Identification of novel ligands for the Gabapentin binding site on the α2δ subunit of a calcium channel and their evaluation as anticonvulsant agents

Article abstract of DOI:10.1021/jm970649n

As part of a program to investigate the structure-activity relationships of Gabapentin (Neurontin), a number of alkylated analogues were synthesized and evaluated in vitro for binding to the Gabapentin binding site located on the α₂δ subunit of

Full text of DOI:10.1021/jm970649n

1838
J . Med. Chem. 1998, 41, 1838-1845
Id en tifica tion of Novel Liga n d s for th e Ga ba p en tin Bin d in g Site on th e r2δ
Su bu n it of a Ca lciu m Ch a n n el a n d Th eir Eva lu a tion a s An ticon vu lsa n t Agen ts
J ustin S. Bryans,* Natasha Davies, Nicolas S. Gee, Visaka U. K. Dissanayake, Giles S. Ratcliffe,
David C. Horwell, Clare O. Kneen, Andrew I. Morrell, Ryszard J . Oles, J ohn C. O’Toole, Gemma M. Perkins,
Lakhbir Singh, Nirmala Suman-Chauhan, and J acqueline A. O’Neill
Parke Davis Neuroscience Research Centre, Cambridge University Forvie Site, Robinson Way, Cambridge CB2 2QB, U.K.
Received September 25, 1997
As part of a program to investigate the structure-activity relationships of Gabapentin
(Neurontin), a number of alkylated analogues were synthesized and evaluated in vitro for
binding to the Gabapentin binding site located on the R2δ subunit of a calcium channel. A
number of other bridged and heterocyclic analogues are also reported along with their in vitro
data. Two compounds showing higher affinity than Gabapentin were selected for evaluation
in an animal model of epilepsy. One of these compounds, cis-(1S,3R)-(1-(aminomethyl)-3-
methylcyclohexyl)acetic acid hydrochloride (19), was shown to be effective in this model with
a profile similar to that of Gabapentin itself.
In tr od u ction
antinociceptive actions via interaction with this site. It
is interesting to note that (S)-isobutylgaba (4), a novel
anticonvulsant agent undergoing clinical trials, also
binds to the Gabapentin binding site with slightly better
affinity than Gabapentin itself-67 nM compared to 140
nM for Gabapentin. Moreover, neither Vigabatrin nor
Baclofen show any significant binding to the Gabapentin
binding site at 10 mM.
Gabapentin (Neurontin) (1) has been introduced as
an anticonvulsant agent which is useful as add-on
therapy in the treatment of epileptic seizures.1 It has
also recently been shown to be a potential treatment
for neurogenic pain.2-4 Gabapentin was originally
designed as a lipophilic γ-aminobutyric acid (GABA)
analogue,5 similarly to other GABA analogues such as
Vigabatrin (2) and Baclofen (3). However, unlike Vi-
gabatrin (an irreversible GABA transaminase inhibi-
tor)6 and Baclofen (a GABAB agonist),7 Gabapentin does
not interact with any of the enzymes on the GABA
metabolic pathway, nor does it interact directly with the
GABAA or GABAB receptors.8 However, it is able to
efficiently cross the blood-brain barrier by an L-system
amino acid transporter.9 Gabapentin shows few, if any,
toxic side effects at clinically relevant doses.10 It does,
however, possess a relatively short half-life, being
excreted unchanged, possibly due to its very high water
solubility and apparent lack of protein binding in vivo.11
Resu lts a n d Discu ssion
Gabapentin and (S)-isobutylgaba are both GABA
analogues which bind to the Gabapentin binding site
located on the R2δ subunit of a calcium channel. It has
recently been reported that the likely binding conforma-
tion of Gabapentin is such that the aminomethyl moiety
sits in an equatorial position relative to the cyclohexane
ring.15 It has also been reported that a number of
neutral R-amino acids (for example (S)-leucine) also bind
to the Gabapentin binding site,13 but in all known cases
the (S)-enantiomer of these amino acids is the eutomer.
Thus it was proposed that the GABA portion of Gaba-
pentin probably sits in a conformation which positions
the acid and amine moieties such that they mimic an
R-amino acid with the aminomethyl moiety existing in
the equatorial position relative to the cyclohexane ring.
If such a conformation of Gabapentin is modeled,
together with a similar conformation of (S)-isobutylgaba,
and the two are overlayed using the acid, amine, and
â-carbon of the GABA backbone, it is clear that the
isobutyl side chain of (S)-isobutylgaba overlaps with the
cyclohexane ring of Gabapentin except that one of the
terminal methyl groups extends from the (pro-R) 3eq-
position of Gabapentin.16 This led to an investigation
based on alkyl-substituted Gabapentin analogues.
For each of the monoalkylated Gabapentin analogues,
either the cis or trans isomer may exist. These could
be accessed individually using the routes exemplified
for cis-(1-(aminomethyl)-4-methylcyclohexyl)acetic acid
hydrochloride (10) and trans-(1-(aminomethyl)-4-meth-
ylcyclohexyl)acetic acid hydrochloride (14) in Schemes
117 and 2, respectively. In Scheme 1 the attack of
Recently, it has been shown that Gabapentin binds
with high affinity to a novel binding site on the R2δ
subunit of a calcium channel.12-14 It is currently
proposed that Gabapentin exerts its anticonvulsant and
S0022-2623(97)00649-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/29/1998

Novel Ligands for the Gabapentin Binding Site
J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11 1839
Sch em e 1a
Ta ble 1. R2δ Subunit Binding Site Affinities for 4- and
3-Substituted Gabapentin Analogues14
compd
R
stereochemistry
IC50 (nM)a
1
10
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
H
140 (11)
>10000
440 (35)
700 (18)
7020 (510)
>10000
4-Me
4-Me
4-Et
4-iPr
4-Ph
4,4-(Me)2
3-Me
3-Me
cis
trans
trans
trans
trans
a
(i) NCCH2CO2Et, NH4OAc, PhMe; (ii) KCN, EtOH, H2O; (iii)
1312 (344)
42 (6)
73 (11)
EtOH, HCl; (iv) H2, Raney Ni, MeOH; (v) HCl.
1S,3R
1R,3S
Sch em e 2a
3-Me
1S,3S and 1R,3R
1S,3R and 1R,3S
1S,3R and 1R,3S
1S,3R and 1R,3S
1S,3R and 1R,3S
(
>10000
3-Et
280 (45)
6270 (456)
3300 (512)
>10000
3-iPr
3-nPr
3-Ph
3,3-(Me)2
3,5-(Me)2
3,5-(Me)2
3,3,5,5-(Me)4
880 (300)
140 (11)
>10000
cis-(3R,5S)
trans-(3R,5S)
>10000
a
IC50 is the concentration (nM) producing half-maximal inhibi-
tion of the specific binding of [3H]Gabapentin to Gabapentin
binding sites located on the R2δ subunit of a calcium channel.
Values shown represent the geometric mean of at least three
experiments with the standard error of the mean in parentheses.14
a
(i) (EtO)2P(O)CH2CO2Et, NaH, THF; (ii) MeNO2, Bu4N+F-,
was recorded in deuteriomethanol. At ambient tem-
perature this revealed a single compound, but on cooling
the sample to -80 °C the spectrum showed two distinct
conformations in a ratio of 2:1. These were assigned
as the conformers with the aminomethyl moiety equato-
rial and axial. By comparison of the pair of methylene
signals from the aminomethyl moiety and the analogous
pair from the acetic acid moiety with the relative shifts
of the methylene protons adjacent to the amine and
carboxyl moieties in compounds 10 and 14, it was
possible to assign the peaks of the major conformer of
Gabapentin to that with the aminomethyl in the equa-
torial position and the peaks from the minor conformer
to that with the aminomethyl in the axial frame.
It can be seen from Table 1 that appending substit-
uents to the 4-position of Gabapentin led to a reduction
of binding affinity. Even trans-(1-(aminomethyl)-4-
methylcyclohexyl)acetic acid hydrochloride (14) showed
a 3-fold decrease in affinity, and larger groups reduced
the binding still further. Evaluation of the 4,4-gem-
dimethyl analogue 18 showed a significant decrease
beyond the monomethylated analogue 14. We investi-
gated the preferred conformation of 18 in a manner
similar to the methods used for Gabapentin, by record-
THF, 70 °C; (iii) Raney Ni, H2, MeOH; (iv) HCl.
cyanide is reversible,18 and so the reaction proceeds to
minimize the 1,3-diaxial interactions between the axial
protons on C-3 and C-5 and the axial group on C-1 by
placing the cyanide moiety on C-1 in the axial position
as opposed to the bulkier acetonitrile moiety. This gave
rise to Gabapentin analogues where the aminomethyl
ends up in the axial position.
Conversely, in Scheme 2, nitromethane attacks the
R,â-unsaturated ester 11 from the least hindered equa-
torial direction giving rise to analogues where the
aminomethyl lies in the equatorial position. The con-
formations of each of the products 10 and 14 were
determined by NMR spectroscopy in DMSO-d6.15 The
1H and 13C chemical shifts of compounds 10 and 14 were
assigned using H-1H double-quantum-filtered COSY
1
and 1H observed heteronuclear multiple quantum co-
herence (HMQC) for the 13C resonances. The relative
stereochemistries of the alkyl side chains to the ami-
nomethyl and acetyl groups were determined by NOE
difference. In both compounds the methyl moieties
occupy equatorial positions, as expected, shown by the
large coupling of the adjacent proton to nearby axial
protons. From previous data15 it was not expected that
any of the analogues where the aminomethyl moiety
existed in the axial conformation would have good
affinity for the Gabapentin binding site on the R2δ
subunit of a calcium channel, and indeed this was found
to be the case. Thus this investigation centered mainly
on compounds where the aminomethyl moiety adopted
an equatorial conformation.
1
ing low-temperature H NMR spectra in deuteriometha-
nol. Analogously to the experiments with Gabapentin,
it was determined that the preferred (2:1) solution
conformation of 18 was that with the aminomethyl
moiety in the equatorial frame. Thus it was deemed
likely that the inactivity of 18 in the binding assay was
due to steric factors around the 4-position and not to
conformational biasing of the compound in favor of an
inactive conformer.
In addition to the experiments described above, we
investigated the preferred solution conformation for
Gabapentin itself. A 1H NMR spectrum of Gabapentin
Molecular modeling suggested that (S)-isobutylgaba
could be overlayed with Gabapentin such that one of
the terminal methyl groups of (S)-isobutylgaba over-

1840 J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11
Bryans et al.
Ta ble 2. Summary of R2δ Subunit Binding Site Affinities
binding site was significantly reduced when compared
to that for Gabapentin itself.
(nM) for 2-Substituted Gabapentin Analogues14
To investigate the 2- and 6-axial positions when the
aminomethyl moiety is in the equatorial position, it was
necessary to join the 2- and 6-axial substituents into a
second ring, otherwise the structure would simply
undergo a ring flip to place the two 2- and 6-position
substituents in an equatorial position with concomitant
placement of the aminomethyl group in the axial posi-
tion, which would lead to poor affinity for the Gabap-
entin binding site.15 The bridged compound 33 was
derived from bicyclo[3.3.1]nonan-9-one in the usual
manner. At the same time the bridged compounds
derived from bicyclo[2.2.1]heptan-7-one19 and adaman-
tanone, 34 and 35, respectively, were also investigated.
It was noticed that hydrolysis of the lactam precursor
(36) of the bicyclic compound 33 gave two compounds,
compd
R
stereochemistry
IC50 (nM)a
30
31
32
2-Me
2-OMe
2-cC6H11
1S,2R and 1S,2S
1S,2S and 1R,2R
1R,2R and 1S,2S
1300 (246)
>10000
>10000
a
See footnote to Table 1.
layed with the (pro-R) 3-position of Gabapentin.16 Thus
the 3-position of Gabapentin was also investigated
(Table 1). The compounds were synthesized in the same
manner as the 4-position analogues, which gave rise to
the same degree of stereocontrol during the addition of
cyanide or nitromethane. The conformations were
elucidated in the same manner utilizing NMR tech-
niques. It can be seen (Table 1) that appending a
methyl group to the 3-position of Gabapentin to generate
cis-(1S,3R)-(1-(aminomethyl)-3-methylcyclohexyl)ace-
tic acid hydrochloride (19) gave a significant improve-
ment in affinity (3-fold) for the Gabapentin binding site.
The binding site shows a preference for this isomer over
all the other possibilities, with its enantiomer, the
(1R,3S)-isomer (20), having a reduced affinity of 73 nM
compared to the (1S,3R)-isomer (19) which showed an
affinity of 42 nM. Moreover, the (1S,3S)- and (1R,3R)-
isomers have much lower affinities for the binding site
(>10 µM) showed by the racemate (21). However,
synthesis of the gem dimethylated 3,3-dimethylgaba-
pentin (26) led to a large decrease in in vitro binding.
A second methyl group was also introduced in the
5-position to produce the achiral analogue 1R,3R,5R-(1-
(aminomethyl)-3,5-dimethylcyclohexyl)acetic acid hy-
drochloride (27) which showed comparable affinity to
Gabapentin at 143 nM (its trans-isomer (28) was more
than 100 times less active, as expected). The 3,3,5,5-
tetramethyl analogue (29) was devoid of any in vitro
activity at 10 µM.
the desired amino acid hydrochloride 33 and unreacted
lactam 36 in a 1:9 ratio. The lactam could be separated
from the amino acid by trituration with ethyl acetate.
Further hydrolysis of the recovered lactam (48 h) again
provided a mixture of amino acid and lactam in a 1:9
ratio suggesting that there was an equilibrium under
these conditions. To confirm this, a sample of the pure
amino acid 33 was subjected to the same hydrolysis
conditions again giving rise to amino acid and lactam
in a ratio of 1:9. A similar scenario was discovered for
the bicyclic compound 34 which gave a lactam:amino
acid ratio of 1:1. However, in the case of the adaman-
tane derivative 35, no amino acid was detected, indicat-
ing that the equilibrium lay completely in favor of the
lactam in this case. We repeated these experiments for
Gabapentin and (S)-isobutylgaba and found that the
equilibrium mixtures gave lactam:amino acid ratios of
1:9 and greater than 1:20, respectively. Thus it is clear
that as the bulk of the lipophilic moiety increases, the
equilibrium is displaced in favor of the lactam.
As cis-(1S,3R)-(1-(aminomethyl)-3-methylcyclohexyl)-
acetic acid hydrochloride (19) showed a significant
improvement over its parent, the methyl group was
extended to investigate larger groups. It can be seen
from Table 1 that as the group increases in size beyond
a methyl group, the affinity for the Gabapentin binding
site on the R2δ subunit of a calcium channel decreases,
with a large decrease progressing from ethyl (22) to
isopropyl (23) or n-propyl (24).
The 2-position of Gabapentin was also examined
(Table 2). Synthesis was undertaken using the two
routes outlined above (Schemes 1 and 2), but here much
less cis/trans control was observed; in all the cases
examined in this study a single pair of enantiomers was
obtained by crystallization from an ethyl acetate/
methanol mixture. These compounds were found to
have the aminomethyl moiety in the equatorial confor-
mation via use of analogous NMR techniques to those
used for the 4- and 3-substituted analogues.15 All the
compounds examined existed in a conformation placing
the 2-substituent in the equatorial position. In every
case we found that the affinity for the Gabapentin
The two bridged compounds 33 and 34 were evaluated
for their binding to the Gabapentin binding site located
on the R2δ subunit of a calcium channel and were shown
to have good affinity at 40 nM ((8 nM) and 44 nM ((9
nM), respectively, which compares to 140 nM for Gaba-
pentin. This indicated that substitutions at the 2- and
6-axial positions in the preferred binding conformation
of Gabapentin were well-tolerated.
Heterocyclic Gabapentin analogues have also been
examined with incorporation of oxygen (37), sulfur (38),
or nitrogen (39) at the 4-position. For the tetrahy-
drothiopyran analogue 38, the synthetic strategy had
to be modified owing to incompatibility of the sulfur

Novel Ligands for the Gabapentin Binding Site
J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11 1841
Sch em e 3a
a
(i) (EtO)2P(O)CH2CO2Et, NaH, THF; (ii) MeNO2, tetrameth-
ylguanidine, reflux; (iii) SnCl2, H2O HCl.
Ta ble 3. Summary of R2δ Subunit Binding Site Affinities
(nM) for Heterocyclic Gabapentin Analogues14
F igu r e 1. Effect of icv administration of doses of Gabapentin
(1-100 µg/animal) on latency to seizure in a semicarbazide-
induced seizure model of epilepsy in the mouse. (Each data
point is the mean of at least seven experiments; *P < 0.05,
**P < 0.01.)
compd
X
IC50 (nM)a
37
38
39
O
S
NH
2380 (537)
385 (95)
>10000
a
See footnote to Table 1.
moiety with the previously utilized catalytic hydrogena-
tion. Thus a chemical reduction of the nitro group was
used (Scheme 3). As can be seen from Table 3 the
affinity for the Gabapentin binding site is in the order
S > O > N. Thus as the heteroatom becomes more
polar, the affinity for the binding site decreases. This
implies that the 4-position of the Gabapentin molecule
is sitting in a hydrophobic part of the R2δ subunit.
F igu r e 2. Effect of icv administration of doses of cis-(1S,3R)-
(1-(aminomethyl)-3-methylcyclohexyl)acetic acid hydrochloride
(19) (1-30 µg/animal) on latency to seizure in a semicarbazide-
induced seizure model of epilepsy in the mouse. (Each data
point is the mean of at least seven experiments; **P < 0.01.)
P h a r m a cologica l Eva lu a tion . Two of the highest-
affinity compounds, cis-(1S,3R)-(1-(aminomethyl)-3-me-
thylcyclohexyl)acetic acid hydrochloride (19) and the
bicyclic analogue 33, were selected for in vivo evaluation
in an animal model of epilepsy. The two compounds
were examined to see if they were able to block semi-
carbazide (SEZ)-induced tonic seizures in mice. Groups
of male TO mice (20-25 g) were injected subcutaneously
with SEZ (750 mg/kg). The latency to the tonic exten-
sion of forepaws was noted. Any mice not convulsing
within 2 h after SEZ were considered protected and
given a maximum latency score of 120 min. Test
compounds were administered intracerebroventricullar-
ly (icv) or subcutaneously (sc) prior to the administration
of SEZ. The time difference between administration of
the test compound and SEZ is referred to as the
pretreatment time. Gabapentin (Figure 1) when ad-
ministered icv at doses of 1-100 µg/animal 0.5 h before
SEZ produced a dose-dependent anticonvulsant effect
with a minimum effective dose of 3 µg/animal. (1S,3R)-
3-Methylgabapentin (19) was examined icv (Figure 2)
at doses of 1-30 µg/animal and shown to have a
minimum effective dose of 3 µg/animal as well. The
bicyclic compound 33 was examined icv at doses of
3-100 µg/animal (Figure 3) and found to have a
minimum effective dose of 30 µg/animal. The minimum
effective dose values obtained compare to a minimum
F igu r e 3. Effect of icv administration of doses of the gaba-
pentin analogue 33 (3-100 µg/animal) on latency to seizure
in a semicarbazide-induced seizure model of epilepsy in the
mouse. (Each data point is the mean of at least seven
experiments; *P < 0.05, **P < 0.01.)
effective dose for Vigabatrin of 30 µg when administered
icv with a pretreatment time of 120 min. The minimum
effective dose found for the bicyclic analogue 33 is
somewhat worse than expected based on its binding
affinity. This could be due to its propensity to cyclize
to the spirolactam 36, which does not bind to the
Gabapentin binding site at any significant concentra-
tion.
Compound 19 was then evaluated alongside Gabap-
entin with both compounds being administered subcu-
taneously. Each compound was dosed at 30 mg/kg
subcutaneously with pretreatment times of 30-240 min

1842 J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11
Bryans et al.
of theoretical values and were determined by Medac Ltd.,
Uxbridge, U.K. Normal phase silica gel used for chromatog-
raphy was Merck no. 9385 (230-400 mesh), and reverse-phase
silica gel used was Lichroprep RP-18 (230-400 mesh); both
were supplied by E. Merck, AG, Darmstadt, Germany. An-
hydrous solvents were purchased in Sureseal bottles from
Fluka Chemicals Ltd., Glossop, U.K. All in vivo experiments
were conducted in accordance with the U.K. Home Office
Animals (Scientific Procedures) Act 1986.
Gen er a l Rou te A (Sch em e 1). Cya n o(4-m eth ylcyclo-
h exylid en e)a cetic Acid Eth yl Ester (6). 4-Methylcyclo-
hexanone (5) (15.30 mL, 125 mmol), ethyl cyanoacetate (13.20
mL, 124 mmol), ammonium acetate (0.96 g, 12.4 mmol), and
glacial acetic acid (1.40 mL, 24.4 mmol) were dissolved in
toluene (150 mL) and heated to reflux with azeotropic removal
of water by a Dean-Stark trap for 24 h. The mixture was
cooled and washed with H2O (3 × 50 mL). The H2O washes
were combined and extracted with toluene (2 × 30 mL). The
toluene extracts were combined with the original organic layer
and dried over MgSO4, and the solvent was evaporated in
vacuo. The crude oil was purified by Kugelrohr distillation to
give 21.05 g (82%) of 6 as an oil: 1H NMR (CDCl3) (400 MHz)
δ 0.95 (3H, d, J ) 6.8 Hz, -CHCH3), 1.20-1.31 (2H, m, CH2-
CHCH2), 1.35 (3H, t, J ) 7.2 Hz, CH2CH3), 1.80-1.90 (1H, m,
CH2CH(Me)CH2), 1.90-2.10 (2H, m, CH2CHCH2), 2.15, 2.34,
3.02, 3.84 (each 1H, m, CH2C(CN)(CH2CO2Et)CH2), 4.27 (2H,
q, J ) 7.2 Hz, CH2CH3); IR (film) 2927, 2225, 1728 cm-1; MS
(CI) m/e 236, 209, 208 ([MH]+, 100%). Anal. (C12H17NO2) C,
H, N.
F igu r e 4. Effect of sc administration of Gabapentin (30 mg/
kg) with different pretreatment times on the latency to seizure
in a semicarbazide-induced seizure model of epilepsy in the
mouse. (Each data point is the mean of at least 7 experiments;
**P < 0.01.)
ci s-1-(C y a n o m e t h y l)-4-m e t h y lc y c lo h e x a n e c a r b o -
n itr ile (7). A solution of 6 (6.21 g, 30 mmol) in 95% ethanol
(60 mL) was added to a solution of NaCN (1.47 g, 30 mmol) in
H2O (6 mL) and 95% ethanol (100 mL) and the resulting
mixture heated to reflux. After 22 h the mixture was cooled
and filtered. The solvent was removed in vacuo, and the crude
oil was purified by flash chromatography (silica, heptane/ethyl
acetate, 3:1) to give 3.208 g (66%) of 7 as a pale-yellow oil which
solidified on standing: 1H NMR (CDCl3) (400 MHz) δ 0.98 (3H,
d, J ) 5.6 Hz, CHCH3), 1.30-1.40 (3H, m, CH2CH(Me)CH2),
1.50 (2H, m, CH2CH(Me)CH2), 1.73-1.92 (2H, m, CH2C(CN)-
(CH2CN)CH2), 2.10 (2H, m, CH2C(CN)(CH2CN)CH2), 2.68 (2H,
s, CH2CN); IR (film) 2859, 2252, 2238, 1170 cm-1; MS (CI) m/e
95, 136, 163 (100%, MH+), 164, 182. Anal. (C10H14N2) C, H,
N.
F igu r e 5. Effect of sc administration of cis-(1S,3R)-(1-
(aminomethyl)-3-methylcyclohexyl)acetic acid hydrochloride
(19) (30 mg/kg) with different pretreatment times on the
latency to seizure in a semicarbazide-induced seizure model
of epilepsy in the mouse. (Each data point is the mean of at
least seven experiments; **P < 0.01.)
(Figures 4 and 5). As can be seen, compound 19 gave a
profile which was not significantly different from that
of Gabapentin based on efficacy and duration of action.
cis-(1-Cya n o-4-m eth ylcycloh exyl)a cetic Acid Eth yl Es-
ter (8). The bisnitrile 7 (2.00 g, 12.4 mmol) was dissolved in
absolute ethanol (30 mL) and added to dry toluene (30 mL).
The solution was chilled in an ice bath while saturating the
solution with gaseous HCl. The flask was stoppered and the
solution left to stand at room temperature for 24 h. The
solvent was removed in vacuo, and the residue was triturated
with diethyl ether (50 mL) to obtain a white solid which was
collected by filtration and dried under vacuum. No purification
was attempted, and the solid was used in next step. The crude
product was dissolved in ice-cold H2O (40 mL) and the pH
adjusted with 1 N HCl to pH 1.5. The solution was stirred at
room temperature for 20 h. The solution was extracted with
ethyl acetate (3 × 30 mL), the organic extracts were washed
with H2O (30 mL) and dried (MgSO4), and the solvent was
removed in vacuo to give 1.94 g (75%) of 8 as a low-melting
solid: 1H NMR (CDCl3) (400 MHz) δ: 0.92-1.01 (3H, d, J ) 7
Hz, CHCH3), 1.27-1.31 (3H, t, J ) 7.2 Hz, CH2CH3), 1.37 (5H,
m, CH2CH(Me)CH2), 1.70-1.73 (2H, m, 2 of CH2C(CN)(CH2-
CO2Et)CH2), 2.10-2.13 (2H, m, 2 of CH2C(CN)(CH2CO2Et)-
CH2), 2.54 (2H, s, CH2CO2Et), 4.21 (2H, q, J ) 7.2 Hz,
CH2CH3); IR (CH2Cl2) 2926, 2856, 2235, 1735 cm-1; MS (CI)
m/e 210 ([MH]+, 12%), 182 ([MH - C2H5]+, 100%), 164, 122.
Anal. (C12H19NO2) C, H, N.
Con clu sion
The structure-activity relationships of a series of
Gabapentin analogues have been examined, and from
this it has been possible to identify a number of
compounds having greater affinity for the Gabapentin
binding site on the R2δ subunit of a calcium channel
than Gabapentin itself. Two of these have been exam-
ined in an animal model of epilepsy using icv adminis-
tration, and from this cis-(1S,3R)-(1-(aminomethyl)-3-
methylcyclohexyl)acetic acid hydrochloride (19) has been
identified as a potent novel anticonvulsant agent pos-
sessing a similar profile to that of Gabapentin in the
animal model utilized. Compound 19 was further
examined alongside Gabapentin utilizing sc administra-
tion which showed no significant difference between the
two compounds.
Exp er im en ta l Section
Melting points were determined with a Mettler FP80 or a
Reichert Thermovar hot-stage apparatus. Proton NMR spec-
tra were recorded on a Varian Unity +400 spectrometer;
chemical shifts were recorded in ppm downfield from tetra-
methylsilane. IR spectra were recorded on a Perkin-Elmer
System 2000 Fourier transform spectrophotometer. Mass
spectra were recorded with a Finnigan MAT TSQ70 or Fisons
VG Trio-2A instrument. Elemental analyses are within (0.4%
cis-8-Meth yl-2-a za sp ir o[4.5]d eca n -3-on e (9). The ester
8 (606 mg, 2.9 mmol) was dissolved in NH3/EtOH (7%, 40 mL)
and shaken over Raney nickel (catalytic) under an atmosphere
of hydrogen gas (46 psi) at 30 °C. After 24 the catalyst was
removed by filtration through Celite. The filtrate was collected
and the solvent removed in vacuo to give 460 mg (95%) of 9 as

Novel Ligands for the Gabapentin Binding Site
J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11 1843
a white solid: 1H NMR (DMSO-d6) (400 MHz) δ 0.86 (3H, d,
J ) 6 Hz, CH3CH), 0.93-1.06 (2H, m, 2 of CH2CHCH2), 1.27-
1.30 (3H, m, 2 of CH2CHCH2 and CH3CH), 1.51 (2H, m, 2 of
CH2CCH2), 1.62 (2H, m, 2 of CH2CCH2), 1.92 (2H, s, CH2CO),
3.02 (2H, s, CH2NH), 7.43 (1H, br s, NH); IR (CH2Cl2) 3189,
12.39 (1H, br s, CO2H); MS (APCI) m/e 186 ([MH - HCl]+,
100%). Anal. (C10H19NO2‚HCl) C, H, N.
tr a n s-(1-(Am in om eth yl)-4-eth ylcycloh exyl)a cetic Acid
Hyd r och lor id e (15). Synthesized via route B in 16% overall
yield from 4-ethylcyclohexanone: 1H NMR (400 MHz) (DMSO-
d6) δ 0.85 (3H, t, J ) 6 Hz, CH3), 1.06 (3H, m, CHEt and 2 of
CH2CHCH2), 1.21 (4H, m, CH2CH3 and 2 of CH2CHCH2), 1.54
(4H, m, CH2CCH2), 2.44 (2H, s, CH2CO2H), 2.85 (2H, s, CH2N),
8.03 (3H, br s, NH3+), 12.37 (1H, br s, CO2H); MS (APCI) m/e
200 ([MH - HCl]+, 100%). Anal. (C11H21NO2‚HCl), C, H, N.
tr a n s-(1-(Am in om et h yl)-4-isop r op ylcycloh exyl)a ce-
tic Acid Hyd r och lor id e (16). Synthesized via route B in
13% overall yield from 4-isopropylcyclohexanone: 1H NMR
(400 MHz) (DMSO-d6) δ 0.85 (6H, d, J ) 6 Hz, CH(CH3)2),
0.90-1.28 (5H, m, CH2CHCH2), 1.35-1.63 (5H, m, CH(CH3)2
and CH2CCH2), 2.43 (2H, s, CH2CO2H), 2.84 (2H, s, CH2N),
8.00 (3H, br s, NH3+), 12.28 (1H, br s, CO2H); MS (APCI) m/e
214 ([MH - HCl]+, 100%). Anal. (C12H23NO2‚HCl), C, H, N.
tr a n s-(1-(Am in om eth yl)-4-ph en ylcycloh exyl)acetic Acid
Hyd r och lor id e (17). Synthesized via route B in 12% overall
yield from 4-phenylcyclohexanone: 1H NMR (400 MHz) (DMSO-
d6) δ 1.20 (2H, m, 2 of CH2CHCH2), 1.67 (6H, m, CH2CCH2
and 2 of CH2CHCH2), 2.43 (1H, CHPh), 2.60 (2H, CH2CO2H),
2.91 (2H, m, CH2N), 7.16-7.33 (5H, m, Ph), 8.02 (3H, br s,
NH3+), 12.44 (1H, br s, CO2H); MS (APCI) m/e 248 ([MH -
HCl]+, 100%). Anal. (C15H21NO2‚HCl), C, H, N.
(1-(Am in om eth yl)-4,4-d im eth ylcycloh exyl)a cetic Acid
Hyd r och lor id e (18). Synthesized via route A in 23% overall
yield from 4,4-dimethylcyclohexanone: 1H NMR (400 MHz)
(DMSO-d6) δ 0.89 (6H, s, 2 CH3), 1.24 (4H, m, 2 CH2), 1.40
(4H, m, 2 CH2), 2.39 (2H, s, CH2), 2.92 (2H, s, CH2), 7.94 (3H,
br s, NH3+), 12.36 (1H, br s, CO2H); IR (film) 3409, 2946, 1713,
1594, 1498, 1408, 1234, 1218, 1186 cm-1; MS (APCI) m/e 200
([MH - HCl]+, 100%). Anal. (C11H22NO2Cl) C, H, N.
cis-(1S,3R)-(1-(Am in om eth yl)-3-m eth ylcycloh exyl)a ce-
tic Acid Hyd r och lor id e (19). Synthesized via route B in
33% overall yield from (R)-3-methylcyclohexanone: 1H NMR
(400 MHz) (DMSO-d6) δ 0.74-0.91 (5H, m, CH2CH(CH3)CH2),
1.02-1.18 (1H, m, CHCH3), 1.38-1.65 (6H, m, CH2CH2CCH2),
2.46 (2H, s, CH2CO2H), 2.84 (2H, s, CH2NH3+), 7.97 (3H, br s,
NH3+), 12.37 (1H, br s, CO2H); IR (KBr disk) 1187, 1214, 1400,
1515, 1710, 2922, 3370 cm-1; MS (APCI) m/e 186 ([MH -
HCl]+, 100%). Anal. (C11H21NO2‚HCl) C, H, N.
cis-(1R,3S)-(1-(Am in om eth yl)-3-m eth ylcycloh exyl)a ce-
tic Acid Hyd r och lor id e (20). (()-(20) was synthesized via
route B in 30% overall yield from 3-methylcyclohexanone: 1H
NMR (400 MHz) (DMSO-d6) δ 0.74-0.91 (5H, m, CH2CH(CH3)-
CH2), 1.02-1.18 (1H, m, CHCH3), 1.38-1.65 (6H, m, CH2CH2-
CCH2), 2.46 (2H, s, CH2CO2H), 2.84 (2H, s, CH2NH3+), 7.97
(3H, br s, NH3+), 12.37 (1H, br s, CO2H); IR (KBr disk) 1187,
1214, 1400, 1515, 1710, 2922, 3370 cm-1; MS (APCI) m/e 186
([MH - HCl]+, 100%). Anal. (C11H21NO2‚HCl) C, H, N. Pure
20 was separated from 19 by HPLC using a Chirobiotic T
(macrocyclic glycopeptide antibiotic) column, 250 × 4.6 mm,
supplied by BAS Technicol, eluting with EtOH/H2O (95:5
isocratic). Compound 20 had a retention time of 25-29 min,
and compound 19 had a retention time of 30.5-36 min.
(()-tr a n s-(1-(Am in om et h yl)-3-m et h ylcycloh exyl)a ce-
tic Acid Hyd r och lor id e (21). Synthesized via route A in
13% overall yield from 3-methylcyclohexanone: 1H NMR
(CDCl3) (400 MHz) δ 0.69-0.79 (1H, m, 1 of CH2CH(CH3)CH2),
0.82 (3H, d, J ) 6 Hz, CHCH3), 0.87-0.90 (1H, m, CH2CH-
(CH3)CH2), 1.12-1.20 (1H, m, CHCH3), 1.34-1.50 and 1.60-
1.63 (each 3H, m, CH2CH2CCH2), 2.30 (2H, s, CH2CO2H), 3.01
(2H, s, CH2NH3+), 7.93 (3H, br s, NH3+), 12.04 (1H, br s,
CO2H); IR (MeOH) 2924, 2353, 1708, 1599, 1523, 1454, 1216
cm-1; MS (CI) m/e 186 ([MH - HCl]+, 13%), 168 (100%), 109.
Anal. (C10H19NO2‚1.1HCl) C, H, N. Sample shown to be a
homogeneous peak by HPLC utilizing a C18 prodigy ODS3
column eluting with a gradient of 40% MeCN/60% H2O to
100% MeCN and also, using the same column, eluting with a
gradient of 30% MeOH/70% H2O to 100% MeOH.
1679 cm-1; MS (CI) m/e 168 ([MH]+, 100%). Anal. (C10H17
-
NO) C, H, N.
cis-(1-(Am in om eth yl)-4-m eth ylcycloh exyl)a cetic Acid
Hyd r och lor id e (10). The lactam (418 mg, 2.5 mmol) was
heated to reflux in 5 N HCl (10 mL) for 5 h. The cooled
solution was diluted with H2O (10 mL) and washed with
dichloromethane (2 × 15 mL). The aqueous layer was collected
and the water removed in vacuo to give 551 mg (92%) of 10 as
a white solid: 1H NMR (DMSO-d6) (400 MHz) δ 0.88 (3H, d,
J ) 6 Hz, CH3CH), 1.02-1.12 (2H, m, 2 of CH2CHCH2), 1.25-
1.32 (3H, m, 2 of CH2CHCH2 and CH3CH), 1.43-1.47 (4H, m,
CH2CCH2), 2.33 (2H, s, CH2CO2H), 2.99 (2H, s, CH2NH3+), 8.03
(3H, br s, NH3+), 12.33 (1H, br s, CO2H); IR (MeOH) 3393,
2925, 2862, 1714, 1613 cm-1; MS (CI) m/e 186 ([MH - HCl]+,-
100%), 168, 109. Anal. (C10H19NO2‚HCl) C, H, N.
Gen er a l Rou te B (Sch em e 2). tr a n s-(4-Meth yl-1-(n i-
tr om eth yl)cycloh exyl)a cetic Acid Eth yl Ester (12). So-
dium hydride (60% dispersion in oil, 0.98 g, 24.45 mmol) was
suspended in dry tetrahydrofuran (50 mL) and cooled to 0 °C
in an ice bath. Triethylphosphonoacetate (5.12 mL, 25.67
mmol) was added dropwise over 5 min. After the addition was
complete the mixture was stirred at 0 °C for an additional 15
min. 4-Methylcyclohexanone (3 mL, 24.45 mmol) was then
added and the mixture allowed to warm to room temperature.
After 1.5 h the mixture was partitioned between 2 N HCl (50
mL) and ethyl acetate (150 mL). The organic phase was
separated, washed with brine (3 × 50 mL), and dried (MgSO4)
and the solvent removed in vacuo to give 5.05 g of a clear oil
which was used without purification. The crude R,â-unsatur-
ated ester (11) (2.94 g) was dissolved in tetrahydrofuran (20
mL) and stirred at 70 °C with nitromethane (1.75 mL, 32.3
mmol) and tetrabutylammonium fluoride (1 M in tetrahydro-
furan, 24 mL, 24.0 mmol). After 18 h the mixture was cooled
to room temperature, diluted with ethyl acetate (40 mL), and
washed with 2 N HCl (20 mL) and then brine (2 × 30 mL).
The organic phase was collected and dried (MgSO4) and the
solvent removed in vacuo. The residue was purified by flash
chromatography (silica, ethyl acetate/heptane, 1:9) to give 2.74
g (70%) of 12 as a clear oil: 1H NMR (CDCl3) (400 MHz) δ
1.27 (3H, d, J ) 6 Hz, CHCH3), 1.14 (2H, m, 2 of CH2CHCH2),
1.22-1.43 (6H, m, CH2CH3, CHCH3, and 2 of CH2CHCH2), 1.60
and 1.75 (2H each, m, CH2CCH2), 2.59 (2H, s, CH2CO2Et), 4.15
(2H, q, J ) 6 Hz, CH2CH3), 4.60 (2H, s, CH2NO2); IR (film)
1549, 1732 cm-1; MS (APCI) m/e 244 ([MH]+, 10%), 198 (100%).
Anal. (C12H21NO4) C, H, N.
tr a n s-8-Meth yl-2-a za sp ir o[4.5]d eca n -3-on e (13). The
nitro ester 12 (2.70 g, 11.1 mmol) was dissolved in methanol
(50 mL) and shaken over Raney nickel (catalytic) under an
atmosphere of hydrogen gas (40 psi) at 35 °C. After 18 h the
catalyst was removed by filtration through Celite and the
solvent removed in vacuo. The residue was purified by flash
chromatography (silica, ethyl acetate/heptane, 1:1) to give 0.72
g (39%) of 13 as a white solid: 1H NMR (CDCl3) (400 MHz) δ
0.91 (3H, d, J ) 6 Hz, CHCH3), 1.01 (2H, m, 2 of CH2CHCH2),
1.36 (3H, m, CHCH3 and 2 of CH2CHCH2), 1.58-1.79 (4H, m,
CH2CCH2), 2.20 (2H, s, CH2CO), 3.09 (2H, s, CH2N), 5.75 (1H,
br s, NH); IR (film) 1668, 1693, 3219 cm-1; MS (APCI) m/e
168 ([MH]+, 100%). Anal. (C10H17NO) C, H, N.
tr a n s-(1-(Am in om eth yl)-4-m eth ylcycloh exyl)acetic Acid
Hyd r och lor id e (14). The lactam (715 mg, 4.0 mmol) was
heated to reflux in a mixture of 6 N HCl (15 mL) and 1,4-
dioxane (5 mL). After 4 h the solvent was removed in vacuo
and the solid residue recrystallized from a methanol/ethyl
acetate/heptane mixture to give 664 mg (70%) of a white
solid: 1H NMR (400 MHz) (DMSO-d6) δ 0.88 (3H, d, J ) 6 Hz,
CH3CH), 1.10 (2H, m, 2 of CH2CHCH2), 1.22 (3H, m, 2 of CH2-
CHCH2 and CH3CH), 1.48-1.53 (4H, m, CH2CCH2), 2.43 (2H,
s, CH2CO2H), 2.85 (2H, s, CH2NH3+), 7.92 (3H, br s, NH3+),
(()-cis-(1-(Am in om eth yl)-3-eth ylcycloh exyl)acetic Acid
Hyd r och lor id e (22). Synthesized via route B in 15% overall

1844 J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11
Bryans et al.
yield from 3-ethylcyclohexanone: 1H NMR (400 MHz) (DMSO-
d6) δ 0.75 (2H, m, CH2), 0.84 (3H, t, J ) 7 Hz, CH3), 1.15 (3H,
m), 1.38 (2H, m), 1.54 (3H, m), 1.69 (1H, m), 2.45 (2H, s, CH2),
2.84 (2H, s, CH2), 8.02 (3H, br s, NH3+), 12.36 (1H, br s, CO2H);
12.36 (1H, br s, CO2H); MS (APCI) m/e 186 ([MH - HCl]+,
90%). Anal. (C11H21NO2‚HCl) C, H, N.
(()-(1-(Am in om eth yl)-2-m eth oxycycloh exyl)acetic Acid
Hyd r och lor id e (31). Synthesized via route A in 12% overall
yield from 2-methoxycyclohexanone: 1H NMR (DMSO-d6) (400
MHz) δ 1.17-1.90 (8H, m, 4 CH2), 2.46 (2H, s, CH2CO2H), 2.98
(2H, m, CH2NH3+), 3.09 (1H, m, CHOMe), 3.24 (3H, s, OCH3),
7.90 (3H, br s, NH3+), 12.25 (1H, br s, CO2H); MS (APCI) m/e
202 ([MH - HCl]+, 85%). Anal. (C10H19NO3‚HCl), C, H, N.
(()-(1-(Am in om et h yl)-2-cycloh exylcycloh exyl)a cet ic
Acid Hyd r och lor id e(32). Synthesized via route A in 13%
overall yield from 2-cyclohexylcyclohexanone: 1H NMR (DMSO-
d6) (400 MHz) δ 0.87-1.75 (2H, m, 9 CH2 and 2 CH), 2.47 (2H,
s, CH2CO2H), 2.97 (2H, ABq, J ) 13.5 and 13 Hz, CH2NH3+),
7.96 (3H, br s, NH3+), 12.38 (1H, br s, CO2H); MS (CI+) m/e
254 ([MH - HCl]+, 7%), 236 (100%). Anal. (C15H27NO2‚HCl)
C, H, N.
IR (film) 3400, 2926, 1710, 1607, 1505, 1457, 1403, 1205 cm-1
;
MS (APCI) m/e 200 ([MH - HCl]+, 100%). Anal. (C11H21NO2‚
HCl) C, H, N.
(()-cis-(1-(Am in om et h yl)-3-isop r op ylcycloh exyl)a ce-
tic Acid Hyd r och lor id e (23). Synthesized via route B in
17% overall yield from 3-isopropylcyclohexanone: 1H NMR
(400 MHz) (DMSO-d6) δ 0.83 (6H, d, J ) 6 Hz, 2 CH3), 0.86
(1H, m), 1.15 (1H, m), 1.20-1.45 (3H, m), 1.49-1.66 (4H, m),
2.45 (2H, s, CH2), 2.85 (2H, s, CH2), 7.98 (3H, br s, NH3+), 12.36
(1H, br s, CO2H); IR (film) 3402, 2930, 2871, 1710, 1608, 1505,
1460, 1403, 1286, 1207 cm-1; MS (APCI) m/e 214 ([MH -
HCl]+, 100%). Anal. (C12H23NO2‚HCl) C, H, N.
(()-cis-(1-(Am in om et h yl)-3-n -p r op ylcycloh exyl)a ce-
tic Acid Hyd r och lor id e (24). Synthesized via route B in
19% overall yield from 3-n-propylcyclohexanone: 1H NMR (400
MHz) (DMSO-d6) δ 0.65-0.90 (2H, m), 0.84 (3H, t, J ) 7 Hz,
CH3), 1.11 (3H, m), 1.26 (2H, m), 1.35-1.69 (6H, m), 2.46 (2H,
s, CH2), 2.83 (2H, s, CH2), 8.01 (3H, br s, NH3+), 12.36 (1H, br
s, CO2H); IR (film) 3393, 2926, 2870, 1711, 1607, 1507, 1458,
1403, 1286, 1259, 1206 cm-1; MS (APCI) m/e 214 ([MH -
HCl]+, 100%). Anal. (C12H23NO2‚HCl) C, H, N.
(()-cis-(1-(Am in om et h yl)-3-p h en ylcycloh exyl)a cet ic
Acid Hyd r och lor id e (25). Synthesized via route B in 30%
overall yield from 3-phenylcyclohexanone: 1H NMR (400 MHz)
(DMSO-d6) δ 1.22-1.85 (8H, m, 4 CH2 ring), 2.60 (2H, s, CH2-
CO2H), 2.78 (1H, m, CHPh), 2.90 (2H, s, CH2N), 7.11-7.35
(5H, m, Ar), 7.96 (3H, br s, NH3+), 12.45 (1H, br s, CO2H); IR
(KBr disk) 1406, 1497, 1592, 1715, 2927 cm-1; MS (ES+) m/e
248 ([MH - HCl]+, 100%). Anal. (C15H21NO2‚HCl) C, H, N.
(1-(Am in om eth yl)-3,3-d im eth ylcycloh exyl)a cetic Acid
Hyd r och lor id e (26). Synthesized via route A in 21% overall
yield from 3,3-dimethylcyclohexanone: 1H NMR (400 MHz)
(DMSO-d6) δ 0.90 (3H, s, Me), 0.92 (3H, s, Me), 1.15-1.49 (8H,
m, 4 CH2), 2.45 (2H, s, CH2CO2H), 2.90 (2H, br q, J ) 13.5
Hz, CH2NH3), 7.96 (3H, br s, NH3), 12.36 (1H, br s, CO2H); IR
(film) 2930, 1728, 1272, 1123 cm-1; MS (APCI) m/e 200 ([MH
- HCl]+, 100%). Anal. (C11H21NO2‚HCl) C, H, N.
1r,3r,5r-(1-(Am in om et h yl)-3,5-d im et h ylcycloh exyl)-
a cetic Acid Hyd r och lor id e (27). Synthesized via route B
in 11% overall yield from cis-3,5-dimethylcyclohexanone: 1H
NMR (400 MHz) (DMSO-d6) δ 0.47 (1H, m, 1 of CHCH2CH),
0.77-0.91 (8H, m, 2-Hax, 6-Hax, and 2 CH3), 1.46-1.63 (5H,
m, 2-Heq, 6-Heq, 2 CHCH3, and 4-Heq), 2.45 (2H, s, CH2CO2H),
2.84 (2H, s, CH2NH3+), 8.00 (3H, br s, NH3+), 12.37 (1H, br s,
CO2H); MS (APCI) m/e 200 ([MH - HCl]+, 100%). Anal.
(C11H21NO2‚HCl), C, H, N.
1r,3â,5â-(1-(Am in om et h yl)-3,5-d im et h ylcycloh exyl)-
a cetic Acid Hyd r och lor id e (28). Synthesized via route A
in 10% overall yield from cis-3,5-dimethylcyclohexanone: 1H
NMR (400 MHz) (DMSO-d6) δ 0.43 (1H, m, 1 of CHCH2CH),
0.75-0.90 (8H, m, 2-Hax, 6-Hax, and 2 CH3), 1.47-1.64 (5H,
m, 2-Heq, 6-Heq, 2 CHCH3, and 4-Heq), 2.31 (2H, s, CH2CO2H),
3.00 (2H, s, CH2NH3+), 7.94 (3H, br s, NH3+), 12.32 (1H, br s,
CO2H); MS (APCI) m/e 200 ([MH - HCl]+, 100%). Anal.
(C11H21NO2‚HCl) C, H, N.
(1-(Am in om et h yl)-3,3,5,5-t et r a m et h ylcycloh exyl)a ce-
tic Acid Hyd r och lor id e (29). Synthesized via route A in
6% overall yield from 3,3,5,5-tetramethylcyclohexanone: 1H
NMR (DMSO-d6) (400 MHz) δ 0.94 (6H, s, 2 CH3), 1.01 (6H, s,
2 CH3), 1.20 (4H, m, CH2), 1.40 (2H, d, CH2), 2.53 (2H, s, CH2),
2.93 (2H, br s, CH2), 7.85 (3H, br s, NH3+), 12.38 (1H, br s,
CO2H); IR (film) 3429, 2954, 1714 cm-1; MS (APCI) m/e 228
([MH - HCl]+, 100%). Anal. (C13H24NO2‚HCl), C, H, N.
(()-(1-(Am in om eth yl)-2-m eth ylcycloh exyl)a cetic Acid
Hyd r och lor id e (30). Synthesized via route A in 15% overall
yield from 2-methylcyclohexanone: 1H NMR (DMSO-d6) (400
MHz) δ 0.78 (3H, d, J ) 6 Hz, CH3), 1.18-1.65 (9H, m, 4 CH2,
CHCH3), 2.22 (1H, J ) 15 Hz, 1 of CH2CO2H), 2.49 (1H, J )
15 Hz, 1 of CH2CO2H), 2.91 (1H, d, J ) 14 Hz, 1 of CH2NH3+),
3.15 (1H, d, J ) 14 Hz, 1 of CH2NH3+), 7.90 (3H, br s, NH3+),
(9-(Am in om eth yl)bicyclo[3.3.1]n on -9-yl)acetic Acid Hy-
d r och lor id e (33). Synthesized via route A in 6% overall yield
from bicyclo[3.3.1]nonan-9-one: 1H NMR (DMSO-d6) (400
MHz) δ 1.24-1.66 (8H, m), 1.74-2.16 (6H, m), 2.63 (2H, s,
CH2CO2H), 3.22 (2H, s, CH2NH3+), 7.90 (3H, br s, NH3+), 12.43
(1H, br s, CO2H); IR (MeOH) 3419, 3172, 2934, 1717, 1614
cm-1; MS (CI) m/e 194 (100%, MH+ - H2O). Anal. (C12H21
-
NO2‚1.8HCl) C, H, N. Sample shown to be a homogeneous
peak by HPLC utilizing a C18 prodigy ODS3 column eluting
with a gradient of 40% MeCN/60% H2O to 100% MeCN and
also, using the same column, eluting with a gradient of 30%
MeOH/70% H2O to 100% MeOH.
(7-(Am in om eth yl)bicyclo[2.2.1]h ept-7-yl)acetic Acid Hy-
d r och lor id e (34). Synthesized via route A in 7% overall yield
from bicyclo[2.2.1]heptanan-7-one: 1H NMR (DMSO-d6) (400
MHz) δ 1.24 (4H, m, 4 CH), 1.73 (4H, m, 4 CH), 2.00 (2H, s, 2
CH), 2.50 (2H, s, CH2), 3.31 (2H, s, CH2), 7.78 (3H, br s, NH3+),
12.36 (1H, br s, CO2H); IR (film) 3439, 2957, 2887, 1703, 1675,
1610 cm-1; MS (APCI) m/e 184 ([MH - HCl]+, 90%). Anal.
(C10H18NO2Cl) C, H, N.
Sp ir o[p yr r olid in e -3,2′-t r icyclo[3.3.1.13,7]d e ca n e ]-5-
on e (36). Synthesized via route A in 19% overall yield from
adamantan-2-one: 1H NMR (CDCl3) (400 MHz) δ 1.60-1.87
(14H, m, 4 CH and 5 CH2), 2.37 (2H, s, CH2), 3.35 (2H, s, CH2),
5.67 (1H, br s, NH); IR (film) 3402, 3236, 1674, 1487, 1452
cm-1; MS (ES+) m/ e: 206 ([MH]+, 100%). Anal. (C13H19NO)
C, H, N.
(4-(Am in om eth yl)tetr ah ydr opyr an -4-yl)acetic Acid Hy-
d r och lor id e (37). Synthesized via route A in 3% overall yield
from tetrahydro-4H-pyran-4-one: 1H NMR (DMSO-d6) (400
MHz) δ 1.40-1.60 (4H, m, 2 CH2CH2-O-), 2.53 (2H, s, CH2-
CO2H), 3.02 (2H, s, CH2-NH3+), 3.50-3.70 (4H, m, CH2-O-
CH2), 8.02 (3H, br s, NH3+), 12.45 (1H, br s, CO2H); IR (film)
1026, 1514, 1611, 1712, 2936 cm-1; MS (ES+) m/ e: 174 ([(M
- HCl)H]+, 95%). Anal. (C8H15NO3‚HCl) C, H, N.
(Tetr a h yd r oth iop yr a n -4-ylid en e)a cetic Acid Eth yl Es-
ter (41). A solution of tetrahydrothiopyran-4-one (2.50 g, 21.6
mmol) and (carbethoxymethylene)triphenylphosphorane (9.0
g, 25.9 mmol) was heated to reflux in toluene (30 mL) for 18
h. The mixture was cooled to room temperature and evapo-
rated to dryness in vacuo. The residue was purified by flash
chromatography (silica, ether/hexane, 1:1) to give 3.77 g (94%)
of 42 as an oil: 1H NMR (CDCl3) (400 MHz) δ 1.28 (3H, t, J )
7.2 Hz, CH3), 2.50-2.55 (2H, m, CH2CH2S), 2.74-2.80 (4H,
m, CH2-S-CH2), 3.18-3.21 (2H, m, CH2CH2S), 4.15 (2H, q,
J ) 7.2 Hz, CH2CH3), 5.67 (1H, s, vinylic); IR (film) 1649, 1713,
2908, 2981 cm-1; MS (CI) m/e 187 ([M + H]+, 15%). Anal.
(C9H14O2S) C, H, N.
(4-(Nitr om eth yl)tetr a h yd r oth iop yr a n -4-yl)a cetic Acid
Eth yl Ester (42). The unsaturated ethyl ester 42 (1.00 g,
5.3 mmol) was heated to reflux under nitrogen in nitromethane
(50 mL) with tetramethylguanidine (0.5 mL) for 10 h. The
mixture was cooled to room temperature, diluted with ethyl
acetate (100 mL), and washed with 1 N HCl (3 × 50 mL). The
organic solution was separated and dried (MgSO4), and the
solvent was removed in vacuo. The residue was purified by
flash chromatography (silica, ethyl acetate/heptane, 1:1) to give

Novel Ligands for the Gabapentin Binding Site
J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 11 1845
0.41 g (31%) of 43 as a colorless oil: 1H NMR (CDCl3) (400
MHz) δ 1.28 (3H, t, J ) 7.2 Hz, CH3), 1.85-2.00 (4H, m, 2
CH2CH2S), 2.54 (2H, s, CH2CO2Et), 2.60-2.75 (4H, m, CH2-
S-CH2), 4.17 (2H, q, J ) 7.2 Hz, CH2CH3), 4.72 (2H, s, CH2-
NO2); IR (film) 1374, 1458, 1549, 1728 cm-1; MS (EI) m/e 247
([M]+, 100%). Anal. (C10H17NO4S), C, H, N.
(5) Satzinger, G. Antiepileptics from gamma-aminobutyric acid.
Arzneimittelforschung 1994, 44, 261-6.
(6) Nanavati, S. M.; Silverman, R. B. Design of potential anticon-
vulsant agents: mechanistic classification of GABA aminotrans-
ferase inactivators. J . Med. Chem. 1989, 32, 2413-21.
(7) Bowery, N. G.; Pratt, G. D. GABAB receptors as targets for drug
action. Arzneimittelforschung 1992, 42 (2A), 215-23.
(8) Taylor, C. P. Emerging perspectives on the mechanism of action
of Gabapentin. Neurology 1994, 44, S10-16.
(9) Thurlow, R. J .; Brown, J . P.; Gee, N. S.; Hill, D. R.; Woodruff,
G. N. [3H]Gabapentin may label a system-L-like neutral amino
acid carrier in brain. Eur. J . Pharmacol. 1993, 247, 341-55.
(10) McLean, M. J . Gabapentin. Epilepsia 1995, 36, S73-86.
(11) Beydoun, A.; Uthman, B. M.; Sackellares, J . C. Gabapentin:
pharmacokinetics, efficacy, and safety. Clin. Neuropharmacol.
1995, 18 (6), 469-81.
(12) Suman-Chauhan, N.; Webdale, L.; Hill, D. R.; Woodruff, G. N.
Characterisation of [3H]gabapentin binding to a novel site in rat
brain: homogenate binding studies. Eur. J . Pharmacol. 1993,
244, 293-301.
(13) Hill, D. R.; Suman-Chauhan, N.; Woodruff, G. N. Localization
of [3H]Gabapentin to a novel site in rat brain: autoradiographic
studies. Eur. J . Pharmacol. 1993, 244, 303-9.
(14) Gee, N. S.; Brown, J . P.; Dissanayake, V. U. K.; Offord, J .;
Thurlow, R.; Woodruff, G. N. The novel anticonvulsant drug,
Gabapentin (Neurontin), binds to the R2δ subunit of a calcium
channel. J . Biol. Chem. 1996, 271 (10), 5768-76.
(15) Bryans, J . S.; Davies, N.; Gee, N. S.; Horwell, D. C.; Kneen, C.
O.; Morrell, A. I.; O’Neill, J . A.; Ratcliffe, G. S. Investigation into
the preferred conformation of gabapentin for interaction with
its binding site on the R2δ subunit of a calcium channel. BioOrg.
Med. Chem. Lett. 1997, 7 (19), 2481-4.
(16) Bryans, J . S.; Horwell, D. C.; Ratcliffe, G. S. Unpublished results.
(17) Griffiths, G.; Mettler, H.; Mills, L. S.; Previdoli, F. Novel
syntheses of Gabapentin via addition of hydrocyanic acid to
cyclohexylidenemalonate or cyano(cyclohexylidene)acetate. Helv.
Chim. Acta 1991, 74, 309-14.
(18) Nagata, W.; Yoshioka, M. Hydrocyanation of conjugated carbonyl
compounds. Org. React. 1977, 25, 255-476.
(4-(Am in om eth yl)tetr ah ydr oth iopyr an -4-yl)acetic Acid
Hyd r och lor id e (38). The nitro ester 43 (0.40 g, 1.62 mmol)
was dissolved in concentrated hydrochloric acid (20 mL) with
tin(II) chloride (1.50 g). The mixture was heated to 100 °C
for 2 h. The mixture was then evaporated to dryness in vacuo.
The residue was purified by reverse-phase chromatography
to give 0.10 g (26%) of 39 as colorless crystals: 1H NMR
(DMSO-d6) (400 MHz) δ 1.65-1.80 (4H, m, 2 CH2CH2-S), 2.44
(2H, s, CH2CO2H), 2.54-2.67 (4H, m, CH2-S-CH2), 2.95 (2H,
s, CH2NH3+), 7.99 (3H, br s, NH3+), 12.42 (1H, br s, CO2H);
IR (film) 1525, 1582, 1712, 2959, 3382 cm-1; MS (ES-) m/e 188
([(M - HCl) - H]-, 100%). Anal. (C8H15NO2S‚HCl) C, H, N.
(4-(Am in om eth yl)p ip er id in -4-yl)a cetic Acid Dih yd r o-
ch lor id e (39). Synthesized via route A in 3% overall yield
from N-(tert-butoxycarbonyl)-4-piperidone: 1H NMR (DMSO-
d6) (400 MHz) δ 1.70 (4H, m, CH2CCH2), 2.14 (2H, s, CH2-
+
CO2H), 2.9-3.2 (6H, m, CH2NH3 and CH2NH2+CH2), 8.15
(3H, br s, NH3+), 8.81 (2H, br s, NH2+), 12. 14 (br s, CO2H);
MS (ES+) m/e 173 ([(MH - 2HCl)]+, 80%). Anal. (C8H16N2O2‚
2HCl) C, H, N.
Refer en ces
(1) Taylor, C. P. The Role of Gabapentin. In New Trends in Epilepsy
Management; Chadwick, D., Ed.; Royal Society of Medicine
Services Ltd.: London, 1993; pp 13-40.
(2) Singh, L.; Field, M. J .; Ferris, P.; Hunter, J . C.; Oles, R. J .;
Williams, R. G.; Woodruff, G. N. The antiepileptic agent Gaba-
pentin (Neurontin) possesses anxiolytic-like and antinociceptive
actions that are reversed by D-serine. Psycopharmacol. Berl.
1996, 127 (1), 1-9.
(3) Rosner, H.; Rubin, L.; Kestenbaum, A. Gabapentin adjunctive
therapy in neuropathic pain states. Clin. J . Pain 1996, 12 (1),
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(4) Zapp, J . J . Postpoliomyelitis pain treated with Gabapentin. Am.
Fam. Physician 1996, 53 (8), 2442, 2445.
(19) Gassman, P. G.; Marshall, J . L. Bicyclo[2,2,1]hepten-7-one.
Organic Syntheses; Wiley: New York, 1973; Coll. Vol. 5, pp 91,
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