Synthesis and structural and pharmacological properties of cyclopropane-based conformationally restricted analogs of 4-methylhistamine as histamine H3/H4 receptor ligands

Article abstract of DOI:10.1016/j.bmc.2009.12.046

On the basis of the previous results on a histamine H₄ receptor agonist 4-methylhistamine and a cyclopropane-based conformationally restricted analog CEIC (3) with potent H₃/H₄ receptor antagonistic effect, 4-methylhistamine analogs 4 and 5 of CEIC were designed and synthesized. Compound 4 showed strong affinity (K_i = 38.7 nM) for the H₃ receptor, which was more potent than a well-known H₃ antagonist thioperamide. Stable tautomer and conformation of 3 and 4, which can affect the pharmacological activity, were analyzed by ab initio calculations.

Full text of DOI:10.1016/j.bmc.2009.12.046

Bioorganic & Medicinal Chemistry 18 (2010) 1076–1082
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Synthesis and structural and pharmacological properties of cyclopropane-based
conformationally restricted analogs of 4-methylhistamine as histamine H3/H4
receptor ligands
Takaaki Kobayashi a, Mizuki Watanabe a, Akira Yoshida b, Shizuo Yamada b, Mika Ito c, Hiroshi Abe c,
Yoshihiro Ito c, Mituhiro Arisawa a, Satoshi Shuto a,
*
a Faculty of Pharmaceutical Sciences, Hokkaido University, Kita-12, Nishi-6, Kita-ku, Sapporo 060-0812, Japan
b Department 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
c RIKEN, Advanced Science Institute, 2-1, Hirosawa, Wako 351-0198, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
On the basis of the previous results on a histamine H4 receptor agonist 4-methylhistamine and a cyclo-
propane-based conformationally restricted analog CEIC (3) with potent H3/H4 receptor antagonistic
effect, 4-methylhistamine analogs 4 and 5 of CEIC were designed and synthesized. Compound 4 showed
strong affinity (Ki = 38.7 nM) for the H3 receptor, which was more potent than a well-known H3 antago-
nist thioperamide. Stable tautomer and conformation of 3 and 4, which can affect the pharmacological
activity, were analyzed by ab initio calculations.
Received 7 December 2009
Revised 16 December 2009
Accepted 17 December 2009
Available online 23 December 2009
Keywords:
Histamine
Ó 2009 Elsevier Ltd. All rights reserved.
H3 receptor
Cyclopropane
Conformational restriction
1. Introduction
GPCRs is the structural data of the target biomolecule are lacking
or poorly documented.
Homeostatic processes related to the neurotransmitter hista-
mine (Fig. 1) are mediated by at least four receptor subtypes
termed H1, H2, H3, and H4 receptors. Recent years, much attention
has been focused on histamine H3 and H4 receptor ligands due to
their medicinal chemical interest. Antagonists to the histamine
H3 receptor are considered to be potential drugs for various dis-
eases, such as Alzheimer’s disease, attention-deficit/hyperactivity
disorder (ADHD), schizophrenia, depression, dementia, and epi-
lepsy.1 On the other hand, the histamine H4 receptor antagonists
may be effectively used in new therapeutic modalities for the
treatment of allergic diseases.2
G protein-coupled receptors (GPCRs), including the histamine
H3 and H4 receptors, are major target biomolecules for drug devel-
opment,3 and accordingly, the process of identifying therapeutic
agents targeting GPCRs has generated much interest. However,
structural analysis of GPCRs is tremendously 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.4 Therefore, a drawback in drug development targeting
Thus, a method for effectively identifying compounds targeting
GPCRs is required in drug development. We have devised a stereo-
chemical diversity-oriented conformational restriction strategy to
develop compounds that bind selectively to target proteins of un-
known structure such as GPCRs.5 In order to realize the strategy,
we developed versatile chiral cyclopropane units with different ste-
reochemistries, the structures of which are shown in Figure 2.5b,6
Using these units, we designed and synthesized a series of stereo-
chemically diverse conformationally restricted analogs of hista-
mine.5a,b,6 We hypothesized that some of these conformationally
restricted analogs might assume a conformation superimposed on
the bioactive conformations of histamine for H3 receptor binding
or H4 receptor binding, since the imidazole moiety and the amino
side-chain moiety are located in a variety of spatial arrangements
due to the conformational restriction in these analogs.
Throughout these studies, we showed that several of these
analogs were potent H3 and/or H4 receptor ligands which are, for
examples, the first highly selective H3 receptor agonist (1S,2S)-
2-(2-aminoethyl)-1-(1H-imidazol-4-yl)cyclopropane [AEIC, 1: Ki =
1.3 nM] with the (1S)-cis-cyclopropane structure5a and a potent
H3/H4 receptor antagonist (1R,2S)-2-[2-(4-chlorobenzylamino)-
ethyl]-1-(1H-imidazol-4-yl)cyclopropane [(R)-CEIC, 3: Ki = 8.4 nM
* Corresponding author. Tel.: +81 11 706 3229; fax: +81 11 706 4980.
E-mail address: shu@pharm.hokudai.ac.jp (S. Shuto).
0968-0896/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2009.12.046

T. Kobayashi et al. / Bioorg. Med. Chem. 18 (2010) 1076–1082
1077
3
1
NH2
NH2
N
N
2
2'
1
5
4
2
N
N
1'
5'
3'
N
H
N
H
3
CH3
NH2
CH3
NH
R
N
H
N
H
4'
1 (AEIC)
2
4-methylhistamine
histamine
(1S)-cis
(1S)-cis
H
N
H
H
4'
Cl
N
N
CH3
3'
2'
5'
3
N
N
N
H
1'
N
R
R
N
2
1
R
N
H
H
R =
5
4
3 (CEIC)
(1R)-trans
(1R)-trans
(1R)-trans
Figure 1. Histamine, 4-methylhistamine, and their cyclopropane-based conformationally restricted analogs.
for H3 receptor, 7.6 nM for H4 receptor] with the (1R)-trans-cyclo-
propane structure.5b In these studies, introduction of a hydrophobic
group at the side-chain amino group of an cyclopropane-based H3
and/or H4 receptor agonists changes them into an antagonist to
the receptor. For example, derivatization of AEIC (1) into its N-(4-
chlorobenzyl) derivative provided a potent H3 receptor antagonist
2 (Ki = 42 nM).5b
sition of 4-methylhistamine, of the potent H3/H4 antagonist (R)-
CEIC (3) might change it into the H4 receptor selective antagonist.
Thus, we planned to synthesize 50-methylimidazole analog 4 of (R)-
CEIC (3) and also its one carbon-reduced analog 5.
2.2. Synthesis
In the course of these studies, in this paper, we describe design,
synthesis, structural analysis, and pharmacological effects of the
cyclopropane-base conformationally restricted analogs of 4-meth-
ylhistamine, namely, 4 and 5, structure of which are shown in
Figure 1.
Although much effort has been devoted to developing practical
methods for preparing chiral cyclopropanes, for example, enantio-
selective cyclopropanations, chemical or enzymatic optical resolu-
tions, and transformations from chiral synthons, synthesis of
cyclopropane derivatives of a desired stereochemistry is often
troublesome.10 We recently developed the chiral units (Fig. 2) for
cyclopropane-based conformational restriction, which were com-
posed of four stereoisomeric cyclopropane derivatives bearing
two adjacent carbon substituents in a cis or a trans relationship,
namely 6 and 7, and their enantiomers ent-6 and ent-7.6 These
units are generally useful for synthesizing various compounds hav-
ing an asymmetric cis- or trans-cyclopropane structure, and there-
fore, in this study, the chiral unit 7 was employed as a synthon.
Synthesis of the target compound 4 and 5 from the unit 75b,8
with the (1R)-trans cyclopropane structure is summarized in
Scheme 1. The methylimidazole ring was constructed by treating
7 with 1-methyl-1-tosylmethyl isocyanide and t-BuOK in THF, fol-
lowed by heated in saturated NH3/EtOH in sealed tube at 125 °C.11
The resulting 4-methylimidazole product was further treated with
TrCl and Et3N in CH2Cl2 to give (N-trityl-4-methylimidazolyl)cyclo-
propane derivative 8, of which TBDPS group was subsequently re-
moved with TBAF/THF to give cyclopropanemethanol 9 in 44%
2. Results and discussion
2.1. Design of compounds
The H4 receptor has significant sequence homology (about 40%)
to the H3 receptor cDNA.7 Furthermore, the H3 and the H4 receptors
share about 60% sequence identity in their transmembrane re-
gions,7 which would make it difficult to develop H4 receptor selec-
tive ligands.2,8,9 In fact, (R)-CEIC (3) is highly but non-selectively
active to both the H3 and the H4 receptors.5b
In 2005, Leurs and co-workers identified 4-methylhistamine as
a H4 receptor agonist that has a >100-fold selectivity for the H4
receptor over the other histamine receptor subtypes including
the H3 receptor.8 Considering these results, we thought that H4
receptor selective antagonists might be developed: introduction
of a methyl group at the 50-position, which corresponds the 4-po-
H
N
3
[folded]
chiral cyclopropane units
R
2
1
N
NH
N
OHC
NH
OTBDPS
n
n
R
N
H
(1S)-cis
(1R)-cis
OHC
OHC
OTBDPS
OTBDPS
ent-6
6
R
H
N
[extended]
NH
OTBDPS
n
N
N
OHC
ent-7
N
H
NH
R
7
n
(1S)-trans
(1R)-trans
n = 0—3
R = H, 4-chlorobenzyl, cyclohexylmethyl
Figure 2. A series of conformationally restricted analogs of histamine with stereochemical diversity synthesized from the chiral cyclopropane units.

1078
T. Kobayashi et al. / Bioorg. Med. Chem. 18 (2010) 1076–1082
O
S
O
NC
Me
1)
H3C
Tr
N
CH3
OHC
t-BuOK, THF
TBAF, THF
N
OTBDPS
2) NH3, EtOH
125 ºC, sealed tube
3) TrCl, Et3N, CH2Cl2
7
OTBDPS
Cl
8
1) H2N
NaBH(OAc)3
Tr
N
H
N
CH3
CH3
CH2Cl2, MS4A
2) HCl, aq. EtOH
78 ºC
Tr
CH3
N
DMP, CH2Cl2
98%
N
N
N
68%
OH
9: 44% from 7
NH
CHO
10
Cl
5 (2HCl salt)
1) MeOCH2PPh3Cl
NaHMDS
1) H2N
THF, 0 ºC
77%
2) HCl aq.THF, 0ºC
Cl
NaBH(OAc)3
CH2Cl2, MS4A
2) HCl, aq. EtOH
78 ºC
H
Tr
CH3
N
CH3
N
N
Cl
N
45%
CHO
11
HN
4 (2HCl salt)
Scheme 1.
overall yield. Dess–Martin oxidation of 9 afforded aldehyde 10,
introduction of a 4-chlorobenzylamino function to which at the
10-position was next investigated under reductive-amination con-
ditions. As a result, when aldehyde 10 was treated with 4-chloro-
benzylamine in the presence of NaBH(OAc)3 and MS4A in CH2Cl2,
the desired reductive-amination product was effectively obtained.
Subsequent acidic treatment of the product finally gave the target
compound 5 in 68% yield.
Wittig reaction of aldehyde 10 with MeOCH2PPh3Cl/NaN(TMS)2
in THF, followed by acidic treatment gave the one-carbon elon-
gated aldehyde 11. As shown in Scheme 1, the other target com-
pound 4 was synthesized from aldehyde 11, according to the
procedure same to that used for the synthesis of 5 described above.
s-cis
a
b
s-trans
R
X
X = O, CR'2
C
C
R
X
I
II
s-cis
N2'
s-trans
C5'
C1'
C1'
2.3. Conformational analysis by calculations
N2'
C5'
A number of theoretical and experimental studies on the confor-
mation of cyclopropanes attached to an unsaturated bond, such as
vinylcyclopropanes, cyclopropyl ketones, or cyclopropanecarboxal-
dehydes, have been carried out.12,13 We also investigated the
conformation of cyclopropyl ketones, cyclopropanecarboxaldehy-
des, C-cyclopropylaldonitrones and explained the stereochemical
outcomes of their hydride reductions and Grignard additions based
on the bisected conformation-dependent stereoelectronic effects.14
These studies showed that unsaturated group-attached cyclopro-
panes preferentially exist in the bisected s-trans and s-cis conforma-
tions, as shown in Figure 3a, due to the effective hyperconjugation
between the unsaturated bond orbital and the strong electron-
donating orbitals of the cyclopropane ring.12–14 Accordingly, the
imidazolylcyclopropane moiety of the cyclopropane-based con-
formationally restricted histamine analogs may also be stable in
III
IV
Figure 3. Bisected s-cis and s-trans conformations of
panes (a) and imidazolylcyclopropanes (b).
a
,b-unsaturated cyclopro-
their bisected s-trans and s-cis conformations, as shown in Figure
3b. This kind of conformational features is possible to affect the
pharmacological activity of cyclopropane-based conformationally
restricted analogs. Compared with parent compound CEIC (3) and
its 50-methyl derivatives 4 and 5, their orientation around the imid-
azole and the cyclopropane may be different due to the steric effect
of the methyl group in 4 and 5. Since, in these imidazole-containing
histamine receptor ligands, three-dimensional location of the imid-
azole moiety relative to the basic nitrogen can be important for their

T. Kobayashi et al. / Bioorg. Med. Chem. 18 (2010) 1076–1082
1079
binding to the receptors, we decided to investigate the conforma-
tional stability of 3 and 4 by theoretical calculations.
Histamine is in an equilibrium between the two differently N-
protonated tautomers A and B (Fig. 4a), and relative stability of
these tautomers would affect the binding affinity for the receptors.
Thus, we first investigated relative stability of these tautomers by
cally minimum bisected s-trans conformers at 0° were about
2.5 kcal/mol more stable than the maximum energy conformers
around 90° and 270°. Thus, the hyperconjugational effect can sig-
nificantly affect the conformation of tautomer A0 to stabilize the bi-
sected s-trans form.
For the 50-methyl derivative 4, similar to 3, minimum energy
value was also observed in the bisected s-trans conformation of
the tautomer A0. It is interesting that, differently from 3, the energy
of A0 tautomer in 4 was not at a maximum in the perpendicular
conformers but rather at angle of 150° (Fig. 6c) and at 210°
(Fig. 6d), where C10@C50 bond of the imidazole eclipsed to the
C1–C3 bond or the C1–C2 bond of the cyclopropane ring. In these
eclipsed conformations, striking steric repulsion due to the 50-
methyl group would make it significantly unstable. The energeti-
cally maximum eclipsed conformation at 150° of the tautomer A0
was about 4 kcal/mol more unstable than the bisected s-trans con-
formers at 0° in 4.
These calculations showed that the conformational stability of 3
and 4 appears to be determined by the hyperconjugational stabil-
ization by the molecular orbital interaction between the imidazole
ring and the electron-donating cyclopropane ring and also by the
steric effect, particularly caused by the imidazole C50-moiety. How-
ever, the most stable conformations in 4 and the parent compound
3 is the same bisected s-trans form of the N40-protonated tautomer
A0, and therefore, conformational change by introducing the
methyl group at the imidazole 50-position is likely to be small.
*
ab initio calculations at B3LYP/6-31G with 4-methylimidazole
(Fig. 4a, R = CH3) as a model compound. As a result, it appeared that
the two tautomers are almost equally stable (A is only 0.11 kcal/
mol more stable than B).
Likely to histamine, imidazolylcyclopropane derivatives 3 and 4
can also equilibrate between two tautomers A0 and B0 (Fig. 4).
However, the N40-protonated form A0 might be more stable than
the N20-protonated form B0, since the steric repulsion between
the proton at the N20-position and cyclopropane moiety might oc-
cur in B0.
Thus, the rotational barrier energy around the C50–C10–C1–H1
dihedral angle of the both tautomers A0 and B0 in 3 and 4 was cal-
culated based on density functional theory (DFT). The dihedral an-
gle was rotated from 0° to 360° at intervals of 10°, and the single
point energies of the optimized conformers were calculated at
*
B3LYP/6-31G to obtain the energy profiles. As shown in Figure
5, in both 3 and its 50-methyl derivative 4, the N40-protonated form
A0 (blue line) was relatively more stable than the N20-protonated
form B0 (red line). It is interesting that in both 3 and 4 the tautomer
A0 is especially stable at an angle of 0°, where it just assumes the
bisected s-trans conformations (III, Fig. 3b). Since the C10–N20 bond
of A0 has more double bonded character compared with that of B
due to effective C50–C10–N20–C30 conjugation in A0, as shown in Fig-
ure 4b, hyperconjugation between the cyclopropane and the imid-
azole moieties significantly more effective in the bisected
conformation of tautomer A0 than that in B0. Therefore, the bisected
s-trans conformation of tautomer A0 is especially stable.
2.4. Pharmacological effects and discussion
Binding affinities of the target compound 4 and 5 for the human
a
H3 receptor subtype using [3H]N -methylhistamine5a,b and also for
the human H4 receptor subtype using [3H]histamine5a,b were
investigated, and were compared with those of the parent com-
pounds CEIC (3). The results are summarized in Table 1. In these
system, Ki values of the reference H3 receptor antagonist thio-
peramide were 51.1 nM for the H3 receptor and 124 nM for the
H4 receptor, respectively, and the parent compounds 3 showed
much higher but non-selective binding affinity as shown by the
Ki values of 8.4 nM for H3 receptor and 7.6 nM for H4 receptor,
respectively.
Hereafter, we mainly discuss the conformational features using
relatively stable tautomer A0 in 3 and 4, which can be important in
the binding to the receptors. For the imidazolylcyclopropane com-
pound 3, the minimum energy value was observed at the bisected
s-trans conformations described above. On the other hand, the bi-
sected s-cis conformation (180°) was not so stable, probably due to
the steric repulsion due to the 50-proton for the cyclopropane moi-
ety. The energy maxims for 3 were observed in the perpendicular-
like conformations around angles of 90° (Fig. 6a) and 270° (Fig. 6b),
where the conjugational stabilization is minimum. The energeti-
In the two newly synthesized methylimidazole-type com-
pounds, 5 did not show any significant binding to the H3 receptor
as well as to the H4 receptor (Ki >1000 nM). However, compound 4
showed remarkable binding affinity for the H3 receptor with a Ki
value of 38.7 nM, which tended to be more potent than the well-
known H3 receptor antagonist thioperamide (Ki = 51.1 nM). How-
ever, the potency was somewhat decreased compared with that
of the parent compound 3. The binding affinity of 4 for the H4
receptor was further reduced (Ki = 148 nM). Thus, compound 4
was not selective to the H4 receptor, but rather selective to the
H3 receptor.
H
R
R
a
b
N
N
N
H
N
A
B
histamine: R = CH2CH2NH2
4-methylimidazole: R = CH3
As described above, 50-methylated derivative 4 is stable in the
bisected s-trans conformation analogous to the parent compound
3, so that the two compounds might show similar pharmacological
feature, while the steric effect of the methyl group of 4 might
weaken the affinity for the H3 and H4 receptors to some extent.
Although 4-methylhistamine is a H4 receptor-selective agonist,8
the cyclopropane-based conformationally restricted analog 4 is
not, in spite of their having the same methylimidazole ring. These
results suggested that binding modes of the two compounds to the
receptors might be different, probably due to the hydrophobic 4-
chlorobenzyl moiety and/or the rigid cyclopropane moiety in 4,
which are not exist in 4-methylhistamine.
H
4'
4'
N
N
R
R
N
2'
H
N
1'
2'
1'
B'
A'
H
N
3: R = H
4: R = CH3
4'
4'
N
R
R
N
2'
H
N
2'
1'
1'
The structural analysis showed that cyclopropane-based
conformationally restricted histamine analogs significantly
stable in the N40-protonated form than the N20-protonated form.
Figure 4. Equilibrium between the two tautomes A (A0) and B (B0) in histamine and
4-methylimidazole (a) and imidazolylcycropanes (b).

1080
T. Kobayashi et al. / Bioorg. Med. Chem. 18 (2010) 1076–1082
a
b
H1
C5'
C1'
N2'
N
H
dihedral angle
Cl
(C5'—C1'—C1—H1)
0
0
0
*
Figure 5. Rotational barrier energies around the C5 –C1 –C1–H1 dihedral angle of 3 (a) and its 5 -methyl derivative 4 (b). Calculations were carried out at B3LYP/6-31G level,
where 3A0 and 4A0 are protonated at 40-N and 3B0 and 4B0 are protonated at 20-N, respectively.
protonated tautomer A0. These results suggest that histamine also
a
b
d
perpendicular
perpendicular
270º
H1
bind to these receptors in the analogous N40-protonated tautomer A.
90º
H1
3. Experimental
N2'
c
C1'
C1' N2'
C5'
C5'
1H NMR and 13C NMR spectra were obtained on JEOL JMM-ECA-
500 spectrometers with tetramethylsilane as an internal standard
and the resonance patterns are reported with notations as the fol-
lowing: br (broad), s (singlet), d (double), t (triplet) and m (multi-
plet). Mass spectra were obtained using a JEOL JMS-700TZ. Thin-
layer chromatography was done on Merck coated plate 60F254. Sil-
ica gel chromatography was done on silica gel 5715 (Merck), silica
gel 60 N (Kanto Chemical Co.) or NH silica gel (ChromatorexÒ, Fuji
Silysia Chemical). Reactions were carried out under an argon
atmosphere.
R
R
eclipsed
eclipsed
210º
N2'
150º
H1
H1
N2'
C1'
C1'
C5'
R
C5'
R
3.1. (1R,2R)-2-Hydoxymethyl-1-(5(4)-methyl-1-
triphenylmethyl-1H-imidazol-4(5)-yl)cyclopropane (9)
N
H
R =
Cl
A solution of 75b,7 (4.14 g, 12.2 mmol), 1-methyl-1-tosylmethyl
isocyanide (3.15 g, 14.9 mmol), and potassium tert-butoxide
(386 mg, 3.44 mmol) in THF (150 mL) was stirred at room tem-
perature for 2 h. After evaporation of the solvent, the residue in
ammonia-saturated absolute EtOH (200 mL) was heated at
125 °C in a stainless steal tube for 24 h. The resulting reaction
mixture was evaporated, and the residue was purified by silica
gel column chromatography (CHCl3/MeOH, 1:0–9:1) to give the
crude 4-methylimidazole derivative as a brown oil. A solution
of the residual oil, Et3N (1.78 mL 12.7 mmol) and TrCl (3.56 g,
12.7 mmol) in CH2Cl2 (65 mL) was stirred at room temperature
for 2 h. After addition of MeOH, the mixture was partitioned be-
tween AcOEt and aqueous HCl (1 M), and the organic layer was
washed with saturated aqueous NaHCO3 and brine, dried over
Na2SO4, and evaporated. The residue was purified by silica gel
column chromatography (hexane/AcOEt, 5:1) to give 1-triphenyl-
methyl-1H-4-methylimidazole derivative 8 as a pale yellow oil. A
Figure 6. Newman projection of the maximum energy conformers of 3 (a and b)
and its 5’-methyl derivative 4 (c and d).
Table 1
Binding affinities of compounds on the human H3 and H4 receptor subtypesa
Compound
H3, Ki (nM)
H4, Ki (nM)
4
38.7 4.9
>103
8.4 1.5
51.1 3.8
148 51
>103
7.6 0.4
124 14
5
3b
Thioperamideb
a
Assay was carried out with cell membranes expressing human H3, or H4
receptor subtype (n = 3).
b
Data were taken from Ref. 5b.
Accordingly, the highly potent cyclopropane based analogs such as
1, 2, 3 or 5, would bind to the H3 and H4 receptors in the stable N40-

T. Kobayashi et al. / Bioorg. Med. Chem. 18 (2010) 1076–1082
1081
solution of the oil and TBAF (1.0 M THF solution, 8.76 mL,
3.4. (1R,2S)-2-Formylmethyl-1-(5(4)-methyl-1-
8.76 mmol) in THF (58 mL) was stirred at room temperature for
24 h. The solvent was evaporated, and the residue was purified
by silica gel column chromatography (hexane/AcOEt, 1:2) to give
triphenylmethyl-1H-imidazol-4(5)-yl)cyclopropane (11)
To a suspension of (methoxymethyl)triphenylphosphonium
chloride (312 mg, 0.910 mmol) in THF (2 mL) was added sodium
hexamethyldisilazide (1.9 M THF solution, 42 mL, 0.79 mmol) at
0 °C, and the mixture was stirred at the same temperature for
9 as a white solid (2.10 g, 5.37 mmol, 44% from 7): ½a D22
ꢁ30.6 (c
ꢀ
1.01, CHCl3); 1H NMR (500 MHz, CDCl3) d 7.32–7.26 (m, 9H),
7.14–7.12 (m, 7H), 3.62–3.55 (m, 2H), 3.12 (br s, 1H), 1.65–1.59
(m, 2H), 1.45 (s, 3H), 1.05–1.01 (m, 1H), 0.78–0.75 (m, 1H); 13C
1 h. To the resulting mixture was added
a solution of 10
NMR (125 MHz, CDCl3)
d
141.9, 139.6, 136.8, 130.0, 127.9,
(155 mg, 0.395 mmol) in THF (2 mL) at 0 °C, and the mixture
was stirred at the same temperature for 4 h. After addition of sat-
urated aqueous NH4Cl, the reaction mixture was evaporated, and
the residue was partitioned between AcOEt and saturated aque-
ous NH4Cl, and the organic layer was washed with brine, dried
over Na2SO4, and evaporated. The residue was purified by silica
gel column chromatography (hexane/AcOEt, 1:1) to give the Wit-
tig reaction product (138 mg) as a colorless oil. A solution of the
product and aqueous HCl (12 M, 1 mL) in THF (14 mL) was stirred
at 0 °C for 30 min, and then saturated aqueous NaHCO3 was
added. The reaction mixture was concentrated and partitioned
between AcOEt and saturated aqueous NaHCO3, and the organic
layer was washed with brine, dried over Na2SO4, and evaporated.
The residue was purified by silica gel column chromatography
(hexane/AcOEt, 1:1) to give 11 as an amorphous pale yellow solid
127.7, 125.2, 74.7, 66.2, 22.3, 13.9, 11.6, 11.5; LRMS (EI) m/z
394 (M+); Anal. Calcd for C26H27N2Oꢂ0.3H2O: C, 81.09; H, 6.70;
N, 7.00. Found: C, 80.72; H, 6.81; N, 7.01.
3.2. (1R,2R)-2-Formyl-1-(5(4)-methyl-1-triphenylmethyl-1H-
imidazol-4(5)-yl)cyclopropane (10)
A solution of 9 (500 mg, 1.27 mmol) and Dess–Martin period-
inane (589 mg, 1.39 mmol) in CH2Cl2 (12 mL) was stirred at room
temperature for 1 h. The resulting mixture was partitioned be-
tween saturated aqueous NaHCO3/saturated aqueous Na2S2O3
(1:1) and CH2Cl2, and the organic layer was washed with brine,
dried over Na2SO4, and evaporated. The residue was purified by sil-
ica gel column chromatography (hexane/AcOEt, 1:1) to give 10 as
an amorphous white solid (485 mg, 1.24 mmol, 98%):
½
a 2D4
ꢀ
(123 mg, 0.303 mmol, 77%): ½a D20
ꢀ
ꢁ32.2 (c 1.01, CHCl3); 1H NMR
ꢁ106.9 (c 1.02, CHCl3); 1H NMR (500 MHz, CDCl3) d 9.35 (d,
J = 4.5 Hz, 1H), 7.35–7.32 (m, 9H), 7.16–7.12 (m, 7H), 2.44–2.35
(m, 2H), 1.79–1.75 (m, 1H), 1.66–1.63 (m, 1H), 1.46 (s, 3H); 13C
NMR (125 MHz, CDCl3)d 200.7, 141.6, 137.4, 136.8, 130.0, 128.0,
127.9, 126.2, 74.9, 32.2, 20.2, 15.5, 11.5; LR-MS (FAB) m/z 393
((M+H)+); HR-MS (FAB) calcd for C27H25N2O 393.1967. Found
393.1947 ((M+H)+).
(500 MHz, CDCl3) d 9.82 (t, J = 2.3 Hz, 1H), 7.33–7.30 (m, 7H),
7.15–7.13 (m, 6H), 2.52 (ddd, J = 13.6, 5.9, 2.3 Hz, 1H), 2.35
(ddd, J = 13.6, 7.2, 2.3 Hz, 1H), 1.57–1.53 (m, 2H), 1.46 (s, 3H),
1.26–1.19 (m, 1H), 0.78–0.74 (m, 1H); 13C NMR(125 MHz, CDCl3)
d 202.2, 141.7, 139.1, 136.9, 130.0, 127.9, 127.8, 125.3, 74.8, 47.9,
15.0, 13.6, 13.2, 11.5; HR-MS (EI) calcd for C28H26N2O 406.2045.
Found 406.2043 (M+).
3.3. (1R,2R)-2-(4-Chlorobenzylamino)methyl-1-(5(4)-methyl-
1H-imidazol-4(5)-yl)cyclopropane (5)
3.5. (1R,2S)-2-[2-(4-Chlorobenzylamino)ethyl]-1-(5(4)-methyl-
1H-imidazol-4(5)-yl)cyclopropane (4)
To a solution of 10 (13 mg, 34 mmol), 4-chlorobenzylamine
(21 mL, 17 mmol), and MS4A powder (13 mg) in CH2Cl2 (1.0 mL)
was added sodium triacetoxyborohydride (7.83 mg, 180 mmol),
and the resulting mixture was stirred at room temperature for
2 h. After addition of MeOH, the mixture was filtered through Cel-
ite 545, and the filtrate was evaporated. The residue was parti-
tioned between AcOEt and saturated aqueous NaHCO3, and the
organic layer was washed with brine, dried over Na2SO4, and evap-
orated. The residue was purified by NH silica gel column chroma-
tography (hexane/AcOEt, 3:1) to give the 1-triphenylmethyl-1H-
imidazole derivative 5 as a colorless oil. A solution of the oil in
aqueous HCl (1.5 M, 0.60 mL) and EtOH (0.3 mL) was heated under
reflux for 6 h, and the reaction mixture was evaporated. The resi-
due was partitioned between CH2Cl2 and aqueous HCl (1 M). After
addition of aqueous NaOH (1 M) to the aqueous layer, the resulting
basic solution was extracted with CH2Cl2, and the organic layer
was washed with brine, dried over Na2SO4, and evaporated. The
residue was purified by NH silica gel column chromatography
(CHCl3/MeOH, 99:1) to give a free amine 5. The free amine 5 was
dissolved in aqueous HCl (4 M), and the solution was evaporated.
The resulting residue was triturated with Et2O to give a white hyd-
Compound 4 was obtained as dihydrochloride (white hydro-
scopic amorphus solid, 16 mg, 43 mmol) from 11 (39 mg,
96 mmol) as described for the preparation of dihydrochloride of
5:
½
a 2D1
ꢀ
ꢁ52.6 (c 1.04, CH3OH, dihydrochloride); 1H NMR
(500 MHz, CD3OD) d 8.63 (s, 1H), 7.56 (d, J = 8.6 Hz, 1H) 7.47 (d,
J = 8.6 Hz, 1H) 4.24 (s, 2H), 3.25–3.22 (m, 2H), 2.33 (s, 3H), 1.98–
1.93 (m, 1H), 1.83–1.73 (m, 2H), 1.30–1.23 (m, 1H), 1.09–1.05
(m, 1H), 1.02–0.98 (m,1H); 13C NMR(125 MHz, CD3OD) d 136.7,
132.9, 132.4, 131.4, 130.5, 130.4, 127.2, 51.6, 48.1, 30.9, 18.3,
13.4, 12.4, 9.1; LR-MS (EI) m/z 289 ((M-2HCl)+); HR-MS (EI) calcd
for C16H20ClN3 289.1346. Found 289.1350 ((M-2HCl)+); Anal. Calcd
for C16H20ClN3ꢂ0.2H2O (free amine): C, 65.50; H, 7.01; N, 14.32.
Found: C, 65.66; H, 7.09; N, 14.32.
3.6. Calculations
All ab initio and DFT calculations were performed using the
GAUSSIAN 03 W. The C50–C10–C1–H1dihedral angle of the compounds
was rotated from 0° to 360° at the intervals of 10°, and the confor-
*
mations were optimized at B3LYP/6-31G . Finally, single point
energies were calculated at RB3LYP/6-31G*.
roscopic amorphous solid of
5 as dihydrochloride (7.97 mg,
22.8 mmol, 68% from 11): ½a D21
ꢀ
ꢁ41.0 (c 0.76, CH3OH); 1H NMR
3.7. Binding assay with human histamine receptors
(500 MHz, CD3OD) d 8.66 (s, 1H), 7.61–7.58 (m, 2H), 7.49–7.46
(m, 2H), 4.28 (s, 2H), 3.34–3.30 (m, 1H) 3.11 (dd, J = 13.2, 8.0 Hz,
1H), 2.36 (s, 3H), 2.16–2.11 (m, 1H), 1.70–1.63 (m, 1H), 1.27–1.20
(m, 2H); 13C NMR (125 MHz, CD3OD)d 136.7, 132.9, 132.7, 131.3,
130.3, 129.3, 127.7, 51.8, 51.4, 17.1, 13.1, 12.5, 9.2; HR-MS (EI)
calcd for C15H18ClN3 275.1189. Found 275.1194 ((M-2HCl)+). Anal.
Calcd for C15H20Cl3N3ꢂ0.55H2O: C, 50.24; H, 5.93; N, 11.72. Found:
C, 50.63; H, 6.12; N, 11.33.
The assay was performed according to the method described
previously.5b
Acknowledgment
This investigation was supported by a Grant-in-Aids for Scien-
tific Research (21390028) from the Japan Society for the Promotion

1082
T. Kobayashi et al. / Bioorg. Med. Chem. 18 (2010) 1076–1082
Yoshida, K.; Yamaguchi, K.; Mizuno, A.; Unno, Y.; Asai, A.; Sone, T.; Yokosawa,
H.; Matsuda, A.; Arisawa, M.; Shuto, S. Org. Biomol. Chem. 2009, 7, 1868.
6. Kazuta, Y.; Matsuda, A.; Shuto, S. J. Org. Chem. 2002, 67, 1669.
of Science. We are grateful to Daiso Co., Ltd for the gift of the chiral
epichlorohydrins.
7. Hough, L. B. Mol. Pharmacol. 2001, 59, 415.
8. Lim, H. D.; van Rijn, R. M.; Ling, P.; Bakker, R. A.; Thurmond, R. L.; Leurs, R. J.
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