Carbon-14 labeling of a trifluoromethoxy group: Synthesis of a substance P antagonist

Article abstract of DOI:10.1002/jlcr.502

Synthesis of a carbon-14 labeled trifluoromethoxy group has been accomplished using the stepwise oxidative fluorination-desulfurization of a readily prepared [¹⁴C]xanthate (5). This novel labeling procedure allowed a rapid synthesis of substanc

Full text of DOI:10.1002/jlcr.502

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Labelled Cpd Radiopharm 2001; 44: 815–829.
DOI: 10.1002/jlcr.502
Research Article
Carbon-14 labeling of a trifluoromethoxy
group: synthesis of a substance P antagonist
Conrad E. Raab*, Dennis C. Dean and David G. Melillo
Merck Research Laboratories, Department of Drug Metabolism, RY80R-104,
Rahway, NJ 07065, USA
Summary
Synthesis of a carbon-14 labeled trifluoromethoxy group has been accom-
plished using the stepwise oxidative fluorination–desulfurization of a readily
prepared [14C]xanthate (5). This novel labeling procedure allowed a rapid
synthesis of substance P antagonist candidate 1 that avoided potentially more
complex ring-labeling procedures. Similar procedures have been used to
prepare C-14 labeled trifluoromethyl and trifluoromethylamine groups.
Copyright # 2001 John Wiley & Sons, Ltd.
Key Words: [14C]trifluoromethoxy; substance P antagonist; synthesis
Introduction
Substance P, a peptide neurotransmitter which binds preferentially to
the NK1 receptor, is widely distributed in the central and peripheral
nervous systems, as well as in gastrointestinal tissue.1 Substance P
receptor antagonists may be of significant therapeutic use in the
treatment of a wide range of clinical conditions, including migraine,
pain, nausea and depression.2 During the development of compounds of
this type at Merck,3–5 synthesis of a carbon-14 labeled analog (1) for
animal metabolism studies was required.
With the exception of the cyclopropyl ring (due to potential
O-dealkylation), all carbon positions of 1 appeared to be metabolically
stable and suitable for labeling (Scheme 1). Since labeling of the
*Correspondence to: C. E. Raab, Merck Research Laboratories, Department of Drug Metabolism,
RY80R-104, Rahway, NJ 07065, USA. E-mail: conrad raab@merck.com
Copyright # 2001 John Wiley & Sons, Ltd.
Received 26 January 2001
Revised 30 May 2001
Accepted 5 June 2001

816
C. E. RAAB ET AL.
Scheme 1.
spirobicyclic ring system or aryl rings of 1 would probably require
multiple steps, a shorter route to the target molecule was envisioned
through labeling of the trifluoromethoxy group. Considering the
metabolicinertness of CF O groups,6 as well as the availability of
3
spiroalkene 2 from Merck Process Research for use in a Heck coupling
reaction, synthesis of the [14C]trifluoromethoxy-labeled aryl iodide 3
was initiated.
Trifluoromethoxy groups can be prepared by a number of means: (1)
7
halogen exchange of CCl3O with SbF3/SbCl5 or HF,8 (2) alkylation
of phenols with CCl4/HF,9 (3) fluorination of ArOC(O)F or
10
ArOC(S)Cl with SF4 or MoF6,11 respectively, and (4) the oxidative
desulfurization–fluorination of dithiocarbonates (xanthates), Ar-
OC(S)SMe, with HF/pyridine and 1,3-dibromo-5,5-dimethylhydan-
toin.12 Since xanthates are readily prepared by the reaction of
phenoxides with carbon disulfide13 (a commercially available carbon-
14 source) the Kuroboshi approach,12 known for high yields and mild
conditions, appeared to be the most convenient for the rapid synthesis
of the 14CF3O group.
There have been no previous reports of 14CF3O-containing
compounds, and only a handful of other 14CF3-containing compounds
was found in the literature. These include [14CF3]Trifluralin and
related herbicides prepared by fluorination of the [14C]trichloromethy-
lated precursors with SbF3, 14 and several 5-[14CF3]-substituted
uracil derivatives.15,y Given the frequency with which CF3 groups
are represented in pharmaceutical compounds, it is surprising that
there have been so few previous reports of C-14 labeling of this
functionality.
y
[
14C]5-Trifluoromethyluracil is available commercially from Moravek.
J Labelled Cpd Radiopharm 2001; 44: 815–829
Copyright # 2001 John Wiley & Sons, Ltd.

[
14C] TRIFLUOROMETHYLATION
817
Results and Discussion
The synthesis of key labeled trifluoromethoxy intermediate 3 was
initiated with the formation of [14C]xanthate 5 in 43% radiochemical
yield (>99% radiochemical purity without chromatographic purifica-
tion) by reaction of [14C]carbon disulfide (1.2 eq, 150 mCi) with
4-benzyloxyphenol 4 (1.0 eq) and methyl iodide under phase transfer
conditions (Scheme 2).13 The moderate radiochemical yield is assumed
to be due to loss of 14CS2 in transfer. Kuroboshi et al.12 proposed that
the mechanism of their fluorination–desulfurization procedure involved
serial oxidation of the xanthate sulfur atoms by Br+ followed by
nucleophilic attack of FÀat the carbon center, leading to stepwise
replacement of each C–S bond with a C–F bond.
For the conversion of [14C]xanthate 5 to [14C]trifluoromethyl ether 3,
it was envisaged that replacing the bromonium ion source with an
analogous iodonium ion source would allow conversion of the
dithiocarbonate group to the trifluoromethoxy group whilst iodinating
the aromaticring ortho to the electron-rich benzyloxy group at the same
time, to yield 3. The oxidant 1,3-diiodo-5,5-dimethylhydantoin (DIH, 6)
was thus prepared by iodination of 5,5-dimethylhydantoin with ICl.16
It was found that the effectiveness of 6 in the trifluoromethylation
reaction was dependent on how thoroughly the final, solid DIH was
washed with water and EtOAc(Table 1). DIH washed with large
excesses of water and EtOAc, as described in the experimental section,
resulted in the highest yields of products 3 and 7. Variation of the
DIH/FÀ ratio (Table 1) allowed optimization of the yields of 3 and 7,
and reduction in the amounts of radioactive byproducts.
Using the optimum conditions, reaction of 62 mCi of 5 with DIH and
HF/pyridine gave 17.1 mCi (28%) of [14C]aryl iodide and 6.5 mCi (11%)
of [14C]thioether 7 after purification to remove radioactive byproducts.
Scheme 2.
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829

818
C. E. RAAB ET AL.
Table 1.
Equiv of FÀ
Equiv of DIH (6)
Area % of 3y
Area % of 7y
80
300
80
100
120
4z
4z
4}
5}
7}
7
18
20
48
19
25
17
24
20
28
y Area % determined by radioactive HPLC.
z DIH washed with H2O (5 ml) and EtOAc(5 ml).
} DIH prepared as described in experimental section.
Scheme 3.
Compound 7, a product of incomplete substitution, appears to support
the mechanism of stepwise C–S bond replacement proposed by
Kuroboshi.12 This material was converted to 3 by subjecting it to
similar reaction conditions giving an additional 3.2 mCi (49%) of
[14C]aryl iodide 3. In our hands, use of DBH in the oxidative
desulfurization–fluorination of 5 generally gave 55% of each of the
corresponding [14C]bromo analogs of 3 and 7.
Having obtained 3, in-house procedures for the preparation of 1 were
followed.3,4 This entailed the reductive Heck coupling of 3 (20.3 mCi)
with spiroalkene 2, to give the desired benzyl-protected phenol 9 (78%
by radioactive HPLC) with 90% diastereomeric excess, as well as the
reduced aryl iodide 8 (20%) (Scheme 3). The reaction mixture was
purified on silica gel and by preparative HPLC to yield 12.9 mCi (67%)
of 9. Deprotection of 9 with Pearlman’s catalyst/H2 gave 10.0 mCi of
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829

[
14C] TRIFLUOROMETHYLATION
819
Scheme 4.
Scheme 5.
[14C]phenol 10, which was alkylated with chloroethyl tosylate yielding
11.3 mCi (93%) of [14C]alkyl chloride 11.
Dehydrohalogenation of 11 with potassium t-butoxide gave 11.1 mCi
(98%) of [14C]vinyl ether 12. Cyclopropanation of 12 with
Zn(CH2Cl)2,17 formed in situ, gave 8.1 mCi (73%) of [14C]cyclopropyl
ether 13 after prep. HPLC purification (Scheme 4). Deprotection of 13
with HBr in EtOAcgave crude 1, which was purified by prep. HPLC to
yield 4 mCi (59 mCi/mmol) of tracer.
The application of related desulfurization–fluorination proce-
dures18,19 to the labeling of other types of trifluoromethyl groups was
illustrated by the preparation of [14C]aryl trifluoromethyl 16 and
[14C]trifluoromethylamine 19 (Scheme 5). Aryl dithiocarboxylate 15 was
prepared from 14CS2,20 and converted to 16 in moderate yield using
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829

820
C. E. RAAB ET AL.
HF/py and DBH.18 Dithiocarbamate 18 was prepared in a similar
manner, and reacted with nBu4NH2F3 and DBH to give the first
instance of a C-14 labeled trifluoromethylamine compound (19).19
Conclusion
In conclusion, we have shown that a C-14 label can readily be
incorporated into a trifluoromethoxy group, using a two step procedure
from 14CS2 via a [14C]xanthate. The application of this methodology to
the synthesis of a drug candidate has been demonstrated. This labeling
approach has been extended to the synthesis of [14C]aryl trifluoromethyl
and [14C]trifluoromethylamine groups as well. Although Kuroboshi and
co-workers12,18,19 have shown that electron rich aromatic rings may be
halogenated under the desulfurization–fluorination conditions, this
could facilitate further synthetic elaboration of the trifluoromethylated
product. The high occurrence of trifluoromethyl groups in pharmaceu-
tical and agrochemical products, together with their metabolic stability,
makes the desulfurization–fluorination approach to 14CF3 groups a
potentially valuable labeling procedure.
Experimental
Radioactivity measurements were carried out using a Bioscan Lumi-
Scint liquid scintillation counter with Packard Ultima GoldTM
scintillant. Analytical HPLC measurements were performed on a system
consisting of Shimadzu LC-10ADVP pumps, SPD-10AVP UV detector,
CTO-10ASVP column oven, SIL-10ADVP auto-injector, SCL-10AVP
system controller and Packard RadiomaticTM 150TR flow monitor
controlled by a Compaq computer running Shimadzu Class-VP
software. Preparative and semi-preparative HPLC were performed on
a system comprising of Rainin Dynamax SD-200 pumps and Shimadzu
SPD-10AVP UV detector controlled by a Dell computer running
Dynamax PC software.
Mass spectra were recorded on an HP1100 LCMSD instrument in
API-ES positive ionization mode.
1H NMR spectra were measured at 400 MHz on a Varian Unity-400
spectrometer. Chemical shifts are reported in ppm (d) and are referenced
to the residual solvent peak (chloroform at 7.26 ppm).
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829

[
14C] TRIFLUOROMETHYLATION
821
Spiroalkene 2 was obtained from Merck Process Research. All other
reagents were obtained from Aldrich Chemical Co. and used as is.
Commercial anhydrous CH2Cl2, DMF and 1,2-DCE were dried over 4
(
A molecular sieves before use. All reactions were carried out under a
nitrogen atmosphere.
The identities of all intermediates and the final product were
1
established by H NMR and/or by co-elution on HPLC with authentic
standards obtained either in-house or form Aldrich.
[dithiocarbonate-14C]Methyl 4-benzyloxyphenyl dithiocarbonate (5)
The title compound was prepared by an adaptation of the procedure of
Lee et al.13 4-Benzyloxyphenol 4 (2.235 mmol, 0.447 g), benzene (15 ml),
tetrabutylammonium phosphate (250 ml), and 50% aq. NaOH (8 ml)
were placed in a 50 ml flask, frozen in liquid N2, and evacuated on a
manifold. [14C]Carbon disulfide (150.0 mCi, 2.685 mmol) was trans-
ferred to the vessel under vacuum. The yellow reaction mixture was
allowed to warm to room temp, and MeI (7.23 mmol, 450 ml) was added
under N2. The white, biphasicmixture was stirred vigorously for 8 h at
room temp, at which stage 51% of the 4-benzyloxyphenol remained by
HPLC (analytical sample blown dry to remove benzene, then taken up
in acetonitrile) (Zorbax Rx-C18 column, 4.6 ꢀ 250 mm, 1 ml/min, 210/
254 nm, 70% acetonitrile/0.1%aq. HClO4 isocratic for 20 min, 4-
benzyloxyphenol tR=3.5 min, xanthate tR=12.2 min,>99% radioche-
mical purity). The reaction mixture was diluted with benzene (15 ml)
and H2O (30 ml), and the layers separated. The aqueous layer was
extracted with benzene (3 ꢀ 30 ml), and the combined organic layers
washed with H2O (3 ꢀ 15 ml). The organiclayer was dried over MgSO 4,
filtered and concentrated. The residue was taken up in benzene (10 ml),
counted (65.6 mCi, 43.7% radiochemical yield) and analyzed by HPLC
(Zorbax Rx-C18 column, same conditions as above, xanthate tR=12.2
min, >99% radiochemical purity). The [14C]xanthate 5 solution in
benzene was used without purification.
1,3-Diiodo-5,5-dimethylhydantoin (6)
The title compound was prepared by adapting the procedure of Orazi
et al.16 5,5-dimethylhydantoin (2.929 g, 22.86 mmol) was dissolved in a
solution of NaOH (1.829 g, 45.71 mmol) in H2O(20 ml) cooled in an ice
bath. Ice (25 g) was added, and then a solution of ICl (8.164 g,
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829

822
C. E. RAAB ET AL.
50.28 mmol) in CHCl3 (25 ml) was added dropwise with vigorous
stirring over 20 min at 0 8C. The slurry was stirred for a further 15 min at
0 8C, then quickly filtered, rinsing thoroughly with ice-cold H2O
(200 ml) and anhydrous EtOAc(100 ml). The pale yellow solid was
pumped under vacuum to constant weight (4.158 g, 48%) and stored in
the dark under N2.
[trifluoromethy1-14C]4-Benzyloxy-3-iodophenyl trifluoromethyl
ether (3)
The title compound was prepared by adapting the procedure of
Kuroboshi et al. for the synthesis of the 3-bromo analog of 3.12 70%
HF/pyridine solution (106 mmol FÀ, 2.65 ml) was added dropwise to a
mixture of DIH 6 (6.018 mmol, 2.287 g) in CH2Cl2 (20 ml) cooled to
À78 8C over 2 min. The benzene solution of [14C]xanthate 5 (61.7 mCi,
1.063 mmol, 9.40 ml) was concentrated in vacuo in CH2Cl2 (10 ml), and
the solution added dropwise to the reaction mixture over 5 min. The
reaction mixture was warmed to 0 8C, and stirred for 80 min. HPLC
analysis at this time showed complete consumption of 5 (Zorbax Rx-
C18 column, 4.6 ꢀ 250 mm, 1 ml/min, 210/254 nm, 60% acetonitrile/
0.1%aq. HClO4 isocratic for 20 min, linear gradient to 100%
acetonitrile over 5 min, 100% acetonitrile isocratic for 15 min, xanthate
tR=16.2 min). The reaction mixture was quenched by the addition of
10% NaHSO3/10% NaHCO3 aq. solution (20 ml). The mixture was
diluted with Et2O (30 ml) and H2O (30 ml), the layers separated, and the
aqueous solution extracted with Et2O (5 ꢀ 15 ml). The combined
organiclayers were washed with H O (3 ꢀ 15 ml), separated, dried over
2
Na2SO4, and filtered. The solution was concentrated to a brown oil
which was dissolved in MeCN (20 ml), counted (57.3 mCi), and
analyzed by HPLC (ZorbaxRx-C18 column, same conditions as above,
aryl iodide tR=17.6 min, thioether tR=20.4 min), which revealed a
51% : 14% [14C]aryl iodide 3/[14C]thioether 7 product ratio. Other
unidentified radioactive impurities observed in the HPLC assay
included the following peaks: tR=12.9 min (7%), tR=19.6 min (4%),
tR=27.5 min (17%), tR=32.0 min (5%) as well as a number of peaks of
51%. The crude mixture was purified by preparative HPLC (Zorbax
SB-Phenyl column, 20 ꢀ 250 mm, 20 ml/min, 210/254 nm, 52% acetoni-
trile/0.1%aq. HClO4 isocratic) to yield 17.1 mCi of [14C]aryl iodide and
6.5 mCi (11%) of [14C]thioether 7. For 7 : 1H NMR d 7.68 (d, 1H,
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829

[
14C] TRIFLUOROMETHYLATION
823
J=2.7 Hz), 7.51–7.33 (m, 5H), 7.16 (dd, 1H, J=8.9, 2.6 Hz), 6.81
(d, 1H, J=8.9 Hz), 5.13 (s, 2H) and 2.38 (s, 3H).
[14C]Thioether 7 (6.5 mCi, 0.116 mmol) was converted to the desired
[14C]aryl iodide 3 by reaction with DIH (0.232 mmol, 88.2 mg) and 70%
HF/pyridine (0.116 mmol, 0.29 ml) in CH2Cl2 (5 ml) using the condi-
tions described above. The crude reaction mixture was purified by
preparative HPLC (Zorbax SB-Phenyl column, same method as above)
to give 3.2 mCi of [14C]aryl iodide. The purified batches of 3 were
combined, concentrated, and taken up in toluene to yield 20.3 mCi
1
(33%) of [14C]aryl iodide 3 at 92.2% radiochemical purity. H NMR d
7.70 (d, 1H, J=2.5 Hz), 7.51–7.34 (m, 5H), 7.17 (dd, 1H, J=9.0,
2.2 Hz), 6.83 (d, 1H, J=9.0 Hz) and 5.15 (s, 2H).
[trifluoromethyl-14C][3R,5R,6S]-7-tButyloxycarbonyl-3-(2-benzyloxy-
5-trifluoromethoxyphenyl)-6-phenyl-1-oxa-7-azaspiro[4.5]decane (9)
The above toluene solution of [14C]aryl iodide 3 (19.2 mCi, 0.343 mmol,
18.9 ml) was transferred to a 4 ml reaction vial. The toluene was
evaporated in a stream of N2, and the residue taken up in 10% H2O/
DMF (780 ml). To the solution were added spiroalkene 2 (0.514 mmol,
167.1 mg), LiCl (1.713 mmol, 72.6 mg), nBu4NCl (0.171 mmol, 47.6 mg),
potassium formate (0.514 mmol, 43.2 mg) and Pd2(dba)3 (0.017 mmol,
15.7 mg), stirring the reaction mixture at 30 8C. After 86 h, HPLC
analysis showed complete consumption of 3 (Phenomenex Customsil
ODS-4 column, 4.6 ꢀ 250 mm, 1 ml/min, 210/254 nm, 70% acetonitrile/
H2O to 95% acetonitrile/H2O linear gradient over 40 min, reduced aryl
iodide 8 tR=14.7 min, aryl iodide 3 tR=20.8 min, benzyl-protected
phenol 9 tR=37.2 min, 3-epimer of 9 tR=37.8 min). The reaction
mixture was diluted with H2O (2 ml) and extracted with EtOAc
(5 ꢀ 2 ml). The combined organic layers were washed with H2O
(2 ꢀ 2 ml), dried over Na2SO4 and concentrated. The brown residue
was purified by silica gel chromatography (eluted with 5% EtOAc/
hexanes) and preparative HPLC (Zorbax Rx-C18 column, 20 ꢀ 250 mm,
20 ml/min, 210/254 nm, 74% acetonitrile/0.1%aq. HClO4 isocratic),
then taken up in MeOH (10 ml) to give 12.9 mCi (67%) of benzyl-
protected phenol 9 at 95.0% radiochemical purity by HPLC (Phenom-
enex Customsil ODS-4 column, 4.6 ꢀ 250 mm, 1 ml/min, 210/254 nm,
70% acetonitrile/H2O to 95% acetonitrile/H2O linear gradient over
40 min, 9 tR=37.2 min). The identity of 9 was established by co-elution
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829

824
C. E. RAAB ET AL.
with an authenticsample of unlabeled material obtained from Merck
Process Research.
[trifluoromethyl-14C][3R,5R,6S]-7-tButyloxycarbonyl-3-(2-hydroxy-5-
trifluoromethoxyphenyl)-6-phenyl-1-oxa-7-azaspiro[4.5]decane (10)
To the above MeOH solution of [14C]benzyl-protected phenol 9 (12.9
mCi, 0.222 mmol, 10.0 ml) were added Et3N (0.417 mmol, 58 ml) and Pd
(OH)2 (29 mg), and the mixture stirred at room temp under an
atmosphere of H2 (1 atm). After 4 h 40 min of reaction, HPLC analysis
(Zorbax Rx-C18 column, 4.6 ꢀ 250 mm, 1 ml/min, 210/254 nm, 80%
acetonitrile/0.1% aq. HClO4 isocratic for 20 min, 80% acetonitrile/
0.1% aq. HClO4 to 100% acetonitrile over 5 min, benzyl-protected
phenol tR=24.0 min, phenol tR=7.3 min) showed complete consump-
tion of starting material. The reaction mixture was filtered through a
Celite pad and was washed with MeOH, to give 12.2 mCi (95%) of
[14C]phenol 10 at 92% radiochemical purity by HPLC. The identity of
10 was established by co-elution with an authentic sample of unlabeled
material obtained from Merck Process Research.
[trifluoromethyl-14C][3R,5R,6S]-7-tButyloxycarbonyl-3-[2-(2-chlor-
ethoxy)-5-trifluoromethoxyphenyl]-6-phenyl-1-oxa-7-azaspiro[4.5]
decane (11)
The MeOH solution of [14C]phenol 10 (12.2 mCi, 0.210 mmol) was
concentrated in vacuo, and the clear residue taken up in DMF (5 ml).
Chloroethyl tosylate (0.424 mmol, 77 ml) and cesium carbonate
(0.435 mmol, 142.0 mg) were added to the solution, which was then
stirred at 50 8C for 7 h. HPLC analysis at this stage indicated that the
reaction was complete (Zorbax Rx-C18 column, 4.6 ꢀ 250 mm, 1 ml/
min, 210/254 nm, 80% acetonitrile/0.1% aq. HClO4 isocratic for 20 min,
phenol 10 tR=7.3 min, alkyl chloride 11 tR=14.8 min). The reaction
mixture was cooled to 5 8C, diluted with EtOAc(10 ml) and cold H 2O
(5 ml) and the layers separated. The aqueous layer was extracted with
EtOAc(6 ꢀ 10 ml), and the combined organic layers washed with water
(2 ꢀ 10 ml), dried over Na2SO4, and filtered to provide 11.3 mCi (93%)
of [14C]alkyl chloride 11 at 93% radiochemical purity by HPLC. The
identity of 11 was established by co-elution with an authentic sample of
unlabeled material obtained from Merck Process Research.
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829

[
14C] TRIFLUOROMETHYLATION
825
[trifluoromethyl-14C][3R,5R,6S]-7-tButyloxycarbonyl-3-(2-vinyloxy-5-
trifluoromethoxyphenyl)-6-phenyl-1-oxa-7-azaspiro[4.5]decane (12)
The above EtOAcsolution of [ 14C]alkyl chloride 11 (11.3 mCi,
0.195 mmol) was concentrated in vacuo, and the clear oil taken up in
THF (15 ml). The solution was cooled 5 8C, and KOtBu (0.774 mmol,
86.8 mg) was added in a single portion. The reaction mixture was stirred
at 5–8 8C for 15 min, and then at room temp for 2 h, at which time
reaction was deemed to be complete by HPLC analysis (Zorbax Rx-C18
column, 4.6 ꢀ 250 mm, 1 ml/min, 210/254 nm, 80% acetonitrile/0.1%
aq. HClO4 isocratic for 20 min, alkyl chloride 11 tR=14.8 min, vinyl
ether 12 tR=18.1 min). The reaction mixture was then cooled in an ice
bath, and H2O (20 ml) was added. The THF was removed in vacuo, and
the remaining aqueous solution extracted with EtOAc (3 ꢀ 15 ml). The
combined organic layers were washed with 1 M NH4Cl (30 ml) and H2O
(2 ꢀ 20 ml), then dried over MgSO4, filtered, concentrated, and taken up
in toluene to provide 11.1 mCi (98%) of [14C]vinyl ehter 12 at 93.7%
radiochemical purity by HPLC. The identity of 12 was established by
co-elution with an authentic sample of unlabled material obtained from
Merck Process Research.
[trifluoromethyl-14C][3R,5R,6S]-7-tButyloxycarbonyl-3-(2-cyclopropy-
loxy-5-trifluoromethoxyphenyl)-6-phenyl-1-oxa-7-azaspiro[4.5]decane
benzenesulfonate (13)
Et2Zn (1 M in hexanes, 1.0 mmol, 1.0 ml) was added dropwise over
10 min to 1,2-DCE (2 ml) cooled to À28 8C. The toluene solution of
[14C]vinyl ether 12 (11.1 mCi, 0.191 mmol) prepared as described above
was concentrated in vacuo in a separate flask, and the clear oil taken up
in 1,2-DCE (5 ml). This solution was added dropwise to the Et2Zn
solution over 5 min, maintaining a temperature of À24 to À18 8C.
Chloroiodomethane (1.922 mmol, 140 ml) was added dropwise over
5 min, and the reaction mixture stirred at À16 to À7 8C for 8 h. HPLC
analysis at this stage showed complete consumption of [14C]vinyl ether
(Zorbax Rx-C18 column, 4.6 ꢀ 250 mm, 1 ml/min, 210/254 nm, 83%
acetonitrile/0.1% aq. HClO4 isocratic for 20 min, vinyl ether 12
tR=13.8 min, cyclopropyl ether 13 tR=15.5 min). The reaction was
diluted with EtOAc(5 ml) at À12 8C, followed by 2M NH4Cl (5 ml) and
H2O (10 ml). The layers were separated, and the aqueous extracted with
EtOAc(5 ꢀ 7 ml). The combined organic layers were washed with 4%
aq. NaHCO3 (2 ꢀ 5 ml) and H2O (4 ꢀ 5 ml), then concentrated in vacuo,
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829

826
C. E. RAAB ET AL.
and purified by preparative HPLC (Zorbax Rx-C18 column, 20 ꢀ 250 mm,
20 ml/min, 210/254 nm, 72% acetonitrile/H2O isocratic), to give 8.1 mCi
(73%) of 13 at 94.9% radiochemical purity by HPLC (Zorbax Rx-C18
column, 4.6 ꢀ 250 mm, 1 ml/min, 210/254 nm, 80% acetonitrile/0.1% aq.
HClO4 isocratic for 30 min, cyclopropyl ether tR=20.7 min). The identity
of 13 was established by co-elution with an authentic sample of unlabeled
material obtained from Merck Process Research.
[trifluoromethyl-14C][3R,5R,6S]-3-(2-Cyclopropyloxy-5-trifluoro-
methoxyphenyl)-6-phenyl-1-oxa-7-azaspiro[4.5]decane benzenesulfo-
nate (1)
[14C]Cyclopropyl ether 13 (8.1 mCi, 0.140 mmol) was taken up in EtOAc
(4 ml), cooled to 0 8C, and HBr (7.072 mmol, 800 ml) was added. The
reaction mixture was warmed to room temp with stirring. After 5 h,
reaction was deemed to be complete by HPLC (Zorbax Rx-C18 column,
4.6 ꢀ 250 mm, 1 ml/min, 210/254 nm, 80% acetonitrile/0.1% aq. HClO4
isocratic for 30 min, 1 tR=2.8 min. cyclopropyl ether 13 tR=20.7 min).
The reaction was cooled in an ice-bath, diluted with EtOAc (15 ml) and
basified to pH 11 by the dropwise addition of 2.5 M NaOH. After
further dilution with H2O (10 ml), the layers were separated, and the
aqueous extracted with EtOAc (3 ꢀ 5 ml). The combined organic layers
were washed with brine (2 ꢀ 10 ml) and H2O (3 ꢀ 10 ml), then
concentrated in vacuo. The residue was purified by semi-prep HPLC
(Phenomenex Luna C8 column, 9 ꢀ 250 mm, 5 ml/min, 210/254 nm,
40% acetonitrile/0.1% aq. HClO4 isocratic) to yield 4.0 mCi of [14C]1 at
98.9% radiochemical purity (Phenomenex Luna C8 column,
4.6 ꢀ 250 mm, 1 ml/min, 210/254 nm, 45% acetonitrile/0.15% aq.
1
HClO4 isocratic for 40 min, 1 tR=28.1 min). H NMR d 8.94 (d, 1 H,
J=11.5 Hz), 8.34–8.29 (m, 1H), 7.56–7.53 (m, 1H), 7.45 (d, 2H, J=7.5 Hz),
7.36–7.31 (m, 2H), 7.26–7.20 (m, 4H), 7.03 (d, 1H, J=8.9 Hz), 6.96 (d, 1H,
J=8.9, 1.9 Hz), 6.90 (d, 1H, J=2.6 Hz), 4.08 (d, 1H, J=10.2 Hz), 3.86–3.82
(m, 1H), 3.57–3.51 (m, 2H), 3.17 (d, 1H, J=11.2 Hz), 2.99–2.89 (m, 1H),
2.30–2.20 (m, 1H), 2.14–2.04 (m, 2H), 1.92–1.88 (m, 2H), 1.70–1.59
(m, 3H), 0.75–0.67 (m, 2H) and 0.59–0.49 (m, 2H).
[trifluoromethyl-14C]1-Bromo-4-trifluoromethylbenzene (16)
The [14C]methyl 1-bromo-4-dithiocarboxybenzene intermediate (15)
was prepared by modification of the procedure of Verkruijsse and
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829

[
14C] TRIFLUOROMETHYLATION
827
Brandsma.20 In a flame-dried 15 ml 2-neck flask attached to a gas
transfer manifold, 1.0 M n-BuLi in hexane (0.39 mmol, 390 ml) was
added to anhyd. THF (250 ml) at À20 8C under N2 atmosphere. The
solution was cooled to À80 8C, and 1,4-dibromobenzene (0.372 mmol,
87.8 mg, 47.6 ml) was added dropwise over 10 min. After stirring for
10 min at À80 8C, the reaction mixture was warmed to À65 8C, and a
solution of CuBr (0.074 mmol, 10.6 mg) and anhyd. LiBr (0.149 mmol,
12.9 mg) in THF (170 ml) was added dropwise over 5 min, before stirred
at À65 8C for 15 min. The mixture was then frozen in liquid N2, and
14CS2 being (25 mCi, 0.446 mmol) transferred to the reaction vessel. The
reaction was warmed to À40 8C for 30 min, and MeI (0.6 mmol,
85.2, mg, 37 ml) was added over 5 min. The reaction mixture was allowed
to warm to rt, then quenched with 5% aq. KCN (10 ml). Et2O (20 ml)
was added, the layers separated, the aqueous extracted with Et2O
(3 ꢀ 10 ml). The combined organics were washed with brine, dried over
MgSO4, and concentrated to give 10.5 mCi of a red–brown oil with 85%
radiochemical purity containing 8.9 mCi (36%) of 15 (Zorbax Rx-C18
column, 4.6 ꢀ 250 mm, 1 ml/min, 210/254 nm, 50% acetonitrile/0.1%
aq. HClO4 linear gradient to 100% MeCN over 20 min, 15
tR=20.4 min). Found: m/z 249.0. Calculated for 15 : 248.92.
The title compound was prepared using the procedure of Furuta
et al.18 A portion of the unpurified material (3 mCi, 0.054 mmol) was
taken up in anhyd. CH2Cl2 (1.0 ml), cooled to 0 8C, and 70% HF/pyr
(4.32 mmol of FÀ, 108 ml) was added. Dibromohydantoin (0.216 mmol,
61.8 mg) was added in one portion, and the reaction mixture was stirred
at room temperature for 1 h, before diluting with 10 : 1 hexane/Et2O.
The material was passed through a silica gel plug, eluting with 10 : 1
hexane/Et2O. The eluent was washed with a 5% aq. NaHCO3/NaHSO3
solution, then dried over Na2SO4, filtered and concentrated to give a pale
brown oil (1.95 mCi) with 70% radiochemical purity containing 1.27 mCi
of 16 (50%), which was co-eluted with an authentic sample of 1-bromo-4-
trifluoromethylbenzene purchased from Aldrich (Zorbax Rx-C18 column,
4.6 ꢀ 250 mm, 1 ml/min, 210/254 nm, 50% acetonitrile/0.1% aq. HClO4
linear gradient to 100% MeCN over 20 min, 16 tR=14.8 min).
[trifluoromethyl-14C]N-Benzyl-N-(2-pyridyl)trifluoromethylamine
(19).19
The [14C]methyl N-benzyl-N-(2-pyridy)dithiocarbamate (18) intermedi-
ate was prepared by adapting the procedure of Kanie et al.19 To a
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829

828
C. E. RAAB ET AL.
solution of N-(2-pyridyl)benzylamine (0.353 mmol, 65.1 mg) in anhyd.
THF (5 ml) was added dropwise at À15 8C, 1.6 M n-Buli in hexane
(0.424 mmol, 265 ml). After stirring the orange–brown reaction mixture
at À15 to À5 8C for 1 h, the reaction vessel was placed on a gas transfer
manifold, frozen in liquid N2, and 14CS2 (25 mCi, 0.446 mmol)
transferred. The reaction was stirred at room temperature overnight,
then cooled to 0 8C and MeI (0.706 mmol, 100.2 mg, 44 ml) added
dropwise over 5 min. The reaction was stirred at room temperature for
6 h, then quenched by the addition of sat. aq. NaHCO3 (5 ml), and
diluted with CH2Cl2 (10 ml). The layers were separated, and the
aqueous layer extracted with CH2Cl2 (3 ꢀ 10 ml). The combined
organics were dried over MgSO4, filtered and concentrated to give
17.2 mCi of a pale brown oil with 93% radiochemical purity containing
16.0 mCi (64%) of 18 (Zorbax Rx-C8 column, 4.6 ꢀ 250 mm, 1 ml/min,
210/254 nm, 5% acetonitrile/0.1% aq. HClO4 linear gradient to 100%
MeCN over 30 min, 18 tR=25.1 min). 1H NMR d 7.52 (d, 2H
J=7.9 Hz) and 7.64 (d, 2H J=7.9 Hz). Found: m/z 277.0. Calculated
for 18 : 277.06.
The title compound was prepared using the procedure of Kanie et al.19
To
a
suspension of tetrabutylammonium dihydrogentrifluoride
(0.146 mmol, 85.2 ml of 1,2-DCE solution) and dibromohydantoin
(0.117 mmol, 33.3 mg) in anhyd. CH2Cl2 (1.0 ml) was added dropwise
a solution of 18 (0.029 mmol, 1.72 mCi) in toluene (0.5 ml) at 0 8C. The
reaction was stirred at 0 8C for 4 h then poured into pH 10 NaOH/
NaHCO3 buffer (10 ml). The mixture was extracted with Et2O
(3 ꢀ 5 ml), and the combined organic layers dried over MgSO4, filtered
and concentrated to give a pale brown oil (1.29 mCi) with 90%
radiochemical purity containing 1.16 mCi (72%) of 19 (Zorbax XDB-C8
column), 3.5 ꢀ 150 mm, 0.75 ml/min, 220/254 nm, 5% acetonitrile/pH
3.5 ammonium formate buffer linear gradient to 95% MeCN over
12 min, 19 tR=11.0 min). 1H NMR d 8.35 (ddd, 1H, J=1.0, 2.0, 5.0 Hz),
7.60 (ddd, 1H, J=2.1, 6.9, 7.0 Hz), 7.34–7.18 (m, 5H), 7.10–6.98 (m,
1H), 6.99 (ddd, 1H, J=0.8, 4.9, 6.8 Hz) and 5.01 (q, 2H, J=2.2 Hz).
Found: m/z 255.2. Calculated for 19 : 255.09.
Acknowledgements
The authors would like to acknowledge the contributions of the
following Merck co-workers: the NK-1 Medicinal Chemistry Team
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829

[
14C] TRIFLUOROMETHYLATION
829
(Terlings Park, UK) for the discovery and SAR of this novel class of
substance P antagonists; Dr. Mark Cameron and Mr. Ben Marcune of
Process Research for providing materials and procedures, and Dr. Allen
Jones and Ms. Yolanda Jakubowski of the Labeled Compound
Synthesis group for analytical support.
References
1. Nicoll RA, Schenker C, Leeman SE. Annu Rev Neurosci 1980; 3: 227.
2. Takeuchi Y, Berkley Shands EF, Beusen DD, Marshall GR. J Med Chem
1998; 41: 3609 and references therein.
3. Elliott JM, Cascieri MA, Chicchi GG, Curtis N, Hargreaves R, Harrison
T, Hollingworth GJ, Kulagowski J, Kurtz MM, Owen S, Rupniak NMJ,
Rycroft W, Sadowski S, Seward E, Swain CJ, Tattersall FD, Williams B,
Williams AR. A new class of spirocyclic NK1 antagonists. Tachykinins
2000, Montpellier, France, October 17–20, 2000. Abstract No. P-84.
4. Kulagowski JJ, Curtis NR, Swain CJ, Williams B. J Org Lett 2001; 3: 667.
5. Wallace DJ, Goodman JM, Kennedy DJ, Davies AJ, Cowden CJ,
Ashwood MS, Cottrell IF, Dolling U-H, Reider P. J Org Lett 2001; 3: 671.
6. Fillar R, Kobayashi Y. Biomedical Aspects of Fluorine Chemistry.
Kodansya Ltd. and Elsevier Biomedical: Tokyo, Amsterdam, 1982.
7. Iarovenko NN, Vasileva AS. J Gen Chem USSR 1958; 28: 2539.
8. Farb. Hoechst AG. Brevet Brit 1957; 765527.
9. Feiring AE. J Org Chem 1979; 44: 2907.
10. Scheppard WA. J Org Chem 1964; 29: 1.
11. Mathey P, Bensoam. J Tetrahedron Lett 1973; 24: 2253.
12. Kuroboshi M, Suzuki K, Hiyama T. J Tetrahedron Lett 1992; 33: 4173.
13. Lee AWM, Chan WH, Wong HC, Wong SM. Synth Commun 1989; 19:
547.
14. Marshall FJ, McMahon RE, Jones RG. J Agric Food Chem 1966; 14: 498.
15. Danenberg PV, Langenbach RJ, Heidelberger C. Biochemistry 1974; 13:
926.
16. Orazi OO, Corral RE, Bertorello HE. J Org Chem 1965; 30: 1101.
17. Denmark SE, Edwards JP. J Org Chem 1991; 56: 6976.
18. Furuta S, Kuroboshi M, Hiyama T. Bull Chem Soc Jpn 1999; 72: 805.
19. Kanie K, Katsuya M, Kuroboshi M, Hiyama T. Bull Chem Soc Jpn 1998;
71: 1973.
20. Verkruijsse HD, Brandsma L. J Organomet Chem 1987; 332: 95.
Copyright # 2001 John Wiley & Sons, Ltd.
J Labelled Cpd Radiopharm 2001; 44: 815–829