Chiral bicyclic guanidine-catalysed conjugate addition of α-fluoro-β-ketoesters to cyclic enones

Article abstract of DOI:10.1071/CH14187

By utilising a chiral bicyclic guanidine as catalyst and triethylamine as additive, the first asymmetric Michael addition of α-fluoro-β- ketoesters to various cyclic enones has been successfully developed, affording a variety of Michael adducts with potential synthetic utilities with satisfactory stereoselectivity (up to 94% ee and 4.3:1 dr). CSIRO 2014.

Full text of DOI:10.1071/CH14187

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2014, 67, 1119–1123
Communication
http://dx.doi.org/10.1071/CH14187
Chiral Bicyclic Guanidine-Catalysed Conjugate Addition
of a-Fluoro-b-Ketoesters to Cyclic Enones
A
A
B
A
Zhenzhong Jing, Jin Liu, Kek Foo Chin, Wenchao Chen,
B
A C
,
Choon-Hong Tan, and Zhiyong Jiang
A
Key Laboratory of Natural Medicine and Immuno-Engineering of Henan Province,
Henan University, Kaifeng, Henan 475004, China.
B
Division of Chemistry and Biological Chemistry, Nanyang Technological University,
21 Nanyang Link, 637371, Singapore.
C
Corresponding author. Email: chmjzy@henu.edu.cn
By utilising a chiral bicyclic guanidine as catalyst and triethylamine as additive, the first asymmetric Michael addition of
a-fluoro-b-ketoesters to various cyclic enones has been successfully developed, affording a variety of Michael adducts
with potential synthetic utilities with satisfactory stereoselectivity (up to 94 % ee and 4.3 : 1 dr).
Manuscript received: 29 March 2014.
Manuscript accepted: 30 April 2014.
Published online: 23 May 2014.
Exchange of hydrogenwitha fluorine atom in biologically active
compounds, namely monofluorination, has received increasing
interest particularly in pharmaceutical research. Monofluori-
nated analogues are bioisosteres of the parent molecules, often
with enhanced desirable properties and do not require significant
modifications to the molecular structures.[1] The development of
an efficient method for the specific incorporation of fluorine in a
stereoselective manner has thus attracted considerable attention
in organic and medicinal chemistry in the past fewdecades.[2]
Stereoselective introduction[2] of a fluorine atom into a chal-
lenging quaternary stereogenic centre[3] of chiral compoundshas
been demonstrated as one of the most efficient protocols to
obtain chiral monofluorinated compounds.
be an efficient catalyst for its high pKa value (Fig. 1, Eqn 1).
The catalytic ability of the guanidine I to activate cyclic
enones was verified once more when 3-benzyl oxindoles were
used as nucleophiles.[11n] Indeed, the chiral bicyclic guanidines
have exhibited strong catalytic and stereoselective activities in
many asymmetric reactions, including the Diels–Alder reac-
tion,[13a] protonation,[13b-d] the Mannich reaction,[9b] decarb-
oxylative reaction,[13e] the Michael reaction,[6,11d–e,11n,13f–h]
vinylogous amination,[13i] amination,[13j] and alkylation.[13k]
Therefore, we envisioned that the Michael addition of
a-fluoro-b-ketoesters to cyclic enones could be addressed by
utilising this kind of well-established guanidines as catalysts.
Initially, Michael addition of a-fluoro-b-ketoester 1a to
2-cyclopentenone 2 was selected as a model reaction to
investigate the reaction conditions (Table 1). The reaction
worked smoothly at 258C, in toluene and in the presence of
10 mol-% of guanidine I as catalyst, affording the desired
adduct 3a in 92 % yield with 75 % ee and 2 : 1 dr (Table 1,
entry 1). This satisfactory result prompted us to evaluate other
guanidines II–IV catalysts containing different side chains
(Table 1, entries 2–4). However, no improvement in the results
was obtained and guanidine I proved to be the best catalyst.
Subsequently, the solvent effect was evaluated in the process
(Table 1, entries 5–10). The use of THF as a solvent improved
the ee value to 87 % with the same diastereoselectivity (Table 1,
entry 6). To our surprise, no reaction was detected when
diethyl ether and ethyl acetate were used as solvents (Table 1,
entries 7–8). Lowering the temperature to 108C resulted in no
improvement in the stereoselectivity when THF was used as a
solvent (Table 1, entry 10), however, toluene did give better
results (90 % ee, 3 : 1 dr, Table 1, entry 11). The reaction in
toluene at À108C was thus conducted, and the adduct 3a was
obtained in 40 % yield with 93 % ee and 3 : 1 dr after 22 h
(Table 1, entry 13). In our previous work,[11e] we have demon-
strated that Et3N could accelerate the reaction rate, without
In 2008, Maruoka and co-workers presented an original
application of a-fluoro-b-ketoesters, acting as fluorocarbon
nucleophiles, in asymmetric amination with moderate enan-
tioselectivity.[4] Later in 2009, Lu and co-workers introduced
a highly enantioselective Michael addition of a-fluoro-b-
ketoesters to nitroalkenes with moderate diastereoselectivity.[5]
Almost simultaneously, we reported the asymmetric Michael
additions of a-fluoro-b-ketoesters with N-maleimides and
trans-4-oxo-4-arylbutenamides with excellent enantio- and dia-
stereoselectivities.[6] Since then, a-fluoro-b-ketoesters and their
analogues have been successively applied in a series of asym-
metric reactions, such as Robinson annulation,[7] amination,[8]
Mannich reaction,[9] and allylic alkylation with Morita–Baylis–
Hillman carbonates.[10] However, cyclic enones, which have
been utilised as electrophiles in many asymmetric reactions to
access various chiral compounds with important biological
activities,[11] has never been attempted to react with a-fluoro-
b-ketoesters, probably due to their formidable poor reactivity as
electrophiles.[11d–e]
In 2007, our group[11d] introduced highly enantioselective
Michael reactions of dithiomalonates with cyclic enones, in
which the chiral bicyclic guanidine[12] I was demonstrated to
Journal compilation Ó CSIRO 2014
www.publish.csiro.au/journals/ajc

1120
Z. Jing et al.
compromising ee values in guanidine I-catalysed Michael
reactions. Therefore, 0.1 equiv. of Et3N was used as an additive
under the same reaction conditions. We were delighted that
the reaction finished within 45 h to afford the adduct 3a in
93 % yield with 92 % ee and 3 : 1 dr (Table 1, entry 14).
Although similar improved results were obtained when pyri-
dine or iPr2EtN were used as additives (Table 1, entries
15–16), Et3N was considered to be the best additive due to its
low boiling point and convenient removal.
Subsequently, we began to investigate the versatility of this
protocol by examining the Michael reactions of various
a-fluoro-b-ketoesters 1 with 2-cyclopentenone 2 under the
optimal reaction conditions (10 mol-% of guanidine I, 0.1 equiv.
of Et3N in toluene at À108C). The results are summarised in
Table 2. The corresponding adducts were obtained in 74–95 %
yield with 81–94 % ee and a dr of 1.5 : 1 to 4.3 : 1. It was found
that a-fluoro-b-ketoester 2b, containing ethyl as a bulkier ester
group, afforded the corresponding adduct 3b with similar
N
tBu
tBu
O
O
O
O
SR
SR
N
H
N
SR
I
(1)
ϩ
n
SR
n
O
Rϭ
O
up to 97 % ee
N
H
n
tBu
tBu
CH2Ar
O
ArH2C
O
N
N
H
I
O
ϩ
O
N
up to 94 % ee
N
(2)
n
CH2ArЈ
CH2ArЈ
Fig. 1. Our preliminary work in the investigations of Michael addition reactions using cyclic ketones as
electrophiles.
Table 1. Screening studies on reaction conditions
N
O
R
R
O
O
N
H
N
O
ϩ
I–IV (10 mol-%)
OMe
COPh
F
F
MeOOC
3a
1a
2
N
N
N
N
Bn
Bn
N
H
N
H
N
N
N
N
H
N
N
H
I
II
III
IV
Entry
Catalyst
Solvent
T [8C]
t [h]
YieldA [%]
eeB [%]
drC
1
I
Toluene
Toluene
Toluene
Toluene
CH2Cl2
THF
25
25
44
27
25
25
44
44
44
44
44
23
22
22
22
45
45
45
92
94
75
73
33
67
0
2 : 1
2 : 1
2 : 1
3 : 1
1 : 1
2 : 1
N.A.
N.A.
1 : 1
2 : 1
3 : 1
3 : 1
3 : 1
3 : 1
3 : 1
3 : 1
2
II
III
IV
I
3
25
86
4
25
93
5
25
55
6
I
25
96
87
N.A.
N.A.
7
7
I
Et2O
25
Trace
Trace
64
8
I
EtOAc
CH3CN
THF
25
9
I
25
10
11
12
13
14D
15E
16F
I
10
83
89
90
91
93
92
91
92
I
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
10
83
I
0
47
I
À10
À10
À10
À10
40
I
93
I
93
I
91
AYield of isolated product.
BDetermined by HPLC method.
CDetermined by 1H NMR analysis of the crude material.
DEt3N (0.1 equiv.) was used.
EPyridine (0.1 equiv.) was used.
FiPr2EtN (0.1 equiv.) was used.

Organocatalysis
1121
enantio- and diastereoselectivities (90 % ee, 3 : 1 dr, Table 1,
entry 1). The a-fluoro-b-ketoesters (1c–f) with electron-
withdrawing groups on the para- or meta-positions of the phenyl
rings (Table 2, entries 2–5) gave higher enantioselectivities
than those with electron-withdrawing groups (1g–i) in the
ortho-position (Table 2, entries 6–8). A similar trend regarding
enantioselectivity was found with a-fluoro-b-ketoesters (1j–o)
with the electron-neutral and electron-donating groups on the
phenyl rings (Table 2, entries 9–14). Moreover, excellent ee
value was obtained when the phenyl ring of a-fluoro-b-ketoester
was replaced witha heteroaromaticgroup, suchasfuryl(Table2,
entry 15). To evaluate the versatility of this protocol, other
cyclic ketones, such as 2(5H)-furanone 4 and 2-cyclopentenone
6 were attempted in succession (Scheme 1). 2(5H)-Furanone 4
exhibited lower reactivity than 2-cyclopentenone 2; the adduct 5
was obtained in 43 % yield with 82 % ee and 2.3 : 1 dr after 72 h
(Eqn 1). Good yield with good enantioselectivity and moderate
diastereoselectivity was achieved when using 2-cyclopentenone
6 as the electrophile (Eqn 2).
Indeed, we have attempted to synthesise these monofluori-
nated compounds from b-ketoester 8. As shown in Scheme 2,
the reaction between b-ketoester 8 and 2-cyclopentenone 2 was
conducted in the presence of 10 mol-% of guanidine I at À108C
in Et3N. As we anticipated,[11e] the reaction finished after 70 h
Table 2. Michael addition of various a-fluoro-b-ketoesters 1 to 2-cyclopentenone 2
catalysed by bicyclic guanidine I
N
O
N
H
N
O
O
O
ϩ
Ar
I (10 mol-%)
OR
COAr
F
F
Et3N (0.1 equiv.)
toluene, Ϫ10ЊC
ROOC
3
1
2
Entry
1 (Ar, R)
1b (Ph, Et)
t [h]
3
YieldA [%]
eeB [%]
drC
1D
2
71
68
69
51
70
54
48
63
69
70
47
69
72
74
72
3b
3c
3d
3e
3f
92
90
90
77
79
84
91
74
87
91
64
93
92
95
92
90
93
93
91
91
85
82
81
93
93
94
87
84
92
90
3.0 : 1
3.6 : 1
3.5 : 1
3.8 : 1
2.9 : 1
1.5 : 1
1.6 : 1
1.5 : 1
3.6 : 1
3.6 : 1
4.3 : 1
3.0 : 1
1.8 : 1
3.7 : 1
2.8 : 1
1c (4-FPh, Me)
1d (4-ClPh, Me)
1e (4-BrPh, Me)
1f (3-BrPh, Me)
1g (2-FPh, Me)
1h (2-ClPh, Me)
1i (2-BrPh, Me)
3
4
5
6
3g
3h
3i
7
8
9
1j (4-MePh, Me)
3j
10
11
12
13
14
15
1k (4-MeOPh, Me)
1l [4-(4-BrPh)Ph, Me]
1m (2-MePh, Me)
1n (2-MeOPh, Me)
1o (3-MeOPh, Me)
1p (2-furyl, Me)
3k
3l
3m
3n
3o
3p
AYield of isolated product.
BDetermined by HPLC method.
CDetermined by 1H NMR analysis of the crude material.
D5.0 mmol scale, 80 h, 86 % yield, 90 % ee and 3 : 1 dr.
N
O
O
O
N
H
N
O
O
ϩ
OMe
(1)
I (10 mol-%)
Ph
COPh
O
Et3N (0.1 equiv.)
toluene, Ϫ10ЊC, 72h
F
F
MeOOC
5
1
4
43 % yield
82 % ee, dr ϭ 2.3:1
O
N
O
O
N
H
N
O
ϩ
I (10 mol-%)
(2)
OMe
Ph
COPh
Et3N (0.1 equiv.)
toluene, Ϫ10ЊC, 120h
F
F
7
MeOOC
88 % yield
86 % ee, dr ϭ 2.4:1
1
6
Scheme 1. Bicyclic guanidine-catalysed Michael addition of a-fluoro-b-ketoester 1a to 2(5H)-furanone 4
(Eqn 1); Michael addition of a-fluoro-b-ketoester 1a to 2-cyclohexenone 6 (Eqn 2).

1122
Z. Jing et al.
O
N
N
H
O
N
O
O
O
ϩ
I (10 mol-%)
Ph
OMe
Ph
H
OMe
Et3N, Ϫ10ЊC
70 h, 86 % yield
O
8
2
9
88 % ee, 1:1 dr
O
O
F
NFSI, TMG (2 mol-%)
(3)
toluene, 25ЊC, 18h
Ph
H
18 % yield
OMe
O
10: 88 % ee, 2:3 dr
ϩ
N
O
N
N
O
H
H
O
COPh
O
MeOOC
F
OMe
F
3a
Side-on model
Scheme 2. Preparation of monofluorinated adduct 10 from a Michael addition–fluorination reaction and proposed
transition state model.
affording adduct 9 in 86 % yield with 88 % ee and 1 : 1 dr.
However, the fluorination of 9 using N-fluorobenzenesulfo-
nimide (NFSI) as the fluorine source and 1,1,3,3-tetramethyl-
guanidine (TMG) as catalyst at 258C was very slow, and only
18 % yield of the product 10 was obtained. HPLC analysis
indicated that compound 10 was a diastereomer of 3a with the
same enantiomeric identity (see the Supplementary Material for
details). Combining these results with the currently understood
mechanism of guanidine I,[6] a transition state with a side-on
manner was proposed and the absolute configuration of these
monofluorinated adducts was reasonably assigned. Although
the role of Et3N in the reaction is not clear, a plausible
mechanism is that the Et3N decreases the activation energy of
the transition state.
In conclusion, we have developed the first asymmetric
Michael addition of a-fluoro-b-ketoesters to various cyclic
enones. The bicyclic guanidine I was verified to be an efficient
chiral catalyst for the reactions. The direct protocol provides a
readily method to construct a monofluorinated quaternary chiral
centre on cyclic ketones with high stereoselectivity. Other
reactions utilising a-fluoro-b-ketoesters as nucleophiles to
access a variety of monofluorinated compounds are in progress
and will be reported in due course.
References
[1] (a) Organofluorine Compounds in Medicinal Chemistry and Bio-
medical Applications (Eds R. Filler, Y. Kobayashi, L. M. Yagupolskii)
1993 (Elsevier: Amsterdam).
(b) T. Hiyama, Organofluorine Compounds: Chemistry and Applica-
tions 2000 (Springer: Berlin).
(c) K. Uneyama, Organo-fluorine Chemistry 2006 (Blackwell:
Oxford).
(d) D. O’Hagan, Chem. Soc. Rev. 2008, 37, 308. doi:10.1039/
B711844A
(e) T. Furuya, A. S. Kamlet, T. Ritter, Nature 2011, 473, 470.
doi:10.1038/NATURE10108
[2] (a) For selected reviews, see: J.-A. Ma, D. Cahard, Chem. Rev. 2004,
104, 6119. doi:10.1021/CR030143E
(b) H. Ibrahim, A. Togni, Chem. Commun. 2004, 1147. doi:10.1039/
B317004G
(c) P. M. Pihko, Angew. Chem. Int. Ed. 2006, 45, 544. doi:10.1002/
ANIE.200502425
(d) V. A. Brunet, D. O’Hagan, Angew. Chem. Int. Ed. 2008, 47, 1179.
doi:10.1002/ANIE.200704700
(e) S. Lectard, Y. Hamashima, M. Sodeoka, Adv. Synth. Catal. 2010,
352, 2708. doi:10.1002/ADSC.201000624
(f) D. Cahard, X. Xu, S. Couve-Bonnaire, X. Pannecoucke, Chem. Soc.
Rev. 2010, 39, 558. doi:10.1039/B909566G
(g) C. Hollingworth, V. Gouverneur, Chem. Commun. 2012, 48, 2929.
doi:10.1039/C2CC16158C
[3] (a) J. Christoffers, A. Mann, Angew. Chem. Int. Ed. 2001, 40, 4591.
doi:10.1002/1521-3773(20011217)40:24,4591::AID-ANIE4591.3.0.
CO;2-V
Supplementary Material
Experimental procedures and compound characterisation data
are available on the Journal’s website.
(b) For
a recent book, see: Quaternary Stereocenters (Eds
J. Christoffers, A. Baro) 2005 (Wiley-VCH: Weinheim).
[4] R. He, X. Wang, T. Hashimoto, K. Maruoka, Angew. Chem. Int. Ed.
2008, 47, 9466. doi:10.1002/ANIE.200804140
Acknowledgements
We are grateful for financial support from the Program for New Century
Excellent Talents in University of Ministry of Education (NCET-11–0938).
[5] X. Han, J. Luo, C. Liu, Y. Lu, Chem. Commun. 2009, 2044.
doi:10.1039/B823184B

Organocatalysis
1123
[6] Z. Jiang, Y. Pan, Y. Zhao, T. Ma, R. Lee, Y. Yang, K.-W. Huang,
M. W. Wong, C.-H. Tan, Angew. Chem. Int. Ed. 2009, 48, 3627.
doi:10.1002/ANIE.200900964
[7] H.-F. Cui, Y.-Q. Yang, Z. Chai, P. Li, C.-W. Zheng, S.-Z. Zhu,
G. Zhao, J. Org. Chem. 2010, 75, 117. doi:10.1021/JO902081W
[8] X. Han, F. Zhong, Y. Lu, Adv. Synth. Catal. 2010, 352, 2778.
doi:10.1002/ADSC.201000562
(b) M. P. Coles, Chem. Commun. 2009, 3659. doi:10.1039/B901940E
(c) D. Leow, C.-H. Tan, Synlett 2010, 1589.
(d) X. Fu, C.-H. Tan, Chem. Commun. 2011, 47, 8210. doi:10.1039/
C0CC03691A
(e) P. Selig, Synthesis 2013, 45, 703. doi:10.1055/S-0032-1318154
(f) For selected examples, see: E. J. Corey, M. J. Grogan, Org. Lett.
1999, 1, 157. doi:10.1021/OL990623L
[9] (a) X. Han, J. Kwiatkowski, F. Xue, K.-W. Huang, Y. Lu, Angew.
Chem. Int. Ed. 2009, 48, 7604. doi:10.1002/ANIE.200903635
(b) Y. Pan, Y. Zhao, T. Ma, Y. Yang, H. Liu, Z. Jiang, C.-H. Tan,
Chem. – Eur. J. 2010, 16, 779. doi:10.1002/CHEM.200902830
(c) Y. Kang, D. Y. Kim, Tetrahedron Lett. 2011, 52, 2356.
doi:10.1016/J.TETLET.2011.02.087
(g) M. Terada, H. Ube, Y. Yaguchi, J. Am. Chem. Soc. 2006, 128, 1454.
doi:10.1021/JA057848D
(h) M. Terada, M. Nakano, H. Ube, J. Am. Chem. Soc. 2006, 128,
16044. doi:10.1021/JA066808M
(i) M. Terada, T. Ikehara, H. Ube, J. Am. Chem. Soc. 2007, 129, 14112.
doi:10.1021/JA0746619
(d) S. J. Yoon, Y. K. Kang, D. Y. Kim, Synlett 2011, 420.
[10] L. Yan, Z. Han, B. Zhu, C. Yang, C.-H. Tan, Z. Jiang, Beilstein J. Org.
Chem. 2013, 9, 1853. doi:10.3762/BJOC.9.216
[11] (a) A. Prieto, N. Halland, K. A. Jørgensen, Org. Lett. 2005, 7, 3897.
doi:10.1021/OL051301M
(j) H. Ube, N. Shimada, M. Terada, Angew. Chem. Int. Ed. 2010, 49,
1858. doi:10.1002/ANIE.200906647
(k) M. Terada, K. Ando, Org. Lett. 2011, 13, 2026. doi:10.1021/
OL200415U
(l) S. Dong, X. Liu, Y. Zhang, L. Lin, X. Feng, Org. Lett. 2011, 13,
5060. doi:10.1021/OL2018888
(b) M. Shi, W. Zhang, Adv. Synth. Catal. 2005, 347, 535. doi:10.1002/
ADSC.200404321
(c) S. Hanessian, Z. Shao, J. S. Warrier, Org. Lett. 2006, 8, 4787.
doi:10.1021/OL0618407
(d) W. Ye, Z. Jiang, Y. Zhao, S. L. M. Goh, D. Leow, Y.-T. Soh,
C.-H. Tan, Adv. Synth. Catal. 2007, 349, 2454. doi:10.1002/ADSC.
200700326
(e) Z. Jiang, W. Ye, Y. Yang, C.-H. Tan, Adv. Synth. Catal. 2008, 350,
2345. doi:10.1002/ADSC.200800423
(m) X. Xiao, X. Liu, S. Dong, Y. Cai, L. Lin, X. Feng, Chem. – Eur. J.
2012, 18, 15922. doi:10.1002/CHEM.201203216
[13] (a) J. Shen, T. T. Nguyen, Y.-P. Goh, W. Ye, X. Fu, J. Xu, C.-H. Tan,
J. Am. Chem. Soc. 2006, 128, 13692. doi:10.1021/JA064636N
(b) D. Leow, S. Lin, S. K. Chittimalla, X. Fu, C.-H. Tan, Angew. Chem.
Int. Ed. 2008, 47, 5641. doi:10.1002/ANIE.200801378
(c) H. Liu, D. Leow, K.-W. Huang, C.-H. Tan, J. Am. Chem. Soc. 2009,
131, 7212. doi:10.1021/JA901528B
(f) K. Kawamura, H. Fukuzawa, M. Hayashi, Org. Lett. 2008, 10, 3509.
doi:10.1021/OL801259U
(g) Y. Tanaka, M. Kanai, M. Shibasaki, J. Am. Chem. Soc. 2008, 130,
6072. doi:10.1021/JA801201R
(d) Y. Zhao, X. Lin, Y. Pan, L. Zong, W. Feng, C.-H. Tan,
K.-W. Huang, Chem. Commun. 2012, 48, 5479. doi:10.1039/
C2CC31443F
(e) Y. Pan, C. W. Kee, Z. Jiang, T. Ma, Y. Zhao, Y. Yang, H. Xue,
(h) P. Li, Y. Wang, X. Liang, J. Ye, Chem. Commun. 2008, 3302.
doi:10.1039/B804540B
C.-H. Tan, Chem. – Eur. J. 2011, 17, 8363. doi:10.1002/
CHEM.201100687
(i) A. J. Smith, L. K. Abbott, S. F. Martin, Org. Lett. 2009, 11, 4200.
doi:10.1021/OL9013975
(f) X. Fu, Z. Jiang, C.-H. Tan, Chem. Commun. 2007, 5058.
doi:10.1039/B713151H
(j) F. Pesciaioli, X. Tian, G. Bencivenni, G. Bartoli, P. Melchiorre,
Synlett 2010, 1704.
(k) M. H. Freund, S. B. Tsogoeva, Synlett 2011, 503.
(l) J. Streuff, Chem. – Eur. J. 2011, 17, 5507. doi:10.1002/
CHEM.201100501
(g) Z. Jiang, Y. Yang, Y. Pan, Y. Zhao, H. Liu, C.-H. Tan, Chem. –
Eur. J. 2009, 15, 4925. doi:10.1002/CHEM.200802601
(h) L. Li, W. Chen, W. Yang, Y. Pan, H. Liu, C.-H. Tan, Z. Jiang,
Chem. Commun. 2012, 48, 5124. doi:10.1039/C2CC31587D
(i) J. Wang, J. Chen, C. W. Kee, C.-H. Tan, Angew. Chem. Int. Ed.
2012, 51, 2382. doi:10.1002/ANIE.201107317
(j) Y. Zhao, Y. Pan, H. Liu, Y. Yang, Z. Jiang, C.-H. Tan, Chem. –
Eur. J. 2011, 17, 3571. doi:10.1002/CHEM.201003761
(k) W. Chen, W. Yang, L. Yan, C.-H. Tan, Z. Jiang, Chem. Commun.
2013, 49, 9854. doi:10.1039/C3CC46111D
(m) N. Shibata, M. Yoshimura, H. Yamada, R. Arakawa, S. Sakaguchi,
J. Org. Chem. 2012, 77, 4079. doi:10.1021/JO300472R
(n) C. Yang, W. Chen, W. Yang, B. Zhu, L. Yan, C.-H. Tan, Z. Jiang,
Chem. Asian J. 2013, 8, 2960. doi:10.1002/ASIA.201301011
[12] (a) For selected reviews, see: D. Leow, C.-H. Tan, Chem. Asian J.
2009, 4, 488. doi:10.1002/ASIA.200800361