Stereoselective synthesis of highly substituted enamides by an oxidative heck reaction

Article abstract of DOI:10.1002/anie.201101550

Functionalization of enamides under Heck conditions has been limited to those with unsubstituted vinyl groups. By tuning reaction parameters that allow for the balance between stability and reactivity of reactants, the oxidative Heck cross-coupling to produce highly substituted enamides in good to excellent yields was achieved (see scheme). Copyright

Full text of DOI:10.1002/anie.201101550

DOI: 10.1002/anie.201101550
Cross-Coupling
Stereoselective Synthesis of Highly Substituted Enamides by an
Oxidative Heck Reaction**
Yu Liu, Dan Li, and Cheol-Min Park*
Dedicated to Professor Eun Lee
The Heck arylation has proven to be among the most versatile
In our efforts to develop a synthetic route for the synthesis
of structurally diverse b-amino acids, we envisioned that Heck
cross-coupling of b-amidoacrylates would provide b-aryl b-
amidoacrylates which could be subsequently converted into
b-amino acid derivatives by asymmetric hydrogenation
[Eq. (1)]. Thus, we began by surveying Heck conditions
À
reactions for C C bond formation owing to its excellent
chemoselectivity, wide functional group tolerability, and
simplicity.[1] The palladium(0)-mediated catalytic process
allows for facile cross-coupling of alkenes with various aryl
and heteroaryl halides/pseudohalides. The oxidative Heck
reaction has drawn significant attention where arylpalladiu-
m(II) species are generated by transmetalation with organ-
ometallic counterparts followed by undergoing insertion with
alkenes.[2] Among the organometallic coupling partners,
organoboronic acids have been extensively explored in
various transition-metal-mediated reactions owing to their
stability, wide availability, and low toxicity. Since the first
demonstration of catalytic, oxidative Heck cross-coupling
using arylboronic acids by Cho and Uemura,[2b] significant
progress has been made. Despite the recent advances in the
field, the limited substrate scope including necessitating
steric/electronic bias prompt further improvements. For
example, a literature survey shows that examples of Heck
cross-coupling with electron-rich alkenes such as enamides
are limited to those with simple unsubstituted vinyl groups.[3]
b-Amidoacrylate moiety represents an important motif
that has been widely utilized as synthetic intermediates in the
total synthesis of natural products[4] as well as preparation of
heterocycles[5] and b-amino acids through asymmetric hydro-
genation.[6] These compounds are typically prepared by
condensation of b-ketoesters with amides,[7] and acylation of
b-aminoacrylates.[8] Also, transition-metal-mediated reac-
tions have been reported including oxidative amidation of
acrylates[9] and addition of amides to terminal alkynes,[10]
which typically provide disubstituted enamides. However,
the limitations of these reactions include intolerance for
sterically demanding substrates. Thus, finding an efficient
synthesis of sterically hindered enamides, such as trisubsti-
tuted enamides bearing tertiary amides, remains a challenge.
reported in the literature employing 1a as a substrate (see
Table 1 for structure). To our surprise, none of the conditions
that we have attempted afforded the Heck products presum-
ably owing to steric and electronic deactivation (see the
Supporting Information). Tuning the balance between reac-
tivity and stability of reactants in catalytic reactions is deemed
to be among the key factors. The outcomes of the attempted
reactions led us to seek the reaction parameters where aryl
metal species possess sufficient stability under the reaction
conditions, yet activation of which provides the reactivity to
participate in the catalytic cycle. Herein, we describe our
efforts to develop oxidative Heck conditions that allow for the
stereoselective synthesis of b-substituted b-amidoacrylates
and their derivatives in high yields.
We commenced with a brief screening of solvents employ-
ing 1a and potassium phenyltrifluoroborate[11] as the coupling
partner in the presence of Pd(OAc)2 (10 mol%), Cu(OAc)2
(3 equiv), and K2CO3 (2 equiv): we quickly identified 20%
AcOH in tert-BuOH as an optimal solvent (see the Support-
ing Information). Interestingly, while the use of either 1,4-
dioxane or tert-BuOH afforded moderate yields when a
stoichiometric amount of Cu(OAc)2 was employed (50% and
54%, respectively), they were found to be detrimental to the
reaction during our screening of oxidants where a catalytic
amount of Cu(OAc)2 under 1 atm oxygen was used (1,4-
dioxane; 18% and tert-BuOH; 0%). On the other hand, pure
AcOH as a solvent also resulted in a poor yield (23%).[12] The
structure of 3aa was unequivocally determined by X-ray
crystallographic analysis.[13] In the screening of bases, the
effect of counter cations clearly stood out with larger cations
such as potassium and cesium preferred over sodium (Table 1,
entry 1 vs. 2–4).
[*] Y. Liu, D. Li, Prof. Dr. C.-M. Park
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University, Singapore 637371 (Singapore)
Fax: (+65)6513-2748
E-mail: cmpark@ntu.edu.sg
[**] We gratefully acknowledge a Nanyang Technological University Start
Up Grant for the funding of this research. We thank Dr. Yongxin Li
for X-ray crystallographic analysis.
With these results in hand, we turned our attention to the
screening of oxidants. Although common oxidants such as
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101550.
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Communications
Table 1: Optimization of oxidative Heck cross-coupling reaction.[a–d]
Entry
R
M
Oxidant
Base
Yield [%][e]
Entry
R
M
Oxidant
Base Ligand[f]
Additive[h] Yield [%][e]
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
BF3K
BF3K
BF3K
BF3K
BF3K
BF3K
BF3K
BF3K
BF3K
BF3K
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Ag2O
AgF
BQ
K2S2O8
K3Fe(CN)6
Na2CO3
K3PO4
K2CO3
6
30
43
13
14
15
16
17
18
19
20
21
22
23
24
H
H
H
H
H
B(OH)2 Cu(OAc)2/O2 K2CO3
–
KHF2
52
26
58
18
64
31[j]
61
48
61
74
42
75[j]
B(OH)2 Cu(OAc)2/O2 K2CO3 dmphen[g] KHF2
B(OH)2 Cu(OAc)2/O2 K2CO3 dppf[g]
B(OH)2 Cu(OAc)2/O2 K2CO3 binap[g]
B(OH)2 Cu(OAc)2/O2 K2CO3 bpPCy2
KHF2
KHF2
KHF2
KHF2
AgF[i]
Cs2CO3 39
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
40
39
11
32
4
68
38[j]
23
MeO B(OH)2 Cu(OAc)2/O2 K2CO3 bpPCy2
H
H
H
H
H
B(Pin) Cu(OAc)2/O2 K2CO3 bpPCy2
B(Pin) Cu(OAc)2/O2 K2CO3 bpPCy2
B(Pin) Cu(OAc)2/O2 K2CO3 bpPCy2
[i]
CuF2
CsF[i]
KHF2
KHF2
KHF2
9
10
11
12
Cu(OAc)2/O2 K2CO3
Cu(OAc)2/O2 K2CO3
B(Pin)
Cu(OAc)2/O2 K2CO3 bpPCy2
MeO BF3K
H
B(Pin) Cu(OAc)2/O2 K2CO3
–
B(OH)2 Cu(OAc)2/O2 K2CO3
MeO B(Pin)
Cu(OAc)2/O2 K2CO3 bpPCy2
[a] ArBF3K and ArB(Pin); 2 equiv used, and ArB(OH)2; 1.5 equiv used. [b] entries 1–9; 3 equiv of oxidant, 808C. [c] entries 10–18; 20 mol% of
Cu(OAc)2, 1 atm of O2, 808C. [d] entries 19–24; 5 mol% of Cu(OAc)2, 1 atm of O2, 908C, 24 h. [e] Yield determined by GC methods. [f] 30 mol% of
ligand. [g] 15 mol% of ligand. [h] 3 equiv of additive. [i] 8 equiv of additive. [j] Yield determined by 1H NMR spectroscopy. Pin=pinacolate, dmphen=
2,9-dimethylphenanthroline, dppf=1,1’-bis(diphenylphosphanyl)ferrocene, BINAP=rac-2,2’-bis(diphenylphosphino)-1,1’-binaphthyl, bpPCy2 =2-
(dicyclohexylphosphino)biphenyl, BQ=benzoquinone.
Ag2O and AgF showed comparable results to Cu(OAc)2, it is
of note that the use of a catalytic amount of Cu(OAc)2
(20 mol%) in conjunction with oxygen (1 atm) as a terminal
oxidant significantly improved the product yield (68%,
entry 10). Careful analysis of the two reactions employing
stoichiometric and catalytic amounts of Cu(OAc)2 revealed
that a large amount of Cu(OAc)2 turns out to be detrimental,
and promoted rapid protodeborylation (entry 3). However,
the reaction conditions employing a catalytic amount of
Cu(OAc)2 also gave a poor result when electron-rich 4-
methoxyphenyltrifluoroborate was used as a coupling part-
ner. This reaction produced a substantial amount of the
protodeborylation product (entry 11).
Thus, with the anticipation that the use of different forms
of arylboron derivatives would lead to an improvement by
altering factors such as the stability of arylboron compounds
and rate of reaction of transient intermediates in the catalytic
cycle, we examined arylboronic acids and arylboronates.[14]
Although phenylboronic acid gave a poor result (23%),
addition of KHF2 improved the coupling yield (52%; entry 12
vs. 13). Encouraged by this result, we initiated the screening
of ligands. The use of nitrogen-based ligands such as dmphen
appeared to inhibit the catalytic cycle, and afforded the
product in 26% yield (entry 14). Among the phosphine-based
ligands screened, bpPCy2 (2-(dicyclohexylphosphino)bi-
phenyl)[15] afforded the Heck product in 64% yield
(entry 17). While considering the possibility of the corre-
sponding phosphine oxide bpP(O)Cy2 serving as the active
ligand under the oxidative conditions, we found that it failed
to give any product (see the Supporting Information). Other
phosphine ligands such as binap and dppf showed either
inhibition or only marginal improvement (an extensive list of
ligand screening can be found in the Supporting Information).
However, 4-methoxyphenylbororic acid continued to give a
poor yield under the reaction conditions (31%, entry 18).
Noting that the electron-rich aryl trifluoroborate and
arylboronic acid rapidly undergo protodeborylation under the
reaction conditions, we attempted the use of arylboronate
anticipating an improved life span of arylboron species under
the reaction conditions. Indeed, the use of pinacol 4-
methoxyphenylboronate significantly improved the yield of
the coupling product (75%, entry 24) in contrast to the
corresponding trifluoroborate and boronic acids (38% and
31%, respectively). Consistently, the use of pinacol phenyl-
boronate also resulted in an improved yield (74%, entry 22).
A control experiment lacking the ligand underscores its
pivotal role in the reaction (42%, entry 23). Screening of
fluoride sources identified KHF2 as an optimal additive
(entries 19–22).
Having identified the optimized reaction conditions, we
began to survey the substrate scope of the reaction. Substrates
used to investigate the scope of the reaction were E isomers
except for 1d. Notably, regardless of the geometry of alkenes
in substrates, the coupling reactions result in Z isomers.
Examination of the electronic influence of aryl groups
revealed that those with both electron-donating and -with-
drawing groups are well-tolerated and gave products in good
to excellent yields (Table 2, 3ab–ae). However, the reactions
of arylboronates with ortho-substituents were sluggish. To
gain access to structurally diverse enamides, we examined
substitution of pyrrolidinone with a variety of amide groups
including secondary, tertiary, cyclic, acyclic, and aromatic
amides. As shown in Table 2, the method allows for the
synthesis of diverse enamides in high yields. In addition, those
containing oxazolidinones in place of amides also afforded
products in high yields (Table 2, 3 fa, 3ga, 3ia, 3ja). More-
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Table 2: Scope of oxidative Heck cross-coupling reaction.[a,b]
substrate. While directing groups are routinely employed to
[16]
À
facilitate C H bond activation,
their presence is not
required in Heck reaction. Nevertheless, we cannot rule out
the possibility in more challenging substrates.
3aa (70%)
3ad (63%)[c]
3ag (80%)
3ab (72%)
3ae (81%)
3ah (52%)
3ac (81%)
3af (80%)
3ba (81%)
Next, we turned our attention to the transformation of
enamides into b-amino acid derivatives. Subjection of enam-
ide 3aa to the asymmetric hydrogenation conditions employ-
ing the catalyst generated from [Rh(cod)2]BF4 and (R)-
Binaphane[17] smoothly produced compound 4 in 99% yield
and 93% ee (see the Supporting Information).
In summary, we have developed efficient oxidative Heck
cross-coupling conditions that allow for the synthesis of highly
substituted enamides that are important synthetic intermedi-
ates with a broad utility in various applications. It is notable
that modulation of stability and reactivity of arylboron
species was found to be the key for the reaction such that
the increased life span of arylboron species leads to a
decreased background reaction and yet sufficiently reactive
to participate in the catalytic cycle upon activation. This
reasoning allowed us to identify the described reaction
parameters.
3ca (57%)
3ea (81%)
3ha (59%)
3cb (51%)
3 fa (71%)
3ia (77%)
3da (69%)[d]
3ga (59%)
3ja (75%)[e]
Experimental Section
1a (42 mg, 0.25 mmol), 4-methoxyphenylboronic acid pinacol ester
2b (117 mg, 0.50 mmol), Pd(OAc)2 (5.6 mg, 10 mol%), Cu(OAc)2
(2.3 mg, 5 mol%), 2-(dicyclohexylphosphino)biphenyl (26 mg, 30
mol%), K2CO3 (69 mg, 2 equiv), and KHF2 (78 mg, 4 equiv). tert-
BuOH—AcOH (4:1, 2.5 mL) were added to a flask that was
subsequently evacuated and back-filled with O2 three times before
being heated to 908C under O2 (1 atm). The progress of the reaction
was monitored by TLC and GC analysis. Upon completion, the
reaction mixture was cooled to RT, diluted with ethyl acetate, and
filtered through a small pad of Celite. The filtrate was concentrated
in vacuo, and the crude material was purified by flash chromatog-
raphy on silica gel (eluent: 40% hexanes/ethyl acetate) to afford the
product 3ab (50 mg, 72%, white solid, mp 88–898C). 1H NMR
(400 MHz, CDCl3): d = 7.41 (d, J = 8.8 Hz, 2H), 6.91 (d, J = 8.8 Hz,
2H), 6.21 (s, 1H), 3.84 (s, 3H), 3.74 (s, 3H), 3.56 (t, J = 7.0 Hz, 2H),
2.60 (t, J = 8.0 Hz, 2H), 2.23–2.15 ppm (m, 2H); 13C NMR (100 MHz,
CDCl3): d = 175.44, 165.13, 161.68, 148.32, 128.56, 126.94, 114.38,
112.38, 55.42, 51.46, 49.26, 31.72, 19.21; HRMS (ESI): m/z calcd for
C15H18NO4 [M+H]+: 276.1236; found: 276.1239.
[a] 2 equiv of pinacol arylboronates. [b] Yield of isolated products.
[c] Reaction time was 4 days. [d] Z isomer used as the substrate.
[e] 11% of b,b-diphenyl product 3ja’ was also isolated.
over, despite the steric hindrance, the reaction of substrate 1g
with Evans chiral oxazolidinone gave the product in 59%
yield. This outcome shows potential in an application of
asymmetric reduction based on a chiral auxiliary. We were
also gratified to find that the reaction with highly deactivated
alkene 1h also proceeded smoothly to give product 3ha in
59% yield. Next, we examined functional group tolerability
of substrates with 1i bearing an amide group in place of ester
groups. The reaction also proceeded smoothly and afforded
3ia in an excellent yield. Likewise, 1j with an aryl group gave
the coupling product in a high yield. This result indicates that
an electron-withdrawing group is not a requisite, although the
regioselectivity of a/b decreases for those lacking an electron-
withdrawing group (6.8:1 favoring a substitution in this case).
To probe if the amide carbonyl groups serve as directing
groups in the reaction, we attempted a reaction by employing
a substrate lacking a carbonyl group. However, the instability
of the substrate in the acidic medium led to hydrolysis of the
Received: March 3, 2011
Revised: May 23, 2011
Published online: June 28, 2011
Keywords: arylation · cross-coupling · enamides · Heck reaction ·
.
palladium
[1] a) I. P. Beletskaya, A. V. Cheprakov, Chem. Rev. 2000, 100,
3009 – 3066; b) V. Coeffard, P. J. Guiry, Curr. Org. Chem. 2010,
14, 212 – 229; c) P. J. Guiry, D. Kiely, Curr. Org. Chem. 2004, 8,
Angew. Chem. Int. Ed. 2011, 50, 7333 –7336
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7335

Communications
781 – 794; d) A. B. Dounay, L. E. Overman, Chem. Rev. 2003,
[5] a) S. Rakshit, F. W. Patureau, F. Glorius, J. Am. Chem. Soc. 2010,
132, 9585 – 9587; b) R. Martꢃn, A. Cuenca, S. L. Buchwald, Org.
Lett. 2007, 9, 5521 – 5524; c) J. U. Jeong, X. Chen, A. Rahman,
D. S. Yamashita, J. I. Luengo, Org. Lett. 2004, 6, 1013 – 1016.
103, 2945 – 2964; e) M. Oestreich, The Mizoroki–Heck Reaction,
Wiley, Chichester, 2009; f) A. de Meijere, F. Diederich, Metal-
Catalyzed Cross-Coupling Reactions, Wiley-VCH, Weinheim,
2004; g) E. Negishi, A. de Meijere, Handbook of Organo-
palladium Chemistry for Organic Synthesis, Wiley-Interscience,
New York, 2002; h) L. Ackermann, Modern Arylation Methods,
Wiley-VCH, Weinheim, 2009.
´
[6] B. Weiner, W. Szymanski, D. B. Janssen, A. J. Minnaard, B. L.
Feringa, Chem. Soc. Rev. 2010, 39, 1656.
[7] P. Dupau, P. Le Gendre, C. Bruneau, P. H. Dixneuf, Synlett 1999,
1832 – 1834.
[2] a) B. Karimi, H. Behzadnia, D. Elhamifar, P. F. Akhavan, F. K.
Esfahani, A. Zamani, Synthesis 2010, 1399 – 1427; b) C. S. Cho, S.
Uemura, J. Organomet. Chem. 1994, 465, 85 – 92; c) X. Du, M.
Suguro, K. Hirabayashi, A. Mori, T. Nishikata, N. Hagiwara, K.
Kawata, T. Okeda, H. F. Wang, K. Fugami, M. Kosugi, Org. Lett.
2001, 3, 3313 – 3316; d) A. Inoue, H. Shinokubo, K. Oshima, J.
Am. Chem. Soc. 2003, 125, 1484 – 1485; e) Y. C. Jung, R. K.
Mishra, C. H. Yoon, K. W. Jung, Org. Lett. 2003, 5, 2231 – 2234;
f) M. M. S. Andappan, P. Nilsson, M. Larhed, Chem. Commun.
2004, 218 – 219; g) H. Zhang, E. M. Ferreira, B. M. Stoltz,
Angew. Chem. 2004, 116, 6270 – 6274; Angew. Chem. Int. Ed.
2004, 43, 6144 – 6148; h) K. S. Yoo, C. H. Yoon, K. W. Jung, J.
Am. Chem. Soc. 2006, 128, 16384 – 16393; i) J. Lindh, P.-A.
Enquist, ꢀ. Pilotti, P. Nilsson, M. Larhed, J. Org. Chem. 2007, 72,
7957 – 7962; j) J. H. Delcamp, A. P. Brucks, M. C. White, J. Am.
Chem. Soc. 2008, 130, 11270 – 11271; k) J. Ruan, X. Li, O. Saidi, J.
Xiao, J. Am. Chem. Soc. 2008, 130, 2424 – 2425; l) Y. Su, N. Jiao,
Org. Lett. 2009, 11, 2980 – 2983; m) E. W. Werner, M. S. Sigman,
J. Am. Chem. Soc. 2010, 132, 13981 – 13983; n) C. Jia, D. Piao, J.
Oyamada, W. Lu, T. Kitamura, Y. Fujiwara, Science 2000, 287,
1992 – 1995; o) S. Sakaguchi, K. S. Yoo, J. OꢁNeill, J. H. Lee, T.
Stewart, K. W. Jung, Angew. Chem. 2008, 120, 9466 – 9469;
Angew. Chem. Int. Ed. 2008, 47, 9326 – 9329.
[3] a) Z. Liu, D. Xu, W. Tang, L. Xu, J. Mo, J. Xiao, Tetrahedron Lett.
2008, 49, 2756 – 2760; b) Z. Hyder, J. Ruan, J. Xiao, Chem. Eur. J.
2008, 14, 5555 – 5566; c) J. Mo, J. Xiao, Angew. Chem. 2006, 118,
4258 – 4263; Angew. Chem. Int. Ed. 2006, 45, 4152 – 4157;
d) A. L. Hansen, T. Skrydstrup, J. Org. Chem. 2005, 70, 5997 –
6003; e) J. Mo, L. Xu, J. Xiao, J. Am. Chem. Soc. 2005, 127, 751 –
760; f) P. Harrison, G. Meek, Tetrahedron Lett. 2004, 45, 9277 –
9280; g) M. M. S. Andappan, P. Nilsson, H. von Schenck, M.
Larhed, J. Org. Chem. 2004, 69, 5212 – 5218; h) W. Cabri, I.
Candiani, A. Bedeschi, R. Santi, J. Org. Chem. 1993, 58, 7421 –
7426; i) A. Nejjar, C. Pinel, L. Djakovitch, Adv. Synth. Catal.
2003, 345, 612 – 619.
[8] G. Zhu, Z. Chen, X. Zhang, J. Org. Chem. 1999, 64, 6907 – 6910.
[9] a) J. M. Lee, D.-S. Ahn, D. Y. Jung, J. Lee, Y. Do, S. K. Kim, S.
Chang, J. Am. Chem. Soc. 2006, 128, 12954 – 12962; b) T.
Hosokawa, M. Takano, Y. Kuroki, S.-I. Murahashi, Tetrahedron
Lett. 1992, 33, 6643 – 6646; c) V. I. Timokhin, N. R. Anastasi, S. S.
Stahl, J. Am. Chem. Soc. 2003, 125, 12996 – 12997.
[10] a) T. Kondo, A. Tanaka, S. Kotachi, Y. Watanabe, J. Chem. Soc.
Chem. Commun. 1995, 413 – 414; b) L. J. Gooßen, J. E. Rauhaus,
G. Deng, Angew. Chem. 2005, 117, 4110 – 4113; Angew. Chem.
Int. Ed. 2005, 44, 4042 – 4045.
[11] a) G. Molander, B. Canturk, Angew. Chem. 2009, 121, 9404 –
9425; Angew. Chem. Int. Ed. 2009, 48, 9240 – 9261; b) R. D.
Chambers, T. Chivers, D. A. Pyke, J. Chem. Soc. 1965, 5144 –
5145; c) S. Darses, J.-P. Genet, J.-L. Brayer, J.-P. Demoute,
Tetrahedron Lett. 1997, 38, 4393 – 4396.
[12] a) Y. Fujiwara, C. Jia, Pure Appl. Chem. 2001, 73, 319 – 324;
b) M. D. K. Boele, G. P. F. van Strijdonck, A. H. M. de Vries,
P. C. J. Kamer, J. G. de Vries, P. W. N. M. van Leeuwen, J. Am.
Chem. Soc. 2002, 124, 1586 – 1587; c) N. P. Grimster, C. Gaun-
tlett, C. R. A. Godfrey, M. J. Gaunt, Angew. Chem. 2005, 117,
3185 – 3189; Angew. Chem. Int. Ed. 2005, 44, 3125 – 3129.
[13] CCDC 815201 (3aa) contains the supplementary crystallo-
graphic 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.
[14] D. M. T. Chan, K. L. Monaco, R. Li, D. Bonne, C. G. Clark,
P. Y. S. Lam, Tetrahedron Lett. 2003, 44, 3863 – 3865.
[15] T. E. Barder, S. L. Buchwald, J. Am. Chem. Soc. 2007, 129, 5096 –
5101.
[16] a) B.-J. Li, S.-D. Yang, Z.-J. Shi, Synlett 2008, 949 – 957; b) T.
Nishikata, A. R. Abela, S. Huang, B. H. Lipshutz, J. Am. Chem.
Soc. 2010, 132, 4978 – 4979; c) T. Vogler, A. Studer, Org. Lett.
2008, 10, 129 – 131; d) H. Zhou, W.-J. Chung, Y.-H. Xu and T.-P.
Loh, Chem. Commun. 2009, 3472 – 3474.
[17] D. Xiao, Z. Zhang, X. Zhang, Org. Lett. 1999, 1, 1679 – 1681.
[4] a) B. H. Lipshutz, B. Amorelli, J. Am. Chem. Soc. 2009, 131,
1396 – 1397; b) H. Shojaei, Z. Li-Bꢂhmer, P. von Zezschwitz, J.
Org. Chem. 2007, 72, 5091 – 5097.
7336
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