An improved synthesis of [2-14C]2, 5-dichloropyrimidine

Article abstract of DOI:10.1002/jlcr.2937

A previously described, five-step synthesis of [2-¹⁴C]2, 5-dichloropyrimidine was based on condensation of [¹⁴C]urea with an acetal, followed by bromination, chlorination, boronic acid formation, and finally chlorination. This improved synthesis also started from readily available [¹⁴C]urea, which was condensed with 2-chloromalonaldehyde, followed by chlorination with POCl₃ yielding [2-¹⁴C]2, 5-dichloropyrimidine with a radiochemical purity of 99% in an overall radiochemical yield of 72%. Copyright

Full text of DOI:10.1002/jlcr.2937

Research Article
Received 27 March 2012,
Revised 1 May 2012,
Accepted 16 May 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.2937
An improved synthesis of [2-14C]2, 5-
dichloropyrimidine
*
Kai Cao, Scott B. Tran, Brad D. Maxwell, and Samuel J. Bonacorsi Jr.
A previously described, five-step synthesis of [2-14C]2, 5-dichloropyrimidine was based on condensation of [14C]urea
with an acetal, followed by bromination, chlorination, boronic acid formation, and finally chlorination. This improved
synthesis also started from readily available [14C]urea, which was condensed with 2-chloromalonaldehyde, followed by
chlorination with POCl3 yielding [2-14C]2, 5-dichloropyrimidine with a radiochemical purity of 99% in an overall radiochemical
yield of 72%.
Keywords: pyrimidine; 2,5-dichloropyrimidine; urea; C-14; isotopic labeling
A, 3 min 100% A, 20 min 95% B, 25 min 100% A. Radioactivity was measured with
a PerkinElmer TriCarb Model 2900Tr liquid scintillation counter (PerkinElmer Life
Introduction
Substituted pyrimidines are useful intermediates for many mole- Sciences Inc., Boston, MA, USA), using Ultima Gold (PerkinElmer Life Science) as
liquid scintillation cocktail.
cules of pharmaceutical interest such as buspirone (antianxiety),
dasatinib (tyrosine kinases inhibitor), bosentan (antihypertensive),
and sildenafil (vasodilator).1 Labeling these compounds with
carbon-14 can be readily achieved from carbon-14 labeled
pyrimidines. Historically, labeled pyrimidines have been difficult
to prepare in high yields. Previously, our group published a five-
[2-14C]5-Chloropyrimidin-2-ol, 6
[
14C]urea (21mg, 0.34 mmol, 18 mCi, 56 mCi/mmol), unlabeled urea (15 mg,
0.25 mmol) (total 36mg, 0.58mmol), and 2-chloromalonaldehyde (89mg,
0.84 mmol) were placed in a 7-ml microwave tube and acetic acid (1.6ml)
step synthesis of [2-14C]2, 5-dichloropyrimidine from [14C]urea by was added. The solution was subjected to microwave conditions: 130ꢀC,
40 min, PowerMax on, stirrer on. HPLC analysis showed approximately
80% conversion to the desired product. The crude mixture was added to
water (10ml) and injected onto the preparative HPLC system. Pooled HPLC
fractions containing product were concentrated under reduced pressure to
give [2-14 C]5-chloropyrimidin-2-ol 6 as a white solid (56mg, 13mCi,
radiochemical yield 72%). 1H NMR (300 MHz, DMSO-d6) d 8.43 (s, 2H). MS
ESI+ [M+ H]+ = 131, 133.
way of a boronic acid intermediate with an overall radiochemical
yield of 22%.2 To avoid volatile and difficult to handle intermedi-
ates in that sequence and shorten the number of steps, we inves-
tigated an alternative synthesis starting from the same [14C]urea
precursor. Here, we describe a new, more efficient two-step
synthesis of [2-14C]2, 5-dichloropyrimidine.
[2-14C]2, 5-Dichloropyrimidine, 5
Experimental
[2-14C]5-Chloropyrimidin-2-ol (6, 28mg, 0.25 mmol, 6.5mCi) and unlabeled
5-chloropyrimidin-2-ol (28 mg, 0.25mmol) (total 56 mg, 0.49mmol) were
dissolved in MeOH (1ml) and carefully transferred to a 7-ml microwave
tube then evaporated under a gentle stream of nitrogen. POCl3 (700 ml,
7.5mmol) was added and the mixture subjected to microwave conditions:
130ꢀC, 20min, PowerMax on, stirrer on. After cooling to room temperature,
the reaction mixture was added dropwise to a 20-ml vial containing ice-cold
water (5ml) with stirring in an ice-bath. KOH pellets (~1g) were added to
adjust the pH to 9. The mixture was extracted with Et2O (5 x 10 ml). Pooled
organic extracts were dried over Na2SO4, filtered and concentrated under a
steady stream of nitrogen for 2h to a volume of ~0.5ml. The vial was capped
and stored in the refrigerator overnight. The crude product solidified to
form a light yellow solid, [2-14 C]2, 5-dichloropyrimidine, 5 (64mg, 6.5mCi,
radiochemical yield: 100%, radiochemical purity: 99%). The product
co-eluted with an authentic standard.
General: All reagents and solvents used were of ACS grade or higher. [14C]
urea was purchased from Quotient Bioresearch, Cardiff, UK. Microwave
reactions were conducted using a CEM Discovery system with magnetic
stirring. Liquid chromatography–mass spectrometry (LC–MS) data were
obtained on a Thermo LXQ 2.0 Mass Spectrometer System (Thermo Fisher
Scientific Inc. 81 Wyman StreetWaltham, MA 02454) with electrospray ioniza-
tion. 1H NMR spectra were recorded on a Bruker AVANCE II 300 MHz Spectro-
meter with an Ultrashield™ Magnet (Bruker Scientific Instruments, Billerica,
Massachusetts). Analytical HPLC analyses were performed on an Agilent
1100 HPLC system including solvent degasser, quaternary pump, autosam-
pler, and diode array detector connected to an IN/US BetaRam flow detector
with a 0.5-ml detector (LabLogic Systems, Inc., Brandon, FL) cell. Analytical
HPLC method: Phenomenex LUNA C18 (Phenomenex Inc., Torrance, CA,
USA), 5 mm, 4.6 Â 150 mm, UV detection at 220, 320, and 280 nm, flow rate
1 ml/min. Mobile phase A: 0.1% trifluoroacetic acid in water, mobile phase
B: 0.1% trifluoroacetic acid in acetonitrile. Gradient: 0 min 100% A, 3 min
100% A, 10 min 40% B, 15 min 95% B, 20 min 95% B, 25 min 100% A.
Preparative HPLC was performed on a Varian HPLC system with two
PrepStar 218 pumps and ProStar 320 variable UV detector. Preparative HPLC
method: Phenomenex LUNA C18, 5mm, 21.2 Â 250 mm, UV detection at
323 nm, flow rate 20 ml/min. Mobile phase A: 0.1% trifluoroacetic acid in water,
mobile phase B: 0.1% trifluoroacetic acid in acetonitrile. Gradient: 0 min 100%
Bristol–Myers Squibb Research and Development, Radiochemical Synthesis,
Route 206 and Province Line Road, Princeton, NJ 08540, USA
*Correspondence to: Kai Cao, Bristol-Myers Squibb, Radiochemical Synthesis,
Route 206 and Province Line Road, Princeton, NJ 08540, USA.
E-mail: kai.cao@bms.com
J. Label Compd. Radiopharm 2012
Copyright © 2012 John Wiley & Sons, Ltd.

K. Cao et al.
Scheme 1. Previously reported preparation of [2-14C]2, 5-dichloropyrimidine 5 by our group.
To avoid this intermediate and develop a more efficient
synthesis, we reviewed the synthetic strategy and decided to focus
on the preparation of key intermediate 5-chloropyrimidin-2-ol.
Retrosynthetic analysis indicated that direct condensation of
urea with 2-chloromalonaldehyde could give the desired
5-chloropyrimidin-2-ol as shown in Scheme 2.
It was previously reported that 2-chloromalonaldehyde reacted
with different benzamidine acetates in pyridine to form pyrimidine
derivatives in good yields.3 First, we tried the condensation
reaction of urea with 2-chloromalonaldehyde in pyridine. A new
product with a [M+ 1]+ = 149/151 was observed with a UV max
of 280 nm, the data suggests that this product is the result of
condensation without the desired dehydration to form the
pyrimidine system. (Table 1)
Results and discussion
A five-step synthesis of [2-14C]2,5-dichloropyrimidine was recently
published by Tran et al.2 In this synthesis, [14C]urea was condensed
with malonaldehyde bis(dimethyl acetal) to yield [2-14C]pyrimidin-
2-ol hydrochloride 1, followed by bromination and chlorination to
give [2-14C]5-bromo-2-chloropyrimidine 3. Subsequently, 3 was
converted to [2-14C]2-chloropyrimidin-5-ylboronic acid 4. Finally,
boronic acid 4 was reacted with N-chlorosuccinimide and copper
chloride to give [2-14C]2, 5-dichloropyrimidine in 22% overall
radiochemical yield. The key intermediate [2-14C]5-bromo-2-
chloropyrimidine 3 was observed to be volatile and decomposed
at elevated temperature.2 (Scheme 1)
Modifying the reaction conditions with a Dean Stark trap at
120ꢀC yielded the same product. We switched the solvent to acetic
acid and a mixture of two products with [M+ 1]+ = 131/133 and
149/151 were observed by LC–MS. The UV maximum of the new
product was 320 nm, matching the product standard. The 1H
NMR spectrum also matched that of the standard. Unfortunately,
at an elevated reaction temperature (120ꢀC), extended reaction
times did not improve conversion of the condensation product
to the pyrimidine. Therefore, we switched to microwave radiation
of the mixture in acetic acid to form the desired product as the
Scheme 2. Retrosynthetic analysis of 5-chloropyrimidin-2-ol.
Table 1. Condensation of urea with 2-chloromalonaldehyde under different reaction conditions
Reaction conditions
Results
Pyridine, 90–105ꢀC
[M + H]+: 149/151 (ratio 3:1, 131/133 + water), UV max 280 nm
[M + H]+: 149/151, UV max 280 nm
Pyridine, 120ꢀC w/ dean stark trap
Acetic acid, 100ꢀC
Mixture of products with [M + H]+: 149/151, 131/133
Mostly desired product + hydrate ([M + H]+: 149/151)
Desired product, unidentified black particles
No product
Acetic acid, microwave 120ꢀC, 20 min
Conc. HCl, microwave 120ꢀC, 20 min
Conc. HCl/DMF (1:1), microwave 120ꢀC, 20 min
Acetic acid, microwave 130ꢀC, 40 min
Complete conversion to form a clear solution
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K. Cao et al.
Scheme 3. Synthesis of [2-14C]2, 5-dichloropyrimidine 5.
major component of the reaction mixture but still some undehy- unlabeled compounds (urea and 5-chloropyrimidin-2-ol) to fit our
drated, non-conjugated intermediate was observed. Concentrated specific activity requirement. Repeated run without any dilution
HCl was also used in place of acetic acid, but black particles were with unlabeled compounds gave same result.
formed because of the poor solubility of 2-chloromalonaldehyde
in the aqueous medium. However, analysis of the aqueous phase
showed complete conversion to the desired product. Next, a 1:1
mixture of concentrated HCl and Dimethylformamide (DMF) was
used to address the solubility issue of 2-chloromalonaldehyde.
Conclusion
An efficient and improved two-step synthesis of [14C]2, 5-
dichloropyrimidine was developed. [2-14C]2, 5-Dichloropyrimidine
was prepared in 72% radiochemical yield. This chemistry should
readily apply to the preparation of other 5-substituted pyrimidines,
such as 5-bromo, 5-alkyl or 5-acetyl.
Unfortunately, no desired product was observed. Eventually, we
settled for the optimal conditions of acetic acid, microwave
130ꢀC, 40 min, PowerMax on, stirrer on. LC–MS showed complete
conversion to the desired product, 5-chloropyrimidin-2-ol.
As shown in Scheme 3,
[
14C]urea reacted with 2-
Acknowledgements
chloromalonaldehyde yielding 72% of [2-14C]5-chloropyrimidin-2-
ol 6 after preparative HPLC purification. The final chlorination was
conducted in POCl3 under microwave conditions at 130ꢀC for
20 min with PowerMax on, giving the desired [2-14C]2, 5-
dichloropyrimidine 5 in quantitative yield. Because of the volatility
of the 2, 5-dichloropyrimidine, a modified workup procedure was
adapted. The reaction mixture was added dropwise to a small
volume of water in an ice-bath. The reverse quench is important
to ensure the safe consumption of excess POCl3. KOH pellets were
added directly to the quenched solution to adjust the pH to 9. Et2O
was then added to extract the product. After drying, most of the
Et2O was removed under a very gentle stream of nitrogen. The
resulting solid product containing small amounts of Et2O was used
directly in the next reaction without further purification. In both
reactions, carbon 14-labeled compounds were diluted with
The authors would like to thank the members of the Bristol–Myers
Squibb Radiochemistry Group for the helpful discussions and
suggestions.
Conflict of Interest
The authors did not report any conflict of interest.
References
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[2] S. B. Tran, B. D. Maxwell, H. Wu, S. J. Bonacorsi, Jr., J. Label. Compd.
Radiopharm. 2011, 54, 813–815.
[3] D. Dorsch, F. Stieber, O. Schadt, A. Blaukat, PCT Int. Appl, 2009, WO
2009006959.
J. Label Compd. Radiopharm 2012
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