Microwave-assisted one-step synthesis of acetophenones via palladium-catalyzed regioselective arylation of vinyloxytrimethylsilane

Article abstract of DOI:10.1002/adsc.201200334

The regiochemistry of the palladium-mediated arylation (Heck arylation) of enol ethers is sensitive to the structure of the enol ether, the arylating agent and the catalytic system. In this study, an effective and practical method was successfully developed for the synthesis of acetophenones with high regioselectivity under palladium-catalyzed conditions. A variety of acetophenones was readily prepared from aryl iodides in good to excellent yields under microwave irradiation in a single step. The key feature of our new protocol is the use of vinyloxytrimethylsilane as a highly regioselective acylation reagent. Copyright

Full text of DOI:10.1002/adsc.201200334

FULL PAPERS
DOI: 10.1002/adsc.201200334
Microwave-Assisted One-Step Synthesis of Acetophenones
via Palladium-Catalyzed Regioselective Arylation of
Vinyloxytrimethylsilane
Wangke Qian,a,b Lei Zhang,a Haifeng Sun,b Hualiang Jiang,a and Hong Liua,*
a
State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Shanghai Institutes for Biological
Sciences, Chinese Academy of Sciences, 555 Zu Chong Zhi Road, Shanghai 201203, Peopleꢀs Republic of China
Fax : (+86)-21-5080-7042; phone: (+86)-21-5080-7042; e-mail: hliu@mail.shcnc.ac.cn
School of Pharmaceutical Engineering, Shenyang Pharmaceutical University, Shenyang, Liaoning 110016, Peopleꢀs
b
Republic of China
Received: May 19, 2012; Revised: September 10, 2012; Published online: November 12, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200334.
Abstract: The regiochemistry of the palladium-medi- aryl iodides in good to excellent yields under micro-
ated arylation (Heck arylation) of enol ethers is sen- wave irradiation in a single step. The key feature of
sitive to the structure of the enol ether, the arylating our new protocol is the use of vinyloxytrimethylsi-
agent and the catalytic system. In this study, an effec- lane as a highly regioselective acylation reagent.
tive and practical method was successfully developed
for the synthesis of acetophenones with high regiose-
lectivity under palladium-catalyzed conditions. A va- Keywords: acetophenones; acylation reagents; Heck
riety of acetophenones was readily prepared from arylation; palladium; regioselectivity
Introduction
under normal Heck conditions, mixtures of a- and b-
arylated products are typically obtained.[14,15] Thus,
Acetophenones are widely used in the pharmaceuti- controlling the regioselectivity of this arylation pro-
cal, fragrance, dye, and agrochemical industries, and cess remains a challenge.
are common structural units of a variety of biological-
The problem of regioselectivity arises because there
ly active compounds.[1] Acetophenones are classically are two competing reaction pathways in the Heck re-
synthesized by Friedel–Crafts acylation catalyzed by action, as illustrated in Scheme 1.[13,14] In previous
Lewis acids.[2] However, the Friedel–Crafts acylation studies, Larhed,[14a,b] Hallberg[14] and Cabri[15] have im-
is neither effective for electron-deficient arenes nor proved the poor regioselectivity of this reaction by
sufficiently regioselective to introduce an acetyl employing aryl triflates, adding stoichiometric silver
group. Over the years, various Wacker-type reactions or thallium salts. However, a significant drawback of
for the synthesis of acetophenones from styrenes have this approach is that the use of silver raises the cost,
been reported. In these reactions, many catalytic sys- thallium salts are toxic, and triflates are base sensi-
tems have been developed, using solvents such as su-
percritical carbon dioxide (CO2),[3] polyethylene
glycol (PEG),[4] and ionic liquids;[5] phase-transfer cat-
alysts;[6] reoxidants such as nitrous oxide[7] and tert-
butyl hydroperoxide (TBHP);[8] metal catalysts such
as Co,[9a,b] Rh,[9c] and Mn[9d] instead of Pd; and metal-
free oxidation.[10] Besides, a number of palladium-cat-
alyzed cross-coupling reactions have been developed
to generate ketones.[11,12] Among these reported meth-
ods, the Pd-catalyzed Heck reaction of aryl halides
with electron-rich olefin vinyl ethers is attractive due
to its simplicity, tolerance of various functional groups
Scheme 1. The two competing pathways in the Heck reac-
and the easy availability of reagents.[13] Unfortunately, tion.
Adv. Synth. Catal. 2012, 354, 3231 – 3236
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3231

Wangke Qian et al.
FULL PAPERS
tive, thermally labile, and generally commercially un- our protocol, we were able to restrict the reaction
available. More recently, Hallberg[16] and Xiao[17] have time to 2 h using microwave heating. The key feature
developed effective methods to deal with the issue of of our new protocol is the use of vinyloxytrimethylsi-
regioselectivity with electron-rich olefins. The devel- lane as a highly regioselective and novel acylation re-
oped catalytic systems make use of solvents such as agent to obtain acetophenones in a single step.
ionic liquids,[17a] and ethylene glycol (EG),[17b–e] and li-
gands such as 1,3-bis(diphenylphosphino)propane
(DPPP),[16,17a,b,d,e] and 1,1’-bis(diphenylphosphino)fer- Results and Discussion
rocene (DPPF).[17c,f] These methods can promote the
ionic pathway A which leads to internal a-arylated Using 2-iodobenzyl acetate 1a and vinyloxytrimethyl-
vinyl ethers. Unfortunately, these reactions typically silane 2 as the model substrates, we initially optimized
take hours or days to reach completion and valuable the reaction conditions under microwave irradiation.
ligands such as DPPP are needed to achieve high re- A series of catalytic systems was screened including
gioselectivity. Furthermore, the a-arylated vinyl the catalyst/ligand, base, solvent and temperature
ethers need to be hydrolyzed in acid to generate ace- (Table 1). No formation of 3a or 4a was observed
tophenones.
with CuI as the catalyst and most of the starting mate-
Few studies have focused on changing the substrate rial remained unchanged (Table 1, entry 1). Among
enol ethers to improve the regioselectivity of the ary- the palladium catalytic systems (Table 1, entries 2–5),
lation process. Silyl enol ether, as an electron-rich PdACTHNUTRGEN(UNG OAc)2/PPh3 was found to be the best in terms of
vinyl ether, has been used in many electrophilic addi- isolated yield and regioselectivity of the Heck aryla-
tion reactions.[18] We therefore investigated vinyloxy- tion. Substrate 1a was completely arylated after 2 h to
trimethylsilane as an acetyl donor for the direct syn- give exclusive a-substituted product 3a in the pres-
thesis of acetophenones from aryl halides. Microwave ence of 1 mol% of PdACTHNUGTRNEUNG(OAc)2 and 2 mol% of PPh3
heating has been shown to significantly expedite tran- using Et3N as the base in DMF at 1008C heated by
sition metal-catalyzed reactions in most cases.[19] In microwave (Table 1, entry 5). The b-substituted prod-
Table 1. Survey of the reaction conditions for the regioselective Heck reaction.[a]
Entry
Catalyst/Ligand
CuI
Base
Solvent
3a/4a[b]
Yield [%][c]
1
2
3
4
5
6
7
8
Et3N
Et3N
Et3N
Et3N
Et3N
DBU
K2CO3
Cs2CO3
Et3N
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
CH3CN
–
0
7
Pd
Pd
U
>99/1
>99/1
>99/1
>99/1
–
10
86
87
trace
0
PdCl
C
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
N
–
–
0
9
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
82
56
30
59
0
89
86
78
10
11
12
13
14
15[e]
16[f]
Et3N
Et3N
DMF/Et3N=1/1[d]
Et3N
DMF/H2O=1/1
Et3N
Et3N
Et3N
DMF/DMSO=1/1
DMF/DMSO=1/1
DMF/DMSO=1/1
[a]
Unless otherwise stated, all reactions were carried out under microwave irradiation using 1a (0.36 mmol), 2 (0.72 mmol),
catalyst (3.6 mmol), ligand (7.2 mmol) and base (1.08 mmol) in the indicated solvent at 1008C for 2 h.
Molar ratio of 3a/4a. When product 4a could not be detected, a value of >99/1 was assigned. The same applies for
[b]
Table 2.
Isolated yield.
1/1 refers to v/v.
This reaction was carried out at 1108C.
This reaction was carried out at 908C.
[c]
[d]
[e]
[f]
3232
asc.wiley-vch.de
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 3231 – 3236

Microwave-Assisted One-Step Synthesis of Acetophenones via Palladium-Catalyzed Regioselective Arylation
uct 4a was not isolated, suggesting that the ionic path-
Lowering of the reaction temperature from 1008C
way A is operative with vinyloxytrimethylsilane 2 as to 908C resulted in a decrease in product yield from
the acylation reagent. The base additive plays an im- 89% to 78%. When the reaction was carried out at
portant role in the overall efficiency of the reaction. 908C, the starting material 1a was not consumed after
When we changed the base from Et3N to other bases 2 h. Besides, no significant increase in the yield of
such as DBU, K2CO3, and Cs2CO3, yields decreased product 3a was observed when the reaction tempera-
sharply (Table 1, entries 6–8). When an inorganic base ture was raised to 1108C (Table 1, entries 15 and 16).
was used, no product was observed (Table 1, entries 7
With the optimized conditions in hand, we started
and 8). We then investigated the effects of different to examine the substrate scope of this reaction. The
solvents on this reaction. DMSO was found to be an- arylation of 2 was undertaken with a variety of aryl
other good choice, providing good yield and regiose- halides under the optimum conditions. The results are
lectivity (>99/1) (Table 1, entry 9). Lower yields were summarized in Table 2. As can be seen, most of the
obtained when reactions proceeded in acetonitrile, reactions proceeded smoothly, leading to relatively
Et3N, DMF/Et3N (v/v=1/1) and DMF/H2O (v/v=1/1) clean products 3. The neutral path B (Scheme 1) was
(Table 1, entries 10–13). The best result was obtained completely suppressed using vinyloxytrimethylsilane 2
in DMF/DMSO (v/v=1/1), where the product yield as the acylation reagent. The reaction can be equally
reached 89% while the regioselectivity remained ex- applied to ortho-, meta-, and para-substituted iodo-
cellent (Table 1, entry 14).
benzenes bearing electron-withdrawing or electron-
Table 2. Palladium-catalyzed regioselective synthesis of acetophenones from aryl halides and vinyloxytrimethylsilane.[a]
Entry
1
Substrate
Product
Yield [%][c]
89
1a
1b
3a
3b
2
84
85
96
91
3[d]
1c
1d
1e
3c
3d
3e
4
5
6
1f
1g
3f
3g
56
90
7[d]
8
9
1h
1i
3h
3i
85
92
Adv. Synth. Catal. 2012, 354, 3231 – 3236
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3233

Wangke Qian et al.
FULL PAPERS
Table 2. (Continued)
Entry
Substrate
Product
Yield [%][c]
75
10
1j
3j
11
1k
3k
71
12
13
1l
3l
80
82
1m
3m
14
15
16
1n
1o
1p
3n
3o
3p
86
72
0
17
18
1q
3q
0
1r
1s
3r
3s
56
75
19
[a]
Unless otherwise stated, all reactions were carried out under microwave irradiation using 1 (0.36 mmol), 2 (0.72 mmol),
Pd(OAc)2 (3.6 mmol), PPh3 (7.2 mmol) and Et3N (1.08 mmol) in DMF/DMSO (1:1) at 1008C for 2 h.
Compound 4 could not be detected.
Isolated yield.
ACHTUNGTRENNUNG
[b]
[c]
[d]
Reaction using xantphos as ligand.
À
À
À
donating groups, such as NO2, CN, CH2OAc, bromo-2-iodobenzene gave 2-bromoacetophenone 3o
À
À
À
À
À
CH2CN, COOMe, OMe, Br, and F, all of under our standard reaction conditions, indicating
which furnished the desired products in moderate to that this catalytic system has good selectivity for iodo-
excellent isolated yields with excellent a/b ratios of benzenes over bromobenzenes and chlorobenzenes
>99/1 (Table 2, entries 1–11, 15, 19). The protocol (Table 2, entries 15–17).
also worked well for sterically hindered substrates,
such as 1l (Table 2, entry 12). The substrate of this ef- mechanism for the selective formation of acetophen-
ficient catalytic system could be extended to other ones 3 is proposed in Scheme 2. There is general
On the basis of the above observations, a plausible
AHCTUNGTRENNUNG
heteroaryl halides, as demonstrated by substrates 1m agreement on the basic chemistry involved in Heck
and 1n, where the reactions were completed with ex- arylation of olefins.[17e,20] As in most palladium-cata-
cellent regioselectivity and good isolated yields lyzed cross-coupling cases, the first step of the reac-
(Table 2, entries 13 and 14). Furthermore, aryl bro- tion is the reduction of the Pd(II) precursor to pro-
mide 1p and aryl chloride 1q were unreactive while 1- vide the active Pd(0) catalyst 5. Then, aryl iodide with
3234
asc.wiley-vch.de
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 3231 – 3236

Microwave-Assisted One-Step Synthesis of Acetophenones via Palladium-Catalyzed Regioselective Arylation
Conclusions
In summary, we have developed an effective and
practical protocol to generate acetophenones from
aryl iodides using vinyloxytrimethylsilane as a highly
regioselective and novel acylation reagent. This syn-
thetic approach has a number of attractive features
such as operational simplicity, a short reaction time,
a single-step process, broad substrate scope, good
yields and excellent regioselectivity. The new method-
ology reported in this work is complementary to
those previously reported for the synthesis of impor-
tant aromatic ketones, particularly Friedel–Crafts acy-
lation, which is neither effective for electron-deficient
arenes nor sufficiently regioselective to introduce an
acetyl group.
Experimental Section
General Procedure for the Synthesis of 2-
Acetylbenzyl Acetate (e.g., 3a)
Scheme 2. Proposed mechanistic pathway for the regioselec-
tive arylation of vinyloxytrimethylsilane.
A tube containing a stir bar was charged with aryl halide
1
Pd
(0.36 mmol), vinyloxytrimethylsilane
2
(0.72 mmol),
U
and DMF/DMSO=1/1 (4 mL) at room temperature. The
tube was placed in a microwave reactor. The mixture was
stirred and heated at 1008C. After 2 h, heating was stopped
and the tube was cooled to room temperature. The reaction
mixture was poured to 20 mL water, extracted with EtOAc
(2ꢂ20 mL), and the combined organic layer was washed
with saturated brine, dried over Na2SO4, filtered, and con-
centrated under vacuum. The aryl methyl ketone 3 was iso-
lated out of the crude product by flash chromatography on
silica gel (ethyl acetate/hexane=1/40–1/4). 1H NMR
(300 MHz, CDCl3): d=7.79 (d, J=7.5 Hz, 1H), 7.52 (m,
2H), 7.43–7.37 (m, 1H), 5.45 (s, 2H), 2.61 (s, 3H), 2.13 (s,
3H); 13C NMR (100 MHz, CDCl3): d=200.76, 170.58,
136.81, 136.31, 132.08, 129.70, 128.12, 127.58, 64.55, 28.91,
20.94; MS (ESI): m/z=191 [MÀH]À; HR-MS (EI): m/z=
192.0796, calcd. for C11H12O3 M+: 192.0786.
Scheme 3. Attempted palladium-catalyzed arylation of 10.
Pd(0) catalyst 5 forms the arylpalladium reagent 6 by
oxidative addition, followed by formation of inter-
mediate 7 with vinyloxytrimethylsilane 2. The b-
carbon of vinyloxytrimethylsilane 2 is the site of
greatest electron density. Pd(II) inclines to form
a bond with the b-carbon to give 8. Finally, the col-
lapse of 8 generates the a-substituted intermediate 9,
then affords product 3.
Acknowledgements
Within this general scheme some regioselectivity
and mechanistic details remain obscure. To gain fur-
ther insight into the a-arylation mechanism using
vinyloxytrimethylsilane, we performed additional ex-
periments (Scheme 3). Under the same conditions,
using silyl enol ether reagent 10 with the a-position
blocked, product 11 was not observed. The result sug-
gests that the a-position of vinyloxytrimethylsilane is
the only site that can be arylated in this Heck reac-
tion, which accounts for the high regioselectivity of
our protocol.
We gratefully acknowledge financial support from National
Basic Research Program of China (Grants 2009CB940903
and 2012CB518000), the National Natural Science Founda-
tion of China (Grants 20721003, 20872153 and 81025017),
National S&T Major Projects (2012ZX09103-101-072) and
Silver Project (260644).
References
[1] a) I. A. Cotgreave, P. Moldꢃus, R. Brattsand, A. Hall-
berg, C. M. Andersson, L. Engman, Biochem. Pharma-
col. 1992, 43, 793–802; b) P. Piyachaturawat, N. Chain-
gam, A. Chuncharunee, P. Komaratat, A. Suksamrarn,
Adv. Synth. Catal. 2012, 354, 3231 – 3236
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3235

Wangke Qian et al.
FULL PAPERS
Eur. J. Pharmacol. 2000, 387, 221–227; c) A. Sala, R. M.
Giner, J. L. Rꢄos, J. Nat. Prod. 2001, 64, 1360–1362;
d) J. Montes-Avila, S. P. Dꢄaz-Camacho, J. Sicairos-
Fꢃlix, F. Delgado-Vargas, I. A. Rivero, Bioorg. Med.
Chem. 2009, 17, 6780–6785; e) S. Attar, Z. OꢀBrien,
M. L. Golden, H. Alhaddad, Bioorg. Med. Chem. 2011,
19, 2055–2073.
[13] a) W. Cabri, I. Candiani, Acc. Chem. Res. 1995, 28, 2–7;
b) G. T. Crisp, Chem. Soc. Rev. 1998, 27, 427–436;
c) G. D. Daves, A. Hallberg, Chem. Rev. 1989, 89,
1433–1445.
[14] a) M. Larhed, A. Hallberg, J. Org. Chem. 1996, 61,
9582–9584; b) M. Larhed, A. Hallberg, J. Org. Chem.
1997, 62, 7858–7862; c) P. Nilsson, M. Larhed, A. Hall-
berg, J. Am. Chem. Soc. 2001, 123, 8217–8225.
[15] a) W. Cabri, I. Candiani, A. Bedeschi, S. Penco, R.
Santi, J. Org. Chem. 1992, 57, 1481–1486; b) W. Cabri,
I. Candiani, A. Bedeschi, R. Santi, J. Org. Chem. 1992,
57, 3558–3563; c) W. Cabri, I. Candiani, A. Bedeschi,
R. Santi, J. Org. Chem. 1993, 58, 7421–7426.
[2] G. A. Olah, Friedel–Crafts Chemistry, Wiley, New York,
1973.
[3] X. Wang, N. S. Venkataramanan, H. Kawanami, Y.
Ikushima, Green Chem. 2007, 9, 1352–1355.
[4] J. Q. Wang, F. Cai, E. Wang, L. N. He, Green Chem.
2007, 9, 882–887.
[5] V. V. Namboodiri, R. S. Varma, E. Shale-Demessie,
[16] S. A. Karl, M. Larhed, A. Hallberg, J. Org. Chem.
U. R. Pillai, Green Chem. 2002, 4, 170–173.
[6] H. Ito, T. Kusukawa, M. Fujita, Chem. Lett. 2000, 6,
598–599.
[7] J. Ettedgui, R. Neumann, J. Am. Chem. Soc. 2009, 131,
4–5.
[8] C. N. Cornell, M. S. Sigman, J. Am. Chem. Soc. 2005,
127, 2796–2797.
[9] a) K. Sugamoto, Y. Matsushita, T. J. Matsui; J. Chem.
Soc. Perkin Trans. 1. 1998, 3989–3998; b) Y. Matsunaga,
T. Matsui, K. Sugamoto, Chem. Lett. 1992, 1381–1384;
c) J. W. McMillan, H. E. Fischer, J. Schwartz, J. Am.
Chem. Soc. 1991, 113, 4014–4016; d) T. Okamoto, Y.
Sasaki, K. Sasaki, S. Oka, Bull. Chem. Soc. Jpn. 1987,
60, 4449–4450.
2001, 66, 4340–4343.
[17] a) J. Mo, L. J. Xu, J. L. Xiao, J. Am. Chem. Soc. 2005,
127, 751–760; b) Z. Hyder, J. W. Ruan, J. L. Xiao,
Chem. Eur. J. 2008, 14, 5555–5566; c) D. Xu, Z. H. Liu,
W. J. Tang, L. J. Xu, Z. Hyder, J. L. Xiao, Tetrahedron
Lett. 2008, 49, 6104–6107; d) M. C. Liu, Z. Hyder, Y. W.
Sun, W. J. Tang, L. J. Xu, J. L. Xiao, Org. Biomol.
Chem. 2010, 8, 2012–2015; e) J. W. Ruan, J. A. Iggo,
N. G. Berry, J. L. Xiao, J. Am. Chem. Soc. 2010, 132,
16689–16699; f) T. M. Gøgsig, J. Kleimark, S. O. N. Lill,
S. Korsager, A. T. Lindhardt, P. O. Norrby, T. Skrydstr-
up, J. Am. Chem. Soc. 2012, 134, 443–452.
[18] a) B. P. Ying, B. G. Trogden, D. T. Kohlman, Y. C. Xu,
Org. Lett. 2004, 6, 1523–1526; b) J. Chae, J. Yun, S. L.
Buchwald, Org. Lett. 2004, 6, 4809–4812; c) T. Iwama,
V. H. Rawal, Org. Lett. 2006, 8, 5725–5728.
[19] a) M. Baghbanzadeh, C. Pilger, C. O. Kappe, J. Org.
Chem. 2011, 76, 1507–1510; b) H. Huang, H. Liu, K. X.
Chen, J. Org. Chem. 2008, 73, 6037–6040.
[20] a) J. P. Knowles, A. Whiting, Org. Biomol. Chem. 2007,
5, 31–44; b) C. Amatore, M. Azzabi, A. Jutand, J. Am.
Chem. Soc. 1991, 113, 8375–8384; c) C. M. Andersson,
A. Hallberg, J. Org. Chem. 1987, 52, 3529–3536;
d) H. T. Kalinoski, U. Hacksell, E. Barofsky, G. D.
Daves Jr, J. Am. Chem. Soc. 1985, 107, 6476–6482;
e) T. D. Lee, G. D. Daves Jr, J. Org. Chem. 1983, 48,
399–402; f) S. Cacchi, A. Arcadi, J. Org. Chem. 1983,
48, 4236–4240; g) B. M. Trost, in: Comprehensive Or-
ganometallic Chemistry, Vol. 8, Pergamon Press, New
York, 1982, pp 867–874; h) R. F. Heck, Acc. Chem. Res.
1979, 12, 146–151.
[10] T. Nobuta; S. Hirashima; N. Tada; T. Miura; A. Itoh,
Org. Lett. 2011, 13, 2576–2579.
[11] a) L. R. Odell, J. Lindh, T. Gustafsson, M. Larhed, Eur.
J. Org. Chem. 2010, 2270–2274; b) J. Ruan, O. Saidi,
J. A. Iggo, J. L. Xiao, J. Am. Chem. Soc. 2008, 130,
10510–10511; c) J. Ruan, X. Li, O. Saidi, J. L. Xiao, J.
Am. Chem. Soc. 2008, 130, 2424–2425; d) O. Rahman,
T. Kihlberg, B. Langstrçm, Eur. J. Org. Chem. 2004,
474–478; e) S. Cacchi, G. Fabrizi, F. Gavazza, A. Gog-
giamani, Org. Lett. 2003, 5, 289–291.
[12] a) M. Jean, J. Renault, P. Uriac, M. Capet, P. van de
Weghe, Org. Lett. 2007, 9, 3623–3625; b) A. Takemiya,
J. F. Hartwig, J. Am. Chem. Soc. 2006, 128, 14800–
14801; c) B. Xin, Y. H. Zhang, K. Cheng, J. Org. Chem.
2006, 71, 5725–5731; d) R. Lerebours, A. Camacho-
Soto. C. Wolf, J. Org. Chem. 2005, 70, 8601–8604;
e) S. W. Lee, K. Lee, D. Seomoon, S. Kim, P. H. Lee, J.
Org. Chem. 2004, 69, 4852–4855; f) R. K. Dieter, Tetra-
hedron 1999, 55, 4177–4236.
3236
asc.wiley-vch.de
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 3231 – 3236