Synthesis and SAR of substituted tetrahydrocarbazole derivatives as new NPY-1 antagonists

Article abstract of DOI:10.1016/j.bmcl.2005.11.104

The SAR of a new series of tetrahydrocarbazole derivatives is described: the appropriate decoration of this template led to the identification of a new class of NPY-1 antagonists showing good in vitro potency and a promising in vivo pharmacokinetic profil

Full text of DOI:10.1016/j.bmcl.2005.11.104

Bioorganic & Medicinal Chemistry Letters 16 (2006) 1749–1752
Synthesis and SAR of substituted tetrahydrocarbazole
derivatives as new NPY-1 antagonists
Romano Di Fabio,* Riccardo Giovannini, Barbara Bertani, Manuela Borriello,
Andrea Bozzoli, Daniele Donati, Alessandro Falchi, Damiano Ghirlanda, Colin P. Leslie,
Angelo Pecunioso, Giovanna Rumboldt and Simone Spada
GlaxoSmithKline Medicines Research Centre, Via A. Fleming 4, 37135 Verona, Italy
Received 23 September 2005; revised 28 November 2005; accepted 29 November 2005
Available online 20 December 2005
Abstract—The SAR of a new series of tetrahydrocarbazole derivatives is described: the appropriate decoration of this template led
to the identification of a new class of NPY-1 antagonists showing good in vitro potency and a promising in vivo pharmacokinetic
profile in rat.
Ó 2005 Elsevier Ltd. All rights reserved.
O
5
Neuropeptide Y (NPY) is a 36-amino acid peptide iso-
lated from porcine brain in 1982 by Tatemoto et al.1
NPY belongs to a family of biologically active polypep-
tides such as peptide YY (PYY) and pancreatic polypep-
tide (PP)2 widely distributed in the central and
peripheral nervous system of many mammalian species
including humans.
2
R
4
2
N
1
O
1
R
Cl
1
Different receptor subtypes exhibiting high affinity for
NPY have been identified.3 These receptors have been
hypothesized to be involved in different physiological
processes4 such as cardiovascular regulation, food in-
take, and pain. However, their real pharmacological po-
tential remains to be fully elucidated, depending upon
the availability of suitable receptor ligands exhibiting
appropriate in vitro potency and pharmacokinetics. In
particular, as far as the NPY-1 receptor is concerned,
although several in vitro potent antagonists have been
reported in the literature, namely BIBP3226,5
SR120819A,6 PD-1601707 and LY357897,8a compounds
with an appropriate pharmacokinetic profile have not
yet been identified.
Figure 1. The tetrahydrocarbazole series explored.
rocarbazole template, as depicted in Figure 1, suitably
designed according to the proposed pharmacophore
model for the NPY-1 antagonists.8b In particular, we
managed to optimize in parallel both the in vitro activity
at the NPY-1 receptor binding site as well as the physi-
cochemical properties/pharmacokinetic profile depend-
ing upon the decoration of the tetrahydrocarbazole
scaffold by the appropriate choice of the substituents
R1 and R2. This exploration was performed in two steps:
a preliminary phase to understand the potential of the
template designed, gathering basic elements of SAR on
substituents R1 and R2 (compounds 1a–g, shown in
Table 1); an optimization phase aimed at maximizing
the affinity to the receptor and improve the drug-like
profile, fine-tuning the physicochemical properties of
the substituents R1 and R2 (compounds 1h–s, shown
in Table 2).
As part of a broad chemical strategy aimed toward the
discovery of new series of NPY-1 antagonists, we fo-
cussed our attention on the exploration of the tetrahyd-
Keywords: NPY-1; Antagonists; GPCR.
*
The synthetic route set up to prepare compounds 1a–s is
shown in Scheme 1. Oxidation of the commercially
Corresponding author. Tel.: +39 045 8218879; fax: +39 045
8218196; e-mail: rdf26781@gsk.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.11.104

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R. Di Fabio et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1749–1752
Table 1. Preliminary exploration of R1 and R2 substituents
Table 2. Optimization strategy using constrained R1 and R2
substituents
Entry
R1
R2
pIC50
4.60
Entry
R1
R2
pIC50
7.03
N
CH2
1a
CH3
N
N
N
N
1h
(CH2)3
N
N
1b
1c
1d
1e
1f
5.88
5.14
4.80
5.47
6.19
6.48
NH
CH2
(CH2)3
N
N
N
1ia
7.31
7.43
7.65
7.00
5.73
5.41
7.47
H
N
H
CH2
(CH2)3
N
OH
O
N
N
1j
N
H
N
(CH2)3
N
CH2
1ka
N
N
N
H
N
N
(CH2)3
N
CH2
F
1l
(CH2)3
N
H
N
N
H
1g
CH2
N
F
F
1m
1n
1o
N
H
N
available tetrahydrocarbazole9,10 2 at the C-2 position
with H5IO4 in THF/H2O 3:1 at 0 °C gave the corre-
sponding ketone derivative, which was reduced with
NaBH4 in MeOH to afford the alcohol intermediate 3
in 68% total yield from 2. Then, the aryl ether derivative
4 was prepared by the Mitsunobu-type reaction; in par-
ticular, after some preliminary promising attempts, a
careful analysis of the reaction conditions was per-
formed to optimize the chemical yield. To this end, the
use of polymer-bound triphenylphosphine gave the best
yield (65%, after purification by flash chromatography),
avoiding laborious work-up when the reaction was run
on multigram scale.11 The following oxidation reaction
at the C-5 position, performed with DDQ, furnished
smoothly the 6-keto derivative 5 (R1 = H), which was
efficiently transformed into compounds 6 by treatment
with NaH and suitable alkylating agents in DMF at
60 °C for 12 h. Then, the bromination reaction at the
position a to the ketone (LHMDS, THF, À70 °C, 1 h,
then Br2, 30 min) gave a 9:1 mixture of anti/syn diaster-
oisomers obtained in 70% and 8% yield, respectively,
after purification by flash chromatography. The anti
diasteroisomer 7 was then reacted with a selected num-
ber of tertiary amines. Final compounds 1a–g and 1h–
s were obtained in good yield with complete inversion
of configuration at the C-4 position.12
CH2
N
N
O
N
H
CH2
N
N
N
CH2
F
F
N
N
1p
1q
6.62
6.36
N
H
CH2
N
N
N
CH2
F
F
N
N
1r
6.73
6.77
N
H
CH2
N
N
1sb
N
The fluorinated monomers R1 (see compounds 1p, 1q, 1r
and 1s in Table 2) were prepared in four steps and good
total yield starting from the known fluoro ketone deriv-
ative 8,13 as shown in Scheme 2. The sequential addition
of the Grignard reagent to the carbonyl group, followed
by the Peterson-type elimination reaction afforded the
olefin intermediate 10 in good yield. Then, the hydrobo-
a pIC50 = 6.11 and 7.01 for the anti diastereoisomers of 1i and 1k.
b pKi = 7.00.15
ration reaction of the double bond furnished a mixture
of cis and trans isomers 11 and 12, which were isolated
in 47% and 25% yield, respectively, after separation by

R. Di Fabio et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1749–1752
1751
On the basis of the results obtained the following com-
ments can be addressed: (a) affinity at the receptor is
improved when final compounds exhibited a di-basic
character (entries 1b vs 1a and 1c in Table 1); (b) the
in vitro affinity of the sub-series of compounds shown
in Table 1 was maximized modulating both the position-
ing of the nitrogen within the R2 substituent and/or its
basic character; (c) more constrained R1 substituents en-
abled further increases of the in vitro affinity (see com-
pounds 1h, 1i and 1o in Table 2 vs 1b in Table 1); (d)
both substituted piperidines and N-substituted pipera-
zines are tolerated at the C-4 position; in particular,
the 4-methyl piperidine seems to be the R2 substituent
of choice to maximize the in vitro potency (entry 1k);
(e) the nitrogen atoms present both in the R1 and R2
substituents seem to be key pharmacophoric points, as
demonstrated by the decrease of pIC50 values observed
reducing their basicity (entry 1k vs 1l, 1m, and 1n; 1p
and 1q vs 1i and 1o, respectively, in Table 2); (f) syn dia-
stereoisomers are more potent than the corresponding
anti (see footnote entries 1i and 1k); (g) as far as the
R1 substituent is concerned both secondary and tertiary
amines are tolerated; tertiary amines are preferred for
the better pharmacokinetic profile16 (entry 1o vs 1i).
a,b
N
H
N
H
OH
2
3
c
O
d,e
N
N
H
O
1
O
R
Cl
Cl
4
5,6
f
O
O
2
Br
R
g, h
N
R
N
O
1
O
1
R
Cl
Cl
In summary, an appropriate choice of substituents R1
and R2 enabled to get in vitro potent NPY-1 antagonists
associated with improved drug-like properties with re-
spect to previous series of known NPY-1 antagonists.
According to these features the in vivo pharmacokinetic
profile of compound 1o was assessed in CD male rats.
Moderate oral bioavailability (F = 27%) was observed
when the compound was given po at 5 mg/kg and at
1 mg/kg, iv, associated to moderate/high Clp (59 ml/
min/kg) and high Vd (>20 L), probably due to the di-ba-
sic character of the molecule. The brain penetration was
evaluated 5 min after the administration of 1 mg/kg
dose, iv, giving promising results (B/P = 0.8).16
7
1a-s
Scheme 1. Reagents and conditions: (a) H5IO6, THF/H2O, 0 °C, 80%;
(b) NaBH4, MeOH, 0 °C, 85%; (c) DEAD, PPh3, p-Cl-phenol, THF,
rt, 2 h, 65%; (d) DDQ, THF/H2O, rt, 60%; (e) NaH, DMF, R1X,
60 °C, 12 h, 60–80%; (f) LHMDS, THF, À70 °C, 1 h, then Br2, 30 min,
70%; (g) amine, K2CO3, CH3CN, 70 °C, 70%; (h) HCl, Et2O, rt, 12 h.
Si(CH3)3
O
OH
F
F
F
a
b
N
N
N
Boc
Boc
Boc
10
8
9
In conclusion, a novel class of drug-like NPY-1 antago-
nists has been identified by exploration of the tetrahyd-
rocarbazole template by suitable decoration at the
position N-1, C-2 and C-4. The preliminary exploration
performed allowed to get positive results both in terms
of in vitro potency and improved developability features.
Additional studies to expand the SAR information and to
optimize pharmacokinetic properties are currently ongo-
ing and results will be reported in due course.
c
RO
RO
F
F
+
N
N
Boc
Boc
12 R = H
11 R = H
e
d
14 R = Ms
13 R = Ns
Scheme 2. Reagents and conditions: (a) (CH3)3SiCH2MgCl, THF,
0 °C, 1.5 h, 57%; (b) KH, THF, 0 °C, 1 h, 89%; (c) BH3ÆTHF, THF,
0 °C, 3 h, then NaOH, H2O2, rt, 72%; (d) NsCl, CH2Cl2, TEA, rt, 5 h,
96%; (e) MsCl, CH2Cl2, TEA, 0 °C to rt, 4 h, 90%.
Acknowledgments
We thank both Dr. Carla Marchioro and Dr. Mahmoud
Hamdan and their co-workers for analytical support. A
special thanks is due to Dr. Giovanna Dal Forno for the
in vitro affinity studies and to Dr. Franca Pugnaghi for
the in vivo PK studies.
flash chromatography. Finally, the trans isomer 12 was
transformed into the mesylate derivative 14, whereas
the cis isomer 11 was transformed into nosylate deriva-
tive 13, more reactive than the corresponding mesilate in
the following alkylation reaction. All the compounds
synthesised were characterized as racemates in terms
of their in vitro binding affinities in CHO cells express-
ing human recombinant NPY-1 receptors.14
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