Direct oxidation of β-aryl substituted aldehydes to α,β-unsaturated aldehydes promoted by an o-anisidine-Pd(OAc) 2 Co-catalyst

Article abstract of DOI:10.1002/asia.200900238

An o-anisidine-Pd(OAc)₂ catalytic system for the direct co-catalytic Saegusa oxidation of β-aryl substituted aldehydes to α,β-unsaturated aldehydes has been developed. The use of o-anisidine in place of (S)-diphenylprolinol made the process more simply and cost-effective. The process not only features the use of unmodified aldehydes rather than enol silyl ethers, but also gives moderate to good yields (44-72%).

Full text of DOI:10.1002/asia.200900238

FULL PAPERS
DOI: 10.1002/asia.200900238
Direct Oxidation of b-Aryl Substituted Aldehydes to a,b-Unsaturated
Aldehydes Promoted by an o-Anisidine–PdACTHNURTGNENG(U OAc)2 Co-catalyst
Jie Liu,[a] Jin Zhu,[a] Hualiang Jiang,[a, c] Wei Wang,*[a, b, c] and Jian Li*[a]
Abstract: An o-anisidine-PdACHTNUTGRNEUNG(OAc)2 catalytic system for the direct co-catalytic Sae-
gusa oxidation of b-aryl substituted aldehydes to a,b-unsaturated aldehydes has
been developed. The use of o-anisidine in place of (S)-diphenylprolinol made the
process more simply and cost-effective. The process not only features the use of
unmodified aldehydes rather than enol silyl ethers, but also gives moderate to
good yields (44–72%).
Keywords: aldehydes · anisidine ·
organocatalysis
·
palladium
·
Saegusa reaction
Introduction
diversity. With the increasing demand for the enals, current
synthetic methods must be reevaluated and optimized or
supplanted with new, direct, and more efficient strategies.
Saegusa oxidation is a convenient, efficient process for
the transformation of silyl enol silanes to corresponding a,b-
unsaturated aldehydes and ketones (Scheme 1).[17,18] The dis-
Interest in the use of a,b-unsaturated aldehydes has been
growing tremendously in the recent past.[1–3] Enals have ar-
guably become the most widely used substrates in the
emerging field of organocatalysis.[4–8] During the past several
decades, there have been certain advances in the prepara-
tion of enals, such as the oxidation of allylic alcohols,[9] Pe-
terson olefinations,[10] formylation,[11,12] cross-metathesis re-
actions of acrolein,[13] cross-aldol condensations,[14] Wittig
olefination,[15] and oxidation–elimination of selenium alde-
hydes.[16] Most of these methods either produce equimolar
amounts of by-products and are thus not atom-economic, or
suffer from several side reactions under relatively strong
basic conditions, self-aldolization, and the narrow substrate
[a] J. Liu, Dr. J. Zhu, Prof. Dr. H. Jiang, Prof. Dr. W. Wang,
Prof. Dr. J. Li
Scheme 1. Pd
co-catalyzed Saegusa reaction (bottom).
ACHUTGTNERN(NUG OAc)2-catalyzed (top) and (S)-diphenylprolinol/PdACHTUNGTRENNUNG(OAc)2
School of Pharmacy
East China University of Science & Technology
130 Meilong road, Shanghai 200237 (P.R. China)
Fax : (+86)21-64252584
tinguishing characteristics of the oxidation process include
the use of a catalytic amount of Pd(OAc)2 as the catalyst
E-mail: jianli@ecust.edu.cn
AHCTUNGTRENNUNG
[b] Prof. Dr. W. Wang
and O2 as the oxidant. However, the method requires the
preformation of silyl enol ethers from corresponding alde-
hydes and ketones.[17,18] Moreover, longer reaction time is
generally required for aldehyde derived silyl enol ethers
than for ketone derivatives in the classic Saegusa oxidation
reactions.[18] Therefore, the development of a new improved
protocol for aldehyde related substrates is highly desired.
Toward this end, recently we have developed a new
method for direct preparation of a,b-unsaturated aldehydes
from readily available aldehydes. The process is co-catalyzed
Department of Chemistry & Chemical Biology
University of New Mexico
MSC03 2060, Albuquerque, NM 87131-0001 (USA)
Fax : (+1)505-277-2609
E-mail: wwang@unm.edu
[c] Prof. Dr. H. Jiang, Prof. Dr. W. Wang
Drug Discovery and Design Center
Shanghai Institute of Materia Medica
Chinese Academy of Sciences
555 Zuchongzhi Road
Shanghai 201203 (P.R. China)
1712
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Chem. Asian J. 2009, 4, 1712 – 1716

Table 1. Results of exploratory studies of primary amine/PdACTHNUTRGNEUNG(OAc)2 co-
by (S)-diphenylprolinol/PdACHTNUTRGNENG(U OAc)2 to give a,b-unsaturated
catalyzed Saegusa reaction of 3-phenylpropionaldehyde (1a) to cinnamal-
aldehydes in moderate to good yields (41–62%) (Scheme 1,
bottom).[19] In view of the facts that the chiral (S)-diphenyl-
prolinol co-catalyst is expensive and not readily available,
and the yields are relatively low, we recently sought a
simple, cost-effective amine catalyst as a replacement to im-
prove the process. Herein, we disclose a significantly im-
proved Saegusa oxidation reaction. Notably, the simple o-
anisidine was found to be an effective co-catalyst in the
dehyde (2a).[a]
presence of PdACHTUNGTRENNUNG(OAc)2 for a direct oxidation of unmodified
aldehydes to enals without requiring the preformation of
silyl enol ethers.
Results and Discussion
To identify a simple amine as a co-catalyst, we screened var-
ious primary amines I–XIV instead of (S)-diphenylprolinol
(20 mol%) (Table 1) under the optimal conditions—Pd-
ACHTUNGTRENNUNG(OAc)2 (10 mol%) and O2 (1 atm) in DMSO 608C—we de-
Entry
Cat
Yield [%][b]
Entry
Cat
Yield [%][b]
veloped earlier for the reaction of 3-phenylpropionaldehyde
(1a; Table 1).[19] It appeared that most of the primary
amines displayed catalytic activity, but their catalytic activi-
ties varied to some extent. Among them, p-bromoaniline
(IX), p-anisidine (XII), and o-anisidine (XIII) demonstrated
encouraging outcomes in high yields (53–54%, Table 1, en-
tries 9, 12, and 13).
We next probed the effect of solvents on the process with
p-bromoaniline (IX), p-anisidine (XII), and o-anisidine
(XIII) as organocatalysts (Table 2). Commonly used sol-
vents, including DMSO, DME, DMF, THF, CH3CN, and tol-
uene, were employed. It was found that DMSO still was the
best medium for the reaction (Table 1, entries 9, 12, and
13).[19] We also probed the effect of acid additives on the
process (Table 2, entries 16 and 17). Benzoic acid and acetic
acid were not beneficial to the process. It was found that
under the reaction conditions, no oxidized carboxylic acid
from the corresponding aldehyde was observed.
1
2
3
4
5
6
7
I
II
37
33
33
49
40
45
24
8
9
VIII
IX
X
31
53
9
45
53
54
34
III
IV
V
VI
VII
10
11
12
13
14
XI
XII
XIII
XIV
[a] Reaction conditions: see the Experimental Section. [b] Yields of iso-
lated products.
Table 2. Solvent effect on the IX-, XII-, and XIII-catalyzed conversion
of 3-phenylpropionaldehyde into cinnamaldehyde.[a]
Having established the optimal reaction conditions, we
then determined the scope of the direct Saegusa oxidation
reaction using o-aniline as an organocatalyst (Table 3).
Analysis of the results reveal that the aromatic rings of cin-
namaldehydes bearing electron-neutral (Table 3, entry 1),
electron-donating (Table 3, entries 2–6), or electron-with-
drawing (Table 3, entries 7–12) substituents underwent reac-
tion smoothly to afford the desired products in moderate to
Entry
Cat
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
IX
IX
IX
IX
DME
DMF
THF
MeCN
toluene
DME
DMF
23
31
14
13
26
20
19
20
21
18
33
21
14
40
27
52
50
IX
XII
XII
XII
XII
XII
XIII
XIII
XIII
XIII
XIII
XIII
XIII
THF
9
MeCN
toluene
DME
DMF
10
11
12
13
14
15
16[c]
17[d]
Abstract in Chinese:
THF
MeCN
toluene
DMSO
DMSO
[a] Reaction conditions: unless specified, see the Experimental Section.
DME=1,2-dimethoxyethane; DMF=N,N-dimethylformamide; THF=
tetrahydrofuran. [b] Yields of isolated products. [c] 10 mol% PhCO2H
added for 16 h. [d] 10 mol% HOAc added for 16 h.
Chem. Asian J. 2009, 4, 1712 – 1716
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W. Wang, J. Li et al.
FULL PAPERS
Table 3. Scope of o-aniline/PdACTHNURGTNEU(GN OAc)2 co-catalyzed direct Saegusa reac-
tions.[a]
Entry
R
Product
t [h]
Yield [%][b]
1
2
3
4
5
6
7
8
C6H5
2-MeC6H4
2-MeOC6H4
3-MeC6H4
4-MeC6H4
2a
2b
2c
2d
2e
2 f
2g
2h
2i
2j
2k
2l
2m
2n
16
21
19
16
2
13
2
10
12
18
9
54
54
45
58
58
62
44
47
55
56
51
68
72
62
4-EtC6H4
4-MeCOC6H4
4-MeOCOC6H4
4-CNC6H4
4-NO2C6H4
2,4-Cl2C6H3
2-Me-4-NO2C6H3
1-biphenyl
1-naphthyl
9
10
11
12
13
14
11
13
13
Scheme 2. Proposed reaction mechanism for amine/PdACTHUNGTERNNU(G OAc)2 co-cata-
lyzed conversion of 3-substitued propionaldehydes to 3-substitued acro-
leins.
[a] Reaction conditions: see the Experimental Section. [b] Yields of iso-
lated products.
Conclusions
good yields (44–68%). These outcomes implied that elec-
tronic features had marginal effect on the processes. Exami-
nation of the results of the investigation also revealed that
the steric effect (o- versus p- substituted pattern) also
played a minimal role in governing the reaction efficiency.
Remarkably, 3-(4-phenylphenyl)propionaldehyde and naph-
thalene-1-propionaldehyde gave the corresponding a,b-un-
saturated aldehyde in yields of 72% and 62%, respectively,
which is in good agreement with our previous putative
mechanism, namely, that the conjugation effect can more ef-
ficiently facilitate the Saegusa oxidation reaction.[19] The
same limitation is also realized for the primary amine/Pd-
In conclusion, we have developed an improved amine/Pd-
AHCTUNGRTEG(NUNN OAc)2 co-catalytic system for the direct Saegusa oxidation
reaction of unmodified aldehydes to a,b-unsaturated alde-
hydes. The use of simple o-anisidine in place of (S)-diphe-
nylprolinol made the process more simple and cost-effective.
Moreover, the protocol, which is effective for aldehydes, is
complementary to that of the classic Saegusa reaction with
ketone derived silyl enone ethers. The application of the
process in developing new cascade reactions is currently
being pursued in our laboratory.
ACHTUNGTRENNUNG(OAc)2 co-catalyzed Saegusa oxidation reaction. Several ke-
Experimental Section
tones and esters cannot be converted into the corresponding
a,b-unsaturated carbonyl compounds; nevertheless, silyl
enol ethers can finish these conversions.
Commercial reagents were used as received, unless otherwise stated.
Qingdao Haiyang Chemical HG/T2354–92 silica gel was used for chroma-
tography, and Huanghai silica gel plates with fluorescence F254 were used
for thin-layer chromatography (TLC) analysis. 1H and 13C NMR spectra
were recorded on Bruker AMX-400, and tetramethylsilane (TMS) was
used as a reference. Mass spectra were recorded on a MAT-95 spectrom-
eter. Melting points were tested on a melting point apparatus (SGW X-4)
and are uncorrected. 3-Phenylpropanal (Table 3, entry 1) was purchased
from Alfa Chemical Reagent Company, and used without further purifi-
cation. Literature procedures were adapted for the other 3-aryl propanals
(Table 3, entries 2–14).[20]
A reaction mechanism of the primary amine/PdACTHNUTRGNEUNG(OAc)2
co-catalyzed Saegusa oxidation reaction is proposed on the
basis of the established Saegusa oxidation reaction[18] and
our previous study (Scheme 2).[19] Complexation of enamine
3 generated from the corresponding aldehyde with Pd-
ACHTUNGTRENNUNG(OAc)2 leads to alkene/Pd complex 4. The intermediate sub-
sequently converts into palladium adduct 5, a species similar
to that in a classic Saegusa oxidation.[18] The iminium 6, gen-
erated by b-hydride elimination of HPdOAc, is hydrolyzed
to afford the enone and release the amino catalyst. Mean-
while, Pd(0), decomposed from reductive elimination of
HPdOAc is subsequently oxidized by O2 to Pd(II) for the
next cycle reaction. It is reasonably argued that 3-aryl pro-
pionaldehydes can more effectively facilitate the elimination
reaction through the conjugation effect, and accordingly in
our studies it is observed that only such substrates can effec-
tively engage in the process.
General procedure for Pd
conversion of 3-aryl propionaldehydes into cinnamaldehydes (Table 3): A
mixture of (0.45 mmol), o-anisidine (XIII) (0.09 mmol), Pd(OAc)2
ACHTUNGRTENN(UNG OAc)2 and o-anisidine (XIII) co-catalyzed
1
ACHTUNGTRENNUNG
(0.045 mmol), and DMSO (1 mL) was stirred for 2–21 h at 608C in an at-
mosphere of oxygen. The crude product was purified by column chroma-
tography on silica gel to give the desired product 2.
Cinnamaldehyde (2a) (Table 3, entry 1): 54% yield; oil; 1H NMR
(CDCl3, 400 MHz): d=6.73 (dd, 1H, J=7.6 and 16.0 Hz), 7.43–7.45 (m,
4H), 7.49 (d, 1H, J=16.0 Hz), 7.56–7.59 (m, 2H), 9.72 ppm (d, 1H, J=
7.6 Hz); 13C NMR (CDCl3, 400 MHz): d=128.5, 128.6, 129.1, 131.3, 134.0,
152.8, 193.7 ppm; MS (EI) m/z 132 [M+], 131 (100%); HRMS (EI) m/z
calcd C9H8O [M+]: 132.0575, found: 132.0573.
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Direct Oxidation of b-Aryl Substituted Aldehydes
2-Methylcinnamaldehyde (2b) (Table 3, entry 2): 54% yield; oil;
1H NMR (CDCl3, 400 MHz): d=2.41 (s, 3H), 6.73 (dd, 1H, J=7.6 and
16.0 Hz), 7.27 (m, 1H), 7.32–7.36 (m, 1H), 7.39–7.40 (m, 2H), 7.46 (d,
1H, J=16.0 Hz), 9.71 ppm (d, 1H, J=7.6 Hz); 13C NMR (CDCl3,
400 MHz): d=21.3, 125.7, 128.5, 129.0, 129.1, 132.1, 134.0, 138.8, 153.0,
193.7 ppm; MS (EI) m/z 146 [M+], 131 (100%); HRMS (EI) m/z calcd
C10H10O [M+]: 146.0732, found: 146.0733.
7.6 Hz); 13C NMR (CDCl3, 400 MHz): d=127.9, 128.6, 130.2, 130.7, 130.8,
135.6, 137.4, 148.5, 193.7 ppm; MS (EI) m/z 200 [M+], 165 (100%);
HRMS (EI) m/z calcd C9H6OCl2 [M+]: 199.9796, found: 199.9803.
2-Methyl-4-nitrocinnamaldehyde (2l) (Table 3, entry 12): 68% yield; m.p.
106–1088C; 1H NMR (CDCl3, 400 MHz): d=2.60 (s, 3H), 6.76(dd, 1H,
J=7.2 and 15.6 Hz), 7.72–7.78 (m, 2H), 8.11–8.14 (m, 2H,), 9.81 ppm (d,
1H, J=7.6 Hz); 13C NMR (CDCl3, 400 MHz): d=19.9, 121.6, 125.7,
127.7, 132.6, 139.1, 139.2, 146.8, 148.6, 192.9 ppm; MS (EI) m/z 191 [M+],
176 (100%); HRMS (EI) m/z calcd C10H9NO3 [M+]: 191.0582, found:
191.0583.
2-Methoxycinnamaldehyde (2c) (Table 3, entry 3): 45% yield; m.p. 41–
438C (lit. m.p. 41–428C);[21] 1H NMR (CDCl3, 400 MHz): d=3.94 (s, 3H),
6.82 (dd, 1H, J=7.6 and 16.0 Hz), 7.02 (m, 2H), 7.42–7.46 (m, 1H), 7.57
(m, 1H), 7.86 (d, 1H, J=16.0 Hz), 9.71 ppm (d, 1H, J=8.0 Hz);
13C NMR (CDCl3, 400 MHz): d=55.6, 110.3, 120.9, 123.0, 128.9, 129.1,
132.7, 148.2, 158.3, 194.6 ppm; MS (EI) m/z 162 [M+], 131 (100%);
HRMS (EI) m/z calcd C10H10O2 [M+]: 162.0681, found: 162.0683.
4-Phenylcinnamaldehyde (2m) (Table 2, entry 13): 72% yield; m.p. 122–
1248C (lit. m.p. 121–1228C);[21] 1H NMR (CDCl3, 400 MHz): d=6.78 (dd,
1H, J=7.6 and 16.0 Hz), 7.39–7.55 (m, 4H), 7.63–7.70 (m, 6H), 9.75 ppm
(d, 1H, J=8.0 Hz); 13C NMR (CDCl3, 400 MHz): d=127.1, 127.7, 128.1,
128.5, 129.0, 133.0, 139.9, 144.0, 152.2, 193.6 ppm; MS (EI) m/z 208 ([M+
], 100%); HRMS (EI) m/z calcd C15H12O [M+]: 208.0888, found:
208.0890.
3-Methylcinnamaldehyde (2d) (Table 3, entry 4): 58% yield; oil;
1H NMR (CDCl3, 400 MHz): d=2.50
ACTHNUTRGNE(NUG s, 3H), 6.69 (dd, 1H, J=7.6 and
16.0 Hz), 7.25–7.29ACHTUNGTRENNUNG(m, 2H), 7.34 (m, 1H), 7.61 (d, 1H), 7.79 ppm (d, 1H,
J=15.6 Hz); 13C NMR (CDCl3, 400 MHz): d=24.4 130.8, 131.3, 131.5,
134.2, 135.8, 137.5, 142.7,155.0,198.4 ppm; MS (EI) m/z 146 [M+], 131-
(E)-3-(naphthalen-1-yl)acrylaldehyde (2n) (Table 3, entry 14): 62% yield;
m.p. 122–1248C (lit. mp 123–1248C);[24] 1H NMR (CDCl3, 400 MHz): d=
5.85 (dd, 1H, J=7.6 and 15.6 Hz), 6.54–6.65 (m, 3H), 6.91–7.03 (m,
3H),7.32 (d, 1H),7.52 (d,1H, J=15.6 Hz), 8.88 ppm (d,1H, J=7.6 Hz);
13C NMR (CDCl3,400 MHz): d=123.2, 125.8, 125.8, 126.6, 127.4, 129.0,
130.8, 131.0, 131.2, 131.6,133.7, 149.5, 194.2 ppm; MS (EI) m/z 182 [M+],
181 (100%); HRMS (EI) m/z calcd C13H10O [M+]: 182.0732, found:
182.0729.
ACHTUNGTRENNUNG
(100%); HRMS (EI) m/z calcd C10H10O [M+]: 146.0732, found:
146.0728.
4-Methylcinnamaldehyde (2e) (Table 3, entry 5): 58% yield; m.p. 46–
488C (lit. m.p. 42–438C);[21] 1H NMR (CDCl3, 400 MHz): d=2.41
ACHTUNGTRENNUNG
6.70 (dd, 1H, J=7.6 and 16.0 Hz), 7.26ACTHUNGTRENNNUG
9.70 ppm (d, 1H, J=7.6 Hz); 13C NMR (CDCl3, 400 MHz): d=21.6,
127.7, 128.5, 129.9, 131.4, 142.0, 152.9, 193.7 ppm; MS (EI) m/z 146 [M+],
131ACHTUNGTRENNUNG
(100%); HRMS (EI) m/z calcd C10H10O [M+]: 146.0732, found:
146.0735.
4-Ethylcinnamaldehyde (2f) (Table 3, entry 6): 62% yield; oil; 1H NMR
(CDCl3, 400 MHz): d=1.27(t, 3H, J=7.6 Hz), 2.71 (q, 2H, J=7.6 Hz)
6.71 (dd, 1H, J=7.6 and 16.0 Hz), 7.28 (d, 2H, J=7.2 Hz), 7.46–7.52 (m,
3H), 9.71 ppm (d, 1H, J=8.0 Hz); 13C NMR (CDCl3, 400 MHz): d=15.2,
28.9, 127.8, 128.7, 131.6, 148.3, 153.0, 193.8 ppm; MS (EI) m/z 160 [M+],
Acknowledgements
We gratefully acknowledge the financial support from the National Natu-
ral Science Foundation of China (Grant 90813005), the 863 Hi-Tech Pro-
gram of China (Grant 2006AA020404), the China 111 Project (Grant
B07023), and Shanghai Rising-Star Program (A type) of the Shanghai
Ministry of Science and Technology (Grant 07A14013).
131ACHTUNGTRENNUNG
(100%); HRMS (EI) m/z calcd C11H12O [M+]: 160.0888, found:
160.0891.
4-Acetylcinnamaldehyde (2 g) (Table 3, entry 7): 44% yield; m.p. 49–
518C (lit. m.p. 49–508C);[21] 1H NMR (CDCl3, 400 MHz): d=2.65 (s, 3H),
6.80 (dd, 1H, J=7.6 and 16.0 Hz), 7.52 (d, 1H, J=16.0 Hz), 7.68 (d, 2H,
J=8.4 Hz), 8.03 (d, 2H, J=8.4 Hz), 9.77 ppm (d, 1H, J=7.6 Hz);
13C NMR (CDCl3, 400 MHz): d=26.7, 128.5, 128.9, 130.4, 138.1, 138.6,
150.7, 193.3, 197.2 ppm; MS (EI) m/z 174 ([M+], 100%); HRMS (EI)
m/z calcd C11H10O2 [M+]: 174.0681, found: 174.0683.
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4-Methoxycarbonylcinnamaldehyde (2h) (Table 3, entry 8): 47% yield;
m.p. 102–1048C (lit. m.p. 1908C);[22] 1H NMR (CDCl3, 400 MHz): d=3.96
(s, 3H), 6.80 (dd, 1H, J=8.0 and 16.0 Hz), 7.52 (d, 1H, J=16.0 Hz), 7.65
(d, 2H, J=8.4 Hz), 8.11 (d, 2H, J=8.4 Hz), 9.76 ppm (d, 1H, J=7.6 Hz);
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150.7, 166.1, 193.2 ppm; MS (EI) m/z 190 [M+], 131 (100%); HRMS
(EI) m/z calcd C11H10O3 [M+]: 190.0630, found: 190.0629.
4-Cyanocinnamaldehyde (2i) (Table 3, entry 9): 55% yield; m.p. 133–
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8.0 Hz), 7.74 (d, 2H, J=8.0 Hz), 9.78 ppm(d, 1H, J=7.2 Hz); 13C NMR
(CDCl3, 400 MHz): d=114.3, 118.1, 128.7, 131.2, 132.8, 138.2, 149.4,
192.9 ppm; MS (EI) m/z 157 [M+], 156 (100%); HRMS (EI) m/z calcd
C10H7NO [M+]: 157.0528, found: 157.0522.
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4-Nitrocinnamaldehyde (2j) (Table 3, entry 10): 56% yield; m.p. 140–
1428C (lit. m.p. 139–1408C);[21] 1H NMR (CDCl3, 400 MHz): d=6.82 (dd,
1H, J=7.6 and 16.0 Hz), 7.54 (d, 1H, J=16.0 Hz), 7.75 (d, 2H, J=
8.8 Hz), 8.31 (d, 2H, J=8.8 Hz), 9.80 ppm (d, 1H, J=7.6 Hz); 13C NMR
(CDCl3, 400 MHz): d=124.3, 129.1, 131.8, 140.0, 148.8, 192.8 ppm; MS
(EI) m/z 177 [M+], 160 (100%); HRMS (EI) m/z calcd C9H7NO3 [M+]:
177.0426, found: 177.0427.
2,4-Dichlorocinnamaldehyde (2k) (Table 3, entry 11): 51% yield; m.p.
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(dd, 1H, J=7.6 and 16.0 Hz), 7.31 (d, 1H, J=8.4 Hz), 7.48 (s, 1H), 7.59
(d, 1H, J=8.4 Hz), 7.85 (d, 1H, J=16.0 Hz), 9.76 ppm (d, 1H, J=
Chem. Asian J. 2009, 4, 1712 – 1716
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www.chemasianj.org
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Received: July 2, 2009
Revised: July 24, 2009
Published online: September 11, 2009
1716
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2009, 4, 1712 – 1716