Synthesis, potency, and in vivo profiles of quinoline containing histamine H3 receptor inverse agonists

Article abstract of DOI:10.1021/jm0705051

A new structural series of histamine H3 receptor antagonist was developed. The new compounds are based on a quinoline core, appended with a required basic aminoethyl moiety, and with potency- and property-modulating heterocyclic substituents. The analogs

Full text of DOI:10.1021/jm0705051

J. Med. Chem. 2007, 50, 5439-5448
5439
Synthesis, Potency, and In Vivo Profiles of Quinoline Containing Histamine H3 Receptor
Inverse Agonists
Robert J. Altenbach,* Huaqing Liu, Patricia N. Banfor, Kaitlin E. Browman, Gerard B. Fox, Ryan M. Fryer,
Victoria A. Komater, Kathleen M. Krueger, Kennan Marsh, Thomas R. Miller, Jia Bao Pan, Liping Pan, Minghua Sun,
Christine Thiffault, Jill Wetter, Chen Zhao, Deliang Zhou, Timothy A. Esbenshade, Arthur A. Hancock, and Marlon D. Cowart
Neuroscience Research, Global Pharmaceutical Research and DeVelopment, Abbott Laboratories, 100 Abbott Park Road,
Abbott Park, Illinois 60064-6123
ReceiVed May 1, 2007
A new structural series of histamine H3 receptor antagonist was developed. The new compounds are based
on a quinoline core, appended with a required basic aminoethyl moiety, and with potency- and property-
modulating heterocyclic substituents. The analogs have nanomolar and subnanomolar potency for the rat
and human H3R in various in vitro assays, including radioligand competition binding as well as functional
tests of H3 receptor-mediated calcium mobilization and GTPγS binding. The compounds possessed favorable
drug-like properties, such as good PK, CNS penetration, and moderate protein binding across species. Several
compounds were found to be efficacious in animal behavioral models of cognition and attention. Further
studies on the pharmaceutic properties of this series of quinolines discovered a potential problem with photo-
chemical instability, an issue which contributed to the discontinuation of this series from further development.
Introduction
rigidify this moiety led to the design of a new structural class
based on a benzofuran skeleton; compounds in the class retained
in vitro activity, as exemplified by analogs such as ABT-239
(1; Figure 1).13 This compound has shown efficacy in several
behavioral models of cognition.13a To further investigate the
SAR and build chemically distinct series of analogs, we carried
out a campaign to replace the benzofuran core with other
bicyclic moieties.14 In these derivatives, incorporation of an ethyl
group as a linker and of (R)-2-methylpyrrolidine as the amine
component generated many compounds with high potency.
Based on an early hypothesis that compounds with higher
lipophilicity would have increased CNS levels, a series of
naphthalenes depicted in structure 2 was designed.15 As
anticipated, these more lipophilic naphthalene derivatives were
indeed found to have a somewhat higher brain-to-plasma
concentration ratio than homologous benzofurans such as 1 and,
as an unexpected bonus, were generally more potent in vitro as
well.15 Like the benzofurans, specific naphthalene analogs were
tested and found efficacious in cognition models.16 The favorable
properties of these two classes, the benzofuran and naphthylene
series, motivated a search for other series that would be able to
greatly expand the structural diversity while retaining good drug-
like properties overall.
Currently, there are four known major classes of histamine
receptors, namely, H1, H2, H3, and H4.1 The H1 and H2 receptors
are involved with the allergic response and gastric acid secretion,
respectively,2 and histamine H1 and H2 antagonists such as
fexofenadine and famotidine are in clinical use. The recently
discovered H4 receptor is expressed in mast cells, eosinophils,
and tissues involved in immune responses and may play a role
in mediating inflammation.3
The histamine H3 receptor (H3R) was identified in 1983 as
an autoreceptor controlling the release of histamine.4 The
histamine H3 receptor subtype has been shown to also be a
heteroreceptor, and by virtue of its predominant expression in
neurons, it has been shown to be able to regulate the release of
other neurotransmitters such as acetylcholine, dopamine, nore-
pinephrine, and serotonin.5 These neurotransmitters are known
to play important roles in modulating vigilance, attention, and
cognition enhancement. In addition to in vitro studies, in vivo
blockade of the H3 receptor with synthetic compounds has been
demonstrated to increase neurotransmitter release.6 Overall, the
body of research on H3 supports that antagonism/inverse
agonism of the receptor has strong potential for utility in treating
a variety of CNS diseases, including ADHD, Alzheimer’s
disease, mild cognitive impairment, and schizophrenia.7
Many of the earliest generations of H3R antagonists were
analogs of the endogenous neurotransmitter histamine and,
therefore, contained an imidazole group.8 In general, these
imidazole containing compounds suffered potential liabilities
of low CNS penetration9 and the propensity to inhibit cyto-
chrome P450 metabolism.10 These liabilities appear to be less
common with the more recent classes of nonimidazole antago-
nists, results which have been reviewed.11
A new series of quinoline-based compounds (3) was targeted
to address an opportunity highlighted during the SAR investiga-
tions of the naphthylene series (2), where it was found that the
R group had a strong and beneficial effect on increasing the in
vitro potency. Analogs of 2 were generated via organometallic
coupling reactions (Suzuki, Stille, Ullmann) to install to the R
group using as an intermediate compounds of structure 2,
wherein R was a bromine or a borate. The quinoline analogs
(3) offered a unique opportunity to greatly expand the variety
of the R moiety because of the different chemistry used to
construct this series.17 Instead of appending R groups by cross-
coupling technologies, as in 2, the quinolines were synthesized
using the Freidlander cyclization between an aminoaldehyde
and a methyl ketone (see Scheme 1). Because a large variety
of methyl ketones were available from commercial and in-house
sources, the Freidlander cyclization allowed for the synthesis
A propyloxy chain linker group is present in many reported
imidazole and nonimidazole H3R antagonists.12 A strategy to
* To whom correspondence should be addressed. Abbott Laboratories,
Dept. R4MN, Bldg AP9A/2, 100 Abbott Park Rd., Abbott Park, IL 60064-
6123. Phone: 847-935-4194. Fax: 847-937-9195. E-mail: robert.j.
altenbach@abbott.com.
10.1021/jm0705051 CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/06/2007

5440 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 22
Altenbach et al.
The problems described above motivated us to develop an
alternative synthesis (Scheme 2) using alcohol 13 as the key
intermediate. This route was used for resynthesis of target
compounds 3b, 3c, and 3f on larger scale. Although the
compounds that are reported here only contain (R)-2-meth-
ylpyrrolidine as the basic amine moiety, in subsequent studies
with other amines, 13 was a versatile intermediate because it
allows for modification of either the amine or the R group at a
late stage in the synthesis. But overall, the route shown in
Scheme 1 was considered superior for generating larger quanti-
ties of compounds due to its higher overall yield and fewer
number of steps.
Figure 1.
Scheme 1a
The scale-up of pyrazole 3e (see Table 1) demanded larger
quantities of methyl ketone 9e. Initially, we synthesized 9e using
reported literature procedures,21 but these routes resulted in the
desired 1,3-dimethylpyrazole isomer (ketone 9e) being con-
taminated with substantial amounts of the unwanted isomer, 1,5-
dimethyl-4-acetylpyrazole, which was separable from 9e only
with great difficulty. Therefore, an alternative synthesis was
developed as shown in Scheme 3. In this route, 1-methylhy-
drazine was first protected as the hydrazone, 16,22 then reacted
with 3-ethoxymethylenepentane-2,4-dione,23 which generated
the key intermediate 17. Treatment of compound 17 with 1 M
HCl in ethanol resulted in deprotection of the hydrazine and
subsequent in situ intramolecular cyclization. This process
cleanly provided the 1,3-dimethyl isomeric pyrazole 9e in high
yield and without contamination by the 1,5-dimethyl isomer.
a
Reagents and conditions: (i) 4-nitrophenethyl bromide, K2CO3, DMF,
60 °C, 60 h; (ii) H2, Pd/C, MeOH; (iii) pivaloyl chloride, Et3N, CH2Cl2, 16
h; (iv) 3.3 equiv n-BuLi, TMEDA, Et2O, -78 °C, rt, 4 h, 0 °C; 6 equiv
DMF, rt, 16 h (78% for 4 steps); (v) 2 M HCl, ∆; NaOH (60-80% yield);
(vi) RCOMe (9a, 9d, 9e), KOH, EtOH, ∆.
Results
of a wider range of compounds that were not readily accessible
via the typical coupling reactions used in the naphthylene series
(2). As an additional benefit, this facilitated a broader exploration
of the SAR and a larger compound set from which to search
for agents with high in vitro potency, good PK, and in vivo
activity. This report highlights compounds 3a-3f (Table 1) as
representative examples of the quinoline series.14,17b These
compounds all possessed good overall drug-like properties and
were subsequently tested in more advanced assays.
The new compounds 3a-3f were all found to bind potently
to the human and rat H3R in competition binding assays (see
Table 1), with the analogs generally 3-fold more potent for the
human receptor over the rat H3R.24 High potency at the rat H3R
is an especially important attribute because the behavioral
models are carried out in rat. These compounds (Table 1) were
highly selective for the H3R over the other histamine subtypes,
as well as a battery of other receptors and the hERG channel.25
The compounds were also evaluated for their functional
blockade of the H3R in separate in vitro cellular assays that
probed for receptor antagonism and inverse agonism. Target
compounds 3a-3f blocked agonist-induced ((R)-R-methylhis-
tamine, RAMH) increases in intracellular calcium levels in HEK
cells expressing the human H3R. The new compounds were
potent, with Kb values ranging from 1.5 to 8.7 nM (see Table
2).24 In separate assays, H3R antagonism was also demonstrated
through blockade of agonist-induced (RAMH) stimulation of
GTPγS binding. As seen in Table 2, the compounds were more
potent at the human receptor than the rat H3R in the functional
assays, mirroring the findings in the radioligand displacement
assays shown in Table 1. In yet another assay (Table 3), the
new compounds were able to potently reduce the basal levels
of GTPγS binding to the human and rat H3 receptor, supporting
that 3a-3f are inverse agonists of the H3 receptor. This inverse
agonism is potentially significant, because H3Rs have been
shown to be constitutively active (partly activated in the absence
of endogenous agonist histamine) in both in vitro and in vivo
studies.26
Chemistry
The key step in the synthesis of the quinolines was the
Friedlander cyclization,18 wherein we coupled aminoaldehyde
8 with diverse heterocyclic methyl ketones (see Scheme 1).
Intermediate 8 was itself synthesized in a five step sequence
using as a key starting material the L-tartaric acid salt of (R)-
2-methylpyrrolidine.19 In the sequence, the only problematic step
was the deprotection of 7. To obtain good yields, this step
required heating a very dilute solution of 7 in aqueous acid for
a short period of time. This requirement for high dilution and
short reaction time made it difficult to generate intermediate 7
on a large scale because of the heating and cooling of the
required large reaction volumes in a timely manner and the large
quantities of sodium hydroxide needed to basify the mixture
during workup. Extended heating or more concentrated condi-
tions resulted in lower yields of compound 8, due to the
formation of a side product with a lower Rf than compound 8,
with a structure that is believed to be a trimer of compound
8.20 In an attempt to circumvent this problem, the pivaloyl amide
of compound 6 was replaced by the more easily removable Boc
protecting group. Unfortunately, this Boc derivative of 6 was
more difficult to synthesize from the intermediate aniline on
large scale, and also, the ortho-directed metalation step on the
Boc analog of 6 was found to require use of tert-butyl lithium
as a base, and even this provided only modest yields of the
corresponding aldehyde.
Before testing analogs in rodent behavioral assays in vivo,
PK properties were determined in rat, the species ultimately
used for behavioral models (see Table 4). The new compounds
3a-3f had very good PK profiles, with moderate half-lives (t1/2
) 1.7-8.3 h) and good oral bioavailabilities (F ) 30-98%).
The reduced volume of distributions (Vâ) for the quinolines
compared to 1 may be due to their increased hydrophilic nature,

Histamine H3 Receptor InVerse Agonists
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 22 5441
Table 1. Binding Affinities of Compounds 1 and 3a-3f at Human and Rat H3Ra
a n g 4. Assessed by displacement of [3H]-N-R-methyl histamine from cell membranes isolated from C6 cells expressing cloned rat or human H3R.24 The
pKi (-log Ki) ( the standard error of the mean (SEM) are reported.
Scheme 2a
Table 2. In Vitro Functional Antagonism of Compounds 1 and 3a-3f
in Calcium Mobilization and GTPγS Binding Assaysa
inhibition of [35S]GTPγS bindingb
calcium
mobilizationc
(human)
human
rat
Kb
Kb
Kb
cmpd pKb ( SEM (nM) pKb ( SEM (nM) pKb ( SEM (nM)
1
7.87 ( 0.15 13.49 9.04 ( 0.04 0.91 8.34 ( 0.14 4.57
3a
3b
3c
3d
3e
3f
8.06 ( 0.14
8.34 ( 0.13
8.46 ( 0.19
8.47 ( 0.09
8.77 ( 0.10
8.82 ( 0.12
8.71 8.88 ( 0.18 1.32 9.00 ( 0.12 1.00
4.57 9.27 ( 0.03 0.54 8.15 ( 0.29 7.08
3.47 9.05 ( 0.18 0.89 8.71 ( 0.10 1.95
3.39 9.41 ( 0.11 0.39 9.04 ( 0.13 0.91
1.70 9.13 ( 0.07 0.74 8.51 ( 0.11 3.09
1.51 9.19 ( 0.09 0.65 8.45 ( 0.05 3.55
a Reagents and conditions: (i) pivaloyl chloride, Et3N, CH2Cl2, 16 h
(78%); (ii) TBDMSCl, imidazole, CH2Cl2 (98%); (iii) 2.3 equiv n-BuLi,
TMEDA, Et2O, 0 °C, rt, 2 h, -10 °C; 6 equiv DMF, rt, 16 h (83%); (iv)
2 M HCl in EtOH, ∆ (35%); (v) RCOMe (9b, 9c, 9f), KOH, EtOH, ∆; (vi)
MsCl, Et3N, CH2Cl2; (vii) (R)-2-Me-pyrrolidine, Cs2CO3, MeCN, PhMe,
∆ (80% for 2 steps).
a n g 3. b Inhibition of RAMH-induced binding of [35S]GTPγS.24
c Functional activity determined in HEK-239 cell line, coexpressing hH3R
and GRq/i5, measuring reduction of 30 nM RAMH-induced increases in
intracellular calcium.24
Scheme 3a
Table 3. In Vitro Inverse Agonism of Compounds 1 and 3a-3f in
GTPγS Binding Assaysa
reduction of basal [35S]GTPγS bindingb
human
pEC50 ( SEM
rat
pEC50 ( SEM
cmpd
EC50 (nM)
EC50 (nM)
1
8.89 ( 0.06
9.23 ( 0.09
8.85 ( 0.11
9.11 ( 0.07
8.95 ( 0.22
8.70 ( 0.11
8.80 ( 0.09
1.29
0.59
1.41
0.78
1.12
2.00
1.58
7.76 ( 0.20
9.13 ( 0.14
8.44 ( 0.08
8.52 ( 0.10
8.30 ( 0.08
8.24 ( 0.07
8.14 ( 0.10
17.4
0.74
3.63
3.02
5.01
5.75
7.24
a Reagents and conditions: (i) acetone, rt; ∆; then distill (44%); (ii)
3-ethoxymethylenepentane-2,4-dione, Et2O (84%); (iii) 1 M HCl, EtOH
(96%).
3a
3b
3c
3d
3e
3f
consistent with their lower calculated CLogP values (see Table
5). With the exception of the more lipophilic ABT-239 (1),
lower clearance values (CLb) in the new compounds correlated
a n g 3. b Reduction of basal [35S]GTPγS binding (efficacy > 20%).24
well with longer t1/2
.
Brain penetration is of course an important property for agents
targeting CNS diseases. Although the processes governing brain
entry are multifactorial involving biological processes such as
efflux transporters,27 the physicochemical properties have a very
strong influence on CNS penetration. Properties thought favor-
able to blood-brain barrier (BBB) penetration include weak
hydrogen-bonding potential, moderate-to-high lipophilicity
(CLogP), lower polar surface area (PSA), reduced flexibility,
and small size.27,28 Most CNS active drugs that efficiently
penetrate the brain have low polar surface areas (<70 Å).29
However, a caveat is that as lipophilicity rises, there is often a
concomitant reduction in the fraction of unbound drug in brain
because much of the compound is nonspecifically bound to brain
tissue.30 Optimal design involves finding an appropriate balance

5442 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 22
Altenbach et al.
Table 4. PK Properties of Compounds Tested in Rats at 1 mg/kg
Table 7. In Vivo Resultsa
PKa
cmpd
SRb
5-trial IAc
i.v.b
p.o.c
1
0.01-0.1
0.1d
0.1-1
3a
3b
3c
3d
3e
1
cmpd
t
1/2 (h)
Vâ
CLb
F (%)
0.01e
1
5.3
8.3
2.4
4.0
5.3
1.7
2.0
11.6
7.4d
4.1
8.5
6.3
1.50
0.53
1.18
1.45
0.84
3.7
53
70
75
98
90
58
30f
0.003
0.003-0.01
0.003-0.01
3a
3b
3c
3d
3e
3f
0.3-1
0.1-1
a Activity of compounds in two rodent behavioral assays: the social
recognition memory (SR) test in adult rats31 and the 5-trial inhibitory
avoidance (5-trial IA) in spontaneously hypertensive rat (SHR) pups.32
Efficacious doses are in mg/kg, following i.p. and s.c. administration,
respectively. b n g 12 animals; P < 0.05, Dunnett’s post hoc analysis. c n
g 9 animals; P< 0.05 Mann-Whitney analysis. d n ) 6. e A trend toward
efficacy was seen at this dose for compound 3b, but it did not reach statistical
significance, P > 0.05, Dunnett’s post hoc analysis.
9.0
9.2e
2.54
a Data expressed as mean; n ) 3 animals; SEM < 20% of mean unless
stated. b Compounds dosed intravenously; n ) 3 animals; Vâ (volume of
distribution, L/h); CLb (clearance, Lh-1kg-1). c Compounds dosed orally;
F (oral bioavailabilty, %). d SEM ( 1.9. e SEM ( 3.1. f SEM ( 10.
Table 5. Physicochemical Properties and CNS Penetration
CNS penetrationa
mouseb
ratc
brain brain-to- brain brain-to-
concn
blood
ratio
concn
(ng/g)
plasma
ratio
cmpd PSA CLogPd MW (ng/g)
1
40.2
69.0
46.8
45.2
46.3
34.0
41.9
5.15
4.36
4.20
4.14
4.12
3.44
4.32
330
443
398
402
386
335
383
1728
3242
1659
718
19×
42×
23×
16×
1.5×
3082 36-52×
3a
3b
3c
3d
3e
3f
174
440 1.6×
425 1.8×
50e 0.4×
0
Figure 2. Compound 3d in SR. The effect of compound 3d on
enhancing SR memory in adult rats is shown graphically as the mean
( SEM.31 With 3d, or with the standard reference H3 antagonist
ciproxifan (ciprox, 1.0 mg/kg), recall of the initial social encounter is
improved, with a consequent reduction in the time spent reinvestigating
the juvenile, expressed as the investigation ratio. *P < 0.05, Dunnett’s
post hoc analysis; n ) 12 animals, dosed i.p.
a All compounds n g 3. Compounds dosed 1 mg/kg, measured 1 h after
dosing, concentrations were determined by HPLC-MS; SEM < 11% of
mean unless stated. b Dosed i.p. c Dosed i.v. d Calculated.47 e SEM ( 22.
Table 6. Plasma Protein Binding (% Bound)a
cmpd
dog
human
monkey
rat
be rationalized by their lower lipophilicity as assessed by CLogP
values (see Table 5). Compounds 3a-3e had good overall drug-
like characteristics and were advanced to behavioral models.
Two in vivo behavioral models were used to assess the
compounds’ ability to facilitate memory and learning in rodents
(see Table 7). The social recognition (SR) memory model
evaluated the memory enhancing effects by testing the ability
of agents to improve the retention of encounters between
animals.31 In this model, an adult rat is allowed to explore a
juvenile rat for 5 min during a period of social investigation
and then, after a 2 h separation, is reintroduced to the juvenile
for a second 5 min period. Test compounds or saline vehicle
were administered intraperitoneally (i.p.) to the adult rat
immediately after the first 5 min exposure period. The duration
of investigation (grooming, sniffing, close following) was
recorded over each 5 min period. Social memory was quantified
by determining the investigation ratio, which is the ratio of the
time the adult rat spends investigating the juvenile in the second
encounter divided by the time the adult spends investigating
the juvenile during the first encounter. An agent that enhances
memory will reduce the exploration time of the second encounter
relative to the first and, hence, the investigation ratio. The H3R
antagonist ciproxifan dosed at 1 mg/kg was used as a standard,
as this compound reliably enhances SR memory.31 Compounds
3a and 3c-3e were clearly effective in this model. A more
detailed dose response determination was done for compounds
3d and 3e, confirming that they were active over a range of
doses (see Figure 2 for the graph of compound 3d).
1
91
83
78
80
60
58
43
97
83
85
81
64
75
56
94
79
81
70
55
50
50
94
89
77
84
70
62
61
3a
3b
3c
3d
3e
3f
a Compounds (5 µM) were incubated with plasma, and the mixture was
centrifuged in a Microcon membrane filtration unit to separate bound from
free compound. Free compound was quantified against a standard curve
(LC-MS/MS) and reported as the percent bound. n ) 1.
of physicochemical properties, lipophilic enough to allow for
efficient partitioning into brain tissue, but not too lipophilic so
that unbound drug is free to act at the target.
The ability of compounds 3a-3f to enter the brain was
evaluated in rodents (see Table 5). In mouse, compound 3a had
exceptionally high brain levels (3242 ng/g/mg of compound
dosed) and a large brain-to-blood ratio (42×). Compounds 3d
and 3e had more modest brain levels in rat, similar to the plasma
levels. Surprisingly, compound 3f had no detectable levels in
mouse brain and only low levels in rat. The reason for this
finding is undetermined and could not be rationalized on any
physicochemical basis such as CLogP, molecular weight, or
polar surface area. Instead, we suspect there could be some
factor particular to compound 3f itself such as, for example,
the compound might be actively effluxed from the brain.
The plasma protein binding of the analogs was measured in
several species (Table 6). Compounds had moderate protein
binding (43-89%), which is favorable, indicating that a sizable
free fraction of circulating drug is present in the plasma. The
moderate degree of plasma protein binding for these agents can
The five-trial inhibitory avoidance (5-trial IA) acquisition
model assesses aspects of learning as well as impulsive
behavior.32 This model, which uses spontaneously hypertensive

Histamine H3 Receptor InVerse Agonists
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 22 5443
Compounds such as 3d and 3e are cationic amphiphiles, so-
called because at physiological pH they contain both a proto-
nated hydrophilic domain and a hydrophobic domain. Com-
pounds in this class have been reported to have the potential to
bind to and accumulate in the phospholipid bilayers of cells
and induce phospholipidosis, a lipid storage disorder in which
phospholipids accumulate in endosomes and lysosomes as
lamellar bodies in various tissues.34 While phospholipidosis
noted in in vitro assays is in itself not considered a toxic
response, it is often associated with toxicity in targeted organs.35
Compounds 3d and 3e were examined in vitro in rat hepatocytes
for their phospholipid index (PI),36,37 a measurement of their
potential to induce phospholipidosis as normalized to the
standard amiodorane (at 5 µM, PI ) 1), a compound with a
known propensity to induce phospholipidosis.38a Compound 3d
was found to be only a moderate inducer of phospholipidosis,
having PIs of 0.66 ( 0.41 and 0.69 ( 0.08 at 44 µM and 88
µM, respectively, while at a lower concentration (22 µM) this
compound was an even weaker inducer (PI ) 0.21 ( 0.09).38b
Thus, the compound was considered to have a limited ability
to induce phospholipidosis at a concentration below 20 µM.
Compound 3e was a weak inducer of phospholipidosis at 100
µM (PI ) 0.39 ( 0.01) and inactive at lower concentrations
and was considered unlikely to cause phospholipidosis in vivo.
Figure 3. Compound 3d in 5-trial IA. Compound 3d improves
avoidance acquisition in the five-trial, repeated acquisition, inhibitory
avoidance response in SHR pups compared with vehicle.32 The summed
latency shows the improvement in acquisition for trials 2-5 compared
to vehicle-treated controls. Compound 3d or vehicle was dosed s.c. 30
min prior to the first training trials; ciproxifan (ciprox, 3.0 mg/kg) was
included as a positive control. Data are expressed as mean ( SEM. *P
< 0.05 Mann-Whitney analysis. n g 9 animals.
rat pups, is used as a model predictive of efficacy in ADHD.33
The model evaluates the ability of an agent to improve the rate
at which a rat pup learns, during five trials, to avoid entering a
darkened chamber where it receives a mild foot shock.
Compounds were injected subcutaneously (s.c.) in the scruff
of the neck 30 min prior to the training. Behavioral responses
are expressed as the combined transfer latency, which is the
sum of time over the five trials the rat spends before entering
the darkened chamber. Compounds 3a, 3d, and 3e were eval-
uated in this model and compared to the activity of the standard
histamine H3 antagonist ciproxifan (tested at 3 mg/kg). All three
of the new compounds were found to be efficacious in this
model (see Figure 3 for graph of compound 3d). Evaluation of
plasma concentrations sampled at the efficacious doses revealed
that compounds 3d and 3e were very potent, with behavioral
efficacy seen at plasma levels of 19.2 ng/mL at the 0.3 mg/kg
dose for 3d and 2.5 ng/mL at the 0.1 mg/kg dose for 3e.
We were surprised to find that compound 3a was relatively
weak in vivo, despite its excellent potency and PK profile in
rat and very high brain levels in mouse. One rationale for this
could be that compound 3a might have a high propensity for
nonspecific binding to brain tissue, such as cell membranes and
lysosomes, thus accounting for its very high brain levels.30 How-
ever, if there was a very high level of nonspecific binding of
3a to CNS tissues, the actual amount of free unbound 3a in the
brain may be low compared to compounds such as 3d and 3e,
which had lower brain penetration but were more potent in vivo.
The overall favorable behavioral profiles of compounds 3d
and 3e in the SR and 5-trial IA in vivo models led us to assess
these agents further. Administered i.p., no untoward CNS side
effects were seen for 3d in rat at doses up to 2.8 mg/kg in a
general test for CNS side effects.31 At higher doses (9.4 mg/
kg), mild hypoactivity and piloerection were noted; however,
these effects are present at 30-fold above the behaviorally
effective dose (0.3 mg/kg in the 5-trial IA). Compound 3e was
free of overt CNS side effects32 at 2.8 mg/kg and mild tremor
and mild hypoactivity were evident at 9.4 mg/kg, 94-fold above
its active dose (0.1 mg/kg in the 5-trial IA).
Testing for genetoxicity found that compounds 3d and 3e
were nonclastogenic and nonmutagenic.39 Also, no inhibition
of any key CYP-P450 enzymes (1A2, 2A6, 2C19, 2C9, 2D6,
2E1, or 3A4) was observed at 10-20 µM, and so it was judged
that 3d and 3e had low potential for drug-drug related
interactions. Further PK studies indicated good bioavailability
for 3d and 3e in dog (F ) 72 ( 10 and 77 ( 10%, respectively).
The compounds showed high whole-cell permeability in Caco-2
membranes, supporting an expectation of efficient oral absorp-
tion. Both compounds 3d and 3e had good water solubility (8.7
and 21 mg/mL, respectively, at pH 7), consistent with their
moderate polarity and single positive molecular charge arising
from a basic pyrrolidine (measure pKb of 9.7).
Further studies on the pharmaceutic properties of compounds
3d and 3e did uncover an interesting observation: a potential
problem with photochemical stability. Stability studies revealed
that the free bases of compounds 3d and 3e were unstable
toward UV light, especially at high pH and in the solid phase.40
This instability was believed likely to be due to the quinoline
moiety because other nonquinoline containing agents (naphth-
ylenes) of otherwise similar structure did not exhibit this
instability. Salts of compound 3e were examined and displayed
an improvement in the stability relative to the free base.41 In
cardiovascular studies in the anesthetized dog, the principal side
effect seen for compound 3d was an increase in heart contractil-
ity (9 ( 1% increase in dP/dtmax at a blood concentration of
108 ( 24 ng/mL).42 The photo instability of members of the
quinoline series and the cardiovascular liabilities of compound
3d led us to discontinue compounds from this quinoline series
for further evaluation as potential clinical candidates.
In summary, a new series of quinoline-containing H3R
antagonists was discovered. The compounds are potent inverse
agonists at the rat and human H3R (Ki 1.95-0.17 nM) and are
selective for the H3R over other histamine subtypes and a variety
of other receptors and ion channels. The compounds were found
to have desirable drug-like characteristics of cognitive agents
such as good PK, moderate protein binding across species and
the ability to cross the BBB. Analogs 3d and 3e were tested in
vivo in two rodent behavioral models and, consistent with
profiles previously reported for H3R antagonists, both 3d and
An examination of the off-target binding profile was con-
ducted (Cerep and in-house). Tested at a concentration of 10
µM, 3d was found to be highly selective for the H3R. Except
for the sigma receptor (96%), only weak binding was seen (%
inhibition at 10 µM in parentheses) at adenosine-A3 (69%
inhibition), muscarinic-M4 (74%), and L-type calcium channels
(63%). For compound 3e, only sigma receptors (86%) showed
significant affinity at 10 µM.

5444 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 22
Altenbach et al.
8.2, 1.0 Hz, 1H), 7.91 (d, J ) 8.8 Hz, 1H), 7.96 (d, J ) 8.8 Hz,
1H), 8.03-8.10 (m, 1H), 8.33 (d, J ) 8.1 Hz, 1H), 8.39 (s, 1H),
8.57-8.60 (m, 1H); MS (DCI/NH3) m/z 398 (M + H)+; Anal.
(C25H27N5‚C4H6O6‚H2O) C, H, N.
3e were efficacious in these in vivo models of learning and
attention and may be useful as tool compounds.
Experimental Section
6-{2-[(2R)-2-Methyl-pyrrolidin-1-yl]ethyl}-2-(4-methyl-2-pyr-
rolidin-1-ylpyrimidin-5-yl)quinoline phosphate (3c). The title
compound was prepared using the procedure described for com-
pound 14b and 3b, substituting 1-(4-methyl-2-pyrrolidin-1-ylpy-
rimidin-5-yl)ethanone43 (compound 9c) for compound 9b. The free
base of the product was treated with 1 molar equiv of a 0.1 M
solution of H3PO4 in 19:1 MeOH/H2O and concentrated. Upon
addition of CH3CN, the salt crystallized and was collected by
filtration, washed with CH3CN, and dried under vacuum to provide
the corresponding H3PO4 salt. Mp 209-212 °C; 1H NMR (DMSO-
d6) δ 1.14 (d, J ) 6.1 Hz, 3H), 1.33-1.50 (m, 1H), 1.69-1.83 (m,
2H), 1.88-2.04 (m, 5H), 2.53 (s, 3H), 2.61-2.78 (m, 2H), 2.93-
3.14 (m, 2H), 3.17-3.38 (m, 2H), 3.53-3.59 (m, 4H), 7.70 (m,
1H), 7.71 (d, J ) 8.5 Hz, 1H), 7.85 (s, 1H), 7.95 (d, J ) 8.8 Hz,
1H), 8.33 (d, J ) 8.8 Hz, 1H), 8.54 (s, 1H); MS (DCI/NH3) m/z
402 (M + H)+; Anal. (C25H31N5‚H3PO4‚0.1 H2O) C, H, N.
2-(2,7-Dimethylpyrazolo[1,5-a]pyrimidin-6-yl)-6-{2-[(2R)-2-
methylpyrrolidin-1-yl]ethyl}quinoline maleate (3d). Compound
8 (2.08 g, 8.9 mmol) was treated with EtOH (90 mL), treated with
compound 9d (2.03 g, 10.7 mmol), treated with a satd soln of KOH
in EtOH (0.9 mL), heated to 75 °C for 16 h, cooled, concentrated,
and chromatographed, using a gradient of 3:1, 1:1, 1:2, and 1:4
EtOAC/[4:1:1 EtOAc/HCOOH/H2O]. The fractions containing the
product were concentrated to dryness, treated with 1 M NaOH (100
mL), treated with CH2Cl2, and filtered through celite. The layers
were separated, and the aqueous layer was extracted with CH2Cl2
(2 × 100 mL). The combined organic layers were dried (MgSO4),
filtered, concentrated, and chromatographed using a gradient of 2
and 3.5% (9:1 MeOH/satd NH4OH) in CH2Cl2 to provide 2.1 g
(60%) of the title compound. The free base was converted to the
maleic acid salt and crystallized from acetone/EtOAc. Mp 151-
General. Proton NMR spectra were obtained on a Varian
Mercury plus 300 or Varian UNITY plus 300 MHz instrument with
chemical shifts (δ) reported relative to tetramethylsilane as an
internal standard. Melting points were determined on a Thomas-
Hoover capillary melting point apparatus and are uncorrected.
Elemental analyses were performed by Quantitative Technologies,
Inc. Column chromatography was carried out on silica gel 60 (230-
400 mesh). Thin-layer chromatography (TLC) was performed using
250 mm silica gel 60 glass-backed plates with F254 as indicator.
2-[1-(6-Ethoxypyridazin-3-yl)-5-methyl-1H-pyrazol-4-yl]-6-[2-
(2-(R)-methylpyrrolidin-1-yl)ethyl]quinoline L-Tartrate (3a).
Compound 8 (0.66 g, 2.9 mmol) and compound 9a (1.0 g, 4.3
mmol) were treated with a solution of NaOH (0.29 g, 7.2 mmol)
in EtOH (80 mL), refluxed overnight, cooled, concentrated, and
chromatographed using 1 and 2% (9:1 MeOH/satd NH4OH) in CH2-
Cl2 to provide 0.32 g (25%) of the title compound, which was
1
converted to the L-tartaric acid salt. H NMR (CD3OD) δ 1.49 (t,
J ) 7.1 Hz, 3H), 1.48 (d, J ) 6.1 Hz, 3H), 1.78 (m, 1H), 2.12 (m,
2H), 2.34 (m, 1H), 2.99 (s, 3 H), 3.17-3.43 (m, 4H), 3.58 (m,
1H), 3.72 (m, 2H), 4.59 (q, J ) 7.1 Hz, 2H), 7.37 (d, J ) 9.2 Hz,
1H), 7.73 (dd, J ) 8.8, 2.0 Hz, 1H), 7.84 (d, J ) 8.5 Hz, 1H), 7.87
(d, J ) 1.7 Hz, 1H), 8.05 (m, 2H), 8.29 (m, 4H); MS (DCI/NH3)
m/z 443 (M + H)+. Anal. (C26H30N6O‚C4H6O6‚2H2O) C, H, N.
2-(5-Methyl-1-pyridin-2-yl-1H-pyrazol-4-yl)-6-[2-(2-(R)-me-
thylpyrrolidin-1-yl)ethyl]quinoline L-Tartrate (3b). Scheme 2,
Step vi: Compound 14b (3.86 g, 11.7 mmol) was suspended in
CH2Cl2 (120 mL), treated with Et3N (2.45 mL, 17.6 mmol), treated
with MsCl (1.2 mL, 15.2 mmol), stirred at ambient temperature
for 2 h, treated with H2O (50 mL), and the layers were separated.
The aqueous layer was extracted with CH2Cl2 (2 × 25 mL), and
the combined organic layers were dried (MgSO4), filtered, and
concentrated to provide the 4.78 g (100%) of the mesylate
(methanesulfonic acid, 2-[2-(5-methyl-1-pyridin-2-yl-1H-pyrazol-
1
153 °C; H NMR (DMSO-d6) δ 1.39 (d, J ) 6.1 Hz, 3H), 1.52-
1.70 (m, 1H), 1.88-2.07 (m, 2H), 2.16-2.30 (m, 1H), 2.51 (s,
3H), 2.91 (s, 3H), 3.14-3.41 (m, 5H), 3.59-3.75 (m, 2H), 7.82
(dd, J ) 8.8, 1.7 Hz, 1H), 7.92 (d, J ) 8.5 Hz, 1H), 7.99 (d, J )
1.7 Hz, 1H), 8.10 (d, J ) 8.5 Hz, 1H), 8.49 (d, J ) 8.5 Hz, 1H),
8.74 (s, 1H), 9.22 (bs, 1H); MS (DCI/NH3) m/z 386 (M + H)+;
Anal. (C24H27N5‚C4H4O4) C, H, N.
(R)-2-(1,3-Dimethyl-1H-pyrazol-4-yl)-6-(2-(2-methylpyrroli-
din-1-yl)ethyl)quinoline (3e). The title compound was prepared
using the procedure described for compound 3d substituting
compound 9e for compound 9d as the free base. Mp 133-134 °C;
1H NMR (CDCl3) δ 1.14 (d, J ) 6.1 Hz, 3H), 1.46 (m, 1H), 1.65-
2.02 (m, 3H), 2.24 (q, J ) 8.7 Hz, 1H), 2.32-2.46 (m, 2H), 2.64
(s, 3H), 2.91-3.19 (m, 3H), 3.30 (dt, J ) 8.5, 2.71 Hz, 1H), 3.91
(s, 3H), 7.51-7.61 (m, 3H), 7.90 (s, 1H), 7.97 (d, J ) 8.1 Hz,
1H), 8.05 (d, J ) 8.8 Hz, 1H); MS (DCI/NH3) m/z 335 (M + H)+;
Anal. (C21H26N4) C, H, N.
1
4-yl)quinolin-6-yl]ethyl ester) as a white solid. H NMR (CDCl3)
δ 3.05 (s, 3H), 3.14 (s, 3H), 3.22 (t, J ) 6.6 Hz, 2H), 4.55 (t, J )
6.6 Hz, 2H), 7.45-7.50 (m, 1H), 7.74 (dd, J ) 8.6, 1.9 Hz, 1H),
7.86-7.91 (m, 2H), 7.94 (d, J ) 8.5 Hz, 1H), 7.99 (d, J ) 8.5 Hz,
1H), 8.04-8.10 (m, 1H), 8.40 (d, J ) 8.5 Hz, 1H), 8.41 (s, 1H),
8.57-8.61 (m, 1H); MS (DCI/NH3) m/z 409 (M + H)+. Step vii:
(R)-2-Methylpyrrolidine L-tartrate19 (4.1 g, 17.2 mmol) was treated
with 50% NaOH soln (5.5 g, 68 mmol), treated with brine (10 mL),
and extracted with toluene (15 mL). This solution of (R)-2-
methylpyrrolidine in toluene was added to a suspension of the
mesylate (3.5 g, 8.6 mmol) in CH3CN (15 mL). The resulting
mixture was treated with Cs2CO3 (5.6 g, 17 mmol), stirred at
80 °C for 16 h in a sealed flask, cooled, and filtered through
diatomateous earth to remove the solids. The solids were washed
with CH2Cl2, and the combined filtrates were concentrated and
chromatographed using a gradient of EtOAc/HCOOH/H2O (22:1:
1, 10:1:1, 4:1:1, and 3:2:1). The fractions containing the product
were concentrated to dryness, treated with 1 M NaOH (100 mL),
treated with CH2Cl2 (100 mL), filtered through celite, and the layers
were separated. The aqueous layer was further extracted with CH2-
Cl2 (2 × 100 mL). The combined CH2Cl2 extractions were dried
(MgSO4), filtered, concentrated, and chromatographed using a
gradient of 2, 3.5, 4, 4.5, and 5% (9:1 MeOH/satd aq NH4OH) in
CH2Cl2 to provide 2.75 g (81%) of the title compound. This free
base (2.73 g, 6.87 mmol) was treated with L-tartaric acid (1.03 g,
6.87 mmol), treated with 3 mL of H2O, concentrated to dryness,
and crystallized from MeOH/EtOAc. The crystals were collected
by filtration, washed with EtOAc, and dried under vacuum to
provide 3.27 g (87%) of the salt. Mp 123-131 °C; 1H NMR
(DMSO-d6) δ 1.22 (d, J ) 6.1 Hz, 3H), 1.40-1.57 (m, 1H), 1.77-
1.90 (m, 2H), 1.99-2.12 (m, 1H), 3.06 (s, 3H), 2.74-3.14 (m,
5H), 3.31-3.51 (m, 2H), 4.04 (s, 2H), 7.43-7.51 (m, 1H), 7.70
(dd, J ) 8.5, 2.0 Hz, 1H), 7.83 (d, J ) 1.7 Hz, 1H), 7.88 (dt, J )
2-Methyl-3-{6-[2-([2R]-2-methylpyrrolidin-1-yl)-ethyl]quino-
lin-2-yl}-[1,8]naphthyridine Phosphate (3f). The title compound
was prepared using the procedure described for compound 14b and
3b, substituting 1-(2-methyl-[1,8]-naphthyridin-3-yl)ethanone44 (com-
pound 9f) for compound 9b. The free base of the product was
converted to the H3PO4 salt as described for compound 3c. Mp
1
142-153 °C; H NMR (DMSO-d6) δ 1.17 (d, J ) 6.1 Hz, 3H),
1.45 (m, 1H), 1.79 (m, 2H), 2.00 (m, 1H), 2.54-2.87 (m, 3H),
2.80 (s, 3H), 3.10 (m, 2H), 3.23-3.43 (m, 2H), 7.64 (dd, J ) 8.1,
4.4 Hz, 1H), 7.78 (dd, J ) 8.6, 1.5 Hz, 1H), 7.89 (d, J ) 8.5 Hz,
1H), 7.95 (s, 1H), 8.05 (d, J ) 8.8 Hz, 1H), 8.49 (d, J ) 8.8 Hz,
1H), 8.53 (dd, J ) 8.1, 1.7 Hz, 1H), 8.60 (s, 1H), 9.10 (dd, J )
4.2, 1.9 Hz, 1H); MS (DCI/NH3) m/z 383 (M + H)+; Anal.
(C25H26N4‚H3PO4‚0.5H2O‚0.25CH3CN) C, H, N.
(2R)-2-Methyl-1-[2-(4-nitrophenyl)ethyl]pyrrolidine (5). (R)-
2-Methylpyrrolidine L-tartrate (4;19 4.0 g, 17.0 mmol), 1-(2-
bromoethyl)-4-nitrobenzene (9.8 g, 43 mmol), and K2CO3 (12 g,
85 mmol), were combined in DMF (20 mL) in a sealed tube at
50 °C and stirred vigorously for 16 h. The mixture was allowed to

Histamine H3 Receptor InVerse Agonists
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 22 5445
cool to room temperature, diluted with Et2O (100 mL), washed
with water (2×, 100 mL and then 50 mL), and extracted twice
with 1 M HCl (50 mL and 25 mL). The aqueous acidic extractions
were combined, washed with Et2O (50 mL), cooled to 0 °C, adjusted
to pH 14 with 50% NaOH solution, and extracted with CH2Cl2
(3×, 50 mL). The CH2Cl2 extractions were combined, dried
(MgSO4), and filtered, and the filtrate was concentrated to provide
3.85 g (97%) of compound 5. 1H NMR (CDCl3) δ 1.08 (d, J ) 6.1
Hz, 3H), 1.43 (m, 1H), 1.75 (m, 2H), 1.93 (m, 1H), 2.19 (q, J )
8.7 Hz, 1H), 2.34 (m, 2H), 2.91 (m, 2H), 3.03 (m, 1H), 3.22 (td, J
) 8.5, 3.0 Hz, 1H), 7.38 (m, 2H), 8.15 (m, 2H); MS (DCI/NH3)
m/z 235 (M + H)+.
2,2-Dimethyl-N-(4-{2-[(2R)-2-methyl-1-pyrrolidinyl]ethyl}-
phenyl)propanamide (6). Step ii: Compound 5 (3.85 g, 16.4
mmol) was hydrogenated using 10% Pd/C (0.39 g) in MeOH (20
mL) under 1 atm H2 for 16 h. After the H2 was replaced with N2,
the mixture was diluted with MeOH (150 mL), stirred for 15 min,
and filtered, and the filtrate was concentrated to provide 3.24 g
(97%) of the aniline, (R)-4-(2-(2-methylpyrrolidin-1-yl)ethyl)aniline.
1H NMR (CDCl3) δ 1.11 (d, J ) 6 Hz, 3H), 1.43 (m, 1H), 1.74
(m, 2H), 1.90 (m, 1H), 2.25 (m, 3H), 2.70 (m, 2H), 2.97 (m, 1H),
3.24 (td, J ) 9, 3 Hz, 1H), 3.55 (s, 2H), 6.63 (d, J ) 8 Hz, 2H),
7.01 (d, J ) 8 Hz, 2H); MS (DCI/NH3) m/z 205 (M + H)+. Step
iii: The aniline (2.77 g, 14 mmol) was dissolved in anhydrous CH2-
Cl2 (70 mL) under N2, treated with Et3N (2.3 mL, 16 mmol), cooled
to 0 °C, treated with trimethylacetyl chloride (1.9 mL, 15 mmol),
stirred at ambient temperature for 60 h, and treated with 1 M NaOH
(40 mL). The layers were separated, and the aqueous layer was
extracted with CH2Cl2 (2 × 40 mL). The combined CH2Cl2 layers
were dried (MgSO4) and filtered, and the filtrate was concentrated
to provide 4.0 g (99%) of the title compound. 1H NMR (CDCl3) δ
1.10 (d, J ) 6.1 Hz, 3H), 1.31 (s, 9H), 1.44 (m, 1H), 1.76 (m, 2H),
1.92 (m, 1H), 2.18 (q, J ) 8.8 Hz, 1H), 2.27 (m, 2H), 2.78 (m,
2H), 2.99 (m, 1H), 3.23 (td, J ) 9.2, 3.0 Hz, 1H), 7.17 (d, J ) 8.5
Hz, 2H), 7.44 (d, J ) 8.5 Hz, 2H); MS (DCI/NH3) m/z 289 (M +
H)+.
N-(2-Formyl-4-{2-[(2R)-2-methyl-1-pyrrolidinyl]ethyl}phenyl)-
2,2-dimethylpropanamide (7). A mixture of compound 6 (4.0 g,
13.9 mmol) under N2 in anhydrous Et2O (140 mL) was treated with
TMEDA (6.5 mL, 43 mmol), cooled to 0 °C, treated with n-BuLi
(16.7 mL of a 2.5 M solution in hexanes, 41.8 mmol) over 10 min,
stirred for 4 h at ambient temperature, cooled to 0 °C, treated all at
once with anhydrous DMF (6.5 mL, 83 mmol), stirred for 16 h at
ambient temperature, diluted with Et2O (100 mL), washed with
water (75 mL), washed with brine, dried (MgSO4), and filtered,
and the filtrate was concentrated. The residue was purified by
chromatography on silica gel eluting with a gradient of 2, 3.5, 5,
and 7.5% (9:1 MeOH/concd NH4OH) in CH2Cl2 to provide 3.57 g
(81%) of the title compound. 1H NMR (CDCl3) δ 1.10 (d, J ) 6.1
Hz, 3H), 1.35 (s, 9H), 1.44 (m, 1H), 1.75 (m, 2H), 1.93 (m, 1H),
2.19 (q, J ) 8.7 Hz, 1H), 2.31 (m, 2H), 2.85 (m, 2H), 3.01 (m,
1H), 3.23 (td, J ) 8.5, 3.0 Hz, 1H), 7.47 (dd, J ) 8.6, 2.2 Hz, 1H),
7.51 (d, J ) 2.4 Hz, 1H), 8.71 (d, J ) 8.5 Hz, 1H), 9.92 (s, 1H),
11.31 (s, 1H); MS (DCI/NH3) m/z 317 (M + H)+.
2-Amino-5-{2-[(2R)-2-methyl-1-pyrrolidinyl]ethyl}-
benzaldehyde (8). Compound 7 (2.46 g, 7.8 mmol) in 3 M HCl
(40 mL) was heated at 80 °C for 4 h, allowed to cool to room
temperature, and carefully poured into a mixture of 1 M NaOH
(250 mL) and CH2Cl2 (75 mL). The layers were separated, and the
aqueous layer was extracted with CH2Cl2 (2 × 75 mL). The
combined CH2Cl2 layers were dried (MgSO4) and filtered, and the
filtrate was concentrated. The residue was purified by chromatog-
raphy on silica gel eluting with a gradient of 2, 3.5, and 5% (9:1
MeOH/concd NH4OH) in CH2Cl2 to provide 1.23 g (68%) the title
compound. 1H NMR (CDCl3) δ 1.12 (d, J ) 6.1 Hz, 3H), 1.50 (m,
1H), 1.76 (m, 2H), 1.93 (m, 1H), 2.25 (m, 3H), 2.76 (m, 2H), 2.99
(m, 1H), 3.25 (td, J ) 8.7, 2.7 Hz, 1H), 5.99 (s, 2H), 6.60 (d, J )
8.5 Hz, 1H), 7.19 (dd, J ) 8.3, 2.2 Hz, 1H), 7.31 (d, J ) 2.4 Hz,
1H), 9.85 (d, 0.7 Hz, 1H).
14 mmol) and 3-dimethylaminomethylenepentane-2,4-dione21b (2.6
g, 17 mmol) in EtOH (80 mL) was heated to reflux for 3 h and
allowed to stand at ambient temperature overnight. The solid was
collected by filtration, washed with ethanol, and dried under vacuum
to provide the title compound. 1H NMR (DMSO-d6) δ 2.49 (s, 3H),
2.85 (s, 3H), 8.17 (d, J ) 9.5 Hz, 1H), 8.23 (d, J ) 9.2 Hz, 1H),
8.43 (s, 1H); MS (DCI/NH3) m/z 237 (M + H)+.
1-(2,7-Dimethylpyrazolo[1,5-a]pyrimidin-6-yl)ethanone (9d).
A solution of 3-dimethylaminomethylenepentane-2,4-dione21b (4 g,
25.8 mmol) and 3-amino-5-methylpyrazole (2.45 g, 24.5 mmol) in
EtOH (50 mL) was heated to reflux for 2 h, cooled to ambient
temperature, and stirred overnight. The solid precipitate was
collected by filtration, washed with cold EtOH (50 mL), and dried
1
under vacuum to provided 3.8 g (82%) of the title compound. H
NMR (CD3OD) δ 2.52 (s, 3H), 2.68 (s, 3H), 3.07 (s, 3H), 6.55 (s,
1H), 6.87 (s, 1H); MS (DCI/NH3) m/z 190 (M + H)+.
1-(1,3-Dimethyl-1H-pyrazol-4-yl)ethanone (9e). Compound 17
(9.1 g, 46 mmol) was dissolved in EtOH (20 mL), treated with 1
M HCl (20 mL), stirred at ambient temperature for 15 min, con-
centrated to approximately 20 mL total volume under vacuum using
a rotovap, treated with satd NaHCO3 (45 mL), and extracted with
CH2Cl2 (4 × 50 mL). The combined CH2Cl2 layers were dried
(MgSO4), filtered, and concentrated to provide 6.2 g (96%) of the
1
title compound as a solid. H NMR (CDCl3) δ 2.39 (s, 3H), 2.47
(s, 3H), 3.86 (s, 3H), 7.76, (s, 1H); MS (DCI/NH3) m/z 139 (M +
H)+.
N-{4-[2-(tert-Butyldimethylsilanyloxy)ethyl]phenyl}-2,2-dim-
ethylpropionamide (11). Step i: 2-(4-Aminophenyl)ethanol (10;
23.24 g, 0.17 mol) was dissolved in CH2Cl2 (670 mL), treated with
Et3N (28.1 mL, 0.20 mol), cooled to 0 °C, treated with trimethy-
lacetyl chloride (22.8 mL, 0.185 mol) over 10 min, stirred at
ambient temperature for 16 h, and treated with 1 M NaOH (250
mL), and the layers were separated. The NaOH layer was extracted
with CH2Cl2 (100 mL). The combined organic layers were dried
(MgSO4), filtered, and concentrated. The residue was recrystallized
from 100 mL of EtOAc, which was treated slowly with 25 mL of
hexane while still hot. The resulting solid was collected by filtration,
washed with 1:1 hexane/EtOAc, and dried under vacuum to provide
29.1 g (78%) of the alcohol, N-[4-(2-hydroxyethyl)phenyl]-2,2-
1
dimethylpropionamide. H NMR (CDCl3) δ 1.31 (s, 9H), 2.83 (t,
J ) 6.6 Hz, 2H), 3.83 (t, J ) 6.6 Hz, 2H), 7.18 (d, 8.5 Hz, 2H),
7.29 (bs, 1H), 7.46 (d, 8.5 Hz, 2H); MS (DCI/NH3) m/z 222 (M +
H)+. Step ii: The alcohol from above (30.76 g, 0.14 mol) was
treated with CH2Cl2 (195 mL), treated with tert-butyldimethylchlo-
rosilane (25.1 g, 0.17 mol), treated with imidazole (13.2 g, 0.19
mol), stirred at ambient temperature for 2 h, treated with hexane
(195 mL), stirred at ambient temperature for 5 min, and filtered to
remove the solid. The solid was washed with 1:1 CH2Cl2/hexane.
The combined filtrates were concentrated and chromatographed
using a gradient of hexane/EtOAc (50:1, 40:1, 20:1, 10:1, 5:1, and
4:1) to provide 45.7 g (98%) of the title compound. 1H NMR
(CDCl3) δ -0.01 (s, 6H), 0.88 (s, 9H), 1.32 (s, 9H), 2.79 (t, J )
7.1 Hz, 2H), 3.78 (t, J ) 7.0 Hz, 2H), 7.16 (d, J ) 8.5 Hz, 2H),
7.28 (bs, 1H), 7.44 (d, J ) 8.5 Hz, 2H); MS (DCI/NH3) m/z 336
(M + H)+.
N-{4-[2-(tert-Butyldimethylsilanyloxy)ethyl]-2-formylphenyl}-
2,2-dimethylpropionamide (12). Compound 11 (35.24 g, 105
mmol) was treated with anhydrous Et2O (1.05 L) under N2, cooled
to 0 °C, treated with anhydrous TMEDA (38.0 mL, 0.25 mol),
treated with n-BuLi (97 mL of a 2.5 M soln in hexane, 0.24 mol),
stirred at ambient temperature for 2 h, cooled to -10 °C, treated
all at once with anhydrous DMF (49 mL, 0.63 mol), stirred at
ambient temperature for 16 h, washed with H2O (400 mL and then
250 mL), washed with brine (50 mL), dried (MgSO4), filtered,
concentrated, and chromatographed using a gradient of hexane/
EtOAc (40:1, 20:1, and 15:1) to provide 31.87 g (83%) of the title
1
compound. H NMR (CDCl3) δ -0.01 (s, 6H), 0.89 (s, 9H), 1.38
(s, 9H), 2.86 (t, J ) 6.4 Hz, 2H), 3.83 (t, J ) 6.4 Hz, 2H), 7.48
(dd, J ) 8.7, 2.2 Hz, 1H), 7.54 (d, J ) 2.0 Hz, 1H), 8.73 (d, J )
8.5 Hz, 1H), 9.93 (s, 1H), 11.33 (bs, 1H); MS (DCI/NH3) m/z 364
(M + H)+.
1-[1-(6-Chloropyridazin-3-yl)-5-methyl-1H-pyrazol-4-yl]ethan-
1-one (9a). A mixture of 3-chloro-6-hydrazinopyridazine (2.0 g,

5446 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 22
Altenbach et al.
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in rat cerebral cortex. Br. J. Pharmacol. 1996, 118, 2045-2052.
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Hancock, A. A. Medicinal chemistry and biological properties of
non-imidazole histamine H3 antagonists. Mini-ReV. Med. Chem. 2004,
4, 979-992. (b) Stark, H. Recent advances in histamine H3/H4
receptor ligands. Expert Opin. Ther. Pat. 2003, 13, 851-865. (c)
Celanire, S.; Wijtmans, M.; Talaga, P.; Leurs, R.; de Esch, I. J. P.
Histamine H3 receptor antagonists reach out for the clinic. Drug
DiscoVery Today 2005, 10, 1613-1627.
2-Amino-5-(2-hydroxyethyl)benzaldehyde (13). Compound 12
(9.0 g, 24.8 mmol) was treated with EtOH (450 mL), treated with
2 M HCl (900 mL), heated to reflux for 30 min, concentrated by
removing the EtOH by distillation at atmospheric pressure over a
1.5 h period, cooled to 0 °C, treated with NaOH soln (50% by
weight, 150 g), and extracted with CH2Cl2 (5 × 250 mL). The
combined CH2Cl2 layers were dried (MgSO4), filtered, concentrated
1
to provide 2.90 g (35%) of the title compound. H NMR (CDCl3)
δ 2.80 (t, J ) 6.4 Hz, 2H), 3.84 (t, J ) 6.4 Hz, 2H), 6.03 (bs, 2H),
6.63 (d, J ) 8.5 Hz, 1H), 7.20 (dd, J ) 8.5, 2.0 Hz, 1H), 7.34 (d,
J ) 2.0 Hz, 1H), 9.85 (s, 1H); MS (DCI/NH3) m/z 166 (M + H)+.
2-[2-(5-Methyl-1-pyridin-2-yl-1H-pyrazol-4-yl)-quinolin-6-yl]-
ethanol (14b). Compound 13 (3.83 g, 23.2 mmol) was treated with
1-(5-methyl-1-pyridin-2-yl-1H-pyrazol-4-yl)ethanone45 (compound
9b; 6.06 g, 30.1 mmol), treated with EtOH (230 mL), treated with
a satd soln of KOH in EtOH (2 mL), heated to reflux for 9 h, stirred
at ambient temperature for 7 h, concentrated, and chromatographed
using a gradient of EtOAc/CH2Cl2 (1:1, 2:1, and 2:0) to provide
4.67 g (61%) of the title compound. 1H NMR (CDCl3) δ 2.93 (t, J
) 6.8 Hz, 2H), 3.06 (s, 3H), 3.68-3.78 (m, 2H), 4.71 (t, J ) 5.1
Hz, 1H), 7.41-7.52 (m, 1H), 7.64 (dd, J ) 8.5, 2.0 Hz, 1H), 7.76
(d, J ) 1.4 Hz, 1H), 7.86-7.93 (m, 3H), 8.01-8.12 (m, 1H), 8.31
(d, J ) 8.1 Hz, 1H), 8.39 (s, 1H), 8.56-8.62 (m, 1H); MS (DCI/
NH3) m/z 331 (M + H)+.
1-Methyl-2-(propan-2-ylidene)hydrazine (16).46 Acetone (100
mL) was treated dropwise with methyl hydrazine (15; 20 mL, 0.38
mol), stirred at ambient temperature for 45 min, heated to 55 °C
for 15 min, cooled to ambient temperature, dried (MgSO4), filtered,
concentrated, and distilled (bp 110-116 °C at atmospheric pressure)
1
to provide 14.2 g (44% yield) of the title compound. H NMR
(CDCl3) δ 1.75 (s, 3H), 1.94 (s, 3H), 2.92 (s, 3H), 3.82 (s, 1H).
3-((1-Methyl-2-(propan-2-ylidene)hydrazinyl)methylene)pen-
tane-2,4-dione (17). 3-(Ethoxymethylene)pentane-2,4-dione23 (8.6
g, 55 mmol) was dissolved in ether (30 mL), cooled to 0 °C, treated
with compound 16 (4.8 g, 55 mmol) dropwise over 5 min, and
stirred at ambient temperature overnight. The mixture was directly
chromatographed on silica gel using a gradient (1:1:0, 1:2:0, 0:1:
0, 0:9:1, and 0:4:1) of hexane/EtOAC/EtOH to provide 9.1 g (84%)
(12) Stark, H.; Sadek, B.; Krause, M.; Huels, A.; Ligneau, X.; Ganellin,
C. R.; Arrang, J.-M.; Schwartz, J.-C.; Schunack, W. Novel histamine
H3-receptor antagonists with carbonyl-substituted 4-(3-(phenoxy)-
propyl)-1H-imidazole structures like ciproxifan and related com-
pounds. J. Med. Chem. 2000, 43, 3987-3994.
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Y. L.; Black, L. A.; Pan, L.; Marsh, K. C.; Sullivan, J. P.; Esbenshade,
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histamine-3 receptor ligands. U.S. Patent US7094790, 2006; B2. (b)
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R.; Gfesser, G. A.; Ku, Y.-Y.; Liu, H.; Lukin, K. A.; Nersesian, D.
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Marsh, K. C.; Browman, K. E.; Fox, G. B.; Miller, T. R.; Esbenshade,
T. A.; Hancock, A. A.; Cowart, M. D. 6-Substituted-2-naphthylethy-
lamines as selective histamine H3 receptor antagonists. International
Chemical Congress of Pacific Basin Societies (Pacifichem 2005)
Medicinal Chemistry, 2005; Poster 262. (b) Black, L. A.; Nersesian,
D. L.; Sharma, P.; Bennani, Y. L.; Marsh, K. C.; Miller, T. R.;
Esbenshade, T. A.; Hancock, A. A.; Cowart, M. 4-[6-(2-Aminoethyl)-
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(16) Black, L. A.; Nersesian, D. L.; Gfesser, G. A.; Curtis, M. P.; Faghih,
R.; Bennani, Y. L.; Browman, K. E.; Fox, G. B.; Wetter, J.; Marsh,
K. C.; Miller, T. R.; Esbenshade, T. A.; Hancock, A. A.; Brioni, J.;
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as selective histamine H3 receptor antagonists. 30th National
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(17) (a) Altenbach, R. J.; Liu, H.; Esbenshade, T. A.; Fox, G. B.; Hancock,
A. A.; Cowart, M. Bicyclic heteroaromatic histamine H3 antago-
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the American Chemical Society, Aug 28-Sept 1, 2005, MEDI-102.
(b) Liu, H.; Altenbach, R. J.; Miller, T. R.; Esbenshade, T. A.;
1
of the title compound. H NMR (CDCl3) δ1.96 (s, 3H), 2.07 (s,
3H), 2.28 (s, 6H), 3.12 (s, 3H), 7.34 (s, 1H); MS (DCI/NH3) m/z
197 (M + H)+.
Supporting Information Available: Combustion analysis of
compounds 3a-3f and bar graphs of in vivo results from Table 7.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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