Efficient ligand-free copper-catalyzed C-S cross-coupling of thiols with aryl iodides using KF/Al2O3 as base

Article abstract of DOI:10.1016/j.tetlet.2010.02.155

We report a mild, convenient, environmentally friendly, and ligand-free synthetic protocol for the cross-coupling reaction of aryl iodides and thiols using 10 mol % CuI with KF/Al₂O₃ as the base, in DMF at 110 °C. Using this protocol, we have shown that a variety of aryl sulfides can be synthesized in excellent yields from readily available iodides and thiols.

Full text of DOI:10.1016/j.tetlet.2010.02.155

Tetrahedron Letters 51 (2010) 2489–2492
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Efficient ligand-free copper-catalyzed C–S cross-coupling of thiols
with aryl iodides using KF/Al2O3 as base
Yi-Si Feng a,b, Yuan-Yuan Li a, Lin Tang a, Wei Wu a, Hua-Jian Xu a,b,
*
a School of Chemical Engineering, Hefei University of Technology, Hefei 230009, PR China
b Anhui Key Laboratory of Controllable Chemical Reaction & Material Chemical Engineering, Hefei 230009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report a mild, convenient, environmentally friendly, and ligand-free synthetic protocol for the cross-
coupling reaction of aryl iodides and thiols using 10 mol % CuI with KF/Al2O3 as the base, in DMF at
110 °C. Using this protocol, we have shown that a variety of aryl sulfides can be synthesized in excellent
yields from readily available iodides and thiols.
Received 26 January 2010
Revised 23 February 2010
Accepted 26 February 2010
Available online 2 March 2010
Ó 2010 Elsevier Ltd. All rights reserved.
The formation of C(aryl)–S bonds represents a key step in the
synthesis of many molecules that are of biological, pharmaceutical,
and material interest.1 However, traditional methods for the syn-
thesis of aryl-sulfur bonds often require harsh reaction conditions2
such as high temperature, the usual requirement of stoichiometric
amounts of copper reagents, long reaction times, and low yields.
The development of transition metal-catalyzed coupling has over-
come these difficulties to a great extent. On the other hand, mod-
ern transition-metal (palladium,3 nickel,4 copper,5 cobalt,6 iron,7
and indium8) in combination with appropriate ligands for cata-
lyzed cross-coupling reactions of aryl halides with thiols enable
the synthesis of aryl sulfides in good yields under milder reaction
conditions. However, the high cost and air sensitivity of Pd cata-
lysts and often tedious procedure for the preparation of ligands re-
strict their applications in large-scale processes. On the other hand,
in general, Co and Ni catalysts are associated with toxicity. Thus,
development of alternative inexpensive, ligand-free, non-air sensi-
tive catalysts for this useful reaction is highly desirable in the con-
text of environmental and industrial concerns.
The selection of the correct base is crucial for the success of C–S
coupling processes. This is particularly true with respect to func-
tional group tolerance and reaction rate. Nevertheless, in almost
all documented methods, Cs2CO3, NaOtBu, KOtBu, K2CO3, and
K3PO4 were chosen as bases. In fact, these bases with high sensitiv-
ity to moisture reduce their capability as bases in moisture sensi-
tive reactions. Solid base has enjoyed growing attention due to
its environmentally friendly nature and unique properties. Of a
variety of solid bases, KF/Al2O3 is a typical example. It has been
employed as an effective base in a variety of organic reactions,
many of which are staples of synthetic organic chemistry.9 On
the other hand, KF/Al2O3 is a mild base that is effective for a wide
range of substrates of coupling reactions and it has been applied
extensively in Suzuki reaction10 and C–N and C–O bond forma-
tion.11 However, the application in the formation of C(aryl)–S bond
has not been explored. Herein, we will fill this gap with our devel-
oped protocol.
Table 1
Optimization of reaction conditiona
I
SH
S
[Cu], KF/Al2O3
+
º
Solvent, 110 C, N2
Entry
[Cu]
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
CuI
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
Dioxane
Toluene
DMF
8
8
8
8
8
8
8
8
8
6
4
2
8
8
8
8
8
99
76
76
93
87
67
49
73
25c
94
91
74
95
40c
3c
CuBr
CuCl
Cu2O
CuO
CuBr2
CuSO4
Cu(OAc)2
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
—
50d
5
DMF
a
General reaction conditions: iodobenzene (1.0 mmol), thiophenol (1.2 mmol),
Cu catalyst (0.1 mmol), and 2.5 equiv KF/Al2O3 as base in DMF (2.5 ml) at 110 °C
under N2 for 8 h.
b
GC yield.
c
The reaction was carried out at 80 °C.
The reaction was carried out in air.
* Corresponding author.
E-mail address: hjxu@ustc.edu (H.-J. Xu).
d
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.155

2490
Y.-S. Feng et al. / Tetrahedron Letters 51 (2010) 2489–2492
Table 2
The C–S cross-coupling reaction of aryl iodides with thiols using KF/Al2O3 as basea
I
SR2
10 mol% CuI, 2.5 equiv KF/Al2O3
R1
R1
+ R2SH
º
DMF, 110 C, N2
Entry
1
ArI
R2SH
Product
Yieldb (%)
87
I
I
HS
S
I
HS
HS
HS
S
2
3
60
78
S
S
I
4
5
6
93
99
94
F
F
I
HS
HS
S
Cl
Cl
I
I
S
O2N
O
O2N
O
HS
S
7
8
70
91
I
I
HS
HS
S
O
O
S
9
83
85
H2N
N
H2N
N
I
HS
HS
S
10
S
I
11
99
I
I
HS
HS
S
S
S
12
13
97
91
I
I
HS
HS
14
15
86
62
S
NO2
NO2
F
I
I
I
I
HS
HS
HS
S
S
S
16
17
18
92
87
88
F
Cl
O
Cl
O
HS
HS
S
S
19
20
92
43
I

Y.-S. Feng et al. / Tetrahedron Letters 51 (2010) 2489–2492
2491
Table 2 (continued)
Entry
ArI
R2SH
Product
Yieldb (%)
I
I
I
I
SC8H17
21
22
23
n-C8H17SH
95
75
57
SC12H25
n-C12H25SH
HS
N
S
S
N
N
N
HS
24
64
S
S
a
Reaction conditions: aryl iodide (1 mmol), thiol (1.2 mmol), CuI (0.10 mmol), and KF/Al2O3 (2.5 mmol) in DMF at 110 °C under N2 for 8 h.
Isolated yield.
b
substituted and m-substituted aryl halides (Table 2, entries 1–3).
It is observed that electron-donating substituents in the aryl io-
dides (Table 2, entries 7–9) decreased the yields of the products.
Furthermore, we successfully coupled a variety of readily available
aryl thiols with iodobenzene in good yields. (Table 2, entries 12–
18). As expected, the tolerance of the catalytic system on aryl thiols
counterpart was not remarkable. The reactivity of the substrates
with electron-withdrawing groups was similar to that with elec-
tron-donating groups (Table 2, entries 15–18). We managed to ob-
tain the desired coupling product of aliphatic thiols with aryl
iodide in good yields for the synthesis of phenyl alkyl sulfide (Table
2, entries 19–22). We also extended the scope of thiol substrates to
heterocyclic thiols and desired that the coupling products were
also successfully observed in good yields (Table 2, entries 23 and
24).
Table 3
One-pot fashion synthesis of disulfide with thiols and 1,4-diiodobenzenea
10 mol% CuI
5 equiv KF/Al2O3
DMF, 110oC, N2
I
SR
RSH
I
RS
S
S
R
R
S
S
R = H, 79%a
R = p-Me, 83%a
R = m-Me, 82%a
65%a
a
Yield of the isolated product.
We next turned our attention to apply this protocol to the pro-
cedure of disulfide. Four thiols and 1,4-diiodobenzene (see Table 3,
entries 1–4) were examined. It is interesting to note that the two
iodide groups in 1,4-diiodobenzene can participate in the C–S cou-
pling reaction in a one-pot fashion. The yields are around 65–83%,
which are good enough for many synthetic applications.
The subject of this study was the development of a novel, sim-
ple C–S cross-coupling methodology incorporating the use of com-
mercially available KF/Al2O3 without any ligand. In the first stage
of the study we focused on the reaction of thiophenol with iodo-
benzene. A variety of experimental conditions were examined
(see Table 1). The highest yield (99%) was obtained when
10 mol % CuI was used as the catalyst, 2.5 equiv KF/Al2O3 as the
base, and DMF as the solvent (Table 1, entry 1). Other copper com-
pounds including CuBr, CuCl, Cu2O, CuO, CuBr2, CuSO4, and
Cu(OAc)2 provided lower yields (Table 1, entries 2–8). The control
experiment showed that the corresponding product with only 25%
yield was obtained when the model reaction was performed at
80 °C (Table 1, entry 9). The yield of the product with 94%, 91%,
and 74% was observed when the reaction times were shortened,
respectively, to 6 h, 4 h, and 2 h (Table 1, entries 10–12). DMF
was found to be the best solvent while DMSO, dioxane, and toluene
caused lower yields (Table 1, entries 1, 13–15). In addition, the
yield of the product was reduced to 50% when the reaction was
carried out in air (Table 1, entry 16). Therefore, the optimized con-
ditions employed 10 mol % CuI and 2.5 equiv KF/Al2O3 in DMF at
110 °C under N2. We also found that when KF/Al2O3 was used in
CuI-catalyzed reactions of C–N and C–O bond formation, 1,10-phe-
nanthroline or N,N-dibenzylethylenediamine as ligand was accom-
panied. But, simplicity of no ligand and less reaction time could be
achieved in our protocol compared with other documented
methods.
In conclusion, the use of KF/Al2O3 as a solid base13 in copper-
catalyzed S-arylation of aryl and alkyl thiols with aryl iodide reac-
tions offers a convenient, environmentally benign, ligand-free, and
efficient alternative. We believe that the application to KF/Al2O3
will provide new way to a wide array of organic reactions.
Acknowledgment
This research was financially supported by the National Natural
Science Foundation of China (Nos. 20802015, 20772115).
References and notes
1. (a) Jones, D. N.. In Comprehensive Organic Chemistry; Barton, D. H., Ollis, D. W.,
Eds.; Pergamon: New York, 1979; Vol. 3, (b) Hartwig, J. F. Angew. Chem., Int. Ed.
1998, 37, 2046–2067; (c) Wolfe, J. P.; Wagaw, S.; Marcoux, J. F.; Buchwald, S. L.
Acc. Chem. Res. 1998, 31, 805–818; (d) Hassan, J.; Sevignon, M.; Gozzi, C.;
Schulz, E.; Lemaire, M. Chem. Rev. 2002, 102, 1359–1470; (e) Beletskaya, I. P.;
Cheprakov, A. V. Coord. Chem. Rev. 2004, 248, 2337–2364; (f) Corbet, J. P.;
Mignani, G. Chem. Rev. 2006, 106, 2651–2710.
2. (a) Lindley, J. Tetrahedron 1984, 40, 1433–1456; (b) Sasaki, N. A.; Hashimoto, C.;
Potier, P. Tetrahedron Lett. 1987, 28, 6069–6072; (c) Van Bierbeek, A.; Gingras,
M. Tetrahedron Lett. 1998, 39, 6283–6286.
3. (a) Kosugi, M.; Ogata, T.; Terada, M.; Sano, H.; Migita, T. Bull. Chem. Soc. Jpn.
1985, 58, 3657–3658; (b) Dickens, M. J.; Gilday, J. P.; Mowlem, T. J.;
Widdowson, D. A. Tetrahedron 1991, 47, 8621–8634; (c) Ishiyama, T.; Mori,
M.; Suzuki, A.; Miyaura, N. J. Organomet. Chem. 1996, 525, 225–231; (d) Zheng,
N.; McWilliams, J. C.; Fleitz, F. J.; Armstrong, J. D.; Volante, R. P. J. Org. Chem.
1998, 63, 9606–9607; (e) Mann, G.; Baranano, D.; Hartwig, J. F.; Rheingold, A. L.;
Guzei, I. A. J. Am. Chem. Soc. 1998, 120, 9205–9219; (f) Schopfer, U.; Schlapbach,
A. Tetrahedron 2001, 57, 3069–3073; (g) Li, G. Y. Angew. Chem., Int. Ed. 2001, 40,
1513–1516; (h) Li, G. Y.; Zheng, G.; Noonan, A. F. J. Org. Chem. 2001, 66, 8677–
After the optimized reaction conditions were determined, we
first screened a range of commercially available thiophenols with
a variety of aryl iodides. The corresponding products were ob-
tained in moderate to excellent yields (43–99%) (Table 2, entries
1–11).12 We discovered that the S-arylation reactions of thiophenol
with p-substituted aryl halides were easier than those with o-

2492
Y.-S. Feng et al. / Tetrahedron Letters 51 (2010) 2489–2492
8681; (i) Murata, M.; Buchwald, S. L. Tetrahedron 2004, 60, 7397–7403; (j) Itoh,
10. Kabalka, G. W.; Pagni, R. M.; Hair, C. M. Org. Lett. 1999, 1, 1423–1425.
11. (a) Hosseinzadeh, R.; Tajbakhsh, M.; Mohadjerani, H. Synlett 2004, 1517–1520;
(b) Hosseinzadeh, R.; Tajbakhsh, M.; Mohadjerani, M.; Alikarami, M. Synlett
2005, 1101–1104; (c) Hosseinzadeh, R.; Tajbakhsh, M.; Alikarami, M. Synlett
2006, 2124–2126; (d) Hosseinzadeh, R.; Tajbakhsh, M.; Alikarami, M.
Tetrahedron Lett. 2006, 47, 5203–5205; (e) Hosseinzadeh, R.; Sarrafi, Y.;
Mohadjerani, M.; Mohammadpourmir, F. Tetrahedron Lett. 2008, 49, 840–843.
12. General experimental procedure. All reagents and solvents were pure analytical
grade materials purchased from commercial sources and were used without
further purification. The 1H and 13C NMR spectra were recorded, respectively,
in CDCl3 on a 300 MHz and 75 MHz instrument with TMS as internal standard.
TLC was carried out with 0.2 mm thick silica gel plates (GF254). The columns
were hand packed with silica gel 60 (200–300). All reactions were carried out
in a Schlenk tube equipped with a magnetic stir bar under N2 atmosphere.
A Schlenk tube was charged with CuI (0.1 mmol, 19 mg), KF/Al2O3 (2.5 equiv,
390 mg), and solid substrate, if present. Then liquid reagents (aryl or heteroaryl
halide, 1 mmol; thiol, 1.2 mmol), and solvent (2.5 ml) were added under N2.
The reaction vessel was closed and placed under stirring in a preheated oil bath
at 110 °C. The reaction mixture was stirred for 8 h. The resulting suspension
was cooled to room temperature and filtered through a pad of filter paper with
the help of 10 ml of ethyl acetate. The filtrate was concentrated and the residue
was purified by silica gel chromatography.
T.; Mase, T. Org. Lett. 2004, 6, 4587–4590; (k) Mispelaere-Canivet, C.; Spindler, J.
F.; Perrio, S.; Beslin, P. Tetrahedron 2005, 61, 5253–5259; (l) Fernández-
Rodríguez, M. A.; Shen, Q.; Hartwig, J. F. Chem. Eur. J. 2006, 12, 7782–7796; (m)
Fernández-Rodríguez, M. A.; Shen, Q.; Hartwig, J. F. J. Am. Chem. Soc. 2006, 128,
2180–2181; (n) Barbiéri, R. S.; Bellato, C. R.; Dias, A. K. C.; Massabni, A. C. Catal.
Lett. 2006, 109, 171–174; (o) Cai, L.; Cuevas, J.; Peng, Y. Y.; Pike, V. W.
Tetrahedron Lett. 2006, 47, 4449–4452.
4. (a) Zhang, Y.; Ngeow, K. N.; Ying, J. Y. Org. Lett. 2007, 9, 3495–3499; (b) Saxena, A.;
Kumar, A.; Mozumdar, S. Appl. Catal. A. 2007, 317, 210–215; (c) Jammi, S.; Barua,
P.; Rout, L.; Saha, P.; Punniyamurthy, T. Tetrahedron Lett. 2008, 49, 1484–1487.
5. (a) Palomo, C.; Oiarbide, M.; Lopez, R.; Gomez-Bengoa, E. Tetrahedron Lett. 2000,
41, 1283–1286; (b) Herradura, P. S.; Pendola, K. A.; Guy, R. K. Org. Lett. 2000, 2,
2019–2022; (c) Yee Kwong, F.; Buchwald, S. L. Org. Lett. 2002, 4, 3517–3520; (d)
Bates, C. G.; Gujadhur, R. K.; Venkataraman, D. Org. Lett. 2002, 4, 2803–2806; (e)
Savarin, C.; Srogl, J.; Liebeskind, L. S. Org. Lett. 2002, 4, 4309–4312; (f) Wu, Y. J.;
He, H. Synlett 2003, 1789–1790; (g) Ley, S. V.; Thomas, A. W. Angew. Chem., Int.
Ed. 2003, 42, 5400–5449; (h) Deng, W.; Zou, Y.; Wang, Y. F.; Liu, L.; Guo, Q. X.
Synlett 2004, 1254–1258; (i) Deng, W.; Liu, L.; Guo, Q. X. Chin. J. Org. Chem.
2004, 2, 150–165; (j) Bates, C. G.; Saejueng, P.; Doherty, M. Q.; Venkataraman,
D. Org. Lett. 2004, 6, 5005–5008; (k) Zeni, G. Tetrahedron Lett. 2005, 46, 2647–
2651; (l) Chen, Y. J.; Chen, H. H. Org. Lett. 2006, 8, 5609–5612; (m) Zheng, Y.;
Du, X.; Bao, W. Tetrahedron Lett. 2006, 47, 1217–1220; (n) Zhu, D.; Xu, L.; Wu,
F.; Wan, B. Tetrahedron Lett. 2006, 47, 5781–5784; (o) Verma, A. K.; Singh, J.;
Chaudhary, R. Tetrahedron Lett. 2007, 48, 7199–7202; (p) Ranu, B. C.; Saha, A.;
Jana, R. Adv. Synth. Catal. 2007, 349, 2690–2696; (q) Rout, L.; Sen, T. K.;
Punniyamurty, T. Angew. Chem., Int. Ed. 2007, 46, 5583–5586; (r) Lv, X.; Bao, W.
J. Org. Chem. 2007, 72, 3863–3867; (s) Carril, M.; SanMartin, R.; Domínguez, E.;
Tellitu, I. Chem. Eur. J. 2007, 13, 5100–5105; (t) Rout, L.; Saha, P.; Jammi, S.;
Punniyamurthy, T. Eur. J. Org. Chem. 2008, 4, 640–643; (u) Xu, H. J.; Zhao, X. Y.;
Deng, J.; Fu, Y.; Feng, Y. S. Tetrahedron Lett. 2009, 50, 434–437; (v) Xu, H. J.;
Zhao, X. Y.; Deng, J.; Fu, Y.; Feng, Y. S. Synlett 2008, 3063–3067; (w) Fukuzawa,
S.; Shimizu, E.; Atsuumi, Y.; Haga, M.; Ogata, K. Tetrahedron Lett. 2009, 50,
2374–2376; (x) Feng, Y. S.; Zhao, X. Y.; Wang, J. Y.; Zheng, F. Y.; Xu, H. J. Chin. J.
Chem. 2009, 27, 2423–2425; (y) Jiang, Y.; Qin, Y.; Xie, S.; Zhang, X.; Dong, J.; Ma,
D. Org. Lett. 2009, 11, 5250–5253; (z) Xu, H. J.; Man, Q. S.; Lin, Y. C.; Li, Y. Y.;
Feng, Y. S. Chin. J. Org. Chem. 2010, 30, 9–22; (aa) Firouzabadi, H.; Iranpoor, N.;
Gholinejad, M. Adv. Synth. Catal. 2010, 352, 119–124.
(a) 1,4-Bis(phenylthio)benzene (Table 3, entry 1): 1H NMR (300 MHz, CDCl3) d
7.45–7.33 (m, 5H), 7.31 (s, 2H), 7.27 (d, J = 4.0 Hz, 2.1, 2H), 7.24 (d, J = 9.8 Hz,
6H). 13C NMR (75 MHz, CDCl3): d 135.12, 131.57, 131.28, 129.41, 127.50; (b)
1,4-Bis(m-toluenelthio)benzene (Table 3, entry 2): 1H NMR (300 MHz, CDCl3) d
7.24–7.12 (m, 9H), 7.07 (d, J = 7.1 Hz, 2H), 2.32 (s, 6H). 13C NMR (75 MHz,
CDCl3):
128.45, 21.42; (c) 1,4-Bis(p-toluenethio)benzene (Table 3, entry 3): 1H NMR
(300 MHz, CDCl3) d 7.35–7.22 (m, 4H), 7.13 (d, J = 6.5 Hz, 7H), 2.34 (s, 6H). 13
d 139.31, 135.13, 134.80, 132.29, 131.92, 131.13, 129.26, 128.79,
C
NMR (75 MHz, CDCl3): d 137.91, 135.56, 132.45, 131.12, 130.32, 21.26; (d) 1,4-
Bis(cyclohexylthio)benzene (Table 3, entry 4): 1H NMR (300 MHz, CDCl3) d 7.29
(s, 4H), 3.22–2.93 (m, 2H), 1.98 (d, J = 10.4 Hz, 4H), 1.77 (d, J = 5.2 Hz, 4H), 1.62
(d, J = 6.5 Hz, 2H), 1.47–1.07 (m, 10H). 13C NMR (75 MHz, CDCl3): d 133.75,
132.11, 46.74, 33.45, 26.02, 25.70–25.52; (e) 1,4-Bis(octylthio) benzene (Table
3, entry 5): 1H NMR (CDCl3, 300 MHz): d 7.23 (s, 4H), 2.88 (t, J = 7.3 Hz, 4H),
1.63 (m, J = 6.8 Hz, 4H), 1.40–1.26 (br m, 20H), 0.88 (t, J = 6.4 Hz, 6H). 13C NMR
(75 MHz, CDCl3): d 134.62, 129.85, 34.25, 32.50, 29.32, 29.29, 29.50, 22.84,
14.27.
6. Wong, Y. C.; Jayanth, T. T.; Cheng, C. H. Org. Lett. 2006, 8, 5613–5616.
7. (a) Correa, A.; Carril, M.; Bolm, C. Angew. Chem., Int. Ed. 2008, 47, 2880–2883;
(b) Correa, A.; Mancheño, O. G.; Bolm, C. Chem. Soc. Rev. 2008, 37, 1108–1117.
8. (a) Reddy, V. P.; Swapna, K.; Kumar, A. V.; Rao, K. R. J. Org. Chem. 2009, 74, 3189;
(b) Reddy, V. P.; Kumar, A. V.; Swapna, K.; Rao, K. R. Org. Lett. 2009, 11, 1697.
9. (a) Yamawaki, J.; Ando, T.; Hanafusa, T. Chem. Lett. 1981, 1143–1146; (b) Yadav,
V. K.; Kapoor, K. K. Tetrahedron 1996, 52, 3659–3668; (c) Alloum, B. A.;
Villemin, D. Synth. Commun. 1989, 19, 2567–2571; (d) Kabalka, G. W.; Wang, L.;
Namboodiri, V.; Pagni, R. M. Tetrahedron Lett. 2000, 41, 5151–5154; (e) Blass, B.
E. Tetrahedron 2002, 58, 9301–9320.
13. Other studies also showed the importance of bases in Cu-catalyzed cross-
coupling reactions: (a) Liu, L.; Frohn, M.; Xi, N.; Dominguez, C.; Hungate, R.;
Reider, P. J. J. Org. Chem. 2005, 70, 10135–10138; (b) Zhang, S. L.; Liu, L.; Fu, Y.;
Guo, Q. X. Organometallics 2007, 26, 4546–4554; (c) Yang, C. T.; Fu, Y.; Huang, Y.
B.; Yi, J.; Guo, Q. X.; Liu, L. Angew. Chem., Int. Ed. 2009, 48, 7398–7401; (d) Feng,
Y. S.; Man, Q. S.; Pan, P.; Pan, Z. Q.; Xu, H. J. Tetrahedron Lett. 2009, 50, 2585–
2588; (e) Xu, H. J.; Zheng, F. Y.; Liang, Y. F.; Cai, Z. Y.; Feng, Y. S.; Che, D. Q.
Tetrahedron Lett. 2010, 51, 669–671.