(S)-(+)-2-(3'-carboxybicyclo[1.1.1]pentyl)-glycine, a structurally new group I metabotropic glutamate receptor antagonist

Full text of DOI:10.1021/jm960254o

2874
J . Med. Chem. 1996, 39, 2874-2876
Ch a r t 1
(S)-(+)-2-(3′-Ca r boxybicyclo[1.1.1]p en tyl)-
glycin e, a Str u ctu r a lly New Gr ou p I
Meta botr op ic Glu ta m a te Recep tor
An ta gon ist
Roberto Pellicciari,*,† Mariarosa Raimondo,†
Maura Marinozzi,† Benedetto Natalini,†
Gabriele Costantino,† and Christian Thomsen‡
Istituto di Chimica e Tecnologia del Farmaco,
Universita` di Perugia, Via del Liceo, 1, 06123 Perugia, Italy
and Health Care Discovery, Novo Nordisk A/ S,
Novo Nordisk Park, DK-2760 Måløv, Denmark
Received April 1, 1996
In tr od u ction . Metabotropic glutamate receptors
(mGluRs) are involved in important central nervous
system (CNS) functions such as long term potentiation
(LTP) and long term depression (LTD) of synaptic
transmission and have attracted considerable attention
because of their therapeutic potential for the treatment
of a range of CNS disorders such as stroke, ischemia,
head trauma, and Alzheimer’s disease.1
Sch em e 1a
Cloning and expression studies2 carried out so far
have shown that this receptor family is composed of
eight mGluR subtypes, further subdivided in three
groups according to sequence homology, pharmacology,
and signal transduction mechanisms. When expressed
in transfected cells, group I mGluRs (mGluR1 and
mGluR5) use inositol triphosphate/diacylglycerol as
second messengers, while group II (mGluR2 and
mGluR3) as well as group III (mGluR4, mGluR6,
mGluR7, and mGluR8) are negatively coupled to the
activity of adenylyl cyclase.
Among the ligands which have been utilized for the
pharmacological and physiological characterization of
mGluRs, an important role has been played by (carboxy-
phenyl)glycine (CPG) derivatives, first reported by
Watkins et al. in 1993.3 Among the members of this
class, which are known to produce different and often
opposite effects on signal transduction processes, (S)-
(4-carboxyphenyl)glycine [(S)-4CPG, 1], (S)-(4-carboxy-
3-hydroxyphenyl)glycine [(S)-4C3HPG, 2], and (+)-R-
methyl(4-carboxyphenyl)glycine (M4CPG, 3) have ex-
hibited interesting properties as mGluR1 antagonists
albeit with activity also at mGluR2 receptor subtypes.4
With the aim of finding new more potent and selective
mGluR antagonists, we have addressed ourselves to the
task of defining relevant structural features of this class
of compounds. In this connection, we have previously
reported that (()-1-aminoindan-1,5-dicarboxylic acid
(UPF 523, 4),5 in which the (carboxyphenyl)glycine core
is inserted into a partially rigidified bicyclic structure,
is endowed with an increased potency and selectivity
as a mGluR1 subtype antagonist compared with (S)-
4CPG (1) and M4CPG (3). The pharmacological profile
of UPF 523 (4) demonstrates that conformational tuning
is required in CPGs for achieving selectivity toward
particular mGluR subtypes. In the present paper we
address the still unexplored role played by the phenyl
moiety of CPGs. Thus, while the coplanarity between
the R-amino acidic and the ω-carboxy functionalities is
generally accepted to be a crucial feature of the CPG
a
(a) NaOH, CHBr3, Me4NBr, EtOH, CH2Cl2, rt; (b) i. MeLi,
pentane, -78 °C, ii. biacetyl, hν, 0 °C; (c) Br2, NaOH, 0 °C, then
50 °C; (d) i. SOCl2, reflux, ii. MeOH, refllux; (e) NaOH, MeOH,
reflux; (f) ClCO2iBu, 4-Me-morpholine, THF, -10 °C, then rt; (g)
NaBH4, rt; (h) PCC, CH2Cl2, rt; (m) i. (R)-2-phenylglycinol, MeOH,
rt, ii. TMSCN, 0 °C, then rt, iii. flash chromatography; (n) i.
Pb(OAc)4, CH2Cl2-MeOH (1:1, v/v), 0 °C, ii. 6, N HCl, reflux, iii.
Dowex 50X2-200, 10% Py.
class of compounds, no reports dealing with the specific
role of the aromatic moiety have yet appeared. With
the aim of clarifying this point, we now report the
synthesis and preliminary biological evaluation of
(S)-(+)- and (R)-(-)-2-(3′-carboxybyciclo[1.1.1]pentyl)-
glycine (CBPG). The bicyclo[1.1.1]pentane moiety, which
serves as a spacer in these compounds, has a different
stereoelectronic profile to the phenyl ring but, analo-
gously to the latter, is able to keep the ω-carboxylate
and the R-amino acidic moieties in the coplanar disposi-
tion crucial for activity.
Ch em ist r y. The preparation of (S)-(+)- and
(R)-(-)-2-(3′-carboxybicyclo[1.1.1]pentyl)glycine (16 and
17) involves the key intermediate bicyclo[1.1.1]pentane-
1,3-dicarboxylic acid (8; Scheme 1), obtained essentially
according to the procedure developed by Michl.6 Ac-
cordingly, the bis(dibromocarbene) adduct 6, obtained
† Universita` di Perugia.
‡ Novo Nordisk A/S.
S0022-2623(96)00254-3 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 15 2875
Ta ble 1. Effects of 16 and 17 on Second-Messenger Formation
in BHK Cells Expressing mGluR1a, mGluR2, mGluR4a, or
mGluR5a
(S)-(+)-CBPG (16)
(R)-(-)-CBPG (17)
EC50 (µM)
IC50 (µM)
EC50 (µM)
IC50 (µM)
mGluR1a
mGluR2
mGluR4a
mGluR5a
>300
25 ( 6
>300
>300
>300
>300
>300
>300
>300
>300
>300
>300
>300
>300
>300
>300a
(103 ( 33)
a
16 stimulated basal PI hydrolysis in BHK cells expressing
mGluR5a, but the half-maximal concentration was not reached
at a 300 µM concentration. The calculated maximal response to
16 was 54 ( 4% of the maximal response to glutamate, and the
half-maximal concentration of 16 for obtaining this effect was 103
( 33 µM. Measurements of PI hydrolysis (mGluR1a and mGluR5a)
or cAMP formation (mGluR2 and mGluR4a) were performed as
described in the methods section, and test compounds were applied
alone (agonist effect) or prior to glutamate (antagonist effect) (10
µM for mGluR1a, 5 µM for mGluR5a, and 50 µM for mGluR2 and
mGluR4a) to cells expressing individual mGluR subtypes. The
values are the mean of two to four experiments which were
performed in duplicate or triplicate.
F igu r e 1. Dose-response curves for stimulation of PI hy-
drolysis in BHK cells expressing mGluR1a as stimulated by
glutamate alone (O) or by glutamate in the presence of 100
µM (S)-(+)-CBPG (16) (b). Values are mean ( SEM of two
individual experiments performed in duplicate (basal PI
hydrolysis 4650 ( 350 dpm/mg of protein). In the presence of
100 µM (S)-(+)-CBPG (16), the levels of PI hydrolysis were
4710 ( 460 dpm/mg of protein.
hydrolysis in mGluR1a cells as stimulated by 10 µM
glutamate (to 103 ( 2% of the basal levels; see Figure
1). Furthermore, the dose-response curve for glutamate
was shifted to the right by 100 µM (S)-(+)-CBPG (16)
in a parallel manner, suggesting that 16 is a competitive
antagonist of mGluR1a. Inactive in concentrations up
to 300 µM at mGluR2 and mGluR4, 16 was a weak
partial agonist at mGluR5, showing a maximal effect
which was about one-half of that obtained with a
concentration of glutamate (30 µM) which was optimal
for stimulating PI hydrolysis in these cells. The present
results indicate that this compound is a new potent
mGluR1 antagonist with mixed activity as a mGluR5
partial agonist. (S)-(+)-CBPG (16) was selective for
mGluRs over ionotropic glutamate receptors since (S)-
(+)-CBPG (16) did not displace R-amino-3-hydroxy-5-
methylisoxazole-4-propionate (AMPA), kainate (KA), or
[3-(2-carboxypiperazin-4-yl)prop-1-ylphosphonic acid
(CPP, a competitive NMDA antagonist) binding from
rat cerebral cortical membranes at concentrations up
to 300 µM (data not shown).
(Carboxyphenyl)glycine derivatives such as (S)-4CPG
(1) or (S)-4C3HPG (2) also show a mixed activity as
mGluR1 antagonists and mGluR2 agonists, but it
should be pointed out that (S)-(+)-CBPG (16) is the first
example of a mixed antagonist/partial agonist at mem-
bers of the same group (group I) of mGluRs. Since the
first reports by Watkins et al.,3 (carboxyphenyl)glycines
have been widely used as lead structures on route
toward more potent and selective mGluR antagonists.
Structural elaborations on the (carboxyphenyl)glycine
skeleton have included R-alkylation of the amino acidic
moiety,11 homologation and bioisosteric replacement of
the distal carboxy moiety,4d and conformational con-
straining of the R-amino acidic side chain.5a The
interesting pharmacological properties displayed by 16
broaden the existing structure-activity relationship. In
particular, two interesting features emerge from our
results. First, the presence of an aromatic moiety is not
a requirement for mGluR1 antagonism, providing that
a suitable spacer is able to dispose the pharmacophoric
groups in a well-defined, coplanar orientation. This
implies that the phenyl moiety of (carboxyphenyl)glycine
derivatives neither is involved in specific ligand-
receptor interactions nor plays a role in dispersing the
from 5, undergoes ring closure by treatment with
methyllithium to give [1.1.1]propellane6 which is then
submitted to reaction with biacetyl, as the acetyl radical
source, followed by sodium hypobromite oxidation of the
1,3-diacetylbicyclo[1.1.1]pentane (7) thus obtained. Es-
terification of 8 via the acyl chloride afforded the diester
9 which was hydrolyzed to the monoester 10.6 This
compound was converted to alcohol 12 by sodium
borohydride reduction of the mixed anhydride 11, and
PCC oxidation of 12 provided the aldehyde 13 in 16%
overall yield from 9. Aldehyde 13 was then reacted with
(R)-R-phenylglycinol (MeOH, room temperature, 2 h)7
followed by treatment of the resulting Schiff base with
TMSCN for 10 h to give a 2:1 (1H-NMR) mixture of the
two expected R-aminonitrile derivatives 14 and 15 (34%)
which were separated by medium pressure chromatog-
raphy (MPC) in 22% and 12% yields, respectively.
It is known8 that when employed in diastereoselective
Strecker synthesis, the inductive effect of chiral R-phen-
ylglycinols is such that the newly formed chiral center
is opposite in sign to that of the R-phenylglycinol
employed. Accordingly, an S configuration was tenta-
tively assigned to the more abundant product, the
N-substituted R-aminonitrile 14, and R configuration
to the less abundant isomer, the N-substituted R-ami-
nonitrile 15. The two R-aminonitriles 14 and 15 were
finally submitted to oxidative cleavage with lead tet-
raacetate,9 acidic (6 N HCl) hydrolysis, and ion exchange
chromatography on Dowex 50 × 2-200 resin to afford
respectively (S)-(+)-2-(3′-carboxybicyclo[1.1.1]pentyl)-
glycine (16) and (R)-(-)-2-(3′-carboxybicyclo[1.1.1]pentyl)-
glycine (17) in 76% and 79% yields.10
Resu lts An d Discu ssion . The two stereoisomers of
2-(3′-carboxybicyclo[1.1.1.]pentyl)glycine [(S)-(+)-CBPG,
16, and (R)-(-)-CBPG, 17] were evaluated in an assay
employing baby hamster kidney (BHK) cells individu-
ally expressing mGluR1a, mGluR2, mGluR4, or mGluR5
(Table 1).4b At concentrations up to 300 µM, (R)-(-)-
CBPG (17) was inactive at all the mGluR subtypes
examined. Conversely, (S)-(+)-CBPG (16) dose-depen-
dently antagonized glutamate-evoked PI hydrolysis in
cells expressing mGluR1a with an IC50 ) 25 µM,
showing an almost complete inhibition at 300 µM. In
this assay, 1 mM M4CPG (3) completely blocked PI

2876 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 15
Communications to the Editor
(2) Nakanishi, S. Molecular diversity of glutamate receptors and
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Sunter, D. C.; Udvarhelyi, P. M.; Wharton, B.; Watkins, J . C.
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as antagonists of metabotropic glutamate receptors. Trends
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F igu r e 2. Superimposition of (S)-(+)-CBPG (16) (green) and
(S)-4CPG (1) (red). The R-amino acidic and the distal carboxy-
late moieties of 16 retain the coplanar disposition of 1, but
the relative distance is shorter by about 0.8 Å.
charges of the R-amino acidic moiety and the distalcar-
boxylate group across the whole structure. Second,
previous structure-activity relationship studies have
determined the distance between the R-amino acidic
moiety and the distal carboxy group of R-methyl(4-
carboxyphenyl)glycine to be optimal for mGluR1 an-
tagonism.4d In this regard, 16 is endowed with a
distance between the R-amino acidic group and the
distal carboxylate shorter than that of R-methyl(4-
carboxyphenyl)glycine (of about 0.8 Å; Figure 2) but still
retains potency as a mGluR1 antagonist in the same
order of magnitude as classical (carboxyphenyl)glycines,
thus indicating that a certain degree of tolerance is
allowed for receptor recognition. These observations can
certainly be of help in designing new, structurally
diverse mGluR1 antagonists with increased potency and
selectivity.
(5) (a) Pellicciari, R.; Luneia, R.; Costantino, G.; Marinozzi, M.;
Natalini, B.; J akobsen, P.; Kanstrup, A.; Lombardi, G.; Moroni,
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In summary, we have reported that (S)-(+)-CBPG (16)
is a structurally novel mGluR1 antagonist endowed with
good potency and selectivity toward the mGluR1 recep-
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(7) Chakraborty, T. K.; Reddy, G. V.; Hussain, K. A. Diastereose-
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Ack n ow led gm en t. This work was supported by
Grant PL 95-0228 from the European Commission to
R.P. and C.T.
in the alkylation of chiral dipole-stabilized organolithiums:
a
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(10) Selected spectroscopic data for compounds 16 and 17 are as
follows. 16: mp 260 °C dec; 1H-NMR (D2O) δ 2.00 (6H, s, 3 ×
CH2), 3.75 (1H, s, CH); 13C-NMR (D2O) δ 35.75, 47.47, 49.00,
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures (5 pages). Ordering information is given on any
current masthead page.
53.22, 169.82, 173.09; [R]20 ) +7.8° (c 1, H2O). 17: mp 260 °C
D
dec; [R]20 ) -12.1° (c 1, H2O).
D
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phenylglycine derivatives on activity at metabotropic glutamate
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glutamate receptors. Structure and functions. Neuropharma-
cology 1995, 34, 1-26. (b) Kno¨pfel, T.; Kuhn, R.; Allgeier, H.
Metabotropic glutamate receptors: novel targets for drug de-
velopment. J . Med. Chem. 1995, 38, 1417-1426.
J M960254O