Synthesis and biological evaluation of phospholane and dihydrophosphole analogues of the glutamate receptor agonist AP4

Article abstract of DOI:10.1039/b204891d

The syntheses and preliminary pharmacological characterisation of two novel cyclic phosphinic acid-containing amino acids designed as conformationally restricted analogues of the metabotropic glutamate receptor agonist AP4 (1) are reported.

Full text of DOI:10.1039/b204891d

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Synthesis and biological evaluation of phospholane and dihydro-
phosphole analogues of the glutamate receptor agonist AP4†
a
a
b
c
Stuart J. Conway,‡ Jacqueline C. Miller, Andrew D. Bond, Barry P. Clark and
David E. Jane*
a
1
a
Department of Pharmacology, School of Medical Sciences, University of Bristol, Bristol,
UK BS8 1TD. E-mail: david.jane@bristol.ac.uk; Fax: ϩ44 117 927 9839;
Tel: ϩ44 117 9287639
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge,
UK CB2 1EW
b
c
Eli Lilly & Co., Erl Wood Manor, Windlesham, Surrey, UK GU20 6PH
Received (in Cambridge, UK) 20th May 2002, Accepted 6th June 2002
First published as an Advance Article on the web 18th June 2002
The syntheses and preliminary pharmacological charac-
terisation of two novel cyclic phosphinic acid-containing
amino acids designed as conformationally restricted
analogues of the metabotropic glutamate receptor agonist
AP4 (1) are reported.
Introduction
(
S)-Glutamate is the principal excitatory neurotransmitter in
the mammalian central nervous system. As such, it is involved
with a wide range of processes in both normal and dys-
functional synaptic function. (S)-2-Amino-4-phosphono-
1
butanoic acid (AP4) (1) (Fig. 1) is one of the most potent and
Scheme 1 Reagents and conditions: i) 2 equiv. 5, Et O, 3 h, Ϫ10 ЊC,
2
Fig. 1 AP4 (1), the phospholane analogue 2 and the dihydro-
phosphole analogue 3, with differing spacer units between the amino
acid moiety and the phosphinic acid and thus with a different degree of
rigidity.
79% yield; ii) AlCl , CH Cl , 1 h, RT; iii) buta-1,3-diene, CH Cl , 4 h,
3
2
2
2
2
Ϫ10 ЊC; iv) 0.2 M EDTA–sat. NaHCO 50 : 50, 4 h, Ϫ10 ЊC; steps ii–iv
3
overall yield 44%; v) 5% Pd on carbon, H , MeOH, 99% yield.
2
removed by hydrogenation to give 12 (Scheme 1) or epoxidised
to afford 16 (Scheme 2). To synthesise 2, the relative acidity
of the methylene protons adjacent to the phosphoramide
could be utilised allowing elaboration at this position by treat-
ment with n-butyllithium and quenching with allyl bromide
selective group III metabotropic glutamate (mGlu) receptor
agonists reported. Although AP4 is selective for group III
mGlu receptors over group I and group II, it is not selective for
1
receptor subtypes within group III. Conformationally
restricted analogues of AP4 (1) were designed in an attempt to
synthesise compounds with improved mGlu receptor subtype
selectivity. This principle has been used in the design of many
(
Scheme 3). The ring-opening of epoxide 16 with diethyl acet-
amidomalonate followed by acid hydrolysis could furnish 3
(Scheme 2).
1,2
analogues of glutamic acid and AP4 (1). The aim of this
project was to synthesise analogues of AP4 (1) that were con-
formationally restricted in a novel way by constraining the ter-
minal phosphonic acid group within a ring. A phospholane
analogue (2) and a dihydrophosphole analogue (3) were
designed (Fig. 1) with differing spacer units between the amino
acid and the phosphinic acid and thus with a different degree of
rigidity.
Discussion
5
Synthesis of 1-(diisopropylamino)-1-oxo-2,5-dihydro-1-ꢀ -
phosphole (11)
Diisopropylphosphoramidous dichloride (6) (Scheme 1) was
synthesised from phosphorus trichloride (4) and diisopropyl-
4
Retrosynthesis of 2 and 3 led us to believe that both could be
synthesised from the common intermediate 11. This could be
obtained in two steps via a cheletropic cycloaddition between
buta-1,3-diene and the phosphenium ion 9, in a manner similar
amine (5). The diisopropylphosphoramidous dichloride (6)
was treated with aluminium chloride to form the phosphenium
ion (9) (Scheme 1). A solution of buta-1,3-diene (7), at Ϫ10 ЊC,
was cannulated on to the phosphenium ion-containing
solution and this was stirred for 4 h at Ϫ10 ЊC. The cycloaddi-
tion formed the cyclic phosphoramidous monochloride
derivative 10 (Scheme 1), which was then treated with aqueous
sodium carbonate and EDTA. Purification afforded 1-(diiso-
3
to that reported by Polniaszek. The double bond could then be
†
Electronic supplementry information (ESI) available: mode of
epoxide ring-opening and experimental data for 2 and 3. See http://
www.rsc.org/suppdata/p1/b2/b204891d/
5
propylamino)-1-oxo-2,5-dihydro-1-λ -phosphole (11) as a white
DOI: 10.1039/b204891d
J. Chem. Soc., Perkin Trans. 1, 2002, 1625–1627
This journal is © The Royal Society of Chemistry 2002
1625

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5
afforded 2-[(2-amino-2-carboxy)ethyl]-1-hydroxy-1-oxo-1-λ -
phospholane (1) (13% yield from 13) as a mixture of racemic
diastereoisomers, which was purified using ion exchange resin
7
chromatography.
Synthesis of the dihydrophosphole analogue of AP4 (3)
Due to epoxide 16 being unsuitable for purification by silica gel
column chromatography, a method of epoxidation was required
in which all by-products could be removed by an aqueous work-
up. A solution to this problem was the use of sodium percar-
bonate as a source of hydrogen peroxide. Sodium percarbonate
has previously been used to generate trifluoroperoxyacetic acid
8
for use in the Baeyer–Villiger reaction. This methodology was
used to generate trifluoroperoxyacetic acid, which was then
used to effect the epoxidation of 1-(diisopropylamino)-1-oxo-
5
2
,5-dihydro-1-λ -phosphole (11).
The advantage of this method was that when the trifluoro-
peroxyacetic acid had reacted to form the epoxide and tri-
fluoroacetic acid, the sodium carbonate reacted to form a
salt that could be easily removed by washing with water.
Recrystallisation of the crude product from chloroform fur-
nished the pure epoxide (16) in 75% yield. The structure of 16
determined by X-ray crystallography§ (Fig. 2) shows that the
Scheme 2 Reagents and conditions: i) Na CO ؒ1.5 H O , trifluoro-
2
3
2
2
acetic anhydride, CH Cl , 1.5 h, RT, 75% yield; ii) a, diethyl acet-
2
2
amidomalonate, NaH, DMF; b, add epoxide, 24 h, RT–80 ЊC; iii) 6 M
HCl, sealed vessel, 24 h, 120 ЊC; 81% yield over ii and iii; all
stereochemistry shown is relative; R = Et or H depending on the
mechanism of epoxide opening.
Fig. 2 Molecular unit in epoxide 16 showing displacement ellipsoids
at 50% probability. H atoms are omitted for clarity. The trans
relationship between the epoxide oxygen and the diisopropylamino
group is clearly visible.
epoxide forms exclusively on the opposite face of the ring to
that of the N-diisopropylamino moiety.
Diethyl acetamidomalonate was treated with sodium hydride
to form the anion, which was then added to a solution of
5
1
-(diisopropylamino)-3,4-epoxy-1-oxo-1-λ -phospholane (16)
7
to give 17. Intermediate 17 was not isolated due to its high
polarity making it unsuitable for silica gel column chromato-
graphy, but from the H NMR spectrum of the crude product it
Scheme 3 Reagents and conditions: i) a, n-BuLi, THF, 1 h, Ϫ78 ЊC, b,
allyl bromide, THF, 1 h, Ϫ78 ЊC, 45% yield; ii) a, OsO , 1,4-dioxane :
4
1
H O (75 : 25), 30 min, RT; b, NaIO , 5 h, RT; EtOAc, 30 min, overall
2
4
was possible to see that a double bond was present, as there
were two corresponding peaks at δ 5.7–6.6 ppm. Intermediate
17 was hydrolysed with 6 M aqueous hydrochloric acid in an
acid digestion bomb (Parr Instruments, Illinois) for 24 h at
120 ЊC to yield crude (±)-4-(aminocarboxymethyl)-1-hydroxy-
crude yield 38%; iii) Bucherer–Bergs reaction: (NH ) CO , KCN, 50%
4
2
3
aqueous EtOH, sealed vessel, 80 ЊC; iv) 6 M HCl, sealed vessel, 24 h,
20 ЊC, 13% overall yield for ii–iv.
1
solid (44% yield from 6) which was hydrogenated to give
5
5
1
-(diisopropylamino)-1-oxo-1-λ -phospholane (12) as a white
1-oxo-4,5-dihydro-1-λ -phosphole (3) as a mixture of racemic
solid in 99% yield.
diastereoisomers. This was purified by ion exchange resin
7
chromatography to yield pure 3 (81% yield).
Synthesis of the phospholane analogue of AP4 (2)
Pharmacological data
5
Synthesis of 2-allyl-1-(diisopropylamino)-1-oxo-1-λ -phosphol-
ane (13) proceeded by reaction of 1-(diisopropylamino)-1-oxo-
Preliminary evaluations of the biological activities of 2 and 3
5
1
-λ -phospholane (12) with n-butyllithium and allyl bromide in
were carried out using the neonatal rat spinal cord prepar-
3
1,7,9
a manner similar to that used by Polniaszek (Scheme 3). This
was converted to aldehyde 14 using osmium tetraoxide to fur-
nish an intermediate diol, which was subsequently oxidised
ation.
Both 2 and 3 caused a depolarisation of moto-
neurones when applied to the hemisected neonatal rat spinal
cord in the presence of tetrodotoxin (TTX). This activity is
consistent with agonist action on ionotropic glutamate (iGlu)
receptors or group I mGlu receptors rather than group III
mGlu receptors. Using selective iGlu receptor antagonists,
2 and 3 were shown to act on AMPA receptors making it
difficult to determine whether they have action on group III
mGlu receptors using the neonatal rat spinal cord assay.
Consequently 2 and 3 are being assessed for their biological
5
using sodium periodate (Scheme 3). The aldehyde (14) was not
purified due to its high polarity making it unsuitable for silica
gel column chromatography. However, it could be easily identi-
1
fied by H NMR by the characteristic peak observed for the
aldehyde proton at δ 9.90 ppm.
Hydantoin 15 was formed from aldehyde 14 using a modifi-
6
cation of the Bucherer–Bergs reaction (Scheme 3). Hydrolysis
1
626
J. Chem. Soc., Perkin Trans. 1, 2002, 1625–1627

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activity on cloned mGlu receptors, to determine what activity,
if any, they possess.
Dr Richard Baker, Eli Lilly and Co., UK and Dr Jonathan W.
Burton, University of Cambridge, UK for insightful discus-
sions regarding the alternative mechanism of epoxide open-
ing. ADB thanks the EPSRC for funding and for financial
assistance towards the purchase of a Nonius KappaCCD
diffractometer.
As both 2 and 3 were tested as a mixture of racemic diastereo-
mers it is possible that the AMPA receptor activity resides in
one isomer and that another isomer has activity on group III
mGlu receptors. Work is ongoing to synthesise the separate
isomers of 2 and 3 in order to determine their activity on
individual mGlu receptor subtypes.
References
Conclusion
‡
Current address: Department of Chemistry, University of Cambridge,
Lensfield Road, Cambridge, UK CB2 1EW.
CCDC reference number 186273. See http://www.rsc.org/suppdata/
Our aim was to synthesise analogues of AP4 where the terminal
phosphono group was constrained as part of a ring. This
objective was achieved, although the compounds that were syn-
thesised display agonist activity on AMPA receptors making
assessment of their activity on native group III mGlu receptors
difficult. Work is ongoing to synthesise the individual stereo-
isomers in order to determine their biological activity.
§
p1/b2/b204891d/ for crystallographic files in .cif or other electronic
format.
1
2
D. D. Schoepp, D. E. Jane and J. A. Monn, Neuropharmacology,
1
999, 38, 1431.
S. J. Conway, J. C. Miller, P. A. Howson, B. P. Clark and D. E. Jane,
We have developed a useful synthesis of amino acids that
contain phosphorus heterocycles. This can now be exploited in
the synthesis of analogues of 2 and 3, which will help to
develop a structure–activity profile for these compounds across
a range of glutamate receptor subtypes.
Bioorg. Med. Chem. Lett., 2001, 11, 777.
3 R. P. Polniaszek, J. Org. Chem., 1992, 57, 5189.
4 T. Tanaka, S. Tamatsukuri and M. Ikehara, Tetrahedron Lett., 1986,
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H. T. Bucherer and V. A. Libe, J. Prakt. Chem., 1934, 141, 5.
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Acknowledgements
7 See supplementary information for more details.
H-.J. Kang and H-.S. Jeong, Bull. Korean Chem. Soc., 1996, 17, 5.
9 R. H. Evans, A. A. Francis, A. W. Jones, D. A. S. Smith and
8
We would like to thank the BBSRC (JCM) and Eli Lilly and
Co. (SJC, JCM) for funding this work. We are grateful to
J. C. Watkins, Br. J. Pharmacol., 1982, 75, 65.
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