Microwave-enhanced, solvent-free synthesis of singly and doubly 13C-labelled trans-cinnamic acid at the α- and β-carbon positions

Article abstract of DOI:10.1002/jlcr.878

¹³C-labelled trans-cinnamic acid (3-phenyl-2-propenoic acid) has been synthesized in one step using benzaldehyde-carbonyl-¹³C and malonic acid-2-¹³C in the presence of ammonium acetate under microwave irradiation and solve

Full text of DOI:10.1002/jlcr.878

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2004; 47: 895–901.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.878
Note
Microwave-enhanced, solvent-free synthesis of singly
and doubly C-labelled trans-cinnamic acid at the
a- and b-carbon positions
13
Guibin Ma and Sophia E. Hayes*
Department of Chemistry, Washington University in St. Louis, St. Louis,
MO 63130-4899, USA
Summary
13C-labelled trans-cinnamic acid (3-phenyl-2-propenoic acid) has been synthesized
in one step using benzaldehyde-carbonyl-13C and malonic acid-2-13C in the presence
of ammonium acetate under microwave irradiation and solvent-free conditions.
Copyright # 2004 John Wiley & Sons, Ltd.
Key Words: trans-cinnamic acid; 3-phenyl-2-propenoic acid; 13C label; solid-phase
reaction; microwave synthesis
Introduction
Trans-cinnamic acid (3-phenyl-2-propenoic acid) and its derivatives are widely
used chemicals in a variety of fields. Recently, they have been applied as
antibacterial agents for suppression of bacterial growth1–3 or as a monomer
unit to modify the properties of polymer materials.4,5 Trans-cinnamic acid
undergoes a [2+2] cycloaddition reaction under photoirradiation, and
therefore it has a potential application in memory storage materials.6,7 In
connection with these various applications, 13C-labelled derivatives of
cinnamic acid can serve as valuable mechanistic probes. For example, one
can imagine that the process of polymerization could be monitored by 13C
NMR spectroscopy, or that the interaction between trans-cinnamic acid and a
biological target molecule could be investigated to probe the environment of
the macromolecular domain.
A commonly used route to prepare trans-cinnamic acid is the condensation
of aryl aldehyde and malonic acid, known as the Knoevenagel reaction. The
methods used to synthesize the natural abundance trans-cinnamic acid and its
*Correspondence to: S. E. Hayes, Department of Chemistry, Washington University, 1 Brookings Drive,
Box 1134, St. Louis, MO 63130-4899, USA. E-mail: hayes@wustl.edu
Copyright # 2004 John Wiley & Sons, Ltd.
Received 2 June 2004
Revised 16 July 2004
Accepted 19 July 2004

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G. MA AND S. E. HAYES
derivatives have been very well documented in the literature.8–11 Furthermore,
microwave-assisted synthesis has been utilized for the synthesis of cinnamic
acid,12 for deuteration of cinnamic acid, and for other related derivatives.13
Isotopic 14C labelling of trans-cinnamic acid was first developed by Herbert
et al. in 1968 using the condensation reaction between malonic acid and 14C-
labelled benzaldehyde in pyridine.14 Pichat et al. used the same procedure to
prepare 14C-labelled trans-cinnamic acid that was subsequently reduced by
aluminum hydride to generate 3-14C-cinnamyl alcohol.15 In 1979, this
preparation was modified by using pyridine and piperidine as the condensation
medium.16 By using this method, the preparation of labelled compounds of
2-14C, 2-13C and 2,3-13C cinnamic acids were reported in the literature.17,18
We are investigating [2+2] cycloadditions in the solid-state, where the sp2-
hybridized a- and b-carbons photodimerize with a neighboring molecule to
create four sp3-hybridized cyclobutane carbons. An isotopic label at either, or
both, of the vinyl positions would provide a sensitive probe of that process.
However, few papers address labelling of trans-cinnamic acid,17,18 and no
systematic 13C labelling of either or both the a- and b-carbons of trans-
cinnamic acid has been studied. We report here a rapid, very efficient approach
to 13C-labelled cinnamic acid derivatives using inexpensive ammonium acetate
as a condensation medium.
Results and discussion
Three different 13C isotopomers of trans-cinnamic acid were labelled by the
reaction shown in Scheme 1 using the commercially available compounds,
benzaldehyde-carbonyl-13C and malonic acid-2-13C (Cambridge Isotope
Laboratories, Inc.). The three isotopomers were formed in 67–73% yield
when a mixture of benzaldehyde and malonic acid, or their isotopically
labelled analogues, and ammonium acetate were combined and subjected to
microwave irradiation.
All the reactions were performed in an open Erlenmeyer flask in the absence
of any solvent. The irradiations were carried out in microwave ‘pulses’ of 1 min
each at approximately 600 W with 2 min cooling between pulses. This
procedure enables one to easily monitor the course of the reaction by TLC.
The temperature of the mixture reached a maximum of 110–1208C at the end
of each irradiation, and the conversion was completed within a short time
(approximately 3–4 min after 3–4 microwave ‘pulses’). The glassy reaction
mass was subsequently poured into ice water, and the precipitated trans-
cinnamic acid was isolated by filtration. The melting point was measured to be
132–1348C, indicating that the products were very pure.
1
The 3-13C-labelled trans-cinnamic acids were fully characterized by H and
13C NMR spectroscopy in chloroform-d (positions indicated in Scheme 2).
1
Figure 1(a) depicts the H spectrum of the doubly labelled compound, 3.
Copyright # 2004 John Wiley & Sons, Ltd.
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MICROWAVE-ENHANCED
897
O
O
13C
13C
OH
COOH
H
CH3COONH4
+
H2C
120oC, 3 min
COOH
(1)
O
O
13C
OH
COOH
H
CH3COONH4
H213C
+
120oC, 3 min
COOH
(2)
O
O
13C
13C 13C
OH
COOH
H
CH3COONH4
+
H213C
120oC, 3 min
COOH
(3)
Scheme 1. Synthesis of singly and doubly 13C-labelled trans-cinnamic acid
O
Ha
b
OH
a
Hb
Scheme 2.
Figure 1(b) depicts the 13C spectra also of the doubly labelled compound
(13C{1H} in the lower image and 13C without decoupling in the upper image).
Assignments were made from spectra of singly labelled compounds. The
13C{1H} spectrum of 1 showed a single resonance at 147.6 ppm corresponding
to the a-carbon (Ca) position, and the 1H spectrum showed a multiplet, similar
to that resolved in Figure 1(a) as a doublet of doublets with equal intensities.
The spin–spin coupling observed from the Ha and Hb signals at 7.82 and
1
3
6.44 ppm, respectively, was J (13Ca–1Ha)=156 Hz, J (1Ha–1Hb)=16 Hz and
2J(1Hb–13Ca)=1 Hz. In a similar manner, the 13C{1H} spectrum of 2 exhibited
Copyright # 2004 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2004; 47: 895–901

898
G. MA AND S. E. HAYES
(a)
1J(13Cb-1Hb)
1J(13Ca-1Ha)
3J(1Ha-1Hb)
3J(1Ha-1Hb)
H
H
a
b
8
7.5
7
6.5
6
Chemical Shifts (ppm)
(b)
1J(13Cb-1Hb)
1J(13Ca-1Ha)
1J(13Ca-13Cb)
1J(13Ca-13Cb)
1J(13Ca-13Cb)
1J(13Ca-13Cb)
75
125
Chemical Shifts (ppm)
150
100
1
Figure 1. H (a) and 13C (b) spectra of trans-cinnamic acid doubly 13C-labelled
at the a- and b-carbon positions in CDCl3. (b) (upper expanded image) 13C spectrum
without proton-decoupling; (b) (lower image) proton-decoupled 13C spectrum
1
a single peak at 117.9 ppm corresponding to the b-carbon (Cb), and the H
spectrum produced a similar multiplet pattern with spin–spin couplings of
3
2
1J (13Cb–1Hb)=163 Hz, J (Ha–Hb)=16 Hz, and J(1Ha–13Cb)=3 Hz. The
Copyright # 2004 John Wiley & Sons, Ltd.
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MICROWAVE-ENHANCED
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spin–spin coupling between Ca, Cb, Ha, and Hb were consistent with those
measured for compounds 1 and 2 (Table 1).
The double label allowed us to observe spin–spin coupling between the two
carbons of 1J(13Ca–13Cb)=70.5 Hz at each of the carbon resonances, 147.6 and
117.9 ppm. The 13C spectrum of 3 was also recorded with and without proton
decoupling. The lower spectrum in Figure 1(b) is the proton-decoupled
13C spectrum, and the upper spectrum was recorded without proton
decoupling.
The splitting pattern observed arises from 1J(C–C) coupling (evident in both
13C spectra). In addition, the experiment without proton decoupling shows the
1
effects of J(C–H) coupling, forming a doublet of doublets, as expected. The
same spin–spin coupling constants observed in the proton spectra of
1
1J(13Ca–13Cb)=70.5, 1J (13Cb–1Hb)=163 Hz, and J (13Ca–1Ha)=156 Hz were
detected in the 13C spectrum.
Mass spectroscopy was used to confirm the identity of the products. Mass
spectra of natural abundance trans-cinnamic acid has an intense peak at
148.2 amu. The 13C-labelled compounds 1 and 2 both had spectra that showed
an intense peak at 149.2 amu, clearly demonstrating the incorporation of one
13C isotope. Furthermore, compound 3 exhibited an intense peak at
150.2 amu, indicative of two 13C isotopes being incorporated. Smaller peaks
were observed at 132.2 and 104.2 amu for compounds 1 and 2, and at 133.2
and 105.2 amu of compound 3, arising from the fragment peaks corresponding
to the loss of –OH and –COOH groups, respectively, for these three labelled
compounds.
In summary, three different vinyllic 13C-labelled compounds were prepared
1
in a one-step reaction. The products were fully characterized by H and 13C
NMR and mass spectroscopy in solution. This synthetic route provides an
easy and fast method to produce both singly and doubly 13C-labelled trans-
cinnamic acids that can be used in 13C NMR experiments for biological and
polymerization studies.
Experimental
General procedure
Microwave irradiations were carried out using a commercial microwave oven
1
(Sanyo EMF-3400, 1500 W). H and 13C spectra were recorded on a Varian
300 MHz NMR instrument using a 300 Auto SW PFG probe at resonance
frequencies of 300.119 MHz (1H) and 75.472 MHz (13C). Benzaldehyde-
carbonyl-13C and malonic acid-2-13C were purchased from Cambridge Isotope
Laboratories, Inc. (99% 13C-enriched). 98% ammonium acetate salt was
purchased from ACROS. Mass spectrometry was recorded using a VG
Analytical Mass Spectrometer.
Copyright # 2004 John Wiley & Sons, Ltd.
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G. MA AND S. E. HAYES
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A mixture of benzaldehyde (0.50 ml, 4.9 mmol), malonic acid (0.49 g,
4.7 mmol) and ammonium acetate (0.39 g, 5.1 mmol) was added to an
Erlenmeyer flask and subjected to microwave irradiation at an energy level
of 60% of total power (approximately 600 W, estimated) for approximately
3–4 min. Microwaves were pulsed for units of 1 min, followed by a 2-min
cooling period. The temperature of the reaction flask was recorded to be
ꢀ1208C. After complete conversion as indicated by TLC, the resulting
condensed light-yellow reaction mixture was poured into ice water (25 ml), and
the precipitated solid was filtered and dried under vacuum to afford 0.47–
0.51 g trans-cinnamic acid (yield of 67–73%), melting point 132–1348C.
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J Label Compd Radiopharm 2004; 47: 895–901