Synthesis of deuterium-labelled meloxicam and piroxicam

Article abstract of DOI:10.1002/jlcr.1261

Four step syntheses of deuterium-labelled meloxicam and piroxicam from saccharin via selective CD₃I alkylation are described. Labelled oxicams are of great interest for qualitative and/or quantitative isotope dilution-mass spectrometry, coupled with liquid chromatography, currently performed in anti-doping and forensic laboratories. Copyright

Full text of DOI:10.1002/jlcr.1261

Journal of Labelled Compounds and Radiopharmaceuticals
J Label Compd Radiopharm 2007; 50: 207–210.
Published online in Wiley InterScience
JLCR
(www.interscience.wiley.com). DOI: 10.1002/jlcr.1261
Note
Synthesis of deuterium-labelled meloxicam and piroxicam
´ ´
FREDERIC BALSSA* and YVES BONNAIRE
`
Laboratoire des Courses Hippiques, 15 rue de Paradis, Verrieres le Buisson 91370, France
Received 7 December 2006; Revised 12 January 2007; Accepted 14 January 2007
Abstract: Four step syntheses of deuterium-labelled meloxicam and piroxicam from saccharin via selective CD3I
alkylation are described. Labelled oxicams are of great interest for qualitative and/or quantitative isotope dilution-
mass spectrometry, coupled with liquid chromatography, currently performed in anti-doping and forensic
laboratories. Copyright # 2007 John Wiley & Sons, Ltd.
Keywords: meloxicam; piroxicam; synthesis; labelled; deuterium
Introduction
Three reliable methods for the preparation of 1,2-
benzothiazidic oxicams have been published.14 Of
those, the method starting from saccharin is the most
appropriate for easy and rapid preparation of labelled
meloxicam and piroxicam (Figure 2).13
Meloxicam and piroxicam are members of the
oxicam class of non-steroidal anti-inflammatory drugs
(Figure 1). There is a need for stable labelled meloxicam
and piroxicam as internal standards for LC/MS
analyses of the native drugs in equine urine. These
oxicams should be labelled at non-exchangeable posi-
tions, they should be chemically and isotopically pure
(preferably as a single isotopomer) and should exhibit a
minimum shift of 3 amu on the molecular ion.
Saccharin 1 was readily alkylated with methyl chlor-
oacetate in the presence of sodium hydride in dimethyl-
formamide to give 2. This compound was rearranged to
3 in the presence of sodium methylate. The reaction
conditions (temperature, nature of the solvent, reaction
duration) were found to be critical with respect to the
yield and product purity.13 The labelling was obtained
by alkylation of the benzothiazine nitrogen of 3 with
CD3I. Ester 4 was then reacted with the appropriate
amine in order to obtain the targeted molecule. Melox-
icam-d3 5 was purified by chromatography on a silica
gel column using dichloromethane/methanol (0–2%) as
the eluent. A mixture of toluene/acetic acid (9:1) was
found to be superior in TLC experiments.
Results and discussion
A review of the literature indicates that several oxicams
labelled with 14C or 3H at various positions have been
used for metabolic pathway elucidation (Table 1).
Unfortunately, the reported syntheses are not satisfac-
tory for our purpose: labelling has been performed at
exchangeable positions,2 a too brief description is
given, for example, ‘Isoxicam was radiolabelled [. . .] by
methylation of the desmethyl precursor [. . .] with [14C]-
methyl iodide’,5 and only one labelled isotope is
introduced.1–12 A total synthesis of [14C]-sudoxicam
was described.1 However, the nature of the base used
in the first step is not indicated and the article
describing the preparation of the key precursor13 refers
to a German paper published in 1897, without experi-
mental details. The other articles do not indicate any
preparative procedure.
The disappearance of the 3H signal at 2.91 ppm
indicates complete labelling of the N-methyl. Calcu-
lated ratio (m-H)/z 350/353 ðd0=d3Þ ¼ 0:008.
Piroxicam-d3
6 was also purified by silica gel
chromatography. However, dissolution in aqueous
sodium hydroxide solution followed by precipitation
with HCl was found to be a valuable alternative.
The disappearance of the 3H signal at 2.96 ppm
indicates complete labelling of the N-methyl. Calcu-
lated ratio (m-H)/z 330/333 ðd0=d3Þ ¼ 0:017.
Experimental
*Correspondence to: Fre´de´ric Balssa, Laboratoire des Courses Hippi-
ques, 15 rue de Paradis, Verrie`res le Buisson 91370, France.
E-mail: lch-fbalssa@wanadoo.fr
All reagents were obtained from Sigma-Aldrich (St
Quentin Fallaviers, France) except 2-amino-5-
Copyright # 2007 John Wiley & Sons, Ltd.

208 F. BALSSA AND Y. BONNAIRE
Figure 1 Some nonsterodial anti-inflammatory drugs of the oxicam class.
Table 1 Radiolabelled oxicams
Molecule
Isotope
Labelled position
References
Sudoxicam
Piroxicam
Isoxicam
Meloxicam
Tenoxicam
14C
3H
14C
14C
14C
C-3 of the thiazide ring
Aromatic ring
N-Methyl, C-3 of the thiazide ring
N-Methyl, carbonyl of the amide
C-4 of the thiazide ring
1
2
3–7
8–11
12
methylthiazole (Lancaster, Strasbourg, France). Mass
spectra were recorded using a LCQ (Thermo-Electron)
equipped with a C18AB column (Nucleosil). 1H NMR
spectra were recorded on a Bruker ARX250 instru-
ment.
DMSO (20 mL) maintained at 308C by cooling with ice-
water. After 4 min, the solution was poured into HCl
(3N, 50 mL) and extracted with CH2Cl2 (3 ꢀ 100 mL).
The organic layer was washed with water until deco-
loration, dried with brine (100 mL) and anhydrous
Na2SO4, and distilled to give a yellow oil. Cold water
(100 mL) was added to a well-stirred solution of the
yellow oil in ethanol (10 mL). The precipitate of 3 was
recovered by filtration and dried in air (yellow solid,
3.21 g, 64.2%).
3-Oxo-1,2-benzoisothiazoline-2-acetic acid methyl
ester 1,1-dioxide (2)
Saccharin 1 (10.0 g, 54.6 mmol) in DMF (15 mL) was
added dropwise to
a suspension of NaH (1.43 g,
59.5 mmol) in DMF (5 mL) at 08C. After 10 min stirring,
methyl chloroacetate (4.8 mL, 54.7 mmol) was added.
This mixture was stirred for another 1 h at room
temperature and 2.5 h at 808C. After cooling in an ice
bath, the mixture was poured into a well-stirred ice-
water (100 mL). The precipitate was recovered by
filtration, washed with cold water (50 mL) and dried
under vacuum (white solid, 8.95 g, 65%).
2-Methyl-4-hydroxy-2H-1,2-benzothiazine-3-car-
boxylic acid methyl ester 1,1-dioxide-d3 (4)
A
mixture of 3 (2.0 g, 7.8 mmol), CD3I (1.6 mL,
25.8 mmol) and NaOH (32%, 830 mL, 8.6 mmol) in water
(16 mL) and ethanol (30 mL) was stirred 24 h in a sealed
flask. After the addition of water (50 mL) the precipitate
was recovered by filtration, washed with water (50 mL)
and dried (yellow crystals, 1.12 g, 52.6%).
4-Hydroxy-2H-1,2-benzothiazine-3-carboxylic acid
methyl ester 1,1-dioxide (3)
Meloxicam-d3 (5)
Solid 2 (5.0 g, 19.6 mmol) was added in one portion into
A mixture of 4 (301 mg, 1.1 mmol) and 2-amino-5-
a well-stirred solution of MeONa (2.45 g, 45.4 mmol) in
methylthiazole (262 mg, 2.3 mmol) was heated under
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 207–210
DOI: 10.1002.jlcr

SYNTHESIS OF DEUTERIUM-LABELLED MELOXICAM AND PIROXICAM 209
Figure 2 Syntheses of meloxicam-d3 and piroxicam-d3.
reflux in m-xylene (15 mL) under nitrogen for 24 h. After
12 h stirring at room temperature, the precipitate was
recovered by filtration, washed with m-xylene (20 mL),
hexane (2 ꢀ 20 mL) and dried. This brown solid was
purified by chromatography on a silica gel column
(20 ꢀ 3 cm, dichloromethane/methanol 0–2%) to give 5
(yellow solid, 235 mg, 60%).
1H NMR (CDCl3) ppm: 8.07 (m, 1H), 7.92 (m, 1H),
7.78 (m, 2H), 7.22 (s, 1H), 2.46 (s, 3H). LC/MS (ESI-),
single peak, m/z: 353 (MꢁHꢁ), 289, 213.
J ¼ 5 Hz, 1H). LC/MS (ESI -), single peak, m/z: 333
(MꢁHꢁ), 269, 213.
Conclusion
Simple and efficient syntheses of deuterium-labelled
meloxicam and piroxicam have been described. The
main advantages of these procedures are: excellent
chemical and isotopic purities; facile processing and
purification; low cost and the possibility of labelling
with other isotopes of hydrogen or carbon.
Piroxicam-d3 (6)
Acknowledgements
A mixture of 4 (89.4 mg, 0.33 mmol) and 2-aminopyr-
idine (64.3 mg, 0.68 mmol) was heated under reflux in
m-xylene (15 mL) under nitrogen for 24 h. After distilla-
tion of the solvent, the unreacted starting materials
were removed by chromatography on a silica gel
column (20 ꢀ 3 cm, chloroform). The yellow solid was
recrystallised from boiling methanol to give 6 (light
yellow crystals, 76 mg, 68.5%).
We would like to thank Yves Moulard for LC/MS
analyses, Dr Me´lanie Ethe`ve-Quelquejeu for NMR
spectroscopy and Dr Louis Dehennin for helpful
assistance in editing the manuscript.
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