ACS Medicinal Chemistry Letters
Letter
a
EC50 of 8: 5-HT2A, 894 nM; 5-HT2B, 289 nM; 5-HT2C, 21 nM).
After enlarging our structure−function library, we were able to
combine our data sets so as to allow the design of compounds
of possibly improved selectivity and potency. As shown in
Figure 2a, the overlay of our best first- and second-generation
Scheme 1. Synthesis of Compounds 9, 10, and 11
Figure 2. (a) Modification of the C-2 and C-5 substituents is
suggested by the overlay of compounds 7 and 8. (b) Schematic
representation of postulated interactions of our ligands with the 5-
HT2C receptor.
compounds, 7 and 8, suggested that it might be rewarding to
focus on the substituents at the C-2 and C-5 positions.
Moreover, on the basis of the homology modeling studies20
reported by Jiang’s group, we assume that our ligands interact
with the 5-HT2C receptor by hydrogen bonding to the amino
group, whereas hydrophobic and/or π−π stacking interactions
predominate at the aromatic moiety (Figure 2b). We disclose
herein a new series of compounds (9−11) bearing nonpolar
groups at the C-5 position and different alkoxyl groups at the
C-2 position, some of which show good agonist activity at the
5-HT2C receptor with no agonist activity at the 5-HT2B
receptor.
The target molecules 9−11 were prepared through a concise
route employing the commercially available aldehyde 12 as the
starting material (Scheme 1). By taking advantage of the
Weinreb amide 13, compound 14 was obtained exclusively as
its trans isomer.21 This intermediate was then reduced to
alcohol 15 and condensed with phthalimide under Mitsunobu
reaction conditions. Hydrazinolysis22 and protection with
Boc2O provided the corresponding urethane 17. Standard
Suzuki coupling of 17 with arylboronic acids provided the
biaryls 18, which upon deprotection with hydrogen chloride in
diethyl ether provided the target molecules 9a−h (Scheme 1a).
Similar procedures were adopted for the synthesis of
compounds 10 (Scheme 1b). Demethylation of 19 with BBr3
and reprotection of the amine with Boc2O provided phenol 20,
which afforded in turn 11a−h by alkylation with the requisite
alkyl halide and N-deprotection with hydrogen chloride in
diethyl ether (Scheme 1c).
The functional activity of the three sets of compounds was
determined by measuring Gαq-mediated intracellular calcium
mobilization in HEK-293 cells stably expressing the human 5-
HT2A, 5-HT2B, and 5-HT2C (INI) receptors.23 In over-
expressing cell lines such as those utilized in the current
screening, it is common to observe EC50 potency concen-
trations much lower than the Ki binding constant, particularly
when antagonist radioligands are used for competition binding
studies.24 The results are summarized in Table 1, in which
serotonin (5-HT) is also included for reference purposes.
Compounds 10a and 10b were designed on the basis of
compound 8 in an effort to probe the effect of added
hydrophobic interactions at C-5. Both ligands were much less
potent at the 5-HT2C receptor than compound 8, which
indicates that the envisaged hydrophobic interactions do not
contribute significantly to binding at this site while, in contrast,
a
(a) Ph3P=CHC(O)N(OMe)Me, CH2Cl2, rt. (b) Me3S+(O)I−, NaH,
DMSO. (c) DIBAL-H, THF, −78 °C; then NaBH4, MeOH, 0 °C to rt.
(d) Phthalimide, PPh3, DEAD, THF. (e) N2H4−H2O, EtOH, reflux.
(f) Boc2O, Et3N, CH2Cl2. (g) ArB(OH)2, Pd(PPh3)4 (5 mol %),
DMF, microwave heating, 120 °C, 2 h. (h) HCl (2 M in Et2O). (i) R′I,
K2CO3, DMF. (j) BBr3, CH2Cl2, −78 °C to rt; then Boc2O, Et3N,
CH2Cl2.
the methyl group’s bulk might account for the decreased
potency. To probe the possibility of engaging a 5-substituent in
π−π stacking interactions, the aryl analogues 9a−9h were
synthesized. Unfortunately, most of these compounds also
proved to be inactive at the 5-HT2C receptor or had potencies
above 1 μM at all tested receptor subtypes. The 2-methylphenyl
substituted compound 9b showed very similar potency (3800
nM vs 3470 nM) and efficacy (64% vs 67%) at the 5-HT2C
receptor as the phenyl substituted compound 9a. Altering the
position of the methyl group from C-2′ (9b) to C-3′ (9c) and
then to C-4′ (9d) led to a progressive drop in 5-HT2C potency.
However, when the methyl group was replaced by a smaller
fluorine atom as in compounds 9e−9g, a less obvious trend was
observed. The low potency of this series of methyl- and fluoro-
substituted compounds may be a consequence of steric factors
that render them less able to fit into the binding pocket of the
5-HT2C receptor. Because of the smaller size of the fluorine
atom, its position of attachment has a smaller impact on
functional activity in comparison to a methyl group. This
hypothesis is further supported by the failure of compound 9h,
which bears a bulky butyl group at C-4′, to exhibit agonism at
any 5-HT2 subtype.
If the loss in potency of compounds 9a−9h is indeed due to
the large volume occupied by the C-5 substituent, we
hypothesized that its removal may conversely increase 5-
HT2C receptor potency. As the variable substituent, the C-2
alkoxy group was chosen. For the initially synthesized ligands
11a and 11b, their potencies and efficacies at the 5-HT2C
receptor were found to be higher than those measured at the 5-
HT2B subtype. For both compounds, efficacies decreased with
increasing bulk of R′, and this decrease was more pronounced
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dx.doi.org/10.1021/ml200206z|ACS Med. Chem. Lett. 2011, 2, 929−932