Preparation of [carboxy-13C]4-nitrophenylacetic acid

Article abstract of DOI:10.1002/jlcr.917

Reaction of sodium [¹³C]cyanide with excess benzyl chloride gave ～75% utilization of the isotope. Subsequent nitration, isomer separation and hydrolysis of the nitrile gave the required carboxy-labelled 4-nitrophenylacetic acid. Copyright

Full text of DOI:10.1002/jlcr.917

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2005; 48: 231–233.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.917
Note
Preparation of [carboxy-13C]4-nitrophenylacetic acid
John E. T. Corrie* and V. Ranjit N. Munasinghe
National Institute for Medical Research, The Ridgeway, Mill Hill, London
NW7 1AA, UK
Summary
Reaction of sodium [13C]cyanide with excess benzyl chloride gave ꢀ75% utilization
of the isotope. Subsequent nitration, isomer separation and hydrolysis of the nitrile
gave the required carboxy-labelled 4-nitrophenylacetic acid. Copyright # 2005 John
Wiley & Sons, Ltd.
Key Words: nitration; chromatography; NMR spectra
Introduction
As part of a quantitative study of photodecarboxylation reactions using
rapid-scan IR spectroscopy, we needed a compound that undergoes ef-
ficient photodecarboxylation and that could easily be labelled with 13C in its
carboxy group. The quantity of [12C]CO2 formed from the test substance
could then be related to the quantity of [13C]CO2 formed from the standard
using the relative intensity of the normal and isotopic CO2 bands in the IR
spectra. 4-Nitrophenylacetic acid 1 (as its anion) is known to photo-
decarboxylate with high quantum yield1 and appeared to be a suitable
choice.
Results and discussion
A simple route to isotopically labelled 1 should a priori be available by
displacement on 4-nitrobenzyl chloride or bromide with labelled cyanide ion,
followed by hydrolysis. Although such procedures have been described
(preferably with a mixture of NaCN and HCN, or the equivalent produced by
adding a strong acid such as TFA to NaCN), they are inefficient in terms of
cyanide incorporation, so are not well suited for isotopic synthesis. Kalir and
Mualem2 described these procedures and reviewed relevant literature on
unwanted dimeric by-products that are formed when displacement is
*Correspondence to: Dr J. E. T. Corrie, National Institute for Medical Research, The Ridgeway, Mill Hill,
London NW7 1AA, UK. E-mail: jcorrie@nimr.mrc.ac.uk
Copyright # 2005 John Wiley & Sons, Ltd.
Received 19 November 2004
Accepted 22 November 2004

232
J. E. T. CORRIE AND V. R. N. MUNASINGHE
*
*
*
CH2Cl
CH2CN
CH2CN
CH2CO2H
b
a
c
NO2
NO2
2
3
1
Scheme 1. Reagents: (a) Na[13C]CN–aq. EtOH, then thiourea; (b) NH4NO3–
TFAA; (c) H2O–H2SO4–HOAc, heat
attempted without HCN. The most direct alternative route to 1 involves
preparation of labelled benzyl cyanide 2, followed by nitration, isolation of the
required 4-nitrobenzyl cyanide 3 and acidic hydrolysis (Scheme 1). These steps
have been described previously, but we report here their adaptation to small-
scale isotopic synthesis.
In a modification of a known procedure,3 sodium [13C]cyanide was allowed
to react with excess benzyl chloride in aqueous ethanol, and residual benzyl
chloride was then destroyed with thiourea. Benzyl [13C]cyanide has been
reported by other workers who used either the original aqueous ethanol
procedure3,4 or in better yield in acetonitrile with 18-crown-6 as catalyst.5 Our
modified procedure gave labelled benzyl cyanide with 75% isotope utilization.
Nitration of benzyl cyanide with nitric–sulfuric acid mixture is well known,6
but for small-scale work ammonium nitrate in TFAA7 was more convenient
and gave the 2- and 4-nitro isomers in ꢀ1:3 ratio. Flash chromatography gave
pure 4-nitrobenzyl cyanide 3 in 50% yield. Acidic hydrolysis as described6 then
yielded the required acid 1.
Experimental
Sodium [13C]cyanide (99% isotopic abundance) was from Cambridge Isotope
Laboratories, Andover, MA. Flash chromatography was on Merck 9385
silical gel. NMR spectra were determined in CDCl3 on either JEOL FX90Q or
Varian Unityplus 500 spectrometers.
Benzyl [13C]cyanide 2
A solution of benzyl chloride (4.0 g, 31.6 mmol) in EtOH (5 ml) was added
dropwise over 45 min to a solution of sodium [13C]cyanide (1.0 g, 20 mmol)
in water (1.8 ml) and the mixture was refluxed for 6 h. Thiourea (1.14 g,
15 mmol) and EtOH (10 ml) were added and the mixture was refluxed for a
further 1 h. The solution was concentrated under reduced pressure and part-
itioned between Et2O and water. The organic phase was washed with
water, dried and evaporated to a yellow oil and purified by flash chromato-
Copyright # 2005 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2005; 48: 231–233

[
13C]4-NITROPHENYLACETIC ACID
233
1
graphy [EtOAc–light petroleum, 1:9] to give 2 (1.76 g, 74.6%), H-NMR
2
(90 MHz): d 7.33 (5H, br s, Ar-H), 3.73 (2H, d, JC-H=10.5 Hz, CH2).
4-Nitrobenzyl [13C]cyanide 3
Ammonium nitrate (1.15 g, 14.4 mmol) and trifluoroacetic anhydride (14.4 ml)
were added to benzyl [13C]cyanide 2 (1.7 g, 14.4 mmol) and the solution was
stirred at room temperature. It warmed spontaneously to reflux within 15 min
and was stirred for a further 1.25 h at room temperature, then diluted carefully
with water (70 ml) and the aqueous layer was extracted with Et2O. The organic
phase was washed with aq. NaHCO3 and brine, dried and evaporated. The
mixture of isotopic 2- and 4-nitrobenzyl cyanides was separated by flash
chromatography (EtOAc–light petroleum, 1:1). Silica gel TLC Rf values in the
same solvent were 0.40 and 0.36 for the 2- and 4-nitro isomers, respectively.
4-Nitrobenzyl [13C]cyanide 3 was obtained as a pale yellow solid (1.16 g, 50%)
that was used without further purification. 1H-NMR (90 MHz): d 8.25 (2H, d,
J=8.7 Hz, Ar-H3,5), 7.54 (2H, d, J=8.7 Hz, Ar-H2,6), 3.88 (2H, d,
2JC-H=10.6 Hz, CH2).
[carboxy-13C]4-Nitrophenylacetic acid 1
The cyanide 3 (1.1 g, 6.75 mmol) was mixed with water (2.2 ml), concentra-
ted H2SO4 (2.2 ml) and glacial acetic acid (2.2 ml) and refluxed for 1 h.
The mixture was diluted with water (55 ml) and extracted with Et2O. The or-
ganic extract was dried and evaporated to give [carboxy-13C]4-nitropheny-
lacetic acid 1 as beige crystals (1.1 g, 90%) m.p. 149–1518C (from
aqueous ethanol). UV [EtOH–25 mM Na phosphate, pH 7 (1:9 v/v)]: lmax
1
287 nm (e 10 900 MÀ1 cmÀ1); H-NMR (500 MHz): d 8.21 (2H, d, J=8.7 Hz,
2
Ar-H3,5), 7.47 (2H, d, J=8.7 Hz, Ar-H2,6) 3.78 (2H, d, JC-H=7.8 Hz,
CH2).
Acknowledgements
We are grateful to the MRC Biomedical NMR Centre for access to facilities.
References
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Copyright # 2005 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2005; 48: 231–233