Synthesis and application of isocyanates radiolabeled with carbon-11

Article abstract of DOI:10.1002/chem.201002345

Carbon-11 labeled isocyanates are efficiently prepared by dehydration of [¹¹C]carbamate salts, which in turn are easily formed from cyclotron-produced [¹¹C]CO₂ and amines in the presence of a CO₂ fixation agent. The [¹¹C]isocyanates are useful radiosynthons for the synthesis of a variety of [carbonyl-¹¹C]- labeled asymmetrical ureas and carbamate esters. The method is well suited to incorporate any isotope of carbon, and is especially useful for positron emission tomography (PET) radiotracers for in vivo imaging. This is demonstrated by using the method to make [carbonyl-¹¹C]-6-hydroxy-[1,1′- biphenyl]-3-yl cyclohexylcarbamate which is a novel radiotracer for PET imaging of fatty acid amide hydrolase.

Full text of DOI:10.1002/chem.201002345

FULL PAPER
DOI: 10.1002/chem.201002345
Synthesis and Application of Isocyanates Radiolabeled with Carbon-11
Alan A. Wilson,* Armando Garcia, Sylvain Houle, Oleg Sadovski, and Neil Vasdev[a]
Abstract: Carbon-11 labeled isocya-
nates are efficiently prepared by dehy-
dration of [11C]carbamate salts, which
in turn are easily formed from cyclo-
tron-produced [11C]CO2 and amines in
the presence of a CO2 fixation agent.
The [11C]isocyanates are useful radio-
synthons for the synthesis of a variety
of [carbonyl-11C]-labeled asymmetrical
ureas and carbamate esters. The
method is well suited to incorporate
any isotope of carbon, and is especially
useful for positron emission tomogra-
phy (PET) radiotracers for in vivo
imaging. This is demonstrated by using
the method to make [carbonyl-11C]-6-
hydroxy-[1,1’-biphenyl]-3-yl cyclohexyl-
carbamate which is a novel radiotracer
for PET imaging of fatty acid amide
hydrolase.
Keywords: carbamates · carbon di-
oxide fixation · fatty acids · isotope
labeling · ureas
Introduction
These O-phenylcarbamates terminate endocannabinoid sig-
naling by being potent and specific inhibitors of the recently
characterized enzyme, fatty acid amide hydrolase
(FAAH).[5,6] As routes to this class of compound were essen-
tially unknown and unlabeled O-phenylcarbamates are con-
veniently prepared by base-catalyzed addition of phenols to
isocyanates,[7] we initiated a study of the production of
[11C]isocyanates from cyclotron-produced [11C]CO2, as
[11C]CO2 is the most common and easily produced raw form
of 11C.
Positron emission tomography (PET) is a powerful medical
imaging modality that depends for its successful implemen-
tation on the design and development of positron-emitting
radiotracers for specific biological targets.[1,2] The most
common radionuclides employed in PET are the short lived
carbon-11 (t1/2 20.4 min) and fluorine-18 (t1/2 109.7 min), both
commonly produced by medical cyclotrons. The utilization
of carbon-11 in synthesizing useful PET radiotracers is a del-
icate compromise between the restraints of working with
short-lived material in high radiation fields and the rich di-
versity of organic chemistry reactions. Thus, there is a con-
stant need for novel and improved methods of synthesizing
[11C]labeled radiopharmaceuticals.[3,4] As part of our PET
imaging research program into psychiatric, neurological and
addiction disorders, we required the radiosynthesis of O-
phenyl carbamates labeled with carbon-11 specifically in the
carbonyl position for example, compound 16 (see below).
There are a few reports of [11C]isocyanate production and
utilization in the literature. However, general and practical
methods are sparse. Carbon-11 labeled phosgene[8] has been
used to prepare [11C]isocyanates by reaction with amines in
the presence of base.[9–12] In particular Brown et al.[13] de-
vised a clever way to overcome the formation of symmetri-
cal [11C]ureas, a common but undesirable side-reaction, by
utilizing N-sulfinylamines as precursors. While reactive
enough to add to phosgene, N-sulfinylamines are poor nu-
cleophiles, thus suppressing further addition of amine to the
generated [11C]isocyanate. By this method, a variety of
simple [11C]isocyanates were efficiently produced and
trapped as [11C]ureas by reaction with a model secondary
amine. However, the availability of [11C]phosgene is limited
to only a few radiochemistry laboratories worldwide, as its
production requires extensive upkeep, technical expertise,
and replacement of key components between production
runs. Cyclic [carbonyl-11C]ureas and carbamates have been
synthesized in good yields under high-pressure conditions
from [11C]CO using selenium catalysts and are postulated to
[a] Dr. A. A. Wilson, A. Garcia, Dr. S. Houle, Dr. O. Sadovski,
Dr. N. Vasdev
PET Centre, Centre for Addiction and Mental Health
and University of Toronto, Toronto, Ontario (Canada)
E-mail: alan.wilson@camhpet.ca
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002345: Full experimental
details on the radiosynthesis of [11C]-16; the synthesis and characteri-
zation of standards including 1H and 13C NMR spectroscopy, mps, and
HRMS; X-ray structures for 15 and 16; representative HPLC radio-
chromatograms.
proceed via [11C]isocyanates[14]
.
Chem. Eur. J. 2011, 17, 259 – 264
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
259

reaction with alcohols or phe-
nols). All steps in the sequence
are carried out in one pot at
ambient pressures and tempera-
ture, employing starting amines
of diverse structure (see above,
structures 1–16; * denotes ra-
diolabeled atom). The utility of
the method is demonstrated by
the practical radiosynthesis of
[carbonyl-11C]N-cyclohexyl-5-
hydroxy[1,1’-biphenyl]-3-ylcar-
bamic acid ester, ([11C]-16).
Compound 16 is a potent inhib-
itor of the enzyme fatty acid
amide hydrolase (FAAH),[20–22]
which is being studied as a ra-
ACHUTNGRENNUdG ioACHTUNTGNERtNUGN racer for PET imaging of
the endocannabinoid system.
Results and Discussion
All experiments were carried
out by bubbling cyclotron-pro-
Direct trapping and utilization of [11C]CO2 is an attractive
approach in [11C]-radiochemistry as [11C]CO2 can be pre-
pared efficiently in large quantities directly in the target of a
biomedical cyclotron.[15] Schribel et al. demonstrated that
simple amines could, at low temperatures, react with
[11C]CO2, generating carbamate salts, which could then be
dehydrated to [11C]isocyanates in moderate yields.[16] How-
ever, [11C]isocyanate always reacted with the excess starting
amine, which is required for efficient [11C]CO2 trapping, to
give symmetrical [11C]ureas. It has also been demonstrated
that [11C]phenylisocyanate could be produced by trapping
[11C]CO2 in a solution of phenyltriphenylphosphinimine at
low (À608C) temperatures with subsequent heating.[17] No
additional examples were given and the method may be lim-
ited to aromatic amines and constrained by the lack of struc-
turally diverse triphenylphosphinimines. Recently Hooker
et al. and our group reported that [11C]CO2 can be efficient-
ly trapped at ambient temperature and pressure in small
duced no-carrier-added [11C]CO2 (ca. 5–10 nmol) in a stream
of N2 (10 mLminÀ1) into a 1 mL conical vial containing test
amine and the CO2 fixation agent, BEMP (5 mL, 17.3 mmol),
in acetonitrile (100 mL). Solutions of phosphoryl trichloride
(POCl3) in acetonitrile were then added, followed by
quenching reagents to trap the formed [11C]isocyanate as a
[carbonyl-11C]carbamate ester or a [carbonyl-11C]urea. This
scale of operation is suitable for the practical production of
PET radiotracers as well as for use with other isotopologues
such as 13/14CO2. Radiochemical conversion yields were de-
termined by radio-HPLC analysis (corrected for radioactive
decay) of an aliquot of the quenched reaction mixture and
expressed as: (100 ꢁ area of product)/(area of product +
area of all other peaks). Since trapping of [11C]CO2 in the
acetonitrile solutions was always >95%, the radiochemical
yields reported are effectively based on starting [11C]CO2.
Unless stated otherwise, all reaction steps were carried out
for 2 min at ambient temperature in order to minimize time
and manipulations, which is especially advantageous when
performing radiochemistry with carbon-11. Initial experi-
ments were carried out using benzylamine as a test amine
(Table 1).
volumes of solvent using 1,8-diazabicyclo
(DBU) or 2-tert-butylimino-2-diethylamino-1,3-dimethylper-
hydro-1,3,2-diazaphosphorine (BEMP) as fixation
ACHTUNGTREN[NUNG 5.4.0]undec-7-ene
agents.[18,19] In the presence of even small amounts of
amines, [11C]carbamate salts of the amines are produced.
Subsequent reaction of the carbamate salts with alkylating
agents such as dimethylsulfate or benzyl chloride gave high
radiochemical yields of [carbonyl-11C]methyl and [carbon-
yl-11C]benzylcarbamate esters in one-pot, rapid procedures.
We show here that, by careful control of the stoichiometry
of the reagents, said [11C]carbamate salts can be efficiently
dehydrated to [11C]isocyanates which can in turn be trans-
formed into unsymmetrical [carbonyl-11C]ureas (by reaction
with different amines) or [carbonyl-11C]carbamate esters (by
Using a large excess of POCl3, successful demonstration
of the formation of [11C]benzylisocyanate was achieved by
the production of the carbamate methyl ester [11C]-1, upon
quenching with excess methanol (Table 1, entry 1) or the di-
methyl urea [11C]-2 (Table 1, entry 2) upon quenching with
excess dimethylamine. However, such a large excess of
POCl3 precludes trapping the formed [11C]isocyanate with
more stoichiometric (based on starting amine) amounts of
nucleophiles, as they are consumed by the POCl3. However,
reduction of the amount of POCl3 encouraged the formation
260
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 259 – 264

Radiolabeled Isocyanates
FULL PAPER
Table 1. Parameter optimization.
Such a situation is more conducive to the rapid purification
required in 11C-radiochemistry, and also important if the nu-
cleophile is expensive or in short supply. Applying the same
reaction conditions as in Table 2, but quenching with either
phenol or 4-(2-methoxyphenyl)-piperazine (100 mL of
10 mgmLÀ1 in acetonitrile) produced the desired products
[11C]-11, [11C]-12, and [11C]-13 in radiochemical yields of 81,
67, and 70%, respectively. Since the reactions are conducted
in the presence of the strong base, BEMP, phenol is presum-
ably deprotonated to yield phenoxide as the nucleophilic
species responsible for attack on the [11C]isocyanate.
Entry[a] Benzylamine POCl3 MeXH Radiochemical Radiochemical
[mmol]
[mmol]
yield[b] of 1 or yield[b] of 3
2
1
2
3
4.7
4.7
4.7
54
54
21
MeOH 80
Me2NH 83
MeOH 47
4
<2
27
87
83
91
91
91
87
5
4
5
6
7
8
9
10
11
12
4.7
4.7
9.3
4.7
3.2
2.4
0.93
0.93
0.47
5.4
5.4
MeOH
Me2NH
Me2NH
Me2NH
Me2NH
Me2NH
Me2NH 86
MeOH 84
Me2NH 76
4
5
1
2
1
8
Application of the technique to the radiosynthesis of PET
radiotracers: Fatty acid amide hydrolase is an enzyme re-
sponsible for the metabolism of endocannabinoids such as
N-arachidonyl ethanolamine (anandamide, AEA).[5,6] Inhibi-
tors of FAAH are being actively pursued as therapeutics for
nociceptive and inflammatory pain, anxiety, eating disorders,
and other neurological and psychiatric disorders.[18,21,24] In
vivo imaging of FAAH is important to understand its role in
both central nervous system and peripheral processes. Previ-
ous attempts to prepare radiotracers for in vivo imaging of
FAAH have focused on labeled analogues of AEA, but
have proven unsuccessful upon ex vivo testing due to rapid
metabolism and poor brain penetration.[25,26] Phenyl carba-
mates, including the well characterized URB 597, are an im-
portant recently developed class of FAAH inhibitors.[22,27]
N-6-Hydroxy-[1,1’-biphenyl]-3-yl cyclohexylcarbamate (16,
1.07
1.07
1.07
1.07
1.07
1.07
1.07
7
1
[a] Conditions for all reactions: [11C]CO2 was trapped in a solution of
benzylamine in acetonitrile (100 mL) containing BEMP (17.3 mmol). After
2 min POCl3 in 10 mL acetonitrile was added followed 2 min later by
methanol (500 mL) or 40% aqueous dimethylamine (500 mL). [b] decay
corrected% based on starting [11C]CO2 (see text).
of the symmetrical urea, [11C]-3, to the point that it was the
major radiolabeled product (Table 1, entries 4–8) at the
lowest POCl3 amounts used. To suppress this unwanted
side-reaction, we also investigated the effect of reducing the
concentration of benzylamine. Gratifyingly, this seemed to
have little effect on the production of the intermediate
[11C]isocyanate but enabled good radiochemical yields of
the desired [11C]unsymmetrical urea or carbamate (Table 1,
entries 10–12).
Table 2. Scope of the method.[a]
The scope of the reaction was then explored by using a
variety of amines of varied structure (Table 2). Simple ali-
phatic primary amines incorporated [11C]CO2 efficiently and
gave the [carbonyl-11C]-N,N-dimethylureas in good yields
(Table 2, entries 1–5) upon quenching with dimethylamine.
However, the less nucleophilic amine, aniline, gave very
poor yields of the desired [11C]-8 (Table 2, entry 6). It is of
some interest that the secondary amine, 4-(2-methoxyphe-
nyl)piperazine, also resulted in reasonable amounts of [11C]-
9 (Table 2, entry 7). Since secondary amines are incapable of
forming an isocyanate, the reaction presumably proceeds via
the corresponding carbamoyl chloride.[23] The sterically hin-
dered amine, N-isopropylbenzylamine yielded little [11C]-10
under the chosen conditions; however, increasing its concen-
tration resulted in moderate yields of [11C]-10 (Table 2,
entry 8). It would appear that the steric hindrance also dis-
courages competing formation of symmetrical [11C]urea.
While the results depicted in Tables 1 and 2 demonstrate
that the intermediate [11C]isocyanate can be efficiently cap-
tured by quenching the reaction mixtures with a large excess
of trapping nucleophile (methanol or dimethylamine), we
also explored the possibility that the [11C]isocyanate could
be trapped by more stoichiometric amounts of nucleophile.
Entry
1
Test amine
Radiochemical yield[b]
83
2
3
79
86
4
5
6
90
68
6
7
8
67
2.9,[c] 47[d]
[a] Unless stated otherwise, conditions for all reactions: [11C]CO2 was
trapped in a solution of test amine (0.93 mmol) in acetonitrile (100 mL)
containing BEMP (17.3 mmol). After 2 min POCl3 (1.07 mmol) in 10 mL
acetonitrile was added followed 2 min later by 40% aqueous dimethyl-
AHCTUNGTRENNUNG
amine (500 mL). [b] decay corrected% based on starting [11C]CO2.
[c] 1.9 mmol of test amine used. [d] 9 mmol of test amine used.
Chem. Eur. J. 2011, 17, 259 – 264
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www.chemeurj.org
261

A. A. Wilson et al.
URB694) belongs to a recently reported second generation
of phenyl carbamate FAAH inhibitors with improved phar-
macological characteristics, including higher brain penetra-
tion.[20,28] To demonstrate the utility of our method we syn-
thesized [11C]-16 in quantities and qualities suitable for
animal imaging studies (Scheme 1). The position of the radio-
Supporting Information) and the identities of the two re-
gioisomers to be established.[31]
Initial evaluation of [11C]-16 in rat biodistribution studies
are promising with good brain penetration and appropriate
distribution reflecting the known distribution of FAAH
enzyme.[32] These results warrant further refining of its ra-
ACHUTNGRENNUdG ioACHTUNTGRNEsNUGN ynthesis perhaps using a
suitable protecting group at
the 1-hydroxyl position of
PDQ to eliminate the unde-
sired side product. Nonethe-
less, clinically useful quantities
of [11C]-16 were readily
achieved.
Comparison of carbon-11 and
carbon-12
major difference
chemistry:
The
Scheme 1. Radiosynthesis of [11C]-16).
between
“cold” or carbon-12 chemistry
and “hot” or carbon-11
label at the carbonyl carbon is crucial for tagging FAAH
with this class of inhibitor as this is the point of attachment
to the enzyme with loss of the O-biaryl group.[29] Full experi-
mental details of this three step, one-pot reaction, including
radiochromatograms, are given in the Supporting Informa-
tion.
chemistry in these types of reactions is the stoichiometry.
Under “cold” conditions, reactions are carried out under
CO2 atmospheres, thus in principal all starting amine can be
converted to carbamate salt as demonstrated by McGhee
et al.[23] However, when CO2 is the limiting reagent (typi-
cally less than 10À7 mol employed), as it must be in order to
obtain high specific activity 11C-radiopharmaceuticals, essen-
tially all of the starting amine is still present in the reaction
mixture after CO2 trapping. As isocyanates react rapidly
with amines to generate ureas, conditions must be found
which consumes the starting amine, otherwise symmetrical
[carbonyl-11C]ureas derived from the starting amine will be
the predominant radiolabeled product (e.g., Table 1, en-
tries 4–9). While the electrophilic dehydrating agent, POCl3,
can remove unwanted excess starting amine, either as qua-
ternary salts or phosphoamides, the excess POCl3 itself will
then need to be consumed by even larger amounts of the
second nucleophile added to react with the [11C]isocyanate
(Table 1, entries 1–3). This latter case is unwieldy for purifi-
cation and isolation of the desired [carbonyl-11C]product.
Fortunately, a practical solution is effected by using very
dilute solutions of both starting amine and POCl3 (Table 1,
entries 10–12).
By analogy to the reaction of electrophilic dehydrating re-
agents with carboxylic acids, it would be expected that
POCl3 reacts with the [11C]carbamate anion to give a mixed
anhydride (Scheme 2). As dichlorophosphate is an excellent
leaving group, it is possible that subsequent reaction with
amines or alcohols or phenols proceeds by direct addition to
the mixed anhydride with elimination of dichlorophosphate.
However, in the case of benzylamine at least, [11C]benzyl
isocyanate was detected as the major radiolabeled product
by HPLC analysis of the reaction mixture before addition of
the second nucleophile (Supporting Information, Figure S4).
Thus, elimination of dichlorophosphate to generate
[11C]isocyanate is favored.
Briefly, cyclotron-produced [11C]CO2 was trapped in a so-
lution of cyclohexylamine (0.1 mg) and BEMP (5 mL) in ace-
tonitrile (100 mL in a 1 mL V-vial at ambient temperature.
Sequential addition of solutions of POCl3 and 2-phenyl-1,4-
dihydroquinone (PDQ) followed by HPLC purification and
formulation gave sterile and pyrogen-free solutions of [car-
bonyl-11C]-16 suitable for PET imaging studies. From
850 mCi (31 GBq) of [11C]CO2, (65Æ10) mCi ((0.24Æ
0.04) GBq) of >99% radiochemically pure [11C]-16 was ob-
tained 27 min after the end of radionuclide production while
specific activities were (2500Æ500) mCimmolÀ1 ((92.5Æ
18.5) GBqmmolÀ1) at end-of-synthesis (n=10, Æstd.dev.).
While the radiochemical yield of [11C]-16 is more than ade-
quate for PET studies, it is lower than predicted from the
model reactions because the major product formed, by
about 2:1, is actually the regioisomer [carbonyl-11C]-N-5-hy-
droxy-[1,1’-biphenyl]-2-yl cyclohexylcarbamate ([11C]-15;
Scheme 1). This is surprising as it would be expected that
attack of the intermediate isocyanate by the hydroxyl at the
1-position of PDQ would be unfavorable due to the ortho-
phenyl group. However, in our hands and contrary to re-
ported results,[30] the “cold” (carbon-12) reaction of PDQ
with cyclohexylisocyanate also displayed preferentially
attack by the hydroxyl group at the 1 position of PDQ fol-
lowed quickly by attack by a second molecule of PDQ to
yield the bis addition product 14 (see Supporting Informa-
tion). While both 16 and its regioisomer 15 were eventually
isolated by partial hydrolysis of 14, we were hard pressed to
distinguish the two by 2D-NMR techniques. Fortunately
single-crystals of both URB694 (16) and 15 were grown
which enabled the X-ray structures to be determined (see
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Chem. Eur. J. 2011, 17, 259 – 264

Radiolabeled Isocyanates
FULL PAPER
a solution of trapping nucleophile (300 mL of MeOH, 40% aq. dimethyl-
AHCTUNGERTGaNNUN mine, or as described in Tables 1 and 2). After 2 min, reactions were
quenched by addition of aq. ammonia (0.01n, 0.5 mL), which traps un-
reacted [11C]CO2. Radioactivity in the vial and NaOH trap were mea-
sured and aliquots of the reaction mixture analyzed by HPLC and com-
pared to standards to determine conversion yields to [11C]ureas or
[11C]carbamates. A variety of reverse-phase HPLC conditions were used
depending upon the particular reaction (examples shown in Supporting
Information). All reaction mixtures were analyzed under at least two dif-
ferent HPLC conditions to identify the radiochemical product(s). Radio-
chemical yields were calculated as:
[activity in vial/(activity in vial + activity in NaOH trap)] ꢁ HPLC con-
version yield
Radiosynthesis of [11C]-16 ([11C]-URB694): Full experimental details are
given in the Supporting Information. Briefly, a septum sealed 1 mL coni-
cal vial was charged with a solution of cyclohexylamine (0.1 mg) and
BEMP (5 mL) in anhydrous acetonitrile (100 mL). Carbon-11 labeled CO2
was then bubbled through the solution and, when radioactivity had
peaked, 100 mL of a solution of POCl3 (0.2% v/v) in acetonitrile was
added, followed by addition of a solution of 2-phenyl-1,4-dihydroquinone
(2 mg) in acetonitrile (100 mL). The reaction mixture was diluted with
HPLC eluent and purified by semi-preparative HPLC.
Scheme 2. Proposed mechanism for reaction sequence. * designates 11C.
Conclusion
Judicious choice of reaction conditions enables the efficient
production of [11C]labeled isocyanates from amines and
[11C]CO2. These versatile synthons allow, in turn, the radio-
synthesis of a variety of [carbonyl-11C]labeled carbamate
esters and unsymmetrical [carbonyl-11C]ureas. The attributes
of the method, rapid, one-pot, and efficient incorporation of
[11C]CO2 are in tune with the requirements for radiolabeling
of biologically interesting molecules for PET imaging with
the short-lived isotope, 11C. In addition, the technique is, in
principle, amenable to labeling with other isotopologues of
CO2 such as 13/14CO2. Given that it is now common practice
to use libraries of isocyanates in combinatorial chemistry,[33]
the scope of the technique will expand. The practical utility
of the method has been demonstrated by the production of
a PET radiotracer with potential use for in vivo imaging of
the FAAH enzyme.
Acknowledgements
The authors thank Winston Stableford and Min Wong for [11C]CO2 pro-
ductions, and Dr. Alan Lough for expertise with the X-ray crystallogra-
phy.
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Experimental Section
A Scanditronix MC17 cyclotron was used for radionuclide production.
ACTHNUTRGNEUNG
[11C]CO2, produced by the 14N(p,a)11C nuclear reaction, was concentrated
from the gas target in a stainless steel coil cooled to À1788C. Upon
warming, the [11C]CO2 in a stream of N2 gas was passed through a NOx
trapping column and a drying column of P2O5 prior to use.[34] Purifica-
tions and analyses of radioactive mixtures were performed by high per-
formance liquid chromatography (HPLC) with an in-line UV (254 nm)
detector in series with a NaI crystal radioactivity detector (purifications)
or a Bioscan Flowcount coincidence radioactivity detector (analyses).
Isolated radiochemical yields were determined with a dose-calibrator
(Capintec CRC-712m). Proton and carbon-13 NMR spectra were record-
ed at 258C on a Varian Mercury 400 MHz spectrometer. POCl3 was dis-
tilled under reduced pressure prior to use. Automated radiosyntheses
were controlled by Labview software. Unless stated otherwise, all radio-
activity measurements were normalized for radioactive decay.
[17] E. van Tilburg, A. Windhorst, M. van der Mey, J. Herscheid, J. La-
belled Compd. Radiopharm. 2006, 49, 321.
Procedure for examining reaction parameters for [11C]isocyanate forma-
tion and subsequent reaction with nucleophiles: [11C]CO2 in N2 was bub-
bled (using a 2’’ ꢁ 21G needle at 10 mLminÀ1) into a solution of test
amine (varying concentrations) and carbon dioxide fixation agent
(BEMP, 5 mL) in acetonitrile (100 mL) in a septum-sealed 1 mL conical
vial. The vial was vented directly to a syringe barrel filled with silica-
coated NaOH (Ascarite) to trap any escaping [11C]CO2. Trapping of
[11C]CO2 was >95% in all cases. After 2 min a solution of POCl3 (10 mL,
varying concentrations) in acetonitrile was added, followed 2 min later by
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