K2S2O8-mediated aerobic oxysulfonylation of olefins into β-Keto sulfones in aqueous media

Article abstract of DOI:10.1002/ejoc.201301833

β-Keto sulfones were obtained in good to excellent yields (73-94%) directly from unactivated olefins by employing inexpensive sodium arenesulfinate salts as sulfonylating agents and K₂S₂O₈ as a radical initiator in open flasks and aqueous media at room temperature under transition-metal-free conditions. The reported oxidative sulfonylation protocol is highly practical and environmentally benign, as self-manifested by the extremely mild reaction conditions. Copyright

Full text of DOI:10.1002/ejoc.201301833

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301833
K2S2O8-Mediated Aerobic Oxysulfonylation of Olefins into β-Keto Sulfones in
Aqueous Media
Ruchi Chawla,[a] Atul K. Singh,[a] and Lal Dhar S. Yadav*[a]
Keywords: Synthetic methods / Radical reactions / Alkenes / Oxygen / β-Keto sulfones
β-Keto sulfones were obtained in good to excellent yields
(73–94%) directly from unactivated olefins by employing in-
expensive sodium arenesulfinate salts as sulfonylating
agents and K2S2O8 as a radical initiator in open flasks and
aqueous media at room temperature under transition-metal-
free conditions. The reported oxidative sulfonylation protocol
is highly practical and environmentally benign, as self-mani-
fested by the extremely mild reaction conditions.
The difunctionalization of alkenes is an important syn-
thetic protocol in organic synthesis. Recently, transition-
Introduction
The contribution of radical chemistry in shaping modern metal-catalyzed oxidative difunctionalization of alkenes
organic synthesis is indubitable. This branch of chemistry with dioxygen has gathered considerable attention, which
has made a significant impact in the areas of organic as has resulted in the development of new methods to achieve
well as polymer chemistry.[1,2] Nevertheless, this lush scene sophisticated synthetic frameworks.[3a–3g] Although an oxy-
of radical reactions in most cases is heralded by extremely sulfonylation reaction of alkenes with dioxygen under tran-
cautious reaction conditions, for example, the use of an in- sition-metal-free conditions in aqueous media would be
ert atmosphere and dry degassed solvents. In contrast, quite advantageous, to the best of our knowledge, β-keto
working within the boundaries of green and sustainable sulfones have never been prepared directly from alkenes in
chemistry is no less than a socio-environmental responsibil- this way. However, the preparation of secondary and terti-
ity for an organic chemist. In this context, functionalization ary β-hydroxy sulfones involving aerobic sulfonylation of
of organic molecules with dioxygen has recently upsurged alkenes with sulfinic acids has been described by Lei et al.
as a hot topic of research.[3] Dioxygen is an ideal source of (Scheme 1, a).[3h] Herein, we disclose a direct oxysulfonyl-
oxygen and cost-effective oxidant that very well meets the ation of alkenes with dioxygen and arenesulfinates in aque-
demands of green chemistry.[4] Its activation for introducing ous media under transition-metal-free conditions to give β-
oxygen-containing functional groups in synthetic chemistry keto sulfones in excellent yields (Scheme 1, b).
is a fascinating and sustainable approach. A number of
transition metals have been used for dioxygen activation,[5]
but examples of such activation under transition-metal-free
conditions are rare.[3h,3i,4h] Also, the avoidance of transition
metals in organic synthesis is a pacing trend these days, as
it minimizes waste and curtails reaction expenses signifi-
cantly. In addition, organic reactions in water are another
aspect of green chemistry that has been adopted by acade-
mia and industry with open arms.[6] Developing a synthetic
protocol that encompasses the advantages of all the afore-
mentioned concepts will be intriguing; in fact, green radical
chemistry in open flasks with water as a solvent under tran-
sition-metal-free conditions can emerge as a state-of-the-art
alternative to existing procedures.
Scheme 1. Transition-metal-free aerobic oxysulfonylation of olefins.
β-Keto sulfones are attractive synthetic scaffolds owing
to their useful biological properties[7] and diverse synthetic
applications ranging from natural products[8] to important
organic compounds.[9] Consequently, β-keto sulfones have
been described as the target of synthesis in numerous re-
ports.[3e,3i,10] Recently, Timoshenko and co-workers re-
viewed their methods of synthesis and their chemical prop-
erties in addition to the applications of this important class
of organic compounds.[11] Among the very recent methods
[a] Green Synthesis Lab, Department of Chemistry, University of
Allahabad,
Allahabad 211002, India
E-mail: ldsyadav@hotmail.com
www.allduniv.ac.in
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301833.
2032
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 2032–2036

K2S2O8-Mediated Aerobic Oxysulfonylation of Olefins
for the synthesis of β-keto sulfones are a copper-catalyzed solvent (Table 1, entries 7, 9, and 10). If the stoichiometric
direct oxysulfonylation of alkenes with dioxygen and sulf- coefficient of K2S2O8 was decreased from 1.0 to 0.5 equiv.
onyl hydrazides reported by Wang et al.[3e] and a dioxygen with respect to 2a, there was considerable decrease in the
triggered radical route starting from terminal alkynes devel- yield (Table 1, entries 7 and 11). However, increasing the
oped by Lie et al.[3i] We also recently disclosed a synthesis ratio produced no increase in the yield (Table 1, entries 7
of β-keto sulfones by using a AgNO3/K2S2O8 catalyst sys- and 12). If 1a was treated with 1.6 equiv. each of 2a and
tem in water as well as in DMF by oxysulfonylation of alk- K2S2O8, moderately superior yields were obtained (Table 1,
enes with dioxygen and arenesulfinates[10i] in contrast to the entry 13).
present work, which is transition-metal-free and uses stoi-
chiometric amounts of reagents in water. The previously re-
Table 1. Optimization of the solvent and additive for the synthesis
ported methods suffer from multistep processes, reagents in
large stoichiometric ratios, and the use of transition metals,
high temperatures, and non-green solvents. The presently
reported strategy for the synthesis of β-keto sulfones easily
overcomes these limitations and widens the scope of our
work on environmentally benign organic synthesis[12] and
sulfinate chemistry.[12b,12f]
of β-keto sulfone 3a.[a]
Entry
Solvent
Additive (equiv.)[d]
Yield[b] [%]
[c]
1
2
3
4
5
6
7
8
CHCl3
DMF
H2O
H2O
H2O
H2O
H2O
H2O
DMF
DMSO
H2O
H2O
H2O
pyridine
pyridine
pyridine
–
DTBP (1.0)
TBHP (1.0)
K2S2O8 (1.0)
(NH4)2S2O8 (1.0)
K2S2O8 (1.0)
K2S2O8 (1.0)
K2S2O8 (0.5)
K2S2O8 (1.5)
K2S2O8 (1.0)
–
–
–
Results and Discussion
[c]
[c]
Recently, Lei et al. reported the generation of sulfonyl
radicals from sulfinic acids in the presence of pyridine and
dioxygen. According to their proposed mechanism, benz-
enesulfinic acid reacts with pyridine (Py) to release free sulf-
inyl anion I, which is further oxidized by dioxygen by sin-
gle-electron transfer (SET) to induce the formation of oxy-
gen-centered radical II in resonance with sulfonyl radical
III (Scheme 2).[3h,3i]
trace[c]
35
23
82
78
70
68
40
9
10
11
12
13
82
94[e]
[a] Reaction conditions: styrene (1a, 0.25 mmol), sodium p-toluene-
sulfinate (2a, 0.25 mmol), additive, and solvent (3 mL) in a flask
open to air; DTBP = di-tert-butyl peroxide, TBHP = tert-butyl hy-
droperoxide. [b] Yield of isolated and purified 3a. [c] Reaction con-
ditions of Lei et al.[3h] [d] Equivalents are given with respect to 2a.
[e] Sodium p-toluenesulfinate (2a, 0.40 mmol) was used.
Scheme 2. Lei’s mechanism for the generation of sulfonyl radical.
We envisioned the generation of radical III directly from
I by employing arenesulfinate salts in place of sulfinic acids
Under the stabilized ecofriendly reaction conditions, oxy-
by using the reaction conditions of Lei et al.[3h] Unfortu- sulfonylation of various alkenes 1 was conducted with so-
nately, we could not obtain any encouraging results with dium arenesulfinates 2, and the results are compiled in
styrene (1a) and sodium p-toluenesulfinate (2a) under Lei’s Table 2. As can be seen from this table, the K2S2O8-medi-
conditions (Table 1, entry 1). We attributed this failure to ated oxysulfonylation was successfully extended to various
the insolubility of the sodium arenesulfinate in less polar aryl alkenes by using sulfinate salts 2a (R = p-CH3) or 2b
solvents. So, we performed the next trial in DMF, a highly (R = H) to afford aryl keto sulfones 3a–m in good to excel-
polar organic solvent, which again fetched only a disap- lent yields. A series of functional groups on the olefins in-
pointing result (Table 1, entry 2). Ultimately, we decided to cluding methoxy, methyl, bromo, chloro, fluoro, and cyano
employ water as a solvent, which could solve the solubility were tolerated under the present mild reaction conditions.
problem as well as add to the green aspects of the protocol. Substituents in the ortho, meta, and para positions of the
Even in aqueous medium, 2a in the presence of pyridine phenyl ring of the alkenes did not produce any appreciable
failed to produce product 3a from 1a in an open flask even change in the yield of products 3d–h. Vinylnaphthalene was
after 18 h (Table 1, entry 3). Upon conducting a trial in also compatible with the reaction, and it led to β-keto sulf-
water under pyridine-free conditions, β-keto sulfone 3a was one 3m in 81% yield. However, if aliphatic or alicyclic alk-
formed in a trace amount (Table 1, entry 4). This led us enes such as 1-butene, 1-octene, or cyclohexene were used
to employ additives that could facilitate radical formation as substrates, the desired β-keto sulfones were not obtained,
followed by oxygenation reaction. The results of the screen- which may be attributed to the generation of unstable inter-
ing of the additives, solvents, and optimization of the stoi- mediates.[3h] Also, in the case of an activated alkene, for
chiometric ratios are summarized in Table 1. K2S2O8 example, methyl acrylate, the Michael addition product,
proved to be the best additive for the transformation methyl 3-tosylpropanoate, was obtained instead of the β-
(Table 1, entries 5–8). Water remained the best choice as keto sulfone. Subsequently, we explored the scope of the
Eur. J. Org. Chem. 2014, 2032–2036
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2033

R. Chawla, A. K. Singh, L. D. S. Yadav
SHORT COMMUNICATION
Table 2. Exploring the substrate scope for the synthesis of β-keto reaction for the synthesis of β-keto sulfones 3n–r by using
sulfones 3a.[a]
a set of sodium arenesulfinates 2 (Table 2). A variety of sulf-
inate salts bearing either an electron-donating group (R =
Me or OMe) or an electron-withdrawing group (R = Br, Cl
or F) on the aryl ring reacted smoothly with 1a (Table 2).
Even the bulky 2-naphthylsulfinate salt produced keto sulf-
one 3r efficiently (76%) under the reaction conditions.
Prop-1-enylbenzene, an internal alkene, also proved to be a
suitable reaction partner in this protocol; it afforded desired
3s in a decent yield.
To gain insight into the reaction mechanism, a series of
experiments were conducted. Bearing in mind that acti-
vation of dioxygen mostly involves radical processes,[3] radi-
cal trapping experiments were performed. Under the opti-
mized reaction conditions, styrene did not produce keto
sulfone 3a in the presence of (2,2,6,6-tetramethylpiperidin-
[a] Reaction conditions: A mixture of olefin 1 (0.25 mmol), K2S2O8
(0.40 mmol), sodium sulfinate 2 (0.40 mmol), and water (3 mL) was
taken in a flask open to air and stirred at room temperature for
18 h. [b] Yield of isolated and purified 3.
Scheme 3. Preliminary mechanistic investigations.
Scheme 4. Plausible mechanism for the formation of β-keto sulfones.
2034
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 2032–2036

K2S2O8-Mediated Aerobic Oxysulfonylation of Olefins
1-yl)oxidanyl (TEMPO), which hinted at a radical pathway
(Scheme 3, a). Next, we were interested in knowing the
source of oxygen in the carbonyl group of the β-keto sulf-
ones. Upon performing the reaction in water under an at-
mosphere of nitrogen, the reaction was considerably in-
hibited (Scheme 3, b). Furthermore, the reaction between
1a and 2a in the presence of 18O2/N2 afforded 18O-labeled
product 3aЈ in 89% yield, and this confirmed that the carb-
onyl oxygen atom of the β-keto sulfone originated from di-
oxygen (Scheme 3, c). Also, it was concluded that a β-
hydroxy sulfone was not the intermediate in the reaction,
as representative 1-phenyl-2-tosylethanol was not oxidized
to β-keto sulfone 3a under the optimized reaction condi-
tions (Scheme 3, d). However, if K2S2O8 was used in excess
amount, a small amount of oxidized product was formed.
A postulated mechanistic scenario that is consistent with
the experimental data and literature precedent[3f,3h,13] is de-
picted in Scheme 4. The K2S2O8-mediated formation of a
sulfonyl radical takes place from the sulfinate anion. This
radical attacks the double bond of the olefin to furnish
carbon-centered radical 6, which is ultimately captured by
Acknowledgments
The authors sincerely thank SAIF, Punjab University, Chandigarh,
for providing microanalyses and spectra. A. K. S. is thankful to the
Council of Scientific and Industrial Research (CSIR) for the award
of a Junior Research Fellowship [File Number 09/001(0379)/2013-
EMR-I], and R. C. is thankful to the Department of Science and
Technology (DST) (File Number SR/S1/OC-22/2010) for the award
of a Senior Research Fellowship.
[1] a) B. Giese (Ed.), Radicals in Organic Synthesis: Formation of
Carbon–Carbon Bonds, Pergamon, Oxford, UK, 1986; b) D. P.
Curran, N. A. Porter, B. Giese (Eds.), Stereochemistry of Radi-
cal Reactions, Wiley-VCH, Weinheim, Germany, 1996; c) P. Re-
naud, M. P. Sibi (Eds.), Radicals in Organic Synthesis Wiley-
VCH, Weinheim, Germany, 2001; d) A. Gansäuer (Ed.), Topics
in Current Chemistry, vol. 263: Radicals in Synthesis I, Springer,
Berlin, 2006; e) A. Gansäuer (Ed.), Topics in Current Chemis-
try, vol. 264: Radicals in Synthesis II, Springer, Berlin, 2006.
[2] For recent reviews, see: a) C. Chatgilialoglu, D. Crich, M. Ko-
matsu, I. Ryu, Chem. Rev. 1999, 99, 1991; b) A. Gansäuer, H.
Bluhm, Chem. Rev. 2000, 100, 2771; c) T. R. Rheault, M. P.
Sibi, Synthesis 2003, 803; d) M. P. Sibi, S. Manyem, J. Zimmer-
man, Chem. Rev. 2003, 103, 3263.
oxygen to form oxygen-centered radical 7. Radical 7 may [3] a) B. C. Giglio, V. A. Schmidt, E. J. Alexanian, J. Am. Chem.
follow two possible pathways to form different products.
Soc. 2011, 133, 13320; b) H. Wang, Y. Wang, D. Liang, L. Liu,
J. Zhang, Q. Zhu, Angew. Chem. Int. Ed. 2011, 50, 5678; Angew.
Path A involves the use of sulfinic acid and pyridine and
Chem. 2011, 123, 5796; c) Z.-Q. Wang, W.-W. Zhang, L.-B.
leads to β-hydroxy sulfones,[3h] whereas path B employs so-
Gong, R.-Y. Tang, X.-H. Yang, Y. Liu, J.-H. Li, Angew. Chem.
dium sulfinate in aqueous media, and this could be a
plausible route for the formation of β-keto sulfones 3 in our
case.[3e] A study of the detailed mechanism is underway.
Int. Ed. 2011, 50, 8968; Angew. Chem. 2011, 123, 9130; d) W.
Wei, J. X. Ji, Angew. Chem. Int. Ed. 2011, 50, 9097; Angew.
Chem. 2011, 123, 9263; e) W. Wei, C. Liu, D. Yang, J. Wen, J.
You, Y. Suo, H. Wang, Chem. Commun. 2013, 49, 10239; f) A.
Deb, S. Manna, A. Modak, T. Patra, S. Maity, D. Maiti, An-
gew. Chem. Int. Ed. 2013, 52, 9747; Angew. Chem. 2013, 125,
9929; g) Y. Su, X. Sun, G. Wu, N. Jiao, Angew. Chem. Int. Ed.
2013, 52, 9808; Angew. Chem. 2013, 125, 9990; h) Q. Lu, J.
Zhang, F. Wei, Y. Qi, H. Wang, Z. Liu, A. W. Lei, Angew.
Chem. Int. Ed. 2013, 52, 7156; Angew. Chem. 2013, 125, 7297;
i) Q. Lu, J. Zhang, G. Zhao, Y. Qi, H. Wang, A. W. Lei, J. Am.
Chem. Soc. 2013, 135, 11481.
Conclusions
In conclusion, a very mild and ecofriendly protocol was
developed for the synthesis of β-keto sulfones directly from
olefins by employing inexpensive, commercially available
sodium arenesulfinates through K2S2O8-mediated aerobic
oxysulfonylation in aqueous media. The facile formation of
new C=O and C–S bonds takes place through a radical
pathway in a one-pot procedure. Further work to exploit
the potential of the present strategy is ongoing in our labo-
ratory.
[4] For some reviews on reactions performed with the use of mo-
lecular oxygen as an oxidant, see: a) S. S. Stahl, Science 2005,
309, 1824; b) S. S. Stahl, Angew. Chem. Int. Ed. 2004, 43, 3400;
Angew. Chem. 2004, 116, 3480; c) B. M. Stoltz, Chem. Lett.
2004, 33, 362; d) M. J. Schultz, M. S. Sigman, Tetrahedron
2006, 62, 8227; e) J. Muzart, Chem. Asian J. 2006, 1, 508; f)
M. S. Sigman, D. R. Jensen, Acc. Chem. Res. 2006, 39, 221;
g) E. M. Beccalli, G. Broggini, M. Martinelli, S. Sottocornola,
Chem. Rev. 2007, 107, 5318; h) J. Piera, J.-E. Bäckvall, Angew.
Chem. Int. Ed. 2008, 47, 3506; Angew. Chem. 2008, 120, 3558;
i) T. Punniyamurthy, S. Velusamy, J. Iqbal, Chem. Rev. 2005,
105, 2329; j) Z. Shi, C. Zhang, C. Tang, N. Jiao, Chem. Soc.
Rev. 2012, 41, 3381; k) A. E. Wendlandt, A. M. Suess, S. S.
Stahl, Angew. Chem. Int. Ed. 2011, 50, 11062; Angew. Chem.
2011, 123, 11256.
[5] For selected examples, see: a) Y.-H. Zhang, J.-Q. Yu, J. Am.
Chem. Soc. 2009, 131, 14654; b) A. Wang, H. Jiang, J. Am.
Chem. Soc. 2008, 130, 5030; c) E. A. Lewis, W. B. Tolman,
Chem. Rev. 2004, 104, 1047; d) L. M. Mirica, X. Ottenwaelder,
T. D. P. Stack, Chem. Rev. 2004, 104, 1013; e) C. Zhang, N.
Jiao, J. Am. Chem. Soc. 2010, 132, 28; f) H. Wang, Y. Wang,
C. Peng, J. Zhang, Q. Zhu, J. Am. Chem. Soc. 2010, 132, 13217;
g) S. Chiba, L. Zhang, J.-Y. Lee, J. Am. Chem. Soc. 2010, 132,
7266; h) K. K. Toh, Y.-F. Wang, E. P. J. Ng, S. Chiba, J. Am.
Chem. Soc. 2011, 133, 13942; i) Y.-F. Wang, H. Chen, X. Zhu,
S. Chiba, J. Am. Chem. Soc. 2012, 134, 11980; j) D. Chan-
dra Mohan, S. N. Rao, S. Adimurthy, J. Org. Chem. 2013, 78,
1266; k) T. Ohshima, M. Sodeoka, M. Shibasaki, Tetrahedron
Experimental Section
General Procedure for the One-Pot Synthesis of β-Keto Sulfones 3: A
mixture of olefin 1 (0.25 mmol), K2S2O8 (0.40 mmol), and sodium
sulfinate 2 (0.40 mmol) in water (3 mL) was stirred at room tem-
perature in an open flask for 18 h (Table 2). Upon completion of
the reaction (monitored by TLC), the mixture was extracted with
EtOAc (3ϫ 5 mL). The combined organic phase was dried with
anhydrous Na2SO4, filtered, and concentrated under reduced pres-
sure. The resulting crude product was purified by silica gel column
chromatography (EtOAc/n-hexane, 1:4) to afford an analytically
pure sample of β-keto sulfone 3 (Table 2).
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, characterization data, and copies of
1
the H NMR and 13C NMR spectra.
Eur. J. Org. Chem. 2014, 2032–2036
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2035

R. Chawla, A. K. Singh, L. D. S. Yadav
SHORT COMMUNICATION
Lett. 1993, 34, 8509; l) S. Tategami, T. Yamada, H. Nishino,
K. Kurosawa, Tetrahedron Lett. 1990, 31, 6371; m) C. Zhang,
Z. Xu, T. Shen, G. Wu, L. Zhang, N. Jiao, Org. Lett. 2012, 14,
2362; n) S. O. Kim, C. V. Sastri, M. S. Seo, J. Kim, W. Nam, J.
Am. Chem. Soc. 2005, 127, 4178; o) B. Li, A.-H. Liu, L.-N.
He, Z.-Z. Yang, J. Gao, K.-H. Chen, Green Chem. 2012, 14,
130; p) K. Hasan, N. Brown, C. M. Kozak, Green Chem. 2011,
13, 1230.
Holmquist, E. J. Roskamp, Tetrahedron Lett. 1992, 33, 1131; g)
N. Samakkanad, P. Katrun, T. Techajaroonjit, S. Hlekhlai, M.
Pohmakotr, V. Reutrakul, T. Jaipetch, D. Soorukram, C. Ku-
hakarn, Synthesis 2012, 1693; h) C. Lai, C. Xi, Y. Jiang, R.
Hua, Tetrahedron Lett. 2005, 46, 513; i) A. K. Singh, R.
Chawla, L. D. S. Yadav, J. Mol. Catal. A: Chem. 2014, manu-
script under review.
[11]
[12]
Y. M. Markitanov, V. M. Timoshenko, Y. G. Shermolovich, J.
Sulfur Chem. 2014, 35, 188.
[6] a) C.-J. Li, T.-H. Chan (Eds.), Organic Reactions in Aqueous
Media, John Wiley and Sons, New York, 1997; b) P. A. Grieco
(Ed.), Organic Synthesis in Water, Blackie Academic and Pro-
fessional, London, England, 1998.
[7] C. Curti, M. Laget, A. O. Carle, A. Gellis, P. Vanelle, Eur. J.
Med. Chem. 2007, 42, 880.
[8] H. Yang, R. G. Carter, L. N. Zakharov, J. Am. Chem. Soc.
2008, 130, 9238.
[9] A. Kumar, M. K. Muthyala, Tetrahedron Lett. 2011, 22, 1287,
and references cited therein.
[10] a) G. E. Vennstra, B. Zwaneburg, Synthesis 1975, 519; b) Y.-
Y. Xie, Z.-C. Chen, Synth. Commun. 2001, 31, 3145; c) A. R.
Katritzky, A. A. Abdel-Fattah, M. Y. Wang, J. Org. Chem.
2003, 68, 1443; d) B. M. Trost, D. P. Curran, Tetrahedron Lett.
1981, 22, 1287; e) T. Zweifel, M. Nielsen, J. Overgaard, C. B.
Jacobsen, K. A. Jørgensen, Eur. J. Org. Chem. 2011, 47; f) C. R.
a) A. K. Singh, R. Chawla, A. Rai, L. D. S. Yadav, Chem. Com-
mun. 2012, 48, 3766; b) R. Chawla, R. Kapoor, A. K. Singh,
L. D. S. Yadav, Green Chem. 2012, 14, 1308; c) A. K. Singh,
L. D. S. Yadav, Synthesis 2012, 591; d) R. Chawla, A. K. Singh,
L. D. S. Yadav, Tetrahedron Lett. 2012, 53, 3382; e) A. K.
Singh, R. Chawla, L. D. S. Yadav, Synthesis 2012, 2353; f) R.
Chawla, A. K. Singh, L. D. S. Yadav, Tetrahedron 2013, 69,
1720; g) R. Chawla, A. K. Singh, L. D. S. Yadav, Synlett 2013,
24, 1558; h) A. K. Singh, R. Chawla, L. D. S. Yadav, Tetrahe-
dron Lett. 2013, 54, 5099.
[13] Y.-M. Li, X.-H. Wei, X.-A. Lia, S.-D. Yang, Chem. Commun.
2013, 49, 11701.
Received: December 9, 2013
Published Online: February 20, 2014
2036
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 2032–2036