Catalyst-free system for sulfenylation of free (N-H) indoles with 2,2′-dithiosalicylic acid under alkaline conditions

Article abstract of DOI:10.1039/c3ra46361c

A catalyst-free system for the sulfenylation of free (N-H) indoles using 2,2′-dithiosalicylic acid under alkaline conditions was developed in this study. This system provides a green, convenient and efficient method for sulfenylindole synthesis, with good yields and high selectivity. This simple synthesis was performed in a green solvent (isopropanol/H₂O) under atmospheric and catalyst-free conditions. The procedure was suitable for use with N-unprotected indoles. An electrophilic coupling mechanism was proposed.

Full text of DOI:10.1039/c3ra46361c

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Catalyst-free system for sulfenylation of free (N–H)
indoles with 2,20-dithiosalicylic acid under alkaline
conditions†
Cite this: RSC Adv., 2014, 4, 1241
*
Xiaofei Zhou and Xinhua Li
A catalyst-free system for the sulfenylation of free (N–H) indoles using 2,20-dithiosalicylic acid under
alkaline conditions was developed in this study. This system provides a green, convenient and efficient
method for sulfenylindole synthesis, with good yields and high selectivity. This simple synthesis was
performed in a green solvent (isopropanol/H2O) under atmospheric and catalyst-free conditions. The
procedure was suitable for use with N-unprotected indoles. An electrophilic coupling mechanism was
proposed.
Received 3rd November 2013
Accepted 14th November 2013
DOI: 10.1039/c3ra46361c
www.rsc.org/advances
Introduction
Substituted indoles continue to be a topic of interest because
they form the basic skeletal structure in many biologically active
and medicinally relevant compounds.1 Sulfenylindoles are of
special interest because they are used as precursors for
compounds, such as anti-HIV active substrates,2 anti-nocicep-
tive active compounds,3 5-lipoxygenase and tubulin polymeri-
sation inhibitors,4 and organic nonlinear optical materials.5 A
long-term interest has been noted in the development of new
methods for sulfenylindole synthesis because of their many
important applications. Several useful methods for indole sul-
fenylation have been reported.6 Although many sulfenylindoles
can be obtained using these methods, most of the reactions
require stringent conditions, such as the use of noble or tran-
sition metals (e.g., Pd, Cu, Ni, and Co) as catalyst, toxic solvents
(e.g., DMF, DMSO, acetonitrile, benzene, toluene, etc.) or gas
protection. The use of metals and volatile toxic solvents causes
environmental pollution, which complicates the aer-treatment
of the reaction, and increases reaction cost.
With the development of green chemistry and the increase in
awareness regarding environment protection in the last few
years, chemists have focused more in screening various non-
metal reagents and designing new sulphur reagents for sulfe-
nylindole synthesis.7 These reactions have been conducted
without metal catalyst or gas protection, and resulted in high
yields. Despite the apparent advantages of this reaction, the use
of non-metal catalysts and other solvents also results in many
problems, such as an elaborate post-treatment and greatly
Scheme 1 Synthesis of the sulfenylated indoles with DTSA.
increased production costs, environmental pollution, and waste
emission. These methods that employ metal and non-metal
catalysts suffer from the previously mentioned disadvantages.
Thus, developing a green approach for indole arylthiolation is
necessary.
In our continuous work on the C–S bond forming reactions,8
an efficient catalyst-free system for the sulfenylation of indoles
with 2,20-dithiosalicylic acid (DTSA) was developed. This simple
reaction was performed in an environmentally benign solvent
(isopropanol/H2O) under green and ambient conditions
(Scheme 1), in the absence of both oxygen and a catalyst. The
method described in this study is economical and can be used
for production scale increase.
Results and discussion
In recent years, the use of catalyst-free systems has drawn
considerable attention concerning the yield of various organic
transformations.9 However, to the best of our knowledge, no
studies regarding indole sulfenylation with disulphides using a
catalyst-free system, under green and ambient conditions, has
been reported yet.
Indole sulfenylation was initially examined using commer-
cially available DTSA and 1H-indole (Scheme 1). The reaction
was performed in isopropanol/H2O, under mild conditions and
without the added catalyst. The reaction afforded 3a in 23%
College of Chemistry and Materials Engineering, Wenzhou University, Zhejiang,
Wenzhou 325035, P. R. China. E-mail: lixinhua01@126.com
† Electronic supplementary information (ESI) available: 1H and 13C NMR spectra,
ESI-Qq-TOF mass spectra for the indole products 3a–3m. See DOI:
10.1039/c3ra46361c
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 1241–1245 | 1241

View Article Online
RSC Advances
Paper
Table 1 Optimisation of reaction conditions for indole sulfenylationa
Table 2 Indole sulfenylation with DTSA under optimum conditionsa
Entry
1
Indole
Product
Yieldb (%)
95
Entry
Base
Solvent
Temp (ꢀC)
Yieldb (%)
1
2
3
4
5
6
7
8
No
KOH
K2CO3
KOtBu
K2SO4
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
C2H5OH/H2O
CH3OH
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
90
23
42
14
54
11
36
93
Trace
22
21
51
21
17
60
40
Trace
85
90
2
3
59
64
KCl
NaOH
NaHCO3
NaOtBu
NaCl
Na2SO4
Ethylenediamine
Triethylamine
NaOH
NaOH
NaOH
NaOH
NaOH
9
10
11
12
13
14
15
16
17
18
19
20
4
5
85
73
DMF
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
i-PrOH/H2O
100
110
130
NaOH
NaOH
92
95
a
Reaction conditions: 1a (5 mmol), solvent (100 mL), 48 h, under air
atmosphere. b Yield of isolated product aer column chromatography.
6
7
8
9
86
69
65
42
yield aer column chromatography (Table 1, entry 1). The
transformation was further investigated to obtain the optimal
conditions, and the results are summarised in Table 1.
A series of Lewis bases, such as K2CO3 and NaOH, was
initially tested. 1H-Indole reacted with DTSA and NaOH, which
quantitatively produced the desired substituted indole 3a. The
use of other bases, such as KOH, resulted in lower yields (42%).
We tested different concentrations of different DTSAs and
found that 1 eq. DTSA, along with 1 eq. NaOH, at 120 ꢀC
afforded the highest yield. In terms of NaOH amount, the
reaction solvents were also examined. Initially, several kinds of
solvents were tested, and the highest yield of 93% was obtained
in isopropanol/H2O(v/v ratio ¼ 9 : 6) under air atmosphere
(Table 1, entry 7). The green solvent (isopropanol/H2O) was
chosen for the optimisation studies to avoid the use of envi-
ronmentally harmful solvents.
10
11
89
91
12
13
83
30
Finally, we studied the reaction at various temperatures, and
found that the best results for DTSA at 130 ꢀC were obtained with
1 eq. NaOH. Compound 3a was characterised by 1H and 13C NMR
and ESI-Qq-TOF mass spectrometry (Fig. S1, S2 and S23 in ESI†).
By optimising the reaction conditions, we investigated the
substrate scope of the reaction using various 1H-indole deriva-
tives. The results listed in Table 2 demonstrate that DTSA
reacted smoothly with most of the indole derivatives to afford
the corresponding sulfenylindoles 3 in good yields (Table 2,
entries 1–12). Furthermore, the substitution position on the
1242 | RSC Adv., 2014, 4, 1241–1245
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Table 2 (Contd.)
Table 3 DTSA reactions with 1H-indolea
Entry
14
Indole
Product
Yieldb (%)
No
Entry
DTSA
TSA
NaOH
AIBN
TEMPO
Yield (%)
1
2
3
4
5
6
7
8
1 eq.
No
No
2 eq.
No
No
No
No
No
No
No
No
No
No
No
No
No
2% eq.
No
No
No
No
No
No
No
No
2% eq.
No
No
23
No
10
25
21
95
58
84
0.5 eq.
1 eq.
1 eq.
1 eq.
1 eq.
1 eq.
No
1 eq.
0.5 eq.
1 eq.
15
No
2% eq.
a
DTSA ¼ 2,20-dithiosalicylic acid, TSA ¼ thiosalicylic acid.
a
Reaction conditions: 1 (5 mmol), 2 (10 mmol), NaOH (5 mmol),
b
isopropanol/H2O 100 mL (v/v ratio ¼ 9 : 6), at 130 ꢀC. Isolated yield
based on substrate 1.
employed, suggesting that the RSH or RSꢁ moieties are not
intermediates in the sulfenylation reaction. Stoichiometric
DTSA is also necessary for the thiolation reaction, the yield of
the reaction between 0.5 eq. 2a and indole was only 10%
(Table 3, entry 3); almost one-half of the yields obtained 1 eq.
2a. Notably, the use of 2 eq. benzenethiol was also effective in
a similar reaction.10 Furthermore, under standard conditions,
the addition of the radical inhibitor, 2,2,6,6-tetramethylpiper-
idinyloxy (TEMPO, 2 mol%), or the radical initiator, azodii-
sobutyronitrile (AIBN, 2 mol%), to the reaction afforded the
indole product 3a in 21% and 25% yields, respectively (Table
3, entries 4 and 5). Thus, a radical-mediated mechanism was
also excluded. The stoichiometric addition of NaOH improved
the reaction yield (Table 3, entries 6 and 7). Moreover, when
phenyl ring of 1H-indole exhibited minimal effect on the reac-
tion yield. For example, the F-substituted indoles 1d, 1j, 1k and
1l yielded 3d (85%), 3j (89%), 3k (91%) and 3l (83%), respec-
tively, when subjected to the optimal reaction conditions. On
the contrary, the electron-withdrawing nitro derivatives of 1H-
indole (Table 2, entries 14 and 15) failed to undergo trans-
formation under optimal reaction conditions, indicating an
electrophilic coupling mechanism. The reaction yield between
DTSA and 3-methyl-1H-indole 3m was only 30%, which is also
consistent with the electrophilic coupling mechanism. Indole
derivatives that bear the electron-donating groups were more
reactive than those with electron-withdrawing groups. Although
no metal catalyst was used in these reactions, the target prod-
ucts can be synthesised with high selectivity. Moreover, the
aer-treatment was simplied in these reactions.
Table 2 shows that 1H-indole derivatives more negatively
charged at the 3-C atom position showed increased yield.
However, reducing the negatively charged derivatives at the 3-C
atom position retarded the reaction. The 2-C atom has less
negative charge than the 3-C atom, which reduced the reactivity
and yield. Under alkaline conditions, the S–S bond in DTSA can
be cleaved to provide two sulphur atoms, i.e., one positively and
one negatively charged. A C–S electrophilic coupling reaction
can proceed when an electropositive S atom in DTSA attacks the
nucleophilic 3- or 2-C atom in the 1H-indole derivatives. The
positively charged sulphur is unstable under alkaline condi-
tions; however, when this atom bonds intramolecularly with the
deprotonated 2-carboxyl group, an active lactone species can be
produced.
the
radical
inhibitor,
2,2,6,6-tetramethylpiperidinooxy
(TEMPO, 2 mol%), was introduced into the standard proce-
dure under basic conditions, the product 3a was isolated in
84% yield (Table 3, entry 8), thereby further supporting a non-
radical-mediated mechanism.
Based on these experimental results and other similar
reported mechanisms,11 the reaction mechanism is outlined in
Scheme 2. The S–S bond in 2,20-dithiosalicylic acid is cleaved by
nucleophiles, such as OHꢁ, to generate the active species (cyclic
sulfenate ester, A) and the inactive species (B) (Scheme 2).
NaOH partially deprotonated the carboxylic acid group under
Experiments were conducted to gain insight into the sul-
fenylation mechanism, and the results are summarised in
Table 3. The results showed that the yield of the target
product of the reaction between 1a and 2a (1 eq.) under
standard conditions (isopropanol/H2O, 130 ꢀC) was 23%
(Table 3, entry 1). No corresponding thioether product was
Scheme 2 Proposed electrophilic coupling mechanism for 1H-indole
generated (Table 3, entry 2) when 2 eq. thiosalicylic acid was sulfenylation with 2,20-dithiosalicylic acid.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 1241–1245 | 1243

View Article Online
RSC Advances
Paper
these conditions. The resulting carboxylate intramolecularly material were puried using ash chromatography (PE : EtOAc,
displaces the thiolate anion to generate A, which serves as the 5 : 2) to afford the analytically pure sulfenylindole product 3.
electrophile in the subsequent reaction with indole to yield
intermediate C. Concomitant rearrangement of intermediate C
affords the desired product. The inactive species B showed no
Acknowledgements
reaction with 1H-indole, which results in the equimolar stoi- We gratefully acknowledge the nancial support provided by
chiometric 2,20-dithiosalicylic acid requirement for the thio- the National Natural Science Foundation of China (Grant no.
lation reaction.
21171133 and 21271143). We thank George R. Newkome and
Charles N. Mooreeld for their assistance.
Conclusions
Notes and references
We developed a simple, green and efficient reaction for the
sulfenylation of free (N–H) indoles to the corresponding sulfe-
nylindoles, using 2,20-dithiosalicylic acid in the presence of
NaOH. The reaction showed no involvement with any catalyst,
and was performed in environment-friendly isopropanol/H2O
under ambient conditions. The use of inexpensive starting
material, green solvents and NaOH as the Lewis base signi-
cantly decreased the environmental pollution and operating
costs for large-scale preparation. The reaction provided a
convenient, moisture-insensitive and green method for the
indole sulphide synthesis with good yields.
1 B. Gabriele, L. Veltri, G. Salerno, R. Mancuso and M. Costa,
Adv. Synth. Catal., 2010, 352, 3355; D. Zhang, H. Song and
Y. Qin, Acc. Chem. Res., 2011, 44, 447; M. Bandini and
A. Eichholzer, Angew. Chem., Int. Ed., 2009, 48, 9608;
S. Potavathri, K. C. Pereira, S. I. Gorelsky, A. Pike,
A. P. Lebris and B. DeBoef, J. Am. Chem. Soc., 2010, 132,
14676; D. W. Robbins, T. A. Boebel and J. F. Hartwig, J. Am.
Chem. Soc., 2010, 132, 4068; C. Guo, J. Song, S.-W. Luo and
L.-Z. Gong, Angew. Chem., Int. Ed., 2010, 49, 5558;
Q.-F. Wu, H. He, W.-B. Liu and S.-L. You, J. Am. Chem. Soc.,
2010, 132, 11418; H. T. Klare, M. Oestreich, J.-i. Ito,
H. Nishiyama, Y. Ohki and K. Tatsumi, J. Am. Chem. Soc.,
2011, 133, 3312; Z. Wang, K. Li, D. Zhao, J. Lan and J. You,
Angew. Chem., Int. Ed., 2011, 50, 5365; W. Wu and W. Su,
J. Am. Chem. Soc., 2011, 133, 11924; M. C. Haibach, I. Deb,
C. K. De and D. Seidel, J. Am. Chem. Soc., 2011, 133, 2100;
B. J. Lundy, S. Jansone-Popova and J. A. May, Org. Lett.,
2011, 13, 4958; K. V. Sashidhara, M. Kumar, R. Sonkar,
B. S. Singh, A. K. Khanna and G. Bhatia, J. Med. Chem.,
2012, 55, 2769; R. Finlayson, A. N. Pearce, M. J. Page,
M. Kaiser, M. L. Bourguet-Kondracki, J. L. Harper,
V. L. Webb and B. R. Copp, J. Nat. Prod., 2011, 74, 888;
S. Li, J. M. Fineeld, J. D. Sunderhaus, T. J. McAfoos,
R. M. Williams and D. H. Sherman, J. Am. Chem. Soc.,
2012, 134, 788; X. H. Xu, G. K. Liu, A. Azuma, E. Tokunaga
and N. Shibata, Org. Lett., 2011, 13, 4854; M. W. Robinson,
J. H. Overmeyer, A. M. Young, P. W. Erhardt and
W. A. Maltese, J. Med. Chem., 2012, 55, 1940;
S. K. Guchhait, M. Kashyap and H. Kamble, J. Org. Chem.,
2011, 76, 4753; V. Rauniyar, Z. J. Wang, H. E. Burks and
F. D. Toste, J. Am. Chem. Soc., 2011, 133, 8486; M. Inman,
A. Carbone and C. J. Moody, J. Org. Chem., 2012, 77, 1217;
Experimental
All chemicals (AR grade) were obtained from commercial
sources and used without further purication. Petroleum ether
(PE) refers to the fraction boiled from 60 ꢀC to 90 ꢀC. The
progress of the reactions was monitored by thin layer chroma-
tography (TLC) (silica gel, Polygram SILG/UV 254 plates).
Column chromatography was performed using Silicycle silica
gel (200–300 mesh). The melting points were obtained using a
Yuhua digital melting point apparatus and were uncorrected.
1H and 13C NMR spectra were recorded in CDCl3 or DMSO-d6
using a Bruker DRX 500 (500 MHz) spectrometer, with tetra-
methylsilane as the internal standard. All compounds (known
and unknown) were identied using 1H and 13C NMR and high-
resolution mass spectroscopy. The physical and spectral data of
the known compounds were compared with those reported in
literature.
General procedure for the indole sulfenylation with 2,20-
dithiosalicylic acid
A mixture of indole 1 (5 mmol), 2,20-dithiosalicylic acid 2 (10
mmol) and sodium hydroxide (5 mmol) in 100 mL isopropanol/
H2O (v/v ratio ¼ 9 : 6) was stirred at 130 ꢀC for 48 h or until
complete consumption of the starting materials was observed.
This process was monitored using thin layer chromatography
(TLC). Aer the reaction was completed, the mixture was slowly
cooled to room temperature. The white solid material formed
was collected through ltration. To obtain the pure product, the
ltrate was poured into EtOAc (170 mL) and washed with a
saturated Na2S2O3solution (5 ꢂ 50 mL), followed by aqueous
layer extraction with EtOAc (5 ꢂ 30 mL). The combined organic
layer was dried over anhydrous Na2SO4 and was concentrated
under reduced pressure. The residue and the white solid
M. Baron, E. Metay, M. Lemaire and F. Popowycz, J. Org.
´
Chem., 2012, 77, 3598; M. C. Van Zandt, M. L. Jones,
D. E. Gunn, L. S. Geraci, J. H. Jones, D. R. Sawicki, J. Sredy,
J. L. Jacot, A. T. Dicioccio, T. Petrova, A. Mischler and
A. D. Podjarny, J. Med. Chem., 2005, 48, 3141; B. Bao,
Q. Sun, X. Yao, J. Hong, C. O. Lee, C. J. Sim, K. S. Im and
J. H. Jung, J. Nat. Prod., 2005, 68, 711; G. R. Humphrey and
J. T. Kuethe, Chem. Rev., 2006, 106, 2875; M. Nuth,
H. Guan, N. Zhukovskaya, Y. L. Saw and R. P. Ricciardi,
J. Med. Chem., 2013, 56, 3235.
2 R. Ragno, A. Coluccia, G. La Regina, G. De Martino,
F. Piscitelli, A. Lavecchia, E. Novellino, A. Bergamini,
C. Ciaprini, A. Sinistro, G. Maga, E. Crespan, M. Artico and
1244 | RSC Adv., 2014, 4, 1241–1245
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
R. Silvestri, J. Med. Chem., 2006, 49, 3172; G. De Martino,
RSC Advances
X. M. Feng, Org. Lett., 2012, 14, 2726; P. Barraja, P. Diana,
A. Carbone and G. Cirrincione, Tetrahedron, 2008, 64,
11625; W. L. Ge and Y. Y. Wei, Green Chem., 2012, 14,
2066; G. L. Regina, V. Gatti, V. Famiglini, F. Piscitelli and
R. Silvestri, ACS Comb. Sci., 2012, 14, 258; C. C. Silveira,
S. R. Mendes, L. Wolf, G. M. Martins and L. Muhlen,
Tetrahedron, 2012, 68, 10464.
G. La Regina, A. Coluccia, M. C. Edler, M. C. Barbera,
A. Brancale, E. Wilcox, E. Hamel, M. Artico and R. Silvestri,
J. Med. Chem., 2004, 47, 6120; G. L. Regina, A. Coluccia,
A. Brancale, F. Piscitelli, V. Gatti, G. Maga, A. Samuele,
C. Pannecouque, D. Schols, J. Balzarini, E. Novellino and
R. Silvestri, J. Med. Chem., 2011, 54, 1587.
3 D. Potin, V. Parnet, J.-M. Teulon, F. Camborde, F. Caussade,
J. Meignen, D. Provost and A. Cloarec, Bioorg. Med. Chem.
Lett., 2000, 10, 805.
4 G. De Martino, M. C. Edler, G. La Regina, A. Coluccia,
M. C. Barbera, D. Barrow, R. I. Nicholson, G. Chiosis,
A. Brancale, E. Hamel, M. Artico and R. Silvestri, J. Med.
Chem., 2006, 49, 947; J. H. Hutchinson, D. Riendeau,
C. Brideau, C. Chan, D. Delorme, D. Denis, J. P. Falgueyret,
R. Fortin, J. Guay, P. Hamel, T. R. Jones, D. Macdonald,
C. S. McFarlane, H. Piechuta, J. Scheigetz, P. Tagari,
7 M. Matsugi, K. Murata, K. Gotanda, H. Nambu,
G. Anilkumar, K. Matsumoto and Y. Kita, J. Org. Chem.,
2001, 66, 2434; M. Matsugi, K. Murata, H. Nambu and
Y. Kita, Tetrahedron Lett., 2001, 42, 1077; K. M. Schlosser,
A. P. Krasutsky, H. W. Hamilton, J. E. Reed and K. Sexton,
Org. Lett., 2004, 6, 819; J. A. Campbell, C. A. Broka,
L. Gong, K. A. M. Walker and J.-H. Wang, Tetrahedron Lett.,
2004, 45, 4073; J. S. Yadav, B. V. S. Reddy and Y. J. Reddy,
Tetrahedron Lett., 2007, 48, 7034; G. Wu, J. Wu, J. Wu and
L. Wu, Synth. Commun., 2008, 38, 1036; H.-A. Du,
R.-Y. Tang, C.-L. Deng, Y. Liu, J.-H. Li and X.-G. Zhang,
Adv. Synth. Catal., 2011, 353, 2739; Y. Chen, C.-H. Cho,
F. Shi and R. C. Larock, J. Org. Chem., 2009, 74, 6802;
D. Huang, J. Chen, W. Dan, J. Ding, M. Liu and H. Wu,
Adv. Synth. Catal., 2012, 354, 2123; L. H. Zou, J. Reball,
J. Mottweiler and C. Bolm, Chem. Commun., 2012, 48, 11307.
8 X. Zhang, X. Zhou, H. Xiao and X. Li, RSC Adv., 2013, 3,
22280.
´
M. Therien and Y. Girard, J. Med. Chem., 1993, 36, 2771.
5 Y. Kawamonzen, Y. Mori, Y. Kawamonzen and Y. Mori,
Chem. Abstr., 1993, 119, 237604.
6 J. S. Yadav, B. V. S. Reddy, Y. J. Reddy and K. Praneeth,
Synthesis, 2009, 1520; X.-L. Fang, R.-Y. Tang, P. Zhong and
J.-H. Li, Synthesis, 2009, 4183; C. C. Silveira, S. R. Mendes,
L. Wolf and G. M. Martins, Tetrahedron Lett., 2010, 51,
2014; Z. Li, J. Q. Hong and X. J. Zhou, Tetrahedron, 2011,
67, 3690; Z. Li, L. Hong, R. Liu, J. Shen and X. Zhou,
Tetrahedron Lett., 2011, 52, 1343; Y.-J. Guo, R.-Y. Tang,
9 N. Chandna, N. Chandak, P. Kumar, J. K. Kapoorb and
P. K. Sharma, Green Chem., 2013, 15, 2294.
J.-H. Li, P. Zhong and X.-G. Zhang, Adv. Synth. Catal., 2009, 10 Y. Maeda, M. Koyabu, T. Nishimura and S. Uemura, J. Org.
351, 2615; J. G. Atkinson, P. Hamel and Y. Girard, Chem., 2004, 69, 7688.
Synthesis, 1998, 480; S. Zhang, P. Qian, M. Zhang, M. Hu 11 P. Hamel, J. Org. Chem., 2002, 67, 2854; M. Tudge,
and J. Cheng, J. Org. Chem., 2010, 75, 6732; P. Hamel,
Y. Girard and N. N. Tsou, J. Heterocycl. Chem., 1999, 36,
643; W. L. Ge and Y. Y. Wei, Synthesis, 2012, 934; Y. F. Cai,
J. Li, W. L. Chen, M. S. Xie, X. H. Liu, L. L. Lin and
M. Tamiya, C. Savarin and G. R. Humphrey, Org. Lett.,
2006, 8, 565; Y. Chen, C. H. Cho and R. C. Larock, Org.
Lett., 2009, 11, 173; C. Uyeda, Y. Tan, G. C. Fu and
J. C. Peters, J. Am. Chem. Soc., 2013, 135, 9548.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 1241–1245 | 1245