One-pot, direct incorporation of [11C]CO2 into carbamates

Full text of DOI:10.1002/anie.200900112

Communications
DOI: 10.1002/anie.200900112
Radiotracer Synthesis
One-Pot, Direct Incorporation of [11C]CO2 into Carbamates**
Jacob M. Hooker,* Achim T. Reibel, Sidney M. Hill, Michael J. Schueller, and Joanna S. Fowler
Positron emission tomography (PET) is a prevailing non-
invasive research tool for the investigation of biochemical
processes and the elucidation of molecular interactions
relevant to human health.[1] The enabling technology that
underlies PET and other radiotracer imaging methods is
radiotracer chemistry. Thus, the continued success of PETand
the expansion of its potential relies on the development of
methods to incorporate positron-emitting isotopes into com-
pounds intended, for example, as biomarkers for human
disease. Arguably, one of the most important isotopes for
PET research is carbon-11 (t1/2 = 20.4 min) as a result of its
ubiquity in pharmacologically active compounds and its
favorable physical properties.[2] However, only a limited
number of reactions that meet the special demands and
constraints of carbon-11 chemistry have been developed.[3]
Even fewer are routinely employed owing to process com-
plexity and/or the requirement for special equipment.
Recently, we have placed a focus on the development of
chemical methods for carbon-11 labeling that can be easily
and immediately implemented,[4] and herein we describe a
new method that addresses the carbamate functional group.[5]
Synthesis with carbon-11 begins with the use of a cyclo-
tron that produces 11CO2 or 11CH4 via a nuclear reaction
(typically, [14N(p,a)11C]).[6] In most cases, this “starting
material” must be converted rapidly into a more useful
reagent, for example, 11CH3I,[7] which is then used to label a
precursor compound. The process of reagent synthesis alone
can consume more than half of the radioactivity (if one
conversion step ꢀ trapping efficiency).[8] In this respect,
chemical reactions in which 11CO2 or 11CH4 is used directly
have a clear advantage in terms of radiochemical yield. If
properly designed, direct incorporation strategies can elimi-
nate or reduce the need for special equipment.
We have now developed a one-pot, operationally simple
method for the direct incorporation of 11CO2 into carbamate-
containing compounds. This functional group is an attractive
target for radiochemical incorporation in light of its versatility
for the modular construction of organic compounds and its
chemical and metabolic stability. Other methods to label the
carbamate carbon atom have utilized [11C]phosgene[9] or
[11C]carbon monoxide,[10] both of which present technical
difficulties and equipment needs that currently make their
routine use somewhat impractical.
The development of our method was guided by previous
studies in which excess, typically high-pressure, carbon
dioxide was used in a variety of chemical-fixation reactions,[11]
including carbamate synthesis.[12] However, at the outset we
anticipated significant differences in reactions with trace
carbon dioxide (11CO2) at atmospheric pressure and recog-
nized process differences that would dictate the general
applicability and use of a new radiochemical method.
Previous studies[13] demonstrated the feasibility of using no-
carrier-added 11CO2 in the conversion of amines into isocya-
nates and ureas. Encouraged by these studies, we examined a
variety of reaction conditions by using substoichiometric
12CO2 in model reactions. Furthermore, we optimized the
reaction in terms of 11CO2-trapping efficiency at room
temperature, capture flow rate, the solvent, and the catalyst/
base through screening.[14]
considers decay during reaction time
ꢀ yield of each
[*] Dr. J. M. Hooker, Dr. M. J. Schueller, Prof. Dr. J. S. Fowler
Medical Department, Brookhaven National Laboratory
Upton, NY 11973-5000 (USA)
From these preliminary experiments, we quickly identi-
fied DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) as both a
superior trapping reagent and a catalyst for the reaction.
Solutions of DBU (100 mm) in MeCN, N,N-dimethylforma-
mide (DMF), and dimethyl sulfoxide (DMSO) trapped more
than 95% of the 11CO2 introduced in a constant flow of
helium (50 mLminÀ1), whereas DMAP (4-(dimethylamino)-
pyridine) or DABCO (1,4-diazabicyclo[2.2.2]octane) in the
same solvents (these were also prepared at 100 mM in MeCN,
DMF, and DMSO) trapped less than 10% of the 11CO2.
The reaction with carbon-11 was optimized with opera-
tional simplicity in mind. Conceptually, the reaction occurs as
detailed in Scheme 1; however, it is most likely that many
equilibria exist in solution and that (bi)carbonate salts of
DBU are important reactive intermediates.[14,15] Solutions of
benzylamine (1), benzyl chloride (2), and DBU were com-
bined in a standard cone-bottomed glass vial with a septum
and a screw cap, and the resulting mixture was used to trap
11CO2 (10–40 mCi) from a constant stream of helium (within
2 min). The inlet and outlet lines were then removed or
closed, and the reaction solution was heated. At the end of a
Fax: (+1)631-344-5815
E-mail: hooker@bnl.gov
A. T. Reibel
Johannes-Gutenberg-Universitꢀt Mainz (Germany)
S. M. Hill
North Carolina State University, Raleigh (USA)
Prof. Dr. J. S. Fowler
Department of Chemistry
State University of New York at Stony Brook (USA)
and
Department of Psychiatry
Mount Sinai School of Medicine, New York (USA)
[**] This research was carried out at Brookhaven National Laboratory
(contract DE-AC02-98CH10886 with the US Department of Energy
and supported by its Office of Biological and Environmental
Research). J.M.H. was supported by the NIH (1F32EB008320-01)
and a Goldhaber Fellowship at BNL. A.T.R was supported by
Deutscher Akademischer Austauschdienst (DAAD) and BNL.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900112.
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Chemie
concentration. In the absence of DBU, a small amount of
labeling was observed; however, the trapping efficiency was
poor. In the absence of benzylamine, [11C]dibenzyl carbonate
(3) was formed in less than 1% yield. This result may indicate
that the soluble [11C]carbonate salt of DBU does not
participate or is alkylated reversibly with the release of
11CO2 upon acidification. The only productive (irreversible)
and high-yielding pathway involved the amine. In fact, the use
of a mixture of benzyl alcohol, benzyl chloride, and DBU
under the optimized reaction conditions provided
[11C]dibenzyl carbonate in a radiochemical yield of less than
3%.
Scheme 1. Conceptual mechanism for the DBU-catalyzed incorporation
of [11C]carbon dioxide.
given reaction time, the reaction was quenched by the
addition of excess acid, and the amount of volatile radio-
activity (i.e. unreacted 11CO2) was determined. A sample was
then analyzed by radio-TLC or radio-HPLC for chemical
identity and radiochemical purity. From these data, we
calculated the radiochemical yield under various reaction
conditions (Table 1).
For the reactions reported in Table 1, we used dry solvents
that had been stored over molecular sieves. However, we
found that dry solvents were not necessary, and even in the
presence of 10 mg of water, the reaction yields were
comparably high. We have not eliminated the possibility
that water is important in the reaction and are working to
perform the reaction under rigorously anhydrous conditions
to determine whether water plays an essential role in the
mechanism. We have found that the use of aqueous NaOH in
DMF under the same conditions is not effective, but
mechanisms involving water with DBU cannot be excluded.
Clearly, the role of DBU in sequestering CO2 either as a
zwitterion or a (bi)carbonate salt is important, but the precise
nature of this interaction and related reactive intermediate(s)
cannot yet be discerned, especially with the use of tracer
(nanomolar) quantities of 11CO2.[14]
Table 1: Preliminary reaction optimization.[a]
Entry
[1]
[mm]
[2]
[mm]
[DBU]
[mm]
t
T
[8C]
Yield
[%][b]
[min]
1
2
3
4
5
6
7
8
9
10
11
12
13
100
1
1
1
1
1
1
1
1
10
100
100
100
100
100
100
100
100
100
100
100
100
100
10
100
100
100
100
100
100
100
100
100
100
100
10
5.0
5.0
7.5
7.5
10
10
10
12
12
10
10
10
10
75
100
75
100
25
75
100
75
100
75
75
75
75
60
69
81
83
17
85
75
83
81
73
51
10
7
To probe the scope of this direct 11CO2-incorporation
method, we examined the reaction of a series of amines and
alkyl halides (Table 2). Secondary and a-disubstituted amines
were converted into the corresponding [11C]carbamates in
good yields (Table 2, entries 1 and 2). Less nucleophilic
amines, such as anilines, participated in the reaction but with
lower levels of conversion under these conditions (Table 2,
entries 3–5). Given the simplicity of the reaction, the direct
use of 11CO2, and the short reaction times, we anticipate that
even with these lower radiochemical yields, this method will
be competitive with alternative methods for carbamate
labeling. We are currently investigating reaction conditions
and cocatalysts for the conversion of alcohols and less
nucleophilic amines in increased radiochemical yields.[17]
We examined the reaction of benzylamine with a series of
alkyl halides (Table 2, entries 6–10). For the desired 11CO2-
incorporation reaction to occur in an acceptable radiochem-
ical yield, the reactivity of the electrophile had to be
modulated so that competitive alkylation of the amine or
DBU and elimination of the alkyl halide were minimal. In
general, alkyl chlorides were more suitable than the corre-
sponding bromides. However, in some cases, alkyl bromides
were more effective. In these cases, alkylation of the amine
was not competitive (Table 2, entry 8). Thus, the reactivity can
be “tuned” by the choice of an appropriate nucleophile/
leaving-group pair. We are now investigating the extent to
which we can develop rules to guide the selection of such pairs
and thus expand the scope and predictability of these
reactions.
100
100
0
[a] Reactions were carried out in DMF (300 mL) in a sealed vessel under a
helium atmosphere. No-carrier-added 11CO2 was trapped from a stream
of helium gas. [b] See the Supporting Information for details related to
the calculation of radiochemical yields.
The desired [11C]carbamate was formed in high radio-
chemical yield in less than 10 min at a slightly elevated
temperature (Table 1, entry 6). The yield was lower when the
reaction was carried out at room temperature, but perhaps
sufficient for the labeling of temperature-sensitive com-
pounds. When comparing this method to other carbon-11
labeling methods, one must bear in mind that the entire
process, including trapping and the desired reaction, can be
accomplished in less than 10 minutes. Therefore, yields for
this direct method with 11CO2 are much higher than yields
(without a correction for radioactive decay) for other, more
time intensive, labeling strategies.[16]
From this series of experiments we determined that the
radiochemical yield was more sensitive to the concentrations
of DBU and benzyl chloride than to the benzylamine
Finally, to test the reaction in the context of a pharmaco-
logically active compound with potential as a PET imaging
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Communications
Table 2: Scope of the DBU-mediated incorporation of 11CO2.[a]
currently optimizing additional
parameters to improve the radio-
chemical yield and reaction scope.
However, owing to its simplicity
and efficiency, this reaction should
be immediately useful for the incor-
poration of carbon-11 and expand
the number and types of molecules
that can be used in PET and other
radiotracer applications. We are
currently working to gain a better
understanding of the mechanism of
the reaction and the role played by
DBU under tracer-scale conditions.
Entry
1
Product
Yield [%][b]
60
Entry
6
Product
Yield [%][b]
77Æ8[d]
33[c]
33
2
3
4
5
48
16
7
8
11[c]
12
69[c]
7
9
6
16[c]
Experimental Section
Separate solutions (each 300 mm) of the
amine, DBU, and the alkyl chloride were
prepared in DMF (which had been
sparged previously with helium gas to
remove any CO2 from the atmosphere).
Aliquots (100 mL) of each solution were
combined in a reaction vessel, which was
then sealed with a septum and screw cap.
The resulting solution (300 mL; 0.1m in
4
10
<1
[a] Reactions were carried out with the amine, the alkyl chloride, and DBU, each at a concentration of
100 mm, in DMF (300 mL). [b] Average radiochemical yield for two or more reactions. [c] Radiochemical
yield for the reaction with the corresponding alkyl bromide. [d] Mean value Æstandard deviation (n=
10).
each reagent) was sparged with helium gas for 2–3 min. [11C]Carbon
dioxide was released in a stream of helium (ca. 50 mLminÀ1) from an
automated trap-and-release system and introduced into the reaction
solution under constant flow at room temperature. The amount of
11CO2 trapped was monitored and recorded, and the capture step was
continued until the radioactivity curve reached a maximum (typically
< 2 min). After removing the inlet and outlet needles, the sealed
vessel was transferred to a heating bath. Most reactions were carried
out for 10 min at 758C. Other conditions are noted in Table 1. To
determine the radiochemical yield, reaction solutions were placed in
contact with a miniature radiation detector; inlet and outlet lines
were introduced by using needles inserted through the septa. The
solution was acidified with 1.0m HCl (100 mL; an excess amount
relative to that of DBU and the amine). The residual 11CO2 trapped in
solution was then removed from the solution by sparging with a
stream of helium gas (ca. 50 mLminÀ1). When the radioactivity in the
solution became constant (ignoring decay), an aliquot of the solution
was analyzed by radio-TLC and/or radio-HPLC. The percentage of
radioactivity coincident with that of a reference compound was
multiplied by the decay-corrected percentage of radioactivity that
remained in the solution following the removal of excess 11CO2.
tracer, we labeled metergoline,[18] an antagonist of the
serotonin (5HT) receptor (Scheme 2). Benzyl carbamate
containing molecules, such as metergoline, are attractive
targets for this methodology, because the labeling precursor
Scheme 2. Synthesis of [11C]metergoline. Bn=benzyl, RCY=radio-
chemical yield.
(an amine) can be produced by deprotection with H2 and Pd/
C. (We anticipate similar strategies for other common
carbamates that are used as protecting groups.) The reaction
setup was extremely simple; a single vessel was required.
11CO2 was trapped directly from the cyclotron target gases in
the reaction solution (300 mL), which contained DBU
(100 mm), the amine precursor (1 mg), and benzyl chloride
(100 mm). Given the simplicity of the method and the short
overall process, including the short reaction time from the end
of bombardment (EOB), the radiochemical yield was excel-
lent (32% calculated to EOB). We have now scaled this
reaction in terms of radioactivity to the use of up to 400 mCi
of 11CO2 and can produce [11C]metergoline at a high specific
activity (up to 5 CimmolÀ1) from less than 1 mg of the
precursor. (The evaluation of [11C]metergoline as a radio-
tracer for the 5HT receptor system in the brain will be
reported elsewhere.)
Received: January 8, 2009
Revised: March 4, 2009
Published online: April 6, 2009
Keywords: carbamates · carbon-11 · isotopic labeling ·
.
positron emission tomography · radiochemistry
[1] R. L. Wahl, Principles and Practice of PETand PET/CT, 2nd ed.,
Lippincott Williams&Wilkins, Philadelphia, PA, 2009.
[2] G. Antoni, T. Kihlberg, B. Lꢁngstrꢂm, “Aspects on the Synthesis
of 11C-Labeled Compounds in Handbook of Radiopharmaceut-
icals: Radiochemistry and Applications (Eds.: M. J. Welch, C. S.
Redvanly), West Sussex, England, 2003, p. 141.
In summary, the direct incorporation of 11CO2 into the
carbamate functional group has been demonstrated. We are
[3] a) P. W. Miller, N. J. Long, R. Vilar, A. D. Gee, Angew. Chem.
2008, 120, 9136; Angew. Chem. Int. Ed. 2008, 47, 8998; b) M.
3484
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Angewandte
Chemie
Allard, E. Fouquet, D. James, M. Szlosek-Pinaud, Curr. Med.
Chem. 2008, 15, 235.
[4] J. M. Hooker, M. Schꢂnberger, H. Schieferstein, J. S. Fowler,
Angew. Chem. 2008, 120, 6078; Angew. Chem. Int. Ed. 2008, 47,
5989.
[5] For a new, practical method for the synthesis of methyl
carbamates from 11CH3I, see: B. W. Schoultz, E. ꢃrstad, J.
Marton, F. Willoch, A. Drzezga, H.-J. Wester, G. Henriksen,
Open Med. Chem. J. 2008, 2, 72.
[6] D. R. Christman, R. D. Finn, K. I. Karlstrom, A. P. Wolf, Int. J.
Appl. Radiat. Isot. 1975, 26, 435.
[7] a) B. Lꢁngstrꢂm, H. Lundqvist, Int. J. Appl. Radiat. Isot. 1976, 27,
357; b) J. M. Link, K. A. Krohn, J. C. Clark, Nucl. Med. Biol.
1997, 24, 93.
[8] a) J. S. Fowler, A. P. Wolf, Acc. Chem. Res. 1997, 30, 181; b) B.
Lꢁngstrꢂm, T. Kihlberg, M. Bergstrom, G. Antoni, M. Bjork-
man, B. H. Forngren, T. Forngren, P. Hartvig, K. Markides, U.
Yngve, M. Ogren, Acta Chem. Scand. 1999, 53, 651.
[9] L. Lemoucheux, J. Rouden, M. Ibazizene, F. Sobrio, M. C. Lasne,
J. Org. Chem. 2003, 68, 7289.
[10] a) T. Kihlberg, F. Karimi, B. Lꢁngstrꢂm, J. Org. Chem. 2002, 67,
3687; b) H. Doi, J. Barletta, M. Suzuki, R. Noyori, Y. Watanabe,
B. Lꢁngstrꢂm, Org. Biomol. Chem. 2004, 2, 3063.
[11] a) M. Costa, G. P. Chiusoli, M. Rizzardi, Chem. Commun. 1996,
1699; b) D. Chaturvedi, A. K. Chaturvedi, N. Mishra, V. Mishra,
Synth. Commun. 2008, 38, 4013.
loni, Chem. Rev. 2003, 103, 3857; c) D. Chaturvedi, S. Ray,
Monatsh. Chem. 2006, 137, 127; d) P. Tascedda, E. Dunach,
Chem. Commun. 2000, 449; e) M. Aresta, E. Quaranta, Tetrahe-
dron 1992, 48, 1515; f) E. R. Pꢅrez, M. O. da Silva, V. C. Costa,
U. P. Rodrigues-Filho, D. W. Franco, Tetrahedron Lett. 2002, 43,
4091.
[13] a) A. Schirbel, M. H. Holschbach, H. H. Coenen, J. Labelled
Compd. Radiopharm. 1999, 42, 537; b) E. W. Van Tilburg, A. D.
Windhorst, M. Van der Mey, J. D. M. Herscheid, J. Labelled
Compd. Radiopharm. 2006, 49, 321.
[14] See the Supporting Information for model reactions with CO2,
11CO2-trapping experiments, and additional mechanistic discus-
sion.
[15] Mechanistic details related to the role of DBU and alkyl amines
in similar reactions have been discussed extensively; see, for
example: a) D. J. Heldebrant, P. G. Jessop, C. A. Thomas, C. A.
Eckert, C. L. Liotta, J. Org. Chem. 2005, 70, 5335; b) W. Mcghee,
D. Riley, K. Christ, Y. Pan, B. Parnas, J. Org. Chem. 1995, 60,
2820; c) E. R. Pꢅrez, R. H. A. Santos, M. T. P. Gambardella,
L. G. M. de Macedo, U. P. Rodrigues-Filho, J. C. Launay, D. W.
Franco, J. Org. Chem. 2004, 69, 8005; d) K. Masuda, Y. Ito, M.
Horiguchi, H. Fujita, Tetrahedron 2005, 61, 213.
[16] For further discussion, see: F. R. Wuest, Trends Org. Chem. 2003,
10, 61.
[17] Cocatalysts have been employed successfully in CO2-incorpo-
ration reactions involving epoxides; see, for example: Y. M.
Shen, W. L. Duan, M. Shi, Adv. Synth. Catal. 2003, 345, 337.
[18] C. Beretta, R. Ferrini, A. H. Glasser, Nature 1965, 207, 421.
[12] a) M. Shi, Y. M. Shen, Helv. Chim. Acta 2001, 84, 3357; b) D. B.
DellꢄAmico, F. Calderazzo, L. Labella, F. Marchetti, G. Pampa-
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