Cu-catalyzed decarboxylative coupling of propiolic acids with boronic acids

Article abstract of DOI:10.1016/j.tetlet.2013.01.118

A mild procedure of Cu-catalyzed decarboxylative cross-coupling of aryl- and alkynyl-boronic acids for construction of unsymmetrical substituted alkynes has been developed. The usage of inexpensive copper chloride as catalyst, and employing stable alkynl carboxylic acids and boronic acids as the substrates under oxidative conditions for sp-sp2 coupling, make this method very easy to operate.

Full text of DOI:10.1016/j.tetlet.2013.01.118

Tetrahedron Letters 54 (2013) 1951–1955
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Tetrahedron Letters
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Cu-catalyzed decarboxylative coupling of propiolic acids with boronic acids
Leilei Shi a,b, Wei Jia b, Xun Li a, , Ning Jiao b,c,
⇑
⇑
a Key Laboratory of Chemical Biology (Ministry of Education), School of Pharmaceutical Sciences, Shandong University, Ji’nan, Shandong 250012, China
b State Key Laboratory of Natural and Biomimetic Drugs, Peking University, Xue Yuan Rd. 38, Beijing 100191, China
c Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A mild procedure of Cu-catalyzed decarboxylative cross-coupling of aryl- and alkynyl-boronic acids for
construction of unsymmetrical substituted alkynes has been developed. The usage of inexpensive copper
chloride as catalyst, and employing stable alkynl carboxylic acids and boronic acids as the substrates
under oxidative conditions for sp–sp2 coupling, make this method very easy to operate.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 21 November 2012
Revised 16 January 2013
Accepted 28 January 2013
Available online 4 February 2013
Keywords:
Decarboxylative coupling
Propiolic acids
Boronic acids
Unsymmetrical alkynes
The unsymmetrical substituted alkyne moiety is a ubiquitous
structural motif of various bioactive natural products and synthetic
pharmaceutical compounds.1 Hence, developing efficient methods
to build unsymmetrical alkynes is a meaningful event.2 Sonogash-
ira reaction is a powerful and straightforward methodology for
construction of aryl alkynes.3 However, there are some deficiencies
in the traditional Sonogashira coupling. For example, homo-
coupling reaction can be observed and electron-deficient alkynes
do not work well by the Sonogashira reaction.4 In the past decades,
many modifications have been developed for the synthesis of
unsymmetrical alkynes, for instance, utilizing 2-methylbut-3-yn-
2-ol5 or silane protected alkynes6 as coupling reagents, allowing
further refinement after the first coupling, thus generating the ex-
pected unsymmetrical alkynes.
Carboxylic acids are largely available, stable, and easy to handle
and store. They have shown great capacity in catalytic transforma-
tions and the protodecarboxylation of carboxylic acids has long ex-
isted in organic synthesis.7,8 Substituted propiolic acids have been
employed in the coupling reaction for Csp–C bond and Csp–X bond
formation.9,10 More recently, Loh and co-workers reported a meth-
od of Pd-catalyzed decarboxylative cross-coupling of alkynyl car-
boxylic acids with arylboronic acids in the presence of Ag2O (1,
Scheme 1).11 Our group once reported a strategy of copper(II) chlo-
ride catalyzed oxidative amidation of propiolic acids under air via
decarboxylative coupling.12 Therefore, we selected propiolic acids
as the alkyne candidates for the construction of unsymmetrical
alkyne. Inspired by Loh’s work and combined with our previous
work, we developed an inexpensive Cu-catalyzed decarboxylative
coupling of propiolic acids with boronic acids (2, Scheme 1). The
advantage of the method is that the copper catalyst is low-toxic,
inexpensive, and readily available.13,14
Firstly, the coupling reaction between phenylpropiolic acid and
(4-methoxyphenyl)boronic acid was investigated and the product
3aa was obtained with the isolated yield of 24% when copper(II)
acetate was used as the catalyst in the presence of oxidant and
pyridine (Table 1, entry1). Then different ligands, bases, oxidants,
catalysts, and solvents were screened. As shown in Table 1, when
2,20-bipyridine and 1,10-phenanthroline were used as ligands, no
products were obtained (Table 1, entries 2 and 3), indicating that
pyridine is important for this coupling reaction. Gratifyingly, when
Et3N was employed as an additional base, the decarboxylative
transformation proceeded more efficiently (Table 1, entry 7). Other
bases were then screened, but the reactions exhibited lower effi-
ciencies (Table 1, entries 4–6, 8). After screening different oxidants,
it was demonstrated that BQ is the most positive oxidant for this
coupling reaction (Table 1, entries 10–13). It was found that a cat-
alytic amount of BQ could reduce the yield (Table 1, cf. entries 7
and 9). Furthermore, the control reaction under Ar gave lower yield
(Table 1, cf. entries 14 and 15). These results indicate that both BQ
and air are required for this decarboxylative coupling. Other Cu-
catalysts such as CuBr2, CuCN, and CuCl2 were screened (Table 1,
entries 16–18). When the loading of pyridine was decreased from
2.0 to 1.0 equiv, the product 3aa was obtained in 65% yield (Table 1,
entry 19). When some diketones were used as ligands, no desired
product was detected (see SI). It is found that CuCl2ꢀ2H2O per-
formed the best catalytic efficiency in dichloromethane (Table 1,
entry 14).
⇑
Corresponding authors. Fax: +86 010 8280 5297.
E-mail addresses: tjulx2004@sdu.edu.cn (X. Li), jiaoning@bjmu.edu.cn (N. Jiao).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.01.118

1952
L. Shi et al. / Tetrahedron Letters 54 (2013) 1951–1955
[Pd]
Ag2O
Loh's work
1
R2
OH
COOH
B
OH
R1
+
R1
R2
[Cu]
BQ
This work
2
Scheme 1.
Table 1
Optimization of reaction conditionsa
OCH3
OCH3
COOH
catalyst, base/ligand
oxidant, DCM
HO
B
+
Ph
OH
Ph
1a
3aa
2a
Entry
Catalyst
Oxidant
Base
Ligand
Yieldb (%)
1
2
3
4
5
6
7
8
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
CuCl2ꢀ2H2O
CuCl2ꢀ2H2O
CuBr2
BQ
BQ
BQ
BQ
BQ
BQ
BQ
BQ
BQ
—
—
—
—
Pyridine
24
0
0
2,20-bipyridine
1,10-phenanthroline
Pyridine
Na2CO3
DABCO
DBU
Et3N
NaHCO3
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
59
Trace
19
70
39
22
Trace
0
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
9c
10
11
12
13
14
15d
16
17
18
19e
20
21f
22g
DDQ
Oxone
O2
0
Trace
84
32
75
0
79
65
12
62
Trace
BQ
BQ
BQ
BQ
BQ
BQ
BQ
BQ
BQ
CuCN
CuCl2
CuCl2ꢀ2H2O
CuCl2ꢀ2H2O
CuCl2ꢀ2H2O
CuCl2ꢀ2H2O
Pyridine
a
Unless otherwise noted, the reaction was carried out at room temperature using phenylpropiolic acid (0.25 mmol), phenylboronic acid (0.5 mmol), base (0.5 mmol),
ligands (0.5 mmol), oxidants (0.25 mmol), catalysts (5 mol %), solvents (3 mL) for 12 h under air atmosphere.
b
Isolated yield.
10 mol % BQ was used.
Reaction under Ar.
Pyridine (1.0 equiv) was used in this case.
Benzene was used as solvent.
DMF was used as the solvent.
c
d
e
f
g
With the optimized conditions in hand, the scope of reaction
substrate was investigated. Various arylboronic acids were used to
examine the tolerance of this coupling reaction. The electronic prop-
erties of the substrates appear to affect the reactivity of coupling.
Typical electron-donating groups on the arylboronic acids such as
methoxyl, tert-butyl gave the desired unsymmetrical alkynes in
good yields respectively (Table 2, entries 1 and 8). However, sub-
strates with strong electron-withdrawing groups at the aryl ring
of the aryboronic acids gave the products in lower yields (Table 2,
entries 5 and 6). It is noteworthy that the halogen group substituted
aryboronic acids also could be converted into the desired products
smoothly in good yields (Table 2, entries 2, 7, and 10).
Then, the compatibility of alkynyl carboxylic acids was tested
under the standard conditions. As displayed in Table 3, all the cou-
pling reactions using different alkynyl carboxylic acids proceeded
smoothly to generate the desired alkynes in moderate to good
yields. Phenylpropiolic acid with meta-methyl group can be
transformed to the corresponding product with good isolated yield
(Table 3, entry 1). However, electron-deficient substrates such as
3-(4-cyanophenyl)propiolic acid performed with low efficiency
(Table 3, entry 6). Hetero-aromatic ring substituted propiolic acid
accomplished the coupling reaction successfully with good yield
(Table 3, entry 2). Aliphatic chain substituted propiolic acids are
also tolerant in this transformation but with low yields (Table 3,
entries 3–5). Although the yields are low to moderate in some
cases, no homocoupling products were detected.
A proposed mechanism of this decarboxylative coupling reac-
tion is shown in Scheme 2. The copper(II) intermediate A is initially
generated. Followed decarboxylation occurs to form the alkynyl
copper(II) intermediate B. Subsequent transmetallation between
arylboronic acids and B occurs to form intermediate C. Finally,
reductive elimination ensures to form the alkyne products.
In summary, a practical Cu-catalyzed decarboxylative cross-
coupling of propiolic acids with boronic acids for the construction

L. Shi et al. / Tetrahedron Letters 54 (2013) 1951–1955
1953
Table 2
Decarboxylative coupling of phenylpropiolic acid with various arylboronic acidsa
BQ (1.0 eq)
Et3N (2.0 eq)
pyridine (2.0 eq)
OH
B
R
COOH
OH
R
+
CuCl2 2H2O (5 mol%)
Ph
Ph
OCH3
Cl
DCM
r.t., air
1a
2
3
Entry
1
Product
Yieldb (%)
Entry
6
Product
Yieldb (%)
CF3
84
80
27
78
3aa
3af
F
2
3
7
8
3ag
3ab
3ac
77
85
CH3
3ah
3ai
H
O
4
5
40
32
9
76
53
3ad
CN
Br
10
3aj
3ae
a
Reaction conditions: 1 (0.25 mmol), 2 (0.5 mmol), CuCl2ꢀ2H2O (0.0125 mmol), BQ (0.25 mmol), Et3N (0.5 mmol), pyridine (0.5 mmol), solvent (3 mL), rt, 12 h. For the
detail, see experimental section.
b
Isolated yields.
Table 3
Decarboxylative coupling of arylboronic acids with alkynyl carboxylic acidsa
BQ (1.0 eq)
Et3N (2.0 eq)
pyridine (2.0 eq)
HO
.
CuCl2 2H2O (5 mol%)
R'
B
R
COOH
+
R'
R
HO
DCM
r.t., air
1
2
3
Entry
1
Product
Yieldb (%)
75
OCH3
3k
H3C
O
OCH3
OCH3
2
3
71
37
3l
H3C
H3C
3m
3n
4
38
(continued on next page)

1954
L. Shi et al. / Tetrahedron Letters 54 (2013) 1951–1955
Table 3 (continued)
Entry
Product
Yieldb (%)
33
5
H3C
3o
NC
F
6
26
3p
a
Reaction conditions: 4 (0.25 mmol), 2 (0.5 mmol), CuCl2ꢀ2H2O (0.0125 mmol), BQ (0.25 mmol), Et3N (0.5 mmol), pyridine (0.5 mmol), solvent (3 mL), rt, 12 h. For the
detail, see experimental section.
b
The yield was isolated.
2004, 6, 4917; (e) Ma, L.; Hu, Q.-S.; Pu, L. Tetrahedron: Asymmetry 1996, 7, 3103; (f)
Melissaris, A. P.; Litt, M. H. J. Org. Chem. 1994, 59, 5818.
COOH
R
6. (a) Shultz, D. A.; Gwaltney, K. P.; Lee, H. J. Org. Chem. 1998, 63, 4034; (b)
Nishihara, Y.; Ikegashira, K.; Hirabayashi, K.; Ando, J.; Mori, A.; Hiyama, T. J. Org.
Chem. 2000, 65, 1780; (c) Nishihara, Y.; Ikegashira, K.; Mori, A.; Hiyama, T.
Chem. Lett. 1997, 1233.
7. For some review, see: (a) Gooßen, L. J.; Lange, P. P.; Dzik, W. I. Chem. Sci. 2012, 3,
2671; (b) Gooßen, L. J.; Rodríguez, N. Chem. Soc. Rev. 2011, 40, 5030; (c)
Gooßen, L. J.; Rodríguez, N.; Gooßen, K. Angew. Chem., Int. Ed. 2008, 47, 3100;
(d) Baudoin, O. Angew. Chem., Int. Ed. 2007, 46, 1373; (e) You, S.-L.; Dai, L.-X.
Angew. Chem., Int. Ed. 2006, 45, 5246; (f) Tunge, J. A.; Burger, E. C. Eur. J. Org.
Chem. 2005, 1715.
8. For some selected examples, see: (a) Shang, R.; Huang, Z.; Xiao, X.; Fu, Y.; Liu, L.
Adv. Synth. Catal. 2012, 354, 2465; (b) Shang, R.; Yang, Z.-W.; Zhang, S.-L.; Liu, L.
J. Am. Chem. Soc. 2010, 132, 14391; (c) Shang, R.; Ji, D.-S.; Chu, L.; Fu, Y.; Liu, L.
Angew. Chem., Int. Ed. 2011, 50, 4470; (d) Shang, R.; Huang, Z.; Chu, L.; Fu, Y.;
Liu, L. Org. Lett. 2011, 13, 4240; (e) Liu, X.; Wang, Z.; Cheng, X.; Li, C. J. Am. Chem.
Soc. 2012, 134, 14330; (f) Wang, Z.; Zhu, L.; Yin, F.; Su, Z.; Li, C. J. Am. Chem. Soc.
2012, 134, 4258; (g) Yin, F.; Wang, Z.; Li, Z.; Li, C. J. Am. Chem. Soc. 2012, 134,
10401; (h) Yoshino, Y.; Kurahashi, T.; Matsubara, S. J. Am. Chem. Soc. 2009, 131,
7494; (i) Wang, C.; Piel, I.; Glorius, F. J. Am. Chem. Soc. 2009, 131, 4194; (j)
Yamashita, M.; Hirano, K.; Satoh, T.; Miura, M. Org. Lett. 2009, 11, 2337; (k) Bi,
H.-P.; Zhao, L.; Liang, Y.-M.; Li, C.-J. Angew. Chem., Int. Ed. 2009, 48, 792; (l)
Gooßen, L. J.; Rodríguez, N.; Linder, C. J. Am. Chem. Soc. 2008, 130, 15248; (m)
Wang, C.; Tunge, J. A. J. Am. Chem. Soc. 2008, 130, 8118; (n) Waetzig, S. R.;
Tunge, J. A. J. Am. Chem. Soc. 2007, 129, 4138; (o) Miao, Y.; Pan, D. L.; Jia, W.;
Chen, W.; Jiao, N. Tetrahedron Lett. 2010, 51, 1287; (p) Zhai, H.; Wei, Y.; Xu, J.;
Kan, J.; Su, W.; Hong, M. J. Org. Chem. 2011, 76, 882; (q) Gooßen, L. J.; Deng, G.;
Levy, L. M. Science 2006, 313, 662.
R
O
A
CuIIX2
BQ and O2
CuII
O
XCuII
Product
CO2
CuIIX
R1
R
C
B
R
R1
B
transmetallation
HO
OH
Scheme 2. Proposed mechanism for this transformation.
of unsymmetrical substituted alkynes has been developed. The
usage of inexpensive copper chloride as catalyst, and employing
stable alkynl carboxylic acids and boronic acids as the substrates
under oxidative conditions for sp–sp2 coupling, make this method
very easy to operate. The application of this transformation in or-
ganic synthesis is ongoing in our laboratory.
9. (a) Tartaggia, S.; Lucchi, O.; Gooßen, L. J. Eur. J. Org. Chem. 2012, 1431; (b)
Palani, T.; Park, K.; Kumar, M. J.; Jung, H. M.; Lee, S. Eur. J. Org. Chem. 2012,
5038; (c) Kim, Y.; Park, A.; Park, K.; Lee, S. Tetrahedron Lett. 2011, 52, 1766; (d)
Park, K.; Bae, G.; Park, A.; Kim, Y.; Choe, J.; Song, K. H.; Lee, S. Tetrahedron Lett.
2011, 52, 576; (e) Park, A.; Park, K.; Kim, Y.; Lee, S. Org. Lett. 2011, 13, 944; (f) Li,
T.; Qu, X.; Zhu, Y.; Sun, P.; Yang, H.; Shan, Y.; Zhang, H.; Liu, D.; Zhang, X.; Mao,
J. Adv. Synth. Catal. 2011, 353, 2731; (g) Kim, H.; Lee, P. H. Adv. Synth. Catal.
2009, 351, 2827; (h) Pan, D.; Zhang, C.; Ding, S.; Jiao, N. Eur. J. Org. Chem. 2011,
4751; (i) Brohmer, M. C.; Mundinger, S.; Brase, S.; Bannwarth, W. Angew. Chem.,
Int. Ed. 2011, 50, 6175; (j) Zhao, D.; Gao, C.; Su, X.; He, Y.; You, J.; Xue, Y. Chem.
Commun. 2010, 46, 9049; (k) Qu, X.; Li, T.; Sun, P.; Zhu, Y.; Yang, H.; Mao, J. Org.
Biomol. Chem. 2011, 9, 6938; (l) Umemoto, T.; Singh, R. P.; Xu, Y.; Saito, N. J. Am.
Chem. Soc. 2010, 132, 18199; (m) Rayabarapu, D. K.; Tunge, J. A. J. Am. Chem. Soc.
2005, 127, 13510; (n) Park, J.; Park, E.; Kim, A.; Park, S.-A.; Lee, Y.; Chi, K.-W.;
Jung, Y. H.; Kim, I. S. J. Org. Chem. 2011, 76, 2214; (o) Moon, J.; Jang, M.; Lee, S. J.
Org. Chem. 2009, 74, 1403; (p) Zhang, W.-W.; Zhang, X.-G.; Li, J.-H. J. Org. Chem.
2010, 75, 5259; (q) Park, A.; Park, K.; Kim, Y.; Lee, S. Org. Lett. 2011, 13, 944; (r)
Acknowledgments
Financial support from National Basic Research Program of
China (973 Program) (Grant No. 2009CB825300) and National
Science Foundation of China (No. 21172006) is greatly appreciated.
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
01.118.
ˇ
Zou, G.; Zhu, J.; Tang, J. Tetrahedron Lett. 2003, 44, 8709; (s) Kolarovic, A.;
Fáberová, Z. J. Org. Chem. 2009, 74, 7199.
References and notes
10. (a) Hu, J.; Zhao, N.; Yang, B.; Wang, G.; Guo, L.-N.; Liang, Y.-M.; Yang, S.-D.
Chem. Eur. J. 2011, 17, 5516; (b) Homsi, F.; Rousseau, G. Tetrahedron Lett. 1999,
40, 1495; (c) Das, J. P.; Roy, S. J. Org. Chem. 2002, 67, 7861; (d) Naskar, D.; Roy, S.
J. Org. Chem. 1999, 64, 6896; (e) Feng, H.; Ermolat’ev, D. S.; Song, G.; Van der
Eycken, E. V. J. Org. Chem. 2012, 77, 5149; (f) Feng, H.; Ermolat’ev, D. S.; Song,
G.; Van der Eycken, E. V. Adv. Synth. Catal. 2012, 354, 505.
1. (a) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605; (b) Tykwinski, R. R. Angew. Chem.,
Int. Ed. 2003, 42, 1566; (c) Chinchilla, R.; Nagera, C. Chem. Rev. 2007, 107, 874;
(d) Tour, J. M. Acc. Chem. Res. 2000, 33, 791; (e) Siemsen, P.; Livingston, R. C.;
Diederich, F. Angew. Chem., Int. Ed. 2000, 39, 2632; (f) Martin, R. E.; Diederich, F.
Angew. Chem., Int. Ed. 1999, 38, 1350.
11. Feng, C.; Loh, T.-P. Chem. Commun. 2010, 46, 4779.
2. (a) Negishi, E.; Kotora, M.; Xu, C. J. Org. Chem. 1997, 62, 8957; (b) Kollhofer, A.;
Pullmann, T.; Plenio, H. Adv. Synth. Catal. 2005, 347, 1295; (c) Gelman, D.;
Buchwald, S. L. Angew. Chem., Int. Ed. 2003, 45, 5993.
3. (a) Sonagashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16, 4467; (b)
Sonagashira, K. J. Organomet. Chem. 2002, 653, 46; (c) Sonogashira, K. In Metal-
Catalyzed Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH:
Weinheim, 1998; p 203; (d) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew.
Chem., Int. Ed. 2005, 44, 4442.
12. Jia, W.; Jiao, N. Org. Lett. 2010, 12, 2000.
13. For some reviews, see: (a) Ley, S. V.; Thomas, A. W. Angew. Chem., Int. Ed. 2003,
42, 5400; (b) Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. Rev. 2004, 104, 3079; (c)
Ma, D.; Cai, Q. Acc. Chem. Res. 2008, 41, 1450; (d) Evano, G.; Blanchard, N.;
Toumi, M. Chem. Rev. 2008, 108, 3054.
14. For some Cu-catalyzed Sonogashira cross-coupling, see: (a) Zuidema, E.; Bolm,
C. Chem. Eur. J. 2010, 16, 4181; (b) Thakur, K. G.; Sekar, G. Synthesis 2009, 2785;
(c) Thakur, K. G.; Jaseer, E. A.; Naidu, A. B.; Sekar, G. Tetrahedron Lett. 2009, 50,
2865; (d) Monnier, F.; Turtaut, F.; Doroure, L.; Taillefer, M. Org. Lett. 2008, 10,
3203; (e) Guan, J. T.; Yu, G.-A.; Chen, L.; Weng, T. Q.; Yuan, J. J.; Liu, S. H. Appl.
Organomet. Chem. 2009, 23, 75; (f) Li, J.-H.; Li, J.-L.; Wang, D.-P.; Pi, S.-F.; Xie, Y.-
X.; Zhang, M.-B.; Hu, X.-C. J. Org. Chem. 2007, 72, 2053; (g) Tang, B.-X.; Wang, F.;
4. Hay, A. S. J. Org. Chem. 1962, 27, 3320.
5. (a) Kang, S.-K.; Yoon, S.-K.; Kim, Y.-M. Org. Lett. 2001, 3, 2697; (b) Chow, H.-F.; Wan,
C.-W.; Low, K.-H.; Yeung, Y.-Y. J. Org. Chem. 1910, 2001, 66; (c) Csékei, M.; Novák, Z.;
Kotschy, A. Tetrahedron2008, 64, 8992;(d) Novák, Z.;Nemes, P.; Kotschy, A. Org. Lett.

L. Shi et al. / Tetrahedron Letters 54 (2013) 1951–1955
1955
Li, J.-H.; Xie, Y.-X.; Zhang, M.-B. J. Org. Chem. 2007, 72, 6294; (h) Ma, D.; Liu, F.
Chem. Commun. 1934, 2004; (i) Colacino, E.; Daïch, L.; Martinez, J.; Lamaty, F.
Synlett 2007, 1279; (j) Thathagar, M. B.; Beckers, J.; Rothenberg, G. Green Chem.
2004, 6, 215; (k) He, H.; Wu, Y. J. Tetrahedron Lett. 2004, 45, 3237; (l) Gujadhar,
R. K.; Bates, C. G.; Venkataraman, D. Org. Lett. 2001, 3, 4315; (m) Okuro, K.;
Furuune, M.; Enna, M.; Miura, M.; Nomura, M. J. Org. Chem. 1993, 58, 4716; (n)
Okuro, K.; Furuune, M.; Mirua, M.; Nomura, M. Tetrahedron Lett. 1992, 33, 5363.