Synthesis and structure-affinity relationships of new 4-(6-iodo-H- imidazo[1,2-a]pyridin-2-yl)-N-dimethylbenzeneamine derivatives as ligands for human β-amyloid plaques

Article abstract of DOI:10.1021/jm0702231

A new and extensive set of 4-(6-iodo-H-imidazo[1,2-a]pyridin-2-yl)-N- dimethylbenzeneamine (IMPY) derivatives was synthesized and assayed for affinity toward human Aβ plaques. 6-Ethylthio- (12h), 6-cyano-(12e), 6-nitro- (12f), and 6-p-methoxybenzylthio- (15d) analogues were discovered to have high affinity (K_I < 10 nM). However, introduction of a hydrophilic thioether group in the 6-position (15a-c, 15e-g) reduced or abolished affinity. In secondary N-methyl analogues, a bromo substituent in the adjacent ring position (14a) imparted high affinity (K_I = 7.4 nM) whereas a methyl substituent did not (14c). The tolerance for nonhydrophilic thioether substituents in the 6-position opens up the possibility of developing new sensitive positron emission tomography radioligands for imaging human Aβ plaques in Alzheimer's disease, especially in view of the amenability of thioethers to be labeled with carbon-11 or fluorine-18 through S-alkylation reactions. The structure-activity relationships revealed in this study extends insight into the topography of the binding site for IMPY-like ligands in human Aβ plaques.

Full text of DOI:10.1021/jm0702231

4746
J. Med. Chem. 2007, 50, 4746-4758
Synthesis and Structure-Affinity Relationships of New
4-(6-Iodo-H-imidazo[1,2-a]pyridin-2-yl)-N-dimethylbenzeneamine Derivatives as Ligands for
Human â-Amyloid Plaques
Lisheng Cai,*,† Jessica Cuevas,† Sebastian Temme,† Mary M. Herman,‡ Claudio Dagostin,§ David A. Widdowson,§
Robert B. Innis,† and Victor W. Pike†
Molecular Imaging Branch and Clinical Brain Disorders Branch, National Institute of Mental Health, National Institutes of Health, Bethesda,
Maryland 20892, and Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London, U.K.
ReceiVed February 26, 2007
A new and extensive set of 4-(6-iodo-H-imidazo[1,2-a]pyridin-2-yl)-N-dimethylbenzeneamine (IMPY)
derivatives was synthesized and assayed for affinity toward human Aâ plaques. 6-Ethylthio- (12h), 6-cyano-
(12e), 6-nitro- (12f), and 6-p-methoxybenzylthio- (15d) analogues were discovered to have high affinity (KI
< 10 nM). However, introduction of a hydrophilic thioether group in the 6-position (15a-c, 15e-g) reduced
or abolished affinity. In secondary N-methyl analogues, a bromo substituent in the adjacent ring position
(14a) imparted high affinity (KI ) 7.4 nM) whereas a methyl substituent did not (14c). The tolerance for
nonhydrophilic thioether substituents in the 6-position opens up the possibility of developing new sensitive
positron emission tomography radioligands for imaging human Aâ plaques in Alzheimer’s disease, especially
in view of the amenability of thioethers to be labeled with carbon-11 or fluorine-18 through S-alkylation
reactions. The structure-activity relationships revealed in this study extends insight into the topography of
the binding site for IMPY-like ligands in human Aâ plaques.
Introduction
Alzheimer’s disease (ADa) is characterized by several fea-
tures, including cerebral plaques formed from amyloid â-peptide
(Aâ), neuronal loss, and intracellular deposits denoted as
neurofibrillary tangles, an aggregated form of hyperphospho-
rylated forms of the microtubule-associated tau protein.1 A major
contemporary hypothesis for the biochemical changes underlying
the progression of AD is the “â-amyloid cascade hypothesis”2
in which brain Aâ peptide is processed through various stages
to form the plaques eventually responsible for the morbid clinical
symptoms of the disease. An ability to detect this process in
human brain in vivo at an early stage, ideally preceding the
presentation of clinical symptoms, would be a valuable tool for
Figure 1. Four radioligands for imaging Aâ plaques in clinical
research.
testing the underlying hypothesis and also for monitoring
because animal models of AD, for example, those using
transgenic mice that express various genes for overexpression
of Aâ, do not necessarily present Aâ plaques with the same
potential therapeutic interventions.
Three radioligands for positron emission tomography (PET)3-5
and one for single photon emission computed tomography
molecular properties and density of ligand binding sites as those
(SPECT)6 imaging of Aâ plaques have reached the clinical
stage: ([11C]2-(4-(methylamino)phenyl)benzo[d]thiazol-6-ol ([11C]-
PIB), [11C](E)-4-(4-(methylamino)styryl)phenol ([11C]SB-13),
[18F]2-(1-(6-((2-fluoroethyl)(methyl)amino)naphthalen-2-yl)eth-
ylidene)malononitrile ([18F]FDDNP), and [123I]12a ([123I]IMPY)
(Figure 1). The development of such radioligands is challenging
found in human AD subjects.7 These models therefore turn out
to have limited value in the PET or SPECT radioligand
development process.
It is generally considered that candidate radioligands for Aâ
plaques should have high affinity with KD below 20 nM,8 and
the radioligands shown in Figure 1 meet this criterion. Generally,
the pharmacokinetic properties of high-affinity (KI < 10 nM)
radioligands in normal animals (monkeys, mice, rats) appear
quite predictive of such properties in AD subjects in vivo. Fast
and high entry of radioligand into brain followed by fast washout
is considered desirable, since this should permit long-lasting
strong interactions between radioligand and plaque to be imaged
with high contrast against weaker nonspecific interactions.
Ligand lipophilicity is one important parameter that may
influence brain pharmacokinetics, with ideal lipophilicity usually
lying in the LogP7.4 range 2.5-3.5.9-11 However, ligand brain
pharmacokinetics may also be influenced strongly by the action
of efflux pumps (e.g., P-gp) at the blood-brain barrier and also
by peripheral metabolism and ligand clearance. Ideally, ligands
* To whom correspondence should be addressed. Address: Molecular
Imaging Branch, National Institute of Mental Health, National Institutes of
Health, 10 Center Drive, Building 10, Room B3 C346, Bethesda, Maryland
20892. Phone: (301) 451-3905. Fax: (301) 480-5112. E-mail: cail@-
intra.nimh.nih.gov.
† Molecular Imaging Branch, National Institute of Mental Health.
‡ Clinical Brain Disorders Branch, National Institute of Mental Health.
§ Imperial College of Science, Technology and Medicine.
a Abbreviations: PET, positron emission tomography; AD, Alzheimer’s
disease; SPECT, single photon emission computed tomography; nM, nano
molar; P-gp, P-glycoprotein; SAR, structure-activity relationship; NBS,
N-bromosuccinimide; NCS, N-chlorosuccinimide; cLogP, calculated loga-
rithm of lipophilicity coefficients; ACD, advanced chemistry development;
SD, standard deviation; ppm, parts per million; TLC, thin-layer chroma-
tography; FCC, flash column chromatography; HRMS, high-resolution mass
spectra; SPE, solid-phase extraction; y, year; h, hour.
10.1021/jm0702231 CCC: $37.00 © 2007 American Chemical Society
Published on Web 08/28/2007

Dimethylbenzeneamine DeriVatiVes as Ligands
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 19 4747
ridin-2-yl)-N-methylbenzeneamine], and its 3-fluoropropyl ana-
logue, 18F-labeled FPM-IMPY (Figure 2), as PET radioligands
for Aâ plaques.18 These analogues have somewhat lower affinity
than 12a itself (Table 1). After intravenous injection of either
radioligand into rodent or monkey, there was rapid and high
uptake of radioactivity into the brain followed by biphasic
clearance comprising fast and very slow components. Metabo-
lism was rapid and shown to involve dealkylation of the tertiary
arylamino group plus defluorination, culminating respectively
in residual activity from radiometabolites in brain and high
uptake of radioactivity ([18F]fluoride ion) in bone, including
skull. Both metabolic features would confound attempts to
measure brain radioligand-Aâ binding accurately in vivo. Here,
we report the synthesis and evaluation in vitro of a further
extensive series of 12a derivatives in our continuing effort to
develop more effective PET radioligands for imaging Aâ
plaques in AD patients.
Figure 2. Structures of [18F]FEM-IMPY and [18F]FPM-IMPY.
should not be substrates for brain efflux pumps and should be
cleared rapidly from the periphery. The ratios of the radioactivity
in normal animal brains at 2 and 30 min or between 2 and 60
min have become used as indexes to predict the signal-to-noise
ratio achievable in humans when Aâ plaques are present.8 This
radioactivity should represent the parent radioligand and be
uncontaminated by radiometabolites. [123I]12a is one of few
ligands showing a high ratio of radioactivity between 2 and 30
min (11)12,13 similar to the ratio for [11C]PIB (12), the most
extensively studied radioligand for Aâ plaque imaging.3,14
The study of structure-affinity relationships (SAR) in 12a
analogues is a useful first step for identifying potentially more
sensitive radioligands for imaging Αâ plaques in vivo. Only
limited SAR studies of 12a derivatives have been reported15,16
(for a recent summary of SAR in all classes of Aâ plaque
ligands, see Cai et al.17). Substituents at the 4′- and 6-positions
are critical for maintaining high binding affinity for Aâ
plaques.15 Strong electron-donating groups are needed at the
4′-position. Tertiary arylamino groups are preferred over
secondary amino groups. A 4′-hydroxyl group is not tolerated.18
Heavier halides, such as iodo and bromo, are needed at the
6-position, while electron-donating groups, such as tertiary
amino, are not tolerated. A substituent at the 3-position abolishes
affinity.
Results and Discussion
Chemistry. In all cases the imidazo[1,2-a]pyridine nucleus
was created through condensation of an R-bromoketone with a
2-aminopyridine or 2-aminopyrazine in the manner described
previously for the synthesis of 12a.18 A variety of substituents
are acceptable in either reactant.
Syntheses of r-Bromoketones. R-Bromoketone 4a was
prepared by ortho bromination of 1-(4-methylaminophenyl)-
ethanone (1)18 with N-bromosuccinimide (NBS),19 N-protection
with a trifluoroacetyl group, and finally R-bromination20 with
copper(II) bromide (Scheme 1).
We have previously evaluated the tertiary amines, 18F-labeled
FEM-IMPY [N-(2-fluoroethyl)-4-(6-iodo-H-imidazo[1,2-a]py-
R-Bromoketones 4b and 4c were prepared by protecting the
arylamino groups in 5b and 5c with a trifluoroacetyl group and
Table 1. Binding Affinity (KI Values) of 12a Derivatives (imidazopyridines)
a
ligand
R′
Me
R′′
Me
R3′
R6
R8
cLogD7.4
KI (nM)b
12a (IMPY)
12b
12c
12d
12e
H
H
H
H
H
H
H
H
H
H
H
Br
Me
Me
Br
Me
Me
H
I
Br
Cl
F
CN
NO2
OMe
SEt
Br
Br
OH
Br
H
4.37 ( 0.88
4.11 ( 0.88
3.93 ( 0.84
3.38 ( 0.88
2.80 ( 1.31
3.09 ( 1.30
3.05 ( 1.30
4.24 ( 1.31
5.17 ( 0.93
3.28 ( 1.35
1.26 ( 1.29
4.88 ( 1.16
4.78 ( 1.06
3.44 ( 0.91
4.30 ( 0.94
3.28 ( 0.87
4.16 ( 0.88
1.59 ( 1.33
2.54 ( 1.35
2.02 ( 1.33
4.76 ( 1.35
2.96 ( 1.35
2.12 ( 1.33
3.07 ( 1.35
4.52 ( 0.92
4.90 ( 0.92
8.9 ( 0.7
5.9 ( 0.4
24.2 ( 5.6
13.0 ( 1.6
8.2 ( 1.0
7.6 ( 0.7
38.5 ( 5.0
8.3 ( 0.5
183 ( 61
>180
177 ( 31
>1000
>1000
>1000
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
CF3CO
CF3CO
MeCO
H
H
H
H
H
Me
Me
Me
H
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
H
H
H
H
H
H
H
I
CN
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
12f
12g
12h
12i
12j
12k
13a
13b
13c
14a
14b
14c
15a
15b
15c
15d
15e
Br
Br
Br
Br
Me
Me
H
7.4 ( 0.6
658 ( 47
>1000
Me
Me
Me
Me
Me
Me
Me
Me
(CH2)2F
(CH2)3F
Br
SCH2CONH2
S(CH2)2OH
SCH2CONH2
SCH2C6H4p-OMe
S(CH2)2OH
SCH2CONH2
S(CH2)2OH
I
1840 ( 497
645 ( 75
391 ( 76
8.3 ( 1.8
88 ( 6
H
H
H
H
Me
Me
H
15f
>1000
15g
FEM-IMPY
FPM-IMPY
>1000
31 ( 5
H
I
41 ( 5
a Calculated with ACD/LogD, version 9.02 (Advanced Chemistry Development, Inc., Toronto, Canada). b Measured in triplicate with results given as the
mean ( SD.

4748 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 19
Scheme 1. Synthesis of R-Bromoketonesa
Cai et al.
a Reagents and conditions: (a) NBS, CH2Cl2, room temp, 91%; (b) (CF3CO)2O, Et3N, CH2Cl2, room temp, 96%; (c) CuBr2, EtOAc, reflux, 4 h, 34%; (d)
(RCO)2O, Et3N, CH2Cl2, 90-95% for both 6b and 6c; (e) BrCH2COBr, AlCl3, reflux, overnight, 38% for 4b, 48% for 4c.
Scheme 2. Copper(I) and Ester-Catalyzed Methoxide
Scheme 4. Synthesis of 5-Chloro-2-aminopyrazinea
Substitution
a Reagents and conditions: (a) NCS (0.5 equiv), CHCl3, reflux 2 h, 26%;
(b) NCS (0.6 equiv), CHCl3, microwave, 60 W, 10 min, 70 °C, 45%.
attempted. This method uses a copper(I) salt and a small amount
of ester to form a stabilized tetrahedral adduct that acts as a
powerful methoxide donor (Scheme 2). Unfortunately, because
of the presence of the free amino group in 7a and the use of
ethyl acetate as cocatalyst, the reaction produced the undesired
acetamide 8l (Scheme 3). We considered that the copper(I) had
been inactivated by coordination to the nitrogen atoms of the
substrate. By increasing the number of equivalents of catalyst,
we obtained the expected product 7g in 36% yield after a 14 h
reaction (Scheme 3). A significant amount of starting material
remained together with black polymers. The reaction was
quenched after 14 h to avoid further polymerization and product
degradation. In order to decrease the reaction time, the
transformation in Scheme 3 was also performed with microwave
irradiation but at the cost of lower yield.
A previously reported synthesis of 5-(ethylthio)pyridine-2-
amine 7h26,27 made use of copper as catalyst and methanol as
solvent at 150 °C under pressure.28 The inconvenience of this
procedure prompted us to evaluate other solvents to replace the
low boiling point methanol. Ethylene glycol has been noted to
be beneficial as a solvent in other copper or copper(I) catalyzed
reactions.29 We found ethylene glycol to be an excellent solvent
for the reaction of 7b to give a high yield of 7h (Scheme 3).
The synthesis of 5-chloro-2-aminopyrazine 10 described in
the literature is an inefficient multistep process.30 Direct
chlorination of 2-aminopyrazine with N-chlorosuccinimide
(NCS) in refluxing chloroform gave a black polymer after a
few minutes, with undesired 3,5-dichloro-2-aminopyrazine as
Scheme 3. Synthesis of New 2-Aminopyridine Derivativesa
a Reagents and conditions: (a) For 7g: 7a, RNa, Cu powder, methanol,
135 °C, 14 h, 36%, or RNa, Cu powder, DMF, microwave, 75 W, 140 °C,
30 min, 17%. For 7h: 7b, RNa, Cu powder, ethylene glycol, 150 °C, 26 h,
76%. (b) AcOEt, CuCl, MeONa, MeOH.
acetyl group, respectively, followed by Friedel-Crafts acylation
21-23
with bromoacetyl bromide catalyzed by AlCl3 in CS2
(Scheme 1).
Syntheses of 2-Aminopyridines and 2-Aminopyrazines.
Attempts to prepare 5-methoxypyridin-2-amine (7g) by nucleo-
philic aromatic displacement of a bromo substituent with a
methoxy group, catalyzed conventionally by copper powder24
with microwave heating or with an ultrasound bath, were
frustrated by lack of reactivity in the starting 2-amino-5-
bromopyridine (7a). Another procedure25 for performing aro-
matic substitution with the methoxy anion was therefore

Dimethylbenzeneamine DeriVatiVes as Ligands
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 19 4749
Scheme 5. Synthesis of Tertiary Amino 12a Derivativesa
a Reagents and conditions: (a) NaHCO3, ethanol or acetonitrile, 5-11 h, reflux, yields 23-65%; (b) BBr3, CH2Cl2, room temp for 2 h, 70%.
Scheme 6. Synthesis of Secondary Amino “Isosteres” of 12aa
a Reagents and conditions: (a) NaHCO3, ethanol, 8-12 h, reflux, yields 28-59%; (b) K2CO3 or KOH, EtOH, 8-12 h, reflux, yields 77-80%.
Scheme 7. Synthesis of 6-Thioether 12a Derivatives from 6-Halo-IMPY Derivativesa
a Reagents and conditions: The general conditions are Me3Sn-NMe2, Pd2(dba)3, DiPPF, toluene, and microwave; (a) HSCH2CONH2, 78%; (b)
HSCH2CH2OH, 91%; (c) HSCH2CONH2, 85%; (d) HSCH2C6H4OCH3, 85%; (e) HSCH2CH2OH, 89%; (f) HSCH2CONH2, 69%; (g) HSCH2CH2OH, 69%.
the main product (14%) after 2 h. Only a low yield of 10 (4%)
was isolated. To avoid overchlorination, milder conditions were
examined. No reaction was observed at room temperature, but
even at 50 °C complete transformation into 3,5-dichloro-2-
aminopyrazine occurred. Pretreatment of the solvent, distilled
chloroform, by passage over basic alumina drastically reduced
the formation of black polymers. When NCS was added slowly
into the refluxing 2-aminopyrazine, 10 was isolated in 26%
yield. Moreover, when microwave irradiation was used at
70 °C for 10 min, the yield reached 45% (Scheme 4).
Syntheses of 12a Derivatives. Most of the new 12a deriva-
tives (12a-j, 13a-c) were synthesized through direct conden-
sations of R-bromoketones with 2-amino-pyridines (Scheme 5).18
In examples where a trifluoroacetyl group was used to protect
the arylamino group (13a,b), no base was needed for the
condensation.
The synthesis of the 6-hydroxy compound 12k was through
demethylation of the methoxy group in 12g. The most effec-
tive reagent to perform the reaction was BBr3 in CH2Cl2
(Scheme 5).31
The N-trifluoroacetyl group in 13a,b or acetyl group in 13c
was easily removed under basic conditions (Scheme 6). This
provided an excellent method for the synthesis of 12a derivatives
with a secondary (14a,c) or primary (14b) amino group.
We have recently developed the palladium-catalyzed substitu-
tion of halides by thiolate as a process that avoids halide
reduction.32 This new method was applied to the synthesis of
several 12a derivatives (15a-g) with a thiolate group in the
6-position (Scheme 7).
The imidazopyrazine 12a analogue 17a was synthesized
through condensation of chloropyrazine 10 with the R-bro-
moacetophenone 11 (Scheme 8). Although 10 was very soluble

4750 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 19
Scheme 8. Syntheses of 6-Chloroimino 12a Analoguesa
Cai et al.
well, as reflected in the very close binding affinities of 12a,
12b, and 12h. The polarizability of the sulfur atom is likely
important because previously described 12a derivatives bearing
2-fluoroethyl and 3-fluoropropyl substituents in the 6-position
have a much lower affinity than 12a itself.16
From the standpoint of developing new PET radioligands,
the discovery that even bulky thioether substituents may be
tolerated in the 6-position has considerable importance. A light
alkyl thiolate group imparts hydrophobicity similar to that of
the iodo substituent in 12a (cf. cLogP values for 12a and 12h,
Table 1). Furthermore, a thioalkyl functional group provides a
site for convenient and efficient labeling with a positron emitter,
such as carbon-11 or fluorine-18, for example, by S-alkylation
with such well-known labeling agents as [11C]iodomethane,34,35
[11C]methyl triflate,36 ω-[18F]fluoroalkyl halides,37 or ω-[18F]-
fluoroalkyl tosylates.38
a Reagents and conditions: (a) NaHCO3, MeCN, reflux, 9 h, 7%; (b)
NaHCO3, MeCN-DMF, reflux, 9 h, 23%.
Table 2. Affinity (KI Values) of 12a Derivatives (N for CH Analogues)
Steric bulk in the 6-position is not, however, a requirement
for high affinity, since the 6-fluoro analogue (12d) has only
marginally less affinity than the highest affinity compounds.
Polarizable [e.g., Br (12b), I (12a)] or electron-withdrawing
6-substituents [e.g., F (12d), CN (12e), NO2 (12f)] favor high
affinity, while strongly electron-donating substituents [e.g., OMe
(12g), OH (12k)] reduce affinity, as observed previously for
Me and NMe2 substituents.15 The nitrile 12e is of special interest
with respect to PET radioligand development, since it has a
high affinity similar to that of 12a itself, lower computed
lipophilicity (cLogD7.4, Table 1), and the potential for labeling
with carbon-11, either in the nitrile group with cyclotron-
produced [11C]cyanide39-43 or in the N-methyl position with
[11C]methyl triflate.44
a
compd
cLogD7.4
KI (nM)b
17a
17b
2.54 ( 1.30
3.14 ( 1.43
88 ( 22
147 ( 20
a Calculated with ACD/LogD, version 9.02 (Advanced Chemistry
Development, Inc., Toronto, Canada). b Measured in triplicate with results
given as the mean ( SD.
in acetonitrile, it was very unreactive; 17a was obtained in only
7% yield. Longer reaction times and stronger bases such as
triethylamine failed to improve the yield.
Separation was difficult in the synthesis of the imidazopy-
rimidine 12a analogue 17b. When excess amine was used, the
product could not be separated from the starting amine. When
excess R-bromoketone was used, starting pyrimidine persisted.
A small amount of DMF was added to the reaction mixture to
dissolve the amine completely and to induce precipitation of
the product. By filtering this solution while hot, even before
the reaction was complete, we were able to isolate the desired
17b in a 23% yield (Scheme 8).
In Vitro Assay. Determination of the binding affinities of
new ligands for Aâ plaques is the first step in selecting candidate
radioligands for PET studies in humans. Three types of Aâ
plaques have been used to assay ligand binding affinity in vitro,
namely, (i) synthetic aggregates of Aâ1-40 or Aâ1-42, (ii) Aâ
plaques from transgenic rodents, and (iii) amyloid plaques from
human AD brain tissue. Binding site concentration varies across
the three types of plaques and between different batches of
synthetic aggregates.14 These findings may reflect variations in
binding site architecture, although no significant difference in
ligand binding affinity has been detected between synthetic
aggregates and human Aâ plaques for the binding site typical
of PIB.33 Since our ultimate goal is to develop a sensitive
radioligand for imaging Aâ plaques in the human AD brain, in
vitro evaluation using human AD brain tissue, as described and
used in this study, is clearly the most appropriate.
Generally, the tertiary N,N-dimethylamino analogues of 12a
were found to have a much higher affinity than their secondary
methylamino analogues (cf. 12b vs 14c, 15c vs 15a, 15e vs
15b), in accord with the previously limited data on tertiary and
secondary amino analogues.17 An exceptionally high-affinity
secondary amino analogue was found to be that with a bromo
substituent in the 3′-position (i.e., ortho to the secondary amino
group) (14a). This analogue may be regarded as “isosteric” with
the tertiary amino analogue with hydrogen as the 3′-substituent
(12b). However, the binding affinity is abolished in the
secondary amino analogue bearing a methyl group in the 3′-
position (14c). The detrimental effect of a 3′-methyl group was
repeated in the secondary amino analogues 15f and 15g (cf.
with the corresponding tertiary amines 15c and 15e, respec-
tively). In fact, 15f and 15g had lower affinity than the
corresponding analogues bearing hydrogen in the 3′-position,
namely, 15a and 15b, respectively. Since a methyl group is
almost isosteric with a bromo substituent, the beneficial effect
of the 3′-bromo substituent in 14a must involve one or more
properties other than steric (e.g., substituent electronics or
polarizability). The secondary arylamino group in 14a is
expected to be more robust to oxidative metabolism in vivo
than the tertiary amino group in 12a. Hence, 14a, labeled with
carbon-11 in the N-methyl position, may be of interest for
evaluation as a prospective PET radioligand.
Structure-Affinity Relationships. The binding affinities (KI
values) at human Aâ plaques are listed in Tables 1 and 2 for
12a and its derivatives. Comparison of the KI values in direct
analogues of 12a-h,k and 15c-e in which the 6-substituent
was varied reveals that the binding site in Aâ plaques tolerates
bulky thioether substituents [e.g., SEt (12h), SCH2C6H4-p-OMe
(15d)], provided that they are not hydrophilic [e.g., SCH2-
CONH2 (15c), S(CH2)2OH (15e)]. An alkyl thiolate group
appears to mimic the effect of an iodo or bromo substituent
The effect of a substituent at the 8-position was evaluated in
12i and 12j. Both iodo and cyano substituents reduced the
binding affinity dramatically, indicating low tolerance of the
binding site for even moderately bulky substituents in this
position.
Increasing the size of substituents on the arylamino group
decreases binding affinity, as reflected in 12a, FEM-IMPY, and
FPM-IMPY.18 This shows moderate tolerance for steric bulk at

Dimethylbenzeneamine DeriVatiVes as Ligands
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 19 4751
N-methyl-N-(trimethylstannyl)methanamine, sodium methoxide, and
Pd2dba3 were from Aldrich. 2-Mercaptoacetamide and 6-aminopy-
ridine-3-carbonitrile (7e) were from Acros. 5-Chloropyrimidin-2-
imine (16) was from Alfa Aesar (Ward Hill, MA). 4-(6-Bromo-
H-imidazo[1,2-a]pyridin-2-yl)-N,N-dimethylbenzenamine (12b) was
obtained from Bioassay Systems (Hayward, CA). Water was
purified through a Millipore purification system, comprising two
filters, one Rio, one reservoir, and one Milli-Q synthesis system
(Bedford, MA). Common solvents were obtained from Fisher
Scientific (Pittsburgh, PA). Brain tissue from deceased AD patients
was obtained from the Brain Collection of the Clinical Brain
Disorders Branch, National Institute of Mental Health, National
Institutes of Health (see below).
this location. An amido group at this position as in 13a-c is
not, however, tolerated.
Substitution of aryl CH by an imino group as in 17a and
17b is effective in reducing ligand lipophilicity (compare
cLogD7.4 values with that of the analogue 12c, Table 1) but
drastically reduces binding affinity, regardless of the position
of nitrogen atom insertion.
Binding Site Topography. Knowledge of the topography of
the binding site in Aâ plaques for 12a-like ligands would be
helpful for the design of improved ligands. Although the detailed
structure of Aâ plaques is unknown, a model derived from solid-
state NMR has been proposed.45 Also, computer modeling of
the binding interaction between trans-stilbene derivatives,
similar to SB13, and the Aâ plaque model has been reported
recently.46 A strong quantitative correlation between ligand
binding affinity and molecular properties (HOMO energy,
charge, molecular volume, and cLogP) has been established.
There are multiple binding sites in Aâ plaques,17 and the
computer modeling study only considered one type of binding
site, that for stilbenes. However, the observed SAR of 12a
derivatives has striking similarities with that observed and
computed for the stilbenes. For example, the highest affinity
compounds carry a p-N,N-dimethylamino group. Bulkier amino
groups or those containing ω-fluoroalkyl groups tend to have
lower affinity, as do primary amino groups or nonbasic groups.
This correspondence in SAR supports the notion that 12a and
stilbene-type ligands (e.g., SB13) occupy the same binding site
within the Aâ plaques. By inference, PIB also occupies the same
binding site, since 12a and stilbene derivatives compete with
PIB for binding. According to findings from this and previous
SAR studies, this site appears to be hydrophobic, flat, and
narrow. There is little tolerance for an extra hydrophilic feature
within the molecular core or for substituents that may disrupt
coplanarity. There is a high restriction on the molecular width
of the ligand, since substantial changes in only the 6- or 3′-
position substituents of 12a are tolerated. This study shows that
ligands elongated with hydrophobic 6-substituents bonded to
the molecular core with a polarizable sulfur atom retain high
affinity. This indicates the binding site to be an extensive narrow
channel. The long axis of the ligand most likely aligns with the
long axis of peptide chains, as previously proposed for stilbene
ligands.46 Such a parallel alignment would probably favor π-π
interactions and electron transfer between ligand and plaque.
1-(4-(Methylamino)phenyl)ethanone (1),18 N,2-dimethylbenze-
namine (5c),47,48 2,2,2-trifluoro-N-o-tolylacetamide (6b),49 N-meth-
yl-N-o-tolylacetamide (6c),50 4-bromoacetyl-N,N-dimethylben-
zeneamine (11),18 5-bromo-3-iodopyridin-2-amine (7i),51 2-amino-
5-bromonicotinonitrile (7j),20 IMPY (12a),15 desmethyl-IMPY
(14d),18 and PIB (2-(4′-methylaminophenyl)-6-hydroxybenzothia-
zole)14 were synthesized as reported. [3H]PIB ([N-methyl-3H3]-2-
(4′-methylaminophenyl)-6-hydroxybenzothiazole, 80 Ci per mmol)
was obtained from GE Healthcare (www.customlabelling.com).
Instruments and General Conditions. Analytical HPLC was
performed with a reverse-phase column (X-Terra C18, 5 µm, 10.0
mm × 250 mm; Waters, Milford, MA) eluted with concentrated
ammonia (0.25%) in acetonitrile-water at 6.2 mL/min. The
chromatography system was fitted with an autosampler (System
Gold 508 model, Beckman; Fullerton, CA) and a continuous
wavelength UV-vis detector (System Gold 168 model, Beckman).
For semipreparative HPLC, a Beckman system was fitted with a
manual injector (5 mL injection loop) and a third delivery pump
eluted at 1 mL/min. For reverse-phase semipreparative HPLC, an
X-Terra C18 column (5 µm, 19 mm × 250 mm, Waters) was eluted
with concentrated ammonia (0.25%) in acetonitrile-water at 20
mL/min. Acetonitrile was used as delivery solvent. For normal
phase semipreparative HPLC, a silica gel column (10 µm, 21.2 mm
× 250 mm, Phenomenex; Torrance, CA) was eluted with triethy-
lamine (0.25%) in chloroform-ethyl acetate at 30 mL/min. Ethyl
acetate was used as the delivery solvent. The purity of the
compounds was determined with HPLC monitored for UV absor-
bance at 280 nm (for 12a derivatives) or 254 nm (for other aromatic
compounds) and expressed as an area percentage of all peaks. The
1H and 13C NMR spectra of all compounds were acquired on a
1
1
Jeol GSX 270 (270 MHz H), Bruker DRX 300 (300 MHz H),
Bruker DRX 400 (400 MHz H and 100 MHz 13C), or Bruker
1
AM500 (500 MHz H and 125 MHz 13C) instrument, using the
1
chemical shifts of residual deuterated solvent as the internal
standard. Chemical shift (δ) data for the proton and carbon
resonances were reported in parts per million (ppm) relative to the
internal standard. Thin-layer chromatography (TLC) was performed
with silica gel 60 F254 plates (EM Science), and compounds were
visualized under UV light (250 or 360 nm). Flash column
chromatography (FCC) was performed with a Horizon HPFC
system (Biotage; Charlottesville, VA; column sizes 12 mm × 150
mm, 25 mm × 150 mm, 40 mm × 150 mm) with hexanes-ethyl
acetate mixtures as eluents; all later mentioned solvent proportions
are expressed as volume per volume. Mass spectra were acquired
using either a LCQDECA LC-MS instrument (Thermo Finnigan;
San Jose, CA) fitted with a Luna C18 column (5 µm, 2.0 mm ×
150 mm; Phenomenex) and with elution with a methanol-water
mixture at 150 µL/min or a PolarisQ GC-MS instrument (Thermo
Finnigan; San Jose, CA) equipped with a capillary RTX-5ms
column (30 m × 0.25 mm; flow rate: 1 mL/min, carrier gas: He),
or VG Micromass 7070E and AutoSpec-Q spectrometers. High-
resolution mass spectra (HRMS) were acquired from the Mass
Spectrometry Laboratory, University of Illinois at Urbanas
Champaign (Urbana, IL). Melting points were measured using a
Mel-Temp manual melting point apparatus (Electrothermal, Fisher
Scientific) and are uncorrected. A Discover microwave apparatus
(CEM, Matthews, NC) was used for microwave-promoted
synthesis.
Conclusions
We have identified synthetic strategies to overcome problems
encountered in our initial investigation of 12a derivatives as
PET radioligands for imaging â-amyloid. High-affinity ligands,
including those with thiolate substituents in the 6-position, have
been discovered, opening up possibilities for developing new
easily labeled candidate radioligands for imaging human Aâ
plaques in vivo. These possibilities are now actively pursued.
The SAR findings aid further in demonstrating the topography
of the ligand binding site in human Aâ plaques.
Experimental Procedures
Reagents and Chemicals. Common reagents were purchased
from Aldrich Chemical Co. (Milwaukee, WI), Fluka Chemical
Company (Milwaukee, WI), Acros (Hampton, NH), or Strem
Chemicals (Newburyport, MA) and were used without further
purification unless otherwise indicated. DiPPF (1,1′-bis(diisopro-
pylphosphino)ferrocene) was from Strem Chemicals. o-Toluidine
(5b), 5-bromopyridin-2-amine (7a), 5-iodopyridin-2-amine (7b),
5-chloro-2-aminopyridine (7c), 5-fluoro-2-aminopyridine (7d),
pyrazin-2-amine (9), 2-mercaptoethanol, 2-bromoacetyl bromide,

4752 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 19
Cai et al.
1-(3-Bromo-4-(methylamino)phenyl)ethanone (2). 1-(4-Me-
thylaminophenyl)ethanone18 (1, 2.98 g, 20 mmol) was dissolved
in CH2Cl2 (50 mL) and cooled to 4 °C with an ice-water bath.
N-Bromosuccinimide (NBS, 3.65 g, 20.5 mmol) was added por-
tionwise. The reaction mixture was warmed to room temperature
and stirred for 2 h. Reaction progress was monitored with TLC
until completion. At the end of reaction, water (50 mL) was added
and the aqueous phase extracted with CH2Cl2 (3 × 50 mL). The
combined organic phases were dried with Na2SO4. The solvent was
removed to leave the crude product, which was further purified on
silica gel (ethyl acetate-petroleum ether (1: 3)) to give 2 (4.15 g,
(<1%), 401.9 (<1%, [M + H]+), 401.0 (<1%), 399.9 (<1%), 325.0
(12%), 324.0 (79%), 323.0 (12%), 322.0 (81%), 310.0 (26%), 308.0
(27%), 244.1 (32%), 215.1 (8%), 201.1 (14%), 170.0 (3%), 132.1
(5%), 110.0 (100%), 77.1 (11%). HRMS m/z (EI+): calcd C11H9-
NO2F3Br2 ) 401.8952. Found: 401.8954. Error (ppm): +0.2. Calcd
C11H8NO2F3Br2 ) 401.8874. Found: 401.8874. Error (ppm): 0.0.
N-(4-(2-Bromoacetyl)-2-methylphenyl)-2,2,2-trifluoroaceta-
mide (4b). 2,2,2-Trifluoro-N-o-tolylacetamide (6b, 5.0 g, 24.6
mmol) was dissolved in CS2 (20 mL) and cooled below 10 °C.
Bromoacetyl bromide (10.1 g, 50.0 mmol) was added dropwise.
AlCl3 (10.0 g, 75.0 mmol) was added in portions, with a drying
tube attached to the reaction flask. The reaction mixture was
refluxed overnight. After the reaction was complete as monitored
by TLC, the upper CS2 layer was removed. The lower layer was
added to ice-water (7.0 mL) and concentrated HCl (1.0 mL). The
product was extracted into CH2Cl2 and dried over MgSO4. The
solvent was removed and the solid was dried to afford crude product
(5.0 g), which was purified by silica gel column chromatography
to afford 4b (3.0 g, yield 38%) as a yellow solid: mp 127-130
°C. 1H NMR (400 MHz, CD3OD) δ 8.38 (s, 1H, Ar-H), 7.83 (dd,
3JHH ) 8.0 Hz, 4JHH ) 1.8 Hz, 1H, Ar-H), 7.78 (s, 1H, NH), 7.39
1
91%): mp 80-83 °C. H NMR (400 MHz, DMSO-d6) δ 7.96 (d,
3
4
4JHH ) 2.0 Hz, 1H, Ar-H), 7.81 (dd, JHH ) 8.6 Hz, JHH ) 2.0
Hz, 1H, Ar-H), 6.65 (d, 3JHH ) 8.6 Hz, 1H, Ar-H), 6.22 (m, 1H,
N-H), 2.83 (d, JHH ) 4.8 Hz, 3H, CH3), 2.43 (s, 3H, CH3). 13C
3
NMR (100 MHz, CD3OD) δ 198.1 (s, 1C, CO), 151.9, 134.3, 131.6,
127.4, 110.2, 109.3, 30.3 (CH3), 26.0 (CH3). m/z (EI-MS): 230.0
(5%), 229.0 (48%), 228.0 (7%), 227.0 (50%, [M + H]+), 215.0
(10%), 214.0 (96%), 213.0 (10%), 212.0 (100%), 186.0 (7%), 184.0
(8%), 105.1 (25%), 104.1 (13%), 77.1 (12%), 63.0 (8%). HRMS
m/z (EI+): calcd C9H10ONBr ) 226.9946. Found: 226.9942. Error
(ppm): -0.4.
3
(d, JHH ) 7.8 Hz, 1H, Ar-H), 4.41 (s, 2H, -CH2-Br), 2.37 (s,
3H, CH3). 13C NMR (100 MHz, CDCl3) δ 190.3 (s, 1C, CO), 155.3
N-(4-Acetyl-2-bromophenyl)-2,2,2-trifluoro-N-methylaceta-
mide (3). 1-(3-Bromo-4-(methylamino)phenyl)ethanone (2, 1.4 g,
6.14 mmol) was dissolved in CH2Cl2 (15 mL) and cooled to 0 °C.
Trifluoroacetic anhydride (1.9 g, 9.21 mmol) and Et3N (0.93 g,
9.21 mmol) were added sequentially. The mixture was stirred at
room temperature for 3 h, washed thrice with water, and dried over
Na2SO4. The solvent was removed and the residue dried to afford
3 (1.9 g, yield 96%) as a yellow oil. 1H NMR (400 MHz, CD3OD)
2
(q, JCF ) 37.6 Hz, 1C, CO), 137.1, 133.4, 133.1, 131.6, 127.6,
124.1, 115.8 (q, 1JCF ) 289 Hz, 1C, CF3), 30.6 (s, 1C, CH2), 17.9
(s, 1C, CH3). m/z (ES-MS): 353.4 (5%), 352.9 (23%), 352.4 (12%),
351.9 (50%), 351.4 (8%), 350.9 (24%), 326.0 (94%), 324.0 (100%,
[M + H]+). HRMS m/z (TOF+): calcd C11H10NO2F3Br )
323.9847. Found: 323.9853. Error (ppm): +1.9.
N-(4-(2-Bromoacetyl)-2-methylphenyl)acetamide (4c). The
synthetic procedure was the same as that for 4b but started from
4
δ isomer R (amide rotamers) (82%) 8.32 (d, JHH ) 2.0 Hz, 1H,
1
Ar-H), 8.07 (dd, 3JHH ) 8.2 Hz,4JHH ) 1.9 Hz, 1H, Ar-H), 7.90
6c. Yield 48%; mp 144-147 °C. H NMR (400 MHz, CDCl3) δ
(s, 1H, Ar-H), 7.65 (d, JHH ) 8.2 Hz, 1H, Ar-H), 3.32 (s, 3H,
isomer R (93%) 7.85 (dd, 3JHH ) 7.9 Hz,4JHH ) 1.8 Hz, 1H. Ar-
3
4
4
3
CH3), 2.64 (s, 3H, CH3). Isomer â (18%) 8.31(d, JHH ) 2.0 Hz,
H), 7.74 (d, JHH ) 1.8 Hz, 1H. Ar-H), 7.41 (d, JHH ) 8.0 Hz,
1H. Ar-H), 4.40 (AB, 2JHH ) 11.6 Hz, 1H, CH2), 4.34 (AB, 2JHH
) 11.7 Hz, 1H, CH2), 3.15 (s, 3H, CH3), 2.28 (s, 3H, CH3), 1.73
3
1H, Ar-H), 8.09 (dd, JHH ) 8.2 Hz,4JHH ) 2.0 Hz, 1H, Ar-H),
7.90 (s, 1H, Ar-H), 7.58 (d, 3JHH ) 8.2 Hz, 1H, Ar-H), 3.31 (q,
5JFH ) 1.6 Hz, 3H, CH3), 2.63 (s, 3H, CH3). 13C NMR (100 MHz,
CD3OD) δ isomer R 197.7 (s, 1C, CO), 157.7 (q, 2JCF ) 36.4 Hz,
3
4
(s, 3H, CH3). Isomer â (7%) 7.82 (dd, JHH ) 8.0 Hz, JHH ) 1.9
Hz, 1H. Ar-H), 7.72 (d, 4JHH ) 1.9 Hz, 1H, Ar-H), 7.33 (d, 3JHH
) 8.0 Hz, 1H, Ar-H), 4.65 (AB, 2JHH ) 14.4 Hz, 1H, CH2), 4.61
1
1C, CO), 144.6, 140.6, 134.6, 131.6, 130.0, 124.8, 117.6 (q, JCF
2
) 288 Hz, 1C, CF3), 38.4 (s, 1C, CH3), 26.9 (s, 1C, CH3). Isomer
(AB, JHH ) 14.4 Hz, 1H, CH2), 3.29 (s, 3H, CH3), 2.25 (s, 3H,
â 197.9 (s, 1C, CO), 157.7 (q, JCF ) 36.4 Hz, 1C, CO), 146.2,
CH3), 1.73 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3) δ isomer R
190.3 (s, 1C, CO), 170.5 (s, 1C, CO), 143.8, 142.6, 133.6, 132.2,
128.9, 128.8, 36.0 (s, 1C, CH2), 30.5 (s, 1C, CH3), 22.2 (s, 1C,
CH3), 17.9 (s, 1C, CH3). Isomer â 190.2 (s, 1C, CO), 170.6 (s, 1C,
CO), 143.8, 142.7, 133.9, 131.7, 128.5, 128.4, 45.7 (s, 1C, CH2),
39.1 (s, 1C, CH3), 30.9 (s, 1C, CH3), 18.2 (s, 1C, CH3). m/z (CI-
MS): 358.0 (7%), 332.1 (36%), 330.1 (39%), 302.1 (19%), 300.1
(19%), 287.1 (14%), 286.1 (93%), 285.1 (15%), 284.1 (99%, [M
+ H]+), 250.2 (9%), 236.1 (28%), 218.1 (14%), 206.1 (57%), 205.1
(13%), 204.1 (40%), 190.1 (7%), 176.1 (4%), 148.1 (9%), 134.1
(3%), 120.1 (4%), 91.1 (4%), 77.1 (2%), 55.9 (10%). HRMS m/z
(EI+): calcd C12H14NO3Br ) 283.0208. Found: 283.0210. Error
(ppm): 0.2.
2
1
140.1, 134.7, 131.6, 130.5, 123.1, 117.6 (q, JCF ) 288 Hz, 1C,
CF3), 38.3 (s, 1C, CH3), 26.0 (s, 1C, CH3). m/z (ES-MS): 348.0
(11%), 346 (11%), 343.0 (7%), 341.0 (10%), 326.0 (87%), 324.0
(100%, [M + H]+), 269.1 (4%), 268.1 (61%), 263.1 (13%), 246.1
(83%). HRMS m/z (TOF+): calcd C11H10NO2F3Br ) 323.9847.
Found: 323.9843. Error (ppm): -1.2.
N-(2-Bromo-4-(2-bromoacetyl)phenyl)-2,2,2-trifluoro-N-me-
thylacetamide (4a). CuBr2 (6.0 g, 26.9 mmol) was suspended in
ethyl acetate (20 mL) and heated to reflux. A solution of N-(4-
acetyl-2-bromophenyl)-2,2,2-trifluoro-N-methylacetamide (3, 4.5 g,
13.9 mmol) in CHCl3 (20 mL) was added. The mixture was refluxed
for 4 h and filtered. The solvent was removed from the filtrate to
give an orange solid, which was purified by silica gel column
chromatography (ethyl acetate-petroleum ether from 1:100 to 1:50)
to afford 4a (1.9 g, yield 34%) as a yellow oil. 1H NMR (400 MHz,
5-Methoxypyridin-2-amine (7g). Method a. 2-Amino-5-bro-
mopyridine (7a, 0.10 g, 0.58 mmol), sodium methoxide (0.16 g,
2.9 mmol), and nanosized activated copper powder (0.11 g, 1.74
mmol) were introduced in a screw-cap vial (Pyrex glass) together
with anhydrous MeOH (2.0 mL) and a stirrer bar. The vial was
closed and put in an oil bath at 135 °C and stirred for 14 h. The
mixture was cooled, diluted with MeOH (5.0 mL), and filtered
through an SPE silica gel cartridge, and the product eluted with
AcOEt. The fractions were collected and evaporated giving crude
product (92 mg), which was further purified by FCC (CH2Cl2/
AcOEt ) 1:1) to give 7g (26 mg, yield 36%) as a brown oil.
Method b. 2-Amino-5-bromopyridine (7a, 0.10 g, 0.58 mmol),
sodium methoxide (0.16 g, 2.9 mmol), and nanosized activated
copper powder (0.11 g, 1.74 mmol) were introduced into a
microwave glass tube with anhydrous DMF (1.5 mL) and sealed.
The tube was introduced into the microwave cavity and heated for
30 min at 140 °C (140C30M75W300Psi). Although DMF is a high
boiling solvent, high pressure was observed, probably caused by
4
CDCl3) δ isomer R (80%) 8.33 (d, JHH ) 2.0 Hz, 1H, Ar-H),
3
4
8.02 (dd, JHH ) 8.2 Hz, JHH ) 2.0 Hz, 1H, Ar-H), 7.48 (dd,
3JHH ) 8.2 Hz, 5JHH ) 0.5 Hz, 1H, Ar-H), 4.46 (AB, 2JHH ) 11.8
Hz, 1H, CH2), 4.42 (AB, 2JHH ) 11.8 Hz, 1H, CH2), 3.36 (d, 4JHF
) 0.4 Hz, 3H, CH3). Isomer â (20%) 8.33 (d, 4JHH ) 2.0 Hz, 1H,
Ar-H), 8.05 (dd, 3JHH ) 8.2 Hz, 4JHH ) 2.0 Hz, 1H, Ar-H), 7.44
(dd, 3JHH ) 8.2 Hz, 5JHH ) 0.5 Hz, 1H, Ar-H), 4.46 (AB, 2JHH
)
11.8 Hz, 1H, CH2), 4.42 (AB, 2JHH ) 11.8 Hz, 1H, CH2), 3.49 (q,
4JHF ) 1.6 Hz, 3H, CH3). 13C NMR (100 MHz, CDCl3) δ isomer
2
R 189.0 (s, 1C, CO), 156.5 (q, JCF ) 34.6 Hz, 1C, CO), 143.9,
1
135.7, 134.3, 130.4, 129.0, 124.1, 115.9 (q, JCF ) 287 Hz, 1C,
CF3), 37.9 (s, 1C, CH3), 30.0 (s, 1C, CH2). Isomer â 189.1 (s, 1C,
CO), 156.5 (q, 2JCF ) 34.6 Hz, 1C, CO), 145.5, 135.4, 134.6, 129.5,
129.3, 123.8, 116.2 (q, 1JCF ) 287 Hz, 1C, CF3), 37.7 (s, 1C, CH3),
29.9 (s, 1C, CH2). m/z (EI-MS): 406.0 (<1%), 404.9 (<1%), 403.0,

Dimethylbenzeneamine DeriVatiVes as Ligands
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 19 4753
4
the partial methanolysis of the DMF, resulting in low-boiling
products such as methyl formate and dimethylamine. The mixture
was diluted with 2 M NH4Cl solution (10 mL) and extracted thrice
with AcOEt. The organic phase was washed twice with 2 M NH4-
Cl solution and once with water to remove the remaining DMF,
dried on Na2SO4, and filtered. After the solvent was removed, the
crude product was purified by FCC (AcOEt) to afford 7g (12 mg,
yield 17%) as a brown oil. 1H NMR (270 MHz, CDCl3) δ 7.74 (1
1H, Ar-H), 8.03 (d, JHH ) 0.3 Hz, 1H, Ar-H), 7.76-7.72 (m,
3
4
5
2H, Ar-H), 7.51 (m, JHH ) 9.8 Hz, JFH ) 5.0 Hz, JHH ) 0.6
3
3
4
Hz, 1H, Ar-H), 7.24 (m, JHH ) 9.8 Hz, JFH ) 8.3 Hz, JHH
)
2.4 Hz, 1H, Ar-H), 6.84-6.80 (m, 2H, Ar-H), 2.99 (s, 6H, CH3).
13C NMR (100 MHz, CDCl3) δ 153.4 (d, JFC ) 236.4 Hz, 1C,
1
Ar), 150.2 (s, 1C, Ar), 147.7 (s, 1C, Ar), 143.3 (s, 1C, Ar), 127.2
3
(s, 2C, Ar), 121.4 (s, 1C, Ar), 117.4 (d, JFC ) 9.3 Hz, 1C, Ar),
116.2 (d, 2JFC ) 25.4 Hz, 1C, Ar), 112.6 (s, 2C, Ar), 112.2 (d, 2JFC
) 40.6 Hz, 1C, Ar), 108.1 (d, 4JFC ) 1.8 Hz, 1C, Ar), 40.7 (s, 2C,
CH3). m/z (ES-MS): 257.1 (7%), 256.1 (100%, [M + H]+). HRMS
m/z (TOF+): calcd C15H15N3F ) 256.1250. Found: 256.1261. Error
(ppm): +4.3.
3
3
4
H, d, JHH ) 3.0 Hz), 7.06 (1H, dd, JHH ) 9.0 Hz, JHH ) 3.0
3
Hz), 6.45 (1 H, d, JHH ) 9.0 Hz), 3.95 (2H, bs, NH2), 3.74 (3H,
s, OCH3). m/z (EI-MS): 124 (M+), 109 ([M - CH3]+).
5-(Ethylthio)pyridin-2-amine (7h).28 2-Amino-5-iodopyridine
(7b, 2.20 g, 10 mmol), sodium ethanethiolate 80% (1.7 g, 16 mmol),
and copper powder (190 mg, 3.00 mmol) were loaded into a 100
mL round-bottom flask under nitrogen. Ethylene glycol (40 mL,
0.25 mol) was added, and the solution was stirred at 150 °C for 26
h. The cooled solution was filtered and twice partitioned between
ethyl acetate and water. The organic layer was dried over barium
oxide and filtered and the solvent removed in vacuo to afford 7h
(1.2 g, yield 76%) as a yellow oil. 1H NMR (300 MHz, CDCl3), δ
8.06 (1H, d, 4JHH ) 2.1 Hz, Ar-H), 7.48 (1H, dd, 3JHH ) 8.7 Hz,
2-(4-(Dimethylamino)phenyl)-H-imidazo[1,2-a]pyridine-6-
carbonitrile (12e). The synthetic procedure is analogous to that
used in the synthesis of 12a.15,16 2-Bromo-1-(4-(dimethylamino)-
phenylethanone (11) (1.21 g, 5.00 mmol) and 6-aminopyridine-3-
carbonitrile (7e) (0.63 g, 5.3 mmol) were used to give 12e (0.85 g,
yield 65%): mp 269-271 °C. 1H NMR (400 MHz, CDCl3) δ 8.47
4
5
(dd, JHH ) 1.6 Hz, JHH ) 0.6 Hz, 1H, Ar-H), 7.81-7.78 (m,
2H, Ar-H), 7.77 (s, 1H, Ar-H), 7.63-7.60 (m, 1H, Ar-H), 7.20
3
4
(dd, JHH ) 9.3 Hz, JHH ) 1.7 Hz, 1H, Ar-H), 6.77-6.74 (m,
2H, Ar-H), 3.00 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3) δ
151.1, 149.3, 144.9, 131.1, 127.5 (s, 2C, Ar), 124.2, 120.4, 117.8,
117.0, 112.4 (s, 2C, Ar), 107.4, 98.1, 40.5 (s, 2C, CH3). m/z (ES-
MS): 364.0 (8%), 264.1 (29%), 263.1 (100%, [M + H]+). HRMS
m/z (TOF+): calcd C16H15N4 ) 263.1297. Found: 263.1303. Error
(ppm): +2.3.
3
5
4JHH ) 2.4 Hz, Ar-H), 6.42 (1H, dd, JHH ) 8.7 Hz, JHH ) 0.6
Hz, Ar-H), 2.68 (2H, q, 3JHH ) 7.3 Hz, CH2), 1.15 (3H, t, 3JHH
)
7.2 Hz, CH3).
5-Chloropyrazin-2-amine (10). Method a. In a double-necked
dry flask equipped with condenser and dropping funnel under
dinitrogen flow were added 2-aminopyrazine (9, 0.50 g, 5.25 mmol)
and anhydrous CHCl3 (13 mL) previously passed over basic Al2O3.
This solution was heated at 60 °C with stirring. A solution of
N-chlorosuccinimide (NCS, 0.35 g, 2.12 mmol) in anhydrous CHCl3
(7 mL) was added to the mixture through a dropping funnel over
1.5 h. After another 30 min, the reaction was stopped and the solvent
evaporated off. The residue was dissolved in methanol and adsorbed
on silica gel (2 g). This crude product was purified by FCC (hexane/
CH2Cl2/AcOEt, 1:1) to afford 10 (73 mg, yield 26%) as a yellow
solid.
N,N-Dimethyl-4-(6-nitro-H-imidazo[1,2-a]pyridin-2-yl)benze-
namine (12f). 2-Bromo-1-(4-(dimethylamino)phenylethanone (11)
(0.40 g, 1.65 mmol) and 5-nitropyridin-2-amine (7f) (0.30 g, 2.15
mmol) were stirred in refluxing anhydrous acetonitrile (25 mL) at
90-95 °C and dinitrogen for 2 h. NaHCO3 (0.25 g, 2.97 mmol)
was added to the reaction mixture after cooling (15 min). The
mixture was refluxed for another 9 h. After cooling, the mixture
was filtered through a Busch funnel. The precipitate was washed
with acetonitrile and water to afford 0.11 g of the red product. Yield
Method b. 2-Aminopyrazine (0.10 g, 1.05 mmol) and NCS (80
mg, 0.60 mmol) were added to a microwave tube with anhydrous
CHCl3 (1.5 mL) and sealed. The tube was placed in the microwave
cavity and heated at 70 °C for 10 min (70C10M60W300Psi). After
the workup, the crude product (0.15 g) was purified by FCC
(hexane/CH2Cl2/AcOEt, 1: 1: 1) to afford 10 (35 mg, yield 45%)
1
23%; mp 275-277 °C. H NMR (400 MHz, DMSO-d6) δ 9.76
(dd, 4JHH ) 2.3 Hz, 5JHH ) 0.8 Hz, 1H, Ar-H), 8.41 (s, 1H, Ar-
3
4
H), 7.90 (dd, JHH ) 9.8 Hz, JHH ) 2.3 Hz, 1H, Ar-H), 7.82-
7.79 (m, 2H, Ar-H), 7.67-7.654 (m, 1H, Ar-H), 6.82-6.79 (m,
2H, Ar-H), 2.96 (s, 6H, CH3). 13C NMR (100 MHz, DMSO-d6) δ
150.6, 148.5, 144.9, 136.0, 127.3, 126.9 (s, 2C, Ar), 120.2, 118.4,
115.2, 112.2 (s, 2C, Ar), 109.3, 40.2 (s, 2C, CH3). m/z (ES-MS):
284.1 (29%), 283.1 (100%, [M + H]+). HRMS m/z (TOF+): calcd
C15H15N4O2 ) 283.1195. Found: 283.1206. Error (ppm): +3.9.
1
as a yellow solid. H NMR (270 MHz, DMSO-d6) δ 7.99 (1H, s,
H-6), 7.67 (1H, s, H-3), 6.65 (2H, bs, NH2). m/z (EI-MS): 129
(M+), 99, 94 ([M - Cl]+).
4-(6-Chloro-H-imidazo[1,2-a]pyridin-2-yl)-N,N-dimethylben-
zenamine (12c). The synthetic procedure is analogous to that used
in the synthesis of 12a.15,16 2-Bromo-1-(4-(dimethylamino)phe-
nylethanone (11) (1.21 g, 5.00 mmol) and 5-chloro-2-aminopyridine
(7c) (0.668 g, 5.20 mmol) were used to give 12c (0.65 g, yield
4-(6-Methoxy-H-imidazo[1,2-a]pyridin-2-yl)-N,N-dimethyl-
benzenamine (12g). In a two-necked round-bottom flask equipped
with a condenser and under dinitrogen flow were introduced
5-methoxypyridin-2-amine (7g) 0.15 g, 1.17 mmol), 2-bromo-4′-
dimethylaminoacetophenone (11) (0.31 g, 1.29 mmol), and absolute
EtOH (14 mL). The reaction mixture was stirred at reflux for 2 h.
After the reaction mixture had cooled down, NaHCO3 (0.15 g, 1.75
mmol) was added and the mixture refluxed for another 6 h. Solvent
was removed and the residue dissolved in AcOEt. The organic phase
was washed with water, dried over MgSO4, and filtered. The solvent
was removed to afford crude product (0.271 g), which was purified
by FCC (CH2Cl2/AcOEt, 1:1) to give 12g (0.129 g, yield 41%) as
a yellow solid: mp 182-184 °C. 1H NMR (400 MHz, DMSO-d6)
δ 8.17 (d, 4JHH ) 2.2 Hz, 1H, Ar-H), 8.10 (s, 1H, Ar-H), 7.76-
1
48%): mp 234-236 °C. H NMR (400 MHz, CDCl3) δ 8.15 (s,
1H, Ar-H), 7.81 (d, 3JHH ) 8.9 Hz, 2H, Ar-H), 7.70 (s, 2H, Ar-
3
3
H), 7.19 (d, JHH ) 9.2 Hz, 1H, Ar-H), 6.76 (d, JHH ) 8.6 Hz,
2H, Ar-H), 3.00 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3) δ
150.7, 147.6, 144.0, 127.2 (s, 2C), 125.6, 123.2, 121.4, 120.2, 117.4,
112.6 (s, 2C), 107.1, 40.6 (s, 2C, NCH3). m/z (ES-MS): 275.1 (4%),
274.1 (51%), 273.1 (18%), 272.1 (100%, [M + H]+). HRMS m/z
(TOF+): calcd C15H15N3Cl ) 272.0955. Found: 272.0960. Error
(ppm): + 1.8.
4-(6-Fluoro-H-imidazo[1,2-a]pyridin-2-yl)-N,N-dimethylben-
zenamine (12d). The synthetic procedure is analogous to that used
in the synthesis of 12a.15,16 2-Bromo-1-(4-(dimethylamino)phe-
nylethanone (11) (1.21 g, 5.00 mmol) and 5-fluoro-2-aminopyridine
(7d) (0.583 g, 5.17 mmol) were stirred in reflux ethanol (50 mL)
for 4 h. Pale-yellow precipitate formed. NaHCO3 (0.41 g, 4.9 mmol)
was added to the reaction mixture after cooling (15 min). The
mixture was refluxed again for another 2 h. After the mixture was
cooled, the solvents were removed. The solid was washed with
water, CH2Cl2, and recrystallized from ethyl acetate to give 0.42 g
3
7.72 (m, 2H, Ar-H), 7.44 (d, JHH ) 9.7 Hz, 1H, Ar-H), 6.98
3
4
(dd, JHH ) 9.7 Hz, JHH ) 2.4 Hz, 1H, Ar-H), 6.79-6.75 (m,
2H, Ar-H), 3.79 (s, 3H, O-CH3), 2.94 (s, 6H, CH3). 13C NMR
(100 MHz, CD3OD) δ 152.0, 150.8, 147.0, 143.7, 127.8 (s, 2C),
123.2, 121.2, 116.8, 113.8 (s, 2C), 109.7, 109.4, 56.7 (s, 1C, OCH3),
40.8 (s, 2C, NCH3). m/z (ES-MS): 269.1 (18%), 268.1 (100%, [M
+ H]+). HRMS m/z (TOF+): calcd C16H18N3O ) 268.1450.
Found: 268.1453. Error (ppm): +1.1.
4-(6-(Ethylthio)-H-imidazo[1,2-a]pyridin-2-yl)-N,N-dimethyl-
benzenamine (12h). The synthetic procedure is analogous to that
used in the synthesis of 12a.15,16 2-Bromo-1-(4-(dimethylamino)-
1
of product. Yield 33%; mp 228-229 °C. H NMR (400 MHz,
CD3OD) δ 8.45 (m,3JFH ) 4.2 Hz, 4JHH ) 2.4 Hz, 5JHH ) 0.7 Hz,

4754 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 19
Cai et al.
phenylethanone (11) (1.21 g, 5.00 mmol) and 5-(ethylthio)pyridin-
2-amine (7h) (0.80 g, 5.2 mmol) were used to give 12h (0.91 g,
Cl2 was added and the resultant precipitate filtered off to afford
13a (2.8 g, yield 59%) as a yellow solid: mp 95-98 °C. 1H NMR
(400 MHz, CD3OD) δ isomer R (84%) 8.73-8.72 (m, 1H, Ar-
1
yield 61%): mp 168-171 °C. H NMR (400 MHz, DMSO-d6) δ
8.56 (dd, 4JHH ) 1.9 Hz, 5JHH ) 0.9 Hz, 1H, Ar-H), 8.14 (s, 1H,
Ar-H), 7.78-7.74 (m, 2H, Ar-H), 7.51-7.49 (m, 1H, Ar-H),
7.23 (dd, 3JHH ) 9.3 Hz, 4JHH ) 1.9 Hz, 1H, Ar-H), 6.76 (m, 2H,
Ar-H), 2.94 (s, 6H, CH3), 2.92 (q, 3JHH ) 7.3 Hz, 2H, CH2), 1.21
H), 8.34-8.31 (m, 2H, Ar-H), 8.01 (dd, JHH ) 8.2 Hz, JHH )
3
3
4
2.0 Hz, 1H, Ar-H), 7.58 (d, JHH ) 8.3 Hz, 1H, Ar-H), 7.55-
7.52 (m, 1H, Ar-H), 7.45 (dd, 3JHH ) 9.6 Hz, 4JHH ) 1.9 Hz, 1H,
Ar-H), 2.66 (s, 3H, CH3). Isomer â (16%) 8.73-8.72 (m, 1H,
Ar-H), 8.34-8.31 (m, 2H, Ar-H), 8.03 (dd, 3JHH ) 8.0 Hz, 4JHH
3
(t, JHH ) 7.3 Hz, 3H, CH3). 13C NMR (100 MHz, DMSO-d6) δ
150.1, 145.7, 143.6, 128.0, 127.3, 126.5 (s, 2C, Ar), 121.5, 118.3,
116.1, 112.2 (s, 2C, Ar), 107.1, 40.0 (s, 2C, CH3), 28.4 (s, 1C,
CH2), 14.4 (s, 1C, CH3). m/z (ES-MS): 300.1 (6%), 299.1 (39%),
298.1 (100%, [M + H]+). HRMS m/z (TOF+): calcd C17H20N3S
) 298.1378. Found: 298.1386. Error (ppm): +2.7.
) 1.9 Hz, 1H, Ar-H), 7.55-7.52 (m, 1H, Ar-H), 7.51 (d, 3JHH
)
3 4
8.3 Hz, 1H, Ar-H), 7.44 (dd, JHH ) 9.6 Hz, JHH ) 1.9 Hz, 1H,
Ar-H), 2.66 (s, 3H, CH3). 13C NMR (100 MHz, DMSO-d6) δ
2
isomer R 155.5 (q, JCF ) 35.1 Hz, 1C, CO),143.5, 142.3, 137.9,
1
136.3, 130.8, 129.8, 128.6, 125.8, 122.8, 117.9, 115.9 (q, JCF
)
286 Hz, 1C, CF3), 111.1, 106.5, 37.7 (s, 1C, CH3). Isomer â 155.5
4-(6-Bromo-8-iodoindolizin-2-yl)-N,N-dimethylbenze-
namine (12i). The synthetic procedure is analogous to that used in
the synthesis of 12a.15,16 2-Bromo-1-(4-(dimethylamino)phenyle-
thanone (11) (1.21 g, 5.00 mmol) and 2-amino-3-iodo-5-bromopy-
ridine (7i) (1.58 g, 5.3 mmol) were used to give 12i (1.21 g, yield
2
(q, JCF ) 35.1 Hz, 1C, CO), 143.4, 142.6, 139.8, 135.6, 129.9,
129.4, 128.4, 127.1, 126.3, 121.3, 117.8, 115.9 (q, 1JCF ) 286 Hz,
1C, CF3), 110.8, 109.4, 37.7 (s, 1C, CH3). m/z (ES-MS): 479.9
(27%), 477.9 (100%, [M + H]+), 475.9 (28%). HRMS m/z
(TOF+): calcd C16H11N3OF3Br2 ) 475.9221. Found: 475.9228.
Error (ppm): +1.5.
1
55%): mp 218-220 °C. H NMR (400 MHz, CDCl3) δ 8.17 (d,
4JHH ) 1.7 Hz, 1H, Ar-H), 7.85-7.81 (m, 2H, Ar-H), 7.79 (s,
4
3
1H, Ar-H), 7.66 (d, JHH ) 1.7 Hz, 1H, Ar-H), 6.79 (d, JHH
)
N-(4-(6-Bromoindolizin-2-yl)-2-methylphenyl)-2,2,2-trifluoro-
acetamide (13b). N-(4-(2′-Bromoacetyl)-2-methylphenyl)-2,2,2-
trifluoroacetamide (4b) (3.0 g, 9.26 mmol) and 5-bromopyridin-
2-amine (7b) (1.5 g, 8.67 mmol) were dissolved in ethanol (20
mL). The mixture was refluxed overnight and the solvent removed.
CH2Cl2 was added to precipitate a solid, which was filtered and
washed with CH2Cl2 to afford 13b (1.0 g, yield 28%) as a yellow
7.9 Hz, 2H, Ar-H), 2.99 (s, 3H, CH3). 13C NMR (100 MHz,
CDCl3) δ 150.5, 147.9, 143.9, 135.8, 127.5 (s, 2C, Ar), 125.4, 121.5,
112.8 (s, 2C, Ar), 108.8, 106.0, 84.0 (s, 1C, Ar-I), 40.9 (s, 1C,
CH3). m/z (ES-MS): 444.9 (6%), 443.9 (99%), 442.9 (9%), 441.9
(100%, [M + H]+). HRMS m/z (TOF+): calcd C15H14N3BrI )
441.9416. Found: 441.9398. Error (ppm): -4.1.
1
powder: mp 306-309 °C. H NMR (400 MHz, CD3OD) δ 9.09
6-Bromo-2-(4-(dimethylamino)phenyl)indolizine-8-carboni-
trile (12j). The synthetic procedure is analogous to that used in
the synthesis of 12a.15,16 2-Bromo-1-(4-(dimethylamino)phenyle-
thanone (11) (1.21 g, 5.00 mmol) and 2-amino-5-bromonicotino-
nitrile (7j) (1.03 g, 5.2 mmol) were used to give 12j (0.77 g, yield
(dd, 4JHH ) 1.7 Hz, 5JHH ) 0.9 Hz, 1H, Ar-H), 8.53 (s, 1H, Ar-
3
4
H), 8.08 (dd, JHH ) 9.6 Hz, JHH ) 1.8 Hz, 1H, Ar-H), 7.87 (d,
4JHH ) 1.9 Hz, 1H, Ar-H), 7.84 (dd, JHH ) 9.5 Hz, JHH ) 0.7
3
5
Hz, 1H, Ar-H), 7.78 (dd, 3JHH ) 8.0 Hz, 4JHH ) 2.0 Hz, 1H, Ar-
3
H), 7.56 (d, JHH ) 8.2 Hz, 1H, Ar-H), 2.36 (s, 3H, CH3). 13C
1
45%): mp 240-246 °C. H NMR (400 MHz, DMSO-d6) δ 9.12
NMR (100 MHz, DMSO-d6) δ 155.4 (q, 2JCF ) 36.6 Hz, 1C, CO),
139.7, 136.6, 136.4, 134.9, 134.3, 131.9, 128.8, 125.6, 125.1, 124.5,
116.1 (q, 1JCF ) 289 Hz, 1C, CF3), 113.9, 111.2, 110.1, 17.4 (CH3).
m/z (ES-MS): 401.0 (7%), 400.0 (93%), 399.0 (7%), 398.0 (100%,
[M + H]+). HRMS m/z (TOF+): calcd C16H12N3OF3Br )
398.0116. Found: 398.0099. Error (ppm): -4.3.
(d, 4JHH ) 1.8 Hz, 1H, Ar-H), 8.32 (s, 1H, Ar-H), 8.13 (d, 4JHH
3
) 1.8 Hz, 1H, Ar-H), 7.81 (d, JHH ) 8.9 Hz, 2H, Ar-H), 6.79
(d, 3JHH ) 8.9 Hz, 2H, Ar-H), 2.96 (s, 3H, CH3). 13C NMR (100
MHz, DMSO-d6) δ 150.6, 147.4, 141.3, 133.6, 131.1, 127.0 (s,
2C, Ar), 119.9, 114.8, 112.1 (s, 2C, Ar), 109.0, 103.5, 99.9, 39.9
(s, 2C, NCH3). m/z (ES-MS): 443.9 (4%), 441.9 (3%), 344.0 (11%),
343.0 (98%), 342.0 (11%), 341.0 (100%, [M + H]+). HRMS m/z
(TOF+): calcd C16H14N4Br ) 341.0402. Found: 341.0389. Error
(ppm): - 3.8.
N-(4-(6-Bromoindolizin-2-yl)-2-methylphenyl)-N-methylaceta-
mide (13c). The synthetic procedure is analogous to that used in
the synthesis of 13a. N-(4-(2′-Bromoacetyl)-2-methylphenyl)-N-
methylacetamide (4c) (1.0 g, 3.52 mmol) and 5-bromopyridin-2-
amine (7b) (0.73 g, 4.22 mmol) were used. Yield 37%; mp 197-
2-(4-(Dimethylamino)phenyl)-H-imidazo[1,2-a]pyridin-6-ol
(12k). To a dried 10 mL flask under dinitrogen flow was added
4-(6-methoxy-H-imidazo[1,2-a]pyridin-2-yl)-N,N-dimethylbenze-
namine (12g, 62 mg, 0.22 mmol) and anhydrous CH2Cl2 (2 mL).
The solution was stirred and cooled to -78 °C with a CO2/acetone
bath and then BBr3 (80 mg, 0.32 mmol) was added dropwise from
a disposable syringe. (Caution! BBr3 reacts violently with water).
The reaction mixture was allowed to stir at room temperature for
2 h and poured into ice-water (15 mL). A mixture of CH2Cl2 and
MeOH (15:1) was added and the mixture stirred for 10 min. The
organic phase was separated, dried over MgSO4, and filtered.
Solvent was removed to give 12k (39 mg, yield 70%) as a yellow
1
198 °C. H NMR (400 MHz, DMSO-d6) δ isomer R (90%) 8.88
4
5
5
(dd, JHH ) 2.0 Hz, JHH ) 0.8 Hz, 1H, Ar-H), 8.39 (d, JHH
)
3
4
0.4 Hz, 1H, Ar-H), 7.89 (dd, JHH ) 7.8 Hz, JHH ) 1.8 Hz, 1H,
4
Ar-H), 7.84 (d, JHH ) 1.8 Hz, 1H, Ar-H), 7.58-7.54 (m, 1H,
Ar-H), 7.43 (d, 3JHH ) 8.0 Hz, 1H, Ar-H), 7.37 (dd, 3JHH ) 9.5
4
Hz, JHH ) 2.0 Hz, 1H, Ar-H), 3.11 (s, 3H, NCH3), 2.21 (s, 3H,
Ar-CH3), 1.70 (s, 3H, CH3). Isomer â (10%) 8.87 (dd, 4JHH ) 1.9
Hz, 5JHH ) 0.8 Hz, 1H, Ar-H), 8.32 (d, 5JHH ) 0.3 Hz, 1H, Ar-
3
4
H), 7.76 (dd, JHH ) 7.8 Hz, JHH ) 1.8 Hz, 1H, Ar-H), 7.73 (d,
4JHH ) 1.8 Hz, 1H, Ar-H), 7.56-7.53 (m, 1H, Ar-H), 7.36 (dd,
3JHH ) 9.5 Hz, JHH ) 2.0 Hz, 1H, Ar-H), 7.31 (d, JHH ) 8.0
Hz, 1H, Ar-H), 3.28 (s, 3H, NCH3), 2.13 (s, 3H, Ar-CH3), 1.70
(s, 3H, CH3). 13C NMR (100 MHz, DMSO-d6) δ isomer R 169.0
(s, 1C, CO), 144.2, 143.3, 143.2, 134.7, 133.0, 131.7, 127.9, 126.9,
125.3, 125.1, 117.6, 109.8, 106.0, 35.3 (s, 1C, NCH3), 21.6 (s, 1C,
CH3), 16.7 (s, 1C, Ar-CH3). Isomer â 169.6 (s, 1C, CO), 144.6,
143.8, 143.2, 134.8, 132.2, 130.9, 127.8, 126.8, 124.5, 124.1, 117.5,
109.3, 105.9, 38.7 (s, 1C, NCH3), 21.9 (s, 1C, CH3), 17.1 (s, 1C,
Ar-CH3). m/z (ES-MS): 361.0 (4%), 360.0 (100%, [M + H]+),
359.0 (47%), 358.0 (96%). HRMS m/z (TOF+): calcd C17H17N3-
OBr ) 358.0555. Found: 358.0557. Error (ppm): +0.6.
4
3
1
solid: mp 262-264 °C. H NMR (270 MHz, DMSO-d6), δ 9.70
(s, 1H, OH), 8.16 (s, 1H, H-3), 8.01 (s, 1H, H-5), 7.71 (d, 3JHH
)
3
8.9 Hz, 2H, Ar-H), 7.45 (d, JHH ) 9.6 Hz, 1H, Ar-H), 7.03 (d,
3
3JHH ) 9.4 Hz, 1H, Ar-H), 6.78 (d, JHH ) 8.9 Hz, 2H, Ar-H),
2.95 (s, 6H, NCH3). 13C NMR (125 MHz, DMSO-d6), δ 150.2,
146.2, 143.2, 140.3, 126.4 (Ph-2,6), 120.7 (C7), 120.4, 115.3 (C8),
112.4 (Ph-3,5), 110.5 (C3), 107.8 (C5), 39.9 (NCH3). m/z (CI-
MS): 254 ([M + 1] +), 238 ([M - CH3] +). m/z (ES-MS): 355.1
(6%), 268.1 (4%), 260.1 (6%), 255.1 (12%), 254.1 (100%, [M +
H]+). HRMS m/z (TOF+): calcd C15H16N3O ) 254.1293. Found:
254.1299. Error (ppm): +2.4.
N-(2-Bromo-4-(6-bromo-H-imidazo[1,2-a]pyridin-2-yl)phenyl)-
2,2,2-trifluoro-N-methylacetamide (13a). N-(2-Bromo-4-(2′-bro-
moacetyl)phenyl)-2,2,2-trifluoro-N-methylacetamide (4a) (4.0 g,
9.93 mmol) and 5-bromopyridin-2-amine (7b) (1.7 g, 9.83 mmol)
were dissolved in MeOH (20 mL). The reaction mixture was
refluxed for 8 h and solvent then removed. A small amount of CH2-
2-Bromo-4-(6-bromo-H-imidazo[1,2-a]pyridin-2-yl)-N-meth-
ylbenzenamine (14a). N-(2-Bromo-4-(6-bromo-H-imidazo[1,2-a]-
pyridin-2-yl)phenyl)-2,2,2-trifluoro-N-methylacetamide (13a, 1.0 g,
2.10 mmol) and K2CO3 (2.0 g, 14.5 mmol) were suspended in
ethanol (30 mL) and water (15 mL). The mixture was refluxed for
8 h. The organic solvent was removed and CH2Cl2 added. The

Dimethylbenzeneamine DeriVatiVes as Ligands
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 19 4755
organic phase was washed twice with water and dried over MgSO4.
The solvent was removed to afford 14a (0.60 g, yield 80%) as a
yellow solid: mp 226-228 °C. 1H NMR (400 MHz, DMSO-d6) δ
8.81 (dd, 4JHH ) 1.8 Hz, 5JHH ) 0.9 Hz, 1H, Ar-H), 8.21 (s, 1H,
Ar-H), 8.01 (d, 4JHH ) 2.0 Hz, 1H, Ar-H), 7.78 (dd, 3JHH ) 8.4
8.68 (d, 4JHH ) 4.4 Hz, 1H, Ar-H), 8.11 (s, 1H, Ar-H), 7.68 (d,
3
3JHH ) 8.0 Hz, 2H, Ar-H), 7.48 (d, JHH ) 9.6 Hz, 1H, Ar-H),
7.26 (d, 3JHH ) 9.2 Hz, 1H, Ar-H), 7.13 (s, 1H, Ar-H), 6.59 (d,
3JHH ) 8.0 Hz, 2H, Ar-H), 5.86 (s, 2H, NH2), 3.53 (s, 2H, CH2S),
2.71 (s, 3H, CH3). 13C NMR (100 MHz, DMSO-d6) δ 169.8
(-CO-N), 149.8, 146.2, 143.6, 128.0, 127.7, 126.6, 120.9, 118.2,
115.9, 111.6, 106.8, 38.5 (C-S), 29.6 (NCH3). m/z (LC-MS):
314.3 (21%), 313.2 (100%, [M + H]+), 255.5 (32%, [M - CH2-
CONH2]+). HRMS m/z (TOF+): calcd C16H17N4OS ) 313.1123.
Found: 313.1126. Error (ppm): +0.9.
2-(2-(4-(Methylamino)phenyl)-H-imidazo[1,2-a]pyridin-6-ylth-
io)ethanol (15b). 4-(6-Iodo-H-imidazo[1,2-a]pyridin-2-yl)-N-me-
thylbenzenamine (14d) (100 mg, 0.29 mmol), N-methyl-N-(trime-
thylstannyl)methanamine (60 mg, 0.29 mmol), 2-mercaptoethanol
(23 mg, 0.29 mmol), Pd2(dba)3 (28 mg, 0.031 mmol), DiPPF (22
mg, 0.039 mmol), and toluene (5.0 mL) were used. Yield 91%;
mp 158-161 °C. 1H NMR (400 MHz, CD3OD) δ 8.51 (s, 1H, Ar-
H), 7.95 (s, 1H, Ar-H), 7.68 (d, 3JHH ) 8.6 Hz, 2H, Ar-H), 7.45
4
3
5
Hz, JHH ) 2.0 Hz, 1H. Ar-H), 7.51 (dt, JHH ) 8.9 Hz, JHH
)
3
4
0.7 Hz, 1H, Ar-H), 7.32 (dd, JHH ) 9.5 Hz, JHH ) 1.9 Hz, 1H,
Ar-H), 6.68 (d, 3JHH ) 8.6 Hz, 1H, Ar-H), 5.55 (m, 1H, N-H),
3
2.81 (d, JHH ) 4.8 Hz, 3H, CH3). 13C NMR (100 MHz, DMSO-
d6) δ 146.0, 144.7, 143.1, 129.3, 127.47, 126.5, 126.2, 122.2, 117.1,
110.1, 108.4, 107.9, 105.5, 30.1 (s, 1C, NCH3). m/z (ES-MS): 383.9
(42%), 383.4 (6%), 381.9 (100%, [M + H]+), 379.9 (47%). HRMS
m/z (TOF+): calcd C14H12N3Br2 ) 379.9398. Found: 379.9396.
Error (ppm): -0.5.
4-(6-Bromo-H-imidazo[1,2-a]pyridin-2-yl)-2-methylbenze-
namine (14b). N-(4-(6-Bromo-H-imidazo[1,2-a]pyridin-2-yl)-2-
methylphenyl)-2,2,2-trifluoroacetamide (13b, 1.0 g, 2.51 mmol) and
K2CO3 (4.0 g, 29.0 mmol) were suspended in ethanol (30 mL) and
water (15 mL). The mixture was refluxed overnight. The solvent
was removed, and CH2Cl2 was added. The organic phase was
washed twice with H2O and dried over MgSO4. The solvent was
removed to afford a solid, which was purified by silica gel column
chromatography to give 14b (0.60 g, yield 79%) as a yellow solid:
3
3
(d, JHH ) 8.6 Hz, 1H, Ar-H), 7.34 (d, JHH ) 8.6 Hz, 1H, Ar-
H), 6.67 (d, JHH ) 8.6 Hz, 2H, Ar-H), 3.69 (t,3JHH ) 6.5 Hz,
3
2H, OCH2), 3.01 (t,3JHH ) 6.5 Hz, 2H, SCH2), 2.81 (s, 3H, NCH3).
13C NMR (100 MHz, CD3OD) δ 151.8, 148.0, 145.7, 130.9, 129.5,
128.1, 122.6, 121.2, 116.5, 113.4, 108.5, 61.5 (C-O), 38.8 (C-
S), 30.6 (NCH3). m/z (LC-MS): 302.2 (5%), 301.3 (15%), 300.2
(100%, [M + H]+). HRMS m/z (TOF+): calcd C16H18N3OS )
300.1171. Found: 300.1163. Error (ppm): -2.5.
1
4
mp 183-187 °C. H NMR (400 MHz, CD3OD) δ 8.65 (dd, JHH
5
) 1.9 Hz, JHH ) 0.9 Hz, 1H, Ar-H), 8.06 (s, 1H, Ar-H), 7.46
(dt, 3JHH ) 9.5 Hz, 4JHH ) 5JHH ) 0.76 Hz, 1H, Ar-H), 7.37 (dd,
3JHH ) 9.5, 4JHH ) 1.9 Hz, 1H, Ar-H), 7.26 (d, 4JHH ) 1.8 Hz, 1
2-(2-(4-(Dimethylamino)phenyl)-H-imidazo[1,2-a]pyridin-6-
ylthio)acetamide (15c). 4-(6-Iodo-H-imidazo[1,2-a]pyridin-2-yl)-
N,N-dimethylbenzenamine (12a, 50 mg, 0.14 mmol), N-methyl-N-
(trimethylstannyl)methanamine (30 mg, 0.14 mmol), 2-mercaptoace-
tamide (13 mg, 0.14 mmol), Pd2(dba)3 (13 mg, 0.014 mmol), DiPPF
(6 mg, 0.014 mmol), and toluene (5.0 mL) were used. Yield 85%;
3
4
H, Ar-H), 7.17 (dd, JHH ) 7.6 Hz, JHH ) 1.7 Hz, 1H, Ar-H),
7.06 (d, JHH ) 7.7 Hz, 1H, Ar-H), 2.19 (s, 3H, CH3). 13C NMR
3
(100 MHz, DMSO-d6) δ 146.6, 146.1, 143.0, 131.6, 130.2, 127.3,
126.6, 121.2, 117.4, 113.7, 111.2, 108.6, 105.5, 17.3 (CH3). m/z
(ES-MS): 305.0 (21%), 304.0 (99%), 303.0 (24%), 302.0 (100%,
[M + H]+). HRMS m/z (TOF+): calcd C14H13N3Br ) 302.0293.
Found: 302.0298. Error (ppm): +1.7.
1
mp 180-183 °C. H NMR (400 MHz, DMSO-d6) δ 8.60 (s, 1H,
Ar-H), 8.17 (s, 1H, Ar-H), 7.77 (d, 3JHH ) 8.8 Hz, 2H, Ar-H),
3
3
4-(6-Bromo-H-imidazo[1,2-a]pyridin-2-yl)-N,2-dimethylben-
zenamine (14c). The synthetic procedure is analogous to that used
in the synthesis of 14a. N-(4-(6-Bromoimidazo[1,2-a]pyridin-2-yl)-
2-methylphenyl)-N-methylacetamide (13c, 1.0 g, 2.79 mmol) and
KOH (1.0 g, 17.8 mmol) were used, to afford 0.68 g. Yield 77%;
mp 165-166 °C. 1H NMR (400 MHz, DMSO-d6) δ 8.82 (dd, 4JHH
7.50 (d, JHH ) 7.9 Hz, 1H, Ar-H), 7.27 (d, JHH ) 9.4 Hz, 1H,
Ar-H), 7.13 (brs, 2H, NH2), 6.78 (d, 3JHH ) 8.7 Hz, 2H, Ar-H),
3.54 (s, 2H, CH2S), 2.94 (s, 6H, N(CH3)2). 13C NMR (100 MHz,
DMSO-d6) δ 168.7 (-CO-N), 149.0, 144.7, 142.5, 129.4, 127.8,
126.9, 126.6, 125.3, 120.3, 117.2, 114.9, 111.1, 109.5, 106.1, 38.5
(C-S), 37.3 (NCH3). m/z (LC-MS): 329.3 (7%), 328.2 (18%),
327.2 (100%, [M + H]+), 325.1 (12%), 323.4 (7%), 321.7 (7%),
320.9 (5%), 269.4 (14%, [M - CH2CONH2]+). HRMS m/z
(TOF+): calcd C17H19N4OS ) 327.1280. Found: 327.1278. Error
(ppm): -0.5.
5
) 1.8 Hz, JHH ) 0.7 Hz, 1H, Ar-H), 8.27 (s, 1H, Ar-H), 7.56
3
3
4
(d, JHH ) 9.5 Hz, 1H, Ar-H), 7.33 (dd, JHH ) 9.5 Hz, JHH
)
)
1.9 Hz, 1H, Ar-H), 7.10-7.09 (m, 2H, Ar-H), 7.01 (dd, 3JHH
5
8.3 Hz, JHH ) 0.32 Hz, 1H, Ar-H), 5.12 (s, 1H, N-H), 2.82 (s,
1H, CH3), 2.10 (s, 1H, CH3). 13C NMR (100 MHz, DMSO-d6) δ
147.8, 146.4, 143.1, 131.9, 129.8, 127.3, 126.6, 121.8, 117.5, 113.2,
108.9, 105.7, 105.6, 30.2 (s, 1C, N-CH3), 17.5 (s, 1C, Ar-CH3).
m/z (ES-MS): 319.0 (17%), 318.0 (96%), 317.0 (22%), 316.0
(100%, [M + H]+). HRMS m/z (TOF+): calcd C15H15N3Br )
316.0449. Found: 316.0453. Error (ppm): +1.3.
4-(6-(4-Methoxybenzylthio)-H-imidazo[1,2-a]pyridin-2-yl)-
N,N-dimethylbenzenamine (15d). 4-(6-Iodo-H-imidazo[1,2-a]py-
ridin-2-yl)-N,N-dimethylbenzenamine (12a, 80 mg, 0.22 mmol),
N-methyl-N-(trimethylstannyl)methanamine (46 mg, 0.22 mmol),
(4-methoxyphenyl)methanethiol (34 mg, 0.22 mmol), Pd2(dba)3 (40
mg, 0.044 mmol), DiPPF (18.4 mg, 0.044 mmol), and toluene (5.0
mL) were used. Yield 85%; mp 184-186 °C. 1H NMR (400 MHz,
CD3OD) δ 8.43 (s, 1H, Ar-H), 8.10 (s, 1H, Ar-H), 7.75 (d, 3JHH
General Procedure for Thiolate Substitution. A 10 mL
microwave tube was charged with Pd2dba3 (13-58 mg, 0.014-
0.063 mmol, 10-20 mol % Pd), DiPPF (6-27 mg, 0.014-0.063
mmol, 10-20 mol %), 12a derivative (0.14-0.32 mmol), and tin
thiolate (0.29-0.63 mmol). The tube was capped and put in a
microwave system for the desired temperature and time as specified
in the text. A small sample of the resulting suspension was analyzed
by HPLC to confirm the conversion. The suspension was partitioned
between CHCl3 and K2CO3 solution. The organic layer was dried
over MgSO4 and filtered. The solvent was removed, and the residue
was dissolved in DMSO and loaded on a reversed-phase HPLC.
After the correct band had been collected, the solvents were
removed and the product was dried by addition and evaporation of
MeCN.
3
) 8.4 Hz, 2H, Ar-H), 7.47 (d, JHH ) 8.0 Hz, 1H, Ar-H), 7.19
3
3
(d, JHH ) 8.0 Hz, 1H, Ar-H), 7.16 (d, JHH ) 8.0 Hz, 2H, Ar-
3
3
H), 6.83 (d, JHH ) 8.0 Hz, 1H, Ar-H), 6.77 (d, JHH ) 8.0 Hz,
1H, Ar-H), 4.13 (s, 2H, CH2S), 3.71 (s, 3H, CH3O), 2.94 (s, 6H,
NCH3). 13C NMR (100 MHz, DMSO-d6) δ 158.2, 150.0, 145.6,
143.5, 129.9, 129.2, 128.2, 127.7, 126.3, 121.3, 118.0, 115.8, 113.9,
112.1, 107.0, 54.8 (OCH3), 39.9 (NCH3), 38.3 (C-S). m/z (LC-
MS): 392.1 (7%), 391.1 (23%), 390.1 (100%, [M + H]+). HRMS
m/z (TOF+): calcd C23H24N3OS ) 390.1640. Found: 390.1634.
Error (ppm): -0.6.
2-(2-(4-(Dimethylamino)phenyl)-H-imidazo[1,2-a]pyridin-6-
ylthio)ethanol (15e). 4-(6-Iodo-H-imidazo[1,2-a]pyridin-2-yl)-N,N-
dimethylbenzenamine (12a, 50 mg, 0.14 mmol), N-methyl-N-
(trimethylstannyl)methanamine (29 mg, 0.14 mmol), 2-mercaptoe-
thanol (11 mg, 0.14 mmol), Pd2(dba)3 (13 mg, 0.014 mmol), DiPPF
(6 mg, 0.014 mmol), and toluene (5.0 mL) were used. Yield 89%;
mp 183-185 °C. 1H NMR (400 MHz, CD3OD) δ 8.50 (s, 1H, Ar-
H), 8.05 (s, 1H, Ar-H), 7.78 (d, 3JHH ) 8.9 Hz, 2H, Ar-H), 7.45
2-(2-(4-(Methylamino)phenyl)-H-imidazo[1,2-a]pyridin-6-ylth-
io)acetamide (15a). 4-(6-Iodo-H-imidazo[1,2-a]pyridin-2-yl)-N-
methylbenzenamine (14d) (100 mg, 0.29 mmol), N-methyl-N-
(trimethylstannyl)methanamine (60 mg, 0.29 mmol), 2-mercaptoace-
tamide (26 mg, 0.29 mmol), Pd2(dba)3 (26.2 mg, 0.029 mmol),
DiPPF (12 mg, 0.029 mmol), and 8.0 mL of toluene were used.
1
3
3
Yield 78%; mp 198-202 °C. H NMR (400 MHz, DMSO-d6) δ
(d, JHH ) 8.9 Hz, 1H, Ar-H), 7.35 (d, JHH ) 8.9 Hz, 1H, Ar-

4756 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 19
Cai et al.
3
3
H), 6.83 (d, JHH ) 8.9 Hz, 2H, Ar-H), 3.70 (t, JHH ) 6.7 Hz,
2H, OCH2), 3.03 (t, 3JHH ) 6.7 Hz, 2H, SCH2), 3.00 (s, 6H, NCH3).
13C NMR (100 MHz, DMSO-d6) δ 150.1, 145.7, 143.6, 128.1,
127.4, 126.4, 121.5, 118.5, 116.1, 112.2, 107.1, 59.8 (C-O), 55.9
(C-S), 37.2 (NCH3). m/z (LC-MS): 316.1 (5%), 315.2 (23%),
314.2 (100%, [M + H]+). HRMS m/z (TOF+): calcd C17H20N3OS
) 314.1327. Found: 314.1317. Error (ppm): -3.4.
2.5 Hz, 1H, Ar-H), 8.02 (s, 1H, Ar-H), 7.82-7.79 (m, 2H, Ar-
H), 6.84-6.81 (m, 2H, Ar-H), 3.01 (s, 6H, CH3). 13C NMR (125
MHz, CDCl3), δ 150.5, 149.3, 147.9 (C7), 147.0, 129.6 (C5), 127.5
(Ph-2,6), 117.5, 112.8 (Ph-3,5), 106.2 (C-Cl), 104.8 (C3), 40.7
(NCH3). m/z (CI-MS): 273 ([M ( 1]+), 239 ([M - Cl + 1]+). m/z
(ES-MS): 355.1 (6%), 275.1 (27%), 274.1 (9%), 273.1 (100%, [M
+ H]+). HRMS m/z (TOF+): calcd C14H14N4Cl ) 273.0907.
Found: 273.0920. Error (ppm): +4.8.
In Vitro Binding Assay. Postmortem brain tissue was obtained
from a female AD patient at the age of 80 years (postmortem
interval, 7.5 h). Consent for donation was obtained from the next
of kin, and the brain was removed. The clinical diagnosis was
confirmed after autopsy by macro- and microscopic examination,
including Bielschowsky’s silver stain adapted for paraffin sections.
The cerebrum was sectioned into 1 cm coronal sections, which were
rapidly frozen in a 50:50 mixture of dry ice and isopentane, and
the frozen sections were transferred to plastic bags and stored at
-76 °C until used. One day before the experiment, bags of selected
brain tissue were removed from the freezer and transferred to a
-20 °C freezer; 3 h before the experiment these sections were
placed on wet ice. While on ice, the leptomeninges were first
removed and the white matter was dissected away. Then the gray
matter was cut into 3 mm × 3 mm pieces with a sharp scalpel and
placed into plastic bags. The tissue was rapidly frozen on dry ice
and stored in a freezer (-70 °C) for further use.
The AD brain tissue was homogenized in PBS (1:50 in vol) to
generate a suspension, which was dispensed into 1.0 mL and stored
in a -70 °C freezer for further use. For each assay, a new vial
with 1.0 mL of suspension was thawed, vortexed, and diluted with
PBS until 10.0 mL. An amount of 100 µL of suspension was used
in each tube. [3H]PIB solution (1 mCi/mL) was diluted with PBS
to give an intermediate stock solution (10 nCi/µL), which was
further diluted with PBS to yield a dilute stock solution (1 nCi/
µL). An amount of 100 µL was used in each tube. Nonradioactive
PIB or other displacer was dissolved in DMSO to give a stock
solution (1 mM), which was further diluted with DMSO to give
serial solutions in the concentration range of 1 × 10-4 to 10-10 M,
and 10 µL was used in each tube. Quadruplets were used for each
concentration. The assembled reaction mixtures were vortexed and
incubated for 2 h at 37 °C. After separation using a cell harvester,
the filter paper (GF/B filter paper pretreated with 0.5% polyimine
solution) was washed with PBS (3 × 3 mL). The filters were placed
in 7 mL plastic vials, and scintillation fluid (4 mL each) was added.
The samples were counted after overnight incubation. The data were
analyzed using GraphPad Prism 4.
2-(2-(3-Methyl-4-(methylamino)phenyl)-H-imidazo[1,2-a]py-
ridin-6-ylthio)acetamide (15f). 4-(6-Bromoimidazo[1,2-a]pyridin-
2-yl)-N,2-dimethylaniline (14c) (100 mg, 0.32 mmol), N-methyl-
N-(trimethylstannyl)methanamine (99 mg, 0.48 mmol), 2-mercapto-
acetamide (43 mg, 0.48 mmol), Pd2(dba)3 (43 mg, 0.048 mmol),
DiPPF (20 mg, 0.048 mmol), and toluene (6.0 mL) were used. Yield
1
69%; mp 181-182 °C. H NMR (400 MHz, CD3OD) δ 8.58 (s,
1H, Ar-H), 8.10 (s, 1H, Ar-H), 7.49 (d, 3JHH ) 8.0 Hz, 1H, Ar-
H), 7.41 (d, 3JHH ) 8.0 Hz, 1H, Ar-H), 7.17 (s, 1H, Ar-H), 7.12
3
3
(d, JHH ) 8.0 Hz, 1H, Ar-H), 7.06 (d, JHH ) 8.0 Hz, 1H, Ar-
H), 3.46 (s, 1H, NH), 3.55 (s, 2H, SCH2), 2.93 (s, 3H, NCH3),
2.16 (s, 3H, Ar-CH3). 13C NMR (100 MHz, DMSO-d6) δ 168.7
(-CO-), 146.6, 144.9, 142.4, 131.0, 128.6, 127.1, 126.6, 120.5,
117.5, 115.3, 112.0, 107.5, 104.5, 37.2 (CH2S), 29.0 (NCH3), 16.3
(CH3). m/z (LC-MS): 329.2 (6%), 328.3 (22%), 327.2 (100%,
[M + H]+). HRMS m/z (TOF+): calcd C17H19N4OS ) 327.1280.
Found: 327.1278. Error (ppm): -0.4.
2-(2-(3-Methyl-4-(methylamino)phenyl)-H-imidazo[1,2-a]py-
ridin-6-ylthio)ethanol (15g). 4-(6-Bromoimidazo[1,2-a]pyridin-2-
yl)-N,2-dimethylaniline (14c) (100 mg, 0.32 mmol), N-methyl-N-
(trimethylstannyl)methanamine (132 mg, 0.63 mmol), 2-mercaptoe-
thanol (50 mg, 0.63 mmol), Pd2(dba)3 (58 mg, 0.063 mmol), DiPPF
(27 mg, 0.063 mmol), and toluene (6.0 mL) were used. Yield 69%;
mp 149-153 °C. 1H NMR (400 MHz, CD3OD) δ 8.55 (s, 1H, Ar-
H), 8.10 (s, 1H, Ar-H), 7.50 (d, 3JHH ) 8.0 Hz, 1H, Ar-H), 7.38
(d, 3JHH ) 8.0 Hz, 1H, Ar-H), 7.18 (s, 1H, Ar-H), 7.15 (d, 3JHH
3
) 8.0 Hz, 1H, Ar-H), 7.08 (d, JHH ) 8.0 Hz, 1H, Ar-H), 3.80
(t, 3JHH ) 7.2 Hz, 2H, CH2O), 3.03 (t, 3JHH ) 7.2 Hz, 2H, CH2S),
2.94 (s, 3H, NCH3), 2.16 (s, 3H, Ar-CH3). 13C NMR (100 MHz,
DMSO-d6) δ 147.7, 146.0, 143.4, 132.2, 129.7, 128.2, 127.4, 121.6,
118.8, 116.5, 113.1, 108.6, 105.6, 59.5 (OCH2), 37.1 (CH2S), 30.1
(NCH3), 17.5 (CH3). m/z (LC-MS): 329.2 (6%), 328.3 (22%),
327.2 (100%, [M + H]+). m/z (LC-MS): 316.2 (8%), 315.2 (24%),
314.2 (100%, [M + H]+). HRMS m/z (TOF+): calcd C17H20N3OS
) 314.1327. Found: 314.1316. Error (ppm): -3.5.
4-(6-Chloroimidazo[1,2-a]pyrazin-2-yl)-N,N-dimethylbenze-
namine (17a). 5-Chloro-2-aminopyrazine (10) (61 mg, 0.47 mmol),
2-bromo-4′-dimethylaminoacetophenone (11) (170 mg, 0.71 mmol),
and anhydrous MeCN (7 mL) were introduced under dinitrogen
flow into a double-necked round-bottomed flask equipped with a
condenser. The reaction mixture was refluxed for 3 h and allowed
to cool. NaHCO3 (71 mg, 0.85 mmol) was added. The mixture was
refluxed for another 6 h. The solvent was removed and the crude
product (221 mg) purified by FCC (hexane/CH2Cl2/AcOEt, 2: 2:
1) to afford 17a (9 mg, yield 7%) as a yellow solid. 1H NMR (270
MHz, CDCl3), δ 8.82 (s, 1H, Ar-H), 8.09 (s, 1H, Ar-H), 7.822
(s, 1H, Ar-H), 7.818 (d, 3JHH ) 8.9 Hz, 2H, Ar-H), 6.77 (d, 3JHH
) 8.7 Hz, 2H, Ar-H), 3.02 (s, 6H, NCH3). m/z (CI-MS): 273
([M + 1]+), 261, 239 ([M - Cl +1] +). m/z (ES-MS): 275.1 (41%),
274.1 (10%), 273.1 (100%, [M + H]+). HRMS m/z (TOF+): calcd
C14H14N4Cl ) 273.0907. Found: 273.0908. Error (ppm): +0.4.
4-(6-Chloroimidazo[1,2-a]pyrimidin-2-yl)-N,N-dimethylben-
zenamine (17b). 2-Amino-5-chloropyrimidine (16) (0.150 g, 1.16
mmol), 2-bromo-4′-dimethylaminoacetophenone (11) (0.56 g, 2.32
mmol), and anhydrous MeCN (16 mL) were added to a double-
necked flask equipped with a condenser under dinitrogen. A small
amount of anhydrous DMF (0.5 mL) was added to dissolve the
amine completely. The mixture was stirred and refluxed for 3 h
and cooled and NaHCO3 (0.19 g, 2.32 mmol) added. The reaction
mixture was refluxed for another 6 h and filtered hot on a Busch
funnel. The solid was washed sequentially with EtOH, water, and
EtOH and finally dried overnight to afford 72 mg (yield 23%) of
Acknowledgment. We gratefully thank Drs. Shuiyu Lu and
Umesha Shetty for experimental assistance and Drs. Joel E.
Kleinman and Thomas M. Hyde (CDBD, NIMH) for useful
discussions and aid in obtaining postmortem brain tissue. This
research was performed with support from the Intramural
Research Program of the NIH (NIMH).
Supporting Information Available: A table listing the analyti-
cal techniques used to determine degree of purity for all target
compounds, a table of elemental analysis data, and some NMR
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
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