Cyclopropane-based stereochemical diversity-oriented conformational restriction strategy: Histamine H3 and/or H4 receptor ligands with the 2,3-methanobutane backbone

Article abstract of DOI:10.1039/c1ob06496g

The stereochemical diversity-oriented conformational restriction strategy can be an efficient method for developing specific ligands for drug target proteins. To develop potent histamine H₃ and/or H₄ receptor ligands, a series of con

Full text of DOI:10.1039/c1ob06496g

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PAPER
Cyclopropane-based stereochemical diversity-oriented conformational
restriction strategy: Histamine H3 and/or H4 receptor ligands with the
2,3-methanobutane backbone
Mizuki Watanabe,a Takaaki Kobayashi,a Takatsugu Hirokawa,b Akira Yoshida,c Yoshihiko Ito,c
Shizuo Yamada,c Naoki Orimoto,d Yasundo Yamasaki,d Mitsuhiro Arisawaa and Satoshi Shuto*a
Received 1st September 2011, Accepted 11th October 2011
DOI: 10.1039/c1ob06496g
The stereochemical diversity-oriented conformational restriction strategy can be an efficient method for
developing specific ligands for drug target proteins. To develop potent histamine H3 and/or H4 receptor
ligands, a series of conformationally restricted analogs of histamine with a chiral trans- or
cis-4-amino-2,3-methano-1-(1H-imidazol-4-yl)butane structure was designed based on this strategy.
These stereochemically diverse compounds were synthesized from previously developed versatile chiral
cyclopropane units. Among these analogs, a trans-cyclopropane-type compound, (2S,3R)-4-
(4-chlorobenzylamino)-2,3-methano-1-(1H-imidazol-4-yl)butane (5b), has remarkable antagonistic
activity to both the H3 (Ki = 4.4 nM) and H4 (Ki = 5.5 nM) receptors, and a cis-cyclopropane-type
compound, (2R,3R)-4-amino-2,3-methano-1-(1H-imidazol-4-yl)butane (6a), is a potent and selective
H3 receptor partial agonist (Ki = 5.4 nM). Although (2S,3R)-4-amino-2,3-methano-1-(1H-
imidazol-4-yl)butane (5a) does not have a hydrophobic group which the usual H3 receptor antagonists
have, it was found to be a potent H3 receptor antagonist (Ki = 20.1 nM). Thus, a variety of compounds
with different pharmacological properties depending on the cyclopropane backbones and also on the
side-chain functional groups were identified. In addition to the previously used 1,2-methanobutane
backbone, the 2,3-methanobutane backbone also worked effectively as a cyclopropane-based
conformational restriction structure. Therefore, the combination of these two cyclopropane backbones
increases the stereochemical and three-dimensional diversity of compounds in this strategy, which can
provide a variety of useful compounds with different pharmacological properties.
On the other hand, the histamine H4 receptor, also one of
the GPCRs, is expressed in immunocytes, such as eosinophils or
Introduction
The histamine H3 receptor, a member of the Gi protein-coupled
mast cells, and chemotaxis of these cells via histamine is triggered
through H4 receptor activation.3a Accordingly, H4 receptor antag-
onists may be effectively used in new therapeutic modalities for
the treatment of allergic diseases.3b,c
receptors (GPCRs) distributed mainly in the central nervous
system, is of interest as a potential drug target.1 Agonists and
antagonists to the H3 receptor are considered to be potential drugs
for the treatment of sleep disorders, migraines, asthma, inflamma-
tion, or ulcers,2a and for the treatment of Alzheimer’s disease,
attention-deficit/hyperactivity disorder (ADHD), schizophrenia,
depression, dementia, or epilepsy,2b respectively.
Although GPCRs, including the H3 and H4 receptors, are
important targets for drug development,4 structural analysis of
GPCRs is difficult due to the membranous nature of these
proteins and to their very low natural abundance, compared
with that of proteins soluble in blood or cytosol.5 Therefore,
structural data on the drug target GPCRs are generally poor,
and a method for effectively identifying compounds that tar-
get GPCRs without any structural data is required in drug
development. Thus, we previously reported a stereochemical
diversity-oriented conformational restriction strategy to develop
compounds that bind selectively to structure-unknown target
proteins such as GPCRs.6,7 To realize the strategy, we devised ver-
satile chiral cyclopropane units with different stereochemistries,6a,c
shown in Fig. 1, and, by using these units, a series of
cyclopropane-based conformationally restricted analogs8 with
aFaculty of Pharmaceutical Sciences, Hokkaido University, Kita-ku, Sap-
poro, 060-0812, Japan. E-mail: shu@pharm.hokudai.ac.jp
bComputational Biology Research Center, National Institute of Advanced
Industrial Science and Technology, Aomi, Koutou-ku, Tokyo, 135-0064,
Japan
cDepartment of Pharmacokinetics and Pharmacodynamics and Global
Center of Excellence (COE), School of Pharmaceutical Sciences, University
of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka, 422-8526, Japan
dHanno Research Center, Taiho Pharmaceutical Co. Lrd., Misugidai, Hanno,
357-8527, Japan
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Fig. 1 Conformationally restricted analogs of histamine having the 1,2-methanoalkylimidazole backbone prepared from chiral cyclopropane units.
stereochemical diversity can be designed and synthesized
effectively.
Based on the strategy, we actually designed a series of confor-
mationally restricted analogs of histamine with different stereo-
chemistries, which were synthesized from the chiral cyclopropane
units (Fig. 1).6 In these conformationally restricted analogs having
an aminoalkyl-1,2-methanoimidazole backbone, the imidazole
and the amino side-chain moieties are located in a variety of
spatial arrangements due to the conformationally restricted 1,2-
methanoalkyl backbone. Consequently, a series of these analogs
is not only stereochemically diverse but also three-dimensionally
diverse as a molecule. Some of these analogs shown in Fig. 2 were
Fig. 3 Conformationally restricted analogs of histamine having the
2,3-methanobutylimidazole backbone.
identified as potent histamine receptor ligands; e.g., AEIC (3) with
a (1S)-cis-cyclopropane structure is the first highly selective H3
receptor agonist,6b and (1R,2S)-2-[2-(4-chlorobenzylamino)ethyl]-
describe the design, synthesis, and pharmacological effects of these
1-(1H-imidazol-4-yl)cyclopropane [(R)-CEIC (4)] with a (1R)-
compounds.
trans-cyclopropane structure and its enantiomer (S)-CEIC (ent-
4) with a (1S)-trans-cyclopropane structure were highly potent
antagonists to both the H3 receptor and the H4 receptor.6c
Results and discussion
Design of compounds
Our previous studies demonstrated that conformational restric-
tion of histamine by the 4-amino-1,2-methanobutane backbone
was effective for the H3 and/or H4 receptor binding, where the
folded cis-cyclopropane structure like AEIC (3) and the extended
trans-cyclopropane structure like (R)- and (S)-CEIC (4 and ent-
4) are suitable for the binding as an agonist and an antagonist,
respectively.6b,c They also showed that functional conversion of
an agonist into an antagonist could occur by introducing a
hydrophobic group, such as a chlorobenzyl group,2b at the ter-
minal primary amino moiety of the 4-amino-1,2-methanobutane
backbone.6c Considering these results, we designed the regioiso-
meric derivatives of AEIC (3) and (R)- and (S)-CEIC (4 and
ent-4), which have a 4-amino-2,3-methanobutane backbone, as
shown in Fig. 3, for identifying new H3 and/or H4 receptor
Fig. 2 Histamine and its conformationally restricted analogs having the
1,2-methanobutylimidazole backbone.
ligands. In these compounds, the folded cis-cyclopropane or
the extended trans-cyclopropane structure on the four carbon
(butane) backbone is preserved, and accordingly, the imidazole
In the course of our studies to develop further potent
H3 and H4 receptor ligands, we newly designed a series of
conformationally restricted analogs of histamine based on the
stereochemical diversity-oriented strategy, namely 5a,b and 6a,b,
and their enantiomers, ent-5a,b and ent-6a,b, all having a 2,3-
methanobutylimidazole structure (Fig. 3). In this report, we
and the basic nitrogen moieties, which are key components in these
structures for binding to the histamine receptors, might be located
in space similarly to those of the previously identified potent
analogs having the 4-amino-1,2-methanobutane backbone. Also,
the regioisomeric 2,3-methano structure could change spatial
arrangement and flexibility around the imidazole and the basic
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Scheme 1 Conditions: a) 1) MeOCH2PPh3Cl, NaN(TMS)2, THF, 0 ◦C, 2) HCl, aq. acetone, 0 ◦C, 85% (7), 92% (10); b) 1) TosCH2NC, NaCN, EtOH,
0 ◦C, 2) sat. NH3 in EtOH, steel tube, 120 ◦C, 3) TrCl, pyridine, 48% (8), 51% (11); c) 1) TBAF, THF, 2) Dess–Martin periodinane, CH2Cl2, 78% (9), 65%
(12); d) 1) 4-chlorobenzylamine, 2-picoline borane, AcOH, MeOH, 2) TrCl, Et3N, CH2Cl2, 3) HCl, aq. EtOH, 78 ◦C, 30% (5b), 49% (6b).
nitrogen moieties, compared with those in the 1,2-methano lead
compounds AEIC (3) and (R)- and (S)-CEIC (4 and ent-4), which
would affect the biological activity. We hoped to develop both
agonists and antagonists, and therefore, compounds with a free
primary amino function (a-series, as agonists) and compounds
with a 4-chlorobenzylamino function (b-series, as antagonists)
were designed for synthesis.
Thus, we expected that the 2,3-methanobutane backbone might
be useful as an alternative conformationally restricted struc-
ture in the cyclopropane-based stereochemical diversity-oriented
strategy.
the desired trans-cyclopropane-type target compound 5b (Scheme
1A). By a similar procedure, the cis-cyclopropane-type target
compound 6b was synthesized from the cis-cyclopropane unit
2 (Scheme 1B). The enantiomers, ent-5b and ent-6b, were also
synthesized from ent-1 and ent-2, respectively.
The synthesis of 5a and 6a with a terminal primary amine
(a-series) is shown in Scheme 2. Treatment of the aldehyde 9
with t-BuS(O)NH2 and CuSO4 in CH2Cl2 gave the corresponding
sulfinylimine product, which was, without purification, reduced
with NaBH4/MeOH to afford the sulfinylamide 13. Simultaneous
removal of the trityl and sulfinyl groups by treating 13 with HCl
in EtOH produced the target trans-cyclopropane-type compound
5a (Scheme 2A). Similarly, the cis-cyclopropane-type compound
6a was prepared from the cis-cyclopropane aldehyde 12 (Scheme
2B). The corresponding enantiomers, ent-5a and ent-6a, were also
synthesized from the trans- and cis-cyclopropane aldehydes, ent-9
and ent-12, respectively.
Chemistry
Although much effort has been devoted to developing prac-
tical methods for preparing chiral cyclopropanes, synthesizing
cyclopropane derivatives with a desired stereochemistry is often
troublesome.9 We devised the chiral units (Fig. 1), which are
composed of four stereoisomeric cyclopropane derivatives bearing
two adjacent carbon substituents in a trans or cis relationship,
namely 1 and 2, and their enantiomers, ent-1 and ent-2, for
cyclopropane-based conformational restriction.6a These units,
which are generally useful for synthesizing various compounds
having an asymmetric trans- or cis-cyclopropane structure,6 were
employed as the key intermediates in this study.
The synthesis of the trans-cyclopropane compound 5b and
the cis-cyclopropane compound 6b with a 4-chlorobenzyl group
(b-series) from the chiral cyclopropane unit 1 and 2, respec-
tively, is shown in Scheme 1. These units 1 and 2 were pre-
pared from (R)-epichlorohydrin according to the method re-
ported previously.6a,c The Wittig reaction of the unit 1 with
MeOCH2PPh3Cl/NaN(TMS)2, followed by acidic treatment gave
the one carbon-elongated aldehyde 7. The imidazole ring was
constructed by treating 7 with tosylmethyl isocyanide and NaCN
followed by heating in NH3/EtOH.10 The resulting imidazole
product without purification was further treated with TrCl in pyri-
dine to give the N-tritylimidazolylmethylcyclopropane derivative
8 in 48% overall yield. After removal of the silyl-protecting group
of 8, the resulting cyclopropanemethanol was oxidized to afford
the aldehyde 9. Introduction of a 4-chlorobenzylamino function
at the terminal carbon was next investigated under reductive
amination conditions. Thus, treatment of the aldehyde 9 with 4-
chlorobenzylamine and 2-picoline borane in AcOH/MeOH, and
subsequent acidic removal of the trityl group of the product gave
Scheme 2 Conditions: a) 1) t-BuS(O)NH2, CuSO4, CH2Cl2, 2) NaBH4,
MeOH, 0 ◦C; b) HCl, EtOH, 78 ◦C, 50% from 9 (5a), 59% from 12 (6a).
Pharmacological effects
The binding affinities of the conformationally restricted analogs
with the 2,3-methanobutane backbone for the human H3 receptor
subtype using [3H]Na-methylhistamine and also for the human H4
receptor subtype using [3H]histamine were investigated, according
to the previously reported procedure.6c
The binding affinities of the compounds for the H3 receptor
are summarized in Table 1. All of the synthesized compounds
inhibited the specific binding of [3H]Na-methylhistamine to the
H3 receptor in a concentration-dependent manner. Of these
compounds, all the trans-analogs, 5a, ent-5a, 5b, and ent-5b, had
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Table 1 Effects of compounds on the human H3 and H4 receptor subtypesa
H3
H4
Selectivity
Compound
Structure
Ki (nM)
act.b (%)
inh.c (%)
Ki (nM)
act.b (%)
inh.c (%)
Ki(H3)/Ki(H4)
5a
2,3-Md/(2S)-trans
2,3-M/(2R)-trans
2,3-M/(2R)-cis
2,3-M/(2S)-cis
2,3-M/(2S)-trans
2,3-M/(2R)-trans
2,3-M/(2R)-cis
2,3-M/(2S)-cis
1,2-Md/(1S)-cis
1,2-M/(1R)-trans
1,2-M/(1S)-trans
—
20.1 5.1
9.3 0.8
5.4 1.1
172 39
4.4 0.2
21.1 5.1
110 16
103 13
1.3 0.2
8.4 1.5
3.6 0.4
2.5
17
18
5.0
0
94
72
57
41
99
99
90
86
—
100
100
99
119 25
50.9 11
113 30
222 23
5.5 0.6
23.2 3.6
172 40
33.5 2.9
—
11
30
24
47
0
5.0
3.2
15
—
0
45
40
-1.4
6.4
100
94
72
68
—
0.17
0.18
0.048
0.77
0.80
0.91
0.64
3.1
—
1.1
0.097
0.41
ent-5a
6a
ent-6a
5b
ent-5b
0
6b
1.8
6.7
100
0
0
—
ent-6b
AEIC (3)e
(R)-CEIC (4)f
(S)-CEIC (ent-4)f
Thioperamidef
7.6 0.4
37.2 2.7
124 14
>100
>100
90
0
—
51.1 3.8
a Assay was carried out with cell membranes expressing human H3 or H4 receptor subtypes (n = 3–4). b Relative potency of compound (10-5 M) to
histamine (10-5 M) for the receptor activation. c Inhibitory effect of compound (10-5 M) on the agonistic activity of histamine (10-6 M). d 1,2-M and 2,3-M
mean 1,2-methano and 2,3-methano, respectively. e Data with rat H3 receptor taken from ref. 6b. f Data taken from ref. 6c.
remarkably more potent activity (Ki < 30 nM) than the well-
known H3 receptor antagonist thioperamide (Ki = 51.1 nM). On
the other hand, the cis-analogs showed relatively weaker affinity
for the H3 receptor than the trans-analogs, except for the (2R)-cis-
analog 6a (Ki = 5.4 nM). In order of the binding affinities, these
compounds ranked as 5b, 6a > ent-5a > 5a, ent-5b > ent-6b, 6b >
ent-6a, where ent-5a, 6a, and 5b had a significant nM level Ki.
The binding affinities of the compounds for the human H4
receptor subtype are also summarized in Table 1. The trans-
analogs, 5b, ent-5b, and cis-analog ent-6b had significant activity
(Ki £ 30 nM), with 5b being the most potent (Ki = 5.5 nM).
The relative affinity of these compounds for the H3 and the
H4 receptors would indicate that a hydrophobic group might be
required for high affinity for the H4 receptor but not for the H3
receptor, as shown with the non-hydrophobic analog 6a (Ki =
5.4 nM for H3, Ki = 113 nM for H4) and the hydrophobic analog
5b (Ki = 4.4 nM for H3, Ki = 5.5 nM for H4). Our results are
consistent with the previous reports on the histamine receptor
ligands.11
full agonists. However, the activation potencies for the receptors
of these compounds relative to histamine were less than 50%, as
shown in Table 1. These results indicate that ent-5a, 6a, and ent-
6a work as partial agonists to both of the H3 and H4 receptors.
Furthermore, compound 5a with the (2S)-trans-cyclopropane
structure almost completely inhibited activation of the H3 receptor
by histamine (94% inhibition). Thus, although 5a does not have
a hydrophobic group which H3 receptor antagonists usually have,
unexpectedly, it was shown to be an H3 receptor antagonist.12
Compound 6a is the regioisomeric cis-2,3-methanobutane analog
of the parent compound AEIC (3) with the cis-1,2-methanobutane
backbone, and both 6a and 3 are selectively and highly active at
the H3 receptor (Ki = 5.4 nM and 1.3 nM, respectively). However,
these two regioisomers have functionally different effects on the
receptor, i.e., 6a was a partial agonist (18% activation), while 3 was
a full agonist (100% activation).
Docking simulation by homology modeling
The function of the compounds on human histamine H3 and
H4 receptor subtypes, which were expressed individually in 293-
EBNA cells, was next investigated by luciferase reporter gene
assay.6b The results are also summarized in Table 1.
We previously constructed a three-dimensional model of the H3
receptor6d based on a structural template from the crystal structure
of the human b2-adrenergic GPCR recently reported by Cherezov
and co-workers.5a Using the model, docking simulations of a series
of cyclopropane-based H3 receptor ligands were performed and a
reliable correlation between binding score and pKi was obtained.6d
Therefore, in order to investigate the binding modes of the
conformationally restricted analogs with the 2,3-methanobutane
backbone to the H3 receptor, docking simulations of the three
potent ligands (ent-5a, 6a, and 5b) and also the three less potent
ligands (ent-6a, 6b, and ent-6b) were carried out by using the H3
receptor homology model described above, and the binding modes
were compared with that of AEIC (3) with the 1,2-methanobutane
backbone. As shown in Fig. 4a, the proposed binding modes of
the three potent ligands are well-superimposed with that of the
potent lead compound 3, especially at the imidazole ring and
the basic nitrogen, which are important for the binding to the
receptor. On the other hand, as shown in Fig. 4b, the proposed
binding modes of the three less potent ligands and that of 3 are
not as well superimposed. These results suggest that this homology
All the b-series compounds having
a hydrophobic 4-
chlorobenzyl function were antagonists in accordance with the
previous results of the histamine receptor ligands.2b,6c The (2S)-
trans-analog 5b with the 2,3-methanobutane backbone was a
highly potent antagonist to both the H3 and H4 receptors with
Ki values of 4.4 nM (for H3) and 5.5 nM (for H4), which was
more potent than its regioisomeric parent compound (R)-CEIC
(4) with the 1,2-methanobutane backbone. While (S)-CEIC (ent-4)
with the 1,2-methanobutane backbone was a H3 receptor selective
antagonist (Ki (H3)/Ki (H4) = 0.097), its regioisomeric trans-
analog ent-5b with the 2,3-methanobutane backbone showed non-
selective moderate antagonistic effects on both of the receptors (Ki
(H3)/Ki (H4) = 0.91).
Based on the previous SAR studies on H3 and H4 receptor
ligands,2a,6b we expected that the a-series compounds having the
primary amino side-chain without a hydrophobic group would be
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human H3 and H4 receptors were shown to be different depending
on the stereochemistry of the cyclopropane backbones. Among
the hydrophobic analogs (b-series), a trans-cyclopropane structure
was preferred to a cis-cyclopropane one for both the H3 and H4
receptors. On the other hand, among the non-hydrophobic analogs
(a-series), the structure of the most potent analog 6a to both of the
receptor subtypes was a cis-cyclopropane. In this study, a couple
of potent H3 and/or H4 receptor ligands with a low nM Ki were
identified. Analog 6a, which has a (2R)-cis-2,3-methanobutane
backbone, was the highest selective H3 ligand, and analog 5b,
which has a (2S)-trans-2,3-methanobutane backbone, was the
most potent H3/H4 dual ligand. Thus, the 2,3-methanobutane
backbone worked effectively as the backbone of the conforma-
tionally restricted histamine analogs with stereochemical diversity
as well as the 1,2-methanobutane. These differences in the stereo-
chemistry of these backbones affected the potency and selectivity
of the ligands. Therefore, the stereochemical diversity-oriented
approach can be an effective strategy in medicinal chemistry
studies.
Experimental
Chemical shifts (d) are reported in ppm downfield from Me4Si
or CD2HOD (3.31 ppm) (1H) and CDCl3 (77.0 ppm) or CD3OD
(49.0 ppm) (13C). All of the 1H-NMR assignments described were
in agreement with COSY spectra. Thin-layer chromatography
was done on Merck coated plate 60F254. Silica gel, Iatron beads
and NH silica gel chromatographies were done on Merck silica
gel 5715, Iatron 6RS-8060 (Mitsubishi Kagaku Iatron, Inc),
Fig. 4 Proposed models for the three potent ligands ent-5a, 6a, 5b and the
lead compound 3 (a) and the three less potent ligands 6b, ent-6a, ent-6b and
3 (b) binding to the homology model of the H3 receptor6d from docking
simulation. Carbon atoms are shown in magenta for 3, cyan for 5b and 6b,
yellow for 6a and ent-6a, and orange for ent-5a and ent-6b, respectively.
Hydrogen bonding and salt bridge between side-chain carboxyl group of
Glu206 and an imidazole of the ligands, and between that of Asp114 and
an amino group of the ligands are depicted by red dots.
R
and Chromatorexꢀ (Fuji Silysia Chemical Ltd.), respectively.
Reactions were carried out under an argon atmosphere except
for hydrous reactions.
(2S,3R)-4-tert-Butyldiphenylsilyloxy-2,3-methanobutyraldehyde
(7)
model can be useful for investigation the binding modes of the H3
receptor ligands and that the bioactive conformations of these
potent ligands are analogous.
To a suspension of MeOCH2PPh3Cl (3.63 g, 10.6 mmol) in THF
(50 mL) was added NaN(TMS)2 (1.0 M in THF, 9.12 mL,
9.12 mmol) at 0 ◦C, and the mixture was stirred at the same
temperature for 20 min. To the resulting solution was added a
solution of 16a (2.27 g, 7.60 mmol) in THF (10 mL) at 0 ◦C, and
the reaction mixture was stirred at the same temperature for 5
h. After addition of saturated aqueous NH4Cl, the solvent was
evaporated, and the residue was partitioned between AcOEt and
saturated aqueous NH4Cl. The organic layer was washed with
brine, dried (Na2SO4) and evaporated. The residue was purified
by silica gel column chromatography (3% AcOEt in hexane) to
give the enol ether product as an oil. To a solution of the product
in acetone (40 mL) was added aqueous HCl (12 M, 20 mL), and
the mixture was vigorously stirred at 0 ◦C for 10 s. Immediately, the
mixture was poured into saturated aqueous NaHCO3 (300 mL),
and the resulting solution was extracted with AcOEt. The organic
layer was washed with saturated aqueous NaHCO3, brine, dried
(Na2SO4) and evaporated. The residue was purified by silica gel
column chromatography (5% AcOEt in hexane) to give 7 (2.27 g,
85%) as a colorless oil: [a]2D2 -12.0 (c 0.95, CHCl3); 1H-NMR (400
MHz, CDCl3) d 0.35 (1 H, m, cyclopropyl-CH2), 0.52 (1 H, m,
cyclopropyl-CH2), 0.84 (1 H, m, cyclopropyl-CH), 0.91 (1 H, m,
cyclopropyl-CH), 1.01 (9 H, s, tBu), 2.27 (2 H, m, CH2CHO), 3.49
As described above, the stereochemical diversity-oriented
conformational restriction strategy, employing the 2,3-
methanobutane backbone, was shown to be useful for
developing potent ligands of the H3 and/or H4 receptor in
this study. It is important to note that, in addition to the
previously used 1,2-methanobutane backbone, the regioisomeric
2,3-methanobutane backbone also worked effectively as an
alternative cyclopropane-based conformational restriction
structure. Thus, the combinational use of these two backbones
not only increases the stereochemical diversity but also increases
three-dimensional structural diversity of the compounds in this
strategy, which can provide a variety of active compounds with
different pharmacological properties.
Conclusions
We designed a series of conformationally restricted histamine
analogs with a chiral trans- or cis-2,3-methanobutane backbone
based on the stereochemical diversity-oriented strategy. These four
stereochemical types of analogs were systematically synthesized
from optically active epichlorohydrins via the versatile chiral cyclo-
propane units. Pharmacological properties of these analogs for the
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(1 H, dd, J = 6.4, 10.7 Hz, CH2OTBDPS), 3.69 (1 H, dd, J = 5.7,
10.7 Hz, CH2OTBDPS), 7.35–7.44 (6 H, m, aromatic), 7.51–7.72
(4 H, m, aromatic), 9.75 (1 H, dd, J = 2.1, 2.3 Hz, CHO); 13C-
NMR (100 MHz, CDCl3) d 9.5, 10.2, 19.4, 20.5, 27.0, 47.6, 66.6,
127.5, 129.5, 133.7, 135.4, 201.9; LRMS (FAB) m/z 353 [(M+H)+];
HRMS (FAB) calcd for C22H29O2Si 353.1937, found 353.1928
[(M+H)+]; Found: C, 75.13; H, 8.05. Calc. for C22H28O2Si: C, 74.95;
H, 8.01%.
the residue in vacuo, a solution of the residue and trityl chloride
(954 mg, 3.43 mmol) in pyridine (10 mL) was stirred at room
temperature for 12 h. After the solvent was evaporated, the residue
was partitioned between AcOEt and aqueous HCl (1 M). The
organic layer was washed with saturated aqueous NaHCO3, brine,
dried (Na2SO4), and evaporated. The residue was purified by silica
gel column chromatography (20–50% AcOEt in hexane) to give 8
1
(1.03 g, 48%) as a colorless oil: [a]2D2 -18.7 (c 1.10, CHCl3); H-
NMR (400 MHz, CDCl3) d 0.31–0.39 (2 H, m, cyclopropyl-CH2),
0.81–0.90 (2 H, m, cyclopropyl-CH ¥2), 1.02 (9 H, s, tBu), 2.41
(1 H, dd, J = 6.8, 15.9 Hz, CH2-imidazole), 2.59 (1 H, dd, J =
5.9, 15.9 Hz, CH2-imidazole), 3.40 (1 H, dd, J = 6.3, 10.9 Hz,
CH2OTBDPS), 3.60 (1 H, dd, J = 5.4, 10.9 Hz, CH2OTBDPS),
6.52 (1 H, s, imidazolyl) 7.11–7.13 (6 H, m, aromatic), 7.25–7.39 (16
H, m, aromatic & imidazolyl), 7.64–7.65 (4 H, m, aromatic); 13C-
NMR (100 MHz, CDCl3) d 9.86, 15.8, 19.2, 20.4, 26.8, 32.2, 67.0,
75.0, 117.8, 127.5, 127.9, 129.4, 129.7, 134.0, 135.6, 138.2, 141.4,
142.5; LRMS (FAB) m/z 633 [(M+H)+]; HRMS (FAB) calcd
for C43H45N2OSi 633.3301; found 633.3299 [(M+H)+]; Found: C,
81.55; H, 7.05; N, 4.37. Calc. for C43H44N2OSi: C, 81.60; H, 7.01;
N, 4.43%.
(2R,3S)-4-tert-Butyldiphenylsilyloxy-2,3-methanobutyraldehyde
(ent-7)
Compound ent-7 (2.41 g, 83%, colorless oil) was prepared from
ent-16a (2.80 g, 8.27 mmol) as described for the preparation of 7:
[a]2D2 +11.5 (c 0.98, CHCl3); LRMS (FAB) m/z 353 [(M+H)+];
HRMS (FAB) calcd for C22H29O2Si 353.1937, found 353.1925
[(M+H)+]; Found: C, 75.02; H, 7.98. Calc. for C22H28O2Si: C, 74.95;
1
H, 8.01%. H- and 13C-NMR spectra were consistent with those
of 7.
(2R,3R)-4-tert-Butyldiphenylsilyloxy-2,3-methanobutyraldehyde
(10)
(2R,3S)-4-tert-Butyldiphenylsilyloxy-2,3-methano-1-(1-
triphenylmethyl-1H-imidazol-4-yl)butane (ent-8)
Compound 10 (2.27 g, 92%, colorless oil) was prepared from 26c
(2.37 g, 7.00 mmol) as described for the preparation of 7: [a]D24
-0.8 (c 1.15, CHCl3); H-NMR (400 MHz, CDCl3) d 0.05 (1 H,
1
Compound ent-8 (755 mg, 43%, colorless oil) was prepared from
ent-7 (970 mg, 2.75 mmol) as described for the preparation
of 8: [a]2D2 +18.3 (c 1.35, CHCl3); LRMS (FAB) m/z 633
[(M+H)+]; HRMS (FAB) calcd for C43H45N2OSi 633.3301; found
633.3300 [(M+H)+]; Found: C, 81.38; H, 6.91; N, 4.60. Calc. for
dd, J = 5.4, 10.6 Hz, cyclopropyl-CH2), 0.77 (1 H, m, cyclopropyl-
CH2), 1.04 (9 H, s, tBu), 1.08–1.26 (2 H, m, cyclopropyl-CH ¥2),
2.32 (1 H, m, CH2CHO), 2.51 (1 H, m, CH2CHO), 3.43 (1 H, dd,
J = 8.8, 11.3 Hz, CH2OTBDPS), 3.89 (1 H, dd, J = 5.4, 11.3 Hz,
CH2OTBDPS), 7.35–7.43 (6 H, m, aromatic), 7.64–7.69 (4 H, m,
aromatic), 9.83 (1 H, t, J = 1.8 Hz, CHO); 13C-NMR (100 MHz,
CDCl3) d 8.79, 9.30, 17.1, 19.2, 26.9, 42.9, 63.8, 127.6, 129.6, 133.6,
135.4, 135.5, 202.3; LRMS (FAB) m/z 353 [(M+H)+]; HRMS
(FAB) calcd for C22H29O2Si 353.1937, found 353.1938 [(M+H)+];
Found: C, 74.90; H, 8.01. Calc. for C22H28O2Si: C, 74.95; H,
8.01%.
1
C43H44N2OSi: C, 81.60; H, 7.01; N, 4.43%. H- and 13C-NMR
spectra were consistent with those of 8.
(2R,3R)-4-tert-Butyldiphenylsilyloxy-2,3-methano-1-(1-
triphenylmethyl-1H-imidazol-4-yl)butane (11)
Compound 11 (744 mg, 51%, colorless oil) was prepared from 10
(821 mg, 2.33 mmol) as described for the preparation of 8: [a]D23
1
+4.9 (c 1.01, CHCl3); H-NMR (400 MHz, CDCl3) d 0.04 (1 H,
(2S,3S)-4-tert-Butyldiphenylsilyloxy-2,3-methanobutyraldehyde
(ent-10)
m, cyclopropyl-CH2), 0.66 (1 H, m, cyclopropyl-CH2), 1.03 (9 H,
s, tBu), 1.07–1.18 (2 H, m, cyclopropyl-CH ¥2), 2.39 (1 H, dd,
J = 7.9, 15.8 Hz, CH2-imidazole), 2.75 (1 H, dd, J = 5.4, 15.8
Hz, CH2-imidazole), 3.68 (2 H, d, J = 6.7 Hz, CH2OTBDPS),
6.51 (1 H, s, imidazolyl) 7.11–7.13 (6 H, m, aromatic), 7.26–7.39
(16 H, m, aromatic & imidazolyl), 7.64–7.68 (4 H, m, aromatic);
13C-NMR (100 MHz, CDCl3) d 10.0, 15.6, 18.2, 19.5, 27.2, 28.0,
64.5, 75.3, 118.0, 127.8, 127.9, 128.2, 128.2, 129.7, 130.1, 134.4,
134.4, 135.9, 135.9, 138.6, 142.3, 142.9; LRMS (FAB) m/z 633
[(M+H)+]; HRMS (FAB) calcd for C43H45N2OSi 633.3301; found
633.3300 [(M+H)+]. Found: C, 81.84; H, 6.81; N, 4.21. Calc. for
C43H44N2OSi: C, 81.60; H, 7.01; N, 4.43%.
Compound ent-10 (2.15 g, 80%, colorless oil) was prepared from
ent-26c (2.62 g, 7.68 mmol) as described for the preparation of 7:
[a]2D4 +0.14 (c 1.02, CHCl3); LRMS (FAB) m/z 353 [(M+H)+];
HRMS (FAB) calcd for C22H29O2Si 353.1937, found 353.1940
[(M+H)+]; Found: C, 75.07; H, 8.11. Calc. for C22H28O2Si: C, 74.95;
1
H, 8.01%. H- and 13C-NMR spectra were consistent with those
of 10.
(2S,3R)-4-tert-Butyldiphenylsilyloxy-2,3-methano-1-(1-
triphenylmethyl-1H-imidazol-4-yl)butane (8)
To a suspension of tosylmethyl isocyanide (667 mg, 3.43 mmol)
and 7 (1.21 g, 3.43 mmol) in absolute EtOH (8 mL) was added
sodium cyanide (17 mg, 0.34 mmol) at 0 ◦C, and the resulting
mixture was stirred at the same temperature for 30 min. After
the solvent was evaporated, the residue in a saturated solution of
ammonia in absolute EtOH (60 mL) was heated at 120 ◦C in a
steel tube for 24 h. After cooling, the solvent was evaporated, and
the residue was co-evaporated with pyridine (¥3). After drying
(2S,3S)-4-tert-Butyldiphenylsilyloxy-2,3-methano-1-(1-
triphenylmethyl-1H-imidazol-4-yl)butane (ent-11)
Compound ent-11 (922 mg, 58%, a colorless oil) was prepared
from ent-10 (890 mg, 2.52 mmol) as described for the prepa-
ration of 8: [a]2D3 -4.6 (c 0.96, CHCl3); LRMS (FAB) m/z 633
[(M+H)+]; HRMS (FAB) calcd for C43H45N2OSi 633.3301; found
633.3310 [(M+H)+]; Found: C, 81.77; H, 6.89; N, 4.18. Calc. for
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1
C43H44N2OSi: C, 81.60; H, 7.01; N, 4.43%. H- and 13C-NMR
(2S,3S)-4-Formyl-2,3-methano-1-(1-triphenylmethyl-1H-
spectra were consistent with those of 11.
imidazol-4-yl)butane (ent-12)
Compound ent-12 (290 mg, 65%, a white solid) was prepared from
ent-11 (720 mg, 1.14 mmol) as described for the preparation of 9:
[a]2D2 +25.0 (c 1.05, CHCl3); LRMS (EI) m/z 392 (M+); HRMS
(EI) calcd for C27H24N2O 392.1889; found 392.1888 (M+); Found:
C, 82.78; H, 5.97; N, 6.90. Calc. for C27H24N2O: C, 82.62; H, 6.16;
(2S,3R)-4-Formyl-2,3-methano-1-(1-triphenylmethyl-1H-
imidazol-4-yl)butane (9)
A mixture of 8 (633 mg, 1.00 mmol) and TBAF (1.0 M THF,
2.0 mL, 2.0 mmol) in THF (6 mL) was stirred at room temperature
for 12 h. After the solvent was evaporated, the residue was
purified by silica gel column chromatography (50% AcOEt in
hexane then 3% MeOH in CHCl3) to give an alcohol product.
To a solution of the alcohol in CH2Cl2 (10 mL) was added
Dess–Martin periodinane (509 mg, 1.20 mmol), and the resulting
mixture was stirred at room temperature for 2 h. After addition of
saturated aqueous Na2S2O3/NaHCO3 (1 : 3), the resulting mixture
was stirred vigorously for 10 min. The mixture was extracted with
AcOEt, and the organic layer was washed with saturated aqueous
NaHCO3, brine, dried (Na2SO4), and evaporated. The residue
was purified by silica gel column chromatography (33% AcOEt
in hexane) to give 9 (305 mg, 78%) as a light brown amorphous
1
N, 7.14%. H- and 13C-NMR spectra were consistent with those
of 12.
(2S,3R)-trans-4-(4-Chlorobenzylamino)-2,3-methano-1-(1H-
imidazol-4-yl)butane (5b)
A mixture of 9 (48 mg, 0.12 mmol), 4-chlorobenzylamine (98%,
16 mL, 0.13 mmol) and 2-picoline borane (13 mg, 0.13 mmol) in
MeOH/AcOH (10 : 1, 1.1 mL) was stirred at room temperature for
8 h. After the addition of aqueous HCl (1 M, 1 mL), the mixture
was stirred at 0 ◦C for 10 min and then the solvent was evaporated.
The residue was partitioned between Et2O and aqueous NaOH (2
M), and the organic layer was washed with H2O, brine, dried
(Na2SO4), and evaporated. The residue was purified by neutral
silica gel column chromatography (10–20% MeOH in CHCl3) to
give the crude amine product. A solution of the amine, trityl
chloride (56 mg, 0.20 mmol) and Et3N (28 mL, 0.20 mmol) in
CH2Cl2 (1 mL) was stirred at room temperature for 12 h. After
addition of MeOH (1 mL), the solvent was evaporated. The residue
was partitioned between Et2O and aqueous HCl (0.5 M), and the
organic layer was washed with saturated aqueous NaHCO3 and
brine, dried (Na2SO4), and evaporated. The residue was purified
by neutral silica gel column chromatography (17–33% AcOEt in
hexane) to give the amine as an amorphous solid. A solution of
the amine in EtOH (2.0 mL)/aqueous HCl (4 M, 1.0 mL) was
stirred at 78 ◦C for 2 h, and then the solvent was evaporated. The
residue was partitioned between aqueous HCl (1 M) and CH2Cl2,
and the aqueous layer was neutralized with aqueous NaOH (2
M). The resulting solution was extracted with Et2O (¥3), and the
organic layer was washed with H2O and brine, dried (Na2SO4),
and evaporated. The residue was purified by Iatron beads column
chromatography (0–100% MeOH in CHCl3) to give 5b (10 mg,
1
solid: [a]2D4 -26.0 (c 1.00, CHCl3); H-NMR (400 MHz, CDCl3)
d 1.04 (1 H, m, cyclopropyl-CH2), 1.32 (1 H, m, cyclopropyl-
CH2), 1.68–1.83 (2 H, m, cyclopropyl-CH ¥2), 2.66 (2 H, d, J =
6.3 Hz, CH2-imidazole), 6.57 (1 H, s, imidazolyl) 7.12–7.15 (6
H, m, aromatic), 7.32–7.36 (10 H, m, aromatic & imidazolyl),
9.01 (1 H, d, J = 5.3 Hz, CHO); 13C-NMR (100 MHz, CDCl3) d
12.8, 24.2, 27.2, 27.7, 75.1, 117.9, 127.9, 129.6, 138.4, 140.3, 142.5,
201.9; LRMS (EI) m/z 392 (M+); HRMS (EI) calcd for C27H24N2O
392.1889; found 392.1880 (M+); Found: C, 82.77; H, 6.39; N, 7.18.
Calc. for C27H24N2O: C, 82.62; H, 6.16; N, 7.14%.
(2R,3S)-4-Formyl-2,3-methano-1-(1-triphenylmethyl-1H-
imidazol-4-yl)butane (ent-9)
Compound ent-9 (305 mg, 78%, white amorphous solid) was
prepared from ent-8 (633 mg, 1.00 mmol) as described for the
preparation of 9: [a]2D4 +25.2 (c 1.00, CHCl3); LRMS (EI) m/z 392
(M+); HRMS (EI) calcd for C27H24N2O 392.1889; found 392.1890
(M+); Found: C, 82.83; H, 6.23; N, 7.42. Calc. for C27H24N2O:
C, 82.62; H, 6.16; N, 7.14%. 1H- and 13C-NMR spectra were
consistent with those of 9.
1
30%, colorless amorphous solid) as a free amine: H-NMR (500
MHz, CDCl3) d 0.47–0.54 (2 H, m, cyclopropyl-CH2), 0.79–0.86
(2 H, m, cyclopropyl-CH ¥2), 2.10–2.17 (2 H, m, CH2-imidazole),
3.02–3.09 (2 H, m, CH2NH), 3.75 (1 H, d, J = 13.2 Hz, benzyl-
CH2), 3.84 (1 H, d, J = 13.2 Hz, benzyl-CH2), 6.77 (1 H, s,
imidazolyl), 7.25–7.26 (2 H, m, aromatic), 7.30–7.32 (2 H, m,
aromatic) 7.36 (1 H, s, imidazolyl); 13C-NMR (125 MHz, CDCl3)
d 10.2, 17.2, 19.2, 29.3, 53.0, 53.4, 120.9, 128.7, 128.7, 129.6, 129.6,
133.1, 133.1, 134.1, 137.5; LRMS (EI) m/z 275 (M+); HRMS (EI)
calcd for C15H18ClN3 275.1189, found 275.1190 (M+); Found: C,
65.03; H, 6.63; N, 15.49. Calc. for C15H18ClN3: C, 65.33; H, 6.58;
N, 15.24%; The free amine 5b was dissolved in aqueous HCl (4 M),
and the solvent was evaporated. The residue was triturated with
Et2O to give 5b dihydrochloride (12 mg) as a white amorphous
solid: [a]2D2 -25.2 (c 1.01, MeOH); 1H-NMR (400 MHz, CD3OD)
d 0.76 (2 H, m, cyclopropyl-CH2), 1.18 (1 H, m, cyclopropyl-CH),
1.24 (1 H, m, cyclopropyl-CH), 2.58 (1 H, dd, J = 7.7, 14.5 Hz,
CH2-imidazole), 2.91–2.96 (2 H, m, CH2NH), 3.14 (1 H, dd, J =
6.8, 14.5 Hz, CH2-imidazole), 4.22 (2 H, s, benzyl-CH2), 7.42 (1 H,
(2R,3R)-4-Formyl-2,3-methano-1-(1-triphenylmethyl-1H-
imidazol-4-yl)butane (12)
Compound 12 (259 mg, 65%, white amorphous solid) was
prepared from 11 (637 mg, 1.01 mmol) as described for the
preparation of 9: [a]2D3 -24.5 (c 1.00, CHCl3); 1H-NMR (400 MHz,
CDCl3) d 1.25–1.34 (2 H, m, cyclopropyl-CH2), 1.83–1.97 (2 H,
m, cyclopropyl-CH ¥2), 2.67 (1 H, dd, J = 8.2, 15.5 Hz, CH2-
imidazole), 2.95 (1 H, dd, J = 6.3, 15.5 Hz, CH2-imidazole), 6.53
(1 H, s, imidazolyl) 7.10–7.15 (6 H, m, aromatic), 7.31–7.34 (9 H,
m, aromatic), 7.36 (1 H, s, imidazolyl), 9.38 (1 H, d, J = 5.0 Hz,
CHO); 13C-NMR (100 MHz, CDCl3) d 15.0, 24.2, 27.2, 27.7, 75.1,
117.9, 127.8, 129.6, 138.4, 140.3, 142.2, 200.9; LRMS (EI) m/z 392
(M+); HRMS (EI) calcd for C27H24N2O 392.1889; found 392.1890
(M+); Found: C, 82.91; H, 6.00; N, 6.95. Calc. for C27H24N2O: C,
82.62; H, 6.16; N, 7.14%.
742 | Org. Biomol. Chem., 2012, 10, 736–745
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s, imidazolyl), 7.47 (2 H, d, J = 8.2 Hz, aromatic), 7.55 (2 H, d, J =
8.2 Hz, aromatic), 8.83 (1 H, s, imidazolyl); LRMS (EI) m/z 275
[(M-2HCl)+]; HRMS (EI) calcd for C15H18ClN3 275.1189, found
275.1189 [(M-2HCl)+]; Found: C, 51.52; H, 5.86; N, 11.78. Calc.
for C15H20Cl3N3: C, 51.67; H, 5.78; N, 12.05%.
(2S,3S)-cis-4-(4-Chlorobenzylamino)-2,3-methano-1-(1H-
imidazol-4-yl)butane (ent-6b)
Compound ent-6b (20 mg, 43%, white amorphous solid) was
prepared from ent-12 (69 mg, 0.17 mmol) as described for the
preparation of 5b; LRMS (EI) m/z 275 (M+); HRMS (EI) calcd
for C15H18ClN3 275.1189, found 275.1167 (M+); Found: C, 65.14;
H, 6.76; N, 15.00. Calc. for C15H18ClN3: C, 65.33; H, 6.58; N,
15.24%; 1H- and 13C-NMR spectra were consistent with those of
6b; The free amine ent-6b was dissolved in aqueous HCl (4 M), and
the solvent was then evaporated. The residue was triturated with
Et2O to give ent-6b dihydrochloride (22 mg) as a white solid: [a]D22
+10.8 (c 0.90, MeOH); LRMS (EI) m/z 275 [(M-2HCl)+]; HRMS
(EI) calcd for C15H18ClN3 275.1189, found 275.1188 [(M-2HCl)+];
Found: C, 51.55; H, 5.96; N, 12.13. Calc. for C15H20Cl3N3: C, 51.67;
H, 5.78; N, 12.05%. 1H-NMR spectrum was consistent with that
of 6b dihydrochloride.
(2R,3S)-trans-4-(4-Chlorobenzylamino)-2,3-methano-1-(1H-
imidazol-4-yl)butane (ent-5b)
Compound ent-5b (18 mg, 60%, colorless amorphous solid) was
prepared from ent-9 (45 mg, 0.12 mmol) as described for the
preparation of 5b: LRMS (EI) m/z 275 (M+); HRMS (EI) calcd
for C15H18ClN3 275.1189, found 275.1185 (M+); Found: C, 65.00;
H, 6.79; N, 15.52. Calc. for C15H18ClN3: C, 65.33; H, 6.58; N,
15.24%; 1H- and 13C-NMR spectra were consistent with those of
5b; The free amine ent-5b was dissolved in aqueous HCl (4 M),
and the solvent was then evaporated. The residue was triturated
with Et2O to give ent-5b dihydrochloride (20 mg) as a white
amorphous solid: [a]2D2 +24.6 (c 1.10, MeOH); LRMS (EI) m/z
275 [(M-2HCl)+]; HRMS (EI) calcd for C15H18ClN3 275.1189,
found 275.1191 [(M-2HCl)+]; Found: C, 51.39; H, 5.98; N, 11.81.
(2S,3R)-trans-4-Amino-2,3-methano-1-(1H-imidazol-4-yl)butane
(5a)
1
Calc. for C15H20Cl3N3: C, 51.67; H, 5.78; N, 12.05%. H-NMR
spectrum was consistent with that of 5b dihydrochloride.
A mixture of 9 (136 mg, 0.347 mmol), ( )-tert-butanesulfinamide
(59 mg, 0.49 mmol) and anhydrous CuSO4 (560 mg, 3.47 mmol)
in CH2Cl2 (3 mL) was stirred at room temperature for 24 h.
After filtration of the reaction mixture with Celite, the filtrate
was evaporated, and the residue was partitioned between CHCl3
and cold aqueous HCl (0.5 M). The organic layer was washed
with H2O and brine, dried (Na2SO4), and evaporated. A solution
of the residue and added NaBH4 (17 mg, 0.46 mmol) in MeOH
(3 mL) was stirred at 0 ◦C for 2 h. After the solvent was evaporated,
the residue was purified by silica gel column chromatography (0–
2% MeOH in CHCl3) to give 13 (120 mg, diastereomixture) as a
colorless amorphous solid. A mixture of 13 (99 mg) and an EtOH
solution of HCl (1 M, 3.0 mL) was stirred at 78 ◦C for 3 h. After the
mixture was evaporated, the residue was washed with Et2O. The
residue was purified by NH silica gel column chromatography
(0–20% MeOH in CHCl3) to give 5a (21 mg, 50% for three
steps, colorless amorphous solid) as a free amine: 1H-NMR (500
MHz, CD3OD) d 0.41 (2 H, m, cyclopropyl-CH2), 0.80 (1 H, m,
cyclopropyl-CH), 0.87 (1 H, m, cyclopropyl-CH), 2.45–2.55 (4 H,
m, CH2-imidazole & CH2NH2), 6.78 (1 H, s, imidazolyl), 7.52 (1
H, s, imidazolyl); 13C-NMR (125 MHz, CD3OD) d 11.0, 18.2, 21.0,
31.2, 46.2, 117.8, 135.6, 137.4; LRMS (EI) m/z 151 (M+); HRMS
(EI) calcd for C8H13N3 151.1110, found 151.1100 (M+); Found: C,
63.11; H, 8.89; N, 27.59. Calc. for C8H13N3: C, 63.54; H, 8.67; N,
27.79%; The free amine 5a was dissolved in aqueous HCl (4 M),
and the solvent was then evaporated. The residue was triturated
with Et2O to give 5a dihydrochloride (20 mg) as a white amorphous
solid: [a]2D2 -44.1 (c 1.10, MeOH); 1H-NMR (400 MHz, CD3OD)
d 0.72 (2 H, m, cyclopropyl-CH2), 1.12 (1 H, m, cyclopropyl-CH),
1.19 (1 H, m, cyclopropyl-CH), 2.58 (1 H, dd, J = 8.2, 15.9 Hz,
CH2-imidazole), 2.78 (1 H, dd, J = 7.7, 13.1 Hz, CH2NH2), 2.92 (1
H, dd, J = 6.3, 15.9 Hz, CH2-imidazole), 2.99 (1 H, dd, J = 7.2, 13.1
Hz, CH2NH2), 7.42 (1 H, s, imidazolyl), 8.84 (1 H, s, imidazolyl);
LRMS (EI) m/z 151 [(M-2HCl)+]; HRMS (EI) calcd for C8H13N3
151.1110, found 151.1102 [(M-2HCl)+]; Found: C, 41.50; H, 6.93;
N, 17.99. Calc. for C8H15Cl2N3·0.5H2O: C, 41.21; H, 6.92; N,
18.02%.
(2R,3R)-cis-4-(4-Chlorobenzylamino)-2,3-methano-1-(1H-
imidazol-4-yl)butane (6b)
Compound 6b (27 mg, 49%, colorless amorphous solid) was
prepared from 12 (78 mg, 0.20 mmol) as described for the
preparation of 5b: 1H-NMR (500 MHz, CDCl3) d 0.81 (1 H, dd,
J = 5.7, 10.9 Hz, cyclopropyl-CH2), 0.87 (1 H, m, cyclopropyl-
CH2), 1.02-1.10 (2 H, m, cyclopropyl-CH ¥2), 2.08 (1 H, dd, J =
4.6, 15.5 Hz, CH2-imidazole), 2.40 (1 H, t, J = 12.6 Hz, CH2NH),
3.15 (1 H, dd, J = 2.3, 15.5 Hz, CH2-imidazole), 3.34 (1 H, dd, J =
2.9, 12.6 Hz, CH2NH), 3.78 (1 H, d, J = 12.6 Hz, benzyl-CH2),
3.93 (1 H, d, J = 12.6 Hz, benzyl-CH2), 6.76 (1 H, s, imidazolyl),
7.26 (1 H, s, imidazolyl), 7.30 (2 H, d, J = 8.6 Hz, aromatic), 7.35
(2 H, d, J = 8.6 Hz, aromatic); 13C-NMR (125 MHz, CDCl3) d
8.47, 15.3, 17.1, 24.4, 48.1, 53.0, 123.3, 128.9, 128.9, 129.7, 129.7,
131.3, 133.5, 134.8, 136.9; LRMS (EI) m/z 275 (M+); HRMS
(EI) calcd for C15H18ClN3 275.1189, found 275.1187 (M+); Found:
C, 65.10; H, 6.88; N, 14.93. Calc. for C15H18ClN3: C, 65.33; H,
6.58; N, 15.24%; The free amine 6b was dissolved in aqueous
HCl (4 M), and the solvent was then evaporated. The residue was
triturated with Et2O to give 6b dihydrochloride (30 mg) as a white
solid: [a]2D2 -11.1 (c 0.96, MeOH); 1H-NMR (400 MHz, CD3OD)
d 0.56 (1 H, ddd, J = 5.4, 5.9, 11.3 Hz, cyclopropyl-CH2), 1.06 (1
H, ddd, J = 8.6, 11.3, 12.6 Hz, cyclopropyl-CH2), 1.36 (1 H, m,
cyclopropyl-CH), 1.43 (1 H, m, cyclopropyl-CH), 2.74 (1 H, dd,
J = 8.6, 16.3 Hz, CH2-imidazole), 2.97 (1 H, dd, J = 6.3, 16.3 Hz,
CH2-imidazole), 3.05 (1 H, dd, J = 3.6, 12.7 Hz, CH2NH), 3.41
(1 H, dd, J = 5.4, 12.7 Hz, CH2NH), 4.24 (1 H, d, J = 13.1 Hz,
benzyl-CH2), 4.30 (1 H, d, J = 13.1 Hz, benzyl-CH2), 7.44 (1 H,
d, J = 1.1 Hz, imidazolyl), 7.48 (2 H, d, J = 8.6 Hz, aromatic),
7.58 (2 H, d, J = 8.6 Hz, aromatic), 8.84 (1 H, d, J = 1.1 Hz,
imidazolyl); LRMS (EI) m/z 275 [(M-2HCl)+]; HRMS (EI) calcd
for C15H18ClN3 275.1189, found 275.1192 [(M-2HCl)+]; Found:
C, 51.42; H, 5.95; N, 11.88. Calc. for C15H20Cl3N3: C, 51.67; H,
5.78; N, 12.05%.
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as a white solid: [a]2D2 -2.2 (c 0.58, MeOH); LRMS (EI) m/z
151 [(M-2HCl)+]; HRMS (EI) calcd for C8H13N3 151.1110, found
151.1121 [(M-2HCl)+]; Found: C, 41.31; H, 6.95; N, 17.93. Calc.
(2R,3S)-trans-4-Amino-2,3-methano-1-(1H-imidazol-4-yl)butane
(ent-5a)
Compound ent-5a (23 mg, 61% for three steps, colorless amor-
phous solid) was prepared from ent-9 (99 mg, 0.25 mmol) as
described for the preparation of 5a: LRMS (EI) m/z 151 (M+);
HRMS (EI) calcd for C8H13N3 151.1110, found 151.1121 (M+);
Found: C, 63.20; H, 8.98; N, 27.48. Calc. for C8H13N3: C, 63.54;
1
for C8H15Cl2N3·0.5H2O: C, 41.21; H, 6.92; N, 18.02%. H-NMR
spectrum was consistent with that of 6a dihydrochloride.
Binding assay with human histamine receptors
1
H, 8.67; N, 27.79%; H- and 13C-NMR spectra were consistent
The assay was performed according to the method described
previously.6c The dihydrochloride salts of the final compounds
were used in the assay.
with those of 5a; The free amine ent-5a was dissolved in aqueous
HCl (4 M), and the solvent was then evaporated. The residue was
triturated with Et2O to give ent-5a dihydrochloride (25 mg) as a
white amorphous solid: [a]2D2 +44.9 (c 1.12, MeOH); LRMS (EI)
m/z 151 [(M-2HCl)+]; HRMS (EI) calcd for C8H13N3 151.1110,
found 151.1099 [(M-2HCl)+]; Found: C, 42.49; H, 6.82; N, 18.35.
Luciferase reporter gene assay
The assay was performed according to the method described
previously.6b Briefly, 3 ¥ 104 cells of 293-EBNA (Invitrogen)
were harvested on collagen-coated 48-well plates for 24 h. An
expression plasmid for Gaq/i, chimera Ga protein of Gaq and Gai,
was constructed and cotransfected with an H3- or H4-expression
plasmid and a pSRE-Luc. The following day, the cells were treated
with histamine (10-5 or 10-6 M) and/or each compound (10-5
M) for 5 h, and laid on ice. Intracellular luciferase activity in
aliquots from each lysate was measured using a model ML3000
luminometer (Dynatech Laboratories). The dihydrochloride salts
of the final compounds were used in the assay.
1
Calc. for C8H15Cl2N3·0.1H2O: C, 42.53; H, 6.78; N, 18.60%. H-
NMR spectrum was consistent with that of 5a dihydrochloride.
(2R,3R)-cis-4-Amino-2,3-methano-1-(1H-imidazol-4-yl)butane
(6a)
Compound 6a (18 mg, 59% for three steps, colorless amorphous
solid) was prepared from 12 (79 mg, 0.20 mmol) as described for
the preparation of 5a: 1H-NMR (500 MHz, CD3OD) d 0.18 (1 H,
dd, J = 5.2, 10.9 Hz, cyclopropyl-CH2), 0.84 (1 H, m, cyclopropyl-
CH2), 1.08–1.19 (2 H, m, cyclopropyl-CH ¥2), 2.42 (1 H, dd, J =
8.6, 15.4 Hz, CH2-imidazole), 2.75 (1 H, dd, J = 9.2, 13.5 Hz,
CH2NH2), 2.87 (1 H, dd, J = 5.2, 15.4 Hz, CH2-imidazole), 3.05
(1 H, dd, J = 5.7, 13.5 Hz, CH2NH2), 6.87 (1 H, s, imidazolyl), 7.61
(1 H, d, J = 1.1 Hz, imidazolyl); 13C-NMR (125 MHz, CD3OD)
d 10.1, 17.1, 18.3, 26.6, 41.6, 117.4, 135.8, 138.4; LRMS (EI) m/z
151 (M+); HRMS (EI) calcd for C8H13N3 151.1110, found 151.1109
(M+); Found: C, 63.11; H, 8.89; N, 27.59. Calc. for C8H13N3: C,
63.54; H, 8.67; N, 27.79%; The free amine 6a was dissolved in
aqueous HCl (4 M), and the solvent was then evaporated. The
residue was triturated with Et2O to give 6a dihydrochloride (20
Docking simulation
Using the homology modeling of the H3 receptor that was
constructed previously,6d the docking simulation was performed
according to the method described previously.6d
Acknowledgements
This investigation was supported by a Grant-in-Aids for Scientific
Research (21390028) from the Japan Society for the Promotion of
Science. We are grateful to Sanyo Fine Co., Ltd. for the gift of the
chiral epichlorohydrins.
1
mg) as a white amorphous solid: [a]2D2 +2.3 (c 0.66, MeOH); H-
NMR (500 MHz, CD3OD) d 0.45 (1 H, dd, J = 5.4, 11.3 Hz,
cyclopropyl-CH2), 1.02 (1 H, m, cyclopropyl-CH2), 1.31 (1 H, m,
cyclopropyl-CH), 1.41 (1 H, m, cyclopropyl-CH), 2.71 (1 H, dd,
J = 8.6, 16.3 Hz, CH2-imidazole), 2.88 (1 H, dd, J = 9.0, 13.1 Hz,
CH2NH2), 3.00 (1 H, dd, J = 6.3, 16.3 Hz, CH2-imidazole), 3.25
(1 H, dd, J = 5.9, 13.1 Hz, CH2NH2), 7.44 (1 H, d, J = 0.9 Hz,
imidazolyl), 8.86 (1 H, d, J = 1.4 Hz, imidazolyl); LRMS (EI) m/z
151 [(M-2HCl)+]; HRMS (EI) calcd for C8H13N3 151.1110, found
151.1097 [(M-2HCl)+]; Found: C, 42.60; H, 6.88; N, 18.65. Calc.
for C8H15Cl2N3: C, 42.87; H, 6.75; N, 18.75%.
References
1 (a) J.-M. Arrang, M. Garbarg and J.-C. Schwartz, Nature, 1983, 302,
832–837; (b) The Histamine H3 Receptor: A Target for New Drugs, ed. R.
Leurs and H. Timmerman, Elsevier, Amsterdam, 1998; (c) R. Leurs,
R. A. Bakker, H. Timmerman and I. J. P. de Esch, Nat. Rev. Drug
Discovery, 2005, 4, 107–120; (d) S. Celanire, M. Wijtmans, P. Talaga, R.
Leurs and I. J. P. de Esch, Drug Discovery Today, 2005, 10, 1613–1627;
(e) T. A. Esbenshade, G. B. Fox and M. D. Cowart, Mol. Interventions,
2006, 6, 77–88.
2 (a) M. Krause, H. Stark, and W. Schunack, Medicinal chemistry of
histamine H3 receptor agonists, pp. 175–196. In ref. 1b; (b) K. Onodera,
and T. Watanabe, Histamine H3 antagonists as potential therapeutics
in the CNS, pp. 255–268. In ref. 1b.
3 (a) P. Ling, K. Ngo, S. Nguyen, R. L. Thurmond, J. P. Edwards, L.
Karlsson and W. P. Fung-Leung, Pharmacol., 2004, 142, 161–171; (b) W.
P. Fung-Leung, R. L. Thurmond, P. Ling and L. Karlsson, Curr. Opin.
Investig. Drugs, 2004, 11, 1174–1183; (c) H. D. Lim, R. M. van Rijn,
P. Ling, R. L. Thurmond, R. A. Bakker and R. Leurs, J. Pharmacol.
Exp. Ther., 2005, 314, 1310–1321.
4 (a) T. Klabunde and G. Hessler, ChemBioChem, 2002, 3, 928–944; (b) K.
Palczewski, T. Kumasaka, T. Hori, C. A. Behnke, H. Motoshima, B. A.
Fox, I. Le Trong, D. C. Teller, T. Okada, R. E. Stenkamp, M. Yamamoto
and M. Miyano, Science, 2000, 289, 739–745; (c) V. Sarramegna, F.
Talmont, P. Demange and A. Milon, Cell. Mol. Life Sci., 2003, 60,
1529–1546; (d) S. Schlyer and R. Horuk, Drug Discovery Today, 2006,
11, 481–493; and references therein.
(2S,3S)-cis-4-Amino-2,3-methano-1-(1H-imidazol-4-yl)butane
(ent-6a)
Compound ent-6a (20 mg, 39% for three steps, colorless amor-
phous solid) was prepared from ent-12 (131 mg, 0.334 mmol) as
described for the preparation of 5a: LRMS (EI) m/z 151 (M+);
HRMS (EI) calcd for C8H13N3 151.1110, found 151.1095 (M+);
Found: C, 63.39; H, 9.02; N, 27.36. Calc. for C8H13N3: C, 63.54;
1
H, 8.67; N, 27.79%; H- and 13C-NMR spectrum was consistent
with that of 6a; The free amine ent-6a was dissolved in aqueous
HCl (4 M), and the solvent was then evaporated. The residue
was triturated with Et2O to give ent-6a dihydrochloride (22 mg)
744 | Org. Biomol. Chem., 2012, 10, 736–745
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View Article Online
5 Recently, several X-ray crystallographic analyses of GPCR have been
reported: (a) V. Cherezov, D. M. Rosenbaum, M. A. Hanson, S. G.
Rasmussen, F. S. Thian, T. S. Kobilka, H. J. Choi, P. Kuhn, W. I. Weis,
B. K. Kobilka and R. C. Stevens, Science, 2007, 318, 1258–1265; (b) D.
M. Rosenbaum, V. Cherezov, M. A. Hanson, S. G. Rasmussen, F. S.
Thian, T. S. Kobilka, H. J. Choi, X. J. Yao, W. I. Weis, R. C. Stevens and
B. K. Kobilka, Science, 2007, 318, 1266–1273; (c) S. G. Rasmussen, H.
J. Choi, D. M. Rosenbaum, T. S. Kobilka, F. S. Thian, P. C. Edwards,
M. Burghammer, V. R. Ratnala, R. Sanishvili, R. F. Fischetti, G. F.
Schertler, W. I. Weis and B. K. Kobilka, Nature, 2007, 450, 383–387.
6 (a) Y. Kazuta, A. Matsuda and S. Shuto, J. Org. Chem., 2002, 67,
1669–1677; (b) Y. Kazuta, K. Hirano, K. Natsume, S. Yamada, R.
Kimura, S. Matsumoto, K. Furuichi, A. Matsuda and S. Shuto, J. Med.
Chem., 2003, 46, 1980–1988; (c) M. Watanabe, Y. Kazuta, H. Hayashi,
S. Yamada, A. Matsuda and S. Shuto, J. Med. Chem., 2006, 49, 5787–
5796; (d) M. Watanabe, T. Hirokawa, T. Kobayashi, A. Yoshida, Y. Ito,
S. Yamada, N. Orimoto, Y. Yamasaki, M. Arisawa and S. Shuto, J.
Med. Chem., 2010, 53, 3585–3593.
2254; (d) S. F. Martin, M. P. Dwyer, B. Hartmann and K. S. Knight,
J. Org. Chem., 2000, 65, 1305–1318; (e) T. Sekiyama, S. Hatsuya, Y.
Tanaka, M. Uchiyama, N. Ono, S. Iwayama, M. Oikawa, K. Suzuki,
M. Okunishi and T. Tsuji, J. Med. Chem., 1998, 41, 1284–1298.
9 (a) H. N. C. Wong, M.-Y. Hon, C.-Y. Tse and Y.-C. Yip, Chem. Rev.,
1989, 89, 165–198; (b) V. K. Singh, A. DattaGupta and G. Sekar,
Synthesis, 1997, 137–149; (c) M. P. Doyle and M. N. Protopopova,
Tetrahedron, 1998, 54, 7919–7946; (d) J. Cossy, N. Blanchard and
C. Meyer, Synthesis, 1999, 1063–1075; (e) Small Ring Compounds in
Organic Synthesis VI. Topic in Current Chemistry 207, A. de Meijere,
Ed.; Springer: Berlin, 1999; (f) H. Lebel, J.-F. Marcoux, C. Molinaro
and A. B. Charette, Chem. Rev., 2003, 103, 977–1050; (g) P. Garcia,
D. Diez, A. B. Anton, N. M. Garrido, I. S. Marcos, P. Basabe and J.
G. Urones, Mini-Rev. Org. Chem., 2006, 3, 291–314; (h) P. Muller, Y.
ves F. Allenbach, S. Chappellet and A Ghanem, Synthesis, 2006, 10,
1689–1696.
10 D. A. Horne, K. Yakushijin and G. A. Buchi, Heterocycles, 1994, 39,
3957–3960.
7 The cyclopropane-based stereochemical diversity-oriented strategy was
also effectively used for identifying potent proteasome inhibitors: (a) K.
Yoshida, K. Yamaguchi, T. Sone, Y. Unno, A. Asai, H. Yokosawa, A.
Matsuda, M. Arisawa and S. Shuto, Org. Lett., 2008, 10, 3571–3574;
(b) K. Yoshida, K. Yamaguchi, A. Mizuno, Y. Unno, A. Asai, T. Sone,
H. Yokosawa, A. Matsuda, M. Arisawa and S. Shuto, Org. Biomol.
Chem., 2009, 7, 1868–1877.
11 (a) R. Kitbunnadaj, O. P. Zuiderveld, I. J. P. de Esch, R. C. Vollinga,
R. Bakker, M. Lutz, A. L. Spek, E. Cavoy, M.-F. Deltent, W. M. P.
B. Menge, H. Timmerman and R. Leurs, J. Med. Chem., 2003, 46,
5445–5457; (b) R. Kitbunnadaj, M. Hoffmann, S. A. Fratantoni, G.
Bongers, R. A. Bakker, K. Wieland, A. el Jilali, I. J. P. de Esch, W. M.
P. B. Menge, H. Timmerman and R. Leurs, Bioorg. Med. Chem., 2005,
13, 6309–6323.
8 For examples of the cyclopropane-based conformational restriction, see
the following: (a) P. D. Armstrong, G. J. Cannon and J. P. Long, Nature,
1968, 220, 65–66; (b) K. Shimamoto and Y. Ofune, J. Med. Chem.,
1996, 39, 407–423; (c) S. H. Stammer, Tetrahedron, 1990, 46, 2231–
12 An example of an H3 receptor antagonist having a primary amino
function without a hydrophobic group: R. C. Vollinga, W. M. P. B.
Menge, R. Leurs and H. Timmerman, J. Med. Chem., 1995, 38, 266–
271.
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The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 736–745 | 745
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