Reductive esterification of aromatic aldehydes using Zn/Ac 2O/imidazole or Zn/Yb(OTf)3/(RCO)2O system

Article abstract of DOI:10.1016/j.tet.2003.10.068

Benzaldehydes are reduced by metallic zinc in the presence of Ac ₂O and imidazole, giving the corresponding benzyl acetates in good yields. Reductive esterification of aromatic aldehydes is also carried out via gem-diacetoxy compounds. Carbonyl compounds are readily converted to the gem-diacyloxy compounds in excellent yields on treatment with 2molar amounts of acid anhydride and 10mol% of Yb(OTf)₃ in MeCN at room temperature. Thus-formed diacyloxy compounds derived from aromatic aldehydes are reduced in situ by metallic zinc to afford the corresponding esters.

Full text of DOI:10.1016/j.tet.2003.10.068

Tetrahedron 59 (2003) 10147–10152
Reductive esterification of aromatic aldehydes using
Zn/Ac O/imidazole or Zn/Yb(OTf) /(RCO) O system
2
3
2
*
Toshikazu Hirao, Sirida Santhitikul, Hiroki Takeuchi, Akiya Ogawa and Hidehiro Sakurai
†
Department of Materials Chemistry, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan
Received 20 September 2003; revised 25 October 2003; accepted 27 October 2003
Abstract—Benzaldehydes are reduced by metallic zinc in the presence of Ac O and imidazole, giving the corresponding benzyl acetates in
2
good yields. Reductive esterification of aromatic aldehydes is also carried out via gem-diacetoxy compounds. Carbonyl compounds are
readily converted to the gem-diacyloxy compounds in excellent yields on treatment with 2 molar amounts of acid anhydride and 10 mol% of
Yb(OTf) in MeCN at room temperature. Thus-formed diacyloxy compounds derived from aromatic aldehydes are reduced in situ by
3
metallic zinc to afford the corresponding esters.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
alcohol or its derivatives are not considered to be involved
9
as an intermediate of the Clemmensen reduction. There-
Reductive activation of carbonyl group by one-, two-, or
four-electron reduction process has been widely applied to
organic synthesis. Controlling the reduction conditions
fore, metallic zinc has not been utilized frequently in two-
electron reduction such as transformation of carbonyl
groups to the corresponding alcohols. Most of the reported
procedures for Zn-promoted reduction to alcohols or
reductive esterification of carbonyl compounds are per-
1
such as the choice of reducing metals and additives enables
a highly chemo-/stereo-selective reaction. For example,
combination of one-electron reductant such as a low-valent
vanadium or titanium and metallic zinc as a co-reductant in
the presence of a silylating reagent realizes a catalytic
system for reductive coupling reaction of carbonyl com-
1
0,11
formed under acidic conditions (Scheme 1).
2
pounds including McMurry coupling and pinacol
3
– 5
coupling.
R SiCl/Zn shows excellent diastereoselectivity in the
Our system consisting of cat. Cp VCl /
2 2
3
pinacol coupling reaction of aliphatic aldehydes and
aromatic aldimines, giving the corresponding vicinal
dl-diols and meso-diamines, respectively.
Metallic zinc is widely utilized as a relatively mild reducing
reagent and addition of an activator such as acids or Lewis
acids is required to enhance the reactivity. Reductive
deoxygenation of carbonyl groups, well-known as the
6
Clemmensen reduction, requires four-electron reduction
and a proton source, which is performed by zinc (usually
amalgamated) and strong acids (e.g. HCl). Although several
7
Scheme 1. Reductive activation of carbonyl compounds.
intermediates including zinc carbenoids have been postu-
8
lated, it is very difficult to apply these intermediates to
other transformations, partly due to the multiple reaction
pathways and the severe reaction conditions. In addition, an
We are interested in the utility of metallic zinc in the
reduction reaction, especially under mild, non-amalga-
mated, and neutral conditions. Addition of an appropriate
activator such as metal salts realizes the unique reactivity.
For example, a combination of Zn/ZnI was demonstrated
2
to promote the novel 2:1 coupling of an acrylic acid ester
and an aromatic aldehyde to give the corresponding
Keywords: reduction; zinc.
*
Corresponding author. Tel.: þ81-6-6879-7413; fax: þ81-6-6879-7415;
e-mail: hirao@chem.eng.osaka-u.ac.jp
Previous name; Department of Applied Chemistry, Faculty of
Engineering, Osaka University.
†
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.10.068

10148
T. Hirao et al. / Tetrahedron 59 (2003) 10147–10152
1
2
a-aroyladipate. Recently, we also reported that acylating
reagents such as Ac O assist the vanadium-catalyzed
yields, respectively (entry 2). Imidazole is considered to
work as a mild proton source in the reaction. In fact,
deuterium was introduced at the benzylic position when
deuterized imidazole was employed. In order to prevent the
one-electron reduction process, the reduction was con-
ducted without vanadium catalyst. As expected, pinacol
coupling did not proceed, but 2a was obtained as a sole
product in the absence of the vanadium catalyst. When the
reaction was performed in DME, the yield of 2a was
increased to 97% (entry 3). The reaction also occurred in
apolar solvents such as toluene even at room temperature
despite the slightly lower yield (entries 4 and 5).
2
pinacol coupling reaction as well as chlorosilanes in the
1
3
presence of metallic zinc as a co-reductant. Since Ac O is
2
considered both to activate zinc and to trap the anionic
4
intermediates as well as silylating reagents, zinc reduction
with acid anhydride in an aprotic solvent is expected to
develop the reductive esterification of carbonyl group under
relatively neutral conditions. In this paper we wish to
present two routes to the reductive esterification using Zn/
(
RCO) O. One of them involves Zn/Ac O/imidazole
2 2
reduction system, which enables one-pot aldehyde-selective
reductive acetylation. Another method is based on the
Yb(OTf) -catalyzed diacyloxylation of carbonyl com-
Table 2 summarizes the results of the reaction using
aromatic aldehydes (ketone) in DME. The reaction was
found to be influenced by the electronic effect. Aromatic
aldehydes bearing a weak electron-donating or withdrawing
group such as p-isopropyl (1b) or p-chloro (1d) group
underwent the smooth reductive esterification as well as
benzaldehyde (entries 1, 2 and 4). The stronger electron-
donating substituent such as p-methoxy group (1e)
diminished the yield. On the other hand, pinacol coupling
pathway preferentially proceeded in the reaction of
p-cyanobenzaldehyde (1c) as shown in entry 3. This may
be due to the stabilization of the radical anion intermediate
by the electron-withdrawing effect of cyano group. These
results indicate an electron transfer process. On the contrary,
aliphatic aldehydes such as hydrocinnamaldehyde (1i) did
not effectively undergo the reduction under the similar
reaction conditions (entry 6). No reaction took place in the
case of acetophenone (1j, entry 7).
3
pounds, which is further reduced with zinc to yield the
reductive esterification products (Scheme 2).
Scheme 2. Reductive esterification of aromatic aldehyde.
2
. Results and discussion
2
.1. Zn/Ac O/imidazole reduction system
2
Table 2. Reduction of aromatic aldehyde or ketone using Zn/Ac2-
O/imidazole system
Previous reports from our laboratory demonstrated that the
vanadium-catalyzed pinacol coupling of benzaldehyde 1a
proceeds in the presence of Zn and Ac O, in which benzyl
2
acetate 2a is also detected as a by-product (Table 1, entry
1
3
1
). Pinacol coupling product 3a seems to be formed via
one-electron reduction process, while 2a might be produced
by two-electron reduction. In order to screen the effect of
additives on the reaction pathway, 2 molar amounts of
imidazole were added to the reaction mixture of benzal-
dehyde, 3 mol% of Cp VCl , Zn (200 mol%), and Ac O
Entry
Yields (%)
1
2
3
2
2
2
(
200 mol%) in DME, giving 2a and 3a in 59 and 30%
1
2
3
4
5
6
PhCHO (1a)
97 (2a)
84 (2b)
19 (2c)
66 (2d)
45 (2e)
5 (2i)
0
0
p-Cl–C
6
H
4
CHO (1b)
CHO (1c)
p-Pr –C H CHO (1d)
p-NC–C
6
H
4
76 (3c)
i
Table 1. Formation of benzyl acetate (2a) by Zn/Ac
2
O/imidazole system
0
0
0
0
6
4
p-MeO–C
PhCH CH
2 2
6
H
4
CHO (1e)
CHO (1i)
PhCOMe (1j)
a
7
0
a
The reaction time is 42 h and 71% of 1i was recovered.
As described above, aldehyde-selective reductive esterifica-
tion was carried out under mild and neutral conditions using
Zn/Ac O/imidazole system. Since most of functional groups
a
Entry
Additives (mol%)
Conditions
Yields
%)
(
2
are tolerant under these conditions, the reduction system is
likely to be applied to the highly chemoselective
transformation.
Cp
2
VCl
2
Imidazole Solvent Temperature (8C) 2a 3a
b
1
3
3
None
200
200
200
200
DME
DME
DME
Toluene
Toluene
80
80
80
80
20
5
83
59 30
2
3
4
5
None
None
None
97
76
66
0
0
0
2.2. Zn/Yb(OTf) /(RCO) O reduction system
3
2
In the protic acid conditions such as HCl or AcOH, a
possible reaction intermediate in the zinc reduction of
carbonyl group is postulated as the corresponding
a
Reaction time is 24 h.
Ref. 13.
b

T. Hirao et al. / Tetrahedron 59 (2003) 10147–10152
10149
8
c
15c
15d
15e
15f
gem-dichloro or diacetoxy compound (Scheme 3). Effi-
cient reductive esterification might be expected if the
diacetoxy compound is generated under mild conditions,
followed by two-electron reduction with zinc.
PCl3, Bi(OTf)3, Zn(BF ) , or LiOTf. Since most
4 2
of the reported procedures do not realize effective and
quantitative conversion under mild conditions, new
catalytic system for the diacyloxylation should be
monitored. Some representative results of the reaction of
PhCHO with Ac O or (EtCO) O are listed in Table 3.
2
2
Recently, a catalytic amount of Me SiI, generated in situ
3
from Me SiCl and NaI in MeCN, was reported to promote
3
1
6
the diacetoxylation of aryl aldehyde in a good yield. We
first applied this procedure to the reaction of benzaldehyde
with Ac O or (EtCO) O. Me SiI conditions were proved to
2
2
3
be applied to both acid anhydrides as listed in entries 1 and 2
although that the yields were not satisfactory. Therefore,
more suitable Lewis acid catalyst was chosen. Several
Lewis acids were screened in the reaction of benzaldehyde
Scheme 3. A possible reduction pathway via gem-dichloro or diacetoxy
compounds.
8c
Table 3. Lewis acid catalyzed diacyloxylation of benzaldehyde
with (EtCO) O, resulting in the less yields than those of the
2
Me SiI-catalyzed reaction. By the analogy of Me SiI,
3
3
Me SiOTf was used, but the yield was lowered (entry 3).
3
Standard Lewis acid such as BF ·Et O (entry 4) also showed
3
2
the moderate activity. Rare earth metal triflates have been
utilized as a Lewis acid catalyst as exemplified by the
Sc(OTf) -catalyzed diacetoxylation of benzaldehyde in
3
1
7
Entry
R
Catalyst (mol%)
Time (h)
Yield (%)
MeNO solution. We also found that rare earth metal
2
triflates are efficient catalysts. Especially, with use of
Sc(OTf) or Yb(OTf) , 4 was obtained quantitatively in
1
2
3
4
5
6
7
8
Me
Et
Et
Et
Et
Et
Et
Me
Me
Me
Me
3
3
3
SiCl/NaI (20)
SiCl/NaI (20)
SiOTf (20)
8
8
19
19
8
8
8
8
76
65
50
60
3
3
MeCN solution (entries 6 and 7). The catalytic diacetoxyla-
tion also proceeded quantitatively with 10 mol% of
Yb(OTf) (entry 8).
BF
Sm(OTf)
Sc(OTf)
3
·Et
2
O (20)
3
(20)
80
3
3
(20)
(10)
(10)
.99
.99
.99
Yb(OTf)
Yb(OTf)
3
Next, one-pot reductive esterification via diacyloxy inter-
mediate 4 was attempted. After confirmation of the
3
1
conversion to 4 by TLC and H NMR, zinc powder,
1
8
It has been reported that the conversion of the carbonyl
compounds into the diacetoxy compounds by the reaction
with Ac O is catalyzed by strong acid such as H SO
2
which was purified according to the literature and non-
amalgamated, was added to the reaction mixture and the
1
4a
19
or
15b
mixture was heated for the listed time. Table 4 lists both
1
yields of 4, which were estimated by H NMR, and 2, which
2
4
1
4b
15a
H PO ,
3
or Lewis acid such as ZnCl2,
FeCl3,
4
Table 4. One-pot reductive esterification using Zn/Yb(OTf)
3
2
/(RCO) O system
1
2
Entry
Ar
R
R
Catalyst
Conditions
b
Time (h)
Yields (%)
a
200
c
4
2
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1b)
H (1c)
H (1e)
H (1f)
H (1g)
H (1h)
H (1i)
Me (1j)
Et
Et
Et
Et
Me
Sm(OTf)
Sc(OTf)
3
SiCl/NaI
20
20
20
10
10
10
10
10
10
10
10
10
10
100
10
20
200
200
200
200
200
200
200
200
200
800
400
200
200
200
400
400
24
24
24
24
24
24
24
24
64 (4k)
80 (4k)
.99 (4k)
.99 (4k)
.99 (4a)
91 (4l)
34 (2k)
67 (2k)
61 (2k)
88 (2k)
97 (2a)
66 (2l)
53 (2m)
61 (2n)
89 (2b)
62 (2c)
Complex mixture
77 (2f)
18 (2g)
5 (2h)
Complex mixture
No reaction
3
200
200
200
200
200
200
200
200
200
700
200
200
500
200
200
3
Yb(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
Yb(OTf)
3
Me
i-Pr
t-Bu
Ph
Me
Me
Me
Me
Me
Me
Me
Me
3
3
3
3
3
3
3
3
3
3
3
3
81 (4m)
80 (4n)
6
p-ClC H
4
70
.99 (4b)
.99 (4c)
.99 (4e)
.99 (4f)
.99 (4g)
95 (4h)
10
11
12
13
14
15
16
p-NCC
6
H
4
192
120
120
70
32
32
p-MeOC
o-MeC
2,4,6-C
PhCHvCH
6
H
4
6
H
H
4
6
2
PhCH
Ph
2
CH
2
90 (4i)
.99 (4j)
a
48
a
4j was generated after heating at 708C.

10150
T. Hirao et al. / Tetrahedron 59 (2003) 10147–10152
was total isolated yield based on the corresponding carbonyl
compound 1.
Yb(OTf) did not give the ester 2i with 82% recovery of
3
4i (entry 15). No reaction was observed with 4j as shown in
entry 16.
Effect of Lewis acid was monitored using Zn/Lewis
acid/(EtCO) O system. As shown in entries 1–4, lanthanoid
triflates gave the better results, but the accommodation of
the metals to the reduction process was different, as
compared with that of the diacyloxylation process.
The above-mentioned results suggest that the reduction
process would proceed via the direct contact and insertion of
metallic zinc to the C–OAc bond of 4 and Lewis acid such
2
as Yb(OTf) would assist the insertion process by weaken-
3
Sm(OTf) , which was less effective in the diacyloxylation
3
than Sc(OTf) , gave the better yield for the one-pot
3
reductive esterification. Finally, Yb(OTf) was found to be
3
ing the C–O bond. However, electron transfer mechanism,
including direct or fast successive two-electron transfer
mechanism, cannot be excluded.
the more suitable catalyst for both processes (entry 4). The
above phenomena strongly suggest that Lewis acid catalyst
also affects the Zn-induced reduction process. Indeed, when
Zn-reduction was operated from the isolated compound 4a
3. Conclusion
in the absence of Yb(OTf) catalyst, no reduction took place
3
with almost quantitative recovery of 4a (Scheme 4).
We have demonstrated two types of reductive esterification
of aromatic aldehydes, Zn/Ac O/imidazole and
2
Zn/Yb(OTf) /(RCO) O systems. Features of each reaction
2
3
are as follows:
1. Zn/Ac O/imidazole operates direct conversion to the
2
esters under nearly neutral conditions. Electron transfer
mechanism may be proposed.
2
. Zn/Yb(OTf) /(RCO) O system enables the two-step
2
3
transformation via the corresponding diacyloxy com-
pounds. Yb(OTf) catalyst does not only accelerate the
Scheme 4.
3
diacyloxylation step, but also act as an activator for the
Zn-induced reduction. Judging from the electronic and
steric susceptibility, the reduction process is likely to
proceed by the direct contact of Zn to the acyloxy
compound.
Various kind of acid anhydrides were employed in the
reaction with benzaldehyde under the one-pot Zn/Yb(OTf)3
conditions (entries 4–8). Although the diacyloxylation did
not proceed quantitatively in the reaction with bulky acid
anhydride, 4 and 2 were produced in moderate to excellent
yields. Substituent effect of the aryl aldehydes was
3
. Yb(OTf) is also found to be an excellent catalyst for
3
the transformation of the carbonyl compounds to the
corresponding diacyloxy compounds. Although the
reactivity is slightly dependent on the structure of acid
anhydrides, the reaction has large versatility to widen
the scope and utility of the starting aldehydes and
ketones.
examined using Ac O as an acylating reagent (entries 5,
2
and 9–13) and in every case, diacetylated compounds 4
were obtained quantitatively. With these results, we
concluded that the diacyloxylation does not depend on
either electronic or steric nature of aldehydes but was
influenced by the structure of acid anhydrides. In contrast,
the reduction step was strongly dependent both on electronic
and steric effects. Aldehydes bearing electron-withdrawing
group such as p-Cl (entry 9) or p-CN (entry 10) underwent
the reductive esterification in 89 and 62% yields, respect-
ively. On the other hand, aldehydes bearing electron-
donating group such as p-OMe (entry 11) reacted with Zn to
give the complex mixture without the formation of the
expected ester. Such inclination of the electronic effect
4. Experimental
4.1. General
1
13
H (300 MHz) and C (75 MHz) NMR spectra were
measured on a Varian Mercury 300 spectrometer. CDCl3
was used as a solvent, and residual chloroform
resembles that observed in the Zn/Ac O/imidazole system
2
1
13
as shown in Table 2. Only the difference lies in the reactivity
of p-cyanobenzaldehyde: no pinacol-type product 3c was
detected in the Zn/Yb(OTf) /Ac O system, indicating that a
( H¼7.24 ppm; C¼77.0 ppm) or Me Si was used as an
4
internal standard. Infrared spectra were recorded on
Perkin–Elmer Model 1600 Series FT-IR. Mass spectra
were measured on a JEOL JMS-DX-303 spectrometer using
either electron impact (EI) or chemical ionization (CI)
modes. Elemental analyses were carried out at the
Analytical Center, Graduate School of Engineering, Osaka
University. Column chromatography was conducted on
silica gel (Wakogel C-200). DME and toluene were freshly
3
2
radical intermediate is unlikely to be stable under the above
conditions. As for the steric effect, o-tolylaldehyde (entry
1
2) was reduced to 2f in 77% yield, while mesitylaldehyde
entry 13) was restrictive to be reduced (18% yield).
(
Yb-catalyzed diacetoxylation was also employed in the
reaction of cinnamaldehyde (1h), hydrocinnamaldehyde
1i), and acetophenone (1j) in 95, 90, and .99% yield,
(
distilled from CaH prior to use. MeCN was distilled from
2
respectively. On the contrary, these aldehydes/ketone did
not effectively undergo the reductive esterification. As
shown in entry 14, reduction of 4h with Zn occurred, but 2h
was produced in very low yield (5%) with complex mixture.
Addition of zinc powder into the mixture of 4i and
P O , then from CaH , and dried over Molecular Sieves 4A.
2
5
2
Zinc powder was purified according to the literature without
1
8
amalgamation. There was no significant difference of
reactivity of the following reactions depending on the
commercial resources. Other organic compounds such as

T. Hirao et al. / Tetrahedron 59 (2003) 10147–10152
10151
arylaldehydes, acid anhydrides, and imidazole were
2
purchased and purified according to the standard methods.
4.3.4. Bis(2,2-dimethylpropanoyloxy)phenylmethane
1
0
(4m). H NMR d¼1.23 (s, 18H), 7.31–7.41 (m, 3H),
1
3
7
.42–7.48 (m, 2H), 7.67 (s, 1H); C NMR 26.85, 38.80,
4
reduction system
.2. General procedure for Zn/Ac O/imidazole
2
89.44, 126.40, 128.50, 129.39, 135.94, 176.31 ppm; IR
(neat) 3070, 2975, 2890, 2845, 2800, 1750, 1145, 1109,
2
1
697 cm . HRMS found: m/z 292.1680. Calcd for
C H O : 292.1674.
To a mixture of zinc powder (131 mg, 2 mmol) and
imidazole (136 mg, 2 mmol) in DME (4 mL) was added
acetic anhydride (204 mg, 2 mmol) at room temperature
under argon. The reaction mixture was heated to 808C, and
then aldehyde 1 (1 mmol) was added. After stirring for 24 h
at 808C, the reaction was quenched with ether (10 mL) and
HCl aq (1 M, 10 mL). After the filtration through Celite, the
organic layer was separated and the aqueous phase was
1
7 24 4
4.3.5. Bis(benzoyloxy)phenylmethane (4n) [1459-18-3].
1
H NMR d¼7.42–7.74 (m, 11H), 8.09–8.14 (m, 4H), 8.23
1
3
(s, 1H); C NMR 90.58, 126.66, 128.34, 128.60, 128.99,
129.69, 129.94, 133.48, 135.58, 164.29; IR (neat) 3050,
3020, 1732, 1601, 1451, 1277, 1059, 707 cm2
1
.
extracted with Et O (10 mL£2). The combined organic
4.4. General procedure for Zn/Yb(OTf) /(RCO) O
3 2
reduction system
2
layer was washed with water (10 mL£3), then brine
(10 mL), dried over MgSO , and concentrated in vacuo.
4
The crude product was purified by silica-gel column
chromatography, giving the benzyl acetate derivative 2.
To a MeCN (1 mL) solution of 1 (1 mmol) were added acid
anhydride (2 mmol) and Yb(OTf) (62 mg, 0.1 mmol). The
3
mixture was stirred at room temperature under argon. When
1
the reaction was completed (monitoring by TLC and H
4.3. General procedure for Yb(OTf) -catalyzed
diacyloxylation of carbonyl compounds
3
NMR), zinc powder (2 mmol) was added and the reaction
was refluxed. After stirring for the appropriate time listed in
Table 4, the reaction was quenched with ether (10 mL) and
HCl aq (1 M, 10 mL). After the filtration through Celite, the
organic layer was separated. The aqueous phase was
To a MeCN (1 mL) solution of 1 (1 mmol) were added acid
anhydride (2 mmol) and Yb(OTf)3 (62 mg, 0.1 mmol).
The mixture was stirred at room temperature under argon.
When the reaction was completed (monitoring by TLC and
extracted with Et O (10 mL£2). The combined organic
2
1
H NMR), excess water was added. The product was
extracted with CH Cl . The organic layer was washed with
layer was washed with water (10 mL£3), then brine
(10 mL), dried over MgSO , and concentrated in vacuo.
4
2
2
saturated solution of NaHCO followed by water, dried over
3
NaSO , and concentrated in vacuo. Distillation or flash
4
chromatography, if necessary, gave the diacyloxylated
compound 4.
The crude product was purified by thin layer chromato-
graphy, giving the ester derivative 2. All of the products are
known compounds, and their spectral data are in good
agreement with those of authentic samples.
4
1
4
.3.1. Diacetoxyphenylmethane (4a): [581-55-5];
-chloro-4-diacetoxymethylbenzene (4b): [13086-93-6];
-diacetoxymethylbenzonitrile (4c): [36735-42-9]; 1-dia-
Benzyl acetate (2a): [140-11-4]; 4-chlorobenzyl acetate
(2b): [5046-33-7]; 4-acetoxymethylbenzonitrile (2c):
[21388-95-4]; 4-isopropylbenzyl acetate (2d): [59230-57-
6]; 1-acetoxymethyl-4-methoxybenzene (2e): [104-21-2];
2-methylbenzyl acetate (2f): [17373-93-2]; 2,4,6-trimethyl-
benzyl acetate (2g): [63548-92-5]; trans-cinnamyl acetate
(2h): [21040-45-9]; benzyl propionate (2k): [122-63-4];
benzyl isobutylate (2l): [103-28-6]; benzyl pivalate (2m):
[2094-69-1]; benzyl benzoate (2n): [120-51-4].
cetoxymethyl-4-methoxybenzene (4e): [14202-31-4];
-diacetoxymethyltoluene (4f): [31675-37-3]; 1-dia-
2
cetoxymethyl-2,4,6-trimethylbenzene (4g).
1
H NMR
d¼2.10 (s, 6H), 2.27 (s, 3H), 2.52 (s, 6H), 6.87 (s, 2H),
1
3
8
1
2
.024 (s, 1H); C NMR 20.34, 21.34, 21.44, 88.89, 128.79,
30.10, 138.01, 139.73, 169.02 ppm; IR (neat) 3020, 2925,
872, 2736, 1760, 1612, 1035, 852 cm . Found: C, 67.23;
2
1
H, 7.20%. Calcd for C H O : C, 67.18; H, 7.25%. HRMS
1
4 18 4
found: m/z 250.1214. Calcd for C H O : 250.1205.
14 18 4
Acknowledgements
4
9
1
.3.2. 3,3-Diacetoxy-1-phenyl-1-propene (4h): [37973-54-
]; 1,1-diacetoxy-3-phenylpropane (4i): [85337-09-3];
The use of the facilities of the Analytical Center, Graduate
school of Engineering, Osaka University is acknowledged.
This work was supported by a Grant-in-Aid for Scientific
Research (No. 15750090) from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan. H.S.
thanks the Fujisawa Foundation for financial support.
1
,1-diacetoxy-1-phenylethane (4j): [28153-24-4].
H
NMR d¼2.21 (s, 6H), 2.60 (s, 3H), 7.45–7.62 (m, 3H),
1
3
7
1
.95 (d, 2H, J¼7.2 Hz); C NMR 20.83, 22.26, 26.70,
28.20, 128.45, 133.01, 177.02 ppm.
4
4
.3.3. Phenyl-bis-propanoyloxymethane (4k): [55696-47-
1
]; bis(2-methylpropanoyloxy)phenylmethane (4l).
H
NMR d¼1.15–1.23 (m, 12H), 2.58–2.65 (m, 2H), 7.37–
References
1
3
7
1
1
1
.41 (m, 3H), 7.47–7.51 (m, 2H), 7.68 (s, 1H); C NMR
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74.89 ppm; IR (neat) 3050, 2976, 2895, 2780, 1757, 1690,
092, 697 cm2 . Found: C, 67.92; H, 7.53%. Calcd for
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