Rapid Syntheses of [11C]Arylvinyltrifluoromethanes through Treatment of (E)-Arylvinyl(phenyl)iodonium Tosylates with [11C]Trifluoromethylcopper(I)

Article abstract of DOI:10.1021/acs.orglett.0c01705

We report a method for labeling arylvinyltrifluoromethanes with carbon-11 (t1/2 = 20.4 min) as representatives of a new radiolabeled chemotype that has potential for developing radiotracers for biomedical imaging with positron emission tomography. Treatme

Full text of DOI:10.1021/acs.orglett.0c01705

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Letter
Rapid Syntheses of [11C]Arylvinyltrifluoromethanes through
Treatment of (E)‑Arylvinyl(phenyl)iodonium Tosylates with
[11C]Trifluoromethylcopper(I)
Susovan Jana, Sanjay Telu,* Bo Yeun Yang, Mohammad B. Haskali, Jimmy E. Jakobsson,
and Victor W. Pike
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ABSTRACT: We report a method for labeling arylvinyltrifluoromethanes with carbon-11 (t1/2 = 20.4 min) as representatives of a
new radiolabeled chemotype that has potential for developing radiotracers for biomedical imaging with positron emission
tomography. Treatment of (E)-arylvinyl(phenyl)iodonium tosylates (1a−1k) with [11C[CuCF3 gave the corresponding
[11C]arylvinyltrifluoromethanes ([11C]2a−[11C]2k) in high radiochemical yields (90−97%) under rapid (2 min) and mild (60
°C) conditions.
adiotracers labeled with the cyclotron-produced short-
lived positron-emitter, carbon-11 (t1/2 = 20.4 min), are
the labeling of fluoroform with the longer lived positron-
emitter fluorine-18 (t1/2 = 110 min) by all of several methods.8
[11C]Fluoroform may be converted readily into [11C]-
trifluoromethylcopper(I) ([11C]CuCF3), which is starting to
find diverse applications.7 Here, we aimed to explore the
reactivity of [11C]CuCF3 for creating previously unknown
[11C]arylvinyltrifluoromethanes as a means to enlarge the
chemical space available for PET radiotracer development.
Trifluoromethylation at a vinyl position has gained
importance in medicinal chemistry for producing a variety of
biologically active compounds (Figure 1).9 Numerous
methods have been devised for constructing Cvinyl−CF3
bonds. Major methods are the palladium(II)-mediated
coupling of aryl halides with 1-iodo-3,3,3-trifluoropropane10
or accessible Cvinyl−CF3 reagents, such as 2-(trifluoromethyl)-
acrylic acid11 (Figure 2, eq 1). Togni I reagent (1-
trifluoromethyl-3,3-dimethyl-1,2-benziodoxole), Togni II re-
agent (1-trifluoromethyl-1,2-benziodoxol-3-(1H)-one),12 and
Langlois reagent (CF3SO2Na)13 have been used for iron(II)-
R
widely used in animal and human subjects with positron
emission tomography (PET) for investigating pathophysiology
and for assisting drug development programs.1 Carbon-11 is
especially appealing for labeling PET radiotracers because, in
principle, it may be used to label any organic compound.
Nonetheless, the short half-life of carbon-11 demands that
labeling procedures are rapid, efficient, and amenable to
automation for radiation safety.2
Carbon-11 is generally produced from the high-yield 14N-
(p,α)11C reaction within nitrogen gas in the presence of
oxygen (1%) to give [11C]carbon dioxide or in the presence of
hydrogen (10%) to give [11C]methane.2 Over several decades,
methods have been developed to rapidly convert these primary
radioactive precursors into more useful labeling agents,3 such
as [11C]methyl iodide for 11C-methylation reactions3−5 and
[11C]carbon monoxide for 11C-carbonylation reactions.3,5,6
A
recent advance has been the development of [11C]fluoroform.7
[11C]Fluoroform opens up possibilities for labeling radio-
tracers in trifluoromethyl groups at a high no-carrier-added
(NCA) molar activity (ratio of radioactivity to mass; Am).
[11C]Fluoroform is produced from cyclotron-produced
[11C]methane by passage over heated (270 °C) cobalt(III)
fluoride (CoF3) in good yield within much less than one-half-
life of carbon-11. A high molar activity (200−500 GBq/μmol)
is readily attainable,7 a value which far exceeds that achieved in
Received: May 19, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01705
© XXXX American Chemical Society
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The phenyliodonio group (PhI+−) is even more nucleofugic
than the commonly used triflate leaving group.24 Diary-
liodonium salts (Ar′ArI+X−) are therefore efficient arylation
reagents.25 In the field of PET radiotracer development, these
hypervalent iodine compounds have become valuable sub-
strates for radiofluorination.26 Recently, they have been shown
to be efficient precursors for trifluoromethylation27 and
radiotrifluoromethylation.28 Phenyl(vinyl)iodonium salts are
also reactive towards nucleophilic substitution under mild
conditions, thereby providing a practical route to various vinyl
building blocks.29 Despite their synthetic utility, vinyliodonium
salts have not been exploited as substrates for trifluoromethy-
lation. In this report, we demonstrate that the treatment of
(E)-arylvinyl(phenyl)iodonium tosylates with [11C]CuCF3
under rapid and mild conditions produce a wide range of
representatives of a new radiolabeled chemotype, namely
[11C]arylvinyltrifluoromethanes (Figure 2, eq 4).
Figure 1. Examples of druglike molecules containing a Cvinyl−CF3
moiety.
We prepared a simple model substrate, (E)-phenyl(styryl)-
iodonium tosylate (1a), by treating the corresponding
styrylboronic acid with Koser’s reagent (PhI(OH)OTs) in
DCM−CF3CH2OH.30 We then tested the 11C-trifluoromethy-
lation of 1a by treatment with NCA [11C]fluoroform at rt for
10 min in the presence of K[Cu(DMF)(OtBu)2] generated in
situ in DMF. This reaction gave the 11C-trifluoromethylated
compound [11C]2a in excellent yield (96%) when 50 μmol of
1a was used (Table 1, entry 1). Reducing the precursor
a
Table 1. Labeling Optimization Study with 1a
precursor amount
temp
(°C)
time
(min)
yield of [11C]
bc
,
entry
(μmol)
solvent
2a (%)
1
2
3
4
5
6
7
8
9
50
25
10
25
10
10
10
10
5
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
MeCN
rt
rt
rt
rt
rt
60
60
60
60
60
60
10
10
10
5
5
10
5
2
5
2
2
96
90
78
86
3
9
7
6
Figure 2. Methods for constructing the aryl-Cvinyl−CF3 bond.
73 10
97
94
85
80
96
56
2
8
8
6
1
9
and copper(I)-catalyzed trifluoromethylations of vinylboron
derivatives, respectively (Figure 2, eq 2). Furthermore, Togni
II reagent14 and Langlois reagent15 have been used for the
decarboxylative trifluoromethylation of α,β-unsaturated acids
(Figure 2, eq 2). Togni II reagent has also been used for the
Cu(I)-catalyzed trifluoromethylation of alkenes (Figure 2, eq
2).16 Styrenes have also been trifluoromethylated using
trifluoromethyl halides17 under Cu(I)-, Pd(II)-, or Ir(III)-
catalyzed conditions (Figure 2, eq 2). Other methods include
the Cu(I)-catalyzed trifluoromethylation of vinylboronic acids
with Ruppert−Prakash reagent (TMSCF3)18 or Langlois
reagent,19 and Pd(II)-catalyzed Suzuki−Miyaura coupling
between a boronic acid and an (E)-(β-trifluoromethyl)-
vinyldiphenylsulfonium salt (Figure 2, eq 2).20 The trifluor-
de
,
10
11
10
10
a
Reaction conditions: 1a, CuBr (5 μmol), KOtBu (15 μmol), Et3N·
3HF (5 μmol), solvent (750 μL), The percent yields were
determined from radio-HPLC. Reactions were performed in
duplicate (n = 2). Trace MeCN was used along with DMF. n = 3.
b
c
d
e
quantity to 25 μmol (Table 1, entry 2) and then to 10 μmol
(Table 1, entry 3) progressively reduced yield but not greatly
so. Likewise, shortening the reaction time to 5 min only
slightly reduced yields (Table 1, cf. entry 4 with entry 2, and
entry 5 with entry 3).
An increase in reaction temperature to 60 °C, when using 10
μmol of 1a for 10 min reaction, raised the yield of [11C]2a to
almost quantitative (Table 1, entry 6, cf. entry 3). This elevated
temperature was also beneficial for the shorter reaction time of
5 min (Table 1, entry 7, cf. entry 5). Shortening reaction time
to 2 min (Table 1, entry 8) or lowering precursor amount to 5
μmol (Table 1, entry 9) still gave very high yields. To our
21
omethylation of alkynes with CuCF3 or with trifluoromethyl
halides22 (Figure 2, eq 3) and the trifluoromethylation of
alkenyl halides (Figure 2, eq 2) with CuCF3 reagent9b,23 have
also emerged as useful methods. Despite these impressive
synthetic advances, no method has hitherto been available for
radiolabeling the trifluoromethyl group at an (E)-vinyl
position.
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knowledge, the latter is the highest yield in any radio-
trifluoromethylation yet reported with such a low amount of
precursor. Use of a low amount of precursor is appealing in
PET radiotracer production because it can save precious
precursor, simplify radiotracer HPLC purification, and render
the whole production process more compatible with current
good manufacturing practice (CGMP) regulations. When the
DMF solvent was lightly doped with MeCN (50 μL) a very
high yield was obtained under similar conditions using 10
μmol of precursor (Table 1, entry 10). The results had
minimal deviation when obtained in triplicate. Presumably, in
this reaction, the low proportion of MeCN stabilized the
[11C]CuCF3 by forming a (MeCN)4Cu11CF3 complex,31
which was further stabilized by Et3N·3HF. However, a change
of solvent from DMF to acetonitrile decreased the yield very
appreciably (Table 1, entry 11).
After establishing the optimal labeling conditions for 1a, we
explored substrate scope. Several (E)-arylvinyl(phenyl)-
iodonium tosylates (1b−1k) were synthesized in moderate
to good yields by treating the corresponding arylvinylboronic
acid or tri-n-butylstannane with Koser’s reagent in DCM−
CF3CH2OH at 0 °C followed by warming to rt overnight
(Figure 3).29,30 These salts generally showed excellent stability
Figure 5. Reaction scope leading to [11C](E)-arylvinyltrifluoro-
a
methanes. Reaction conditions: 1a−1k (10 μmol), CuBr (5 μmol),
KOtBu (15 μmol), Et3N·3HF (5 μmol), DMF (0.7 mL), MeCN (50
b
μL). [11C]CHF3 was added in DMF solution. Radio-HPLC yield
shown as mean SD for n = 3. cIsolated yield shown as mean SD
for n = 3.
Figure 3. Synthesis of (E)-arylvinyl(phenyl)iodonium tosylates for
use as labeling precursors.
electronics, or bulk of any substituent on the aryl group in the
β-position of the vinyl group. [11C]2h is interesting as a
potential labeling synthon because of its formyl substituent,
which may be readily converted into other groups or which
may be used for coupling to other biologically significant
molecules, such as peptides. Vinyliodonium salts bearing a
biphenyl (1i) or a bulky naphthyl group (1j) also gave the
corresponding 11C-trifluoromethylated products [11C]2i and
[11C]2j in very high yields. The result from the naphthylvinyl-
(phenyl)iodonium salt (1j) suggests that aryl ortho sub-
stituents would be well tolerated, although this was not further
tested. A vinyl precursor containing a pyridinyl moiety (1k)
gave [11C]2k in very good yield (90%) (Figure 5). Thus, this
method appears highly general in tolerating not only homoaryl
but also heteroaryl substituents in the vinyl β-position.
All of the labeled products showed the same retention times
and peak shapes as reference (E)-compounds in the analytical
reversed-phase HPLC method. This method was not, however,
tested and optimized for separation of (E)- and (Z)-isomers.
Mechanisms may be envisaged that involve radical or cationic
tetrameric intermediates that could give rise to (E)- and (Z)-
isomers from (E)-precursors. Detailed studies are required to
assess whether E to Z isomerism can occur to any significant
extent in these reactions.
to storage for several months at 4 °C. We also synthesized
reference (E)-arylvinyltrifluoromethanes 2a−2k in good yields
by Pd-mediated domino Heck-type coupling of aryl iodides 3
with 1,1,1-trifluoro-3-iodopropane (4) under microwave
conditions (Figure 4).10 1H, 13C, and 19F NMR spectroscopy
Figure 4. Synthesis of reference compounds.
enabled unambiguous assignment of E geometry for all these
products. These reference compounds were needed to identify
labeled products by using radio-HPLC.32
With a range of substrates and reference compounds
available, we explored substrate scope under the optimized
radiolabeling conditions. All of the substrates underwent 11C-
trifluoromethylation to afford the corresponding radiolabeled
arylvinyltrifluoromethanes [11C]2a[11C]2k in excellent
analytical yields (95−97%) and good isolated yields (66−
77%) without appreciable amounts of byproducts (Figure 5;
see SI). Yields were only slightly sensitive to the position,
It should be noted that this method is readily transferable to
use with [18F]fluoroform when radiotracers of longer physical
half-life are required. We confirmed this by producing [18F]2a
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from 1a in 99% yield (n = 2) using [18F]fluoroform produced
by our recently described method8e under the conditions
matching those of entry 1 in Table 1 (see the SI).
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01705.
General information, experimental procedures, charac-
terization of compounds, radiolabeling methods, NMR
spectra, and HPLC chromatograms (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Sanjay Telu − Molecular Imaging Branch, National Institute of
Mental Health, National Institutes of Health, Bethesda,
Maryland 20892, United States; orcid.org/0000-0001-
5300-8725; Email: telus@mail.nih.gov
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Authors
Susovan Jana − Molecular Imaging Branch, National Institute of
Mental Health, National Institutes of Health, Bethesda,
Maryland 20892, United States; orcid.org/0000-0001-
9910-347X
Bo Yeun Yang − Molecular Imaging Branch, National Institute
of Mental Health, National Institutes of Health, Bethesda,
Maryland 20892, United States
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Chem. 2013, 78, 12837.
Mohammad B. Haskali − Molecular Imaging Branch, National
Institute of Mental Health, National Institutes of Health,
Bethesda, Maryland 20892, United States
Jimmy E. Jakobsson − Molecular Imaging Branch, National
Institute of Mental Health, National Institutes of Health,
Bethesda, Maryland 20892, United States
Victor W. Pike − Molecular Imaging Branch, National Institute
of Mental Health, National Institutes of Health, Bethesda,
Maryland 20892, United States; orcid.org/0000-0001-
9032-2553
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Intramural Research Program of the National
Institutes of Health (NIMH; Project ZIAMH002793) for
financial support. We are grateful to the NIH Clinical Center
(Chief Dr. P. Herscovitch) for cyclotron production of carbon-
11 and fluorine-18.
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other details.
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