Magnetically separable Fe3O4 nanoparticles: an efficient catalyst for the synthesis of propargylamines

Article abstract of DOI:10.1016/j.tetlet.2010.02.016

Magnetically separable Fe₃O₄ nanoparticles endow with an efficient and economic route for the synthesis of propargylamines by the three-component coupling of aldehyde, amine, and alkyne through C-H activation. The reaction is especia

Full text of DOI:10.1016/j.tetlet.2010.02.016

Tetrahedron Letters 51 (2010) 1891–1895
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Magnetically separable Fe3O4 nanoparticles: an efficient catalyst
for the synthesis of propargylamines
*
B. Sreedhar , A. Suresh Kumar, P. Surendra Reddy
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology (Council of Scientific and Industrial Research), Hyderabad 500607, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Magnetically separable Fe3O4 nanoparticles endow with an efficient and economic route for the synthesis
of propargylamines by the three-component coupling of aldehyde, amine, and alkyne through C–H acti-
vation. The reaction is especially effective for reactions involving aliphatic aldehydes and no additional
co-catalyst or activator is required. High catalytic activity and ease of recovery using an external mag-
netic field are additional eco-friendly attributes of this catalytic system. The catalyst was recycled for five
times without a significant loss of catalytic activity.
Received 3 November 2009
Revised 30 January 2010
Accepted 5 February 2010
Available online 8 February 2010
Keywords:
Propargylamines
Ó 2010 Elsevier Ltd. All rights reserved.
Magnetically separable Fe3O4 nanoparticles
Terminal alkynes
Amines
Aldehydes
Magnetic nanoparticles are a class of nanostructured materials of
current interest, due largely to their advanced technological and
medical applications, envisioned or realized.1 Among the various
magnetic nanoparticles under investigation, Fe3O4 nanoparticles
are arguably the most extensively studied.1a,e,2 Furthermore, recent
reports show that Fe3O4 nanoparticles are efficient supports for cat-
alysts andcan facilitatetheirseparation effectivelyfrom thereaction
media by magnetization with a permanent magnetic field.3
Propargylamines are versatile synthetic intermediates in organ-
ic synthesis and are also important structural elements in natural
products and therapeutic drug molecules.4 These compounds have
traditionally been synthesized by nucleophilic attack of lithium
acetylides or Grignard reagents on imines or their derivatives.5
However, these reagents must be used in stoichiometric amounts,
are highly moisture sensitive, and require strictly controlled reac-
tion conditions. An alternative atom-economical approach to their
synthesis is to perform this type of reaction by a catalytic coupling
of alkyne, aldehyde, and amine (A3 coupling) by C–H activation,
where water is only the theoretical by-product.6,7
ionic liquids and supported AuIII, AgI, CuI, and so on) conditions have
been successfully used to catalyze three-component coupling reac-
tions. However, heterogenized catalyst generally requires tedious
preparation and/or separation procedures, there is a need to find
new materials with speciality properties such as magnetic in order
to overcome these limitations. Recently, Kidwai et al.15 have re-
ported gold and copper nanoparticles as reusable catalysts for the
synthesis of propargylamines. The main difficulty, however, is that
such small particles are almost impossible to separate by conven-
tional means, which can lead to the blocking of filters and valves
by the catalyst. The efficient separation of suspended magnetic cat-
alyst bodies from the liquid product by using an external magnetic
field offers a solution to this problem and would be of immense
interest.
In the present work, as part of our ongoing interest in the syn-
thesis of propargylamines,9d,14i we report our investigations on the
application of Fe3O4 nanoparticles16 for the practical and atom-
economic synthesis of propargylamines through three-component
coupling of aldehyde, alkyne, and amine via C–H activation
(Scheme 1).
Initially, in an effort to develop an improved catalytic system,
different solvents were screened for the reaction of benzaldehyde,
piperidine, and phenylacetylene in the presence of 20 mol % of
Fe3O4 at their reflux temperatures and the results are summarized
in Table 1. The outcome of the reaction was dependent on the nat-
ure of the solvent and temperature. It was observed that much bet-
ter yield was obtained when the reaction was carried out in
toluene at 110 °C compared to other solvents (Table 1, entry 1).
Among the other solvents screened, acetonitrile and THF gave
In recent years, the scope of direct addition of alkynes to carbon
nitrogen double bonds either from prepared imines or from alde-
hydes and amines in one-pot procedure by several noble transi-
tion-metal catalysts via C–H activation of terminal alkynes both
under homogeneous (AgI salts,6 AuI/AuIII salts,7 AuIII-salen com-
plexes,8 CuI salts,9 Ir complexes,10 InCl3,11 FeCl312 and Cu/RuII bime-
tallic system,13 and so on) and heterogeneous14 (AuI, AgI, and CuI in
* Corresponding author. Tel.: +91 40 27193510; fax: +91 40 27160921.
E-mail address: sreedharb@iict.res.in (B. Sreedhar).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.016

1892
B. Sreedhar et al. / Tetrahedron Letters 51 (2010) 1891–1895
R2
R1
R3
O
N
20 mol % Fe3O4
R2
R3
+
N
H
+
R1
H
Ar
Toluene
Ar
80 - 110 oC, 16h
R1 = Alkyl or Aryl R2, R3 = dialkyl
Scheme 1.
Table 2 (continued)
Entry Aldehyde
Table 1
Screening of various solvents for the three-component coupling reaction between
Product
Yield (%) Ref.
benzaldehyde, piperidene and phenylacetylenea
O
Entry
Solvent
Temp (°C)
Yieldb (%)
1
2
3
4
5
6
Toluene
Acetonitrile
THF
Methanol
Water
110
65
65
65
100
100
75
30
25
60
Trace
Trace
N
H
O
7
92
90
14i
17
Ph
Neat
a
Reaction conditions: benzaldehyde (1 mmol), piperidine (1.2 mmol), phenyl-
N
H
8
acetylene (1.5 mmol) Fe3O4 (20 mol %), solvent 3 mL.
Yields are based on 1H NMR integration.
b
Ph
a
Reaction conditions: aldehyde (1 mmol), piperidine (1.2 mmol), phenylacety-
lene (1.5 mmol), Fe3O4 (20 mol %) in toluene (3 mL) at 80 °C for 16 h.
b
Reaction carried out at 110 °C.21
Table 2
Three-component coupling reaction of various aldehydes with piperidine and phenyl
acetylenea
Table 3
Entry Aldehyde
Product
Yield (%) Ref.
Three-component coupling reaction of various amines with cyclohexanecarboxalde-
hyde and phenylacetylenea
O
Entry Amine
Product
Yield (%) Ref.
H
O
N
1b
75
56
14f
14f
N
N
Ph
1
85
90
6
6
N
H
Ph
Ph
H
N
2b
2
Ph
N
H
O
H
O
N
N
3b
40
7
O
Br
3
80
9a
Ph
N
H
Br
O
Ph
N
H
N
4
85
80
90
14f
N
H
4
15
11
14c
11
Ph
Ph
Ph
Ph
Ph
N
Ph
O
N
H
Ph
Ph
5
N
H
5
6
6
Ph
a
Reaction conditions: cyclohexanecarboxaldehyde (1 mmol), amine (1.2 mmol),
phenylacetylene (1.5 mmol), Fe3O4 (20 mol %) in toluene (3 mL) at 80 °C for 16 h.21
O
N
H
14i
the product in low yield (Table 1, entries 2 and 3), whereas the
reaction in methanol gave the desired product in a moderate yield
(Table 1, entry 4). However, only a trace amount of the product was

B. Sreedhar et al. / Tetrahedron Letters 51 (2010) 1891–1895
1893
formed in water and neat conditions (Table 1, entries 5 and 6). Fur-
ther, the optimum ratio of aldehyde, amine, and alkyne was found
to be 1:1.2:1.5.
reaction time for reaction completion to give the desired product
in good yields (Table 2, entries 1–3). The reactions involving ali-
phatic aldehydes gave both higher conversions and greater yields
(Table 2, entries 4–8). While unwanted trimerization of aldehyde
was a major limitation of the reactions catalyzed by gold and cop-
per, almost no trimer was found with aliphatic aldehydes when
using Fe3O4 as the catalyst.
To expand the scope of the amine substrates, we used cyclohex-
anecarboxaldehyde and phenyl acetylene as model substrates and
examined various secondary amines in the synthesis of propargyl-
amines and the results are given in Table 3. The order of reactivity
Subsequently, a variety of propargylamines were prepared from
various aldehydes, alkynes, and amines using the optimized reac-
tion conditions and the results are summarized in Tables 2–4. Ini-
tially, the scope of the aldehyde substrate was evaluated and the
results are given in Table 2. The aldehydes used in this study in-
cluded aromatic and aliphatic examples and the reaction was
found to be highly effected by the nature of the aldehyde. Aryl
aldehyde decreased the reactivity of the reaction, required longer
Table 4
Three-component coupling of various terminal alkynes with cyclohexanecarboxaldehyde and piperidinea
Entry
Alkyne
Product
Yield (%)
N
1
90
H3CO
OCH3
N
2
92b
N
3
85
F
F
N
4
70
N
N
N
NO2
5
60
NO2
N
6
85
H3CO
OCH3
a
Reaction conditions: cyclohexanecarboxaldehyde (1 mmol), piperidine (1.2 mmol), terminal alkyne (1.5 mmol), Fe3O4 (20 mol %) in toluene (3 mL) at 80 °C for 16 h.21
Ref. 6.
b

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B. Sreedhar et al. / Tetrahedron Letters 51 (2010) 1891–1895
for these amines in terms of yield and reaction time was found to
pyrrolidine > piperidine > morpholine > dialkylamine. Cyclic amines
gave the desired products in good yield when compared to acyclic
amines, such as dibutylamine and dibenzylamine. As can be seen in
the table, acyclic amines are less reactive in reactions with phenyl-
acetylene and cyclohexanecarboxaldehyde, and only trace
amounts of the desired products were isolated.
ucts (4a and 4b) were obtained in the ratio of 1:3 using toluene as
the solvent in 80% yield (Scheme 2).
To check the recyclability of the catalyst, as can be seen from
Table 5, the reaction was performed with both aliphatic and aro-
matic aldehydes and the catalyst was separated from the reaction
mixture by applying external magnetic field and reused without a
significant loss of catalytic activity.
Furthermore, as shown in Table 4, several terminal alkynes
were examined for the synthesis of propargylamines using cyclo-
hexanecarboxaldehyde and piperidine as the model substrates
and good to excellent yields of the desired products were obtained
in each case. The reactions were completed within 16 h affording
60–95% yields. Both aryl and heteroaryl alkynes underwent the
reaction to furnish the desired products in good yields except 2-
nitrophenylacetylene (Table 4, entries 1–6). With aliphatic alkynes
such as octyne and butyne only a trace amount of the product was
obtained. It was observed that when the reaction was carried out
with 1,3-diethynylbenzene using 2 equiv of cyclohexanecarboxal-
dehyde and 2.4 equiv of piperidine, mono and disubstituted prod-
On the basis of these results, together with the literature
reports,6–9 we propose
a plausible mechanism as shown in
Scheme 3. Fe3O4 is of cubic inverse spinel crystal structure, in
which the oxygen anions (O2À) form a closely packed face-centered
cubic (fcc) sublattice with iron cations located in interstitial sites
and there are two different kinds of cation sites: tetrahedrally
coordinated sites occupied by Fe3+ and octahedrally coordinated
sites occupied by Fe3+ and Fe2+ ions in equal numbers.18 The Fe2+
cation can be considered to be Fe3+ plus an ‘extra’ electron, with ra-
pid valence oscillation between the Fe(III) and Fe(II) octahedral
sites. Initially, in the presence of amine, deprotonation of terminal
alkyne occurs resulting in the activation of C–H bond and
N
O
H
4a
20 mol % Fe3O4
+
+
+
Toluene
80 oC, 16 h
N
H
2.4 mmol
N
N
1 mmol
2 mmol
2
1
3
4b
Scheme 2.
Table 5
Recovery and reuse of Fe3O4 nanoparticles for the synthesis of propargylaminesa
Entry
Aldehyde
Yieldb (%)
Third
Average yield (%)
First
Second
Fourth
Fifth
1c
2d
Cyclohexanecarboxaldehyde
Benzaldehyde
85
75
80
72
82
70
80
70
80
65
81
70
a
b
c
Reaction conditions: aldehyde (1 mmol), piperidine (1.2 mmol), phenylacetylene (1.5 mmol), toluene (3 mL), Fe3O4 (20 mol %), 16 h.
Yields are based on 1H NMR integration.
Reaction carried out at 80 °C.
d
Reaction carried out at 110 °C.
R3
R2
+
R3
R2
R3
+
O
+
R2
N
+
N
N
R1
H H
Ar
H
H H
R1
I
H2O
II
Ar
H
R3
R2
N
R1 = alkyl or aryl
R2, R3 = dialkyl
R3
R2
R1
Magnetic
nanoparticles
N
H
Ar
III
Scheme 3.

B. Sreedhar et al. / Tetrahedron Letters 51 (2010) 1891–1895
1895
(e) Reddy, K. M.; Babu, N. S.; Prasad, P. S. S.; Lingaiah, N. Tetrahedron Lett. 2006,
47, 7563; (f) Zhang, X.; Corma, A. Angew. Chem., Int. Ed. 2008, 47, 4358; (g) Li, P.;
Wang, L. Tetrahedron 2007, 63, 5455; (h) Wang, M.; Li, P.; Wang, L. Eur. J. Org.
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Tetrahedron Lett. 2007, 48, 7882.
generation of the terminal iron-acetylide intermediate I, which
could be presumably due to the reduction of Fe3+ to a low valent
Fe2+ oxidation state.19 Iron cations also acts as Lewis acid and play
a significant role in increasing the electrophilic character of the
starting aldehyde and stabilizing the immonium salt by the coordi-
nation of the oxygen or nitrogen lone electron pair.20 The formed
iron-acetylide intermediate I, further undergoes nucleophilic addi-
tion to the immonium ion II, to yield the corresponding propargyl-
amine III and regeneration of the catalyst.
In conclusion, we have developed a simple and efficient method
for the synthesis of propargylamines via C–H activation using mag-
netically separable Fe3O4 as the catalyst under mild reaction condi-
tions. The reaction is especially effective for reactions with
aliphatic aldehydes and no additional co-catalyst or activator is re-
quired. The simple procedure for catalyst preparation, easy recov-
ery and reusability of the catalyst are expected to contribute to its
utilization for the development of benign chemical processes and
products.
15. (a) Kidwai, M.; Bansal, V.; Kumar, A.; Mozumdar, S. Green Chem. 2007, 9, 742;
(b) Kidwai, M.; Bansal, V.; Mishra, N. K.; Kumar, A.; Mozumdar, S. Synlett 2007,
1581.
16. Magnetic Fe3
O4 nanoparticles were prepared according to literature
(Polshettiwar, V.; Baruwati, B.; Varma, R. S. Green Chem. 2009, 11, 127) and
characterized by XRD and TEM.
17. Srinivas, R. A.; Ramu, E.; Ravi, V.; Sreelathaa, N. Tetrahedron Lett. 2007, 48,
7184.
18. Liang, M.; Wang, X.; Liu, H.; Liu, H.; Wang, Y. J. Catal. 2008, 255, 335.
19. (a) Berben, L. A.; Long, J. R. Inorg. Chem. 2005, 44, 8459; (b) Delfs, C. D.; Stranger,
R.; Humphrey, M. G.; McDonagh, A. M. J. Organomet. Chem. 2000, 607, 208; (c)
Nast, R. Coord. Chem. Rev. 1982, 47, 89. and references therein.
20. Bloch, R. Chem. Rev. 1998, 98, 1407.
21. Typical procedure for the synthesis of propargylamines: A mixture of aldehyde
(1 mmol), amine (1.2 mmol), alkyne (1.5 mmol), catalyst (20 mol %), and
toluene (3 mL) was taken in a round-bottomed flask and stirred at 110 °C
(aromatic aldehydes) or 80 °C (aliphatic aldehydes) temperature. After
completion of the reaction (monitored by TLC) the catalyst was easily
separated from the reaction mixture with an external magnet. After
removing the solvent, the crude material was chromatographed on silica gel
to afford the pure product. The spectroscopic data of all known compounds
were identical to those reported in the literature.6–9 Spectroscopic data for all
new compounds synthesized are reported herein.
Acknowledgments
A.S.K. and P.S.R. thank the Council of Scientific and Industrial
Research, New Delhi, for the award of Senior Research Fellowships.
1-(1-Cyclohexyl-3-(4-methoxyphenyl)prop-2-ynyl)piperidine (Table 4, entry 1):
Half-white Solid. Mp: 73–76 °C. IR (neat): 3038, 2930, 2848, 2748, 2213, 2058,
1899, 1605, 1508, 1246, 1108, 1033, 996, 830 cmÀ1 1H NMR (300 MHz, CDCl3):
.
d 0.87–1.78 (m, 15H), 2.00–2.12 (m, 2H), 2.33–2.42 (m, 2H), 2.56–2.65 (m, 2H),
3.04 (d, 1H, J = 10.0 Hz), 3.79 (s, 3H), 6.76 (d, 2H, J = 8.8 Hz), 7.31 (d, 2H,
J = 8.6 Hz). 13C NMR (75 MHz, CDCl3): d 24.6, 26.0, 26.1, 26.7, 30.4, 31.2, 39.5,
55.2, 64.3, 85.8, 85.9, 113.7, 115.8, 132.9, 159.0. ESI MS (m/z): 312 (M+H).
1-(1-Cyclohexyl-3-(3-fluorophenyl)prop-2-ynyl)piperidine (Table 4, entry 3): IR
References and notes
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(neat): 2958, 2851, 2800, 1608, 1483, 1444, 1150, 996, 868, 780 cmÀ1 1H NMR
.
(300 MHz, CDCl3): d 0.86–1.78 (m, 15H), 1.99–2.09 (m, 2H), 2.31–2.41(m, 2H),
2.55–2.64 (m, 2H), 3.06 (d, 1H, J = 9.5 Hz), 6.96 (t, 1H, J = 7.8 Hz), 7.08–7.24 (m,
2H), 7.25 (s, 1H). 13C NMR (75 MHz, CDCl3): d 24.6, 26.0, 26.1, 26.7, 30.3, 31.2,
64.2, 85.0, 88.8, 114.7, 115.0, 118.2, 118.5, 127.5, 129.5 (d, J = 8.7 Hz). ESI MS
(m/z): 300 (M+H).
2-(3-Cyclohexyl-3-piperidin-1-yl)prop-1-ynyl)pyridine (Table 4, entry 4): IR
(neat): 3055, 2929, 2850, 2794, 2219, 1635, 1582, 1458, 1262, 1103, 993,
785 cmÀ1 1H NMR (300 MHz, CDCl3): d 0.89–1.80 (m, 15H), 1.99–216 (m, 2H),
.
2.37–2.47 (m, 2H), 2.61–2.70 (m, 2H), 3.12 (d, 1H, J = 9.8 Hz), 7.14–7.19 (m,
1H), 7.38(d, 1H, J = 7.5 Hz), 7.60(ddd, 1H, J = 2.2 Hz, 7.5 Hz, 9.8 Hz), 8.54 (d, 1H,
J = 6.0 Hz). 13C NMR (75 MHz, CDCl3): d 24.7, 26.2, 26.4, 26.9, 30.5, 31.5, 39.5,
64.3, 86.1, 88.3, 96.2, 122.2, 127.2, 135.7, 143.9, 149.8. ESI MS (m/z): 283
(M+H).
1-(1-Cyclohexyl-3-(2-nitrophenyl)prop-2-ynyl)piperidine (Table 4, entry 5): IR
(neat): 2926, 2851, 2803, 2215, 1699, 1609, 1447, 1344, 1103, 853, 746 cmÀ1
.
1H NMR (300 MHz, CDCl3): d 0.89–1.80 (m, 15H), 2.00–2.16 (m, 2H), 2.27–2.36
(m, 2H), 2.60–2.70 (m, 2H), 3.15 (d, 1H, J = 10.3 Hz), 7.36–7.43 (m, 1H), 7.47–
7.55 (m, 1H), 7.58–7.63 (m, 1H), 7.97–8.02 (m, 1H). 13C NMR (75 MHz, CDCl3):
d 24.7, 26.3, 26.9, 29.8, 30.5, 31.4, 39.5, 64.6, 85.1, 85.3, 120.4, 124.4, 127.8,
132.2, 135.1, 146.0. ESI MS (m/z): 327 (M+H).
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1-(1-Cyclohexyl-3-(6-methoxynapthalen-2-yl)prop-2-ynyl)piperidine (Table 4,
entry 6): White Solid. Mp: 100–103 °C. IR (neat):3057, 2931, 2849, 2795,
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1908, 1626, 1482, 1448, 1241, 1156, 1030, 993, 846, 743 cmÀ1 1H NMR
.
(300 MHz, CDCl3): d 0.88–1.78 (m, 15H), 2.05–2.18 (m, 2H), 2.36–2.47 (m, 2H),
2.60–2.69 (m, 2H), 3.10 (d, 1H, J = 10.2 Hz, 1H), 3.91 (s, 3H), 7.02–7.11 (m, 2H),
7.41 (d, 1H, J = 8.3 Hz), 7.59 (d, 1H, J = 8.5, 7.63 (d, 1H, J = 8.8 Hz), 7.81(s, 1H).
13C NMR (75 MHz, CDCl3): d 24.6, 26.1, 26.2, 26.7, 29.6, 30.4, 31.3, 39.6, 50.7,
64.4, 86.5, 87.3, 105.7, 118.6, 119.1, 126.6, 129.1, 129.4, 130.7, 130.7, 130.9,
157.9. ESI MS (m/z): 362 (M+H).
1-(1-Cyclohexyl-3-(3-ethnylphenyl)prop-2-ynyl)piperidine 4a (Scheme 2): IR
(neat): 3302, 2928, 2851, 2800, 2749, 1593, 1473, 1447, 1229, 1104, 996,
894, 793 cmÀ1 1H NMR (300 MHz, CDCl3): d 0.84–1.78 (m, 15H), 1.99–2.11 (m,
.
10. (a) Fischer, C.; Carreira, E. M. Org. Lett. 2001, 3, 4319; (b) Sakaguchi, S.; Kubo, T.;
Ishii, Y. Angew. Chem., Int. Ed. 2001, 40, 2534; (c) Sakaguchi, S.; Mizuta, T.;
Furuwan, M.; Kubo, T.; Ishii, Y. Chem. Commun. 2004, 1638.
11. Zhang, Y.; Li, P.; Wang, M.; Wang, L. J. Org. Chem. 2009, 74, 4364.
12. (a) Chen, W. W.; Nguyen, R. V.; Li, C. H. Tetrahedron Lett. 2009, 50, 2895; (b) Li,
P.; Zhang, Y.; wang, L. Chem. Eur. J. 2009, 15, 2045.
2H), 2.32–2.41 (m, 2H), 2.55–2.64 (m, 2H), 2.99 (s, 1H), 3.05 (d, 1H, J = 10.0 Hz),
7.19–7.23 (m, 1H), 7.33–7.39 (m, 2H), 7.52 (s, 1H). 13C NMR (75 MHz, CDCl3): d
24.6, 26.0, 26.2, 26.7, 30.3, 31.2, 39.4, 20.6, 64.3, 77.5, 82.9, 85.1, 88.6, 122.1,
124.0, 128.2, 131.1, 131.9, 135.2. ESI MS (m/z): 306 (M +H).
1,3-Bis(3-cyclohexyl-3-(piperidin-1-yl)prop-1-ynyl)benzene 4b (Scheme 2): IR
(neat): 2928, 2852, 2801, 1666, 1447, 1263, 1228, 1103, 997, 755 cmÀ1 1H
.
13. Li, C. J.; Wei, C. Chem. Commun. 2002, 3, 268.
NMR (300 MHz, CDCl3): d 0.86–1.20 (m, 30H), 2.01–2.10 (m, 4H), 2.36–2.48
(m, 4H), 2.58–2.69 (m, 4H), 3.11 (d, 2H, J = 9.8 Hz), 7.16–7.22 (m, 1H), 7.31
(d, 2H, J = 9.8 Hz), 7.44 (s, 1H). 13C NMR (75 MHz, CDCl3): d 24.4, 25.9, 26.1,
26.6, 30.4, 31.2, 39.5, 64.2, 86.7, 87.7, 128.1, 131.1, 134.5, 134.5. ESI MS (m/
z): 485 (M+H).
14. (a) Li, Z.; Wei, C.; Chen, L.; Varma, R. S.; Li, C. J. Tetrahedron Lett. 2004, 45, 2443;
(b) Park, S. B.; Alper, H. Chem. Commun. 2005, 10, 1315; (c) Choudary, B. M.;
Sridhar, C.; Kantam, M. L.; Sreedhar, B. Tetrahedron Lett. 2004, 45, 7319; (d)
Kantam, M. L.; Prakash, B. V.; Reddy, C. R. V.; Sreedhar, B. Synlett 2005, 2329;