S. G. Hammer et al. / Bioorg. Med. Chem. Lett. 26 (2016) 292–300
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were identified. Additionally, the binding mode of the parent
diaminopyrimidine 1 and two selected derivatives were studied
in more detail by means of docking and molecular dynamic (MD)
simulations at hH1R and hH4R.
The reference compound 1 was prepared from 1-tetralone in
four steps according to procedures described by Cowart.31 The
other ligands which were synthesized and evaluated within this
study are derived from three major scaffolds (Schemes 1–3).
The starting material 4 required for the synthesis of the first
group of ligands 6a–e and 7a–c (Scheme 1) was readily obtained
from 1-tetralone and dimethylcarbonate via deprotonation with
sodium hydride.43 Methyl 1-tetralone-2-carboxylate (4) was then
condensed with urea at a high temperature and the resulting
pyrimidine-2,4-dione was converted to its corresponding
dichloropyrimidine derivative 5 using phosphorus oxychloride.43
At 40 °C in dimethylformamide, nucleophilic substitution of 5 with
the first amine (HNR1R2) selectively occurred at the 4-position of
the pyrimidine ring as evinced by NOESY experiments (see Sup-
porting information). The second amino group was introduced in
reactions using the amine HNR3R4 as solvent at 90 °C. According
to this strategy, eight ligands 6a–e and 7a–c were prepared.
The ligands of the second group (Scheme 2) were synthesized
from 2,4-dichloro-6-phenylpyrimidine 9, which was itself obtained
Figure 1. Structures and affinities of some selected H1R and/or H4R ligands:
diaminopyrimidine derivative, 1;31 quinazoline derivative, 2;39 loxapine derivative,
3.40
example, phenylhistamine, histaprodifens and phenoprodifens,
showed selectivity towards the hH1R, except three N-methylated
phenylhistamine derivatives, which showed about 10-fold
selectivity towards hH4R.38 But due to their partial agonism at both
receptor subtypes, those compounds are not relevant in the ther-
apy of H1R/H4R-related diseases.10,11,38 The analysed H1R antago-
nists showed only weak affinity to hH4R.12,32 Some compounds
with high affinity for hH1R and hH4R, but with selectivity either
to hH1R or to hH4R, are described in literature, for example the
quinazoline 2 and loxapine derivative 3 (Fig. 1).39–42
The diaminopyrimidine 1 is described as a highly affine hH4R
antagonist.31 Previous studies, based on astemizole derived com-
pounds, suggest that the flexible aromatic side chain at the aro-
matic core is necessary for affinity to the hH1R.14 Furthermore,
the experimental data suggest that the increase of the spacer
length between the aromatic core and the aromatic moiety of the
side chain from one CH2 moiety to two CH2 moieties leads to a
decrease in affinity to hH1R, but to an increase in affinity to
hH4R, which corresponds to a decrease in selectivity hH1R/hH4R
from ꢀ23,000 to ꢀ65 (compds 9 and 13 in Ref. 14).14 Therefore,
the aim of the present study was to introduce different aromatic
side chains at the amine moiety of the diaminopyrimidine 1. Fur-
thermore, the position of the N-methylpiperazine and the aromatic
side chain were exchanged and the rigidity of the tricyclic core was
removed (Fig. 2).
through
a Suzuki coupling between phenylboronic acid and
trichloropyrimidine 8.44 As in the nucleophilic substitution of 5,
the attack of the first amine HNR1R2 exclusively occurred at the
4-position of the dichloropyrimidine 9, which was again verified
by NOESY experiments (see Supporting information). A reaction
with the amine HNR3R4 at 90 °C provided the final compounds
10a–c and 11a–c. The ligands 12a and 12b bearing a dimethy-
lamino group in 2-position were accessible from 9 through a single
reaction step at 90 °C. In this particular double substitution the
attack of the amine HNR1R2 occurred at the 4-position and the
dimethylamino group, which was most likely transferred from
the solvent dimethylformamide, was subsequently introduced into
the 2-position of the pyrimidine core.
The regioisomeric dichloropyrimidine 14 was used as central
precursor for the ligands of the third group (Scheme 3). Under
conditions identical to those shown in Scheme 1, methyl 2-tetra-
lone-1-carboxylate 13 (this compound was obtained from 2-tetra-
lone and dimethylcarbonate following the procedure reported by
Harris)43 was first condensed with urea and the resulting pyrim-
idine-dione was then reacted with phosphorous oxychloride to give
dichloropyrimidine 14.43 In contrast to the nucleophilic substitu-
tions described above (Schemes 1 and 2), the attack of the first
amine HNR1R2 now occurred at the 2-position of the pyrimidine
moiety (see Supporting information for NOESY experiments). In
the final step, N-methylpiperazine was attached in 4-position
through a reaction at 90 °C to yield 15a–c. Unexpectedly, but possi-
bly related to the stronger nucleophilicity of N-methylpiperazine
compared to the primary aliphatic amines,45 the double substitu-
tion of 14 with N-methylpiperazine to yield 15d was already
observed at 40 °C in dimethylformamide.
Twenty different diaminopyrimidine derived compounds were
synthesized and characterized pharmacologically at hH1R and
hH4R by radioligand competition binding assays. Within the
present study, two new compounds in the one-digit
lM affinity
range at both receptors with no significant subtype selectivity
In the present study, several compounds derived from the
diaminopyrimidine 1 were pharmacologically characterized at
hH1R and hH4R by radioligand competition binding assays with
Sf9 insect cell membranes, expressing either hH1R and RGS4 or
hH4R, Gai2 and Gb1c2 as described previously (see Supporting
information).10,38 The resulting pharmacological data are given in
Table 1.
The diaminopyrimidine derivative
1 has a weak affinity
(pKi = 5.04) to the hH1R, whereas the affinity to hH4R is about 49
fold higher (Table 1). Within the present study, we determined a
pKi value of 6.73 (Table 1) for compound 1 at hH4R, in contrast
to a pKi of 7.81 which is described in literature.31 Similar discrep-
ancies in affinity between literature data and the data determined
Figure 2. Strategy for the design of dual histamine H1/H4-receptor ligands, using
the diaminopyrimidine 1 as lead structure.