A general method for the synthesis of aryl [11C]methylsulfones: Potential PET probes for imaging cyclooxygenase-2 expression

Article abstract of DOI:10.1016/j.bmcl.2005.06.080

A general one-pot method has been developed for the conversion of an aryl thiol moiety masked as the butyrate ester to the corresponding ¹¹C-labeled methylsulfone group. The potential of this methodology has been demonstrated by the successful radiosynthesis of carbon-11 analogues of several highly selective cyclooxygenase-2 (COX-2) inhibitors such as Rofecoxib, Etoricoxib, and 3-(4-methylsulfonylphenyl)-4-phenyl-5-trifluoromethyl isoxazole in high yield. The chemical and radiochemical purities obtained for the ¹¹C-labeled COX-2 inhibitors are >99% with a specific activity >1000 Ci/mmol.

Full text of DOI:10.1016/j.bmcl.2005.06.080

Bioorganic & Medicinal Chemistry Letters 15 (2005) 4268–4271
A general method for the synthesis of aryl [11C]methylsulfones:
Potential PET probes for imaging cyclooxygenase-2 expression
Vattoly J. Majo,a Jaya Prabhakaran,a Norman R. Simpson,b,c Ronald L. Van Heertum,b,c
J. John Manna,b,c and J. S. Dileep Kumara,c,
*
aDepartment of Psychiatry, New York State Psychiatric Institute, New York, NY 10032, USA
bDepartment of Radiology, New York State Psychiatric Institute, New York, NY 10032, USA
cColumbia University College of Physicians and Surgeons and Department of Neuroscience,
New York State Psychiatric Institute, New York, NY 10032, USA
Received 26 May 2005; revised 17 June 2005; accepted 21 June 2005
Available online 27 July 2005
Abstract—A general one-pot method has been developed for the conversion of an aryl thiol moiety masked as the butyrate ester to
the corresponding 11C-labeled methylsulfone group. The potential of this methodology has been demonstrated by the successful
radiosynthesis of carbon-11 analogues of several highly selective cyclooxygenase-2 (COX-2) inhibitors such as Rofecoxib, Etoricox-
ib, and 3-(4-methylsulfonylphenyl)-4-phenyl-5-trifluoromethyl isoxazole in high yield. The chemical and radiochemical purities
obtained for the 11C-labeled COX-2 inhibitors are >99% with a specific activity >1000 Ci/mmol.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Cyclooxygenase-2 (COX-2) enzyme is up-regulated as
part of the inflammatory pathogenesis in conditions
such as cancers, arthritis, ischemic heart disease, stroke,
organ rejection, and neurodegenerative diseases like
Alzheimerꢀs and Parkinsonꢀs disease.1–6 The involve-
ment of COX-2 in these diseases or disease processes
raise the possibility that quantification of COX-2
expression might be a biological marker for early diag-
nosis, monitoring of disease progression, and an indica-
tor of effective treatment. One approach to non-invasive
monitoring of COX-2 in vivo is PET or SPECT. PET
and SPECT tracers specifically targeting COX-2 enzyme
are needed to implement this approach.
and intestine.9 Isakson et al.10 Toyokuni et al.11 and
Marnett et al.12 have reported the synthesis of several
labeled COX-2 inhibitors without demonstrating specif-
ic binding in vivo. More recently, Stille coupling of 4-
[18F]fluoroiodobenzene has been utilized for the synthe-
sis of 18F-labeled PET probes for monitoring COX-2
expression.13 However, in vivo studies have not been
published with these compounds.
In the course of our studies to develop a specific PET trac-
er for imaging COX-2,14 we now report development of a
general method for rapid one-pot conversion of masked
aryl thiols to 11C-labeled methylsulfone group. We pro-
pose that such a methodology could be conveniently
adopted for the facile synthesis of [11C] analogues of sev-
eral highly selective COX-2 inhibitors like 3-(4-meth-
ylsulfonyl-phenyl)-4-phenyl-5-trifluoromethyl isoxazole
(TMI), Rofecoxib, and Etoricoxib. The presence of a
methylsulfone or sulfonamide group attached to an aryl
ring is a characteristic feature of a large number of
COX-2 selective inhibitors (COXIBs) (e.g., Fig. 1). In this
communication, we describe the successful implementa-
tion of our strategy to synthesize aryl-[11C]methylsulfones
in good yield and specific activity.
In spite of considerable efforts from several research
groups, currently no specific COX-2 PET imaging
agents are available for in vivo monitoring of COX-2
expression.7 McCarthy et al. reported the radiosynthesis
of [18F]SC58125 as a COX-2 PET tracer.8 However, spe-
cific binding could not be demonstrated by in vivo
blocking of tracer uptake by carrier SC58125. [18F]-
Desbromo-DuP-697, another COX-2 inhibitor radio-
tracer exhibited substantial non-specific uptake in fat
Keywords: Arylsulfones; Cyclooxygenase-2; Positron emission tomo-
graphy; Pummerer rearrangement.
TMI is considered as a potential imaging agent for COX-
2 due to its high affinity to COX-2 (<1 nM) and excellent
selectivity over COX-1 (COX-1/COX-2 = >100,000) and
*
Corresponding author. Tel.: +1 212 543 5909; fax: +1 212 543
6017; e-mail: dk2038@columbia.edu
0960-894X/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.06.080

V. J. Majo et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4268–4271
4269
O
S
cursor 4 were initially optimized by mimicking the
radiosynthesis with non-radioactive methyl iodide. Opti-
mum yields were obtained by unraveling the free thiol
from thiobutyrate ester 4 using tetrabutylammonium
hydroxide in tetrahydrofuran (THF) followed by addi-
tion of methyl iodide (MeI), stirring for 5 min at rt,
and then heating the reaction mixture for 2 min at
70 ꢁC in presence of excess Oxoneꢂ in methanol/water
(1:1 v/v). Under identical conditions, labeling with
[11C]MeI provided [11C]TMI (5) in 30 min with 37% yield
(n = 3, SD = 3) at end of bombardment (EOB) based
on [11C]MeI and with >99% chemical and radiochemical
purity (Scheme 1). The product was purified by reverse
phase-high performance liquid chromatography (RP-
HPLC, Phenomenex C18, 10 · 250 mm, 10 lm, mobile
phase: acetonitrile/0.1 M ammonium formate solution
60:40, flow rate: 10 mL/min) followed by C-18 Sep-
Pakꢂ purification. The formation of [11C]TMI was
confirmed by co-injecting the [11C]-product with non-
radioactive compound and comparing the HPLC
retention times of the two compounds. Specific activity
of [11C]TMI obtained was 2 · 103 Ci/mmol (n = 3,
SD = 250) based on a standard mass curve.
O
CF3
O
O
Cl
O
N
O
S
O
N
O
S
O
N
TMI
Rofecoxib
Ki = 40 nM
Etoricoxib
Ki = 81 nM
Ki = < 1 nM
Figure 1. Examples of highly selective COX-2 inhibitors having
methylsulfone group.
various competitive brain receptors, transporters, and
proteins (affinity >10 lM).15,16 Our initial strategy was
to synthesize 4-(4-phenyl-5-trifluoromethylisoxazol-3-
yl)benzenethiol, the required free thiol, as the radiolabel-
ing precursor for [11C]TMI. Toward this end, we pursued
a palladium catalyzed coupling of 3-(4-bromophenyl)-4-
phenyl-5-trifluoro-methylisoxazole with potassium
tri(isopropyl)silane thiol17 followed by preparative
thin-layer chromatography generating free thiol in 40%
and corresponding dimer in considerable yield. Howev-
er, the lack of reproducibility of the reaction in accept-
able yield as well as the instability of the free thiol
prompted us to follow an alternative strategy for the syn-
thesis of stable thiobutyrate ester, as shown in Scheme 1.
The thiomethyl ether 2, the intermediate required for the
synthesis of precursor for radiosynthesis of TMI, was
prepared by another modified procedure.16 Accordingly,
an isoxazole ring was established from the prop-2-ene-1-
one 1 by treatment with hydroxylamine followed by the
oxidative cyclization of the adduct in 88% yield. The syn-
thesis of thiol precursor for radiosynthesis by deprotec-
tion of methyl sulfide 2 using several alkyl thiolates18
was unsuccessful. We then pursued an alternative strate-
gy for the synthesis of the stable thiobutyrate ester 4
from methylsulfide 2 via oxidation to sulfoxide 3 fol-
lowed by Pummerer rearrangement19 and quenching of
the intermediate free thiol with butyryl chloride, as
shown in Scheme 1. The desired precursor 4 was formed
in 59% yield along with traces of disulfide.20 Radiolabel-
ing conditions for the synthesis of [11C]TMI (5) from pre-
We then adopted a similar strategy for the synthesis of
[11C]Etoricoxib (10) (Scheme 2). The central pyridine
ring of 8 (74%) was introduced by ring annulation of
a-aryl ketone 6 with vinamidinium hexafluorophos-
phate salt 7.21 The attempted deprotection of methyl-
thio group in 8 by sodium thiomethoxide resulted in
the displacement of chloro substituent in the central
pyridine ring by thiomethyl substituent. However, oxi-
dation of thiomethyl group to the corresponding sulf-
oxide22 followed by Pummerer rearrangement and
protection of the free thiol group as the thiobutyrate
ester proceeded uneventfully with an overall yield of
48% in three steps to give compound 9.23 The radio-
synthesis of [11C]Etoricoxib was achieved in 28% yield
[(n = 3, SD = 4), EOB based on 11CH3I] by an analo-
gous procedure adopted for the synthesis of
[11C]TMI.24
S
CF3
c
a, b
O
CF3
N
MeS
S
O
N
N
Cl
S
2
1
Cl
a
+
N
O
CF3
CF3
PF6
N
N
d
O
8
O
6
7
N
N
O
O
S
S
S
O
3
H311C
d
4
S
O
Cl
CF3
O
Cl
b,c
O
e
N
N
N
N
O
H311C
N
9
S
O
[
11C]5
10
Scheme 1. Radiosynthesis of [11C]TMI. Reagents and conditions: (a)
NH2OHÆHCl, NaOAc, EtOH–H2O, reflux, 90 ꢁC, 65%; (b) KI, I2,
NaHCO3, Water–THF, reflux, 7 h, rt, 12 h, 88%; (c) m-CPBA,
CH2Cl2, ꢀ20 ꢁC, 1 h, 84%; (d) (i) (CF3CO)2O, 2,6-lutidine, CH3CN,
ꢀ20 ꢁC, 1 h; (ii) butyryl chloride, pyridine, CH2Cl2, 0 ꢁC, 30 min, 59%
(two steps); (e) (i) [11C]CH3I, tetrabutylammonium hydroxide, THF,
rt, 5 min; (ii) Oxoneꢂ, MeOH/H2O (1:1), 70 ꢁC, 2 min, 37% (EOB).
Scheme 2. Radiosynthesis of [11C]Etoricoxib. Reagents and condi-
tions: (a) (i) KOt-Bu, THF, rt, (ii) AcOH, TFA, rt, (iii) concd NH3,
reflux, 6 h, 74% (three steps); (b) (i) m-CPBA, CH2Cl2, ꢀ20 ꢁC, 1 h,
84%; (c) (i) (CF3CO)2O, 2,6-lutidine, CH3CN, ꢀ20 ꢁC, 1 h; (ii) butyryl
chloride, pyridine, CH2Cl2, 0 ꢁC, 30 min, 57% (two steps); (d) (i)
[
11C]CH3I, tetrabutylammonium hydroxide, THF, rt, 5 min; (ii)
Oxoneꢂ, MeOH/H2O (1:1), 75 ꢁC, 3 min, 28% (EOB).

4270
V. J. Majo et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4268–4271
2. Fajardo, M.; Di Cesare, P. E. Drugs Aging 2005, 22, 141.
O
O
O
c-d
a-b
Br
Br
3. Saito, T.; Rodger, I. W.; Shennib, H.; Hu, F.; Tayara, L.;
Giaid, A. Can. J. Physiol. Pharmacol. 2003, 81, 114.
4. Hull, M.; Lieb, K.; Fiebich, B. L. Curr. Med. Chem. 2002,
9, 83.
5. Teismann, P.; Vila, M.; Choi, D. K.; Tieu, K.; Wu, D. C.;
Jackson-Lewis, V.; Przedborski, S. Ann. N.Y. Acad. Sci.
2003, 991, 272.
O
12
11
MeS
O
O
O
e
O
O
O
H311C
S
6. Hausknecht, B.; Voelkl, S.; Riess, R.; Gauer, S.; Goppelt-
Struebe, M. Transplantation 2003, 76, 109.
S
O
14
13
7. Herschman, H. R.; Talley, J. J.; DuBois, R. Mol. Imaging
Biol. 2003, 5, 286.
8. McCarthy, T. J.; Sheriff, A. U.; Graneto, M. J.; Talley,
J. J.; Welch, M. J. J. Nucl. Med. 2002, 43, 117.
9. de Vries, E. F.; van Waarde, A.; Buursma, A. R.;
Vaalburg, W. J. Nucl. Med. 2003, 44, 1700.
10. Isakson, P. C.; Seibert, K.; Talley, J. J. PCT Int. Appl.
9714679, 1997; Chem. Abstr. 1997, 126, 343566.
11. Toyokuni, T.; Satyamurthy, N.; Herschman, H. R.;
Phelps, M. E.; Barrio, J. R. PCT. Int. Appl. 2003089013,
2003; Chem. Abstr. 2003, 139, 334913.
Scheme 3. Radiosynthesis of [11C]Rofecoxib. Reagents and condi-
tions: (a) 4-thiomethylphenylboronic acid, PdCl2(PPh3)2, CsF,
BnEt3N+Clꢀ, 3 d, 74%; (b) phenylboronic acid, PdCl2(PPh3)2, CsF,
BnEt3N+Clꢀ, 3 d, 71%; (c) m-CPBA, CH2Cl2, ꢀ30 ꢁC, 1 h, 93%; (d) (i)
(CF3CO)2O, 2,6-lutidine, CH3CN, ꢀ20 ꢁC, 1 h; (ii) butyryl chloride,
pyridine, CH2Cl2, 0 ꢁC, 30 min, 54% (two steps); (e) (i) [11C]CH3I,
pyrrolidine, DMF, rt, 5 min; (ii) Oxoneꢂ, MeOH/H2O (1:1), 70 ꢁC,
2 min, 20% (EOB).
12. Marnett, L. J.; Timofeevski, S.; Prudhomme, D. U.S.
Patent 2005002859, 2005; Chem. Abstr. 2005, 142, 100330.
13. Wust, F. R.; Hohne, A.; Metz, P. Org. Biomol. Chem.
2005, 3, 503.
14. Kumar, J. S. D.; Prabhakaran, J.; Underwood, M. D.;
Parsey, R. V.; Arango, V.; Majo, V. J.; Simpson, N. R.;
Arcement, J.; Cooper, A. R.; Van Heertum, R. L.; Mann,
J. J. Abstract of Papers, 226th National Meeting of the
American Chemical Society, New York, NY, 2003;
Abstract MEDI-375.
15. Kumar, J. S. D.; Underwood, M. D.; Prabhakaran, J.;
Parsey, R. V.; Arango, V.; Majo, V. J.; Simpson, N. R.;
Cooper, A. R.; Arcement, J.; Van Heertum, R. L.; Mann,
J. J. Abstract of Papers, 227th National Meeting of the
American Chemical Society, Philadelphia, PA, 2004;
Abstract MEDI-244.
The a,b-unsaturated c-butyrolactone ring in Rofecoxib
provided an unexpected challenge for its radiosynthesis.
The precursor for radiosynthesis was prepared by ini-
tially establishing the diaryl lactone ring in 12 by two
consecutive Suzuki couplings of mucobromic acid 11,
as shown in Scheme 3.25 Selective mono-oxidation of
12, Pummerer rearrangement, and protection of the
thiol group proceeded with an overall yield of 50% to
give the thiobutyrate ester 13. However, attempted
deprotection of the free thiol group by adding tetrabuty-
lammonium hydroxide resulted in the cleavage of
c-butyrolactone ring. A host of other mild bases also
failed to release the thiol group without destroying the
unsaturated lactone. The in situ deprotection and
methylation of the thiobutyrate ester was finally
achieved successfully by carrying out [11C]methylation
in the presence of excess of pyrrolidine in DMF at room
temperature.26 Oxidation with Oxoneꢂ proceeded as
expected to furnish [11C]Rofecoxib (14) in 20% yield
[(n = 3, SD = 4), based on 11C-CH3I at EOB].
16. Habeeb, A. G.; Rao, P. N. P.; Knaus, E. E. Drug Dev. Res.
2000, 51, 273.
17. Rane, A. M.; Miranda, E. I.; Soderquist, J. A. Tetrahedron
Lett. 1994, 35, 3225.
18. Pinchart, A.; Dallaire, C.; Van Bierbeek, A.; Gingras, M.
Tetrahedron Lett. 1999, 40, 5479.
19. Gryko, D. T.; Zhao, F.; Yasseri, A. A.; Roth, K. M.;
Bocian, D. F.; Kuhr, W. G.; Lindsey, J. S. J. Org. Chem.
2000, 65, 7356.
20. The disulfide could be converted to TMI by the cleavage
of S–S bond on treatment with aqueous solution of Na2S
nanohydrate in acetone followed by in situ addition of
methyl iodide to the resultant thiol and oxidation with
oxone in methanol–water.
In conclusion, we have designed a one-pot method for
the synthesis of 11C-labeled methylsulfone group from
the corresponding aryl thiol protected as the butyrate es-
ter. The methodology was successfully exploited for the
radiosynthesis of carbon-11 analogues of three highly
selective COX-2 inhibitors Rofecoxib, Etoricoxib, and
TMI in high radiochemical purity. In vivo studies with
these potential PET probes for imaging COX-2 expres-
sion are currently in progress.
21. Marcoux, J. F.; Corley, E. G.; Rossen, K.; Pye, P.; Wu, J.;
Robbins, M. A.; Davies, I. W.; Larsen, R. D.; Reider, P. J.
Org. Lett. 2000, 2, 2339.
22. 5-Chloro-3-(4-methanesulfinylphenyl)-60-methyl-[2,30]bipyri-
dine. A solution of thioether 6(147 mg, 0.45 mmol) in CH2Cl2
(2 mL) was cooled to ꢀ40 ꢁC and stirred vigorously. Then
a solution of m-CPBA (106 mg of 77 % water suspension,
0.47 mmol) in CH2Cl2 (2 mL) was added dropwise. The
mixture was stirred at ꢀ20 ꢁC for 40 min. Then Ca(OH)2
(60 mg, 0.81 mmol) and MgSO4 (200 mg) were added, and
stirring was continued for 30 min. After filtration and
evaporation, the resultant colorless oil was column chro-
matographed (4% MeOH in CH2Cl2) to yield the desired
sulfoxide 8 as a colorless puffy solid (130 mg, 84%).
Acknowledgments
The authors thank Dr. Bryan Roth and the NIMH–
PDSP program for the competitive receptor–transporter
assay of TMI, and Ms. Julie Arcement for her assistance
in the radiolabeling studies.
1
Compound 8: mp: 123 ꢁC; H NMR: d 2.54 (s, 3H), 2.75
(s, 3H), 7.07 (d, J = 8.0 Hz, 1H), 7.35–7.37 (m, 2H), 7.56
(dd, J = 8.0, 2.3 Hz, 1H), 7.61–7.63 (m, 2H), 7.74 (d,
J = 2.4 Hz, 1H), 8.42 (d, J = 2.1 Hz, 1H), 8.70 (d,
References and notes
1. Crosby, C. G.; DuBois, R. N. Expert Opin. Emerg. Drugs
2003, 8, 1.

V. J. Majo et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4268–4271
4271
J = 2.4 Hz, 1H); HRMS Calcd for C18H16ClN2OS (MH+):
343.0672; Found: 343.0679.
to stand for 2 min. High specific active [11C]CO2 was
produced from RDS112 Cyclotron (ꢁ37 GBq) and sub-
sequently converted into [11C]CH3I. [11C]Methyl iodide
was transported by a stream of argon (20–30 mL/min) into
the vial over approximately 5 min at room temperature.
At the end of the trapping, a suspension of Oxoneꢂ (5 mg)
in MeOH/H2O (1:1 v/v; 200 lL) was introduced into the
reaction mixture and was heated on a waterbath at 75 ꢁC
for 3 min. The suspension was filtered through a Nylon
filter and the dark yellow solution then directly injected
into a semi-preparative RP-HPLC (Phenomenex C18,
10 · 250 mm, 10 lm) and eluted with acetonitrile/0.1 M
ammonium formate solution (35:65) at a flow rate of
10 mL/min. The precursor appeared after 5–6 min reten-
tion time during the HPLC analysis. The product fraction
with a retention time of 9–10 min based on c-detector was
collected, diluted with 100 mL of deionized water, and
passed through a classic C-18 Sep-Pakꢂ cartridge. Recon-
stitution of the product in 1 mL of absolute ethanol
afforded [11C]10 (28% yield, based on 11CH3I at EOB). A
portion of the ethanol solution was analyzed by analytical
RP-HPLC (Phenomenex, Prodigy ODS(3) 4.6 · 250 mm,
5 lm; mobile phase: acetonitrile/0.1 M ammonium for-
mate solution 40:60, flow rate: 2 mL/min, retention time:
6.9 min, wavelength: 254 nm) to determine the specific
activity and radiochemical purity. Then the final solution
of the [11C]10 in 10% ethanol-saline was analyzed to
confirm the chemical and radiochemical purities.
23. Thiobutyric acid S-[4-(5-chloro-60-methyl-[2,30]bipyridi-
nyl-3-yl)-phenyl] ester. To a solution of sulfoxide 8
(50 mg, 0.15 mmol) in acetonitrile (0.75 mL), 2,6-lutidine
(60 lL, 0.52 mmol) was added, and the mixture was
cooled to ꢀ20 ꢁC. To the resulting suspension was added
TFAA (60 lL, 0.46 mmol) dropwise to give a clear yellow
solution. The reaction mixture was stirred at ꢀ10 ꢁC for
1 h and then cooled to 0 ꢁC. All volatile materials were
evaporated under inert atmosphere and the residue was
dissolved in a precooled (0 ꢁC) mixture of triethylamine
(0.3 mL) and methanol (0.3 mL). After 30 min at 0 ꢁC, all
volatile materials were evaporated at low temperature.
The residual yellow oil was dissolved in dichloromethane
(1 mL), treated with pyridine (0.29 mmol, 25 lL) and
cooled to 0 ꢁC. Butyryl chloride (0.22 mmol, 23 lL) was
added to the reaction mixture and allowed to warm to
room temperature over 30 min. The solution was then
poured into cold water and extracted with dichlorometh-
ane. The combined organic phases were dried over MgSO4
and concentrated under reduced pressure and column
chromatographed (70:30 hexane/EtOAc) to yield the
thiobutyric ester 9 as a viscous liquid (32 mg, 57%).
1
Compound 9: H NMR: d 1.00 (t, J = 7.4 Hz, 3H), 1.75
(sextet, J = 7.4 Hz, 2H), 2.53 (s, 3H), 2.65 (t, J = 7.4 Hz,
2H), 7.05 (d, J = 7.9 Hz, 1H), 7.20–7.22 (m, 2H), 7.36–7.38
(m, 2H), 7.53 (dd, J = 8.0, 2.3 Hz, 1H), 7.75 (d, J = 2.4 Hz,
1H), 8.46 (d, J = 2.0 Hz, 1H), 8.67 (d, J = 2.4 Hz, 1H),
HRMS Calcd for C21H20ClN2OS (MH+): 383.0985;
Found: 383.0992.
25. Zhang, J.; Blazecka, P. G.; Belmont, D.; Davidson, J. G.
Org. Lett. 2002, 4, 4559.
26. The precursor thiobutyrate ester 13 (1.0 mg) was dissolved
in 400 lL of anhydrous DMF in a capped 5 mL V-vial.
Pyrrolidine (50 lL) was added and the resultant solution
was treated with [11C]CH3I followed by Oxoneꢂ in an
analogous procedure adopted for the synthesis of
[11C]Etoricoxib.
24. Radiosynthesis of [11C]Etoricoxib (10): The precursor
thiobutyrate ester 9 (1.0 mg) was dissolved in 400 lL of
freshly distilled anhydrous THF in a capped 5 mL V-vial.
Tetrabutylammonium hydroxide (10 lL, 1 M in MeOH)
was added and the resultant pale yellow solution allowed