(2S,4S)-2-amino-4-(4,4-diphenylbut-1-yl)-pentane-1,5-dioic acid: A potent and selective antagonist for metabotropic glutamate receptors negatively linked to adenylate cyclase

Full text of DOI:10.1021/jm9508144

814
J . Med. Chem. 1996, 39, 814-816
(2S,4S)-2-Am in o-4-(4,4-d ip h en ylbu t-1-yl)-
p en ta n e-1,5-d ioic Acid : A P oten t a n d
Selective An ta gon ist for Meta botr op ic
Glu ta m a te Recep tor s Nega tively Lin k ed
to Ad en yla te Cycla se
Camille G. Wermuth,† Andre´ Mann,†
Ange`le Schoenfelder,† Rebecca A. Wright,‡
Bryan G. J ohnson,‡ J . Paul Burnett,‡
Nancy G. Mayne,‡ and Darryle D. Schoepp*,‡
UPR 421, Centre de Neurochimie du CNRS,
Strasbourg, France, and Lilly Research Laboratories, A
Division of Eli Lilly and Company, Lilly Corporate Center,
Indianapolis, Indiana 46285
F igu r e 1. Displacement of ligand binding to glutamate
receptors in membranes of the rat forebrain by compound 2.
Data represent mean ( SE of three experiments performed
in triplicate.
Received November 2, 1995
Metabotropic glutamate receptors (mGluRs) are a
heterogeneous and novel family of G-protein linked
receptors that couple to multiple second messengers,
including inhibition of adenylate cyclase, activation of
phosphoinositide-specific phospholipase C, and modula-
tion of ion channels currents.1-4 Subtypes of mGluRs
have been cloned and fall into three groups. Group 1
mGluRs couple to phosphoinositide hydrolysis when
expressed and include mGluR1 (R, â, and c splice
variants) and mGluR5 (a and b splice variants). Group
2 mGluRs are coupled to inhibition of cyclic adenosine
5′-monophosphate (cAMP) formation and include mGluR2
and mGluR3. Both group 1 and group 2 mGluRs can
be selectively activated by compound (1S,3R)-aminocy-
clopentane-1,3-dicarboxylic acid (ACPD). Group 3
mGluRs (mGluR4, mGluR6, mGluR7, and mGluR8) also
couple negatively to cAMP. But unlike the others, group
3 mGluRs are insensitive to ACPD and potently acti-
vated by L-2-amino-4-phosphonobutanoic acid (L-AP4).
Studies of metabotropic glutamate receptor function
are greatly hampered by the lack of potent and selective
antagonists. The first antagonist described for mGluRs
was L-2-amino-3-phosphonopropanoic acid (L-AP3). L-
AP3 will block ACPD-stimulated phosphoinositide hy-
drolysis in brain slices5 and cells expressing group 1
mGluRs,6 but requires high micromolar to millimolar
concentrations. A series of phenylglycine derivatives
which act as competitive agonists/antagonists have more
recently been described.7 The most accepted mGluR
antagonist of this series appears to be (+)-R-methyl-4-
carboxyphenylglycine (MCPG). However, the useful-
ness of MCPG is limited by its low potency (high micro-
to millimolar concentrations are required) and lack of
selectivity for group 1 versus group 2 mGluRs.8
F igu r e 2. Concentration-dependent reversal by compounds
2 (panel A) and 3 (panel B) of ACPD (20 µM) inhibition of
forskolin (15 µM)-stimulated cAMP formation in human
mGluR2 expressing non-neuronal cells. Data are mean ( SE
of three experiments performed in triplicate. An asterisk (*)
indicates significantly different when compared to the ACPD
control (p < 0.05, Duncan’s Procedure).
In this paper we provide preliminary studies on a
structurally novel and highly stereoselective antagonist
for mGluRs which binds selectively to mGluRs at low
micromolar concentrations and potently antagonized a
group 2 mGluR (human mGluR2), but with no effects
on group 1 mGluRs (human mGluR1R and mGluR5a).
Ch em istr y. As shown in Scheme 1, the synthesis of
compound 1 began with the known aldehyde 4, which
was obtained from (R)-serine.9 The Wittig-Horner
reaction of 4 and triethyl phosphonoacetate under protic
conditions gave a 95:5 mixture of the corresponding E
and Z enoates 5, which were reduced via catalytic
hydrogenation to ester 6. After some experimentation
it was found that alkylation next to the carboxylate with
4,4-diphenyl-1-bromobutane10 occurred smoothly in THF/
HMPT, with 2 equiv of LDA, and gave rise to 7 as an
inseparable mixture of diastereomers. To get the fully
protected glutamate 9, the restoration of the amino acid
function was performed as follows:11 (i) the oxazolidine
ring was opened in refluxing ethanol in the presence of
* Address correspondence to: CNS Research, drop 0510, Eli Lilly
and Company, Lilly Corporate Center, Indianapolis, IN 46285. Phone
(317) 276-6316. FAX (317) 276-5546. E-mail address: SCHOEPP
DARRYLE D@Lilly.Com.
† Centre de Neurochimie du CNRS.
‡ Lilly Research Laboratories.
0022-2623/96/1839-0814$12.00/0 © 1996 American Chemical Society

Communications to the Editor
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 4 815
Sch em e 1a
a
Reagents, conditions, and yields: (i) (EtO)2POCH2CO2Et, aqueous K2CO3 (3M), n-Bu4N+I-, 12 h (88%); (ii) H2, Pd/C (100%); (iii)
LDA, THF/HMPT, 10/1, -78 °C, then Br(CH2)3CHPh2 (91%); (iv) p-PTOH, EtOH (63%); (v) pyridinium dichromate, DMF, 12 h, then
CH2N2 in Et2O (65% for two steps); (vi) LiOH in dimethoxyethane, then HCl(g) in AcOEt, 12 h (56%).
Sch em e 2a
a
Reagents, conditions, and yields: (i) TFA, then K2CO3, then toluene, 110 °C; then Boc2O, TEA, DMAP, CH2Cl2 (80%); (ii) Boc2O,
TEA, DMAP, CH2Cl2 (80%), then separation by column chromatography of the mixture of 11 + 12 (40% for 11, 35% for 12); (iii) LiOH in
dimethoxyethane, then AcOH in HCl (86% for 2, 75% for 3).
pPTSA to afford alcohol 8; (ii) the primary alcohol was
oxidized to the carboxylic acid using PDC in DMF; and
(iii) diazomethane esterification was used to produce the
methyl ester 9. Finally, compound 1 was obtained as a
mixture of epimers at carbon C-4 after hydrolysis.
In order to obtain the two enantiomerically pure
compounds 2 and 3 the following sequence was devel-
oped (Scheme 2). The Boc protection of compound 9 was
removed by a brief treatment with TFA. The corre-
sponding base 10 was lactamized in refluxing toluene.
At this stage the two epimeric lactams could not be
separated by column chromatography. The nitrogen
was Boc-protected, and only then a separable mixture
of 11 and 12 was obtained. The assignment of their
relative stereochemistry, respectively cis for 11 and
trans for 12, was performed by NOE experiments and
correlation with literature data.12,13 Finally, removal
of the N- and O-protecting groups, under successively
basic and acidic conditions, gave the fully deprotected
γ-glutamates 2 (2S,4S) and 3 (2S,4R) with diastereomer
excesses greater than 95% as determined by HPLC.14
Resu lts. Amino acid 1, as a mixture of 2S,4S and
2S,4R diastereomers, was initially evaluated for its
ability to displace metabotropic glutamate receptor
binding to rat brain membranes (as measured by
displacement of ACPD-sensitive [3H]glutamate bind-
ing).15 1 was relatively potent in displacing this binding
with an IC50 ) 4.2 ( 1.2 µM (n ) 3). The metabotropic
glutamate receptor binding affinity of 1 (Figure 1)
resided in the 2S,4S isomer (2, IC50 ) 4.1 ( 1.7 µM),
with the 2S,4R isomer (3) being much less potent (IC50
) 55.3 ( 10.6 µM, n ) 3). 2 was also evaluated for
affinity at ionotropic glutamate receptors (NMDA,
AMPA, and kainate) using radioligand binding assays.
No appreciable displacement of [3H]AMPA,16 [3H]kain-
ate,17 or [3H]CGS 19755 (NMDA receptor ligand)18
binding was observed at concentrations which displaced
ACPD-sensitive [3H]glutamate binding (IC50s >100 µM)
(Figure 1). 2 and 3 were each evaluated in a functional
assay of a group 2 mGluR. Both compounds produced
concentration-dependent reversal of ACPD-induced in-
hibition of forskolin-stimulated cAMP formation in
human mGluR2 expressing non-neuronal cells.19. The
2S,4S isomer (2) fully reversed ACPD at a concentration
of 50 µM with an IC50 ) 18.1 ( 1.7 µM (Figure 2). The
2S,4R isomer (3) was much less potent (IC50 ) 171 ( 9
µM), and 500 µM was needed to fully reverse ACPD.
Compound 2 was further examined for activity at
phosphoinositide coupled mGluRs using cells expressing
human mGluR1R and human mGluR5a receptors.20,21
When tested in either cell line, compound 2 (tested at
300 µM) did not stimulate phosphoinositide hydrolysis
or significantly antagonize stimulation by ACPD (100
µM) (Figure 3).
Discu ssion . The initial activity of 1 was discovered
by the observation that this compound displaced [3H]-
glutamate binding to membranes of the rat forebrain.
This assay was performed under conditions where [3H]-
glutamate binding is ACPD-sensitive, but insensitive
to the mGluR1 and -5 (group 1) agonist quisqualate or

816 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 4
Communications to the Editor
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails for the compounds in this paper (6 pages). Ordering
information is given on any current masthead page.
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