Electrochemical carboxylation of benzylic carbonates: Alternative method for efficient synthesis of arylacetic acids

Article abstract of DOI:10.1016/j.tet.2010.07.067

Electrochemical carboxylation of benzylic carbonates was successfully performed as an alternative method for the synthesis of phenylacetic acids by using a one-compartment cell equipped with a Pt plate cathode and an Mg rod anode in CH₃CN to afford the corresponding phenylacetic acids in good yields.

Full text of DOI:10.1016/j.tet.2010.07.067

Tetrahedron 66 (2010) 7732e7737
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Electrochemical carboxylation of benzylic carbonates: alternative method for
efficient synthesis of arylacetic acids
*
Masashi Ohkoshi, Jun-ya Michinishi, Shoji Hara, Hisanori Senboku
Laboratory of Organic Reaction, Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2010
Received in revised form 26 July 2010
Accepted 26 July 2010
Available online 1 August 2010
Electrochemical carboxylation of benzylic carbonates was successfully performed as an alternative
method for the synthesis of phenylacetic acids by using a one-compartment cell equipped with a Pt plate
cathode and an Mg rod anode in CH3CN to afford the corresponding phenylacetic acids in good yields.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Electrochemical reduction
Fixation of carbon dioxide
Benzylic carbonates
Phenylacetic acids
Magnesium anode
1. Introduction
the other hand, has been unexpectedly limited to transition metal-
catalyzed coupling and related reactions8 and only a few applica-
Since arylacetic acids and 2-arylpropanoic acids are well known
to have biological activities, their efficient syntheses have been
widely investigated. Electroorganic synthesis1 is one effective
method for the synthesis of these compounds. Electrochemical
carboxylation (EC) of benzylic halides is known to proceed effi-
ciently under mild conditions even under atmospheric pressure of
carbon dioxide to afford phenylacetic acids in good yields when
a reactive metal, such as magnesium or aluminum, is used as an
anode in the electrolysis.2,3 The reaction involves CeC bond for-
mation between benzylic halides and carbon dioxide at the benzylic
position under electroreductive conditions.1b Carbon dioxide used
as a source of a carboxyl group is not only abundant and economical
but also non-toxic and attractive as an environmentally benign C1
chemical reagent for organic synthesis.4 Carbon monoxide and cy-
anide ion are also effective as sources of a carboxyl group for syn-
thesis of phenylacetic acids and their esters from benzylic halides.
However, theyare unfortunately toxic and must be used with special
caution. While various ECs of benzylic halides,5 including benzylic
fluorides6 have been reported to synthesize phenylacetic acids, only
one example of EC using benzyl carbonate, instead of benzylic ha-
lides, for the synthesis of phenylacetic acid in low yield has been
reported.7 The use of benzylic carbonates for organic synthesis, on
tions to organic synthesis have been reported.9 During the course of
our continuous studies on electroorganic synthesis,5j,6b,c,10 we re-
cently directed our attention to benzylic carbonate as an alternative
substrate for EC yielding phenylacetic acid and found that EC of
various benzylic carbonates took place efficiently under neutral and
mild conditions by constant current electrolysis using a Pt cathode
and an Mg anode in the presence of carbon dioxide in CH3CN or DMF.
We report herein EC of benzylic carbonates as an alternative method
for the synthesis of phenylacetic acids in good yields.
2. Results and discussion
2.1. Screening of reaction conditions
The results of screening of reaction conditions, including sol-
vent, current density, and electricity, using benzyl methyl carbon-
ate (1a-Me) as a substrate are summarized in Table 1. When
constant current electrolysis (25 mA/cm2) of 1a-Me in DMF con-
taining 0.1 M Bu4NBF4 was carried out with 3 F/mol of electricity by
using an undivided cell equipped with a Pt cathode and an Mg
anode in the presence of CO2 at 0 ꢀC, phenylacetic acid (2a) was
obtained in 38% yield and 57% of 1a-Me was recovered (entry 1 in
Table 1). Electrolyses at a higher current density resulted in slight
increases of the yields of 2a (entries 2 and 3). Electrolysis in CH3CN,
instead of DMF, gave a slightly better result (entries 2 and 4), while
EC at a higher current density (65 mA/cm2) in CH3CN was not
* Corresponding author. Tel./fax: þ81 11 706 6555; e-mail address: senboku@
eng.hokudai.ac.jp (H. Senboku).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.07.067

M. Ohkoshi et al. / Tetrahedron 66 (2010) 7732e7737
7733
Table 1
observed, and in all cases, the EC of methyl (1c-Me), ethyl (1c-Et),
isopropyl (1c-i-Pr), tert-butyl (1c-t-Bu), and phenyl carbonates (1c-
Ph) gave carboxylic acid 2c in high yields (entries 5e9). These re-
sults indicated that another alkyl group of benzyl carbonate does
not affect reaction efficiency and product yield in the EC of benzyl
carbonate. p-Cyano- and m-cyanobenzyl carbonates 1d-Me, 1d-Et,
and 1e-Me were also applicable to the present EC to give the cor-
responding carboxylic acids 2d and 2e in high yields.
Screening of reaction conditions in EC of 1a-Me
Entry
Solvent
Current Density
[mA/cm2]
Electricity
[F/mol]
Yield
of 2a [%]
Recovery of
1a-Me [%]
We also investigated EC of benzyl carbonates 3 derived from
secondary benzyl alcohols (a-phenethyl alcohols) as substrates,
1
2
3
4
5
6
7
DMF
DMF
DMF
CH3CN
CH3CN
CH3CN
CH3CN
25
45
65
45
65
45
45
3
3
3
3
3
38
42
46
51
46
72
89
57
46
44
40
45
15
0
and the results are summarized in Table 3. The EC of secondary
benzyl carbonate 3a took place less efficiently than that of primary
benzyl carbonate 1a-Me. While the EC of 1a-Me gave phenylacetic
acid (2a) in 89% yield (entry 7 in Table 1 and entry 1 in Table 2),
similar EC of secondary benzyl carbonate 3a under the same con-
ditions gave 2-phenylpropanoic acid (4a) in only 56% yield along
with 31% recovery of 3a (entry 1 in Table 3). On the other hand,
2-phenylpropanoic acids 4bed were obtained in high yields by
similar EC of secondary benzyl carbonates 3bed having electron-
withdrawing groups on the phenyl ring (entries 2e4). When car-
bonate 3b having an ester group was used as a substrate, 3.7 F/mol
of electricity was sufficient for consumption of the starting car-
bonate 3b to yield carboxylic acid 4b in 95% yield (entry 2). Al-
though the EC of carbonates 3ced having a cyano group needed
4.5 F/mol of electricity for consumption of the substrates, the yields
of carboxylic acids 4ced increased up to 96%.
6
10
effective for improvement of the yield and conversion (entry 5).
Supplying 6 F/mol of electricity resulted in an increase of the yield
of 2a and conversion of 1a-Me (entry 6). Finally, when the elec-
trolysis was carried out at a current density of 45 mA/cm2 with
10 F/mol of electricity at 0 ꢀC, phenylacetic acid (2a) was obtained
in 89% yield with consumption of 1a-Me (entry 7).
2.2. Scope of the EC of benzyl carbonates
We next investigated the scope of the present EC using several
benzylic carbonates derived from primary benzyl alcohols as sub-
strates, and the results are summarized in Table 2. As well as benzyl
methyl carbonate (1a-Me), benzyl ethyl carbonate (1a-Et) was ef-
ficiently electrocarboxylated under the same conditions (entry 2 in
Table 2). When benzyl carbonates 1b-Me and 1b-Et, having an alkyl
group at their para position, were used as substrates, remarkable
decreases of reaction efficiency were observed, and carboxylic acid
2b was obtained in only about 45% yield along with over 50% re-
covery of the substrates (entries 3 and 4). On the other hand, the EC
of benzyl carbonates 1cee having an electron-withdrawing group
on the phenyl group took place more efficiently than that of 1a.
Even 4.5 F/mol of electricity was sufficient for consumption of the
substrate to yield carboxylic acid in high yields. The effect of an-
other alkyl group of benzyl carbonates in the present EC was also
investigated by using benzyl carbonates 1c, having a methoxy-
carbonyl group at the para position of the phenyl group, as sub-
strates. No remarkable influence of another alkyl group was
Table 3
Scope of secondary benzyl methyl carbonate 3 in the present EC
Entry
Substrate
X
Electricity [F/mol]
Product and yield [%]
1
2
3
4
3a
3b
3c
3d
H
10
4a (56)a
4b (95)
4c (95)
4d (96)
p-CO2CH3
p-CN
m-CN
3.7
4.5
4.5
a
Carbonate 3a was recovered in 31% yield.
2.3. EC of cinnamyl carbonates
Similar EC of cinnamyl carbonate 5 was also investigated, and
the results are summarized in Table 4. In all cases, a reductive CeO
Table 2
Scope of primary benzyl carbonate 1 in the present EC
Table 4
EC of cinnamyl carbonates 5
Entry
Substrate
X
R
Electricity
[F/mol]
Product and
yield [%]
1
2
3
4
5
6
7
8
1a-Me
1a-Et
1b-Me
1b-Et
1c-Me
1c-Et
1c-i-Pr
1c-t-Bu
1c-Ph
1d-Me
1d-Et
H
H
CH3
C2H5
CH3
C2H5
CH3
C2H5
i-C3H7
t-C4H9
C6H5
CH3
10
10
10
10
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
2a (89)
2a (93)
2b (44)a
2b (45)b
2c (94)
2c (86)
2c (93)
2c (86)
2c (90)
2d (79)
2d (85)
2e (81)
p-t-C4H9
p-t-C4H9
p-CO2CH3
p-CO2CH3
p-CO2CH3
p-CO2CH3
p-CO2CH3
p-CN
Entry
Substrate
X
R
Product and
yield [%]
Isomer
ratioa (6/7)
1
2
3
4
5
5a
5b
5c
5d
5e
H
F
CH3O
C6H5
H
H
H
H
H
6aþ7a (81)
6bþ7b (84)
6cþ7c (81)
6dþ7d (92)
6eþ7e (87)
31/69
38/62
51/49
18/82
9/91b
9
10
11
12
p-CN
m-CN
C2H5
CH3
1e-Me
CH3
a
a
Carbonate 1b-Me was recovered in 55% yield.
Carbonate 1b-Et was recovered in 53% yield.
Isomer ratio was determined by 1H NMR.
Compound 7e consists of a 82/18 mixture of E and Z isomers.
b
b

7734
M. Ohkoshi et al. / Tetrahedron 66 (2010) 7732e7737
bond cleavage of cinnamyl carbonate 5 at the cinnamylic position
followed by a fixation of CO2 took place smoothly and efficiently
even supply of 3 F/mol of electricity and without any electron-
withdrawing group on their phenyl ring to yield 4-phenyl-3-
butenoic acid 6 and 2-phenyl-3-butenoic acid 7 as an inseparable
mixture in high combined yields.11,12 EC of secondary cinnamyl
carbonate 5e with 3 F/mol of electricity afforded 6e and 7e in 87%
combined yield (entry 5 in Table 4), while similar EC of secondary
benzyl carbonate 3a with 10 F/mol of electricity gave 4a in only 69%
conversion and 56% yield (entry 1 in Table 3). Regioselectivity of
reduction peak at ꢁ2.7 V versus Ag/Agþ in its cyclic voltammetry in
CH3CN. On the other hand, a reduction peak potential of carbon di-
oxide in CH3CN containing 0.1 M Bu4NBF4 appeared at ꢁ3.3 V versus
Ag/Agþ. These results indicate that electrochemical reduction of
benzyl and cinnamyl carbonates would take place more easily than
that of carbon dioxide. It also seems likely that electrochemical re-
duction of carbon dioxide would competitively proceed in the EC of
1a-Me, resulting in an excess amount of electricity (10 F/mol) for
a high yield. The generated anion C attacks carbon dioxide to give
carboxylate ion D. On the other hand, at the anode, dissolution of
magnesium takes place to produce magnesium cation, which cap-
tures carboxylate ion D to give magnesium carboxylate E and/or
F. Acid treatment in workup gives carboxylic acid.
CO2 fixation yielding a-adduct 6 and g-adduct 7 can be rationalized
by considering the relative stability of two anion forms for cinnamyl
anion as shown in Figure 1, which are generated by a two-electron
reduction of cinnamyl carbonate 5, as well as EC of allyl bromides.13
Thus, the anion B generated from 5a is more stable than the anion A
under the present reaction conditions. The addition of more stable
anion B to carbon dioxide gives the corresponding carboxylate
anion of 7a as a major product (entry 1). A fluorine atom at the para
position on the phenyl ring has almost no effect on the stability of
anions A and B, resulting in products in a similar ratio (entry 2). In
the case of 5c, the generated anion B is destabilized by an electron-
donating methoxy group at the para position and the stability of
anions A and B becomes approximately equal to give the same
amount of 7c and 6c (entry 3). On the other hand, in the case of 5d,
the generated anion B is stabilized by a phenyl group at the para
position to yield 7d with a higher ratio than that of 7a (entry 4). In
the reaction using 5e as a substrate, the generated anion A is
a secondary anion and is more unstable than the anion B, resulting
in the formation of 7e with a much higher ratio than that of 7a
(entry 5).
3. Conclusion
In conclusion, we have successfully carried out electrochemical
carboxylation of benzyl carbonates by using a Pt cathode and an Mg
anode to afford phenylacetic acids in high yields. We have also
shown that the present method is applicable to cinnamyl carbon-
ates. The results indicate that the present EC should be useful and
attractive as an alternative method for synthesis of phenylacetic
acids and b,g-unsaturated carboxylic acids.
4. Experimental section
4.1. General
Melting points were measured on a Yanagimoto micro melting
point apparatus and are uncorrected. IR spectra were determined
with a JASCO FT/IR-410 spectrometer in neat form unless otherwise
stated. 1H (270 MHz or 400 MHz) and 13C (67.5 MHz or 100 MHz)
NMR spectra were recorded in CDCl3 with a JEOL EX-270, JEOL ECX-
400P or JEOL A400II FT NMR spectrometer. Unless otherwise stated,
NMR spectra were performed at 400 MHz (1H) and 100 MHz (13C),
Figure 1.
respectively. The chemical shifts, d, are given in parts per million
2.4. Reaction mechanism
with tetramethylsilane as a reference. J values are in hertz. Peak
multiplicitieswere givenas follows: s, singlet; d, doublet; t, triplet;q,
quartet; quint, quintet; m, multiplet. MS spectra were determined
using a JEOL JMS-FABmate, JEOL JMS-T100GC or Thermo Scientific
Exactive. Elemental analyses were performed at Instrumental
Analysis Division, Equipment Management Center, Creative Re-
search Institution Sousei in Hokkaido University. Electrochemical
reactions were carried out using a Constant Current Power Supply
(model 5944), Metronix Corp. Tokyo. Cyclic voltammetry was per-
formed by a Hokuto Denko HSV-100 in CH3CN containing 0.1 M
A probable reaction mechanism in the present EC of benzyl and
cinnamyl carbonates is shown in Scheme 1. At the cathode, two-
electron reduction of the carbonate results in CeO bond cleavage at
the benzylic position to generate benzylic anion C as an in-
termediate. Although cyclic voltammetry of 1a-Me in CH3CN
showed no obvious reduction peak at >ꢁ3.5 V versus Ag/Agþ, cyclic
voltammetry of 1c-Me in CH3CN showed two irreversible reduction
peaks at ꢁ2.8 V versus Ag/Agþ and ꢁ2.4 V versus Ag/Agþ, re-
spectively. Cinnamyl carbonate 5a also showed an irreversible
Bu4NBF4 using a Pt disk electrode (f 1.6 mm) as a working electrode,
Scheme 1.

M. Ohkoshi et al. / Tetrahedron 66 (2010) 7732e7737
7735
a Pt wire (
f
0.5 mm) as a counter electrode, and Ag/Agþ/CH3CN/
J¼8.2 Hz), 8.07 (2H, d, J¼8.2 Hz). 13C NMR:
d
52.2, 69.3, 120.9, 126.1,
Bu4NClO4 (0.01 M AgNO3 in 0.1 M Bu4NClO4 in CH3CN), purchased
from BAS (product code; RE-7), as a reference electrode, re-
spectively, with a scan rate 0.1 V/s. Column chromatography was
carried out using Kanto Kagaku Silica gel 60 N with hexane/EtOAc as
an eluant. Allreagents and solvents werecommerciallyavailable and
were used as received without further purification.
127.9, 129.5, 129.9, 130.3, 139.6, 151.0, 153.5, 166.6. IR (KBr): 1764,
1714, 1247 cmꢁ1. HRMS (ESI): m/z [MþNa]þ calcd for C16H14O5Na
309.0733. Found 309.0733.
4.2.9. p-Cyanobenzyl methyl carbonate (1d-Me). Yield: 89%. Mp:
91e93 ꢀC. 1H NMR:
d
3.82 (3H, s), 5.21 (2H, s), 7.49 (2H, d, J¼8.2 Hz),
7.67 (2H, d, J¼8.2 Hz). 13C NMR:
d 55.1, 68.1,112.2,118.4,128.2, 132.3,
4.2. Preparation of benzyl carbonates 1, 3, and 5; general
procedure except for 1c-t-Bu
140.4, 155.4. IR (KBr): 2965, 2225, 1746, 1383, 1293 cmꢁ1. HRMS
(EI): m/z [M]þ calcd for C10H9NO3 191.0582. Found 191.0581. Anal.
Calcd for C10H9NO3: C, 62.82; H, 4.74; N, 7.33. Found: C, 62.87; H,
4.74; N, 7.33.
To a solution of an alcohol (10 mmol) and pyridine (11 mmol) in
60 mL of CH2Cl2 was added dropwise an appropriate chloroformate
(ClCO2R; R¼CH3, C2H5, i-C3H7, or C6H5, 18 mmol) at 0 ꢀC. After stirring
at room temperature for 2 h, 1 M HCl (30 mL) was added to the solu-
tion. Then the resulting mixture was extracted with CH2Cl2 (30 mLꢂ3)
and the combined organic layer was washed with 1 M HCl (50 mL).
After drying over MgSO4, evaporation of the solvent gave a crude
product, which was purified by recrystallization with hexane/EtOAc or
by column chromatography on silica gel to obtain a pure carbonate.
4.2.10. p-Cyanobenzyl ethyl carbonate (1d-Et). Yield: 87%. Mp:
56e58 ꢀC. 1H NMR:
d
1.33 (3H, t, J¼7.3 Hz), 4.24 (2H, q, J¼7.3 Hz),
5.20 (2H, s), 7.49 (2H, d, J¼8.2 Hz), 7.67 (2H, d, J¼8.2 Hz). 13C NMR:
d
14.2, 64.5, 68.0, 112.2, 118.5, 128.2, 132.4, 140.6, 155.8. IR (KBr):
2225, 1738, 1264, 1020 cmꢁ1. HRMS (EI): m/z [M]þ calcd for
C11H11NO3 205.0739. Found 205.0739. Anal. Calcd for C11H11NO3: C,
64.38; H, 5.40; N, 6.83. Found: C, 64.42; H, 5.38; N, 6.83.
4.2.1. Benzyl methyl carbonate (1a-Me)14. Yield: 97%. 1H NMR
4.2.11. m-Cyanobenzyl methyl carbonate (1e-Me). Yield: 86%. Mp:
(270 MHz):
d
3.79 (3H, s), 5.16 (2H, s), 7.34e7.37 (5H, m).
43e44 ꢀC. 1H NMR:
d
3.83 (3H, s), 5.19 (2H, s), 7.50 (1H, t, J¼7.8 Hz),
7.65e7.61 (2H, m), 7.69 (1H, s). 13C NMR:
d
55.1, 68.0, 112.8, 118.3,
4.2.2. Benzyl ethyl carbonate (1a-Et)14. Yield: 96%. 1H NMR
129.5, 131.5, 132.0, 132.3, 136.8, 155.4. IR (KBr): 2970, 2233, 1746,
1441, 1282 cmꢁ1. HRMS (EI): m/z [M]þ calcd for C10H9NO3 191.0582.
Found 191.0582. Anal. Calcd for C10H9NO3: C, 62.82; H, 4.74; N, 7.33.
Found: C, 62.80; H, 4.68; N, 7.33.
(270 MHz):
s), 7.34e7.39 (5H, m).
d
1.31 (3H, t, J¼7.0 Hz), 4.21 (2H, d, J¼7.0 Hz), 5.16 (2H,
4.2.3. p-tert-Butylbenzyl methyl carbonate (1b-Me). Yield: 86%. 1H
NMR:
d
1.32 (9H, s), 3.79 (3H, s), 5.14 (2H, s), 7.33 (2H, d, J¼8.3 Hz),
4.2.12. Methyl (1-phenyl)ethyl carbonate (3a)15. Yield: 93%. 1H
7.39 (2H, d, J¼8.3 Hz). 13C NMR:
d
31.2, 34.6, 54.8, 69.5, 125.5, 128.2,
NMR:
7.29e7.39 (5H, m).
d
1.59 (3H, d, J¼6.6 Hz), 3.75 (3H, s), 5.73 (1H, q, J¼6.6 Hz),
132.2, 151.6, 155.7. IR: 3041, 2790, 2361, 2341, 1750, 1270 cmꢁ1
.
HRMS (EI): m/z [M]þ calcd for C13H18O3 222.1256. Found 222.1260.
4.2.13. 1-(p-Methoxycarbonylphenyl)ethylmethylcarbonate(3b). Yield:
4.2.4. p-tert-Butylbenzyl ethyl carbonate (1b-Et). Yield: 92%. 1H
60% (67% conversion). 1H NMR:
d
1.59 (3H, d, J¼6.6 Hz), 3.77 (3H, s),
NMR:
d
1.30 (3H, t, J¼7.3 Hz), 1.31 (9H, s), 4.20 (2H, q, J¼7.3 Hz), 5.13
3.91 (3H, s), 5.76 (1H, q, J¼6.6 Hz), 7.44 (2H, d, J¼8.3 Hz), 8.03 (2H,
(2H, s), 7.33 (2H, d, J¼8.3 Hz), 7.39 (2H, d, J¼8.3 Hz). 13C NMR:
d
14.0,
d, J¼8.3 Hz). 13C NMR:
d 22.1, 51.9, 54.5, 75.5, 125.6, 129.6, 129.7,
31.1, 34.3, 63.7, 69.0, 125.2, 128.0, 132.3, 151.2, 154.9. IR: 2963, 1746,
1379,1364,1261,1010 cmꢁ1. HRMS (EI): m/z [M]þ calcd for C14H20O3
236.1412. Found 236.1416.
145.9, 154.8, 166.4. IR: 2988, 2952, 1749, 1719, 1267 cmꢁ1. HRMS
(EI): m/z [M]þ calcd for C12H14O5 238.0841. Found 238.0849.
4.2.14. 1-(p-Cyanophenyl)ethyl methyl carbonate (3c). Yield: 45%
4.2.5. p-Methoxycarbonylbenzyl methyl carbonate (1c-Me). Yield:
(52% conversion). Mp: 40e43 ꢀC. 1H NMR:
d
1.59 (3H, d, J¼6.6 Hz),
93%. Mp: 100e103 ꢀC. 1H NMR:
d
3.82 (3H, s), 3.92 (3H, s), 5.22 (2H,
3.78 (3H, s), 5.74 (1H, q, J¼6.6 Hz), 7.47 (2H, d, J¼8.2 Hz), 7.66 (2H, d,
s), 7.45 (2H, d, J¼8.3 Hz), 8.04 (2H, d, J¼8.3 Hz). 13C NMR:
d
52.1,
J¼8.2 Hz). 13C NMR:
d 22.2, 54.9, 75.31, 111.9, 118.5, 126.5, 132.5,
55.0, 68.7, 127.6, 129.8, 130.1, 140.2, 155.5, 166.6. IR (KBr): 2959,
2360, 2341, 1748, 1716, 1269 cmꢁ1. HRMS (EI): m/z [M]þ calcd for
C11H12O5 224.0685. Found 224.0681. Anal. Calcd for C11H12O5: C,
58.93; H, 5.39. Found: C, 58.83; H, 5.36.
146.3, 154.9. IR (KBr): 2985, 2227, 1753, 1442, 1267, 1056, 1010, 940,
892, 836, 790 cmꢁ1. HRMS (EI): m/z [M]þ calcd for C11H11NO3
205.0739. Found 205.0737. Anal. Calcd for C11H11NO3: C, 64.38; H,
5.40; N, 6.83. Found: C, 64.44; H, 5.38; N, 6.84.
4.2.6. Ethyl p-methoxycarbonylbenzyl carbonate (1c-Et). Yield: 96%.
4.2.15. 1-(m-Cyanophenyl)ethyl methyl carbonate (3d). Yield: 71%
1H NMR:
d
1.32 (3H, t, J¼7.2 Hz), 3.92 (3H, s), 4.23 (2H, q, J¼7.2 Hz),
(76% conversion). 1H NMR:
d
1.60 (3H, d, J¼6.6 Hz), 3.78 (3H, s), 5.73
5.21 (2H, s), 7.45 (2H, d, J¼8.2 Hz), 8.04 (2H, d, J¼8.2 Hz). 13C NMR:
(1H, q, J¼6.6 Hz), 7.48 (1H, t, J¼7.7 Hz), 7.62e7.60 (2H, m), 7.67 (1H,
d
14.2, 52.1, 64.3, 68.5, 127.6, 129.8, 130.0, 140.3, 154.9, 166.6. IR:
s). 13C NMR:
d 21.8, 54.5, 74.7, 112.3, 118.1, 129.1, 129.2, 130.1, 131.3,
2989, 2959, 1745, 1719, 1256 cmꢁ1. HRMS (EI): m/z [M]þ calcd for
142.3, 154.5. IR: 2986, 2959, 2230, 1746, 1264 cmꢁ1. HRMS (EI): m/z
C12H14O5 238.0841. Found 238.0840.
[M]þ calcd for C11H11NO3 205.0739. Found 205.0739.
4.2.7. Isopropyl p-methoxycarbonylbenzyl caebonate (1c-i-Pr). Yield:
4.2.16. Methyl 3-phenyl-2-propenyl carbonate (5a)16. Yield: 96%. 1H
85%. Mp: 30e33 ꢀC. 1H NMR:
d
1.31 (6H, d, J¼6.2 Hz), 3.92 (3H, s),
NMR (270 MHz):
(1H, dt, J¼6.4 and 16.0 Hz), 6.69 (1H, d, J¼16.0 Hz), 7.23e7.42 (5H, m).
d
3.81 (3H, s), 4.79 (2H, dd, J¼1.2 and 6.4 Hz), 6.30
4.90 (1H, sept, J¼6.2 Hz), 5.19 (2H, s), 7.45 (2H, d, J¼8.3 Hz), 8.04
(2H, d, J¼8.2 Hz). 13C NMR:
d 21.7, 52.1, 68.4, 72.4, 127.7, 129.8, 130.0,
140.4, 154.4, 166.7. IR (KBr): 2973, 1744, 1726, 1276, 1110 cmꢁ1
.
4.2.17. 3-(p-Fluorophenyl)-2-propenyl methyl carbonate (5b). Yield:
HRMS (EI): m/z [M]þ calcd for C13H15O5 252.0998. Found 252.1004.
90%. 1H NMR (270 MHz):
d
3.82 (3H, s), 4.78 (2H, d, J¼4.4 Hz), 6.22
(1H, dt, J¼4.4 and 10.8 Hz), 6.66 (1H, d, J¼10.8 Hz), 7.02 (2H, t,
4.2.8. p-Methoxycarbonylbenzyl phenyl carbonate (1cePh). Yield:
J¼5.9 Hz), 7.36 (2H, dd, J¼3.7 and 5.9 Hz). 13C NMR:
d 54.8, 68.2,
93%. Mp: 80e83 ꢀC. 1H NMR:
d
3.93 (3H, s), 5.32 (2H, s), 7.17e7.19
82.2, 115.5 (2C, d, J¼21.5 Hz), 122.1 (d, J¼2.1 Hz), 128.2 (2C, d,
J¼8.2 Hz), 133.5, 155.6, 162.6 (d, J¼246.3 Hz). IR: 1751, 1509, 1442,
(2H, m), 7.23e7.27 (1H, m), 7.37e7.41 (2H, m), 7.50 (2H, d,

7736
M. Ohkoshi et al. / Tetrahedron 66 (2010) 7732e7737
1281, 954, 852, 795 cmꢁ1. HRMS (GC): m/z [M]þ calcd for C11H11O3F
210.0692. Found 210.0688.
194.0570. Anal. Calcd for C10H10O4: C, 61.85; H, 5.19. Found: C,
61.69; H, 5.01.
4.2.18. 3-(p-Methoxyphenyl)-2-propenyl
methyl
3.80 (3H, s), 3.81 (3H, s),
carbonate
4.4.4. p-Cyanophenyl acetic acid (2d). Mp: 152e154 ꢀC. 1H NMR:
d
(5c). Yield: 93%. Mp: 81e83 ꢀC. 1H NMR:
d
3.73 (2H, s), 7.41 (2H, d, J¼8.2 Hz), 7.64 (2H, d, J¼8.2 Hz). 13C NMR:
41.1, 111.0, 118.7, 130.3, 132.3, 139.4, 173.7. IR (KBr): 3600e2400,
4.77 (2H, dd, J¼1.2 and 6.7 Hz), 6.17 (1H, dt, J¼6.6 and 15.9 Hz), 6.64
d
(1H, d, J¼15.9 Hz), 6.86 (2H, d, J¼8.7 Hz), 7.33 (2H, d, J¼8.7 Hz). 13C
2229, 1696, 1251 cmꢁ1. HRMS (EI): m/z [M]þ calcd for C9H7NO2
161.0477. Found 161.0477. Anal. Calcd for C9H7NO2: C, 67.07; H,
4.38; N, 8.69. Found: C, 66.90; H, 4.38; N, 8.59.
NMR:
d 54.8, 55.3, 68.7, 114.0, 120.0, 127.9, 128.7, 134.7, 155.7, 159.7.
IR: 1751, 1607, 1515, 1441, 1386, 1274, 1177, 1032, 953, 795 cmꢁ1
.
Anal. Calcd for C12H14O4: C, 64.85; H, 6.35. Found: C, 64.71; H, 6.33.
4.4.5. m-Cyanophenyl acetic acid (2e). Mp: 116e118 ꢀC. 1H NMR:
4.2.19. Biphenyl-4-yl methyl carbonate (5d)16a. Yield: 90%. 1H NMR:
d
3.71 (2H, s), 7.46 (1H, t, J¼7.8 Hz), 7.52e7.56 (1H, m), 7.59e7.61 (2H,
d
3.82 (3H, s), 4.81 (2H, dd, J¼1.0 and 6.3 Hz), 6.34 (1H, dt, J¼6.3 and
m).13C NMR:
d 40.3,112.7,118.4,129.4,131.1,133.0,134.0,134.5,176.7.
16.0 Hz), 6.73 (1H, d, J¼16.0 Hz), 7.34e7.61 (9H, m).
IR (KBr): 3700e2400, 2228, 1697, 1412, 1223 cmꢁ1. HRMS (EI): m/z
[M]þ calcd for C9H7NO2 161.0477. Found 161.0476. Anal. Calcd for
C9H7NO2: C, 67.07; H, 4.38; N, 8.69. Found: C, 66.89; H, 4.41; N, 8.62.
4.2.20. Methyl 1-methyl-4-phenyl-2-propenyl
carbonate
(5e)17. Yield: 85%. 1H NMR:
d
1.47 (3H, d, J¼6.6 Hz), 3.78 (3H, s),
5.37 (dquint, J¼0.9 and 6.6 Hz), 6.20 (1H, dd, J¼7.0 and 15.9 Hz),
4.4.6. 2-Phenylpropanoic acid (4a). 1H NMR:
J¼7.2 Hz), 3.74 (1H, q, J¼7.2 Hz), 7.24e7.35 (5H, m).
d
1.51 (3H, d,
6.65 (1H, d, J¼15.9 Hz), 7.21e7.41 (5H, m).
4.3. Preparation of p-methoxycarbonylbenzyl tert-butyl
carbonate (1c-t-Bu)
4.4.7. 2-(4-Methoxycarbonylphenyl)propanoic acid
(4b). Mp:
85e87 ꢀC. 1H NMR:
d
1.54 (3H, d, J¼7.2 Hz), 3.81 (1H, q, J¼7.2 Hz),
3.91 (3H, s), 7.39 (2H, d, J¼8.3 Hz), 8.01 (2H, d, J¼8.3 Hz). 13C NMR:
A solution of methyl 4-(hydroxymethyl)benzoate (10 mmol),
Boc2O (11 mmol), and Zn(OAc)2$2H2O (1 mmol, 10 mol %) in CH2Cl2
(10 mL) was heated under reflux for 16 h. After the solution has
been poured into ice-water, the resulting mixture was extracted
with CH2Cl2 (30 mLꢂ3). The combined organic layer was dried over
MgSO4 and the solvent was evaporated to give a crude product,
which was subjected to column chromatography on silica gel to
give t-butyl carbonate 1c-t-Bu (2.23 g, 84%).
d 18.0, 45.3, 52.1, 127.7, 129.3, 130.0, 144.7, 166.8, 179.8. IR (KBr):
3700e2400, 1722, 1703, 1287 cmꢁ1. HRMS (EI): m/z [M]þ calcd for
C11H12O4 208.0736. Found 208.0738. Anal. Calcd for C11H12O4: C,
63.45; H, 5.81. Found: C, 63.33; H, 5.82.
4.4.8. 2-(4-Cyanophenyl)propanoic acid (4c). Mp: 103e105 ꢀC. 1H
NMR:
d
1.54 (3H, d, J¼7.2 Hz), 3.81 (1H, q, J¼7.2 Hz), 7.44 (2H, d,
J¼8.3 Hz), 7.63 (2H, d, J¼8.3 Hz). 13C NMR:
d 17.9, 45.3, 111.4, 118.5,
1H NMR:
d
1.50 (9H, s), 3.92 (3H, s), 5.14 (2H, s), 7.44 (2H, d,
128.5, 132.5, 144.8, 179.4. IR (KBr): 3500e2400, 2228, 1700,
1228 cmꢁ1. HRMS (EI): m/z [M]þ calcd for C10H9NO2 175.0633.
Found 175.0633. Anal. Calcd for C10H9NO2: C, 68.56; H, 5.18; N, 8.00.
Found: C, 68.27; H, 5.05; N, 7.89.
J¼8.2 Hz), 8.03 (2H, d, J¼8.2 Hz). 13C NMR:
d 27.6, 52.0, 67.7, 82.4,
127.5, 129.7, 129.8, 140.6, 153.1, 166.5. IR: 2981, 1744, 1727,
1277 cmꢁ1
.
HRMS (ESI): m/z [MþNa]þ calcd for C14H18O5Na
289.1046. Found 289.1048.
4.4.9. 2-(3-Cyanophenyl)propanoic acid (4d). Mp: 85e87 ꢀC. 1H
4.4. Electrochemical carboxylation of benzyl carbonates 1, 3
and cinnamyl carbonates 5; general procedure
NMR:
d
1.55 (3H, d, J¼7.2 Hz), 3.79 (1H, q, J¼7.2 Hz), 7.46 (1H, t,
J¼7.8 Hz), 7.56e7.59 (2H, m), 7.63 (1H, s). 13C NMR:
d 18.0, 44.9,
112.8, 118.5, 129.5, 131.2, 131.3, 132.2, 140.9, 179.5. IR (KBr):
3600e2400, 2229, 1697 cmꢁ1. HRMS (EI): m/z [M]þ calcd for
C10H9NO2 175.0633. Found 175.0633. Anal. Calcd for C10H9NO2: C,
68.56; H, 5.18; N, 8.00. Found: C, 68.64; H, 5.17; N, 7.97.
A solution of benzyl carbonate 1 (1.0 mmol) in anhyd CH3CN
(10 mL for 1 and 3; 12 mL for 5) containing Bu4NBF4 (0.1 M) was
electrolyzed at 0 ꢀC with a constant current (45 mA/cm2) under
atmospheric pressure of bubbling carbon dioxide. An undivided cell
equipped with a Pt plate cathode (2ꢂ2 cm2) and an Mg rod anode
4.4.10. 4-Phenyl-3-butenoic acid (6a)19 and 2-phenyl-3-butenoic
(f
3 mm) was used for the electrolysis. After an appropriate
acid (7a)20. Yield: 81%. 1H NMR (270 MHz):
d
3.29 (0.6H, dd, J¼1.1
amount of electricity was passed (shown in tables), the electrolyzed
solution was poured into 1 M HCl (50 mL) and then extracted with
Et2O (3ꢂ30 mL). The combined ethereal solution was washed with
satd NaHCO3 (3ꢂ40 mL). The resulting aqueous solution was acid-
ified with 3 M HCl and then extracted with Et2O (3ꢂ30 mL). The
combined ethereal solution was washed with satd brine and dried
over MgSO4. Evaporation of the solvent gave an almost pure car-
boxylic acid 2.
and 7.2 Hz), 4.33 (0.7H, d, J¼7.8 Hz), 5.16e5.27 (1.4H, m), 6.15e6.33
(1H, m), 5.52 (0.3H, d, J¼15.9 Hz), 7.16e7.39 (5H, m).
4.4.11. 4-(p-Fluorophenyl)-3-butenoic acid (6b)19 and 2-(p-fluo-
rophenyl)-3-butenoic acid (7b). Yield: 84%. 1H NMR (270 MHz):
d
3.28 (0.8H, dd, J¼1.3 and 7.1 Hz), 4.31 (0.6H, d, J¼7.8 Hz), 5.17
(0.6H, dt, J¼1.1 and 17.1 Hz), 5.26 (0.6H, dt, J¼1.1 and 10.2 Hz),
6.11e6.24 (1H, m), 6.48 (0.4H, d, J¼16.0 Hz), 6.96e7.36 (5H, m).
4.4.1. Phenylacetic acid (2a). 1H NMR (270 MHz):
7.27e7.37 (5H, m).
d
3.65 (2H, s),
4.4.12. 4-(p-Methoxyphenyl)-3-butenoic acid (6c)19 and 2-(p-me-
thoxyphenyl)-3-butenoic acid (7c). Yield: 81%. 1H NMR (270 MHz):
d
3.27(1H,dd, J¼1.4and7.1 Hz), 3.79(1.5H, s), 3.80(1.5H, s), 4.28(0.5H,
4.4.2. p-tert-Butylphenyl acetic acid (2b)18
3.62 (2H, s), 7.22 (2H, d, J¼8.4 Hz), 7.35 (2H, d, J¼8.4 Hz).
.
1H NMR:
d
1.31 (9H, s),
d, J¼7.9 Hz), 5.17 (0.5H, dt, J¼1.1 and 17.1 Hz), 5.23 (0.5H, dt, J¼1.1 and
10.2 Hz), 6.12 (0.5H, dt, J¼7.1 and 15.9 Hz), 6.19 (0.5H, ddd, J¼7.9, 10.2
and 17.1 Hz), 6.46 (0.5H, d, J¼15.9 Hz), 6.85 (1H, d, J¼8.9 Hz), 6.88 (1H,
d, J¼8.9 Hz), 7.24 (1H, d, J¼8.9 Hz), 7.31 (1H, d, J¼8.9 Hz).
4.4.3. p-Methoxycarbonylphenyl acetic acid (2c). Mp: 112e115 ꢀC.
1H NMR:
d
3.72 (2H, s), 3.91 (3H, s), 7.36 (2H, d, J¼8.2 Hz), 8.01 (2H,
d, J¼8.2 Hz). 13C NMR:
d
40.8, 52.2, 129.3, 129.5, 129.9, 138.2, 166.8,
4.4.13. 4-(Biphenyl-4-yl)-3-butenoic acid (6d) and 2-(biphenyl-4-
176.8. IR (KBr): 3500e2500, 1718, 1426, 1409, 1285, 1238, 1107, 763,
714 cmꢁ1. HRMS (EI): m/z [M]þ calcd for C10H10O4 194.0579. Found
yl)-3-butenoic acid (7d). Yield: 92%. 1H NMR (270 MHz):
d
3.30
(0.4H, dd, J¼0.9 and 7.0 Hz), 4.37 (0.8H, d, J¼8.0 Hz), 5.19e5.29

M. Ohkoshi et al. / Tetrahedron 66 (2010) 7732e7737
7737
Zapf, A.; Beller, M. Angew. Chem., Int. Ed. 2005, 44, 238; (e) Iovel, I.; Mertins, K.;
Kischel, J.; Zapf, A.; Beller, M. Angew. Chem., Int. Ed. 2005, 44, 3913; (f) Kuwano, R.;
Yokogi, M. Org. Lett. 2005, 7, 945; (g) Mertins, K.; Iovel, I.; Kischel, J.; Zapf, A.;
Beller, M. Adv. Synth. Catal. 2006, 348, 691; (h) Molander, G. A.; Elia, M. D. J. Org.
Chem. 2006, 71, 9198; (i) Kuwano, R.; Yu, J.-Y. Heterocycles 2007, 74, 233; (j)
Nakao, Y.; Ebata, S.; Chen, J.; Imanaka, H.; Hiyama, T. Chem. Lett. 2007, 36, 606; (k)
Kuwano, R.; Shige, T. J. Am. Chem. Soc. 2007, 129, 3802; (l) Kuwano, R.; Kusano, H.
Org. Lett. 2008, 10, 1979; (m) Ohsumi, M.; Kuwano, R. Chem. Lett. 2008, 37, 796;
(n) Ueno, S.; Ohtsubo, M.; Kuwano, R. J. Am. Chem. Soc. 2009, 131, 12904; (o) Xu,
X.; Xu, X.; Li, H.; Xie, X.; Li, Y. Org. Lett. 2010, 12, 100; (p) Mukai, T.; Hirano, K.;
Satoh, T.; Miura, M. Org. Lett. 2010, 12, 1360.
(1.6H, m), 6.24 (0.8H, ddd, J¼8.0, 10.1 and 17.1 Hz), 6.30 (0.2H, dt,
J¼7.0 and 15.9 Hz), 6.54 (0.2H, d, J¼15.9 Hz), 7.22e7.60 (9H, m).
4.4.14. 4-Phenyl-2-methyl-3-butenoic acid (6e)21 and (E)- and (Z)-2-
phenyl-3-pentenoic acid (7e)22. Yield:87%.1H NMR (270 MHz):
d1.38
(0.3H, d, J¼7.1 Hz),1.67e1.73 (2.7H, m), 3.33 (0.1H, m), 4.27 (0.73H, d,
J¼8.3 Hz), 4.62 (0.17H, d, J¼9.4 Hz), 5.55e5.94 (1.8H, m), 6.26 (0.1H,
J¼7.9 and 16.0 Hz), 6.51 (0.1H, d, J¼16 Hz), 7.23e7.39 (5H, m).
9. Perosa, A.; Selva, M.; Tundo, P.; Zordan, F. Synlett 2000, 272.
10. (a) Kamekawa, H.; Senboku, H.; Tokuda, M. Electrochim. Acta 1997, 42, 2117; (b)
Tokuda, M.; Yoshikawa, A.; Suginome, H.; Senboku, H. Synthesis 1997, 1143; (c)
Kamekawa, H.; Kudo, H.; Senboku, H.; Tokuda, M. Chem. Lett. 1997, 26, 917; (d)
Kamekawa, H.; Senboku, H.; Tokuda, M. Tetrahedron Lett. 1998, 39, 1591; (e)
Senboku, H.; Fujimura, Y.; Kamekawa, H.; Tokuda, M. Electrochim. Acta 2000, 45,
2995; (f) Senboku, H.; Komatsu, H.; Fujimura, Y.; Tokuda, M. Synlett 2001, 418;
(g) Senboku, H.; Kanaya, H.; Fujimura, Y.; Tokuda, M. J. Electroanal. Chem. 2001,
507, 82; (h) Senboku, H.; Kanaya, H.; Tokuda, M. Synlett 2002, 140; (i)
Chowdhury, M. A.; Senboku, H.; Tokuda, M. Tetrahedron 2004, 60, 475; (j)
Kuang, C.; Yang, Q.; Senboku, H.; Tokuda, M. Chem. Lett. 2005, 34, 528; (k)
Senboku, H.; Yamauchi, Y.; Fukuhara, T.; Hara, S. Electrochemistry 2006, 74, 612;
(l) Senboku, H.; Takahashi, M.; Fukuhara, T.; Hara, S. Chem. Lett. 2007, 36, 228;
(m) Senboku, H.; Nakahara, K.; Fukuhara, T.; Hara, S. Tetrahedron Lett. 2010, 51,
435.
References and notes
1. Recent reviews: (a) Yoshida, J.; Kataoka, K.; Horcajada, R.; Nagaki, A. Chem. Rev.
2008, 108, 2265; (b) Torii, S. Electroorganic Reduction Synthesis; Wiley-VCH:
Weinheim, 2006.
2. (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.
3. (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.
4. (a) Sakakura, T.; Choi, J.-C.; Yasuda, H. Chem. Rev. 2007, 107, 2365; (b) Louie,
J. Curr. Org. Chem. 2005, 9, 605; (c) Aresta, M.; Dibenedettob, A. Dalton
Trans. 2007, 2975.
5. Representative papers: (a) Folest, J.-C.; Duprilot, J.-M.; Périchon, J.; Robin, Y.;
Devynck, J. Tetrahedron Lett. 1985, 26, 2633; (b) Fauvarque, J. F.; Jutand, A.;
Francois, M. Nouv. J. Chim. 1986, 10, 119; (c) Chaussard, J.; Troupel, M.; Robin, Y.;
Jacob, G.; Juhasz, J. P. J. Appl. Electrochem.1989,19, 345; (d) Isse, A. A.; Gennaro, A.;
Vianello, E. J. Chem. Soc., DaltonTrans.1996,1613; (e) Isse, A. A.; Gennaro, A. Chem.
Commun. 2002, 2798; (f) Stepanov, A. A.; Volodin, Y. Y.; Grachev, M. K.;
Kurochkina, G. I.; Syrtsev, A. N.; Grinberg, V. A. Russ. J. Electrochem. 2002, 38,
1346; (g) Damodar, J.; Krishna Mohan, S.; Khaja Lateef, S.; Jayarama Reddy, S.
Synth. Commun. 2005, 35, 1143; (h) Ramesh Raju, R.; Khaja Lateef, S.; Krishna
Mohan, S.; Jayarama Reddy, S. Arkivoc 2006, 76; (i) Scialdone, O.; Galia, A.;
Errante, G.; Isse, A. A.; Gennaro, A.; Filardo, G. Electrochim. Acta 2008, 53, 2514; (j)
Yamauchi, Y.; Hara, S.; Senboku, H. Tetrahedron 2010, 66, 473.
11. Torii, S.; Tanaka, H.; Hamatani, T.; Morisaki, K.; Jutand, A.; Pfluger, F.; Fauvarque,
J.-F. Chem. Lett. 1986, 15, 169.
12. Characterization of carboxylic acids 6 and 7 was carried out by comparison
with reported 1H NMR data of 6aec,19 6e,21 7a,20 (E)-7e22a and ester of 7e,22b
respectively.
13. Tokuda, M.; Kabuki, T.; Katoh, Y.; Suginome, H. Tetrahedron Lett. 1995, 36, 3345.
14. Bakhtiar, C.; Smith, E. H. J. Chem. Soc., Perkin Trans. 1 1994, 239.
15. Verdecchia, M.; Feroci, M.; Palombi, L.; Rossi, L. J. Org. Chem. 2002, 67, 8287.
16. (a) Lehmann, J.; Lloyd-Jones, G. C. Tetrahedron 1995, 51, 8863; (b) Glorius, F.;
Pfaltz, A. Org. Lett. 1999, 1, 141.
17. Trost, B. M.; Richardson, J.; Yong, K. J. Am. Chem. Soc. 2006, 128, 2540.
18. Bachir, L.; Wood, E.; Casida, J. E. Chem. Res. Toxicol. 1996, 9, 445.
19. Thibonnet, J.; Abarbri, M.; Parrain, J.-L.; Duchêne, A. Tetrahedron 2001, 59, 4433.
20. Friedrich, L. E.; Cormier, R. A. J. Org. Chem. 1971, 36, 3011.
21. (a) Salamonczyk, G. M.; Han, K.; Guo, Z.-W.; Sih, C. J. J. Org. Chem. 1996, 61, 6839;
(b) Hanekamp, J. C.; Rookhuizen, R. B.; Bos, H. J. T.; Brandsma, L. Tetrahedron
1992, 48, 5151.
6. (a) Saboureau, C.; Troupel, M.; Sibille, S.; Périchon, J. J. Chem. Soc., Chem.
Commun. 1989, 1138; (b) Yamauchi, Y.; Fukuhara, T.; Hara, S.; Senboku, H.
Synlett 2008, 438; (c) Yamauchi, Y.; Sakai, K.; Fukuhara, T.; Hara, S.; Senboku, H.
Synthesis 2009, 3375.
7. Gal, J.; Folest, J. C.; Troupel, M.; Moingeon, M. O.; Chaussard, J. New J. Chem.
1995, 19, 401.
22. (a) For (E)-7e: van Noort, P. C. M.; Cerfontain, H. J. Chem. Soc., Perkin Trans. 2
1979, 249; (b) For ester of 7e: Takimoto, M.; Mori, M. J. Am. Chem. Soc. 2001, 123,
2895.
8. (a) Kuwano, R.; Kondo, Y.; Matsuyama, Y. J. Am. Chem. Soc. 2003, 125, 12104; (b)
Kuwano, R.; Kondo, Y. Org. Lett. 2004, 6, 3545; (d) Mertins, K.; Iovel, I.; Kischel, J.;