Synthesis and pharmacology of new enantiopure Δ3-4-arylkainoids

Article abstract of DOI:10.1016/S0960-894X(00)00093-7

Seven Δ³-4-arylkainoids possessing various 4-position aromatic and heteroaromatic groups were synthesized and their apparent affinities were measured in order to explore the influences of 4-position electron density and stereochemistry on receptor affinity and specificity. Kainoids 1a-f were shown to be selective agonists at the NMDA receptor and the electron rich furanyl and thienyl analogues exhibited the highest affinities. Naphthylkainoid 1g proved to be a nonselective antagonist at the iGluRs. (C) 2000 Elsevier Science Ltd. All rights reserved.

Full text of DOI:10.1016/S0960-894X(00)00093-7

Bioorganic & Medicinal Chemistry Letters 10 (2000) 771±773
Synthesis and Pharmacology of New Enantiopure
3
D-4-Arylkainoids
Dany Rondeau, a Patrice Gill, a Monique Chan, a Kenneth Curry b
and William D. Lubell a,*
a
    Â
Departement de chimie, Universite de Montreal C.P. 6128, Succursale Centre Ville, Montreal, Quebec, Canada H3C 3J7
bCNS Research Division, IGT Pharma Inc. 311-2386 East Mall, Vancouver, British Columbia, Canada V6T 1Z3
Received 3 September 1999; accepted 4 February 2000
AbstractÐSeven Á3-4-arylkainoids possessing various 4-position aromatic and heteroaromatic groups were synthesized and their
apparent anities were measured in order to explore the in¯uences of 4-position electron density and stereochemistry on receptor
anity and speci®city. Kainoids 1a±f were shown to be selective agonists at the NMDA receptor and the electron rich furanyl and
thienyl analogues exhibited the highest anities. Naphthylkainoid 1g proved to be a nonselective antagonist at the iGluRs. # 2000
Elsevier Science Ltd. All rights reserved.
Introduction
kainoids relative to kainic acid has led to the suggestion
that ``the excitatory activities of the kainoids depend on
the height of HOMO (highest occupied molecular orbi-
tal) energies of the p-systems existing in the substituents
at C4''.11 Finally, the natural kainic acid ring stereo-
chemistry manifests maximal activity and inversion
of the 4-position results in a more dramatic reduction
of activity than inversion of the 3-position stereo-
chemistry.11
Glutamic acid mediates synaptic transmission in the
mammalian central nervous system at two classes of
receptors: the ionotropic glutamate receptors (iGluRs)
and the metabotropic glutamate receptors (mGluRs).1,2
Glutamate receptor ligands that act as selective agonists
(Fig. 1) have been used to de®ne three subtypes of
ionotropic receptors: kainate, AMPA and NMDA.3 5
Five kainate receptors have also been identi®ed: KA,
KA1, GluR5, GluR6, and GluR7.6 Biological evalua-
tion of natural and synthetic ligands has shown that
kainoid neuroexcitatory activity is contingent on the
combination of a glutamate-like structure, the nature of
the C-4 substituent and the ring stereochemistry.5,7,8
For example, trans-carboxy-3-pyrrolidineacetic acid
possesses the Glu core structure characteristic of kainic
acid and exhibits agonist activity at kainate and NMDA
receptor subtypes.9 p-Electron density at the C-4 posi-
tion is essential for excitatory activity as demonstrated
by the complete loss of neurotoxicity upon reduction of
the isopropenyl double bond of kainic acid.10 In addi-
tion, the naturally occuring kainoids, domoic acid,
and acromelic acids A and B exhibit potent neuroexci-
tatory activity.5 Furthermore, the higher depolarizing
activity exhibited by 4-o-methoxy- and 4-o-hydroxyphenyl
In this report, seven novel Á3-4-arylkainoids12 1a±g
were synthesized and their apparent anities were
measured in order to explore further the in¯uences of
the 4-position electron density and stereochemistry on
receptor anity and speci®city. The synthesis and
examination of Á3-4-arylkainoids possessing electron
rich heteroaromatic 4-position substituents has pro-
vided additional evidence to suggest that increased
electron density at the p-system augments receptor a-
nity. The in¯uence of the Á3-unsaturation on receptor
speci®city has also illustrated the importance of struc-
ture on receptor selectivity.13,14
Results and Discussion
Enantiopure (2S)-Á3-4-arylkainoids 1a±g were synthe-
sized by Pd(0)-catalyzed cross-couplings of di€erent
arylboronic acids15 17 to (2S)-benzyl-3-methoxycarbonyl-
methyl-Á3-4-tri¯yloxy-N-(PhF)prolinate (2) that was
conveniently prepared by a ®ve step sequence in 64%
*Corresponding author. Tel.: +1-514-343-7339; fax: +1-514-343-
7586; e-mail: lubell@ere.unmontreal.ca
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00093-7

772
D. Rondeau et al. / Bioorg. Med. Chem. Lett. 10 (2000) 771±773
Figure 1. Representative neuroexcitatory amino acids.
overall yield from (2S,4R)-hydroxyproline as an inex-
pensive chiral educt (Scheme 1, Table 1, PhF=9-(9-
phenyl¯uorenyl)).12,18 (2S)-Benzyl-3-methoxycarbonyl-
methyl-Á3-4-aryl-N-(PhF)prolinates 3a±g were obtained
in 72±99% yields by heating 2 and the respective bor-
onate with 10 mol% of Pd(PPh3)4 and LiCl (300 mol%)
in a two phase system of toluene and 2 M Na2CO3, fol-
lowed by puri®cation by silica gel chromatography.
Deprotection of 3 was accomplished in two steps. First,
(2S)-Á3-4-aryl-N-(PhF)kainoids 4a±g were isolated in
71±99% yields from ester hydrolysis on heating 3 in a
1:1 solution of 3 M NaOH: dioxane and subsequent
chromatography. Finally, (2S)-Á3-4-arylkainoids 1a±g19
were isolated in 71±99% yields as solids after PhF group
solvolysis on stirring a dilute solution of 4 in a mixture
of TFA and anisole in CH2Cl2 at rt, followed by
removal of the hydrocarbon impurities by trituration
with hexane. Arylkainoids 1 are presumed to be of
>99% enantiomeric purity based on an analysis invol-
ving conversion of (2S)-Á3-4-phenylkainic acid 1a into
(S)- and (R)-N-a-methylbenzylurea dimethyl esters and
observation of the diastereomeric methyl ester singlets
incremental additions of the opposite isomer which
demonstrated a >99% de.12
Compounds 1a±g were tested on the rat cortical wedge
preparation by a method modi®ed from Simmons and
Harrison.20 Á3-4-Arylkainoids 1a±f exhibited weak
agonist activity at the NMDA receptor subtype. The
introduction of the electron-rich thienyl and furanyl
heteroaromatic 4-position substituents in 1c±f resulted
in a signi®cant increase in the excitatory e€ect in com-
parison with aromatic analogues 1a and 1b. The 2-fur-
anyl and 2-thienyl analogues were respectively more
potent that their 3-substituted counterparts. On the
other hand, the increased size of the 4-position sub-
stituent led to antagonistic activity, as 1g proved to
be a nonselective antagonist at ionotropic glutamate
receptors.
Conclusion
Enantiopure Á3-4-arylkainoids 1a±g were synthesized in
28% to 42% overall yields from (2S, 4R)-hydroxyproline.
1
by 300 MHz H NMR spectroscopy in C6D6 during
Scheme 1. Synthesis of (2S)-Á3-4-Arylkainoids 1a±g.
Table 1. Synthesis yields and anity of Á3-4-arylkainoids 1a±g19
Ar=
R1
R2
R3
A
b
c
d
e
f
g
% Yield 3
4
1
PhF
PhF
H
Me
H
Bn
H
H
92
99
82
560
82
88
76
380
80
87
78
20
93
81
99
45
99
78
72
70
72
88
95
85
99
71
71
250
H
Apparent KD (mM)a
aNMDA receptor.

D. Rondeau et al. / Bioorg. Med. Chem. Lett. 10 (2000) 771±773
773
The presence of the Á3-double bond altered the receptor
selectivity such that 1a±f bound to the NMDA receptor
subtype. Increased receptor anity was observed in the
4-thienyl and furanyl analogues relative to their phenyl
and methoxyphenyl counterparts indicating that greater
electron density at the 4-position substituent increases
receptor anity. The combination of the Á3-unsatura-
tion and naphthyl 4-position substituent led to a non-
selective antagonist. Because similar relative anities
may be obtained at the kainate receptor on saturation
of the Á3-double bond, we are now examining hydro-
genation conditions to a€ord new arylkainoids.
10. Ishida, M.; Shinozaki, H. Br. J. Pharmacol. 1991, 104, 873.
11. Hashimoto, K.; Horikawa, M.; Ishida, M.; Shinozaki, H.;
Shirahama, H. Bioorg. Med. Chem. Lett. 1992, 2, 743.
12. Gill, P.; Lubell, W. D. J. Org. Chem. 1995, 60, 2658.
13. (a) Shimamoto, K.; Ohfune, Y. J. Med. Chem. 1996, 39,
407. (b) Shimamoto, K.; Shigeri, Y.; Nakajima, T.; Yumoto,
N.; Yoshikawa, S.; Ohfune, Y. Bioorg. Med. Chem. Lett. 1996,
6, 2381.
14. Sonnenberg, J. D.; Koch, H. P.; Willis, C. L.; Bradbury,
F.; Dauenhauer, D.; Bridges, R. J.; Chamberlin, A. R. Bioorg.
Med. Chem. Lett. 1996, 6, 1607.
15. Thompson, W. J.; Gaudino, J. J. J. Org. Chem. 1984, 49,
5237. Furanyl and thienyl boronic acids were synthesized by
the method for furanyl boronic acids.
16. Diorazio, L. J.; Widdowson, D. A.; Clough, J. M. Tetra-
hedron 1992, 48, 8073. 2-Naphthyl boronic acid was prepared
as described.
References and Notes
1. Hollman, M.; Heinemann, S. Annu. Rev. Neurosci. 1994, 17,
31.
2. Reviewed recently in: Moloney, M. V. Nat. Prod. Rep. 1998,
15, 205.
3. Excitatory amino acid related agents. Bioorg. Med. Chem.
Lett., Johnson, G., Ed., 1993, 3, 9.
4. Watkins, J. C.; Krogsgaard-Larsen, P.; Honore, T. Trends
Pharm. Sci. 1990, 11, 25.
17. Morgan, J.; Pinhey, J. T. J. Chem. Soc. Perkin Trans. 1
1990, 3, 715. o-Methoxyphenyl boronic acid was prepared as
described.
18. Sharma, R.; Lubell, W. D. J. Org. Chem. 1996, 61, 202.
19. Á3-Pyrrolidines 1±4 exhibited correct spectroscopic and
1
spectrometric data as ascertained by H and 13C NMR spec-
troscopy and HRMS.
20. Harrison N. L.; Simmonds, M. A. Br. J. Pharmacol. 1985,
84, 381. Male Wistar rats were decapitated following CO2
narcosis. The brains were removed to ice cold bu€er at pH 7.4
and mounted on a vibratome platform. The brains were sliced
in 500 mm sections and then further hemisected to give wedge
shaped slices. These were placed in a grease seal chamber such
that the cortical side was separated from the callosal by the
seal. The cortex was perfused with bu€er at rt while the other
side was kept in bu€er containing 2 mM Mg2+. DC ®eld
potentials were recorded from each side using Ag/AgCl elec-
trodes and the di€erence ampli®ed and recorded for analysis.
Apparent KD values were obtained from dose±response curves
from at least six points.
5. Hashimoto, K.; Shirahama, H. Trends Org. Chem. 1991, 2,
1.
6. Cantrell, B. E.; Zimmerman, D. M.; Monn, J. A.; Kamboj,
R. K.; Hoo, K. H.; Tizzano, J. P.; Pullar, I. A.; Farrell, L. N.;
Bleakman, D. J. Med. Chem. 1996, 39, 3617.
7. Hashimoto, K.; Ohfune, Y.; Shirahama, H. Tetrahedron
Lett. 1995, 36, 6235.
8. For reviews see: (a) Parsons, A. F. Tetrahedron 1996, 52,
4149. (b) Bleakman, D.; Lodge, D. Neuropharmacology 1998,
37, 1187.
9. Tsai, C.; Schneider, J. A.; Lehmann, J. Neurosci. Lett. 1988,
92, 298.