Iron-promoted tandem reaction of anilines with styrene oxides via C-C cleavage for the synthesis of quinolines

Article abstract of DOI:10.1021/ol300391t

A novel iron-promoted tandem reaction of anilines with styrene oxides via C-C cleavage and C-H activation has been developed. The reaction utilizes an inexpensive FeCl₃ as promoter and is suitable for forming a variety of 3-arylquinolines from

Full text of DOI:10.1021/ol300391t

ORGANIC
LETTERS
2012
Vol. 14, No. 9
2206–2209
Iron-Promoted Tandem Reaction of
Anilines with Styrene Oxides via CꢀC
Cleavage for the Synthesis of Quinolines
Yicheng Zhang,† Min Wang,† Pinhua Li,† and Lei Wang*,‡
Department of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000,
P.R. China, and State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry, Shanghai 200032, P. R. China
leiwang@chnu.edu.cn
Received February 18, 2012
ABSTRACT
A novel iron-promoted tandem reaction of anilines with styrene oxides via CꢀC cleavage and CꢀH activation has been developed. The reaction utilizes
an inexpensive FeCl3 as promoter and is suitable for forming a variety of 3-arylquinolines from the simple and readily available starting materials.
Transition-metal-catalyzed selective cleavage of CꢀC
bonds and their applications as versatile tools in modern
organic synthesis have attracted much attention.1 Several
modes of the catalytic process have been reported to
activate inert CꢀC bonds, such as the relief of ring strain,2
the formation of a stable complex by chelation,1a,c,3 and
other means.4 In general, the transition metals used in the
cleavage of CꢀC reactions are Rh, Ru, Pd,1ꢀ4 and Cu.5 In
comparison with the catalytic selective CꢀC cleavage of
tertiary1g,i,5b and secondary alcohols1f,j via β-carbon elim-
ination, transition-metal-catalyzed CꢀC bond cleavage of
epoxides has not been described.
Quinolines have received considerable interest because
they are widely found in natural products with biological
activity.6 They have also played an important role in
medicinal chemistry due to their pharmacological prop-
erties.7 In addition, quinolines as building blocks have
been used for preparing functional materials with en-
hanced electronic and photonic properties.8 Recently, a
† Huaibei Normal University.
‡ Shanghai Institute of Organic Chemistry.
(1) For selected reviews, see: (a) Park, Y. J.; Park, J.-W.; Jun, C.-H.
Acc. Chem. Res. 2008, 41, 222. (b) Rybtchinski, B.; Milstein, D. Angew.
Chem., Int. Ed. 1999, 38, 870. (c) Jun, C.-H.; Moon, C. W.; Lee, D.-Y.
Chem.;Eur. J. 2002, 8, 2422. (d) Jun, C.-H. Chem. Soc. Rev. 2004, 33,
610. For selected examples, see: (e) Nishimura, T.; Uemura, S. J. Am.
Chem. Soc. 1999, 121, 11010. (f) Jun, C.-H.; Lee, D.-Y.; Kim, Y.-H.; Lee,
H. Organometallics 2001, 20, 2928. (g) Zhao, P.; Incarvito, C. D.;
Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 3124. (h) Iwasaki, M.;
Hayashi, S.; Hirano, K.; Yorimitsu, H.; Oshima, K. J. Am. Chem. Soc.
2007, 129, 4463. (i) Niwa, T.; Yorimitsu, H.; Oshima, K. Angew. Chem.,
Int. Ed. 2007, 46, 2643. (j) Li, H.; Li, Y.; Zhang, X.-S.; Chen, K.; Wang,
X.; Shi, Z.-J. J. Am. Chem. Soc. 2011, 133, 15244 and references cited
therein.
(5) For selected examples, see: (a) He, C.; Guo, S.; Huang, L.; Lei, A.
J. Am. Chem. Soc. 2010, 132, 8273. (b) Sai, M.; Yorimitsu, H.; Oshima,
K. Angew. Chem., Int. Ed. 2011, 50, 3294.
(2) For selected examples, see: (a) Seiser, T.; Cramer, N. J. Am. Chem.
Soc. 2010, 132, 5340. (b) Winter, C.; Krause, N. Angew. Chem., Int. Ed.
2009, 48, 2460. (c) Matsumura, S.; Maeda, Y.; Nishimura, T.; Uemura,
S. J. Am. Chem. Soc. 2003, 125, 8862. (d) Bart, S. C.; Chirik, P. J. J. Am.
Chem. Soc. 2003, 125, 886.
(3) For selected examples, see: (a) Chatani, N.; Ie, Y.; Kakiuchi, F.;
Murai, S. J. Am. Chem. Soc. 1999, 121, 8645. (b) Liou, S.-Y.; van der
Boom, M. E.; Milstein, D. Chem. Commun. 1998, 687.
(4) For selected examples, see: (a) Sugiishi, T.; Kimura, A.; Nakamura,
H. J. Am. Chem. Soc. 2010, 132, 5332. (b) Chiba, S.; Xu, Y.-J.; Wang, Y.-F.
J. Am. Chem. Soc. 2009, 131, 12886. (c) Takahashi, T.; Song, Z.; Hsieh,
Y.-F.; Nakajima, K.; Kanno, K.-i. J. Am. Chem. Soc. 2008, 130, 15236.
(d) Gunay, A.; Jones, W. D. J. Am. Chem. Soc. 2007, 129, 872. (e) Najera,
C.; Sansano, J. M. Angew. Chem., Int. Ed. 2009, 48, 2452.
(6) (a) Habermehl, G.; Hammann, P. E. Naturstoffchemie; Springer:
Berlin, 1992; p 192. (b) Michael, J. P. Nat. Prod. Rep. 2001, 18, 543.
(c) Funayama, S.; Murata, K.; Noshita, T. Heterocycles 2001, 54, 1139.
(7) (a) Balasubramanian, M.; Keay, J. G. Isoquinoline Synthesis. In
Comprehensive Heterocyclic Chemistry II; McKillop, A. E., Katrizky,
A. R., Rees, C. W., Scrivem, E. F. V., Eds.; Elsevier: Oxford, UK, 1996;
Vol. 5, p 245. (b) Claret, P. A.; Osborne, A. G. In The Chemistry of
Heterocyclic Compounds, Quinolines; Jones, G., Ed.; John Wiley and Sons:
Chichester, 1982, Vol. 32, Part 2, p 31. (c) Sawada, Y.; Kayakiri, H.; Abe,
Y.; Imai, K.; Katayama, A.; Oku, T.; Tanaka, H. J. Med. Chem. 2004,
47, 1617.
(8) (a) Zhang, X.; Shetty, A. S.; S. Jenekhe, A. Macromolecules 1999,
32, 7422. (b) Jenekhe, S. A.; Lu, L.; Alam, M. M. Macromolecules 2001,
34, 7315.
r
10.1021/ol300391t
Published on Web 04/17/2012
2012 American Chemical Society

number of modifications for preparing quinolines have
€
been reported, including Skraup, Combes, Miller, Friedlander,
Table 1. Optimization of the Reaction Parametersa
ConradꢀLimpach, and Pfitzinger syntheses,9 and transition-
metal-catalyzed synthetic methodologies.10 However, the
existing methods suffer from limited availability of sub-
strates, complicated multistep procedures, and low regio-
selectivity in most cases. Compared to the traditional
stepwise synthesis, the tandem (domino) reaction is the
most facile and economic synthetic approach. It enhances
the reaction efficiency and avoids the tedious step-by-step
separations and purifications of intermediates.11 An ex-
ample of preparing 2,3-disubstituted quinolines through a
rhodium-catalyzed tandem amination of aromatic olefins
with anilines was developed.12
entry
Fe (mol %)
solvent/T (°C)
ligandb
yieldc (%)
1
2
3
4
5
6
7
8
9
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
dioxane/110
CH3CN/80
C2H5OH/80
CH3NO2/100
toluene/110
(CH2Cl)2/80
(CH2Br)2/110
PEG-400/110
DMSO/110
DMF/110
38
26
18
20
17
25
13
23
0
Over the past decade, iron-based catalysts have signifi-
cantly risen to promote a broad range of organic transfor-
mations, such as cross-couplings, allylations, hydrogena-
tions, and direct CꢀH bond functionalizations owing to
their abundance, affordability, and environmental friend-
liness.13 Most recently, an FeCl3-catalyzed A3-reaction of
aldehydes, alkynes, and amines via a tandem process in
one-pot for preparing 2,4-disubstituted quinolines was
reported.14 As part of our continuing effort for accessing
natural-product-like compounds and the development of
iron-catalyzed organic transformations,15 we report herein
a novel FeCl3-promoted tandem reaction of anilines with
styrene oxides via CꢀC cleavage and CꢀH activation for
10 FeCl3 (10)
11 FeCl3 (10)
12 FeCl3 (10)
13 FeCl3 (10)
14 FeCl2 (10)
15 FeBr3 (10)
16 FeBr2 (10)
0
dioxane/110
dioxane/110
ꢀ/110
15d
38e
22f
16
29
15
10
8
dioxane/110
dioxane/110
dioxane/110
17 Fe(OTf)3 (10) dioxane/110
18 Fe(ClO4)3
dioxane/110
3
xH2O (10)
19 Fe2O3 (10)
dioxane/110
0
0
20 Fe2(acac)3 (10) dioxane/110
21 FeCl3 (10)
22 FeCl3 (10)
23 FeCl3 (10)
24 FeCl3 (10)
25 FeCl3 (10)
26 FeCl3 (10)
27 FeCl3 (10)
28 FeCl3 (10)
29 FeCl3 (10)
30 FeCl3 (20)
31 FeCl3 (25)
32 FeCl3 (35)
dioxane/110 PPh3
dioxane/110 PCy3
dioxane/110 Dppf
dioxane/110 DMEDA
dioxane/110 TMEDA
dioxane/110 Phen
dioxane/110 L-proline
dioxane/110 TMHD
dioxane/110 2-acetylcyclohexanone
dioxane/110
34
37
36
33
35
35
33
45
49
70
81
80
(9) (a) Jones, G. In Comprehensive Heterocyclic Chemistry, Katritzky,
A. R., Rees, A. R., Eds.; Pergamon, New York, 1984; Vol. 2, Part 2A, p 395.
(b) Chan, B. K.; Ciufolini, M. A. J. Org. Chem. 2007, 72, 8489. (c) Zong,
R.; Zhou, H.; Thummel, R. P. J. Org. Chem. 2008, 73, 4334. (d) Powell,
D. A.; Batey, R. A. Org. Lett. 2002, 4, 2913. (e) Cho, I. S.; Gong, L.;
Muchowski, J. M. J. Org. Chem. 1991, 56, 7288. (f) Riesgo, E. C.; Jin, X.;
Thummel, R. P. J. Org. Chem. 1996, 61, 3017. (g) Wu, Y.; Liu, L.; Li, H.;
Wang, D.; Chen, Y. J. Org. Chem. 2006, 71, 6592. (h) Denmark, S. E.;
Venkatraman, S. J. Org. Chem. 2006, 71, 1668. (i) Akbari, J.; Heydari,
A.; Kalhor, H. R.; Kohan, S. A. J. Comb. Chem. 2010, 12, 137.
(10) (a) Liang, Z.; Wu, J. Adv. Synth. Catal. 2007, 349, 1047.
(b) Tokunaga, M.; Eckert, M.; Wakatsuki, Y. Angew. Chem., Int. Ed.
1999, 38, 3222. (c) Korivi, R. P.; Cheng, C. J. Org. Chem. 2006, 71, 7079.
(d) McNaughton, B. R.; Miller, B. L. Org. Lett. 2003, 5, 4257. (e) Zhang,
Z.; Tan, J.; Wang, Z. Org. Lett. 2008, 10, 173. (f) Wu, J. L.; Cui, X. L.;
Chen, L. M.; Jiang, G. J.; Wu, Y. J. J. Am. Chem. Soc. 2009, 131, 13888.
(g) Takahashi, T.; Li, Y.; Stepnicka, P.; Kitamura, M.; Liu, Y.;
Nakajima, K.; Kotora, M. J. Am. Chem. Soc. 2002, 124, 576. (h) Cho,
C. S.; Kim, B. T.; Kim, T. J.; Shim, S. C. Chem. Commun. 2001, 2576.
(11) Tietze, L. F.; Brasche, G.; Gericke, K. Domino Reactions in
Organic Synthesis; Wiley-VCH: Weinheim, 2006.
dioxane/110
dioxane/110
a Reaction conditions: styrene oxide (1a, 2.0 mmol), aniline (2a,
1.0 mmol), Fe source (10ꢀ35 mol %), ligand (20 mol %) if necessary,
solvent (2.0 mL) at the temperature indicated in Table 1 for 12 h.
b PCy3 = tricyclohexylphosphine, Dppf = 1,10-bis(diphenylphosphino)-
ferrocene, DMEDA = N,N0-dimethylethylenediamine, TMEDA = N,N,
N0,N0,-tetramethylethylenediamine, Phen = 1,10-phenanthroline, TMHD =
2,2,6,6-tetramethyl-3,5-heptanedione. c Isolated yield. d 1a (1.0 mmol),
2a (1.0 mmol). e 1a (3.0 mmol), 2a (1.0 mmol). f 1a (5.0 mmol), 2a
(1.0 mmol) without additional solvent. Cu(OAc)2 was used instead of
FeCl3; no 3a was detected.
(12) Beller, M.; Thiel, O.; Trauthwein, H.; Hartung, C. G. Chem.;
Eur. J. 2000, 6, 2513.
(13) For selected reviews, see: (a) Enthaler, S.; Junge, K.; Beller, M.
Angew. Chem., Int. Ed. 2008, 47, 3317. (b) Correa, A.; Garcıa Mancheno,
´
the preparation of quinolines. It is important to note that
this is an inexpensive, regioselective, alternative, and effi-
cient approach to 3-arylquinolines from the simple and
readily available starting materials.
This novel tandem reaction of styrene oxide with aniline
was observed during our investigation on the preparation
of a 2-phenylindole framework via FeCl3-catalyzed ring
opening of styrene oxide (1a) with aniline (2a).16 Instead of
the anticipated 2-phenylindole product, 3-phenyquinoline
(3a) was isolated in 38% yield when the reaction was
€
O.; Bolm, C. Chem. Soc. Rev. 2008, 37, 1108. (c) Sherry, B. D.; Furstner,
A. Acc. Chem. Res. 2008, 41, 1500. (d) Sun, C.-L.; Li, B.-J.; Shi, Z.-J.
Chem. Rev. 2011, 111, 1293. (e) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L.
€
Chem. Rev. 2004, 104, 6217. For selected examples, see: (f) Furstner, A.;
Majima, K.; Martin, R.; Krause, H.; Kattnig, E.; Goddard, R.; Christian,
W. L. J. Am. Chem. Soc. 2008, 130, 1992. (g) Chen, M. S.; White, M. C.
€
Science 2007, 318, 783. (h) Furstner, A. Angew. Chem., Int. Ed. 2009, 48,
1364. (i) Buchwald, S. L.; Bolm, C. Angew. Chem., Int. Ed. 2009, 48, 5586.
(j) Fan, J.; Gao, L.; Wang, Z. Chem. Commun. 2009, 5021. (k) Fan, J.;
Wang, Z. Chem. Commun. 2008, 5381.
(14) (a) Cao, K.; Zhang, F.-M.; Tu, Y.-Q.; Zhou, X.-T.; Fan, C.-A.
Chem.;Eur. J. 2009, 15, 6332. (b) Zhang, Y.; Li, P.; Wang., L.
J. Heterocycl. Chem. 2011, 48, 153.
(15) (a) Li, P.; Zhang, Y.; Wang, L. Chem.;Eur. J. 2009, 15, 2045.
(b) Liu, J.; He, T.; Wang, L. Tetrahedron 2011, 67, 3420. (c) Wang, B.;
Wu, H.; Li, P.; Wang, L. Chem. Commun. 2010, 46, 5891. (d) Wang, M.;
Ren, K.; Wang, L. Adv. Synth. Catal. 2009, 351, 1586.
(16) Cho, C. S.; Kim, J. H.; Choi, H.-J.; Kim, T.-J.; Kim, T.-J.; Shim,
S. C. Tetrahedron Lett. 2003, 44, 2975.
Org. Lett., Vol. 14, No. 9, 2012
2207

considerably affected by the molar ratio of 1a to 2a. The
optimal ratio of 1a to 2a was found to be 2:1. As expected,
two molecules of 1a reacted with one molecule of 2a to
form a quinoline skeleton. The product yield of 3a did not
improve when increasing the molar ratio of 1a to 2a.
Conversely, reaction of 2a (1.0 equiv) with 1a (5.0 equiv),
without additional solvent, led to a lower yield of 3a
(Table 1, entries 11ꢀ13).
Scheme 1. Iron-Promoted Tandem Reaction of Styrene Oxides
with Anilines for the Synthesis of Quinolinesa
The effect of the Fe source on the model reaction was
examined. Various iron salts were tested; FeCl3 was found
to be the best one. FeCl2, FeBr3, FeBr2, Fe(OTf)3, and
Fe(ClO4)3 xH2O were less effective, while Fe2O3 and
3
Fe2(acac)3 were inactive (Table 1, entries 14ꢀ20). Further-
more, when a Cu source, such as CuI, CuO, or Cu(OAc)2
was used instead of FeCl3, no 3a was detected.
Next, the ligand was added to improve the product yield.
When P-ligands, such as triphenylphosphine, tricyclo-
hexylphosphine, and 1,10-bis(diphenylphosphino)ferrocene,
N-ligands, such as N,N0-dimethylethylenediamine, N,N,
N0,N0-tetramethylethylenediamine, and 1,10-phenanthro-
line, and an N,O-ligand, i.e., L-proline, were used (20 mol %)
in the model reaction, the reaction afforded 3a in slightly
lower yields than obtained under ligand-free conditions
(Table 1, entries 21ꢀ27). When bidentate O-ligands 2,2,
6,6-tetramethyl-3,5-heptanedione and 2-acetylcyclohex-
anone were employed, 3a was obtained in 45 and 49%
yields, respectively (Table 1, entries 28 and 29).
To further improve the product yield, we added up to
35 mol %of FeCl3, considering its low cost. To our sur-
prise, 81% yield of3awasisolatedwhenthe model reaction
was performed in the presence of FeCl3 (25 mol %) with-
out ligand. Thus, the optimal amount of FeCl3 used in the
reaction was 25 mol % (Table 1, entries 30ꢀ32).
Under the optimized reaction conditions, the reaction
scope of various styrene oxides with a variety of anilines
was investigated. As shown in Scheme 1, for the mono-
substituted arylamines, a series of substituents on anilines,
including o-Me, m-Me, p-Me, p-Et, p-(i-Pr), p-MeO, o-Cl,
m-Cl, o-Br, p-Br, p-F, and p-CF3 groups were tolerated,
and the corresponding quinolines were obtained in 50ꢀ
86% yields (Scheme 1, 3bꢀm). Anilines with an electron-
donating group on their ortho-, meta-, or para-position
(3bꢀg) gave yields superior to that of the anilines with an
electron-withdrawing group on their corresponding posi-
tion (3hꢀm). The ortho-position effect was obviously ob-
served in the formation of 3b, 3h, and 3j. It should be noted
that only 7-methyl-3-phenylquinoline (3c) or 7-chloro-3-
phenylquinoline (3i) as sole regioisomer was isolated when
m-methylaniline or m-chloroaniline reacted with styrene
oxide. Further investigation revealed that disubstituted ar-
ylamines bearing electron-rich or electron-poor functional
groups could react with styrene oxide to form the correspond-
ing products in moderate to good yields (3nꢀr). The ortho-
position effect was also observed (3nꢀq vs 3r). A more bulky
aniline substrate, 1-naphthylamine, reacted with styrene
oxide to generate 3s in 67% yield. Trisubstituted arylamine,
such as 3,4,5-trichloroaniline, reacted with styrene oxide to
form 3t in 62% yield. On the other hand, a variety of styrene
oxides bearing substituents on the benzene rings were
a Reaction conditions: 1 (2.0 mmol), 2 (1.0 mmol), FeCl3 (0.25 mmol),
1,4-dioxane (2.0 mL) at 110 °C for 12 h. b Isolated yields.
performed in 1,4-dioxane at 110 °C for 12 h. This pre-
liminary finding prompted us to develop a facile, practical,
and economic approach to 3-aryl-quinolines.
To improve the yield of 3a, a variety of reaction param-
eters were optimized. At first, the effect of solvent on the
model reaction of styrene oxide (1a) with aniline (2a) in the
presence of FeCl3 (10 mol %) was examined. Among the
solvents tested, 1,4-dioxane was found to be the most
suitable one, although only 38% yield of 3a was obtained
(Table 1, entry 1). Lower yields of 3a were observed when
using other solvents, such as CH3CN, C2H5OH, CH3NO2,
toluene, 1,2-dichloroethane, 1,2-dibromoethane, and PEG-
400 (Table 1, entries 2ꢀ8). However, no desired 3a was
detected when the reaction was performed in DMSO or
DMF (Table 1, entries 9 and 10). The product yield was
2208
Org. Lett., Vol. 14, No. 9, 2012

underwent dehydration to generate enamine (II). Subse-
quently, the obtained II reacted with another styrene oxide
assisted by FeCl3 to form III, which could undergo β-H
elimination from iron alkoxide linkage, leading to the for-
mation of a ketone intermediate (IV) and iron hydride species
(which might eliminate HCl to reach FeI oxidation state). The
ketone intermediate (IV) could then undergo FeI-mediated,
directed CꢀH bond activation at the ortho-position of aniline
moiety to generate (V),19 followed by chelation-assisted CꢀC
bond activation and cleavage to form the 7-membered
cyclometalate (VI) and eliminate PhCHO. After reductive
elimination of FeCl from VI, VII was obtained.20 Finally, the
desired quinoline was generated through dehydrogenation of
VII in the presence of oxygen.14 It should be noted that
benzaldehyde and intermediate (I) was detected by HPLC
during the reaction of 1a with 2a by controlling the reaction
conditions. Moreover, the structure of 3w was further con-
firmed by X-ray crystallography.21
Scheme 2. Proposed Reaction Mechanism
In summary, we have developed a novel iron-promoted
tandem reaction for the synthesis of quinoline derivatives.
The reactions of anilines and styrene oxides via CꢀC
cleavage and CꢀH activation in the presence of FeCl3
generated3-substitutedquinolines withgood yields. This is
an inexpensive and convenient approach to 3-arylquino-
lines from simple and readily available starting materials
and reagents. Detailed mechanistic study and further
investigation are currently underway.
examined, and the results in Scheme 1 indicated that electron-
withdrawing groups (p-F, p-Cl, p-Br) and electron-donating
one (p-Me) were tolerated, and the corresponding products
(3uꢀx) were obtained in high yields (77ꢀ82%). It is obvious
that the electronic effect of substituted groups on the benzene
rings in styrene oxides had little impact on the yields of the
products. It is worth noting that no desired product was
isolated when 2-methyl styrene oxide reacted with aniline
under the present reaction conditions. However, aliphatic
epoxides, such as 3-chloro-1,2-epoxypropene and 1,2-epoxy-
butane, did not react with aniline. It is important to note that
quinolines with halo groups on the benzene rings (3hꢀk, 3n,
3t, 3v, and 3w) could be utilized in the further transition-
metal-catalyzed organic transformations.17
Acknowledgment. This work was financially supported
by the National Science Foundation of China (Nos.
21172092, 20972057).
Supporting Information Available. Analytical data and
spectra (1H and 13C NMR) for all products, CIF of 3w,
typical procedure. This material is available free of charge
via the Internet at http://pubs.acs.org.
Although the reaction mechanism has not been clear up to
now, a possible process was proposed in Scheme 2. First,
aniline reacted with styrene oxide in the presence of FeCl3
as Lewis acid, to form β-amino alcohol (I),16,18 which
(21) X-ray crystal structure of 3-(4-bromophenyl)quinoline (3w):
(17) Nolt, M. B.; Zhao, Z. J.; Wolkenberg, S. E. Tetrahedron Lett.
2008, 49, 3137.
(18) Onaka, M.; Kawai, M.; Izumi, Y. Chem. Lett. 1985, 779.
(19) Garcia-Cuadrado, D.; Braga, A. A. C.; Maseras, F.; Echavarren,
A. M. J. Am. Chem. Soc. 2006, 128, 1066.
ꢀ
ꢀ
(20) (a) Echavarren, A. M.; Gomez-Lor, B.; Gonzalez, J. J.; de Frutos,
ꢀ
O. Synlett 2003, 585. (b) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem.
Res. 2001, 34, 633.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 9, 2012
2209