Molecular sieve mediated decarboxylative Mannich and aldol reactions of β-ketoacids

Article abstract of DOI:10.1016/j.tetlet.2013.06.030

A molecular sieve mediated decarboxylative Mannich reaction of β-ketoacids with sulfonyl imines is reported; this protocol leads to an efficient preparation of synthetically useful β-amino ketones. An analogous molecular sieve promoted decarboxylative aldol reaction between β-ketoacids and isatins is also described, which affords bioactive 3-substituted-3-hydroxy-oxindoles in excellent yields.

Full text of DOI:10.1016/j.tetlet.2013.06.030

Accepted Manuscript
Molecular sieve mediated decarboxylative Mannich and aldol reactions of β-
ketoacids
Fangrui Zhong, Chunhui Jiang, Weijun Yao, Li-Wen Xu, Yixin Lu
PII:
S0040-4039(13)00986-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.06.030
TETL 43083
Reference:
To appear in:
Tetrahedron Letters
Please cite this article as: Zhong, F., Jiang, C., Yao, W., Xu, L-W., Lu, Y., Molecular sieve mediated decarboxylative
Mannich and aldol reactions of β-ketoacids, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.
2013.06.030
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Molecular sieve mediated decarboxylative
Mannich and aldol reactions of -ketoacids
Leave this area blank for abstract info.
Fangrui Zhong, Chunhui Jiang, Weijun Yao, Li-Wen Xu, Yixin Lu*
R2
R2
O
O
S
O
S
NH
O
23 examples
up to 96% yield
N
O
O
O
R1
or
R3
4 Å MS
EtOAc, r.t.
R1
O
or
+
R3
OH
O
O
HO
R4
R3
19 examples
up to 97% yield
R4
O
N
R
N
R

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Molecular sieve mediated decarboxylative Mannich and aldol reactions of -ketoacids
Fangrui Zhong,a Chunhui Jiang,a Weijun Yao,a Li-Wen Xu,b Yixin Lua*
aDepartment of Chemistry & Medicinal Chemistry Program, Life Sciences Institute, National University of Singapore, 3 Science Drive 3, Singapore 117543,
Singapore
bKey Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 310012, P. R. of
China
*Corresponding author. Tel.: +65 6516 1569. Fax: +65 6779 1691. E-mail: chmlyx@nus.edu.sg
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A molecular sieve mediated decarboxylative Mannich reaction of -ketoacids with sulfonyl
imines is reported; this protocol leads to an efficient preparation of synthetically useful -amino
ketones. An analogous molecular sieve promoted decarboxylative aldol reaction between -
ketoacids and isatins is also described, which affords bioactive 3-substituted-3-hydroxy-
oxindoles in excellent yields.
Available online
2013 Elsevier Ltd. All rights reserved.
Keywords:
Molecular Sieves
Decarboxylation
Mannich Reaction
Aldol Reaction
Ketoacids
The catalytic Mannich reaction of carbonyl compounds with
imines represents a powerful approach for the synthesis of
same decarboxylative process can be extended to the aldol
reaction of -ketoacids with isatins.
-
amino ketones, which are not only important due to their
biological activities, but are also extremely useful as versatile
building blocks for the construction of 1,3-amino alcohols, 1,3-
amino acids and related bioactive molecules.1 Therefore, not
surprisingly, intense efforts have been devoted to these types of
reactions.2
Table 1. Decarboxylative Mannich reaction of -ketoacid 2a
with imines 1a
Aryl methyl ketones are interesting donors in the Mannich
reaction. However, due to their intrinsically low reactivity, pre-
activation is often required. For instance, the Lewis acid
promoted Mukaiyama–Mannich reaction between silyl enol
ethers and sulfonyl aldimines represents one of the most common
approaches.3 In recent years, biomimetic approaches using
malonic acid half thioesters (MAHTs), as ester enolate
equivalents in the decarboxylative reactions, have drawn much
attention.4 In the decarboxylative processes of MAHTs, a wide
range of electrophiles, including aldehydes, ketones, imines,
activated alkenes and azodicarboxylates have been employed as
reaction partners under either metal5 or organic6 catalysis. From a
more practical viewpoint, the employment of -ketoacids as
ketone surrogates is highly desirable, and such decarboxylative
processes have been realized under the catalysis of a suitable
Brønsted base or metal.7 With ever-growing concerns of
performing organic reactions in a green and sustainable manner,
we became interested in developing an efficient synthetic
protocol involving decarboxylation of -ketoacids. Herein, we
report a molecular sieve-mediated decarboxylative Mannich
reaction of -ketoacids with N-sulfonyl imines. Moreover, the
Entry
Imine Promoter
Loading
(mg/mmol)
Solvent
Time Conv.
(h)
24
1
(%)b
<20
>99
>99
>99
>99
80
1
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
none
-
THF
2
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
300
300
300
300
300
300
300
300
100
50
THF
3
CH3CN
EtOAc
acetone
CHCl3
CH2Cl2
toluene
Et2O
3
4
0.5
1
5
6
24
24
24
24
2
7
85
8
<50
90
9
10
11
12
13
EtOAc
EtOAc
EtOAc
EtOAc
>99
>99
58
4
50
24
24
50
mixture

2
Tetrahedron
a Reactions were performed with 1 (0.1 mmol), 2a (0.12 mmol) and the
24c
25d
C6H5
C6H5
Me
Me
C6H5
C6H5
3a
3a
4
4
86
promoter in the solvent specified (1.0 mL).
86-92
b Determined by 1H NMR spectroscopic analysis of the crude reaction
mixture. MS: molecular sieves.
a Reactions were performed with 1 (0.1 mmol), 2 (0.12 mmol) and 4 Å MS (5
mg) in EtOAc (1.0 mL).
We initiated our investigation by performing a model reaction
between tosylimine 1a and -ketoacid 2a, and the results are
summarized in Table 1. The background reaction was found to be
very weak, and less than 20% conversion was observed after 24
hours at room temperature (Table 1, entry 1). We were delighted
to discover that the reaction could be promoted effectively by
molecular sieves. In the presence of 4 Å molecular sieves, full
conversion of the Mannich product was observed in just 1 hour
(entry 2). Further solvent screening revealed that ethyl acetate
gave the best result (entry 4). To make the method greener and
more synthetically useful, the loading of molecular sieves was
reduced to 50 mg/mmol, with reference to the imine substrate,
and full conversion was reached within 4 hours (entry 11).
Notably, the sulfonyl protecting group was found to be crucial,
and sluggish reactions were observed when N-Cbz imine 1b or
N-CO2Et imine 1c were employed under otherwise identical
reaction conditions (entries 12 and 13).
b Isolated yield.
c Reaction was performed with 5 mmol of 1a.
d Reaction results with recycled MS: cycle 1, 91% yield; cycle 2, 86% yield;
cycle 3, 88% yield; cycle 4, 89% yield; cycle 5, 92% yield. MS = molecular
sieves.
The effects of different sulfonamides on the reaction were
next examined. Substrates with electron-donating groups on the
aryl ring of the sulfonamide reacted slightly better than those
with an electron-withdrawing substituent (Table 2, entries 1 3).
The reaction tolerated a wide range of aryl imine substrates,
regardless of the electronic nature and the substitution pattern of
the aromatic rings (entries 4–14). Moreover, the reaction was
applicable to linear and branched alkyl tosylimines, although
longer reaction times were required for the reactions to reach
completion (entries 15 and 16). The -ketoacids could also be
varied, and both aryl and alkyl substituted -ketoacids were found
to be suitable for the decarboxylative process (entries 17 23).
This decarboxylative Mannich reaction could be performed on
gram-scale. Under the optimized conditions, the reaction
proceeded smoothly to afford the product in high yield (entry
24). The practicality of our method was further demonstrated by
the successful recycling of the molecular sieves. The recovered
molecular sieves were able to catalyse the reaction for up to five
runs without any decrease in reactivity (entry 25).
Table 2. Scope of the molecular sieve-mediated Mannich
reactiona
R2
R2
O
O
O
O
+
4 Å MS
S
S
N
R3
OH
NH
O
O
EtOAc, r.t.
O
2
R1
R1
R3
1
3
Entry
R1
R2
R3
Product
Time Yield
To gain a better insight into the reaction mechanism, a series
of control experiments was performed (Table 3). As expected,8
the decarboxylative process took place smoothly in the presence
of an organic or inorganic base (entries 2−4). We believe the
ability of molecular sieves to promote the reaction is independent
of their desiccating property, as anhydrous sodium sulfate failed
to promote the process (entry 5). The 4 Å molecular sieves
without pre-activation worked equally well (entry 6). Moreover,
the size of the molecular sieves proved to be important for the
reaction: 4 Å and 3 Å molecular sieves were more effective,
while 5 Å molecular sieves displayed much lower catalytic
activity (entries 7 and 8). Since all the molecular sieves were too
small to accommodate the reactants within their cavity, the
reaction took place mostly likely on the surface of the molecular
sieves. It seems that the basic alkaline metal ions were
responsible for the catalytic performance of the molecular sieves,
as no catalytic activity was observed when common silica or
aluminosilicate (Celite) was used (entries 9 and 10). We also
found the basicities of molecular sieves were 4 Å>3 Å>5 Å,
which agreed well with the observed catalytic activities (entries 7
and 8).9 A moderate isolated yield was obtained when the
mixture was heated, probably due to significant decomposition of
the reactants under the harsh conditions (entry 11).
(h)
(%)b
91
96
86
94
88
91
90
88
92
75
89
93
84
92
89
82
84
1
C6H5
C6H5
C6H5
Me
C6H5
3a
3b
3c
3d
3e
3f
4
2
OMe C6H5
4
3
NO2
Me
C6H5
C6H5
C6H5
4
4
4-Br-C6H4
4-F-C6H4
4
5
Me
4
6
4-MeO-C6H4
3-Br-C6H4
3-Cl-C6H4
3-NO2-C6H4
2-Me-C6H4
2-Br-C6H4
2,4-Cl2-C6H3
3-furyl
OMe C6H5
4
7
Me
Me
C6H5
C6H5
3g
3h
3i
4
8
5
9
OMe C6H5
OMe C6H5
OMe C6H5
OMe C6H5
3
10
11
12
13
14
15
16
17
3j
12
6
3k
3l
4
Me
C6H5
3m
3n
3o
3p
3q
24
6
2-naphthyl
hexyl
OMe C6H5
Me
Me
Me
C6H5
C6H5
12
12
4
cyclohexyl
C6H5
Table 3. The Effect of Different Promotersa
4-Me-
C6H4
NTs
NHTsO
O
O
promoter
+
18
19
C6H5
C6H5
Me
4-F-C6H4
3r
3s
24
48
81
66
EtOAc, r.t.
Ph
Ph
Ph
Ph
OH
OMe
3-Cl-
C6H4
3a
1a
2a
Entry
Promoter
4 Å MS
Et3N
Time (h)
Yield (%)b
20
C6H5
Me
2-MeO-
C6H4
3t
4
91
1
1
94
60
90
2c
24
12
21
22
23
C6H5
C6H5
C6H5
Me
Me
Me
Me
Pr
3u
3v
3w
24
24
24
82
86
77
3
NaHCO3
t-Bu

3
4
Na2CO3
Na2SO4
4 Å MS
3 Å MS
5 Å MS
silica gel
Celite
1
90
5
H
C6H5
H
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
5e
5f
5g
5h
5i
4
87
82
94
91
83
85
95
96
5
24
1
<20
92
6
5-OMe
5-Br
5-NO2
7-Cl
7-F
12
2
6d
7
H
7
3
83
8
H
2
8
24
24
24
24
72
9
H
6
9e
10f
11g
26
10
11
12
H
5j
6
<10
58
H
H
4-F-C6H4 5k
1
none
H
H
4-Me-
C6H4
5l
1
a Reactions were performed with 1a (0.1 mmol), 2a (0.12 mmol), promoter
(30 mg) in EtOAc (1.0 mL).
13
14
H
H
H
H
3-Cl-C6H4 5m
6
2
71
93
b Isolated yield.
2-MeO- 5n
C6H4
c10 mol% of Et3N was used.
d 4 Å MS were used without preactivation.
15
16
17
18
19
H
H
H
H
H
H
H
H
H
H
2-naphthyl5o
2-thienyl 5p
1
98
95
91
88
83
1
e Purchased from Aldrich, technical grade, pore size 60 Å, 230-400 mesh
particle size, 40-63 µm particle size.
Me
Pr
5q
5r
5s
2
f Fluka Analytical, Celite® 500 fine.
g Reaction mixture was heated at 50 oC.
4
tBu
12
a Reactions were performed with 4 (0.1 mmol), 2 (0.12 mmol) and 4 Å MS (5
mg) in EtOAc (1.0 mL).
b Isolated yield. MS = molecular sieves.
In conclusion, we have successfully developed a molecular
sieve mediated decarboxylative Mannich reaction of -ketoacids
with sulfonylimines. We have also developed a decarboxylative
aldol reaction between -ketoacids and isatins. The described
synthetic protocols were highly efficient, and were also broad in
substrate scope, affording the desired products in high yields.
Notably, the above reactions were carried out under very mild,
base-free conditions, and molecular sieves were the sole activator
for the reaction. We believe our methods represent highly
efficient protocols for the practical synthesis of useful molecular
scaffolds.
Fig. 1. Selected examples of 3-hydroxy-3-acyloxindole-containing
bioactive molecules.
Having developed the above efficient decarboxylative
Mannich reaction of -ketoacids, our next goal was to extend this
process to the analogous aldol reaction. When benzaldehyde was
employed as an acceptor, only a trace amount of the desired aldol
product was obtained. Given the high reactivity of isatins and the
prevalence of 3-hydroxyoxindoles10 in natural products and
bioactive molecules (Figure 1), we examined the feasibility of
molecular sieves in promoting the aldol reaction of isatins. To
our delight, the aldol reaction of -ketoacid 2a with isatin
proceeded very rapidly in the presence of 4 Å molecular sieves,
affording 3-hydroxyoxindole 5a in 93% yield (Table 4, entry 1).
Different protecting groups on the NH of isatin worked equally
well (entries 2 5). The reaction was also applicable to isatins with
different aromatic moieties, including those with electron-
donating and electron-withdrawing groups and halogen atoms on
the phenyl ring (entries 6 10). The -ketoacid substrates could
also be varied, both aryl and alkyl -ketoacids were well-
tolerated, and the desired aldol products were obtained in
excellent yields in all the examples examined (entries 11 19).
Acknowledgments
We thank the National University of Singapore (R-143-000-
469-112) and the Ministry of Education (MOE) of Singapore (R-
143-000-362-112), the Singapore Peking Oxford Research
Enterprise (COY-15-EWI-RCFSA/N197-1), and GSK EDB (R-
143-000-491-592) for generous financial support.
References and notes
1.
2.
(a) Baktharaman, S.; Hili R.; Yudin, A. K. Aldrichim. Acta 2008,
41, 109; (b) Joshi, N. S.; Whitaker, L. R.; Francis, M. B. J. Am.
Chem. Soc. 2004, 126, 15942.
For selected reviews, see: (a) Kobayashi, S.; Ishitani, H. Chem.
Rev. 1999, 99, 1069; (b) Liu, M.; Sibi, M. P. Tetrahedron 2002,
58, 7991; (c) Arend, M.; Westermann B.; Risch, N. Angew.
Chem., Int. Ed. 1998, 37, 1044; (d) Xu, L-W.; Xia, C.-G. Eur. J.
Org. Chem. 2005, 633; (e) Weiner, B.; Szymanński, W.; Janssen,
D. B.; Minnaard A. J.; Feringa, B. L. Chem. Soc. Rev. 2010, 39,
1656; (f) Cai, X.-H.; Guo H.; Xie, B. Eur. J. Chem. 2012, 3, 258.
(a) Ojima, I.; Inaba, S.; Yoshida, K. Tetrahedron Lett. 1977, 18,
3643; (b) Ishitani, H.; Ueno, M.; Kobayashi, S. J. Am. Chem. Soc.
1997, 119, 7153; (c) Hagiwara, E.; Fujii, A.; Sodeoka, M. J. Am.
Chem. Soc. 1998, 120, 2474; (d) Taggi, A. E.; Hafez, A. M.;
Leckta, T. Acc. Chem. Res. 2003, 36, 10; (e) Akiyama, T.; Itoh, J.;
Yokota, K.; Fuchibe, K. Angew. Chem., Int. Ed. 2004, 43, 1566;
(f) Fujisawa, H.; Takahashi, E. and Mukaiyama, T. Chem. Eur. J.
2006, 12, 5082.
Table 4. Molecular sieve mediated decarboxylative aldol
reaction of -ketoacids with isatinsa
O
O
HO
O
O
4 Å MS
R3
R1
O
+
R1
R3
OH
O
N
R2
EtOAc, r.t.
3.
N
2
R2
4
5
Entry
R1
H
H
H
H
R2
R3
Product
Time (h) Yield (%)b
1
2
3
4
H
C6H5
C6H5
C6H5
C6H5
5a
5b
5c
5d
1
1
1
3
93
96
95
93
Boc
Ac
Bn
4.
5.
Pan. Y.; Tan, C.-H. Synthesis, 2011, 2044.
(a) Lalic, G.; Aloise, A. D.; Shair, M. D. J. Am. Chem. Soc. 2003,
125, 2852; (b) Orlandi, S.; Benaglia, M.; Cozzi, F. Tetrahedron

4
Tetrahedron
Lett. 2004, 45, 1747; (c) Magdziak, D.; Lalic, G.; Lee, H. M.;
Fortner, K. C.; Aloise, A. D.; Shair, M. D. J. Am. Chem. Soc.
2005, 127, 7284; (d) Fortner, K. C.; Shair, M. D. J. Am. Chem.
Soc. 2007, 129, 1032; (e) Furutachi, M.; Mouri, S.; Matsunaga, S.;
Shibasaki, M. Chem. Asian J. 2010, 5, 2351. For examples of
other decarboxylative additions, see: (f) van Tamelen, E. E.;
Baran, J. S. J. Am. Chem. Soc. 1956, 78, 2913; (g) Yin, L.; Kanai,
M.; Shibasaki, M. J. Am. Chem. Soc. 2009, 131, 9610; (h) Yin, L.;
Kanai, M.; Shibasaki, M. Tetrahedron 2012, 68, 3497; (i)
Albrecht, S.; Adam, J.-M.; Bromberger, U.; Diodone, R.; Fettes,
A.; Fischer, R.; Goeckel, V.; Hildbrand, S.; Moine, G.; Weber, M.
Org. Proc. Res. Develop. 2011, 15, 503.
6.
(a) List, B.; Doehring, A.; Fonseca, M.; Wobser, K.; van Thienen,
H.; Torres, R.; Galilea, P. Adv. Synth. Catal. 2005, 347, 1558; (b)
Blanchet, J.; Baudoux, J.; Amere, M.; Lasne, M. C.; Rouden, J.
Eur. J. Org. Chem. 2008, 5493; (c) Blaquiere, N.; Shore, D. G.;
Rousseaux, S.; Fagnou, K. J. Org. Chem. 2009, 74, 6190; (d)
Ricci, A.; Pettersen, D.; Bernardi, L.; Fini, F.; Fochi, M.; Herrera,
R. P.; Sgarzani, V. Adv. Synth. Catal. 2007, 349, 1037; (e)
Lubkoll, J.; Wennemers, H. Angew. Chem., Int. Ed. 2007, 46,
6841; (f) Pan, Y.; Kee, C. W.; Jiang, Z.; Ma, T.; Zhao, Y.; Yang,
Y.; Xue, H.; Tan, C.-H. Chem. Eur. J. 2011, 17, 8363; (g) Bae, H.
Y.; Some, S.; Lee, J. H.; Kim, J.-Y.; Song, M. J.; Lee, S. ; Zhang
Y. J.; Song, C. E. Adv. Synth. Catal. 2011, 353, 3196; (h)
Baudoux, J.; Lefebvre, P.; Legay, R.; Lasne M.-C.; Rouden, J.
Green Chem. 2010, 12, 252; (i) Hara, N.; Nakamura, S.;
Funahashi Y.; Shibata, N. Adv. Synth. Catal. 2011, 353, 2976.
(a) Stiles, M.; Wolf, D.; Hudson, G. V. J. Am. Chem. Soc. 1959,
81, 628; (b) Kourouli, T.; Kefalas, P.; Ragoussis, N.; Ragoussis,
V. J. Org. Chem. 2002, 67, 4615; (c) Tsuda, T.; Okada, M.; Nishi,
S.; Saegusa, T. J. Org. Chem. 1986, 51, 421; (d) Tsuda, T.; Tokai,
M.; Ishida, T.; Saegusa, T. J. Org. Chem. 1986, 51, 5216; (e)
Yang, C.-Y.; Wang, J.-Y.; Tian, S.-K. Chem. Commun. 2011, 47,
8343; (f) Evans, D. A.; Mito, S.; Seidel, D. J. Am. Chem. Soc.
2007, 129, 11583; (g) Rohr, K.; Mahrwald, R. Org. Lett. 2011, 13,
1878; (h) Yang, C.-F.; Shen, C.; Wang, J.-Y.; Tian, S.-K. Org.
Lett. 2012, 14, 3092; (i) Zhong, F.; Yao, W.; Dou, X.; Lu, Y. Org.
Lett. 2012, 14, 4018; (j) Zheng, Y.; Xiong, H.-Y.; Nie, J.; Hua,
M.-Q.; Ma, J.-A. Chem. Commun. 2012, 48, 4308; (k) Jiang, C.;
Zhong, F.; Lu, Y. Beilstein J. Org. Chem. 2012, 8, 1279; (l) Yuan,
H.-N.; Wang, S.; Nie, J.; Meng, W.; Yao, Q.; Ma, J.-A. Angew.
Chem., Int. Ed. 2013, 52, 2869.
7.
8.
9.
(a) Singh, G. S.; Desta, Z. Y. Chem. Rev. 2012, 112, 6104. (b)
Guo, Q.; Bhanushali, M.; Zhao, C.-G. Angew. Chem., Int. Ed.
2010, 49, 9460. (c) Allu, S.; Molleti, N.; Panem, R.; Singh, V. K.
Tetrahedron Lett. 2011, 52, 4080. (d) Chen, W.-B.; Liao, Y.-H.;
Du, X.-L.; Zhang, X.-M.; Yuan, W.-C. Green Chem. 2009, 11,
1465.
The pH values of the molecular sieves were measured by stirring a
suspension of molecular sieves (50 mg) in H2O (1 mL) for 1 h
prior to measurements with pH paper. The results are: 3 Å
molecular sieves, 9<pH<10; 4 Å molecular sieves, pH~10; 5 Å
molecular sieves, 7<pH<8. The basicities agreed well with their
catalytic reactivities.
10. For reviews on oxindoles, see: (a) Marti, C.; Carreira, E. M. Eur.
J. Org. Chem. 2003, 2209; (b) Galliford, C. V.; Scheidt, K. A.
Angew. Chem., Int. Ed. 2007, 46, 8748; (c) Dounay, A. B.;
Overman, L. E. Chem. Rev. 2003, 103, 2945; (d) Zhou, F.; Liu,
Y.-L.; Zhou, J. Adv. Synth. Catal. 2010, 352, 1381; (e) Lin, H.;
Danishefsky, S. J. Angew. Chem., Int. Ed. 2003, 42, 36; (f) Ball-
Jones, N. R.; Badillo, J. J.; Franz, A. K. Org. Biomol. Chem. 2012,
10, 5165. For examples reported by us, see: (g) Zhu, Q. Lu, Y.
Angew. Chem., Int. Ed. 2010, 49, 7753; (h) Zhong, F.; Chen, G.-
Y.; Lu, Y. Org. Lett. 2011, 13, 82; (i) Zhong, F.; Han, X.; Wang,
Y.; Lu, Y. Angew. Chem., Int. Ed. 2011, 50, 7837; (j) Han, X.;
Wang, S.-X.; Zhong, F.; Lu, Y. Synthesis 2011, 1859; (k) F.
Zhong, X. Han, Y. Wang, Y. Lu, Chem. Sci., 2012, 3, 1231; (l)
Chen, G.-Y.; Zhong, F.; Lu, Y. Org. Lett. 2012, 14, 3955; (m)
Dou, X.; Lu, Y. Chem. Eur. J. 2012, 18, 8315; (n) Zhong, F.; Dou,
X.; Han, X.; Yao, W.; Zhu, Q.; Meng, Y.; Lu, Y. Angew. Chem.,
Int. Ed. 2013, 52, 943.