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G. R. Naumiec et al. / Bioorg. Med. Chem. Lett. 25 (2015) 225–228
Table 1
X
Inhibitions of [3H]TCP binding (%) at the PCP site of the NMDA receptor, binding
X
affinities (Ki), and clogD values for 1 and 2
a
R
O
Me
N
Y
H2N
Y
NH
ArNH2
N
H
N
Me
d
b
e
ArNCN
Me
ArNHCN
Ligand
R
Inhibitiona (%)
[3H]TCP Ki (nM)b
clogDc
X
X
X
1 (CNS1261)
I
106
31.9
39.8
2.76
2.49
2
SMe
93.1
c
HCl
H2N
HCl
HN
Me
Method 2
a
b
c
f
f
Inhibition at 1.0 lM concentration (mean from two experiments).
Y
N
Me
O
Mean from two experiments.
Calculated using ACD Labs program.
Y
Y
Method 1
Subsequently, the methylthio group was retained while changes
X
were made to the N-(1-naphthyl) group of 2. First, the 1-naphthyl
group was changed to a 2-naphthyl group, as in 3, but this change
was highly detrimental to binding affinity (Table 2).
NH
Ar
N
H
N
Me
Y
We next explored whether other bicyclic aryl groups, namely an
isoquinolinyl or a quinolinyl group, would be advantageous over
the 1-naphthyl group in 2. Five compounds that were formally
isosteric with 2 were prepared in which the position of the benzo-
pyridinyl nitrogen atom was varied (4–8) (Table 2). Compound 4
showed weaker inhibition than 2. Compounds 5–8, and especially
5 and 6, showed high inhibition of [3H]TCP binding at micromolar
concentrations. The Ki values of 5 and 6 were however found to be
much higher than that of 2 (Table 2).
1-43
Scheme 1. Syntheses of N-aryl-N0-(3-substituted)-N0-methylphenyl-guanidines.
Reagents and conditions: (a) 1.5 equiv trimethylorthoformate, cat. H2SO4, 120 °C,
2 h; (b) 170 °C, 30 min; (c) 1. 10% aq HCl, reflux, 3 h; 2. Et2O/HCl; (d) 0.67 equiv
CNBr, Et2O, reflux, 24 h; (e) 1. 1.5 equiv NaH, THF, reflux, 3 h; 2. 1.5 equiv MeI, rt,
overnight; (f) toluene, 130 °C, 21 h.
an N-methyl-N-aryl cyanamide was heated to 130 °C with the req-
uisite arylamine hydrochloride salt for 21 h in toluene. In method 2
an N-methyl arylamine was used instead of an N-methyl-N-aryl
cyanamide in order to reduce steric bulk at the cyanamide. This
change led to increased yield, as demonstrated by the synthesis of
20, where Methods 1 and 2 gave 14% and 35% isolated yields,
respectively. When the N0-methylguanidines could not be made
by Method 1, we used Method 2 (see Supplementary data).
The requisite N-aryl cyanamides were synthesized by refluxing
cyanogen bromide (5 M in acetonitrile) with the appropriate aryl-
amine in diethyl ether. The purified N-aryl cyanamides were used
either directly or converted into their respective N-methyl-N-aryl
cyanamides by treatment with sodium hydride and then iodo-
methane in tetrahydrofuran.21
We noted that the computed lipophilicities of compounds 3ꢀ8
(Table 2) were lower than those of 1 and 2 (Table 1). Therefore, we
attempted to compensate for this lower lipophilicity by adding a
halogen to the heterobicyclic rings, as in compounds 9–11. Inser-
tion of chlorine in ortho position to the isoquinolinyl nitrogen of
6, as in compound 9, retained some affinity, but insertion of halo-
gen (Cl or Br) into the benzenoid ring, as in 10 and 11, was highly
detrimental to affinity (Table 2).
Among compounds 1ꢀ8 no relationship between affinity and
lipophilicity emerged. Therefore, we next explored replacing the
naphthyl substituent in 2 with monocyclic aryl substituents. The
compound 12 having a 2,4-dibromophenyl substituent, intended
to mimic the bulkiness of the naphthyl group in 2, displayed
69.7% inhibition of [3H]TCP binding at micromolar concentration
(Table 2), but clearly had lower affinity than 2. When the naphthyl
substituent was replaced with a 4-bromopyridin-3-yl substituent,
as in 13, binding affinity was even lower (23.7% inhibition; Table 2).
Therefore no improvement over the 1-naphthyl ring of 2 was found.
Our attention returned to varying substitution on the N0-phenyl
ring of 1. Following on from the methylthio group replacement
described earlier, we explored the effect of introducing various
other substituents at the 3-position.20 Commercially available 3-
substituted anilines were used to prepare compounds 14–33
(Table 3). From the assay data, it is apparent that many other sub-
stitutions are well tolerated at the 3-position. At micromolar con-
centration, over 60% of the monosubstituted compounds displayed
appreciable (>50%) inhibition of [3H]TCP binding at the PCP site of
the NMDA receptor (15–17, 19–26, and 28–29). Lower affinity was
observed in compounds with bulky aromatic (30–33) or small alco-
holic substituents (18, 27). Also, small and highly electronegative
substituents were not well tolerated as shown in the halo deriva-
tives (14–16 and 1); % inhibition/affinity increased across this ser-
ies with halogen size.
Ligands were screened for affinity at the PCP site of the NMDA
receptor in rat brain membrane suspensions with an assay using
[3H]TCP as radioligand (Caliper Life Sciences). Initially, ligands
were tested at 1.0 lM concentration. Usually, if the inhibition of
[3H]TCP binding exceeded 80%, a Ki value was determined, also
with [3H]TCP as radioligand. In this assay, 1 inhibited [3H]TCP
binding fully at 1.0 lM concentration with a Ki of 31.9 nM (Table 1).
This value is appreciably higher than that reported when using
[3H]MK801 as radioligand (4.65 nM).14,15
Initially, we focused on preparing non-iodinated derivatives of 1
where the metabolically labile 3-iodo substituent9,15 was replaced
with another. We noted that a few such compounds have been
reported previously, where the substituent at 3 position is Me,
Et, NO2, NH2, or azido.20 However, only the 3-ethyl analog had
shown a quite low IC50 value (36 nM) against [3H]MK801 binding
to NMDA receptors. We first considered methylthio as an alterna-
tive to an iodo substituent because of its similar size and polariz-
ability. The direct methylthio analog (2) was synthesized and
was found to inhibit 93.1% of [3H]TCP binding at micromolar
concentration and to exhibit a Ki value of 39.8 nM, which is very
similar to that of 1 (31.9 nM) in the same assay (Table 1). Thus, a
methylthio substituent was well tolerated in place of iodine.
From the Ki data, the preferred substituent at the 3-position is a
CF3 group (20, 18.3 nM), which gave a nearly two-fold increase in
binding affinity over 1. The Ki for the compound with an N(Me)2