Facile synthesis of 2,3-dihydrobenzofuran-3-ylacetic acids by novel electrochemical sequential aryl radical cyclization-carboxylation of 2-allyloxybromobenzenes using methyl 4-tert-butylbenzoate as an electron-transfer mediator

Article abstract of DOI:10.1055/s-0030-1260794

Facile synthesis of 2,3-dihydrobenzofuran-3-ylacetic acids and related analogues was successfully carried out by a novel electrochemical aryl radical generation and its 5-exo cyclization followed by a carboxylation sequence of 2-allyloxybromobenzenes by u

Full text of DOI:10.1055/s-0030-1260794

LETTER
1567
Facile Synthesis of 2,3-Dihydrobenzofuran-3-ylacetic Acids by Novel Electro-
chemical Sequential Aryl Radical Cyclization–Carboxylation of 2-Allyloxy-
bromobenzenes Using Methyl 4-tert-Butylbenzoate as an Electron-Transfer
Mediator
F
acile Synthesis of
i
2,3-D
s
a
zofuran-3-y
n
lacetic
A
cid
o
s
ri Senboku,* Jun-ya Michinishi, Shoji Hara
Laboratory of Organic Reaction, Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University,
Sapporo, Hokkaido 060-8628, Japan
Fax +81(11)7066555; E-mail: senboku@eng.hokudai.ac.jp
Received 29 March 2011
zenes by electrochemical reduction using methyl 4-tert-
Abstract: Facile synthesis of 2,3-dihydrobenzofuran-3-ylacetic ac-
butylbenzoate as an electron-transfer mediator. We also
ids and related analogues was successfully carried out by a novel
found that thus-generated aryl radicals were applicable to
radical cyclization, and after further one-electron reduc-
electrochemical aryl radical generation and its 5-exo cyclization fol-
lowed by a carboxylation sequence of 2-allyloxybromobenzenes by
using methyl 4-tert-butylbenzoate as an electron-transfer mediator. tion of the resulting cyclized radical, the resulting anion
efficiently captured carbon dioxide to yield 3,4-dihy-
drobenzofuran-3-ylacetic acids in high selectivities and
good yields. Herein, we report a facile synthesis of 3,4-di-
hydrobenzofuran-3-ylacetic acids and related analogues
by novel electrochemical sequential aryl radical cycliza-
tion–carboxylation (ERCC) of 2-allyloxybromobenzenes
using methyl 4-tert-butylbenzoate as an electron-transfer
mediator.
Key words: electrochemical reduction, radical cyclization, fixation
of carbon dioxide, radical-anion tandem reaction, 2,3-dihydroben-
zofuran-3-ylacetic acids
Electroreductive generation of carbon radicals from or-
ganic halides is one of the useful and eco-friendly alterna-
tives to conventional radical generation using toxic
organotin reagents such as tributyltin hydride. Successful
electroreductive generation of carbon-centered radicals
without Bu3SnH has been carried out by direct,1,2 metal-
complex-catalyzed,3–6 or mediated7–9 electrochemical re-
duction of organic halides and diazonium salts, and its ap-
plication to radical cyclization reactions has also been
performed to synthesize carbo- and heterocycles. On the
other hand, the termination step of these radical reactions
under electroreductive conditions usually proceeds by the
reaction of anion species, which are produced by further
one-electron reduction of the resulting cyclized radical.
However, most of the termination steps in electroreduc-
tive radical reactions are merely protonation of the result-
ing anion, yielding protonated cyclic compounds, and
little attention has been given to the use of the resulting
anion for further reactions, especially carbon–carbon
bond-forming reactions. While this sequential radical
generation–cyclization can be achieved by the use of
samarium(II) iodide, a representative one-electron reduc-
tant, followed by terminal anion reaction with electro-
philes such as CO2,10,11 to the best of our knowledge, Ni-
catalyzed tandem cyclization–carboxylation of unsaturat-
ed haloaryl ethers is the only example in electroreductive
‘radical-type’ cyclization followed by C–C bond forma-
tion with CO2.12 During the course of our studies in elec-
troorganic synthesis,13 we recently succeeded in selective
generation of aryl radicals from 2-allyloxybromoben-
The results of screening of an additive as an electron-
transfer mediator for ERCC of 2-allyloxybromobenzene
(1a) are summarized in Table 1. Electrolysis of 1a was
carried out using a test tube-like undivided cell equipped
with a Pt plate cathode (2 × 2 cm2) and an Mg rod anode
(6 mm f) in a DMF solution containing 0.1 M Bu4NBF4
as a supporting electrolyte in the presence of CO2 at
0 °C.14 When 3 F/mol of electricity was passed by 10 mA/
cm2 constant current without any additives, expected
ERCC product 2a was obtained as a mixture of directly
carboxylated benzoic acid 3a in moderate yield and low
selectivity (Table 1, entry 1). We next tried phenanthrene8
and 9,9-diethylfluorene,9 which were reported to be effec-
tive for generation of aryl radicals from aryl halides, as
additives. However, in both cases, carboxylic acids were
obtained as a complex mixture, probably due to direct
electrochemical carboxylation of additives (Table 1, en-
tries 2 and 3). After several attempts, methyl benzoate was
found to be favorable to yield ERCC product 2a (Table 1,
entry 4). Although the yield and selectivity were accept-
able in this case, other carboxylic acids were also detected
1
in its H NMR as impurities, probably due to direct car-
boxylation of methyl benzoate. Finally, we found that
methyl 4-tert-butylbenzoate (4) was effective, and expect-
ed 2a was obtained in 54% isolated yield with high selec-
tivity and no contaminants (Table 1, entry 5). We next
investigated reaction conditions, including equivalents of
mediator, current density, and electricity, using 1a as a
substrate, and the results are also summarized in Table 1.
In contrast to the electrolysis at 20 mA/cm2 with 3 F/mol
of electricity in entry 6 (Table 1), electrolyses at 10 mA/
SYNLETT 2011, No. 11, pp 1567–1572
x
x
.x
x
.2
0
1
1
Advanced online publication: 15.06.2011
DOI: 10.1055/s-0030-1260794; Art ID: U02911ST
© Georg Thieme Verlag Stuttgart · New York

1568
H. Senboku et al.
LETTER
cm2 and 50 mA/cm2 current densities resulted in slight de- drobenzofuran-3-ylacetic acids 2b and 2c in 85% and
creases of the yields of 2a (Table 1, entries 5 and 7). Elec- 79% yields, respectively (Table 2, entries 1 and 2). The
tron-transfer mediator 4, methyl 4-tert-butylbenzoate, product 2b was obtained as a 9:1 mixture of diastere-
was able to be reduced to 0.5 equivalents at a current den- omers. Recrystallization from hexane gave the major
sity of 20 mA/cm2 without any decreases in yield or selec- product as a single isomer, whose stereochemistry ana-
tivity of 2a (Table 1, entry 8). However, further reducing lyzed by 1H NMR NOESY revealed it to be a trans isomer
of the electron-transfer mediator resulted in lower product of
2,3-dihydro-2-methylbenzofuran-3-ylacetic
acid
selectivities of 2a toward 3a with almost same product (trans-2b). In a similar manner, the present ERCC reac-
yields. Finally, when 7 F/mol of electricity was passed in tion of bromide 1d having a methyl group at the internal
a constant current electrolysis of 1a at 20 mA/cm2 in the carbon of the C–C double bond provided the correspond-
presence of CO2 and 0.5 equivalents of mediator 4, con- ing dihydrobenzofuranylacetic acid 2d in 78% yield
version reached 85%, and the desired radical cyclization– (Table 2, entry 3). In all cases, aryl radical cyclization of
carboxylation product, 2,3-dihydrobenzofuran-3-ylacetic ERCC proceeded exo-selectively, and no endo-cycliza-
acid (2a), was obtained in 82% yield with high selectively tion products were observed. On the other hand, methyl
(Table 1, entry 9). No more electricity with a stable con- groups at the terminal carbon of the C–C double bond in
stant current, however, could not be supplied, probably the allyloxy group in 1 strongly affected the yields of
due to precipitation of magnesium salts of both the prod- ERCC product 2. When crotyloxybromobenzene (1e) was
ucts and produced oxalic acid, which could be observed as subjected to the ERCC reaction, desired product 2e was
white precipitates at the cathode surface. It is noteworthy selectively obtained in only 48% yield along with 3-ethyl-
that a mediator 4 could be recovered quantitatively, and 2,3-dihydrobenzofuran (5) and a formylated, instead of
after purification by column chromatography, the recov- carboxylated, product 6 in 15% and 4% 1H NMR yields,
ered mediator 4 was reusable for the present reaction.
respectively (Table 2, entry 4 and Figure 1). Similar re-
sults were obtained when prenyloxy derivative 1f having
two methyl groups at the terminal carbon was used as a
substrate. Desired ERCC product 2f was selectively ob-
tained in only 33% yield along with 3-isopropyl-2,3-dihy-
drobenzofuran (7) and a formylated 8 in 25% and 3% 1H
NMR yields, respectively (Table 2, entry 7 and Figure 1).
In contrast to the results for 1e, the yield of the desired
product 2f became lower than that of 2e, and the yield of
protonated product 7 increased.
Synthesis of substituted dihydrobenzofuranylacetic acids
2 from bromobenzenes 1 having methyl groups on the al-
lyloxy group as substrates was next investigated, and the
results are summarized in Table 2. The present ERCC was
found to be also applicable to the synthesis of substituted
benzofuranylacetic acids 2. From 2-allyloxybromoben-
zenes 1b and 1c having methyl groups at their allylic po-
sition, the corresponding aryl radicals could also be
generated with high selectivity, and efficiently underwent
sequential radical cyclization–carboxylation through one- These results would be due to the relative stability of an-
electron reduction to yield the corresponding 2,3-dihy- ion intermediates A, B, and C, which were generated by
Table 1 Screening of Reaction Conditions in ERCC of 1a
CO2H
Br
O
CO2H
O
Pt
Mg,
CO2
+
0.1 M Bu4NBF4, DMF
0 °C
O
1a
2a
3a
Entry
Additive (equiv)
Current density
[mA/cm2]
Electricity
[F/mol]
Yield (%)a and
Conversion
(%)b
ratio (2a/3a)b
1
2
3
4
5
6
7
8
9
none
10
10
10
10
10
20
50
20
20
3
3
3
3
3
3
3
3
7
56b (62:38)
a complex mixture
a complex mixture
71b (97:3)
75
44
69
73
64
68
62
69
85
phenanthrene (2)
9,9-diethylfluorene (2)
methyl benzoate (2)
methyl 4-tert-butylbenzoate (2)
methyl 4-tert-butylbenzoate (2)
methyl 4-tert-butylbenzoate (2)
methyl 4-tert-butylbenzoate (0.5)
methyl 4-tert-butylbenzoate (0.5)
54 (98:2)
59 (98:2)
55 (98:2)
59 (99:1)
82 (98:2)
a Isolated yield.
b Determined by 1H NMR.
Synlett 2011, No. 11, 1567–1572 © Thieme Stuttgart · New York

LETTER
Facile Synthesis of 2,3-Dihydrobenzofuran-3-ylacetic Acids
1569
one-electron reduction of the radicals produced by aryl 7 and formylation products 6 and 8, respectively. Basicity
radical cyclization (Figure 1). Anion A, generated from of tertiary anion C would be, in general, stronger than that
1a by aryl radical cyclization followed by one-electron re- of secondary anion B, resulting in the increase in yield of
duction of the resulting cyclized radical, is a primary an- protonated product 7 in contrast to that of 5. On the other
ion, which is sufficiently stable to react with carbon hand, nucleophilicity of tertiary anion C would be lower
dioxide selectively yielding 2a as a sole cyclized product. than that of secondary anion B because of its bulkiness,
On the other hand, anions B and C, generated in a similar resulting in the decrease in yield of carboxylated product
manner from 1e and 1f, respectively, are secondary and 2f compared to that of 2e.
tertiary anions, which are relatively unstable compared
To improve the yield and selectivity of the ERCC product
with primary anion A. Therefore, side reactions, such as
2e, the reaction of 1e was carried out at lower tempera-
proton abstraction from tetrabutylammonium cation and
tures, and the results are also summarized in Table 2 (en-
reaction with DMF, used as a solvent, would also take
tries 5 and 6). When the ERCC reaction was performed at
place competitively to provide protonation products 5 and
–20 °C, the yield of protonated product 5 decreased, and
Table 2 Synthesis of 2,3-Dihydrobenzohuran-3-ylacetic Acids 2 by the Present ERCC
R4 R5
R3
CO2H
Br
CO2H
R1 R2
R4
R1
R2
R4
Pt
Mg,
CO2
R1 R2
+
0.1 M Bu4NBF4, DMF
20 mA/cm2, 7 F/mol
mediator 4 (0.5 equiv)
O
R5
O
R5
O
R3
R3
1
2
3
Entry
1c
Substrate 1
Product 2
Temp (°C)
0
Yield (%)a and ratio (2/3)b Conversion (%)b
CO2H
Br
85 [90] (97:3)
80 [89] (98:2)
78 [92] (98:2)
94
90
85
O
O
1b
2b (dr = 9:1)b,d
CO2H
Br
2e
0
0
O
O
O
1c
2c
Br
CO2H
CO2H
CO2H
3
O
2d
1d
4f
0
–20
–40
48 [56] (>99:1)
49 [79] (>99:1)
31 [86] (>99:1)
86
71
36
Br
5g
6h,i
O
O
O
2ej
1e
Br
7k,l
0
33 [45] (>99:1)
73
O
1f
2f
a Isolated yields. The yields based on reacted 1 are shown in brackets.
b Determined by 1H NMR.
c 10 F/mol of electricity was passed.
d Diastereomer ratio is shown as dr. Major isomer is shown.
e 9.2 F/mol of electricity was passed.
f Compounds 5 and 6 were also obtained in 15% and 4% 1H NMR yields, respectively.
g Compounds 5 and 6 were also obtained in 10% and 3% 1H NMR yields, respectively.
h 3.2 F/mol of electricity was passed.
i Compounds 5 and 6 were also obtained in 3% and 1% 1H NMR yields, respectively.
j Compound 2e consists of a 6:4 mixture of diastereomers.
k 6.2 F/mol of electricity was passed.
l Compounds 7 and 8 were also obtained in 25% and 3% 1H NMR yields, respectively.
Synlett 2011, No. 11, 1567–1572 © Thieme Stuttgart · New York

1570
H. Senboku et al.
LETTER
based on reacted 1e reached 86%, and the yields of 5 and
6 successfully decreased (Table 2, entry 6). These results
indicated that reaction temperature strongly affected
yields of the ERCC product when a substrate has alkyl
substituents at the terminal carbon of the allyloxy group.
It is thought that when the reaction using 1e was carried
out at a lower temperature, sufficient stability of anion in-
termediate B to react selectively with carbon dioxide
could be achieved, providing 2e in higher yields. By low-
ering the reaction temperature, a higher concentration of
carbon dioxide than that at 0 °C would be realized, and ac-
celeration of the reaction of anion B with carbon dioxide
would also be a reasonable explanation to improve the
yield and selectivity of the product 2e.
CHO
CHO
O
O
O
O
5
6
7
8
O
O
O
A
B
C
Figure 1 Byproducts in ERCC of 1e and 1f and anion intermediate
The present ERCC reaction was also applicable to con-
structing not only dihydrobenzofurans but also indane,
indoline, and chromane skeletons followed by carboxyla-
tion, yielding acetic acids substituted by those. The results
are summarized in Table 3.
the yield of 2e based on reacted 1e increased (Table 2, en-
try 5) in contrast to the results at 0 °C (Table 2, entry 4). It
was found that lowering the temperature to –40 °C was ef-
fective for improving the yield and selectivity of ERCC
product 2e. Although only 3.2 F/mol of electricity could
be passed, probably due to passivation by precipitation of
magnesium salts on the cathode surface, the yield of 2e
ERCC of bromobenzene 9 having an N-allyl group on the
phenyl ring provided indolin-3-ylacetic acid (10) in 40%
yield with high product selectivity (Table 3, entry 1). Car-
Table 3 Synthesis of Acetic Acid Analogues by the Present ERCC
CO2H
CO2H
Br
X
Pt
Mg,
CO2
+
( )n
( )n
X
( )n
0.1 M Bu4NBF4, DMF
20 mA/cm2, 10 F/mol, 0 °C
mediator 4 (0.5 equiv)
X
direct carboxylation
(DC) product
ERCC product
Entry
1c
Substrate
ERCC product
Yield (%)a and ratio (ERCC/DC)b Conversion (%)b
CO2H
Br
40 [67] (>99:1)
68 [74] (94:6)
60
92
N
N
Boc
Boc
9
10
12
CO2H
Br
2
11
CO2H
Br
3
58 [91] (98:2)
64
71
O
O
O
13
14
16
HO2C
CO2H
Br
4d
60 [85] (>99:1)
O
15
a Isolated yields. The yields based on reacted substrate are shown in brackets.
b Determined by 1H NMR.
c 5.7 F/mol of electricity was passed with 10 mA/cm2 of current density.
d 3 F/mol of electricity was passed.
Synlett 2011, No. 11, 1567–1572 © Thieme Stuttgart · New York

LETTER
Facile Synthesis of 2,3-Dihydrobenzofuran-3-ylacetic Acids
1571
bocycles could also be constructed by the present ERCC –3.2 V vs. Ag/Ag+ in CV of 2-allyloxybromobenzene (1a)
reaction. Thus, indan-1-ylacetic acid (12) was successful- in DMF. When CV of 4 was carried out in the presence of
ly obtained in 68% yield from 2-(3-butenyl)bromoben- 0.5 equivalents of 1a, an increase in reduction peak cur-
zene (11) through cyclopentane formation by aryl radical rent and a decrease in oxidation peak current of 4 were ob-
cyclization followed by a capture of carbon dioxide by the served. Moreover, when an additional 0.5 equivalents of
resulting anion species (Table 3, entry 2). Formation of a 1a (totally a stoichiometric amount for 4) was added to the
six-membered ring by ERCC yielding a chromane skele- electrolyte solution, CV of 4 resulted in a further increase
ton was also carried out. When 2-(3-butenyloxy)bro- in reduction peak current and a disappearance of the oxi-
mobenzene (13) was subjected to ERCC reaction under dation peak of 4.14 These results indicate that methyl 4-
similar conditions, the expected six-membered ring could tert-butylbenzoate (4) works as an electron-transfer medi-
be constructed, and the subsequent carboxylation also ator for highly selective generation of aryl radicals from
took place efficiently to yield chroman-4-lyacetic acid aryl bromides under electroreductive conditions.
(14) in 58% yield with high product selectivity (Table 3,
entry 3). 2-(2-Propynyloxy)bromobenzene (15) was final-
ly tried as a substrate to investigate the possibility of a C–
C triple bond as a radical acceptor yielding a,b-unsaturat-
ed carboxylic acid. Electrolysis of 15 under similar condi-
tions with 3 F/mol of electricity provided not a,b-
unsaturated carboxylic acid but dicarboxylic acid 16 as a
Probable reaction pathways of the present ERCC reaction
are shown in Scheme 1. A one-electron reduction of aryl
halide selectively occurs by a radical anion of mediator 4
to generate aryl radical. In the absence of mediator 4, two-
electron reduction of aryl halide competitively occurs to
generate the corresponding aryl anion, providing directly
carboxylated benzoic acid. On the other hand, dissolution
sole product (Table 3, entry 4). The formation of dicar-
of an Mg anode as magnesium ion proceeds to result in
boxylic acid 16 would be a result of overcarboxylation of
prevention of any species from oxidizing at the an-
early-produced a,b-unsaturated carboxylic acid at the b-
position. A similar b-position-selective carboxylation of
ethyl cinnamate by using Mg metal yielding diethyl 2-
phenylsuccinate after esterification has recently been re-
ported.15
ode.13,16,17
In conclusion, facile synthesis of 2,3-dihydrobenzofuran-
3-ylacetic acids and related analogues was achieved by a
novel electrochemical aryl radical cyclization–carboxyla-
tion sequence (ERCC) of 2-allyloxybromobenzenes by
using methyl 4-tert-butylbenzoate as an electron-transfer
mediator. The results indicate that the present ERCC will
become a powerful tool for the synthesis of 2,3-dihy-
drobenzofuran-3-ylacetic acids and related analogues by
From the viewpoint of reaction pathways, cyclic voltam-
metry (CV) of 1a and 4 was carried out.14 In CV of methyl
4-tert-butylbenzoate (4) alone in DMF, a reversible reduc-
tion peak appeared at –2.9 V vs. Ag/Ag+. On the other
hand, one irreversible reduction peak was observed at
•
CO2Me
Br
O
t-Bu
1a
Mg
anode
Pt
cathode
– Br
CO2Me
e
•
t-Bu
O
Mg2+
4
•
CO2
+ e
CO2
O
O
O
Mg2+
CO2 MgBr
CO2
Mg
CO2H
2
H3O
work-up
and/or
O
O
O
2a
Scheme 1 Reaction pathways
Synlett 2011, No. 11, 1567–1572 © Thieme Stuttgart · New York

1572
H. Senboku et al.
LETTER
M. Chem. Lett. 2003, 32, 720. (b) Kurono, N.; Honda, E.;
Komatsu, F.; Orito, K.; Tokuda, M. Tetrahedron 2004, 60,
1791.
using CO2 without metal reagents, such as Bu3SnH and
SmI2.
(9) At the present stage, they concluded that 9,9-diethylfluorene
does not work as a ‘typical’ mediator in their electro-
reductive radical generation.9d See: (a) Nakagawa, Y.;
Mitsudo, K.; Tanaka, H. The 89th Annual Meeting of the
Chemical Society of Japan Funabashi, March 27th–30th,
2009, Abstr. No. 1F4-34. (b) Mitsudo, K.; Nakagawa, Y.;
Tanaka, H. The 216th Electrochemical Society Meeting
Vienna, Austria, Oct. 4th–9th, 2009, Abstr. No. 1644.
(c) Mizukawa, Z.; Nakagawa, Y.; Mitsudo, K.; Tanaka, H.;
The 77th Annual Meeting of the Electrochemical Society of
Japan Toyama, March 29th–31st, 2010, Abstr. No. 1J08.
(d) Mitsudo, J.; Nakagawa, Y.; Mizukawa, J.; Suga, S.;
Akaba, R.; Tanaka, H. The 217th Electrochemical Society
Meeting Vancouver, BC, Canada, April 25th–30th, 2010,
Abstr. No. 818.
(10) Molander, G. A.; Harris, C. R. Chem. Rev. 1996, 96, 307.
(11) Representative papers: (a) Curran, D. P.; Totleben, M. J.
J. Am. Chem. Soc. 1992, 114, 6050. (b) Molander, G. A.;
McKie, J. A. J. Org. Chem. 1992, 57, 3132. (c) Nomoto,
A.; Koji, Y.; Shiino, G.; Tomisaka, Y.; Mitani, I.; Tatsumi,
M.; Ogawa, A. Tetrahedron Lett. 2010, 51, 6580.
(12) Olivero, S.; Duñach, E. Eur. J. Org. Chem. 1999, 1885.
(13) Recent papers: (a) Senboku, H.; Yamauchi, Y.; Fukuhara,
T.; Hara, S. Electrochemistry 2006, 74, 612. (b) Senboku,
H.; Takahashi, M.; Fukuhara, T.; Hara, S. Chem. Lett. 2007,
36, 228. (c) Yamauchi, Y.; Fukuhara, T.; Hara, S.; Senboku,
H. Synlett 2008, 438. (d) Yamauchi, Y.; Sakai, K.;
Fukuhara, T.; Hara, S.; Senboku, H. Synthesis 2009, 3375.
(e) Yamauchi, Y.; Hara, S.; Senboku, H. Tetrahedron 2010,
66, 473. (f) Senboku, H.; Nakahara, K.; Fukuhara, T.; Hara,
S. Tetrahedron Lett. 2010, 51, 435. (g) Ohkoshi, M.;
Michinishi, J.; Hara, S.; Senboku, H. Tetrahedron 2010, 66,
7732.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
H.S. deeply appreciates partial support by Grant-in-Aid for Scienti-
fic Research (B: No. 22360327) from the Japan Society for the Pro-
motion of Science.
References and Notes
(1) (a) Grimshaw, J.; Trocha-Grimshaw, J. Tetrahedron Lett.
1974, 15, 993. (b) Grimshaw, J.; Haslett, R. J.; Trocha-
Grimshaw, J. J. Chem. Soc., Perkin Trans. 1 1977, 2448.
(c) Grimshaw, J.; Mannus, D. J. Chem. Soc., Perkin Trans.
1 1977, 2456. (d) Grimshaw, J.; Hamilton, R.; Trocha-
Grimshaw, J. J. Chem. Soc., Perkin Trans. 1 1982, 229.
(e) Donnelly, S.; Grimshaw, J.; Trocha-Grimshaw, J.
J. Chem. Soc., Chem. Commun. 1994, 2171.
(2) LeStrat, F.; Murphy, J. A.; Hughes, M. Org. Lett. 2002, 4,
2735.
(3) (a) Ozaki, S.; Matsushita, H.; Ohmori, H. J. Chem. Soc.,
Chem. Commun. 1992, 1120. (b) Ozaki, S.; Matsushita, H.;
Ohmori, H. J. Chem. Soc., Perkin Trans. 1 1993, 2339.
(c) Ozaki, S.; Horiguchi, J.; Matsushita, H.; Ohmori, H.
Tetrahedron Lett. 1994, 35, 725.
(4) Ihara, M.; Katsumata, A.; Setsu, F.; Tokunaga, Y.;
Fukumoto, K. J. Org. Chem. 1996, 61, 677.
(5) (a) Olivero, S.; Clinet, J. C.; Duñach, E. Tetrahedron Lett.
1995, 36, 4429. (b) Clinet, J. C.; Duñach, E. J. Organomet.
Chem. 1995, 503, C48. (c) Gómez, M.; Muller, G.;
Penyella, D.; Rocamora, M.; Duñach, E.; Olivero, S.; Clinet,
J. C. Organometallics 1997, 16, 5900. (d) Olivero, S.;
Rolland, J. P.; Dunãch, E. Organometallics 1998, 17, 3747.
(6) (a) Torii, S.; Inokuchi, T.; Yukawa, T. J. Org. Chem. 1985,
50, 5875. (b) Inokuchi, T.; Kawafuchi, H.; Aoki, K.;
Yoshida, A.; Torii, S. Bull. Chem. Soc. Jpn. 1994, 67, 595.
(7) Munusamy, R.; Dhathathreyan, K. S.; Balasubramanian,
K. K.; Venkatachalam, C. H. J. Chem. Soc., Perkin Trans. 2
2001, 1154.
(14) See Supporting Information.
(15) Maekawa, H.; Murakami, T.; Miyazaki, T.; Nishiguchi, I.
Chem. Lett. 2011, 40, 368; and references cited therein.
(16) (a) Silvestri, G.; Gambino, S.; Filardo, G.; Gulotta, A.
Angew. Chem., Int. Ed. Engl. 1984, 23, 979. (b) Silvestri,
G.; Gambino, S.; Filardo, G. Acta Chem. Scand. 1991, 45,
987.
(17) (a) Sock, O.; Troupel, M.; Périchon, J. Tetrahedron Lett.
1985, 26, 1509. (b) Chaussard, J.; Folest, J. C.; Nédélec, J.
Y.; Périchon, J.; Sibille, S.; Troupel, M. Synthesis 1990, 369.
(8) (a) Kurono, N.; Honda, E.; Komatsu, F.; Orito, K.; Tokuda,
Synlett 2011, No. 11, 1567–1572 © Thieme Stuttgart · New York