Spiro[pyrrolidine-3,3′-oxindoles] as 5-HT7 receptor ligands

Article abstract of DOI:10.1016/j.bmcl.2018.06.019

Here we report the design and synthesis of spiro[pyrrolidine-3,3′-oxindole] derivatives representing a novel scaffold of 5-HT₇ receptor ligands. The synthesized analogues were validated as low nanomolar ligands showing selectivity in a panel of

Full text of DOI:10.1016/j.bmcl.2018.06.019

Bioorganic&MedicinalChemistryLettersxxx(xxxx)xxx–xxx
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Spiro[pyrrolidine-3,3′-oxindoles] as 5-HT7 receptor ligands
Ádám Andor Kelemena, Grzegorz Satałab, Andrzej J. Bojarskib, György M. Keserűa,⁎
a
Medicinal Chemistry Research Group, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar tudósok körútja 2, H3660, 23-3650, 96, 1117
Budapest, Hungary
b
Department of Medicinal Chemistry, Institute of Pharmacology, Polish Academy of Sciences, 12 Smętna Street, 31-343 Krakow, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords:
Oxindole
5-HT7R
G-protein coupled receptor
Oxidative spiro-rearrangement
Pharmacophore model
Here we report the design and synthesis of spiro[pyrrolidine-3,3′-oxindole] derivatives representing a novel
scaffold of 5-HT7 receptor ligands. The synthesized analogues were validated as low nanomolar ligands showing
selectivity in a panel of related serotonin receptor subtypes including 5-HT1AR, 5-HT2AR and 5-HT6R.
and a positive ionizable moiety located at 5–6 Å distance from HYD1
that contacts Asp3.32 (Fig. 2).
The serotonin receptor subtype-7 (5-HT7R) is expressed in both the
central nervous system (CNS) and peripheral tissues, and coupled po-
sitively to Gαs (activation raises cAMP levels)1 or Gα12 protein. The 5-
HT7R plays role in the regulation of body temperature, sleep-wake
rhythm, circadian rhythm, and mood. Thus the receptor has become
rapidly an important target for several important CNS-related indica-
tions with proved in vivo efficacy in the animal models of depression,2,3
sleep disorders,4 anxiety,5 learning and memory deficits,6 and autism
spectrum disorders.7
To date the candidate drug with the pyrazolo[3,4-d]azepine core
(JNJ-18308683 1) has reached the clinic (a phase 2 study is currently
recruiting8), and a number of selective and non-selective antagonists1
(e.g. SB-656104 2, DR-4004 3) and agonists have been proved as in-
vestigational compounds (see Fig. 1 for examples).
An early 3D pharmacophore model was built on the basis of thirty
known 5-HT7R antagonists including DR-4004 and analogues, SB-
258719 (structure not shown) and analogues, SB-269970 (structure not
shown) analogue etc.9 The 5-HT7R binding pharmacophore has been
updated10 using broad structure-activity relationship (SAR) data and
structure-based docking validated by site-directed mutagenesis experi-
ments. Pharmacophore features of 5-HT7R ligands include an aromatic
ring (in stacking interactions with Phe3.28 and Tyr7.43), two further
hydrophobic regions HYD1 and HYD2 (facing towards Phe6.52), a hy-
drogen bond acceptor (binding to Ser5.42 and/or Thr5.43) next to HYD1,
Tetrahydrobenzindoles (e.g. DR-4004 (3))11,12 and corresponding
oxindoles13,14 were validated as potent 5-HT7R scaffolds. As an attempt
to synthesize new, selective and less lipophilic compounds we designed
spiro[pyrrolidine-3,3′-oxindoles] (4) as potential 5-HT7R ligands. Aryl-
piperazine analogues attached to the common spiro[pyrrolidine-3,3′-
oxindole] core could provide ligands with more favourable ADME
characteristics by exchanging the carbocycle of the tetra-
hydrobenzindole to the more polar and less rigid pyrrolidine ring. In
order to design selective compounds against other serotonin receptors
(5-HT1AR, 5-HT2AR, 5-HT6R) we used the pharmacophore models by
López et al.9,10 and further modified by Medina et al.15 These models
suggest that decreasing the distance between PI (positive ionizable) and
HBA (H-bond acceptor) features, facilitating interactions to Ser6.55
(Ala6.55 at 5-HT1AR), introducing polar substituents at HYD1-HYD2 to
contact Arg7.36 (that is absent in 5-HT1AR) would be beneficial. Fol-
lowing these suggestions we set our objective for exploring selectivity
drivers around the spiro[pyrrolidine-3,3′-oxindoles] core.
The 5-HT7R pharmacophore suggested that we need a four-atom
linker between the spiro carbon C-3 to the pyrrolidine-nitrogen and
therefore connected this with two methylenes (Fig. 2) to the basic
moiety of compounds with General Structure 4. Thus in line with other
studies12,11,16–19 we focused on compounds with two atom spacer
(Fig. 3, compound 5, although compound 6 was also synthesized in
⁎
Corresponding author.
E-mail address: kelemen.adam@ttk.mta.hu (G.M. Keserű).
https://doi.org/10.1016/j.bmcl.2018.06.019
Received 15 April 2018; Received in revised form 1 June 2018; Accepted 11 June 2018
0960-894X/©2018PublishedbyElsevierLtd.
Pleasecitethisarticleas:Kelemen,Á.A.,Bioorganic&MedicinalChemistryLetters(2018),https://doi.org/10.1016/j.bmcl.2018.06.019

Á.A. Kelemen et al.
Bioorganic&MedicinalChemistryLettersxxx(xxxx)xxx–xxx
Scheme 1. Synthesis of the aryl-piperazine derivatives (5–10, 14). Reagents
and conditions: (A) 2-bromoethanol (or 3-bromopropanol in case of 6), K2CO3,
MeCN, RT, overnight,; (B) MsCl, TEA, DCM, reflux, 2 h; (C) tryptoline/tetra-
hydro-β-carboline, K2CO3, MeCN, reflux, overnight; (D) NBS, cc. AcOH, THF,
water, 1.5 h, 0 °C.
Fig. 1. Structures of the most prominent 5-HT7R antagonists.
order to investigate its effect on flexibility and HBA-PI distances.
Halo-scan, however, was planned to explore the substituent effects
around the phenyl ring of the aryl-piperazine moiety comparing the
profile of the unsubstituted 7 and the meta-Cl 8, ortho-Cl 9, para-Cl 5,
and 3,4-dichloro-derivatives 10. Halo-scan methodology explores the
functionalization of the ligands thus filling subpockets, and probing the
impact of introducing lipophilic, and possibly H-bonding halogen
atoms.20
In order to explore the impact of the HYD2/HYD3 features on the 5-
HT7R affinity and selectivity we replaced the canonical aryl-piperazine
by phenyl-, and phenoxy-piperidines 11, 12, 15, 5,6-dihydropyridine
13, and benzoyl-piperazine 16 moieties.
Aryl-piperazine derivatives 5–10, 14 were synthesized from the
corresponding secondary amines 17a–g. First, cyclic secondary amines
were alkylated by 2-bromoethanol (in case of 1-(4-chlorophenyl)pi-
perazine 17a either by 3-bromopropanol) followed by the mesylation of
the appropriate alcohols using mesyl chloride. A protecting group on
the basic nitrogen of the tryptoline is necessary to avoid decomposition
during the spiro-rearrangement reaction. As depicted in Scheme 1, N-
alkylation by the corresponding mesylates leading to intermediates
20a–g provided the desired protection of the tetrahydro-β-carboline
nitrogen for the final spirocyclization step to afford derivatives 5–10
and 14.21
Fig. 2. Design concept for the SAR analysis of spiro[pyrrolidine-3,3′-oxindoles]
(4).
Fig. 3. Spiro[pyrrolidine-3,3′-oxindoles] designed for SAR studies.
2

Á.A. Kelemen et al.
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Scheme 2. Synthesis of aryl-piperidine (11–12),
and 5,6-dihydropyridine (13), and phenoxy (15)
analogues. Reagents and conditions: (A) 2-bro-
moethanol, K2CO3, MeCN, RT, overnight; (B)
MsCl, TEA, DCM, reflux, 2 h; (C) tryptoline/tetra-
hydro-β-carboline, K2CO3, MeCN, reflux, over-
night; (D) NBS, cc. AcOH, THF, water, 1.5 h, 0 °C.
Scheme 3. Synthetic route leading to the amide-analogue (16). Reagents and conditions: (A) HATU, DIPEA, DMF, RT, overnight; (B) MsCl, TEA, DCM, reflux, 2 h; (C)
tryptoline/tetrahydro-β-carboline, K2CO3, MeCN, reflux, overnight; (D) NBS, cc. AcOH, THF, water, 1.5 h, 0 °C.
The synthesis of the aryl-piperidine 11–12, the 5,6-dihydro-pyridine
13, and the phenoxy 15 analogues (Scheme 2) was also started from the
corresponding secondary amines 21a–d, followed by the alkylation
with 2-bromoethanol, to afford the alcohols 22a–d. The crude mesy-
lated derivatives 23a–d were then used for alkylation of the tetrahydro-
β-carboline to get 24a–d that was spirocyclized to 11, 12, 13, 15, re-
spectively.
1-(2-Hydroxyethyl)piperazine (25) was used as starting material for
the synthesis of the benzoyl-piperazinyl analogue 16 (Scheme 3). The
acylation22 of 25 with the 4-chlorobenzoic acid 26 gave the corre-
sponding amide 27, which was treated with mesyl-chloride to yield the
mesylate 28. After the alkylation of tetrahydro-β-carboline with 28, the
spirocyclization of intermediate 29 afforded 16.
HT7R. Selectivity against 5-HT1AR decreased, however the selectivity
against the 5-HT2AR subtype has improved. 6 showed lower selectivity
(65.6-fold) towards 5-HT6R than 7 (163.3-fold).
Starting from the most active 4-fluorine derivative (14) the 5,6-di-
hydropyridine derivative (13) showed decreased selectivity (91.8-fold)
against 5-HT1AR. Replacing the piperazine ring by piperidine (12) gave
10-times lower affinity that was further confirmed by the corresponding
4-chloro analogue (11). Selectivities against 5-HT1AR and 5-HT2AR did
not change significantly, but the 5-HT6R selectivity decreased ten-
times. Similar to 11 and 12, introduction of the phenoxy-piperidine
moiety (15) improved the affinity towards 5-HT6R. Finally, the ben-
zoyl-piperazine derivative (16) showed reduced affinity to 5-HT7R,
however, it has the highest selectivity against 5-HT2AR (32.8-fold).
In summary, preliminary SAR data demonstrates that spiro[pyrro-
lidine-3,3′-oxindoles] are potent and selective 5-HT7R ligands. We
confirmed that the 2-methylene linker ensures the ideal distance be-
tween HYD1 and HYD2 and therefore it is beneficial for the 5-HT7R
affinity. Although previous findings13,23 suggested that the para sub-
stitution at the aryl-piperazine moiety may abolish 5-HT7R affinity we
showed that it provides spiro[pyrrolidine-3,3′-oxindoles] with reason-
able affinity and selectivity against 5-HT1AR and 5-HT6R. Actually, the
4-fluoro analogue (14) showed the best affinity and most remarkable
The synthesized compounds were investigated in competition
binding assays against 5-HT7R and other closely related serotonin re-
ceptor subtypes 5-HT1AR, 5-HT2AR and 5-HT6R (Table 1).
The unsubstituted phenylpiperazine derivative 7 of the spiro[pyr-
rolidine-3,3′-oxindole] core showed reasonably high affinity towards
the target, however its selectivity was moderate. Halo-scan around the
phenyl ring revealed that the 5-HT7R affinity and 5-HT1A and 5-HT6R
selectivities are increasing from ortho 9 – meta 8 – para 5 direction. In
fact, the para-Cl derivate (5) showed low nanomolar affinity for the
target and more than hundred-fold selectivity against two of the three
other serotonin receptors. Selectivity against these receptors was fur-
ther improved for the para-F analogue 14. As compared to compound 5,
the longer ([CH2]3) alkyl chain in 6 increased the distance between the
PI and HBA features, accounting for an overall loss of affinity for the 5-
selectivity against 5-HT1AR and 5-HT6R. Selectivity against 5-HT2A
R
might be improved by replacing the phenyl substituent of the piper-
azine by a benzoyl group. The present results show the potential of this
novel chemotype and validate its further optimization for more detailed
in vivo characterization in diseases models.
3

Á.A. Kelemen et al.
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Table 1
Acknowledgments
Serotonergic activity of the spirooxindole derivatives as measured in binding
assays of four serotonin receptors (Ki values are in µM). 5-HT7R affinities and
selectivities are shown in bold and italics, respectively. Assay reference com-
The authors are grateful to Ágnes Gömöry and László Drahos (MS
Proteomics Research Group, Core Technologies Centre) for exact mass
(MS) measurements.
This work was supported by the National Brain Research Program
grant (2017-1.2.1-NKP-2017-00002) and by statutory funds of the
Institute of Pharmacology, Polish Academy of Sciences in Krakow.
pounds: serotonin in 5-HT1AR and 5-HT7R assays, chlorpromazine in 5-HT2A
R
and methiothepine in 5-HT6R assay. Results were expressed as means of at least
three separate experiments (SD ≤ 24%).
ID
Structure
5-HT1A
5-HT2A
5-HT6
5-HT7
A. Supplementary data
Supplementary data (synthetic procedures, analytical data, de-
scription of the cell-based assays) associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.bmcl.2018.
06.019.
5
6
3.755
129.5
0.068
2.3
4.737
163.3
0.029
0.067
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