Binding of isotryptamines and indenes at h5-HT6 serotonin receptors

Article abstract of DOI:10.1016/j.bmcl.2005.02.070

To determine if the indolic nitrogen atom is required for the binding of N₁-benzyltryptamines at h5-HT₆ serotonin receptors, several isotryptamines and indene analogs were examined. The affinity of 3-benzyl-N₁-(N,N-dimethy

Full text of DOI:10.1016/j.bmcl.2005.02.070

Bioorganic & Medicinal Chemistry Letters 15 (2005) 1987–1991
Binding of isotryptamines and indenes at h5-HT6
serotonin receptors
Renata Kolanos,a Uma Siripurapu,a Manik Pullagurla,a Mohamed Riaz,a Vince Setola,b
Bryan L. Roth,b,c Małgorzata Dukata and Richard A. Glennona,*
aDepartment of Medicinal Chemistry, School of Pharmacy, Virginia Commonwealth University, Richmond, VA 23298-0540, USA
bDepartment of Biochemistry, School of Medicine, Case Western Reserve University, Cleveland, OH 44106-4935, USA
cDepartment of Psychiatry and Neurosciences, School of Medicine, Case Western Reserve University, Cleveland, OH 44106-4935, USA
Received 3 February 2005; revised 18 February 2005; accepted 22 February 2005
Abstract—To determine if the indolic nitrogen atom is required for the binding of N1-benzyltryptamines at h5-HT6 serotonin recep-
tors, several isotryptamines and indene analogs were examined. The affinity of 3-benzyl-N1-(N,N-dimethylaminoethyl)indole (5,
Ki = 32 nM) and 1-benzyl-3-(N,N-dimethylaminoethyl)indene (11, Ki = 3 nM) indicates that the indolic nitrogen atom is not essen-
tial for binding.
Ó 2005 Elsevier Ltd. All rights reserved.
Serotonin (5-HT) receptors are classified as belonging to
one of seven families (5-HT1–5-HT7).1,2 Human 5-HT6
receptors were identified about a decade ago3,4 and have
been implicated in depression and psychosis, based in
large part on their high affinities for many psychiatric
medications including antipsychotics and antidepres-
sants.4–6 Evidence suggests these receptors might also
be involved in convulsive disorders, cognition, appetite
control, and possibly drug abuse.5–7 Within the past
few years several 5-HT6 antagonists have been re-
ported,6–8 and among these is one developed in our lab-
oratories: MS-245 (1).9,10 MS-245 (1; Ki = 2.1 nM) binds
with high affinity at human 5-HT6 (h5-HT6) receptors; it
was also shown that a 4ꢀ-amino group is tolerated (2,
Ki = 2.0 nM), an intact tryptamine side chain is not re-
quired for binding (3, Ki = 12 nM), and that the sulfonyl
moiety could be replaced by a methylene group (4,
Ki = 6.5 nM).10–12
employed to address this issue. First, because isotrypt-
amines have been shown to mimic tryptamines at certain
5-HT receptors,13,14 an isotryptamine analog of 4 (i.e.,
5) was examined. Second, the indole nitrogen atom of
4 was replaced by an sp2-hybridized carbon atom to af-
ford indene analog 6. It was reasoned that the sp2-
hybridized carbon atom to which the ꢁbenzylꢀ substituent
would be attached, both in the isotryptamine and indene
series, might mimic the electronic character of the indole
nitrogen atom of 4. Because 3 retains affinity for 5-HT6
receptors, we also targeted abbreviated structures such
as 7 for evaluation (see Fig. 1).
Compound 5 was prepared by reaction of 3-benzylin-
dole15 with N,N-dimethylaminoethyl chloride in the
presence of NaH.
Indene 6 was prepared by reaction of 3-(2-dimethylami-
noethyl)indene17 with benzaldehyde in the presence of
ethanolic KOH to yield the trans isomer.18,19
In continuing efforts to identify a binding pharmaco-
phore for MS-245-type compounds at h5-HT6 receptors,
a question of interest was whether an indolic N1 nitro-
gen atom is required for high-affinity binding of trypt-
amine-related compounds. Two strategies were
Reaction of the indenyl anion with benzyl chloride affor-
ded nearly equal mixture of products 9 and 10 (Scheme
1). Treatment of the mixture with NaH and reaction with
N,N-dimethylaminoethyl chloride can, in theory, result
in the formation of three products (i.e., 11–13). Such re-
sults have been reported for related compounds.20 The
reaction was performed several times. Despite apparent
homogeneity upon thin-layer chromatographic analysis,
Keywords: Serotonergic ligands; 5-HT6; Binding requirements.
*
Corresponding author. Tel.: +1 804 828 8487; fax: +1 804 828
7404; e-mail: glennon@hsc.vcu.edu
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.02.070

1988
R. Kolanos et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1987–1991
H3C
H3C
CH3
CH3
N
N
CH3
H3CO
H3CO
N
N
N
S
S
O
O
O
O
X
NH2
1 X = -H
2 X = -NH2
4
3
H3C
H3C
CH3
N
CH3
N
N
NH2
5
7
6
Figure 1. Structures of known (1–4) and target (5–7) 5-HT6 ligands.
Scheme 2 shows the synthesis of compounds 7, 19, 21,
and 23. Reaction of the indenyl anion with 4-acetamido-
benzyl chloride gave compound 19 following hydrolysis
of the protecting group. However, when 4-nitrobenzyl
chloride was used as the reactant under similar condi-
tions, compound 21 was isolated following reduction
of the nitro intermediate; compound 19, identified as a
minor product of the reaction, was removed by recrys-
tallization. Condensation of 8 with 4-acetamidobenzal-
dehyde afforded 20; deprotection of 20 led to 7,
whereas reduction of 20 prior to deprotection led to in-
dane 23.
a
+
(
)
(
)
n
n
8
9 n = 1
10 n = 1
15 n = 2
14 n = 2
NCH3
NCH3
b
+
+
(
n
)
(
n
)
( )
n
CH2Cl
AcHN
(H3C)2N
a,b
11 n = 1
16 n = 2
12 n = 1
17 n = 2
13 n = 1
18 n = 2
8
19
21
NH2
Scheme 1. Reagents and conditions: (a) n-BuLi, THF, Ph(CH2)nCl,
0 °C; (b) NaOH, N,N-dimethylaminoethyl chloride.
CH2Cl
O2N
CHO
c,d
b
e
NH2
the 1H NMR spectrum indicated that a mixture of prod-
ucts had been formed (predominantly 11 with approxi-
mately 3–10% 12 depending upon the particular
reaction); 12 could not be completely separated from
11 but the two products could be differentiated by their
benzylic proton signals (i.e., a doublet of doublets at d
2.63 and 3.13 for 11, and a singlet at d 3.87 for 12). In
one instance, 1321 was isolated in low yield by column
chromatography.
NHAc
7
20
NH2
NHAc
b
f
Alkylation of the homologous phenylethyl derivative
(14 and 15) with N,N-dimethylaminoethyl chloride
(Scheme 1) gave three products as identified by TLC
and NMR analysis. Only compounds 16 and 1821
could be isolated in pure form by column
chromatography.
NH2
NHAc
23
22
Scheme 2. Reagents and conditions: (a) n-BuLi, hexanes; (b) HCl,
absolute EtOH; (c) n-BuLi, Et2O; (d) SnCl2, absolute EtOH; (e) KOH,
absolute EtOH; (f) H2, 5% Pd/C, MeOH.

R. Kolanos et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1987–1991
1989
Attempts were made to prepare the benzenesulfonyl
counterpart of 21 (i.e., 29) to obtain an analog similar
in structure to 3. Compound 2622 (Scheme 3) was ob-
tained from 24,23 but all efforts to obtain 29 by reduc-
tion of the nitro group (under a variety of conditions
including catalytic reduction, SnCl2, Na2S2O4) were
unsuccessful. Although it was possible to obtain the
Boc-protected amine 27, and to oxidize the mercapto
group to sulfone 28,22 attempts to deprotect 28 under
a variety of acidic or basic conditions resulted in nearly
immediate decomposition of the crude product that was
obtained from the reaction.
The 40-amino derivative of 5, (i.e., 36; Fig. 2) was ob-
tained by reaction of N,N-dimethylisotryptamine14 with
4-(benzotriazol-1-ylmethyl)aniline16 in methanolic HCl.
Radioligand binding data are provided in Table 1. Iso-
tryptamines 5 (Ki = 32 nM) and 36 (Ki = 50 nM) were
found to bind with high affinity, but with lower affinity
than N1-benzyltryptamine 4 (Ki = 6.5 nM). Four expla-
nations are possible for this somewhat lower affinity:
(a) compounds 5 and 36 lack a 5-methoxy group, (b)
the double bond is in the ꢁwrongꢀ position, (c) relocation
of the ring nitrogen atom is not well tolerated, and/or (d)
the tryptamine N1-nitrogen atom is optimal for binding.
The des-methoxy analog of 4 (i.e., 37, Ki = 6 nM) retains
affinity arguing that the methoxy group is not a major
contributor to binding. Benzylindene 6 (Ki = 57 nM),
which lacks the indole nitrogen atom, binds with an
affinity similar to that of 5 suggesting that an N1 indolic
nitrogen atom might not be required. However, with 6
there is an additional complicating factor. Although
the presence of the sp2-hybridized ring carbon atom of
6 might electronically mimic the tryptamine nitrogen
atom, it also imposes conformational constraint that
could ꢁlockꢀ the substituent into a conformation that is
not particularly favored for binding. Consequently,
benzylindene 11 was examined. The enhanced affinity
of 11 (Ki = 3.0 nM) indicates that the sp2-hybridized
ring carbon atom is not required for binding. Indeed,
racemic 11 binds with an affinity between that of MS-
245 (1) and N1-benzyltryptamine 4. These results suggest
that neither the tryptamine N1-nitrogen atom, nor an
sp2-hybridized carbon atom, is essential at this position
for binding to 5-HT6 receptors. The results with 11 fur-
ther suggest that the somewhat lower affinity of 5 rela-
tive to 4 (or 37), might be due to the new location of
the ring nitrogen atom and/or to the location of the dou-
ble bond.
An alternative approach involved the oxidation of 32.
Compound 3122 was obtained from 30,24 and success-
fully reduced to 32; however, it was not possible to dehy-
drogenate/oxidize 32 to 29. Oxidation of 31 afforded
33,22 which was reduced to 34; but, here too, attempts
to dehydrogenate 34 to 29 led to decomposition. In
one instance, oxidation of 31 with oxone resulted in sulf-
oxide 35.25 In the course of these studies, crude 29 was
1
isolated in several instances as evidenced by H NMR;
however, the product quickly decomposed.
a
b
O
Br
S
S
O
24
25
26
O2N
O2N
c
b
O
O
O
S
S
S
O
Boc-HN
Boc-HN
H2N
28
29
27
Interestingly, although homologation of the benzyl sub-
stituent of 37 to a phenylethyl group (i.e., 38, Ki =
12 nM) only halved affinity, compound 16 (Ki =
100 nM), a homolog of 11, binds with 30-fold lower
affinity than 11—an observation that cannot be satisfac-
torily explained at this time, but that might be related to
stereochemistry. That is, the benzylic carbon atom of 38
is in the plane of the aromatic ring whereas that of chiral
16 is either behind or in front of the plane; this would
place the pendent phenyl substituent in different posi-
tions relative to that in 38. This effect could be more
exaggerated with 16 than with 11 due to chain length.
d
e
OH
S
S
30
O2N
H2N
31
32
g
f
e
H3C
H3C
O
O
O
CH3
CH3
N
N
S
O
S
S
O
N
O2N
O2N
H2N
N
35
33
34
n(H2C)
Scheme 3. Reagents and conditions: (a) Hunigꢀs base, CH2Cl2, THF,
4-nitrobenzenethiol, ꢀ60 °C; (b) oxone, H2O, MeOH, rt; (c) Hunigꢀs
base, CH2Cl2, THF, t-Bu N-(4-mercapto-phenyl)carbamate, ꢀ60 °C;
(d) (i) Tf2O, CH2Cl2, ꢀ60 °C; (ii) 4-nitrobenzenethiol; (e) SnCl2, H2O,
EtOH, D; (f) mCPBA, 0 °C; (g) oxone.
NH2
36
37 n=1
38 n=2
Figure 2. Structures of arylalkyl analogs 36–38.

1990
R. Kolanos et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1987–1991
Table 1. Physicochemical properties and h5-HT6 serotonin receptor affinities of the compounds investigated
Melting point (°C)
Recrystallization solvent
Empirical formulaa
h5-HT6 Ki, nM ( SEM)b
5
182–184
211
CH2Cl2/Et2O
2-PrOH
C19H22N2ÆHClc
C20H21NÆHCl
C16H15NÆHClc
32
57
( 5)
6
( 14)
( 100)
( 1)
7
202–205
167–168
138–139
220–222
190–192
189–190
195–196
210–213
164–166
155–156
144–146
169–170
EtOH/Et2O
MeOH
640
3
11d
16
19
21
23
32
34
36
37
38
39g
C20H23NÆC2H2O4
e
Abs EtOH
EtOH/Et2O
EtOH/Et2O
EtOH/Et2O
EtOH/Et2O
EtOH/Et2O
CH2Cl2/Et2O
MeOH
C
21H25NÆC2H2O4
100
11,800
4470
6100
4200
740
50
( 10)
( 2570)
( 620)
( 1300)
( 600)
( 60)
( 4)
C16H15NÆHCl
C16H13NÆHCl
C16H17NÆHCl
C15H15NSÆHCl
C15H15NO2SÆHCl
C19H23N3Æ2HClf
C
19H22N2ÆC2H2O4
6
12
( 2)
( 3)
e
MeOH
—
C20H24N2ÆC2H2O4
C15H14N2ÆHClc
2530
( 200)
a Compounds analyzed within 0.4% of theory for C, H, and N. C2H2O4 = oxalate salt.
b The radioligand binding assay was performed in triplicate as previously reported.28
c Crystallized with 0.25 mol H2O.
d Homogeneous by thin-layer chromatography, but contains approximately 3% of 12 as identified by 1H NMR.
e Crystallized with 0.5 mol H2O.
f Crystallized with 1.0 mol H2O.
g Prepared according to a literature procedure29 and converted to HCl salt; the salt was washed with anhydrous Et2O.
An intact tryptamine moiety is not required for the bind-
ing of 2; that is, the abbreviated tryptamine 3
(Ki = 12 nM) binds with only 6-fold reduced affinity.
The abbreviated isotryptamine 39 (Fig. 3) (Ki =
2530 nM), however, binds with 50-fold lower affinity
than isotryptamine 36 (Ki = 50 nM). Indene 7 (Ki =
640 nM) also binds with low affinity; again, this might
be partly attributable to conformational constraint im-
posed by the exocyclic double bond. But, its reduced
counterpart 1-(4-aminobenzyl)indene (21, Ki = 4470
nM) binds with even lower affinity. These results indicate
that in the abbreviated series the indolic nitrogen atom or
the benzenesulfonyl group of 3 might contribute to bind-
ing. The low affinity of 3-(4-aminobenzyl)indene (19,
Ki = 11,800) provides evidence that the location of ring
unsaturation can also influence affinity.
Human 5-HT6 receptors possess an aspartate moiety in
the third transmembrane helix (TM3).26 It is likely that
analogs possessing the N,N-dimethylaminoethyl side
chain (e.g., 4–6, 11, 37) bind in a common manner inter-
acting with this aspartate. For these compounds, the
presence of the ring nitrogen atom does not seem to be
required for binding. This conclusion is consistent with
recent findings on 1-(1-naphthyl)piperazine (1-NP)
derivatives that appear to behave as tryptamine-mimics
at 5-HT6 receptors.27 For example, 4-benzyl-1-NP (40;
Fig. 3) (Ki = 38 nM), which lacks an indolic nitrogen
atom (and an indolic nucleus) binds at 5-HT6 receptors
with an affinity comparable to that of 5.
In contrast, those compounds lacking the basic amine
side chain (i.e., the abbreviated analogs 7, 19, 21, 23,
32, 34) must bind in an altogether different orientation
if they interact with the same aspartate residue; that is,
with the latter compounds it must be the anilino amine
function that interacts with the aspartate. Given the
high affinity of 3, the presence of the indolic nitrogen
atom, the presence and location of ring unsaturation,
and/or the nature of the substituent (i.e., benzyl vs benz-
enesulfonyl) seem to play a greater role in binding for
the latter compounds. The influence of each of these fac-
tors will require further investigation.
To examine the possible contribution of the benz-
enesulfonyl group, attempts were made to prepare benz-
enesulfonyl indene 29. Repeated trials employing several
different methods were unsuccessful; although crude
product was obtained in several instances, it quickly
decomposed. The indane counterpart of 29 (i.e., 34,
Ki = 740 nM), however, was stable. The nearly 10-fold
higher affinity of 34 relative to 23 argues that in the
abbreviated series, the benzenesulfonyl moiety might
contribute to binding.
Acknowledgements
NH
N
The present work was supported in part by MH60599,
and studies on the abbreviated indene analogs were
funded by BTG International.
H
N
NH2
References and notes
39
40
Figure 3. Structures of 3-(4-aminobenzyl)indole (39) and naphthyl-
piperazine 40.
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18 (mp 129–130 °C; C21H25NÆ1.3C2H2O4) were isolated as
their oxalate salts following recrystallization from abso-
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and N.
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studies: 25 mp 120–122 °C, C15H11NO2S; 26 mp 135–
136 °C, C15H11NO4S; 28 mp 164–166 °C, C20H21NO4S; 31
mp 91–93 °C, C15H13NO2S; 33 mp 135–137 °C,
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