Bicyclic piperazinylbenzenesulphonamides are potent and selective 5-HT6 receptor antagonists

Article abstract of DOI:10.1016/S0960-894X(02)00172-5

The synthesis of novel 3-(octahydropyrido[1,2-a]pyrazin-2-yl)- and 3-(hexahydropyrrolo[1,2-a]pyrazin-2-yl)phenyl-2-benzo[b]thiophene sulphonamide derivatives 3, (S)-4 and (R)-4 is described. The compounds show high affinity for the 5-HT₆ recept

Full text of DOI:10.1016/S0960-894X(02)00172-5

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1357–1360
Bicyclic Piperazinylbenzenesulphonamides are Potent and
Selective 5-HT6 Receptor Antagonists
Steven M. Bromidge,a Stephen E. Clarke,b Frank D. King,c Peter J. Lovell,a
Helen Newman,d Graham Riley,d Carol Routledge,a Halina T. Serafinowska,a,*
Douglas R. Smithb and David R. Thomasb
aDepartment of Psychiatry, GlaxoSmithKline, New Frontiers Science Park, Third Avenue, Harlow, Essex CM19 5AW, UK
bDepartment of Drug Metabolism and Pharmacokinetics, GlaxoSmithKline, New Frontiers Science Park, Third Avenue, Harlow,
Essex CM19 5AW, UK
cDepartment of Neurology, GlaxoSmithKline, New Frontiers Science Park, Third Avenue, Harlow, Essex CM19 5AW, UK
dDepartment of Discovery Research, GlaxoSmithKline, New Frontiers Science Park, Third Avenue, Harlow, Essex CM19 5AW, UK
Received 26 July 2001; revised 27 February 2002; accepted 15 March 2002
Abstract—The synthesis of novel 3-(octahydropyrido[1,2-a]pyrazin-2-yl)- and 3-(hexahydropyrrolo[1,2-a]pyrazin-2-yl)phenyl-2-
benzo[b]thiophene sulphonamide derivatives 3, (S)-4 and (R)-4 is described. The compounds show high affinity for the 5-HT6
receptor, excellent selectivity against a range of other receptors and good brain penetration. # 2002 Elsevier Science Ltd. All rights
reserved.
The 5-HT6 receptor is one of the most recently identified
serotonin receptors. It was first isolated from rat striatal
mRNA1,2 in 1993 and subsequently the human 5-HT6
gene was cloned and characterised.3 The 5-HT6 receptor
is positively coupled to the adenylate cyclase second-
messenger system and is primarily found in the central
nervous system.4 The exact functional role of the recep-
tor has yet to be ascertained, however its unique dis-
tribution in the brain and high affinity for therapeutic
atypical antipsychotics and antidepressants suggest a
possible role for 5-HT6 receptor antagonists in the
therapy of schizophrenia and depression.5,6
A number of studies7À9 have also indicated that 5-HT6
antagonists could be of value in the treatment of mem-
ory dysfunction.10 Recently, we reported that the selec-
tive 5-HT6 antagonist
1
(SB-271046)11 improved
cognitive performance in animal models12 providing fur-
ther support for this hypothesis. Moreover, this com-
pound has been demonstrated to selectively enhance
excitatory transmission within the frontal cortex in
rats.13
We now report the preparation and biological profile of
some bicyclic piperazine derivatives of 1 with improved
CNS penetration.
Chemistry
The synthesis of the 3-(octahydropyrido[1,2-a]pyrazin-
2-yl)phenyl analogue 3 was carried out according to
Scheme 1.14 Thus, alkylation of 2-methoxy-5-nitroanil-
ine with 2-bromomethylpiperidine afforded 5 which in
turn was alkylated with ethyl bromoacetate in refluxing
ethanol to give the piperidinyl-1-acetic acid ethyl ester 6
in 64% yield. Cyclisation of 6 using sodium in dioxane15
gave the required lactam 7. Sequential reduction of the
nitro and carbonyl groups in 7 provided the bicyclo-
piperazinylaniline 8 in 82% yield for the two steps.
Finally, 8 was coupled with 5-chloro-3-methyl[b]ben-
*Corresponding author. Tel.: +44-1279-627683; fax: +44-1279-
622550; e-mail: halina_serafinowska-1@gsk.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00172-5

1358
S. M. Bromidge et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1357–1360
Scheme 1. Reagents and conditions: (i) 2-(bromomethyl)piperidine, DMF or chlorobenzene, reflux (47%); (ii) ethyl bromoacetate/Et3N, ethanol,
reflux (64%); (iii) Na, dioxane, reflux (31%); (iv) H2/Pd/C, ethanol, rt (91%); (v) BH3, THF, reflux (90%); (vi) 5-chloro-3-methylbenzo[b]thiophene-
2-sulphonyl chloride/iPr2EtN, CH2Cl2, rt (32%).
zothiophene-2-sulphonyl chloride to afford the target
sulphonamide 3.16
similar affinity (pKi 8.9) to that of its N-methylated
counterpart, it was somewhat less brain penetrant.11
Therefore, analogues with constrained bicyclic systems
such as 3 and 4 were targeted in the hope of enhancing
metabolic stability and CNS penetration by precluding
dealkylation to the active NH metabolite.
Initial attempts at the preparation of the corresponding
3-(hexahydropyrrolo[1,2-a]pyrazin-2-yl)phenyl analo-
gue (S)-4 using a similar synthetic approach were
unsuccessful. Rather unexpectedly, alkylation of 2-
methoxy-5-nitroaniline with 2-bromo-1-((S)-2-bromo-
methylpyrrolidin-1-yl) ethanone 9 led to the formation
of the hydroxymethyl derivative 10 as a major product:
presumably via amide carbonyl assisted hydrolysis
during aqueous work up (Scheme 2). An alternative
strategy based on the use of enantiomerically pure
diketopiperazine 13 was therefore adopted (Scheme 3).
Thus, acylation of 2-methoxy-5-nitroaniline with the
mixed anhydride of Boc-l-proline provided the amide
11 in 75% yield. After removal of the Boc group, the
pyrrolidinyl nitrogen was selectively acylated with bro-
moacetyl bromide to give the diamide 12. Cyclisation of
12 using NaH in DMF afforded the desired diketopi-
perazine 13 which was 98% pure by chiral HPLC.
Sequential reduction of the nitro and keto groups in 13
yielded the amino intermediate 14 which was converted
into the sulphonamide (S)-4,16 in a similar way to that
described for 3. The corresponding R-enantiomer (R)-4
was prepared from Boc-d-proline by the same synthetic
sequence that was employed for (S)-4.
The binding profile of 3, (S)-4 and (R)-4 across a range
of receptor subtypes was determined and is shown in
Table 1 along with that of 1 and 2 for comparison.11
The 3-(octahydropyrido[1,2-a]pyrazin-2-yl)phenyl ana-
logue 3 had good 5-HT6 affinity (pKi 8.3) and selectivity
profile (>80-fold). The more compact (S)-3-(hexa-
hydropyrrolo[1,2-a]pyrazin-2-yl)phenyl derivative (S)-4
was found to have almost 10-fold increased affinity for
the 5-HT6 receptor (pKi 9.1) and greater than 200-fold
selectivity against a range of other receptors (totaling 13
subtypes). The enhanced target affinity of (S)-4 relative
to 3 points towards a size constraint for the bicyclo-
piperazines in the 5-HT6 receptor binding pocket.
Interestingly, the R-enantiomer (R)-4 had identical 5-
HT6 affinity and a very similar selectivity profile to that
of (S)-4 suggesting that both the enantiomers had
almost identical binding modes.
Similar binding pattern was also observed by us for
(R) and (S) enantiomers of the 3-methylpiperazinyl
arylsulphonamide in the structure–activity study of
conformationally restricted analogues of 1 reported
recently.17
Results and Discussion
5-Chloro -N- [4-methoxy-3-(4-methylpiperazin-1-yl-
phenyl]-2-benzo[b]thiophene sulphonamide 2, developed
in our laboratory,11 showed subnanomolar binding affi-
nity (pKi 9.2) for the 5-HT6 receptor and moderate
brain penetration (18%) combined with low clearance
(12.5 mL/min/kg) in rats. However, this compound was
found to be metabolised by N-dealkylation to yield the
NH-piperazine 1. Although the latter analogue had
Compound (S)-4 was evaluated in a functional model of
5-HT6 receptor activation.11 In the presence of (S)-4, the
5-HT concentration-response curve had the same maxi-
mal response but was shifted rightward in a parallel
manner with an apparent pKb of 7.7Æ0.1 (n=3). The
reason for the difference between the affinity value (pKi)
for (S)-4, derived from radioligand binding assays and
antagonist potency (pKb), derived from functional
Scheme 2. Reagents and conditions: (i) bromoacetyl bromide/iPr2EtN, CH2Cl2, 10 ꢀC (95%); (ii) 2-methoxy-5-nitroaniline/K2CO3, NMP, rt (22%).

S. M. Bromidge et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1357–1360
1359
Scheme 3. Reagents and conditions: (i) EtOCOCl/2-methoxy-5-nitroaniline/4-methylmorpholine, THF, À10 ꢀC to rt (75%); (ii) CF3COOH,
CH2Cl2, rt (91%); (iii) bromoacetyl bromide/iPr2EtN, CH2Cl2, À10 ꢀC; (iv) NaH, DMF, rt (34% for two steps); (v) H2/Pd/C, ethanol/ethyl acetate,
rt (89%); (vi) BH3/THF, reflux (51%); (vii) 5-chloro-3-methylbenzo[b]thiophene-2-sulphonyl chloride/pyridine, CH2Cl2, rt (75%).
Table 1. 5-HT6 Receptor binding affinity and selectivitya of compounds 1, 2, 3, (S)-4 and (R)-4
Affinity (pKi)
Receptor
1
2
3
(S)-4
(R)-4
5-HT1A
5-HT1B
6.4
6.1
6.3
6.1
6.2
5.4
6.9
6.2
6.7
6.2
5-HT1D
5-HT1E
5-HT1F
5-HT2A
5-HT2B
5-HT2C
5-HT4
5-HT6
5-HT7
6.6
6.7
5.6
6.6
6.0
6.0
6.3
5.5
9.2 (n=3)
5.5
5.7
6.1
6.7
6.5
7.0
5.8
6.5
6.6
5.9
6.4
5.3
9.1 (n=9)
6.0
5.8
5.8
6.5
6.9
6.0
6.7
5.9
5.9
6.2
5.8
9.1 (n=3)
6.2
6.0
5.7
6.2
<5.0
<6.0
<5.6
<5.4
5.7
5.4
8.9 (n=3)
5.4
5.7
5.6
6.3
<6.0
<6.0
6.2
5.7
<5.5
<5.1
8.3 (n=3)
<5.1
5.7
6.4
Adrenergic a1B
Dopaminergic D2
Dopaminergic D3
6.0
aAll values represent the mean of at least two determinations, with each determination lying within 0.2 log unit of the mean. Receptors and radio-
ligands used in binding assay: 5-HT1A (human cloned receptors in HEK 293 cells, [3H]-8-OH-DPAT); 5-HT1B (human cloned receptors in CHO
cells, [3H]-5-HT); 5-HT1D (human cloned receptors in CHO cells, [3H]-5-HT); 5-HT1E (human cloned receptors in CHO cells, [3H]-5-HT); 5-HT1F
(human cloned receptors in CHO cells, [3H]-5-HT); 5-HT2A (human cloned receptors in HEK 293 cells, [3H]ketanserin); 5-HT2B (human cloned
receptors in HEK 293cells, [3H]-5-HT);5-HT2C (human cloned receptors in HEK 293 cells, [3H]mesulergine); 5-HT6 (human cloned receptors in
HeLa cells, [3H]-LSD); 5-HT7 (human cloned receptors in HEK 293 cells, [3H]-5-CT); D2 (human cloned receptors in CHO cells, [125I]iodosulpride);
D3 (human cloned receptors in CHO cells, [125I]iodosulpride).
assays is unclear, but may be due to methodological
differences between the binding and functional assays.
Pharmacokinetic studies at steady state in rats (n=3)
following a 16 h infusion demonstrated that (S)-4 was
slightly more brain penetrant (24%) than 1, but was
subject to increased blood clearance (38 mL/min/kg).
We have previously observed a positive correlation
between in vivo clearance and CNS penetration in rats
with a related series of compounds.18
with our recent findings,17 the enantiomers (S)-4 and
(R)-4 had very similar 5-HT6 affinities and selectivity
profiles. The 5,6-bicyclic piperazine (S)-4 showed mar-
ginally improved CNS penetration but increased in vivo
clearance in rat relative to 1. In conclusion, this study
together with our previous work11,17,18 adds sig-
nificantly to the knowledge of selective, brain penen-
trant 5-HT6 antagonists and their rational design.
References and Notes
Summary
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chromatography on silica gel and were isolated as amorphous
solids. 3: dH (250 MHz, CDCl3-d6), 1.28 (3H, m), 1.73 (3H, m),
2.08 (3H, m), 2.19 (3H, s), 2.43 (1H, m), 2.68 (1H, m), 2.84
(2H, m), 3.00 (1H, m), 3.21 (1H, m), 3.82 (3H, s), 6.46 (1H, d,
J=2.34 Hz), 6.73 (2H, m), 7.42 (1H, m), 7.65 (1H, d,
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d, J=8.60 Hz). MS: m/z (MH+)=492.
(R)-4: dH (250 MHz, CDCl3-d6), 1.30 (1H, m), 1.81 (3H, m),
2.11 (3H, m), 2.22 (3H, s), 2.38 (1H, m), 2.60 (1H, m), 3.01
(1H, m), 3.18 (3H, m), 3.80 (3H, s), 6.50 (1H, d, J=2.16 Hz),
6.70 (2H, m), 7.44 (1H, m), 7.66 (1H, d, J=1.90 Hz), 7.74 (1H,
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