1372
M. Isaac et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1371–1373
Suzuki cross-couplingto afford
5. The benzophe-
ted in a decrease in activity (see 1b and 1d in
none moiety was then removed by trifluoroacetic
acid to give 6 followed by basic hydrolysis with
sodium hydroxide in methanol to afford the sodium
salts 1.
Table 1). The positional isomer 1c, containingthe (3-
isopropylphenyl) also led to a decrease in GlyT-2 trans-
porter potency. However, in the cis biphenyl series, the
3-biphenyl substituent 1g (IC50=1.340 mM) was more
potent than the corresponding2- and 4-biphenyl iso-
mers.
In the correspondingcases where the aryl substituent
Ar2 is fixed, the alternative synthesis is illustrated in
Scheme 2. The propargyl alcohol 76 was transformed
into the correspondingpropargyl bromide 8, which was
then utilized in the propargylation of 2 providing 9.
Regioselective hydrostannation7 of 9 followed by
tin–iodine exchange afforded the intermediate 10,
which was then subjected to Suzuki coupling, tri-
fluoroacetic acid mediated deprotection followed by
hydrolysis.
In contrast, the sterically less demandingsubstituents
such as the 4-methylphenyl and the 4-ethylphenyl
groups maintained reasonable potency at GlyT-2 trans-
porter with IC50 values of 1.17 and 0.46 mM, respec-
tively.
Despite the favorable nature of small substituent in the
para position of the phenyl ring, electron-donating sub-
stituents such as the methoxy group provided ligands
with reduced activity (1k with IC50=2.03 mM). The
electron-withdrawinggroup, exemplified by the 4-cyano
substituent, was found to be inactive (1l with IC50
>10 mM). Unfortunately, cis substituted heterocyclic
aromatic groups (1m and 1n) had a negative effect on
GlyT-2 transporter affinity giving rise to ligands with
dramatically reduced activity.
Compounds 1a–z were evaluated in vitro for their abil-
ity to block the reuptake of tritiated glycine in cells
stably expressingthe GlyT-1c and GlyT-2 transporters.
The cells were incubated at 37 ꢀC for 30 min with 10
different concentrations of each test compound in the
presence of 50 nM tritiated glycine. Estimated IC50
values of each test compound were subsequently deter-
mined for both transporter subtypes.
Disubstitution in the 3- and 4-position, for example 3,4-
diethylphenyl, 1h (IC50=0.66 mM) and 2-naphthyl, 1j
(IC50=0.77 mM) showed good potency at the GlyT-2
transporter. With the 4-isopropylphenyl group as the
optimal cis substituent, this group was kept fixed while
several trans substituted phenyl groups were examined.
Similar to the cis substitutions, trans bulky aryl para
substituents such as phenyl, tert-butyl, and isopropyl
(1q, 1u, and 1o in Table 1) resulted in reduced potency
when compared to the trans 2,4-difluorophenyl group.
Nevertheless, trans aryl groups such as 2-fluorophenyl
1t (IC50=0.29 mM) and phenyl 1s (IC50=0.4 mM)
retained good GlyT-2 potency. Interestingly, the geo-
metric isomer 1v (isomer of 1a) was found to be inactive
(IC50 >10 mM) at both GlyT-1c and GlyT-2 suggesting
that the geometric integrity of the double bond can be
crucial for transporter activity. trans-Electron-donating
substituted phenyls such as the 4-methoxyphenyl group
provided ligands with slightly reduced activity (1p with
IC50=1.17 mM). Similarly, for the electron-withdrawing
group, exemplified by the 4-cyano substituent, was
found to also have reduced activity compared to
1a (1w with IC50=2.64 mM). The trans substituted
heterocyclic aromatic analogues (1x and 1y) showed
significantly reduced GlyT-2 transporter affinity
and dramatically reduced selectivity over GlyT-1c
transporter.
Compound 1a, which contains a 2,4-difluorophenyl
group trans to the glycine moiety and the cis 4-iso-
propylphenyl substituent, had good potency at GlyT-2
(IC50=0.330 mM) with good selectivity over the GlyT-1
transporter. With this early lead, the 2,4-difluorophenyl
group was fixed and the cis aryl substituents were sys-
tematically explored. Increasingthe steric bulk of
the para substituent to phenyl and tert butyl resul-
Scheme 1. (a) LDA, THF, HMPA, À78 ꢀC; (b) Ar2-B(OH)2,
Pd(PPh3)4, DME/Na2CO3, 110 ꢀC; (c) TFA, H2O, CH2Cl2, rt; (d) 1 N
NaOH, MeOH.
To further determine the effect of the chirality in this
series of compounds on GlyT-2 potency, compounds
1a, 1h, and 1j were resolved usinga chiral column
(Chirobiotic TTM) of an analytical HPLC and the
resolved components for each compound were sepa-
rated and evaluated in vitro. For each racemic com-
pound, only one enantiomer (the S-isomer, the
eutomer) of the pair was active and the activity was
about half that of the racemic mixture. The abso-
lute stereochemistry (shown in Fig. 1) of the active
Scheme 2. (a) PBr3, Et2O, 0 ꢀC; (b) 2, LDA, THF, HMPA, À78 ꢀC;
(c) i. Bu3SnH, PdCl2(PPh3)2, THF; ii. I2, CH2Cl2; iii. Ar1- B(OH)2,
Pd(PPh3)4, DME/Na2CO3, 110 ꢀC; (d) TFA, H2O, CH2Cl2, rt; (d) 1 N
NaOH, MeOH.