Palladium-catalyzed aromatization of β-bromovinyl aldehydes with alkenes

Article abstract of DOI:10.1016/j.tet.2005.08.001

β-Bromovinyl aldehydes, which are readily available from ketones and PBr₃/DMF/CHC1}, are aromatized with suitably electron withdrawing group substituted alkenes in THF at 125 °C in the presence of a catalytic amount of a palladium catalyst along with a base.

Full text of DOI:10.1016/j.tet.2005.08.001

Tetrahedron 61 (2005) 9490–9494
Palladium-catalyzed aromatization of b-bromovinyl aldehydes
with alkenes
b
b,
Chan Sik Cho,a, Daksha B. Patel and Sang Chul Shim
*
*
aResearch Institute of Industrial Technology, Kyungpook National University, Daegu 702-701, South Korea
bDepartment of Applied Chemistry, Kyungpook National University, Daegu 702-701, South Korea
Received 13 July 2005; revised 29 July 2005; accepted 1 August 2005
Available online 19 August 2005
Abstract—b-Bromovinyl aldehydes, which are readily available from ketones and PBr3/DMF/CHCl3, are aromatized with suitably electron
withdrawing group substituted alkenes in THF at 125 8C in the presence of a catalytic amount of a palladium catalyst along with a base.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
transition metal-catalyzed cyclization reactions, we recently
reported on palladium-catalyzed synthesis of various cyclic
compounds from o-halo aromatics.2–6 Among them, in
connection with this report, 2-bromobenzaldehydes were
found to be cyclized with suitably functionalized alkenes in
the presence of a palladium catalyst and a base via tandem
Heck and intramolecular aldol reactions to give naphtha-
lenes.7 This protocol led us to extend to the reaction with
b-bromovinyl aldehydes, which are readily prepared from
ketones via the bromo analogue of Vilsmeier reaction
(Scheme 1).8 Herein, we report a successful palladium-
catalyzed aromatization of b-bromovinyl aldehydes with
alkenes via intrinsic tandem Heck and aldol reactions.
Palladium-catalyzed annulation technology has been widely
introduced as an useful synthetic tool for carbo- and
heterocycles, which play an important role as a basic unit
for the design of many pharmacologically and biologically
active compounds.1 As part of our continuing studies toward
Scheme 1.
Table 1. Palladium-catalyzed aromatization of 1a with 2a under several conditions
Run
Pd catalysts
Bases
Temperature (8C)
Isolated yield (%)
3a
4
1
2
3
4
5
6
7
Pd(OAc)2/2PPh3
Pd(OAc)2/2PPh3
Pd(OAc)2/2PPh3
PdCl2(PPh3)2
Pd(OAc)2/2PPh3
Pd(OAc)2/2PPh3
PdCl2(PPh3)2
NaOAc
NaOAc
NaOAc
NaOAc
Et3N
70
0
60
39
19
13
36
0
100
125
125
100
100
100
14
50
47
15
7
K2CO3
K2CO3
14
0
Reaction conditions: 1a (1 mmol), 2a (1 mmol), palladium catalyst (0.05 mmol), base (3 mmol), THF (5 mL), for 20 h.
Keywords: Alkenes; Aromatization; b-Bromovinyl aldehydes; Heck–aldol reaction; Palladium catalyst.
*
Corresponding authors. Tel.: C82 53 950 7318; fax: C82 53 950 6594; e-mail addresses: cscho@knu.ac.kr; scshim@knu.ac.kr
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.08.001

C. S. Cho et al. / Tetrahedron 61 (2005) 9490–9494
9491
2. Results and discussion
Based on our recent report on palladium-catalyzed synthesis
of naphthalenes from 2-bromobenzaldehydes and function-
alized alkenes,7 Table 1 shows several attempted results for
the aromatization of 2-bromocyclohex-1-enecarbaldehyde
(1a) with dimethyl itaconate (2a). Generally, treatment of
equimolar amounts of 1a and 2a in THF in the presence of a
catalytic amount of a palladium catalyst along with a base
afforded dimethyl 5,6,7,8-tetrahydronaphthalene-2,3-
dicarboxylate (3a) with concomitant formation of dimethyl
Scheme 2.
Table 2. Palladium-catalyzed aromatization of b-bromovinyl aldehydes 1 with alkenes 2
b-Bromovinyl aldehydes 1
Alkenes 2
Products 3a
Yield (%)
50
2a
1a
3a
3b
3c
3d
3e
3f
50
49
60
53
49
46
54
55
35
2b
2c
2d
2e
2a
1b
1c
2a
2d
2a
2a
3g
3h
3i
1d
1e
3j
RZMe(3k)
RZEt(3l)
2a
2b
46
60
1f
2a
2a
3k
45
21
1g
1h
3m
Reaction conditions: 1 (1 mmol), 2 (1 mmol), Pd(OAc)2 (0.05 mmol), PPh3 (0.1 mmol), NaOAc (3 mmol), THF (5 mL), 125 8C, for 20 h.
a Although non-aromatized products like 4 are present in a small amount in several cases on GLC and TLC analyses, we did not focus our attention on the
isolation of such non-aromatized products.

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C. S. Cho et al. / Tetrahedron 61 (2005) 9490–9494
2-(2-formylcyclohex-1-enylmethyl)maleate (4). As has
been depicted in our recent report,7 it appears that precursor
7 for 3a and 4 are produced, respectively, according to the
direction of b-hydrogen elimination on alkylpalladium 6,
which is formed via vinylpalladium 5 by conventional Heck
reaction between 1a and 2a. (Scheme 2). The reaction
temperature was critical for the distribution of aromatized
product 3a and non-aromatized product 4 under Pd(OAc)2
combined with PPh3, higher reaction temperature resulting
in preferential formation of 3a (runs 1–3). Absolute
formation of 4 was achieved with the reaction temperature
70 8C shown in run 1 of Table 1. However, the overall yield
of both products did not show such a significant change.
Similar yield and distribution of products were observed
with PdCl2(PPh3)2 comparing with Pd(OAc)2/2PPh3 cata-
lytic system (runs 3 and 4). Among bases examined, Et3N
could be alternatively used under the employed conditions
(runs 2 and 5), but the reaction did not proceed effectively
with K2CO3 (runs 6 and 7).
via column (silica gel 60, 70–230 mesh, Merck) and thin
layer (silica gel 60 GF254, Merck) chromatography.
b-Bromovinyl aldehydes 18 and alkenes (2c,9 2d,10 and
2e10) were prepared by the reported methods. Commercially
available organic and inorganic compounds were used
without further purification except for THF, which was
distilled by known method before use.
3.1. Typical procedure for palladium-catalyzed aroma-
tization of b-bromovinyl aldehydes with alkenes
A mixture of 2-bromocyclohex-1-enecarbaldehyde (1a)
(0.189 g, 1 mmol), dimethyl itaconate (2a) (0.158 g,
1 mmol), Pd(OAc)2 (0.011 g, 0.05 mmol), PPh3 (0.026 g,
0.1 mmol) and NaOAc (0.246 g, 3 mmol) in THF (5 mL)
was placed in a 50 mL pressure vessel. After the system was
flushed with argon, the mixture was stirred at 125 8C for
20 h. The reaction mixture was passed through a short silica
gel column (ethyl acetate) to eliminate inorganic salts.
Removal of the solvent left a crude mixture, which was
separated by thin-layer chromatography (silica gel, ethyl
acetate/hexaneZ1:5) to give dimethyl 5,6,7,8-tetrahydro-
naphthalene-2,3-dicarboxylate (3a) (0.124 g, 50%) along
with dimethyl 2-(2-formylcyclohex-1-enylmethyl)maleate
(4) (0.051 g, 19%). The compounds prepared by the above
procedure were characterized spectroscopically as shown
below. 3a, 3g, 3k, and 3m are known.
Given the controlled conditions, the reactions of various
b-bromovinyl aldehydes 1 and suitably electron with-
drawing group substituted alkenes 2 were screened in
order to investigate the reaction scope and several
representative results are summarized in Table 2. From
the reactions between 1a and dialkyl itaconate (2a–c),9 the
corresponding dialkyl 5,6,7,8-tetrahydronaphthalene-2,3-
dicarboxylates (3a–c) were produced in similar yields
(49–50%). The reaction proceeds likewise with ethyl
3-butenoates (2d and 2e)10 having acyl group at position 3
to give the corresponding aromatized products (3d and 3e).
2-Bromo-5-methylcyclohex-1-enecarbaldehyde (1b) reacts
similarly with 2a to give dimethyl 6-methyl-5,6,7,8-
tetrahydronaphthalene-2,3-dicarboxylate (3g) in 49%
yield. Several cyclic b-bromovinyl aldehydes (1c–e) were
also reacted with 2a and 2d, the corresponding aromatized
products (3g–j) being obtained in the range of 35–55%
yields. However, lower reaction yield was observed with
higher membered cyclic b-bromovinyl aldehyde 1e. To test
for the effect of the position of formyl group and bromide on
cyclic b-bromovinyl aldehydes, 1f and 1g were employed.
However, the aromatization took place similarly irrespec-
tive of the position. Performing the reaction of acyclic
b-bromovinyl aldehyde 1h with 1a did not proceed
satisfactorily toward aromatization compared with cyclic
one.
3.1.1. Dimethyl 5,6,7,8-tetrahydronaphthalene-2,3-
1
dicarboxylate (3a).11 Oil; H NMR (400 MHz, CDCl3) d
1.79–1.82 (m, 4H), 2.78–2.81 (m, 4H), 3.88 (s, 6H), 7.42 (s,
2H); 13C NMR (100 MHz, CDCl3) d 22.82, 29.41, 52.59,
129.16, 129.89, 140.95, 168.44.
3.1.2. Dimethyl 2-(2-formylcyclohex-1-enylmethyl)mal-
eate (4). Oil; 1H NMR (400 MHz, CDCl3) d 1.66–1.71 (m,
4H), 2.23–2.28 (m, 4H), 3.28 (s, 2H), 3.67 (s, 3H), 3.81 (s,
3H), 7.50 (s, 1H), 9.70 (s, 1H); 13C NMR (100 MHz,
CDCl3) d 21.52, 21.99, 22.04, 30.98, 33.61, 52.57, 52.84,
129.49, 136.80, 140.55, 153.29, 166.64, 170.83, 192.48.
Anal. Calcd for C14H18O5: C 63.15; H 6.81. Found: C 62.94;
H 6.75.
3.1.3. Diethyl 5,6,7,8-tetrahydronaphthalene-2,3-dicarb-
oxylate (3b). Oil; 1H NMR (400 MHz, CDCl3) d 1.36 (t, JZ
7.04 Hz, 6H), 1.78–1.82 (m, 4H), 2.78–2.81 (m, 4H), 4.34
(q, JZ7.04 Hz, 4H), 7.42 (s, 1H); 13C NMR (100 MHz,
CDCl3) d 14.53, 23.05, 29.63, 61.77, 129.60, 130.05,
141.01, 168.32. Anal. Calcd for C14H20O4: C 69.54; H
7.30. Found: C 69.26; H 7.27.
In summary, it has been shown that b-bromovinyl aldehydes
undergo an aromatization with various suitably electron
withdrawing group substituted alkenes in the presence of a
palladium catalyst and a base via domino Heck and aldol
processes. We believe that the present reaction will work as
an useful procedure for the synthesis of aromatics from
ketones.
3.1.4. 2-Ethyl 3-methyl 5,6,7,8-tetrahydronaphthalene-2,
3-dicarboxylate (3c). Oil; H NMR (400 MHz, CDCl3) d
1
1.35 (t, JZ7.0 Hz, 3H), 1.78–1.80 (m, 4H), 2.78 (br s, 4H),
3.87 (s, 3H), 4.34 (q, JZ7.0 Hz, 2H), 7.40 (s, 1H), 7.42 (s,
1H); 13C NMR (100 MHz, CDCl3) d 14.51, 23.1 (!2), 29.6
(!2), 52.71, 61.75, 129.35, 129.58, 130.03, 130.08, 141.04,
141.08, 168.19, 168.74. Anal. Calcd for C15H18O4: C 68.68;
H 6.92. Found: C 68.49; H 6.81.
3. Experimental
1H and 13C NMR (400 and 100 MHz) spectra were recorded
on a Bruker Avance Digital 400 spectrometer using TMS as
an internal standard. Melting points were determined on a
Thomas-Hoover capillary melting point apparatus and were
uncorrected. The isolation of pure products was carried out
3.1.5. Ethyl 3-acetyl-5,6,7,8-tetrahydronaphthalene-2-
carboxylate (3d). Oil; H NMR (400 MHz, CDCl3) d 1.36
1
(t, JZ7.0 Hz, 3H), 1.80–1.81 (m, 4H), 2.50 (s, 3H), 2.80 (s,

C. S. Cho et al. / Tetrahedron 61 (2005) 9490–9494
9493
4H), 4.34 (q, JZ7.0 Hz, 2H), 7.10 (s, 1H), 7.54 (s, 1H); 13
C
oxylate (3k). Solid (hexane–chloroform); mp 82–83 8C
(lit.12 84 8C); 1H NMR (400 MHz, CDCl3) d 2.87–2.98 (m,
4H), 3.92 (s, 3H), 3.94 (s, 3H), 7.25–7.36 (m, 3H), 7.60 (s,
1H), 7.79–7.81 (m, 1H), 8.09 (s, 1H); 13C NMR (100 MHz,
CDCl3) d 28.04, 28.39, 52.33 (!2), 123.97, 124.39, 125.22,
126.98, 128.04, 128.48, 129.26, 129.96, 130.39, 132.33,
137.20, 140.24, 167.88, 168.21.
NMR (100 MHz, CDCl3) d 14.42, 23.08 (!2), 29.55, 29.80,
30.43, 61.79, 126.80, 127.76, 130.80, 139.99, 140.20,
142.02, 167.68, 203.37. Anal. Calcd for C15H18O3: C
73.15; H 7.37. Found: C 72.81; H 7.28.
3.1.6. Ethyl 3-benzoyl-5,6,7,8-tetrahydronaphthalene-2-
carboxylate (3e). Solid (hexane–chloroform); mp 79–
1
80 8C; H NMR (400 MHz, CDCl3) d 1.01 (t, JZ7.0 Hz,
3.1.13. Diethyl 9,10-dihydrophenanthrene-2,3-dicarb-
oxylate (3l). Oil; H NMR (400 MHz, CDCl3) d 1.38 (t,
1
3H), 1.83–1.85 (m, 4H), 2.81–2.86 (m, 4H), 4.02 (q, JZ
7.0 Hz, 2H), 7.09 (s, 1H), 7.41 (t, JZ7.5 Hz, 2H), 7.51–7.55
(m, 1H), 7.76–7.78 (m, 3H); 13C NMR (100 MHz, CDCl3) d
14.03, 23.09, 23.16, 29.58, 29.85, 61.58, 126.91, 128.79,
128.84, 129.71, 131.24, 133.26, 137.95, 139.02, 139.39,
142.76, 166.58, 197.74. Anal. Calcd for C20H20O3: C 77.90;
H 6.54. Found: C 77.53; H 6.75.
JZ7.0 Hz, 3H), 1.39 (t, JZ7.0 Hz, 3H), 2.86–2.94 (m, 4H),
4.38 (q, JZ7.0 Hz, 2H), 4.40 (q, JZ7.0 Hz, 2H), 7.24–7.35
(m, 3H), 7.60 (s, 1H), 7.79–7.81 (m, 1H), 8.08 (s, 1H); 13C
NMR (100 MHz, CDCl3) d 14.59 (!2), 28.81, 29.13, 62.01,
62.05, 124.67, 124.71, 127.69, 128.75, 129.14, 129.15,
130.96, 131.38, 133.15, 137.66, 137.96, 140.79, 168.12,
168.28. Anal. Calcd for C20H20O4: C 74.06; H 6.21. Found:
C 73.76; H 6.17.
3.1.7. Dimethyl 6-methyl-5,6,7,8-tetrahydronaphtha-
lene-2,3-dicarboxylate (3f). Solid (hexane–chloroform);
1
3.1.14. Dimethyl 4-phenylphthalate (3m).13 Oil; 1H NMR
(400 MHz, CDCl3) d 3.93 (s, 3H), 3.94 (s, 3H), 7.41–7.49
(m, 3H), 7.60–7.62 (m, 2H), 7.74 (dd, JZ2.0, 8.0 Hz, 1H),
7.84 (d, JZ8.0 Hz, 1H), 7.91 (d, JZ1.5 Hz, 1H); 13C NMR
(100 MHz, CDCl3) d 53.04, 53.13, 127.61, 127.77, 128.86,
129.42, 129.70, 130.11, 130.28, 133.46, 139.39, 144.75,
168.05, 168.74.
mp 66–67 8C; H NMR (400 MHz, CDCl3) d 1.06 (d, JZ
6.5 Hz, 3H), 1.34–1.44 (m, 1H), 1.81–1.94 (m, 2H), 2.37–
2.44 (m, 1H), 2.80–2.88 (m, 3H), 3.88 (s, 6H), 7.41 (s, 1H),
7.43 (s, 1H); 13C NMR (100 MHz, CDCl3) d 22.00, 29.16,
29.38, 31.19, 38.16, 52.83 (!2), 129.35, 129.38, 129.86,
130.06, 140.77, 141.01, 168.73 (!2). Anal. Calcd for
C15H18O4: C 68.68; H 6.92. Found: C 68.43; H 7.15.
3.1.8. Dimethyl indan-5,6-dicarboxylate (3g). Solid
(hexane–chloroform); mp 67–68 8C; (lit.11 68–69 8C); H
1
Acknowledgements
NMR (400 MHz, CDCl3) d 2.87–2.98 (m, 4H), 3.92 (s, 3H),
3.94 (s, 3H), 7.25–7.36 (m, 3H), 7.60 (s, 1H), 7.79–7.81 (m,
1H), 8.09 (s, 1H); 13C NMR (100 MHz, CDCl3) d 2.09–2.16
(m, 2H), 2.95 (t, JZ7.5 Hz, 4H), 3.88 (s, 6H), 7.56 (s, 2H);
13C NMR (100 MHz, CDCl3) d 25.47, 32.89, 52.70, 125.00,
130.46, 148.12, 168.73.
The present work was supported by BK-21 in 2003 and a
Korea Research Foundation Grant (KRF-2002-070-
C00055). C.S.C. gratefully acknowledges a Research
Professor Grant of Kyungpook National University (2004).
3.1.9. Ethyl 6-acetylindan-5,6-dicarboxylate (3h). Oil; 1H
NMR (400 MHz, CDCl3) d 1.36 (t, JZ7.0 Hz, 3H), 2.09–
2.16 (m, 2H), 2.51 (s, 3H), 2.95 (t, JZ7.5 Hz, 4H), 4.34 (q,
JZ7.0 Hz, 2H), 7.25 (s, 1H), 7.69 (s, 1H); 13C NMR
(100 MHz, CDCl3) d 14.42, 25.68, 30.55, 32.92, 33.21,
61.81, 122.82, 125.86, 127.84, 141.68, 147.01, 149.20,
167.80, 203.78. Anal. Calcd for C14H16O3: C 72.39; H 6.94.
Found: C 72.15; H 6.82.
References and notes
1. For recent reviews, see: (a) Thebtaranonth, C.; Thebtaranonth,
Y. Cyclization Reactions; CRC: London, 1994; pp 255–330.
(b) Tsuji, J. Palladium Reagents and Catalysts; Wiley:
´
Chichester, 1995. (c) Negishi, E.; Coperet, C.; Ma, S.; Liou,
S.-Y.; Liu, F. Chem. Rev. 1996, 96, 365. (d) Grigg, R.;
Sridharan, V. Pure Appl. Chem. 1998, 70, 1047. (e) Larock,
R. C. J. Organomet. Chem. 1999, 576, 111.
3.1.10. Dimethyl 6,7,8,9-tetrahydro-5H-benzocyclo-
heptene-2,3-dicarboxylate (3i). Solid (hexane–chloro-
form); mp 86–87 8C; H NMR (400 MHz, CDCl3) d 1.61–
1.66 (m, 4H), 1.82–1.88 (m, 2H), 2.82–2.85 (m, 4H), 3.88
(s, 6H), 7.45 (s, 2H); 13C NMR (100 MHz, CDCl3) d 28.07,
32.81, 36.75, 52.85, 129.88 (!2), 147.45, 168.69. Anal.
Calcd for C15H18O4: C 68.68; H 6.92. Found: C 68.36; H
7.09.
1
2. For the synthesis of isoindolinones: (a) Cho, C. S.; Lee, J. W.;
Lee, D. Y.; Shim, S. C.; Kim, T. J. Chem. Commun. 1996,
2115. (b) Cho, C. S.; Chu, D. Y.; Lee, D. Y.; Shim, S. C.; Kim,
T.-J.; Lim, W. T.; Heo, N. H. Synth. Commun. 1997, 27, 4141.
(c) Cho, C. S.; Jiang, L. H.; Shim, S. C. Synth. Commun. 1998,
28, 849. (d) Cho, C. S.; Jiang, L. H.; Lee, D. Y.; Shim, S. C.;
Lee, H. S.; Cho, S.-D. J. Heterocycl. Chem. 1997, 34, 1371.
(e) Cho, C. S.; Wu, X.; Jiang, L. H.; Shim, S. C.; Choi, H.-J.;
Kim, T.-J. J. Heterocycl. Chem. 1998, 35, 265. (f) Cho, C. S.;
Wu, X.; Jiang, L. H.; Shim, S. C.; Kim, H. R. J. Heterocycl.
Chem. 1999, 36, 297. (g) Cho, C. S.; Shim, H. S.; Choi, H.-J.;
Kim, T.-J.; Shim, S. C.; Kim, M. C. Tetrahedron Lett. 2000,
41, 3891.
3.1.11. Dimethyl 5,6,7,8,9,10-hexahydrobenzocyclo-
octene-2,3-dicarboxylate (3j). Oil; H NMR (400 MHz,
1
CDCl3) d 1.33–1.36 (m, 4H), 1.69–1.70 (m, 4H), 2.78–2.81
(m, 4H), 3.89 (s, 6H), 7.47 (s, 2H); 13C NMR (100 MHz,
CDCl3) d 26.07, 32.18, 32.50, 52.83, 130.02, 130.12,
145.42, 168.75. Anal. Calcd for C16H20O4: C 69.54; H
7.30. Found: C 69.64; H 7.36.
3. For the synthesis of b-lactams: Cho, C. S.; Jiang, L. H.; Shim,
S. C. Synth. Commun. 1999, 29, 2695.
4. For the synthesis of indoles: Cho, C. S.; Shim, H. S.; Choi,
3.1.12. Dimethyl 9,10-dihydrophenanthrene-2,3-dicarb-

9494
C. S. Cho et al. / Tetrahedron 61 (2005) 9490–9494
H.-J.; Kim, T.-J.; Shim, S. C. Bull. Korean Chem. Soc. 2004,
25, 441.
8. (a) Coates, R. M.; Senter, P. D.; Baker, W. R. J. Org. Chem.
1982, 47, 3597. (b) Zhang, Y.; Herndon, J. W. Org. Lett. 2003,
5, 2043. (c) Mal, S. K.; Ray, D.; Ray, J. K. Tetrahedron Lett.
2004, 45, 277.
5. For the synthesis of phthalides: (a) Lee, D. Y.; Cho, C. S.;
Jiang, L. H.; Wu, X.; Shim, S. C. Synth. Commun. 1997, 27,
3449. (b) Cho, C. S.; Baek, D. Y.; Shim, S. C. J. Heterocycl.
Chem. 1999, 36, 1101. (c) Cho, C. S.; Baek, D. Y.; Kim, H. Y.;
Shim, S. C.; Oh, D. H. Synth. Commun. 2000, 30, 1139.
(d) Cho, C. S.; Lim, D. K.; Kim, T.-J.; Shim, S. C. J. Chem.
Res. (S) 2002, 550.
9. Kimura, R.; Ichihara, A.; Sakamura, S. Synthesis 1979, 516.
10. Ayed, T. B.; Amri, H. Synth. Commun. 1995, 25, 3813.
11. Hillard, R. L.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1977, 99,
4058.
12. Bussenius, J.; Laber, N.; Mu¨ller, T.; Eberbach, W. Chem. Ber.
1994, 127, 247.
6. For the synthesis of 1-aryl-1H-indazoles: Cho, C. S.; Lim,
D. K.; Heo, N. H.; Kim, T.-J.; Shim, S. C. Chem. Commun.
2004, 104.
13. Isobe, H.; Sato, S.; Tanaka, T.; Tokuyama, H.; Nakamura, E.
Org. Lett. 2004, 6, 3569.
7. Cho, C. S.; Lim, D. K.; Zhang, J. Q.; Kim, T.-J.; Shim, S. C.
Tetrahedron Lett. 2004, 45, 5653.