3H solid materials in
order to optimize these properties, and the overall performance of
the catalysts.10d,11 Recently, among solid acid catalysts, special
attention has been paid to the preparation, characterization, and
catalytic investigation of magnetic sulfonic acids.12
In continuation of our investigations on the hydrophobic,
organosulfonic acid functionalized silica-coated magnetic
nanoparticle catalyst, Fe3O4@SiO2@Me&Et-PhSO3H (Figure
1),12a,b in this paper, we report the synthesis of β-amino ketones
by two one-pot approaches: aza-Michael-type (pathway a) and
Mannich-type (pathway b) reactions of aryl aldehydes, ketones
and amines in EtOH at room temperature employing this reusable
catalyst (Scheme 1).
*Corresponding author. Tel.:+98 21 8802 5459; fax:+98 21 2285
3650. E-mail: bmovass1178@yahoo.com (B. Movassagh).

2
Tetrahedron
HO3S
Table 1
OH
H3C
O
SO3H
Optimization of the Fe3O4@SiO2@Me&Et-PhSO3H-catalyzed
one-pot reaction of cyclohexanone, benzaldehyde, and
anilinea
Si CH3
O
Si
Si
O
Si
O
O
O
O
O
HO
O
SiO2
Fe3O4
NP
Si
O
OH
O
CH3
Yield (%)b
Catalyst
Si
H3C
O
O
Entry
Solvent
T (°C)
O
(mol%)
0.3
0.3
0.3
0.6
0.6
0.6
0.4
0.6
0.7
0.9
0.9
0.9
0.9
O
Aza-Michael
Mannich
86
O
O
Si
O
O
O
O
1
2
EtOH
EtOH
EtOH
H2O
25
50
80
25
25
25
25
25
25
25
25
80
25
–
62c
–
H3C
O
O
51c
25c
68
O
Si
Si
O
CH3
3
H3C
O
SO3H
HO
4
45
–
5
THF
–
56
80
6
85
–
7
EtOH
EtOH
EtOH
EtOH
MeOH
CH3CN
CH2Cl2
87
99
99
–
SO3H
8
9
87
96
96
95
N.R.
15
10
11
12
13
Figure 1. The hydrophobic magnetic solid sulfonic acid catalyst,
Fe3O4@SiO2@Me&Et-PhSO3H.
–
N.R.
–
a Reaction conditions: cyclohexanone (3.0 mmol), benzaldehyde (2.0 mmol),
aniline (2.0 mmol), catalyst, solvent (4 mL), 2 h.
b Isolated yield. N.R = no reaction.
c Several by-products were detected.
Scheme 1. Two approaches for the synthesis of β-amino ketones.
To find the optimum conditions which suit the two approaches,
the reaction of cyclohexanone, benzaldehyde and aniline, via
different sequences of addition, was examined under various
Scheme 2. Two possible pathways for the formation of 3-(4-
nitrophenyl)-1-phenyl-3-(phenylamino)propan-1-one in the presence
of Fe3O4@SiO2@Me&Et-PhSO3H.
conditions
(Table 1).
catalyzed
by Fe3O4@SiO2@Me&Et-PhSO3H
The effect of different molar ratios of the reactants, the amount
of catalyst, the solvent, and temperature on the yield, was
studied. The best results (96% for aza-Michael and 99% for
Mannich reactions) were obtained by carrying out the reaction
with a 1.5:1:1 molar ratio of cyclohexanone, benzaldehyde, and
aniline at room temperature in the presence of 0.7 and 0.6 mol%
(based on benzaldehyde) of the catalyst for the aza-Michael and
Mannich reactions, respectively, in 4 mL of absolute EtOH for
two hours (Table 1, entries 9 and 8, respectively). We observed
that 0.7 and 0.6 mol% of the catalyst could catalyze the aza-
Michael and Mannich reactions efficiently; increasing or
decreasing the amount of the catalyst did not lead to any
improvement in the yields. To investigate the aza-Michael
addition reaction of amines with α,β-unsaturated carbonyl
compounds (pathway a), and the reaction of enolates with imines
(Mannich reaction, pathway b), we performed two experiments
as shown in Scheme 2.
minutes and was separated as a yellow solid [mp 86-88 °C, Lit.14
mp 88-90 °C; IR (KBr): νmax = 1597 and 1517 cm-1]. The
addition of acetophenone to this intermediate gave the
corresponding β-amino ketone after one hour.
This catalyst is composed of Brønsted acidic groups and
hydrophobic methyl moieties. The aza-Michael reaction
(pathway a) proceeds through the formation of intermediate 3
(Scheme 1) followed by nucleophilic attack by the amine. The
Fe3O4@SiO2@Me&Et-PhSO3H-catalyzed Mannich reaction of the
aldehyde and amine proceeds through imine formation (6) with
subsequent attack of the enol derived from the ketone on the
protonated imine (pathway b, Scheme 1). The characteristic feature
of both reactions is the formation of water as a by-product in the first
step.
Using the optimized reaction conditions, the one-pot aza-
Michael and Mannich reactions of various aromatic ketones,
aldehydes and amines were investigated and the results are
shown in Table 2.15 It was found that the reaction of various
aromatic aldehydes with aromatic amines and acetophenones
bearing electron-withdrawing and electron-donating groups
proceeded smoothly using both approaches, and gave the
corresponding β-amino ketones in high to excellent yields (Table
2, entries 1-20). Amines with electron-withdrawing groups, such
as 4-nitroaniline and 4-chloroaniline gave the desired products in
good to excellent yields (Table 2, entries 3, 5, 6 and 11). The
aliphatic amine, benzylamine was also studied, and was found to
give the corresponding products in high yields (Table 2, entries
In the first experiment (pathway a), an ethanolic mixture of
acetophenone and 4-nitrobenzaldehyde was stirred at room
temperature in the presence of the catalyst (0.7 mol%); after 30
minutes, the corresponding chalcone was separated as a light
orange solid [mp 157-158 °C, Lit.13 mp 158-160 °C; IR (KBr):
ν
max =1677 and 1596 cm-1]. The reaction of this intermediate with
aniline, after 1.5 hours, led to the corresponding β-amino ketone.
In another experiment (pathway b), aniline and 4-
nitrobenzaldehyde were stirred in the presence of the catalyst
(0.6 mol%). The corresponding imine was formed in 10

3
Table 2
Fe3O4@SiO2@Me&Et-PhSO3H-catalyzed one-pot aza-Michael-type and Mannich reactions of various aldehydes, ketones and
amines
O
O
NHR4
R2
Cat. (0.6 or 0.7 mol%)
EtOH, r.t.
R3CHO
R4NH2
R1
R1
R3
+
+
R2
Aldehyde
Amine
R4
Yield (%)a/ Time (h)b
Ketone
R1
Entry
R2
H
R3
Ph
Ph
Pathwaya
Pathway b
91/1.5
1
2
Ph
Ph
Ph
95/27e
H
4-MeOC6H4
4-O2NC6H4
96/1.57e
90/1
3
Ph
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
–
–
95/17e
93/216a
98/17e
92/1.57e
91/2
4
Ph
Ph
4-MeC6H4
4-ClC6H4
4-ClC6H4
Ph
5
Ph
Ph
–
6
Ph
4-ClC6H4
4-ClC6H4
4-MeC6H4
4-HOC6H4
–
7
Ph
96/27e
90/416b
–
8
Ph
4-MeC6H4
4-MeC6H4
Ph
90/3
9
Ph
90/316b
99/1
10
11
12
13
14
15
16
17
18
19
20
21
Ph
4-O2NC6H4
4-MeOC6H4
4-MeC6H4
4-MeOC6H4
4-ClC6H4
2-MeOC6H4
3-pyridyl
2-thienyl
Ph
99/1.57e
90/27e
–
Ph
4-ClC6H4
Ph
89/1.5
92/27e
–
Ph
Ph
Ph
Ph
90/27e
90/27e
87/316c
86/216d
85/216e
–
4-MeOC6H4
Ph
–
Ph
–
Ph
Ph
90/1
Ph
Ph
92/1
4-ClC6H4
4-MeOC6H4
4-MeC6H4
Ph
98/1.57e
93/1.57e
90/1
Ph
Ph
–
Ph
Ph
92/1.57e
85/47e
Ph
Ph
H
H
4-ClC6H4
CH2Ph
86/3
22
23
24
4-MeC6H4
Ph
CH2Ph
Ph
88/416f
96/116a
94/116g
90/3
cyclohexanone
cyclohexanone
99/1.5
99/0.5
4-O2NC6H4
Ph
90/116h
92/1
cyclohexanone
25
4-MeOC6H4
Ph
a Isolated yields.
b References are provided for known compounds.
of 0.6 and 0.7 mol%, respectively, of the catalyst in absolute
EtOH at room temperature. Upon completion of each reaction,
the catalyst was easily separated and recovered by applying an
external magnet. The catalyst was washed with EtOH and finally
dried at 100 °C for 30 minutes prior to the next run. The crude
product was recrystallized from EtOH. During the recycling
experiment with fresh reactants, under the same reaction
conditions, no considerable change was observed in the activity
of the catalyst over four successive runs (Table 3).
21 and 22). The successful results obtained with these protocols
prompted us to use cyclohexanone under these conditions (Table
2, entries 23-25). The results indicated that cyclohexanone was
more active than acetophenones (Table 2, entries 1, 10 and 13),
because formation of its enol was much faster than that of the
acetophenones.
The catalytic activity and the reusability of this hydrophobic
magnetic solid acid catalyst were also studied for the Mannich
and aza-Michael reactions of 4-nitrobenzaldehyde (2.0 mmol),
acetophenone (3.0 mmol), and aniline (2.0 mmol) in the presence
In conclusion, we have successfully developed a magnetic
solid sulfonic acid modified with hydrophobic regulators,

4
Tetrahedron
J. Org. Chem. 2001, 66, 9052-9055; (f) Chaudhuri, M. K.;
Table 3
Recycling of catalyst for the reaction of 4-nitrobenzaldehyde,
acetophenone, and anilinea
Hussain, S.; Kantam, M. L.; Neelima, B. Tetrahedron Lett. 2005,
46, 8329-8331; (g) Das, B.; Chawdhury, N. J. Mol. Catal. A:
Chem. 2007, 263, 212-216; (h) Wabnitz, T. C.; Spencer, J. B. Org.
Lett., 2003, 5, 2141-2144; (i) Basu, B.; Das, P.; Hossain, I. Synlett
2004, 2630-2632.
Entry
Cycle
Aza-Michael yield(%)b
Mannich yield(%)b
1
2
3
4
1
2
3
4
99
97
95
90
99
93
91
88
7. (a) Ai, X.; Wang, X.; Liu, J.-M.; Ge Z.–M.; Cheng, T.–M.; Li, R.–T.
Tetrahedron 2010, 66, 5373-5377; (b) Yang, L.; Xu. L. W.; Xia, C.–G.
Tetrahedron Lett. 2005, 46, 3279-3282; (c) Yang, L.; Xu, L.–W.; Zhou,
W.; Li, L.; Xia, C.-G. Tetrahedron Lett. 2006, 47, 7723-7726; (d) Yeom,
C.–E.; Kim, M. J.; Kim, B. M. Tetrahedron 2007, 63, 904-909; (e)
Movassagh, B.; Khosousi, S. Monatsh. Chem. 2012, 143, 1503-1506.
a Reaction conditions were the same as that of entry 10, Table 2 for both
reactions.
b Isolated yields.
8. (a) Wong, W.–P.; Ho K. –P.; Lee, L. Y. S.; Lam, K.–M.; Zhou, Z.–Y.;
Chan, T. H.;Wong, K.–Y. ACS Catal. 2011, 1, 116-119; (b) Corma, A.;
Garcia, H. Chem. Rev. 2003, 103, 4307-4366; (c) Davis, M. E. Nature
2002, 417, 813-821.
Fe3O4@SiO2@Me&Et-PhSO3H, which was used efficiently as a
heterogeneous catalyst in one-pot aza-Michael and Mannich
reactions for the synthesis of β-amino carbonyl compounds under
mild conditions. The high reactivity of the catalyst is probably
due to synergistic effects between sufficient hydrophobicity and
acidity of siliceous networks, which in turn results in: (a)
remarkable shielding effects against polar molecules, good
accessibility of the active sites and easier diffusion of reaction
partners within the network resulting from the presence of
organic methyl groups on the surface of catalyst, and (b) mild
acidic conditions for the preparation of β-amino ketones.10,11,12b
The catalyst can be recovered and reused, thus making these
procedures more environmentally acceptable.
9. (a) Nikolla, E.; Roman-Leshkov, Y.; Moliner, M.; Davis, M. E. ACS
Catal. 2011, 1, 408-410; (b) Barbaro, P.; Liguori, F. Chem. Rev. 2009,
109, 515-529.
10. (a) Liu, F. J.; Meng, X. J.; Zhang, Y. L.; Ren, L. M.; Nawaz, F.; Xiao, F.–
S. J. Catal. 2010, 271, 52-58; (b) Mondal, J.; Sen, T.; Bhaumik, A. Dalton
Trans. 2012, 41, 6173-6181; (c) Melero, J. A.; Bautista, L. F.; Morales,
G.; Iglesias, J.; Briones, D. Energy Fuels 2009, 23, 539-547; (d) Mbaraka,
I. K.; Shanks, B. H. J. Catal. 2005, 229, 365-373; (e) Molares, G.; Athens,
G.; Chmelka, B. F.; Van Grieken, R.; Melero, J. A. J. Catal. 2008, 254,
205-217; (f) Okuhara, T. Chem. Rev. 2002, 102, 3641-3666.
11. (a) Karimi, B.; Mirzaei, H. M.; Mobaraki, A. Catal. Sci. Technol. 2012, 2,
828-834; (b) Karimi, B.; Mobaraki, A.; Mirzaei, H. M.; Zareyee, D.; Vali,
H. Chem. Catal. Chem. 2014, 6, 212-219.
12. (a) Mobaraki, A.; Movassagh, B.; Karimi, B. ACS Comb. Sci. 2014, 16,
352-358; (b) Mobaraki, A.; Movassagh, B.; Karimi, B. Appl. Catal. A:
Gen. 2014, 472, 123-133; (c) Khalafi-Nezhad, A.; Mohammadi, S. ACS
Comb. Sci. 2013, 15, 512-518; (d) Wang, S.; Zhang, Z.; Liu, B.; Li, J.
Catal. Sci. Technol. 2013, 3, 2104-2112; (e) Liu, Y. H.; Deng, J.; Gao, J.
W.; Zhang, Z. H. Adv. Synth. Catal. 2012, 354, 441-447; (f) Zhang, Q.;
Su, H.; Luo, J.; We, Y. Green Chem. 2012, 14, 201-208.
Acknowledgments
This work was supported by the Research Council of the K. N.
Toosi University of Technology and the Iranian National Science
Foundation (INSF, Grant No. 92013520). The authors express
their gratitude to Dr. Sogand Noroozizadeh for revising the
English language of the manuscript.
13. Lin, P.; Li, B.; Li, J.; Wang, H.; Bian, X. B.; Wang, X. Catal. Lett. 2011,
141, 459-466.
14. Suresh, R.; Kamalakkannan, D.; Ranganathan, K.; Arulkumaran, R.;
Sundararajan, R.; Sakthinathan, S. P.; Vijayakumar, S.; Sathiyamoorthi,
K.; Mala, V.; Vanangamudi, G.; Thirumurthy, K.; Mayavel, P.;
Thirunarayanan, G. Spectrochim. Acta Part A: Mol. Biomol. Spectrosc.
2013, 101, 239-248.
References and notes
1. (a) Juaristi, E. Enantioselective Synthesis of β-Amino Acids; Wiley-VCH:
New York, 1991; (b) Davies, S. G.; Smith, A. D.; Price, P. D.
Tetrahedron: Asymmetry 2005, 16, 2833-2891; (c) Prakasch, M.;
Srivastava, S.; Leek, D. M.; Arya, P. J. Comb. Chem. 2006, 8, 762-773;
(d) Dorbec, M.; Florent, J.–C.; Monneret, C.; Rager, M.–N.;
Bertounesque, E. Synlett 2006, 591-594; (e) Chandrasekhar, S.; Reddy, N.
R.; Rao, Y. S. Tetrahedron 2006, 62, 12098-12107; (f) Dorbec, M.;
Florent, J.–C; Monneret, C.; Rager, M.–N.; Bertounesque, E. Tetrahedron
2006, 62, 11766-11781.
15. General procedure for the one pot aza-Michael-type reaction: A mixture
of aldehyde (2.0 mmol), ketone (3.0 mmol) in absolute EtOH (4 mL) was
placed in a flask. While stirring magnetically at room temperature,
Fe3O4@SiO2@Me&Et-PhSO3H (0.1 g, 0.72 mol%) was added to the above
mixture and stirring was continued for 30-60 min. The course of the
reaction was monitored by TLC until the starting materials had completely
disappeared, and the corresponding chalcone had formed. Next, the amine
(2 mmol) was added, and the mixture was stirred for the appropriate
amount of time (Table 2). After the completion of the reaction (TLC), the
catalyst was separated by magnetic decantation, followed by evaporation
of the solvent. The crude product was either recrystallized from EtOH or
subjected to preparative TLC (silica gel, eluent EtOAc/pet. ether = 1:4).
All the products were characterized by IR, 1H and 13C NMR spectroscopy.
2. (a) Abele, S.; Seebach, D. Eur. J. Org. Chem. 2000, 6, 1-15; (b) Hagiwara,
E.; Fujii, A.; Sodeoka, M. J. Am. Chem. Soc. 1998, 120, 2474-2475; (c)
Seebach, D.; Matthews, J. L. Chem. Commun. 1997, 2015-2022; (d)
Drury, W. J.; Ferraris, D.; Cox, C.; Young, B.; Lectka, T. J. Am. Chem.
Soc. 1998, 120, 11006-11007; (e) Kleinmann, E. F. Comprehensive
Organic Synthesis, Pergamon, New York, 1991; (f) Corey, E. J.; Reichard,
G. A. Tetrahedron Lett. 1989, 30, 5207-5210.
General procedure for the one-pot Mannich-type reaction: To a mixture of
aldehyde (2.0 mmol) and amine (2.0 mmol) in absolute EtOH (4 mL), was
added the catalyst (0.06 g, 0.6 mol%). The mixture was stirred at room
temperature for 10-15 min until the starting materials had almost
disappeared (TLC). The ketone (3.0 mmol) was then added, and the
mixture was stirred for the appropriate amount of time (Table 2) until the
reaction was complete as monitored by TLC. An analytical sample was
obtained by the same method described for the aza-Michael-type reaction.
3. (a) Prukala, D. Tetrahedron Lett. 2006, 47, 9045-9047; (b) Pandey, G.;
Singh, R. P.; Garg, A.; Singh, V. K. Tetrahedron Lett. 2005, 46, 2137-
2140; (c) Periasamy, M.; Suresh, S.; Ganesan, S. S. Tetrahedron Lett.
2005, 46, 5521-5524; (d) Badorrey, R.; Cativiela, C.; Diaz-de-Villegas, M.
D.; Galvez, J. A. Tetrahedron Lett. 2003, 44, 9189-9192; (e) Loh, T. P.;
Wei, L. L. Tetrahedron Lett. 1998, 39, 323-326.
4. (a) Sahoo, S.; Joseph, T.; Halligudi, S. B. J. Mol. Catal. A: Chem. 2006,
244, 179-182; (b) Akiyama, T.; Matsuda, K.; Fuchibe, K. Synlett 2005,
322-324; (c) Akiyama, T.; Takaya, J.; Kagoshima, H. Synlett 1999, 1426-
1428; (d) Manabe, K.; Kobayashi, S. Org. Lett. 1999, 1, 1965-1967; (e)
Iimura, S.; Nobutou, D.; Manabe, K.; Kobayashi, S. Chem. Commun.
2003, 1644-1645.
16. (a) Bigdeli, M. A.; Nemati, F.; Mahdavinia, G. H. Tetrahedron Lett. 2007,
48, 6801-6804; (b) Li, Z.; Ma, X.; Liu, J.; Feng. X.; Tian, G.; Zhu, A. J.
Mol. Catal. A: Chem. 2007, 272, 132-135; (c) Yang, H.–M.; Li, L.; Li, F.;
Jiang, K.–Z.; Shang, J.–Y.; Lai, G.–Q.; Xu, L.–W. Org. Lett. 2011, 13,
6508-6511; (d) Ollevier, T.; Nadeau, E. Org. Biomol. Chem. 2007, 5,
3126-3134; (e) Ranu, B. C.; Samanta, S.; Guchhait, S. K. Tetrahedron
2002, 58, 983-988; (f) Zahouily, M.; Bahlaouan, B.; Rayadh, A.; Sebti, S.
Tetrahedron Lett. 2004, 45, 4135-4138; (g) Nemati, F.; Fakhaei, A. S.;
Amoozadeh, A.; Hayeniaz, Y. S. Synth. Commun. 2011, 41, 3695-3702;
(h) Kidwai, M.; Mishra, N. K.; Bansal, V.; Kumar, A.; Mozumdar, S.
Tetrahedron Lett. 2009, 50, 1355-1358.
5. Takahashi, E.; Fujisawa, H.; Mukaiyama, T. Chem. Lett. 2004, 33, 936-
937.
6. (a) Azizi, N.; Saidi, M. R. Tetrahedron 2004, 60, 383-387; (b)
Varala, R.; Alam, M. M.; Adapa, S. R. Synlett 2003, 720-722; (c)
Jenner, G. Tetrahedron Lett. 1995, 36, 233-236; (d) Srivastava, N.;
Banik, B. K. J. Org. Chem. 2003, 68, 2109-2114; (e) Bartoli, G.;
Bosco, M.; Marcantoni, E.; Petrini, M.; Sambri, L. Torregiani, E.