Highly enantioselective aldol reaction of acetone with β,γ- unsaturated α-keto esters promoted by simple chiral primary-tertiary diamine catalysts

Article abstract of DOI:10.1039/c1ob05607g

A series of primary-tertiary diamine catalysts were successfully applied to promote the enantioselective aldol reaction of acetone with β,γ-unsaturated α-keto esters in excellent yields (up to 99%) and enantioselectivities (up to 96% ee). The Royal Societ

Full text of DOI:10.1039/c1ob05607g

View Article Online / Journal Homepage / Table of Contents for this issue
Organic &
Biomolecular
Chemistry
Dynamic Article Links
Cite this: Org. Biomol. Chem., 2011, 9, 4774
www.rsc.org/obc
COMMUNICATION
Highly enantioselective aldol reaction of acetone with b,c-unsaturated a-keto
esters promoted by simple chiral primary–tertiary diamine catalysts
Lin Peng,a,b Liang-Liang Wang,a,b Jian-Fei Bai,a,b Li-Na Jia,a,b Yun-Long Guo,a,b Xi-Ya Luo,a,b
Fei-Ying Wang,a Xiao-Ying Xu*a and Li-Xin Wang*a
Received 15th April 2011, Accepted 11th May 2011
DOI: 10.1039/c1ob05607g
A series of primary–tertiary diamine catalysts were success-
fully applied to promote the enantioselective aldol reaction of
acetone with b,c -unsaturated a-keto esters in excellent yields
(up to 99%) and enantioselectivities (up to 96% ee).
ple and commercially available, are widely used in asymmetric
transformation,10 and can be easily transformed to tunable
primary–tertiary diamine bifunctional catalysts.11 As a part
of our continuing interest in asymmetric synthesis based on
chiral primary amine catalysis,12 herein we wish to report a
highly enantioselective (up to 96% ee) aldol reaction of acetone
with b,g -unsaturated a-keto esters catalyzed by simple chiral
primary–tertiary diamines to construct types of chiral tertiary
alcohols.
The reaction of acetone with b,g -unsaturated a-keto ester 2a
was chosenas a modelreaction tooptimize the reaction conditions.
A series of catalysts (Fig. 1) and acid additives13 were examined
and the results are summarized in Table 1, and moderate yield
and enantioselectivity (46% yield and 40% ee, Table 1, entry 1)
was observed in the presence of 20 mol% 1a in CH2Cl2 at room
temperature. A remarkable improvement was achieved when 4-
nitrobenzoic acid was added as an acid additive (Table 1, entry 2).
In the presence of the same additive, a variety of primary–tertiary
diamine catalysts 1b–1f were further evaluated, and all the cases
afforded the adducts with better enantioselectivities than their
counterparts without additive (Table 1, entries 3–7). Particularly,
primary–tertiary diamine 1a gave better results (64% yield and
73% ee; Table 1, entry 2), and was selected for further screening of
acid additives. All the additives afforded moderate to good yields
and enantioselectivities except acetic acid, trifluoroacetic acid and
p-toluenesulfonic acid (Table 1, entries 8–21). 3,5-Dinitrobenzoic
acid gave the best yield and enantioselectivity (70% yield and 81%
ee; Table 1, entry 16), and was selected for other optimizations.
The chiral tertiary alcohol framework occurs prevalently in a
variety of natural products and biologically active compounds,
as well as being a class of useful synthetic building blocks in
organic synthesis.1 Tremendous efforts have been devoted to the
synthesis of these attractive subunits, with methods including
asymmetric oxidation,2 enantioselective desymmetrization,3 stere-
ospecific insertion of carbene into secondary alcohols4 and nucle-
ophilic addition.5 Among them, the aldol reaction of carbonyl
compounds (ketones and aldehydes) with a-keto esters, especially
organocatalytic versions, is a straightforward way to access chiral
tertiary alcohols.6 Up till now, the functional reactants of this
transformation included mainly the aromatic a-keto esters (or a-
keto acids) and b,g -unsaturated a-keto esters. The reaction of
cyclohexanone with b,g -unsaturated a-keto ester has been well-
documented and excellent results have been obtained.7 However,
the highly efficient reaction of acetone with b,g -unsaturated a-
keto ester has been less exploited. Tang7a and Zhao7b reported the
reaction of acetone with b,g -unsaturated a-keto esters catalyzed
by L-proline derivatives individually and only poor enantioselec-
tivities (up to 45% ee) were achieved. Very recently, Chan and
Kwong8 revealed a Cinchona alkaloid derivative primary amine
promoted the enantioselective aldol reaction of acetone with b,g -
unsaturated a-keto esters in excellent enantioselectivities (up to
94% ee), the only and the highest enantioselective example up to
the present. For further and deeper understanding and expanding
of this useful and asymmetric transformation, it is still highly
desirable to develop more and efficient organocatalytic protocols
for the syntheses of optically active chiral tertiary alcohols.
Over the past decades, chiral aminocatalysis has emerged as
a powerful synthetic strategy in organic synthesis.9 Chiral 1,2-
diphenylethane-1,2-diamine and cyclohexane-1,2-diamine, sim-
Fig. 1 Chiral catalysts evaluated.
aKey Laboratory of Asymmetric Synthesis and Chirotechnology of Sichuan
Province, Chengdu Institute of Organic Chemistry, Chinese Academy of
Sciences, Chengdu 610041, People’s Republic of China. E-mail: wlxioc@
cioc.ac.cn, xuxy@cioc.ac.cn; Fax: +86-028-8525-5208
The effects of solvents, the amount of acetone, catalyst loading,
reaction temperature and reactant concentration were then studied
and the results are summarized in Table 2, and solvents exerted
a significant effect on the yields and enantioselectivities. Polar
bGraduate School of Chinese Academy of Sciences, Beijing 10039, People’s
Republic of China
4774 | Org. Biomol. Chem., 2011, 9, 4774–4777
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Table 1 The effects of additivesa
Table 2 Optimization of reaction conditionsa
Entry Catalyst Additive
Time/h Yield (%)b ee (%)c
Acetone/ Catalyst/
equiv.
Entry Solvent
mol (%) Time/h Yield (%)b ee (%)c
1
2
3
4
5
6
7
8
1a
1a
1b
1c
1d
1e
1f
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
—
112
112
112
112
112
112
112
112
72
46
64
44
46
61
56
66
20
79
78
64
61
71
50
81
70
55
51
68
71
72
40
73
48
48
44
61
65
49
34
43
72
66
53
65
77
81
71
68
77
66
55
4-NO2PhCOOH
4-NO2PhCOOH
4-NO2PhCOOH
4-NO2PhCOOH
4-NO2PhCOOH
4-NO2PhCOOH
AcOH
1
CH2Cl2
10
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
10
30
20
20
20
20
20
20
20
112
112
112
112
112
112
112
112
72
70
84
73
80
90
94
92
99
82
83
89
85
47
90
80
84
86
83
96
99
98
99
98
99
99
99
99
95
93
81
79
77
79
78
84
84
82
74
82
82
72
64
10
44
38
37
75
85
66
84
85
83
86
85
91
92
94
93
2
3
ClCH2CH2Cl 10
CHCl3
10
10
10
4
5
6
PhCH3
p-xylene
cyclohexane 10
7
8
hexane
Et2O
10
10
10
10
10
10
10
10
10
10
10
5
9
CF3COOH
p-TsOH
10
11
12
13
14
15
16
17
18
19
20
21
72
9
1,4-dioxane
MTBE
EtOAc
(CH3)2CO
THF
CH3OH
CH3CN
DMF
2-OHPhCOOH
2-NO2PhCOOH
4-COOHPhCOOH
2,6-di-FPhCOOH
2,3-di-OHPhCOOH
3,5-di-NO2PhCOOH
2-NO2-4-ClPhCOOH
2-CH3-3-NO2PhCOOH
3-OH-4-NO2PhCOOH
112
112
112
112
112
112
112
112
112
10
11
12
13
14
15
16
17
18
19
20
21
22
23d
24e
25f
112
112
48
112
48
112
48
48
DMSO
cyclohexane
112
72
96
72
72
48
48
48
48
D-camphor-10-sulfonic acid 112
L-camphor-10-sulfonic acid 112
cyclohexane 20
cyclohexane 40
cyclohexane 20
cyclohexane 20
cyclohexane 20
cyclohexane 20
cyclohexane 20
a Unless otherwise specified, all reactions were carried out with acetone (2.0
mmol), (E)-methyl 2-oxo-4-phenylbut-3-enoate (2a, 0.20 mmol), additive
(0.04 mmol, 20 mol%), the catalyst 1 (0.04 mmol, 20 mol%) in 1 mL
CH2Cl2 at room temperature. b Isolated yield after silica gel column
chromatography. c The ee values were determined by HPLC using Chiral
OD-H column.
26e,g cyclohexane 20
27e,h cyclohexane 20
28e,i cyclohexane 20
29e,j cyclohexane 20
48
72
96
solvents such as methanol, CH3CN, DMF and DMSO gave
excellent yields (80–90%) and poor enantioselectivities (10–44%
ee, Table 2, entries 14–17), while nonpolar or low polarity solvents
such as alkylhalides, toluene, alkanes and ethers gave good
enantioselectivities (74–84% ee, Table 2, entries 1–10), and the
best results (94% yield and 84% ee) were obtained in cyclohexane
(Table 2, entry 6). When 20 equivalents of acetone in cyclohexane
were used, 85% ee and 96% yield were obtained (Table 2, entry 19).
When the catalyst loading was lowered to 10 mol%, the enantiose-
lectivity was slightly decreased to 84% ee without loss of reactivity
(Table 2, entry 21). Still, excellent yields were obtained without
any observable effects of the substrate concentration (98%–99%
yield, Table 2, entries 23–25). In general, the reaction temperature
has significant effects on the enantioselectivities. Lowering the
reaction temperature to -20 ◦C, the best enantioselectivity was
obtained (94% ee, Table 2, entry 28). When it was further lowered
to -25 ◦C, the yield and enantioselectivity were slightly decreased,
and a longer reaction time was required (93% and 93% ee, Table
2, entry 29). Through these screenings, the optimized reaction
conditions were found to be a combination of 20 mol% of 1a with
20 mol% of 3,5-dinitrobenzoic acid, 20 eq. acetone, in cyclohexane
(0.6 mL) at -20 ◦C.
a Unless otherwise specified, all reactions were carried out with acetone
(4.0 mmol), (E)-methyl 2-oxo-4-phenylbut-3-enoate (2a, 0.20 mmol), 3,5-
dinitrobenzoic acid (0.04 mmol, 20 mol%), the catalyst 1a (0.04 mmol, 20
mol%) in 1 mL solvent at room temperature. b Isolated yield after silica
gel column chromatography. c The ee values were determined by HPLC
using Chiral OD-H column. d Cyclohexane: 0.4 mL. e Cyclohexane: 0.6
◦
mL. f Cyclohexane:◦0.8 mL. g Temperature: 0 C. h Temperature: -10 ◦C.
i Temperature: -20 C. j Temperature: -25 ◦C.
(up to 95% yield and 94% ee, Table 3, entries 1–4). Almost in
each case, moderate to excellent yields (up to 99%) and excellent
enantioselectivities (up to 96% ee) were achieved with various
b,g -unsaturated a-keto ester derivatives bearing either electro-
donating or electron-withdrawing substituents at para- (Table
3, entries 5–8), meta- (Table 3, entries 9–14) or ortho-positions
(Table 3, entries 15, 16) in aromatic rings. Further broadening the
substrates to heteroaromatic b,g -unsaturated a-keto esters, the
reaction also gave excellent enantioselectivity with only moderate
yield (40% yield and 90% ee, Table 3, entry 18). To understand the
potential applications of our catalyst system, the present procedure
was scaled up to 1 mmol of the starting a-keto ester and the
corresponding product was also smoothly obtained in moderate
yield (68%) without deleterious loss of the enantioselectivity (92%
ee vs. 94% ee, Table 3, entry 19 vs. entry 1).
Under the optimal reaction conditions, the scopes and lim-
itations of the reaction of acetone and b,g -unsaturated a-
keto esters 2 were further investigated. As shown in Table 3,
the steric hindrances of the ester groups slightly affected the
yields and enantioselectivities and good results were obtained
Cyclic ketones, such as cyclopentanone and dihydro-2H-pyran-
4(3H)-one were also investigated under the optimized reaction
conditions. As shown in Scheme 1, the corresponding products
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 4774–4777 | 4775
©

View Article Online
Table 3 Aldol reaction between acetone and 2a
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 20802075 and No. 21042006) and the
Chinese Academy of Sciences.
References
Entry R1
R2
Time/h Yield (%)b ee (%)c
1 Representative examples of tertiary carbinol containing bioactive
compounds: Fostriecin, Camptothecin, Integerrimine, Erythromycin
and Zaragozic acid.
2 For selected examples of asymmetric epoxidation: (a) V. J. Subramana-
ian, J. Ind. Microbiol., 1986, 1, 119–127; (b) J. T. Rossiter, S. R. Williams,
A. E. G. Cass and D. W. Ribbons, Tetrahedron Lett., 1987, 28, 5173–
5174; (c) R. W. Murray, Chem. Rev., 1989, 89, 1187–1201; (d) M. Takagi,
K. Uemura and K. Furuhashi, Ann. N. Y. Acad. Sci., 1990, 613, 697–
701; (e) H. C. Kolb, M. S. VanNieuwenhze and K. B. Sharpless, Chem.
Rev., 1994, 94, 2483–2547.
3 For selected examples of enantioselective desymmetrization of prochi-
ral tertiary alcohols: (a) B. Jung and S. H. Kang, Proc. Natl. Acad.
Sci. U. S. A., 2007, 104, 1471–1475; (b) B. Jung, M. S. Hong and S. H.
Kang, Angew. Chem., Int. Ed., 2007, 46, 2616–2618.
4 For selected examples of stereospecific insertion of carbene into
secondary alcohols: (a) Y. Masaki, H. Arasaki and M. Shiro, Chem.
Lett., 2000, 29, 1180–1181; (b) Y. Masaki, H. Arasaki and M. Iwata,
Chem. Lett., 2003, 32, 4–5; (c) J. L. Stymiest, V. Bagutski, R. M. French
and V. K. Aggarwal, Nature, 2008, 456, 778–783.
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Me
Et
i-Pr 72
n-Bu 72
72
72
95 (3a)
91 (3b)
90 (3c)
84 (3d)
96 (3e)
64 (3f)
85 (3g)
90 (3h)
99 (3i)
95 (3j)
54 (3k)
98 (3l)
98 (3m)
98 (3n)
91 (3o)
98(3p)
93 (3q)
40 (3r)
68 (3s)
94
92
91
93
91
93
95
92
92
93
93
93
93
90
91
91
96
90
92
4-FC6H4
4-ClC6H4
4-NO2C6H4
4-CH3C6H4
3-FC6H4
3-ClC6H4
3-BrC6H4
3-NO2C6H4
3-CH3C6H4
3-CH3OC6H4
2-ClC6H4
2-BrC6H4
1-naphthyl
2-thienyl
Ph
Me
Me
Me
Me
Me
Me
Me
Et
Me
Me
Me
Me
Me
Et
60
72
72
72
60
60
60
60
60
60
36
36
72
72
72
9
10
11
12
13
14
15
16
17
18
19d
Me
5 For selected examples of nucleophilic addition to ketones: (a) J. S.
Johnson and D. A. Evans, Acc. Chem. Res., 2000, 33, 325–335; (b) S.
E. Denmark and Y. Fan, J. Am. Chem. Soc., 2002, 124, 4233–4235;
(c) S.-K. Tian, R. Hong and L. Deng, J. Am. Chem. Soc., 2003, 125,
9900–9901; (d) D. E. Fuerst and E. N. Jacobsen, J. Am. Chem. Soc.,
2005, 127, 8964–8965; (e) E. Canales, K. G. Prasad and J. A. Soderquist,
J. Am. Chem. Soc., 2005, 127, 11572–11573; (f) V. J. Forrat, O. Prieto,
D. J. Ramon and M. Yus, Chem.–Eur. J., 2006, 12, 4431–4445; (g) Z.
Tang, L.-F. Cun, X. Cui, A.-Q. Mi, Y.-Z. Jiang and L.-Z. Gong, Org.
Lett., 2006, 8, 1263–1266; (h) X.-Y. Xu, Z. Tang, Y.-Z. Wang, S.-W.
Luo, L.-F. Cun and L.-Z. Gong, J. Org. Chem., 2007, 72, 9905–9913;
(i) J. Nie, G.-W. Zhang, L. Wang, D.-H. Zheng, Y. Zheng and J.-A.
Ma, Eur. J. Org. Chem., 2009, 3145–3149; (j) F. Wang, X.-H. Liu, Y.-L.
Zhang, L.-L. Lin and X.-M. Feng, Chem. Commun., 2009, 7297–7299;
(k) S.-L. Shi, L.-W. Xu, K. Oisaki, M. Kanai and M. Shibasaki, J. Am.
Chem. Soc., 2010, 132, 6638–6639; (l) H Kawai, K Tachi, E Tokunaga,
M Shiro and N Shibata, Org. Lett., 2010, 12, 5104–5107; (m) V. B.
Gondi, K. Hagihara and V. H. Rawal, Chem. Commun., 2010, 46, 904–
906; (n) H.-H. Xu and C. Wolf, Chem. Commun., 2010, 46, 8026–8028;
(o) K. Aikawa, Y. Hioki and K. Mikami, Org. Lett., 2010, 12, 5716–
5719; (p) K. Aikawa, S. Mimura, Y. Numata and K. Mikami, Eur. J.
Org. Chem., 2011, 62–65; (q) M. I. Antczak, F. Cai and J. M. Ready,
Org. Lett., 2011, 13, 184–187.
a Unless otherwise specified, all reactions were carried out with acetone
(4.0 mmol), 2 (0.20 mmol), 3,5-dinitrobenzoic acid (0.04 mmol, 20 mol%),
the catalyst 1a (0.04 mmol, 20 mol%) in 0.6 mL cyclohexane at -20 ◦C.
b Isolated yield after silica gel column chromatography. c The ee values
were determined by HPLC using Chiral OD-H, chiralpak AD-H and
chiralpak AS-H column; the absolute configurations were determined to
be S-configuration by comparison with literature data.7,8 d In 1.0 mmol
scale: acetone (20.0 mmol), (E)-methyl 2-oxo-4-phenylbut-3-enoate (2a,
1.0 mmol), 3,5-dinitrobenzoic acid (0.20 mmol, 20 mol%), the catalyst 1a
(0.20 mmol, 20 mol%) in 3.0 mL cyclohexane at -20 ◦C.
6 Selected examples: (a) J. M. White and S. J. Ohira, J. Org. Chem., 1986,
51, 5492–5494; (b) E Vedejs, S. Ahmad, S. D. Larsen and S. Westwood,
J. Org. Chem., 1987, 52, 3937–3938; (c) F. Wang, Y. Xiong, X.-H. Liu
and X.-M. Feng, Adv. Synth. Catal., 2007, 349, 2665–2668; (d) J. Liu,
Z.-G. Yang, Z. Wang, F. Wang, X.-H. Chen, X.-H. Liu, X.-M. Feng, Z.-
S. Su and C.-W. Hu, J. Am. Chem. Soc., 2008, 130, 5654–5655; (e) Z.-Q.
Jiang and Y.-X. Lu, Tetrahedron Lett., 2010, 51, 1884–1886.
7 (a) C.-L. Cao, X.-L. Sun, Y.-B. Kang and Y. Tang, Org. Lett., 2007, 9,
4151–4154; (b) C.-W. Zheng, Y.-Y. Wu, X.-S. Wang and G. Zhao, Adv.
Synth. Catal., 2008, 350, 2690–2694.
Scheme 1 Reaction of cyclopentanone and dihydro-2H-pyran-4(3H)-one
with 2a.
were obtained in excellent yields (93%–99%) and satisfactory
enantioselectivities (81%–95% ee).
8 P.-F. Li, J.-L. Zhao, F.-B. Li, A. S. C. Chan and F. Y. Kwong, Org. Lett.,
2010, 12, 5616–5619.
In summary, we have developed a highly enantioselective aldol
reaction of acetone with b,g -unsaturated a-keto esters catalyzed
by simple chiral primary–tertiary diamines in excellent yields (up
to 99%) and enantioselectivities (up to 96% ee). This highly enan-
tioselective protocol presents a chiral quaternary carbon center
together with four functional groups (hydroxy, carbon–carbon
double bond, ester, and carbonyl functional groups), which may
be further transformed to types of potential chiral intermediates
and functional materials. The protocol is practical and efficient.
Further investigations on the applications in pharmaceutical
preparations and in other asymmetric catalysis are in progress.
9 Special issues on organocatalysis and selected reviews: (a) P. I. Dalko,
Enantioselective Organoctalysis, Wiley-VCH: Weinheim, 2007; (b) A.
Berkessel, Asymmetric Organocatalysis: From Biomimetic Concepts to
Applications in Asymmetric Synthesis, Wiley-VCH: Weinheim, 2005;
(c) P. I. Dalko and L. Moisan, Angew. Chem., Int. Ed., 2004, 43, 5138–
5175; (d) A. Dondoni and A. Massi, Angew. Chem., Int. Ed., 2008, 47,
4638–4660.
10 Selected examples: (a) H. Kim, C. Yen, P. Preston and J. Chin, Org.
Lett., 2006, 8, 5239–5242; (b) Y. Inokoishi, N. Sasakura, K. Nakano,
Y. Ichikawa and H. Kotsuki, Org. Lett., 2010, 12, 1616–1619.
11 Selected examples: (a) M. Raj, G. S. Parashari and V. K. Singha, Adv.
Synth. Catal., 2009, 351, 1284–1288; (b) J.-H. Lin, C.-P. Zhang and J.-C.
4776 | Org. Biomol. Chem., 2011, 9, 4774–4777
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Xiao, Green Chem., 2009, 11, 1750–1753; (c) F.-Z. Peng, Z.-H. Shao,
X.-W. Pu and H.-B. Zhang, Adv. Synth. Catal., 2008, 350, 2199–2204;
(d) S.-Z. Luo, H. Xu, J.-Y. Li, L. Zhang and J.-P. Cheng, J. Am. Chem.
Soc., 2007, 129, 3074–3075.
13 Selected examples: (a) A. Alexakis and O. Andrey, Org. Lett., 2002, 4,
3611–3614; (b) T. Ishii, S. Fujioka, Y. Sekiguchi and H. A. Kotsuki, J.
Am. Chem. Soc., 2004, 126, 9558–9559; (c) O. Andrey, A. Alexakis, A.
Tomassini and G. Bernardinelli, Adv. Synth. Catal., 2004, 346, 1147–
1168; (d) H.-B. Huang and E. N. Jacobsen, J. Am. Chem. Soc., 2006,
128, 7170–7171; (e) N. Mase, Y. Nakai, N. Ohara, H. Yoda, K. Takabe,
F. Tanaka and C. F. Barbas III, J. Am. Chem. Soc., 2006, 128, 734–735;
(f) N. Mase, K. Watanabe, H. Yoda, K. Takabe, F. Tanaka and C. F.
Barbas III, J. Am. Chem. Soc., 2006, 128, 4966–4967; (g) Vishnumaya
and V. K. Singh, Org. Lett., 2007, 9, 1117–1119; (h) H. Chen, Y. Wang,
S. Wei and J. Sun, Tetrahedron: Asymmetry, 2007, 18, 1308–1312; (i) X.-
X. Jiang, Y.-F. Zhang, A. S. C. Chan and R. Wang, Org. Lett., 2009,
11, 153–156; (j) A.-D. Lu, P. Gao, Y. Wu, Y.-M. Wang, Z.-H. Zhou and
C.-C. Tang, Org. Biomol. Chem., 2009, 7, 3141–3147.
12 (a) L. Peng, X.-Y. Xu, L.-L. Wang, J. Huang, J.-F. Bai, Q.-C. Huang
and L.-X. Wang, Eur. J. Org. Chem., 2010, 1849–1853; (b) L.-L. Wang,
L. Peng, J.-F. Bai, Q.-C. Huang, X.-Y. Xu and L.-X. Wang, Chem.
Commun., 2010, 46, 8064–8066; (c) L.-L. Wang, L. Peng, J.-F. Bai,
L.-N. Jia, X.-Y. Luo, Q.-C. Huang, X.-Y. Xu and L.-X. Wang, Chem.
Commun., 2011, 47, 5593; (d) J.-F. Bai, L. Peng, L.-L. Wang, X.-Y. Xu
and L.-X. Wang, Tetrahedron, 2010, 66, 8928–8932; (e) R.-Q. Mei, X.-
Y. Xu, Y.-C. Li, J.-Y. Fu, Q.-C. Huang and L.-X. Wang, Tetrahedron
Lett., 2011, 52, 1566–1568; (f) L.-L. Wang, X.-Y. Xu, J. Huang, L. Peng,
Q.-C. Huang and L.-X. Wang, Lett. Org. Chem., 2010, 7, 367–372.
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 4774–4777 | 4777
©