In situ formed bifunctional primary amine-imine catalyst for asymmetric aldol reactions of α-keto esters

Article abstract of DOI:10.1002/chem.201100200

In prime condition: The hydrolysis of a chiral diimine precursor can be carried out by a Bronsted acid to form an in situ primary amine-imine intermediate as a bifunctional primary aminocatalyst, which promoted direct asymmetric aldol reactions between α-

Full text of DOI:10.1002/chem.201100200

COMMUNICATION
DOI: 10.1002/chem.201100200
In Situ Formed Bifunctional Primary Amine–Imine Catalyst for Asymmetric
Aldol Reactions of a-Keto Esters
Xi Zhu,[a] Aijun Lin,[a] Ling Fang,[a] Wei Li,[b] Chengjian Zhu,*[a] and Yixiang Cheng*[a]
Asymmetric organocatalysis has become the main focus
of research in asymmetric synthesis since the independent
pioneering work of List, Barbas, and MacMillan.[1] Among
the different kinds of organocatalysts, chiral secondary
Scheme 1. Hydrolysis equilibrium conversion of diimine and primary
amine–imine.
amines have been extensively used as amino catalysts in the
past several years.[2] Recently, there has been growing inter-
est in chiral primary amine catalysts because they are effec-
tive promoters for aldol reactions,[3] Michael additions,[4] a-
aminations,[5] and cycloaddition reactions.[6] In asymmetric
synthesis catalyzed by primary amines, the amido moiety is
normally employed to form the enamine intermediates,
which activate the nucleophile and facilitates the reaction.[7]
Furthermore, the electronic effect and steric shielding are
commonly utilized to afford a mutifunctional environment
and improve the stereoselectivity. Chemists have succeeded
in developing several mutifunctional primary amine catalysts
for asymmetric synthesis. Jacobsen and Tsogoeva independ-
ently reported chiral primary amine–thiourea organocata-
lysts in 2006.[8] Luo and Cheng successfully developed a new
chiral primary–tertiary diamine for aldol reactions in 2007.[9]
Despite these significant advancements,[10] the chiral primary
aminocatalysts can only be obtained by complex synthesis
procedures, which limited their further applications. There-
fore, it is of significance to develop more facile syntheses of
chiral primary aminocatalysts.
converted to a C2-symmetric diimine (Scheme 1); therefore,
methods to isolate and take advantage of the primary
amine–imine were challenging. In our study, we found that
the diimine (such as the salen ligand) was easy to hydrolyze
in acidic conditions and convert to the primary amine–
imine. In the preliminary investigation, chiral diimine 1d
was prepared and acidified by AcOH in CH2Cl2 (Scheme 2).
Scheme 2. Structure of diimines.
Delightfully, the non-C2-symmetric primary amine–imine
could be detected by analysis of the mixture by using ESI-
MS (Figure 1a). Other diimines (1a–1c and 1e) also gave
the similar results, and the data remained unchanged even
after the mixtures had been stirred for five days. This
showed that the primary amine–imine could be prepared
from the diimine and was well-tolerated in acidic conditions.
Inspired by this finding, we herein report the in situ prep-
aration of a series of novel bifunctional primary amine–
imine catalysts from chiral diimine precusors. The bifunc-
tional primary amine–imine catalysts can exhibit excellent
enantioselectivities for the direct asymmetric aldol reactions
of ketones with a-keto esters in high yields.
The aldol reaction of methyl phenylglyoxylate and ace-
tone was selected as model reaction with diimine 1a as pre-
catalyst. The results are summarized in Table 1. Initially, the
reaction failed to proceed without any additive (Table 1,
entry 1). Surprisingly, when compound 1a (10 mol%) was
added with AcOH (0.1 mL), the desired product was afford-
ed in 79% yield with 65% enantiomeric excess (ee) under
solvent-free conditions at 08C (Table 1, entry 2). Chiral di-
imines 1b–1e were also tested and 1d showed the best
enantioselectivity of 83% ee (Table 1, entries 3–6). The
Although the C2-symmetric chiral salens are among the
most widely studied ligands in asymmetric catalysis,[11] the
non-C2-symmetric chiral primary amine–imine has never
been employed in chemistry. To the best of our knowledge,
a reliable strategy to prepare a stable non-C2-symmetric pri-
mary amine–imine has not been reported.[12] Due to the
thermal instability, the primary amine–imine was inevitably
[a] X. Zhu, A. Lin, L. Fang, Prof. Dr. C. Zhu, Prof. Dr. Y. Cheng
School of Chemistry and Chemical Engineering
State Key Laboratory of Coordination Chemistry
Nanjing University, Nanjing, 210093 (China)
Fax : (+86)25-83594886
E-mail: cjzhu@nju.edu.cn
yxcheng@nju.edu.cn
[b] Dr. W. Li
National Standard Laboratory of Pharmacology
for Chinese Medicine
Research Institute of Chinese Medicine
Nanjing University of Chinese Medicine, Nanjing, 210029 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100200.
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8281

that steric hindrance and the
hydrophilic group were crucial
for high enantioselectivity.
To further improve the enan-
tioselectivity, the effects of re-
action temperature and the cat-
alyst loading were investigated.
As summarized in Table 2, low-
ering the temperature to
À208C led to an improvement
of the enantioselectivity but a
slight loss of yield (Table 2,
entry 2). Further lowering of
the temperature caused very
poor
reactivity
(Table 2,
entry 3). Decreasing catalyst
loading from 20 to 2 mol% pro-
vided comparable enantioselec-
tivities but had a considerable
negative impact on the yield
(Table 2, entries 2 and 4–6).
The loading of AcOH was also
studied; 24 equiv of AcOH to
methyl phenylglyoxylate was
found to be suitable for the re-
Figure 1. Mass spectra (ESI-MS) of mixture of 1d with AcOH.[13] a) In
CH2Cl2 b) In acetone.
Table 2. Optimization of the reaction conditions.
Table 1. Direct aldol reaction of acetone and methyl phenylglyoxylate.
Entry[a]
T
[8C]
1d
[mol%]
t
AcOH
[equiv][b]
Yield
[%][c]
ee
G
[h]
N
[%][d]
Entry[a]
Precatalyst
Additive
Yield
[%][b]
ee
[%][c]
1
2
3
4
5
6
7
8
0
10
10
10
2
48
72
120
96
96
72
72
72
72
72
18
18
18
18
18
18
21
24
27
30
86
80
33
19
45
86
81
85
80
74
83
93
93
94
92
86
92
96
93
88
À20
À30
À20
À20
À20
À20
À20
À20
À20
1
2
3
4
5
6
7
8
1a
1a
1b
1c
1d
1e
1d
1d
1d
1d
–
–
79
84
82
86
84
60
64
–
65
70
72
83
77
73
70
AcOH
AcOH
AcOH
AcOH
AcOH
HCOOH
TFA
TfOH
PhCOOH
–/AcOH
5
20
10
10
10
10
9
10
9
10
11
trace
trace
87/86
n.d.[d]
n.d.[d]
41/43
[a] Carried out with methyl phenylglyoxylate (0.1 mmol) in acetone
(0.3 mL). [b] Equiv of AcOH to methyl phenylglyoxylate. [c] Yield of iso-
lated product. [d] Determined by chiral HPLC.
ACHTUNGTRENNUNG(R, R)-diaminocyclohexane
[a] Carried out with methyl phenylglyoxylate (0.1 mmol), Brønsted acid
(18 equiv to methyl phenylglyoxylate) and 10 mol% loading of diimine
in acetone (0.3 mL) at 08C for 48 h. TFA=trifluoroacetic acid. TfOH=
trifluoromethanesulfonic acid. [b] Yield of isolated product. [c] Deter-
mined by chiral HPLC. The absolute configuration was established as S
by comparison to the literature.[14] [d] n.d.=Not determined.
action scale (Table 2, entry 8). It appeared that the amount
of AcOH might affect the hydrolysis reaction equilibrium of
the chiral diimine. Optimization of the reaction conditions
revealed that a 10 mol% loading of 1d and AcOH
(24 equiv) added in acetone (0.3 mL) at À208C afforded the
best reactivity (85% yield) and enantioselectivity (96% ee)
in 72 h. The absolute configuration of the product was deter-
mined to be S by comparison with the optical rotation of its
known counterpart.[14]
effect of the additive was also investigated. Among the
Brønsted acids probed (Table 1, entries 5 and 7–10), it ap-
peared that AcOH was the most effective, achieving the
highest enantioselectivity. In addition, (R, R)-diaminocyclo-
hexane was also investigated as the catalyst but poor enan-
tioselectivity was obtained (41% ee), even with the addition
of AcOH (43% ee, Table 1, entry 11). The results indicated
With the optimized conditions, the substrate generality
was investigated. In general, the aldol reactions proceeded
smoothly with a wide range of a-keto esters and ketones to
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Chem. Eur. J. 2011, 17, 8281 – 8284

In Situ Formed Bifunctional Primary Amine–Imine Catalyst
COMMUNICATION
generate the corresponding products with high enantioselec-
tivities (Table 3). The methyl, ethyl, isopropyl, and cyclohex-
yl phenylglyoxylate underwent the reaction with acetone in
high yields and 91–96% ee (Table 3, entries 1–4). a-Keto
esters that had electron-donating substituents (Table 3, en-
tries 5–10) and electron-withdrawing substituents (Table 3,
entries 11–14) on the aromatic rings were introduced and
well-tolerated. The results revealed that the electronic
termediates were detected significantly in the spectra (Fig-
ure 1b). Compared with the catalysts 1a–1c (Table 1), 1d
showed an obviously higher enantioselectivity. On the other
hand, catalyst 1e with protection of the alcohol gave some
loss of ee value, showing that the alcohol is a key moiety for
high enantioselectivity. The structure of 1d was considered
to contribute to the enantio-induction not only due to steric
hindrance, but also due to the electronic effect (hydrogen
bonding with the a-keto ester).
Based on this result, we pro-
posed a mechanism of the aldol
reaction as depicted in
Table 3. Calalyst 1d-promoted asymmetric aldol reaction of ketones with a-keto esters under optimum condi-
tions.
Scheme 3. The chiral diimine
was hydrolyzed under acidic
conditions and converted to pri-
mary amine–imine (A), which
promotes the aldol reaction as
3
t
Yield
[%][b]
ee
Entry[a]
2
Product
R3
R4
[h]
[%][c]
a
bifunctional organocatalyst
1
2
3
4
5
6
7
8
acetone
acetone
acetone
acetone
acetone
acetone
acetone
acetone
acetone
acetone
acetone
acetone
acetone
acetone
acetone
acetone
Ph
Ph
Ph
Ph
Me
Et
iPr
72
72
72
96
96
96
96
96
96
96
72
72
72
72
72
96
96
96
96
4a
4b
4c
4d
4e
4 f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
85
82
75
81
88
86
82
79
76
82
84
86
87
85
71
65
81
82
67
96 (S)[d]
96
96
91
95
97
95
96
87
through the enamine. The inter-
action between ketone and
amine gives the active enamine
(B), which activates the a-keto
ester by hydrogen bonding (C).
The aromatic moiety and the
active enamine are bent up-
wards and downwards respec-
tively. The active enamine is
much more accessible to attack
the methyl phenylglyoxylate
from the Re face, affording the
major stereoisomer.
In conclusion, we have devel-
oped a new bifunctional pri-
mary amine–imine catalyst,
which is prepared in situ from a
chiral diimine under acidic con-
ditions. The catalyst promotes
the direct aldol reactions be-
tween ketones and a-keto
esters in high yields and with
excellent enantioselectivity. Ef-
cyclohexyl
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
2-CH3C6H4
3-CH3C6H4
4-CH3C6H4
3,4-(CH3)2C6H4
4-CH3OC6H4
9
10
11
12
13
14
15
16
17
18
19
2,5-(CH3O)2C6H4
3-ClC6H4
4-ClC6H4
4-BrC6H4
4-FC6H4
2-Naphthyl
2-Thiophenyl
Ph
99
96
96
96
96
94
95
87
2-butanone
cyclohexanone
acetophenone
Ph
Ph
89/93[e]
87[f]
4s
[a] Unless otherwise noted, the reaction was carried out with methyl phenylglyoxylate (0.1 mmol), AcOH
(24 equiv to methyl phenylglyoxylate), and 10 mol% loading of 1d in ketone (0.3 mL) at À208C. [b] Yield of
isolated product. [c] Determined by chiral HPLC. [d] The absolute configurations were established as S by
comparison to the literature.[14] [e] Diastereomeric ratio (d.r.)=2.6:1, for major product (89% ee) and for
minor product (93% ee). The absolute configuration of the major diastereoisomer was established as (S, R) by
comparison to the literature.[14] [f] The reaction was carried out at 08C with CH2Cl2 (0.3 mL) to dissolve aceto-
phenone.
nature of the aromatic ring has no effect on the enantiose-
lectivity. Methyl 2-(2,5-dimethoxyphenyl)-2-oxoacetate was
examined for the first time as a substrate and showed the
best enantioselectivity of 99% ee (Table 3, entry 10). The a-
keto ester with a naphthyl in the b position showed good
enantioselectivity but the reactivity was inferior to other a-
keto esters (Table 3, entry 16). The heteroaromatic com-
pound methyl 2-oxo-2-(tetrahydrothiophen-2-yl) acetate was
also a good aldol acceptor and gave the product with high
enantioselectivity and only
a small decrease in yield
(Table 3, entry 16). Other ketones were tested and provided
the products with a slight decrease in yield and enantioselec-
tivity (Table 3, entries 17–19).
For further demonstration of the reaction process, diimine
1d was acidified by AcOH in acetone and the ESI-MS anal-
ysis was used to study the reaction mixture. The enamine in-
Scheme 3. Proposed catalytic mechanism.
Chem. Eur. J. 2011, 17, 8281 – 8284
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8283

C. Zhu, Y. Cheng et al.
2006, 348, 826–832; d) Z. G. Yang, J. Liu, X. H. Liu, Z. Wang, X. M.
Feng, Z. S. Su, C. W. Hu, Adv. Synth. Catal. 2008, 350, 2001–2006.
[5] T.-Y. Liu, H. L. Cui, Y. Zhang, K. Jiang, W. Du, Z.-Q. He, Y. C.
Chen, Org. Lett. 2007, 9, 3671–3674.
forts are focused on a more detailed mechanism and further
application of the catalyst system in our laboratory.
[6] a) A. Sakakura, K. Suzuki, K. Ishihara, Adv. Synth. Catal. 2006, 348,
2457–2465; b) W. Chen, W. Du, Y. Duan, Y. Wu, S. Yang, Y. Chen,
Angew. Chem. 2007, 119, 7811–7814; Angew. Chem. Int. Ed. 2007,
46, 7667–7670; c) Y. Wang, H. M. Li, Y. Q. Wang, Y. Liu, B. M.
Foxman, L. Deng, J. Am. Chem. Soc. 2007, 129, 6364–6365; d) R. P.
Singh, K. Bartelson, Y. Wang, H. Su, X. J. Lu, L. Deng, J. Am.
Chem. Soc. 2008, 130, 2422–2423.
[7] L. W. Xu, J. Luo, Y. X. Lu, Chem. Commun. 2009, 1807–1821.
[8] a) M. P. Lalonde, Y. Chen E. N. Jacobsen, Angew. Chem. 2006, 118,
6514–6518; Angew. Chem. Int. Ed. 2006, 45, 6366–6370; b) H.
Huang, E. N. Jacobsen, J. Am. Chem. Soc. 2006, 128, 7170–7171;
c) S. B. Tsogoeva, S. Wei, Chem. Commun. 2006, 1451–1453.
[9] S. Z. Luo, H. Xu, J. Li, L. Zhang, J. Cheng, J. Am. Chem. Soc. 2007,
129, 3074–3075.
Experimental Section
General procedure: To
a stirred solution of diimine 1d (4.2 mg,
0.01 mmol) in anhydrous acetone (0.3 mL) was added methyl phenyl-
glyoxylate (15.0 mL, 0.1 mmol) and then AcOH (24 equiv to methyl phe-
nylglyoxylate) at À208C. The resulting mixture was stirred at À208C for
72 h. The mixture was directly purified by flash column chromatography
on silica gel (EtOAc/petroleum ether=1:5) to afford the product as a
white solid.
[10] a) F. Xue, S. Zhang, W. Duan, W. Wang, Adv. Synth. Catal. 2008,
350, 2194–2198; b) Y. Ju, L. Xu, L. Li, G. Lai, H. Qiu, J. Jiang, Y.
Lu, Tetrahedron Lett. 2008, 49, 6773–6777; c) S. B. Tsogoeva, S. W.
Wei, Tetrahedron: Asymmetry 2005, 16, 1947–1951; d) D. A. Yalalov,
S. B. Tsogoeva, T. E. Shubina, I. M. Martynova, T. Clark, Angew.
Chem. 2008, 120, 6726–6730; Angew. Chem. Int. Ed. 2008, 47, 6624–
6628; e) F. Z. Peng, Z. H. Shao, X. W. Pu, H. B. Zhang, Adv. Synth.
Catal. 2008, 350, 2199–2204.
Acknowledgements
We gratefully acknowledge the National Natural Science Foundation of
China (20832001, 20972065, 21074054, 81072542) and the National Basic
Research Program of China (2007CB925103, 2010CB92330) for their fi-
nancial support.
[11] a) W. Zhang, J. L. Loebach, S. R. Wilson, E. N. Jacobsen, J. Am.
Chem. Soc. 1990, 112, 2801–2803; b) C. Baleiz¼o, H. Garcia, Chem.
Rev. 2006, 106, 3987–4043; c) S. J. Wezenberg, A. W. Kleij, Angew.
Chem. 2008, 120, 2388–2399; Angew. Chem. Int. Ed. 2008, 47, 2354–
2364; d) A. W. kleij, Eur. J. Inorg. Chem. 2009, 193–205; e) Y. J.
Huang, F. Y. Yang, C. J. Zhu, J. Am. Chem. Soc. 2005, 127, 16386–
16387; f) J. T. Sun, C. J. Zhu, Z. Y. Dai, M. H. Yang, Y. Pan, H. W.
Hu, J. Org. Chem. 2004, 69, 8500–8503; g) M. H. Yang, C. J. Zhu, F.
Yuan, Y. J. Huang, Y. Pan, Org. Lett. 2005, 7, 1927–1930; h) J. T.
Sun, M. H. Yang, F. Yuan, X. F. Jia, X. Yang, Y. Pan, C. J. Zhu, Adv.
Synth. Catal. 2009, 351, 920–930.
[12] a) Q. Xia, H. Ge, C. Ye, Z. Liu, K. Su, Chem. Rev. 2005, 105, 1603–
1662; b) J. Lopez, S. Liang, X. Bu, Tetrahedron Lett. 1998, 39, 4199–
4202; c) J. Lopez, E. A. Mintz, F. L. Hsu, X. Bu, Tetrahedron: Asym-
metry 1998, 9, 3741–3744; d) E. J. Campbell, S. T. Nguyen, Tetrahe-
dron Lett. 2001, 42, 1221–1225; e) Y. Sun, N. Tang, J. Mol. Catal. A:
Chem. 2006, 255, 171–179; f) A. M. Daly, C. T. Dalton, M. F. Rene-
han, D. G. Gilheany, Tetrahedron Lett. 1999, 40, 3617–3620; g) M. F.
Renehan, H. J. Schanz, E. M. McGarrigle, C. T. Dalton, A. M. Daly,
D. G. Gilheany, J. Mol. Catal. A: Chem. 2005, 231, 205–220.
[13] For more comprehensive figures, see the Supporting Information.
[14] a) A. Bøgevig, N. Kumaragurubaran, K. A. Jørgensen, Chem.
Commun. 2002, 620–621; b) N. Mase, F. Tanaka, C. F. Barbas III,
Angew. Chem. 2004, 116, 2474–2477; Angew. Chem. Int. Ed. 2004,
43, 2420–2423; c) O. Tokuda, T. Kano, W. G. Gao, I. Ikemolo, K.
Maruoka, Org. Lett. 2005, 7, 5103–5105; d) W. Wang, H. Li, J.
Wang, Tetrahedron Lett. 2005, 46, 5077–5079; e) Z. Tang, L. F. Cun,
X. Cui, A. Q. Mi, Y. Z. Jiang, L. Z. Gong, Org. Lett. 2006, 8, 1263–
1266; f) F. Wang, Y. Xiong, X. Liu, X. M. Feng, Adv. Synth. Catal.
2007, 349, 2665–2668; g) Q. Z. Liu, X. L. Wang, S. W. Luo, B. L.
Zheng, D. B. Qin, Tetrahedron Lett. 2008, 49, 7434–7437; h) J. Liu,
Z. Yang, Z. Wang, F. Wang, X. Chen, X. Liu, X. M. Feng, J. Am.
Chem. Soc. 2008, 130, 5654–5655; i) M. Raj, G. S. Parashari, V. K.
Singh, Adv. Synth. Catal. 2009, 351, 1284–1288; j) X. Y. Xu, Z. Tang,
Y. Z. Wang, S. W. Luo, L. F. Cun, L. Z. Gong, J. Org. Chem. 2007,
72, 9905–9913.
Keywords: aldol reaction · asymmetric catalysis · ketones ·
organocatalyst · primary amine–imine
[1] a) B. List, R. A. Lerner, C. F. Barbas III, J. Am. Chem. Soc. 2000,
122, 2395–2396; b) K. A. Ahrendt, C. J. Borths, D. W. C. MacMillan,
J. Am. Chem. Soc. 2000, 122, 4243–4244; c) K. Sakthivel, W. Notz,
T. Bui, C. F. Barbas III, J. Am. Chem. Soc. 2001, 123, 5260–5267.
[2] For recent reviews, see: a) S. Bertelsen, K. A. Jørgensen, Chem. Soc.
Rev. 2009, 38, 2178–2189; b) D. W. C. MacMillan, Nature 2008, 455,
304–308; c) S. Mukherjee, J. Yang, S. Hoffmann, B. List, Chem. Rev.
2007, 107, 5471–5569; d) A. Dondoni, A. Massi, Angew. Chem.
2008, 120, 4716–4739; Angew. Chem. Int. Ed. 2008, 47, 4638–4660;
e) P. I. Dalko, L. Moisan, Angew. Chem. 2004, 116, 5248–5286;
Angew. Chem. Int. Ed. 2004, 43, 5138–5175; f) T. Marcelli, J. H. van.
Maarseveen, H. Hiemstra, Angew. Chem. 2006, 118, 7658–7666;
Angew. Chem. Int. Ed. 2006, 45, 7496–7504; g) D. Enders, C. Gron-
dal, M. R. M. Huttl, Angew. Chem. 2007, 119, 1590–1601; Angew.
Chem. Int. Ed. 2007, 46, 1570–1581; h) X. Yu, W. Wang, Org.
Biomol. Chem. 2008, 6, 2037–2046; i) W. Notz, F. Tanaka, C. F.
Barbas. III, Acc. Chem. Res. 2004, 37, 580–591; j) B. List, Acc.
Chem. Res. 2004, 37, 548–557.
[3] For aldol reactions, see: a) A. Bassan, W. B. Zou, E. Reyes, F.
Himo, A. Cꢁrdova, Angew. Chem. 2005, 117, 7190–7194; Angew.
Chem. Int. Ed. 2005, 44, 7028–7032; b) A. Cꢁrdova, W. B. Zou, P.
Dziedzic, I. Ibrahem, E. Reyes, Y.-M. Xu, Chem. Eur. J. 2006, 12,
5383–5397; c) X. Wu, Z. Jiang, H. Shen, Y. Lu, Adv. Synth. Catal.
2007, 349, 812–816; d) S. S. V. Ramasastry, H. L. Zhang, F. Tanaka,
C. F. Barbas III, J. Am. Chem. Soc. 2007, 129, 288–289; e) X. Y. Xu,
Y. Z. Wang, L. Z. Gong, Org. Lett. 2007, 9, 4247–4249; f) L. W. Xu,
Y. D. Ju, L. Li, H. Y. Qiu, J. X. Jiang, G. Q. Lai, Tetrahedron Lett.
2008, 49, 7037–7041; g) J. Zhou, V. Wakchaure, P. Kraft, B. List,
Angew. Chem. 2008, 120, 7768–7771; Angew. Chem. Int. Ed. 2008,
47, 7656–7658.
[4] a) K. Liu, H. Cui, J. Nie, K. Dong, X. Li, J. Ma, Org. Lett. 2007, 9,
923–925; b) S. H. McCooey, S. J. Connon, Org. Lett. 2007, 9, 599–
602; c) D. A. Yalalov, S. B. Tsogoeva, S. Schmatz, Adv. Synth. Catal.
Received: January 19, 2011
Published online: June 7, 2011
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