A two-step one-pot radiosynthesis of the potent dopamine D 2/D3 agonist PET radioligand [11C]MNPA

Article abstract of DOI:10.1002/jlcr.1583

(R)-(-)-2-[¹¹C]Methoxy-N-n-propylnorapomorphine ([ ¹¹C]MNPA ([¹¹C]2)) is an agonist radioligand of interest for imaging D₂/ D₃ receptors in vivo. Here we sought to develop an improved radiosynthesis of this radioligand. Reference 2 was synthesized in nine steps with an overall yield of about 5%, starting from codeine. Trimethylsilyldiazomethane proved to be a practical improvement in comparison to diazomethane in the penultimate methylation step. A protected precursor for radiolabeling ((R)-(-)-2-hydroxy-10,11-acetonide-N-n- propylnoraporphine, 4) was prepared from (R)-(-)-2-hydroxy-N-n- propylnorapomorphine (1) in 30% yield. [¹¹C]2 was prepared from 4 via a two-step one-pot radiosynthesis. The first step, methylation of 4 with [ ¹¹C]methyl triflate, occurred in quantitative radiochemical yield. The second step, deprotection of the catechol moiety with HCl and heat, yielded 60-90% of [¹¹C]2 giving an overall incorporation yield from [ ¹¹C]methyl triflate of 60-90%. In a typical run more than 1 GBq of [¹¹C]2, was produced from carbon-11 generated from a 10-min proton irradiation (16 MeV; 35 μA) of nitrogen-hydrogen target gas. The radiochemical purity of [¹¹C]2 was > 99% and specific radioactivity at the time of injection was 901±342 GBq/μmol (n = 10). The total synthesis time was 35-38 min from the end of radionuclide production. The identity of [¹¹C]2 was confirmed by comparing its LC-MS/MS spectrum with those of reference 2 and (R)-(-)-10-methoxy-2,11-dihydroxy-N-n- propylnoraporphine. Copyright

Full text of DOI:10.1002/jlcr.1583

Research Article
Received 19 September 2008,
Revised 8 December 2008,
Accepted 23 December 2008
Published online 23 January 2009 in Wiley Interscience
(www.interscience.wiley.com) DOI: 10.1002/jlcr.1583
A two-step one-pot radiosynthesis of the
potent dopamine D2/D3 agonist PET
radioligand [11C]MNPA
Ã
C. Steiger,a S. J. Finnema,a L. Raus,b M. Schou,a,c R. Nakao,d K. Suzuki,d
e
V. W. Pike, H. V. Wikstrom, and C. Halldin
f
a
¨
(R)-(À)-2-[11C]Methoxy-N-n-propylnorapomorphine ([11C]MNPA ([11C]2)) is an agonist radioligand of interest for imaging D2/
D3 receptors in vivo. Here we sought to develop an improved radiosynthesis of this radioligand. Reference 2 was
synthesized in nine steps with an overall yield of about 5%, starting from codeine. Trimethylsilyldiazomethane proved to be
a practical improvement in comparison to diazomethane in the penultimate methylation step. A protected precursor for
radiolabeling ((R)-(À)-2-hydroxy-10,11-acetonide-N-n-propylnoraporphine, 4) was prepared from (R)-(À)-2-hydroxy-N-n-
propylnorapomorphine (1) in 30% yield. [11C]2 was prepared from 4 via a two-step one-pot radiosynthesis. The first step,
methylation of 4 with [11C]methyl triflate, occurred in quantitative radiochemical yield. The second step, deprotection of
the catechol moiety with HCl and heat, yielded 60–90% of [11C]2 giving an overall incorporation yield from [11C]methyl
triflate of 60–90%. In a typical run more than 1 GBq of [11C]2, was produced from carbon-11 generated from a 10-min
proton irradiation (16 MeV; 35 lA) of nitrogen–hydrogen target gas. The radiochemical purity of [11C]2 was 4 99% and
specific radioactivity at the time of injection was 9017342 GBq/lmol (n = 10). The total synthesis time was 35–38 min from
the end of radionuclide production. The identity of [11C]2 was confirmed by comparing its LC-MS/MS spectrum with those of
reference 2 and (R)-(À)-10-methoxy-2,11-dihydroxy-N-n-propylnoraporphine.
Keywords: PET; MNPA; dopamine D2/D3 agonist; radiosynthesis; LC-MS/MS analysis
reported methylenedioxy-protecting group needs harsh reac-
tion with BCl3 to be removed.6–9 The conditions of this
Introduction
The binding of agonist radioligands to dopamine D2/D3-
receptors has been found to be more sensitive to altered
endogenous dopamine levels, than that of antagonist radi-
oligands.1–3 [11C]MNPA ((R)-(À)-2-[11C]methoxy-N-n-propylnora-
pomorphine, [11C]2, Scheme 1) is an agonist radioligand that
has been used to visualize dopamine D2/D3-receptors in living
non-human primate brain.4 [11C]2 was initially prepared by
direct alkylation of (R)-(À)-2-hydroxy-N-n-propylnorapomor-
phine (TNPA, 1, Scheme 2) with [11C]methyl iodide.4 The
selectivity for O-alkylation at the 2-hydroxy position was
determined by post-derivatization of the radioactive product.
deprotection reaction would very likely hydrolyse some of the
product, as indicated by tritiation attempts.5
The aim of this work was (i) to develop a two-step one-pot
radiosynthesis that selectively produces [11C]2 and (ii) to
synthesize the acetonide precursor 4 (Scheme 3) and reference
standard 2 (Scheme 4), each from codeine.
aDepartment of Clinical Neuroscience, Karolinska Institutet, Psychiatry Section,
However, recent attempts to produce [3H]MNPA following this Karolinska University Hospital, S-17176 Stockholm, Sweden
direct methylation strategy, produced a mixture of both
[3H]MNPA and (R)-(À)-10-[3H]methoxy-2,11-dihydroxy-N-n-pro-
pylnoraporphine.5 Furthermore, our subsequent evaluation of
the direct methylation of 1 with [13C]methyl iodide by LC-MS/MS
indicated that this alkylation was not completely selective for
the 2-hydroxy position, as the product ion spectrum of the
labeled product differed from that of the reference standard 2.
For future use of [11C]2 in human studies, we considered it
necessary to modify the radiosynthesis in such a way that [11C]2
would be formed selectively. Protection of the catechol moiety
would ensure selective labeling at the 2-hydroxy position. Such
protection would need to be stable under the methylation
conditions but easily removable afterwards. The previously
bPharmasynth AS and Institute of Chemistry, University of Tartu, Tartu, Estonia
c
¨
¨
Present affiliation: Astra Zeneca R&D, Medicinal Chemistry, S-15185 Sodertalje,
Sweden
dNational Institute of Radiological Sciences, Molecular Imaging Center, 4-9-1
Anagawa, Inage-ku, Chiba 263-8555, Japan
eMolecular Imaging Branch, National Institute of Mental Health, National
Institutes of Health, Bethesda, MD 20892, USA
fAllbay AB, Gamla va¨gen 15, 45070 Hamburgsund, Sweden
*Correspondence to: C. Steiger, Department of Clinical Neuroscience, Karolinska
Institutet, Psychiatry Section, Karolinska University Hospital, Building R5:U1, S-
17176 Stockholm, Sweden.
E-mail: carsten.steiger@ki.se
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C. Steiger et al.
Scheme 1. Radiosynthesis of [11C]2. Reagents: (a) [11C]methyl triflate, 0.5 M NaOH, MeCN and (b) 6 M HCl, 1501C, 8 min.
Scheme 2. Synthesis of 1. Reagents: (a) acetic anhydride, reflux; (b) 1-chloroethyl chloroformate, proton sponges, 1,2-dichloroethane, reflux; (c) MeOH, reflux; (d)
propionyl chloride, CH2Cl2, 2.5 M NaOH; (e) LiAlH4, THF, 01C; (f) MnO2, toluene; (g) 48% HBr, reflux.
the general strategy of Gao et al. and Neumeyer et al.,7,10 but
started from codeine phosphate hemihydrate instead of
Results and discussion
Chemistry
thebaine. The synthesis of 1 is depicted in Scheme 2. Refluxing
codeine in acetic anhydride gave 5 in high yield (98%). Reaction
Both reference compound 2 and precursor 4 were synthesized of 5 with 1-chloroethyl chloroformate in the presence of proton
from 1 (Schemes 3 and 4). For the synthesis of 1, we followed sponges and subsequent reflux in methanol yielded the
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C. Steiger et al.
Scheme 3. Synthesis of 4. Reagents: (a) TFA, 2,2-dimethoxypropane, 01C and (b) acetone (excess), 10 eq. P2O5, THF, reflux.
Scheme 4. Synthesis of reference 2. Reagents: (a) DMSO, NaOH (solid), dibromomethane, 801C, 4 h; (b) MeCN/MeOH (50/50), DIPEA, 2 M TMS-diazomethane in hexane,
36 h; (c) CH2Cl2, 1 M BCl3 in hexane, 20 h; (d) MeOH, reflux.
secondary amine 6. Compound 6 was treated with propionyl
The synthesis of reference compound 2 is outlined in
chloride to yield propionamide 7. Reduction of 7 with LiAlH4 Scheme 4. Reaction of 1 with CH2Br2 gave 10.6–8,10 Methylation
gave 8. Oxidation of 8 with MnO2 yielded ketone 9. Compound 9 of 10 with trimethylsilyldiazomethane (TMS-diazomethane) gave
was rearranged by reflux in freshly distilled 48% HBr solution to 11, which was purified by column chromatography on SiO2.12
give 1. The overall yield of the synthesis of 1 was approximately Finally, deprotection of 11 with BCl3 gave 2, which was purified
22% starting from codeine phosphate hemihydrate.
by HPLC and subsequent recrystallization.7–10 The overall yield
All chemical reactions in this sequence were quite convenient of reference compound 2 starting from 1 was approximately
and gave high yields. The only practical problem was purifica- 20%. With codeine phosphate hemihydrate as starting point the
tion of the ketone 9, which was later found to be unnecessary. overall yield of 2 was about 5%. The use of TMS-diazomethane
Using the crude compound 9 in the rearrangement reaction instead of diazomethane was a great improvement. No special
caused no further problems since we were able to purify 1 by glassware or any other special precautions were needed. Stirring
precipitation using cold trifluoroacetic acid in the next step.
the TMS-diazomethane mixture at room temperature, in a
The synthesis of 4 is depicted in Scheme 3. Two methods of regular round-bottomed flask for 1.5 days yielded 88% 11.
synthesizing 4 are presented of which the latter one is currently
in use, due to its more consistent and higher yields. Method 1:
Radiochemistry
Solid 1 was cooled on an ice-bath. TFA (5 ml) was added with
stirring, followed by addition of the same volume of 2,2-
dimethoxypropane. This procedure was repeated until TLC
The two-step one-pot radiosynthesis of [11C]2 is shown in
Scheme 1. [11C]methyl triflate was trapped at room temperature
in a vessel containing 4, acetonitrile and sodium hydroxide.
Methylation was instantaneous and quantitative. Subsequent
addition of hydrochloric acid and heating of the mixture, yielded
the crude [11C]2. A sodium acetate solution was added to the
crude reaction mixture after which the mixture was injected
showed no more increase in 4. The best result with this method
was a 10% yield after work-up procedures. Method 2:11 1 was
dissolved in THF. After addition of P2O5 and acetone, both in
large excess, the mixture was refluxed for 5 h, yielding 4 in
moderate yields (25–30%) after work-up and purification.
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C. Steiger et al.
Figure 1. HPLC chromatogram (UV absorbance and radioactivity vs. time) of the purification of [11C]2 with a semi-preparative reversed-phase HPLC column. (1) TNPA and
([11C]2) [11C]MNPA.
onto the semi-preparative HPLC column. [11C]2 eluted after spectrum of the carrier of [11C]2 was somewhat noisier, because
8–9 min (Figure 1). The total synthesis time was 35–38 min from of the lower concentration of the sample.
the end of radionuclide production. The yield of the deprotec-
tion step varied between 60 and 90% giving an overall Experimental
incorporation yield from [11C]methyl triflate of 60–90%. From a
10-min bombardment more than 1 GBq of [11C]2 was collected
at the end of synthesis. The radiochemical purity was better
Chemistry
All chemicals, except codeine phosphate hemihydrate, were
than 99%. The specific radioactivity of the product was
purchased from commercial companies such as Sigma Aldrich
9017342 GBq/mmol (n = 10) at the time of injection into the
subject for PET study.
and Fluka Chemicals and were used without further purification.
Codeine phosphate hemihydrate was purchased from the
Karolinska Hospital Pharmacy. All NMR spectra were recorded
Identification of carrier associated with [11C]MNPA by LC-
MS/MS
on a Bruker Avance 500 spectrometer (500.1 MHz for [1H]) with
tetramethylsilane as an internal standard. Chemical shifts (d) are
given in ppm. The electrospray ionization (ESI) mass spectra of
the intermediates and final compounds in the synthesis of
reference compound 2 and precursor 4 were obtained on an
LCQ ion trap mass spectrometer equipped with an ESI source
(Finnigan LTQ, San Jose, CA, USA) using 90% acetonitrile plus
10% 10 mM-ammonium acetate as eluent.
For identification of the carrier associated with [11C]2 by LC-MS/
MS, a sample of the purified and filtered product was taken after
radioactive decay without further dilution. For analysis of
reference standard 2 and (R)-(À)-10-methoxy-2,11-dihydroxy-N-
n-propylnoraporphine, solutions of 1 mg/ml were prepared. The
samples were analysed and the ion spectrum of the carrier of
[11C]2 was compared to that of the reference compound 2
(Figure 2) and (R)-(À)-10-methoxy-2,11-dihydroxy-N-n-propyl-
noraporphine (Figure 3) to identify the product. The ion
spectrum of the product was identical to that of the reference
compound 2. All the major fragments present in the ion
spectrum of the decayed product were also present in the
ion spectrum of reference compound 2. Furthermore, the ion
spectrum of (R)-(À)-10-methoxy-2,11-dihydroxy-N-n-propylnora-
porphine (Figure 3) showed two ions m/z 235 and 207, which
are absent in the ion spectrum of the carrier of [11C]2. The
6-Acetoxy-7,8-didehydro-4,5-epoxy-3-methoxy-17-methylmorphi-
nan (5)
Codeine phosphate hemihydrate (40.0 g; 98 mmol) was sus-
pended in acetic anhydride (200 ml). The reaction mixture was
heated to reflux for 3 h. During heating the suspension became
a solution and turned red. The reaction mixture was cooled to
room temperature and the solvent was evaporated off under
reduced pressure. The residue was dissolved in CH2Cl2 and
washed twice with saturated NaHCO3 and twice with water. The
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C. Steiger et al.
Figure 2. LC-MS/MS fragmentation spectrum of the carrier associated with [11C]2 (a) and reference compound 2 (b).
organic layer was dried on MgSO4, filtered and evaporated to solvent was evaporated off under reduced pressure. Methanol
dryness, yielding 5 (33.0 g, 96.7 mmol, 98%) as a pinkish solid. A (200 ml) was added to the residue. The solution was heated to
sample was recrystallized using CH2Cl2 and hexane, yielding off- reflux for 2 h. The solution was then cooled down to about 301C
1
white needles. H-NMR (CDCl3): d: 1.85–1.88 (m, 1H); 2.01–2.07 and diethyl ether (100 ml) was added while stirring. The mixture
(m, 1H); 2.16 (s, 3H); 2.27–2.40 (m, 2H); 2.44 (s, 3H); 2.56–2.60 (m, was cooled further to room temperature. A precipitate formed,
1H); 2.73 (t, 1H); 3.04 (d, 1H); 3.34–3.36 (m, 1H); 3.85 (s, 3H); 5.07 which was filtered off yielding 6 (26.3 g; 80 mmol, 83%) as an off-
(d, 1H); 5.17–5.20 (m, 1H); 5.42–5.45 (m, 1H); 5.63 (d, 1H); 6.54 (d, white powder. 1H-NMR (CD3OD): d: 2.07 (m, 1H); 2.13 (s, 3H); 2.25
1H); 6.66 (d, 1H). MS: ESI [MH1] Calcd: 342.17; found: 342.18.
(m, 1H); 2.97–3.09 (m, 4H); 3.31 (m, 2H); 3.83 (s, 3H); 4.26 (t, 1H);
5.16 (d, 1H); 5.22 (m, 1H); 5.50 (m, 1H); 5.75 (m, 1H); 6.67 (d, 1H);
6.79 (d, 1H). MS: ESI [MH1] Calcd: 328.16; found: 328.12.
6-Acetoxy-7,8-didehydro-4,5-epoxy-3-methoxy-17-normorphinan
(6)
6-Acetoxy-7,8-didehydro-4,5-epoxy-3-methoxy-17-propionylmor-
phinan (7)
A solution of 5 (33 g; 96.7 mmol), proton sponges (21 g;
96.7 mmol) and 1-chloroethyl chloroformate (13.2 ml; 115 mmol)
in 1,2-dichloroethane (200 ml) was heated to reflux under 6 (26.3 g; 80 mmol) was dissolved in CH2Cl2 (180 ml). A 2.5 M
nitrogen for 90 min. After cooling to room temperature, the NaOH solution (140 ml) was added. While the two-layer reaction
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C. Steiger et al.
Figure 3. LC-MS/MS fragmentation spectrum of reference compound (R)-(À)-10-methoxy-2,11-dihydroxy-N-n-propylnoraporphine.
mixture was stirred vigorously, propionyl chloride (88 mmol, (R)-(À)-2-Hydroxy-N-n-propylnorapomorphine (1)
8.0 ml) was added portion-wise to the organic layer. When the
9 (14.2 g; 44 mmol) was suspended in freshly distilled HBr
reaction was complete, the organic layer was separated from
the basic layer and washed with 1 M HCl, dried on MgSO4
and evaporated to dryness, yielding 7 (29.4 g; 77 mmol, 96%)
as an orange foam. 1H-NMR (CDCl3): d: 1.18 (t, 3H); 1.86–1.97 (m,
2H); 2.16 (s, 3H); 2.30–2.96 (m, 6H); 3.23 (m, 1H); 3.73 (m, 1H);
3.86 (s, 3H); 5.09 (d, 1H); 5.19 (m, 1H); 5.48 (m, 1H); 5.70 (m, 1H);
6.55 (d, 1H); 6.69 (d, 1H). MS: ESI [MH1] Calcd: 384.18, found:
384.14.
solution (48%, ꢀ250 ml). The reaction mixture was left to reflux
for 3 h and then cooled to room temperature. Solvent was
removed under reduced pressure. Cold (À5 to À101C)
trifluoroacetic acid (ꢀ150 ml) was added to the residue. The
suspension was stirred for 15 min after which the precipitate
was filtered off, yielding 1 TFA-salt (9.0 g; 21 mol, 48%) as a
pinkish grey powder. 1H-NMR (DMSO-d6): d: 1.02 (t, 3H);
1.69–1.87 (m, 2H); 2.71 (t ,1H); 2.93 (m, 1H); 3.12–3.40 (m, 4H);
3.51 (m, 1H); 3.79 (m, 1H); 4.25 (m, 1H); 5.50–6.00 (br s 1H); 6.54
(s, 1H); 6.67 (d, 1H); 6.76 (d, 1H); 7.83 (s, 1H); 8.6–9.0 (br s, 1H);
7,8-Didehydro-4,5-epoxy-3-methoxy-17-propylmorphinan-6-ol (8)
7 (29.4 g; 77 mmol) was dissolved in THF (500 ml) on an ice-bath. 9.81 (s, 1H). MS: ESI [MH1] Calcd: 312.16, found: 312.12.
Slowly 2.0 eq. of LiAlH4 (154 mmol, 5.9 g) was added portion-
wise to the reaction mixture. The reaction was quenched by (R)-(À)-2-Hydroxy-10,11-acetonide-N-n-propylnoraporphine
(4)
(method 1)
cautious addition of H2O (6.0 ml), aq. NaOH (2.5 M; 6.0 ml) and
again H2O (18.0 ml) (in that order!). The precipitate was filtered
off over celite. The filtrate, a clear colourless solution, was dried
on MgSO4. Evaporation of the solvent under reduced pressure
yielded 8 (23.8 g; 73 mmol, 95%) as an orange foam. 1H-NMR
(DMSO-d6): d: 1.02 (t, 3H); 1.77–1.92 (m, 3H); 2.42 (m, 1H); 2.76
(m, 1H); 3.08–3.40 (m, 7H); 3.75 (s, 3H); 4.10 (s, 1H); 4.21 (m, 1H);
4.82 (m,1H); 5.30 (d, 1H); 5.65 (d, 1H); 6.56 (d, 1H); 6.72 (d, 1H).
MS: ESI [MH1] Calcd: 328.19, found: 328.18.
1 TFA-salt (600mg; 1.4 mmol) was placed in a round-bottomed
flask on an ice-bath. TFA (5 ml) was added to the solid, followed
by 2,2-dimethoxypropane (5 ml). The reaction mixture was stirred
for 10min, after which the additions were repeated. This
sequence was repeated 20 times, after which the reaction
mixture was evaporated almost to dryness. The crude mixture
was dissolved in dichloromethane. This solution was washed
twice with saturated NaHCO3 solution and subsequently dried on
MgSO4. The organic layer was evaporated to dryness. The residue
was refluxed in hexane and left to cool to room temperature. The
formed precipitate, which was a mixture of starting material and
product, was filtered off. The product was purified with column
chromatography (Al2O3, eluent CH2Cl2 gradient up to 20% MeOH)
yielding 4 (49 mg; 0.14mmol, 10%) as a brown solid. Mass analysis
showed identical results to the product of method 2 (see below).
7,8-Didehydro-4,5-epoxy-3-methoxy-17-propylmorphinan-6-one (9)
8 (23.8 g; 73 mmol) was dissolved in toluene (250 ml). MnO2
(50 g) was added portion-wise to the reaction mixture (every 10
to 20 min). The MnO2 was filtered off and the procedure was
repeated once more. After filtering off the MnO2 a second time,
the solvent was removed under reduced pressure. The
generated dark red oil (25 g) was further purified with column
chromatography (SiO2, eluent CH2Cl2 gradient up to 10%
MeOH), yielding 9 (14.2 g; 44 mmol, 60%) as a crude, brown
solid. This crude product was not analysed, but was used
directly in the next reaction.
(R)-(À)-2-Hydroxy-10,11-acetonide-N-n-propylnoraporphine
(method 2)
(4)
1 TFA-salt (600 mg; 1.4 mmol) was dissolved in dry THF (8 ml).
Acetone (20 ml) and P2O5 (ꢀ10 eq.) was added and the mixture
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C. Steiger et al.
was then heated to reflux. After 2.5 h more P2O5 (ꢀ5 eq.) was 7.93 (d, 1H); 8.83 (s, 1H); 9.73 (s, 1H). MS: ESI [MH1] Calcd:
added. After 5 h the reaction mixture was left to cool to room 326.18, found: 326.14. LC-MS/MS: m/z: 326, 267, 249, 238, 221,
temperature. Solvents were removed under reduced pressure. 193 and 72.
The residue was basified with saturated NaHCO3 and extracted
with diethyl ether. The organic layer was dried on MgSO4 and Radiochemistry
evaporated to dryness. The residue was purified with flash
Acetonitrile and 0.5 M NaOH were obtained from VWR Interna-
chromatography (SiO2, eluent CH2Cl2 gradient up to 4% MeOH).
tional and the Swedish Pharmacy, respectively. Thirty-seven
After evaporation of the solvents under reduced pressure, the
percent HCl was purchased from Sigma Aldrich, which was
diluted to a concentration of 6 M using sterile water obtained
residue was recrystallized using CH2Cl2, yielding 4 (150 mg;
0.4 mmol, 30%) as grey crystals. 1H-NMR (DMSO-d6): d: 0.92
from Fresenius Kabi. Additional chemicals were obtained from
(t, 3H); 1.46–1.56 (m, 2H); 1.64 (s, 3H); 1.76 (s, 3H); 2.29–2.32 (m,
various commercial sources and were, whenever possible, of
3H); 2.60 (d, 1H); 2.88 (m, 2H); 3.12 (m, 3H); 6.47 (s, 1H); 6.68 (d,
analytical grade. Target product [11C]methane was made using a
1H); 6.76 (d, 1H); 7.28 (s, 1H); 9.24 (s, 1H). MS: ESI [MH1] Calcd:
GEMS PETtrace cyclotron at the Karolinska University Hospital,
352.19, found: 352.14.
Stockholm, Sweden, using 16.4-MeV protons in the 14N(p,a)11C
reaction on nitrogen gas containing 10% H2. The target gas was
(R)-(À)-2-Hydroxy-10,11-methylenedioxy-N-n-propylnoraporphine
irradiated for 10 min with a beam intensity of 35 mA.
(10)
The radiosynthesis and purification of [11C]2 were performed
in a fully automated system in which [11C]methyl iodide was
Compound 10 was synthesized according to the method of
Ramsby et al. and Gao et al.6,7 1 TFA-salt (5.0 g; 11.7 mmol) was
used as starting material. The product was converted into an
HCl-salt with 2 M HCl in diethyl ether. Recrystallization from
MeOH yielded 10 HCl-salt (2.2 g; 6.1 mmol, 52%) as grey crystals.
1H-NMR and MS analysis yielded results that agreed with
synthesized from [11C]methane by gas-phase iodination.13
Subsequently, the [11C]methyl iodide was transformed on-line
into [11C]methyl triflate by means of a heated silver triflate
column.14 [11C]2 was purified with a semi-preparative reversed-
phase HPLC system, containing a Waters XBridges C-18 column
(250 Â 10 mm, 5 mm) and an absorbance detector (l = 300 nm) in
series with a GM tube for radiation detection. The mobile phase
consisted of acetonitrile and sodium acetate buffer (100 mM, pH
4.7) containing 0.1% ascorbic acid (22:78 v/v), which was used at
a flow rate of 5 ml/min. The radiochemical purity of [11C]2 was
analysed by reversed-phase HPLC using a Waters m-Bondapak C-
18 column (300 Â 3.9 mm, 10 mm) and an absorbance detector
(l = 270 nm) in series with a Beckman 170 b-flow radio detector
for radiation detection. The mobile phase consisted of
acetonitrile and phosphoric acid (10 mM) (25:75 v/v) used at a
flow rate of 3 ml/min. [11C]2 was identified by coinjection with
unlabeled reference standard 2.
1
previously reported data. H-NMR (DMSO-d6 1 CD3OD): d: 1.00
(t ,3H); 1.82 (m, 2H); 2.86–2.97 (m, 2H); 3.16 (m, 2H); 3.36 (m, 2H);
3.50 (m, 2H); 3.80 (d, 1H); 4.40 (d, 1H); 6.08 (s, 1H); 6.20 (s, 1H);
6.63 (s, 1H); 6.84–6.91 (m, 2H); 7.47 (s, 1H). MS: ESI [MH1] Calcd:
324.16, found: 324.12.
(R)-(À)-2-Methoxy-10,11-methylenedioxy-N-n-propylnoraporphine
(11)
10 HCl-salt (1.65 g; 4.6 mmol) was dissolved in acetonitrile/
methanol (70 ml; 50:50 v/v). To the solution were added di-
isopropylethylamine (DIPEA; 3 ml) and 2 M trimethylsilyl-diazo-
methane in hexane (10 ml). The reaction mixture was stirred at
room temperature. After 12 h, more 2 M trimethylsilyl-diazo-
methane in hexane (8 ml) was added to the reaction mixture,
which was stirred at room temperature for another 24 h. The
solvents were evaporated off under reduced pressure. The
residue was purified by flash chromatography (SiO2, eluent
CH2Cl2 gradient up to 2% MeOH). Evaporation of the solvents
under reduced pressure yielded yellow oil (1.5 g). This product
was treated with 2 M HCl in diethyl ether, yielding 11 HCl-salt
(1.5 g; 4.0 mmol, 88%) as an off-white solid. 1H-NMR and MS
analysis gave results that agreed with previously reported data.7
1H-NMR (DMSO-d6 1 CD3OD): d: 1.02 (t, 3H); 1.85 (m, 2H);
2.90À3.20 (m, 3H); 3.40 (m, 2H); 3.52 (m, 2H); 3.80 (s, 3H); 3.83
(m, 1H); 4.46 (d, 1H); 6.07 (s, 1H); 6.20 (s, 1H); 6.84–6.92 (m, 3H);
7.54 (s, 1H). MS: ESI [MH1] Calcd: 338.18, found: 338.14.
(R)-(À)-2-[11C]Methoxy-N-n-propylnorapomorphine ([11C]2)
[11C]Methyl triflate was trapped at room temperature in a vessel
containing 4 (0.3 mg), acetonitrile (100 ml) and sodium hydroxide
(0.5 M, 4 ml). Subsequently, hydrochloric acid (6 M, 150 ml) was
added to the reaction mixture. The vessel was heated to 1501C
for 8 min, after which a sodium acetate solution (2 M, 1 ml) was
added to the crude reaction mixture. This was followed by
injection into the semi-preparative HPLC column. After [11C]2
eluted (8–9 min) and the fraction was collected, the mobile
phase was evaporated off. The residue was dissolved in sterile
phosphate-buffered saline (pH 7.4, 7 ml). The product was then
filtered through a sterile millipore filter (0.22 mm), yielding a
sterile solution free of pyrogens. Irradiation of the target gas for
10 min with a beam intensity of 35 mA yielded more than 1 GBq
of 499% radiochemical pure [11C]2, with a specific radioactivity
of 9017342 GBq/mmol (n = 10) at the time of injection. LC-MS/
MS: m/z: 326, 267, 249, 238, 221, 193 and 72.
(R)-(À)-2-Methoxy-N-n-propylnorapomorphine (MNPA, 2)
Compound 2 was synthesized according to the method of Gao
et al. and Teitel et al.7,9 Crude 2 (900 mg) was purified with HPLC
and converted into an HCl-salt with 2 M HCl in diethyl ether.
Subsequent recrystallization using methanol and diethyl ether,
LC-MS/MS analysis
1
A Waters Acquity ultra performance LCTM system (Waters,
Milford, MA, USA) was used for pumping the mobile phase,
injection of the samples and heating of the column. The mobile
phase consisted of 0.1% formic acid plus 5% acetonitrile in water
(A) and 0.1% formic acid in acetonitrile (B). The pumps were
yielded 2 (460 mg; 1.3 mmol) as off-white crystals. H-NMR and
MS analysis yielded results that agreed with previously reported
data.7 1H-NMR (DMSO-d6): d: 0.99 (t, 3H); 1.81 (m, 2H); 2.87
(t, 1H); 2.96 (m, 1H); 3.13 (m, 1H); 3.28–3.55 (m, 4H); 3.74 (m, 1H);
3.77 (s, 3H); 4.22 (m, 1H); 6.68 (d, 1H); 6.73 (s, 1H); 6.79 (d, 1H);
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J. Label Compd. Radiopharm 2009, 52 158–165

C. Steiger et al.
programmed to deliver a linear gradient running from 5 to 80% technical assistance. Dr Victor W. Pike was supported by the
B in A, from 0 to 2.50 min, at a flow rate of 0.3 ml/min. The Intramural Research Program of NIH (NIMH).
composition was returned to 5% B in A from 2.50 to 2.60 min.
The end time of the program was set at 5 min. The column, a
Waters Acquity UPLCTM BEH C18 (50 mm  2.1 mm, 1.7 mm) was
References
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mode, with the following settings: capillary voltage 3.0 kV; cone
voltage 40 V; source temperature 801C; desolvation temperature
1801C and collision energy ramp ranging from 5 to 25 V.
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J. Label Compd. Radiopharm 2009, 52 158–165
Copyright r 2009 John Wiley & Sons, Ltd.
www.jlcr.org