An efficient chemoselective strategy for the preparation of (E)-cinnamic esters from cinnamaldehydes using heterogeneous catalyst and DDQ

Article abstract of DOI:10.1016/j.tet.2006.11.011

An efficient chemoselective protocol is developed for the synthesis of (E)-cinnamic esters from substituted cinnamaldehydes or cinnamyl alcohols using a combination of DDQ and heterogeneous catalyst under microwave irradiation. The method showed remarkable selectivity for cinnamaldehydes over aliphatic and aromatic aldehydes, which is a novel finding. The results demonstrate that the developed protocol can be a useful synthetic tool for chemoselective esterification in total synthesis of complex organic compounds.

Full text of DOI:10.1016/j.tet.2006.11.011

Tetrahedron 63 (2007) 1000–1007
An efficient chemoselective strategy for the preparation
of (E)-cinnamic esters from cinnamaldehydes using
heterogeneous catalyst and DDQ*
*
Arun K. Sinha, Anuj Sharma, Anand Swaroop and Vinod Kumar
Natural Plant Products Division, Institute of Himalayan Bioresource Technology, Palampur 176061, Himachal Pradesh, India
Received 6 July 2006; revised 19 October 2006; accepted 2 November 2006
Available online 29 November 2006
Abstract—An efficient chemoselective protocol is developed for the synthesis of (E)-cinnamic esters from substituted cinnamaldehydes or
cinnamyl alcohols using a combination of DDQ and heterogeneous catalyst under microwave irradiation. The method showed remarkable
selectivity for cinnamaldehydes over aliphatic and aromatic aldehydes, which is a novel finding. The results demonstrate that the developed
protocol can be a useful synthetic tool for chemoselective esterification in total synthesis of complex organic compounds.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
of various complex natural products.8 The above method
continued to attract the attention of researchers for direct
conversion of aldehydes or alcohols into esters and various
reports have surfaced with some modifications in the
original method using a range of reagents such as MnO2–
NaCN,9 chromium oxide–pyridine,10 TMSCN11 and tert-
butyl hypochlorite,12 etc. For example, Foot et al.9c,d
described a facile and direct oxidative esterification of cin-
namyl alcohols to the corresponding cinnamic esters using
a combination of MnO2–NaCN in alcohol. Similarly, Bal
et al.11 accomplished the direct transformation of a,b-un-
saturated aldehydes to the corresponding esters utilizing
TMSCN.
(E)-Cinnamic esters are immensely important organic com-
pounds due to their application in a wide range of industrial
products such as plasticizers, graphics, lubricants, flavours,
perfumes and cosmetics.1 For example, 2-ethylhexyl-4-
methoxycinnamate is a UV absorbing sunscreen agent and
a common ingredient in most of the new sunscreen lotions
and many other cosmetic formulations.1 Further, these
a,b-unsaturated esters possess various pharmacological ac-
tivities including antioxidant, antimicrobial and anticancer
activities,2 besides being useful in other synthetic appli-
cations.3 Numerous methods4 have been reported for the
preparation of a,b-unsaturated esters, however, most of
the reported procedures require strong acids5 like sulfuric
acid, hydrochloric acid, and toxic chemicals6 such as di-
methylsulfate, methyl iodide and diazomethane, which are
environmentally hazardous and hence unacceptable. More
importantly, none of the methods disclose a single-step
approach towards conversion of cinnamaldehydes into
cinnamic esters, while simultaneously taking care of the
compatibility of the process with other sensitive functional
groups.4–6 In this context, the protocol developed by Corey
et al.7 assumes special significance as it demonstrated a
mild and direct conversion of cinnamaldehydes into cin-
namic esters without affecting any other sensitive functional
groups.
Though meticulous, the above protocols for direct conver-
sion of cinnamaldehydes or cinnamyl alcohols (C6–C3
unit) into cinnamic esters (C6–C3 unit) remain impregnated
with some limitations. Apart from expensive reagents and
delicate reaction conditions, some of these methods7–9,11
also pass through an intermediate C6–C4 unit, formed by
the combination of substrate (C6–C3 unit) and an extra C1
unit in the form of toxic sodium/potassium cyanide7–9 or
TMSCN,11 thereby conferring lack of atom economy13 on
these protocols. Further, the above methods7–9 produce toxic
hydrogen cyanide gas as a by-product, which severely limits
their industrial applications. Consequently, there have been
some reports disclosing the direct esterification of cinnamyl
alcohols utilizing dimethyl acetate/trimethyl formate/meta-
chloroperbenzoic acid14 or hypervalent iodine (III) reagent15
in lieu of toxic sodium/potassium cyanide.7–9 Recently, some
reports employing selective,16 atom economical methodolo-
gies17 or utilizing heterogeneous recyclable catalyst18 for the
synthesis of (E)-cinnamic esters have also surfaced. Though
the above-mentioned methodologies are environmentally
This revolutionary method made a remarkable impact in
organic synthesis and has been instrumental in the synthesis
*
IHBT communication no. 0532.
* Corresponding author. Tel.: +91 1894 230426; e-mail: aksinha08@
rediffmail.com
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.11.011

A. K. Sinha et al. / Tetrahedron 63 (2007) 1000–1007
1001
benign, the expensive nature of the reagent limits their wide-
spread utility.
with DDQ (1.2 equiv) and methanol in catalytic amount of
Amberlyst-15 for 24 h or refluxed for 10 h, which provided
methyl cinnamate (1b) in 81% and 83% yield, respectively
(Scheme 1). To further increase the yield of the product,
various alterations in the reaction conditions were made.
Oxidants such as chloranil, manganese dioxide and pyridi-
nium chlorochromate24 were tried, but these greatly reduced
yields of 1b up to 10–15%. After a lot of experimentation, it
was found that performing the above reaction with the addi-
tion of high boiling solvent toluene to a mixture of 1a, DDQ
(1.5 equiv), Amberlyst-15 and methanol under Dean–Stark
not only reduced the reaction time from 10 to 6 h but also
increased the yield of 1b from 83% to 92%. Our desire to fur-
ther reduce the reaction time prompted us to undertake the
above reaction under microwave irradiation. To our delight,
microwave not only curtailed the reaction time from 6 h to
30 min, but also increased the yield up to 96%. After success
of 1b, the method was extended to various other substrates
like substituted cinnamaldehydes (entries 2a–10a), cinnamyl
alcohols (entries 11a and 12a), aromatic and aliphatic alde-
hydes (entries 13a–16a, Table 1). It is clear from Table 1
that yield of all the methoxy substituted cinnamic esters
(entries 2b–9b) is comparable among themselves as well
as with unsubstituted ester (entry 1b).
It is thus evident that there remains a wide scope for the
development of clean and efficient19 methodologies for
direct transformation of cinnamaldehydes or cinnamyl alco-
hols into the corresponding esters. The recent years have wit-
nessed a tremendous upsurge of interest in using recyclable
reagents and heterogeneous catalysts20 for various chemical
transformations due to their inherent economic and environ-
mental benefits.21 Reactions assisted by heterogeneous cata-
lysts have revolutionized the domain of organic synthesis
due to higher yields, easy work up, recyclability of the
catalysts and consequent minimization of waste production.
Similarly, microwave19 induced synthesis has become the
flavour of contemporary organic research due to its spectac-
ular benefits like enhancement in reaction rates and yields,
and the attendant advantages of energy efficiency. In this
context, we herein wish to report an efficient and chemo-
selective methodology for the preparation of various (E)-
cinnamic esters from cinnamaldehydes or cinnamyl alcohols
in one pot using 2,3-dichloro-5,6-dicyanobenzoquinone
(DDQ)21 and heterogeneous catalyst under microwave.
R1
O
R1
O
2. Results and discussion
R2
R3
R2
R3
H
Amberlyst-15/DDQ
R'OH/Toluene
OR'
In continuation of our unflinching strive for microwave-
assisted synthesis of bioactive compounds,22 it was fascinat-
ing to develop an effective methodology for conversion of
cinnamaldehydes into cinnamates in one step with the help
of DDQ as an oxidizing agent21,23 and a heterogeneous
catalyst. To begin with, cinnamaldehyde (1a) was treated
R5
where
R5
R4
R4
R
1 to R5 = OMe, O-CH2-O, H, Br, NO2 etc.
R'= methyl, ethyl, amyl, benzyl etc.
Scheme 1.
Table 1. Synthesis of (E)-cinnamic esters from cinnamaldehydes or cinnamyl alcoholsa
R1
O
R1
R4
O
R2
R3
R2
R3
H
Amberlyst/DDQ
OR'
R5
where
R'OH/Toluene, MW
R5
R4
R
1 to R5 = OMe, O-CH2-O, H, Br, NO2 etc.
R' = methyl, ethyl, amyl, benzyl etc.
Entry
1
Substrate (a)
Reaction
time (min)
Product (b)
Yieldb (%)
Reference
4i,25j
O
O
30
96
OMe
O
H
O
H
H
2
3
4
35
35
40
92
91
89
25b,c,j
4a,25c
25j
OMe
MeO
MeO
O
O
O
MeO
MeO
MeO
MeO
OMe
OMe
O
O
O
O
O
OMe
(continued)

1002
A. K. Sinha et al. / Tetrahedron 63 (2007) 1000–1007
Table 1. (continued)
Entry
Substrate (a)
Reaction
time (min)
Product (b)
Yieldb (%)
Reference
25c,i
O
O
OMe
OMe
OMe
H
H
H
5
35
35
40
92
MeO
MeO
OMe
OMe
OMe
OMe
O
O
O
O
MeO
MeO
MeO
MeO
OMe
OMe
6
7
91
89
26
26
OMe
OMe
OMe
OMe
MeO
MeO
O
O
8
9
60
50
51
63
25e
26
H
OMe
O
Me
Me
O
O
O
O
O
H
OMe
Br
O
Br
O
10
11
12
H
120
50
OMe
7
25f
NO2
NO2
O
90c
87c
4i,25j
25b,c,j
OH
OMe
O
OH
50
OMe
MeO
MeO
O
H
O
13
120
0
9b
OMe
O
O
14
15
120
120
0
0
9b
26
OMe
H
MeO
MeO
O
O
OMe
H
16
120
0
26
COOMe
CHO
a
The reaction was carried out under microwave fitted with Dean–Stark apparatus, using 1a (37.5 mmol), DDQ (56.3 mmol), MeOH (4–5 mL), toluene (25 mL)
and Amberlyst-15 (0.1–0.2 g).
Yield of the isolated product based on 1a.
Amount of DDQ used, 75 mmol.
b
c

A. K. Sinha et al. / Tetrahedron 63 (2007) 1000–1007
1003
Further, the yield was low in the case of a-methyl and
2-bromo substituted cinnamic esters (entries 8b and 9b),
while the ester (entry 10b) was formed only in traces (7%
based upon NMR analysis of crude product) in the case of
2-nitrocinnamaldehyde even upon irradiation for 120 min.
It is worthwhile to note that a couple of the above reactions
were also performed in DDQ alone, i.e., without any
Amberlyst-15, however, no product was formed even after
120 min of irradiation. This proves beyond doubt, the utility
of heterogeneous catalyst.
evident indispensable requirement of an aromatic ring with
a,b-unsaturated aldehydic side chain is an interesting facet,
and to the best of our knowledge has been reported for the
first time. It is pertinent to mention here that this pronounced
selectivity can be a significant addition to the growing
arsenal for chemoselective esterification in total synthesis
of complex organic compounds including natural products.
In order to ascertain the efficacy of the heterogeneous cata-
lyst, a variety of them were scanned for esterification of 1a
with methanol into 1b and the results are shown in Table 3.
The order of the activities of various catalysts was found
to be Amberlyst-15>neutral alumina>Amberlyte IR-120>
acidic alumina>silica gel (60–120 mesh). It may be men-
tioned here that organic acids like acetic acid and p-toluene-
sulfonic acid were also tried and found effective (88% yield).
However, the approach suffers from several limitations
It is also evident from Table 1 that benzaldehydes (entries
13a and 14a) as well as the aliphatic aldehydes (entries
15a and 16a) did not undergo the above reaction, thus indi-
cating a probable chemoselectivity towards cinnamalde-
hydes as compared to aromatic or aliphatic aldehydes. In
order to test this hypothesis, the above method was subjected
to an equimolar mixture of cinnamaldehyde and aromatic
aldehyde (Table 2). To our surprise, cinnamaldehyde got
preferentially esterified in comparison to the aromatic alde-
hyde. Similarly, cinnamaldehyde was selectively esterified
in comparison to the aliphatic aldehyde (Table 2). The
ꢀ
vis-a-vis the use of heterogeneous catalyst, as not only does
it entail subsequent neutralization and tedious workup, but
also the use of non-recyclable chemicals, which goes against
the basic tenets of ecofriendly chemical practices.13
Similarly, the impact of nature of alcohol (Table 4) on the
esterification reaction with cinnamaldehyde (1a) was also
examined and it was found that MeOH provided maximum
yield of the corresponding methyl esters in comparison to
other alcohols. Also that branching in the structure of the
alcohol seems to reduce the yield of the ester as in the
case of isopropyl and isoamyl alcohols (entries 6 and 7)
while tertiary butanol (entry 8) did not give any product. Fur-
ther, the diol (entry 9) provided poor yield of the ester while
its mono methylated counterpart (entry 10) gave excellent
yield. Interestingly, cinnamyl alcohol (entry 11) gave only
5% of the expected esterification product, with the bulk of
the alcohol being oxidized by DDQ to cinnamaldehyde as
is evident from Table 4. Further, no formation of the ester
was observed in the case of 1-phenyl-1-propanol (entry 12)
or benzyl alcohol (entry 13), rather the alcohols were
oxidized to 1-phenyl-1-propanone and benzaldehyde, re-
spectively, under these conditions and 1a was recovered
unreacted. However, 3-phenyl-1-propanol (entry 14) did give
3-phenylpropyl-3-phenylpropenoate, an important cytotoxic
ester,2a in 89% yield. These observations apparently reflect
the relative tendency of DDQ to oxidize benzylic alcohols.
It may be mentioned here that the applicability of this
method to wide range of alcohols (Table 4) is a significant
improvement over the earlier methods, which were mostly
limited to methanol, ethanol or n-butanol.7–9,17,18
Table 2. Esterification of equimolar mixtures of cinnamaldehyde and benz-
aldehyde (entry 1), cinnamaldehyde and crotonaldehyde (entry 2)a
O
O
Amberlyst/DDQ
R'OH/Toluene, MW
R
R
OR'
H
Entry
Substrate
Product
Reaction Yield Conversion
time
(min)
(%) (%)
92 100
CHO
COOMe
1
50
+
+
CHO
COOMe
0
0
CHO
COOMe
90 100
2
50
+
+
COOMe
CHO
0
0
a
The reaction was carried out under microwave fitted with Dean–Stark
apparatus using both the substrates in 1:1 ratio (18.75 mmol each), DDQ
(56.3 mmol), MeOH (4–5 mL), toluene(20–25 mL) and Amberlyst (0.2 g).
Table 3. Effect of heterogeneous catalyst on the synthesis of methyl cinnamate from cinnamaldehydea
O
CHO
DDQ-heterogeneous catalyst, MeOH, toluene
OMe
MW
Support
Reaction time (min)
Product yield (%)
Amberlyst-15
30
120
40
50
120
96
94
93
89
85
Alumina (neutral)
Amberlyte IR-120
Alumina (acidic)
Silica gel
a
The reaction was carried out under microwave fitted with Dean–Stark apparatus using 1a (37.5 mmol), DDQ (56.3 mmol), MeOH (4–5 mL), toluene
(20–25 mL) and heterogeneous catalyst (0.1–0.2 g).

1004
A. K. Sinha et al. / Tetrahedron 63 (2007) 1000–1007
Table 4. Effect of nature of alcohols on the synthesis of (E)-cinnamic esters from cinnamaldehyde
Entry
Alcohol
Reaction
time (min)
Product
Yield (%)
96
Reference
4i,25j
O
1
MeOH
30
35
30
40
OMe
O
O
2
3
4
EtOH
89
92
91
4h,25h
25g
OEt
n-BuOH
O-Bu
OC8H17
OC12H25
O
CH3(CH2)6CH2OH
25l
O
5
CH3(CH2)10CH2OH
40
60
90
31
79
0
26
O
iPr-OH
25d
25a
4k
i
6
O-Pr
O
7
iAmyl-OH
40
i
O-amyl
O
tBu-OH
120
60
t
8
O- Bu
O
9
(CH2OH)2
35
92
5a
25k
25k,26
2a
O-CH2-CH2OH
O
10
11
(OMe)CH2CH2OH
30
O-CH2CH2(OMe)
O
O
O
40
O
O
OH
12
13
14
40
40
0a
26
26
O
O
0a
OH
O
O
OH
OH
40
40
89
2a
—
O
O
15
88b
O
a
Instead of expected esters, the alcohols (entries 11–13) were oxidized to cinnamaldehyde, 1-phenyl-l-propanone and benzaldehyde, respectively.
Synthetic procedure and spectral data of this new compound is given in Section 4.
b

A. K. Sinha et al. / Tetrahedron 63 (2007) 1000–1007
1005
O
OR'
CHO
OR'
DDQ
amberlyst
R'OH
OR'
R
R
R
R' = methyl, i-propyl, n-butyl, dodecyl etc.
R = OMe, O-CH2-O, H, Br, NO2
Scheme 2.
Table 5. Trapping of intermediate from cinnamaldehydesa
efforts to extend the scope of the developed protocol are
currently underway.
Entry
Substrate
Reaction
time
(min)
Product
Yield
(%)
4. Experimental section
4.1. General remarks
O
OMe
1
30
30
30
71
73
73
H
OMe
Melting points were determined with a Mettler FP80 micro-
melting point apparatus and are uncorrected. Column
chromatography was performed on neutral alumina. 1H
(300 MHz) and 13C (75.4 MHz) NMR spectra were recorded
on a Bruker Avance-300 spectrometer. CEM DiscoverÓ
focused microwave (2450 MHz, 300 W) was used wherever
mentioned.
O
OMe
OMe
OMe
OMe
OMe
OMe
H
2
MeO
MeO
O
OMe
OMe
MeO
MeO
MeO
MeO
3
H
4.2. General procedure for the chemoselective esterifi-
cation of a,b-unsaturated cinnamaldehydes and
cinnamyl alcohols (Tables 1 and 4)
a
Reaction conditions: cinnamaldehyde (37.5 mmol), Amberlyst-15 (0.2 g),
MeOH (2–3 mL) and toluene (10–15 mL) refluxed under microwave fitted
with Dean–Stark. Reaction mixture is filtered, concentrated, directly
loaded on a neutral alumina column and eluted with toluene. Product
analysed on the basis of NMR.
The synthesis of 20-cyclohexylethyl 3-phenylpropanoate
(Table 4, entry 15) as given below is representative of
the general procedure employed for the synthesis of (E)-
cinnamic esters (Tables 1 and 4). The mixture containing
cinnamaldehyde (37.5 mmol), DDQ (56.3 mmol), 2-cyclo-
hexyl-ethanol (75 mmol) and toluene (10–15 mL) was taken
in a round-bottom flask and heterogeneous catalyst (Amber-
lyst-15, 0.1–0.2 g) was added to it. The flask was shaken
well and irradiated under focused monomode microwave
system (100 W, 110 ꢀC) fitted with Dean–Stark apparatus
for 40 min. Thereafter, the precipitated DDQH2 and Amber-
lyst-15 were filtered. The filtrate was passed over a bed of
neutral alumina column and eluted with 2–5% mixture of
methanol in toluene. The obtained organic layer after evap-
oration under vacuum provided 8.51 g (88%) of 20-cyclo-
hexylethyl 3-phenylpropanoate as viscous liquid; IR (KBr)
Mechanistically, the reaction is believed to occur by an
Amberlyst-promoted attack of alcohol on cinnamaldehyde
to form the hemiacetal,14 which subsequently gets oxidized
with DDQ to form the cinnamic ester (Scheme 2). To rein-
force the above postulate, cinnamaldehydes were reacted
with MeOH in Amberlyst-15 alone, i.e., without DDQ and
the product was confirmed to be hemiacetal14 (Table 5)
on the basis of NMR investigations. It was also noticed
that the addition of DDQ after formation of intermediate
hemiacetal in the reaction mixture provided the required
(E)-cinnamic ester. Overall, the process not only provides
the esters25,26 (Tables 1 and 4) in high yield after just passing
the crude mixture through a bed of alumina, but also gives an
opportunity to reuse the catalyst and oxidant.27
1685 (C]O); 1H NMR (CDCl3)
d 7.64 (1H, d,
J¼15.64 Hz), 7.46 (2H, m), 7.30 (3H, m), 6.40 (1H, d,
J¼15.64 Hz), 4.20 (2H, t, J¼6.86 Hz), 1.70 (4H, m), 1.57
(3H, m), 1.35 (1H, m), 1.20 (3H, m), 0.94 (2H, m);
13C NMR (CDCl3) d 167.1, 144.5, 134.5, 130.2, 128.8,
128.0, 118.3, 62.8, 36.1, 34.6, 33.2, 26.5 and 26.2. HREIMS
data: m/z [M+H]+ for C17H23O2 calculated 259.3698,
observed 259.3664. The spectral data and physical pro-
perties of all other (E)-cinnamic esters (Tables 1 and 4)
were found matching with the reported values2,4,25,26 (see
the Supplementary data for details).
3. Conclusion
In conclusion, heterogeneously catalyzed oxidation of sub-
stituted cinnamaldehydes or cinnamyl alcohols with DDQ
is a clean and atom economical method and to the best of
our knowledge, reported for the first time for the chemoselec-
tive synthesis of various (E)-cinnamic esters in high yield in
an energy efficient manner without either using strong acids
or hazardous reagents. Moreover, the pronounced chemo-
selectivity of this method towards a,b-unsaturated aromatic
aldehydes over aromatic or aliphatic aldehydes has the
potential to eliminate the tedious protection–deprotection
sequences in complex natural product synthesis. Further,
In the above reaction, the filtered DDQH2 and Amberlyst-15
were dissolved in alcohol (methanol/ethanol), which led to
the complete separation of DDQH2 and Amberlyst-15. The
recovered Amberlyst-15 was consecutively reused three

1006
A. K. Sinha et al. / Tetrahedron 63 (2007) 1000–1007
5618; (b) Schlessinger, R. H.; Iwanowitz, E. J.; Springer, J. P.
J. Org. Chem. 1986, 51, 3070.
times with a minimum variation in the yield of cinnamic
esters. Regeneration of DDQ from DDQH2 has been
reported earlier.27 In addition, the solvent mixture (alcohol
and toluene) recovered from reaction mixture or after
column purification was found suitable for the next batch
as our protocol did not require aqueous work up.
9. (a) Walia, J. S.; Vishwakarma, L. C. J. Chem. Soc., Chem.
Commun. 1969, 396a; (b) Smith, A. B.; Sulikowski, G. A.;
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11. Bal, B. S.; Childers, W. E.; Pannick, H. W. Tetrahedron 1981,
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Acknowledgements
Two of us (A.S. and V.K.) are indebted to CSIR and UGC
Delhi, respectively, for the award of SRF. The authors grate-
fully acknowledge the Director of IHBT, Palampur for his
kind cooperation and encouragement.
12. Milovanovic, J. N.; Vasojevic, M.; Gojkovic, S. J. Chem. Soc.,
Perkin Trans. 2 1991, 1231.
13. (a) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and
Practice; Oxford University Press: New York, NY, 1998; (b)
Mason, T. J.; Cintas, P. Handbook of Green Chemistry and
Technology; Clark, J., Macquarrie, D., Eds.; Blackwell:
London, 2002; p 372.
14. Rhee, H.; Kimb, J. Y. Tetrahedron Lett. 1998, 39, 1365.
15. Tohma, H.; Maegawa, T.; Kita, Y. Synlett 2003, 723.
16. List, B.; Doehring, A.; Fonseca, M. T. H.; Job, A.; Torres, R. R.
Tetrahedron 2006, 62, 476.
Supplementary data
Supporting information available: spectral data of some
representative (E)-cinnamic esters. Supplementary data
associated with this article can be found in the online ver-
sion, at doi:10.1016/j.tet.2006.11.011.
17. Zeitler, K. Org. Lett. 2006, 8, 637.
18. Wang, R.; Twamley, B.; Shreeve, J. M. J. Org. Chem. 2006,
71, 426.
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