Structural variations of 1-(4-(phenoxymethyl)benzyl)piperidines as nonimidazole histamine H3 receptor antagonists

Article abstract of DOI:10.1016/j.bmc.2004.03.009

Recent bioisoteric replacements in histamine H₃ receptor ligands with an exchange of the imidazole moiety by a piperidino group as well as of the trimethylene chain in 4-((3-phenoxy)propyl)-lH-imidazole derivatives (proxifan class) by an α,α^′-xylendiyl linker represents the starting point in the development of 1-(4-(phenoxymethyl)benzyl) piperidines as a new class of nonimidazole histamine H₃ receptor antagonists. According to different strategies in optimization of imidazole-containing antagonists the central benzyl phenyl ether moiety was replaced by numerous other polar functionalities. Additionally, the ortho- and meta-analogues of the lead were synthesized to determine the influence of the position of the piperidinomethyl substituent. The new compounds were tested in an in vitro binding assay for their affinities for cloned human H₃ receptors stably expressed in CHO-K1 cells and for their oral in vivo potencies brain in a functional screening assay in the brain of mice. Additionally, activities of selected compounds were determined in the guinea-pig ileum functional test model. In contrast to the analogues ortho-substituted compounds all other compounds maintained respectable affinities for the human H ₃ receptor (-logK_i values 6.3-7.5). Despite the results from other classes of compounds the 4-methyl substituted derivatives generally displayed higher affinities than the corresponding 4-chloro substituted compounds. In vivo only the inverse phenyl benzyl ether (3) showed worthwhile antagonist potencies.

Full text of DOI:10.1016/j.bmc.2004.03.009

Bioorganic & Medicinal Chemistry 12 (2004) 2727–2736
Structural variations of 1-(4-(phenoxymethyl)benzyl)piperidines
as nonimidazole histamine H3 receptor antagonists
b
a
c
d
ꢀ a
Tibor Miko, Xavier Ligneau, Heinz H. Pertz, Jean-Michel Arrang, C. Robin Ganellin,
Jean-Charles Schwartz,b,c Walter Schunacka and Holger Starke,*
aInstitut f€ur Pharmazie, Freie Universit€at Berlin, K€onigin-Luise-Strasse 2+4, 14195 Berlin, Germany
bBioproject Biotech, 4 rue du Chesnay Beauregard BP 96205, 35762 Saint Grꢀegoire Cedex, France
cUnitꢀe de Neurobiologie et Pharmacologie Molꢀeculaire (U. 573), Centre Paul Broca de l’INSERM, 2ter rue d’Alꢀesia,
75014 Paris, France
dDepartment of Chemistry, Christopher Ingold Laboratories, University College London, 20 Gordon Street, London WC1H 0AJ, UK
eJohann Wolfgang Goethe-Universit€at, Biozentrum, Institut f€ur Pharmazeutische Chemie, Marie-Curie-Strasse 9, 60439
Frankfurt am Main, Germany
Received 1 August 2003; accepted 5 March 2004
Available online 2 April 2004
Abstract—Recent bioisoteric replacements in histamine H3 receptor ligands with an exchange of the imidazole moiety by a pip-
eridino group as well as of the trimethylene chain in 4-((3-phenoxy)propyl)-lH-imidazole derivatives (proxifan class) by an a,a0-
xylendiyl linker represents the starting point in the development of 1-(4-(phenoxymethyl)benzyl)piperidines as a new class of
nonimidazole histamine H3 receptor antagonists. According to different strategies in optimization of imidazole-containing antag-
onists the central benzyl phenyl ether moiety was replaced by numerous other polar functionalities. Additionally, the ortho- and
meta-analogues of the lead were synthesized to determine the influence of the position of the piperidinomethyl substituent. The new
compounds were tested in an in vitro binding assay for their affinities for cloned human H3 receptors stably expressed in CHO-K1
cells and for their oral in vivo potencies brain in a functional screening assay in the brain of mice. Additionally, activities of selected
compounds were determined in the guinea-pig ileum functional test model. In contrast to the analogues ortho-substituted com-
pounds all other compounds maintained respectable affinities for the human H3 receptor ()log Ki values 6.3–7.5). Despite the results
from other classes of compounds the 4-methyl substituted derivatives generally displayed higher affinities than the corresponding 4-
chloro substituted compounds. In vivo only the inverse phenyl benzyl ether (3) showed worthwhile antagonist potencies.
Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction
activity in vivo,12 and very recently a related fourth
histamine receptor subtype have been identified.13
The neurotransmitter histamine inhibits its own release1
and synthesis2 in the central nervous system (CNS) via
presynaptically located H3 autoreceptors and modulates
the release of numerous other neurotransmitters.3–6
Since the recent successful cloning of rodent7 and espe-
cially human H3 receptors8 many important new find-
ings in this research field have been reported. Multiple
isoforms of the H3 receptor,9 differential isoform dis-
tribution in the brain of rodent10 and man,11 constitutive
The involvement of histamine in several (patho)physio-
logical processes is discovered in more detail. Since
cipralisant (GT-2331) has entered phase II clinical trials
for the treatment of attention-deficit hyperactivity dis-
order and the Abbott compound ABT-834 is also in
early clinical trials for cognitive disorders or as an anti-
obesity agent,14–16 the formerly predicted therapeutic use
of H3 receptor antagonists in psychiatric diseases and
other disorders of the CNS17 is eagerly awaited to be
confirmed. Therefore, it is of growing interest to dis-
cover ligands with high potency and selectivity, not only
to characterize receptor subtypes and to determine
structure–activity relationships, but also to maintain
their practical use by performing CNS availability after
Keywords: Histamine H3; Antagonist; Nonimidazole; Receptor; Med-
icinal chemistry.
* Corresponding author. Tel.: +49-69-798-29302; fax: +49-69-798-
29258; e-mail: h.stark@pharmchem.uni-frankfurt.de
0968-0896/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2004.03.009

2728
T. Mikꢀo et al. / Bioorg. Med. Chem. 12 (2004) 2727–2736
per oral application.16 Since many imidazole-containing
compounds, for example, cimetidine, possess unwanted
interactions with CYP 450 isoenzymes we focused on
the replacement of the imidazole moiety by alicyclic
amines in many classes of H3 receptor ligands.18 This led
to the discovery of UCL 2190 (Ki (human) ¼ 1.5 nM,
ED50 (mouse) ¼ 0.18 mg/kg p.o.) as a potent non-
imidazole analogue of ciproxifan.19–21 Interestingly, it
was recently reported that some nonimidazole H3
receptor antagonists do not possess affinity for H4
receptors whereas most imidazole-containing antago-
nists tested displayed more or less comparable affinities
for H3 and H4 receptors.22
2. Results and discussion
2.1. Chemistry
The inverse ether compounds 3 and 4 were obtained by
reaction of the corresponding phenol 1a either with
4-chlorobenzyl alcohol using Mitsunobu procedure29 or
with 4-methylbenzyl bromide by performing a classical
Williamson ether synthesis (Scheme 1). 4-(Piperidino-
methyl)phenylmethanol (3a) was prepared from
4-formylbenzoic acid methyl ester as reported previ-
ously.24 Amines 5 and 6 were prepared from tereph-
thalaldehyde mono(diethyl acetal). The aldehyde was
reductively aminated with either piperidine or 4-
chloroaniline, and then the protecting group was cleaved
under acidic work-up conditions resulting in the inter-
mediate products 5a and 6a, respectively. In the second
similar step these intermediates were again reductively
aminated with 4-methylaniline or piperidine to the final
compounds, respectively. Using phase transfer catalysis
3a reacted with the corresponding benzyl chlorides to
the dibenzyl ether derivatives 7–10.30 Esters (11, 12) and
carbamates (13, 14) were synthesized, respectively, from
3a and the corresponding benzoyl chlorides and isocy-
anates, respectively (Scheme 1).31
Based on a 4-((1H-imidazol-4-yl)methyl)benzyl tem-
plate23 reported for a new class of imidazole-containing
antagonists we have designed related nonimidazole
compounds by simple replacement of the imidazole ring
by a piperidino moiety leading to 1-(4-(phenoxy-
methyl)benzyl)piperidines.24 In an extension of previous
investigations on the substitution pattern of these com-
pounds here we report on variations of the functionality
and of substituents (Fig. 1).
By analogy with previous concepts to improve imidaz-
ole-containing antagonists, inverse ethers, amines,
esters, carbamates and dibenzyl ethers were synthesized.
Furthermore the ortho- and meta-analogues of the lead
compounds 1 and 2 were prepared to determine the
influence of the position of the piperidinomethyl
substituent. Thus we have studied a novel series of
piperidines in which the chemical structures possess
two lipophilic moieties connected by a polar function-
ality.
For compounds with the amino moiety in the meta-
position (15, 16) the meta-analogue of 3a was required:
this was prepared by analogous reductive amination of
isophthaldialdehyde, which resulted in the phenyl
methanol precursor 15a through simultaneous reduction
of the remaining aldehyde functionality to the alcohol
under the conditions selected (Scheme 2). Compound
15a was then used to prepare the final compounds 15
and 16 via Mitsunobu reaction. The same synthetic
route using phthaldialdehyde to synthesize the ortho-
substituted precursor 17a led to compound 17. Unex-
pectedly this method failed to provide the chloro
compound 18. So an alternative route was selected. 4-
Chlorophenol was treated with an excess of ortho-a,a0-
dibromoxylene in a SN reaction to give the intermediate
18a, which was then used to alkylate piperidine (Scheme
2).
All compounds were screened for their affinities at the
human histamine H3 receptor stably expressed in CHO-
K1 cells.25;26 In vivo potency was determined by
measuring the level of the main histamine metabolite,
Ns-methylhistamine, in the cerebral cortex after per oral
administration to mice.27 Selected compounds were also
tested in a functional model for H3 receptor potency on
isolated organs of guinea-pig.28
O
O
N
N
O
O
N
H
Ciproxifan
UCL 2190
N
N
H
H
O
Cl
N
N
N
H
O
R
Cl
1-(4-(Phenoxymethyl)benzyl)piperidines
Compound A
Figure 1. Imidazole and nonimidazole histamine H3 receptor antagonists.

T. Mikꢀo et al. / Bioorg. Med. Chem. 12 (2004) 2727–2736
2729
R
Z
O
NH
H
N
O
3, 4
Z = OH
(i)
(ii)
O
N
Z = CH
(iii)
2OH
R
R
7 - 10
11, 12
O
Z
Z = CH2OH
(iv)
O
O
N
N
Z = CH2OH
(v)
R
O
N
Z = CHO
R = CH3 (vi)
H
N
O
13, 14
R
Cl
N
N
H
R = Cl
(i)
H
N
H
5, 6
6a
Scheme 1. Synthesis of precursors 1a, 5a, 6a and final compounds 3–14 (for residue R please refer to Table 1). Reagents and conditions: (i)
Ti[OCH(CH3)2]4, NaBH3CN, EtOH, 12 h, rt; (ii) 3: 4-methylbenzyl bromide, KOH, EtOH, 48 h, reflux; 4: 4-chlorobenzyl alcohol, DEAD, PPh3,
THF, 3 d, rt; (iii) NaOH (40%), TBAHSO4, toluol, 12 h, rt; (iv) triethylamine, THF, 5 h, rt; (v) corresponding isocyanate, THF, N2, 4 h, rt; (vi) 4-
methylaniline, Ti[OCH(CH3)2]4, NaBH4, EtOH, 12 h, rt; 3–4: 1-(4-hydroxybenzyl)piperidine (1a); 7–14: 4-(piperidinomethyl)phenylmethanol (3a); 5:
4-(piperidinomethyl)benzaldehyde (5a); 6: piperidine; 6a: terephthalaldehyde mono(diethyl acetal), 4-chloroaniline, Ti[OCH(CH3)2]4, NaBH3CN,
EtOH, 12 h, rt.
O
O
H
OH
O
R
(i)
(ii)
N
N
H
15a, 17a
15-18
o- or m-
(iv)
Cl
Br
Br
(iii)
Br
O
18a
Scheme 2. Synthesis of precursors 15a, 17a, 18a and final compounds 15–18 (for residue R please refer to Table 1). Reagents and conditions:
(i) piperidine, Ti[OCH(CH3)2]4, NaBH4, EtOH, 9 h, rt; (ii) DEAD, PPh3, corresponding phenol, THF, 3 d, rt; (iii) 4-chlorophenol, K2CO3, EtOH,
12 h reflux; (iv) piperidine, KOH, EtOH, 12 h, reflux.
2.2. Pharmacological results and discussion
receptor. The analogous ortho-substituted compounds
17 and 18 showed a dramatic loss of affinities, whereas
the meta-substituted derivatives 15 and 16 still showed
moderate, but lower affinities than those of the corre-
sponding para-substituted lead compounds 1 and 2.
2.2.1. In vitro binding assay at cloned human H3
receptors. The affinities of compounds were determined
by measuring the displacement curves of [125I]iodo-
proxyfan at human histamine H3 receptors stably ex-
pressed in CHO cells (Table 1).25;26
Comparing the different polar functionalities (–Y–X–)
connecting the lipophilic moieties for the para-substi-
tuted compounds it is seen that the inverse ether com-
pounds 3 and 4, the esters 11 and 12 and the carbamates
13 and 14 show surprisingly small variation in affinity
All compounds having a para- or meta-substituted
phenyl spacer instead of the trimethylene chain showed
moderate to good affinities for the human histamine H3

2730
T. Mikꢀo et al. / Bioorg. Med. Chem. 12 (2004) 2727–2736
Table 1. Chemical structures and antagonist potencies of benzylpiperidines at histamine H3 receptors
3
4
2
N
X
Y
R
Pa
Y
X
R
)logKib
ED50 ꢀ SEMc
pA2
d
No
(mg/kg) p.o.
1e
4
4
4
4
4
4
4
4
4
4
4
4
4
4
3
3
2
2
CH2
CH2
O
O
CH3
Cl
6.9
6.9
7.4
7.1
7.1
6.9
6.7
6.6
6.8
6.7
7.4
6.6
7.1
6.7
ꢁ6.3
6.6
<6
ꢁ6
8.6g
7.3g
8.8
7.2g
1.8 ꢀ 0.4
>10
<6.5
2e
O
3
CH2
CH2
NH
NH
CH3
Cl
2.4 ꢀ 0.5
ꢁ25
6.3
6.4
6.3
4
O
5
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH3
Cl
12 ꢀ 2
>10
>10
6
7
O–CH2
O–CH2
H
8
F
Cl
>10
>10
9
10
O–CH2
O–CH2
6.3
6.3
Br
>10
11
12
O–C(@O)
O–C(@O)
O–C(@O)–NH
CH3
Cl
P10
>10
13
14
CH3
Cl
>10
>10
6.1
O–C(@O)–NH
15
16
O
O
O
O
CH3
Cl
10 ꢀ 2
>10
>10
Ndf
Ndf
17
18
CH3
Cl
>10
Proxyfan
Ciproxifan
UCL 2190
Thioperamid
>10h
0.14i
0.18j
1.0i
8.4i
7.9j
8.3k
a P position of substitution.
b
[
125I] Iodoproxyfan binding assay with membranes from CHO-K1 cells stably expressing the human H3 receptor (SEM 6 0.2).25;26
c Central H3 receptor assay in vivo after p.o. administration to mice.27
d Functional H3 receptor in vitro assay on guinea-pig ileum (SEM 6 0.2).28
e Ref. 24.
f Nd not determinable.
g Ref. 32.
h Ref. 34.
i Ref. 20.
j Ref. 18.
k Ref. 33.
values. Despite previous findings in other imidazole-
containing classes of H3 receptor antagonists34–36 a slight
preference for methyl instead of chloro substitution was
found with all compounds having a para-substituted
phenyl spacer. The most potent compounds in this series
(3, 5, 11 and 13) have similar affinities to that of the
reference compound ciproxifan, but do not reach the
high affinities of UCL 2190 or proxyfan. In the series of
ethers with dibenzyl partial structure (7–10) no influence
of the different halogen substituents was observed when
compared to compound 9.
ble in vitro results of the para-methyl (1) and para-
chloro substituted phenoxy (2) compounds, compound 1
only exhibited good potency in mice. The chloro com-
pound 2 was dramatically less active in vivo. Compa-
rable results were obtained but at a lower level with the
meta-substituted phenyl spacer compounds 15 and 16.
The inverse ethers (3, 4) showed a similar pattern,
whereas all other functionalities tested (5–14) led to
inactive compounds in vivo. It is unclear at the moment
if pharmacokinetic reasons or different kinds of receptor
antagonism (neutral antagonists vs inverse agonists) are
the reason for this failure of in vivo activity. Compa-
rable observations were made in the series of proxifans.34
2.2.2. In vivo screening on Swiss mice. Since compounds
are required to express activities in the CNS, it is of
major importance for their drug-like properties that they
reach their target after oral absorption. All novel com-
pounds were screened for their modulating effects on
levels of the main metabolite of histamine, Ns-methyl-
histamine, in the brain cortex of Swiss mice after oral
administration (Table 1).27 In contrast to the compara-
2.2.3. Screening of selected compounds at the guinea-pig
H3 receptor. Based on the results of the methyl substi-
tuted compounds in the in vitro binding and/or in the in
vivo experiments selected compounds were additionally
screened for their ability to block histamine H3 receptor
agonist-induced relaxation of field-stimulated guinea-

T. Mikꢀo et al. / Bioorg. Med. Chem. 12 (2004) 2727–2736
2731
pig ileum segments in a functional assay.28 Apart from
compounds 1, 15 and 17, which were not exactly
determinable due to strong cross interaction with mus-
carinic M3 receptors in this test model, all compounds
tested were found to be antagonists with lower affinities
for the guinea-pig H3 receptor than were obtained for
the human receptor. The affinities were also all similar
having pA2-value ranging from 6.1 to 6.4.
were recorded on a Bruker DPX 400 Avance spec-
trometer (400 MHz). 1H NMR chemical shifts are
expressed in ppm downfield from internal tetramethyl-
silane as reference. Data are reported in the following
order: multiplicity (br, broad; s, singlet; d, doublet; t,
triplet; m, multiplet; approximate coupling constants in
hertz (Hz); number of protons; Hax, axial proton; Heq,
equatorial proton; Anil, aminophenyl; Pip, piperidino;
Ph, phenyl; Xyl, para-a,a0-xylenediyl). Mass spectra
were obtained on Finnigan MAT CH7A (EI-MS) and
Finnigan MAT 711 (high-resolution mass spectra),
spectrometer resolving power 12,500. Elemental ana-
lyzes (C, H, N) for all compounds were measured on
Perkin–Elmer 240 B or Perkin–Elmer 240 C instru-
ments and are within ꢀ0.4% of theoretical values (Table
2).
3. Conclusion
Recently developed 1-(4-phenoxymethyl)benzyl)piperi-
dines, which are a new class of nonimidazole histamine
H3 receptor antagonists, have provided the basis for an
investigation of different structural aspects in this series.
Substitution patterns of the phenyl spacer seems to be
less favourable with meta- and even worse with ortho-
relative to para-substitution: Replacement of the meth-
oxy linker of the leads 1 and 2 by other structural groups
led to compounds with higher affinities at the human H3
receptor in the case of oxymethyl and carboxylic ester
compounds having a methyl substituent in the phenyl
group (3, 11). Their affinities for the hH3 receptor are in
the same range as that of the reference antagonist
ciproxifan. Chloro substitution seems to be less
favourable in this class. Preference for the methyl sub-
stitution was also seen with the in vivo screening.
Unfortunately, the amino (5, 6), dibenzyl (7–10), ester
(11, 12) and carbamate (13, 14) compounds were found
to be inactive in vivo in mice. The most promising
compound in this series was the inverse ether com-
pound 3 representing nanomolar affinity at the human
histamine H3 receptor and good antagonist potency
in vivo.
4.1.2. 4-Piperidinomethylphenol (1a). 4-Hydroxybenzal-
dehyde (3.66 g, 30 mmol), piperidine (3 mL, 30 mmol)
and titanium(IV)-isopropoxide (11.2 mL, 37.5 mmol)
were dissolved in 50 mL of dry ethanol and stirred for
4 h at room temperature. NaBH3CN (1.26 g, 20 mmol)
was added and the mixture was additionally stirred for
12 h. After adding water the solution was filtered and
concentrated in vacuo. The residue was purified by flash
column chromatography (eluent: EtOAc/hexane/triethyl-
amine 1:1:0.02) and a yellow solid was obtained (67.5%).
Mp 139 °C.37 1H NMR (CF3COOD): d 7.37 (d,
J ¼ 8:5 Hz, 2H, Ph-3-H, Ph-5-H), 7.11 (d, J ¼ 8:5 Hz,
2H, Ph-2-H, Ph-6-H), 6.66 (br, 1H, Nþ–H), 4.30 (d,
J ¼ 4:9 Hz, 2H, CH2–N), 3.67 (d, J ¼ 12:0 Hz, 2H,
Pip-2-Heq, Pip-6-Heq), 2.97–3.05 (m, 2H, Pip-2-Hax, Pip-
6-Hax), 2.09 (d, J ¼ 15:1 Hz, 2H, Pip-3-Heq, Pip-5-Heq),
1.99 (d, J ¼ 13:9 Hz, 1H, Pip-4-Heq), 1.80–1.90 (m,
2H, Pip-3-Hax, Pip-5-Hax), 1.54–1.63 (m, 1H, Pip-4-
Hax); MS (70 eV), m=z (%) 191 (Mþꢀ, 7). Anal. (C12H17
NO).
4. Experimental
4.1. Chemistry
4.1.1. General procedures. Melting points were deter-
4.1.3. 1-(4-(4-Methylbenzyloxy)benzyl)piperidine (3).
Compound 1a (0.57 g, 3 mmol), 4-methylbenzylbro-
mide (0.74 g, 4 mmol) and KOH (0.5 g, 9 mmol) were
€
mined on an Electrothermal IA 9000 digital or a Buchi
512 apparatus and are uncorrected. H NMR spectra
1
Table 2. Physical properties and elemental analysis of final compounds 3–18
No
Formula
Mw (g/mol)
Mp (°C)
Calcd (%)
H
Found (%)
H
C
N
C
N
3
C
20H25NOÆC2H2O4Æ0.25H2O
19H22ClNOÆC2H2O4Æ0.25H2O
389.7
410.1
433.7
404.6
389.7
403.2
419.7
464.1
413.2
433.6
428.2
453.1
385.2
405.6
389.7
405.6
149
137
67.81
61.46
63.70
61.56
67.81
65.54
62.96
56.94
66.86
60.94
64.51
58.31
68.60
62.18
67.81
62.18
7.06
5.97
6.80
6.09
7.06
6.45
6.19
5.60
6.53
5.54
6.54
5.62
7.01
5.92
7.06
5.92
3.59
3.41
6.46
6.77
3.59
3.47
3.34
3.02
3.39
3.23
6.54
6.18
3.63
3.45
3.59
3.45
67.67
61.39
63.83
61.95
67.66
65.37
62.95
56.99
66.68
60.98
64.19
58.30
68.47
62.17
67.99
62.01
6.77
5.82
6.82
6.20
7.37
6.24
5.93
5.52
6.59
5.38
6.55
5.48
6.91
5.81
6.75
5.92
3.54
3.33
6.12
6.63
3.64
3.44
3.28
2.92
3.35
3.24
6.26
6.14
3.85
3.43
3.55
3.33
4
C
5
C20H25N2Æ1.5C2H2O4Æ0.25H2O
154–156
153
6
C19H23ClN2ÆC2H2O4
7
C20H25NOÆC2H2O4Æ0.25H2O
128–129
125
161
8
C20H24FNOÆC2H2O4
C20H24ClNOÆC2H2O4
9
10
11
12
13
14
15
16
17
18
C20H24 BrNOÆC2H2O4
165–166
160–161
140
C21H25NO2ÆC2H2O4
C20H22ClNO2ÆC2H2O4
C21H26N2O2ÆC2H2O4
156–157
154–155
175
C
20H23ClN2O2ÆC2H2O4Æ0.25H2O
20H25NOÆC2H2O4
C
C19H22ClNOÆC2H2O4
20H25NOÆC2H2O4Æ0.25H2O
C19H22ClNOÆC2H2O4
163
155
C
177–178

2732
T. Mikꢀo et al. / Bioorg. Med. Chem. 12 (2004) 2727–2736
dissolved in 30 mL of ethanol and heated for 48 h under
reflux. The solvent was removed in vacuo and the resi-
due purified by flash column chromatography (eluent:
EtOAc/triethylamine 99.5:0.5). The resulting orange oil
was crystallized as a salt of oxalic acid from ethanol/
4.1.6. p-(4-(Piperidinomethyl)benzyl)tolyl amine (5).
Compound 5a (2.03 g, 10 mmol), 4-methylaniline
(1.07 g, 10 mmol) and titanium(IV)-isopropoxide (3 mL,
12 mmol) were dissolved in 60 mL of dry ethanol and
stirred at room temperature for 4 h. NaBH4 (0.37 g,
10 mmol) was added and the mixture was stirred for
additional 12 h. Water was added and the solution was
filtered. The aqueous solution was added dropwise to
200 mL of 2 N HCl and stirred for 12 h. The alcohol was
evaporated and the remaining aqueous solution
extracted with ethyl acetate (3 ꢂ 50 mL). The organic
layers were combined, dried (Na2SO4) and the solvent
was removed in vacuo. The residue was purified by flash
column chromatography (eluent: EtOAc/hexane/triethyl-
amine 1:1:0.02) and the product obtained was crystal-
lized as a salt of oxalic acid from ethanol/diethyl ether
(4%). 1H NMR (CF3COOD): d 9.97 (s, 1H, CHO),
7.54–7.62 (m, 4H, Xyl-2-H, Xyl-3-H, Xyl-5-H, Xyl-6-
H), 7.40 (d, J ¼ 8:1 Hz, 2H, Ph-3-H, Ph-5-H), 7.31 (d,
J ¼ 8:1 Hz, 2H, Ph-2-H, Ph-6-H), 7.01 (br, 1H, Nþ–H),
4.77 (s, 2H, Ph-N-CH2), 4.41 (d, J ¼ 4:4 Hz, 2H, CH2–
N), 3.67 (d, J ¼ 11:7 Hz, 2H, Pip-2-Heq, Pip-6-Heq),
3.02–3.10 (m, 2H, Pip-2-Hax, Pip-6-Hax), 2.46 (s, 3H,
CH3), 2.09 (d, J ¼ 14:7 Hz, 2H, Pip-3-Heq, Pip-5-Heq),
1.99 (d, J ¼ 13:6 Hz, 1H, Pip-4-Heq), 1.89–1.91 (m, 2H,
Pip-3-Hax, Pip-5-Hax), 1.54–1.64 (m, 1H, Pip-4-Hax); MS
(70 eV), m=z (%) 294 (Mþꢀ, 100).
1
diethyl ether (17%). H NMR (CF3COOD): d 7.38 (d,
J ¼ 8:3 Hz, 2H, benzyl-2-H, benzyl-6-H), 7.32 (d,
J ¼ 7:9 Hz, 2H, Ph-2-H, Ph-6-H), 7.24 (d, J ¼ 7:9 Hz,
2H, Ph-3-H, Ph-5-H), 7.11 (d, J ¼ 8:3 Hz, 2H, benzyl-3-
H, benzyl-5-H), 6.69 (br, 1H, Nþ–H), 5.44 (s, 2H, CH2–
O), 4.30 (d, J ¼ 5:1 Hz, 2H, CH2–N), 3.67 (d,
J ¼ 12:3 Hz, 2H, Pip-2-Heq, Pip-6-Heq), 2.96–3.08 (m,
2H, Pip-2-Hax, Pip-6-Hax), 2.37 (s, 3H, CH3), 2.09 (d,
J ¼ 14:6 Hz, 2H, Pip-3-Heq, Pip-5-Heq), 1.99 (d,
J ¼ 13:5 Hz, 1H, Pip-4-Heq), 1.81–1.91 (m, 2H, Pip-3-
Hax, Pip-5-Hax), 1.47–1.53 (m, 1H, Pip-4-Hax); MS
(70 eV), m=z (%) 295 (Mþꢀ, 12).
4.1.4. 1-(4-(4-Chlorobenzyloxy)benzyl)piperidine (4).
Compound 1a (0.57 g, 3 mmol), 4-chlorobenzylalcohol
(0.43 g, 3 mmol) and triphenylphosphine (0.78 g,
3 mmol) were dissolved in 20 mL of dry tetrahydrofuran
under argon atmosphere and cooled in an ice bath.
Diethyl azodicarboxylate (DEAD, 0.57 mL, 3.6 mmol)
was added dropwise followed by stirring for 3 d at room
temperature. The solvent was removed in vacuo and the
residue was purified by flash column chromatography
(eluent: EtOAc/hexane/triethylamine 1:1:0.02). The
product obtained was crystallized as a salt of oxalic
acid from ethanol/diethyl ether (16%). 1H NMR
(CF3COOD): d 7.34–7.44 (m, 6H, benzyl-2-H, benzyl-3-
H, benzyl-5-H, benzyl-6-H, Ph-3-H, Ph-5-H), 7.17 (d,
J ¼ 8:5 Hz, 2H, Ph-2-H, Ph-6-H), 6.68 (br, 1H, Nþ–H),
5.21 (s, 2H, CH2–O), 4.30 (d, J ¼ 4:9 Hz, 2H, CH2–N),
3.67 (d, J ¼ 11:9 Hz, 2H, Pip-2-Heq, Pip-6-Heq), 2.95–
3.03 (m, 2H, Pip-2-Hax, Pip-6-Hax), 2.08 (d, J ¼ 14:3 Hz,
2H, Pip-3-Heq, Pip-5-Heq), 1.99 (d, J ¼ 13:6 Hz, 1H, Pip-
4-Heq), 1.80–1.90 (m, 2H, Pip-3-Hax, Pip-5-Hax), 1.53–
1.62 (m, 1H, Pip-4-Hax); MS (70 eV), m=z (%) 315 (Mþ,
10).
4.1.7.
4-((4-Chlorophenylamino)methyl)benzaldehyde
hydrochloride (6a). Terephthalaldehyde mono(diethyl
acetal) (2.1 mL, 10 mmol), 4-chloroaniline (1.27 g,
10 mmol) and titanium(IV)-isopropoxide (3.7 mL,
12.5 mmol) were dissolved in 50 mL of ethanol and
stirred for 6 h at room temperature. NaBH3CN (0.42 g,
6.7 mmol) was added and the mixture was additionally
stirred for 12 h. After adding 10 mL of water the solu-
tion was filtered and the alcohol was evaporated under
reduced pressure. The aqueous solution was added
dropwise to 20 mL of 2 N HCl and stirred for 12 h. The
resulting yellow precipitate was filtered and dried (21%).
Mp 192 °C. 1H NMR (CF3COOD):
d 8.27 (d,
J ¼ 8:0 Hz, 2H, Ph-2-H, Ph-6-H), 7.99 (d, J ¼ 8:0 Hz,
2H, Ph-3-H, Ph-5-H), 7.60 (d, J ¼ 8:7 Hz, 2H, Anil-3-H,
Anil-5-H), 7.44 (d, J ¼ 8:7 Hz, 2H, Anil-2-H, Anil-6-H),
4.81 (s, 2H, CH2–N); MS (70 eV), m=z (%) 245 (Mþꢀ,
100). Anal. (C14 H12ClNO), (Mr 245.58).
4.1.5. 4-(Piperidinomethyl)benzaldehyde (5a). Tereph-
thalaldehyde mono(diethyl acetal) (20 mL, 96 mmol),
piperidine (8.2 mL, 96 mmol) and titanium(IV)-iso-
propoxide (35.7 mL, 120 mmol) were dissolved in 50 mL
of dry ethanol and stirred for 12 h at room temperature.
NaBH4 (3.7 g, 100 mmol) was added in and the mixture
was stirred for additional 12 h. After adding 50 mL of
water the solution was filtered and concentrated in vacuo.
The residue was purified by flash column chromato-
graphy (eluent: EtOAc/hexane/triethylamine 1:1:0.02) to
4.1.8. N-(4-Chlorophenyl)-(4-(piperidinomethyl)phenyl)-
methanamine (6). Synthesized, as described for 5 from
6a (0.56 g, 2 mmol), piperidine (0.51 mL, 6 mmol),
titanium(IV)-isopropoxide (1.2 mL, 4 mmol) and
NaBH3CN (0.08 g, 1.3 mmol) (24.5%). 1H NMR
(CF3COOD): d 7.53–7.64 (m, 6H, Ph-3-H, Ph-5-H, Xyl-
2-H, Xyl-3-H, Xyl-5-H, Xyl-6-H), 7.44 (d, J ¼ 8:7 Hz,
2H, Ph-2-H, Ph-6-H), 7.00 (br, 1H, Nþ–H), 4.81 (s, 2H,
Ph–CH2–N), 4.40 (d, J ¼ 4:5 Hz, 2H, CH2–N), 3.67 (d,
J ¼ 11:7 Hz, 2H, Pip-2-Heq, Pip-6-Heq), 3.02–3.10 (m,
2H, Pip-2-Hax, Pip-6-Hax), 2.09 (d, J ¼ 14:2 Hz, 2H,
Pip-3-Heq, Pip-5-Heq), 1.99 (d, J ¼ 13:7 Hz, 1H, Pip-4-
1
give a yellow oil (87%). H NMR (CF3COOD): d 10.12
(s, 1H, CHO), 8.28 (d, J ¼ 8:1 Hz, 2H, Ph-2-H, Ph-6-H),
8.0 (d, J ¼ 8:1 Hz, 2H, Ph-3-H, Ph-5-H), 3.99 (d,
J ¼ 11:4 Hz, 1H, CH2–N), 3.85 (d, J ¼ 11:4 Hz, 1H,
CH2–N), 3.41 (t, J ¼ 12:0 Hz, 1H, Pip-2-Heq), 3.30 (t,
J ¼ 12:0 Hz, 1H, Pip-6-Heq), 2.15–2.25 (m, 2H, Pip-2-
Hax, Pip-6-Hax), 1.92–2.09 (m, 5H, Pip-3-H, Pip-4-Heq,
Pip-5-H), 1.60–1.79 (m, 1H, Pip-4-Hax); MS (70 eV), m=z
(%) 203 (Mþ, 83). Anal. (C13H17NO). (Mr 203.12).

T. Mikꢀo et al. / Bioorg. Med. Chem. 12 (2004) 2727–2736
2733
Heq), 1.82–1.92 (m, 2H, Pip-3-Hax, Pip-5-Hax), 1.54–1.65
(m, 1H, Pip-4-Hax ); MS (70 eV), m=z (%) 314 (Mþꢀ, 74).
4.2.4. 1-(4-(4-Bromobenzyloxymethyl)benzyl)piperidine
(10). 37%. 1H NMR (CF3COOD):
7.55 (d,
d
J ¼ 7:7 Hz, 4H, Xyl-2-H, Xyl-6-H, Ph-3-H, Ph-5-H),
7.47 (d, J ¼ 7:9 Hz, 2H, Xyl-3-H, Xyl-5-H), 7.28 (d,
J ¼ 8:3 Hz, 2H, Ph-2-H, Ph-6-H), 6.79 (br, 1H, Nþ–H),
4.78 (s, 4H, (CH2–O)2), 4.37 (d, J ¼ 5:2 Hz, 2H, CH2–
N), 3.67 (d, J ¼ 11:9 Hz, 2H, Pip-2-Heq, Pip-6-Heq),
2.99–3.08 (m, 2H, Pip-2-Hax, Pip-6-Hax), 2.09 (d,
J ¼ 14:9 Hz, 2H, Pip-3-Heq, Pip-5-Heq), 1.99 (d,
J ¼ 13:8 Hz, 1H, Pip-4-Heq), 1.81–1.91 (m, 2H, Pip-3-
Hax, Pip-5-Hax), 1.54–1.64 (m, 1H, Pip-4-Hax); MS
(70 eV), m=z (%) 373 (Mþꢀ, 21).
4.2. General procedure for the preparation of dibenzyl-
ethers 7–10
4-(Piperidinomethyl)phenylmethanol
(3a)
(0.41 g,
2 mmol), benzylbromide (for 7), 4-fluorobenzylchloride
(for 8), 4-chlorobenzylchloride (for 9), 4-bromobenzyl-
chloride (for 10) (6 mmol) and tetrabutylammoniumhy-
drogensulfate (TBAHSO4, 0.34 g, 1 mmol) were
dissolved in 10 mL of toluol, 10 mL of NaOH solution
(40%) was added and the mixture was vigourously stir-
red at room temperature for 12 h (7: 6 h). After adding
20 mL of water the mixture was extracted with ethyl
acetate (3 ꢂ 20 mL). The organic layers were combined,
dried (NaSO4) and the solvent removed in vacuo. The
residue was purified by flash column chromatography
(eluent: ethyl acetate saturated with ammonia) and the
product obtained was crystallized as a salt of oxalic acid
from ethanol/diethyl ether.
4.2.5. 4-Methylbenzoic acid 4-piperidinomethylbenzyl
ester (11). 4-(Piperidinomethyl)phenylmethanol (3a)
(0.41 g, 2 mmol) and triethylamine (0.2 g, 3.5 mmol) were
dissolved in 5 mL of dry tetrahydrofuran. The solution
was cooled to 0 °C in an ice bath and p-toluoyl chloride
(0.4 mL, 3 mmol) was slowly added. The mixture was
allowed to warm to room temperature and was stirred
for additional 4 h. The white precipitate obtained was
filtered and the solution was concentrated in vacuo. The
residue was purified by flash column chromatography
(eluent: EtOAc/hexane 1:1 saturated with ammonia).
The resulting colorless oil was crystallized as a salt of
4.2.1. 1-(4-(Benzyloxymethyl)benzyl)piperidine (7). 76%.
1H NMR (CF3COOD): d 7.55 (d, J ¼ 7:9 Hz, 2H, Xyl-
2-H, Xyl-6-H), 7.47 (d, J ¼ 7:9 Hz, 2H, Xyl-3-H, Xyl-5-
H), 7.35–7.43 (m, 5H, Ph-2-H, Ph-3-H, Ph-4-H, Ph-5-H,
Ph-6-H), 6.78 (br, 1H, Nþ–H), 4.80 (s, 4H, (CH2–O)2),
4.37 (d, J ¼ 5:3 Hz, 2H, CH2–N), 3.66 (d, J ¼ 11:9 Hz,
2H, Pip-2-Heq, Pip-6-Heq), 2.99–3.07 (m, 2H, Pip-2-Hax,
Pip-6-Hax), 2.08 (d, J ¼ 14:6 Hz, 2H, Pip-3-Heq, Pip-5-
Heq), 1.98 (d, J ¼ 13:8 Hz, 1H, Pip-4-Heq), 1.81–1.91 (m,
2H, Pip-3-Hax, Pip-5-Hax), 1.54–1.63 (m, 1H, Pip-4-Hax);
MS (70 eV), m=z (%) 295 (Mþꢀ, 47).
1
oxalic acid from ethanol/diethyl ether (75%). H NMR
(CF3COOD): d 8.01 (d, J ¼ 8:0 Hz, 2H, Ph-2-H, Ph-6-
H), 7.64 (d, J ¼ 7:9 Hz, 2H, Xyl-2-H, Xyl-6-H), 7.50 (d,
J ¼ 7:9 Hz, 2H, Xyl-3-H, Xyl-5-H), 7.34 (d, J ¼ 8:0 Hz,
2H, Ph-3-H, Ph-5-H), 6.80 (br, 1H, Nþ–H), 5.54 (s, 2H,
CH2–O), 4.38 (d, J ¼ 5:3 Hz, 2H, CH2–N), 3.71 (d,
J ¼ 12:0 Hz, 2H, Pip-2-Heq, Pip-6-Heq), 3.00–3.09 (m,
2H, Pip-2-Hax, Pip-6-Hax), 2.45 (s, 3H, CH3), 2.09 (d,
J ¼ 14:9 Hz, 2H, Pip-3-Heq, Pip-5-Heq), 1.99 (d,
J ¼ 13:7 Hz, 1H, Pip-4-Heq), 1.82–1.92 (m, 2H, Pip-3-
Hax, Pip-5-Hax), 1.54–1.64 (m, 1H, Pip-4-Hax); MS
(70 eV), m=z (%) 323 (Mþꢀ, 40).
4.2.2. 1-(4-(4-Fluorobenzyloxymethyl)benzyl)piperidine
(8). 67%. 1H NMR (CF3COOD):
d
7.55 (d,
J ¼ 7:9 Hz, 2H, Xyl-2-H, Xyl-6-H), 7.48 (d, J ¼ 7:9 Hz,
2H, Xyl-3-H, Xyl-5-H), 7.41 (q, J1 ¼ 5:4 Hz,
J2 ¼ 2:8 Hz, 2H, Ph-2-H, Ph-6-H), 7.09 (t, J ¼ 8:7 Hz,
2H, Ph-3-H, Ph-5-H), 6.79 (br, 1H, Nþ–H), 4.79 (s, 4H,
(CH2–O)2), 4.37 (d, J ¼ 5:3 Hz, 2H, CH2–N), 3.67 (d,
J ¼ 11:9 Hz, 2H, Pip-2-Heq, Pip-6-Heq), 3.00–3.08 (m,
2H, Pip-2-Hax, Pip-6-Hax), 2.09 (d, J ¼ 14:9 Hz, 2H,
Pip-3-Heq, Pip-5-Heq), 1.99 (d, J ¼ 13:6 Hz, 1H, Pip-4-
Heq), 1.81–1.91 (m, 2H, Pip-3-Hax, Pip-5-Hax), 1.54–1.64
(m, 1H, Pip-4-Hax); MS (70 eV), m=z (%) 313 (Mþꢀ, 40).
4.2.6. 4-Chlorobenzoic acid 4-piperidinomethylbenzyl
ester (12). Synthesized, as described for 11 from 4-
chlorobenzoyl chloride (0.6 mL, 5 mmol) (32%). 1H
NMR (CF3COOD): d 8.06 (d, J ¼ 7:2 Hz, 2H, Ph-2-H,
Ph-6-H), 7.64 (d, J ¼ 7:4 Hz, 2H, Xyl-2-H, Xyl-6-H),
7.50 (d, J ¼ 7:2 Hz, 4H, Ph-3-H, Ph-5-H, Xyl-3-H, Xyl-
5-H), 6.79 (br, 1H, Nþ–H), 5.56 (s, 2H, CH2–O), 4.38 (d,
J ¼ 5:0 Hz, 2H, CH2–N), 3.71 (d, J ¼ 11:9 Hz, 2H, Pip-
2-Heq, Pip-6-Heq), 3.00–3.08 (m, 2H, Pip-2-Hax, Pip-6-
Hax), 2.09 (d, J ¼ 14:9 Hz, 2H, Pip-3-Heq, Pip-5-Heq),
1.99 (d, J ¼ 14:2 Hz, 1H, Pip-4-Heq), 1.82–1.92 (m, 2H,
Pip-3-Hax, Pip-5-Hax), 1.54–1.63 (m, 1H, Pip-4-Hax); MS
(70 eV), m=z (%) 343 (Mþꢀ, 35).
4.2.3. 1-(4-(4-Chlorobenzyloxymethyl)benzyl)piperidine
(9). 51%. 1H NMR (CF3COOD):
d
7.55 (d,
J ¼ 7:9 Hz, 2H, Xyl-2-H, Xyl-6-H), 7.48 (d, J ¼ 7:9 Hz,
2H, Xyl-3-H, Xyl-5-H), 7.39 (d, J ¼ 8:5 Hz, 2H, Ph-3-H,
Ph-5-H), 7.35 (d, J ¼ 8:5 Hz, 2H, Ph-2-H, Ph-6-H), 6.80
(br, 1H, Nþ–H), 4.79 (s, 4H, (CH2–O)2), 4.37 (d,
J ¼ 5:4 Hz, 2H, CH2–N), 3.67 (d, J ¼ 11:9 Hz, 2H, Pip-
2-Heq, Pip-6-Heq), 3.00–3.08 (m, 2H, Pip-2-Hax, Pip-6-
Hax), 2.09 (d, J ¼ 14:9 Hz, 2H, Pip-3-Heq, Pip-5-Heq),
1.99 (d, J ¼ 13:6 Hz, 1H, Pip-4-Heq), 1.81–1.91 (m, 2H,
Pip-3-Hax, Pip-5-Hax), 1.54–1.64 (m, 1H, Pip-4-Hax); MS
(70 eV), m=z (%) 329 (Mþꢀ, 31).
4.2.7. p-Tolyl-carbamic acid 4-piperidinomethylbenzyl
ester (13). p-Tolyl isocyanate (0.53 g, 4 mmol) and
4-(piperidinomethyl)phenylmethanol
(3a)
(0.41 g,
2 mmol) were dissolved together in 20 mL of dry tetra-
hydrofuran and stirred at room temperature for 3 h
under nitrogen atmosphere. The solvent was removed
and the residue purified by flash column chromatogra-
phy (eluent: EtOAc/hexane/triethylamine 1:1:0.02). The

2734
T. Mikꢀo et al. / Bioorg. Med. Chem. 12 (2004) 2727–2736
resulting white solid was crystallized as a salt of oxalic
acid from ethanol/diethyl ether (60%). 1H NMR
(CF3COOD): d 7.58 (d, J ¼ 7:9 Hz, 2H, Ph-2-H, Ph-6-
H), 7.48 (d, J ¼ 7:9 Hz, 2H, Ph-3-H, Ph-5-H), 7.18 (d,
J ¼ 8:6 Hz, 4H, Xyl-2-H, Xyl-3-H, Xyl-5-H, Xyl-6-H),
2-Heq, Pip-6-Heq), 2.92–3.01 (m, 2H, Pip-2-Hax, Pip-6-
Hax), 2.31 (s, 3H, CH3), 1.96–2.07 (m, 3H, Pip-3-Heq,
Pip-4-Heq, Pip-5-Heq), 1.79–1.90 (m, 2H, Pip-3-Hax, Pip-
5-Hax), 1.51–1.60 (m, 1H, Pip-4-Hax); MS (70 eV), m=z
(%) 295 (Mþꢀ, 42).
6.80 (br, 1H, N –H), 5.39 (s, 2H, CH2–O), 4.37 (d,
þ
J ¼ 5:4 Hz, 2H, CH2–N), 3.70 (d, J ¼ 11:4 Hz, 2H, Pip-
2-Heq, Pip-6-Heq), 3.00–3.09 (m, 2H, Pip-2-Hax, Pip-6-
Hax), 2.34 (s, 3H, CH3), 2.09 (d, J ¼ 14:8 Hz, 2H, Pip-3-
Heq, Pip-5-Heq), 1.99 (d, J ¼ 13:2 Hz, 1H, Pip-4-Heq),
1.82–1.92 (m, 2H, Pip-3-Hax, Pip-5-Hax), 1.54–1.64 (m,
1H, Pip-4-Hax); MS (70 eV), m=z (%) 338 (Mþꢀ, 44).
4.2.11. 1-(3-(4-Chlorophenoxymethyl)benzyl)piperidine
(16). Synthesized, as described for 15 from 4-chloro-
phenol (0.26 g, 2 mmol) (60%). 1H NMR (CF3COOD): d
7.65 (d, J ¼ 7:4 Hz, 1H, Xyl-4-H), 7.59 (t, J ¼ 7:4 Hz,
1H, Xyl-5-H), 7.54 (s, 1H, Xyl-2-H), 7.45 (d, J ¼ 7:4 Hz,
1H, Xyl-6-H), 7.30 (d, J ¼ 7:9 Hz, 2H, Ph-3-H, Ph-5-H),
7.00 (d, J ¼ 7:9 Hz, 2H, Ph-2-H, Ph-6-H), 6.79 (br, 1H,
Nþ–H), 5.29 (s, 2H, CH2–O), 4.37 (d, J ¼ 4:1 Hz, 2H,
CH2–N), 3.61 (d, J ¼ 12:0 Hz, 2H, Pip-2-Heq, Pip-6-
Heq), 2.92–3.01 (m, 2H, Pip-2-Hax, Pip-6-Hax), 2.07 (d,
J ¼ 14:9 Hz, 2H, Pip-3-Heq, Pip-5-Heq), 1.98 (d,
J ¼ 13:8 Hz, 1H, Pip-4-Heq), 1.80–1.90 (m, 2H, Pip-3-
Hax, Pip-5-Hax), 1.51–1.61 (m, 1H, Pip-4-Hax); MS
(70 eV), m=z (%) 315 (Mþꢀ, 34).
4.2.8. (4-Chlorophenyl)carbamic acid 4-piperidinometh-
ylbenzyl ester (14). Synthesized, as described for 13 from
4-chlorophenyl isocyanate (0.61 g, 4 mmol). The white
solid was directly crystallized as a salt of oxalic acid
from ethanol/diethyl ether without prior purification
1
(36%). H NMR (CF3COOD): d 7.57 (d, J ¼ 7:6 Hz,
2H, Ph-2-H, Ph-6-H), 7.48 (d, J ¼ 7:6 Hz, 2H, Ph-3-H,
Ph-5-H), 7.33 (d, J ¼ 8:7 Hz, 2H, Xyl-2-H, Xyl-6-H),
7.28 (d, J ¼ 8:7 Hz, 2H, Xyl-3-H, Xyl-5-H), 6.80 (br,
1H, Nþ–H), 5.38 (s, 2H, CH2–O), 4.37 (d, J ¼ 4:3 Hz,
2H, CH2–N), 3.69 (d, J ¼ 12:2 Hz, 2H, Pip-2-Heq, Pip-6-
Heq), 3.00–3.08 (m, 2H, Pip-2-Hax, Pip-6-Hax), 2.09 (d,
J ¼ 14:6 Hz, 2H, Pip-3-Heq, Pip-5-Heq), 1.99 (d,
J ¼ 13:6 Hz, 1H, Pip-4-Heq), 1.82–1.92 (m, 2H, Pip-3-
Hax, Pip-5-Hax), 1.57–1.64 (m, 1H, Pip-4-Hax); MS
(70 eV), m=z (%) 358 (Mþꢀ, 48).
4.2.12. (2-(Piperidinomethyl)phenyl)methanol (17a).
Synthesized, as described for 15a from phthaldialde-
hyde (3 g, 22 mmol). The product was obtained as an
orange solid (41%). Mp 72–73 C.38 1H NMR
(CF3COOD): d 7.51–7.60 (m, 3H, Xyl-3-H, Xyl-4-H,
Xyl-5-H), 7.49 (d, J ¼ 7:3 Hz, 1H, Xyl-6-H), 5.02 (s, 2H,
CH2–O), 4.3 (d, J ¼ 3:5 Hz, 2H, CH2–N), 3.64 (d,
J ¼ 12:2 Hz, 2H, Pip-2-Heq, Pip-6-Heq), 3.01–3.07 (m,
2H, Pip-2-Hax, Pip-6-Hax), 2.10 (d, J ¼ 14:8 Hz, 2H,
Pip-3-Heq, Pip-5-Heq), 1.98 (d, J ¼ 13:3 Hz, 1H, Pip-4-
Heq), 1.73–1.80 (m, 2H, Pip-3-Hax, Pip-5-Hax), 1.57–1.64
(m, 1H, Pip-4-Hax); MS (70 eV), m=z (%) 205 (Mþꢀ, 16).
Anal. (C13H19O). (Mr 205.12).
4.2.9. (3-(Piperidinomethyl)phenyl)methanol (15a). Iso-
phthaldialdehyde (3 g, 22 mmol), piperidine (1 mL,
9 mmol) and titanium(IV)-isopropoxide (2.6 mL,
9 mmol) were dissolved in 20 mL of dry ethanol and
stirred for 1 h at room temperature. NaBH4 (0.84 g,
22 mmol) was added and the mixture was additionally
stirred for 8 h. After adding 10 mL of water the solution
was filtered and concentrated in vacuo. The residue was
purified by flash column chromatography (eluent:
EtOAc/hexane/triethylamine 1:1:0.02) and a colourless
oil was obtained (46%).38 1H NMR (CF3COOD): d 7.65
(d, J ¼ 7:7 Hz, 1H, Xyl-5-H), 7.56–7.59 (m, 2H, Xyl-4-
H, Xyl-6-H), 7.51 (d, J ¼ 7:7 Hz, 1H, Xyl-2-H), 5.51 (s,
2H, CH2–O), 4.38 (d, J ¼ 3:7 Hz, 2H, CH2–N), 3.69 (d,
J ¼ 11:9 Hz, 2H, Pip-2-Heq, Pip-6-Heq), 2.99–3.09 (m,
2H, Pip-2-Hax, Pip-6-Hax), 2.09 (d, J ¼ 14:8 Hz, 2H,
Pip-3-Heq, Pip-5-Heq), 1.99 (d, J ¼ 13:8 Hz, 1H, Pip-4-
Heq ), 1.82–1.92 (m, 2H, Pip-3-Hax, Pip-5-Hax), 1.55–
1.65 (m, 1H, Pip-4-Hax); MS (70 eV), m=z (%) 205 (Mþꢀ,
45). Anal. C13H19NO. (Mr 205.12).
4.2.13. 1-(2-p-Tolyloxymethyl)benzylpiperidine (17).
Synthesized, as described for 15 from 17a (0.41 g,
1
2 mmol) (7%). H NMR (CF3COOD): d 7.59–7.65 (m,
3H, Xyl-3-H, Xyl-4-H, Xyl-5-H), 7.52 (d, J ¼ 7:3 Hz,
1H, Xyl-6-H), 7.27 (d, J ¼ 8:5 Hz, 2H, Ph-3-H, Ph-5-H),
7.01 (d, J ¼ 8:5 Hz, 2H, Ph-2-H, Ph-6-H), 6.79 (br, 1H,
Nþ–H), 5.30 (s, 2H, CH2–O), 4.44 (d, J ¼ 5:6 Hz, 2H,
CH2–N), 3.58 (d, J ¼ 12:3 Hz, 2H, Pip-2-Heq, Pip-6-
Heq), 3.01–3.10 (m, 2H, Pip-2-Hax, Pip-6-Hax), 2.36 (s,
3H, CH3), 2.07 (d, J ¼ 12:8 Hz, 2H, Pip-3-Heq, Pip-5-
Heq), 1.98 (d, J ¼ 13:6 Hz, 1H, Pip-4-Heq), 1.55–1.70 (m,
3H, Pip-3-Hax, Pip-4-Hax, Pip-5-Hax); MS (70 eV), m=z
(%) 295 (Mþꢀ, 34).
4.2.14. 2-(4-Chlorophenoxymethyl)benzyl bromide (18a).
ortho-a,a0-Dibromoxylene (5.28 g, 20 mmol) and 4-
chlorophenol (0.65 g, 5 mmol) were dissolved in 150 mL
of ethanol and K2CO3 (1.38 g, 10 mmol) was added. The
mixture was heated for 12 h under reflux and filtered.
The filtrate was concentrated in vacuo and the residue
was purified by flash column chromatography (eluent:
dichloromethane/petrol ether 2:8) to give a colourless oil
(32%). 1H NMR (CDCl3): d 7.39–7.44 (m, 2H, Xyl-3-H,
Xyl-5-H), 7.32–7.35 (m, 2H, Xyl-4-H, Xyl-6-H), 7.25 (d,
4.2.10. 1-(3-(4-Methylphenoxymethyl)benzyl)piperidine
(15). Synthesized, as described for 4 from 15a (0.51 g,
2.5 mmol) and p-cresol (0.27 g, 2.5 mmol) (34%). 1H
NMR (CF3COOD): d 7.64 (d, J ¼ 7:8 Hz, 1H, Xyl-5-
H), 7.56–7.59 (m, 2H, Xyl-4-H, Xyl-6-H), 7.44 (d,
J ¼ 7:5 Hz, 1H, Xyl-2-H), 7.17 (d, J ¼ 8:4 Hz, 2H, Ph-3-
H, Ph-5-H), 6.96 (d, J ¼ 8:4 Hz, 2H, Ph-2-H, Ph-6-H),
6.75 (br, 1H, Nþ–H), 5.31 (s, 2H, CH2–O), 4.36 (d,
J ¼ 5:3 Hz, 2H, CH2–N), 3.61 (d, J ¼ 12:0 Hz, 2H, Pip-

T. Mikꢀo et al. / Bioorg. Med. Chem. 12 (2004) 2727–2736
2735
J ¼ 8:9 Hz, 2H, Ph-3-H, Ph-5-H), 6.93 (d, J ¼ 8:9 Hz,
2H, Ph-2-H, Ph-6-H), 5.17 (s, 2H, CH2–O), 4.59 (s, 2H,
CH2–Br); MS (70 eV), m=z (%) 310 (Mþꢀ, 5). Anal.
(C14H12BrClO). (Mr 311.48).
according to Garbarg et al.27 Brain histaminergic neu-
ronal activity was assessed by measuring the main
metabolite of histamine, Ns-methylhistamine. Mice were
fasted for 24 h before per oral treatment. Animals were
decapitated 90 min after treatment, and the cerebral
cortex was isolated. The cerebral cortex was homoge-
nized in 10 vol of ice-cold perchloric acid (0.4 M). The
Ns-methylhistamine level was measured by radioimmu-
noassay.41 By treatment with 3 mg/kg ciproxifan the
maximal increase in Ns-methylhistamine level was ob-
tained and related to the level reached with the admin-
istered drug. Each experiment was performed at least in
triplicate. The ED50 value was calculated as mean value
with SEM.39
4.2.15. 1-(2-(4-Chlorophenoxymethyl)benzyl)piperidine
(18). Compound 18a (0.5 g, 1.6 mmol), piperidine
(0.26 mL, 3 mmol) and KOH (0.2 g, 3 mmol) were dis-
solved in 20 mL of ethanol and heated for 12 h under
reflux. The mixture was filtered and concentrated in
vacuo. The residue was purified by flash column chro-
matography (eluent: dichloromethane/petrol ether 3:7).
The resulting white solid was crystallized as a salt of
oxalic acid from ethanol (96%). 1H NMR (CF3COOD):
d 7.60–7.68 (m, 3H, Xyl-3-H, Xyl-4-H, Xyl-5-H), 7.54
(d, J ¼ 7:3 Hz, 1H, Xyl-6-H), 7.41 (d, J ¼ 8:9 Hz, 2H,
Ph-3-H, Ph-5-H), 7.28 (br, 1H, Nþ–H), 7.07 (d,
J ¼ 8:9 Hz, 2H, Ph-2-H, Ph-6-H), 5.32 (s, 2H, CH2–O),
4.47 (d, J ¼ 5:6 Hz, 2H, CH2–N), 3.61 (d, J ¼ 12:1 Hz,
2H, Pip-2-Heq, Pip-6-Heq), 3.05–3.14 (m, 2H, Pip-2-Hax,
Pip-6-Hax), 2.10 (d, J ¼ 15:0 Hz, 2H, Pip-3-Heq, Pip-5-
Heq), 1.99 (d, J ¼ 13:7 Hz, 1H, Pip-4-Heq), 1.61–1.71 (m,
3H, Pip-3-Hax, Pip-4-Hax, Pip-5-Hax); MS (70 eV), m=z
(%) 315 (Mþꢀ, 15).
4.3.3. Histamine H3 receptor antagonist potency on
guinea-pig ileum.28 Histamine H3 receptor antagonist
potency was determined by concentration-dependent
inhibition of (R)-a-methylhistamine induced relaxation
of field-stimulated isolated guinea-pig ileum segments
(longitudinal muscle with adhering plexus myentericus)
in the presence of the antagonist under study according
to Ligneau et al.26 Each experiment was performed at
least in triplicate. Logarithmic data are presented as
mean with SEM. Longitudinal muscle strips were pre-
pared from the small intestine, 20–50 cm proximal to the
ileocaecal valve. The muscle strips were mounted be-
tween two platinum electrodes (4 mm apart) in 20 mL of
Krebs buffer, containing 1 lM mepyramine, connected
to an isometric transducer, continuously gassed with
oxygen containing 5% CO2 at 37 °C. After equilibration
of the muscle segments for 1 h accompanied by washing
every 10 min, they were stimulated continuously with
rectangular pulses of 15 V and 0.5 ms at a frequence of
0.1 Hz. After 30 min of stimulation, a cumulative con-
centration-response curve to (R)-a-methylhistamine was
recorded. Subsequently the preparations were washed
three times every 10 min without stimulation. The
antagonist was incubated 20–30 min before redetermi-
nation of the concentration-response curve of (R)-a-
methylhistamine. The new antagonists were tested at
concentrations that did not block ileal muscarinic M3
receptors (data not shown), unless otherwise stated
(please see the results and discussion section for details).
4.3. Pharmacology
4.3.1. [125I]Iodoproxyfan binding assay on stably trans-
fected CHO-K1 cells. Potency of the novel compounds
7–18 was investigated in a radioligand binding assay
described by Ligneau et al.20 In brief, transfected CHO-
K1 cells were washed and harvested with a PBS med-
ium. They were centrifuged (140g, 10 min, þ4 °C) and
then homogenized with a Polytron in the ice-cold
binding buffer (Na2HPO4/KH2PO4, c ¼ 50 mmol/L,
pH ¼ 7.5). The homogenate was centrifuged (23,000g,
30 min, þ4 °C) and the pellet obtained resuspended in
the binding buffer to constitute the membrane prepara-
tion used for the binding assays. Aliquots of the mem-
brane suspension (5–15 lg protein) were incubated for
60 min at 25 °C with [125I]iodoproxyfan (c ¼ 25 pmol/L)
alone, or together with competing drugs dissolved in the
same buffer to give a final volume of 200 lL. Incubations
were performed in triplicate and stopped by four addi-
tions (5 mL) of ice-cold medium, followed by rapid fil-
tration through glass microfibre filters (GF/B Whatman,
Clifton, NJ) presoaked in polyethylene imine (x ¼
0:3%). Radioactivity trapped on the filters was measured
with a LKB (Rockville, MD) gamma counter (efficiency:
82%). Specific binding was defined as that inhibited by
imetit (c ¼ 1 lmol/L), a specific H3 receptor agonist.26
IC50 values were determined using an iterative least
squares method,39 and assuming a competitive antago-
nism Ki values were calculated from their IC50 values by
using the Cheng–Prusoff equation.40 Data are presented
as the mean of experiments performed at least in triplicate.
4.3.4. Muscarinic M3 receptor assay on guinea-pig ileum.
The procedure was performed as described in the liter-
ature.42
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