The 14C, 13C and 15N syntheses of MON 37500, a sulfonylurea wheat herbicide

Article abstract of DOI:10.1002/jlcr.934

[¹⁴C] and [¹³C]MON 37500 labeled in the imidazopyridine (Im) ring system were synthesized in seven steps in an overall yield of 39 and 28%, respectively, from [2-¹⁴C] and [2- ¹³C]chloroacetic acid. [¹⁴

Full text of DOI:10.1002/jlcr.934

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2005; 48: 397–406.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.934
Research Article
14
13
15
The C, C and N syntheses of MON 37500,
a sulfonylurea wheat herbicide
Brad D. Maxwell1,*, Olivier G. Boye1 and Kazunari Ohta2
´
1 Monsanto Company, 700 Chesterfield Parkway North, St. Louis, MO 63198, USA
2 Takeda Chemical Industries, Ltd., Agricultural Research Laboratories, Agro Division,
Wadai, Tsukuba 300-42, Japan
Summary
[14C] and [13C]MON 37500 labeled in the imidazopyridine (Im) ring system
were synthesized in seven steps in an overall yield of 39 and 28%, respectively,
from [2-14C] and [2-13C]chloroacetic acid. [14C]MON 37500 labeled in the pyri-
midine (Pd) ring system was synthesized from [2-14C]diethyl malonate in four
steps in 48% yield. Lastly, [15N]MON 37500 was prepared in three steps in an
overall yield of 28% from [15N]ammonium chloride. Copyright # 2005 John Wiley
& Sons, Ltd.
Key Words: sulfonylurea; wheat herbicide; MON 37500; Maverick1; Monitor1
Introduction
MON 37500 or sulfosulfuron is a sulfonylurea herbicide co-developed
by Monsanto Company and Takeda Chemical Industries, Ltd. under
the brand names Monitor1 in Europe and Maverick1 in the US.1 It has
been found to effectively control several grassy weeds in wheat. The mode
of action involves the inhibition of acetolactate synthase, an enzyme in
the biosynthesis of essential amino acids leucine, valine and isoleucine.2
To complete environmental plant, animal, soil and water studies for
registration, the syntheses of [14C], [13C] and [15N]MON 37500 were
required. Material radiolabeled in both ring systems was required in
order to follow the metabolism of each ring system. Stable labels
were required to make identification of metabolites easier by NMR and
MS. This paper describes the radiolabel and stable label syntheses and
analysis of the active ingredient, MON 37500, in Monitor1 and Maverick1
herbicides.
*Correspondence to: B. D. Maxwell Pfizer Inc. 0925-300-361.1, 301 Henrietta St. Kalamazoo, MI 49007,
USA. E-mail: brad.d.maxwell@pfizer.com
Copyright # 2005 John Wiley & Sons, Ltd.
Received 4 January 2005
Revised 28 January 2005
Accepted 31 January 2005

398
B. D. MAXWELL ET AL.
Results and discussion
Four separate labeled compounds were synthesized according to the routes
illustrated in Schemes 1–3. In Scheme 1 is shown the synthesis of [14C] and
[13C]MON 37500 labeled in the imidazopyridine (Im) ring system. The label
was introduced by reacting [2-14C] or [2-13C]chloroacetic acid with 2-
aminopyridine to generate product 2. Substituted acetic acid 2 was cyclized
to the chloroimidazopyridine 3 in excellent yield.3 Product 3 was converted to
the sulfonic acid and then to the acid chloride in a single flask with
chlorosufonic acid and phosphorous oxychloride. Addition of ammonia to the
acid chloride formed amide 6 in good yield for the three steps. Chloroamide 64
was converted to sulfide 75 which was oxidized to the sulfone using sodium
percarbonate for the 13C synthesis and Oxone1 for the 14C synthesis. We
found that the sodium percarbonate lost activity over time and the Oxone1
worked as well without decomposition. The final step involved the reaction of
product 8 with phenyl chloroformate to form the carbonyl portion of the
sulfonylurea bridge followed by the attachment of 2-amino-4,6-dimethox-
ypyrimidine.6 Overall yields were 39 and 28% for the 14C and 13C syntheses,
respectively.
NH
NH
POCl
2
3
Cl*CH CO H
N
*
2
2
ClSO H
3
Toluene
115oC
Cl
Et N, H O
3
2
ClCH CH Cl
N
90oC
N
2
2
N
14
o
14
13
*CH CO H
90 C
C: 76%
C: 79%
C: 99%
C: 90%
2
2
1
2
3
13
14
13
* = C or
C
N
NH OH
4
N
N
Et N
3
CH CN
3
Cl
Cl
Cl
POCl
N
N
N
RT
C: 88%
C: 78%
3
14
*
95 - 100oC
*
*
4
5
6
SO Cl
SO NH
2 2
13
SO H
2
3
CH CH SH
3
2
N
N
Oxone or Sodium
percarbonate
NaH, DMF
SO CH CH
3
SCH CH
3
2
2
o
2
N
N
115 C
H O, CH CN, r.t.
2
3
14
*
*
14
13
C: 77%
C: 76%
7
8
SO NH
13
SO NH
C: 84%
C: 82%
2
2
2
2
N
SO CH CH
3
OCH
OCH
2
2
3
N
2-Amino-4,6-di
methoxypyrimidine
ClCO Ph
2
N
N
Et N
3
*
9
SO NHCONH
2
CH CN
3
CH SO H
3
3
MON 37500
14
C: 92%
C: 81%
Overall Y ields
13
3
14
C: 39%
13
C: 28%
Scheme 1.
Copyright # 2005 John Wiley & Sons, Ltd.
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SYNTHESES OF LABELED MON 37500
399
Scheme 2 shows the synthesis of [14C-Pd]MON 37500. Guanidine nitrate
was reacted with [2-14C]diethyl malonate and 25% NaOMe in MeOH to form
2-amino-4,6-[5-14C]dihydroxypyrimidine 12 in 99% yield. Product 12 was
Cl
OH
NH
C
*CH (CO Et) , (11)
2
2
2
HNO
N
N
3
N
N
POCl , Et N
3
3
25% NaOCH
3
H N
2
*
H N
2
NH
NH
*
2
2
MeOH, Reflux
CH CN, Reflux
3
10
13
12
99%
14
88%
Cl
OH
* =
C
N
SO CH CH
2
2
3
N
25% NaOCH
OCH
3
3
N
15
CH OH
3
Reflux
SO NH
2
N
N
2
SO CH CH
3
OCH
2
2
3
N
H N
2
*
Et N, CH CN
N
N
3
3
74%
ClCO Ph
2
SO NHCONH
2
*
14
OCH
16
3
CH SO H
3
3
MON 37500
OCH
74%
3
Overall Yield
48%
Scheme 2.
Cl
Cl
Cl
Cl
Cl
15NH4Cl
NaOH
Initial Ratio of
N
N
N
N
N
N
18:19 =1:1. Use of
Soxhlet Extraction
increased ratio to
82 : 18
Cl
17
15NH2
18
+
Cl
EtOH
94%
15NH2
19
OCH3
OCH3
N
N
N
25% NaOCH3
CH3OH, Reflux
15NH2
OCH3
20
+
N
15NH2
21
88%
OCH3
N
N
SO2Et
N
OCH3
OCH3
SO2CH2CH3
N
N
N
15
SO2NH2
SO2NHCO15NH
MON 37500
Overall Yield
28%
Et3N, CH3CN
ClCO2Ph
22
CH3SO3H
67%
Scheme 3.
Copyright # 2005 John Wiley & Sons, Ltd.
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400
B. D. MAXWELL ET AL.
converted to the dichloride 13 in 88% yield with POCl3 and Et3N in refluxing
MeCN. The dichloride was reacted with 25% NaOMe in refluxing MeOH to
produce 2-amino-4,6-[5-14C]dimethoxypyrimidine 14 in 74% yield. Product 14
was reacted under the same conditions for the last step described in Scheme 1
to give [14C-Pd]MON 37500 in 74% yield.6 The overall yield for the four steps
was 48%.
[15N]MON 37500 was prepared as shown in Scheme 3. The label was
introduced by condensing 15NH3 generated from solid 15NH4Cl and NaOH
into EtOH. To this was added trichloropyrimidine, 17. An equal mixture of
regioisomers 18 and 19 was formed. The ratio of products was determined by
integrating the 1H-NMR signal for the protons attached to C5 of products 18
and 19. The chemical shift of the C5 proton in product 18 appears at 6.88 ppm
and that of product 19 appears at 6.46 ppm. The mixture of the two
regioisomers was placed in a Soxhlet Extractor and was refluxed with benzene
overnight. This increased the ratio of products 18/19 to 82/18. This mixture
was reacted with 25% NaOMe in MeOH at reflux to give the two isomeric
dimethoxypyrimidines 20 and 21 which were separated by flash column
chromatography. Product 21 was reacted with phenyl chloroformate and
product 15 to give [15N]MON 37500 in 67% yield for the last step.6 The overall
yield for the three step synthesis was 28%.
Experimental
Radioactivity was determined using Tracor Analytic Mark III counters, which
were interfaced with a Monsanto developed software package. Flash column
chromatography was performed using Merck grade silica gel, 230–400 mesh.
HPLC analysis was performed using a Waters HPLC system consisting of a
model 680 gradient controller, two model 510 pumps, a model U6 K injector, a
model 481 UV detector set at 254 nm, a Packard Radiomatic Flo-One Beta
radioactivity detector, and a Beckman Ultrasphere column, C18, 5 mm,
4.6 Â 250 mm. Proton NMR was completed on a Varian 300 MHz instrument
and chemical shifts were referenced against TMS. GC-CIMS and DEP-CIMS
analyses were completed on a Finnigan model 4515 instrument using
isobutane. Electrospray MS was performed on a Finnigan 4535 quadrupole
mass spectrometer with a Vestec Model 601B electrospray interface. LC-FAB
MS was completed using a VG ZAB-HF double-focusing MS via a dynamic
FAB probe. All labeled materials were identified by HPLC, TLC, MS and
NMR comparison with the corresponding unlabeled materials.
[14C-Im]MON 37500
[2-14C]2-(2-Imino-1,2-dihydropyridin-1-yl)acetic acid, 2. To [2-14C]chloroace-
tic acid (1.84 g, 19.4 mmol, 605 mCi, specific activity=31 mCi/mmol, NEN,
Boston, MA) in water (3 ml) was added Et3N (3.1 ml, 22 mmol) dropwise at
Copyright # 2005 John Wiley & Sons, Ltd.
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SYNTHESES OF LABELED MON 37500
401
room temperature (RT). After stirring for 10 min, 2-aminopyridine 1 (2.2 g,
23 mmol) was added and the resulting brown solution was warmed to 908C for
5 h. After cooling to RT, EtOH (2 ml) was added and the suspension was
stirred at 58C for 2 h. The precipitate was collected by filtration and rinsed
with cold EtOH and dried under vacuum at RT for 24 h to produce 2.24 g
(76% yield) of product 2.
[3-14C]2-Chloroimidazo[1,2-a]pyridine, 3:3 To 2 (2.24 g, 14.7 mmol) in toluene
(10 ml) at reflux was added POCl3 (4.1 ml, 44 mmol) dropwise. After
refluxing for 16 h and cooling to RT, cold water (50 ml) was added and the
solution was stirred for 15 min. The layers were separated. In an ice bath, the
aqueous layer was neutralized with 10% NaOH (aq). The precipitate was
filtered, dissolved in CH2Cl2 (50 ml) and dried over Na2SO4. The aqueous
filtrate was extracted with CH2Cl2 (20 ml  4). The combined organic layers
were washed with brine and dried over Na2SO4. The two CH2Cl2 solutions
were filtered, combined and the solvent was removed by rotary evaporation to
give 4.7 g of a yellow powder. This material was purified by flash column
chromatography on silica using 60% EtOAc/40% Hexane resulting in 2.23 g
(99%) of 3 as white crystals. TLC analysis produced a single spot with an Rf of
0.70 when co-spotted with reference material on silica using 60% EtOAc/40%
hexane.
[3-14C]2-Chloroimidazo[1,2-a]pyridine-3-sulfonamide, 6:4 To 3 (2.19g, 14.3mmol)
in DCE (20 ml) was added a solution of ClSO3H (1.7 ml, 26 mmol) in DCE (8 ml).
The solution was warmed to 908C for 5.5 h to produce 4. Et3N (3.8 ml, 27 mmol)
was added, refluxed for 15 min, POCl3 (2.53 ml, 27 mmol) was added dropwise and
the solution refluxed for 5 h. After cooling in an ice water bath, cold water (60 ml)
was added and the mixture was vigorously stirred. The organic layer was separated
and the aqueous layer was extracted with CH2Cl2 (100 ml  2). The combined
organic phases were dried over Na2SO4, filtered and the solvent was removed by
rotary evaporation to give 5 which was dissolved in 30ml MeCN. To this solution
at 58C was added aqueous 28% NH4OH (4 ml). The solution was warmed to RT
overnight. The solvent was partially removed and water (20 ml) was added. The
suspension was stirred in an ice water bath, filtered, washed with cold water (10 ml)
and dried under high vacuum to give 2.24 g of off-white crystals. Analysis by TLC
showed a single spot, Rf=0.40, on silica using 60% EtOAc/40% hexane.
Additional crops of crystals were recovered (440 mg) and extraction of the filtrate
with EtOAc produced an additional 170 mg of product. The 610 mg of 6 were
purified by flash chromatography on silica gel using 60% EtOAc/40% hexane to
produce 510 mg of product 6 as white crystals. A total of 2.75 g of product 6 (88%
yield) was obtained.
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B. D. MAXWELL ET AL.
[3-14C]2-Ethylthioimidazo[1,2-a]pyridine-3-sulfonamide, 7:5 To NaH (1.51 g,
63.1 mmol) and DMF (15 ml) in an ice water bath was added EtSH (4.70 ml,
63.1 mmol) dissolved in DMF (5 ml) dropwise. The solution was warmed to
RT and stirred for 1.3 h. Product 6 (2.75 g, 12.6 mmol) in DMF (5 ml) was
added dropwise. The mixture was stirred at 1158C for 14 h, cooled to RT,
poured into water (250 ml) and acidified with HCl to pH=1. The aqueous
layer was extracted with CH2Cl2 (100 ml  4). The combined organic extracts
were dried over Na2SO4, filtered and the solvent was removed by rotary
evaporation to give 2.85 g of a yellow powder. Recrystallization from EtOAc
produced 2.5 g (77% yield) of product 7 as white crystals. Analysis by TLC on
silica with EtOAc gave a single spot with an Rf=0.80 when co-spotted with
reference material. At this stage, 0.6 g of product 7 was removed to serve as a
standard for future metabolism studies.
[3-14C]2-Ethylsulfonylimidazo[1,2-a]pyridine-3-sulfonamide, 8. To 7 (1.9 g,
7.4 mmol) in MeCN (100 ml) in an ice water bath was added dropwise a
solution of Oxone1 (19.3 g, 31.4 mmol) in 100 ml of water. The suspension was
stirred at RT for 16 h and water (100 ml) and CH2Cl2 (100 ml) were added. The
layers were separated and the aqueous layer was extracted with CH2Cl2
(100 ml  3). The combined organic extracts were dried over Na2SO4, filtered
and the solvent was removed by rotary evaporation to give 1.9 g of 8. This
material was dissolved in EtOAc (150 ml) with gentle heating and was passed
through a short column of silica gel. Removal of the solvent by rotary
evaporation produced 1.8 g (84% yield) of white crystals.
[3-14C]N-[[(4,6-Dimethoxy-2-pyrimidinyl)amino]carbonyl]-2-(ethylsulfonyl)-
imidazo[1,2-a]pyridine-3-sulfonamide, 9:6 Phenyl chloroformate (0.97 ml,
7.7 mmol) in MeCN (5 ml) was added dropwise to a solution of 8 (1.8 g,
6.4 mmol) and Et3N (2.06 ml, 14.8 mmol) in MeCN (22 ml) in a saltwater ice
bath at À58C at a rate such that the reaction temperature remained below
108C. The reaction mixture was warmed to RT and stirred for 1 h. In one
portion, 2-amino-4,6-dimethoxypyrimidine (1.5 g, 9.7 mmol) was added and
the mixture was warmed to 558C. CH3SO3H (0.63 ml, 9.7 mmol) in MeCN
(5 ml) was added dropwise. The white product precipitated from solution after
1.5 h. The precipitate was filtered, washed with cold water (5 ml) and dried
under high vacuum to give 2.11 g of 9 as a white crystalline material. The
filtrate was added to 100 ml of water and then was extracted with CH2Cl2
(80 ml  3). The organic layers were combined, washed with brine and dried
over Na2SO4. Filtration and evaporation of the solvent produced a residue
that was dissolved in EtOAc. Upon standing, the 2-amino-4,6-dimethoxypyri-
midine precipitated and was removed by filtration. The filtrate was purified by
flash column chromatography. The product was eluted first with 60% EtOAc/
Copyright # 2005 John Wiley & Sons, Ltd.
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SYNTHESES OF LABELED MON 37500
403
40% hexane and then with CH2Cl2 followed by 90% CH2Cl2/10% MeOH to
provide 700 mg of additional product. The final material was recrystallized
from MeCN to give 2.7 g (92% yield) of white crystalline product. The
radiochemical purity was determined to be 99.8% by HPLC and the specific
activity (SA) was measured gravimetrically to be 30.5 mCi/mmol. Co-spotting
of the product with MON 37500 standard on silica using 90% CH2Cl2/10%
MeOH produced a single spot with an Rf value of 0.40. MS analysis by
FAB (À) ion produced the expected ion clusters at m/z=469/471 and by
electrospray (+) ion produced the expected ion clusters at m/z=471/473.
1H-NMR (300 MHz, DMSO-D6) d 13.14 (s, 1 H, N-H); 10.76 (s, 1 H, N-H);
9.19 (d, J=7.1 Hz, 1 H, Ar-H); 7.99 (d, J=9.1 Hz, 1 H, Ar-H); 7.80 (t, J=
8.0 Hz, 1 H, Ar-H); 7.49 (t, J=7.1 Hz, 1 H, Ar-H); 6.00 (s, 1 H, Ar-H); 3.95
(s, 6 H, OCH3); 3.60 (q, J=7.4 Hz, 2 H, CH2); 1.16 (t, J=7.4 Hz, 3 H, CH3).
[13C-Im]MON 37500
The synthesis of [13C-Im]MON 37500 was completed using the same reaction
conditions as that used in the preparation of [14C-Im]MON 37500 with two
minor differences. First, the 13C synthesis was completed on a larger scale than
that of the 14C synthesis. The 13C synthesis began with 12.5 g of [2-13C]
chloroacetic acid (99.1% 13C enriched, Isotec Inc., Miamisburg, OH). Second,
the oxidation of 7 to 8 in the 13C synthesis, used sodium percarbonate whereas
Oxone1 was used in the 14C synthesis. The overall yield for the preparation of
the 13C material was 28% and the chemical purity was 99.9% by HPLC. The
melting point of the 13C material was 196–1988C (reference MP=197–1998C).
Direct probe CIMS showed a molecular ion at m/z=472 as expected for
[13C-Im]MON 37500. 1H-NMR (300 MHz, DMSO-D6) d 13.14 (s, 1 H, N-H);
10.76 (s, 1 H, N-H); 9.19 (d, J=7.1 Hz, 1 H, Ar-H); 7.99 (d, J=9.1 Hz, 1 H,
Ar-H); 7.80 (t, J=8.0 Hz, 1 H, Ar-H); 7.49 (t, J=7.1 Hz, 1 H, Ar-H); 6.00
(s, 1 H, Ar-H); 3.95 (s, 6 H, OCH3); 3.60 (q, J=7.4 Hz, 2 H, CH2); 1.16
(t, J=7.4 Hz, 3 H, CH3). 13C-NMR (300 MHz, DMSO-D6) d 104.8.
[14C-Pd]MON 37500
[4-14C]2-Amino-4,6-dihydroxypyrimidine, 12. To guanidine nitrate, 10 (1.25 g,
10.2 mmol) in 7 ml EtOH was added [2-14C]diethyl malonate 11 (1.65 g,
10.2 mmol, 600 mCi, SA=58.5 mCi/mmol, NEN, Boston, MA) and 4.7 ml
25% NaOMe in MeOH. The solution was refluxed for 3.5 h. After cooling, the
solvent was removed by rotary evaporation. The resulting solid was dissolved
in 10 ml water and was neutralized with glacial AcOH. The resulting solid was
filtered, washed with cold water and dried under vacuum at 608C to give 1.31 g
of product 12 (99% yield) as an off-white powder.
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B. D. MAXWELL ET AL.
[4-14C]2-Amino-4,6-dichloropyrimidine, 13. To [4-14C]2-amino-4,6-dihydroxy-
pyrimidine 12 (1.31 g, 10.1 mmol) in 6 ml MeCN was added Et3N (2.86 ml,
20.5 mmol) followed by the dropwise addition of POCl3 (1.97 ml, 21.1 mmol).
The solution was refluxed for 1 h, cooled to RT and poured into ice water. The
resulting brown solid was filtered and rinsed with cold water. The combined
crops were dried under vacuum with mild heating to give 1.49 g of product 13
(88% yield) as a brown powder.
[4-14C]2-Amino-4,6-dimethoxypyrimidine, 14. To the [4-14C]2-amino-4,6-di-
chloropyrimidine 13 (1.49 g, 9.00 mmol) in 6.8 ml MeOH was added 25%
NaOMe in MeOH (6.16 ml, 26.9 mmol) dropwise over 15 min. The solution
was refluxed for 4 h, cooled and the MeOH was removed by rotavap. The solid
was washed with 9 ml of water and was filtered. Additional crops of product
were recovered from the water washes. The combined crops were dried under
vacuum at 508C to produce 1.04 g of product 14 (74% yield).
N-[[4,6-Dimethoxy-2-pyrimidinyl-[5-14C])amino]carbonyl]-2-(ethylsulfonyl)-
imidazo[1,2a]pyridine-3-sulfonamide, 16:4 To a flame dried flask containing
2-ethylsulfonylimidazo[1,2a]pyridine-3-sulfonamide 15 (1.93 g, 6.68 mmol) was
added 7 ml MeCN and Et3N (1.87 ml, 13.4 mmol). The solution was cooled to
38C with an ice water bath. To the reaction was added phenyl chloroformate
(0.86 ml, 6.85 mmol) in 1.6 ml MeCN dropwise over 20 min keeping the
temperature at 3–58C. After the addition was complete, the solution was
stirred at 38C for 25 min and then warmed to RT and stirred for 1 h. To the
reaction was added [4-14C]2-amino-4,6-dimethoxypyrimidine 14 (1.04 g,
6.64 mmol). After 2 min, the solution was warmed to 558C with an oil bath
and CH3SO3H (0.43 ml, 6.63 mmol) in 1.6 ml MeCN was added dropwise
keeping the temperature between 50 and 558C. The reaction mixture was
stirred at 50–558C for 45 min, cooled in an ice water bath for 1.5 h and the
resulting beige solid product was filtered. The solid was rinsed with cold
MeCN, cold water and was dried under vacuum to give 2.31 g, 286 mCi (74%
yield) of product 16. The overall yield was 48%. The SA of 16 was measured
gravimetrically to be 58.5 mCi/mmol. MS analysis using LC-FAB (–) ion
1
produced a molecular ion at m/z=471. H-NMR (300 MHz, DMSO-D6)
d 13.13 (s, 1 H, N-H); 10.77 (s, 1 H, N-H); 9.19 (d, J=7.1 Hz, 1 H, Ar-H); 7.99
(d, J=9.0 Hz, 1 H, Ar-H); 7.80 (t, J=8.0 Hz, 1 H, Ar-H); 7.49 (t, J=7.1 Hz,
1 H, Ar-H); 6.00 (s, 1 H, Ar-H); 3.95 (s, 6 H, OCH3); 3.60 (q, J=7.4 Hz, 2 H,
CH2); 1.16 (t, J=7.4 Hz, 3 H, CH3).
[15N]MON 37500
[2-15N]2-Amino-4,6-dichloropyrimidine, 18 and [4-15N]4-Amino-2,6-dichloro-
pyrimidine, 19. To a flask equipped with a dry ice condenser was added EtOH
(60 ml) at –788C. NaOH (18.0 g, 450 mmol) and 15NH4Cl (12.6 g, 231 mmol,
Copyright # 2005 John Wiley & Sons, Ltd.
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SYNTHESES OF LABELED MON 37500
405
99.3% 15N, Isotec Inc., Miamisburg, OH) were ground in separate mortars.
The resulting powders were placed together in a flask that was connected
to the dry ice condenser via tubing. Upon intermittent shaking, the 15NH3
gas was condensed into the EtOH. After 45 min, no additional 15NH3
was generated. The reaction was warmed to 58C and a solution of 2,4,6-
trichloropyrimidine 17 in EtOH (6 ml) was added. The temperature of the
reaction increased to 508C over 30 min and then was cooled to RT over 45 min.
The precipitate was filtered, washed with water and dried to give 11.5 g (94%
yield) of a white powder as a 50/50 mixture of 18 and 19 as determined by
1H-NMR by integration of the protons attached to C5. The mixture was
enriched in the desired isomer, 18 by Soxhlet Extraction with 250 ml of
benzene for 22 h. The white precipitate that formed in the benzene solution
1
after cooling to RT was filtered to give 5.66 g. H-NMR analysis showed the
material to be 82% of the desired [2-15N]2-amino-4,6-dichloropyrimidine
18 and 18% of the undesired [4-15N]4-amino-2,6-dichloropyrimidine 19.
Analysis of the material left in the thimble indicated less than 3% of product
18 remained.
[2-15N]2-Amino-4,6-dimethoxypyrimidine, 20 and [4-15N]4-Amino-2,6-di-
methoxypyrimidine, 21. To the 82/18 mixture of products 18 and 19 (5.65 g,
29.0 mmol) was added 65 ml of anhydrous MeOH and 25% NaOMe (34.4 ml,
168 mmol). The reaction was refluxed for 7.5 h, cooled to RT and AcOH (7 ml)
was added. The mixture was diluted with water (150 ml), Na2CO3 was added
until the pH=9 and was extracted with ether (100 ml  3). The combined ether
extracts were dried over Na2SO4, filtered and the solvent was removed by
rotary evaporation to give 5.15 g of a white powder. The product was purified
by silica gel flash column chromatography with 40% EtOAc/60% hexane to
give 3.97 g (88% yield) of white crystalline solid. 1H-NMR (300 MHz, DMSO-
D6) d 6.56 (d, J=89.0 Hz, 2 H, 2-15NH2), 5.36 (s, 1 H, 5-H), 3.77 (s, 6 H,
1
OCH3). None of the 4-amino isomer was present by H-NMR or by TLC
analysis on silica using 40% EtOAc/60% hexane. The Rf of the 2-amino
isomer 21 was 0.54 whereas the 4-amino isomer 20 had an Rf of 0.26.
N-[4,6-Dimethoxy-2-pyrimidinyl[15N-amino]carbonyl]-2-(ethylsulfonyl)-imidazo
[1,2a]pyridine-3-sulfonamide, 22. To a flame-dried flask containing 2-ethyl-
sulfonylimidazo[1,2a]pyridine-3-sulfonamide 15 (8.99 g, 31.0 mmol) was added
50 ml anhydrous MeCN and Et3N (8.64 ml, 62.0 mmol). The solution was
cooled to À58C in a saltwater ice bath. To the reaction was added phenyl
chloroformate (3.97 ml, 31.6 mmol) in 5.0 ml MeCN dropwise keeping the
temperature below 108C. The solution was warmed to RT and stirred for 1 h.
[2-15N]2-Amino-4,6-dimethoxypyrimidine 21 (3.58 g, 31.0 mmol) was added in
one portion and the mixture was warmed to 558C. CH3SO3H (2.00 ml,
Copyright # 2005 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2005; 48: 397–406

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B. D. MAXWELL ET AL.
31.0 mmol) in 3 ml MeCN was added dropwise keeping the temperature
between 50 and 558C. The reaction mixture was stirred at 50–558C for 1.2 h
and then cooled in an ice water bath for 2 h. The precipitate was filtered,
washed with cold water (5 ml) and cold MeCN (5 ml) and then dried under
vacuum to give 9.82 g (67% yield, overall yield=28% from 2,4,6-trichloro-
pyrimidine) of the desired product 22 as a white powder. The isotopic
enrichment was determined to be 97.8% 15N and the chemical purity was
determined to be 100% by HPLC. The melting point of the 15N material was
1988C (reference MP=197–1998C). MS analysis using LC-FAB (–) ion agreed
1
with that expected with a molecular ion at m/z=470. H-NMR (300 MHz,
DMSO-D6) d 13.15 (s, 1 H, 14N-H); 10.77 (d, J=90 Hz, 1 H, 15N-H); 9.19
(d, J=7.1 Hz, 1 H, Ar-H); 7.99 (d, J=9.1 Hz, 1 H, Ar-H); 7.80 (t, J=8.0 Hz,
1 H, Ar-H); 7.49 (t, J=7.1 Hz, 1 H, Ar-H); 6.00 (s, 1 H, Ar-H); 3.94 (s, 6 H,
OCH3); 3.59 (q, J=7.4 Hz, 2 H, CH2); 1.16 (t, J=7.4 Hz, 3 H, CH3).
Acknowledgements
The authors thank Mr Robert Chott, Mr Mark Cooper, Dr Hideji Fujiwara
and Dr Tom Solsten for analysis of products by Mass Spectrometry.
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Copyright # 2005 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2005; 48: 397–406