Microwave-assisted synthesis of (RS) methyl-2-([2′- 14C]4,6-dimethoxypyrimidin-2′-yloxy)-2-phenyl [1- 14C]ethanoate

Article abstract of DOI:10.1002/jlcr.1078

We report here a facile synthesis of (RS) methyl-2-([2′- ¹⁴C]4,6-dimethoxypyrimidin-2′-yloxy)-2-phenyl [1- ¹⁴C]ethanoate under microwave irradiation. Copyright

Full text of DOI:10.1002/jlcr.1078

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2006; 49: 699–705.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.1078
Research Article
Microwave-assisted synthesis of (RS) methyl-2-
([20-14C]4,6-dimethoxypyrimidin-20-yloxy)-2-phenyl
[1-14C]ethanoate
K. M. Mathew, S. Ravi*, V. K. P. Unny and N. Sivaprasad
Labelled Compounds, Radiopharmaceuticals, Immunoassay, Labelled Compounds, and
Jonaki Board of Radiation and Isotope Technology, BARC Vashi Complex, Navi,
Mumbai 400705, India
Summary
We report here a facile synthesis of (RS) methyl-2-([20-14C]4,6-dimethoxypyrimidin-20-
yloxy)-2-phenyl [1-14C]ethanoate under microwave irradiation. Copyright # 2006
John Wiley & Sons, Ltd.
Received 1 April 2006; Revised 13 April 2006; Accepted 13 April 2006
Key Words: [14C]thiourea,K14CN; [2-14C]4,6-dihydroxy-2-mercaptopyrimidine; [2-
14C]-4,6-dimethoxy-2-methylsulphonylpyrimidine; [1-14C]2-phenyl ethanoic acid;
[1-14C]2-hydroxy-2-phenyl ethanoic acid; mandelic acid; (RS) methyl-2-([20-14C]4,
6-dimethoxypyrimidin-20-yloxy)-2-phenyl [1-14C]ethanoate
Introduction
(RS) methyl-2-(4,6-dimethoxypyrimidin-20-yloxy)-2-phenyl ethanoate is a
herbicide showing excellent herbicidal effects on annual and perennial weed
and high-safety crops, especially rice and wheat and is applied to paddy fields,
ploughed fields and nonagricultural land.1 We report here a facile synthesis
(RS) methyl-2-([20-14C]4,6-dimethoxypyrimidin-20-yloxy)-2-phenyl [1-14C]
ethanoate for its absorption, translocation and metabolism studies in plants.
In recent times, use of microwave technology in organic synthesis have
received wide acceptance as microwave irradiation leads to fast heating of the
chemicals at or above their boiling points thus enhancing the reaction rates
*Correspondence to: S. Ravi, Labelled Compounds, Radiopharmaceuticals, Immunoassay, Labelled
Compounds, and Jonaki Board of Radiation and Isotope Technology, BARC Vashi Complex, Navi,
Mumbai 400705, India. E-mail: lcbrit@rediffmail.com
Copyright # 2006 John Wiley & Sons, Ltd.

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K. M. MATHEW ET AL.
and dramatically reducing the reaction times in comparison with
conventional heating. The combination of solvent-free reaction conditions
and microwave irradiation leads to large reductions in reaction times and
enhancement of yields with several advantages of an eco-friendly approach,
termed ‘green chemistry’. Examples of this technology in organic synthesis are
numerous.2,3
Results and discussion
Condensation of [14C]thiourea 1 with diethyl malonate 2 in the presence of
sodium methoxide gave [2-14C]4,6-dihydroxy-2-mercaptopyrimidine 3.
The product 3 showed a characteristic -SH band at 2570 cmꢀ1 and bands
due to hydroxyl group at 3500–3300 cmꢀ1 and 1050 cmꢀ1 in the IR spectrum.
The product also exhibited, PMR signal at d 5.5–5.7, which on integration
accounted for single proton, and thus confirmed the formation of 3.
Methylation of 3 with diazomethane4,5 furnished [2-14C]4,6-dimethoxy-
2-methylmercaptopyrimidine 4. The structure was confirmed by the
absence of band at 2570 cmꢀ1 due to -SH group and by the absence of
bands at 3500–3300 cmꢀ1 and 1050 cmꢀ1 due to hydroxyl group in IR
spectrum. The structure was additionally confirmed by the presence of
methoxyl group band at 1250 cmꢀ1. Further, it was evident from the
presence of PMR signal at d 3.8, (integrating for six protons) that two
methoxyl groups were present. In addition, presence of PMR signal at d 3.5
was observed. On integration the signal accounted for three protons due to -
SCH3 group, which in turn confirmed the formation of 4. Oxidation of 4 with
H5IO6/CrO3 in ethyl acetate6,7 gave [2-14C]4,6-dimethoxy-2-methylsulphonyl-
pyrimidine 5. The formation of 5 was confirmed by HPLC analysis, by
comparison with the authentic sample. The results obtained were in good
agreement (the product eluted out at 4.980 min, mobile phase was 30%
acetonitrile in water, column ODS 5m). Presence of bands at 1350–1300 cmꢀ1
and at 1160–1110 cmꢀ1 (sulphonyl group bands) in IR spectrum confirmed the
formation of 5.
Toluene 6 was brominated in presence of manganese dioxide8 to give
quantitative yield of benzyl bromide 7. The formation was confirmed by IR
(presence of band at 710 cmꢀ1 due to bromine), boiling point and density. It
was then reacted with K14CN under microwave irradiation to furnish [1-14C]
2-phenyl ethanoic acid 8. The formation was confirmed by absence of band at
710 cmꢀ1 due to bromine and presence of band at 1710 cmꢀ1 due to acid
carbonyl group. This on a-bromination followed by hydroxylation furnished
[1-14C]2-hydroxy-2-phenyl ethanoic acid (mandelic acid) 10. The product on
methylation with diazomethane followed by condensation with 5 in presence
of NaH in toluene under microwave heating (1300 W) for 4 min furnished the
title compound 12 (Scheme 1).
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm. 2006; 49: 699–705
DOI: 10.1002/jlcr

MICROWAVE-ASSISTED SYNTHESIS
701
N
N
N
OCH3
EtO
EtO
O
OCH3
NH2
*
NH2
OH
H3CO2S
HS
H3CS
+
*
*
ii
*
S
C
1
i
iii
N
N
N
O
OH
OCH3
5
OCH3
4
2
3
Br
*
OH
OH
CH3
*
CH2Br
CH2COOH
CHCOOH
*
*
IV
CHCOOH
ii
CHCOOCH3
v,vi
viii
viii
7
6
8
9
10
11
N
OCH3
ix
CH
O
*
5 + 11
N
COOCH3
*
OCH3
12
* denotes 14C label
(i) Sodium, methanol (ii) N-methyl-N-nitrosotoluene-p-sulphonamide, NaOH (iii) periodic acid / chromium trioxide
(iv) MnO2, Br2, (v) TBAB, K*CN, (vi) HCl ; propionic acid, 1 : 2 v/v, MW, (vii) PCl3, Br2 ,
(viii) Aq sodium carbonate, MW, (ix) 60% NaH, toluene, MW
Scheme 1.
Experimental
K14CN and 14C-thiourea were procured from M/s Board of Radiation and
Isotope Technology, Mumbai, India.
The PMR spectra were recorded with a Brucker AC-200 (200 MHz)
spectrometer using CDCl3 as the solvent. The chemical shift (d) values were
expressed in ppm. The IR spectra were recorded with a Jasco FT/IR 460
spectrophotometer. Analytical HPLC performed (Dionex HPLC system) with
C18 5m, BDS Hypersil (100 ꢁ 4.6 mm) column.
LG domestic microwave oven (1300 W) was used for the experiments.
TLC was done on silica gel plates (silica gel 60 F254, Merck), TLC plates
were scanned with Bioscan system, AR 2000 TLC Scanner. All yields were
reported based on radioactivity.
[2-14C]4,6-dihydroxy-2-mercaptopyrimidine 3
Sodium methoxide was generated in situ by adding metallic sodium (55 mg) to
dry methanol (1 ml), followed by stirring for 10 min. The temperature of the
reaction mixture was raised to 608C and [14C]thiourea (1 mmol, specific
activity 185 MBq/mmol) was added to it. The reaction mixture was stirred for
5 min at 608C. To this reaction mass diethyl malonate (1.2 mmol, 182 ml) was
added and the reaction mixture was stirred continuously for 30 min at 608C.
The product formation was confirmed by TLC (silica, solvent: ethyl acetate)
followed by radioactivity scanning. Rf for [14C]thiourea was 0.26 and for
[2-14C]4,6-dihydroxy-2-mercaptopyrimidine was 0.06. The reaction mixture
Copyright # 2006 John Wiley & Sons, Ltd.
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K. M. MATHEW ET AL.
was loaded on to a cation exchanger column (Dowexꢀ50 ꢁ 8, 50–100 mesh) in
H+ form. The column was eluted with water to elute [2-14C]4,6-dihydroxy-2-
mercaptopyrimidine 3. Yield: 80%, IR: (KBr film) 3500–3300 cmꢀ1
2530 cmꢀ1, 1050 cmꢀ1 PMR (CD3C(O)CD3) d 6.4 ppm (1H, s).
,
[2-14C]4,6-dimethoxy-2-mercaptomethylpyrimidine 4
To a cooled solution of N-methyl-N-nitrosotoluene-p-sulphonamide (6 mmol,
1.28 gm) in ether (30 ml), alcoholic sodium hydroxide solution (0.4 gm in 96%
ethanol (10 ml) was added drop wise and the solution stirred for 5 min. The
resulting ethereal solution of diazomethane was distilled into the flask
containing [2-14C]4,6-dihydroxy-2-mercaptopyrimidine (0.5 mmol, specific
activity 185 MBq/mmol) in dry methanol (6 ml). The reaction mixture was
evaporated to dryness under reduced pressure and loaded on to a silica
column. The column was eluted with ethyl acetate to obtain [2-14C]4,
6-dimethoxy-2-mercaptomethylpyrimidine. Yield 85%, TLC (silica, solvent:
ethyl acetate) followed by radioactivity scanning. Rf for [2-14C]4,6-dimethoxy-
2-mercaptomethylpyrimidine was 0.9 and for [2-14C]4,6-dihydroxy-2-mercap-
topyrimidine was 0.06. IR: (KBr film) 2835 cmꢀ1, 1250 cmꢀ1, 717–625 cmꢀ1
PMR (CD3C(O)CD3) d 3.5 (3H, s), d 3.0–4.0 (6H, s) d 6.1 (1H, s).
[2-14C]4,6-dimethoxy-2-methylsulphonylpyrimidine 5
Periodic acid (2.63 mmol, 600 mg) was dissolved in acetonitrile (6 ml) by
stirring at room temperature for 60 min. To this solution, chromium trioxide
(0.125 mmol, 12.5 mg) was added and stirred for 5 min, to give a clear orange
solution. H5IO6/CrO3 solution (1.7 ml) was added to the solution of [2-14C]4,
6-dimethoxy-2-methylmercaptopyrimidine (0.23 mmol, specific. activity
185 MBq/mmol) in ethyl acetate and was stirred at room temperature for
30 min. The reaction mixture was quenched with saturated sodium sulphite
and was loaded on to a silica column. The column was eluted with acetone to
obtain [2-14C]4,6-dimethoxy-2-methylsulphonylpyrimidine. The product was
analysed by HPLC, (30% acetonitrile in water, column ODS 5m, Rt 4.9 min).
Yield 75%. TLC (silica, solvent: chloroform) followed by radioactivity
scanning. Rf for [2-14C]4,6-dimethoxy-2-mercaptomethylpyrimidine was 0.8
and for [2-14C]4,6-dimethoxy-2-methylsulphonyl-pyrimidine was 0.56, IR:
(KBr film) 2835 cmꢀ1, 1350–1300 cmꢀ1, 1250 cmꢀ1, 1160–1110 cmꢀ1, PMR
(CD3C(O)CD3), d 3.5 (3H, s), d 3.8–4.0 (6H, s), d 6.3 (1H, s).
Benzyl bromide
Toluene (47 mmol, 5 ml), MnO2 (47 mmol, 4 g), bromine (48 mmol, 2.5 ml) and
dichloromethane (0.75 ml) were stirred at room temperature for 10 min. After
the reaction was complete, the reaction mixture was filtered over a silica bed
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm. 2006; 49: 699–705
DOI: 10.1002/jlcr

MICROWAVE-ASSISTED SYNTHESIS
703
and washed with hexane. The combined filtrate was distilled off to remove
hexane and to give pure benzyl bromide. Yield: Quantitative (3.5 ml). bp
198–1998C, d 1.44.
[1-14C]2-phenyl ethanoic acid 8
Benzyl bromide (0.18 ml, 1.5 mmol), potassium [14C]-cyanide (0.6 mmol,
specific activity 185 MBq/mmol), tetrabutylammonium bromide (5 mg) and
water (0.10 ml) were sealed in a glass ampoule. The reaction mixture was
heated in a domestic microwave oven (1300 W) for 4 min. The reaction mixture
was cooled and the ampoule was cut open. To it was added a mixture of
concentrated hydrochloric acid:propionic acid (1:2 v/v, 300 ml) and the glass
ampoule was again sealed. It was heated using a domestic microwave oven for
3 min (6 ꢁ 30 s). The ampoule was cut open and the reaction mixture was
extracted with ether. The obtained product was purified by column
chromatography (silica, 15% ethyl acetate in hexane) furnishing [1-14C]
2-phenyl ethanoic acid in a yield of 99.8%, with respect to K14CN. The
radiochemical purity of [1-14C]2-phenyl ethanoic acid was confirmed by TLC
(silica gel, 30% ethyl acetate in hexane, v/v) followed by radioactivity scanning
(Rf for [1-14C]2-phenyl ethanoic acid was 0.2 and for benzyl bromide was 0.8)
IR (KBr film) 1710 cmꢀ1. PMR: (D2O), d 3.64–3.71 (2H, s), d 7.32–7.58
(5H, m) ppm.
[1-14C]2-bromo-2-phenyl ethanoic acid 9
A mixture of [1-14C]2-phenyl ethanoic acid (0.55 mmol, specific activity
185 MBq/mmol), bromine (0.1 ml, 0.5 mmol) and PCl3 (0.1 ml) was initially
heated at 808C for 10 min and then at 1008C for 20 min (till the colour of
bromine disappeared). The reaction mixture was cooled in an ice bath.
Saturated sodium bicarbonate solution was added in drops with stirring until
neutral and the solution was concentrated under reduced pressure. The residue
was purified by column chromatography (silica gel, 0–5% ethyl acetate/
hexane). Yield: 0.5 mmol (90%). The purity was ascertained by TLC (silica gel,
5% ethyl acetate in hexane v/v) followed by radioactivity scanning (Rf for
[1-14C]2-phenyl ethanoic acid was 0.1 and for [1-14C]2-bromo-2-phenyl
ethanoic acid was 0.7). IR (KBr, film) 1720 cmꢀ1, 710 cmꢀ1, PMR: (D2O), d
4.70 (1H, s), d 7.32–7.58 (5H, m) ppm.
[1-14C]2-hydroxy-2-phenyl ethanoic acid (mandelic acid) 10
[1-14C]2-bromo-2-phenyl ethanoic acid (0.5 mmol, specific activity 185 MBq/
mmol) was heated with aqueous saturated sodium carbonate solution (1 ml)
under microwave condition for 2 min (4 ꢁ 30 s) to furnish [1-14C]2-hydroxy-2-
phenyl ethanoic acid. The reaction mixture was purified by column
Copyright # 2006 John Wiley & Sons, Ltd.
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chromatography (silica). The column was first eluted with 5% ethyl acetate
in hexane and then with methanol to elute [1-14C]2-hydroxy-2-phenyl
ethanoic acid. Yield 99.9%. TLC (silica gel, 20% ethyl acetate: hexane v/v)
followed by radioactivity scanning showed the product to be 99% radio-
chemically pure (Rf for [1-14C]2-bromo-2-phenyl ethanoic acid was 0.9 and for
[1-14C]2-hydroxy-2-phenyl ethanoic acid was 0.2). IR (KBr film) 3500–
3000 cmꢀ1, 1720 cmꢀ1, 1050 cmꢀ1, PMR: (D2O), d 4.50 (1H, s), d 7.32–7.58
(5H, m) ppm.
Methyl [1-14C]2-hydroxy-2-phenyl ethanoate (methyl mandelate) 11
To a cooled solution of N-methyl-N-nitrosotoluene-p-sulphonamide (0.5mmol,
106 mg) in 15 ml ether, alcoholic sodium hydroxide solution (0.1 g in
96% ethanol, 5 ml) was added dropwise and stirred for 5 min. The resulting
ethereal solution of diazomethane was distilled into the flask containing
[1-14C]2-hydroxy-2-phenyl ethanoic acid (0.25 mmol, specific activity
185 MBq/mmol) in ether (3 ml). The reaction mixture was evaporated to
dryness under reduced pressure and loaded onto a silica column. The column
was eluted with 20% ethyl acetate in n-hexane to elute methyl-[1-14C]2-
hydroxy-2-phenyl ethanoate. Yield: 98%, TLC (silica, solvent: 20% ethyl
acetate in hexane) followed by radioactivity scanning revealed the product to
be 99% radiochemically pure (Rf for [1-14C]2-hydroxy-2-phenyl ethanoic acid
was 0.2. and methyl-[1-14C]2-hydroxy-2-phenyl ethanoate was 0.8). IR (KBr
film), 3500 cmꢀ1, 1750 cmꢀ1, 1050 cmꢀ1, PMR: (D2O), d 3.60 (3H, s), d 4.50
(1H, s), d 7.32–7.58 (5H, m) ppm.
(RS) methyl-2([20-14C]4,6-dimethoxypyrimidin-20-yloxy)-2-phenyl-[1-14C]ethanoate
Methyl-[1-14C]2-hydroxy-2-phenyl ethanoate 11 (0.15 mmol, specific activity
185 MBq/mmol)) was stirred with 20 mg of 60% NaH in toluene for 15 min in
a glass ampoule. To it was added [2-14C]4,6-dimethoxy-2-mercaptomethylpyr-
imidine 5 (0.15 mmol, specific activity 185 MBq/mmol), sealed and heated in a
domestic microwave oven (1300 W) for 4 min (8 ꢁ 30 s). The ampoule was cut
open and the reaction mixture was taken up in methanol. The product was
purified by column chromatography (silica). The column was washed with 5%
methanol in chloroform. (RS) methyl-2-([20-14C]4,6-dimethoxypyrimidin-20-
yloxy)-2-phenyl [1-14C]ethanoate was eluted with methanol. Yield: 98.55%.
The product was analysed and confirmed by TLC (silica, solvent: 5%
methanol in chloroform) followed by radioactivity scanning. Rf for (RS)
methyl-2-([20-14C]4,6-dimethoxypyrimidin-20-yloxy)-2-phenyl [1-14C]ethanoate
is 0.2, Rf for methyl-[1-14C]2-hydroxy-2-phenyl ethanoate 0.92 and [2-14C]4,
6-dimethoxy-2-mercaptomethyl-pyrimidine 0.92 IR (KBr film) 2835 cmꢀ1
,
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm. 2006; 49: 699–705
DOI: 10.1002/jlcr

MICROWAVE-ASSISTED SYNTHESIS
705
1680 cmꢀ1, 1250 cmꢀ1, 680 cmꢀ1, PMR: (D2O), d 3.6 (3H, s), d 3.73 (6H, s), d
4.5 (1H), d 5.92 (1H, s), d 7.32–7.58 (5H, m).
Column: C-18, ODS 5m, 100 ꢁ 4.6 mm
Mobile Phase: 30% Acetonitrile in water (v/v)
Detector UV: 263 nm
Flow rate: 1.00 ml
Retention time for (RS) methyl-2-([20-14C]-4,6-dimethoxypyrimidin-20-yloxy)-
2-phenyl [1-14C]ethanoate was 3.5 min and [2-14C]4,6-dimethoxy-2-mercapto-
methyl-pyrimidine was 7.9 min. Under these conditions it was observed that
methyl-[1-14C]-2-hydroxy-2-phenyl ethanoate did not elute from the column.
The fraction corresponding to the product was collected and assayed by
liquid scintillation counter for its radioactive content. (A purified product
having radioactivity concentration of 16 000 dpm was injected and the fraction
at Rt 3.50 contained 15 928 dpm.) Radiochemical purity 99.55%.
Acknowledgement
The authors wish to express their thanks to Dr A. K. Kohli, Chief Executive,
BRIT for his support and encouragement.
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Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm. 2006; 49: 699–705
DOI: 10.1002/jlcr