Enantioselective radiosynthesis of positron emission tomography (PET) tracers containing [18F]fluorohydrins

Article abstract of DOI:10.1021/ja5025645

Herein, we describe an operationally straightforward radiosynthesis of a chiral transition metal fluoride catalyst, [¹⁸F](salen)CoF, and its use for late-stage enantioselective aliphatic radiofluorination. We demonstrate the utility of the method by preparing single enantiomer experimental and clinically validated PET tracers that contain base-sensitive functional groups, epimerizable stereocenters, and nitrogen-rich motifs. Unlike the conventional radiosyntheses of these targets with [¹⁸F]KF, labeling with (salen)CoF is possible in the last step and under exceptionally mild conditions. These results constitute a rare example of a nucleophilic radiofluorination using a transition metal fluoride and highlight the potential of such reagents to enhance traditional methods for labeling aliphatic hydrocarbons.

Full text of DOI:10.1021/ja5025645

Communication
pubs.acs.org/JACS
Enantioselective Radiosynthesis of Positron Emission Tomography
(PET) Tracers Containing [18F]Fluorohydrins
Thomas J. A. Graham,† R. Frederick Lambert,† Karl Ploessl,‡ Hank F. Kung,‡,§ and Abigail G. Doyle*,†
†Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
‡Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
§Department of Pharmacology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
S
* Supporting Information
ABSTRACT: Herein, we describe an operationally
straightforward radiosynthesis of a chiral transition metal
fluoride catalyst, [18F](salen)CoF, and its use for late-stage
enantioselective aliphatic radiofluorination. We demon-
strate the utility of the method by preparing single
enantiomer experimental and clinically validated PET
tracers that contain base-sensitive functional groups,
epimerizable stereocenters, and nitrogen-rich motifs.
Unlike the conventional radiosyntheses of these targets
with [18F]KF, labeling with (salen)CoF is possible in the
last step and under exceptionally mild conditions. These
results constitute a rare example of a nucleophilic
radiofluorination using a transition metal fluoride and
highlight the potential of such reagents to enhance
traditional methods for labeling aliphatic hydrocarbons.
Figure 1. (A) Asymmetric fluoride ring opening of epoxides catalyzed
by (salen)Co; DBN: 1,5-diazabicyclo(4.3.0)non-5-ene. (B) Proposed
homobimetallic mechanism. (C) Strategy for direct radiosynthesis of
[18F]fluorohydrins.
possessing protic functional groups (e.g., alcohols) and
functionality prone to elimination are generally not tolerated
under these reaction conditions due to the high temperatures
(>100 °C) necessary for labeling and the basicity of [18F]KF/
he use of [18F]-labeled small molecules for positron
T
emission tomography (PET) represents one of the most
promising approaches to detect disease progression and evaluate
therapeutic effectiveness in vivo.1−3 However, the radiochemical
methods available to introduce [18F]fluoride into bioactive
probes severely limit the potential scope of the imaging
modality.4,5 The short half-life (110 min) and low available
concentrations of 18F (ranging from nM to μM), compounded
with the general difficulties posed by C−19F bond formation,
make the identification of broadly applicable radiofluorinations
of complex molecules incredibly challenging.6,7 Nevertheless, the
past five years have witnessed the discovery of new methods that
begin to address the limited scope of radiolabeling with
[18F]fluoride. The majority of these solutions have focused on
the challenge of [18F]aryl fluoride synthesis.8−10 In contrast,
methods for improving the scope of aliphatic radiofluorination
remain significantly underdeveloped.11,12 Although numerous
modern synthetic methods have been reported that achieve mild
and selective aliphatic carbon−fluorine bond formation, these
methods utilize electrophilic 19F sources.13 Nucleophilic fluoride
is currently the only practical and generally available source of 18F
to prepare PET tracers in high specific activity.13 As such, these
electrophilic methods have proven less useful for applications in
PET.
K222.
6,7 Furthermore, despite the importance of stereochemistry
with regard to biological activity, the preparation of single
stereoisomer PET probes is often challenging owing to the
propensity of [18F]fluoride reagents to induce epimerization. As
such, PET tracers are often evaluated as racemic mixtures or they
are subjected to time-consuming chiral HPLC separation.14,15
To the best of our knowledge, methods capable of late-stage
enantioselective labeling with [18F]fluoride are completely
unknown. Herein, we report an asymmetric, no-carrier-added
radiosynthesis of [18F]fluorohydrins by ring opening of epoxides
with chiral cobalt catalysts. In addition to offering direct access to
single enantiomer tracers in the last synthetic step, the method
also addresses many of the noted deficiencies associated with
aliphatic labeling using [18F]KF.
[18F]Fluorohydrins represent a useful motif in probe design
and are featured in several experimental and clinically validated
PET tracers.14,16−18 They are typically prepared through
selective displacement of differentially protected diols followed
by deprotection of the remaining protecting group (vide infra).
As such, preparation of a single enantiomer PET probe
containing an [18F]fluorohydrin requires that stereochemistry
be set within an organic molecule prior to labeling. Asymmetric
PET tracers containing aliphatic C−18F labels are typically
prepared using a substitution reaction with activated alcohol
derivatives (i.e., tosylate, mesylate) and [18F]KF in the presence
of cryptands such as Kryptofix 2.2.2. (K222). Substrates
Received: March 13, 2014
Published: March 17, 2014
© 2014 American Chemical Society
5291
dx.doi.org/10.1021/ja5025645 | J. Am. Chem. Soc. 2014, 136, 5291−5294

Journal of the American Chemical Society
Communication
a
We recently demonstrated that a Lewis acidic chiral (salen)Co
and an isothiourea or amidine cocatalyst promote enantiose-
lective, nucleophilic fluoride ring opening of epoxides (Figure
1A).21 This approach represents a rare example of a catalyst-
controlled enantioselective nucleophilic fluorination and features
remarkably mild and functional group tolerant conditions. Key to
its success is the use of benzoyl fluoride (PhCOF) and an alcohol
additive as a latent source of HF, which avoids an unselective
background reaction and catalyst poisoning. Based on these
promising attributes, we sought to translate the methodology to a
radiofluorination protocol. However, a number of issues
inhibited direct application, not the least of which was that use
of [18F]PhCOF would require its radiosynthesis and purification
prior to labeling. Instead, we relied on insight gathered from our
mechanistic analysis of the catalytic reaction, which revealed that
a (salen)CoF(HF) species generated in situ from PhCOF serves
as the active fluorinating agent and that fluorination occurs
through a homobimetallic pathway in which one (salen)Co
center activates the epoxide while the second delivers fluoride
(Figure 1B).22 As such, we envisioned that an alternative
synthesis of (salen)CoF by counterion metathesis between
[18F]fluoride and a suitable (salen)Co(III)X (X = anionic
counterion) species could lead to the development of a successful
[18F]fluoride ring opening (Figure 1C).23 In the event, we found
that, by using (R,R)-(salen)CoOTs (1) as a precursor, a
[18F](salen)CoF (2) species suitable for the radiofluorination
of epoxides was generated by elution of [18F]fluoride from an
ion-exchange cartridge containing a quaternary ammonium
cation (QMA). Importantly, the preparation of 2 is directly
analogous to the preparation of [18F]KF and can be carried out
under air without the use of rigorously dried solvents or
glassware.6,7
Table 1. Scope of Radiofluorination with 2
a
Radiochemical yields (RCYs) are the average of n runs and are based
on activity added to each reaction. RCY was determined by radioTLC.
The identity and ee of the product were determined by radio-HPLC.
See SI for details; MTBE: methyl tert-butyl ether.
ring-opening of epoxides is an attractive alternative approach to
[18F]fluorohydrins because it obviates the need for multistep
preparation of differentially protected enantiopure 1,2-diols and
it introduces the radioisotope in the last step. However, ring-
opening reactions of epoxides with [18F]fluoride generally
cannot be achieved due to poor rates and regioselectivity.19,20
We next investigated the scope and generality of the epoxide
radiofluorination with 2 (Table 1). Representative epoxides were
Figure 2. (A) Linked-catalyst employed in asymmetric radiofluorination of complex epoxides. (B) Comparative radiosyntheses of [18F]THK-5105. (C)
Comparative radiosyntheses of [18F]isatin sulfonamides. Radiochemical yields (RCYs) are the average of n runs and are based on activity added to each
reaction. RCY was determined by radioTLC. The identity and ee of the product were determined by radio-HPLC. The dr of 15 was determined used
Mosher ester analysis on [19F]15. See SI for details.
5292
dx.doi.org/10.1021/ja5025645 | J. Am. Chem. Soc. 2014, 136, 5291−5294

Journal of the American Chemical Society
Communication
added to the MTBE solution of 2 at 50 °C for 20 min, and the
radiofluorination reactions were analyzed by radio-TLC for
radiochemical yield and by chiral HPLC for product identity and
ee determination. Using this procedure, a variety of racemic
terminal epoxides could be radiolabeled in 23−68% RCY and
>90% ee. Ether, aliphatic, and styrenyl epoxides were all well
tolerated (3, 4, 5, 7). Additionally an epoxide adjacent to
relatively acidic C−H bonds, which undergo undesired
elimination under standard conditions with KF, provided 6 in
23% RCY and >95% ee (see Supporting Information (SI) for
details). Furthermore, a meso epoxide underwent desymmetri-
zation under the same conditions in 25% RCY and >95% ee,
allowing the generation of 8, bearing a stereogenic C−18F
center.24 Disappointingly, labeling of epoxides possessing Lewis
basic nitrogens or α-branching with 2 provided poor radio-
isotope incorporation (<1% RCY). This presented a significant
limitation due to the prevalence of such functionality among
bioactive molecules.
In order to address this limitation we explored the use of
dimeric cobalt catalyst 9 (Figure 2A), reasoning that previously
observed rate enhancements induced by 9 for the fluorination of
epoxides may enable increased scope.22 The radiosynthesis of
dimeric catalyst 10 was achieved in a manner directly analogous
to 2 (see SI for details). In an effort to evaluate the reactivity of
dimeric catalyst 10, we selected an array of clinically validated
and recently disclosed preclinical PET tracers as case studies.25
To examine the compatibility of 10 with N-containing
substrates, [18F]THK-5105 (12) was selected (Figure 2B).
[18F]THK-5105 (12) is an experimental PET tracer for imaging
of tau pathology, which is an important biomarker for
Alzheimer’s disease.14 The reported preparation of 12 relies on
the commonly employed strategy of differential protection of a
1,2-diol as a tosylate and THP-ether (Figure 2B). Radiosynthesis
is achieved by nucleophilic displacement of the tosylate 11,
followed by acidic cleavage of THP-protected alcohol to provide
racemic 12. In contrast, we found that dimeric cobalt catalyst 10
labeled racemic epoxide 13 in 25% RCY and 85% ee to directly
deliver enantioenriched [18F]THK-5105 (Figure 2B, (S)-12).
Although the radiochemical yield is diminished compared to that
in the reported preparation of [18F]THK-5105, this result
demonstrates the utility of the method with respect to tracer
candidates containing Lewis basic functionality.
Figure 3. (A) Comparative radiosyntheses of [18F]FMISO. (B)
Comparative radiosyntheses of [18F]FETNIM. (C) Direct radiosyn-
thesis of [18F]threonine precursor. Radiochemical yields (RCYs) are the
average of n runs and are based on activity added to each reaction. RCY
was determined by radioTLC. The identity and ee of the product were
determined by radio-HPLC. See SI for details.
[18F]Isatin sulfonamide 15 was selected in order to probe the
regioselectivity of [18F]fluoride delivery with catalyst 10.
[18F]Isatin sulfonamide 15 is an experimental tracer for imaging
of caspases-3 and -7, which are implicated in the process of
apoptosis.16 It was reported that the stereochemistry of the
fluorohydrin is integral to the selectivity of caspase inhibition.
The reported strategy to prepare 15 utilizes a two-step protocol
employing ring opening of diastereomerically pure cyclic
sulfonate 14 followed by deprotection to gain access to
[18F]fluorohydrin 15 (Figure 2C). Unfortunately, under the
described labeling conditions, cyclic sulfonate 14 undergoes
unselective ring opening and results in the formation of a
separable mixture of both possible regioisomers, (R)-15 and 16.
In contrast, we observed that dimeric catalyst (S,S,S,S)-10 was
capable of labeling a 1:1 mixture of diastereomers (17),
producing (R)-15 as a single regioisomer in 62% RCY with
high levels of diastereocontrol (>20:1), demonstrating the
unique selectivity of 10 relative to traditional [18F]fluoride
sources.
°C), we sought to demonstrate the utility of the new process for
the preparation of nitroimidazole based tracer [18F]FMISO (20),
which was developed to image hypoxia within tumors (Figure
3A).18 Reported attempts at direct radiochemical synthesis of
[18F]FMISO (20) from epoxide precursor 18 with [18F]KF
require 3 h at 100 °C and lead primarily to SNAr chemistry
between the nitroimidazole and the pendant alcohol formed as a
result of ring opening with fluoride (Figure 3A, 19).26 In contrast,
we found that 10 facilitates the direct radiochemical synthesis of
enantioenriched [18F]FMISO (20) in 67% RCY and 90% ee at
50 °C. Given these results, we anticipated that the related tracer
[18F]FETNIM (23), which bears an additional hydroxyl group,
might be amenable to labeling without the need for protection of
epoxy alcohol 22 (Figure 3B).17 Gratifyingly, formation of
[18F]FETNIM (23) was accomplished in 74% RCY directly from
racemic epoxy alcohol 22 (2:1 syn/anti mixture of diaster-
eomers), and the desired syn diastereomer was generated in
>95% ee. These examples demonstrate that the low temperature
and attenuated basicity of this new radiofluorination protocol
enable efficient product formation in the presence of reactive
functionality.
Given the mildness of these radiofluorinations (50 °C) in
comparison to the fluorohydrin syntheses with [18F]KF (>100
5293
dx.doi.org/10.1021/ja5025645 | J. Am. Chem. Soc. 2014, 136, 5291−5294

Journal of the American Chemical Society
Communication
Under the conditions utilized for labeling with [18F]KF,
substrates prone to base induced epimerization frequently
undergo loss of stereochemical information.15 Given the reduced
basicity of 10, we expected that the integrity of epimerizable
stereocenters within substrates could be maintained during
labeling. We selected 4-fluorothreonine, a rare example of a
fluorine-containing natural product, as a substrate bearing a
potentially epimerizable stereocenter. Prepared as a mixture of
enantiomerically pure diastereomers (4:1 syn:anti), epoxide 24
was subjected to 10, affording the desired product in 60% RCY
(Figure 3C, 25). It was also observed that the matched catalyst
preferentially reacted with the desired syn-diastereomer produc-
ing 25 as a single diastereomer (>99:1).
Having shown the utility of this method for the manual
preparation of known tracers, we next sought to demonstrate the
feasibility of a remote, semiautomated radiosynthesis of
[18F]FMISO. Utilizing a remote-controlled microwave cavity
integrated into an automated liquid handler, 12.3 mCi
[18F]FMISO was isolated in 10.6% nondecay corrected RCY
(from activity delivered, 37 min total synthesis time) following
semipreparative-HPLC. The isolated tracer possessed a Co-
content of 5 ppb (ICP-MS) and specific activity of 3.7 Ci/umol
(EOB), indicating that this method is capable of providing a PET
tracer useful for in vivo studies.16
In conclusion, we have demonstrated that commercially
available catalyst 2 and dimeric catalyst 10 are capable of
radiolabeling a diverse array of complex epoxides with high levels
of stereocontrol and functional group compatibility under mild
conditions. The protocol makes use of an air-stable catalyst and is
operationally simple to carry out. We anticipate that it will
facilitate the synthesis of novel PET tracers and also allow
investigators to better understand the relationship between
stereochemistry and radiotracer imaging properties. Further-
more, the utility of 10 and other transition metal fluorides for
mild and selective radiofluorination is a topic of ongoing research
in our laboratory.
Fellow, Eli Lilly Grantee, an Amgen Young Investigator, and a
Roche Early Excellence in Chemistry Awardee.
REFERENCES
■
(1) Ametamey, S. M.; Honer, M.; Schubiger, P. A. Chem. Rev. 2008,
108, 1501−1516.
(2) Matthews, P. M.; Rabiner, E. A.; Passchier, J.; Gunn, R. N. Br. J.
Clin. Pharmacol. 2012, 73, 175−186.
(3) Phelps, M. E. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 9226−9233.
(4) Zhu, L.; Ploessl, K.; Kung, H. F. Science 2013, 342, 429−430.
(5) Furuya, T.; Kamlet, A. S.; Ritter, T. Nature 2011, 473, 470−477.
(6) Cai, L.; Lu, S.; Pike, V. W. Eur. J. Org. Chem. 2008, 2853−2873.
(7) Miller, P. W.; Long, N. J.; Vilar, R.; Gee, A. D. Angew. Chem., Int. Ed.
2008, 47, 8998−9033.
(8) Lee, E.; Kamlet, A. S.; Powers, D. C.; Neumann, C. N.; Boursalian,
G. B.; Furuya, T.; Choi, D. C.; Hooker, J. M.; Ritter, T. Science 2011, 334,
639−642.
(9) Lee, E.; Hooker, J. M.; Ritter, T. J. Am. Chem. Soc. 2012, 134,
17456−17458.
(10) Gao, Z.; Lim, Y. H.; Tredwell, M.; Li, L.; Verhoog, S.; Hopkinson,
M.; Kaluza, W.; Collier, T. L.; Passchier, J.; Huiban, M.; Gouverneur, V.
Angew. Chem., Int. Ed. 2012, 51, 6733−6737.
(11) Topczewski, J. J.; Tewson, T. J.; Nguyen, H. M. J. Am. Chem. Soc.
2011, 133, 19318−19321.
(12) Benedetto, E.; Tredwell, M.; Hollingworth, C.; Khotavivattana,
T.; Brown, J. M.; Gouverneur, V. Chem. Sci. 2013, 4, 89−96.
(13) Liang, T.; Neumann, C. N.; Ritter, T. Angew. Chem., Int. Ed. 2013,
52, 8214−8264.
(14) Okamura, N.; Furumoto, S.; Harada, R.; Tago, T.; Yoshikawa, T.;
Fodero-Tavoletti, M.; Mulligan, R. S.; Villemagne, V. L.; Akatsu, H.;
Yamamoto, T.; Arai, H.; Iwata, R.; Yanai, K.; Kudo, Y. J. Nucl. Med. 2013,
54, 1420−1427.
(15) Qu, W.; Zha, Z.; Ploessl, K.; Lieberman, B. P.; Zhu, L.; Wise, D. R.;
Thompson, C. B.; Kung, H. F. J. Am. Chem. Soc. 2011, 133, 1122−1133.
(16) Limpachayaporn, P.; Wagner, S.; Kopta, K.; Hermann, S.;
Schafers, M.; Haufe, G. J. Med. Chem. 2013, 56, 4509−4520.
(17) Gronroos, T.; Eskola, O.; Lehtio, K.; Minn, H.; Marjamaki, P.;
̈
̈
̈
Bergman, J.; Haaparanta, M.; Forsback, S.; Solin, O. J. Nucl. Med. 2001,
42, 1397−1404.
(18) Lee, S. T.; Scott, A. M. Semin. Nucl. Med. 2007, 37, 451−461.
(19) Podichetty, A. K.; Wagner, S.; Schroer, S.; Faust, A.; Schafers, M.;
Schober, O.; Kopka, K.; Haufe, G. J. Med. Chem. 2009, 52, 3484−3495.
(20) Schirrmacher, R.; Lucas, P.; Schirrmacher, E.; Wangler, B.;
Wangler, C. Tetrahedron Lett. 2011, 52, 1973−1976.
(21) Kalow, J. A.; Doyle, A. G. J. Am. Chem. Soc. 2010, 132, 3268−
̈
ASSOCIATED CONTENT
* Supporting Information
■
S
̈
Experimental procedures, additional reaction optimization and
spectroscopic data for all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
̈
3269.
(22) Kalow, J. A.; Doyle, A. G. J. Am. Chem. Soc. 2011, 133, 16001−
16012.
AUTHOR INFORMATION
Corresponding Author
agdoyle@princeton.edu
■
(23) Under radiolabeling conditions with low concentrations of [18F]
fluoride, it is unlikely that a [18F]Co(III)F(HF) species is the active
nucleophile as was proposed in our cold catalytic system. We therefore
describe this species as a [18F]Co(III)F; however, the exact structure is
unknown at this time. Furthermore, it is unclear if the OTs counterions
undergo additional exchange (e.g., with MeOH or H2O) during the
preparation of 10.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(24) To date, attempts at employing nonsymmetrical 1,2-disubstituted
epoxides have met with failure with both the mono- and dimeric catalyst
systems. See SI for further details.
(25) Our radiochemical yields are decay-corrected and based on
activity added to each substrate. In contrast, the published radiochemical
yields for 12, 15, 20, and 23 are based on initial activity delivered from a
cyclotron and total activity isolated following purification and dose
formulation.
Dr. Julia A. Kalow is acknowledged for helpful discussions. Drs.
Wenping Li and Eric Hostetler (Merck & Co., Inc., West Point
PA) are acknowledged for providing the facilities utilized for the
semiautomated synthesis of [18F]FMISO and assistance with
specific activity determinations. We thank Princeton University,
the National Science Foundation (CAREER-1148750), the
National Institute of Health (CA164490 and DK081342), and
the PA Health department for financial support. T.J.A.G. thanks
Bristol-Myers Squibb and Eli Lilly for graduate fellowships. This
material is based upon work supported by the National Science
Foundation Graduate Research Fellowship under Grant No.
DGE-1148900 (R.F.L.). A.G.D. is an Alfred P. Sloan Foundation
(26) Bejot, R.; Kersemans, V.; Kelly, C.; Carroll, L.; King, R. C.;
Gouverneur, V. Nucl. Med. Biol. 2010, 37, 565−575.
5294
dx.doi.org/10.1021/ja5025645 | J. Am. Chem. Soc. 2014, 136, 5291−5294