Synthesis and Structure-Activity Relationships of Conformationally Constrained Histamine H3 Receptor Agonists

Article abstract of DOI:10.1021/jm030905y

Immepip, a conformationally constrained analogue of the histamine congener imbutamine, shows high affinity and functional activity on the human H ₃ receptor. Using histamine and its homologues as prototypes, other rigid analogues containing eit

Full text of DOI:10.1021/jm030905y

J . Med. Chem. 2003, 46, 5445-5457
5445
Syn th esis a n d Str u ctu r e-Activity Rela tion sh ip s of Con for m a tion a lly
Con str a in ed Hista m in e H3 Recep tor Agon ists
Ruengwit Kitbunnadaj,†,‡ Obbe P. Zuiderveld,† Iwan J . P. De Esch,† Roeland C. Vollinga,† Remko Bakker,†
Martin Lutz,§ Anthony L. Spek,§ Emile Cavoy,|| Marie-France Deltent,|| Wiro M. P. B. Menge,†
Henk Timmerman,† and Rob Leurs*,†
Leiden/ Amsterdam Center of Drug Research (LACDR), Division of Medicinal Chemistry, Department of
Pharmacochemistry, Faculty of Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands,
Department of Pharmaceutical Chemistry and Pharmacognosy, Faculty of Pharmaceutical Sciences, Naresuan University,
Phitsanulok, Thailand, Bijvoet Center for Biomolecular Research, Department of Crystal and Structural Chemistry,
Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands, and UCB Pharma, Chemin du Foriest,
B-1420 Braine-1’ Alleud, Belgium
Received May 23, 2003
Immepip, a conformationally constrained analogue of the histamine congener imbutamine,
shows high affinity and functional activity on the human H3 receptor. Using histamine and its
homologues as prototypes, other rigid analogues containing either a piperidine or pyrrolidine
ring in the side chain were synthesized and tested for their activities at the human H3 receptor
and the closely related H4 receptor. In the series of piperidine containing analogues, immepip
was found to be the most potent H3 receptor agonist, whereas its propylene analogue 13a was
identified as a high-affinity neutral antagonist for the human H3 receptor. Moreover,
replacement of the piperidine ring of immepip by a pyrrolidine ring led to a pair of enantiomers
that show a distinct stereoselectivity at the human H3 and H4 receptor.
In tr od u ction
related diseases have also been published.19-23 In 1999,
the cDNA of the human H3 receptor has been success-
fully cloned by Lovenberg et al.24 Tissue distribution
analysis indicated that the expression of the receptor
is predominantly restricted to the brain.25
Histamine, an autocoid as well as a neurotransmitter
in the brain,1 is endogenously synthesized from L-
histidine by the enzyme L-histidine decarboxylase (HDC).
The biogenic amine neurotransmitter is implicated in
a wide range of physiological effects through the activa-
tion of distinct subtypes of histamine receptors. The
histamine receptors, members of G-protein-coupled
receptors (GPCRs) families, were initially categorized
based on their pharmacological characterization, but are
currently classified as H1, H2, H3, and H4 receptors
owing to the diversity of their amino acid sequences.2,3
The existence of a histamine H3 receptor was reported
for the first time in 1983 by Arrang and colleagues.4 The
receptor was identified as a presynaptic autoreceptor;
activation of the H3 receptor inhibits the synthesis and
release of histamine from neurons. Subsequently, it has
been found that the H3 receptor also acts as a hetero-
receptor on nonhistaminergic axon terminals controlling
the release of several neurotransmitters, e.g., acetyl-
choline,5,6 dopamine,7 noradrenaline,8 serotonin,9 in both
the central and peripheral nervous systems. Therapeu-
tic applications of histamine H3 receptor antagonists
have been proposed for several diseases and CNS
disorders, for example, attention-deficit hyperactivity
disorder (ADHD),10,11 Alzheimer’s disease,12 epilepsy,13-15
schizophrenia,15,16 and obesity,17,18 whereas the thera-
peutic potential for histamine H3 receptor agonists for
myocardial ischemia, inflammatory, and gastric acid
In 2000, the histamine H4 receptor, mainly expressed
on leukocytes, was identified.26 The receptor is sug-
gested to be a new therapeutic target for the regulation
of immune functions with possible uses in allergy and
asthma.27 An alignment of deduced amino acid se-
quences of the human histamine H4 receptor indicates
an overall 43% identity homology to the histamine H3
receptor.28 Consequently, many imidazole-based hista-
mine H3 ligands also act at the H4 receptor.
In a preliminary study from our laboratory, histamine
and various homologues showed a high affinity and full
agonist potency on the human H3 receptor. Yet, the high
flexibility of the ligand side chain does not seem to result
in an optimal interaction between the ligand and the
H3 receptor. A recent study demonstrated that immepip,
a conformationally constrained analogue of imbutamine
(the butylene analogue of histamine), possesses a much
higher affinity and functional activity on the human
histamine H3 receptors than the flexible imbutamine.29
In this study, other conformationally constrained ligands
were designed using histamine and its homologues as
prototypes. All ligands were tested for their activities
at both the human H3 and H4 receptor. The variation
of the spacer length between the piperidine and the
imidazole ring and/or alteration of nitrogen position in
the piperidine ring will provide useful information for
structure activity relationships (SAR) and molecular
modeling studies of histamine H3 receptor ligands.
Moreover, characterization of these ligands on the
* Corresponding author. Tel.: (+31) 20 4447600. Fax: (+31) 20
4447610. E-mail: leurs@few.vu.nl.
† Vrije Universiteit.
‡ Naresuan University.
§ Utrecht University.
|| UCB Pharma.
10.1021/jm030905y CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/01/2003

5446 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 25
Sch em e 1. Synthetic Pathway for 9a
Kitbunnadaj et al.
a
Reagents and conditions: (i) LiAlH4, ether, reflux 3 h; (ii) DEAD, phthalimide, triphenylphosphine, THF, rt, 2 h; (iii) hydrazine,
EtOH, reflux, 6 h; (iv) 0.5 M HCl, reflux, 1.5 h.
Sch em e 2. Synthetic Pathway for 12a -ca
a
Reagents and conditions: (i) 5% Rh/C, 10% Pd/C, glacial acetic acid, H2 100 atm, 24 h; (ii) benzyl bromide, K2CO3, EtOH, rt, 1 h,
reflux, 2 h; (iii) oxalyl chloride, dimethyl sulfoxide, triethylamine, DCM, -78 °C to rt, 5 h; (iv) TosMIC, NaCN, EtOH; (v) sat. NH3 in
EtOH, 90-110 °C, 10-12 atm, 24 h; (vi) 10% Pd/C, ammonium formate, MeOH, reflux, 24 h.
histamine H4 receptor might provide a key for the
development of histamine H4 receptor ligands.
Compounds 12a-c were prepared according to Scheme
2. The key intermediates, 3-piperidinepropanals (26a -
c), were easily obtained from the corresponding py-
ridinepropanols via catalytic hydrogenation under acidic
conditions, using Pd/C and Rh/C as catalysts. Next, the
piperidine nitrogen was protected by treatment with
benzylbromide33,34 and the hydroxyl group was subse-
quently transformed into the corresponding aldehyde
via a Swern oxidation.35 The aldehyde group was readily
converted to an imidazole ring using TosMIC chemis-
try.36 Some of the intermediate 4-tosyloxazolines (27a -
c) precipitated from the solution and were used after
filtration. Otherwise, the mixture was concentrated and
used immediately in the next step without further
purification. The 4-tosyloxazolines were heated in a
saturated solution of ammonia in ethanol to give the
imidazole compounds in moderate to good yields. De-
benzylation using ammonium formate in the presence
Ch em istr y
The synthesis of all compounds is outlined in Schemes
1-6. Compound 9 was synthesized (Scheme 1) from a
known intermediate, (()-trans-ethyl 2-(1-triphenyl-
methylimidazol-4-yl)cyclopropanecarboxylate.30 Reduc-
tion of the intermediate using LiAlH4 gave an alcohol
(21), which was subsequently transformed into amine
(9) via a Mitsunobu reaction and hydrolysis in a good
yield (90%).31,32

Histamine H3 Receptor Agonists
J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 25 5447
Sch em e 3. Synthetic Pathway for 13a -ca
a
Reagents and conditions: (i) n-BuLi, THF, [2-(1,3-dioxolan-2-yl)ethyl]triphenyl phosphonium bromide, -30 °C, 3 h; (ii) H2, 5% Pd/C,
1 atm, 24 h; (iii) 1 N HCl, rt, 10 min; (iv) TosMIC, NaCN, EtOH, rt, 2 h; (v) sat. NH3 in EtOH, 90-110 °C, 10-12 atm, 24 h; (vi) HBr,
10% Pd/C, 5% Rh/C, H2, 100 atm, 48 h.
Sch em e 4. Synthetic Pathway for 14a
a
Reagents and conditions: (i) N-benzylpyrrolidone, n-BuLi, -60 °C, THF; (ii) (1) POCl3, pyridine, rt, 1 h, (2) DBU, rt, 3 h; (iii) LiAlH4,
THF, 0 °C, H2SO4; (iv) (1) Pd(OH)2, MeOH, H2, 50 atm, 72 h, (2) di-tert-butyl dicarbonate, triethylamine, MeOH, rt, (3) 2 N HBr.
Sch em e 5. Synthetic Pathway for 16a
a
Reagents and conditions: (i) EtMgBr, DCM, 4-pyridinecarboxaldehyde, rt, 16 h; (ii) HBr, 10% Pd/C, 5% Rh/C, 100 atm H2, 48 h; (iii)
di-tert-butyl dicarbonate, triethylamine, MeOH, rt; (iv) methanesulfonyl chloride, triethylamine, DCM; (v) CaCO3, DMF, 140 °C; (vi) 1 N
HCl.
of 10% Pd/C as a catalyst resulted in the final products
(12a -c) in moderate yields (45-65%).
Many attempts to hydrolyze the dioxolane ring either
in an acidic solution or in the presence of ferric chloride
hexahydrate38 in this step failed. However, after hydro-
genation of 29 using Pd/C as catalyst, the resulting
3-(1,3-dioxolan-2-yl)propyl pyridines were easily hydro-
lyzed under mildly acidic conditions to give the corre-
sponding pyridinebutanals (31a -c). The aldehyde group
was converted to an imidazole ring via the aforemen-
The synthesis of 3-(1H-imidazol-4-yl)propyl-piperi-
dines (13a -c) followed Scheme 3. The intermediates,
ω-pyridinebutanals, were synthesized using [2-(1,3-
dioxolan-2-yl)ethyl]triphenylphosphonium bromide and
pyridinecarboxaldehydes as starting reagents and gave
3-(1,3-dioxolan-2-yl)propen-1-yl pyridines in high yields.37

5448 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 25
Sch em e 6. Synthetic Pathway for 17 and 18a
Kitbunnadaj et al.
a
Reagents and conditions: (i) oxalyl chloride, dimethyl sulfoxide, triethylamine, DCM, -78 °C to rt, 5 h; (ii) methyl triphenylphosphonium
bromide, potassium-tert-butoxide, toluene; 1 N HCl, rt; (iii) 1 N HCl, rt.
F igu r e 1. Chromatogram of enantiomers of bis-protected 14. The enantiomers were separated by preparative liquid
chromatography using a Chiralpak AD 100 × 500 mm column and eluted with a solution of 10% EtOH, 90% isohexane, and 0.1%
DEA (diethylamine) at a flow rate of 300 mL/min. The collected peaks were controlled by HPLC using a Chiralpak AD-H column
and eluted with a solution of 10% EtOH, 90% isohexane, and 0.1% DEA; the retention times of the (S)- and (R)-enatiomers were
7.4 and 9.1 min, respectively.
tioned TosMIC chemistry. Catalytic hydrogenation of
the pyridine rings gave the final products (13a -c) in
moderate yields (40-50%).
give 39. The intermediate 39 was mesylated using
methanesulfonyl chloride and subsequently heated un-
der reflux in dimethylformamide in the presence of
calcium carbonate to give 41. In this step, deprotection
of the imidazole nitrogen occurred, whereas the piperi-
dine nitrogen remained protected. Deprotection of the
piperidine nitrogen under mildly acidic conditions re-
sulted in 16 in a good yield (90%).
Compounds 17 and 18 were synthesized according to
Scheme 6. The intermediate alcohol (39) was trans-
formed to ketone (42) via a Swern oxidation reaction.
Treatment of 42 with methyl triphenylphosphonium
bromide in the presence of potassium tert-butoxide gave
43 in a moderate yield (50%). Deprotection of the
nitrogen atoms of 42 and 43 under mildly acidic condi-
tions resulted in 18 and 17, respectively.
En a n tiom er Sep a r a tion . The racemate 14 was
protected by treatment with di-tert-butyl dicarbonate
(using the procedure for 39). The enantiomers were
separated by preparative liquid chromatography using
a Chiralpak AD 100 × 500 mm column and eluted with
a solution of 10% EtOH, 90% isohexane, and 0.1% DEA
(diethylamine) at a flow rate of 300 mL/min. The
collected peaks were controlled by HPLC using a
Chiralpak AD-H column and eluted with a solution of
10% EtOH, 90% isohexane, and 0.1% DEA, to give (S)-
and (R)-14 at the retention time of 7.4 and 9.1 min,
Compound 14 was prepared using 1-(triphenylmeth-
yl)imidazol-4-ylcarboxaldehyde39 as starting chemical
(Scheme 4). The substance was reacted with N-ben-
zylpyrrolidinone in the presence of n-BuLi to give an
intermediate alcohol (34).40 Dehydration of the alcohol
using POCl3 and DBU resulted in 35. Lithium alumi-
num hydride reduction of the pyrrolidone provided the
desired pyrrolidine (36). Hydrogenation of 36 using Pd-
(OH)2 as catalyst, in a one-pot procedure via simulta-
neous deprotection, debenzylation, and reduction of the
double bond, gave the product 14 in a good yield (90%).
For the synthesis of 4-(1H-imidazol-4-ylmethylene)-
piperidine (16), the intermediate (1H-imidazol-4-yl)-
piperidin-4-ylmethanol (38) was required. Following
Scheme 5, treatment of 4-iodo-1-trityl-1H-imidazole41
with 4-pyridinecarboxaldehyde in the presence of eth-
ylmagnesium bromide42 and subsequently catalytic
hydrogenation of the pyridine ring gave the desired
intermediate (38) in a good yield (85%). Dehydration of
the resulting intermediate under acidic conditions,
however, did not provide 16. In contrast, many uniden-
tified products were observed. An alternative route was
therefore considered: the compound 38 was first pro-
tected by treatment with di-tert-butyl dicarbonate to

Histamine H3 Receptor Agonists
J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 25 5449
ing ligands; cell homogenates of H4-receptor-expressing
cells (166 ( 26 fmol/mg of protein) were incubated for
60 min at 37 °C with 9-11 nM [3H]histamine (23.2 Ci/
mmol) in 50 mM Tris HCl (pH 7.4), with or without
competing ligands.
Incubations were terminated by the addition of 3 mL
of ice-cold wash buffer (H3: 25mM Tris HCl, 145 mM
NaCl, pH 7.4 at 4 °C; H4: 50mM Tris HCl, pH 7.4 at 4
°C), and filtered through 0.3% polyethyleneimine-pre-
treated Whatman GF/C filters. Filters were subse-
quently washed twice with wash buffer. Retained
radioactivity was determined by liquid scintillation
counting. Nonspecific binding was defined with 1 µM
thioperamide as competing ligand. Competition iso-
therms were analyzed with the GraphPad Prism soft-
ware (GraphPad, Intuitive Software for Science, San
Diego, CA). Ki values were determined with the equa-
tion Ki ) IC50/(1 + ([ligand]/Kd)). Protein concentrations
were determined spectrophotometrically by a Packard
Argus 400 microplate reader using the Bradford re-
agent,48 with bovine serum albumin as a standard.
F igu r e 2. (A) Displacement ellipsoid plot of (S)-VUF4848 (19),
drawn at the 50% probability level. Selected torsion angles
(deg): C5-C1-C6-C7, 93.9(3); N2-C1-C6-C7, 84.5(2); C1-
C6-C7-C8, 174.05(17); C1-C6-C7-C11, 70.1(2). (B) A table
outlining the hydrogen bonding.
Color im etr ic cAMP Assa y. SK-N-MC cells stably
24
expressing either the human histamine H3
or the
human histamine H4 receptor,27 as well as a cyclic AMP
responsive element (CRE)-responsive â-galactosidase
reporter-gene were grown overnight in 96-well plates
before the assay. To start the assay, the cells were
incubated for 6 h with 1 µM forskolin and respective
ligands at 37 °C. Thereafter, the medium was aspirated
and cells were incubated overnight at 4 °C with 100 µL
of assay buffer (100 mM NaH2PO4, 100 mM Na2HPO4,
pH 8, 2 mM MgSO4, 0.1 mM MnCl2, 0.5% Triton, 40
mM â-mercaptoethanol, and 4 mM o-nitrophenyl-â-D-
galactopyranoside (ONPG). The absorbance at 405 nm
was determined by using a Victor2 plate reader (Perkin-
Elmer).
respectively (Figure 1). The compounds 19 and 20 were
obtained after deprotection of the nitrogen atoms in 1
N HBr and recrystallization from EtOH/(Et)2O.
Cr ysta l Str u ctu r e of (S)-VUF 4848 (19). Single
crystals of 19 for structure deternimation were obtained
from EtOH and ether using the vapor phase diffusion
technique:43 C8H15Br2N3, Fw ) 313.05, colorless needle,
0.36 × 0.15 × 0.09 mm3, orthorhombic, P212121 (no. 19),
a ) 8.2324(1) Å, b ) 11.6971(1) Å, c ) 12.0629(1) Å, V
) 1161.60(2) Å3, Z ) 4, Dx ) 1.790 g/cm3, µ ) 6.942
mm-1. A total of 17 802 reflections were measured on a
Nonius KappaCCD diffractometer with rotating anode
(λ ) 0.71073 Å) at a temperature of 150(2) K up to a
resolution of (sin θ/λ)max ) 0.65 Å-1; 2662 reflections
were unique (Rint ) 0.046). An absorption correction
based on multiple measured reflections was applied
(0.30-0.49 transmission). The structure was solved with
Patterson methods (DIRDIF-97) and refined with
SHELXL-97 against F2 of all reflections.44,45 Non-
hydrogen atoms were refined freely with anisotropic
displacement parameters; hydrogen atoms were refined
freely with isotropic displacement parameters (178
refined parameters, no restraints). R-values [I > 2σ(I)]:
R1 ) 0.0158, wR2 ) 0.0363. R-values (all refln): R1 )
0.0174, wR2 ) 0.0369. GoF ) 1.045. Flack parameter
x ) -0.013(8).46 Residual electron density was between
-0.26 and 0.40 e/Å3. Molecular illustration, structure
checking, and calculations were performed with the
PLATON package47 (Figure 2).
Resu lts a n d Discu ssion
Characterization of histamine and its homologues (1-
5) on the human histamine H3 receptor showed that all
longer homologues of histamine (3-5) display affinities
as high as histamine, except compound 2, which exhibits
a lower affinity (pKi ) 7.31). Compared to histamine,
impentamine exhibits a 4-fold increase in the functional
activity (pEC50 ) 8.63), whereas the shorter and longer
homologues are somewhat less effective. The optimum
chain length between the imidazole ring and the side
chain nitrogen in the series is five methylene units.
Although the longer homologue (5) still shows moderate
affinity and functional activity on the histamine H3
receptor, its intrinsic activity is decreased (R ) 0.61).
As previously reported, (R)-R-methylhistamine (6)
possesses an affinity as high as histamine for the human
H3 receptor.25 Interestingly, the compound exhibited a
10-fold increase in functional activity (pEC50 ) 9.17).
The activities of two rigid analogues of histamine (7 and
8) confirmed that a small group on the chain between
the imidazole ring and the side chain amine is tolerated.
The (S,S)-diastereomer 8, however, has higher affinity
and functional activity on the receptor (pKi ) 8.03,
pEC50 ) 8.23) than the (R,R)-diastereomer 7 (pKi ) 7.17,
pEC50 ) 7.24). On the histamine H4 receptor, an
additional group on the ethylene chain of histamine led
to a decrease in affinity and functional activity. For
P h a r m a cology
Ra d ioliga n d Disp la cem en t Stu d ies. Homogenates
of SK-N-MC cells, stably expressing either the human
24
histamine H3 or the human histamine H4 receptor,27
were used for determining ligand affinities for the H3
and H4 receptor, respectively. Cell homogenates of H3-
receptor-expressing cells (131 ( 11 fmol/mg of protein)
were incubated for 40 min at 25 °C with 0.9-1.1 nM
[3H]-NR-methylhistamine (82 Ci/mmol) in 50 mM so-
dium phosphate buffer (pH 7.4) with or without compet-

5450 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 25
Kitbunnadaj et al.
F igu r e 3. Affinities and functional activities of immepip and
its homologues with variations in the spacer between the
imidazole and the piperidine ring, on the histamine H3
receptor. (A) Receptor affinities were determined by displace-
ment of [3H]-NR-methylhistamine binding to membranes of SK-
N-MC cells stably expressing the human H3 receptor. (B)
Histamine H3 agonistic responses were measured by CRE-
mediated â-galactosidase reporter gene assay.
F igu r e 4. (A) Rightward shift of concentration-response
curves of immepip in the presence of various concentration of
13a determined by the inhibition of the CRE-stimulated
â-galactosidase transcription in SK-N-MC cell expressing the
human H3 receptor. (B) Schild plot demonstrating the deter-
mination of the antagonist potency (pA2 value) of 13a at the
human H3 receptor.
instance (R)-R-methylhistamine (6) possessed a 15-fold
decrease in affinity and a 20-fold decrease in functional
activity (pKi ) 6.62, pEC50 ) 5.95) at histamine H4
receptor.
mouse brain but acts as a full agonist at the human H3
receptor.49,51
In view of the high affinity and lack of pharmacologi-
cal responses, we hypothesized that compound 13a could
be a novel neutral antagonist at the human H3 receptor.
As can be seen in Figure 4, compound 13a effectively
antagonizes the response to immepip, yielding a pA2
value of 8.1 ( 0.1 (n ) 6). In our current experimental
model system, the known inverse agonistic actions of
thioperamide and clobenpropit are easily detectable
(data not shown), indicating that compound 13a acts
as a high-affinity neutral antagonist on the human H3
receptor, indeed. Moreover, characterization of 13a on
the human H4 receptor (pKi ) 6.21) implied selectivity
of the ligand on the H3 receptor. On the basis of these
observations, we conclude that elongation of the spacer
between the imidazole ring and the side chain nitrogen
does not significantly influence the affinity but obviously
reduces the agonistic activity of the ligand.
Alterations of the nitrogen position in the piperidine
ring resulted in a decrease of affinity and functional
activity independent of the length of the spacer in the
side chain (11a fbfc, 12a fbfc, and 13a fbfc).
Although in 11a , 12b, 13c, and 3, the distances between
the side chain nitrogen and the imidazole ring are not
significantly different (five methylene units), 12b and
13c showed a 100-fold decrease in affinity at both the
H3 and H4 receptor (Tables 1 and 3). The results not
only indicate the crucial role of the side chain nitrogen
in the binding, but also imply that the steric environ-
ment around the side chain nitrogen affects the interac-
tion between the piperidine nitrogen and the binding
In the series of ligands containing a 4-piperidine ring,
immepip (11a ) exhibited the highest affinity and func-
tional activity on both histamine H3 and H4 receptors.
Increasing or decreasing in the spacer length between
the imidazole and piperidine ring of immepip (10, 12a ,
13a ) led to a decrease in affinity and functional activity
(Figure 3). The shorter homologue (10), however, re-
tained full agonistic activity on the H3 receptor, whereas
the longer homologues behaved as partial agonist (12a ,
R ) 0.60) or did not show any agonistic response (13a ).
Nevertheless, compound 13a exhibited a nanomolar
affinity for the H3 receptor (pKi ) 8.35).
The histamine H3 receptor shows a high level of
constitutive activity, i.e., it signals via Gi proteins
without the presence of an H3 agonist.49,50 Currently,
the H3 receptor is one of the few examples of GPCRs
for which constitutive activity has been reported to
appear in vivo (rat and mouse brain). The occurrence
of constitutive activity leads to a reclassification of H3
antagonists into inverse agonists (i.e. they inhibit
constitutive H3 receptor signaling) and neutral antago-
nists (i.e. they do not affect constitutive signaling, but
competitively block the effect of agonists and inverse
agonists). So far only two ligands have been recognized
as neutral antagonists. We previously reported on
N-isopropylimpentamine as a neutral antagonist at the
human H3 receptor (pKi ) 7.89),50 while proxyfan has
been shown to be a neutral antagonist in both rat and

Histamine H3 Receptor Agonists
J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 25 5451
Ta ble 1. Affinities and Functional Activities of Histamine,
Homologues of Histamine, and Some Conformationally
Constrained Ligands on the Human Histamine H3 Receptorsd
Ta ble 2. Affinities and Functional Activities of the Two
Enantiomers of 14 on the Human Histamine H3 Receptor
a
The pKi values were measured by [3H]-NR-methylhistamine
binding to membranes of SK-N-MC cells expressing the human
b
H
3 receptor. The pEC50 values were determined by the inhibition
of the cAMP-stimulated â-galactosidase transcription in SK-N-
MC cell expressing the human H3 receptor. The results are
presented as the mean ( SEM of at least three independent
experiments.
Ta ble 3. Affinities and Functional Activities of Histamine,
Homologues of Histamine and Some Conformationally
Constrained Ligands on the Human Histamine H4 Receptors
human histamine H4 receptor
intrinsic
selectivity
no.
pKi ( SEMa pEC50 ( SEMb activity (R)
(∆)c
1
2
3
4
5
6
7
8
7.84 ( 0.03
7.46 ( 0.12
7.84 ( 0.09
6.43 ( 0.08
6.12 (1)
7.26 ( 0.12
6.67 ( 0.17
7.05 ( 0.04
1.00
0.82
0.96
n.dd
n.d.
1.04
n.d.
0.61
0.56
n.d.
0.73
n.d.
n.d.
0.64
0.48
n.d.
n.d.
n.d.
n.d.
0.83
n.d.
n.d.
n.d.
n.d.
0.80
0.85
1.6
0.7
3.6
72.4
75.8
55.0
56.2
117.5
3.2
<5
<5
5.95 ( 0.09
<5
5.15 ( 0.27
5.96 ( 0.26
<5
7.25 ( 0.16
<5
<5
5.40 ( 0.17
5.69 ( 0.15
<5
6.62 ( 0.04
5.42 ( 0.04
5.96 ( 0.01
6.95 ( 0.10
5.66 ( 0.05
7.66 ( 0.04
6.43 ( 0.05
6.21 ( 0.02
6.21 ( 0.04
6.49 ( 0.04
6.41 ( 0.02
5.05 ( 0.10
5.35 ( 0.01
6.24 ( 0.04
7.29 ( 0.13
3.58 ( 0.25
5.76 ( 0.03
5.55 ( 0.03
3.32 ( 0.22
6.85 ( 0.06
7.43 ( 0.13
9
10
11a
12a
13a
11b
12b
13b
11c
12c
13c
14
15
16
17
18
19
20
2.5
45.7
18.6
138.0
9.1
1.8
4.7
7.8
1.6
6.2
14.1
61.6
295.1
707.9
123.0
100.0
7.1
<5
<5
<5
6.13 ( 0.18
<5
<5
<5
<5
5.99 ( 0.07
6.38 ( 0.05
The pKi values were measured by [3H]histamine binding to
membranes of SK-N-MC cells expressing the human H4 receptor.
b The pEC50 values were determined by the inhibition of the cAMP-
stimulated â-galactosidase transcription in SK-N-MC cell express-
ing the human H4 receptor. The results were presented as the
mean ( SEM of at least three independent experiments, otherwise
the number of experiments is mentioned in parentheses. c Selec-
a
a
The pKi values were measured by [3H]-NR-methylhistamine
binding to membranes of SK-N-MC cells expressing the human
3 receptor. The pEC50 values were determined by the inhibition
d
b
tivity was calculated from equation ∆ ) 1/[Ki(H3)/Ki(H4)]. n.d. )
not determined.
H
of the cAMP-stimulated â-galactosidase transcription in SK-N-
MC cell expressing the human H3 receptor. The results were
presented as the mean ( SEM of at least three independent
experiments, otherwise the number of experiments is mentioned
in parentheses. c The pA2 value was determined from the Schild
plot (Figure 4). The results are represented as the mean ( SEM
tional activity (pKi ) 8.44, pEC50 ) 8.88). Characteriza-
tion of the enantiomers of 14 (19 and 20) on the
histamine H3 receptor showed that the (S)-enantiomer
(19), the absolute configuration of which was identified
by X-ray crystallography (Figure 2), exhibits a higher
affinity and functional activity than (R)-enantiomer (20).
Intriguingly, reverse results were obtained on the H4
receptor, since the (R)-enantiomer 20 possesses higher
affinity and functional activity than (S)-enantiomer 19.
d
of three independent experiments. n.d. ) not determined.
site on both the H3 and H4 receptors. Replacement of
the piperidine ring of 11b by a pyrrolidine ring (14),
which due to the reduced ring size provides less steric
interference, leads to an improved affinity and func-

5452 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 25
Kitbunnadaj et al.
terated solvent peak as reference. Optical rotations were
measured on a Perkin-Elmer 241 MC polarimeter. Elemental
analyses were performed at Mikroanalytisches Labor Pascher
(Remagen-Bandorf, Germany).
A further reduction of the conformational flexibility
as in 15-18 led to a decrease in affinity and functional
activity both on the human H3 and H4 receptors (Tables
1 and 3) compared to their prototypes (10 and 11a ).
Although compounds 16 and 17 exhibited decreased
affinity and functional activity at the histamine H3
receptor as full agonists, the selectivity for the hista-
mine H3 receptor over the H4 receptor is increased (300-
and 700-fold, respectively; Table 3). In contrast, com-
pounds 15 and 18 lost affinity and functional activity
at both the human histamine H3 and H4 receptors.
Comparing 18 with 17 might indicate the crucial role
of the basic properties of the imidazole ring in the
binding, since an earlier report shows that incorporation
of a carbonyl adjacent to an imidazole ring results in
massive decreased basicity.52 Surprisingly, this modi-
fication does not affect the (full) agonistic activity at the
histamine H3 receptor (R ) 0.98) but only affects the
binding affinity.
Meth od s. 2-(1-Tr ip h en ylm eth ylim id a zol-4-yl)cyclop r o-
p ylm eth a n ol (21). Ethyl 2-(1-triphenylmethylimidazol-4-yl)-
cyclopropanecarboxylate (13.1 mmol) was added to a suspen-
sion of 25.8 mmol of LiAlH4 in 100 mL of dry ether and heated
under reflux for 3 h. Slowly, 100 mL 0.1 M NaOH was added
and the mixture was extracted with CHCl3 (3 × 100 mL). The
organic layers were washed with 100 mL of NaHCO3 and 100
mL of brine and dried with Na2SO4. Concentration in vacuo
gave 21 (70%) as white crystals. 1H NMR (CDCl3): δ 7.03-
7.37 (m, 16H), 6.50 (s, 1H), 3.41-3.64 (m, 1H), 2.15 (br, 1H),
1.67-1.78 (m, 1H), 1.38-1.49 (m, 1H), 0.89-0.98 (m, 1H),
0.68-0.77 (m, 1H).
N -[2-(1-Tr ip h e n ylm e t h ylim id a zol-4-yl)cyclop r op yl-
m eth yl]p h th a lim id e (22). Diethyl diazodicarboxylate (3.65
mmol) was added to a solution of 2.26 mmol of 21, 3.64 mmol
of phthalimide, and 3.46 mmol of triphenylphosphine in 50
mL of THF and stirred for 2 h at ambient temperature under
an atmosphere of dry nitrogen. The mixture was washed with
50 mL water and the aqueous layer was extracted DCM (2 ×
50 mL). The combined organic layers were washed with 50
mL of brine and dried with Na2SO4. Concentration in vacuo
gave an oil which was purified by column chromatography
using DCM/EtOAc (4:1) as eluent to give 22 (99%) as a white
Con clu sion s
Characterization of a series of compounds containing
a basic amine in the side chain indicates that additional
substituents on R and/or â positions are tolerated at the
human H3 receptor but not at the human H4 receptor.
On the H3 receptor, the optimum chain length between
an imidazole ring and the basic nitrogen in the side
chain is five methylene units. Elongation of the spacer
between the imidazole ring and the nitrogen in the side
chain did not tremendously affect the affinity and
functional activity, but a reduction of intrinsic activity
was clearly observed. In most cases, a more rigid side
chain improved the affinity and functional activity of
the ligands. The 4-piperidinyl analogues possess the
highest affinity and functional activity on the receptor
compared to other isomers. In this study immepip (11a ),
a conformationally constrained analogue of 3, is the
most potent agonist on both human H3 and H4 receptors.
Elongation of the spacer between the imidazole ring and
the piperidine ring results in a decrease of intrinsic
activity at the histamine H3 receptor. Compound 13a ,
a propylene homologue of immepip, indeed behaves as
a high-affinity neutral antagonist on the human hista-
mine H3 receptor. Although more conformationally
constrained analogues of 11a (16-18) do not show an
improved affinity and efficacy, they could become valu-
able for molecular modeling studies. The loss of affinity
of 18 at both the histamine H3 and H4 receptors might
indicate the crucial role of the basic properties of the
imidazole ring in the binding.
1
solid. H NMR (CDCl3): δ 7.78-7.86 (m, 2H), 7.64-7.72 (m,
2H), 7.01-7.31 (m, 16H), 6.49 (s, 1H), 3.83 (dd, 1H, J ) 7.0
and 13.9 Hz), 3.50 (dd, 1H, J ) 7.0 and 13.9 Hz), 1.81-1.91
(m, 1H), 1.40-1.50 (m, 1H), 0.88-0.98 (m, 2H).
2-(1-Tr ip h en ylm eth ylim id a zol-4-yl)cyclop r op ylm eth -
yla m in e (23). Compound 22 (1.49 mmol) and 0.29 mL of
hydrazine hydrate in 50 mL of EtOH were heated under reflux
for 6 h. The reaction mixture was cooled to 0 °C and filtered.
Evaporation of the mother liquor gave 23 (88%) as an off-white
solid. 1H NMR (CDCl3): δ 7.03-7.35 (m, 16H), 6.50 (s, 1H),
2.67 (d, 2H, J ) 7.2 Hz), 2.10 (br, 2H), 1.57-1.68 (m, 1H),
1.19-1.31 (m, 1H), 0.82-0.94 (m, 1H), 0.61-0.72 (m, 1H).
2-(1H -Im id a zol-5-yl)cyclop r op ylm et h yla m in e Dih y-
d r och lor id e (9). Compound 23 (1.24 mmol) was dissolved in
25 mL of EtOH and 25 mL of 0.5M HCl and heated at reflux
for 1.5 h. The ethanol was evaporated completely and the solid,
triphenylmethanol, was filtered. The mother liquor was further
concentrated and the product (9) was recrystallized from
1
2-propanol as an off-white solid (44%). H NMR (D2O): δ 8.5
(s, 1H), 7.2 (s, 1H), 3.0-3.1 (m, 2H), 2.0 (m, 1H), 1.5 (m, 1H),
1.1-1.2 (m, 2H).
3-(P ip er id in yl)p r op a n -1-ol Aceta te Sa lt (24a -c). Py-
ridinepropanol (0.15 mol) was dissolved in 100 mL of glacial
acetic acid, and 1 g of 10% Pd/C and 0.5 g of 5% Rh/C were
added. The suspension was hydrogenated under 100 atm of
H2 for 2 h in a stainless steel bomb. The mixture was filtered
through a short column of Hyflo super cel and the solvent was
evaporated under reduced pressure to obtain the product as
an oil, which was used in the next step without further
purification. 1H NMR (CDCl3): 24a (95%), δ 3.7 (t, 2H), 3.0
(m, 2H), 2.7 (m, 2H), 2.0 (s, 3H), 1.8 (m, 2H), 1.3-1.6 (m, 7H);
24b (92%), δ 3.7 (t, 2H), 3.3 (m, 2H), 2.7 (m, 1H), 2.4 (m, 1H),
2.0 (s, 3H), 1.8 (m, 4H), 1.5 (m, 2H), 1.3 (m, 2H), 1.1 (m, 1H);
24c (95%), δ 3.7 (t, 2H), 3.4 (m, 1H), 2.9 (m, 1H), 2.7 (m, 1H),
1.9 (s, 3H), 1.4-1.8 (m, 10H).
Exp er im en ta l Section
Ma ter ia ls. Reagents were obtained from commercial sup-
pliers and used without further purification, except the py-
ridinepropanols and the pyridinecarboxylaldehydes, which
were purified by distillation under reduce pressure. Solvents
used were either AR or HPLC grade. Dry THF and DCM were
distilled from LiAlH4 and CaH2, respectively. Compounds 1-8
were obtained from laboratory stock,30,53 compounds 10 and
15 were synthesized according to the method of Lange et al.54
All structure confirmations and purities are presented in Table
4. Thin-layer chromatography was carried out on Merck
Kieselgel 60 F254 on aluminum sheets, and flash chromatog-
raphy was performed using J . T. Baker Kieselgel 60 under
pressure. Melting points were determined on an Electrother-
mal IA9200 apparatus. 1H spectra were recorded on a Bruker
AC-200 spectrometer at 200 MHz using the residual undeu-
3-(N-Ben zylp ip er id in yl)p r op a n -1-ol (25a -c). Compound
24 (0.11 mol), 0.11 mol of benzyl bromide, and 0.50 mol of
potassium carbonate were stirred in 150 mL of absolute
ethanol at room temperature for 1 h and then heated at reflux
for 2 h. The solvent was removed under reduced pressure.
Saturated potassium carbonate solution (100 mL) was added
and the mixture was extracted with DCM (3 × 100 mL). The
organic layers were dried with anhydrous sodium sulfate and
concentrated in vacuo. The crude product was purified using
flash column chromatography (EtOAc/EtOH, 1:2), and the
1
product was obtained as a yellow oil. H NMR (CDCl3): 25a
(75%), δ 7.2-7.3 (m, 5H), 3.6 (t, 2H), 3.5 (s, 2H), 2.8 (m, 2H),

Histamine H3 Receptor Agonists
J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 25 5453
Ta ble 4. Structural Characterization and Purity of Compounds
no. mp
NMR (D2O)
[R]
formula
elemental analysis
7a
207 δ 8.6 (s, 1H), 7.2 (s, 1H), 3.0 (ddd, 1H, J ) 3.6, 4.5, and
8.2 Hz), 2.5 (ddd, 1H, J ) 3.4, 6.5, and 10.1 Hz),
1.6 (ddd, 1H, J ) 4.7, 7.2 and 10.2 Hz),
+54.4
1.4 (ddd, 1H, J ) 6.7, 6.9 and 8.2 Hz)
8a
207 δ 8.6 (s, 1H), 7.2 (s, 1H), 3.0 (ddd, 1H, J ) 3.6, 4.5, and
8.2 Hz), 2.5 (ddd, 1H, J ) 3.4, 6.5, and 10.1 Hz),
1.6 (ddd, 1H, J ) 4.7, 7.2 and 10.2 Hz),
-54.2
1.4 (ddd, 1H, J ) 6.7, 6.9 and 8.2 Hz)
9
265 δ 8.5 (s, 1H), 7.2 (s, 1H), 3.0-3.1 (m, 2H),
2.0 (m, 1H), 1.5 (m, 1H), 1.1-1.2 (m, 2H)
279 δ 8.8 (s, 1H), 7.5 (s, 1H), 3.6 (m, 2H),
3.3 (m, 3H), 2.4 (m, 2H), 2.0 (m, 2H)
C7H13N3Cl2
C8H15N3Br2
C9H17N3Br2
Anal. (40.27% C, 6.33% H, 19.9% N)
Calcd (40.0% C, 6.2% H, 20.0% N)
Anal. (30.50% C, 4.73% H, 13.3% N)
Calcd (30.7% C, 4.8% H, 13.4% N)
Anal. (33.11% C, 5.13% H, 12.8% N)
Calcd (33.1% C, 5.2% H, 12.8% N)
Anal. (35.34% C, 5.53% H, 12.4% N)
Calcd (35.2% C, 5.6% H, 12.3% N)
Anal. (37.43% C, 6.11% H, 11.6% N)
Calcd (37.2% C, 6.0% H, 11.8% N)
Anal. (32.94% C, 5.26% H, 12.9% N)
Calcd (33.1% C, 5.2% H, 12.8% N)
Anal. (35.54% C, 5.79% H, 12.8% N)
Calcd (35.2% C, 5.6% H, 12.3% N)
Anal. (37.96% C, 6.22% H, 11.6% N)
Calcd (37.2% C, 6.0% H, 11.8% N)
Anal. (33.27% C, 5.48% H, 12.9% N)
Calcd (33.1% C, 5.2% H, 12.8% N)
Anal. (35.83% C, 5.85% H, 12.9% N)
Calcd (35.2% C, 5.6% H, 12.3% N)
Anal. (37.29% C, 6.14% H, 11.9% N)
Calcd (37.2% C, 6.0% H, 11.8% N)
Anal. (30.68% C, 4.82% H, 13.6% N)
Calcd (30.7% C, 4.8% H, 13.4% N)
Anal. (30.58% C, 4.19% H, 13.3% N)
Calcd (30.9% C, 4.2% H, 13.5% N)
Anal. (45.51% C, 6.45% H, 17.6% N)
Calcd (45.8% C, 6.4% H, 17.8% N)
Anal. (48.06% C, 6.90% H, 17.1% N)
Calcd (48.0% C, 6.8% H, 16.8% N)
Anal. (42.50% C, 6.00% H, 16.7% N)
Calcd (42.9% C, 6.0% H, 16.7% N)
Anal. (30.75% C, 4.78% H, 13.5% N)
Calcd (30.7% C, 4.8% H, 13.4% N)
Anal. (30.62% C, 4.77% H, 13.4% N)
Calcd (30.7% C, 4.8% H, 13.4% N)
10
11a 230 δ 8.5 (s, 1H), 7.3 (s, 1H), 3.4 (m, 2H), 2.8-3.0 (m, 2H),
2.7 (d, 2H, J ) 7 Hz), 1.8-2.0 (m, 3H), 1.3-1.5 (m, 2H)
12a 224 δ 8.5 (s, 1H), 7.2 (s, 1H), 3.4 (m, 2H), 2.8-2.9 (m, 2H),
2.7 (m, 2H), 1.9 (m, 2H), 1.6 (m, 3H), 1.4 (m, 2H)
C
C
10H19N3Br2
11H21N3Br2
13a 179 δ 8.5 (s, 1H), 7.2 (s, 1H), 3.4 (m, 2H), 2.9 (m, 2H),
2.7 (t, 2H), 1.9 (d, 2H), 1.6 (m, 3H), 1.3 (m, 4H)
11b 244 δ 8.6 (s, 1H), 7.3 (s, 1H), 3.3-3.6 (m, 2H), 2.6-3.0 (m, 4H),
2.0-2.2 (m, 1H), 1.5-2.0 (m, 3H), 1.2-1.4 (m, 1H)
12b 162 δ 8.5 (s, 1H), 7.2 (s, 1H), 3.3 (m, 2H), 2.6-2.9 (m, 4H),
1.8 (m, 2H), 1.6 (m, 4H), 1.1-1.3 (m, 1H)
13b 157 δ 8.5 (s, 1H), 7.2 (s, 1H), 3.4 (m, 2H), 2.9 (m, 1H),
2.5-2.7 (m, 3H), 1.9 (d, 2H), 1.7 (m, 4H), 1.1-1.3 (m, 3H)
11c 197 δ 8.6 (s, 1H), 7.4 (s, 1H), 3.3-3.6 (m, 2H),
2.9-3.2 (m, 3H), 1.4-2.0 (m, 6H)
12c 200 δ 8.5 (s, 1H), 7.2 (s, 1H), 3.4 (m, 1H), 3.2 (m, 1H),
2.7-3.0 (m, 3H), 1.8-2.0 (m, 5H), 1.4-1.7 (m, 3H)
13c 167 δ 8.5 (s, 1H), 7.2 (s, 1H), 3.4 (m, 1H), 3.2 (m, 1H),
3.0 (m, 1H), 2.8 (t, 2H), 1.2-1.8 (m, 10H)
C9H17N3Br2
C
C
10H19N3Br2
11H21N3Br2
C9H17N3Br2
C
C
10H19N3Br2
11H21N3Br2
14
15
16
17
18
19
20
147 δ 8.6 (s, 1H), 7.2 (s, 1H), 3.2-3.5 (m, 3H), 2.8-3.0 (m, 3H),
2.7 (m, 1H), 2.1 (m, 1H), 1.7 (m, 1H)
C8H15N3Br2
C8H13N3Br2
C9H15N3Cl2
273 δ 8.7 (s, 1H), 7.5 (s, 1H), 6.4 (m, 1H),
3.9 (s, 2H), 3.5 (t, 2H, J ) 7 Hz), 2.8 (m, 2H)
250 δ 8.7 (s, 1H), 7.5 (s, 1H), 6.4 (s, 1H),
3.3 (m, 4H), 2.8 (m, 4H)
257 δ 8.6 (s, 1H), 7.5 (s, 1H), 5.6 (s, 1H), 5.3 (s, 1H), 3.5 (m, 2H),
3.1 (m, 2H), 2.7 (m, 1H), 2.1 (m, 2H), 1.5-1.7 (m, 2H)
328 δ 8.8 (s, 1H), 8.3 (s, 1H), 3.5 (m, 3H),
3.1 (m, 2H), 2.1 (m, 2H), 1.0 (m, 2H)
147 δ 8.6 (s, 1H), 7.2 (s, 1H), 3.2-3.5 (m, 3H),
2.8-3.0 (m, 3H), 2.7 (m, 1H), 2.1 (m, 1H), 1.7 (m, 1H)
147 δ 8.6 (s, 1H), 7.2 (s, 1H), 3.2-3.5 (m, 3H),
2.8-3.0 (m, 3H), 2.7 (m, 1H), 2.1 (m, 1H), 1.7 (m, 1H)
C
10H17N3Cl2
C9H15N3OCl
2
+5.0b C8H15N3Br2
-5.0b C8H15N3Br2
a
b
22
See ref 30. [R]D (c ) 0.52, MeOH).
1.9 (m, 2H), 1.5-1.6 (m, 5H), 1.1-1.2 (m, 4H); 25b (83%), δ
7.2-7.3 (m, 5H), 3.6 (t, 2H), 3.5 (s, 2H), 2.7 (m, 2H), 1.5-1.9
(m, 8H), 1.2 (m, 2H), 0.9 (m, 1H); 25c (75%), δ 7.2-7.3 (m,
5H), 4.0 (d, 1H, J ) 14 Hz), 3.6 (t, 2H), 3.3 (d, 1H, J ) 14 Hz),
2.8 (m, 1H), 2.4 (m, 1H), 1.2-2.1 (m, 11H).
26 (40 mmol) in 500 mL of absolute ethanol. In a moment the
reaction mixture became clear. For 27a , the product precipi-
tated. Filtration and washing with 50 mL of ether/n-hexane
(1:1 v/v) yielded 27a as a pale yellow solid. For the synthesis
of 27b and 27c, the mixture was stirred for 2 h and the solvent
was evaporated under reduced pressure. The residue was
dissolved in CHCl3 (250 mL) and washed with saturated
sodium carbonate solution (3 × 250 mL). The organic layer
was collected and concentrated in vacuo. The product was
obtained as a yellow oil and used immediately for the next
step without further purification. 1H NMR (CDCl3): 27a (88%),
δ 7.8 (d, 2H), 7.4 (d, 2H), 7.2-7.3 (m, 5H), 6.9 (s, 1H), 5.1 (dt,
1H), 4.8 (d, 1H), 3.5 (s, 2H), 2.8-2.9 (m, 2H), 2.5 (s, 3H), 1.8-
1.9 (m, 2H), 1.4-1.6 (m, 6H), 1.1-1.3 (m, 3H); 27b (99%), δ
7.8 (d, 2H), 7.4 (d, 2H), 7.2-7.3 (m, 5H), 6.9 (s, 1H), 5.0 (dt,
1H), 4.7 (d, 1H), 3.5 (s, 2H), 2.8 (m, 2H), 2.4 (s, 3H), 1.1-1.9
(m, 11H); 27c (97%), δ 7.8 (d, 2H), 7.4 (d, 2H), 7.2-7.3 (m,
5H), 6.9 (d, 1H), 5.0 (m, 1H), 4.7 (m, 1H), 3.9 (m, 1H), 3.2 (m,
1H), 2.8 (m, 1H), 2.5 (s, 3H), 2.3 (m, 1H), 1.3-2.1 (m, 11H).
1-Ben zyl-[2-(1H-Im id a zol-5-yl)eth yl]p ip er id in e (28a -
c). In a stainless steel bomb, a solution of 17.5 mmol of 27 in
120 mL of absolute ethanol saturated with ammonia was
heated at 90-110 °C for 24 h. After cooling to room temper-
ature, the solvent was removed under reduced pressure. The
residue was dissolved in DCM (100 mL) and washed with a
5% K2CO3 solution (100 mL) and water (2 × 100 mL). The
organic layer was dried with anhydrous sodium sulfate. After
evaporation of the solvent under reduced pressure, a dark
brown oil was obtained. Purification using flash column
3-(N-Ben zylp ip er id in yl)p r op ion a ld eh yd e (26a -c). A
solution of oxalyl chloride (80 mmol) in 100 mL of DCM, was
cooled to -60 °C under nitrogen atmosphere. A solution of 160
mmol of dimethyl sulfoxide in 100 mL of DCM was added
dropwise and stirred for an additional 15 min at -60 °C. A
solution of 64 mmol of 25 in 100 mL of DCM was added
dropwise and stirred at -60 °C for 45 min. Subsequently, 200
mmol of triethylamine was added and the mixture was
warmed to ambient temperature. The reaction mixture was
washed with water (3 × 200 mL) and dried over anhydrous
sodium sulfate. The solvent was evaporated under reduced
pressure. After the purification by flash column chromatog-
raphy (EtOAc), a yellow oil was obtained. 1H NMR (CDCl3):
26a (64%), δ 9.7 (t, 1H), 7.2-7.3 (m, 5H), 3.5 (s, 2H), 2.8 (m,
2H), 2.4 (m, 2H), 1.9 (m, 2H), 1.5 (m, 4H), 1.2 (m, 3H); 26b
(63%), δ 9.7 (t, 1H), 7.2-7.3 (m, 5H), 3.5 (s, 2H), 2.7 (m, 2H),
2.4 (m, 2H), 1.9 (m, 1H), 1.5-1.8 (m, 7H), 0.9 (m, 1H); 26c
(65%), δ 9.7 (t, 1H), 7.2-7.3 (m, 5H), 4.0 (d, 1H, J ) 14 Hz),
3.3 (d, 1H, J ) 14 Hz), 2.8 (m, 1H), 2.5 (m, 2H), 2.3 (m, 1H),
1.8-2.1 (m, 3H), 1.3-1.7 (m, 6H).
1-Ben zyl-{2-[4-(tolu en e-4-su lfon yl)-4,5-d ih yd r ooxa zol-
5-yl]eth yl}p ip er id in e (27a -c). Finely powdered sodium
cyanide (6.5 mmol) was added in one portion to a stirred
suspension of tosylmethyl isocyanide (TosMIC) (40 mmol) and

5454 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 25
Kitbunnadaj et al.
1
chromatography (MeOH) gave the product as a brown oil. H
the procedure described for the synthesis of 27b and 27c, but
9 mmol of 31 was used as starting chemical. 1H NMR
(CDCl3): 32a (97%), δ 8.5 (d, 2H), 7.8 (d, 2H), 7.3 (d, 2H), 7.1
(d, 2H), 6.9 (s, 1H), 5.1 (dd, 1H), 4.7 (d, 1H), 2.6 (t, 2H), 2.4 (s,
3H), 1.7 (m, 4H); 32b (98%), δ 8.5 (s, 2H), 7.8 (d, 2H), 7.5 (d,
1H), 7.3 (d, 2H), 7.2 (m, 1H), 6.9 (s, 1H), 5.1 (dd, 1H), 4.7 (d,
1H), 2.6 (t, 2H), 2.4 (s, 3H), 1.7 (m, 4H); 32c (97%), δ 8.5 (d,
1H), 7.8 (d, 2H), 7.5 (t, 1H), 7.3 (d, 2H), 7.1 (m, 2H), 6.9 (s,
1H), 5.1 (dd, 1H), 4.7 (d, 1H), 2.8 (t, 2H), 2.4 (s, 3H), 1.7-1.9
(m, 4H).
NMR (CDCl3): 28a (57%), δ 7.5 (s, 1H), 7.2-7.3 (m, 5H), 6.7
(s, 1H), 3.5 (s, 2H), 2.8-2.9 (m, 2H), 2.5-2.6 (m, 2H), 1.8-1.9
(m, 2H), 1.4-1.6 (m, 6H), 1.1-1.3 (m, 3H); 28b (54%), δ 7.5
(s, 1H), 7.2-7.3 (m, 5H), 6.7 (s, 1H), 3.5 (s, 2H), 2.8-2.9 (m,
2H), 2.5-2.6 (m, 2H), 1.4-1.8 (m, 11H); 28c (85%), δ 7.5 (s,
1H), 7.2-7.3 (m, 5H), 6.7 (s, 1H), 3.9 (d, 1H, J ) 14 Hz), 3.2
(d, 1H, J ) 14 Hz), 2.6-2.9 (m, 3H), 2.5 (m, 1H), 1.3-2.1 (m,
8H).
2-(1H-Im id a zol-5-yl)eth ylp ip er id in e Dih yd r ogen Br o-
m id e (12a -c). A suspension of 28 (8 mmol in 100 mL of
MeOH), 0.25 g of 10% Pd/C, and 80 mmol of ammonium
formate was heated under reflux for 5 h. The mixture was
filtered through short column of Hyflo super cel and the filtrate
was concentrated under reduced pressure. To remove excess
ammonium formate, 2-propanol was added to the residue and
the ammonium formate was filtered off. The filtrate was
titrated with 1 N hydrobromic acid solution until the pH of
the solution was acidic (pH ≈ 3). The solvents were evaporated
under reduced pressure and the product was recrystallized
3-(1H-Im id a zol-5-yl)p r op ylp yr id in e (33a -c). The com-
pounds were synthesized using the procedure described for the
synthesis of 28a -c, but 9 mmol of 32 was used as starting
1
chemical. H NMR (CDCl3): 33a (62%), δ 8.5 (d, 2H), 7.6 (s,
1H), 7.1 (d, 2H), 6.8 (s, 1H), 2.6 (m, 4H), 1.9 (tt, 2H); 33b (67%),
δ 8.5 (d, 2H), 7.6 (s, 1H), 7.5 (d, 1H), 7.2 (m, 2H), 6.8 (s, 1H),
2.7 (m, 4H), 2.0 (tt, 2H); 33c (64%), δ 8.5 (d, 1H), 7.5 (m, 2H),
7.2 (m, 2H), 6.8 (s, 1H), 2.7 (t, 2H), 2.6 (t, 2H), 2.0 (tt, 2H).
3-(1H-Im idazol-5-yl)pr opylpiper idin e Dih ydr ogen Br o-
m id e (13a -c). A mixture of 5 mmol of 33, 50 mL of EtOH,
and 10 mL of a 1 N HBr solution was stirred with 0.1 g of
10% Pd/C and 0.05 g of 5% Rh/C under 100 atm of H2 for 48
h in a stainless steel bomb. Next, the mixture was filtered
through a short column of Hyflo super cel and concentrated
under reduced pressure. The product was obtained by tritu-
1
from EtOH/(Et)2O. H NMR (D2O): 12a (61%), δ 8.5 (s, 1H),
7.2 (s, 1H), 3.4 (m, 2H), 2.8-2.9 (m, 2H), 2.7 (m, 2H), 1.9 (m,
2H), 1.6 (m, 3H), 1.4 (m, 2H); 12b (45%), δ 8.5 (s, 1H), 7.2 (s,
1H), 3.3 (m, 2H), 2.6-2.9 (m, 4H), 1.8 (m, 2H), 1.6 (m, 4H),
1.1-1.3 (m, 1H); 12c (55%), δ 8.5 (s, 1H), 7.2 (s, 1H), 3.4 (m,
1H), 3.2 (m, 1H), 2.7-3.0 (m, 3H), 1.8-2.0 (m, 5H), 1.4-1.7
(m, 3H).
1
ration with acetone. H NMR (D2O): 13a (52%), δ 8.5 (s, 1H),
7.2 (s, 1H), 3.4 (m, 2H), 2.9 (m, 2H), 2.7 (t, 2H), 1.9 (d, 2H),
1.6 (m, 3H), 1.3 (m, 4H); 13b (42%), δ 8.5 (s, 1H), 7.2 (s, 1H),
3.4 (m, 2H), 2.9 (m, 1H), 2.5-2.7 (m, 3H), 1.9 (d, 2H), 1.7 (m,
4H), 1.1-1.3 (m, 3H); 13c (48%), δ 8.5 (s, 1H), 7.2 (s, 1H), 3.4
(m, 1H), 3.2 (m, 1H), 3.0 (m, 1H), 2.8 (t, 2H), 1.2-1.8 (m, 10H).
(3-[1,3]Dioxola n -2-ylp r op en yl)p yr id in e (29a -c). A solu-
tion of 15 mmol of [2-(1,3-dioxolan-2-yl)ethyl]triphenylphos-
phonium bromide in 100 mL of THF was cooled to -30 °C and
15 mmol of 1.6 M n-BuLi in hexane was added dropwise. The
orange solution was stirred for 45 min at -30 °C. Next, a
solution of 14 mmol of the corresponding pyridinecarboxalde-
hyde in 25 mL of THF was added dropwise and stirred at -30
°C for 30 min. The cooling bath was removed and the mixture
was stirred at room temperature for 3 h, after which time 25
mL of CHCl3 was added. The resulting mixture was washed
with saturated ammonium chloride solution (3 × 25 mL). The
organic layer was separated and dried with anhydrous sodium
sulfate and the solvent was evaporated under reduced pres-
sure. The crude product was purified by flash column chro-
matography (ether) to give a yellow oil. 1H NMR (CDCl3): 29a
(80%), δ 8.5 (d, 2H), 7.2 (d, 2H), 6.5 (d, 1H), 5.9 (dt, 1H), 5.0
(t, 1H), 3.9 (m, 4H), 2.7 (m, 2H); 29b (83%), δ 8.6 (s, 1H), 8.5
(m, 1H), 7.5 (m, 1H), 7.2 (m, 1H), 6.5 (d, 1H), 5.9 (dt, 1H), 5.0
(t, 1H), 3.9 (m, 4H), 2.7 (m, 2H); 29c (80%), δ 8.6 (d, 1H), 7.6
(m, 1H), 7.1-7.3 (m, 2H), 6.5 (d, 1H), 5.9 (dt, 1H), 5.0 (t, 1H),
3.9 (m, 4H), 2.7 (m, 2H).
N-Ben zyl-3-[h yd r oxy(t r ip h en ylm et h ylim id a zol-4-yl)-
m eth yl]p yr r olid in -2-on e (34). N-Benzylpyrrolidinone (35.4
mmol) dissolved in 100 mL of THF was cooled to -60 °C under
an atmosphere of dry nitrogen gas, 22.2 mL of n-BuLi (1.6M
in hexanes) was added dropwise, and the mixture stirred for
another 30 min at -60 °C. 1-(Triphenylmethyl)imidazol-4-yl-
carboxaldehyde (29.5 mmol) in 100 mL of THF was added
dropwise and the mixture was allowed to warm to ambient
temperature. Water (100 mL) was added and the THF was
evaporated under reduced pressure. The residue was extracted
with DCM (3 × 50 mL) and dried with Na2SO4. Concentration
in vacuo gave an oil which was purified by crystallization from
EtOAc to give 34 (76%) as an off-white crystals. 1H NMR
(CDCl3): δ 7.00-7.40 (m, 16H), 6.76+6.83 (s, 1H, ImH5
diastereomers), 5.25 + 5.42 (s, 1H, CH-O diastereomers), 4.46
(d, 1H, J ) 14.0 Hz), 4.33 (d, 1H, J ) 14.0 Hz), 3.45 (br, 1H),
2.95-3.20 (m, 2H), 1.70-2.10 (m, 3H).
(3-[1,3]Dioxola n -2-ylp r op yl)p yr id in e (30a -c). Com-
pound 29 (10 mmol) and 0.25 g of 5% Pd/C were added to 25
mL of EtOH. The suspension was stirred under an atmosphere
of hydrogen gas at room temperature for 24 h. The mixture
was filtered through a short column of Hyflo super cel and
concentrated under reduced pressure. The product was ob-
tained as a yellow oil and used directly in the next step. 1H
NMR (CDCl3): 30a (96%), δ 8.5 (d, 2H), 7.1 (d, 2H), 4.8 (t,
1H), 3.9 (m, 4H), 2.7 (t, 2H), 1.7 (m, 4H); 30b (95%), δ 8.5 (m,
2H), 7.5 (d, 1H), 7.2 (m, 1H), 4.9 (t, 1H), 3.9 (m, 4H), 2.7 (t,
2H), 1.7 (m, 4H); 30c (95%), δ 8.5 (d, 1H), 7.5 (m, 1H), 7.1-
7.3 (m, 2H), 4.9 (t, 1H), 3.9 (m, 4H), 2.7 (t, 2H), 1.7 (m, 4H).
4-P yr id in ylbu tyr a ld eh yd e (31a -c). Compound 30 (9
mmol) was dissolved in 50 mL of 1 N HCl solution. The
solution was stirred for 30 min and subsequently washed with
CHCl3 (3 × 50 mL). The aqueous layer was basified with
sodium carbonate (pH ≈ 12) and extracted with CHCl3 (3 ×
50 mL). The organic layer was dried with anhydrous sodium
sulfate and evaporated under reduced pressure to give a yellow
oil. 1H NMR (CDCl3): 31a (99%), δ 9.8 (t, 1H), 8.5 (d, 2H), 7.1
(d, 2H), 2.7 (t, 2H), 2.5 (t, 2H), 2.0 (tt, 2H); 31b (99%), δ 9.8 (t,
1H), 8.5 (m, 2H), 7.5 (d, 1H), 7.2 (m, 1H), 2.7 (t, 2H), 2.5 (t,
2H), 2.0 (tt, 2H); 31c (99%): δ 9.8 (t, 1H), 8.5 (d, 1H), 7.5 (t,
1H), 7.2 (m, 2H), 2.7 (t, 2H), 2.5 (t, 2H), 2.0 (tt, 2H).
N-Ben zyl-3-(tr ip h en ylm eth ylim id a zol-4-ylm eth ylen e)-
p yr r olid in -2-on e (35). Compound 34 (22.66 mmol) was
dissolved in 75 mL of dry pyridine and cooled in an ice-bath.
POCl3 (25 mmol) was added dropwise and the mixture was
stirred at ambient temperature for 1 h. Next, 68 mmol of DBU
was added and the mixture was stirred for 3 h. Then, 250 mL
of water was added and the mixture was extracted with DCM
(3 × 50 mL). The organic layers were washed with 50 mL of
1 M NaOH and 50 mL of water and dried with Na2SO4.
Concentration in vacuo gave an oil which was dissolved in 200
mL of DCM/EtOAc (1:1) and filtered through 150 g of silica
gel. The column was washed with DCM/EtOAc until the entire
product was recovered. The crude product was recrystallized
from EtOAc to give 35 (87%) as white crystals. 1H NMR
(CDCl3): δ 7.49 (s, 1H), 7.05-7.40 (m, 15H), 6.98 (s, 1H), 4.59
(s, 2H), 3.32 (t, 2H, J ) 6.6 Hz), 2.95-3.15 (m, 2H).
N-Ben zyl-3-(tr ip h en ylm eth ylim id a zol-4-ylm eth ylen e)-
p yr r olid in e (36). LiAlH4 (50 mmol) was suspended in 100
mL of THF and cooled to 0 °C. H2SO4 (25 mmol, 97%) was
added dropwise over a period of 30 min and stirred an
additional 30 min. Compound 35 (19.6 mmol) in 100 mL of
THF was added dropwise and the mixture was stirred at
ambient temperature for 1 h. Next, the mixture was cooled in
an ice-bath and the excess LiAlH4 was destroyed by an
addition of 5 mL of THF/water (1:1) and 30 mL of 5% NaOH.
The white solids were filtered and washed with THF (2 × 50
3-[4-(Tolu en e-4-su lfon yl)-4,5-d ih yd r ooxa zol-5-yl]p r o-
p ylp yr id in e (32a -c). The compounds were synthesized using

Histamine H3 Receptor Agonists
J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 25 5455
mL). The filtrate was dried with Na2SO4. Concentration in
vacuo gave 36 in a quantative yield as an oil. 1H NMR
(CDCl3): δ 7.38 (s, 1H), 7.00-7.50 (m, 15H), 6.63 (s, 1H), 6.22
(s, 1H), 3.62 (s, 2H), 3.35 (s, 2H), 2.21 (t, 2H, J ) 6.5 Hz),
2.40-2.60 (m, 2H).
NMR (CDCl3): δ 7.9 (s, 1H), 7.2 (s, 1H), 4.7 (d, 1H), 4.1 (m,
2H), 2.6-2.7 (m, 5H), 2.3 (m, 1H), 1.8-1.9 (m, 2H), 1.6 (s, 9H),
1.5 (s, 9H), 1.1 (m, 2H).
4-(1H-Im id a zol-5-ylm eth ylen e)p ip er id in e-1-ca r boxyl-
ic Acid ter t-Bu tyl Ester (41). A mixture of 40 (6 mmol) and
calcium carbonate (15 mmol) in 50 mL of dimethylformamide
was heated at reflux temperature for 2 h. After cooling, the
solvent was evaporated under reduced pressure. Ether (50 mL)
was added and the suspension was washed with saturated
sodium carbonate solution (3 × 50 mL). The ether layer was
dried with anhydrous sodium sulfate and concentrated in
vacuo. After purification using flash column chromatography
(EtOAc), the product was obtained as a white solid (85%). NMR
(CDCl3): δ 7.6 (s, 1H), 6.9 (s, 1H), 6.1 (s, 1H), 3.5 (m, 4H), 2.7
(t, 2H), 2.3 (t, 2H), 1.5 (s, 9H).
4-(1H-Im id a zol-5-ylm eth ylen e)p ip er id in e Dih yd r ogen
Ch lor id e (16). A solution of 41 (5 mmol) in 10 mL of 2 N HCl
solution was stirred at room temperature for 2 h and concen-
trated under reduced pressure. The product was recrystallized
from EtOH/(Et)2O as a pale yellow solid (45%). NMR (D2O):
δ 8.7 (s, 1H), 7.5 (s, 1H), 6.4 (s, 1H), 3.3 (m, 4H), 2.8 (m, 4H).
4-(1-t er t -Bu t oxyca r b on yl-1H -im id a zol-4-ca r b on yl)-
p ip er id in e-1-ca r boxylic Acid ter t-Bu tyl Ester (42). The
compound was synthesized using the procedure described for
the synthesis of 26, but 7.1 mmol of 39 was used as starting
chemical. The crude product was purified by flash column
(EtOAc:hexane, 1:3), to give a white solid (95%). 1H NMR
(CDCl3): δ 8.1 (s, 1H), 8.0 (s, 1H), 4.1 (m, 2H), 3.5 (m, 1H),
2.9 (m, 2H), 1.5-2.0 (m, 4H), 1.6 (s, 9H), 1.4 (s, 9H).
3-[(1H-Im id a zol-5-yl)m eth yl]p yr r olid in e Dih yd r ogen
Br om id e (14). Compound 36 (18.7 mmol) was dissolved in
100 mL of MeOH with 1 g of Pd(OH)2 (20% on carbon) and
hydrogenated under 50 atm hydrogen gas for 72 h. The
mixture was filtered through Hyflo super cel and concentrated
in vacuo. The residue was dissolved in 100 mL of 2 M HCl,
washed with ether (3 × 50 mL), and concentrated in vacuo to
give an oil. The oil was subsequently dissolved in 100 mL of
MeOH and 10 mL of triethylamine and treated with 45 mmol
of di-tert-butyl dicarbonate. After stirring of the mixture at
ambient temperature overnight, the solvents were evaporated,
and the residue was extracted with 100 mL of water and
EtOAc (3 × 75 mL). The organic layer was washed with brine
and dried with Na2SO4. Concentration in vacuo gave an oil
which was purified by column chromatography using a gradi-
ent of EtOAc and MeOH to give tert-butyl 4-(1-tert-butoxycar-
bonylpyrrolidin-3-yl)imidazole-1-carboxylate (66%) as a white
solid. The solid was dissolved in 10 mL of 2 M HBr and stirred
overnight. The mixture was evaporated and recrystallized from
1
EtOH to give 14 (96%) as a white solid. H NMR (D2O): δ 8.6
(s, 1H), 7.2 (s, 1H), 3.2-3.5 (m, 3H), 2.8-3.0 (m, 3H), 2.7 (m,
1H), 2.1 (m, 1H), 1.7 (m, 1H).
P yr id in -4-yl-(1-tr ityl-1H-im id a zol-4-yl)m eth a n ol (37).
1-Trityl-4-iodo imidazole41 (12 mmol) was dissolved in 100 mL
of DCM, 13.2 mmol of ethylmagnesium bromide (3.0 M in THF)
was added, and the mixture was stirred at room temperature
for 45 min. 4-Pyridinecarboxaldehyde (12 mmol) was added
dropwise and stirred for 16 h. The mixture was washed with
saturated sodium carbonate solution (3 × 100 mL) and dried
over anhydrous sodium sulfate. Evaporation of the solvent and
recrystallization from EtOAc yielded a pale yellow solid (89%).
NMR (CDCl3): δ 8.5 (d, 2H), 7.4 (s, 1H), 7.1-7.3 (m, 17H), 6.6
(s, 1H), 5.7 (s, 1H).
(1H-Im id a zol-5-yl)p ip er id in -4-ylm eth a n ol Dih yd r ogen
Ch lor id e (38). Compound 37 (10 mmol) was dissolved in 10
mL of 5 N HCl and added to a suspension of 0.5 g of 10% Pd/C
and 0.25 g of 5% Rd/C in 100 mL of MeOH. The mixture was
hydrogenated under 100 atm of hydrogen gas for 48 h in a
stainless steel bomb. The organic solvent was evaporated
under reduced pressure. The residue was dissolved in 100 mL
of water and washed with ether (3 × 100 mL). The aqueous
layer was concentrated in vacuo and recrystallized from EtOH
to give a pale yellow solid (85%). NMR (MeOH-d4): δ 8.7 (s,
1H), 7.4 (s, 1H), 4.8 (d, 1H), 3.5 (t, 2H), 3.0 (t, 2H), 2.0 (d, 2H),
1.5-1.7 (m, 3H).
4-[1-ter t-Bu t oxyca r b on yl-1H -im id a zol-4-yl]h yd r oxy-
m et h yl]p ip er id in e-1-ca r b oxylic Acid ter t-Bu t yl E st er
(39). To a solution of 38 (8 mmol) and triethylamine (30 mmol)
in 100 mL of MeOH was added di-tert-butyl dicarbonate (17
mmol) and the mixture stirred at room temperature for 24 h.
The solvent was evaporated and the residue was suspended
in 100 mL of ether. The mixture was washed with a saturated
sodium carbonate solution (100 mL) and subsequently with
water (2 × 100 mL). The organic layer was dried with
anhydrous sodium sulfate. The product was obtained as a
yellow oil and used directly in the next step (95%). NMR
(CDCl3): δ 7.9 (s, 1H), 7.2 (s, 1H), 4.3 (d, 1H), 4.1 (m, 2H), 2.6
(t, 2H), 1.8-2.0 (m, 3H), 1.5 (s, 9H), 1.4 (s, 9H), 1.1 (m, 2H).
4-[(1-ter t-Bu toxyca r bon yl-1H-im id a zol-4-yl)m eth a n e-
su lfon yloxym eth yl]p ip er id in e-1-ca r boxylic Acid ter t-Bu -
tyl Ester (40). A solution of 39 (7 mmol) and triethylamine
(17.5 mmol) in 50 mL of dry DCM was cooled in an ice bath
and methanesulfonyl chloride (7.7 mmol) was added. The ice
bath was removed and the mixture was stirred at room
temperature for 12 h. The mixture was washed with brine (3
× 50 mL) and the organic layer was dried with anhydrous
sodium sulfate. After evaporation under reduced pressure, the
residue was purified using flash column chromatography
(EtOAc). The product was obtained as a white solid (89%).
(1H-Im id a zol-5-yl)p ip er id in -4-ylm eth a n on e Dih yd r o-
gen Ch lor id e (18). Compound 42 (1.2 mmol) was dissolved
in 10 mL of acetone. The solution was added to 10 mL of 1 N
HCl and stirred at room temperature for 2 h. After concentra-
tion under reduced pressure, a white solid was obtained
1
(100%). H NMR (D2O): δ 8.8 (s, 1H), 8.3 (s, 1H), 3.5 (m, 3H),
3.1 (m, 2H), 2.1 (m, 2H), 1.0 (m, 2H).
4-[1-(1-ter t-Bu t oxyca r b on yl-1H -im id a zol-4-yl)vin yl]-
p ip er id in e-1-ca r boxylic Acid ter t-Bu tyl Ester (43). To a
solution of methyl triphenylphosphonium bromide (15 mmol)
in 150 mL of toluene was added potassium-tert-butoxide (15
mmol). After stirring for 1 h at room temperature, a solution
of 42 (7.5 mmol) in 100 mL of toluene was added dropwise.
The solution was stirred for 3 h and subsequently quenched
with a saturated solution of Na2CO3 (150 mL). The water layer
was extracted with toluene (3 × 100 mL), and the combined
organic layers were washed with brine (2 × 150 mL), dried
over anhydrous sodium sulfate, filtered, and concentrated
under reduced pressure. The crude product was purified using
flash column (EtOAc:hexane, 1:2) to yield a white solid (50%).
1H NMR (CDCl3): δ 8.0 (s, 1H), 7.3 (s, 1H), 5.7 (s, 1H), 5.0 (s,
1H), 4.0-4.3 (m, 2H), 2.6-2.9 (m, 2H), 2.4-2.6 (m, 1H), 1.7-
1.9 (m, 2H), 1.2-1.5 (m, 2H), 1.6 (s, 9H), 1.4 (s, 9H).
4-[1-(1H -Im id a zol-5-yl)vin yl]p ip er id in e Dih yd r ogen
Ch lor id e (17). Compound 43 (1.2 mmol) was dissolved in 10
mL of acetone. The solution was added to 10 mL of 1 N HCl
and stirred at room temperature for 2 h. After concentration
under reduced pressure, a white solid was obtained (100%).
1H NMR (D2O): δ 8.6 (s, 1H), 7.5 (s, 1H), 5.6 (s, 1H), 5.3 (s,
1H), 3.5 (m, 2H), 3.1 (m, 2H), 2.7 (m, 1H), 2.1 (m, 2H), 1.5-
1.7 (m, 2H).
Ack n ow led gm en t . This work was supported in
part (M.L., A.L.S.) by The Netherlands Foundation
for Chemical Sciences (CW) with financial aid from
The Netherlands Organization for Scientific Research
(NWO). The cells stably expressing the human hista-
mine H3 and H4 receptors were a gift from Dr. T.
Lovenberg, San Diego, CA.
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