Potential CRF1R PET imaging agents: N-Fluoroalkyl-8-(6-methoxy- 2-methylpyridin-3-yl)-2,7-dimethyl-N-alkylpyrazolo[1,5-a][1,3,5]triazin-4-amines

Article abstract of DOI:10.1016/j.bmcl.2011.02.050

A series of N-fluoroalkyl-8-(6-methoxy-2-methylpyridin-3-yl)-2,7-dimethyl- N-alkylpyrazolo[1,5-a][1,3,5]triazin-4-amines were prepared and evaluated as potential CRF₁R PET imaging agents. Optimization of their CRF ₁R binding potencies and octanol-phosphate buffer phase distribution coefficients resulted in discovery of analog 7e (IC₅₀ = 6.5 nM, log D = 3.5).

Full text of DOI:10.1016/j.bmcl.2011.02.050

Bioorganic & Medicinal Chemistry Letters 21 (2011) 2484–2488
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Potential CRF1R PET imaging agents: N-Fluoroalkyl-8-(6-methoxy-2-methyl-
pyridin-3-yl)-2,7-dimethyl-N-alkylpyrazolo[1,5-a][1,3,5]triazin-4-amines
Dmitry Zuev a, , Ronald J. Mattson a, , Hong Huang a, Gail K. Mattson a, Larisa Zueva b, Julia M. Nielsen b,
Edward S. Kozlowski a, Xiaohua Stella Huang a, Dedong Wu a, , Qi Gao a, Nicholas J. Lodge a,
Joanne J. Bronson a, John E. Macor a
⇑
⇑
a Bristol-Myers Squibb Research and Development, 5 Research Parkway, Wallingford, CT 06492, USA
b Spectrix Analytical Services, LLC, PO Box 234, Middletown, CT 06457, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of N-fluoroalkyl-8-(6-methoxy-2-methylpyridin-3-yl)-2,7-dimethyl-N-alkylpyrazolo[1,5-a][1,3,-
5]triazin-4-amines were prepared and evaluated as potential CRF1R PET imaging agents. Optimization of
their CRF1R binding potencies and octanol–phosphate buffer phase distribution coefficients resulted in
discovery of analog 7e (IC50 = 6.5 nM, log D = 3.5).
Received 28 December 2010
Revised 9 February 2011
Accepted 14 February 2011
Available online 17 February 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Corticotropin-releasing factor
Imaging agents
Positron emission tomography
Anxiety
Depression
Corticotropin-releasing factor (CRF), a 41-amino acid neuro-
peptide produced by hypothalamic nuclei in brain, plays a central
role in the coordination of neuroendocrine, autonomic, behavioral
and immune responses to stress.1,2 Secreted from hypothalamus in
response to acute physical or psychological stress, CRF activates
the transcription of the pro-opiomelanocortin gene, resulting in
the secretion of adrenocorticotropin hormone (ACTH) from the
pituitary gland. In turn, ACTH enters the circulation and stimulates
the release of cortisol, the principal primate adrenal steroid hor-
mone, from the adrenal gland. To restore the homeostasis of the
hypothalamic-pituitary-adrenal (HPA) axis, cortisol exerts nega-
tive feedback control on the secretion of CRF in the hypothalamus.3
One hypothesis is that severe or prolonged stress results in an
uncontrolled secretion of CRF and a long-term activation of the
HPA axis, which may lead to a variety of stress-related illnesses,
such as anxiety, depression, obsessive–compulsive and posttrau-
matic stress disorders.4a–c
selective CRF1R antagonists may be useful for the treatment of the
psychiatric disorders. In the past decade and a half, a number of
potent nonpeptide CRF1R antagonists have been reported, reflect-
ing significant efforts of many research groups in this area.6,7
In light of this progress, the development of a selective CRF1R
Positron Emission Tomography (PET) radioligand could provide
scientists with a powerful tool both to assess receptor occupancy
in clinical trials of CRF1R receptor antagonists, and to monitor
changes in CRF concentration in normal and diseased patients.
However, the discovery of such a ligand has proved to be
challenging.
In 2000, Rice8 published the synthesis of unlabelled fluorinated
pyrrolo[2,3-d]pyrimidines as high-affinity potential CRF1R PET li-
gands 1 (Fig. 1). A year later, Martarello et al.9 described the radio-
synthesis, in vitro binding studies and in vivo rat tissue
distribution of [18F] FBPPA 2, the first reported CRF1R PET ligand.
Although the initial level of accumulation of radioactivity of 2 in
the rat pituitary was fairly high (5.59%), the ligand exhibited fast
washout (79.2% after 60 min). Very poor levels and retention of
radioactive ligand were displayed by the hypothalamus, amygdala
and cerebellum, which are the brain areas rich in CRF1 receptor
sites. The authors speculated that the high lipophilicity of 2
(c log P >6) and/or the insolubility of the radiopharmaceutical in
blood could account for its exceedingly low blood–brain barrier
penetration. In 2003, Eckelman10 disclosed the synthesis of
CRF mediates its function through the CRF1 and CRF2 receptor
subtypes,5 of which the CRF1 receptor appears to play a significant
role in the stress-related responses. It has been hypothesized that
⇑
Corresponding authors. Fax: +1 203 677 7702.
E-mail addresses: dmitry.zuev@bms.com (D. Zuev), ronald.mattson@bms.com
(R.J. Mattson).
Present address: AstraZeneca PLP, 1800 Concord Pike, Wilmington, DE 19850,
USA.
[
76Br]-MJL-1-109-2 3,
a high affinity CRF1R PET radioligand
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.02.050

D. Zuev et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2484–2488
2485
18F
O
O
O
N
Cl
N
R
N
F
N
N
N
N
N
N
N
N
HN
N
N
N
N
N
N
N
a
b
N
N
Cl
N
N
O
O
76Br
18F] FBPPA
[
76Br]-MJL-1-109-2
8
9 100%
[
R = Et, Pr, Bu
2
1
3
N
N
OH
N
OMs
n
n
11
CH3
N
N
O
O
N
N
N
N
N
N
N
O
c
d
N
N
N
N
N
N
N
N
N
N
N
N
N
O
O
N
11CH3
N 11
N
10a n = 0 78%
10b n = 1 83%
11a n = 0
11b n = 1
O
CH3
11C] DMP696
[
11C] SN003
[
11C] R121920
[
N
N
N
N
4
5
6
n
4
N
N
18
N
N
N
N
Figure 1.
2
X
X
(Ki ꢀ 2 nM) with appropriate lipophilicity (c log P = 3). In rat bio-
distribution studies, this compound proved to be able to penetrate
the blood–brain barrier with cerebellum and cortex uptakes of
0.29 0.01% ID/g and 0.32 0.03% ID/g after 30 min. More recently,
N
N
X = OMs or Cl
O
O
12a n = 0 64%
12b n = 1 43%
13
Dileep Kumar and Sullivan evaluated [11C]SN003 4,11 11C]R121920
[
5 and [11C]DMP696 612 in baboons. The lack of detectable specific
binding in each study was attributed to the lower density of CRF1R
receptors in primate brain as compared to rat or human brain. This
group also determined that all three radioligands underwent rapid
metabolism in baboon, with compounds 5 and 6 showing just 50%
of the parent molecule remaining after 9 and 13 min, respectively.
In the course of our search for a suitable CRF1 PET radioligand,
we viewed the amino side chain of pyrazolo[1,5-a][1,3,5]triazin-
4-amines13 as suitable site for the introduction of a radiolabel, par-
ticularly an 18F isotope. We also believed that the introduction of a
Scheme 1. Reagents and conditions: (a) POCl3, DIPEA, toluene, 115 °C, 4 h, 100%;
(b) 2-(ethylamino)ethanol or 3-(ethylamino)propanol, THF, rt, overnight; (c) MsCl,
Et3N, CH2Cl2, rt, overnight; (d) spontaneous cyclization, rt.
desired fluoroethylamines 7a and 7c. Instead, 11a and 11b under-
went spontaneous intramolecular cyclization to form pyrazolo
[1,5-a][1,3,5]triazinium salts 12a and 12b. While the cyclization
of 11a to 12a was almost instantaneous, the conversion of 11b to
12b proceeded slower, as was observed by the LCMS method.15
We initially attempted to determine the structure of 12a by
X-ray crystallographic analysis, however all our efforts to crystal-
lize this salt were unsuccessful. Thus, we took advantage of a
proton–carbon correlation HMBC NMR experiment to elucidate
the structure of the molecule. Indeed, the observed C–H long range
correlations between H18 and C2 as well as H18 and C4 (Scheme 1)
were consistent with structure 12a, not structure 13, indicating
that the cyclization took place via the nitrogen atom N3 of the
triazine ring.
polar 8-(6-methoxy-2-methylpyridin-3-yl) substituent and
a
methoxy- or a cyano-group in the amino side chain could provide
compounds with reduced lipophilicity. In the present publication,
we describe the synthesis of analogs 7a–e, and their evaluation
as potential radioligands by optimization of CRF1R binding poten-
cies and phase distribution coefficients (Fig. 2).
Synthesis of fluorinated amines 7a–e commenced from known
pyrazolo[1,5-a][1,3,5]triazin-4(3H)-one 8,14 which was converted
to chloride 9 by reaction with phosphorus oxychloride and DIPEA
in toluene (Scheme 1). Subsequent treatment of 9 with 2-(ethyl-
amino)ethanol and 3-(ethylamino)propanol afforded amines 10a
and 10b in 78% and 83% respective yields. We anticipated that
the conversion of 10a and 10b to mesylates 11a and 11b, followed
by a fluoride displacement of the mesyl group would provide
Treatment of 12a and 12b with potassium fluoride in ethylene
glycol or TBAF in THF did not result in conversion to 7a and 7c,
which necessitated introduction of the fluorine earlier in the
synthesis.
2-Fluoroethylamine hydrochlorides 14a–b were prepared by
a
literature method,16 and 3-fluoropropylamines 15a–c were
prepared by reaction of 1-bromo-3-fluoropropane with the corre-
sponding ethylamine hydrochlorides. Fluoroalkylamines 7a–e
were ultimately synthesized by reaction of chloride 9 with 14a–b
and 15a–c (Scheme 2).
We found that upon concentration of the acetonitrile–
water–TFA fractions of purified 2-fluoroethylamines 7a and 7b,
partial N3-cyclization (9–12%) to the corresponding pyrazolo-
[1,5-a][1,3,5]triazinium salts took place. In contrast, fluoropropyl
derivatives 7c–e were stable to the solvent evaporation conditions
and were isolated without any noticeable cyclization. The CRF1R
X
N
N
F
n
N
7a n = 0, X = H
7b n = 0, X = OCH3
7c n = 1, X = H
7d n = 1, X = OCH3
7e n = 1, X = CN
N
N
N
O
Figure 2.

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D. Zuev et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2484–2488
F
X
a
CN
CN
F
Br
N
H
F
N
H
H2N
hydrochloride
hydrochloride
15c
14a X = H
14b X = OCH3
NC
Cl
N
N
F
a
X
X
F
Br
F
N
H
H2N
N
N
N
N
N
N
hydrochloride
b
15a X = H
N
15b X = OCH3
15c X = CN
N
N
X
O
O
Cl
N
N
N
F
n
N
N
7a 32%
7b 15%
7c 100%
7d 79%
7e 47%
7e
N
N
N
N
9
b
Scheme 3. Reagents and conditions: (a) Ag2CO3, sulfolane, 172 °C, 10 min; (b) 15c,
DIPEA, THF, rt, 5 min, 45% for 2 steps.
N
N
O
O
developed an appropriately rapid synthesis of the unlabelled mol-
ecule (Scheme 3). Amine 7e was prepared in two steps in 45%
overall yield by initial alkylation of 3-aminopropanenitrile hydro-
chloride with 1-bromo-3-fluoropropane in the presence of silver(I)
carbonate in sulfolane at 172 °C, followed by a reaction of the
resulting amine 15c with chloride 9 and DIPEA in THF at room tem-
perature. This two step sequence was complete within 15 min,
with no aqueous work-up required, and the product was quickly
purified by a reverse-phase preparative HPLC method.20 The prep-
aration of [18F]-1-bromo-3-fluoropropane, a proposed starting
material for the radiosynthesis, has been previously reported.21
To determine if we could reduce the lipophilicity of 7d
while retaining its high binding affinity, we prepared a series of
8-heteroarylsubstituted analogs 21a–c (Scheme 4). Treatment of
9
Scheme 2. Reagents and conditions: (a) DIPEA, DMF, 60 °C, 3 days; (b) amine
hydrochlorides 14a–b or amines 15a–c, DIPEA, THF, rt, overnight.
Table 1
CRF1R binding affinities and phase distribution coefficients of amines 7a–e
X
N
N
F
n
N
N
N
N
O
O
H
H
H
N
N
N
a
b
H2N
Compound
n
X
IC50, nM
c log P
log D
N
HN
N
N
N
HN
7a
7b
7c
7d
7e
0
0
1
1
1
H
OCH3
H
OCH3
CN
5.8
5.5
1.6
1.9
6.5
3.0
2.5
3.3
2.8
2.5
4.5
4.6
4.8
4.1
3.5
N
Br
16
Br
17
Br
18
O
N
N
F
O
N
Cl
N
N
N
N
d
c
N
N
HN
N
N
N
binding affinities of 7a–e were determined by measuring the inhi-
bition of specific binding of [125I]-o-CRF in a CRF1 receptor binding
assay using rat frontal cortex homogenate.17 The calculated log P
values were obtained using the in-house computer program. The
log D values of 7a–e were determined by a classical octanol-phos-
phate buffer (pH ꢀ 7.4) partitioning of the compounds.18 Despite
relatively good correlation, the calculated log P values were about
1–2 logarithmic units lower than the corresponding log D values.
These sets of data are presented in Table 1.
Unsubstituted ethylamines 7a and 7c displayed high binding
affinity to the CRF1 receptor. The introduction of a methoxy-group
(as in 7b and 7d) did not significantly affect the binding affinity
values. Within the fluoropropyl series, cyano-derivative 7e was
somewhat less potent than 7c and 7d, but still had reasonable
potency. In addition, the presence of a polar cyano-group in 7e
lowered the log D by 1.3 units compared to 7c, and 0.6 units com-
pared to 7d. Thus, in the series, analog 7e offered a good compro-
mise between binding potency and lipophilicity.19
Br
Br
Br
20
18
19
O
N
N
F
N
X
e
N
N
B(OH)2
R
R
N
X
N
O
22a-c
O
21a X = CH, R = H
21b X = N, R = H
21c X = N, R = OCH3 11%
17%
47%
Scheme 4. Reagents and conditions: (a) CH3C(NH)OC2H5, AcOH–CH3CN, 97%; (b)
(C2H5O)2CO, C2H5ONa, C2H5OH, 75%; (c) POCl3, DIPEA, toluene, 115°C, 5h, 100%; (d)
15b, DIPEA, THF, rt, 45 min, 31%; (e) Pd(PPh3)2Cl2, Et3N, DME–H2O–EtOH, micro-
wave, 95 °C, 45 min.
Since the preparation of radiolabelled 7e must be carried out
within the half-life of
a fluorine-18 isotope (110 min), we

D. Zuev et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2484–2488
2487
Acknowledgement
The authors are grateful to Dr. Paul J. Gilligan and Dr. Douglas
D. Dischino for useful discussions. The authors thank Dr. James
P. Sherbine for providing them with a sample of 8-(6-methoxy-
2-methylpyridin-3-yl)-2,7-dimethylpyrazolo[1,5-a][1,3,5]triazin-
4(3H)-one.
References and notes
1. Owens, M.; Nemeroff, C. B. Pharmacol. Rev. 1991, 43, 425.
2. Grigoriadis, D. E.; Haddach, M.; Ling, N.; Saunders, J. Curr. Med. Chem., CNS
Agents 2001, 1, 63.
3. Plotsky, P. M. J. Neuroendocrinol. 1991, 3, 1.
4. (a) Banki, C. M.; Karmasci, L.; Bissette, G.; Nemeroff, C. B. Eur.
Neuropsychopharmacol. 1992, 2, 107; (b) Holsboer, F. J. Psychiatric Res. 1999,
33, 181; (c) Kaskow, J. W.; Baker, D.; Geracioti, T. D. Peptides 2001, 22, 845.
5. Takahashi, L. K. Neurosci. Behav. Rev. 2001, 25, 627.
Figure 3.
6. Dzierba, C. D.; Hartz, R. A.; Bronson, J. J. Annu. Rep. Med. Chem. 2008, 43, 3.
7. Gilligan, P. J.; Robertson, D. W.; Zaczek, R. J. Med. Chem. 2000, 43, 1641.
8. Hsin, L.-W.; Webster, E. L.; Chrousos, G. P.; Gold, P. W.; Eckelman, W. C.;
Contoreggi, C.; Rice, K. C. Bioorg. Med. Chem. Lett. 2000, 10, 707.
9. Martarello, L.; Kilts, C. D.; Ely, T.; Owens, M. J.; Nemeroff, C. H.; Camp, M.;
Goodman, M. M. Nucl. Med. Biol. 2001, 28, 187.
10. Jagoda, E.; Contoreggi, C.; Lee, M.-J.; Kao, C.-H. K.; Szajek, L. P.; Listwak, S.; Gold,
P.; Chrousos, G.; Greiner, E.; Kim, B. M.; Jacobson, A. E.; Rice, K. C.; Eckelman,
W. J. Med. Chem. 2003, 46, 3559.
11. Dileep Kumar, J. S.; Majo, V. J.; Sullivan, G. M.; Prabhakaran, J.; Simpson, N. R.;
Van Heertum, R. L.; Mann, J. J.; Parsey, R. V. Bioorg. Med. Chem. 2006, 14, 4029.
12. Sullivan, G. M.; Parsey, R. V.; Dileep Kumar, J. S.; Arango, V.; Kassir, S. A.; Huang,
Y.; Simpson, N. R.; Van Heertum, R. L.; Mann, J. J. Nucl. Med. Biol. 2007, 34, 353.
13. Gilligan, P. J.; Clarke, T.; He, L.; Lelas, S.; Li, Y.-W.; Heman, K.; Fitzgerald, L.;
Miller, K.; Zhang, G.; Marshall, A.; Krause, C.; McElroy, J. F.; Ward, K.; Zeller, K.;
Wong, H.; Bai, S.; Saye, J.; Grossman, S.; Zaczek, R.; Hartig, P.; Robertson, D.;
Trainor, G. J. Med. Chem. 2009, 52, 3084.
Table 2
CRF1R binding affinities and phase distribution coefficients of amines 7d and 21a–c
O
N
N
F
N
X
N
N
R
N
O
14. Gilligan, P. J. 4-(2-Butylamino)-2,7-dimethyl-8-(2-methyl-6-methoxypyrid-3-
yl)[1,5-a]-1,3,5-triazine, its enantiomers and pharmaceutically acceptable salts
as a corticotropin releasing factor Receptor ligands, US 7157578 B2 (2007).
15. Analytical LCMS conditions: column PHENOMENEX–LUNA 4.6 mm  50 mm S5;
Compound
X
R
IC50, nM
log D
7d
CH
CH
N
CH3
H
H
1.9
35
390
>10,000
4.1
3.4
3.2
1.8
injection volume, 5 lL; solvent A, CH3OH (10%)/H2O (90%)/TFA (0.1%); solvent
21a
21b
21c
B, CH3OH (90%)/H2O (10%)/TFA (0.1%); starting% B, 0; final% B, 100; flow rate
5 mL/min; gradient time, 2 min; end time, 3 min.
16. Dubowchik, G. M.; Michne, J. A.; Zuev, D. Bioorg. Med. Chem. Lett. 2004, 14,
3147.
N
OCH3
17. CRF1R binding assay: Frozen rat frontal cortex (source of CRF1 receptor) was
thawed rapidly in assay buffer containing 50 mM HEPES (pH 7.0 at 23 °C)
10 mM MgCl2, 2 mM EGTA, 1 lg/mL aprotinin, 1 lg/mL leupeptin, 1 lg/mL
pepstatin A, 0.005% Triton X-100, 10 U/mL bacitracin and 0.1% ovalbumin and
homogenized. The suspension was centrifuged at 32,000 g for 30 min. The
resulting supernatant was discarded and the pellet resuspended by
homogenization in assay buffer and centrifuged again. The supernatant was
discarded and the pellet resuspended by homogenization in assay buffer and
frozen at À70 °C. On the day of the experiment aliquots of the homogenate
4-bromo-3-methyl-1H-pyrazol-5-amine 16 with ethyl acetoimi-
date in acetonitrile-acetic acid provided acetamidine 17 in 97%
yield. Reaction of 17 with ethyl carbonate provided 8-bromo-2,
7-dimethylpyrazolo[1,5-a][1,3,5]triazin-4-ol 18 in 75% yield.
The structure of the molecule was confirmed by an X-ray crys-
tallographic analysis (Fig. 3).22 Treatment of 18 with phosphorus
oxychloride, followed by amination of resulting chloride 19 with
3-fluoro-N-(2-methoxyethyl)propan-1-amine 15b provided 20 in
31% yield. Analogs 21a–c were obtained by a palladium catalyzed
microwave-assisted coupling of 20 with boronic acids 22a–c.23
Binding affinities of analogs 21a–c and their phase distribution
coefficients are summarized in Table 2.
The introduction of polar groups or atoms in the 8-(6-methoxy-
2-methylpyridin-3-yl) ring of 7d led to a dramatic decrease in
potency. This observation was in agreement with the known
preference of the CRF1 receptor for lipophilic ligands. The presence
of at least one ortho-substituent at the ring was important for high
activity of the compounds. Particularly, analog 7d was 18-fold
more potent than 21a.
were thawed quickly and homogenate (25
lg/well rat frontal cortex) added to
ligand (150 pM 125I-o-CRF) and drugs in a total volume of 100
l
L of assay
buffer. The assay mixture was incubated for 2 h at 21 °C. Bound and free
radioligand were then separated by rapid filtration, using glass fiber filters
(Whatman GF/B, pretreated with 0.3% PEI) on a Brandel Cell Harvester. Filters
were then washed multiple times with ice cold wash buffer (PBS w/o Ca2+ and
Mg2+, 0.01% Triton X-100 (pH 7.0 at 23 °C)). Nonspecific binding was defined
using 1
lM DMP696. Filters were then counted in a Wallac Wizard gamma
counter.
Materials: Rat frontal cortex was obtained from Analytical Biological Services,
Inc. (Willmington, DE). 125I-o-CRF (2200 Ci/mmol) was obtained from
PerkinElmer Life Sciences, Inc. (Boston, MA).
18. Shake-flask log D determination assay for lipophilicity: In a 250 mL separatory
funnel were added 25 mM phosphate buffer (200 mL) and octanol (10 mL). The
two-phase system was mixed well and let stand overnight to allow complete
saturation and separation of both layers. A sample (1.0 mg) was dissolved in
octanol (1 mL) and transferred to a volumetric flask, containing 50 mL of the
phosphate buffer, saturated with octanol, as described above. The resulting
mixture was shaken intensely for 30–40 min and was allowed to stand until
two layers separated completely. A sample of each layer was analyzed by an
HPLC method twice. The sample area counts were used to calculate the shake-
flask log D.
In conclusion, we prepared
a series of N-fluoroalkyl-8-
(6-methoxy-2-methylpyridin-3-yl)-2,7-dimethyl-N-alkylpyraz-
olo[1,5-a][1,3,5]triazin-4-amines as potential CRF1R PET imaging
agents. Analog 7e (IC50 = 6.5 nM, log D = 3.5) was found to possess
a good balance between binding potency and lipophilicity. Its
calculated log P value (2.3) is lower than the c log P values of refer-
ence compounds 1–6.24 The radiosynthesis and biodistribution
studies of 7e will be the subject of a future disclosure.
19. All new compounds gave satisfactory analytical data. For 7e: 1H NMR (CD3OD,
500 MHz) 7.91 (d, J = 8.5 Hz, 1H), 7.11 (d, J = 8.5 Hz, 1H), 4.68 (t, J = 6.0 Hz, 1H),
4.59 (t, J = 6.0 Hz, 1H), 4.47 (br s, 2H), 4.33 (br s, 2H), 4.12 (s, 3H), 3.11 (t,
J = 6.5 Hz, 2H), 2.49 (s, 3H), 2.40 (s, 3H), 2.33 (s, 3H), 2.27 (m, 2H). Mass spec.:
Calcd 398.20; found 398.40 (MH+).
20. Preparative HPLC conditions: column XTERRA 30 mm  150 mm S5; injection
volume, 2000 lL; solvent A, CH3OH (10%)/H2O (90%)/TFA (0.1%); solvent B,

2488
D. Zuev et al. / Bioorg. Med. Chem. Lett. 21 (2011) 2484–2488
CH3OH (90%)/H2O (10%)/TFA (0.1%); starting% B, 30; final% B, 100; flow rate
25 mL/min; gradient time, 20 min; end time, 25 min.
22. The X-ray crystal structure of 18 was deposited with Cambridge
Crystallographic Data Centre (deposition number—CCDC 810575).
21. Kämäräinen, E.-L.; Kyllönen, T.; Airaksinen, A.; Lundkvist, C.; Yu, M.; Någren,
K.; Sandell, J.; Langer, O.; Vepsäläinen, J.; Hiltunen, J.; Bergström, K.; Lötjönen,
S.; Jaakkola, T.; Halldin, C. J. Labelled Compd. Radiopharm. 1995, 37, 55.
23. Heck, R. F. Palladium Reagents in Organic Synthesis; Academic: London, 1987.
24. The log D values of compounds 1–6 had not been reported and, therefore, were
unavailable for comparison with the log D value of 7e.