Asymmetric hydroxyamination of oxindoles catalyzed by chiral bifunctional tertiary amine thiourea: Construction of 3-amino-2-oxindoles with quaternary stereocenters

Article abstract of DOI:10.1039/c1ob06413d

Chiral bifunctional tertiary amine thiourea was applied to catalyze the asymmetric hydroxyamination of 3-subsituted oxindoles with nitrosobenzene to construct 3-amino-2-oxindoles with quaternary stereocenters in good yields (up to 91%) and enantioselectiv

Full text of DOI:10.1039/c1ob06413d

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Asymmetric hydroxyamination of oxindoles catalyzed by chiral bifunctional
tertiary amine thiourea: construction of 3-amino-2-oxindoles with quaternary
stereocenters†
Li-Na Jia,a,b Jun Huang,a,b Lin Peng,a Liang-Liang Wang,a,b Jian-Fei Bai,a,b Fang Tian,a Guang-Yun He,a
Xiao-Ying Xu*a and Li-Xin Wang*a
Received 18th August 2011, Accepted 26th September 2011
DOI: 10.1039/c1ob06413d
Chiral bifunctional tertiary amine thiourea was applied to cat-
alyze the asymmetric hydroxyamination of 3-subsituted oxin-
doles with nitrosobenzene to construct 3-amino-2-oxindoles
with quaternary stereocenters in good yields (up to 91%) and
enantioselectivities (up to 90% ee).
droxyamination of 3-substituted 2-oxindoles with nitrosobenzene
catalyzed by a chiral Lewis acid complex of Sc(OTf)3/N,N-
dioxide, and excellent results were obtained.5 To date, there
is only one metal-free catalytic asymmetric version of hydrox-
yamination of 3-substituted 2-oxindoles, which gives no more
than 72% ee enantioselectivity.3l Considering the different cat-
alytic mechanisms and features of metal catalysis and small
molecule catalysis, it is still desirable to develop more and
effective new organocatalytic protocols to achieve this trans-
formation for the preparation of optically active 3-amino-2-
oxindoles.
Recently, the development of chiral bifunctional thioureas as
powerful hydrogen-bond-donating organocatalysts has received
growing attention.6 Among them, tertiary amine thioureas have
been demonstrated effectively to activate both donors and accep-
tors simultaneously.7,8a Based on this concept and background,
we envisioned that the tertiary amine group would activate
the oxindoles 1 as a base to produce zwitterionic enolate A,
while the thiourea group would direct the nitroso-group through
two hydrogen bonds with the oxygen of the nitrosobenzene 2.
Thus, the synergistic interactions through chiral tertiary amine
thiourea bifunctional catalysis would facilitate the zwitterionic
enolate A to N-specific nucleophilic attack of nitrosobenzene 2
(Scheme 1). For our continuing interests in asymmetric catalysis8
and further endeavors in the synthesis of structurally diverse
oxindoles with quaternary stereocenters,9 herein, we wish to
report the first example of organocatalytic asymmetric hydrox-
yamination of oxindoles 1 with nitrosobenzene 2 catalyzed by
tertiary amine thioureas, affording 3-amino-2-oxindoles 3 in
satisfactory yields and enantioselectivities (up to 91 yield and
90% ee).
Introduction
Optically active 3,3-disubstituted oxindole skeletons constitute
important structural motifs in a variety of natural products and
biologically active drugs.1 Particularly, chiral 3-amino-2-oxindoles
are versatile and useful building blocks for the preparation
of natural products and candidate drugs, such as chartelline
C and psychotrimine, as well as the potent gastrin/CCK-B
receptor antagonist AG-041R, the effective anti-malarial drug
candidate NITD609 and the vasopressin VIb receptor antagonist
SSR-149415.2 For the syntheses of those interesting structures,
several asymmetric strategies have been developed for the syn-
thesis of chiral quaternary 3-aminooxindoles, such as cyclization
of o-chlorinated anilines,3a alkylation of 3-aminooxindoles,3b
addition of imines derived from isatins,3c,d amination3e–k and
hydroxyamination3l of oxindoles. Among them, hydroxyami-
nation of 3-substituted oxindoles with nitrosobenzene is an
important and straightforward way to access those interest-
ing molecules. In 2010, Barbas and coworkers first reported
dimeric quinidinine-catalyzed enantioselective aminoxylations of
N-substituted 3-substituted 2-oxindoles with nitrosobenzene to
give 3-hydroxyoxindole with good results.4 By contrast, Chen and
coworkers reported a N-nitroso aldol reaction of 3-substituted 2-
oxindoles with nitrosobenzene catalyzed by Cinchona alkaloids,
giving 3-amino-2-oxindoles in good to excellent yields (up to
100%) and moderate enantioselectivities (59–72% ee).3l Subse-
quently, Feng and coworkers developed an enantioselective hy-
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
bGraduate University of Chinese Academy of Sciences, Beijing 10039,
People’s Republic of China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1ob06413d
Scheme 1 Proposed transition state.
236 | Org. Biomol. Chem., 2012, 10, 236–239
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Table 1 Effect of bifunctional chiral organocatalystsa
Table 2 Screening of the solvents for the reactiona
Yield (%)b
ee (%)c
Entry
Cat.
Time (h)
Yield (%)b
ee (%)c
Entry
Solvent
Time (h)
1
2
3
4
5
6
7
8
9
10
11
12
13
4a
4b
4c
4d
4e
4f
4g
4h
4i
16
15
117
15
67
15
21
24
16
17
17
17
17
53
63
33
69
43
73
44
14
34
72
42
65
61
55
55
56
64
57
70
27
30
43
35
44
-30
-37
1
2
3
4
5
6
7
8
Toluene
15
17
16
16
16
16
12
16
15
16
16
16
16
16
16
16
16
2.3
73
65
65
70
75
60
66
88
73
78
75
55
81
44
60
90
67
87
70
62
62
61
57
65
55
56
67
66
63
61
61
58
56
28
24
7
o-Xylene
m-Xylene
p-Xylene
Mesitylene
Chlorobenzene
n-Hexane
Cyclohexane
Ether
MTBE
DME
THF
1,4-Dioxane
DCM
9
4j
4k
4l
10
11
12
13
14
15
16
17
18
4m
a All the reactions were performed using 1a (0.2 mmol), 2 (0.22 mmol)
and 4 (0.04 mmol) in toluene (1 ml). b Isolated yield after silica gel
column chromatography. c Enantiomeric excess (ee) was determined by
chiral HPLC.
EtOAc
Acetonitrile
DMAd
MeOH
a All the reactions were performed using 1a (0.2 mmol), 2 (0.22 mmol)
and 4 (0.04 mmol) in solvent (1 ml). b Isolated yield after silica gel
column chromatography. c Enantiomeric excess (ee) was determined by
chiral HPLC. d N,N-Dimethylacetamide.
Results and discussion
To validate our hypothesis, a series of catalysts with various
substituents 4a–4m (Fig. 1) was investigated and the reaction
of oxindole 1a with nitrosobenzene 2 was chosen as the model
reaction, and the results are summarized in Table 1. All the
catalysts tested could give the desired N-addition product 3a
in moderate yields (14–73%) and enantioselectivities (27–70%
ee) in the presence of 20 mol% catalyst loading. Catalysts with
cyclohexanediamine as chiral scaffold afforded better yields and
enantioselectivities than those with diphenyldiamine scaffolds
(Table 1, entries 1 vs. 7, 4 vs. 8, 6 vs. 9). The acidity of the N–
H of the thiourea group has a positive effect on the catalytic
activity. Catalysts 4d and 4f with 3,5-(CF3)2 substituents on the
phenyl ring afforded better yields and enantioselectivities than
those without substituents (Table 1, entries 4 vs. 3, 6 vs. 5). The
substituents on the tertiary amine also have variable effects on
the results. The N-methyl protected 4a gave a lower yield than
N-propyl 4b with the same enantioselectivity (Table 1, entries 1
vs. 2). Cyclic substituents of the tertiary amine could increase the
yields and enantioselectivities, and the six-membered ring is more
efficient than the five-membered one (Table 1, entries 6 vs. 4 vs. 1).
Cinchona alkaloid-derived thioureas did not improve the results
(Table 1, entries 10–13). Relatively, catalyst 4f gave better yield and
enantioselectivity (73% yield and 70% ee, Table 1, entry 6), and
was selected for further optimization.
With 4f as the optimal catalyst, the effect of the solvent was
then investigated and the results are listed in Table 2. Solvents
showed significant effects on the yields and enantioselectivities. In
non-polar and less polar aprotic solvents such as toluene, alkanes
and ethers, the reactions proceeded smoothly and gave moderate
to good yields (44–88% yield) and enantioselectivities (Table 2,
entries 1–15, 55–70% ee). In CH3CN, the highest yield (90% yield)
was obtained, but only 28% ee was detected (Table 2, entry 16).
For more polar solvents, relatively lower enantioselectivities were
observed, which may be due to the potential destruction of H-
bonding interactions (Table 2, entries 16–18, 7–28% ee). The
screening of solvents showed that toluene was a more suitable
solvent for this conversion (Table 2, entry 1, 73% yield, 70% ee).
To further optimize the reaction conditions, the reaction
temperature, substrate loading and the concentration of reactants
were then screened, and the results are listed in Table 3. The
reaction temperature affected the results dramatically. When the
reaction was carried out at 0 ◦C, the enantioselectivity decreased to
61% ee (Table 3, entry 2). At the lower temperature of -20 ◦C, the
yield and enantioselectivity both decreased obviously (34% yield,
39% ee, Table 3, entry 3). Excessive loading of nitrosobenzene 2 is
unfavourable and the enantioselectivities decreased dramatically
(Table 3, entries 4 and 5 vs. 1). By contrast, with excess 3-subsituted
oxindoles 1a, excellent yields were obtained (up to 99%) and
the enantioselectivities slightly increased (up to 75% ee, Table 3,
entries 6–8). The screening of the substrate loading showed that
Fig. 1 Organocatalysts in this study.
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Table 4 Enantioselective hydroxyamination of oxindolesa
Table 3 Optimization of the reaction conditionsa
Conc.
Entry Solvent
1a : 2
(mol L-1) Time (h) Yield (%)b ee (%)c
1
Toluene 1 : 1.1 0.2
Toluene 1 : 1.1 0.2
Toluene 1 : 1.1 0.2
15
96
96
24
34
6
5
5
2
2
73
73
34
36
20
79
99
99
90
95
98
86
70
61
39
37
27
75
74
75
68
72
79
83
2d
3e
4
Toluene 1 : 2
Toluene 1 : 3
Toluene 2 : 1
Toluene 3 : 1
Toluene 4 : 1
Toluene 3 : 1
Toluene 3 : 1
Toluene 3 : 1
0.2
0.2
0.2
0.2
0.2
0.4
0.3
0.1
0.05
5
6
7
8
9
10
11
12
5
6
Toluene
3 : 1
a Unless otherwise noted, reactions were performed with 0.2 mmol scale,
0.04 mmol 4f in toluene at room temperature. b Isolated yield after silica
gel column chromatography. c Enantiomeric excess (ee) was determined by
chiral HPLC. d At 0 ◦C. e At -20 ◦C.
Entry
Substrate
Time (h)
Product
Yield (%)b
ee (%)c,e
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
6
5
5
6
4
7
9
11
5
5
3
3
3
2
1.5
1.5
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
86
75
87
71
83
89
71
51
85
89
86
85
91
84
71
71
83
80
81
82
80
77
77
81
75
77
90
82
82
76
86
78
3.0 equiv. of 1a to 2 was a suitable molecular ratio. Furthermore,
the concentration of nitrosobenzene 2 also affected the yields and
enantioselectivities, and the lower concentration resulted in a slight
improvement in enantioselectivity (entries 7 vs. 9–12). Good yield
and enantioselectivity were obtained at 0.05 M concentration (86%
yield, 83% ee, Table 3, entry 12). Based on the above screenings, the
recommended reaction conditions of 3.0 equiv. 1a and 1.0 equiv.
2 in toluene with 20 mol% catalyst 4f at 25 ◦C were established.
Under the optimal conditions, the scopes of the substrates were
finally investigated, and the results are listed in Table 4. The
effects of substituents at the C3 position of N-benzyl-substituted
oxindoles 1a–g were evaluated first, and the yields ranged from
71% to 89% and the enantioselectivities ranged from 77% ee to
83% ee (Table 4, entries 1–7). The electronic features and position
of the substituents on the C3-subsituted aromatic ring slightly
affected the enantioselectivities; both electron-withdrawing (Table
4, entries 2–5) and electron-donating groups (Table 4, entries
6, 7) at the meta- (Table 4, entries 3, 6) or para-position (Table
4, entries 2, 4, 5, 7) in the aromatic ring at the C3 position
gave good yields and enantioselectivities (up to 89% yield and
83% ee), whereas meta-substituted groups in the aromatic ring of
C3-substituents afforded higher yields than the ones with para-
substituted groups (Table 4, entries 3 vs. 4, entries 13 vs. 14).
N-Ethyl oxindole 1h and N-methyl oxindoles 1i and 1j gave
moderate to good yields and good enantioselectivities (51–89%
yield, 75–81% ee, Table 4, entries 8–10). This optimized protocol
was then expanded to N-unsubstituted oxindoles 1k–p, and good
yields and enantioselectivities were obtained (71–91% yield, 76–
90% ee, Table 4, entries 11–16). Particularly, C3-benzyl-substituted
N-unprotected oxindoles 1k gave the corresponding 3-amino-
2-oxindoles in the highest enantioselectivity (90% ee, Table 4,
entry 11).
9
10
11d
12d
13d
14d
15d
16d
a All the reactions were performed using 1a (0.6 mmol), 2 (0.2 mmol)
and 4f (0.04 mmol) in toluene (4 ml) at room temperature. b Isolated
yield after silica gel column chromatography. c Enantiomeric excess (ee)
was determined by chiral HPLC. d Isolated yield after filtration. e The
configuration (R) was determined by comparison of the rotation of 1k, 1l
and 1n with the literature.3l,5
Scheme 2 Cleavage of N–O bond.
Conclusions
In conclusion, we have presented an efficient regioselective
and enantioselective bifunctional chiral thiourea–tertiary amine-
catalyzed hydroxyamination of C3-subsituted oxindoles. 3-
Amino-2-oxindoles with chiral quaternary stereocenters were
obtained in good yields and enantioselectivities (up to 91%
yield, up to 90% ee) for a series of C3-subsituted oxindoles, the
best results for metal-free catalytic hydroxyamination to date.
Further investigations of the mechanism and other variants of
The hydroxyamination product 3k could easily transformed into
3-amino oxindole 5k in 71% yield and 92% ee (Scheme 2).
238 | Org. Biomol. Chem., 2012, 10, 236–239
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enantioselective hydroxyamination reaction are underway in our
laboratory.
D.-H. Chen, X.-H. Liu, L.-L. Lin and X.-M. Feng, Chem.–Eur. J., 2010,
16, 6632; (k) T. Bui, G. Herna´ndez-Torres, C. Milite and C. F. Barbas
III, Org. Lett., 2010, 12, 5696; (l) T. Zhang, L. Cheng, L. Liu, D. Wang
and Y.-J. Chen, Tetrahedron: Asymmetry, 2010, 21, 2800.
4 T. Bui, N. R. Candeias and C. F. Barbas III, J. Am. Chem. Soc., 2010,
132, 5574.
Acknowledgements
5 K. Shen, X.-H. Liu, G. Wang, L.-L. Lin and X.-M. Feng, Angew. Chem.,
Int. Ed., 2011, 50, 4684.
6 For selected reviews concerning bifunctional-thiourea organocatalysis:
(a) Y. Takemoto, Org. Biomol. Chem., 2005, 3, 4299; (b) S. J. Connon,
Chem.–Eur. J., 2006, 12, 5418; (c) M. S. Taylor and E. N. Jacobsen,
Angew. Chem., Int. Ed., 2006, 45, 1520; (d) A. G. Doyle and E. N.
Jacobsen, Chem. Rev., 2007, 107, 5713.
We are grateful for financial support from National Natural
Science Foundation of China (No. 20802075 and No.21042006).
Notes and references
1 For selected examples: (a) X. Zhang and C. D. Smith, Mol. Pharmacol.,
1996, 49, 228; (b) H. C. Malinakova and L. S. Liebeskind, Org. Lett.,
2000, 2, 4083; (c) S. Hibino and T. Choshi, Nat. Prod. Rep., 2001, 18, 66;
(d) T. Tokunaga, W. E. Hume, T. Umezome, K. Okazaki, Y. Ueki, K.
Kumagai, S. Hourai, J. Nagamine, H. Seki, M. Taiji, H. Noguchi and R.
Nagata, J. Med. Chem., 2001, 44, 4641; (e) T. D. Bagul, G. Lakshmaiah,
T. Kawabata and K. Fuji, Org. Lett., 2002, 4, 249; (f) K. C. Nicolaou,
P. B. Rao, J. Hao, M. V. Reddy, G. Rassias, X. Huang, D. Y.-K. Chen
and S. A. Snyder, Angew. Chem., Int. Ed., 2003, 42, 1753; (g) C. Marti
and E. M. Carreira, Eur. J. Org. Chem., 2003, 12, 2209; (h) K. Ding,
Y. Lu, Z. Nikolovska-Coleska, S. Qiu, Y. Ding, W. Gao, J. Stuckey, K.
Krajewski, P. P. Roller, Y. Tomita, D. A. Parrish, J. R. Deschamps and
S. Wang, J. Am. Chem. Soc., 2005, 127, 10130; (i) A. H. Abadi, S. M.
Abou-Seri, D. E. Abdel-Rahman, C. Klein, O. Lozach and L. Meijer,
Eur. J. Med. Chem., 2006, 41, 296; (j) C. V. Galliford and K. A. Scheidt,
Angew. Chem., Int. Ed., 2007, 46, 8748; (k) S. E. Reisman, J. M. Ready,
M. M. Weiss, A. Hasuoka, M. Hirata, K. Tamaki, T. V. Ovaska, C. J.
Smith and J. L. Wood, J. Am. Chem. Soc., 2008, 130, 2087.
2 For selected reviews and examples: (a) L. Chevolot, A. M. Chevolot,
M. Gajhede, M. C. Larsen, U. Anthoni and C. Christophersen, J. Am.
Chem. Soc., 1985, 107, 4542; (b) U. Anthoni, L. Chevolot, C. Larsen, P.
H. Nielsen and C. Christophersen, J. Org. Chem., 1987, 52, 4709; (c) H.
Takayama, I. Mori and M. Kitajima, Org. Lett., 2004, 6, 2945; (d) S. L.
G. Claudine, D. Sylvain, B. Gabrielle, M. Maurice, S. Jacques, G. Rolf,
G. Guy and G. Gilles, Stress, 2003, 6, 199; (e) K. Bernard, S. Bogliolo
and J. Ehrenfeld,, Br. J. Pharmacol., 2005, 144, 1037; (f) R. M. Williams
and R. J. Cox, Acc. Chem. Res., 2003, 36, 127; (g) A. K. Franz, P. D.
Dreyfuss and S. L. Schreiber, J. Am. Chem. Soc., 2007, 129, 1020; (h) J.
T. Mohr, M. R. Krout and B. M. Stoltz, Nature, 2008, 455, 323; (i) F.
Zhou, Y.-L. Liu and J. Zhou, Adv. Synth. Catal., 2010, 352, 1381; (j) M.
Rottmann et al., Science, 2010, 329, 1175.
3 For selected examples: (a) Y.-X. Jia, J. M. Hillgren, E. L. Watson, S.
P. Marsden and E. P. Ku¨ndig, Chem. Commun., 2008, 4040; (b) T. E
mura, T. Esaki, K. Tachibana and M. Shimizu, J. Org. Chem., 2006, 71,
8559; (c) G. Lesma, N. Landoni, T. Pilati, A. Sacchetti and A. Silvani,
J. Org. Chem., 2009, 74, 4537; (d) H.-H. Jung, A. W. Buesking and J.
A. Ellman, Org. Lett., 2011, 13, 3912; (e) L. Cheng, L. Liu, D. Wang
and Y.-J. Chen, Org. Lett., 2009, 11, 3874; (f) T. Bui, M. Borregan
and C. F. Barbas III, J. Org. Chem., 2009, 74, 8935; (g) Z.-Q. Qian,
F. Zhou, T.-P. Du, B.-L. Wang, M. Ding, X.-L. Zhao and J. Zhou,
Chem. Commun., 2009, 6753; (h) S. Sato, M. Shibuya, N. Kanoh and
Y. Iwabuchi, Chem. Commun., 2009, 6264; (i) S. Mouri, Z. Chen, H.
Mitsunuma, M. Furutachi, S. Matsunaga and M. Shibasaki, J. Am.
Chem. Soc., 2010, 132, 1255; (j) Z.-G. Yang, Z. Wang, S. Bai, K. Shen,
7 For selected examples: (a) T. Okino, Y. Hoashi and Y. Takemoto, J. Am.
Chem. Soc., 2003, 125, 12672; (b) T. Okino, Y. Hoashi, T. Furukawa, X.
Xu and Y. Takemoto, J. Am. Chem. Soc., 2005, 127, 119; (c) B.-J. Li, L.
Jiang, M. Liu, Y.-C. Chen, L.-S. Ding and Y. Wu, Synlett, 2005, 603;
(d) A. Berkessel, F. Cleemann, S. Mukherjee, T. N. Muller and J. Lex,
Angew. Chem., Int. Ed., 2005, 44, 807; (e) A. Berkessel, F. Cleemann and
S. Mukherjee, Angew. Chem., Int. Ed., 2005, 44, 7466; (f) J. Wang, H.
Li, X. Yu, L. Zu and W. Wang, Org. Lett., 2005, 7, 4293; (g) J. Wang, H.
Li, L. Zu,W. Jiang, H. Xie, W. Duan and W. Wang, J. Am. Chem. Soc.,
2006, 128, 12652; (h) T.-Y. Liu, J. Long, B.-J. Li, L. Jiang, R. Li, Y. Wu,
L.-S. Ding and Y.-C. Chen, Org. Biomol. Chem., 2006, 4, 2097; (i) T.-Y.
Liu, R. Li, Q. Chai, J. Long, B.-J. Li, Y. Wu, L.-S. Ding and Y.-C. Chen,
Chem.–Eur. J., 2007, 13, 319; (j) T.-Y. Liu, H.-L. Cui, J. Long, B.-J. Li,
Y. Wu, L.-S. Ding and Y.-C. Chen, J. Am. Chem. Soc., 2007, 129, 1878;
(k) E. G. Klauber and D. Seidel, J. Am. Chem. Soc., 2008, 130, 12248;
(l) Y.-H. Liao, W.-B. Chen, Z.-J. Wu, X.-L. Du, L.-F. Cun, X.-M. Zhang
and W.-C. Yuan, Adv. Synth. Catal., 2010, 352, 827; (m) Y.-H. Liao, X.-
L. Liu, Z.-J. Wu, L.-F. Cun, X.-M. Zhang and W.-C. Yuan, Org. Lett.,
2010, 12, 2896; (n) X.-L. Liu, Y.-H. Liao, Z.-J. Wu, L.-F. Cun, X.-M.
Zhang and W.-C. Yuan, J. Org. Chem., 2010, 75, 4872; (o) J.-W. Xie, L.-P.
Fan, H. Su, X.-S. Li and D.-C. Xu, Org. Biomol. Chem., 2010, 8, 2117;
(p) W.-B. Chen, Z.-J. Wu, J. Hu, L.-F. Cun, X.-M. Zhang and W.-C.
Yuan, Org. Lett., 2011, 13, 2472.
8 (a) J.-F. Bai, L.-L. Wang, L. Peng, Y.-L. Guo, J.-N. Ming, F.-Y. Wang,
X.-Y. Xu and L.-X. Wang, Eur. J. Org. Chem., 2011, 4472; (b) J.-Y. Fu,
Q.-C. Yang, Q.-L. Wang, J.-N. Ming, F.-Y. Wang, X.-Y. Xu and L.-X.
Wang, J. Org. Chem., 2011, 76, 4661; (c) L. Peng, L.-L. Wang, J.-F. Bai,
L.-N. Jia, Y.-L. Guo, X.-Y. Luo, F.-Y. Wang, X.-Y. Xu and L.-X. Wang,
Org. Biomol. Chem., 2011, 9, 4774; (d) L. Peng, L.-L. Wang, J.-F. Bai, L.-
N. Jia, Q.-C. Yang, Q.-C. Huang, X.-Y. Xu and L.-X. Wang, Tetrahedron
Lett., 2011, 52, 1157; (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; (f) J.-Y. Fu,
X.-Y. Xu, Y.-C. Li, Q.-C. Huang and L.-X. Wang, Org. Biomol. Chem.,
2010, 8, 4524; (g) 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; (h) J.-F.
Bai, X.-Y. Xu, Q.-C. Huang, L. Peng and L.-X. Wang, Tetrahedron Lett.,
2010, 51, 2803; (i) J.-F. Bai, L. Peng, L.-L. Wang, X.-Y. Xu and L.-X.
Wang, Tetrahedron, 2010, 66, 8928; (j) J.-Y. Fu, Q.-C. Huang, Q.-W.
Wang, X.-Y. Xu and L.-X. Wang, Tetrahedron Lett., 2010, 51, 4870.
9 (a) 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; (b) L.-L.
Wang, L. Peng, J.-F. Bai, Q.-C. Huang, X.-Y. Xu and L.-X. Wang, Chem.
Commun., 2010, 46, 8064.
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