Asymmetric cyclopropanation of β,γ-unsaturated α-ketoesters with stabilized sulfur ylides catalyzed by C 2-symmetric ureas

Article abstract of DOI:10.1021/jo101699r

A novel organocatalytic asymmetric cyclopropanation of β,γ- unsaturated α-ketoesters with stabilized sulfur ylides using C ₂-symmetric urea as a hydrogen-bond catalyst has been described. This reaction allows an efficient access to 1,2,3-trisubstituted cyclopropane derivatives in moderate to good yields with up to 16:1 dr and 90:10 er under mild reaction conditions. The mechanism study proved that the high stereoinduction originated from the cooperative effect of the hydrogen-bond catalyst.

Full text of DOI:10.1021/jo101699r

pubs.acs.org/joc
SCHEME 1. Catalytic Asymmetric Cyclopropanation of Electron-
Deficient Olefins with Sulfur Ylides
Asymmetric Cyclopropanation of β,γ-Unsaturated
r-Ketoesters with Stabilized Sulfur Ylides Catalyzed
by C2-Symmetric Ureas
Ying Cheng, Jing An, Liang-Qiu Lu, Lan Luo,
Zheng-Yi Wang, Jia-Rong Chen,* and Wen-Jing Xiao*
Key Laboratory of Pesticide & Chemical Biology, Ministry of
Education, College of Chemistry, Central China Normal
University, 152 Luoyu Road, Wuhan, Hubei 430079, China
wxiao@mail.ccnu.edu.cn; jiarongchen2003@yahoo.com.cn
Received August 30, 2010
decades.2 Of the most important methods available, asym-
metric cyclopropanation of electron-deficient olefins with
ylides, pioneered by Corey in 1965,3 is an attractive strategy.
For example, Dai and Tang4 have elegantly developed an
enantioselective synthesis of vinylcyclopropanes using chiral
telluronium and sulfonium ylides. However, catalytic asym-
metric cyclopropanation of electron-deficient olefins with
ylides is very rare. In this regard, the groups of Aggarwal5
and Gaunt6 first disclosed enantioselective cyclopropanations
by the catalytic generation of chiral sulfonium and ammonium
ylides. Recently, Shibasaki7 successfully developed a La-Li
(biphenyldiolate)3/NaI complex-promoted asymmetric cyclo-
propanation of enones with dimethyloxosulfonium methylide
(Scheme 1A). Notably, MacMillan ingeniously achieved a
highly enantioselective organocatalytic cyclopropanation of
R,β-unsaturated aldehydes with stabilized sulfonium ylides
based on iminium catalysis and directed electrostatic activa-
tion by the use of 2-carboxylic acid dihydroindole as the
catalyst (Scheme 1B).8,9 Despite advances, the further devel-
opment of efficient and practical organocatalytic systems10
A novel organocatalytic asymmetric cyclopropanation of
β,γ-unsaturated R-ketoesters with stabilized sulfur ylides
using C2-symmetric urea as a hydrogen-bond catalyst has
been described. This reaction allows an efficient access to
1,2,3-trisubstituted cyclopropane derivatives in moderate
to good yields with up to 16:1 dr and 90:10 er under mild
reaction conditions. The mechanism study proved that
the high stereoinduction originated from the cooperative
effect of the hydrogen-bond catalyst.
(5) (a) Aggarwal, V. K.; Smith, H. W.; Hynd, G.; Jones, R. V. H.;
Fieldhouse, R.; Spey, S. E. J. Chem. Soc., Perkin Trans. 1 2000, 3267.
(b) Aggarwal, V. K.; Alsono, E.; Fang, G.; Ferrara, M.; Hynd, G.; Porcelloni,
M. Angew. Chem., Int. Ed. 2001, 40, 1433. (c) For a recent comprehensive
mechanistic study, see: Riches, S. L.; Saha, Filgueira, N. F.; Grange, E.;
McGarrigle, E. M.; Aggarwal, V. K. J. Am. Chem. Soc. 2010, 132, 7626.
(6) Papageorgiou, C. D.; Cubillo de Dios, M. A.; Ley, S. V.; Gaunt, M. J.
Angew. Chem., Int. Ed. 2004, 43, 4641.
(7) (a) Kakei, H.; Sone, T.; Sohtome, Y.; Matsunaga, S.; Shibasaki, M.
J. Am. Chem. Soc. 2007, 129, 13410. With stoichiometric amount of Lewis
acids and a chiral ligand: (b) Mamai, A.; Madalengoita, J. S. Tetrahedron
Lett. 2000, 41, 9009.
The cyclopropane motif is a common structural subunit in
many natural products and biologically active molecules and
serves as a versatile and important building block in organic
synthesis because of its unique combination of reactivity and
structural properties.1 Consequently, great research efforts
have been directed toward the stereoselective construction
of such three-membered carbocyclic rings over the last few
(1) (a) Taber, D. F. In Comprehensive Organic Sythesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon: New York, 1991; Vol. 3, p 1045. (b) Donaldson,
W. A. Tetrahedron 2001, 57, 8589. (c) Pietruszka, J. Chem. Rev. 2003, 103,
1051. (d) Reissig, H.-U.; Zimmer, R. Chem. Rev. 2003, 103, 1151.
(2) Lebel, H.; Marcoux, J. F.; Molinaro, C.; Charette, A. B. Chem. Rev.
2003, 103, 977.
(8) (a) Kunz, R. K.; MacMillan, D. W. C. J. Am. Chem. Soc. 2005, 127,
3240. For similar works, see: (b) Hartikka, A.; Arvidsson, P. I. J. Org. Chem.
ꢀ
ꢀ
2007, 72, 5874. (c) Hartikka, A.; Slosarczyk, A. T.; Arvidsson, P. I. Tetra-
hedron: Asymmetry 2007, 18, 1403. (d) Zhao, Y.-H.; Zhao, G.; Cao, W.-G.
Tetrahedron: Asymmetry 2007, 18, 2462.
(9) For other organocatalytic approaches, see also: (a) Arai, S.; Nakaya-
ma, K.; Ishida, T.; Shioiri, T. Tetrahedron Lett. 1999, 40, 4215. (b) Rios, R.;
(3) (a) Corey, E. J.; Chaykovsky, M. J. Am. Chem. Soc. 1965, 87, 1353.
(b) Payne, G. B. J. Org. Chem. 1967, 32, 3351.
ꢀ
ꢀ
Sunden, H.; Vesely, J.; Zhao, G.-L.; Dziedzic, P.; Cordova, A. Adv. Synth.
Catal. 2007, 349, 1028. (c) Xie, H.-X.; Zu, L.-S.; Wang, J.; Wang, W. J. Am.
Chem. Soc. 2007, 129, 10886.
(4) (a) Ye, S.; Huang, Y.-Z.; Xia, C.-A.; Tang, Y.; Dai, L.-X. J. Am.
Chem. Soc. 2002, 124, 2432. (b) Liao, W.-W.; Li, K.; Tang, Y. J. Am. Chem.
Soc. 2003, 125, 13030. For more related work, see: (c) Zheng, J.-C.; Liao,
W.-W.; Tang, Y.; Sun, X.-L.; Dai, L.-X. J. Am. Chem. Soc. 2005, 127, 12222.
(d) Deng, X.-M.; Cai, P.; Ye, S.; Sun, X.-L.; Liao, W.-W.; Li, K.; Tang, Y.;
Wu, Y.-D.; Dai, L.-X. J. Am. Chem. Soc. 2006, 128, 9730. (e) Zhu, B.-H.;
Zhou, R.; Zheng, J.-C.; Deng, X.-M.; Sun., X.-L.; Sheng, Q.; Tang, Y. J. Org.
Chem. 2010, 75, 3454.
(10) For recent general reviews on organocatalysis, see: (a) Berkessel, A.;
€
Groger, H. Asymmetric Organocatalysis; Wiley-VCH: Weinheim, 2005.
(b) Dalk, P. I. Enantioselective Organocatalysis: Reactions and Experimental
Procedures; Wiley-VCH: Weinheim, 2007; Vol. 1. (c) Yu, X.; Wang, W.
Chem. Asian J. 2008, 3, 516. (d) Bertelsen, S.; Jørgensen, K. A. Chem. Soc.
Rev. 2009, 38, 2178. (e) Melchiorre, P. Angew. Chem., Int. Ed. 2009, 48, 1360.
DOI: 10.1021/jo101699r
r
Published on Web 12/08/2010
J. Org. Chem. 2011, 76, 281–284 281
2010 American Chemical Society

Cheng et al.
JOCNote
TABLE 1. Condition Optimization for the Asymmetric Cyclopropanation
between β,γ-Unsaturated r-Ketoesters and Sulfur Ylide 3aa
entry
cat.
solvent
R
temp (°C)
yieldb (%)
erc
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1a
1a
1a
1a
1a
1a
1a
1a
toluene
toluene
toluene
toluene
xylenes
THF
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
0
0
0
0
0
0
0
0
0
77
27
53
50
35
45
61
27
46
48
53
81
80:20
75:25
80:20
54:46
70:30
53:47
60:40
53:47
50:50
73:27
72:28
85:15
FIGURE 1. C2-Symmetric hydrogen-bond catalysts employed in
this study.
Et2O
CH2Cl2
MeOH
toluene
toluene
toluene
for the synthesis of optically active cyclopropanes with func-
tional diversity is still highly desirable.11
10
11
12
0
0
iPr
Me
-40
As part of our ongoing program on carbon- and heterocycle-
oriented methodology development based on sulfur ylides,12
we recently disclosed highly chemo- and stereoselective [4 þ 1]
cycloaddition/rearrangement of nitroolefins,12a [4 þ 1]/[3 þ 2]
cycloaddition cascade of alkene-tethered nitroolefins,12b and
[4 þ 1] cycloaddition of R,β-unsaturated imines, providing
a variety of nitrogen-containing heterocycles.12c Inspired by
these achievements, we describe herein the first example of
hydrogen-bond catalyzed cyclopropanation of β,γ-unsaturated
R-ketoesters with stabilized sulfur ylides (Scheme 1, C),13,14
the products of which can be readily transformed into the
corresponding amino acids or R-hydroxy acids.15
aExperimental conditions: a mixture of 2 (0.4 mmol), 3a (0.44 mmol),
and 1 (10 mol %) in solvent (2.0 mL) was stirred at the indicated
temperature. bIsolated yield of the pure diastereoisomer 4. cDetermined
by chiral HPLC analysis.
TABLE 2. Asymmetric Cyclopropanations of β,γ-Unsaturated
r-Ketoester 2a with Sulfur Ylides 3a-ka
Initially, we investigated the possible cyclopropanation
reaction between β,γ-unsaturated R-ketoester 2a and sulfur
ylide 3a with four common and readily available C2-symmetric
hydrogen-bond catalysts (Figure 1).16 As revealed in Table 1,
all of these catalysts can efficiently promote the desired cyclo-
addition reactions at 10 mol % catalyst loading in toluene,
giving the corresponding cyclopropane 4aa with variable
enantioselectivity (Table 1, entries 1-4). Initial catalyst
screening indicated that urea 1a displayed the highest cata-
lytic activity in terms of reaction efficiency (77% isolated yield)
and enantioselectivity (80:20 er) (Table 1, entry 1). With
the use of 1a, we then simply examined a series of solvents
and revealed that toluene was the optimal reaction media
entry
R
yieldb (%)
drc
erd
1
2
3
4
5
6
7
8
Ph (3a)
4aa
4ab
4ac
4ad
4ae
4af
4ag
4ah
4ai
81
85
85
73
74
86
60
54
61
37
85
79
8:1
12:1
8:1
10:1
13:1
12:1
10:1
5:1
4:1
5:1
10:1
12:1
85:15
78:22
81:19
87:13
84:16
83:17
77:23
71:29
87:13
78:22
72:28
90:10
p-MeOC6H4 (3b)
p-MeC6H4 (3c)
p-FC6H4 (3d)
p-ClC6H4 (3e)
p-BrC6H4 (3f)
m-BrC6H4 (3g)
o-MeOC6H4 (3h)
2-Thioenyl (3i)
PhCHdCH (3j)
PhCH2CH2 (3k)
p-ClC6H4 (3e)
9
10
11
12e
4aj
4ak
4ae
aExperimental conditions: a mixture of 2 (0.4 mmol), 3 (0.44 mmol),
and 1a (10 mol %) in toluene (2.0 mL) was stirred at -40 °C for 3 days.
bIsolated yield of the pure diastereoisomer 4. cDetermined by 1H NMR.
dDetermined by chiral HPLC analysis. eSee the Supporting Information
for details.
(11) Bhella, S. S.; Elango, M.; Ishar, M. P. S. Tetrahedron 2009, 65, 240
and references cited therein.
(12) (a) Lu, L.-Q.; Cao, Y.-J.; Liu, X.-P.; An, J.; Yao, C.-J.; Ming, Z.-H.;
Xiao, W.-J. J. Am. Chem. Soc. 2008, 130, 6946. (b) Lu, L.-Q.; Li, F.; An, J.;
Zhang, J.-J.; An, X.-L.; Hua, Q.-L.; Xiao, W.-J. Angew. Chem., Int. Ed. 2009,
48, 9542. (c) Lu, L.-Q.; Zhang, J.-J.; Li, F.; Cheng, Y.; An, J.; Chen, J.-R.;
Xiao, W.-J. Angew. Chem., Int. Ed. 2010, 49, 4495.
(Table 1, entries 5-9 vs entry 1). Further tuning of the steric
hindrance of the ester moiety of β,γ-unsaturated R-ketoester
led to no positive effect on the enantioselectivity (Table 1,
entries 10 and 11). Finally, it was found that lowering the
reaction temperature to -40 °C increased the enantiomeric
ratio to 85:15 (Table 1, entry 12).
With the optimal conditions in hand (Table 1, entry 12,
10 mol % of urea 1a in toluene at -40 °C), we first investigated
the substrate scope for this asymmetric cyclopropanation by
employing a variety of sulfur ylides. As summarized in Table 2,
various sulfur ylides proved to be suitable for this catalytic
asymmetric cyclization reaction. For example, both electron-
donating and electron-withdrawing substituents in the benzene
(13) For recent reviews on H-bond catalysis, see: (a) Schreiner, P. R. Chem.
Soc. Rev. 2003, 32, 289. (b) Connon, S. J. Chem. Eur. J. 2006, 12, 5418.
;
(c) Doyle, A. G.; Jacobsen, E. N. Chem. Rev. 2007, 107, 5713. (d) Takemoto, Y.
Org. Biomol. Chem. 2005, 3, 4299. (e) Pihko, P. M., Ed. Hydrogen Bonding in
Organic Synthesis; Wiley: Weinheim, 2009.
(14) For a recent excellent review on C2-symmetric H-bond catalysis, see:
(a) Sohtome, Y.; Nagasawa, K. Synlett 2010, 1, 1. For selected examples
about C2-symmetric H-bond catalysts promoted reactions, see: (b) Sohtome,
Y.; Tanatani, A.; Hashimoto, Y.; Nagasawa, K. Tetrahedron Lett. 2004, 45,
5589. (c) Sohtome, Y.; Takemura, N.; Takagi, R.; Hashimoto, Y.; Nagasawa,
K. Tetrahedron 2008, 64, 9423. (d) Zhang, Y.; Liu, Y.-K.; Kang, T.-R.; Hu,
Z.-K.; Chen, Y.-C. J. Am. Chem. Soc. 2008, 130, 2456. (e) Allu, S.; Selvakumar,
S.; Singh, V. K. Tetrahedron Lett. 2010, 51, 446.
(15) Rueping, M.; Nachtsheim, B. J.; Moreth, S. A.; Bolte, M. Angew.
Chem., Int. Ed. 2008, 47, 593.
(16) Fleming, E. M.; McCabe, T.; Connon., S. J. Tetrahedron Lett. 2006,
47, 7037 and references 14b-14d.
282 J. Org. Chem. Vol. 76, No. 1, 2011

Cheng et al.
JOCNote
TABLE 3. Asymmetric Cyclopropanation of Sulfur Ylide 3a with
SCHEME 2. Variation of Chemical Shifts of N-H of
Thiourea 1b in Different Combinations (in CDCl3, for b-d:
donor/acceptor=1:1)
Various β,γ-Unsaturated r-Ketoesters 2a-ja
entry
R
yieldb (%)
drc
erd
1
2
3
4
5
6
7
8
9
10
Ph (2a)
4aa
4ba
4ca
4da
4ea
4fa
4ga
4ha
4ia
81
54
61
55
57
52
49
56
58
35
8:1
16:1
9:1
7:1
7:1
7:1
10:1
5:1
85:15
80:20
83:17
83:17
81:19
80:20
77:23
81:19
85:15
90:10
p-MeOC6H4 (2b)
p-MeC6H4 (2c)
p-FC6H4 (2d)
p-ClC6H4 (2e)
p-BrC6H4 (2f)
m-BrC6H4 (2g)
o-FC6H4 (2h)
o-ClC6H4 (2i)
PhCHdCH (2j)
9:1
2:1
4ja
aExperimental conditions: a mixture of 2 (0.4 mmol), 3a (0.44 mmol),
and 1a (10 mol %) in toluene (2.0 mL) was stirred at -40 °C for 3 days.
bIsolated yield of the pure diastereoisomer 4. cDetermined by 1H NMR.
dDetermined by chiral HPLC analysis.
SCHEME 3. Variation of Chemical Shifts of C-H of Sulfur
Ylide 3a upon Combination with Catalysts (in CDCl3)
ring of sulfur ylide 3a-h are well tolerated, and the correspond-
ing products were obtained in moderate to good isolated
yields (54-86%) and stereoselectivities (up to 13:1 dr, 87:13 er)
(Table 2, entries 1-8). Introducing an electron-donating
group (e.g., methoxyl) on the ortho-position of the aromatic
ring led to somewhat decreased yield and diastereoselectivity
(Table 2, entry 8). Note that the heterocycle-substituted
sulfur ylide can also participate in this cyclization to give
the desired product in good yield with 4:1 dr and 87:13 er
(Table 2, entry 9). A vinyl-type group was also tolerated,
albeit with diminished yield (Table 2, entry 10). Importantly,
the reaction with alkyl-substituted sulfur ylide, such as 3k,
proceeded successfully to give cyclopropane 4ak in 85%
yield with 10:1 dr and 72:28 er (Table 2, entry 11). Notably,
each of the products 4 can be easily separated by column
chromatography on silica gel to obtain the major diastereo-
isomer as a pure compound. To demonstrate preparative
utility, the reaction of 3e and 2a was performed on a larger
scale (2.0 mmol vs 0.40 mmol). As expected, the reaction
afforded the corresponding cyclopropane 4ae in 79% iso-
lated yield with good stereoselectivities (Table 2, entry 12).
Significantly, structural variation in the β,γ-unsaturated
R-ketoester components can also be realized. As shown in
Table 3, various electron-poor and -rich β,γ-unsaturated
R-ketoesters with different substitution patterns on the aromatic
ring reacted smoothly with sulfur ylide 3a under optimal con-
ditions, affording the corresponding cyclopropanes 4aa-ia
in high yields with high diastereo- (up to 16:1 dr) and
enantioselectivities (up to 85:15 er) (Table 3, entries 1-9).
Vinyl-subsituted β,γ-unsaturated R-ketoester, such as 2j, can
also be successfully employed in this transformation, and
high er (90:10) was obtained, albeit with moderate yield and
dr (35% yield, 2:1 dr) (Table 3, entry 10). The products
4aa-ak and 4ba-4ja were fully characterized by 1H NMR
and 13C NMR, as well as HRMS. Furthermore, the absolute
configuration of 4af was unambiguously determined to be
(1S,2R,3R) by X-ray analysis.17
To gain insight into the mechanism of the asymmetric cyclo-
1
propanation, we carried out a series of H NMR spectro-
scopic studies with catalysts 1a and 1b, and the representative
results are shown in Schemes 2 and 3.18 It was demonstrated
that two reaction components and the product have a
substantial effect on the chemical shift of N-H in catalysts,
and the H-bond interaction between sulfur ylide 3a and 1b
was the strongest (Scheme 2b, Δδ 1.37 ppm; 1.20 ppm).
Moreover, when the sulfur ylide 3a was mixed with thiourea
1b and urea 1a, respectively, in CDCl3, urea 1a resulted in a
remarkable change in chemical shift of C-H of ylide 3a,
which indicated a stronger H-bond interaction between urea
1a and sulfur ylide 3a (Scheme 3c: Δδ 0.22 ppm). Consequently,
the hydrogen-bond catalyst urea 1a was believed to provide a
multi-hydrogen-bond cooperative substrate activation.14
Based on the above observations and the X-ray structure
of the product 4af (Figure 2), we proposed a possible path-
way to account for the observed stereoinduction. As depicted
in Scheme 4, catalyst 1a interacted with ylide 3a to form
the complex A through a strong hydrogen-bond effect. Then,
the resulting complex A incorporated with β,γ-unsaturated
(18) In the 1H NMR experiments, we used thiourea 1b other than urea 1a
because of the poor solubility of urea 1a in CDCl3. Interestingly, we found
that the mixture of urea 1a and ylide 3a was completely dissolved in CDCl3
possibly due to the strong H-bond interaction. Please see more details in the
Supporting Information.
(17) CCDC 776429 for product 4af contains supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk /
data_request/cif.
J. Org. Chem. Vol. 76, No. 1, 2011 283

Cheng et al.
JOCNote
ylides by cooperative hydrogen-bond catalysis. The process
provided a complementary method to previously documented
Lewis acid or chiral amine catalyzed asymmetric cyclo-
propanation.7,8 Although the stereoselectivity awaits further
improvement, the current methodology provide a new strategy
for catalytic asymmetric cyclopropanation reactions. Addi-
tional mechanistic studies and extension of this methodology
to more functionalized substrates are now ongoing in our
research group.
Experimental Section
FIGURE 2. Optimized structure and free energy gap of the transi-
tion states.
General Procedure for Asymmetric Cyclopropanation between
β,γ-Unsaturated r-Ketoesters and Stabilized Sulfur Ylides. β,γ-
Unsaturated R-ketoester 2a (76.1 mg, 0.40 mmol) and catalyst
1a (28.9 mg, 0.04 mmol) were mixed in dry toluene (2.0 mL).
After the mixture was stirred at -40 °C for 0.5 h, sulfur ylide 3a
(79.3 mg, 0.44 mmol) was introduced directly. The reaction
mixture was stirred at -40 °C for 3 days. After complete
consumption of the β,γ-unsaturated R-ketoester 2a (as moni-
tored by TLC), the mixture was purified by flash column
chromatography on silica gel (petroleum ether/ethyl ether
(5:1-3:1)) to give the corresponding pure product 4aa in 81%
yield as white solid. The diastereoisomer ratio (8:1) was directly
determined by 1H NMR of the reaction mixture. The enantio-
meric ratio (85:15) was determined by chiral HPLC analysis
(Daicel Chirapak OD-H, hexane/2-propanol 90/10, flow rate
1.0 mL/min, T 25 °C, 254 nm, tR 21.71 min (major), tR 19.29 min
SCHEME 4. Proposed Catalytic Pathway
(minor)): [R]19 þ67.65 (c 2.5, CHCl3).
D
Methyl 2-(2-benzoyl-3-phenylcyclopropyl)-2-oxoacetate (4aa):
1H NMR (600 MHz, CDCl3) δ 7.96 (d, J = 7.6 Hz, 2H), 7.57
(t, J = 7.3 Hz, 1H), 7.44 (t, J = 7.7 Hz, 2H), 7.36 (t, J = 7.5 Hz,
2H), 7.30 (t, J = 7.3 Hz, 1H), 7.25 (d, J = 7.3 Hz, 2H), 3.88 (s,
3H), 3.47-3.42 (m, 2H), 3.31 (dd, J = 8.7, 6.7 Hz, 1H); 13C NMR
(150 MHz, CDCl3) δ 194.09, 188.50, 160.88, 137.32, 135.96,
133.67, 128.81, 128.66, 128.52, 127.43, 126.37, 53.06, 39.35,
34.73, 32.09; MS m/z 308.2 (Mþ); HRMS m/z calcd for C19
H
-
17O4þ [M þ H]þ 309.1127, found 309.1111.
R-ketoester 2a to give a ternary complex B, wherein the
hydrogen bonds directed the reactants in the proper posi-
tions and induced the observed facial selectivities in inter-
mediate C.14,16 Finally, the intermediate C underwent an
intramolecular SN2 displacement to afford the desired pro-
pane 4aa with release of the urea 1a for the next catalytic
cycle. Note that the prediction of possible interaction in
complex B was in agreement with the NMR studies and DFT
calculation (Figure 2).19
Acknowledgment. We are grateful to the Program for
Academic Leader in Wuhan Municipality (200851430486)
and the National Natural Science Foundation of China (No.
21072069 and 21002036) for support of this research. We
also sincerely thank Dr. L. Peng and Prof. F.-L. Gu of South
China Normal University for the DFT calculation.
In conclusion, we have developed a catalytic asymmetric
cyclopropanation of β,γ-unsaturated R-ketoesters with sulfur
Supporting Information Available: Full experimental details,
spectroscopic data of 4, CIF file for 4af, HPLC spectra of 4, and
details of mechanism. This material is available free of charge
via the Internet at http://pubs.acs.org.
(19) See the Supporting Information for details.
284 J. Org. Chem. Vol. 76, No. 1, 2011