Enantioselective synthesis of quaternary stereogenic centers through catalytic asymmetric addition of dimethylzinc to α-ketoesters with chiral cis-cyclopropane-based amide alcohol as ligand

Article abstract of DOI:10.1016/j.tetasy.2009.07.050

A new amino alcohol with a chiral cyclopropane backbone has been developed and used in the catalytic asymmetric diethylzinc addition to various types of α-ketoesters. This cyclopropane-based chiral amino alcohol shows moderate enantioselectivity in the ad

Full text of DOI:10.1016/j.tetasy.2009.07.050

Tetrahedron: Asymmetry 20 (2009) 2125–2129
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Enantioselective synthesis of quaternary stereogenic centers through
catalytic asymmetric addition of dimethylzinc to a-ketoesters with chiral
cis-cyclopropane-based amide alcohol as ligand
*
*
Bing Zheng, Shicong Hou, Zhiyuan Li, Hongchao Guo, Jiangchun Zhong , Min Wang
Department of Applied Chemistry, China Agricultural University, Beijing 100193, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2009
Accepted 31 July 2009
Available online 28 September 2009
A new amino alcohol with a chiral cyclopropane backbone has been developed and used in the catalytic
asymmetric diethylzinc addition to various types of -ketoesters. This cyclopropane-based chiral amino
alcohol shows moderate enantioselectivity in the addition of organozinc to -ketoesters. For dimethyl-
zinc addition to -ketoesters, up to 81% ee are obtained, respectively.
a
a
a
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
morpholinoisobornane-10-thiol 53e (ꢀ)-MITH have been used with
success for the enantioselective addition of dialkylzinc to ketoesters
(Fig. 1).
The stereoselective creation of quaternary stereogenic centers in
organic chemistry, especially in the synthesis of complex organic
molecules, is important and highly desired. Approaches based on
C–C bond formation provide a basic strategy for this target.1 Over
the past ten years, asymmetric organozinc additions to carbonyl
compounds has been one of the most useful approaches. In principle,
a variety of organometallic reagents can be used for the catalytic
enantioselective transfer of alkyl groups. However, most of them
are too reactive, and background reactions always deteriorate the
enantioselectivity. The enantioselective catalytic process becomes
quite difficult. In contrast, organozinc reagents are quite unreactive
and can tolerate the presence of many reactive functional groups.
They are highly selective in nucleophilic addition reactions to car-
bonyl compounds. However, because of the low reactivity of these
reagents toward the ketone carbonyl group, the highly reactive cat-
alyst has to be used to strongly activate the carbonyl group or the
organometallic reagent. Therefore, although hundreds of chiral cat-
alysts have been used for catalytic enantioselective organozincaddi-
tions to aldehydes,2 much less work on asymmetric organozinc
additions to ketones, especially additions to ketoesters, has been re-
ported. Compared with aldehydes and ketones, a ketoester can act as
a chelating ligand by itself and activate the alkylzinc reagent, which
results in a strong background reaction. It is more difficult to find an
excellent catalyst to afford sufficient activity, consequently, only
Kozlowski‘s titanium–salen complex 1,3a Shibashaki’s proline-de-
rived aminodiol-based ligands 2,3b Hoveyda’s amino acid-based
ligands 3,3c Pedro’s amide alcohol 4,3d and Uang’s (ꢀ)-2-exo-
In Pedro’s work,3d deprotonated N-monosubstituted carboxy
amides have been used to suppress the competition between the
chiral ligand and the substrate to coordinate the zinc metal ion,
and favor the formation of the chiral ligand–Zn complex. This re-
sults in a highly enantioselective addition reaction. Following this
strategy, as our continuous efforts on the development of new
enantioselective catalytic reactions by using cyclopropane-based
ligands,4 we herein report the synthesis of a new type of amide
alcohol ligand 9 based on substituted cyclopropane and its applica-
tion in Me2Zn addition to
a-ketoesters.
2. Results and discussion
The amide alcohol ligand 9 was prepared from commercially
available (1R,5S)-4-hydroxy-6,6-dimethyl-3-oxa-bicyclo-[3.1.0]-
hexan-2-one 6 (Fig. 2), which is a key intermediate of producing
pyrethroid insecticides. Compound 6 reacted with diazomethane
in diethyl ether to quantitatively give aldehydoester 7 followed by
NaBH4 reduction and refluxing in benzene to afford enantiomeri-
cally pure cyclopropane lactone 8 in 80% yield. Initially, we exam-
ined the direct amidation of lactone
8 by treatment with
benzylamine. Unfortunately, no reaction occurred at room tempera-
turein thepresenceorabsenceof 4-dimethylaminopyridine (DMAP)
as a catalyst, even if reaction time was prolonged. Finally, the DIBAL-
H-H2NR, whichisahighlyefficientandmildamidatingreagentinthe
conversion of lactones to amides, was used to furnish the synthesis
of ligand 9 in more than 87% yield.5 The structure of 9 was estab-
lished unequivocally via X-ray crystallographic analysis. The abso-
lute configuration of 9 was assigned as 1R,3S by its crystal
structure (Fig. 3).6 The –OH group and N atom are situated on the
* Corresponding authors. Tel.: +86 010 62731356; fax: +86 010 62815939.
E-mail addresses: dr_zhongjiangchun@yahoo.com.cn (J. Zhong), wangmincau@
yahoo.com.cn (M. Wang).
0957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2009.07.050

2126
B. Zheng et al. / Tetrahedron: Asymmetry 20 (2009) 2125–2129
i-Pr
N
O
N
Ti
O
t-Bu
O
i-Pr
O
t-Bu
MeO
AA2
AA1
N
NHn-Bu
N
N
OH
1
3
HO
Ph
Ph
N
O
O
OH
N
SH
HO
NHBn
5
4
2
Figure 1. Catalysts of catalytic addition of dialkylzinc to a-ketoesters.
Table 1
Catalytic asymmetric addition of dimethylzinc to methyl oxo(phenyl)acetate 10,
CH2N2
Et2O, rt
using cyclopropane-based amide alcohols as ligands 9a
OHC
COOCH3
O
H3C OH
HO
O
O
10mol% 9
OCH3
OCH3
7
6
O
Me2Zn
O
1) NaBH4, EtOH, rt
2) Reflux, benzene
DIBAl-H-H2NR
THF, rt
Entry
Solvent
Temp (°C)
Time (h)
Yieldb (%)
eec (%)
O
1
2
3
4
Toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
20
20
0
ꢀ20
ꢀ20
ꢀ20
ꢀ20
ꢀ35
0
24
24
24
48
48
48
60
48
24
48
94
93
90
70
62
77
70
55
64
49
40
54
60
70
66
59
63
28
57
69
O
8
O
5d
6e
7
OH
HN
Bn
8
9
9f
10f
ꢀ20
Figure 2. Synthesis of amide alcohol ligand.
a
b
c
Me2Zn: 1.8 equiv; ligand: 10 mol %.
Isolated yields after column chromatography.
Determined by chiral GLC and the absolute configuration of major enantiomer
was assigned as (R)-configuration by the comparison with literature data.
d
5% Ligands was used.
20% Ligands was used.
5 mol % DiMPEG was used.
e
f
reaction was carried out at ꢀ20 °C in CH2Cl2 (entry 4). Lower reac-
tion temperature (ꢀ35 °C) or more ligand (20 mol %) did not im-
prove the enantioselectivity. Because polyethers could improve
enantioselectivity of the aryl transfer reaction in several reports,
the DiMPEG was used as an additive in this reaction.7 However,
when 5 mol % of DiMPEG (M = 2000 g/mol) was used, ee value re-
mained nearly the same (70% vs 69%), and the yield decreased sig-
nificantly from 70% to 49%. (Table 1, entry 10 vs 4).
Figure 3. ORTEP diagram of amino alcohol (1R,3S)-9.
same side of cyclopropane backbone. This clearly indicated that me-
tal can easily chelate to both of the groups despite the highly rigid
cyclopropane backbone.
Initial experiments were performed with compound 10 as sub-
strate for catalyst evaluation. Generally, 10 mol % catalyst and
1.8 equiv of Me2Zn had been used. Some screening results are
listed in Table 1. The reaction of Me2Zn and 10 in toluene at
20 °C gave the corresponding product in 94% yield and 40% ee (Ta-
ble 1, entry 1). The ee value was further improved to 70% when the
Under the optimized conditions, a series of a-ketoesters were
examined for the enantioselective Me2Zn additions in the presence
of ligand 9 as shown in Table 2. The catalyst exhibited a remarkably
broad substrate scope, and the corresponding products were ob-
tained in good yield (60–87%) and moderate enantioselectivities
(41–81%) (entries 1–9). Phenyl groups in the substrates featuring
electron-withdrawing or -donating substituents worked well (en-
tries 1–7), as did a thiophen-2-yl or 2-naphthyl group (entries 8
and9). It is worthnotingthatmethyl(phenyl)oxoacetate andisopro-

B. Zheng et al. / Tetrahedron: Asymmetry 20 (2009) 2125–2129
2127
Table 2
Catalytic asymmetric addition of dimethylzinc to
a
-ketoestersa
O
H3C OH
10mol% 9
OR
OR
Ar
O
Ar
Me2Zn
O
Entry
1
Ketoesters
Temp (°C)
Time (h)
48
Yieldb (%)
70
eec (%)
H3C OH
OMe
OEt
ꢀ20
ꢀ20
ꢀ20
ꢀ20
70
54
70
81
O
H3C OH
2
3
4
48
48
48
75
70
85
O
H3C OH
Oi-Pr
O
H3C OH
Ot-Bu
O
H3C OH
O
OMe
5
6
ꢀ20
ꢀ20
48
48
66
68
41
61
Cl
H3C OH
O
OMe
Me
H3C OH
O
OMe
7
ꢀ20
48
60
67
MeO
S
H3C OH
OMe
8
9
ꢀ20
ꢀ20
48
48
87
69
45
64
O
H3C OH
O
OMe
a
b
c
Me2Zn: 1.8 equiv, 10 mol % ligands. CH2Cl2.
Isolated yields after column chromatography.
Determined by chiral GC.
pyl (phenyl)oxoacetate can afford the same enantiomeric excesses
(70% vs 70%), while in sharp contrast, ethyl (phenyl)oxoacetate re-
acted with Me2Zn at ꢀ20 °C to give 54% ee (Table 2, entry 2), and
tert-butyl (phenyl)oxoacetate provided 81% ee (Table 2, entry 4).
4. Experimental
4.1. General experimental method
All reactions were carried out under an atmosphere of dry ar-
gon. Dimethylzinc (1.2 M solution in toluene), was purchased from
Acros. CH2Cl2 was distilled from LiAlH4 under argon. All ketoesters
were either commercially available or prepared according to the
literature.8 NMR spectra were recorded by a 500 MHz NMR instru-
ment; Mass spectra were recorded on an Agilent instrument by the
TOF MS technique. Enantiomeric excesses (ee) were determined by
3. Conclusion
In conclusion, we have designed and prepared a new cis-cyclo-
propane-based amide alcohol ligand. With this ligand, the addition
of dimethylzinc to
moderate enantioselectivities and moderate to good yields.
a-ketoesters under mild conditions can give

2128
B. Zheng et al. / Tetrahedron: Asymmetry 20 (2009) 2125–2129
chiral GC analysis. The optical rotations were measured on PERKIN
ELEMER 341 Polarimeter.
NH4Cl (10 mL) was added to stop the reaction and extracted with
ether (10 mL ꢂ 3). The organic layer was dried over anhydrous
Na2SO4, then, concentrated, and purified by Flash column chroma-
tography (ethyl acetate/hexanes = 1:3) to give a colorless oil.
4.2. General procedure for synthesis of cis-cyclopropane
aminoalcohol 9
4.3.1. (R)-Methyl 2-hydroxy-2-phenylpropanoate
4.2.1. (1R,3S)-Methyl-3-formyl-2,2-dimethylcyclopropanecarbo-
xylate 7
70% Yield, 70% ee determined by chiral GC analysis (CYCLODEX-
B column, 115 °C). Retention time: tmajor = 31.9 min, tminor = 33.1
(1R,5S)-4-Hydroxy-6,6-dimethyl-3-oxa-bicyclo[3.1.0]-hex-an-
2-one 6 (2.8 g, 20 mmol) was dissolved in the mixture of 30 mL of
Et2O and 5 mL of MeOH. This solution was cooled to 0 °C and a solu-
tion of diazomethane in ether was slowly added with stirring, until
the solution did not bubble and the color of solution remained yel-
low. The solution was slowly allowed to warm to room temperature
without additional heating. The reaction mixture was concentrated
under reduced pressure. The crude productwas passed through a sil-
ica gel column (ethyl acetate/hexanes = 1:6) to afford 7 as a colorless
min. ½a 2D0
ꢁ
¼ ꢀ44:2 (c 3.2, CHCl3). 1H NMR (300 MHz, CDCl3) d:
7.56–7.53 (m, 2H), 7.36–7.25 (m, 3H), 3.77 (s, 3H), 3.75 (s, 1H),
1.79 (s, 3H). 13C NMR (75 MHz, CDCl3) d: 176.1, 142.7, 128.3,
127.8, 125.1, 75.8, 53.2, 26.7. HRMS (TOF) calcd for C10H12O3+Na+:
203.0684, found: 203.0679.
4.3.2. (R)-Ethyl 2-hydroxy-2-phenylpropanoate
75% Yield, 54% ee determined by chiral GC analysis (CYCLODEX-
B column, 115 °C). Retention time: tmajor = 38.3 min, tminor = 39.4
oil(2.9 g, 93%yield). ½a D18
ꢁ
¼ ꢀ73:4(c0.99, CHCl3). 1H NMR(500 MHz,
min. ½a 2D0
ꢁ
¼ ꢀ29:6 (c 2.55, CHCl3). 1H NMR (300 MHz, CDCl3) d:
CDCl3): d 9.75 (d, 1H, J = 9.5 Hz), 3.71 (s, 3H), 2.12 (d, 1H, J = 9.5 Hz),
1.86–1.83 (m, 1H), 1.55 (s, 3H), 1.27 (s, 3H). 13C NMR (125 MHz,
CDCl3): d 200.4, 170.4, 52.2, 40.9, 36.1, 29.8, 28.3, 15.5. HRMS
(TOF) calcd for C8H13NO3 [M+H+]: 157.0859, found: 157.0865.
7.58–7.54 (m, 2H), 7.38–7.26 (m, 3H), 4.29–4.17 (m, 2H), 3.79 (s,
1H), 1.78 (s, 3H), 1.26 (t, J = 7.1 Hz, 3H). 13C NMR (75 MHz, CDCl3)
d: 175.6, 142.9, 128.3, 127.7, 125.1, 75.6, 62.4, 26.7, 14.0. HRMS
(TOF) calcd for C11H14O3+Na+: 217.0841, found: 217.0834.
4.2.2. (1R,5S)-6,6-Dimethyl-3-oxa-bicyclo[3.1.0]hexan-2-one 8
At first, NaBH4 (3.8 g, 100 mmol) was dissolved in 60 mL of dry
MeOH, and a solution of 7 (15.6 g, 100 mmol) in MeOH was added
slowly into the above mixture. Next, the reaction was kept for
30 min. Then, concentrated HCl (1 mL) was added to stop the reac-
tion and extracted with ether (40 mL ꢂ 3). The organic layer was
dried over anhydrous Na2SO4 and then, concentrated to give a light
yellow oil (13.7 g, 87% yield).
To a solution of benzene (60 mL) was added the light yellow oil
(8.0 g, 50 mmol). The mixture was refluxed for 2 h, then, concen-
trated under reduced pressure (2 mmHg, 56 °C) to give 8 (90%
yield). 1H NMR (500 MHz, DMSO): d 4.38–4.35 (m, 1H), 4.16–4.14
(d, 1H, J =10 Hz), 2.06–2.03 (m, 1H), 1.96–1.94 (m, 1H), 1.18 (s,
3H), 1.17 (s, 3H). 13C NMR (125 MHz, DMSO): d 174.9, 66.5, 30.5,
30.0, 25.2, 23.0, 14.4.
4.3.3. (R)-Isopropyl 2-hydroxy-2-phenylpropanoate
70% Yield, 70% ee determined by chiral GC analysis (G-TA col-
umn, 95 °C). Retention time: tmajor = 76.0 min, tminor = 82.4 min.
½
a 2D0
ꢁ
¼ ꢀ42:5 (c 1.85, CHCl3). 1H NMR (300 MHz, CDCl3) d: 7.58–
7.54 (m, 2H), 7.38–7.26 (m, 3H), 5.10–5.01 (m, 1H), 3.81 (s, 1H),
1.76 (s, 3H), 1.29 (d, J = 6.2 Hz, 3H), 1.18 (d, J = 6.2 Hz, 3H). 13C
NMR (75 MHz, CDCl3) d: 175.1, 143.0, 128.2, 127.6, 125.1, 75.5,
70.2, 26.6, 21.6, 21.4. HRMS (TOF) calcd for C12H16O3+Na+:
231.0997, found: 231.0995.
4.3.4. (R)-tert-Butyl 2-hydroxy-2-phenylpropanoate
85% Yield, 81% ee determined by chiral GC analysis (G-TA col-
umn, 100 °C). Retention time: tmajor = 57.5 min, tminor = 60.0 min.
½
a 2D0
ꢁ
¼ ꢀ54:0 (c 4.35, CHCl3). 1H NMR (300 MHz, CDCl3) d: 7.57–
7.54 (m, 2H), 7.37–7.26 (m, 3H), 3.87 (s 1H), 1.79 (s, 3H), 1.44 (s,
9H). 13C NMR (75 MHz, CDCl3) d: 174.8, 143.3, 128.1, 127.5,
125.2, 83.0, 75.6, 27.8, 26.6. HRMS (TOF) calcd for C13H18O3+Na+:
245.1154, found: 245.1152.
4.2.3. (1R,3S)-N-Benzyl-3-(hydroxymethyl)-2,2-dimethylcyclo-
propanecarboxamide 9
Benzylamine (2.2 mL, 20 mmol) was dissolved in dry THF
(10 mL), cooled to ꢀ15 °C, and DIBAL-H (13 mL, 20 mmol) was in-
jected. After the mixture was stirred for 20 min, the solution was
allowed to warm to 30 °C and left to react for 3 h. Then, it was
cooled to ꢀ5 °C, and a solution of 8 (2.2 mL) in THF (7.5 mL) was
added with stirring for 10 min. After 20 h at room temperature,
the reaction was quenched with water (10 mL) and 4 N HCl,
(15 mL) and the mixture was extracted several times with Et2O.
The combined organic phases were washed with 1 N HCl, then,
dried over Na2SO4 and concentrated under reduced pressure to
4.3.5. (R)-Methyl 2-(4-chlorophenyl)-2-hydroxypropanoate
66% Yield, 41% ee determined by chiral GC analysis (CYCLODEX-
B column, 150 °C). Retention time: tmajor = 21.5 min, tminor = 22.2
min. ½a 2D0
ꢁ
¼ ꢀ22:6 (c 3.15, CHCl3). 1H NMR (300 MHz, CDCl3) d:
7.51–7.48 (m, 2H), 7.34–7.26 (m, 2H), 3.78 (s, 3H), 3.77 (s, 1H),
1.76 (s, 3H). 13C NMR (75 MHz, CDCl3) d: 175.8, 141.2, 133.8,
128.4, 126.8, 75.4, 53.4, 26.9. HRMS (TOF) calcd for C10H11O3Cl+
Na+: 237.0397, found: 237.0293.
give the products 9 (3.5 g, 87% yield). ½a D20
ꢁ
¼ þ41:0 (c 1.14, CHCl3).
4.3.6. (R)-Methyl 2-hydroxy-2-p-tolylpropanoate
1H NMR (500 MHz, DMSO): d 7.36–7.33 (m, 2H), 7.30–7.26 (m, 3H),
6.07 (br, 1H), 4.46–4.44 (m, 2H), 4.03–3.97 (m, 1H), 3.88–3.84 (m,
1H), 3.15–3.12 (m, 1H), 1.42–1.37 (m, 2H), 1.20 (s, 3H), 1.16 (s, 3H).
13C NMR (125 MHz, DMSO): d 171.3, 138.2, 128.7, 127.8, 127.6,
59.1, 43.8, 32.6, 31.8, 28.6, 24.2, 15.5.
68% Yield, 61% ee determined by chiral GC analysis (CYCLODEX-
B column, 130 °C). Retention time: tmajor = 30.1 min, tminor = 31.2
min. ½a 2D0
ꢁ
¼ ꢀ34:0 (c 2.15, CHCl3). 1H NMR (300 MHz, CDCl3) d:
7.44–7.41 (m, 2H), 7.18–7.15 (m, 2H), 3.77 (s, 3H), 3.69 (s, 1H),
2.34 (s, 3H), 1.77 (s, 3H). 13C NMR (75 MHz, CDCl3) d: 176.2,
139.8, 137.5, 129.0, 125.0, 75.6, 53.1, 26.6, 20.9. HRMS (TOF) calcd
for C11H14O3+Na+: 217.0841, found: 217.0845.
4.3. General procedure for the asymmetric addition of dimethyl-
zinc to a-ketoesters
4.3.7. (R)-Methyl 2-hydroxy-2-(4-methoxyphenyl) propanoate
60% Yield, 67% ee determined by chiral GC analysis (CYCLODEX-
B column, 155 °C). Retention time: tmajor = 25.2 min, tminor = 25.5
A solution of Me2Zn (1.5 mL, 1.2 M in toluene, 1.8 mmol,
1.8 equiv) was added to a solution of amine alcohol ligand 9
(23 mg, 0.1 mmol, 0.1 equiv) at 0 °C under an atmosphere of argon.
After 30 min, the mixture was cooled to ꢀ20 °C and ketoester
(1 mmol) was added. The reaction was kept for 48 h. Then, saturated
min. ½a 2D0
ꢁ
¼ ꢀ38:3 (c 2.4, CHCl3). 1H NMR (300 MHz, CDCl3) d:
7.47–7.44 (m, 2H), 6.89–6.86 (m, 2H), 3.80 (s, 3H), 3.78 (s, 3H),
3.70 (s, 1H), 1.77 (s, 3H). 13C NMR (75 MHz, CDCl3) d: 176.3,

B. Zheng et al. / Tetrahedron: Asymmetry 20 (2009) 2125–2129
2129
159.2, 134.9, 126.4, 113.7, 75.4, 55.2, 53.1, 26.7. HRMS (TOF) calcd
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87% Yield, 45% ee determined by chiral GC analysis (CYCLODEX-
B column, 115 °C). Retention time: tmajor = 32.7 min, tminor = 33.4
min. ½a 2D0
ꢁ
¼ ꢀ25:4 (c 2.0, CHCl3). 1H NMR (300 MHz, CDCl3) d:
7.26–7.23 (m, 1H), 7.08–7.06 (m, 1H), 6.98–6.95 (m, 1H), 4.00 (s,
1H), 3.82 (s, 3H), 1.83 (s, 3H). 13C NMR (75 MHz, CDCl3) d: 175.1,
147.5, 126.9, 125.0, 124.0, 74.3, 53.4, 27.8. HRMS (TOF) calcd for
C8H10O3S+Na+: 209.0248, found: 209.0248.
4.3.9. (R)-Methyl 2-hydroxy-2-(naphthalen-2-yl) propanoate
69% Yield, 64% ee determined by chiral GC analysis (CYCLODEX-
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2009, 15, 3069–3071.
B
column, 145 °C). Retention time: tmajor = 152.0 min, tminor =
155.6 min. ½a 2D0
ꢁ
¼ ꢀ36:0 (c 1.2, CHCl3). 1H NMR (300 MHz, CDCl3)
d: 8.03–8.02 (m, 1H), 7.84–7.81 (m, 3H), 7.64 (dd, J = 8.7, 1.9 Hz,
1H), 7.49–7.46 (m, 2H), 3.88 (s, 1H), 3.78 (s, 3H), 1.88 (s, 3H). 13C
NMR (75 MHz, CDCl3) d: 176.0, 140.0, 133.0, 132.8, 128.3, 128.1,
127.5, 126.2, 124.1, 123.4, 76.0, 53.3, 26.7. HRMS (TOF) calcd for
C14H14O3+Na+: 253.0831, found: 253.0841.
5. Huang, P.-Q.; Zheng, X.; Deng, X.-M. Tetrahedron Lett. 2001, 42, 9039–
9041.
6. CCDC 646165 contains the supplementary crystallographic data for 9. These
data can be obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/conts/retrieving.html [or from the CCDC,
Cambridge, CB21EZ, UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.
cam.ac.uk].
7. (a) Oezcubukcu, S.; Schmidt, F.; Bolm, C. Org. Lett. 2005, 7, 1407–1409; (b) Ji, J.-X.;
Wu, J.; Au-Yeung, T. T. L.; Yip, C.-W.; Haynes, R. K.; Chan, A. S. C. J. Org. Chem. 2005,
70, 1093–1095; (c) Bolm, C.; Rudolph, J. J. Am. Chem. Soc. 2002, 124, 14850–14851.
8. Creary, X. J. Org. Chem. 1987, 52, 5026–5030.
Acknowledgment
Financial support from the National Natural Science Foundation
of China (Grant No. 20742004) is gratefully acknowledged.