Multistep solution-phase parallel synthesis of spiperone analogues

Article abstract of DOI:10.1016/S0960-894X(00)00483-2

A flexible, multistep parallel synthesis of spiperone analogues is described. A library of 4-substituted piperidines, assembled utilizing reductive amination and acylation protocols, was alkylated either homogeneously or heterogeneously, exploiting a product release only concept, to afford an oxa-series of spiperone analogues. Screening of the products at 5-HT₂ and D₂ receptors revealed 5-HT(2A) antagonists with improved selectivity compared to spiperone and AMI-193. (C) 2000 Published by Elsevier Science Ltd.

Full text of DOI:10.1016/S0960-894X(00)00483-2

Bioorganic & Medicinal Chemistry Letters 10 (2000) 2435±2439
Multistep Solution-Phase Parallel Synthesis of Spiperone
Analogues
Henrik C. Hansen,a,y Roger Olsson,a Glenn Crostonb
and Carl-Magnus Anderssona,*
aSynthetic Chemistry, ACADIA Pharmaceuticals A/S, Fabriksparken 58, DK-2600 Glostrup, Denmark
bHigh Throughput Pharmacology, ACADIA Pharmaceuticals Inc., 3911 Sorrento Valley Blvd, San Diego, CA 92121-1402, USA
Received 30 May 2000; revised 11 August 2000; accepted 17 August 2000
AbstractÐA ¯exible, multistep parallel synthesis of spiperone analogues is described. A library of 4-substituted piperidines,
assembled utilizing reductive amination and acylation protocols, was alkylated either homogeneously or heterogeneously, exploit-
ing a product release only concept, to a€ord an oxa-series of spiperone analogues. Screening of the products at 5-HT2 and D2
receptors revealed 5-HT2A antagonists with improved selectivity compared to spiperone and AMI-193. # 2000 Published by
Elsevier Science Ltd.
The antipsychotic agent spiperone (Fig. 1), a potent
mixed dopamine D2 and serotonin antagonist, is a rare
example of a 5-HT ligand binding with any selectivity at
the 5-HT2A subclass of serotonin receptors.1 Surprisingly,
little e€ort has been devoted to studying and optimizing
this interesting feature of spiperone. Recently, Glennon et
al.2 investigated the in¯uence of imidazolidinone sub-
stituents on anity and selectivity at 5-HT2 and D2
receptors. Analogues with smaller substituents replacing
the phenyl group, especially the methyl analogue, were
found to possess lower anity than spiperone itself, but
higher selectivity. Introduction of substituents at the
lactam nitrogen had little in¯uence on the 5-HT2A anity,
but reduced the 5-HT2A/D2 ratio, a trend also observed by
others.3 In an important earlier study,4 Glennon et al.
reported that improved 5-HT2A selectivity (versus 5-HT2C
and D2 receptors) resulted upon replacing the aminobu-
tyrophenone portion of spiperone with an aryl ether
functionality. The p-¯uorophenoxy analogue (AMI-193;
Fig. 1) was reported to have the highest selectivity.4
The current interest in selective 5-HT2A antagonists5 as
potential antipsychotic agents prompted us to revisit the
spiperone pharmacophore in a combinatorial context.
In particular, we reasoned that oxa-analogues based on
AMI-193 would constitute an interesting family of tar-
gets. In this communication, our synthetic strategy is
exempli®ed by the solution-phase synthesis of a 240-
membered (5Â8Â6) rehearsal library.
The preparation of the required series of 4-substituted
piperidines is outlined in eqn (1) and Figure 2. Com-
Figure 1.
*Corresponding author. Tel.: +45-4329-3015; fax: +45-4329-3030; e-mail: cma@acadia-pharm.com
yPresent address: Department of Chemistry, University of California, Berkeley, CA 94720-1460, USA.
0960-894X/00/$ - see front matter # 2000 Published by Elsevier Science Ltd.
PII: S0960-894X(00)00483-2

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H. C. Hansen et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2435±2439
Figure 2. Building blocks.
mercially available Boc-protected piperidone was chosen
as a convenient starting material enabling facile extrac-
tive puri®cation of the intermediates as the synthesis
progressed. Alternately, basic and neutral products were
encountered, permitting simple and ecient liquid/liquid
extraction for puri®cation of the proceeding library.
The 4-aminopiperidines 1 were synthesized via reductive
amination6 with an excess of the piperidone in order to
ensure complete consumption of the amine (A1±A5).
Extractive puri®cation returned 1 with high purities
(Table 1) and in yields of over 85% except for 1-A3,
which was obtained in 40% yield. Acylation of the
amines 1 was performed with acid chlorides7 (B1, B2),
sulfonyl chlorides7 (B3, B4), and isocyanates8 (B5±B8),
resulting in a library of Boc-protected piperidines 2. The
products were obtained in acceptable purities, provided
that sequestration of excess isocyanates was undertaken
in the latter cases (Table 1). Deprotection by treatment
with 4 M HCl in dioxane for 2 h left building blocks 3
ready for further alkylation.
In general, the products derived from isocyanate acyla-
tion were of higher purity than the amides or sulfon-
amides (Table 1). In these latter series, a number of entries

H. C. Hansen et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2435±2439
Table 1. Puritiesa for selected compounds
2437
Compound
Purity (UV/MS)
Compound
Purity (UV/MS)
Compound
Purity (UV/MS)
Compound
Purity (UV/MS)
1-A1
1-A2
1-A3
1-A4
1-A5
2-A1-B1
2-A1-B2
2-A1-B3
2-A1-B4
2-A1-B5
100/97
100/97
90/78
95/99
100/95
68/93
100/100
77/61
12/91
2-A1-B6
2-A1-B7
2-A1-B8
3-A1-B1
3-A1-B2
3-A1-B3
3-A1-B4
3-A1-B5
3-A1-B6
3-A1-B7
100/89
100/92
100/89
65/78
99/80
68/63
82/73
89/92
92/90
100/98
3-A1-B8
4-C1
4-C2
4-C3
4-C4
4-C5
7-A1-B5-C4
7-A1-B6-C4
7-A1-B7-C4
7-A1-B8-C4
93/93
89/93
66/79
67/81
97/84
7-A4-B6-C1
7-A1-B7-C3
7-A1-B8-C2
8-A4-B1
8-A4-B2
8-A4-B3
8-A3-B5
8-A3-B6
8-A3-B7
8-A3-B8
100/81
97/83
Ð/100
95/81b
100/82b
100/73b
100/90b
100/80b
100/90b
100/92b
96/Ð
100/88b
92/94b
88/89b
95/85b
100/81
aPurities were measured on an HP1100 LC-MS using DAD/MSD electrospray detection. Eluent: 8 mM ammoniumacetate in MeCN/water. UV-
data were measured at UVmax. Matrix characterization by NMR, performed on a quarter of the products, corroborated LC-MS results.
bPurity after ion-exchange chromatography on ISOLUTE SCX cartridges.
contained minor amounts of acylated 4-hydroxy-
piperidine, obviously arising from reduction of the ketone
in the ®rst step. Diacylated piperidines were also observed
in some cases, the appearance of which was attributed to
partial premature hydrolysis of the Boc-group.
ciently scavenged, since only the primary sulfonates reac-
ted under the nucleophilic conditions employed in the
releasing step (illustrated for the synthesis of 9a,b in eqn
(3)). Alternatively, crude 4 could be passed through a
short plug of silica gel to give products of high purity.
The coupling to solid support was performed in
CH2Cl2/pyridine 1:1 as reported.12 For 4-C1 and 4-C3
the anchoring was con®rmed by IR spectroscopy.
The 3-aryloxy-1-propanols 4 (eqn (2)) were synthesized
in parallel by alkylation of an excess of the selected
phenols (C1±C5).10 Extraction with aqueous base was
used to remove the bulk of excess phenol. However, by
activating the crude alcohols 4 via attachment to solid-
supported sulfonyl chloride,11 residual phenols were e-
Before their use in parallel synthesis, the ®ve solid-sup-
ported tosylates 5 were evaluated in a reaction with 4-
butylpiperidine.13 All ®ve expected products were

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H. C. Hansen et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2435±2439
obtained in good yields (average 75%), and high purities
(average 90%). Surprisingly, as we utilized the hydro-
chlorides 3 as nucleophiles towards 5 in the presence of N-
ethyldiisopropylamine, the alkylation reaction failed. The
major products isolated were the corresponding N-(aryl-
oxypropyl)pyridinium salts,14 presumably formed during
activation of the alcohols on the solid support, and only
released in the presence of chloride counter ions. In
accordance with this assumption, when the piperidines 3
were alkylated as the free bases, the desired products 7
(eqn (4)) were obtained in high purities (Table 1). Good
conversion was accomplished by heating 3 at 60 ꢀC for
8±12 h with an excess of 5 in acetonitrile.
A procedure for alkylation in solution was also devel-
oped.15 The piperidines 3 were readily alkylated with an
excess of the commercially available 6 (eqn (5)), and the
target compounds 8 were successfully puri®ed by ion-
exchange chromatography (Table 1).
Figure 3. 5-HT2A Inverse Agonism.
The library of spiperone analogues 7 and 8 was assayed
for antagonism and inverse agonism at the 5-HT2A
receptor using R-SATTM (Fig. 3).16 R-SATTM is a mam-
malian cell-based assay technology that provides a sensi-
tive measure of receptor function. Cells are transiently
transfected with vectors encoding the receptor target and
a reporter enzyme gene, incubated with drug, and the
reporter enzyme quanti®ed after incubation. The degree
of receptor activation correlates with the quantity of
reporter enzyme produced by cells. In the case of mea-
suring receptor inverse agonist activity, receptor con-
stitutive activity is elevated and the ability of compounds
to reverse the constitutive activation is measured.
For example, the cyclooctyl derivative 8-A5-B2, AC-
165024, displayed reasonable potency as an inverse
agonist with an average pIC50 of 7.9 from three deter-
minations at the 5-HT2A receptor, and yet did not have
signi®cant inverse agonist activity at the 5-HT2B or 5-
HT2C receptors up to 3000 nM. This compound also
lacked D2 R-SATTM antagonist activity when tested up
to a high dose of 3000 nM. In antagonist testing at the
5-HT2A receptor, using 5-HT as the agonist, potent and
ecacious antagonism of 5-HT2A activity was observed,
with a Ki of 20 nM. AC-165024 lacked antagonist activ-
ity against the muscarinic m1 and histamine H1 recep-
tors and lacked agonist activity at the 5-HT1A receptor.
The analogue AC-165043 also displayed strong repres-
sion of 5-HT2A activity (pIC50=7.1 as inverse agonist)
without signi®cant e€ects at the other receptors tested.
Gratifyingly, a number of analogues turned out to pos-
sess activity as 5-HT2A inverse agonists and antagonists.
Even though the new compounds disclosed herein were
generally less potent (2±3 orders of magnitude) than the
reference AMI-193 (pIC50=9.5 as inverse agonist),
In conclusion, we have developed a practical, multistep,
solution-phase protocol for parallel synthesis of spiper-
one analogues. A combination of homogeneous and
heterogeneous extractive puri®cation, reagent seques-
preliminary pro®ling at the 5-HT2A, 5-HT2B, 5-HT2C
,
and D2 receptors using R-SATTM revealed analogues
with improved and potentially explorable selectivities.

H. C. Hansen et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2435±2439
2439
tration and polymer-anchored reagents was employed in
order to secure adequate product purities. Preliminary in
vitro pharmacological characterization of a 240-mem-
bered rehearsal library revealed new compounds with
greatly improved selectivity for the 5-HT2A receptor
compared to spiperone and AMI-193.
2Â20 mL), HCl (0.1 M, 20 mL), dried (Na2SO4) and con-
centrated to give the products 2.
8. Acylation with isocyanates: The amine
1 (approx.
0.45 mmol) was dissolved in dry CH2Cl2 (1 mL). The isocya-
nate (2 equiv) was added, and the mixture was stirred at rt.
After 20 h, N,N-dimethylaminoethylamine (5 equiv)9 was
added, and the mixture was stirred for another 20 h. Ether
(20 mL) was added, and the mixture was washed with HCl (0.1
N, 5Â10 mL), water (1Â10 mL), dried (MgSO4) and con-
centrated to give the products 2.
Acknowledgements
9. Siegel, M. G.; Hahn, P. J.; Dressman, B. A.; Fritz, J. E.;
Grunwell, J. R.; Kaldor, S. W. Tetrahedron Lett. 1997, 38, 3357.
10. Synthesis of 4: The phenol (10 mmol) was dissolved in
KOH/EtOH (1.2 M, 10 mL) and stirred at rt. After 1 h, 3-
bromo-1-propanol (0.9 equiv) was added. The mixture was
stirred at 50 ꢀC. After 16 h, the mixture was cooled to rt, con-
centrated, and redissolved in EtOAc (20 mL), washed with
NaOH (2 N, 4Â10 mL), dried (MgSO4) and concentrated to
give the alcohol 4.
11. The polystyrene sulfonyl chloride was purchased from
Argonaut Technologies.
12. Argonaut Technologies Catalogue `Resins and Reagents',
2000; p. 71. For a similar procedure, see: Rueter, J. K.; Nor-
tey, S. O.; Baxter, E. W.; Leo, G. C.; Reitz, A. B. Tetrahedron
Lett. 1998, 39, 975.
13. Mimura, M.; Hayashida, M.; Nomiyama, K.; Ikegami, S.;
Iida, Y.; Tamura, M.; Hiyama, Y.; Ohishi, Y. Chem. Pharm.
Bull. 1993, 41, 1971.
14. Pyridinium analogues 10 were released from the solid
support in the presence of chloride. A con®rmatory experi-
ment employing the hydrochloride of Hunigs base as a chlor-
ide source gave the same result. 10 could be removed by ion-
exchange chromatography.
We thank Tina Jensen and Monica Jùrgensen for HPLC
analyses and Marissa Suarez and Aisha Suleiman for
RSATTM assays.
References and Notes
1. Zifa, E.; Fillion, G. Pharmacol. Rev. 1992, 44, 401.
2. Metwally, K. A.; Dukat, M.; Egan, C. T.; Smith, C.;
Dupre, A.; Gauthier, C. B.; Herrick-Davis, K.; Teitler, M.;
Glennon, R. A. J. Med. Chem. 1998, 41, 5084.
3. (a) Mach, R. H.; Jackson, J. R.; Luedtke, R. R.; Ivins, K.
J.; Molino€, P. B.; Ehrenkaufer, R. L. J. Med. Chem. 1992,
35, 423. (b) Lever, J. R.; Sche€el, U. A.; Stathis, M.; Musa-
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4. Ismaiel, A. M.; De Los Angeles, J.; Teitler, M.; Ingher, S.;
Glennon, R. A. J. Med. Chem. 1993, 36, 2519.
5. For a recent review, see: Meltzer, H. Y. Neuropsycho-
pharmacology 1999, 21, S106.
6. Reductive amination: tert-Butyl-4-oxo-1-piperidinecarboxyl-
ate (10 mmol) was dissolved in MeOH (5 mL). The amine
(5 mmol) in MeOH (3 mL) was added, followed by AcOH in
MeOH (1 M, 6.7 mL) and NaCNBH3 in MeOH (0.3 M,
22 mL). The mixture was stirred at rt. After 48 h, water (2 mL)
was added, and the mixture was concentrated and redissolved
in ether. The organic solution was extracted with HCl (0.1 N,
3±4 times). The combined aqueous layers were treated with
NaOH (0.2 N) until pH>8, then extracted with ether
(2Â10 mL). The combined organic layers were dried (Na2SO4),
®ltered and concentrated to give the amines 1.
7. Acylation with acid chlorides and sulfonyl chlorides: The
amine 1 (approx. 0.45 mmol) was dissolved in dry CH2Cl2
(1 mL). N-Ethyldiisopropylamine (3.3 equiv) and the acid
chloride (2.2 equiv) were added. The mixture was stirred at rt.
After 20 h, the mixture was diluted with ether (20 mL),
sequentially washed with water (20 mL), NaOH (1 N,
15. Alkylation in solution: The amine 3 (approx. 0.45 mmol)
was dissolved in MeCN (2 mL). N-Ethyldiisopropylamine
(10 equiv) and 1-(3-chloropropoxy)-4-¯uorobenzene (3±
5 equiv) were added. The mixture was stirred at 60 ꢀC. After
19 h, the mixture was concentrated and puri®ed on ISOLUTE
SCX to give the product 8.
16. Brann, M. R. US Patent 5,707,798, 1998; Chem. Abstr.
1998, 128, 111548.