Highly enantioselective allylation of α-ketoesters catalyzed by N,N′-dioxide-in(III) complexes

Article abstract of DOI:10.1021/jo701491r

(Chemical Equation Presented) An efficient asymmetric allylation of α-ketoesters was catalyzed by the N,N′-dioxide-In(III) complex in excellent yields (up to 99%) and high enantioselectivities (up to 94% ee) for a variety of substrates under mild reaction

Full text of DOI:10.1021/jo701491r

Highly Enantioselective Allylation of r-Ketoesters Catalyzed by
N,N′-Dioxide-In(III) Complexes
Ke Zheng,† Bo Qin,† Xiaohua Liu,† and Xiaoming Feng*,†,‡
Key Laboratory of Green Chemistry & Technology (Sichuan UniVersity), Ministry of Education,
College of Chemistry, Sichuan UniVersity, Chengdu 610064, China, and State Key Laboratory of
Biotherapy, Sichuan UniVersity, Chengdu 610041, China
xmfeng@scu.edu.cn
ReceiVed July 9, 2007
An efficient asymmetric allylation of R-ketoesters was catalyzed by the N,N′-dioxide-In(III) complex
in excellent yields (up to 99%) and high enantioselectivities (up to 94% ee) for a variety of substrates
under mild reaction conditions. On the basis of experimental results, a possible catalytic cycle including
a transition state has been proposed to explain the origin of the reactivity and asymmetric inductivity,
and a bifunctional catalysis was described with Lewis base N-oxide activating tetraallylstannane and
Lewis acid indium activating R-ketoester.
Introduction
asymmetric reactions. Kozlowski et al. reported the addition of
Et2Zn to R-ketoesters catalyzed by chiral Ti complexes.3b,c
Snapper and Hoveyda et al. have developed a related method
of Al-catalyzed asymmetric additions of dialkylzinc.3a Asym-
metric nitroaldol reaction of R-ketoesters catalyzed by cinchona
alkaloids was reported by Deng and co-workers.5d Jiang and
co-workers have employed chiral amino alcohol as ligands in
Zn-catalyzed enantioselective alkynylations of aromatic R-ke-
toesters.4 However, the asymmetric allylation of R-ketoesters
Optically active R-hydroxy acids and their derivatives are very
important structural motifs in numerous biologically interesting
compounds1 and are often utilized as resolving agent.2 R-Ke-
toesters, as a new type of prochiral substrate, can be readily
transformed to a series of R-hydroxyesters by alkylation,3
alkynylation,4 and nitroaldol5 reaction. Accordingly, consider-
able effort has been devoted to their preparation via catalytic
† Key Laboratory of Green Chemistry & Technology.
(6) For recent reports about allylation of ketones see: (a) Prieto,
O.; Woodward, S. J. Organomet. Chem. 2006, 691, 1515-1519. (b)
Tietze, L. F.; Kinzel, T.; Schmatz, S. J. Am. Chem. Soc. 2006, 128,
11483-11495. (c) Miller, J. J.; Sigman, M. S. J. Am. Chem. Soc. 2007,
129, 2752-2753. (d) Wadamoto, M.; Yamamoto, H. J. Am. Chem. Soc.
2005, 127, 14556-14557. (e) Wooten, A. J.; Kim, J. G.; Walsh, P. J. Org.
Lett. 2007, 9, 381-384. (f) Wada, R.; Oisaki, K.; Kanai, M.; Shibasaki,
M. J. Am. Chem. Soc. 2004, 126, 8910-8911. (g) Yamada, K.; Tozawa,
T.; Nishida, M.; Mukaiyama, T. Bull. Chem. Soc. Jpn. 1997, 70, 2301-
2308.
‡ State Key Laboratory of Biotherapy.
(1) (a) In ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N., Pfaltz,
A., Yamamoto, H., Eds.; Speringer: Berlin, Germany 1999; Vols. I-III.
(b) In Catalytic Asymmetric Synthesis, 2nd ed.; Ojima, I., Ed.; Wiley-
VCH: New York, 2000. (c) Coppola, G. M.; Schuster, H. F. Chiral
R-Hydroxy Acids in EnantioselectiVe Synthsis; Wiley-VCH; Weinheim,
Germany, 1997.
(2) For a review see: Kinbara, K. Synlett 2005, 732-743.
(3) (a) Wieland, L. C.; Deng, H. B.; Snapper, M. L.; Hoveyda, A. H. J.
Am. Chem. Soc. 2005, 127, 15453-15456. (b) DiMauro, E. F.; Kozlowski,
M. C. Org. Lett. 2002, 4, 3781-3784. (c) DiMauro, E. F.; Kozlowski, M.
C. J. Am. Chem. Soc. 2002, 124, 12668-12669.
(4) Jiang, B.; Chen, Z. L.; Tang, X. X. Org. Lett. 2002, 4, 3451-3453.
(5) (a) Christensen, C.; Juhl, K.; Hazell, R. G.; Jørgensen, K. A. J. Org.
Chem. 2002, 67, 4875-4881. (b) Christensen, C.; Juhl, K.; Jørgensen, K.
A. Chem. Commun. 2001, 2222-2223. (c) Du, D. M.; Lu, S. F.; Fang, T.;
Xu, J. X. J. Org. Chem. 2005, 70, 3712-3715. (d) Li, H. M.; Wang, B.
M.; Deng, L. J. Am. Chem. Soc. 2006, 128, 732-733.
(7) For a review on N-oxide in the asymmetric synthesis see: (a) Malkov,
A. V.; Kocˇovsky´, P. Eur. J. Org. Chem. 2007, 29-36. (b) Denmark, S. E.;
Fan, Y. J. Am. Chem. Soc. 2002, 124, 4233-4235. (c) Shimada, T.; Kina,
A.; Hayashi, T. J. Org. Chem. 2003, 68, 6329-6337. (d) Traverse, J. F.;
Zhao, Y.; Hoveyda, A. H.; Snapper, M. L. Org. Lett. 2005, 7, 3151-3154.
(e) Hrdina, R.; Valterova´, I.; Hodacˇova´, J.; C´ısaˇrova´, I.; Kotora, M. AdV.
Synth. Catal. 2007, 349, 822-826.
(8) Wen, Y. H.; Huang, X.; Huang, J. L.; Xiong, Y.; Qin, B.; Feng, X.
M. Synlett 2005, 2445-2448.
10.1021/jo701491r CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/06/2007
8478
J. Org. Chem. 2007, 72, 8478-8483

Allylation of R-Ketoesters
FIGURE 1. Ligands employed for asymmetric allylation.
TABLE 1. Effect of Allylic Reagents in Enantioselective Allylation
TABLE 3. Effect of Metal Reagents and Solvent on the
of r-Ketoester 3a
Asymmetric Allylation of r-Ketoester 3a
entry
metal
InBr3
solvent
time (h)
yield (%)b
ee (%)c
entry
allylic reagent
yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
9
DMF
DMF
DMF
DMF
DMF
Et2O
24
18
72
72
72
48
48
48
48
36
90
99
73
86
1
2
3
4
5
allyltrichlorosilane
allyltrimethylsilane
allyltrimethoxysilane
allyltributylstannane
tetraallylstannane
NDd
NDd
NDd
NDd
15
In(OTf)3
Sc(OTf)3
Ti(OiPr)4
Cu(OTf)2
In(OTf)3
In(OTf)3
In(OTf)3
In(OTf)3
In(OTf)3
NDd
NDd
NDd
60
0
10
22
5
12
15
a Conditions: All reactions were performed with R-ketoester 3 (0.1 mmol)
and allylic reagent (1.1 equiv) in DMF (1.0 mL) at 0 °C for 72 h. b Isolated
yield. c Determined by HPLC on a Chiralcel OJ-H column. d Not detected.
THF
46
15
21
82
toluene
CH2Cl2
CH3CN
10
a Conditions: All reactions were performed with R-ketoester 3 (0.1 mmol)
and tetraallylstannane (1.1 equiv) in solvent (1.0 mL) at 0 °C for 18 to
72 h. Catalyst was composed of a 1:1 molar ratio of 1e to metal. b Iso-
lated yield. c Determined by HPLC on a Chiralcel OJ-H column. d Not
detected.
TABLE 2. Allylation of r-Ketoester 3 Catalyzed by In(III)
Complexesa
TABLE 4. Effect of Ester Group on the Asymmetric Allylation of
r-Ketoestersa
entry
N,N′-dioxide
time (h)
yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
2a
2b
1g
1h
48
48
48
10
18
12
12
10
12
24
80
78
87
93
99
95
85
99
95
67
27
60
71
80
86
80
84
83
77
10
entry
R-ketoesters
time (h)
yield (%)b
ee (%)c
1
2
3
4
3
4a
7
18
6
12
12
99
99
92
85
86
93
89
87
10
8
a Conditions: All reactions were performed with R-ketoester 3 (0.1 mmol)
and tetraallylstannane (1.1 equiv) in DMF (1.0 mL) at 0 °C for 10 to 48 h.
Catalyst was composed of a 1:1 molar ratio of ligand to In(OTf)3. b Isolated
yield. c Determined by HPLC on a Chiralcel OJ-H column.
a Conditions: All reactions were performed with R-ketoesters (0.1 mmol)
and tetraallylstannane (1.1 equiv) in DMF (1.0 mL) at 0 °C for 6 to 18 h.
Catalyst was composed of a 1:1 molar ratio of 1e to In(OTf)3. b Isolated
yield. c Determined by HPLC on a Chiralcel OJ-H column.
was rarely described6g although the resultant R-allyl-R-hydrox-
yesters are useful starting components for the synthesis of
pharmaceutical agents and natural products.1,6
Results and Discussion
It has been demonstrated in the asymmetric cyanation8,9 and
allylation13 that the N,N′-dioxide acted as Lewis base to activate
the silicon atom and tin atom. So the N,N′-dioxide 1a derived
from L-proline (Figure 1) was employed as an organocatalyst
for the enantioselective allylation of R-ketoester 3 with different
As a versatile catalyst, chiral N-oxides play a significant
role in many asymmetric procedures.7 A series of C2-symmetric
chiral N,N′-dioxides as highly efficient organocatalysts
have been successfully used in the cyanation of aldehydes,8
aldimines,9 ketones,10 and ketoimines.11 Recently, the chiral
N,N′-dioxide metal complexes also showed good results in the
cyanation12 and allylation13 of carbonyl compounds. Herein, we
wish to report the asymmetric allylation of R-ketoesters
catalyzed by N,N′-dioxide-In(III) complex in excellent yields
with high enantioselectivities.
(9) Jiao, Z. G.; Feng, X. M.; Liu, B.; Chen, F. X.; Zhang, G. L.; Jiang,
Y. Z. Eur. J. Org. Chem. 2003, 3818-3826.
(10) Qin, B.; Liu, X. H.; Shi, J.; Zheng, K.; Zhao, H. T.; Feng, X. M. J.
Org. Chem. 2007, 72, 2374-2378.
(11) (a) Huang, J. L.; Liu, X. H.; Wen, Y. H.; Qin, B.; Feng, X. M. J.
Org. Chem. 2007, 72, 204-208. (b) Huang, X.; Huang, J. L.; Wen, Y. H.;
Feng, X. M. AdV. Synth. Catal. 2006, 348, 2579-2584.
J. Org. Chem, Vol. 72, No. 22, 2007 8479

Zheng et al.
TABLE 5. Scope of the Enantioselective Allylation of r-Ketoestersa
a Conditions: All reactions were performed with R-ketoesters (0.1 mmol) and tetraallylstannane (1.1 equiv) in DMF (1.0 mL) at 0 °C for 6 to 36 h.
Catalyst was composed of a 1:1 molar ratio of 1e to In(OTf)3. b Isolated yield. c Determined by chiral HPLC or GC analysis as described in the Supporting
Information. d The absolute configuration of the product 5a was determined by comparison with the reported value16 of optical rotation (see the Supporting
Information). e The absolute configurations were assigned by comparing the Cotton effect of the CD spectra with that of 5a.
TABLE 6. Control Experimentsa
allylic reagent (Table 1). Unfortunately, only tetraallylstannane
gave a racemate product in low yield (Table 1, entry 5).
We speculated that the N,N′-dioxide coordinated with a
suitable metal reagent might increase the chiral induction. So
In(III), which had been effective in allylation of the carbonyl
compounds,13-15 was selected as a promising candidate. For-
tunately, In(OTf)3 combined with N,N′-dioxide 1a could suc-
cessfully catalyze the reaction with 27% ee and 80% yield
(Table 2, entry 1). Encouraged by this result, a series of N-oxides
1 and 2 were prepared and evaluated in the asymmetric allylation
of R-ketoester 3 (Table 2, entries 2-9). Excitingly, the enan-
tiomeric excess was improved to 60% and 71% when the
L-proline-derived N,N′-dioxide 1a was replaced by (S)-pipecolic
ligand
(mol %)
In(OTf)3
(mol %)
tetraallylstannane
(equiv)
yield
(%)b
ee
entry
(%)c
1
2
3
4
5
6
7
6 (10)
6 (10)
1.1
1.1
1.1
1.1
1.1
0.5
0.25
NDd
67
18
53
99
10
0
0
8
93
92
81
1e (10)
9 (10)
1e (10)
1e (10)
1e (10)
10
10
10
10
83
42
a Conditions: All reactions were performed with R-ketoester 4a (0.1
mmol) and tetraallylstannane in DMF (1.0 mL) at 0 °C for 36 h. b Isolated
yield. c Determined by HPLC on a OJ-H column. d Not detected.
(12) (a) Li, Q. H.; Liu, X. H; Wang, J.; Shen, K.; Feng, X. M.
Tetrahedron Lett. 2006, 47, 4011-4014. (b) Li, Q. H.; Chang, L.; Liu,
X. H.; Feng, X. M. Synlett 2006, 1675-1678. (c) Shen, Y. C.; Feng,
X. M.; Li, Y.; Zhang, G. L.; Jiang, Y. Z. Eur. J. Org. Chem. 2004, 129-
137.
acid-derived N,N′-dioxides 1b and 1c (Table 2, entries 2 and
3). The enantioselectivity was further increased by changing
the R group of the ligand’s amide moiety from the aromatic
group to the aliphatic group (Table 2, entries 4-6). The
enantiomeric excess was increased to 86% with the cyclopen-
(13) Zhang, X.; Chen, D. H.; Liu, X. H.; Feng, X. M. J. Org. Chem.
2007, 72, 5227-5233.
(14) Lu, J.; Hong, M. L.; Ji, S. J.; Teo, Y. C.; Loh, T. P. Chem. Commum.
2005, 4217-4218.
(15) Teo, Y. C.; Goh, J. D.; Loh, T. P. Org. Lett. 2005, 7, 2743-2745.
8480 J. Org. Chem., Vol. 72, No. 22, 2007

Allylation of R-Ketoesters
tanylamine-base N,N′-dioxide 1e (Table 2, entry 5). However,
no better results were obtained by using aliphatic amine based
N,N′-dioxides (2a, 2b) with ramipril acid backbone (Table 2,
entries 7 and 8). Moreover, the linker length of the catalyst was
also screened and the results indicated that the three-carbon
linkage was the best (Table 2, entries 5 vs 9, 10).
Following these results, different metal reagents were inves-
tigated to improve the enantioselectivity. As shown in Table 3,
only InBr3 or In(OTf)3 coordinated with ligand 1e could produce
the product with a high enantioselectivity of 73% ee and 86%
ee, respectively (Table 3, entries 1 and 2); other metal reagents
coordinated with ligand 1e could not catalyze the enantiose-
lective allylation of R-ketoester 3 (Table 3, entries 3-5).
Subsequently, we examined the effect of solvents under the
condition of 10 mol % of 1e-In(OTf)3 (1:1) complex as catalyst
and 1.1 equiv of tetraallylstannane. As shown in Table 3, the
enantioselectivity and reactivity were deeply dependent on the
solvents (Table 3, entries 2 and 6-10). DMF was chosen as
the optimal solvent, in which 99% yield and 86% ee were
obtained (Table 3, entry 2).
FIGURE 2. Precursor of the chiral N,N′-dioxide 1e and chiral N-
oxide 9.
Under the conditions of 10 mol % of 1e and 10 mol % of
In(OTf)3 in DMF, further improvement was achieved by
changing the ester group of the R-ketoester. When the ester
group was changed from ethyl to methyl, the ee value was
increased to 93% (Table 4, entry 2). However, the ee value was
not increased when the ester group was changed to larger groups
such as i-Pr and t-Bu (Table 4, entries 3 and 4). These results
showed that the smaller ester group was suitable for this catalytic
system.
Next, the concentration of R-ketoester, molar ratios of metal
to ligand, temperature, and catalyst loading were investigated,
but the reactivity and enantioselectivity could not be improved
(see the Supporting Information). Moreover, this allylation could
be conducted under air without any decrease in yield and
enantiomeric excess. Hence, the optimal condition was 10 mol
% of 1e-In(OTf)3 and 0.1 M concentration of R-ketoester with
1.1 equiv of tetraallylstannane in DMF at 0 °C.
Encouraged by the results obtained from R-ketoester 4a under
the optimized conditions, a variety of R-ketoesters were
investigated. As summarized in Table 5, aromatic R-ketoesters
as well as heterocyclic R-ketoester afforded R-allyl-R-hydrox-
yesters 5 in excellent yields (up to 99%) with high enantiomeric
excess (up to 94% ee). Comparison of the experimental results
(Table 5, entries 2-8) revealed the negative effect of ortho-
substitution on aromatic R-ketoesters for the enantioselectivity.
The o-methyl- and o-methoxylphenyl R-ketoesters (4b and 4g)
afforded the products with 77% ee and 83% ee, respectively
(Table 5, entries 2 and 7), which were lower than those obtained
from m- or p-substituted-phenyl R-ketoesters (89-94% ee, Table
5, entries 3-6 and 8). In particular, R-ketoester 4h gave the
best ee value (94% ee, Table 5, entry 8). However, the aliphatic
R-ketoester 4k only gave a moderate enantioselectivity (69%
ee, Table 5, entry 11).
FIGURE 3. 1H NMR spectra of the NH group of N,N′-dioxide 1e in
diverse reaction stage: (a) 1e in CDCl3 and (b) 1e and In(OTf)3 (ratio
1/1) in DMSO-d6.
compounds exhibited a similar (-) Cotton effect in their CD
spectra (see the Supporting Information). It could be deduced
that these compounds possess the same configuration (R) as
5a.
Mechanism Studies. Control experiments were performed
to provide insight into the mechanism. As shown in Table 6,
no corresponding R-hydroxyester was obtained with amide 6
(Figure 2) as catalyst (Table 6, entry 1), which suggested that
the reaction could not occur with only chiral amide 6 activating
tetraallylstannane. The In(OTf)3 coordinated with amide 6 as
the catalyst could catalyze the allylation of R-ketoesters with
67% yield, but no enantiomeric excess was observed (Table 6,
entry 2). The result demonstrates that the N-oxide played a key
role in the reaction. To compare with N,N′-dioxide 1e, N-oxide
9 with one dipolar group was prepared and evaluated in the
reaction (Table 6, entry 4). However, only 8% ee and 53% yield
were observed in the allylation of R-ketoester. This important
piece of evidence suggested that tetraallylstannane was simul-
taneously activated by the two oxygen atoms of N,N′-dioxide
1e. In addition, by decreasing the amount of tetraallylstannane
to 0.5 and 0.25 equiv, the reaction proceeded smoothly with
81% yield and 92% ee and 42% yield and 81% ee, respectively
(Table 6, entries 6 and 7). This result showed that one molecular
tetraallylstannane could provide more than one allyl group.
To determine the status of the coordination between In(OTf)3
and N,N′-dioxide 1e, the 1H NMR and ESI-HRMS studies have
been carried out (Figure 3). The NH proton showed a deshield-
ing effect at 10.30 ppm due to the characteristic strong
intramolecular hydrogen bond between N-oxide and the NH
proton (Figure 3a). However, upon combination of In(OTf)3 and
N,N′-dioxide 1e in a ratio of 1:1, a new upfield shift was
observed at 5.76 ppm (Figure 3b). Furthermore, the catalyst
compositions and molecular weight studies by the positive mode
ESI-HRMS spectrum showed that the major peak corresponded
to the 1:1 complex of In(III) and N,N′-dioxide 1e (727.0610,
calcd for [N,N′-dioxide 1e + In(OTf) - H]+ ) 727.1843), which
might be the major active species (Figure 4).
The absolute configuration of 5a was determined as R by
comparison with literature data.16 To determine the absolute
configurations of the other products, the CD (circular dichroism)
spectra of the products 5a-f,h were measured in ethanol. These
(16) (a) Ooi, T.; Fukumoto, K.; Maruoka, K. Angew. Chem., Int. Ed.
2006, 45, 3839-3842. (b) Moorlag, H.; Kellogg, R. M.; Kloosterman, M.;
Kaptein, B.; Kamphuis, J.; Schoemaker, H. E. J. Org. Chem. 1990, 55,
5878-5881. (c) Fra´ter, G.; Mu¨ller, U.; Gu¨nther, W. Tetrahedron Lett. 1981,
22, 4221-4224.
J. Org. Chem, Vol. 72, No. 22, 2007 8481

Zheng et al.
FIGURE 4. The proposed catalytic cycle.
FIGURE 5. Plausible transition state.
On the basis of the experimental results, the possible catalytic
cycle was proposed in Figure 4. We speculated the N,N′-dioxide
1e coordinated with In(OTf)3 to form complex A.17 Then the
N-oxide as the Lewis base actived the tin atom by formation of
a possible hypervalent tin species (complex B) and enhanced
the nucleophilicity of the allylic reagent.18 One of the activated
allyl groups might be transferred from the tin atom to the indium
atom so as to form an allylic indium complex (complex C).13,19
8482 J. Org. Chem., Vol. 72, No. 22, 2007

Allylation of R-Ketoesters
As a Lewis acid, the indium activated the R-ketoester to form
the possible complex D.3a,5a The allylic group in indium
preferred to attack the activated R-ketoester to afford the product
5a′. The triallylstannane was eliminated from complex D as 5a′,
and another tetraallylstannane was coordinated by the N-oxide
to regenerate complex B.
On the basis of the observed absolute configuration of 5a,16
we proposed a possible transition state I. In transition state I,
the allylic group was much more accessible to attack the Si-
face of the carbonyl of R-ketoester 4a than the Re-face, since
the Re-face attack was likely to increase the steric repulsion
between the phenyl group of R-ketoester 4a and the cyclo-
pentyl subunit of the catalyst in proposed transition state II
(Figure 5).
devoted to search for effective catalyst systems that tolerate a
range of R-ketoesters with higher yield and enantioselectivity.
Experimental Section
Typical Experimental Procedure for the Enantioselective
Allylation of R-Ketoesters. The (S)-pipecolic acid-based N,N′-
dioxide 1e (4.7 mg, 10 mol %) and In(OTf)3 (5.6 mg, 10 mol %)
in anhydrous DMF (1.0 mL) was stirred at ambient temperature
for 10 min, then R-ketoester 4a (16.4 mg, 0.1 mmol) was added to
the mixture. The tetraallylstannane (25 µL, 0.11 mmol) was added
at 0 °C. After being stirred for 6 h, the reaction mixture was directly
purified by column chromatography on silica gel eluted ether/
petroleum ether (1/10) to give (R)-methyl 2-hydroxy-2-phenylpent-
4-enoate 5a in 99% isolated yield with 93% ee, [R]28 -36.8 (c
D
0.38, CH2Cl2); 1H NMR (400 MHz, CDCl3) δ 7.58-7.60 (m, 2H),
7.27-7.37 (m, 3H), 5.74-5.85 (m, 1H), 5.12-5.19 (m, 2H), 3.76
(s, 4H), 2.98 (dd, J ) 14.0, 7.6 Hz, 1H), 2.77 (dd, J ) 14.0, 7.2
Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 174.0, 140.2, 131.3, 127.3,
126.8, 124.5, 118.4, 77.1, 52.2, 43.1; ESI-HRMS calcd for
(C12H14O3 + Na+) 229.0835, found 229.0841. HPLC (DAICEL
CHIRALCEL OJ-H, hexane/2-propanol 80/20, 1.0 mL/min) tR-
(minor) ) 6.77 min, tR(major) ) 13.36 min.
Conclusions
In summary, we have developed the first enantioselective
allylation of R-ketoesters by chiral N,N′-dioxide-In(III) com-
plex. Under the optimized conditions, excellent yields (up to
99%) and high enantioselectivities (up to 94% ee) were obtained
for a range of R-ketoesters. Attractive features of the current
method included the catalyst preparation with readily available
material, mild reaction conditions, and convenient procedure
with the tolerance of moisture and air. On the basis of the
experimental results, a possible catalytic cycle and favorable
transition state have been proposed. Future efforts will be
Acknowledgment. This work was financially supported by
the National Natural Science Foundation of China (Nos.
20732003 and 20602025) and the Student Innovation Fund of
Sichuan University (No. 2006 L011). We also thank Sichuan
University Analytical & Testing Center for NMR and CD
spectra analysis and State Key Laboratory of Biotherapy for
HRMS analysis.
(17) Delpech, F.; Guzei, I. A.; Jordan, R. F. Organometallics 2002, 21,
1167-1176.
Supporting Information Available: Experimental precedures
and characterization of products for catalysts and racemates, CD
spectra, 1H NMR and 13C NMR spectra, HRMS and HPLC
conditions, etc. This material is available free of charge via the
Internet at http://pubs.acs.org.
(18) Denmark, S. E.; Fu, J. J. Am. Chem. Soc. 2003, 125, 2208-2216.
(19) (a) Takita, R.; Fukuta, Y.; Tsuji, R.; Ohshima, T.; Shibasaki. M.
Org. Lett. 2005, 7, 1363-1366. (b) Yasuda, M.; Miyai, T.; Shibata, I.; Baba,
A.; Nomura, R.; Matsuda, H. Tetrahedron Lett. 1995, 36, 9497-9500. (c)
Shibata, I.; Kato, H.; Ishida, T.; Yasuda, M.; Baba, A. Angew. Chem., Int.
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JO701491R
J. Org. Chem, Vol. 72, No. 22, 2007 8483