Indium(III) bromide-catalyzed hydroarylation of alkynes with indoles

Article abstract of DOI:10.1016/j.tetlet.2010.04.036

Indium(III) bromide-catalyzed hydroarylation of alkynes with indoles leading to the synthesis of 3-vinylindoles in good to excellent yields was achieved. The E/Z ratio of the products depends on the substituents on the indole. This protocol is efficient a

Full text of DOI:10.1016/j.tetlet.2010.04.036

Tetrahedron Letters 51 (2010) 3141–3145
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Indium(III) bromide-catalyzed hydroarylation of alkynes with indoles
*
G. Bhaskar, C. Saikumar, P. T. Perumal
Organic Chemistry Division, Central Leather Research Institute, Adyar, Chennai 600 020, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 February 2010
Revised 8 April 2010
Accepted 10 April 2010
Available online 14 April 2010
Indium(III) bromide-catalyzed hydroarylation of alkynes with indoles leading to the synthesis of 3-viny-
lindoles in good to excellent yields was achieved. The E/Z ratio of the products depends on the substitu-
ents on the indole. This protocol is efficient and atom-economic from the synthetic point of view.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Indium(III) bromide is known to catalyze intramolecular cycliza-
tion of 2-alkynylanilines.27 As part of our ongoing interest in the
Development of heteroarene functionalization has attracted
much attention in view of the wide range of useful applications
such as fluorescent dyes, synthetic analogues of natural products,
and pharmaceuticals. Addition of an aromatic C–H bond to an
unsaturated C–C bond is a useful functionalization method for
introducing carbon framework on to the heteroarenes and leads
to the formation of multi-substituted olefins. The success of such
processes would provide convenient, clean, and economic method-
ologies for the synthesis of aryl-substituted compounds directly
from simple arenes without requiring arene prefunctionalization
such as halogenation.1 Moreover the regio- and stereoselective
synthesis of multi-substituted olefins has been a challenge for syn-
thetic organic chemists for many years.2 Lewis acid catalysts,3
transition metal catalysts such as rhodium,4 ruthenium,5–7 plati-
num,8,9 palladium,10 gold,11–13 rhenium,14 rare-earth metals,15
FeCl3,16 and transition metal triflates17–21 in the presence of ionic
liquid media also promote the alkenylation of arenes. However,
some of these methods employ expensive, moisture sensitive
catalysts and also produce undesired side products. Moreover
the alkynes involved in the hydroarylation process need to be
electron-deficient and the reaction requires longer time.
The indole ring system exists ubiquitously in natural products,
and exhibits important biological and pharmaceutical properties.22
Echavarren and co-workers reported a systematic investigation on
the gold-catalyzed intra- and intermolecular addition of indoles to
alkynes.23 Cheng and co-workers investigated the reaction of in-
doles with alkynyl alcohols employing platinum as catalyst.24
These studies demonstrate that a range of substituted indoles
could be accessed through the reaction of indoles with alkynes. Re-
cently, there has been considerable interest in the catalytic use of
indium(III) halides in organic synthesis,25 due to their unique prop-
erties such as non-toxicity, stability in air, and water tolerance.26
catalytic applications of indium(III) halides for various organic
transformations,28 herein we report a hydroarylation reaction of
unactivated internal and terminal alkynes with various substituted
indoles in the presence of indium(III) bromide. (Scheme 1).
2. Results and discussion
To begin our study, 1-methylindole (1b) and diphenyl acetylene
(2a) were allowed to react in equimolar quantities in toluene at
room temperature for 24 h. The reaction does not proceed at all
as indicated by TLC. Refluxing for 10 h under N2 atmosphere also
did not yield fruitful results. However, the use of indium(III) bro-
mide (5 mol %) at reflux condition drove the reaction to form the
desired product 3-vinylindole (3b) in 67% yield within 24 h. The
1H NMR spectrum of the product exhibited two sets of signals cor-
responding to two diastereomers (E/Z, 90:10). The major E-isomer
was separated by crystallization and the stereochemistry was con-
firmed by X-ray diffraction analysis. The ORTEP diagram for the
major isomer is shown in Figure 1. In order to improve the yield
of the product, we increased the catalyst to substrate ratio to
10 mol % and obtained 3b in 78% in 3.5 h. We noticed that when
the same reaction was carried out with 1.5 equiv of diphenylacet-
ylene, for 2 h, it gave 89% yield. Further increasing of alkyne ratio
had no significant effect on the reaction time and yield. Hence this
optimized condition was executed for other indole derivatives to
explore the substrate scope of the reaction (Scheme 1, Table 1).
All the compounds were obtained in good to excellent yields
R
H
R
R3
InBr3
R3
R
R2
10 mol%
R2
+
N
R
Toluene, 110 oC,
N2
N
R1
R1
2
1
3
* Corresponding author. Tel.: +91 44 24911329; fax: +91 44 24911539.
E-mail address: ptperumal@gmail.com (P.T. Perumal).
Scheme 1.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.04.036

3142
G. Bhaskar et al. / Tetrahedron Letters 51 (2010) 3141–3145
(74–92%). These results are summarized in Table 1. There is no
obvious effect on the yield of the products by changing the substit-
uents on the indole.
The stereochemistry of the products was highly dependent on
the nature of the substituents on the indole (3a–k). The reaction
of 2-methylindole with diphenylacetylene gave Z isomer as the
major product (E/Z, 6/94), which was confirmed by X-ray diffrac-
tion analysis (Fig. 2). This observation was controversial to that
of the previously noticed stereoselectivity for 1-methylindole.
The similar selectivities were observed for other 2-substituted in-
doles (1g and 1h). These results prompted us to proceed with other
substituted indoles.
The indole, N-substituted, and 5-substituted indoles (1a,1d, 1e,
1f, 1i, and 1j) reacted with diphenylacetylene (2a) to afford good
yield of products (3a, 3d, 3e, 3f, 3j, and 3k) and the stereoselectiv-
ity was same as in the case of 1-methylindole. The E-isomer was
obtained as the major product, when indole, N-substituted, and
5-substituted indoles were used, and the Z isomer was obtained
as the major product, when 2-substituted indoles were used. There
was no change of E/Z selectivity and product (3i) yield, when
Figure 1. ORTEP diagram of 3b.
Table 1
InBr3-catalyzed hydroarylation of internal alkynes (2) with indoles (1)
Entry
Indole (1)
Alkyne (2)
Producta (3)
Time (h)
4.0
(E/Z)b
Yieldc (%)
Ph
Ph
Ph
Ph
2a
N
H
H
1
78/22
74
1a
3a
N
H
Ph
Ph
N
H
Me
1b
1c
2
3
2a
2a
2.0
2.0
90/10
6/94
89
92
3b
N
Me
Ph
Ph
Me
N
H
H
Me
N
3c
H
Cl
Br
Ph
H
Ph
Cl
N
4
5
2a
2a
2.5
2.5
84/16
86/14
86
90
H
1d
3d
N
H
Ph
Ph
H
N
Br
H
3e
1e
N
H
NC
Ph
H
Ph
N
NC
6
7
2a
2a
2.5
2.5
82/18
86
81
H
1f
3f
N
H
Ph
H
Ph
Ph
N
H
Ph
Ph
>99 (Z)
1g
N
3g
H

G. Bhaskar et al. / Tetrahedron Letters 51 (2010) 3141–3145
3143
Table 1 (continued)
Entry
Indole (1)
Alkyne (2)
Producta (3)
Time (h)
2.5
(E/Z)b
Yieldc (%)
H
Ph
Cl
N
Ph
H
8
9
2a
>99 (Z)
79
90
1h
Cl
N
H
3h
p-Tolyl
p-Tolyl
p-Tolyl
p-Tolyl
Me
2b
N
H
H
2.0
2.0
8/92
1c
Me
N
H
3i
Ph
Ph
N
H
1i
10
2a
2a
86/14
87
85
Ph
N
3j
Ph
Ph
Ph
N
H
11
2.0
88/12
1j
Bn
N
3k
Bn
a
All products were characterized by IR, 1H, 13C NMR, and Mass.
Isomeric ratio (E/Z) analyzed by GC and 1H NMR.
Isolated yield.
b
c
Indole
_
Ph
+
Ph
InBr3
6
Indole
Figure 3.
Ph
Ph
Ph
H
H
Ph
Ph
InBr3
- InBr3
InBr3
+
Indole
Indole
Ph
Indole
H
9
7
8
Figure 4.
bromide which would induce the isomerization between the two
isomers 8 and 9 this equilibrium allows the formation of the ther-
modynamically favored product.
Figure 2. ORTEP diagram of 3c.
To investigate the scope and generality of this methodology,
various terminal alkynes were tested with indoles (Scheme 2,
Table 2).
substituted diphenylacetylene (2b) was used. The stereoselectivity
of the reaction was in good agreement with other hydroarylation
protocol.10
The ratio of product distribution of E/Z isomers might be ratio-
nalized based on thermodynamic stability of the products. The pro-
posed mechanism involves zwitterionic intermediate3a 6 (Fig. 3),
which leads to the formation with a E/Z ratio of 1:1. But the exper-
imental results indicate that 2-substituted indoles gave Z as the
major isomer and other indoles afford E as the major isomer. This
might be explained through the formation of an alkyl cation 7
(Fig. 4) by the reaction of 8 with indium bromide and subsequent
single bond rotation followed by the elimination of the indium
R3
R4
R3
InBr3
R2
+
N
R1
R4
Toluene,
R2
R5
N
R5
110 oC, N2
1
R
1
4
5
Scheme 2.

3144
G. Bhaskar et al. / Tetrahedron Letters 51 (2010) 3141–3145
Table 2
InBr3-catalyzed hyrdroarylation of terminal alkynes (4) with indoles (1)a
Entry
Indole (1)
Terminal alkyne (4)
Productb (5)
Time (min)
15
Yieldc (%)
87
N
1
1b
4a
N
5a
N
2
3
15
15
91
84
H
1c
N
4a
5b
H
OMe
N
H
1c
OMe
N
H
4b
5c
5d
5e
4
5
15
15
85
87
N
H
1c
1c
n-Pentyl
N
H
4c
n-Pentyl
N
H
N
H
4d
a
b
c
All the reactions were carried out using 1 equiv of indole and 1.2 equiv of terminal alkyne.
All products were characterized by IR, NMR, and Mass.
Isolated yield.
Initially we carried out the reaction between 2-methylindole
7.34 (5H, s) 7.71 (1H, d, J = 8.4 Hz). 13C NMR (125 MHz, CDCl3): d
32.9, 109.2, 109.6, 119.3, 120.0, 121.1, 122.1, 124.4, 125.9, 127.3,
127.9, 128.6, 129.3, 129.7, 130.3, 137.1, 137.7, 138.1, 141.4. MS
(ES) = 310 (M++H+).
(1.0 mmol) and phenylacetylene (1.2 mmol) with 10 mol % of InBr3
in toluene at 110 °C under N2 atmosphere. The reaction proceeded
well and furnished 3-vinylindole product in good yield within
15 min. Terminal alkynes generally gave good yields at a shorter
reaction time than internal alkynes. Similarly 1-methyl and
2-methylindoles reacted very smoothly with various terminal
alkynes to afford the corresponding vinyl indole products in good
to excellent yield (Table 2). There is no obvious effect on the reac-
tion time and yields with substituents present on the indoles and
alkynes.
In summary, we developed a method for the synthesis of 3-
vinylindoles by hydroarylation of indoles with unactivated internal
and terminal alkynes using indium bromide as the catalyst. This
methodology is simple, efficient, and atom-economic from the syn-
thetic point of view.
3.2. 3-((Z)-1,2-Di-p-tolylvinyl)-2-methyl-1H-indole (3i)
Colorless solid. Mp = 131–133 °C. IR (KBr)
m
max: 3404, 3004,
1H
2910, 1911, 1605, 1446, 1288, 1115, 1012, 820, 736 cmÀ1
.
NMR (500 MHz, CDCl3): d 1.99 (3H, s), 2.27 (3H, s), 2.38 (3H, s),
6.93 (2H, d, J = 8.5 Hz), 6.98 (1H, t, J = 7.6 Hz), 7.01–7.06 (2H, m),
7.09–7.17 (5H, m), 7.31–7.37 (3H, m), 7.84 (1H, NH, s). 13C NMR
(125 MHz, CDCl3): d 12.6, 21.3, 110.2, 112.2, 119.6, 120.1, 121.2,
127.1, 127.7, 128.7, 128.8, 129.0, 133.0, 134.0, 135.8, 135.9,
136.0, 137.1, 140.4. MS (ES) = 338 (M++H+).
4. General experimental procedure for compounds 5a–e
3. General experimental procedure for compounds 3a–k
A
mixture of terminal alkyne (4) (2.4 mmol), indole (1)
A mixture of diphenylacetylene (2) (3.0 mmol), indole (1)
(2.0 mmol), and indium bromide (0.2 mmol) in toluene (4 mL)
was stirred at 110 °C for the appropriate time (see Table 1). After
completion of the reaction as indicated by TLC, the reaction mix-
ture was diluted with water and extracted with ethyl acetate.
The combined organic layers were dried over anhydrous Na2SO4,
concentrated in vacuo, and purified by column chromatography
on silica gel (Merck, 100–200 mesh) to afford a mixture of E and
Z isomers. These isomers were separated by crystallization using
ethyl acetate/petroleum ether. Spectroscopic data for selected
products.
(2.0 mmol), and indium bromide (0.2 mmol) in toluene (4 mL)
was stirred at 110 °C for the appropriate time (see Table 2). After
completion of the reaction as indicated by TLC, the reaction mix-
ture was diluted with water and extracted with ethyl acetate.
The combined organic layers were dried over anhydrous Na2SO4,
concentrated in vacuo, and purified by column chromatography
on silica gel (Merck, 100–200 mesh) to afford pure products 5a–e.
4.1. 2-Methyl-3-(1-phenylvinyl)-1H-indole (5b)
Brown viscous liquid. IR (neat)
mmax: 3401, 3055, 2920, 1681,
1609, 1450, 898, 747 cmÀ1 1H NMR (500 MHz, CDCl3): d 2.28
.
3.1. 3-((E)-1,2-Diphenylvinyl)-1-methyl-1H-indole (3b)
(3H, s), 5.41 (1H, d, J = 1.6 Hz), 5.81 (1H, d, J = 1.6 Hz), 7.08 (1H, t,
J = 7.6 Hz), 7.19 (1H, t, J = 6.9 Hz), 7.31 (2H, t, J = 7.6 Hz), 7.35–
7.39 (3H, m), 7.47–7.51 (2H, m), 7.91 (1H, NH, s). 13C NMR
(125 MHz, CDCl3): d 12.9, 110.4, 114.0, 115.0, 119.8 (2C), 121.3,
127.5, 127.7, 128.4, 128.5, 133.4, 135.3, 142.0, 142.7. MS
(ES) = 234 (M++H+).
Colorless solid. Mp = 182–184 °C. IR (KBr):
mmax: 3023, 1952,
1642, 1519, 1452, 1014, 733 cmÀ1.1H NMR (500 MHz, CDCl3): d
3.73 (3H, s), 6.78 (1H, s), 7.03 (2H, d, J = 7.6 Hz), 7.07 (1H, d,
J = 6.9 Hz), 7.10–7.16 (4H, m), 7.27 (1H, d, J = 7.7 Hz), 7.32 (1H, s),

G. Bhaskar et al. / Tetrahedron Letters 51 (2010) 3141–3145
3145
(c) Pasha, Nayeem.; Seshu Babu, N.; Venkateswara Rao, K. T.; Sai Prasad, P. S.;
Lingaiah, N. Tetrahedron Lett. 2009, 50, 1714.
4.2. 3-(1-(2-Methoxyphenyl)vinyl)-2-methyl-1H-indole (5c)
4. Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102, 1731.
5. Murai, S.; Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.; Sonoda, M.; Chatani,
N. Nature 1993, 366, 529.
6. Merlic, C. A.; Pauly, M. E. J. Am. Chem. Soc. 1996, 118, 11319.
7. Murakami, M.; Hori, S. J. Am. Chem. Soc. 2003, 125, 4720.
8. Jia, C.; Piao, D.; Oyamada, J.; Lu, W.; Kitamura, T.; Fujiwara, Y. Science 2000, 287,
1992.
9. Jia, C.; Lu, W.; Oyamada, J.; Kitamura, T.; Matsuda, K.; Irie, M.; Fujiwara, Y. J. Am.
Chem. Soc. 2000, 122, 7252.
10. Lu, W.; Jia, C.; Kitamura, T.; Fujiwara, Y. Org. Lett. 2000, 2, 2927.
11. Reet, M. T.; Sommer, K. Eur. J. Org. Chem. 2003, 3485.
12. Shi, Z.; He, C. J. Org. Chem. 2004, 69, 3669.
13. Ne-vado, C.; Echavarren, A. M. Chem. Eur. J. 2005, 11, 3155.
14. Kuninobu, Y.; Kikuchi, K.; Tokunaga, Y.; Nishina, Y.; Takai, K. Tetrahedron 2008,
64, 5974.
15. Song, C.; Jung, D.; Choung, S.; Roh, E.; Lee, S. Angew. Chem., Int. Ed. 2004, 43,
6183.
16. Nakao, Y.; Kanyiva, K.; Oda, S.; Hiyama, T. J. Am. Chem. Soc. 2006, 128, 8146.
17. Bolm, C.; Legros, J.; Paih, J.; Zani, L. Chem. Rev. 2004, 104, 6217.
18. Jovel, I.; Mertins, K.; Kischel, J.; Zapf, A.; Beller, M. Angew. Chem., Int. Ed. 2005,
44, 3913.
19. Kischel, J.; Jovel, I.; Mertins, K.; Zapf, A.; Beller, M. Org. Lett. 2006, 8, 19.
20. Nakamura, M.; Hirai, A.; Nakamura, E. J. Am. Chem. Soc. 2000, 122, 978.
21. Komeyama, K.; Morimoto, T.; Takaki, K. Angew. Chem., Int. Ed. 2006, 45,
2938.
Brown viscous liquid. IR (neat)
m
max: 3396, 3053, 2936, 1682,
1599, 1452, 1246, 1027, 896, 751, 593 cmÀ1
.
1H NMR (500 MHz,
CDCl3): d 2.20, (3H, s), 3.68 (3H, s), 5.58 (1H, d, J = 2.3 Hz), 5.67
(1H, d, J = 2.0 Hz), 6.92–6.96 (2H, m), 7.02 (1H, t, J = 6.9 Hz), 7.11
(1H, t, J = 7.1 Hz), 7.21 (1H, d, J = 8.4 Hz), 7.29 (2H, d, J = 7.6 Hz),
7.35 (1H, d, J = 6.5 Hz), 7.77 (1H, NH, s). 13C NMR (125 MHz, CDCl3):
d 12.8, 55.9, 110.2, 111.7, 114.9, 117.1, 119.6 (2C), 120.7, 121.0,
128.2, 128.5, 130.9, 132.2, 132.5, 135.1, 139.8, 157.4. MS
(ES) = 264 (M++H+).
4.3. 2-Methyl-3-(1-(4-pentylphenyl)vinyl)-1H-indole (5d)
Yellow viscous liquid. IR (neat)
1676, 1606, 1450, 839, 744 cmÀ1
m
max: 3041, 3039, 2928, 2860,
.
1H NMR (500 MHz, CDCl3): d
0.89 (3H, t, J = 6.8 Hz), 1.32–1.38 (4H, m), 1.58–1.68 (2H, m), 2.28
(3H, s), 2.60 (2H, t, J = 7.6 Hz), 5.27 (1H, d, J = 2.3 Hz), 5.72 (1H, d,
J = 2.2 Hz), 6.99 (1H, t, J = 8.4 Hz), 7.11 (3H, d, J = 8.4 Hz), 7.24
(1H, d, J = 7.6 Hz), 7.29 (1H, d, J = 7.6 Hz), 7.33 (2H, t, J = 8.4 Hz),
7.94 (1H, NH, s). 13C NMR (125 MHz, CDCl3): d 12.8, 14.1, 22.6,
31.2, 31.6, 35.7, 110.2, 114.2, 114.3, 119.6, 119.8, 121.2, 127.2,
128.3, 128.6, 133.1, 135.2, 139.2, 142.3, 142.4. MS (ES) = 304
(M++H+).
22. Sundberg, R. J. Indoles; Academic: London, 1996.
23. Ferrer, C.; Amijs, C. H. M.; Echavarren, A. M. Chem. Eur. J. 2007, 1358.
24. Bhuvaneswari, S.; Jeganmohan, M.; Cheng Chem. Eur. J. 2007, 13, 8285.
25. For review see: (a) Nair, V.; Ros, S.; Jayan, C. N.; Pillai, B. S. Tetrahedron 2004, 60,
1959; (b) Tocco, G.; Begala, M.; Delogu, G.; Picciau, C.; Podda, G. Tetrahedron
Lett. 2004, 45, 6909; (c) Fringuell, F.; Peirmatt, O.; Pizz, O. F.; Vaccaro, L. Curr.
Org. Chem. 2003, 7, 1661; (d) Frost, C. G.; Hartly, J. P. Mini-Rev. Org. Chem. 2004,
1, 1; (e) Ranu, B. C. Eur. J. Org. Chem. 2000, 2347; (f) Auge, J.; Lubin, N.; Germain;
Uziel, J. Synthesis 2007, 1739.
26. (a) Sakai, N.; Annaka, K.; Konakahara, T. Tetrahedron Lett. 2006, 47, 631; (b)
Martins, M. A. P.; Teixeira, M. V. M.; Cunico, W.; Scapin, E.; Mayer, R.; Pereira, C.
M. P.; Zanatta, N.; Bonacorso, H. G.; Peppe, C.; Yua, Y.-F. Tetrahedron Lett. 2004,
45, 8991; (c) Sakai, N.; Annaka, K.; Konakahara, T. J. Org. Chem. 2006, 71, 3653;
(e) Ghosh, R.; Maiti, S. J. Mol. Catal. A: Chem. 2007, 264, 1; (f) Endo, K.;
Hatekeyama, T.; Nakamura, M.; Nakamura, E. J. Am. Chem. Soc. 2007, 129, 5264;
(g) Yasuda, M.; Somyo, T.; Baba, A. Angew. Chem., Int. Ed. 2006, 45, 793. and the
references cited therein.
Acknowledgment
One of the authors, G.B., thanks the Council of Scientific and
Industrial Research, New Delhi, India, for the research fellowship.
References and notes
1. (a) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102, 1731–1769; (b) Dyker,
G. Angew. Chem., Int. Ed. 1999, 38, 1698; (c) Kakiuchi, F.; Chatani, N. Adv. Synth.
Catal. 2003, 345, 1077; (d) Kakiuchi, F.; Murai, S. Activation of C–H bonds:
Catalytic Reactions. In Activation of Unreactive Bonds and Organic Synthesis;
Murai, S., Ed.; Springer: Berlin, 1999; pp 47–79; (e) Shilov; Shul.’pin, G. B. Chem.
Rev. 1997, 97, 2879; (f) Kakiuchi, F.; Kochi, T. Synthesis 2008, 3013.
2. (a) Denmark, S. E.; Amburgey, J. J. Am. Chem. Soc. 1993, 115, 10386; (b) Creton,
I.; Marek, I.; Normant, J. F. Synthesis 1996, 1499; (c) Brown, S. D.; Armstrong, R.
W. J. Am. Chem. Soc. 1996, 118, 6331; (d) Organ, M. G.; Cooper, J. T.; Rogers, L. R.;
Soleymanzadeh, F.; Paul, T. J. Org. Chem. 2000, 65, 7959.
27. Sakai, N.; Annaka, K.; Fujita, A.; Sato, A.; Konakahara, T. J. Org. Chem. 2008, 73,
4160.
28. (a) Elamparithi, E.; Anniyappan, M.; Muralidharan, D.; Perumal, P. T. ARKIVOC
2005, XI, 6–16; (b) Sridharan, V.; Perumal, P. T.; Avendan, O. C.; Mene’ndez, C. J.
Org. Biomol. Chem. 2007, 1351; (c) Hemanth, K.; Muralidharan, D.; Perumal, P.
T. Synthesis 2004, 1, 0063; (d) Shanthi, G.; Perumal, P. T. Tetrahedron Lett. 2007,
48, 6785; (e) Shanthi, G.; Perumal, P. T. Tetrahedron 2007, 63, 2057; (f) Shanthi,
G.; Perumal, P. T. Tetrahedron Lett. 2008, 49, 7139; (g) Shanthi, G.; Perumal, P. T.
Tetrahedron Lett. 2008, 50, 3959; (h) Thirumurugan, P.; Perumal, P. T.
Tetrahedron 2009, 65, 7620.
3. (a) Tsuchimoto, F.; Maeda, T.; Shirakawa, E.; Kawakami, Y. Chem. Commun.
2000, 1573; (b) Reilly, A. J. A.; Nieuwland, J. A. J. Am. Chem. Soc. 1928, 50, 2564;