Bridgehead-methyl analog of SC-53116 as a 5-HT4 agonist

Article abstract of DOI:10.1016/j.bmcl.2004.04.021

Pyrrolizidine benzamide (±)-2, the bridgehead-methyl analog of SC-53116, was prepared and evaluated for 5-HT₄ agonism activity in the rat tunica muscularis (TMM) mucosae assay. Compound (±)-2 has an EC₅₀ of 449nM in the TMM assay, as compared to 23nM for SC-53116, and 66nM for the racemate of SC-53116.

Full text of DOI:10.1016/j.bmcl.2004.04.021

Bioorganic & Medicinal Chemistry Letters 14 (2004) 3073–3075
Bridgehead-methyl analog of SC-53116 as a 5-HT4 agonist
Daniel P. Becker,* Daniel L. Flynn and Clara I. Villamilà
Department of Medicinal Chemistry, Pfizer Research and Development, 4901 Searle Parkway, Skokie, IL 60077, USA
Received 15 March 2004;revised 6 April 2004;accepted 10 April 2004
Abstract—Pyrrolizidine benzamide ( )-2, the bridgehead-methyl analog of SC-53116, was prepared and evaluated for 5-HT4
agonism activity in the rat tunica muscularis (TMM) mucosae assay. Compound ( )-2 has an EC50 of 449 nM in the TMM assay, as
compared to 23 nM for SC-53116, and 66 nM for the racemate of SC-53116.
Ó 2004 Elsevier Ltd. All rights reserved.
H
We have previously reported our discovery of SC-53116,
which was the first selective agonist at the 5-HT4
receptor.1 SC-53116 has an ED50 of 23 nM in the tunica
muscularis mucosae assay of Craig and Clarke,2 and is
selective versus other monoamine receptors with a Ki of
152 nM at the 5-HT3 receptor and Kis of >10,000 nM at
the 5-HT1, 5-HT2, D1, D2, a1, a2, and b-receptors. The
5-HT4 receptor was discovered by Clark2 and Bockaert3
in the brain and gut, respectively, and is expressed in a
wide variety of tissues including brain, heart, bladder,
gut, and kidney.4 Selective ligands for the receptor show
promise in the treatment of diseases including the irri-
table bowel syndrome, atrial arrhythmia, urinary
incontinence, and gastrointestinal motility disorders. An
excellent review of the 5-HT4 receptor and key ligands
was recently published.5
O
O
H
N
Cl
Cl
N
NH
N
H
OMe
OMe
H2N
H2N
SC-53116
SC-52491
O
O
CH3
N
Cl
H2N
Cl
NH
NH
N+
OMe
OMe
1
H2N
( )-2
tested with S9 activation.9 We hypothesized that this
toxicity was due to oxidation of the pyrrolizidine moiety
to the bicyclic iminium ion 1, which can then function as
an alkylating agent. To avoid this liability, we targeted
the bridgehead-methyl SC-53116 analog ( )-2, as the
methyl group would block metabolism to the iminium
species. We targeted the racemate initially to test the
concept and ensure that sufficient potency is maintained
with the additional methyl group.
We have been interested in pursuing 5-HT4 agonists as
gastrointestinal prokinetic agents. Our efforts in this
area6 have also prompted us to develop several azatri-
cyclic benzamides7 that are potent 5-HT4 agonists and
5-HT3 antagonists, particularly SC-52491,8 in addition
to the pyrrolizidine SC-53116.
SC-53116 was a clinical candidate but was halted when
it was observed to be positive in the Ames assay when
We employed general methodology reported by Burgess
and Meyers10 for introduction of the requisite quater-
nary carbon at the pyrrolizidinone bridgehead position.
As outlined in Scheme 1, ketoester 3 was heated under
reflux with ethanolamine to afford ( )-411 in 53% yield.
The bicyclic lactam ( )-4 was treated with t-butyldi-
methylsilyl methyl acetate ketal12 in 2 M lithium per-
chlorate ether13 to afford lactam ( )-5 in 30% yield.
Attempts with titanium tetrachloride in methylene
chloride also afforded ( )-5 but in lower yield. The
primary alcohol of ( )-5 was converted to the tosylate in
86% yield by treating with tosyl chloride in pyridine.
Keywords: Serotonin;5-HT4;Prokinetic;SC-53116;Pyrrolizidine.
* Corresponding author at present address: Department of Chemistry,
Loyola University, 6525 North Sheridan Road, Chicago, IL 60626,
USA. Tel.: +1-8477297455;fax: +1-8477303181;e-mail: dbecke3@
luc.edu
Present address: Deciphera Pharmaceuticals Inc., 1505 Wakarusa
Drive, Lawrence, KS 66047, USA.
Present address: Abbott Labs, 100 Abbott Park Road, Abbott Park,
à
IL 60064-6099, USA.
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.04.021

3074
D. P. Becker et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3073–3075
CH3
OTBDMS
CH3
N
O
reflux/toluene
O
O
OMe
NH2
+
HO
2M LiClO4.Et2O
O
OEt
3
( )-4
CH3
CH3
N
MeO2C
I
MeO2C
HO
1) TsCl/py
KHMDS/THF
N
2) NaI/acetone
O
O
( )-5
( )-6
3% NOE
CH3
H
MeO2C
CH3
MeO2C
CH3
N
CH3
HO
HO
LAH/THF
+
+
N
N
N
O
1 : 2
O
1 : 2
( )-7
( )-8
( )-9
( )-10
O
1) H2NNH2.H2O
CH3
N
Cl
H2N
CH3
N
PhthN
N
H
1) phthalimide
2) 4-acetamido-5-chloro-
2-methoxy benzoic acid
CDI/DMF
Ph3P/DEAD/THF
2) chrom
OMe
( )-11
3) KOH
( )-2
Scheme 1. Synthesis of ( )-2.
Treatment of the tosylate with sodium iodide
under Finkelstein conditions afforded the primary io-
dide ( )-6.
References and notes
1. Flynn, D. L.;Zabrowski, D. L.;Becker, D. P.;Nosal, R.;
Villamil, C. I.;Gullikson, G. W.;Moummi, Ch.;Yang,
Dai-C. J. Med. Chem. 1992, 35, 1486.
A variety of bases were employed to effect closure of ( )-
6, with potassium hexamethyldisilazide giving the best
yield of ( )-7 and ( )-8 in a combined 70% yield. The
exo and endo methyl esters were isolated in a 1:2 ratio,
with the endo ( )-8 as the main component, as deter-
mined by NOE.14 Reduction with lithium aluminum
hydride gave a mixture of alcohols ( )-9 and ( )-10, and
this mixture was converted directly to the phthalimides
under Mitsunobu conditions, allowing chromatographic
isolation of the requisite exo isomer ( )-11 in 24% yield.
Deprotection of ( )-11 with hydrazine gave the free
amine in quantitative yield, which was coupled directly
with 5-acetamido-4-chloro-2-methoxybenzoic acid uti-
lizing carbonyl diimidazole to afford the desired benz-
amide. The acetamide was removed with potassium
hydroxide in ethanol under reflux to afford the desired
benzamide isomer ( )-2 in 66% yield.
2. Baxter, G. S.;Craig, D. A.;Clarke, D. E.
Schmiedeberg’s Arch. Pharmacol. 1991, 343, 439.
3. Dumuis, A.;Sebbon, M.;Bockaert, J. Naunyn Schmiede-
bergs Arch. Pharmacol. 1989, 340, 403.
4. (a) Berque-Bestel, I.;Soulier, J.-L.;Giner, M.;Rivail, L.;
Langlois, M.;Sicsic, S. J. Med. Chem. 2003, 46, 2606;(b)
Eglen, R. M.;Wong, E. H. F.;Dumuis, A.;Bockaert, J.
Trends Pharmacol. Sci. 1995, 16, 391;(c) Hedge, S.;Eglen,
R. FASEB J. 1996, 10, 1398.
Naunyn-
5. Langlois, M.;Fischmeister, R. J. Med. Chem. 2003, 46,
319.
6. Becker, D. P.;Goldstin, B.;Gullikson, G. W.;Loeffler, R.;
Moormann, A.;Moummi, C.;Nosal, R.;Spangler, D.;
Villamil, C. I.;Yang, D.-C.;Zabrowski, D. L.;Flynn,
D. L. Design and Synthesis of Agonists and Antagonists of
the Serotonin 5-HT4 Receptor Subtype. In Perspectives in
Receptor Research;Giardina, D. et al., Eds.; Perspectives in
Receptor Research. Pharmacochemistry Library;Elsevier
Science B.V.: Amsterdam, 1996;Vol. 24, pp 99–120.
7. Becker, D. P.;Nosal, R.;Villamil, C. I.;Gullikson, G.;
Moummi, C.;Yang, D.-C.;Flynn, D. L. Bioorg. Med.
Chem. Lett. 1997, 7, 2149.
8. (a) Becker, D. P.;Husa, R. K.;Moormann, A. E.;
Villamil, C. I.;Flynn, D. L. Tetrahedron 1999, 55, 11787;
(b) Becker, D. P.;Nosal, R.;Zabrowski, D. L.;Flynn, D.
L. Tetrahedron 1997, 53, 1;(c) Flynn, D. L.;Becker, D. P.;
Spangler, D. P.;Nosal, R.;Gullikson, G. W.;Moummi,
C.;Yang, D.-C. Bioorg. Med. Chem. Lett. 1992, 2, 1613.
9. SC-53116 was tested for mutagenic activity in a GLP study
using the Ames Salmonella/microsome assay with five
strains of Salmonella typhimurium (TA1535, TA100,
TA1538, TA98, and TA97) in the presence and absence
of a rat liver homogenate metabolic activation system (S9)
Thus ( )-2, the bridgehead-methyl analog of SC-53116,
was tested for agonist activity at the 5-HT4 receptor in
the rat tunica muscularis mucosae assay. The potency
for the compound was good, at 449 nM ( 185). How-
ever, this was approximately 7X less potent than SC-
49518, the racemate of SC-53116, which has an EC50 of
66 nM 11 nM. Due to the loss of potency, the bridge-
head methyl analog ( )-2 was not pursued further. The
azatricycle benzamide compounds that we developed7;8
are potent, efficacious, and safe, so we turned our
attention to those molecules. Specifically, SC-52491 was
negative in the Ames assay at the highest concentrations
tested, either with or without S9 activation.15

D. P. Becker et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3073–3075
3075
over test article concentrations ranging from 7.2 to 3600 lg/ 12. Heathcock, C. H.;Davidsen, S. K.;Hug, K. T.;Flippin, L.
plate. Significant test article-related increases of 4X in the
number of revertant colonies were observed in strain TA98
with activation at 3600 lg/plate. A 2–3X increase was
observed with activation in strain TA100 between 720 lg
and 3600 lg/plate, and a 2-6X increase was observed with
activation in strain TA1538 also between 720 lg and
3600 lg/plate. Significant increases in numbers of revertant
colonies were not observed in the test without S9 activation.
10. Burgess, L. E.;Meyers, A. I. J. Am. Chem. Soc. 1991, 113,
9858.
A. J. Org. Chem. 1986, 51, 3027.
13. Grieco, P. A.;Nunes, J. J.;Gaul, M. D. J. Am. Chem. Soc.
1990, 112, 4595.
14. Patricia Finnegan is gratefully acknowledged for help with
the NOE experiment.
15. SC-52491 was tested versus five strains of Salmonella
typhimurium (TA97, TA98, TA100, TA1535, and TA1538
in the presence and absence of rat liver homogenated
metabolic activation system (S9) over SC-52491 concen-
trations ranging from 50 to 7500 lg/plate. There was no
evidence of mutagenicity by SC-52491 in any of the strains
tested.
11. Wedler, C.;Schick, H.;Scharfenberg-Pfeiffer, D.;Reck, G.
Liebigs Ann. Chem. 1992, 29.