Synthesis and preliminary pharmacological evaluation of the four stereoisomers of (2S)-2-(2′-phosphono-3′-phenylcyclopropyl)glycine, the first class of 3′-substituted transC1′-2′-2-(2′-phosphonocyclopropyl)glyc ines

Article abstract of DOI:10.1016/j.bmc.2007.02.040

Four stereoisomers of (2S)-2-(2′-phosphono-3′-phenylcyclopropyl)glycine were synthesized by a stereocontrolled synthetic procedure and evaluated as mGluRs ligands. The (2S,1′R,2′S,3′R)-isomer (PPCG-2) showed to be a group III mGluRs selective ligand endow

Full text of DOI:10.1016/j.bmc.2007.02.040

Bioorganic & Medicinal Chemistry 15 (2007) 3161–3170
Synthesis and preliminary pharmacological evaluation
of the four stereoisomers of (2S)-2-(20-phosphono-30-
phenylcyclopropyl)glycine, the first class of 30-substituted
0
0
0
transC1 ꢀ2 -2-(2 -phosphonocyclopropyl)glycines
Maura Marinozzi,a Michaela Serpi,a Laura Amori,a Monica Gavilan Diaz,a,§
Gabriele Costantino,a Udo Meyer,a Peter J. Flor,b Fabrizio Gasparini,b
Roland Heckendorn,b Rainer Kuhn,b Gianluca Giorgi,c Mette Brunsgaard Hermit,d
Christian Thomsend and Roberto Pellicciaria,*
a
`
Dipartimento di Chimica e Tecnologia del Farmaco, Universita di Perugia, Via del Liceo, 1- I-06123 Perugia, Italy
bNovartis Institutes for BioMedical Research, Neuroscience Disease Area, WKL-125.6.08, CH-4002 Basel, Switzerland
c
`
Dipartimento di Chimica, Universita di Siena, Via A. Moro, I-53100 Siena, Italy
dH. Lundbeck A/S, 9 Ottilavej, DK-2500 Valby, Copenhagen, Denmark
Received 30 October 2006; revised 15 February 2007; accepted 20 February 2007
Available online 22 February 2007
Abstract—Four stereoisomers of (2S)-2-(20-phosphono-30-phenylcyclopropyl)glycine were synthesized by a stereocontrolled syn-
thetic procedure and evaluated as mGluRs ligands. The (2S,10R,20S,30R)-isomer (PPCG-2) showed to be a group III mGluRs selec-
tive ligand endowed with a moderate potency as mGluR4/mGluR6 agonist.
ꢀ 2007 Elsevier Ltd. All rights reserved.
1. Introduction
acids as conformationally constrained-bioisosteric ana-
logues of L-glutamic acid (L-Glu), the main excitatory
neurotransmitter in the mammalian central nervous sys-
tem. Thus, phosphonocyclopropyl amino acid 1, charac-
terized by an appropriate intermediate chain and by the
D-configuration at the amino acidic moiety, has been de-
signed by us as a competitive antagonist for the NMDA
receptor,7 whereas (2S,10R,20S)-2-(20- phosphonocyclo-
propyl)glycine (2), a conformationally constrained ana-
logue of L-1-amino-4-phosphonobutanoic acid (L-AP4,
3), the prototypic selective agonist for the group III fam-
ily of the glutamate metabotropic receptors (mGluRs),
has been shown to be a group III mGluRs agonist with
micromolar activity.8 In the frame of our continuing
interest in the development of chemical tools for the
characterization of the glutamate receptors and in par-
ticular for the metabotropic ones, we became interested
in the synthesis of 2,3-disubstituted cyclopropylphosph-
onates with definite stereochemistry, able to be precursors
of 30-substituted-(20-phosphonocyclopropyl)glycines, as
potential ligands for group III mGluR subtypes. In
particular, we reasoned that the introduction of a hydro-
phobic moiety, such as a phenyl ring, in the 30-position
Cyclopropylphosphonate derivatives have been the fo-
cus of interest for chemists because of their potential
biological properties. They have been designed as mim-
ics of aminocyclopropane carboxylic acid (ACC) and as
such showed to be potent inhibitors of alanine racemase
and ACC-deaminase,1 as analogues of (ꢀ)-allonorcoro-
namic acid2 and minalcipran,3 as structural moieties of
nucleotides,4 as potential herbicides or plant growth reg-
ulators5 and as potential insecticides.6 Combining two
of the most important strategies in rational drug design,
namely bioisosterism and reduction of the conforma-
tional flexibility through ring insertion, we and others
turned the attention to phosphonocyclopropyl amino
Keywords: 30-Substituted-2-(20-phosphonocyclopropyl)glycines; Meta-
botropic glutamate receptors; Conformationally constrained amino
acids.
*
Corresponding author. Tel.: +39 0755855120; fax: +39
0755855124; e-mail: rp@unipg.it
§
On leave from Departamento de Quimica Organica, Facultad de
Farmacia, Campus de Cartuja s/n, 18071 Granada, Spain.
0968-0896/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2007.02.040

3162
M. Marinozzi et al. / Bioorg. Med. Chem. 15 (2007) 3161–3170
the preparation of 30-substituted-(20-phosphonocyclo-
NH2
CO2H
propyl)glycines characterized by the trans-disposition
between the pharmacophoric groups, the glycine moiety
and the phosphonate group. The synthesis and prelimin-
ary biological evaluation of the first representatives of
this new class, namely the complete stereolibrary of
H2O3P
1
H2O3P
NH2
CO2H
NH2
0
0
H2O3P
0
0
transC1 ꢀ2 -(2S)-2-(2 -phosphono-3 -phenylcyclopropyl)-
glycines (4–7), are reported herein (Chart 1).
CO2H
2
3
2. Results and discussion
R
3'
S
Although a number of synthetic routes to cyclo-
propylphosphonates have been described in the litera-
ture,11 only few examples of 2,3-disubstituted
cyclopropyl-1-phosphonates have been reported.12
Among them, the stereocontrolled synthesis of 1,2,3-tri-
substituted cyclopropane phosphonic acids, reported by
Hanessian et al.,13 involving the addition of chiral phos-
phonamide-based anions to a,b-unsaturated esters, re-
sulted particularly suitable to our purposes. Because
we required all the possible isomers endowed with
trans-disposition between the glycine and the phospho-
nate moieties, the achiral diisopropyl a-chloro-
methylphosphonate (8) was employed in place of the
optically active Hanessian’s phosphonamide (Scheme
1). Thus, treatment at ꢀ78 ꢁC of E-tert-butyl cinnamate
(9) with the anion of 8, generated at ꢀ78ꢁC (LDA,
THF), afforded two diastereoisomeric diisopropyl (2-
tert-butoxycarbonyl-3-phenylcyclopropyl)phosphonates
[( )-10)] and [( )-11] (2:1 by 1H NMR) in 56% and 19%
yields, respectively, after separation by flash
chromatography.
S
2
'R
NH2
CO2H
R
1'S
H2O3P
H2O3P
S
2
S
CO2H
NH2
PPCG-2
PPCG-1
5
4
S
R
S
R
NH2
CO2H
S
R
H2O3P
H2O3P
S
S
CO2H
NH2
PPCG-3
PPCG-4
6
7
Chart 1.
of 2-(20-phosphonocyclopropyl)glycine can be proved
useful for mapping the presence of still unexplored
accessory areas in the recognition site of members of this
glutamate receptor family.
The relative stereochemistries of the two racemic mix-
tures ( )-10 and ( )-11 were determined by spectro-
scopic analysis. A proof of the relative configuration
Taking advantage from the pharmacophore models for
group III agonists,9,10 we addressed our efforts towards
+
i-Pr2O3P
Cl
a
i-Pr2O3P
CO2t-Bu
( )-10
i-Pr2O3P
CO2t-Bu
( )-11
CO2t-Bu
9
8
b
R
S
d
H
R
S
S
R
CN
N
i-Pr2O3P
OH
S
R
i-Pr2O3P
S
R
HN
CN Ph
i-Pr2O3P
R'
OH
Ph
R = CH2OH, ( )-12
R = CHO, ( )-13
c
14
15
e
e
PPCG-1
PPCG-2
4
5
Scheme 1. Reagents and conditions: (a) (i) LDA, THF, ꢀ78 ꢁC, 900; (ii) flash chromatography ( )-10 (56%) and ( )-11 (19%); (b) LiEt3BH, THF,
ꢀ78 ꢁC, 150, 81%; (c) PCC, CH2Cl2, rt, 24 h, 71%; (d) (i) R-(ꢀ)-a-phenylglycinol, MeOH, rt, 24 h; (ii) TMSCN, 0 ꢁC, 24 h; (iii) MPLC, 14 (32%) and
15 (14%); (e) (i) Pb(OAc)4, CH2Cl2–MeOH, 0 ꢁC, 200; (ii) 6 N HCl, reflux, 24 h; (iii) ion exchange chromatography, 4 (28%) and 5 (35%).

M. Marinozzi et al. / Bioorg. Med. Chem. 15 (2007) 3161–3170
3163
of C-1–C-2 follows from the H–H coupling constants
(J1–2), which are 5.9 and 10.1 Hz for the diastereoiso-
dized by PCC into diisopropyl (2-formyl-3-phenylcyclo-
propyl)phosphonate [( )-13]. The aldehyde ( )-13 was
submitted to condensation with R-a-phenylglycinol
and the Schiff base thus formed reacted with trimethyl-
silyl cyanide to afford a mixture of the four expected
a-amino nitriles, as two major (2S)- and two minor
(2R)-components (85:15 by HPLC).15 Flash chromato-
graphy of the reaction mixture allowed a good separa-
tion of the desired two more abundant (2S)-[(R)-
(phenylglycinyl)amino]nitriles 14 and 15 in 32% and
14% yields, respectively. The derivative 14 was submit-
ted to oxidative cleavage of the chiral auxiliary, acidic
hydrolysis and ion exchange resin chromatography to
afford (2S, 10S, 20R, 30S)-2-(20-phosphono-30-phenylcy-
clopropyl)glycine (PPCG-1, 4) in 28% yield. By an anal-
ogous procedure, starting from the amino nitrile 15,
(2S, 10R, 20S, 30R)-2-(20-phosphono-30-phenylcyclopro-
pyl)glycine (PPCG-2, 5) was obtained in 35% yield.
mers ( )-10) and ( )-11, respectively. Since for the
14
cyclopropane derivatives, JHHcis > JHHtrans
,
the diaste-
reoisomer ( )-10, whose coupling constant value is
5.9 Hz, is endowed with trans-disposition between car-
boxylate and phosphonate groups. On the contrary,
( )-11 has cis-configuration of the same substituents
compatible with a larger coupling constant value. Fur-
thermore, the comparison of H-2–H-3 coupling constant
values which are 8.6 and 6.0 Hz for the diastereoisomers
( )-10 and ( )-11, respectively, allows us to suppose a
cis-disposition between phosphonate and phenyl group
in the diastereoisomer ( )-10 and the opposite one in
( )-11. The stereochemical assignments were supported
by NOESY experiments (Fig. 1): in the case of ( )-10,
strong NOE occurs between H-2 and H-3, whereas
NOEs were observed between H-1 and H-2 in the spec-
trum of ( )-11.
0
0
0
To complete the stereolibrary of transC1 ꢀ2 -(2S)-2-(2 -
phosphono-30-phenylcyclopropyl)glycines, the synthesis
of the derivatives endowed with trans-relationship
between phosphonate and phenyl group remained to
realize. The base-catalyzed intramolecular epoxide open-
ing reaction of c,d-epoxybutylphosphonates has been first
reported by Oh and colleagues,4 as a synthetic methodol-
ogy to produce 2-substituted cyclopropylphosphonates.
We recently reported the application of this procedure
to the synthesis of (2S,10R,20S)-2-(20-phosphonocyclo-
propyl)glycine (2).8 Since the possibility to obtain
2,3-disubstituted cyclopropylphosphonates starting from
b-substituted-c,d-epoxybutylphosphonates has not yet
been explored, we decided to explore this possibility to
synthesize the lacking (2S)-2-(20-phosphono-30-phenylcy-
clopropyl)glycine isomers (6 and 7). To this end, 2-phe-
nylbut-3-en-1-ol [( )-16]16 was treated with carbon
tetrabromide and triphenylphosphine to obtain the
Diisopropyl (2-tert-butoxycarbonyl-3-phenylcyclopro-
pyl)phosphonate [( )-10], characterized by the desired
disposition of the pharmacophoric groups, was then re-
duced by lithium ethylborohydride in THF at ꢀ30 ꢁC to
give the corresponding alcohol ( )-12, subsequently oxi-
H3
H1
CO2t-Bu
H3
H2
CO2t-Bu
H1
H2
i-Pr2O3P
i-Pr2O3P
( )-11
( )-10
Figure 1. NOESY correlations of the two diastereoisomeric diisopro-
pyl (2-tert-butoxycarbonyl-3-phenylcyclopropyl)phosphonates [( )-
10)] and [( )-11].
PO3Et2
O
PO3Et2
Br
OH
c
b
a
19
( )-18
( )-17
( )-16
R
d
f
S
R
CN
H
S
S
Et2O3P
S
R
N
R
OH
Et2O3P
R
S
HN
Et2O3P
R
OH
CN Ph
Ph
R = CH2OH, ( )-20
R = CHO, ( )-21
22
23
e
g
g
PPCG-4
PPCG-3
7
6
Scheme 2. Reagents and conditions: (a) CBr4, PPh3, CH2Cl2, rt, 24 h, 80%; (b) P(OEt)3, 150 ꢁC, 7 days, 51%; (c) MCPA, CH2Cl2, rt, 48 h, 80%; (d)
LiHMDS, THF, ꢀ78 to ꢀ20 ꢁC, 3 h, 51%; (e) PCC, CH2Cl2, rt, 24 h, 67%; (f) (i) R-(ꢀ)-a-phenylglycinol, MeOH, rt, 24 h; (ii) TMSCN, 0 ꢁC, 24 h;
(iii) MPLC, 22 (15%) and 23 (17%); (g) (i) Pb(OAc)4, CH2Cl2–MeOH, 0 ꢁC, 200; (ii) 6 N HCl, reflux, 24 h; (iii) ion exchange chromatography, 6 (46%)
and 7 (37%).

3164
M. Marinozzi et al. / Bioorg. Med. Chem. 15 (2007) 3161–3170
corresponding bromide ( )-17, then refluxed in triethyl
phosphite to afford diethyl 2-phenylbut-3-enylphospho-
nate ( )-18 in 55% yield (Scheme 2). Epoxidation of ( )-
18 with m-chloroperbenzoic acid in dichloromethane
afforded diethyl 2-oxiran-2-yl-2-phenylethylphosphonate
(19) which was next transformed into trisubstituted cyclo-
propane ( )-20 by treatment with LiHMDS at ꢀ78 ꢁC.
(20-phosphono-30-phenylcyclopropyl)glycines (6) and
(7) was achieved following the synthetic protocol
above described for the synthesis of PPCG-1 (4) and
PPCG-2 (5). Thus, oxidation of ( )-20 into the corre-
sponding aldehyde ( )-21, diastereoselective Strecker
reaction, cleavage of the chiral auxiliary and final hydro-
lysis of the a-amino nitriles 22 and 23 provided
(2S,10R,20S,30S)-2-(20-phosphono-30-phenylcyclopropyl)gly-
cine (PPCG-3, 6) and (2S,10S,20R,30R)-2-(20-phos-
phono-30-phenylcyclopropyl)glycine (PPCG-3, 7) in
28% and 13% overall yield, respectively.
The cyclisation was highly stereoselective affording ( )-
20 as the only stereoisomer; indeed, no minor isomers
could be detected by GC-MS analysis. The relative con-
figuration of the derivative ( )-20 was assigned by a
comparison of its spectroscopic data with those previ-
ously obtained from the diastereoisomeric alcohol ( )-
12. The trans-geometry between the phenyl group and
the phosphonate moiety in ( )-20 was supported by
3. Absolute configuration assignment
The absolute configuration assignment to the four dia-
stereoisomeric amino acids 4–7 was based upon single-
crystal X-ray analysis performed on 23, one of the two
major a-amino nitriles derived from the racemic alde-
H3–P coupling constant value (JH –P ¼ 16 Hz) typical
3
of a cis-disposition of the groups. Starting from the alco-
hol ( )-20, the preparation of the other pair of (2S)-2-
CH3
O
N
R
a
b
P
Cl
+
S
O
P
H3C
N
R
CO2t-Bu
CH3
CO2t-Bu
N
N
R
CH3
R
25
24
26
R
S
R
H
N
O
P
d
e
S
O
PPCG-2
H3C
H3C
R
R
OH
P
N
R
R
N
N
5
N
CH3
CH3
Ph
R
R
CN
R
29
R = CH2OH, 27
R = CHO, 28
c
Scheme 3. Reagents and conditions: (a) Ref. 13; (b) LiEt3BH, THF, ꢀ78 ꢁC, 150, 84%; (c) PCC, CH2Cl2, rt, 1 h, 38%; (d) (i) R-(ꢀ)-a-phenylglycinol,
MeOH, rt, 24 h; (ii) TMSCN, 0 ꢁC, 24 h; (iii) MPLC, 80%; (e) (i) Pb(OAc)4, CH2Cl2–MeOH, 0 ꢁC, 200; (ii) 6 N HCl, reflux, 24 h; (iii) ion exchange
chromatography, 45%.
Figure 2. View of the asymmetric unit of the a-amino nitrile 22. Ellipsoids enclose 50% probability.

M. Marinozzi et al. / Bioorg. Med. Chem. 15 (2007) 3161–3170
3165
Table 2. Binding affinity values expressed as Ki (lM), for PPCGs (4-7)
and (2S,10R,20S)-2-(20-phosphonocyclopropyl)glycine (2)
hyde ( )-21, and on the independent, enantioselective
synthesis of (2S,10R,20S,30R)-2-(20-phosphono-30-phe-
nylcyclopropyl)glycine (5) as depicted in Scheme 3.
X-ray analysis of the a-amino nitrile 23 (Fig. 2) confirmed
the S-chirality of the amino nitrilic centre and allowed to
fix the absolute configuration at the three cyclopropylic
carbon atoms (10S,20R,30R). Conversely, the amino acid
6, proceeding from the other major a-amino nitrile
obtained from the aldehyde ( )-21, resulted with the same
absolute configuration S at C-2 position and opposite one
(10R,20S,30S) at the cyclopropylic carbons.
Compound
mGluR4
mGluR8
PPCG-1 (4)
PPCG-2 (5)
PPCG-3 (6)
PPCG-4 (7)
PCG-1 (2)
L-Glutamate
L-AP4 (3)
>1000
—
24 4.2
630 3.2
>1000
130 3.9
190 3.8
—
7.4 2.7
3.4 0.5
0.16 0.02
63 16
3.4 1.1
1.9 0.3
[3H]-L-AP4 and [3H]-LY34149519 binding experiments where per-
formed on rmGluR4 and rmGluR8 receptor-expressing cell mem-
branes (BHK cells) using a SPA assay.18 Data are means SEM of at
least three independent experiments performed in triplicate.
In the impossibility to obtain suitable crystals for X-ray
analysis with amino acids derived from aldehyde ( )-13,
we decided to synthesize the chiral cyclopropyl ester
(+)-(1R,2R,3S)-3-tert-butyl (1,3-dimethyl-2-oxidoocta-
hydro-1H-1,3,2-benzodiazaphosphol-2-yl)-2-phenylcyclo-
propanecarboxylate (26) by following Hanessian’s
enantioselective procedure.13 Starting from 26, by subse-
quent reduction–oxidation protocol, above reported, the
corresponding (1R,2S,3R)-aldehyde 28 was obtained,
which was submitted to the diastereoselective Strecker
reaction giving only two a-amino nitriles (Scheme 3).
centration one order of magnitude higher (Ki = 130 lM)
than mGluR4. PPCG-3 (6) also binds to mGluR8 with
the same order of magnitude (Ki = 190 lM) as PPCG-
2 (5), but it displays more than 20 times weaker binding
at mGluR4 (Ki = 630 lM) than PPCG-2 (5). Functional
measurements (Table 1) revealed that PPCG-2 (5) was
an agonist at mGluR4 subtype. PPCG-2 (5) was also
able to activate mGluR6 subtype with the same order
of potency. The equivalence of the spatial disposition
of the a-amino acidic moiety and the x-phosphonate
group between both the two active isomers of the series,
namely PPCG-2 (5) and PPCG-3 (6), and the corre-
sponding 30-unsubstituted derivative 2 confirmed the
need for a full extended disposition of the pharmaco-
phoric moieties for group III recognition. It is worthy
of notice that the introduction of the bulky phenyl
group at 30-position of 2 did not modify the pharmaco-
logical profile, thus pointing out a peculiar difference
with, for example, group II mGluR ligands, where the
introduction of bulky groups over the rigid 2-(20-carb-
oxycyclopropyl)glycine skeleton invariably changed the
functional profile from agonist to antagonist.15b,20
According to the above considerations on the inductive
effect of (R)-a-phenylglycinol,
a more abundant
(2S,10R,20S,30R)-N-[(R)-a-phenylglycinyl]glycinonitrile
29 was obtained, which by final deprotection led to
(2S, 10R, 20S, 30R)-2-(20-phosphono-30-phenylcyclopro-
pyl)glycine. A comparison of the spectroscopic data as
well as the specific rotatory power values of this com-
pound with those obtained on PPCG-1 (4) and PPCG-
2 (5) enables us to assign to the amino acid 5 the
(2S,10R,20S,30R) absolute configuration.
4. Biological results
The pharmacological profiles of the four stereoisomers
of PPCGs (4–7) were examined at recombinant mGluR
subtypes in functional assays (rmGluR1, rmGluR5,
hmGluR2, rmGluR4, and hmGluR7) and binding
experiments (rmGluR4 and rmGluR8). As shown in
Table 2, only PPCG-2 (5) among the four PPCGs
showed to bind at mGluR4 subtype with an acceptable
affinity (Ki = 24 lM). It should be noticed that com-
pound 5 was also able to bind mGluR8, albeit at a con-
5. Molecular modelling
In order to rationalize the differences in the pharmaco-
logical profile of the four diastereoisomeric amino acids
4–7, we have performed molecular modelling studies.
Thus, by using the crystal structure of the ligand binding
domain (LBD) of mGluR1 as a template (pdb: 1ewk),21
Table 1. EC50 (lM) values for PPCGs (4–7) and (2S,10R,20S)-2-(20-phosphonocyclopropyl)glycine (2)
Compound
Group I
Group II
Group III
mGluR1
mGluR5
mGluR2
mGluR4
mGluR6
mGluR7
PPCG-1 (4)
PPCG-2 (5)
PPCG-3 (6)
PPCG-4 (7)
PCG-1 (2)
>1000
>1000
>1000
>1000
>1000
7.3 1.0
>1000
>1000
>1000
>1000
>1000
>1000
1.2 0.3
>1000
>1000
>1000
320 3.5
>1000
>1000
—
59 4.2
—
>1000
51 4.3
—
>1000
>1000
—
>1000
9.4 3.0
6.1 1.5
0.51 0.2
—
—
13 3.0
7.6 0.5
0.34 0.07
700 140
>1000
188 79
L-Glutamate
L-AP4 (3)
26
>1000
2
Functional data where obtained from CHO cells expressing rmGluR1, rmGluR5 or rmGluR6 or from BHK cells expressing hmGluR2, hmGluR7 or
rmGluR4 using previously described methods.15b,17,18 Potencies were determined by (i) measuring intracellular calcium concentration (group I
mGluRs), (ii) GTP(c)35S stimulation assay (mGluR2) and (iii) measuring cAMP formation (group III mGluRs). The data shown are means SEM
of at least three independent experiments performed in triplicate.

3166
M. Marinozzi et al. / Bioorg. Med. Chem. 15 (2007) 3161–3170
homology models of both the open and closed forms of
the LBD of mGluR4 were constructed. The four ligands
4–7 were built up in MOE and, after energy minimiza-
tion, manually docked into the models of LBD of
mGluR4, by using the crystallographically determined
position of L-Glu as a guide (Fig. 3).
mGluR1. The substitution of the Trp 110 by a serine
in mGluR4 results in the disappearance of a space-filling
hydrophobic residue thus allowing more sterically
demanding ligand side chains to fit in the LBD of
mGluR4 receptor subtype with respect to mGluR1
(Fig. 4).
Inspection of the resulting complexes indicated the pres-
ence, in the binding pocket, of a cavity able to accom-
modate the phenyl ring of PPCG-2 (5). This cavity is
not present in the LBD of mGluR1, thus reflecting the
substitution of the mGluR1 residue tryptophan 110
(W110) by a serine in mGluR4. W110 was recognized
by Jingami and colleagues22 in a site-directed mutagen-
esis study to be involved in hydrophobic van der Waals
interactions with the b- and the c-carbon of L-Glu in
Thus, in the model of the LBD of mGluR4, there is a
pocket large enough to host the phenyl-moiety of
PPCG-type amino acid ligands. This can explain both
the observed micromolar affinity of PPCG-2 (5) to
mGluR4 (Ki = 24 lM) and the stereodiscrimination ob-
served between the diastereomeric amino acids 4–7.
Thus, when PPCG-3 (6) is superimposed on the position
of L-Glu, this cavity is not perfectly accessed by the 30-
substituent. Filling up the cavity by the 30-phenyl group
causes a dislocation of the polar groups NH2, a-COOH
and PO3H, from their effective binding positions. Fur-
thermore, the lack of activity of PPCG-1 (4) and
PPCG-4 (7) can easily be rationalized by steric
clashes of the phenyl moiety with the residues S317,
D318, S319, Q211 and Z236 of the mGluR4 receptor
model.
6. Conclusions
0
0
-
The stereocontrolled synthesis of the four transC1 ꢀ2
(2S)-2-(20-phosphono-30-phenylcyclopropyl)glycine isomers
provided a small library that was employed to assess the
steric accessibility of the binding pocket of group III
mGluRs. Among the members of the series PPCG-2
(5) and PPCG-3 (6) displayed affinities as group III
mGluRs ligands, albeit with differences in potency and
subtype selectivity, while all of them were inactive at
group I/II receptor subtypes. These data could be ratio-
nalized by docking experiments on the basis of the pres-
ence of a cavity in group III receptors that is not present
in group I/II. This observation accounts for the stereo-
specific effect of PPCG-2 (5) over the other isomers;
for the selectivity of PPCG-2 (5) for group III over
group I/II receptor subtypes; and for the functional pro-
file of PPCG-2 (5) as an agonist at group III. Indeed, fill-
ing the cavity with the directionally orientated 30-phenyl
group does not prevent a correct closure of the two lobes
of the ligand binding domain of the receptors.
Figure 3. Superimposition of PPCGs (4–7) on L-Glu (not shown) the
side chains are differently orientated according to the absolute
configuration of the ring carbons 10, 20 and 30 (colour code of the
phenyl groups: PPCG-1 (4), red; PPCG-2 (5), green; PPCG-3 (6),
yellow; PPCG-4 (7), blue).
Figure 4. (Left) L-Glu (yellow) and W110 in the crystal of mGluR1. (Right) docking of PPCG-2 (5) (element and green phenyl ring) into the
homology receptor-model of mGluR4 (in yellow, position of L-Glu in the crystal structure of 1ewk.pdb.) In mGluR4 the substitution of Trp 110 by a
less sterically demanding serine allows PPCG-2 (5) to bind.

M. Marinozzi et al. / Bioorg. Med. Chem. 15 (2007) 3161–3170
3167
7. Experimental
126.85, 128.48, 138.39, 167.78; 31P NMR (80 MHz) d
22.63.
All reagents were of analytical grade and purchased
from Sigma–Aldrich (Milano, Italy). Flash chromatog-
raphy was performed on Merck silica gel (0.040–
0.063 mm). Melting points were determined in open cap-
7.2. ( )-(1S,2R,3R)-Diisopropyl 2-(hydroxymethyl)-3-
phenylcyclopropylphosphonate [( )-12]
illary tubes on a Buchi 535 electrothermal apparatus and
¨
A 1 M solution of lithium triethylborohydride in THF
(7.8 mL) was added to a cooled (ꢀ78 ꢁC), magnetically
stirred solution of ( )-10 (1.00 g, 2.61 mmol) in dry
THF (33 mL). After 15 min, the reaction mixture was
acidified with 2 N HCl (10 mL) and allowed to warm
to room temperature. The aqueous layer was extracted
with EtOAc (2·60 mL), the combined organic phases
dried over anhydrous Na2SO4, filtered and evaporated
to give a residue which was purified by flash chromatog-
raphy. Elution with AcOEt–MeOH (95:5) afforded ( )-
1
are uncorrected. H-, 31P- and 13C NMR spectra were
registered on a Bruker AC 200 or Bruker AC 400 using
CDCl3 as solvent unless otherwise indicated. Chemical
shifts are reported in ppm. The abbreviations used are
as follows: s, singlet; d, doublet; dd, double doublet;
bs, broad signal. Optical rotations were recorded on a
Jasco Dip-360 digital polarimeter. The analytical HPLC
analyses were carried out on a Shimadzu (Kyoto, Japan)
Workstation Class LC-10 equipped with a CBM-10A
system controller, two LC-10AD high pressure binary
gradient delivery systems, a SPD-10A variable-wave-
length UV–vis detector and a Rheodyne 7725i injector
(Rheodyne Inc., Cotati, CA, USA) with a 20-ll stainless
steel loop. Cation- and anion-exchange resin chroma-
tographies were performed with Dowex 50WX2-200
and Dowex 1X8-200, respectively.
1
12 (0.66 g, 81% yield). H NMR (400 MHz) d 1.04 (m,
12H, P[OCH(CH3)2]2), 1.18 (ddd, J = 2.4, 6.2 and
9.6 Hz, 1H, 1-CH), 2.22 (m, 1H, 2-CH), 2.37 (ddd,
J = 6.2, 9.6 and 12.8 Hz, 1H, 3-CH), 3.62 (dd, J = 6.1
and 11.4 Hz, 1H, CHaOH), 3.73 (dd, J = 5.7 and
11.4 Hz, 1H, CHbOH), 4.23 (m, 1H, P[OCH(CH3)2]),
4.45 (m, 1H, P[OCH(CH3)2]), 7.12–7.32 (m, 5H, aromat-
ics); 13C NMR (100 MHz) d 19.20 (d, JCP = 195 Hz),
23.76 (m), 25.19 (d, JCP = 3 Hz), 26.76 (d, JCP = 6
Hz), 64.46, 69.98, 126.49, 127.63, 129.39, 136.20; 31P
NMR (160 MHz) d 25.90.
7.1. ( )-(1R,2S,3R)- and ( )-(1S,2S,3S)-tert-butyl 2-(diiso-
propoxyphosphoryl)-3-phenyl cyclopropanecarboxylates
[( )-10 and ( )-11]
A cold (ꢀ78 ꢁC) solution of 8 (2.40 g, 11.2 mmol) in
THF (25 ml) was added to a magnetically stirred cooled
(ꢀ78 ꢁC) solution of LDA [from addition of 2.5 M nBu-
Li in hexane (8.8 mL) to a solution of dry diisopropyl-
amine (1.80 g, 18.1 mmol) in THF (32 mL)]. After
15 min, a cooled (ꢀ78 ꢁC) solution of 9 (3.00 g,
14.7 mmol) in THF (32 mL) was added dropwise in
15 min. Stirring was continued for 1.5 h at ꢀ78 ꢁC, then
the reaction mixture was quenched with saturated
NH4Cl solution (40 mL) and allowed to warm to room
temperature. The resulting mixture was then extracted
with AcOEt (3·60 mL), the combined organic phases
were dried over anhydrous Na2SO4, filtered and evapo-
rated to give a residue which was purified by flash chro-
matography. Elution with EtOAc–light petroleum (3:7)
afforded ( )-10 (2.40 g, 56% yield). 1H NMR
(200 MHz) d 1.1 (m, 12H, P[OCH(CH3)2]2), 1.42 (s,
9H, CO2C(CH3)3), 1.74 (ddd, J = 2.7, 5.9, and 8.6 Hz,
1H, 2-CH), 2.62 (dt, J = 5.8 and 15.5 Hz, 1H, 1-CH),
2.80 (ddd, J = 5.8, 10.3, and 15.5 Hz, 1H, 3-CH), 4.28
(m, 1H, P[OCH(CH3)2]), 4.45 (m, 1H, P[OCH(CH3)2]),
7.1–7.34 (m, 5H, aromatics); 13C NMR (50 MHz) d
23.05 (d, JCP = 192 Hz), 23.67 (4d, JCP = 4 Hz), 25.10,
27.97, 29.83 (d, JCP = 5 Hz), 70.32 (2d, JCP = 6 Hz),
81.37, 126.91, 127.78, 129.21, 134.80, 171.00; 31P
NMR (80 MHz) d 22.32. Further elution with the same
solvents yielded ( )-11 (0.80 g, 19% yield). 1H NMR
(200 MHz) d 1.26 (m, 12H, P[OCH(CH3)2]2), 1.44 (s,
9H, CO2C(CH3)3), 1.55 (ddd, J = 0.6, 7.3 and 10.1 Hz,
1H, 2-CH), 2.18 (ddd, J = 6.0, 10.1 and 11.2 Hz, 1H,
1-CH), 3.01 (ddd, J = 6.0, 7.3 and 17.4 Hz, 1H, 3-CH),
4.7 (m, 2H, P[OCH(CH3)2]2), 7.0–7.5 (m, 5H, aromat-
ics); 13C NMR (50 MHz) d 24.20 (d, JCP = 192 Hz),
23.91 (3d, JCP = 5 Hz), 27.59, 28.00, 30.03 (d,
JCP = 6 Hz), 70.51 (2d, JCP = 6 Hz), 81.23, 126.23,
7.3. ( )-(1S,2R,3R)-Diisopropyl 2-formyl-3-phenyl-
cyclopropylphosphonate [( )-13]
A solution of ( )-12 (0.66 g, 2.13 mmol) in dry CH2Cl2
(18 mL) was added dropwise in 10 min to a magnetically
stirred suspension of PCC (0.68 g, 1.90 mmol) in dry
CH2Cl2 (12 mL). After 12 h, the reaction mixture was
diluted with Et2O (35 mL), filtered and the solvent
evaporated off. Filtration of the residue through a
Florisilꢂ pad yielded ( )-13 (0.47 g, 71% yield). 1H
NMR (400 MHz)
d
1.09 (m, J = 6.0 Hz, 12H,
P[OCH(CH3)2]2), 1.94 (dd, J = 5.9 and 9.4 Hz, 1H, 1-
CH), 2.94 (m, 2H, 2-CH and 3-CH), 4.29 (m, 1H,
P[OCH(CH3)2]), 4.44 (m, 1H, P[OCH(CH3)2]), 7.22–
7.34 (m, 5H, aromatics), 9.48 (d, J = 4.2 Hz, 1H,
CHO); 13C NMR (100 MHz) d 23.02 (d, JCP = 193 Hz),
23.78 (2d + 1t, JCP = 3 Hz), 30.53 (d, JCP = 5 Hz), 32.36,
70.63, 127.29, 127.95, 129.19, 133.91, 198.14; 31P NMR
(160 MHz) d 20.96.
7.4. (2S,10S,20R,30S)- and (2S,10R,20S,30R)-N-[(R) ꢀ
a-Phenylglycinyl]-2-[20-diisopropylphosphono-30-phenyl-
cyclopropyl]glycinonitriles (14 and 15)
(R)-a-Phenylglycinol (0.92 g, 6.67 mmol) was added to a
solution of ( )-13 (2.07 g, 6.67 mmol) in MeOH
(50 mL), and the resulting solution was magnetically
stirred at room temperature for 24 h. After cooling to
(0 ꢁC), TMSCN (1.31 g, 13.2 mmol) was added, and
the resulting mixture was stirred for 24 h at room tem-
perature. Evaporation of the solvent gave a residue
which was submitted to a preliminary purification by
flash chromatography (EtOAc) in order to remove the
residue (R)-a-phenylglycinol. The mixture (2.8 g) of 14
and 15, thus obtained, was then submitted to MPLC.

3168
M. Marinozzi et al. / Bioorg. Med. Chem. 15 (2007) 3161–3170
Elution with n-hexane–EtOAc (45:55) afforded the cor-
responding N-substituted-a-aminonitrile 14 (0.49 g,
32% yield). 1H NMR (200 MHz) d 1.10 (m, 12 H,
P[OCH(CH3)2]2), 1.41 (dd, J = 6.3 and 9.9 Hz, 1H, 10-
CH), 2.38 (m, 1H, 20-CH), 2.52 (ddd, J = 6.4, 9.9 and
13.1 Hz, 1H, 30-CH), 3.59 (m, 2H, 2-CH and CHaOH),
3.74 (dd, J = 3.8 and 11.0 Hz, 1H, CHbOH), 4.10 (dd,
J = 3.8 and 9.2 Hz, 1H, NHCHPh), 4.26 (m, 1H,
P[OCH(CH3)2]), 4.47 (m, 1H, P[OCH(CH3)2]), 7.15–
7.34 (m, 10H, aromatics); 13C NMR (50 MHz) d 18.47
(d, JCP = 194 Hz), 23.88 (2d, JCP = 3 Hz), 24.82, 26.76
(d, JCP = 5 Hz), 49.94, 63.19, 67.07, 70.67 (2d,
JCP = 6 Hz), 117.98, 127.06, 127.41, 127.74, 128.30,
chromatography. Elution with 2 N AcOH yielded 5
1
(0.049 g, 35% yield). H NMR (200 MHz, D2O) d 1.42
(dd, J = 3.7 and 10.1 Hz, 1H, 20-CH), 2.02 (m, 1H, 10-
CH), 2.71 (m, 1H, 30-CH), 3.55 (d, J = 10 Hz, 1H, 2-
CH), 7.07–7.14 (m, 5H, aromatics); 13C NMR
(50 MHz, D2O) d 18.15, 21.91, 27.54, 56.07, 127.06,
128.20, 129.07, 135.08, 170.69; 31P NMR (80 MHz,
20
D
D2O) d 21.72; ½aꢁ ꢀ 32:20 (c 1.3, H2O).
7.7. ( )-1-Bromo-2-phenyl-3-butene [( )-17]
Carbon tetrabromide (37.87 g, 114.21 mmol) and triphe-
nylphosphine (19.97 g, 76.14 mmol) were added to a
stirred solution of ( )-16 (11.26 g, 76.14 mmol) in dry
CH2Cl2 (39 mL). After 12 h, the solvent was removed
in vacuo and the residue thus obtained, purified by flash
chromatography. Elution with light petroleum afforded
( )-17 (12.84 g, 80% yield). 1H NMR (200 MHz) d
3.69 (m, 3H, 1-CH2 and 2-CH), 5.18 (m, 2H, 4-CH2),
6.03 (m, 1H, 3-CH) 7.2–7.37 (m, 5H, aromatics).
128.87, 129.44, 134.96, 135.08, 138.20; 31P NMR
20
D
elution with the same solvents gave 15 (0.180 g, 14%
(80 MHz) d 23.78; ½aꢁ ꢀ 12:54 (c 2.62, CHCl3). Further
yield). 1H NMR (200 MHz)
d
1.14 (m, 12H,
P[OCH(CH3)2]2), 1.36 (dd, J = 6.2 and 10.2 Hz, 1H,
10-CH), 2.45 (m, 2H, 20-CH and 30-CH), 3.45 (d,
J = 6.4 Hz, 1H, 2-CH), 3.60 (m, 1H, CHaOH), 3.74
(dd, J = 4.0 and 11.0 Hz, 1H, CHbOH), 4.10 (m, 1H,
NHCHPh), 4.27 (m, 1H, P[OCH(CH3)2]), 4.47 (m, 1H,
P[OCH(CH3)2]), 7.14-7.35 (m, 10H, aromatics); 13C
NMR (50 MHz) d 19.67 (d, JCꢀP = 194 Hz), 23.81,
24.63, 26.59–26.68 (d, JC–P = 5.7 Hz), 50.49, 63.13,
67.01, 70.57, 117.95, 127.07, 127.53, 127.73, 127.94,
7.8. ( )-Diethyl-2-phenylbut-3-enylphosphonate [( )-18]
A magnetically stirred mixture of ( )-17 (10.3 g,
48.8 mmol) and triethylphosphite (60 mL) was heated at
150 ꢁC for 7 days. Excess of triethylphosphite was
distilled off and the residue purified by flash chromatogra-
phy. Elution with n-hexane–AcOEt (1:1) gave ( )-18
(6.7 g, 51% yield). 1H NMR (200 MHz) d 1.10 (t,
J = 6.0 Hz, 3H, P(OCH2CH3), 1.16 (t, J = 6.0 Hz, 3H,
P(OCH2CH3)), 2.13 (dd, J = 2.8 and 7.3 Hz, 1H, 1-
CHa), 2.22 (dd, J = 3.2 and 7.4 Hz, 1H, 1-CHb), 3.87
(m, 5H, P(OCH2CH3)2 and 2-CH), 4.99 (m, 2H, 4-
CH2), 5.96 (m, 1H, 3-CH), 7.13–7.29 (m, 5H, aromatics);
13C NMR (50 MHz) d 16.17, 16.27, 30.33–33.12 (d,
JCP = 139.5 Hz), 43.76, 61.28, 61.38, 114.42, 126.64,
127.59, 128.51, 141.07-141.30 (d, JCP = 11.5 Hz),
142.92–143.12 (d, JCP = 10.2 Hz); 31P NMR (80 MHz) d
30.79.
128.48, 128.97, 129.24, 129.44, 134.83, 137.64. 31P-
20
D
NMR (160 MHz) d 23.27; ½aꢁ ꢀ 65:46 (c 1.41, CHCl3).
7.5. (2S,10S,20R,30S)-2-(20-Phosphono-30-phenylcyclopro-
pyl)glycine (PPCG-1, 4)
Lead tetraacetate (0.49 g, 1.1 mmol) was added to a
cooled (0 ꢁC) magnetically stirred solution of 14
(0.42 g, 0.92 mmol) in anhydrous MeOH/CH2Cl2 (2:1,
48 mL). After 20 min, pH 7.7 phosphate buffer
(70 mL) was added and the resulting mixture was fil-
tered on a Celite pad. The solvent was then removed
in vacuo and the residue thus obtained heated at 95 ꢁC
in 6 N HCl (5 mL) for 24 h. After evaporation of the sol-
vent, the residue was purified by ion exchange resin
chromatography. Elution with 2 N AcOH yielded 4
7.9. Diethyl 2-oxiran-2-yl-2-phenylethylphosphonate (19)
1
(0.093 g, 28% yield). H NMR (200 MHz, D2O) d 1.60
A solution of ( )-18 (2.90 g, 10.8 mmol) in CH2Cl2
(45 mL) was added dropwise to a magnetically stirred
solution of 77% m-chloroperbenzoic acid (3.05 g,
14.4 mmol) in CH2Cl2 (25 mL). After 48 h the excess of
peracid was destroyed by addition of 10% Na2SO3
(35 mL). The organic layer was separated and washed
with 5% NaHCO3 (35 mL) then with water (30 mL), dried
over anhydrous Na2SO4 and the solvent removed in va-
cuo. The residue, thus obtained was purified by flash chro-
matography. Elution with EtOAc–light petroleum (7:3)
afforded 19 (2.46 g, 80% yield).1H NMR (200 MHz) d
1.12 (m, 6H, P(OCH2CH3)2), 2.13 (m, 2H, 1-CH2), 2.47
(dd, J = 2.4 and 4.8 Hz, 1H, CHaO of major diastereoiso-
mer), 2.53 (dd, J = 2.5 and 6.7 Hz, 1H, CHaO of minor
diastereoisomer), 2.70 (m, 1H, CHbO of both the diastere-
oisomers), 3.09 (m, 1H, OCH), 3.94 (m, 10H,
P(OCH2CH3)2 and 2-CH), 7.17–7.30 (m, 10H, aromat-
ics); 13C NMR (50 MHz) d 16.16, 27.7 (d, JCP = 160
Hz), 28.6 (d, JCP = 140 Hz), 42.14, 43.2, 46.58, 47.3,
55.22, 55.5, 56.5, 61.45, 127.23, 127.97, 128.49; 31P
NMR (80 MHz) d 30.39, 30.49.
(dd, J = 5.6 and 10 Hz, 1H, 20-CH), 2.08 (m, 1H, 10-
CH), 2.63 (m, 1H, 30-CH), 3.58 (d, J = 10 Hz, 1H, 2-
CH), 7.04–7.11 (m, 5H, aromatics); 13C NMR
(50 MHz, D2O) d 20.07 (d, JCP = 184.5 Hz), 21.73,
27.44-27.54 (d, JCP = 5 Hz), 55.90, 127.03, 128.19,
128.97, 134.92, 170.64; 31P NMR (80 MHz, D2O) d
20
D
21.97. ½aꢁ þ 80:62 (c 2.85, H2O).
7.6. (2S,10R,20S,30R)-2-(20-Phosphono-30-phenylcyclo-
propyl)glycine (PPCG-2, 5)
Lead tetraacetate (0.21 g, 0.47 mmol) was added to a
cooled (0 ꢁC) magnetically stirred solution of 15
(0.18 g, 0.40 mmol) in anhydrous MeOH/CH2Cl2 (2:1,
21 mL). After 20 min, pH 7.7 phosphate buffer
(15 mL) was added and the resulting mixture was fil-
tered on a Celite pad. The solvent was then removed
in vacuo and the residue thus obtained heated at 95 ꢁC
in 6 N HCl (5 mL) for 24 h. After evaporation of the sol-
vent, the residue was purified by ion exchange resin

M. Marinozzi et al. / Bioorg. Med. Chem. 15 (2007) 3161–3170
3169
7.10. ( )-(1S,2R,3S)-Diethyl 2-(hydroxymethyl)-3-phen-
ylcyclopropylphosphonate [( )-20]
J = 4.0 and 12.0 Hz, 1H, CHbOH), 3.89 (dd, J = 4.0
and 12.0 Hz, 1H, CHCH2OH), 4.16 (m, 4H,
P(OCH2CH3)2), 6.90–6.96 (m, 5H, aromatics), 7.09–
7.33 (m, 5H, aromatics); 13C NMR (50 MHz) d 15.49
(d, JCP = 191.4 Hz), 16.39, 26.54, 46.32–46.40 (d,
JCP = 4 Hz), 62.38–62.50 (d, JCP = 6 Hz), 62.57–62.70
(d, JCP = 6 Hz), 62.86, 66.97, 118.71, 127.47, 128.10,
1 M LiHMDS (3.9 mL) was added to a cooled (ꢀ78 ꢁC),
magnetically stirred solution of 19 (0.48 g, 1.69 mmol).
The reaction mixture was then allowed to warm to
ꢀ50 ꢁC and stirred at this temperature for 3 h. The reac-
tion mixture was quenched with saturated NH4Cl solu-
tion (40 mL), allowed to warm to room temperature and
extracted with AcOEt (3·60 mL). The combined organic
phases were dried over anhydrous Na2SO4, filtered and
evaporated to give a residue which was purified by flash
chromatography. Elution with AcOEt–acetone (8:2)
afforded ( )-20 (0.25 g, 51% yield). 1H NMR
(400 MHz) d 1.37 (m, 7H, P(OCH2CH3)2 and 1-CH),
2.01 (m, 1H, 2-CH), 2.16 (br s, 1H, OH), 2.84 (ddd,
J = 6.4, 9.1 and 16.0 Hz, 3-CH), 3.46 (d, J = 6.9 Hz,
1H, CH2OH), 4.18 (m, 4H, P(OCH2CH3)2), 7.26–7.33
(m, 5H, aromatics); 13C NMR (50 MHz) d 14.29 (d,
JCP = 197 Hz), 16.41, 16.47, 25.52 (d, JCP = 5 Hz),
26.02 (d, JCP = 3.5 Hz), 61.04 (d, JCP = 4 Hz), 62.08,
62.18, 126.93, 128.43, 128.86, 136.59; 31P NMR
(80 MHz) d 29.70.
128.38, 128.52, 128.65, 128.87, 133.22, 138.30; 31P
20
D
NMR (80 MHz) d 27.83; ½aꢁ ꢀ 110:9 (c 3.35, CHCl3).
Further elution with the same solvents gave 23 (0.19 g,
17% yield): 1H NMR (200 MHz) d 1.30 (m, 7H,
P(OCH2CH3)2 and 20-CH), 2.06 (m, 1H, 10-CH), 2.63
(d, J = 10.0 Hz, 1H, 2-CH), 2.90 (m, 1H, 30-CH), 3.36
(dd, J = 9.2 and 10.7 Hz, 1H, CHaOH), 3.55 (dd,
J = 4.0 and 10.6 Hz, 1H, CHbOH), 3.90 (dd, J = 4.0
and 9.0 Hz, 1H, CHCH2OH), 4.12 (m, 4H,
P(OCH2CH3)2), 6.61–6.65 (m, 2H, aromatics), 6.91–
7.26. (m, 8H, aromatics); 13C NMR (50 MHz) d13.01
(d, JCP = 191.6 Hz), 16.42, 26.65, 27.03, 47.10–47.03
(d, JCP = 3.5 Hz), 62.31, 62.42, 62.91; 31P NMR
20
D
(80 MHz) d 27.87; ½aꢁ ꢀ 24:99 (c 2.5, CHCl3).
7.13. (2S,10R,20S,30S)-2-(20-Phosphono-30-phenylcyclo-
propyl)glycine (PPCG-3, 6)
7.11. ( )-(1S,2R,3S)-Diethyl 2-formyl-3-phenylcyclopropyl-
phosphonate [( )-21]
Lead (IV) acetate (0.07 g, 0.16 mmol) was added to a
cooled (0 ꢁC) magnetically stirred solution of 22
(0.083 g, 0.19 mmol) in anhydrous CH2Cl2/MeOH (2:1,
9 mL). After 20 min, pH 7.7 phosphate buffer (5 mL)
was added and the resulting mixture was filtered on a
Celite pad. The solvent was then removed in vacuo
and the residue thus obtained heated at 95 ꢁC in 6 N
HCl (5 mL) for 24 h. After evaporation of the solvent,
the residue was purified by ion exchange resin chroma-
tography. Elution with 2 N AcOH yielded 6 (0.018 g,
46% yield). 1H NMR (400 MHz, D2O + HCl) d 1.85
(m, 2H, 10-CH and 20-CH), 2.75 (m, 1H, 30-CH), 3.10
(d, J = 10.0 Hz, 1H, 2-CH), 7.12–7.17 (m, 5H, aromat-
A solution of ( )-20 (1.10 g, 3.06 mmol) in dry CH2Cl2
(35 mL) was added dropwise in 10 min to a stirred sus-
pension of PCC (1.27 g, 5.9 mmol) in dry CH2Cl2
(25 mL). After 12 h, the reaction mixture was diluted
with Et2O (55 mL), filtered and the solvent evaporated
off. Filtration of the residue through a Florisilꢂ pad
yielded ( )-21 (0.73 g, 67% yield). 1H NMR
(200 MHz) d 1.30 (t, J = 7.0 Hz, 3H, P(OCH2CH3),
1.31 (t, J = 7.0 Hz, 3H, P(OCH2CH3)), 2.26 (m, 1H,
1-CH), 2.59 (m, 1H, 2-CH) 3.17 (m, 1H, 3-CH), 4.09
(m, 4H, P(OCH2CH3)2) 7.10–7.30 (m, 5H, aromatics),
8.87 (d, J = 6.0 Hz, 1H, CHO); 13C NMR (100 MHz)
ics); 13C NMR (50 MHz, D2O)
d
15.26 (d,
JCP = 185 Hz), 23.73, 26.57, 51.67, 127.46, 128.32,
d
17.25 (d, JCP = 192.5 Hz), 16.54, 29.77, 32.64,
127.7, 128.8, 129.1, 133.27, 197.6; 31P NMR (80 MHz)
128.89, 133.26, 169.65; 31P NMR (80 MHz, D2O) d
20
D
d 25.76.
21.20; ½aꢁ ꢀ 16:43 (c 0.78, 6 N HCl).
7.12. (2S,10R,20S,30S)- and (2S,10S,20R,30R) ꢀ N-
[(R) ꢀ a-Phenylglycinyl]-2-[20-diethylphosphono-30-
phenylcyclopropyl]glycinonitriles (22 and 23)
7.14. (2S,10S,20R,30R)-2-(20-Phosphono-30-phenylcyclo-
propyl)glycine (PPCG-4, 7)
Lead (IV) acetate (0.150 g, 0.34 mmol) was added to a
cooled (0 ꢁC) magnetically stirred solution of 23
(0.121 g, 0.28 mmol) in anhydrous CH2Cl2/MeOH (2:1,
15 mL). After 20 min, pH 7.7 phosphate buffer
(10 mL) was added and the resulting mixture was fil-
tered on a Celite pad. The solvent was then removed
in vacuo and the residue thus obtained heated at 95 ꢁC
in 6 N HCl (8 mL) for 36 h. After evaporation of the sol-
vent, the residue was purified by ion exchange resin
chromatography. Elution with 2 N AcOH afforded 7
(R)-(a)-Phenylglycinol (0.35 g, 2.59 mmol) was added to
a magnetically stirred solution of ( )-21 (0.73 g,
2.59 mmol) in MeOH (25 mL) and the resulting solution
was stirred at room temperature for 24 h. After cooling
to 0 ꢁC, TMSCN (0.51 g, 5.16 mmol) was added, and
the resulting mixture was stirred for 24 h at room tem-
perature. Evaporation of the solvent gave a residue
which was submitted to a preliminary purification by
flash chromatography (EtOAc) in order to remove the
residue (R)-a-phenylglycinol. The mixture (0.9 g) of 22
and 23, thus obtained, was then submitted to MPLC.
Elution with n-hexane–acetone (6:4) afforded 22
(0.17 g, 15% yield): 1H NMR (200 MHz) d 1.30 (m,
7H, P(OCH2CH3)2 and 20-CH), 2.08 (m, 1H, 10-CH),
2.40 (d, J = 9.8 Hz, 1H, 2-CH), 2.84 (m, 2H, 30-CH
and OH), 3.51 (t, J = 12.0 Hz, 1H, CHaOH), 3.67 (dd,
1
(0.055 g, 37% yield). H NMR (200 MHz, D2O) d 1.66
(m, 1H, 10-CH), 1.82 (m, 1H, 20-CH), 2.60 (m, 1H, 30-
CH), 3.09 (d, J = 12.0 Hz, 1H, 2-CH), 7.15–7.26 (m,
5H, aromatics); 13C NMR (50 MHz, D2O) d 17.11 (d,
JCP = 180 Hz), 24.34, 25.59, 51.05, 127.62, 128.44,
128.89, 133.59, 171.48; 31P NMR (80 MHz, D2O) d
20
D
22.02; ½aꢁ þ 154:50 (c 0.88, H2O).

3170
M. Marinozzi et al. / Bioorg. Med. Chem. 15 (2007) 3161–3170
7.15. (1R,2S,3R)-[2-(1,3-Dimethyl-2-oxidooctahydro-1H-
1,3,2-benzodiazaphosphol-2-yl)-3-phenylcyclopropyl]meth-
anol (27)
20.06 (d, JCP = 150 Hz), 24.12, 26.04, 27.99 (d,
JCP = 4.7 Hz), 28.31, 28.68 (d, JCP = 9.6 Hz), 29.14,
49.90, 62.45, 65.04 (d, JCP = 5.0 Hz), 67.30, 118.46,
127.03, 127.49, 128.10, 128.87, 129.38, 135.70, 138.04;
20
31P NMR (160 MHz) d 41.10; ½aꢁ ꢀ 115:37 (c 0.50,
A 1-M solution of lithium triethylborohydride in THF
(1.48 mL) was added to a cooled (ꢀ78 ꢁC), magnetically
stirred solution of 26 (0.30 g, 0.74 mmol) in dry THF
(12 mL). After 15 min, the reaction mixture was
quenched with saturated aqueous NH4Cl (10 mL) and
allowed to warm to room temperature. The aqueous
layer was extracted with EtOAc (2·12 mL), the com-
bined organic phases dried over anhydrous Na2SO4, fil-
tered and evaporated to give a residue which was
purified by flash chromatography. Elution with
AcOEt–MeOH (90:10) afforded 27 (0.21 g, 84% yield).
1H NMR (200 MHz) d 1.07 (m, 5H, 2·CH2 and 1-
CH), 1.79 (m, 4H, 2 · CH2), 2.02 (d, J = 11.6 Hz, 3H,
N-CH3), 2.42 (m, 7H, N-CH3, 2-CH, 3-CH and 2·
CH), 3.64 (dd, J = 6.4 and 11.6 Hz, 1H, CHaOH), 3.87
(dd, J = 5.3 and 11.6 Hz, 1H, CHbOH), 7.17-7.42 (m,
5H, aromatics); 13C-NMR (50 MHz) d 20.70 (d,
JCP = 153 Hz), 24.17, 26.60 (d, JCP = 4.3 Hz), 28.00,
28.16, 28.36, 28.55, 29.10, 63.65 (d, JCP = 7.4 Hz),
D
CH2Cl2).
References and notes
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3417; (b) Groth, U.; Lehmann, L.; Richter, L.; Scho¨llkopf,
U. Liebigs Ann. Chem. 1993, 427.
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3. Duquenne, C.; Goumain, S.; Jubault, P.; Feasson, C.;
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7. Dappen, M. S.; Pellicciari, R.; Natalini, B.; Monahan, J.
B.; Chiorri, C.; Cordi, A. A. J. Med. Chem. 1991, 34, 161.
8. Amori, L.; Serpi, M.; Marinozzi, M.; Costantino, G.;
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65.05, 126.59, 127.94, 129.53, 136.90; 31P-NMR (80
20
D
MHz) d 57.86; ½aꢁ ꢀ 108:76 (c 0.80, CH2Cl2).
7.16. (1R,2S,3R)-[2-(1,3-dimethyl-2-oxidooctahydro-1H-
1,3,2-benzodiazaphosphol-2-yl)-3-phenylcyclopropane-
carbaldehyde (28)
10. Bessis, A. S.; Jullian, N.; Coudert, E.; Pin, J-P.; Acher, F.
Neuropharmacology 1999, 38, 1543.
11. Davies, H. M. L.; Lee, G. H. Org. Lett. 2004, 6, 2117 and
references cited therein.
A solution of 27 (0.202 g, 0.59 mmol) in dry CH2Cl2
(5 mL) was added dropwise in 2 min to a magnetically
stirred suspension of PCC (0.14 g, 0.65 mmol) in dry
CH2Cl2 (3 mL). After 1 h, the reaction mixture was di-
luted with Et2O (15 mL), filtered and the solvent evapo-
rated off to give crude 28 (0.075 g, 38% yield), used for
the next step without any purification due to its
instability.
12. (a) Shen, Y.; Qi, M. J. Chem. Res. (S) 1996, 328; (b)
Midura, W. H.; Mikołajczyk, M. Tetrahedron Lett. 2002,
´
43, 3061; (c) Waszkuc, W.; Janecki, T. Org. Biomol. Chem.
2003, 1, 2966.
13. Hanessian, S.; Cantin, L.-D.; Roy, S.; Andreotti, D.;
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Data for Structure Determination, 3rd ed.; Springer:
Berlin, Heidelberg, New York, 2000.
15. (a) Chakraborty, T. K.; Hussian, K. A.; Reddy, G. V.
Tetrahedron 1995, 51, 9179; (b) Pellicciari, R.; Marinozzi,
M.; Natalini, B.; Costantino, G.; Luneia, R.; Giorgi, G.;
Moroni, F.; Thomsen, C. J. Med. Chem. 1996, 39, 2259.
16. (a) Seyferth, D. In ‘Organic Synthesis’ Collection; Wiley:
New York, NY, 1963; vol. IV, p. 258; (b) Rose, C. B.;
Taylor, S. K. J. Org. Chem. 1974, 39, 578.
17. Thomsen, C.; Kristensen, P.; Mulvihill, E.; Haldeman, B.;
Suzdak, P. D. Eur. J. Pharmacol. 1992, 227, 361.
18. Mathiesen, J. M.; Svendsen, N.; Bra¨uner-Osborne, H.;
Thomsen, C.; Ramirez, M. T. Br. J. Pharmacol. 2003, 138,
1026.
19. Wright, R. A.; Arnold, M. B.; Wheeler, W. J.; Ornstein, P.
L.; Schoepp, D. D. Naunyn Schmiedebergs Arch. Pharma-
col. 2000, 362, 546.
20. Pellicciari, R.; Costantino, G.; Marinozzi, M.; Macchiar-
ulo, A.; Amori, L.; Flor, P. J.; Gasparini, F.; Kuhn, R.;
Urwyler, S. Bioorg. Med. Chem. Lett. 2001, 11, 3179–3182.
21. <http://www.rcsb.org: mGluR1 brookhaven crystall data
file: 1ewk.pdb>.
7.17. (2S,10R,20S,30R)-N-[(R)-a-Phenylglycinyl]-2-[20-
(1,3-dimethyl-2-oxidooctahydro-1H-1,3,2-benzodiaza-
phosphol-2-yl)-30-phenylcyclopropyl]glycinonitrile (29)
(R)-(a)-Phenylglycinol (0.0562 g, 0.41 mmol) was added
to a magnetically stirred solution of 28 (0.075 g,
0.23 mmol) in MeOH (3 mL) and the resulting solution
was stirred at room temperature for 24 h. After cooling
to 0 ꢁC, TMSCN (0.02 g, 0.23 mmol) was added, and
the resulting mixture was stirred for 24 h at room tem-
perature. Evaporation of the solvent gave a residue
which was purified by flash chromatography. Elution
with AcOEt–MeOH (90:10) afforded 29 (0.044 g, 80%
1
yield). H NMR (400 MHz) d 1.00 (m, 3H, CH2 and
20-CH), 1.48 (m, 2H, CH2), 1.79 (m, 2H, CH2), 1.91
(m, 2H, CH2), 2.05 (d, J = 5.8 Hz, 3H, N-CH3), 2. 36
(d, J = 5.4 Hz, 3H, N-CH3), 2.55 (m, 4H, 20-CH, 30-
CH, and 2·CH2), 3.50 (d, J = 5.62 Hz, 2-CH), 3.58
(m, 1H, CHaOH), 3.75 (m, 2H, CHbOH and CHPh),
7.26–7.40 (m, 10H, aromatics); 13C NMR (100 MHz) d
22. Kunishima, N.; Shimada, Y.; Tsuji, Y.; Sato, T.; Yamam-
oto, M.; Kumasaka, T.; Nakanishi, S.; Jingami, H.;
Morikawa, K. Nature 2000, 407, 971.