Facile aromatic radiofluorination of [18F]flumazenil from diaryliodonium salts with evaluation of their stability and selectivity

Article abstract of DOI:10.1039/c1ob06277h

Aromatic radiofluorination of the diaryliodonium tosylate precursor with [¹⁸F]fluoride ions has been applied successfully to access [ ¹⁸F]flumazenil in high radiochemical yields of 67.2 ± 2.7% (decay corrected). The stability and reactivity of the diaryliodonium tosylate precursor plays a key role in increasing the production of ¹⁸F- labelled molecules under the fluorine-18 labelling condition. Various conditions were explored for the preparation of [¹⁸F]flumazenil from different diaryliodonium tosylate precursors. Optimum incorporation of [ ¹⁸F]fluoride ions in the 4-methylphenyl-mazenil iodonium tosylate precursor (5f) was achieved at 150°C for 5 min by utilizing 4 mg of the precursor, K_2.2.2/K₂CO₃ complex, and the radical scavenger in N,N-dimethylformamide. This approach was extended to a viable method for use in automated synthesis with a radiochemical yield of 63.5 ± 3.2% (decay corrected, n = 26) within 60.0 ± 1.1 min. [ ¹⁸F]Flumazenil was isolated by preparative HPLC after the reaction was conducted under improved conditions and exhibited sufficient specific activity of 370-450 GBq μmol^-1, with a radiochemical purity of >99%, which will be suitable for human PET studies. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1ob06277h

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PAPER
Facile aromatic radiofluorination of [18F]flumazenil from diaryliodonium salts
with evaluation of their stability and selectivity†
Byung Seok Moon,a,b Hee Seup Kil,c Jun Hyung Park,a Ji Sun Kim,a Jimin Park,d Dae Yoon Chi,c
Byung Chul Lee*a,b and Sang Eun Kima,b
Received 28th July 2011, Accepted 22nd September 2011
DOI: 10.1039/c1ob06277h
Aromatic radiofluorination of the diaryliodonium tosylate precursor with [18F]fluoride ions has been
applied successfully to access [18F]flumazenil in high radiochemical yields of 67.2 2.7% (decay
corrected). The stability and reactivity of the diaryliodonium tosylate precursor plays a key role in
increasing the production of 18F-labelled molecules under the fluorine-18 labelling condition. Various
conditions were explored for the preparation of [18F]flumazenil from different diaryliodonium tosylate
precursors. Optimum incorporation of [18F]fluoride ions in the 4-methylphenyl-mazenil iodonium
tosylate precursor (5f) was achieved at 150 ◦C for 5 min by utilizing 4 mg of the precursor, K2.2.2/K2CO3
complex, and the radical scavenger in N,N-dimethylformamide. This approach was extended to a viable
method for use in automated synthesis with a radiochemical yield of 63.5 3.2% (decay corrected, n =
26) within 60.0 1.1 min. [18F]Flumazenil was isolated by preparative HPLC after the reaction was
conducted under improved conditions and exhibited sufficient specific activity of 370–450 GBq mmol-1,
with a radiochemical purity of >99%, which will be suitable for human PET studies.
activity of the 18F-labelled molecules (<37 GBq mmol-1). This
Introduction
level of specific activity is not enough to account for a receptor
A fluorine-18 ion is one of the most attractive positron emitters
with low density in the brain. Given the above reasons, fluorine-
produced by a cyclotron; it can yield up to multi-Curie levels,
18 incorporation into electron-rich arenes remains a particular
even with low-energy1. It also has a relatively moderate half-
challenge due to the fact that classical aromatic nucleophilic
life (t1/2 = 110 min) and is rapidly incorporated into aliphatic
or aromatic sites of organic molecules through a nucleophilic
substitution reaction, normally with a high radiochemical yield
and specific activity.2 However, the incorporation efficiency of
fluorine-18 is significantly dependent upon the type and position
of the substitution group. For instance, nucleophilic substitution at
the aryl position commonly does not work well without electron-
withdrawing groups such as an aldehyde, nitro, cyanide, ester, or
ketone in the ortho- or para-position of the aryl ring, accompanied
by leaving groups such as nitro, halogen, or trialkylammonium
salt. Conversely, aromatic radiofluorination in electron-rich arenes
has to follow the electrophilic fluorine-18 labelling method, e.g.,
using the [18F]F2 gas. However, electrophilic radiofluorination of
aromatic compounds has significant shortcomings, such as low
radiochemical yields with various by-products and low specific
substitution generally gives low or negligible yields.
Recently, diaryliodonium salts have been shown to be useful for
labelling with a [18F]fluoride ion at the aromatic position in small
model compounds that have proven difficult to label using general
aromatic fluorine-18 incorporation precursors.3 This method is
potentially more powerful for the introduction of fluorine-18
into aromatic organic compounds than former methods, such
as the Balz–Schiemann4 or Wallach reactions,5 which are limited
due to low radiochemical yields. Although diaryliodonium salts
have potential advantages in radiofluorination, their application
to give complicated chemical structures had little success. The
main reason is that the diaryliodonium salt precursors that have
functional substitution groups or more complex structures are
difficult to synthesize and isolate from reaction mixtures. In
addition, the diaryliodonium salt is chemically unstable under the
basic conditions and high temperatures required for the fluorine-
18 incorporation reaction. Nevertheless, diaryliodonium salts have
excellent properties such as the possibility of efficient fluorine-18
incorporation in electron-rich arenes; we therefore investigated the
application of diaryliodonium salt precursors for efficient fluorine-
18 incorporation in [18F]flumazenil ([18F]FMZ, Fig. 1).
aDepartment of Nuclear Medicine, Seoul National University Bundang
Hospital, Seoul National University College of Medicine, Seoul, 110-799,
Korea. E-mail: leebc@snu.ac.kr; Fax: 82-31-787-4072; Tel: 82-31-787-2956
bInstitute of Radiation Medicine, Medical Research Center, Seoul National
University, Seoul, 110-744, Korea
cDepartment of Chemistry, Sogang University, Seoul, 121-742, Korea
dDepartment of Materials Science and Engineering, Seoul National Univer-
sity, Seoul, 151-744, Korea
Radioisotope labelled flumazenil is known to be an important
radiopharmaceutical product for the assessment of the central
benzodiazepine receptor (cBZR) concentration in the brain.6 This
† Electronic supplementary information (ESI) available: See DOI:
10.1039/c1ob06277h
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Fig. 1 Structures of flumazenil and the radioisotope labelled flumazenil.
has been extensively studied for the quantitative evaluation of
cBZR in epilepsy,7 panic disorders,8 the evaluation of cortical dam-
age to the brain after an acute stroke,9,10 and others11–13 by positron
emission tomography (PET). Carbon-11 labelled flumazenil,
[11C]flumazenil (ethyl-8-fluoro-5,6-dihydro-5-[11C]methyl-6-oxo-
4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate, [11C]FMZ,
[11C]1, Fig. 1), shows itself to be more suitable than [18F]FDG–
PET (2-deoxy-2-[18F]fluoro-D-glucose) for providing valuable
information in some fields. Its main limitation, however, is the
physical half-life (t1/2 = 20 min) of carbon-11, which means that
only one or two patients can be treated from each production
cycle in cyclotron-equipped centers. On the other hand, the
fluorine atom in the native structure of flumazenil is an attractive
feature which can introduce fluorine-18 by nucleophilic aromatic
substitution without any structural modifications. The fluorine-
18 labelled form, [18F]flumazenil ([18F]FMZ, [18F]1), has the
advantage of a longer half-life compared to the carbon-11-labelled
form and a PET study on monkeys showed similar uptake patterns
between [18F]FMZ and [11C]FMZ.14 [18F]FMZ also showed more
stable estimates of binding potential values and a less noisy image
than [11C]FMZ in the human brain.15 Many research groups have
tried to synthesize [18F]FMZ via a nucleophilic displacement from
nitro-mazenil in order to overcome the short half-life, a limitation
of [11C]FMZ.14,16,17 Direct nucleophilic fluorination showed a
moderate radiochemical yield of about 15–30% (decay corrected,
d.c.). However, other research groups reported that the overall
decay corrected radiochemical yield after HPLC purification
were very low, about 3–10% in contrast to the reported 15–
30% yields.15,17,18 Mandap et al. reported a more efficient method
for [18F]FMZ synthesis using a microwave-assisted system as an
alternative method, but the effort produced a relatively moderate
yield, about 26 4% (d.c.) and microwave-assisted systems are
not common at PET production sites.16 Added to that, no
attempts on high-scale production of flumazenil in commercial
automatic devices have been addressed until now. In other trials,
[18F]flumazenil derivatives 2¢-[18F]fluoroflumazenil ([18F]FFMZ)19
and 5-(2¢-[18F]fluoroethyl)flumazenil ([18F]FEFMZ)20 were devel-
oped. However, these tracers did not achieve the same widespread
use because their brain uptake and kinetics are different from those
of [11C]FMZ or [18F]FMZ; this is because their structures are not
exactly the same as that of native flumazenil.
tosylate precursors, which introduces electron-rich heteroarenes,
and attempted to optimize the reaction conditions based on
their stability and the reactivity in the fluorine-18 incorporation
environment. Here we describe the synthesis of [18F]FMZ using
the diaryliodonium tosylate precursors, which we believe to be
highly efficient.
Results and discussion
The limitations of the general aromatic [18F]fluorination of
flumazenil from nitro-mazenil allowed us to propose an alter-
native approach to aromatic fluorination. Diaryliodonium salts
were already known as suitable precursors for the labelling
of fluorine-18 into the aromatic ring by nucleophilic fluorina-
tion without additional activating groups (electron-withdrawing
groups), such as the carbonyl or nitro groups. This approach
also provides good yields, including electron-rich and electron-
deficient arenes for which classical aromatic nucleophilic sub-
stitution is generally unfavorable and gives low or negligible
yields. It is known that diaryliodonium salts can be used as
the precursors for fluoroarenes,22 and their values correspond
well with those of the fluorine-18-labelled compounds produced
using iodonium salts reported in our previous work.21 In applying
diaryliodonium salts to [18F]FMZ synthesis, several heterodiaryl-
introducing iodonium tosylate precursors were considered. In
preparing the heterodiarene iodonium tosylate precursors, we syn-
thesized the initial 8-bromoimidazobenzodiazepine (2) required
for this study according to the procedures described in the
literature with little modification,23 as summarized in Scheme
1. Stannylation of ethyl-8-bromo-5,6-dihydro-5-methyl-6-oxo-
4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate (2) with a
tributyltin reagent and tetrakis(triphenylphosphine)palladium(0)
[(Ph3P)4Pd(0)]afforded the corresponding stannylated compound
3. This reaction was carried out according to a previously
reported method with little modification.24 Treatment of ethyl-
5,6-dihydro-5-methyl-6-oxo-8-tributylstannyl-4H-imidazo[1,5-
a][1,4]benzodiazepine-3-carboxylate (3) with a commercially avail-
able Koser’s reagent (for 5a) or various hydroxy(tosyloxy)-
iodoarenes (for 5b–f) afforded (8-benzodiazepine)iodonium to-
sylates 5a–f, respectively. The electron-rich hydroxyl(tosyloxy)-
iodoarenes are known to be unstable and to decompose violently
at room temperature. Thus, we prepared 4a–f according to the
literature22,25 and used them without purification under a nitrogen
atmosphere to prepare the diaryliodonium tosylate precursors
5a–f.
We therefore adapted the aromatic [18F]fluorination method,
using a diaryliodonium salt that was recently studied in our group
and by colleagues, to achieve efficient [18F]FMZ synthesis.21 These
radiotracers showed good radiochemical yields by overcoming
the limitations of general aromatic radiofluorination. In order
to obtain high radiochemical yields and a reproducible synthe-
sis of [18F]FMZ, we have synthesized different diaryliodonium
In aromatic [18F]fluorination with the diaryliodonium salt
precursors, the fluoride ion is expected to attack the more electron-
deficient ring in the diaryliodonium tosylate precursor, so we
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Scheme 1 Sxynthesis of various diaryliodonium tosylate precursors.
prepared various electron-rich diaryliodonium salt precursors
with a “dispensable ring” from 4a–f. The electron density with
respect to the dispensable ring was in the following order:
phenyl- (5a) ª 3-methoxyphenyl- (5e) < 4-methylphenyl- (5f)
< 4-methoxyphenyl- (5d) < 3-thiophenyl- (5c) < 2-thiophenyl-
mazenil (5b) (from observed 13C NMR26). In addition to the
effects of an electron-rich heteroaromatic ring, the effects of base
and solvent also play a key role in aromatic radiofluorination with
the diaryliodonium salt. Thus, various conditions were explored
for the preparation of [18F]FMZ, as summarized in Table 1.
In the fluorine-18 incorporation experiments, [18F]fluoride ion
was dried with acetonitrile in the presence of a base (0.8 equiv.
versus amounts of precursor) such as TBAHCO3, TBAOH, or
K2.2.2./K2CO3. The generated [18F]fluoride ion complex was then
dissolved in different solvents (0.5 mL) in the presence of a
diaryliodonium tosylate precursor (4 mg) and a radical scavenger
(1 mg). This solution was heated at the desired temperature for
15 min. We performed a feasibility study for radiofluorination
of the different diaryliodonium tosylate precursors (5a–f)
and monitored their fluorination tendencies. The fluorine-18
incorporation yield was determined by radio-TLC in 10%
MeOH : CH2Cl2 eluent.
Table 1 and Fig. 2 summarize the results of the [18F]fluorine
incorporation yields with different diaryliodonium tosylate
Table 1 Aromatic [18F]fluorination using various heterodiarene iodonium tosylates (5a–f)a
Fluorine-18 incorporation yield of [18F]1 (%)b
Temperature (◦C)
Entry
Precursor
Base
Solvent
100
125
150
1
5a
5a
5b
5c
5d
5d
5e
5f
5a
5b
5c
5d
5e
5f
5a
5a
5b
5c
5c
5c
5d
5e
5f
nBu4NHCO3
nBu4NHCO3
nBu4NHCO3
nBu4NHCO3
nBu4NHCO3
nBu4NHCO3
nBu4NHCO3
nBu4NHCO3
nBu4NOH
DMF
DMF
DMF
DMF
DMF
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
DMF
DMF
CH3CN
DMSO
DMF
DMF
DMF
6.3 1.2
—
1.2 0.6
1.8 0.9
<1
—
<1
15.4 1.6
8.2 2.7
3.6 1.4
4.5 2.9
<1
20.7 3.1
—
24.6 4.4
2.8 0.4
6.3 1.9
3.4 2.2
2.5 1.2
—
2.3 1.0
26.1 4.1
13.2 3.6
10.1 4.1
7.7 2.7
2.3 0.3
2.7 1.2
17.7 4.4
39.2 9.1
6.4 3.2
2.8 1.4
6.8 2.1
—
2c
3
4.1 2.0
4.0 2.5
1.3 0.2
2.1 0.2
1.2 0.2
23.1 2.9
11.0 3.6
5.7 2.0
7.1 1.4
<1
1.4 0.6
15.6 2.7
32.5 5.9
—
2.3 1.0
15.8 5.6
11.7 4.4
1.8 0.3
<1
1.3 0.5
55.1 6.9
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
nBu4NOH
nBu4NOH
nBu4NOH
nBu4NOH
<1
nBu4NOH
11.7 1.7
29.9 6.2
—
1.7 1.2
8.1 2.1
—
—
<1
1.4 0.3
20.7 4.2
K
K
2.2.2/K2CO3
2.2.2/K2CO3
K2.2.2/K2CO3
K2.2.2/K2CO3
K
2.2.2/K2CO3
K2.2.2/K2CO3
2.2.2/K2CO3
K2.2.2/K2CO3
2.2.2/K2CO3
—
K
1.5 0.3
1.4 0.7
74.1 5.8
K
a The reaction was carried out with 0.8 equiv. of base relative to the precursor (4 mg, 5a–f) in 0.5 mL of solvent (DMF (N¢,N¢-dimethylformamide), DMSO
(dimethyl sulfoxide) or CH3CN (acetonitrile)) and a radical scavenger (TEMPO, 1 mg) at the desired temperature for 15 min. b Radiochemical yield
was determined by radio-TLC representing the percentage of [18F]flumazenil in the reaction mixture (developing solvent: methanol : dichloromethane =
10 : 90, v/v; n = 3 or 4). c No TEMPO used.
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Fig. 2 Comparison of the fluorine-18 incorporation yields of 5a–f under different bases (left-handed striped bar: 5a; horizontal striped bar: 5b;
right-handed striped bar: 5c; vertical striped bar: 5d; open bar: 5e; closed bar: 5f).
precursors. Among the various reaction conditions, promising
results were achieved when a combination of cryptand (Kryptofix
2.2.2; K2.2.2)/base (K2CO3) and 4-methylphenyl-mazenil iodonium
tosylate (5f) were used in the presence of TEMPO in DMF as
a reaction solvent (Table 1, entry 23). TEMPO was used as a
radical scavenger and is also one of the important factors in
the fluorine-18 incorporation reactions using a diaryliodonium
salt precursor (Table 1, entry 2) because it is unstable under
basic conditions and generates aromatic hydrocarbons by radical-
induced decomposition.27 Three commonly used solvent systems
for radiolabelling reactions were tested, namely CH3CN, DMF
and DMSO. Among all these solvents under the equivalent
conditions tested, DMF gave better radiolabelling results than
those obtained with CH3CN and DMSO (Table 1 entries 15 versus
16 and 18 versus 19 or 20). The reactions in all of the solvents tested
did not produce any remarkable fluorine-18 labelled by-products,
as determined by the HPLC of the crude reaction mixture. The
reaction temperature also had a considerable effect on the fluorine-
18 incorporation yield. The optimal labelling temperature was
determined to be 150 ◦C under most phase-transfer catalysts (also
called the base) and with most types of precursor. For 5b–e with
bases at high temperature, the fluorine-18 incorporation yield was
poor under most conditions. It could be considered that these
low radiochemical yields might be the result of the stability of
the diaryliodonium salts. Amongst the bases used, K2.2.2/K2CO3
gave better fluorine-18-incorporation yields than TBAOH and
TBAHCO3, in the case of 5a and 5f, but the other precursors
had similarly low incorporation yields. We evaluated the fluorine-
18 incorporation effects according to the base concentration in
the aromatic radiofluorination of [18F]FMZ after the determining
of the precursor, base, solvent, and temperature. The [18F]fluorine
incorporation yield was compared at molar ratios from the 0.4
to 1.0 equivalents of K2CO3 with that of the precursor, as
summarized in Table 2. The results clearly show that the fluorine-
18 incorporation yield is not quantitative (in the case of Table
2, entries 1–4) and that of aromatic radiofluorination from a
diaryliodonium salt depends on the base : precursor ratio. The
result is that diaryliodonium tosylate (5f)/K2CO3 in a ratio of
1 : 0.6 (80.4%) was sufficient to obtain the optimum yield (Table
2, entry 1), while increasing the amount of base resulted in a
significant drop in radiochemical yield (Table 2, entries 3 and
4). The molar ratio of the base seemed to be an important
factor in determining the efficiency of the fluorination because of
the instability of the diaryliodonium salts in basic environments.
Increasing the amount of precursor (5f) from 4 mg to 8 mg (Table
2, entry 6) was not significant in this experiment, while decreasing
the amount resulted in a low fluorine-18 incorporation yield (Table
2, entry 5). Reduction of the heating time resulted in a similar
outcome (Table 2, entries 2 versus 7). These results indicate that
minimal amounts of precursor and short reaction times are readily
available to apply the automated synthetic process for [18F]FMZ
production at low cost.
The results in Table 1 and Table 2 clearly showed the absence
of the electron-rich heteroarene effect on the diaryliodonium
salt. On comparison of the [18F]fluoride incorporation yields for
[18F]flumazenil among the various precursors, the result obtained
from 4-methylphenyl-mazenil iodonium tosylate (8f) was superior
to those of the other precursors, in spite of the fact that 2-
thiophenyl-(8b), 3-thiophenyl-(8c), and 4-methoxyphenyl-mazenil
iodonium tosylate (8d) have relatively high electron densities.
As mentioned above, in the fluoride substitution reactions of
diaryliodonium salts that are functionalized on both aryl rings, the
fluoride anion generally attacks the more activated arene (electron-
deficient ring). This result therefore indicates that another factor,
such as the stability of the diaryliodonium salt under basic
conditions at a high temperature, is important for aromatic
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Table 2 Optimization of [18F]flumazenil preparation under various
Table 3 Instabilities of diaryliodonium tosylatesa
conditionsa
Decomposed
Precursor
(mg)
K2CO3
(equiv.)
Time
(min)
Yieldb
(d.c.c)
compounds (mmol)b
Entry
Entry
Precursor
6
7
Instability (%)c
1
2
3
4
5
6
7
4
4
4
4
2
8
4
0.4
0.6
0.8
1.0
0.6
0.6
0.6
15
15
15
15
15
15
5
51.7 7.8
1
2
3
4
5
6
5a
5b
5c
5d
5e
5f
0.75
0.94
1.07
0.51
1.01
0.45
0.64
0.90
1.16
1.25
0.73
0.78
22.8
30.4
37.1
30.3
29.9
20.8
80.4 3.2 (66.5 2.3)
74.1 5.9 (60.1 3.6)
42.2 4.4
56.6 5.3
76.4 6.5 (56.8 4.3)
81.2 4.1 (67.2 2.7)
a The reaction was performed using 4 mg of precursors (5.81–6.07 mmol),
0.5 mL of DMF, radical scavenger (TEMPO, 1 mg) and 0.8 equivalents of
K2CO3 at 150 ◦C for 15 min. b Progress of the reaction was analyzed by
HPLC (YMC-triart C18 column, 5 mm, 4.6 ¥ 250 mm, acetonitrile : water,
254 nm, flow rate: 1.0 mL min-1) and the values obtained by comparing
the UV response of the sample with that of known concentrations of 6 and
7. c Instability represents the percentage of decomposed products 6 and 7
compared with the concentration of starting materials.
a The reaction was carried out with 0.4–1.0 equiv. of K2CO3 (K2.2.2
=
5.5 mg) relative to the precursor (5f) in 0.5 mL of DMF and a radical
scavenger (TEMPO, 1 mg) at 150 ◦C for the desired reaction time.
b The radiochemical yield is determined by radio-TLC and represents the
percentage of [18F]flumazenil in the reaction mixture (n = 3 or 4). c The
radiochemical yield is determined by isolating pure [18F]flumazenil from
a semi-preparative column using HPLC (20 : 80 acetonitrile : water, UV
(254 nm), gamma-ray, flow rate: 3 mL min-1, n = 3).
fluorination, as well as the electron density of the counter
arene. In fact, diaryliodonium salts are generally unstable at
higher temperatures as previously reported,28 and the fluorine-18
incorporation yields of [18F]flumazenil are also dependent on the
base concentration, as described above (Table 2, entries 1–4). To
demonstrate convincingly the results displayed and as described
above, we were interested in examining further the stabilities and
reactivities of the prepared diaryliodonium salt precursors. If
the diaryliodonium tosylate precursors (5a–f) decompose under
basic conditions at high temperatures, the side products would
be generated via two pathways; “Pathway A” would produce I-
mazenil (6), and “Pathway B” would produce H-mazenil (7), as
shown in Fig. 3. To monitor the stability of the diaryliodonium
salts, each diaryliodonium salt precursor (5a–f) was reacted with
K2.2.2/K2CO3 in DMF at 150 ◦C for 15 min without the presence of
the fluoride ion; the stability was evaluated by tracking the decom-
posed products (6 and 7) in the crude reaction mixture by HPLC.
As shown in Table 3, 4-methylphenyl-mazenil iodonium tosylate
(5f) showed the best stability (20.8%) among the diaryliodonium
tosylate precursors whilst in the presence of the base (K2CO3)
at the optimized temperature (150 ◦C). Phenyl-mazenil iodonium
tosylate (5a) had slightly less stability (Table 3, entry 1). Although
the electron density in the dispensable ring is higher than in the
phenyl ring in the case of the precursors 5b–d, it was noted that the
stability of the precursor was much decreased in comparison with
those of the phenyl and the 4-methyl substituted phenyl rings. This
observation indicates that the more electron-rich diaryliodonium
tosylate precursors (5b–d) have lower stabilities; these results
correspond with the tendency of the [18F]fluoride incorporation
yield (Table 1, entries 15, 17–18, and 21–23).
On the other hand, each diaryliodonium salt (5a–f) was treated
with CsF (no base) in DMF at 90 ◦C for 2 h for selectivity
measurements, thus checking the produced [19F]flumazenil (1), I-
mazenil (6) and H-mazenil (7) concentrations, as shown in Table
4. The crude product mixtures were analyzed by HPLC and then
the molar absorptivity of each compound was examined with
respect to their relative ratio. In all aspects of this experiment,
[19F]flumazenil was the main product, except in the case of precur-
sor 5e. The selectivities of the precursors were all quite different
from each other, and it was verified that 4-methylphenyl-mazenil
Fig. 3 Proposed decomposition pathways.
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Table 4 The reaction of diaryliodonium tosylates with CsFa
our results regarding stability and selectivity will be useful in the
design of diaryliodonium salt precursors that can be applied to
the synthesis of 18F-labelled radiopharmaceuticals.
Product ratiosb
Selectivity for
Overall, 4-methylphenyl-mazenil iodonium tosylate (5f) had
greater stability and selectivity than the electron-rich diaryliodo-
nium tosylates (5b–d) or phenyl-mazenil iodonium tosylate (5a),
it also showed the best radiochemical yield among the di-
aryliodonium tosylate precursors. The best result was obtained
for the reaction of 4 mg of the 4-methylphenyl-iodonium tosylate
precursor with fluorine-18 with K2.2.2/K2CO3 (0.6 equiv.) in DMF
Entry
Precursor
6
7
1
main product, 1
1
2
3
4
5
6
5a
5b
5c
5d
5e
5f
0.18
0.08
0.19
0.07
0.39
0.07
0.02
0.06
0.07
0.10
0.15
0.02
0.80
0.86
0.74
0.83
0.46
0.91
4.00
6.14
2.85
4.88
0.85
10.11
◦
a The reaction was performed using 4 mg of precursors (5.81–6.07 mmol),
0.5 mL of DMF, radical scavenger (TEMPO, 1 mg) and 5.0 equivalents
of CsF at 90 ◦C for 2 h. b The relative ratios obtained from a mole of
each compound, which was analyzed by comparing the UV response of
the sample with that of known concentrations of 6, 7, and 1 (YMC-triart
C18 column, 5 mm, 4.6 ¥ 250 mm, acetonitrile : water, 254 nm, flow rate:
1.0 mL min-1).
at 150 C for 5 min. In separation steps, the crude product was
diluted with water and pre-treated by a C18 plus Sep-pak cartridge
to remove the DMF solvent and other polar decomposed mass
prior to HPLC purification. The life-time of the HPLC column
was shortened to about 4 months when the pretreatment step
was not performed using the Sep-Pak cartridge. Finally [18F]FMZ
was isolated at around 28 min and did not show any remarkable
fluorine-18-labelled by-products or UV detected mass surrounding
this retention time, as determined by the HPLC of the crude
reaction mixture (Fig. 4). Reformulation of the final product in an
ethanol/saline solution can be achieved with the C18 plus Sep-pak
cartridge. Under these conditions, the radiochemical yield (RCY)
was 67.2 2.7% (d.c.) with more than 99% radiochemical purity
(RCP) after reverse HPLC purification. The total synthesis time for
[18F]FMZ ([18F]1) was about 55 min, including HPLC purification,
and the specific activity was in the range of 370–450 GBq
mmol-1. This high radiochemical yield showed the promising
usefulness of diaryliodonium salt precursors which result in
complex compounds as well as other radiopharmaceuticals which
had 40–60% fluorine-18 incorporation yield.21b,25c
iodonium tosylate (5f) is the most attractive precursor for the
efficient fluorine-18 incorporation of [18F]FMZ. Phenyl-mazenil
iodonium tosylate (5a) had a lower selectivity although its stability
was similar to that of 4-methylphenyl-mazenil iodonium tosylate
(5f) (Table 4, entry 1). Conversely, 2-thiophenyl-mazenil iodonium
tosylate (5b) showed a reasonable selectivity value on the basis
of its high electron density in the dispensable ring but it was
unsuitable as a precursor because of its poor stability. It was
well known that the fluorination of diaryliodonium salts with
fluorine-18 occurred preferentially at a more electron-deficient
arene, and showed higher efficiency with an electron-rich arene
as the counter aryl ring. This tendency is in agreement with the
result for the model compounds reported by other research groups.
Our studies showed, however, that the stability and selectivity
of precursors under basic conditions at a high temperature are
important factors in addition to the electron density of the
diaryliodonium salt for efficient aromatic [18F]fluorination. Thus,
In order to explore additional applications, studies were ex-
tended to a high-scale production in the commercial automated
synthetic module which is based on conductive heating. Employ-
ing the optimized conditions described above, we successfully ob-
tained [18F]1 using the commercial automated device, with a high
Fig. 4 The radio-HPLC profile of [18F]1 from the reaction mixture (bottom: UV (254 nm); top: g-ray).
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radiochemical yield of about 63.5 3.2 (n = 26) within 60 1.1 min
which included azeotropic distillation, fluorine-18 incorporation,
HPLC purification, and solid-phase purification without major
modifications. We compared the results of previously reported
syntheses based on conductive heating14,17 or microwave dielectric
heating16 with this study. The elapsed time was shorter than that for
the conductive heating system, which took 75–80 min using nitro-
mazenil as the precursor and this method for [18F]FMZ production
has limitations in automated procedures because of the very low
radiochemical yields, as described by other research groups.15,17,18
Our experimental studies using the commercial automated device
in accordance with literature routes also showed very low yields
of about 0.4–1.1% (n = 6). However, the automated production of
[18F]FMZ using a diaryliodonium salt as the precursor produced
the high-efficiency yield (with no additional devices such as a
microwave system) and the total synthetic time is comparable
to microwave-assisted systems.16 Our experiment also showed the
high specific activity (over 370 GBq mmol-1), reproducible yield
(n = 26 with no failure), simple operation, and without major
modification of a commercial module. These results demonstrate
that the diaryliodonium salt precursor can satisfy our desires for
the high yield production of [18F]FMZ and its applicability for
clinical PET studies.
7.9 ¥ 250 mm) or Thermo Separation Products System (Fremont,
U. S.) (YMC, analytical YMC-triart C18, 5 mm, 4.6 ¥ 250 mm)
equipped with a NaI radiodetector (Raytest) and a UV-detector.
HPLC-grade solvents (J. T. Baker, U. S.) were used for HPLC
purification after membrane filtering (Whatman, 0.22 mm). Radio-
TLC was analyzed on a Bioscan radio-TLC scanner (Washington
DC, USA). All radioactivities were measured using a VDC-505
activity calibrator from Veenstra Instruments (Netherlands).
Ethyl-5,6-dihydro-5-methyl-6-oxo-8-tributylstannyl-4H-imidazo-
[1,5-a][1,4]benzodiazepine-3-carboxylate (3)
To a solution of ethyl 8-bromo-5,6-dihydro-5-methyl-6-oxo-
4H - imidazo[1,5 - a][1 , 4]benzodiazepine - 3 - carboxylate (3.52 g,
9.67 mmol) in toluene (100 mL) was added tetrakis-
(triphenylphosphine)palladium(0) (0.34 g, 0.290 mmol) and
bistributyltin (16.8 g, 29.0 mmol). The reaction mixture was
heated to reflux under an argon atmosphere for 6 h. The mixture
was diluted with ethyl acetate (300 mL) and the extracted
organic layer was washed with water, dried over anhydrous
sodium sulfate, and evaporated to dryness in vacuo. Purifi-
cation by flash column chromatography (5% MeOH : CH2Cl2)
afforded ethyl-5,6-dihydro-5-methyl-6-oxo-8-tributylstannyl-4H-
imidazo[1,5-a][1,4] benzodiazepine-3-carboxylate (3, 3.72 g, 66%)
1
as a white solid: H NMR (400 MHz, CDCl3) d 0.90 (t, J = 7.2
Conclusions
Hz, 9H), 1.09–1.16 (m, 6H), 1.28–1.40 (m, 6H), 1.46 (t, J = 7.2 Hz,
3H), 1.49–1.59 (m, 6H), 3.26 (s, 3H), 4.22–4.58 (m, 2H and 1H),
5.19 (d, J = 14.8 Hz, 1H), 7.35 (d, J = 8.0 Hz, 1H), 7.71 (dd, J =
8.0, 1.2 Hz, 1H), 7.89 (s, 1H), 8.13 (d, J = 0.8 Hz, 1H); 13C NMR
(100 MHz, CDCl3) d 10.1, 13.9, 14.6, 27.6, 29.2, 36.1, 42.6, 61.2,
121.0, 128.2, 128.8, 132.0, 135.2, 136.0, 140.2, 140.8, 144.3, 163.4,
167.3; CAS Registry No. provided by the author: 200408-05-5.
For the efficient radiolabelling of [18F]flumazenil, we developed
an advanced [18F]flumazenil radiosynthesis system using different
diaryliodonium tosylate precursors. Among the various precur-
sors, the 4-methylphenyl-mazenil iodonium tosylate precursor (5f)
showed the highest radiochemical yield with high specific activity.
Further studies showed that the optimized reaction conditions
are well adapted to the high-scale automatic production of
[18F]flumazenil in a commercial module. In addition, stability and
reactivity studies of different diaryliodonium salt precursors could
be applied to the development of various radiopharmaceuticals
having low radiochemical yield of fluorine substituted on the aryl
positions.
General method for the diaryliodonium tosylate precursors (5a–f)
Hydroxyl(tosyloxy)iodo-aryl moieties (4a–f) were prepared
according to the literature.21,25 To a solution of hydroxyl-
(tosyloxy)iodo-aryl moiety (4a–f, 1.5 equiv.) in CH3CN (2 mL)
was added ethyl-5,6-dihydro-5-methyl-6-oxo-8-tributylstannyl-
4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate (3, 1.0
equiv.) in CH2Cl2 (2 mL) at room temperature under argon
atmosphere and stirred for 20 h. The solvent was evaporated by
a stream of nitrogen. The crude mixture was dissolved in ethanol
(1.0 mL) and transferred to the centrifuge tube. The solution was
diluted with excess diethyl ether (20 mL). After centrifuging, the
collected solid was dried in vacuo to give various heterodiarene
iodonium tosylate precursors 5a–f.
Experimental
All commercial reagents and solvents were used without further
purification unless otherwise specified. Reagents and solvents
were commercially purchased from Sigma–Aldrich (U. S.). Flash
column chromatography was performed with silica gel (Merck,
230–400 mesh, ASTM). All reactions were monitored on pre-
coated plates (Merck, silica gel 60F254). 1H and 13C NMR spectra
were recorded on a Varian 400-MR (400 MHz) spectrometer at
ambient temperature. Chemical shifts were reported in parts per
million (ppm, d units). H218O was purchased from Taiyo Nippon
Sanso Corporation (Japan). 18F-Fluoride was produced at Seoul
National University Bundang Hospital by 18O(p,n)18F reaction
through proton irradiation using a KIRAMS-13 cyclotron (Samy-
Ethyl-8-phenyl(iodonium tosylate)-5,6-dihydro-5-methyl-6-oxo-
4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate (5a). M.p.:
◦
1
215.5–217.9 C; H NMR (400 MHz, CD3OD) d 1.41 (t, J =
6.8 Hz, 3H), 2.34 (s, 3H), 3.21 (s, 3H), 4.30–4.51 (m, 2H and 1H),
5.13 (bs, 1H), 7.20 (d, J = 8.0 Hz, 2H), 7.55 (td, J = 7.8, 1.6 Hz,
2H), 7.66 (dd, J = 6.8, 1.6 Hz, 2H), 7.71 (t, J = 7.6 Hz, 1H), 7.81
(d, J = 8.4 Hz, 1H), 8.27 (dd, J = 8.4, 1.2 Hz, 2H), 8.30 (s, 1H),
8.48 (dd, J = 8.8, 2.4 Hz, 1H), 8.72 (d, J = 2.0 Hz, 1H); 13C NMR
(100 MHz, CD3OD) d 14. 7, 21.3, 36.2, 43.2, 62.2, 114.69, 116.5,
127.0, 127.4, 129.7, 129.9, 132.8, 133.4, 134.0, 136.5, 136.7, 136.9,
137.9, 140.2, 140.3, 141.7, 143.6, 163.7, 166.2; MS (FAB) m/z 488
R
oung Unitech Co., Ltd.). Chromafixꢀ PS-HCO3 (45 mg) cartridges
were purchased from Macherey–Nagel Ins. (Germany). The auto-
mated production studies were performed in the TracerLab FXFN
R
(GE Healthcare). C18 plus Sep-Pakꢀ cartridges were purchased
from Waters Corp. (U. S.). HPLC purification was performed
with a Gilson 322 (Waters, semi-Preparative Xterra RP-18, 5 mm,
8352 | Org. Biomol. Chem., 2011, 9, 8346–8355
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(M+–OTs, 100%). HRMS calcd for C21H19IN3O3 488.0466, found
488.0474.
7.37 (d, J = 8.4 Hz, 2H), 7.68 (d, J = 8.0 Hz, 2H), 7.82 (d, J = 8.4
Hz, 1H), 8.12 (d, J = 8.4 Hz, 2H), 8.30 (s, 1H), 8.43 (dd, J = 8.8, 2.4
Hz, 1H), 8.69 (d, J = 2.4 Hz, 1H); 13C NMR (100 MHz, CD3OD)
d 13.2, 19.85, 19.94, 34.7, 41.8, 60.7, 111.2, 113.1, 125.5, 125.8,
128.2, 128.4, 131.3, 132.6, 135.0, 135.2, 135.4, 136.4, 138.5, 138.6,
140.2, 142.1, 144.1, 162.2, 164.8; MS (FAB) m/z 494 (M+–OTs).
HRMS calcd for C19H17IN3O3S 494.0035, found 488.0033.
Ethyl-8-(2-thiophenyl)(iodonium tosylate)-5,6-dihydro-5-methyl-
6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate (5b).
◦
1
M.p.: 103.5–106.2 C; H NMR (400 MHz, CD3OD) d 1.41 (t,
J = 7.0 Hz, 3H), 2.36 (s, 3H), 3.22 (s, 3H), 4.40–4.62 (m, 2H and
1H), 5.15 (bs, 1H), 7.20–7.23 (m, 3H), 7.69 (d, J = 8.4 Hz, 2H),
7.84 (d, J = 8.8 Hz, 1H), 7.94 (dd, J = 5.2, 1.2 Hz, 1H), 8.10
(dd, J = 3.6, 1.2 Hz, 1H), 8.31 (s, 1H), 8.47 (dd, J = 8.8, 2.4 Hz,
1H), 8.73 (d, J = 2.0 Hz, 1H); 13C NMR (100 MHz, CD3OD) d
13.2, 19.9, 34.8, 41.8, 60.7, 97.9, 115.9, 125.5, 125.9, 128.2, 128.4,
129.6, 131.3, 135.0, 135.2, 136.4, 137.7, 138.0, 138.1, 140.3, 141.4,
142.1, 162.2, 164.7.; MS (FAB) m/z 494 (M+–OTs). HRMS calcd
for C19H17IN3O3S 494.0035, found 494.0042.
Ethyl-8-iodo-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a]-
[1,4]benzodiazepine - 3 - carboxylate (6). Ethyl - 5,6 - dihydro - 5 -
methyl-6-oxo-8-tributylstannyl-4H-imidazo[1,5-a][1,4]benzodi-
azepine-3-carboxylate (3, 0.282 g, 0.49 mmol) was dissolved in
ethanol (30 mL). Aqueous sodium iodide (3.0 M, 1.60 mL,
4.90 mmol), peracetic acid (32%, 1.16 g, 4.90 mmol) and acetic
acid (20 mL) were added. The reaction mixture was stirred at room
temperature for 30 h. The solution was quenched with 10 mL of
saturated sodium sulfite solution and basified with conc. ammonia
water. The crude product was extracted with dichloromethane (3 ¥
20 mL). The organic layer was dried over anhydrous sodium sulfate
and evaporated to dryness in vacuo. Purification by flash column
chromatography (5% MeOH/CH2Cl2) afforded iodinate 6 (0.184
Ethyl-8-(3-thiophenyl)(iodonium tosylate)-5,6-dihydro-5-methyl-
6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate (5c).
◦
1
M.p.: 121.2–124.4 C; H NMR (400 MHz, CD3OD) d 1.41 (t,
J = 7.2 Hz, 3H), 2.35 (s, 3H), 3.22 (s, 3H), 4.40–4.61 (m, 2H and
1H), 5.16 (bs, 1H), 7.20 (d, J = 8.0 Hz, 2H), 7.67–7.71 (m, 3H),
7.73–7.75 (m, 1H), 7.82 (d, J = 8.8 Hz, 1H), 8.30 (s, 1H), 8.44 (dd,
J = 8.4, 2.4 Hz, 1H), 8.60 (dd, J = 2.8, 1.2 Hz, 1H), 8.70 (d, J = 2.4
Hz, 1H); 13C NMR (100 MHz, CD3OD) d 14.6, 21.3, 36.2, 43.2,
62.2, 99.8, 115.4, 127.0, 127.3, 129.7, 129.8, 132.1, 132.69, 132.74,
136.5, 136.6, 137.8, 138.0, 139.8, 139.9, 141.7, 143.6, 163.7, 166.2;
MS (FAB) m/z 494 (M+–OTs). HRMS calcd for C19H17IN3O3S
494.0035, found 494.0033.
1
g, 92%) as a white solid: H NMR (400 MHz, CDCl3) d 1.45 (t,
J = 7.2 Hz, 3H), 3.25 (s, 3H), 4.20–4.62 (m, 2H and 1H), 5.06–5.40
(m, 1H), 7.17 (d, J = 8.4 Hz, 1H), 7.88 (s, 1H), 7.95 (dd, J = 8.4, 2.0
Hz, 1H), 8.39 (d, J = 2.0 Hz, 1H); 13C NMR (100 MHz, CDCl3)
d 14.6, 36.2, 42.5, 61.3, 93.7, 123.6, 129.2, 130.8, 131.8, 134.9,
135.5, 141.7, 141.8, 163.2, 165.2; CAS Registry No. provided by
the author: 268566-09-2.
Ethyl-8-(4-methoxyphenyl)(iodonium tosylate)-5,6-dihydro-5-
Ethyl-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzo-
diazepine-3-carboxylate (7). Ethyl-8-iodo-5,6-dihydro-5-methyl-
6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate (6,
60 mg, 0.14 mmol) was dissolved in methanol (2 mL), followed
by the addition of triethylamine (36 mL, 0.26 mmol), 10% Pd/C
(30 mg) and hydrogen gas was bubbled using hydrogen balloon.
After the reaction mixture stirred at room temperature for 1 h, the
solution was filtered and evaporated. Purification by flash column
chromatography (5% MeOH/CH2Cl2) afforded deiodinate 7
(38 mg, 92%) as a colorless oil:1H NMR (400 MHz, CDCl3) d
3.03 (s, 3H), 3.91 (s, 2H), 7.02 (d, J = 7.6 Hz, 1H), 7.29 (t, J =
7.8 Hz, 1H), 7.47 (td, J = 7.6, 1.6 Hz, 1H), 7.97 (dd, J = 8.0, 1.6
Hz, 1H), 8.63 (s, 1H); 13C NMR (100 MHz, CDCl3) d 14.6, 36.1,
42.6, 61.2, 122.8, 128.8, 128.9, 129.3, 132.2, 132.9, 132.9, 135.2,
135.9, 163.3, 166.7; CAS Registry No. provided by the author:
3415-35-8.
methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxy-
◦
1
late (5d). M.p.: 101.3-104.0 C; H NMR (400 MHz, CD3OD)
d 1.41 (t, J = 7.2 Hz, 3H), 2.35 (s, 3H), 2.35 (s, 3H), 3.22 (s, 3H),
3.84 (s, 3H), 4.39–4.61 (m, 2H and 1H), 5.16 (bs, 1H), 7.08 (d, J =
9.2 Hz, 2H), 7.21 (d, J = 8.0 Hz, 2H), 7.68 (d, J = 8.0 Hz, 2H),
7.81 (d, J = 8.4 Hz, 1H), 8.17 (d, J = 8.8 Hz, 2H), 8.30 (s, 1H),
8.41 (dd, J = 8.8, 2.4 Hz, 1H), 8.66 (d, J = 2.0 Hz, 1H); 13C NMR
(100 MHz, CD3OD) d 13.2, 19.9, 34.7, 41.8, 55.0, 60.7, 103.3,
113.6, 117.6, 125.5, 125.8, 128.2, 128.4, 131.2, 134.9, 135.2, 136.4,
137.5, 138.2, 138.4, 140.3, 142.0, 162.2, 163.3, 164.8; MS (FAB)
m/z 518 (M+–OTs). HRMS calcd for C22H21IN3O4 518.0576,
found 518.0580.
Ethyl- 8-(3-methoxypehnyl)(iodonium tosylate)-5,6-dihydro-5-
methyl-6-oxo-4H -imidazo[◦1,5-a][1,4]benzodiazepine-3-carboxy-
1
late (5e). M.p.: 93.7–96.5 C; H NMR (400 MHz, CD3OD) d
1.40 (t, J = 7.2 Hz, 3H), 2.35 (s, 3H), 3.21 (s, 3H), 3.85 (s, 3H),
4.39 –4.61 (m, 2H and 1H), 5.16 (bs, 1H), 7.23–7.26 (m, 3H),
7.46 (t, J = 8.2 Hz, 1H), 7.68 (d, J = 8.4 Hz, 2H), 7.78–7.88 (m,
3H), 8.29 (s, 1H), 8.46 (d, J = 8.8, 2.0 Hz, 1H), 8.74 (d, J = 2.0
Hz, 1H); 13C NMR (100 MHz, CD3OD) d 14.6, 21.3, 36.2, 43.2,
56.6, 62.2, 114.6, 116.1, 120.0, 122.0, 126.9, 127.3, 128.6, 129.4,
129.8, 132.7, 133.9, 136.4, 136.7, 137.9, 140.1, 140.2, 141.7, 143.5,
162.9, 163.5, 166.2; MS (FAB) m/z 518 (M+–OTs). HRMS calcd
for C22H21IN3O4 518.0577, found 518.0576.
General method for aromatic fluorination of [18F]flumazenil. 18F
was prepared by the 18O(p,n)18F reaction using H218O as the
target material. [18F]F-/H218O was isolated from the enriched
water by trapping on Chromafix-HCO3 cartridge (pre-activated
with 2 mL of ethanol and 5 mL of water), and then eluted with
methanol : water (1 : 0.2 mL) dissolved the TBAHCO3, TBAOH or
K2.2.2./K2CO3.29 This solution was dried by azeotropic distillation
with acetonitrile under a nitrogen stream and then the iodonium
tosylate precursor 5a–f and TEMPO (1 mg) in various reaction
solvents (0.5 mL) were added. The reaction mixture was heated
at the desired temperature for 5–15 min. After cooling to room
temperature, the reaction mixture was diluted with 10 mL of water.
This solution was loaded into a C18 plus Sep-Pak, washed with
10 mL of water and eluted with 1 mL of CH3CN. The combined
Ethyl - 8 - (4 - methylphenyl)(iodonium tosylate) - 5,6 - dihydro-5-
methyl-6-oxo-4H -imidazo[1,5-a][1,4]benzodiazepine-3-carboxy-
◦
1
late (5f). M.p.: 223.1–225.4 C; H NMR (400 MHz, CD3OD)
d 1.41 (t, J = 7.2 Hz, 3H), 2.35 (s, 3H), 2.41 (s, 3H), 3.22 (s, 3H),
4.39–4.61 (m, 2H and 1H), 5.16 (bs, 1H), 7.21 (d, J = 8.4 Hz, 2H),
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solution was separated by a semi-prep HPLC system (Waters,
Xterra RP-18, 250 ¥ 7.9 mm, 10 m). The acetonitrile and water
(20 : 80) were used as a mobile phase at a flow rate of 3 mL min-1.
The product fraction was collected at around 28 min.
Acknowledgements
This work was supported in part by the National Research
Foundation of Korea (NRF) grant funded by the Korea gov-
ernment (MEST) (2011-0006320 and NRF-351-2009-2-E00063).
The research was supported in part by the Converging Research
Center Program through the Ministry of Education, Science and
Technology (2010K001055). This study was also supported by
a grant from the Korea Food Research Institute (Project No.
E0112401) and Ministry of Health and Welfare (Project No.
A062254).
Instability of diaryliodonium tosylates. The reaction was per-
formed with 4 mg of heterodiarene iodonium salt precursors (5.81–
6.07 mmol), TEMPO (1 mg), K2.2.2./K2CO3 (5.5 mg/0.8 equiv.)
in DMF (0.5 mL) at 150 ◦C for 15 min. After cooling to room
temperature, the reaction mixture was diluted with 10 mL of
water and then loaded into a C18 plus Sep-Pak, followed by
10 mL of water. The decomposed compounds were collected with
CH3CN (2 mL) and the progress of the reaction was analyzed
by HPLC (YMC-triart C18 column, 5 mm, 4.6 ¥ 250 mm; 25%
acetonitrile–water (0 min), 25% acetonitrile–water (10 min), 80%
acetonitrile–water (20 min), 80% acetonitrile–water (30 min); 254
nm, flow rate: 1.0 mL min-1). The values (mmol) were obtained
from comparing the UV responses of the samples with that of
known concentrations of 6 and 7. The standard curves were
obtained in six points from 1.95 ¥ 10-6 to 6.23 ¥ 10-10 mol (R2 =
1.00) for 6 and 2.81 ¥ 10-6 to 8.98 ¥ 10-10 mol (R2 = 1.00) for 7.
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R
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