Copper-catalyzed enantioselective hetero-diels-alder reaction of Danishefsky's diene with β,γ-unsaturated α-ketoesters

Article abstract of DOI:10.1021/ol5015009

A highly enantioselective hetero-Diels-Alder reaction of Danishefskys diene with β,γ-unsaturated α-ketoesters was developed for the first time by virtue of chiral copper complexes. This protocol provided a facile access to optically active dihydropyranones bearing a quaternary center with high enantioselectivities and good yields. Furthermore, on the basis of the isolated intermediate analysis, the reaction pathway was substrate-dependent.

Full text of DOI:10.1021/ol5015009

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed Enantioselective Hetero-Diels−Alder Reaction of
Danishefsky’s Diene with β,γ-Unsaturated α‑Ketoesters
Yanbin Hu,‡ Kun Xu,‡ Sheng Zhang, Fengfeng Guo, Zhenggen Zha, and Zhiyong Wang*
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Soft Matter Chemistry and Department of
Chemistry & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of
China, Hefei, Anhui 230026, P. R. China
S
* Supporting Information
ABSTRACT: A highly enantioselective hetero-Diels−Alder
reaction of Danishefsky’s diene with β,γ-unsaturated α-
ketoesters was developed for the first time by virtue of chiral
copper complexes. This protocol provided a facile access to
optically active dihydropyranones bearing a quaternary center
with high enantioselectivities and good yields. Furthermore, on
the basis of the isolated intermediate analysis, the reaction
pathway was substrate-dependent.
Besides, chiral rare earth organophosphates were demonstrated
he hetero-Diels−Alder (HDA) reaction provides a
Tpowerful tool to construct aza, oxa-heterocycle molecules to be efficient catalysts for the HDA reaction of phenyl-
glyoxylates by Inanaga et al.8 Recently, the Wolf group reported
a multisubstrate one-pot high-throughput screening method for
the HDA reaction of glyoxylates with the diene.9
in organic synthesis.1 Since Danishefsky et al. identified the
trans-1-methoxy-3-(trimethylsilyloxy)butadiene (Danishefsky’s
diene)2 as an active diene, many useful chiral Lewis acid
catalysts have been utilized to catalyze the HDA cycloaddition
of Danishefsky’s diene with aldehydes or ketones.3 Recently,
the List group developed a chiral disulfonimide as an efficient
catalyst for the HDA reaction of aldehydes with Danishefsky’s
diene or substituted dienes, which led to the efficient synthesis
of 2,5,6-trisubstituted dihydropyrones.4 Compared with
aldehyde/ketone, the HDA reactions of bicarbonyl dienophiles
with Danishefsky’s diene are relatively fewer (Scheme 1a).5
Jørgensen et al. described the first highly enantioselective
HDA reaction of α-ketoesters with Danishefsky’s diene,6 and
this type of reaction was further developed by the Loh group.7
The Ghosh group5a,c and the Mikami group5g reported the
transformation of glyoxylates with Danishefsky’s diene
catalyzed by Cu-BOX, chiral Ti complexes, respectively.
To the best of our knowledge, the Cu-catalyzed asymmetirc
HDA reaction between Danishefsky’s diene and β,γ-unsaturated
α-ketoesters10 has not been reported so far. Herein, we report
the first HDA reaction of Danishefsky’s diene with β,γ-
unsaturated α-ketoesters to afford chiral dihydropyrones
bearing a quaternary center using copper complexes (Scheme
1b). The unsaturated bonds (alkene or alkyne) in the resulting
dihydropyranones allow further derivatization. The mechanism
study showed that the reaction pathway was substrate-
dependent.
First, (E)-isopropyl 2-oxo-4-phenylbut-3-enoate 1a was
selected as a model dienophile to conduct the cycloaddition
with Danishefsky’s diene. Based on our group’s efforts on chiral
Cu−Schiff base and Cu−prolinol derivative complexes,11 three
catalytic systems were tested to obtain the product 3a with high
enantioselectivity (Table 1). When dinuclear copper com-
plex11b (entry 1) and monodentate N-ligand directed zinc
complex11a (entry 2) were employed as the catalysts, a low
yield and poor enantioselectivity were obtained. To our delight,
the ee value was improved when a chiral copper−prolinol (L2)
complex was used (entry 3).
Scheme 1. Previous Study and This Work on Hetero-Diels−
Alder Reaction
Encouraged by these results, the reaction conditions were
further optimized (Table 2). First, the equivalent of
Danishefsky’s diene was varied to enhance the yield (entries
1−5). The use of 2.0 equiv of the diene proved to be the best
condition, giving the dihydropyrone 3a in 77% yield (entry 4).
Received: May 27, 2014
Published: June 23, 2014
© 2014 American Chemical Society
3564
dx.doi.org/10.1021/ol5015009 | Org. Lett. 2014, 16, 3564−3567

Organic Letters
Letter
a
a
Table 1. Screening of Catalytic Systems
Table 3. Scope of β,γ-Unsaturated α-Ketoesters
b
c
entry
R1
R2
time (h)
3
yield (%) ee (%)
1
2
C6H5
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
Me
Et
16
20
24
20
17
17
17
20
17
17
16
16
20
17
20
20
24
20
3a
3b
3c
3d
3e
3f
94
90
90
95
93
95
99
99
81
96
94
97
84
96
97
97
85
97
95
95
97
96
96
96
96
94
97
93
96
96
96
97
95
96
93
96
p-MeC6H4
p-OMeC6H4
p-FC6H4
p-BrC6H4
p-ClC6H4
p-NO2C6H4
m-ClC6H4
o-ClC6H4
(E)-cinnamyl
2-naphthyl
2-thienyl
n-pentyl
d
3
4
5
6
d
e
entry
ligand
metal
CuBr2
base
yield (%)
ee (%)
7
3g
3h
3i
b
c
1
2
3
L1
piperidine
piperidine
Cs2CO3
18
28
39
24
41
93
8
L1
Zn(OTf)2
Cu(OTf)2
9
L2a
10
11
3j
a
3k
3l
Unless otherwise noted, all reactions were performed with 1a (0.1
mmol), 2a (0.11 mmol), L (10 mol %), base (10 mol %), and metal
d
12
b
c
salt (10 mol %) in toluene (1.0 mL) at 0 °C. 2.0 equiv. 3.0 equiv.
13
14
15
16
17
18
3m
3n
3o
3p
3q
3r
d
e
Isolated yield. Determined by chiral HPLC analysis. Tf =
cyclohexyl
C6H5
trifluoromethanesulfonyl.
C6H5
a
Table 2. Optimization of Reaction of 1a with 2a
C6H5
t-Bu
Bn
C6H5
b
c
entry
ligand
base
2a (equiv)
yield (%)
ee (%)
a
Unless otherwise noted, all reactions were performed with 1 (0.2
mmol), 2a (0.4 mmol), L2c (10 mol %), Et3N (10 mol %), and
1
L2a
L2a
L2a
L2a
L2a
L2a
L2a
L2a
L2a
L2a
L2b
L2c
L2d
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
DBU
1.25
1.50
1.75
2.0
3.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
54
65
71
77
76
85
87
80
49
84
77
94
77
90
90
89
91
91
92
93
94
90
95
95
95
95
2
b
Cu(OTf)2 (10 mol %) in toluene (2.0 mL) at 0 °C. Isolated yield.
c
d
3
Determined by chiral HPLC analysis. MTBE as the solvent. MTBE
4
= Methyl tert-butyl ether.
5
6
little influence on the enantioselectivity (entry 7). Substitution
at other positions on the phenyl group was also compatible
with this reaction condition (entries 8−9). Unsaturated (E)-
cinnamyl groups and ring-fused groups were also successfully
employed, giving products 3j and 3k in excellent yields and
enantioselectivities (entries 10−11). When a heterocyclic
group, such as a 2-thienyl group, was employed, an excellent
yield and enantioselectivity were obtained (entry 12). More
importantly, excellent enantioselectivities were realized when
R1 were aliphatic groups (entries 13−14). Then, different ester
groups (R2) were examined under the standard reaction
conditions (entries 15−18), and excellent yields and
enantioselectivities were achieved regardless of the steric and
electronic effects. The absolute configuration of the product 3e
was confirmed by X-ray crystal diffraction.12
As shown in Table 3, HDA reactions of β,γ-unsaturated α-
ketoesters with Danishesky’s diene provided an efficient
approach to various dihydropyrones bearing alkenyl groups
with good yield and excellent enantioseletivity. It would be
more valuable if the alkenyl group could be replaced by an
alkynyl group.13 With this goal in mind, isopropyl 2-oxo-4-
phenylbut-3-ynoate 4a was selected as a model substrate to
examine the substrate scope of the HDA reaction. It was found
that a minor modification of the standard reaction conditions
led to the formation of products 5a with excellent yield and
enantioselectivity, in which ligand L2c was replaced with L2a
(see Supporting Information (SI) for details).
7
8
piperidine
t-BuONa
Et3N
9
10
11
12
13
Et3N
Et3N
Et3N
a
Unless otherwise noted, all reactions were performed with 1a (0.1
mmol), 2a (corresponding equivalent), L2 (10 mol %), base (10 mol
%), and Cu(OTf)2 (10 mol %) in toluene (1.0 mL) at 0 °C. Isolated
yield. Determined by chiral HPLC analysis.
b
c
Further screening of the base showed that Et3N was the best
choice for the reaction with respect to the yield and
enantioselectivity (entry 10). The electronic property of the
aryl moiety of the ligand was examined next (entries 10−13).
The electron-donating group on the aryl moiety of L2 could
give a better result than the electron-withdrawing group (entry
12 vs 10−11, 13). When L2c was employed as the catalyst, the
best result could be obtained, in which dihydropyrone 3a could
be obtained in 94% yield and 95% ee (entry 12).
With the optimal conditions in hand, the substrate scope of
β,γ-unsaturated α-ketoesters for the HDA reaction was
explored (Table 3). The electronic effect was examined by
changing the para-substituents of R1 (entries 2−7). In terms of
electron-donating groups, the methyl and methoxyl groups
were tolerated well, affording the products 3b and 3c with
excellent yield and enantioselectivity (entries 2−3). Electron-
withdrawing groups, such as fluoro-, bromo-, and chloro-
groups, had little influence on the reaction (entries 4−6). The
strong electron-withdrawing effect favored the yield but had
As presented in Table 4, the scope of 2-oxo-3-ynoates was
investigated. Common electron-donating groups in the para
position of the phenyl group (R3) could give a good yield and
excellent ee value (entries 2−5). However, the reaction yield
decreased but the enantioselectivity remained when the p-
3565
dx.doi.org/10.1021/ol5015009 | Org. Lett. 2014, 16, 3564−3567

Organic Letters
Letter
a
Then, treatment of 6 with TFA gave the product 3a. The
Mukaiyama−aldol adduct was not observed in the crude
mixture. This indicated that the HDA reaction of 1a with 2a
proceeded via a concerted pathway. In contrast, a Mukaiyama−
aldol pathway was observed when 4a was employed as the
dienophile and the reaction intermediate 7 was isolated with
96% ee [eq 2]. Then, treatment of 7 with TFA gave the product
5a with 93% ee. Nevertheless, the [4 + 2] cycloaddition adduct
was not observed in the reaction of 4a with 2a. This
demonstrated that a stepwise pathway was involved in the
HDA reaction of 4a with 2a.
Table 4. Scope of 2-Oxo 3-Ynoate
b
c
entry
R3
C6H5
time (h)
5
yield (%)
ee (%)
d
1
24
25
25
24
20
20
17
20
24
21
5a
5b
5c
5d
5e
5f
91
89
88
81
94
83
99
86
82
84
93
92
92
92
93
94
87
92
90
86
2
3
4
5
6
7
8
p-EtC6H4
p-FC6H4
p-ClC6H4
p-MeC6H4
p-OMeC6H4
p-NO2C6H4
m-MeC6H4
m-ClC6H4
TMS
Moreover, when the TMS group of Danishefsky’s diene was
alternated to the TBS group, the HDA reaction of the diene 2b
with 1a yielded cycloaddition adduct 8 and the product 3a.
Then, treatment of 8 with TFA gave 3a with 96% ee [91% yield,
two steps, eq 3]. Similarly, the HDA reaction of 4a with 2b
afforded intermediate 9 with 19:1 dr and 93% ee. Then,
treatment of 9 with TFA gave the final product 5a in overall
93% yield and 93% ee [eq 4]. This indicated that the HDA
reactions of diene 2b with ketoesters 1a and 4a could proceed
smoothly to afford corresponding products via a cycloaddition
pathway. Intermediates 7, 8, 9 were isolated by silica gel
5g
5h
5i
d
9
d
10
5j
a
Unless otherwise noted, all reactions were performed with 4 (0.2
mmol), 2a (0.4 mmol), L2a (10 mol %), Et3N (10 mol %), and
b
Cu(OTf)2 (10 mol %) in toluene (2.0 mL) at 0 °C. Isolated yield.
c
d
Determined by chiral HPLC analysis. MTBE as the solvent.
1
chromatography and confirmed by H NMR, 13C NMR, and
methoxyl group was employed (entry 6). When an electron-
withdrawing group (p-nitro group) was employed, an excellent
yield and 87% ee were achieved (entry 7). The meta-
substitution had a little influence on the reaction yields but
minimally on the enantioselectivities (entries 8, 9). When an
aliphatic group was employed, a moderate yield and
enantioselectivity were obtained (entry 10).
HRMS. Based on the above-mentioned experiments, the
reaction pathway (cycloaddition or Mukaiyama−aldol con-
densation) was substrate-dependent under the reaction
conditions. The reaction pathway not only depended on the
dienes but also relied on the ketoesters, as shown in Scheme 2.
Subsequently, the reaction mechanism was investigated. The
mechanism of the HDA reaction of Danishefsky’s diene with
aldehydes has been reported,4,5a,14 in which two reaction
pathways, the concerted (Diels−Alder cycloaddition) and
stepwise (Mukaiyama−aldol condensation) pathway, were
involved.
Scheme 2. Substrate-Dependent Reaction Pathway
To get a better understanding of the current reaction
pathway, some control experiments were carried out. First, for
the HDA reaction of Danishefsky’s diene with 1a, analysis of 1H
NMR of the crude mixture showed the HDA reaction should
proceed via the cycloaddition adduct 6 [eq 1; see SI for details].
In conclusion, we report the first Cu-catalyzed asymmetric
HDA reaction of β,γ-unsaturated α-ketoesters with Danishef-
sky’s diene under mild reaction conditions. A series of chiral
dihydropyranones bearing a quaternary center were obtained
with excellent enantioselectivities and good to excellent yields.
Furthermore, based on the analysis of the isolated inter-
mediates, the reaction pathway was substrate-dependent.
ASSOCIATED CONTENT
* Supporting Information
■
S
1
Experimental procedures, characterization data, copies of H
NMR, 13C NMR of new compounds; HPLC profiles and
crystallographic data of compound 3e (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: zwang3@ustc.edu.cn.
3566
dx.doi.org/10.1021/ol5015009 | Org. Lett. 2014, 16, 3564−3567

Organic Letters
Letter
(8) (a) Furuno, H.; Hayano, T.; Kambara, T.; Sugimoto, Y.;
Hanamoto, T.; Tanaka, Y.; Jin, Y. Z.; Kagawa, T.; Inanaga, J.
Tetrahedron 2003, 59, 10509. (b) Furuno, H.; Kambara, T.; Tanaka,
Y.; Hanamoto, T.; Kagawa, T.; Inanaga, J. Tetrahedron Lett. 2003, 44,
6129.
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
(9) Wolf, C.; Fadul, Z.; Hawes, P. A.; Volpe, E. C. Tetrahedron:
Asymmetry 2004, 15, 1987.
(10) Desimoni, G.; Faita, G.; Quadrelli, P. Chem. Rev. 2013, 113,
5924.
ACKNOWLEDGMENTS
■
Financial support from the National Nature Foundation of
China (2127222, 91213303, 21172205, J1030412) is greatly
acknowledged.
(11) For developed catalytic systems, see: (a) Guo, F.; Lai, G.; Xiong,
S.; Wang, S.; Wang, Z. Chem.Eur. J. 2010, 16, 6438. (b) Guo, F.;
Chang, D.; Lai, G.; Zhu, T.; Xiong, S.; Wang, S.; Wang, Z. Chem.
Eur. J. 2011, 17, 11127. (c) Lai, G.; Guo, F.; Zheng, Y.; Fang, Y.; Song,
H.; Xu, K.; Wang, S.; Zha, Z.; Wang, Z. Chem.Eur. J. 2011, 17, 1114.
(d) Xu, K.; Lai, G.; Zha, Z.; Pan, S.; Chen, H.; Wang, Z. Chem.Eur.
J. 2012, 18, 12357. (e) Zhang, S.; Xu, K.; Guo, F.; Hu, Y.; Zha, Z.;
Wang, Z. Chem.Eur. J. 2014, 20, 979.
REFERENCES
■
(1) For selected reviews on the hetero-Diels−Alder reaction, see:
(a) Kametani, T.; Hibino, S. Adv. Heterocycl. Chem. 1987, 42, 245.
(b) Waldmann, H. Synthesis 1994, 535. (c) Tietze, L. F.; Kettschau, G.
Top. Curr. Chem. 1997, 189, 1. (d) Jørgensen, K. A. Angew. Chem., Int.
Ed. 2000, 39, 3558. (e) Jørgensen, K. A. In Cycloaddition Reactions in
Organic Synthesis; Kobayashi, S., Jørgensen, K. A., Eds.; Wiley-VCH:
Weinheim, Germany, 2002; p 151. (f) Ojima, I. In Catalytic
Asymmetric Synthesis, 3rd ed.; Soail, K., Kawasakil, T., Shibata, T.,
Eds.; Wiley-VCH: New York, 2010; p 891. (g) Pellissier, H.
Tetrahedron 2009, 65, 2839.
(12) Details of the crystal structure analysis are provided as SI.
CCDC-997035 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
(13) For the synthesis of 2-oxo-3-ynoates, see: Guo, M.; Li, D.;
Zhang, Z. J. Org. Chem. 2003, 68, 10172.
(2) (a) Danishefsky, S.; Kitahara, T. J. Am. Chem. Soc. 1974, 96, 7807.
(b) Danishefsky, S.; Kerwin, J. F.; Kobayashi, S. J. Am. Chem. Soc.
1982, 104, 358. (c) Danishefsky, S.; Larson, E. R.; Askin, D. J. Am.
Chem. Soc. 1982, 104, 6457.
(14) (a) For both types of mechanisms of HDA reactions of
aldehydes and Danishefsky’s diene, see: Roberson, M.; Jepsen, A. S.;
Jørgensen, K. A. Tetrahedron 2001, 57, 907. For the Mukaiyama−
aldol pathway, see: (b) Corey, E.; Cywin, C.; Roper, T. Tetrahedron
Lett. 1992, 33, 6907. (c) Qian, C.; Wang, L. Tetrahedron Lett. 2000, 41,
2203. (d) Yamashita, Y.; Saito, S.; Ishitani, H.; Kobayashi, S. J. Am.
Chem. Soc. 2003, 125, 3793. For the [4 + 2] cycloaddition pathway,
see: (e) Bednarski, M.; Danishefsky, S. J. Am. Chem. Soc. 1983, 105,
3717. (f) Bednarski, M.; Maring, C.; Danishefsky, S. Tetrahedron Lett.
(3) For selected reports on asymmetric HDA reactions of aldehydes
with Danishefsky’s diene, see: (a) Hanamoto, T.; Furuno, H.;
Sugimoto, Y.; Inanaga, J. Synlett 1997, 79. (b) Wang, B.; Feng, X.;
Cui, X.; Liu, H.; Jiang, Y. Chem. Commun. 2000, 1605. (c) Anada, M.;
Washio, T.; Shimada, N.; Kitagaki, S.; Nakajima, M.; Shiro, M.;
Hashimoto, S. Angew. Chem., Int. Ed. 2004, 43, 2665. (d) Chen, I. H.;
Oisaki, K.; Kanai, M.; Shibasaki, M. Org. Lett. 2008, 10, 5151. (e) Yu,
Z.; Liu, X.; Dong, Z.; Xie, M.; Feng, X. Angew. Chem., Int. Ed. 2008, 47,
1308. For the metal-BINOL catalysis, see: (f) Keck, G. E.; Li, X.-Y.;
Krishnamurthy, D. J. Org. Chem. 1995, 60, 5998. (g) Simonsen, K. B.;
Svenstrup, N.; Roberson, M.; Jørgensen, K. A. Chem.Eur. J. 2000, 6,
123. (h) Gong, L.-Z.; Pu, L. Tetrahedron Lett. 2000, 41, 2327. (i) Du,
H.; Long, J.; Hu, J.; Li, X.; Ding, K. Org. Lett. 2002, 4, 4349. (j) Kii, S.;
Hashimoto, T.; Maruoka, K. Synlett 2002, 931. (k) Yamashita, Y.;
Saito, S.; Ishitani, H.; Kobayashi, S. J. Am. Chem. Soc. 2003, 125, 3793.
For the Salen-Mn, Cr catalysis, see: (l) Schaus, S. E.; Branalt, J.;
Jacobsen, E. N. J. Org. Chem. 1998, 63, 403. (m) Aikawa, K.; Irie, R.;
Katsuki, T. Tetrahedron 2001, 57, 845. (n) Sellner, H.; Karjalainen, J.
K.; Seebach, D. Chem.Eur. J. 2001, 7, 2873. (o) Joly, G. D.;
Jacobsen, E. N. Org. Lett. 2002, 4, 1795. For the dirhodium
carboxamidates catalysis, see: (p) Doyle, M. P.; Phillips, I. M.; Hu, W.
J. Am. Chem. Soc. 2001, 123, 5366. (q) Doyle, M. P.; Valenzuela, M.;
Huang, P. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5391.
1983, 24, 3451. (g) Schaus, S.; Branalt, J.; Jacobsen, E. N. J. Org. Chem.
̊
1998, 63, 403. (h) Motoyama, Y.; Koga, Y.; Nishiyama, H. Tetrahedron
2001, 57, 853. (i) Zhang, X.; Du, H.; Wang, Z.; Wu, Y.-D.; Ding, K. J.
Org. Chem. 2006, 71, 2862. (j) Yang, X.-B.; Feng, J.; Zhang, J.; Wang,
N.; Wang, L.; Liu, J.-L.; Yu, X.-Q. Org. Lett. 2008, 10, 1299.
(4) Guin, J.; Rabalakos, C.; List, B. Angew. Chem., Int. Ed. 2012, 51,
8859.
(5) (a) Ghosh, A. K.; Mathivanan, P.; Cappiello, J.; Krishnan, K.
Tetrahedron: Asymmetry 1996, 7, 2165. (b) Qian, C.; Wang, L.
Tetrahedron Lett. 2000, 41, 2203. (c) Ghosh, A. K.; Shirai, M.
Tetrahedron Lett. 2001, 42, 6231. (d) Motoyama, Y.; Koga, Y.;
Nishiyama, H. Tetrahedron 2001, 57, 853. (e) Dalko, P. I.; Moisan, L.;
Cossy, J. Angew. Chem., Int. Ed. 2002, 41, 625. (f) Kwiatkowski, P.;
Asztemborska, M.; Jurczak, J. Tetrahedron: Asymmetry 2004, 15, 3189.
(g) Tonoi, T.; Mikami, K. Tetrahedron Lett. 2005, 46, 6355.
(h) Kanemitsu, T.; Asajima, Y.; Shibata, T.; Miyazaki, M.; Nagata,
K.; Itoh, T. Heterocycles 2011, 83, 2525.
(6) (a) Johannsen, M.; Yao, S.; Jørgensen, K. A. Chem. Commun.
1997, 2169. (b) Jørgensen, K. A.; Johannsen, M.; Yao, S. L.; Audrain,
H.; Thorhauge, J. Acc. Chem. Res. 1999, 32, 605. (c) Yao, S.;
Johannsen, M.; Audrain, H.; Hazell, R. G.; Jørgensen, K. A. J. Am.
Chem. Soc. 1998, 120, 8599. (d) Jørgensen, K. A. Eur. J. Org. Chem.
2004, 2093.
(7) Zhao, B.; Loh, T.-P. Org. Lett. 2013, 15, 2914.
3567
dx.doi.org/10.1021/ol5015009 | Org. Lett. 2014, 16, 3564−3567