Enantioselective 1,3-dipolar cycloadditions of nitrones with unsaturated aldehydes promoted by a recyclable tetraarylphosphonium supported imidazolidinone catalyst

Article abstract of DOI:10.1016/j.molcata.2014.06.015

The tetraarylphosphonium supported chiral imidazolidinone catalyzes the enantioselective 1,3-dipolar cycloadditions of nitrones and α,β- unsaturated aldehydes to provide isoxazolidine aldehydes in good yields with excellent diastereo- and enantioselectivi

Full text of DOI:10.1016/j.molcata.2014.06.015

Journal of Molecular Catalysis A: Chemical 393 (2014) 171–174
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Enantioselective 1,3-dipolar cycloadditions of nitrones with
unsaturated aldehydes promoted by a recyclable
tetraarylphosphonium supported imidazolidinone catalyst
∗
Xin Nie, Cuifen Lu, Zuxing Chen, Guichun Yang, Junqi Nie
Hubei Collaborative Innovation Center for Advanced Organochemical Materials & Ministry-of-Education Key Laboratory for the Synthesis and Application
of Organic Functional Molecules, Hubei University, Wuhan 430062, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The tetraarylphosphonium supported chiral imidazolidinone catalyzes the enantioselective 1,3-dipolar
cycloadditions of nitrones and ␣,␤-unsaturated aldehydes to provide isoxazolidine aldehydes in good
yields with excellent diastereo- and enantioselectivities. Most importantly, the tetraarylphosphonium
supported imidazolidinone catalyst can be readily recovered and recycled for further transformations at
least four cycles without observing significant decrease in yield and stereoselectivity.
Received 16 January 2014
Received in revised form 4 May 2014
Accepted 15 June 2014
Available online 21 June 2014
©
2014 Elsevier B.V. All rights reserved.
Keywords:
Tetraarylphosphonium
Imidazolidinone
1
,3-Dipolar cycloadditions
Recyclable
Organocatalyst
1
. Introduction
tetraarylphosphonium supported imidazolidinone catalyst 1a–c to
the 1,3-dipolar cycloadditions of ␣,␤-unsaturated aldehydes with
nitrones.
In the past few years, there has been a tremendous increase in
the use of organocatalysts in asymmetric synthesis [1–5]. Among
them, MacMillan’s imidazolidinone catalyst has emerged as one
of the most efficient organocatalysts for a variety of highly enan-
tioselective reactions involving ␣,␤-unsaturated aldehydes [6–10].
However, the recovery and recycling of the catalyst are still the
issues that need to be addressed in this area. To solve this problem,
recently several modified methods which allowed the recycling
of the imidazolidinone catalyst by immobilizing MacMillan’s cata-
lyst onto ionic liquid, polymer, fluorous tag, or silica gel have been
developed [11–24].
In a preliminary communication, we have reported the immobi-
lization of chiral imidazolidinones on tetraarylphosphonium salts
to afford the supported catalysts 1a–c (Fig. 1) for the enantio-
selective Diels–Alder reaction of ␣,␤-unsaturated aldehydes with
dienes [25]. In continuation of our efforts in exploring the synthetic
utility of the tetraarylphosphonium supported imidazolidinone in
organic synthesis as well as developing practical organic trans-
formations, herein we describe the extended application of the
2
. Experimental
2.1. General remarks
Reactions were monitored by TLC using precoated plates of sil-
ica gel HF254 (0.5 mm, Yantai, China). Column chromatography was
performed with a silicagel column (200–300 mesh, Yantai, China).
NMR spectra were recorded on Varian Unity Inova 600 spectrome-
1
13
ter ( H at 600 MHz and C at 150 MHz) or WIPM 400 spectrometer
1
13
(
H at 400 MHz and C at 100 MHz). IR spectra were recorded on
an IR-spectrum one (PE) spectrometer. HPLC were performed with
Dionex UltiMate 3000 and equipped with a chiral column (Chiral-
cel OJ-RH, Daicel) using CH OH/H O as an eluent. A UV detector
3
2
(UVD-3000) was used for the peak detection.
2.2. General procedure for the 1,3-dipolar cycloadditions
To a solution of the tetraarylphosphonium supported imidazo-
lidinone catalyst 1a (438 mg, 0.6 mmol) in CH NO (30 mL) and
3
2
∗ Corresponding author. Tel.: +86 27 5086 5322.
E-mail address: jqnie@hubu.edu.cn (J. Nie).
H O (27 ␮L) was added HBF4 (40 ␮L, 0.6 mmol) and N-benzyl-C-
phenyl nitrone (633 mg, 3 mmol). After cooling the solution to 0 C,
2
◦
http://dx.doi.org/10.1016/j.molcata.2014.06.015
1
381-1169/© 2014 Elsevier B.V. All rights reserved.

1
72
X. Nie et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 171–174
O
128.1, 127.5, 127.0, 73.4, 71.5, 71.1, 59.3, 21.0, 20.9. Enantiomeric
R3
ratio was determined by HPLC using Chiracel OJ-RH column after
reduction with NaBH /MeOH (80:20 MeOH/H O, 1 mL/min flow
1
2
3
+
a: R =R =R =Me;
Ph3P
O
N
1
2
3
4
2
R2
b: R =R =Me, R =n-Bu;
PF -
HN
1
2
3
rate), endo isomers tR = 8.5 min (major enantiomer) and 10.0 min
minor enantiomer).
4S, 5R)-2-Benzyl-4-formyl-5-methyl-3-(2-napthyl) isoxazo-
6
c: R =H, R =C(CH ), R =Me.
R1
3
(
1
(
−
1
Fig. 1. Structure of the tetraarylphosphonium supported chiral imidazolidinones.
lidine (2e): IR (NaCl): ꢀ 2866, 1723, 1601, 1496, 1454, 1125 cm
H NMR (600 MHz, CDCl , ppm): ı 9.72 (d, J = 1.9 Hz, 1H, CHO),
;
1
3
7
.77–7.20 (m, 12H, ArH), 4.52–4.48 (m, 1H, CHON), 4.24 (d, J = 7.9 Hz,
E-crotonaldehyde (2.5 mL, 30 mmol) was added, and then the mix-
ture was stirred for 36 h at this temperature. The resulting solution
was then evaporated under vacuum. The residue was dissolved in
the minimum amount of CH Cl (2 mL) and poured in Et O (25 mL),
and then filtered. The filtrate was concentrated under vacuum and
purified by silica gel column chromatography (n-hexane/EtOAc,
1H, CHNO), 3.96 (d, J = 14.3 Hz, 1H, C H5CH ), 3.80 (d, J = 14.3 Hz,
6
2
1H, C H5CH ), 3.13–3.10 (m, 1H, CHCHO), 1.45 (d, J = 6.2 Hz, 3H,
6
2
1
3
OCHCH ); C NMR (150 MHz, CDCl , ppm): ı 198.4, 137.1, 135.8,
3
3
133.4, 133.2, 128.9, 128.6, 128.2, 127.9, 127.7, 127.2, 126.9, 126.4,
126.2, 124.9, 73.7, 71.5, 71.3, 59.7, 20.9. Enantiomeric ratio was
determined by HPLC using Chiracel OJ-RH column after reduc-
tion with NaBH /MeOH (70:30 MeOH/H O, 1 mL/min flow rate),
2
2
2
5
0:1, v/v) to afford the desired products, in which the endo/exo
4
2
1
ratio was determined by H NMR. The precipitate in the sand-core
funnel was dried under vacuum and reused in further reactions as
the recovered catalyst. The recovered catalyst was examined by H
endo isomers tR = 69.7 min (major enantiomer) and 76.0 min (minor
enantiomer).
1
(4S, 5R)-2-Benzyl-4-formyl-5-propyl-3-phenylisoxazolidine
−
1
NMR spectroscopy, which showed unchanged after each recovery.
Spectroscopic data of the 1,3-dipolar cycloaddition products:
(2f): IR (NaCl): ꢀ 2872, 1725, 1495, 1455, 1377, 1047 cm
;
1
H NMR (400 MHz, CDCl , ppm): ı 9.70 (d, J = 2.6 Hz, 1H, CHO),
3
(4S,
5R)-2-Benzyl-5-methyl-3-phenylisoxazolidine-4-
7.35–7.13 (m, 10H, ArH), 4.29–4.21 (m, 1H, CHCH CH CH ), 4.06 (d,
2
2
3
carbaldehyde (2a): IR (NaCl): ꢀ 2928, 1720, 1603, 1495, 1455,
J = 7.8 Hz, 1H, C H5CH), 3.93 (d, J = 14.3 Hz, 1H, C H5CH ), 3.74 (d,
6
6
2
−
1
1
1
373 cm
;
H NMR (400 MHz, CDCl , ppm): ı 9.71 (d, J = 2.3 Hz,
J = 14.3 Hz, 1H, C H5CH ), 3.08–3.04 (m, 1H, CHCHO), 1.94–1.85 (m,
3
6 2
1
H, CHO), 7.37–7.15(m, 10H, ArH), 4.49–4.46 (m, 1H, CHCH ), 4.10
1H, CHCH CH CH ), 1.60–1.54 (m, 1H, CHCH CH CH ), 1.40–1.24
3
2
2
3
2
2
3
13
(
d, J = 7.8 Hz, 1H, C H5CH), 3.95 (d, J = 14.3 Hz, 1H, C H5CH ), 3.77
(m, 2H, CH CH CH ), 0.87–0.84 (t, J = 7.3 Hz, 3H, CH ); C NMR
6
6
2
2 2 3
3
(
d, J = 14.3 Hz, 1H, C H5CH ), 3.06–3.03 (m, 1H, CHCHO), 1.43 (d,
(100 MHz, CDCl , ppm): ı 198.7, 138.2, 137.3, 128.9, 128.4, 128.2,
128.1, 127.6, 127.1, 77.3, 71.0, 70.4, 59.3, 37.6, 19.2, 13.9. Enan-
tiomeric ratio was determined by HPLC using Chiracel OJ-RH
6
2
3
1
3
J = 6.2 Hz, 3H, CH ); C NMR (100 MHz, CDCl , ppm): ı 198.6,
1
7
3
3
38.2, 137.2, 129.0, 128.9, 128.5, 128.3, 127.7, 127.2, 73.6, 71.5,
1.2, 59.5, 21.1. Enantiomeric ratio was determined by HPLC using
column after reduction with NaBH /MeOH (70:30 MeOH/H O,
4
2
Chiracel OJ-RH column after reduction with NaBH /MeOH (80:20
1 mL/min flow rate), endo isomers tR = 7.3 min (major enantiomer)
and 6.0 min (minor enantiomer).
4
MeOH/H O, 1 mL/min flow rate), endo isomers tR = 7.1 min (major
2
enantiomer) and 6.4 min (minor enantiomer).
(S)-2-Benzyl-4-formyl-3-phenylisoxazolidine (2g): IR (NaCl):
−
1
1
(4S,
5R)-2-Benzyl-4-formyl-5-methyl-3-(4-chlorophenyl)
ꢀ 2875, 1722, 1495, 1455, 1047 cm
ppm): ı 9.60 (d, J = 3.7 Hz, 1H, CHO), 7.35–7.11 (m, 10H, ArH),
4.12–4.05 (m, 2H, CH ON), 3.93 (d, J = 7.4 Hz, 1H, C H5CH), 3.86
; H NMR (400 MHz, CDCl3,
isoxazolidine (2b): IR (NaCl): ꢀ 2975, 1723, 1493, 1455, 1372,
−
; H NMR (400 MHz, CDCl , ppm): ı 9.65 (d, J = 2.1 Hz,
3
1
1
1
1
090 cm
H, CHO), 7.26–7.16 (m, 9H, ArH), 4.45–4.39 (m, 1H, CHON), 4.07
2
6
(d, J = 14.1 Hz, 1H, C H5CH ), 3.65 (d, J = 14.1 Hz, 1H, C H5CH ),
6
2
6
2
13
(
d, J = 7.4 Hz, 1H, ClC H CH), 3.87 (d, J = 14.2 Hz, 1H, C H5CH ),
3.26–3.24 (m, 1H, CHCHO); C NMR (100 MHz, CDCl , ppm): ı
6
4
6
2
3
3
.75 (d, J = 14.2 Hz, 1H, C H5CH ), 2.92–2.95 (m, 1H, CHCHO), 1.38
198.9, 138.3, 137.4, 129.1, 128.8, 128.4, 128.3, 127.9, 127.4 70.4,
65.9, 64.4, 59.8. Enantiomeric ratio was determined by HPLC using
6
2
1
3
(
d, J = 6.2 Hz, 3H, CH ); C NMR (100 MHz, CDCl , ppm): ı 198.3,
3
3
1
7
37.3, 137.0, 133.8, 129.1, 128.9, 128.3, 127.3, 122.0, 73.6, 71.6,
0.1, 59.7, 21.0. Enantiomeric ratio was determined by HPLC using
Chiracel OJ-RH column after reduction with NaBH /MeOH (70:30
4
MeOH/H O, 1 mL/min flow rate), endo isomers tR = 18.6 min (major
2
Chiracel OJ-RH column after reduction with NaBH /MeOH (80:20
enantiomer) and 20.8 min (minor enantiomer).
4
MeOH/H O, 1 mL/min flow rate), endo isomers tR = 11.2 min (major
2
enantiomer) and 13.2 min (minor enantiomer).
(4S, 5R)-2-Benzyl-4-formyl-5-methyl-3-(4-methoxyphenyl)
isoxazolidine (2c): IR (NaCl): ꢀ 2836, 1723, 1513, 1455, 1303,
3. Results and discussion
−
; H NMR (400 MHz, CDCl , ppm): ı 9.64 (d, J = 2.3 Hz,
3
1
1
1032 cm
H, CHO), 7.26–7.10 (m, 7H, ArH), 6.80 (d, J = 8.8 Hz, 2H,
1
3.1. Catalytic activity of catalyst 1a–c in the 1,3-dipolar
orthoC H OCH ), 4.44–4.38 (m, 1H, CHON), 3.98 (d, J = 8.0 Hz, 1H,
cycloadditions
6
4
3
CH OC H5CH), 3.90 (d, J = 14.5 Hz, 1H, C H5CH ), 3.68–3.65 (m, 4H,
3
6
6
2
OCH , C H5CH ), 2.95–2.99 (m, 1H, CHCHO), 1.39 (d, J = 6.2 Hz, 3H,
To evaluate the capacity of the tetraarylphosphonium supported
chiral imidazolidinones 1a–c in the 1,3-dipolar cycloadditions,
the synthesis of the isoxazolidine aldehyde via N-benzyl-C-phenyl
nitrone with E-crotonaldehyde was chosen as a model reaction.
Then we sought to establish optimal reaction conditions that would
furnish the isoxazolidine aldehyde products 2 and 3 in good yield
and excellent enantioselectivity.
3
6
2
13
CH ); C NMR (100 MHz, CDCl , ppm): ı 198.8, 159.6, 137.5, 129.9,
3
3
1
28.9, 128.8, 128.5, 128.2, 127.1, 114.4, 73.4, 71.5, 70.9, 59.3, 21.2.
Enantiomeric ratio was determined by HPLC using Chiracel OJ-
RH column after reduction with NaBH /MeOH (80:20 MeOH/H O,
4
2
1
mL/min flow rate), endo isomers tR = 7.3 min (major enantiomer)
and 9.5 min (minor enantiomer).
4S, 5R)-2-Benzyl-4-formyl-5-methyl-3-(4-methylphenyl)
isoxazolidine (2d): IR (NaCl): ꢀ 2925, 1724, 1514, 1454, 1372,
(
As illustrated in Table 1, the choice of solvent is found to have a
significant impact on the outcome of the reaction, not only in terms
of yield, but also involve the enantioselectivity (Table 1, entries
1–8). The reactions were carried out in the presence of 20 mol%
−
; H NMR (600 MHz, CDCl , ppm): ı 9.69 (d, J = 2.4 Hz,
3
1
1
1
1
076 cm
H, CHO), 7.26–7.08 (m, 9H, ArH), 4.47–4.43 (m, 1H, CHON), 4.02
◦
(
d, J = 7.9 Hz, 1H, CH C H5CH), 3.92 (d, J = 14.4 Hz, 1H, C H5CH ),
1a/HBF4 combinations in different solvent system at 4–25 C for
3
6
6
2
3
2
.72 (d, J = 14.4 Hz, 1H, C H5CH ), 3.03–3.00 (m, 1H, CHCHO),
24 h. Among the solvents examined, CH NO /H O provided the
best result (entry 8, 82% yield, 94/6 endo/exo, endo 88% ee). There-
6
2
3
2
2
13
.27 (s, 3H, C H CH ), 1.42 (d, J = 6.2 Hz, 3H, OCHCH ); C NMR
6
4
3
3
(
150 MHz, CDCl , ppm): ı 198.5, 137.9, 137.3, 135.1, 129.5, 128.4,
fore, CH NO /H O was chosen as the solvent system to further
3
3
2
2

X. Nie et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 171–174
173
Table 1
Optimization of 1,3-dipolar cycloaddition reaction conditions between E-crotonaldehyde and nitrone.
Bn
Bn
1
a (20 mol%)
Bn
O
N O
N O
HX (20 mol%)
N
+
+
O
o
Ph
Ph
Solvent/H O, T ( C)
Ph
2
CHO
CHO
2
3
endo
exo
.
◦
Yieldb (%)
endo/exoc
endo eed (%)
Entry
Catalyst
Solvent
HX
T ( C)
Time (h)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1a
1a
1a
1a
1a
CHCl3
CHCl2
THF
CH3CN
DMF
HBF4
HBF4
HBF4
HBF4
HBF4
HBF4
HBF4
HBF4
CF3SO3H
HCl
CF3COOH
HClO4
HPF6
CH3COOH
HBF4
4–25a
4–25
4–25
4–25
4–25
4–25
4–25
4–25
4–25
4–25
4–25
4–25
4–25
4–25
0
24
24
24
24
24
24
24
24
24
24
24
24
24
24
12
24
36
48
72
36
36
36
36
36
36
36
73
68
44
65
27
40
65
82
53
40
51
71
77
32
Trace
75
89
89
65
86
90
72
77
86
85
82
90/10
75/25
78/22
91/9
82/18
74/26
80/20
94/6
63/37
81/19
88/12
91/9
93/7
84/16
—
91/9
94/6
93/7
90/10
92/8
70/30
88/12
92/8
94/6
90/10
90/10
70
63
68
76
82
50
39
88
43
77
72
86
80
72
—
89
94
93
92
76
45
94
92
94
92
92
H2O
Toluene
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
HBF4
HBF4
HBF4
HBF4
0
0
0
−20
HBF4
0
HBF4
0
0
0
0
0
0
e
HBF4
HBF4f
g
HBF4
h
HBF4
HBF4i
a
b
c
d
e
f
◦
◦
Substrates were added at 4 C and then the mixture was stirred for 24 h while the temperature was allowed to rise slowly to 25 C.
Isolated yield of a mixture of endo and exo isomers.
Exo/endo ratios were determined by 1H NMR analysis.
ee values were determined by HPLC of alcohol after reduction of the formyl group.
1
3
2
3
4
0 mol% catalyst.
0 mol% catalyst.
nd run.
rd run.
th run.
g
h
i
optimize the reaction conditions by screening different acids as
reaction co-catalysts.
Seven types of protic acids were screened as the reaction co-
catalysts (Table 1, entries 8–14). Among the protic acids examined,
it was found that better endo/exo selectivities (94/6, 91/9, 93/7)
With the optimized reaction condition in hand, the
tetraarylphosphonium supported chiral imidazolidin-4-ones
1b and 1c were also evaluated for the 1,3-dipolar cycloaddition
to compare with the use of 1a (entries 20 and 21). The chemical
yields by the use of the catalysts 1b and 1c were comparable to 1a,
but the diastereoselectivity and enantioselectivity were lower. So
the catalyst 1a is the best one among them.
were obtained when HBF , HClO4 and HPF6 used as reaction co-
4
catalysts (entries 8, 12 and 13). In contrast, relatively high yield and
excellent endo ee were produced when HBF4 was selected (entry
Overall, the target products 2 and 3 were obtained in 89% yield,
94/6 endo/exo and 94% endo ee by 1a, which is comparable to the
result obtained from the use of non-supported imidazolidinone
[10]. For instance, the same excellent endo enantioselectivity of 94%
was obtained by employing either Macmillan’s imidazolidinone
catalyst or the tetraaryphosphonium supported catalyst.
8
, 82% and 88%). Thus, HBF4 was chosen as the optimal reaction
co-catalyst in further reactions.
Next, the effects of reaction temperature and time, universally
crucial factors, in the 1,3-dipolar cycloaddition was investigated
◦
(
Table 1, entries 8, 15–19). It is manifest that 0 C and 36 h is the
best temperature and time for the reaction, which resulted in 89%
yield, 94/6 endo/exo and 94% endo ee.
3
.2. Recycling and reuse of catalyst 1a
Furthermore, the influence of the catalyst loading in the 1,3-
◦
dipolar cycloaddition was also examined under the condition of 0 C
Isolation of the products by simple precipitation and filtration
for 36 h (Table 1, entries 17, 22 and 23). The best amount of catalyst
used was 20 mol%, as higher or lower catalyst loadings seemed to
be detrimental either for yield or stereoselectivity. So the optimal
was quite easy when using the tetraaryphosphonium supported
chiral imidazolidinone catalyst. Catalyst recycling experiments
could also be readily performed. As shown in Table 1 (entries 17,
reaction condition of the 1,3-dipolar cycloaddition with 1a as cata-
2
4–26), no significant decrease in catalytic activity is observed
◦
lyst is CH NO /H O as solvent system, HBF as co-catalyst, 0 C for
3
2
2
4
when the tetraaryphosphonium supported catalyst 1a was recov-
ered and reused in further reactions. Excellent diastereoselectivity
3
6 h and 20 mol% catalyst amount.

1
74
X. Nie et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 171–174
Table 2
Scope of catalyst 1a in the 1,3-dipolar cycloadditions.
Bn
Bn
1
a (20 mol%)
N O
N O
Bn
O
HBF (20 mol%)
4
N
+
+
R'
O
R
R'
R
R'
R
CH NO /H O
3 2 2
o
CHO
CHO
0
C, 48h
2
3
endo
exo
.
ꢀ
Yielda (%)
endo/exob
endo eec (%)
Entry
R
R
Products
1
2
3
4
5
6
7
Ph
Me
Me
Me
Me
Me
2a + 3a
2b + 3b
2c + 3c
2d + 3d
2e + 3e
2f + 3f
89
72
78
79
81
72
77
94/6
90/10
93/7
91/9
90/10
94/6
94/6
94
90
88
85
92
97
88
4-ClPh
4-OMePh
4-MePh
2-Naphthyl
Ph
Ph
n-Pr
H
2g + 3 g
a
Isolated yield of a mixture of endo and exo isomers.
Exo/endo ratios were determined by 1H NMR analysis.
ee values were determined by HPLC of alcohol after reduction of the formyl group.
b
c
(
90/10) and enantioselectivity (92% endo ee) of the corresponding
Major Scientific Instrument and Equipment Development Project
(No. 2011YQ12003505) for financial support. We are also grateful
to the referees for their valuable suggestions.
products were obtained even in the 4th cycle, making catalyst 1a
synthetically efficient and useful (entry 26).
3.3. Scope of catalyst 1a
References
[
[
[
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to 94/6 endo/exo, 85–97% endo ee). Changes in the structure of
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[
[
6
[
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In conclusion, we have developed an efficient method for
[
[
[
the enantioselective 1,3-dipolar cycloadditions by using the
tetraarylphosphonium supported imidazolidinone catalyst, pro-
viding the desired products in good yields with excellent
diastereo- and enantioselectivities. It is worth mentioning that the
tetraarylphosphonium supported imidazolidinone catalyst can be
readily recovered and recycled for further transformations at least
four cycles without observing significant decrease in yield and
stereoselectivity.
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(
[
(
[
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Acknowledgments
9
41.
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We gratefully acknowledge the National Natural Sciences Foun-
dation of China (No. 21342002 and 21042005) and 2011 National
[