Synthesis of 3-Selenylindoles under Ecofriendly Conditions

Article abstract of DOI:10.1002/ejoc.201500514

The preparation of biologically relevant 3-selenylindoles by a greener protocol by using a catalytic amount of K₂CO₃ and ethanol as a biosolvent in a system open to air was explored. Through this easy approach, 3-selenylindoles with

Full text of DOI:10.1002/ejoc.201500514

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201500514
Synthesis of 3-Selenylindoles under Ecofriendly Conditions
Natasha L. Ferreira,[a] Juliano B. Azeredo,[a] Breno L. Fiorentin,[a] and Antonio L. Braga*[a]
Keywords: Homogeneous catalysis / Green chemistry / Selenium / Nitrogen heterocycles
The preparation of biologically relevant 3-selenylindoles by
a greener protocol by using a catalytic amount of K2CO3 and
ethanol as a biosolvent in a system open to air was explored.
Through this easy approach, 3-selenylindoles with different
functionalities were obtained in good yields by a radical
pathway.
exhibit fascinating biological profiles, notably their ability
to mimic the enzyme glutathione peroxidase, which plays a
crucial role in the detoxification of reactive oxygen species
in mammalian cells.[11]
Introduction
The indole core is one of the most fascinating heterocy-
cles found in nature.[1] It is present in many important bio-
logical structures, such as the amino acid tryptophan, the
neurotransmitter serotonin, and drugs used in several types
of treatments.[2] Owing to their biological and pharmaceuti-
cal properties, chalcogenylindoles have attracted interest
from both industry and academia.
This class of compounds has been shown to have thera-
peutic value in the treatment of different diseases including
cancer,[3] HIV,[4] allergies,[5] and heart-related disorders.[6]
Also, recent studies have shown that 3-arylthioindoles are
potent inhibitors of tubulin polymerization, which is a
proven strategy to combat the growth of tumor cells.[7] For
instance, compound A (Figure 1) is able to interfere with
the tubulin system by acting as a powerful inhibitor of its
polymerization even at low concentrations.[8] Analogously,
compound B shows potent antiviral activity against small-
pox.[9]
Because of the importance of organochalcogen com-
pounds in many bioactive products and because of the wide
spectrum of biologically active indoles, some synthetic
methods for the construction of 3-chalcogenylindoles have
been reported. These compounds can be prepared through
electrophilic cyclization of o-alkynylanilines by using elec-
trophilic species such as organochalcogen agents mediated
by a transition metals[12] or performed in the presence of
iodine reagents.[13] Another process for obtaining this class
of compounds involves the direct chalcogenylation of the
indole moiety through the use of different agents such as
N-chalcogenoimides,[14] quinone mono-O,S-acetals,[15] sulf-
onylhydrazides,[16] dichalcogenides,[17] and sulfonium
salts.[18] On the other hand, the development of environ-
mentally friendly methods has attracted significant atten-
tion in recent years.[7a,19] The aim of this approach is to
minimize environmental impacts, particularly those caused
by the use of toxic solvents and transition metals. In this
regard, the use of green solvents is highly desirable, and
several solvents have been used successfully for this pur-
pose, for instance, water, ionic liquids, glycerol, polyethylene
glycol (PEG), and dimethyl carbonate among others. In this
context, reactions involving the use of ethanol as a solvent
in organic synthesis have emerged, and they are employed
in several transformations including epoxide ring open-
ing,[20] C–C bond formation,[21] and Biginelli cycloconden-
sation.[22] Furthermore, ethanol is considered to be a bio-
solvent, as it is produced from renewable sources.[23] In ad-
dition, ethanol is easy to handle and is associated with low
cost and low environmental and health risks.
Figure 1. Some bioactive 3-arylthioindoles.
Similarly, the activity of organoselenium compounds has
been exploited in different areas of chemistry such as bio-
chemistry and materials.[10] In addition, these compounds
Our research group has focused on the development of
more sustainable methods for the preparation of organose-
lenium compounds. Recently, we reported the synthesis of
3-chalcogenylindoles by using indoles and diorganyl di-
chalcogenides as starting materials catalyzed by I2 under
microwave irradiation.[24]
[a] Department of Chemistry – LabSelen, Federal University of
Santa Catarina (UFSC),
88040-900 Florianópolis, Brazil
E-mail: braga.antonio@ufsc.br
http://labselen.jimdo.com/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500514.
5070
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2015, 5070–5074

Synthesis of 3-Selenylindoles
In 2013, Zhang and co-workers[25] reported the synthesis Table 1. Optimization of the reaction conditions.[a]
of 3-sulfenylindoles by using K2CO3 (0.5 equiv.) and
DMSO as the solvent with an excess amount of disulfides.
After a reaction time of 9 h, they obtained a series of 3-
sufenylindoles in good yields. Inspired by the development
of new environmentally friendly methodologies involving
the preparation of biologically relevant organoselenium
Entry
Base
Solvent
Temp.
[°C]
Time
[h]
Yield[b]
[%]
compounds and considering that the indole nucleus is of
biological interest, we decided to explore the synthesis of 3-
selenylindoles. Despite the success of the synthesis of this
class of compounds reported by our group, we considered
the possibility of developing a simple, rapid, and environ-
mentally adequate method, in which the use of microwave
equipment could be avoided. In this context, we developed
an alternative and greener protocol for the synthesis of
these compounds by treating indoles with diaryl diselenides
promoted by a catalytic amount of K2CO3 and with the use
of ethanol as a green solvent in a short reaction time.
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
–
[bmim]PF6
[bmim]Br
EtOH
100
100
40
100
100
100
100
25
25
60
25
60
60
60
60
25
40
80
60
60
14
14
14
14
2
14
14
3
24
3.5
2
2
1
2
3
2
2
2
2
2
2
83
75
95
PEG-400
PEG-400
glycerol
–
99/98[c]
71
47
25
61
99
99
75
20
63
96
99
31
56
97
0
99
PEG-400[d]
EtOH
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
EtOH
EtOH[d]
EtOH/H2O
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
Results and Discussion
In an attempt to determine the optimal reaction condi-
tions, in our initial study we started from the optimization
of the reaction of 1H-indole (1a) with diphenyl diselenide
(2a) in the presence of a base in an open-to-air system
(Table 1). Initially, the influence of the reaction medium
was evaluated by using different ionic liquids owing to the
properties of these materials, such as their low toxicity,
good stability, and catalytic ability. First, the ionic liquid 1-
butyl-3-methylimidazolium hexafluorophosphate ([bmim]-
PF6) was used, and the product was obtained in 83% yield
(Table 1, entry 1). However, if we used [bmim]Br the yield
decreased to 75%, which indicated that the counterion in-
fluences the formation of product 3a (Table 1, entry 2).
KOH
Na2CO3
Et3N
60
60
60
trace
trace
99
EtOH
EtOH
2
2
Cs2CO3
[a] Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), base
(0.25 mmol), solvent (1 mL), at a specific time. [b] Yield of isolated
product based on 3a. [c] Reused PEG-400. [d] The reaction was ir-
radiated in an ultrasonic bath.
Notably, if the reaction was performed in the presence of yield (Table 1, entry 7). We also investigated the influence
the polymer PEG-400 for 14 h, a significant increase in the of ultrasonic irradiation on this reaction by using PEG-400
yield was observed (99%). This level of yield was main- (Table 1, entry 8) and ethanol (Table 1, entry 11) as sol-
tained even if the solvent was recycled once, as the desired vents, but the results were not satisfactory, as the product
product was delivered in 98% yield (Table 1, entry 4). This was obtained in yields of 61 and 75%, respectively. Finally,
demonstrated that PEG-400 has the potential for use in this we tested a mixture of ethanol and water as the solvent, but
type of transformation. This reaction was also conducted the yield decreased considerably (20%; Table 1, entry 12).
over 2 h; however, this led to a decrease in the yield (71%;
At this point, it is important to note that with our
Table 1, entry 5). We also tested glycerol as a sustainable method the preparation of 3-selenylindole (3a) can be per-
solvent, but in this case the desired product was obtained formed by using PEG-400 at 100 °C or ethanol at 60 °C, as
in only 47% yield (Table 1, entry 6).
both provide the desired product in almost quantitative
In view of the interest in the use of a low-costing solvent yield. However, we decided to continue the investigation
obtained from a renewable source, ethanol was considered and improve this reaction by employing ethanol as the sol-
as an alternative. Upon conducting the reaction with eth- vent, as it is inexpensive, widely available, and obtained
anol overnight at 40 °C, the corresponding product was ob- from renewable sources.
tained in 95% yield (Table 1, entry 3). This result encour-
After screening the solvents, we set out to establish the
aged us to continue exploring the reaction in ethanol me- appropriate reaction time for this transformation. Upon
dium. The reaction was then performed at room tempera- performing the reaction over 1 h, product 3a was obtained
ture for 24 h, which gave product 3a in 99% yield (Table 1, in 63% yield (Table 1, entry 13). Upon increasing the reac-
entry 9). Upon increasing the temperature to 60 °C for tion time to 2 h the yield increased to 96% (Table 1, en-
3.5 h, it was observed that the yield remained at a quantita- try 14), and after 3 h of reaction the yield was 99% (Table 1,
tive level (99%; Table 1, entry 10).
entry 15). Thus, considering a balance of yield versus reac-
The reaction was also conducted in the absence of sol- tion time, 2 h was selected as providing the most adequate
vent, but the desired product was obtained in only 25% combination.
Eur. J. Org. Chem. 2015, 5070–5074
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5071

N. L. Ferreira, J. B. Azeredo, B. L. Fiorentin, A. L. Braga
SHORT COMMUNICATION
In the next step, the effect of temperature on the reaction try 5). Even with the use of a small excess amount of dis-
system was evaluated. Upon performing the reaction at elenide, the preparation of 3-selenylindoles with this proto-
room temperature the yield was 31% (Table 1, entry 16), col is greener than its sulfur counterpart.[25]
whereas at 40 °C the yield increased to 56% (Table 1, en-
With the best conditions in hand, the scope of the reac-
try 17). If the reaction was performed at 60 °C, the desired tion was investigated (Table 3). First, the effect of different
product was obtained in 96% yield, whereas at 80 °C the groups bound to the indole nucleus was evaluated through
yield was 97% (Table 1, entry 18). On the basis of these reaction of various indoles with diphenyl diselenide. The
results, we established that a reaction temperature of 60 °C electronic character of both electron-donating and -with-
was the most suitable.
drawing substituents in the 5-position of the indole did not
The effect of an alkaline medium was also investigated. affect substantially the performance of the transformation.
If the reaction was performed in the absence of base, the For these cases, the reaction proceeded with different
formation of the desired product was not observed (Table 1, groups including, methoxy, bromo, and ester groups, and
entry 19). This result showed that the presence of a base gave desired products 3b–d in good to excellent yields. If 4-
was essential for this transformation. With the use of a
strongly alkaline base, such as KOH, a quantitative yield
Table 3. Synthesis of 3-selenylindoles.
was observed (Table 1, entry 20). A similar result was ob-
tained if Cs2CO3 was employed (Table 1, entry 23). On the
other hand, if Na2CO3 (Table 1, entry 21) was used, only a
trace amount of the desired product was obtained. This re-
sult can be rationalized in terms of the difference in the
solubilities of the bases K2CO3 and Na2CO3 in alcohol me-
dia, as the former is about 20 times more soluble than the
latter.[26] If Et3N (Table 1, entry 22) was used, product 3a
was obtained in only trace amounts. On the basis of these
results, it was established that the most suitable base for
this transformation was K2CO3, mainly as a result of its
availability and low cost.
To promote atom economy, we examined the stoichio-
metry of the starting materials (Table 2). If the reaction was
performed with 50 mol-% of base, product 3a was obtained
in 96% yield (Table 2, entry 1). The same yield was ob-
served upon using 20 mol-% (Table 2, entry 2), which indi-
cated that the reaction occurs in a catalytic way.
Table 2. Study on the stoichiometry of the reactants in the forma-
tion of 3-selenylindole (3a).
Entry
K2CO3 [mol-%]
Diselenide 2a [equiv.]
Yield[a] [%]
1
2
3
4
5
50
20
10
20
20
2
2
2
1.5
1
96
96
66
99
68
[a] Yield of isolated product based on 3a.
However, by using 10 mol-% of base the yield of the
product decreased to 66% (Table 2, entry 3). Once the opti-
mal amount of base was established, the amount of diselen-
ide was varied. The best condition was found to be 1 equiv-
alent of indole (1a) to 1.5 equivalents of diselenide (2a),
which provided the product in 99% yield (Table 2, entry 4).
A decrease in the yield was observed if the reaction was
conducted with 1 equivalent for both reagents (Table 2, en-
5072
www.eurjoc.org
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2015, 5070–5074

Synthesis of 3-Selenylindoles
cyanoindole was used, the corresponding product 3s was
obtained in 43% yield.
The presence of a phenyl substituent at the 2-position led
to a decrease in the yield of product 3e (61%). Substitution
of hydrogen by a methyl group at the 1-position did not
result in the formation of product 3t, which suggests that
this position contains the free amine (N–H).
Next, we examined the influence of different diaryl dis-
elenides. We observed that a substituent in the aryl moiety
of these compounds did not have a great influence on the
reactivity of the reaction. If diselenides containing electron-
withdrawing groups (e.g., Cl, CF3) were employed, corre-
sponding products 3f–g were obtained in quantitative
yields. Electron-donating groups (e.g., Me, OMe) in the or-
tho and para positions also proved to be efficient under the
reaction conditions, and corresponding products 3j, 3h, and
3o were obtained in very good yields. By using a benzylic
diselenide, a decrease in the yield of product 3i was ob-
served. The employment of an aliphatic diselenide did not
result in the formation of product 3k.
To demonstrate the versatility of the protocol developed,
we performed this transformation with the use of indoles
containing different substituents as well as diaryl diselen-
ides. All multifunctionalized products synthesized (i.e., 3l–
n, 3p–r) were obtained in good to excellent yields, which
shows that the reaction has good functional group toler-
ance.
The mechanism of the C-3 substitution was investigated
through control experiments, as shown in Scheme 1. In
Equation (1), the reaction was performed under an inert at-
mosphere and yielded product 3a in 53% yield, which indi-
cated that the presence of oxygen was important for this
transformation. We also evaluated the effect of a radical
trap on the reaction outcome. If 2,2,6,6-tetramethylpiperi-
din-1-oxyl (TEMPO) was employed, we observed a signifi-
cant decrease in the yield (12%), which indicated a radical
pathway [Equation (2), Scheme 1]. The need for a free NH
[Equation (3), Scheme 1] group suggested that this
hydrogen is essential for the formation of the product. Fi-
nally, the reaction was conducted by using 3-methylindole
to block the 3-position and to check its reactivity in this
protocol. In this case, the formation of the corresponding
product was not observed [Equation (4), Scheme 1].
We also performed another reaction [Equation (5),
Scheme 1] under an inert atmosphere by using degassed eth-
anol. In this case, the yield dropped to 18%, which showed
that oxygen was essential for the radical catalytic cycle pro-
posed (Scheme 2). The reaction in the presence of the radi-
cal initiator 2,2Ј-azobisisobutyronitrile (AIBN, 20 mol-%)
was also promoted [Equation (6), Scheme 1]. In this case,
we did not observe the formation of the desired product. It
is possible to conclude that K2CO3 is essential to promote
Scheme 1. Investigation of the reaction mechanism.
Scheme 2. Proposed mechanism for the reaction.
the initiation step with the formation of the indole radical. ole radical is generated from the indole anion in the pres-
According to the literature[27] and on the basis of the ence of atmospheric air. The indole radical reacts with dis-
experimental results, a proposed mechanism is represented elenide to form the 3-selenylindole and a selenide radical.
in Scheme 2. Initially, the hydrogen atom attached to the This species, in turn, removes the hydrogen atom from an-
nitrogen atom of indole is removed by K2CO3, which gener- other indole molecule to complete the catalytic cycle,
ates a negative charge on the indole. Subsequently, the ind- whereas the selenol is oxidized back to the diselenide.
Eur. J. Org. Chem. 2015, 5070–5074
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5073

N. L. Ferreira, J. B. Azeredo, B. L. Fiorentin, A. L. Braga
SHORT COMMUNICATION
cale, E. Wilcox, E. Hamel, M. Artico, R. Silvestri, J. Med.
Chem. 2004, 47, 6120–6123.
Conclusions
[8] G. La Regina, R. L. Bai, W. M. Rensen, E. Di Cesare, A. Col-
uccia, F. Piscitelli, V. Famiglini, A. Reggio, M. Nalli, S. Pellic-
cia, E. Da Pozzo, B. Costa, I. Granata, A. Porta, B. Maresca,
A. Soriani, M. L. Iannitto, A. Santoni, J. J. Li, M. M. Cona,
F. Chen, Y. C. Ni, A. Brancale, G. Dondio, S. Vultaggio, M.
Varasi, C. Mercurio, C. Martini, E. Hamel, P. Lavia, E. Nov-
ellino, R. Silvestri, J. Med. Chem. 2013, 56, 123–149.
[9] M. Nuth, H. C. Guan, N. Zhukovskaya, Y. L. Saw, R. P. Ricci-
ardi, J. Med. Chem. 2013, 56, 3235–3246.
[10] a) G. Perin, E. J. Lenardao, R. G. Jacob, R. B. Panatieri, Chem.
Rev. 2009, 109, 1277–1301; b) D. M. Freudendahl, S. Santoro,
S. A. Shahzad, C. Santi, T. Wirth, Angew. Chem. Int. Ed. 2009,
48, 8409–8411; Angew. Chem. 2009, 121, 8559–8562.
[11] a) C. W. Nogueira, G. Zeni, J. B. T. Rocha, Chem. Rev. 2004,
104, 6255–6285; b) E. E. Alberto, V. do Nascimento, A. L.
Braga, J. Braz. Chem. Soc. 2010, 21, 2032–2041; c) G. Roy, M.
Nethaji, G. Mugesh, J. Am. Chem. Soc. 2004, 126, 2712–2713;
d) A. L. Braga, J. Rafique, The Chemistry of Organic Selenium
and Tellurium Compounds (Ed.: Z. Rappoport), Wiley, Chiches-
ter, UK, 2014, vol. 4, chapter 13–15, p. 989–1174.
In conclusion, we developed alternative greener method-
ology to obtain 3-selenylindoles, a class of compounds of
interest for therapeutic applications, under mild conditions.
It is important to note that ethanol was employed as a re-
newable biosolvent of low environmental risk and K2CO3
was used in catalytic amount. The methodology outlined
herein allowed the synthesis of a library of 3-selenylindoles
with different functionalities in yields of up to 99% in a
short reaction time.
Experimental Section
General Procedure: A round-bottomed flask was charged with the
indole (0.5 mmol), dichalcogenide (0.37 mmol), K2CO3 (20 mol-
%), and ethanol (1 mL). The resulting solution was stirred at 60 °C
for 2 h. The progress of the reaction was monitored by TLC. Upon
completion of the reaction, the crude product was purified by col-
umn chromatography (silica gel, hexane/ethyl acetate) to furnish
the pure product.
[12] a) H. A. Du, R. Y. Tang, C. L. Deng, Y. Liu, J. H. Li, X. G.
Zhang, Adv. Synth. Catal. 2011, 353, 2739–2748; b) A. Sper-
anca, B. Godoi, P. H. Menezes, G. Zeni, Synlett 2013, 24,
1125–1132.
[13] Y. Chen, C. H. Cho, F. Shi, R. C. Larock, J. Org. Chem. 2009,
74, 6802–6811.
Acknowledgments
[14] a) C. C. Silveira, S. R. Mendes, L. Wolf, G. M. Martins, Tetra-
hedron Lett. 2010, 51, 2014–2016; b) X. D. Zhao, Z. K. Yu,
T. Y. Xu, P. Wu, H. F. Yu, Org. Lett. 2007, 9, 5263–5266.
[15] Y. Kita, H. Nambu, N. G. Ramesh, G. Anilkumar, M. Matsugi,
Org. Lett. 2001, 3, 1157–1160.
[16] F. L. Yang, S. K. Tian, Angew. Chem. Int. Ed. 2013, 52, 4929–
4932; Angew. Chem. 2013, 125, 5029–5032.
[17] a) X. F. Zhou, X. H. Li, RSC Adv. 2014, 4, 1241–1245; b) C. D.
Prasad, S. Kumar, M. Satter, A. Adhikary, S. Kumar, Org. Bio-
mol. Chem. 2013, 11, 8036–8040.
[18] S. Jain, K. Shukla, A. Mukhopadhyay, S. N. Suryawanshi, D. S.
Bhakuni, Synth. Commun. 1990, 20, 1315–1320.
The authors gratefully acknowledge Coordenação de Aper-
feiçoamento de Pessoal de Nível Superior (CAPES), Conselho Na-
cional de Desenvolvimento Científico e Tecnológico (CNPq), Fun-
dação de Amparo à Pesquisa do Estado de Santa Catarina (FA-
PESC), Fundação de Amparo à Pesquisa do Estado de São Paulo
(FAPESP-GSK), and Instituto Nacional de Ciência e Tecnologia
(INCT-Catálise) for financial support. CNPq is further acknowl-
edged for the fellowship received by N. L. F. The authors are also
thankful to CEBIME for the HRMS analysis.
[19] L. Vaccaro, D. Lanari, A. Marrocchi, G. Strappaveccia, Green
Chem. 2014, 16, 3680–3704.
[1] a) G. W. Gribble, J. Chem. Soc. Perkin Trans. 1 2000, 1045–
1075; b) D. A. Horton, G. T. Bourne, M. L. Smythe, Chem. [20] J. X. Wang, X. W. Wu, Y. L. Hu, K. Zhao, Z. X. Liu, J. Chem.
Rev. 2003, 103, 893–930.
[2] W. R. Chao, D. Yean, K. Amin, C. Green, L. Jong, J. Med.
Chem. 2007, 50, 3412–3415.
[3] I. Avis, A. Martinez, J. Tauler, E. Zudaire, A. Mayburd, R.
Abu-Ghazaleh, F. Ondrey, J. L. Mulshine, Cancer Res. 2005,
65, 4181–4190.
[4] R. Ragno, A. Coluccia, G. La Regina, G. De Martino, F. Pisci-
telli, A. Lavecchia, E. Novellino, A. Bergamini, C. Ciaprini, A.
Sinistro, G. Maga, E. Crespan, M. Artico, R. Silvestri, J. Med.
Chem. 2006, 49, 3172–3184.
[5] P. C. Unangst, D. T. Connor, S. R. Stabler, R. J. Weikert, M. E.
Carethers, J. A. Kennedy, D. O. Thueson, J. C. Chestnut, R. L.
Adolphson, M. C. Conroy, J. Med. Chem. 1989, 32, 1360–1366.
[6] C. D. Funk, Nat. Rev. Drug Discovery 2005, 4, 664–672.
[7] a) G. La Regina, V. Gatti, V. Famiglini, F. Piscitelli, R. Silvestri,
ACS Comb. Sci. 2012, 14, 258–262; b) G. De Martino, G.
La Regina, A. Coluccia, M. C. Edler, M. C. Barbera, A. Bran-
Res. Synop. 1999, 688–689.
[21] R. Chinchilla, C. Najera, Chem. Soc. Rev. 2011, 40, 5084–5121.
[22] D. S. Bose, L. Fatima, H. B. Mereyala, J. Org. Chem. 2003, 68,
587–590.
[23] a) J. H. Clark, S. J. Tavener, Org. Process Res. Dev. 2007, 11,
149–155; b) C. Capello, U. Fischer, K. Hungerbuhler, Green
Chem. 2007, 9, 927–934.
[24] J. B. Azeredo, M. Godoi, G. M. Martins, C. C. Silveira, A. L.
Braga, J. Org. Chem. 2014, 79, 4125–4130.
[25] P. Sang, Z. K. Chen, J. W. Zoua, Y. H. Zhang, Green Chem.
2013, 15, 2096–2100.
[26] V. A. Stenger, J. Chem. Eng. Data 1996, 41, 1111–1113.
[27] J. M. Richter, B. W. Whitefield, T. J. Maimone, D. W. Lin, M. P.
Castroviejo, P. S. Baran, J. Am. Chem. Soc. 2007, 129, 12857–
12869.
Received: April 22, 2015
Published Online: July 15, 2015
5074
www.eurjoc.org
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2015, 5070–5074