[11C]Dimebon, radiosynthesis and lipophilicity of a new potential PET agent for imaging of Alzheimer's disease and Huntington's disease

Article abstract of DOI:10.1016/j.bmcl.2010.02.094

[¹¹C]Dimebon (2-[¹¹C]methyl-8-methyl-5-(2-(6-methylpyridin-3-yl)ethyl)-2,3 ,4,5-tetrahydro-1H-pyrido[4,3-b]indole), a new potential PET agent for imaging of Alzheimer's disease and Huntington's disease, was prepared by N-[¹¹C]methylation of desmethyl-Domebon precursor with [¹¹C]CH₃OTf and purified with a semi-preparative HPLC method in 30-40% decay corrected radiochemical yield and 222-296 GBq/μmol specific activity at EOB. The measured lipophilicity coefficient (Log P) value of [¹¹C]Dimebon was 2.53.

Full text of DOI:10.1016/j.bmcl.2010.02.094

Bioorganic & Medicinal Chemistry Letters 20 (2010) 2529–2532
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
[11C]Dimebon, radiosynthesis and lipophilicity of a new potential PET agent
for imaging of Alzheimer’s disease and Huntington’s disease
*
Mingzhang Gao, Min Wang, Gary D. Hutchins, Qi-Huang Zheng
Department of Radiology and Imaging Sciences, Indiana University School of Medicine, 1345 West 16th Street, Room 202, Indianapolis, IN 46202, USA
a r t i c l e i n f o
a b s t r a c t
11C]Dimebon (2-[11C]methyl-8-methyl-5-(2-(6-methylpyridin-3-yl)ethyl)-2,3,4,5-tetrahydro-1H-pyr-
Article history:
[
Received 11 February 2010
Revised 23 February 2010
Accepted 25 February 2010
Available online 3 March 2010
ido[4,3-b]indole), a new potential PET agent for imaging of Alzheimer’s disease and Huntington’s disease,
was prepared by N-[11C]methylation of desmethyl-Domebon precursor with [11C]CH3OTf and purified
with a semi-preparative HPLC method in 30–40% decay corrected radiochemical yield and 222–
296 GBq/
bon was 2.53.
l
mol specific activity at EOB. The measured lipophilicity coefficient (Log P) value of [11C]Dime-
Keywords:
Ó 2010 Elsevier Ltd. All rights reserved.
[
11C]Dimebon
Radiosynthesis
Lipophilicity
Positron emission tomography (PET)
Alzheimer’s disease
Huntington’s disease
Dimebon
(2,8-dimethyl-5-(2-(6-methylpyridin-3-yl)ethyl)-
prepared as outlined in Scheme 1 according to the published pro-
2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole), as indicated in Figure
1, is an orally-available, small molecule antihistamine drug that
has been developed and used in Russia since 1983.1–7 This old drug
has been recently proposed to be useful for treating different types
of schizophrenia and various neurodegenerative disorders such as
Alzheimer’s disease (AD) and Huntington’s disease (HD),6–9 since
Dimebon was discovered to inhibit enzymes butyrylcholinesterase
(BChE) and acetylcholinesterase (AChE),10 and mitochondrial per-
cedures with modifications.2–5 4-Methyl-N-(2-(6-methylpyridin-
11CH3
N
CH3
H3C
H3C
N
N
N
meability transition pore opening, to block the N-methyl-D-aspar-
tate (NMDA) receptor signaling pathway, and to display
neuroprotective effects in models for AD and HD.8 Carbon-11-la-
beled Dimebon may serve as a new radioligand for the biomedical
imaging technique positron emission tomography (PET), and en-
able non-invasive monitoring of enzyme such as BChE and AChE
and receptor such as NMDA expression in AD and HD, and AD
and HD response to Dimebon treatment using PET. To facilitate
imaging AD and HD and monitoring the therapeutic efficacy of
the drug Dimebon as a novel treatment for AD and HD, we have de-
signed and synthesized [11C]Dimebon (2-[11C]methyl-8-methyl-5-
(2-(6-methylpyridin-3-yl)ethyl)-2,3,4,5-tetrahydro-1H-pyrido[4,3-
b]indole) (Fig. 1) as a new potential PET agent, for the first time.
The Dimebon standard compound (7b) and its desmethylated
precursor desmathyl-Dimebon (8-methyl-5-(2-(6-methylpyridin-
3-yl)ethyl)-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole, 7a) were
N
N
CH3
CH3
Dimebon, 7b
[
11C]Dimebon, [11C]7b
O
CH3
N
N
11CH3
N
S
11
O
CH
3
N
H
O
HO
[
11C]PIB
[
11C]PBR28
* Corresponding author. Tel.: +1 317 278 4671; fax: +1 317 278 9711.
E-mail address: qzheng@iupui.edu (Q.-H. Zheng).
Figure 1. Chemical structures of Dimebon and
11C]PBR28.
[ [
11C]Dimebon, 11C]PIB and
[
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.02.094

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M. Gao et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2529–2532
H3C
i
H3C
NH
N
N
NH2
3
1
CH3
CH3
2
ii
NH2
NO
H3C
N
iii
H3C
N
N
N
O
5
4
CH3
CH3
R
H3C
N
iv
N
R
6
N
N
N
R
H3C
N
iv
N
N
CH3
CH3
7a, R=H, desmethyl-Dimebon
7b, R=CH3, Dimebon
7c, R=Bn
H
N
Bn
H3C
H3C
N
N
N
v
N
N
CH3
7a
CH3
7c
Scheme 1. Synthesis of desmethyl-Dimebon and Dimebon. Reagents, conditions, and yields: (i) Na, 100 °C, 78%; (ii) HCl, NaNO2, 5 °C, 85%; (iii) LiAlH4, THF, reflux, 73%; (iv)
HAc, reflux, 61–70%; (v) Pd/C, HCO2NH4, MeOH, reflux, 80%.
11CH3
N
H
3-yl)ethyl)aniline (3) was prepared by the condensation of two
H3C
H3C
N
commercially available starting materials p-toluidine (1) and 2-
methyl-5-vinylpyridine (2) under sodium catalysis in 78% yield.
This reaction failed either under acidic catalyst or without catalyst.
Nitration of compound 3 with hydrochloric acid and sodium nitrite
under 5 °C easily provided N-(2-(6-methylpyridin-3-yl)ethyl)-N-
(p-tolyl)nitrous amide (4) in 85% yield. Reduction of compound 4
with LiAlH4 gave 2-methyl-5-(2-(1-(p-tolyl)hydrazinyl)ethyl)pyri-
dine (5) in 73% yield. Several other reduction reagents were tested
in this reaction, including Zn/HAc,4 NaBH4 and Pd/C–H2. NaBH4 and
Pd/C–H2 failed to produce compound 5, and Zn/HAc2 just produced
compound 3 instead of compound 5. Condensation and cyclization
of compound 5 with various 1-substituted piperidin-4-one (6) was
carried out at reflux temperature in 80% HAc to afford desmethyl-
Dimebon precursor (7a), Dimebon (7b) and 2-benzyl-8-methyl-5-
(2-(6-methylpyridin-3-yl)ethyl)-2,3,4,5-tetrahydro-1H-pyrido[4,3-
b]indole (7c) in 61–70% yield. This was a one-pot two-step
reaction. We used 80% HAc instead of benzene and HCl/EtOH in
the literature procedure.2 This modification significantly improved
the work-up procedures. In order to obtain significant amount of
precursor for radiolabeling, an alternate synthetic approach for
desmethyl-Dimebon was developed as shown in Scheme 1. Deben-
N
N
i
N
N
CH3
11C]Dimebon, [11C]7b
CH3
desmethyl-Dimebon, 7a
[
Scheme 2. Synthesis of [11C]Dimebon. Reagents, conditions, and yields: (i) NaH,
11C]CH3OTf, CH3CN, 80 °C, 3 min, 30–40%.
[
zylation of compound 7c with Pd/C and ammonium formate affor-
ded 7a in 80% yield.
Synthesis of the target tracer [11C]Dimebon ([11C]7b) is indicated
in Scheme 2. Desmethyl-Dimebon (7a) was labeled using
[
11C]methyl triflate ([11C]CH3OTf)11,12 through N-[11C]methyla-

M. Gao et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2529–2532
2531
tion13 under basic conditions (NaH) and isolated by a semi-prepara-
bon was labeled with
a
reactive
[
11C]methylating agent
tive HPLC method14 to produce the corresponding pure radiolabeled
compound [11C]7b in 30–40% radiochemical yield, decay corrected
toendofbombardment(EOB), basedon[11C]CO2. Theradiosynthesis
was performed in an in-house automated multi-purpose 11C-radio-
synthesis module, allowing measurement of specific radioactivity
during synthesis.15,16 The overall synthesis, purification and formu-
lation time was 25–30 min from EOB. The specific radioactivity was
[
11C]CH3OTf, and isolated by a semi-preparative HPLC purification
procedure to provide [11C]Dimebon in high radiochemical yields,
short overall synthesis time, and good specific radioactivity. The
Log P value of [11C]Dimebon was calculated, and the data indicated
[
11C]Dimebon has suitable lipophilicity as a candidate brain radio-
ligand. These results facilitate the potential preclinical and clinical
PET studies of [11C]Dimebon in animals and humans.
in a range of 222–296 GBq/lmol at EOB. Chemical purity and radio-
chemical purity were determined by analytical HPLC.17 The chemi-
cal purity of the precursor and reference standard was >97%. The
radiochemical purity of the target tracer was >99% determined by
radio-HPLC through c-ray (PIN diode) flow detector, and the chem-
ical purity of the target tracer was >95% determined by reverse-
Acknowledgments
This work was partially supported by the Indiana Genomics Ini-
tiative (INGEN) of Indiana University, which is supported in part by
Lilly Endowment Inc. The authors would like to thank Dr. Bruce H.
Mock and Barbara E. Glick-Wilson for their assistance in produc-
tion of [11C]CH3OTf. 1H NMR spectra were recorded on a Bruker
Avance II 500 MHz NMR spectrometer in the Department of Chem-
istry and Chemical Biology at Indiana University Purdue University
Indianapolis (IUPUI), which is supported by a NSF-MRI grant CHE-
0619254. The referees’ criticisms and editor’s comments for the
revision of the manuscript are greatly appreciated.
phase HPLC through UV flow detector.
The measured HPLC lipophilicity coefficient (octanol–water
partition coefficient, Log P) is an important parameter in selecting
PET brain ligand candidates (small organic molecules) for further
evaluations. The ability of a brain radioligand to penetrate the
blood brain barrier (BBB) could be due, at least in part, to its lipo-
philicity, and thus we were interested in identifying an analogue
that was significantly lipophilic brain tracer. We calculated Log P
values of [11C]Dimebon in comparison with two other brain imag-
ing agents [11C]PIB and [11C]PBR28 (detecting AD amyloid plaques
and translocator protein formerly known as the peripheral benzo-
diazepine receptor, respectively, in the living human brain,
Fig. 1)18–21 based on their retention times that were measured by
C-18 HPLC method.22–24 Briefly, the chromatographic capacity fac-
References and notes
1. Bachurin, S.; Bukatina, E.; Lermontova, N.; Tkachenko, S.; Afanasiev, A.;
Grigoriev, V.; Grigorieva, I.; Ivanov, Y.; Sablin, S.; Zefirov, N. Ann. N.Y. Acad.
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3. Shadurskii, K. S.; Danusevich, I. K.; Kost, A. N.; Vinogradova, E. V. U.S. Patent No.
1138164, 1985.
tors (k0) were measured by reversed-phase HPLC. k0 = (tR ꢀ tR0)/tR0
,
4. Vinogradova, E. V.; Daut, K.; Kost, A. N.; Terent’ev, A. P. Zhurnal Obshchei Khim.
1962, 32, 1550.
5. Wollowitz, S.; Kataisto, E. W. WO Patent No. 111540, 2009.
6. Ivachtchenko, A. V.; Frolov, E. B.; Mitkin, O. D.; Tkachenko, S. E.; Okun, I. M.;
Khvat, A. V. Bioorg. Med. Chem. Lett. 2010, 20, 78.
7. Ivachtchenko, A. V.; Frolov, E. B.; Mitkin, O. D.; Kysil, V. M.; Khvat, A. V.; Okun, I.
M.; Tkachenko, S. E. Bioorg. Med. Chem. Lett. 2009, 19, 3183.
8. Doody, R. S.; Gavrilova, S. I.; Sano, M.; Thomas, R. G.; Aisen, P. S.; Bachurin, S. O.;
Seely, L.; Hung, D.Dimebon investigators Lancet 2008, 372, 207.
9. Wu, J.; Li, Q.; Bezprozvanny, I. Mol. Neurodegen. 2008, 3, 15.
10. Wang, M.; Wang, J.-Q.; Gao, M.; Zheng, Q.-H. Appl. Radiat. Isot. 2008, 66, 506.
11. Jewett, D. M. Int. J. Radiat. Appl. Instrum. A 1992, 43, 1383.
12. Mock, B. H.; Mulholland, G. K.; Vavrek, M. J. Nucl. Med. Biol. 1999, 26, 467.
13. Zheng, Q.-H.; Mulholland, G. K. Nucl. Med. Biol. 1996, 23, 981.
14. Wang, M.; Lacy, G.; Gao, M.; Miller, K. D.; Sledge, G. W.; Zheng, Q.-H. Bioorg.
Med. Chem. Lett. 2007, 17, 332.
15. Mock, B. H.; Zheng, Q.-H.; DeGrado, T. R. J. Labelled Compd. Radiopharm. 2005,
48, S225.
16. Mock, B. H.; Glick-Wilson, B. E.; Zheng, Q.-H.; DeGrado, T. R. J. Labelled Compd.
Radiopharm. 2005, 48, S224.
17. Zheng, Q.-H.; Mock, B. H. Biomed. Chromatogr. 2005, 19, 671.
18. Klunk, W. E.; Engler, H.; Nordberg, A.; Wang, Y.; Blomqvist, G.; Holt, D. P.;
Bergstrom, M.; Savitcheva, I.; Huang, G.-F.; Estrada, S.; Ausen, B.; Debnath, M.
L.; Barletta, J.; Price, J. C.; Sandell, J.; Lopresti, B. J.; Wall, A.; Koivisto, P.; Antoni,
G.; Mathis, C. A.; Langstrom, B. Ann. Neurol. 2004, 55, 306.
where tR is the compound’s retention time, and tR0 is retention of
an unretained substance determined by injection of an aqueous
solution of potassium nitrite (tR0 = 1.84 min). Log P measurement
is based on the linear relationship, which has been established be-
tween the Log k0 values of most compounds and their Log P values.
Four compounds, benzyl alcohol (Log P 1.16), acetophenone (Log P
1.66), toluene (Log P 2.74) and naphthalene (Log P 3.37), were cho-
sen as a ‘standard’ calibration mixture for the evaluation of the
Log P’s of unknowns. Log Pexp = partition coefficient calculated
from k0 value and calibration curve established by these four com-
pounds. Calibration equation: Log P = 2.222 Log k0 + 1.915. Reten-
tion times in the analytical HPLC system for
11C]PIB, and [11C]PBR28 were 5.32, 4.97, and 4.35 min, respec-
tively. The calculation results showed the Log P values for
11C]Dimebon, [11C]PIB, and [11C]PBR28 were 2.53, 2.43, and 2.21,
[
11C]Dimebon,
[
[
respectively. The Log P value of [11C]Dimebon is higher than that
of either [11C]PIB or [11C]PBR28. The results indicate [11C]Dimebon
is more lipophilic than both [11C]PIB and [11C]PBR28. The calcu-
lated Log P values from ChemDraw Ultra 9.0 (ChemOffice 2005)
for Dimebon, PIB, and PBR28 are 3.48, 3.41, and 2.98, respectively.
Obviously, the measured Log P data sequence of [11C]Dimebon,
19. Wang, M.; Gao, M.; Mock, B. H.; Miller, K. D.; Sledge, G. W.; Hutchins, G. D.;
Zheng, Q.-H. Bioorg. Med. Chem. 2006, 14, 8599.
20. Brown, A. K.; Fujita, M.; Fujimura, Y.; Liow, J. S.; Stabin, M.; Ryu, Y. H.;
Imaizumi, M.; Hong, J.; Pike, V. W.; Innis, R. B. J. Nucl. Med. 2007, 48, 2072.
21. Wang, M.; Yoder, K. K.; Gao, M.; Mock, B. H.; Xu, X.-M.; Saykin, A. J.; Hutchins,
G. D.; Zheng, Q.-H. Bioorg. Med. Chem. Lett. 2009, 19, 5636.
[
11C]PIB, and [11C]PBR28 is consistent with the calculated Log P
data sequence of Dimebon, PIB, and PBR28. The data suggest
11C]Dimebon has similar lipophilicity to [11C]PIB. Therefore, we
22. Haky, J. E.; Young, A. M. J. Liq. Chromatogr. 1984, 7, 675.
23. Fei, X.; Zheng, Q.-H. J. Liq. Chromatogr. Related Technol. 2005, 28, 939.
24. Gao, M.; Mock, B. H.; Hutchins, G. D.; Zheng, Q.-H. Nucl. Med. Biol. 2005, 32, 543.
25. (a) General: All commercial reagents and solvents from Sigma–Aldrich, Fisher
Scientific and ChemPacific Corporation were used without further purification.
[
assume the tracer [11C]Dimebon has suitable ability to pass the
BBB, and is a promising candidate as potential brain imaging agent.
The evaluation of Dimebon in AD and HD represents new appli-
cations of a known drug. However, the synthetic information of
Dimebon was limited in the literature. Thus, the experimental de-
tails and characterization data for compounds 3–5 and 7a–c and
for the tracer [11C]7b are given.25
[
11C]CH3OTf was prepared according to a literature procedure.12 Melting points
were determined on
a MEL-TEMP II capillary tube apparatus and were
uncorrected. 1H NMR spectra were recorded on Varian Gemini 2000 200 MHz
FT-NMR and Bruker Avance II 500 MHz NMR spectrometers using
tetramethylsilane (TMS) as an internal standard. Chemical shift data for the
proton resonances were reported in parts per million (ppm, d scale) relative to
internal standard TMS (d 0.0), and coupling constants (J) were reported in hertz
(Hz). Low resolution mass spectra (LRMS) were obtained using a Bruker Biflex III
MALDI-Tof mass spectrometer, and high resolution mass spectra (HRMS) were
In conclusion, improved and efficient syntheses of Dimebon and
its precursor desmethyl-Dimebon (new compound for carbon-11
labeling) have been developed. An automated multi-purpose 11C-
radiosynthesis module of our own design for fully automated syn-
thesis of [11C]Dimebon has been built, featuring the measurement
of specific activity by the on-the-fly technique. Desmethyl-Dime-
obtained using
a Thermo MAT 95XP-Trap spectrometer. Chromatographic
solvent proportions are indicated as volume: volume ratio. Thin-layer
chromatography (TLC) was run using Analtech silica gel GF uniplates (5 ꢁ 10
cm2). Plates were visualized under UV light. Normal phase flash column

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M. Gao et al. / Bioorg. Med. Chem. Lett. 20 (2010) 2529–2532
chromatography was carried out on EM Science Silica Gel 60 (230–400 mesh)
purified by silica gel column chromatography using eluent (MeOH/CH2Cl2 3–
20%) to give product 7a–c (61–70%). 8-Methyl-5-(2-(6-methylpyridin-3-
yl)ethyl)-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole (7a, desmethyl-Dimebon).
Yellow solid, mp: 93–95 °C; Rf = 0.18(MeOH/CH2Cl2 1:4); 1HNMR (CDCl3)d:1.98
(s, 1H, NH), 2.31 (t, J = 6.0 Hz, 2H, CH2), 2.45 (s, 3H, CH3), 2.50 (s, 3H, CH3), 2.96 (t,
J = 7.0 Hz, 2H, CH2), 3.08 (t, J = 6.0 Hz, 2H, CH2), 4.01(s, 2H, CH2), 4.17(t, J = 7.0 Hz,
2H, CH2), 6.99–7.00 (m, 2H, Ar–H), 7.02 (dd, J = 2.0, 8.0 Hz, 1H, Ar–H), 7.17 (d,
J = 8.0 Hz, 1H, Ar–H), 7.22 (s, 1H, Ar–H), 8.19 (s, 1H, Ar–H). MS (ESI): 306 ([M+H]+,
100%). HRMS (CI), calcd for C20H23N3 (M+), 305.1886, found 305.1894; calcd for
C20H24N3 ([M+H]+), 306.1965, found 306.1964. 2,8-Dimethyl-5-(2-(6-
with a forced flow of the indicated solvent system in the proportions described
below. All moisture- and/or air-sensitive reactions were performed under a
positive pressure of nitrogen maintained by a direct line from a nitrogen source.
Analytical HPLC was performed using a Prodigy (Phenomenex) 5
column, 4.6 ꢁ 250 mm; 3:1:4 CH3CN:MeOH:20 mM, pH 6.7 phosphate (buffer
solution) mobile phase; flow rate 1.5 mL/min; and UV (254 nm) and -ray (PIN
diode) flow detectors. Semi-preparative HPLC was performed using a Prodigy
(Phenomenex) 5
m C-18 column, 10 ꢁ 250 mm; 3:1:4 CH3CN:MeOH:20 mM,
pH 6.7 phosphate (buffer solution) mobile phase; flow rate 5.0 mL/min; and UV
(254 nm) and -ray (PIN diode) flow detectors. Sterile Millex-GS 0.22 m vented
filter unit was obtained from Millipore Corporation, Bedford, MA; (b) 4-Methyl-
N-(2-(6-methylpyridin-3-yl)ethyl)aniline (3). To stirred mixture of new
distilled 2-methyl-5-vinylpyridine (31.2 g, 262 mmol) and p-toluidine
lm C-18
c
l
c
l
methylpyridin-3-yl)ethyl)-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole
(7b,
Dimebon). Yellow solid, mp: 115–116 °C; Rf = 0.34 (MeOH/CH2Cl2 1:4); 1H
NMR (CDCl3) d: 2.44 (s, 3H, CH3), 2.49 (m, 2H, CH2), 2.50 (s, 3H, CH3), 2.51 (s, 3H,
CH3), 2.71 (t, J = 5.5 Hz, 2H, CH2), 2.95 (t, J = 7.0 Hz, 2H, CH2), 3.64 (s, 2H, CH2),
4.17 (t, J = 7.0 Hz, 2H, CH2), 6.97–6.99 (m, 2H, Ar–H), 7.05 (dd, J = 2.0, 8.0 Hz, 1H,
Ar–H), 7.13 (d, J = 8.0 Hz, 1H, Ar–H), 7.20 (s, 1H, Ar–H), 8.23 (s, 1H, Ar–H). MS
(ESI): 320 ([M+H]+, 100%). HRMS (ESI), calcd for C21H26N3 ([M+H]+), 320.2127,
a
2
1
(33.6 g, 314 mmol), small pieces of Na (0.90 g, 39.2 mmol) was added. Then
thereactionmixture washeatedto100 °Cfor7 h. After thereactionwascooled to
room temperature, absolute EtOH (30 mL) was added followed by ice-water
(60 mL). The mixture was extracted with ether (3 ꢁ 150 mL), washed with brine,
and dried over Na2SO4. The solvent was removed by distillation. The residue was
purified by distillation in vacuum to obtain a white solid product 3 (55.0 g, 78%).
Rf = 0.30 (MeOH/CH2Cl2 1:19); bp: 179–181 °C/3 Torr; mp: 64–65 °C; 1H NMR
(CDCl3) d: 2.17 (s, 1H, NH), 2.24 (s, 3H, CH3), 2.53 (s, 1H, 3H, CH3), 2.86 (t,
J = 7.0 Hz, 2H, CH2), 3.36 (t, J = 7.0 Hz, 2H, CH2), 6.55 (d, J = 8.0 Hz, 2H, Ph–H), 6.98
(d, J = 8.0 Hz, 2H, Ph–H), 7.08 (d, J = 8.0 Hz, 1H, Py–H), 7.41 (dd, J = 2.0, 8.0 Hz, 1H,
Py–H), 8.35 (d, J = 2.0 Hz, 1H, Py–H). MS (ESI): 227 ([M+H]+, 100%); (c) N-(2-(6-
Methylpyridin-3-yl)ethyl)-N-(p-tolyl)nitrous amide (4). To a solution of 1 N HCl
(87 mL) and EtOH (96 mL) was added compound 3 (16.95 g, 75 mmol). The
mixture was stirred at room temperature until complete solution was obtained,
and cooled to 0 °C by ice bath. A solution of NaNO2 (6.21 g, 90 mmol) in water
(36 mL) was added dropwise with stirring while the temperature was kept under
5 °C. The reaction mixture was stirred under 5 °C for 2 h, and allowed to stir for
another 3 h at room temperature. Then the mixture was again cooled to 0 °C for
1 h. The mixture was filtered. The filter solid was washed with cold water and
allowed to dry on air for 36 h to give a yellow solid product 4 (16.3 g, 85%).
Rf = 0.40 (MeOH/CH2Cl2 1:19); mp: 73–75 °C; 1H NMR (CDCl3) d: 2.39 (s, 3H,
CH3), 2.51 (s, 1H, 3H, CH3), 2.80 (t, J = 8.0 Hz, 2H, CH2), 4.20 (t, J = 8.0 Hz, 2H, CH2),
7.07 (d, J = 7.5 Hz, 1H, Py–H), 7.25–7.43 (m, 4H, Ph–H), 7.41 (dd, J = 2.5, 7.5 Hz,
1H, Py–H), 8.28 (d, J = 2.5 Hz, 1H, Py–H). MS (ESI): 256 ([M+H]+, 100%); (d) 2-
Methyl-5-(2-(1-(p-tolyl)hydrazinyl)ethyl)pyridine (5). A solution of compound
4 (5.11 g, 20 mmol) in anhydrous THF (50 mL) was added to LiAlH4 (1.52 g,
40 mmol) in anhydrous THF (100 mL), and the reaction mixture was refluxed for
3 h. The solvent was partially removed by distillation and excess LiAlH4 was
destroyed by the careful addition of ice-water. The mixture was filtered through
Celite and washed with ether. The filtered solution was extracted with ether
(3 ꢁ 100 mL), washed with brine, dried over MgSO4, and concentrated. The
residue was purified by silica gel column chromatography using eluent (MeOH/
CH2Cl2 3%) to afford a brown liquid product 5 (3.51 g, 73%). Rf = 0.76 (MeOH/
CH2Cl2 1:9); 1H NMR (CDCl3) d: 2.11 (s, 3H, CH3), 2.25 (s, 2H, NH2), 2.52 (s, 1H, 3H,
CH3), 2.83 (t, J = 7.5 Hz, 2H, CH2), 3.58 (t, J = 7.5 Hz, 2H, CH2), 6.69 (d, J = 8.0 Hz,
2H, Ph–H), 7.02 (d, J = 8.0 Hz, 2H, Ph–H), 7.06 (d, J = 8.0 Hz, 1H, Py–H), 7.42 (dd,
J = 2.0, 8.0 Hz, 1H, Py–H), 8.33 (d, J = 2.0 Hz, 1H, Py–H). MS (ESI): 242 ([M+H]+,
100%); (e) General procedure for preparation of compounds 7a–c. The mixture of
1-substituted piperidin-4-one 6 (2.0 mmol) and 30 mL of 80% HOAc (24 mL of
HOAc and 6 mL of H2O) was stirred at room temperature under N2 for 2 h and
heated to reflux for 2 h. Then the mixture was concentrated under reduced
pressure, added 1 N NaOH to adjust pH to 9, extracted with EtOAc, washed with
brine, and dried over Na2SO4. The solvent was evaporated, and the residue was
found
320.2111.
2-Benzyl-8-methyl-5-(2-(6-methylpyridin-3-yl)ethyl)-
2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole (7c). Brown liquid, Rf = 0.69 (MeOH/
CH2Cl2 1:4); 1H NMR (CDCl3) d: 2.41 (t, J = 5.5 Hz, 2H, 2H, CH2), 2.42 (s, 3H, CH3),
2.51 (s, 3H, CH3), 2.74 (t, J = 5.5 Hz, 2H, CH2), 2.95 (t, J = 7.0 Hz, 2H, CH2), 3.68 (s,
2H, CH2), 3.73 (s, 2H, CH2), 4.15 (t, J = 7.0 Hz, 2H, CH2), 6.97–6.99 (m, 2H, Ar–H),
7.02 (dd, J = 2.0, 8.0 Hz, 1H, Ar–H), 7.14–7.16 (m, 2H, Ar–H), 7.25–7.28 (m, 1H,
Ar–H), 7.31–7.34 (m, 2H, Ar–H), 7.37 (d, J = 7.0 Hz, 2H, Ph–H), 8.23 (d, J = 2.0 Hz,
1H, Py–H). MS (ESI): 396 ([M+H]+, 100%); (f) Alternate synthetic procedure for
compound 7a. Compound 7c (0.395 g, 1.0 mmol) was dissolved in MeOH
(25 mL), Pd/C (10%, 90 mg) and dried HCO2NH4 (0.315 g, 5.0 mmol) were
added. The resulting mixture was stirred and heated to reflux for 3 h. After
additional amount of HCO2NH4 (0.189 g, 3.0 mmol) was added, the mixture was
refluxed for another 2 h. Then the mixture was cooled down to room
temperature, it was filtered through Celite. The solvent was removed, and the
residue was purified by column chromatography using eluent (MeOH/CH2Cl2 5%
to 20%) to afford product 7a (0.245 g, 80%). The analytical data were obtained as
same as above; (g) Production of the tracer [11C]Dimebon ([11C]7b). [11C]CO2 was
produced by the 14N(p, 11C nuclear reaction in small volume (9.5 cm3)
a)
aluminum gas target (CTI) from 11 MeV proton cyclotron on research purity
nitrogen (+1% O2) in a Siemens radionuclide delivery system (Eclipse RDS-111).
In a small reaction vial (5 mL), the precursor 7a (0.3–0.5 mg) was dissolved in
CH3CN (300 lL). To this solution was added NaH (1 mg). No carrier-added (high
specific activity) [11C]CH3OTf that was produced by the gas-phase production
method12 from [11C]CO2 through [11C]CH4 and [11C]CH3Br with silver triflate
(AgOTf) column was passed into the reaction vial at room temperature, until
radioactivity reached a maximum (ꢂ2 min), and then the reaction vial was
isolated and reacted at 80 °C for 3 min. The contents of the reaction vial were
diluted with NaHCO3 (1 mL, 0.1 M), and injected onto the semi-preparative HPLC
column with 2 mL injection loop for purification. The product fraction was
collected, the solvent was removed by rotatory evaporation under vacuum, and
the final product, [11C]Dimebon ([11C]7b), was formulated in saline, sterile-
filtered through a sterile vented Millex-GS 0.22 lm cellulose acetate membrane,
and collected into a sterile vial. Total radioactivity was assayed and total volume
was noted for dose dispensing. The overall synthesis, purification and
formulation time was 25–30 min from EOB. Retention times in the analytical
HPLC were: tR 7a = 2.78 min, tR 7b = 5.32 min, tR
times in the semi-preparative HPLC were: tR 7a = 6.35 min, tR 7b = 8.67 min, tR
11C]7b = 8.67 min. The radiochemical yields were 30–40% decay corrected to
EOB, based on [11C]CO2.
[
11C]7b = 5.32 min. Retention
[