Synthesis and in vivo evaluation of 3,4-disubstituted gababutins

Article abstract of DOI:10.1016/j.bmcl.2009.10.121

A range of 3,4-alkylated five-membered ring derivatives of gabapentin were synthesised. One compound (21) had an excellent level of potency against α₂δ and was profiled in in vivo models of pain and anxiety.

Full text of DOI:10.1016/j.bmcl.2009.10.121

Bioorganic & Medicinal Chemistry Letters 20 (2010) 248–251
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and in vivo evaluation of 3,4-disubstituted gababutins
a
a
a
a
David C. Blakemore a, , Justin S. Bryans , Pauline Carnell , Mark J. Field , Natasha Kinsella ,
*
Jack K. Kinsora b, Leonard T. Meltzer b, Simon A. Osborne a, Lisa R. Thompson a, Sophie C. Williams a
a Sandwich Laboratories, Pfizer Global Research and Development, Ramsgate Road, Sandwich, Kent CT13 9NJ, UK
b Groton Laboratories, Eastern Point Road, Groton, CT 00340, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A range of 3,4-alkylated five-membered ring derivatives of gabapentin were synthesised. One compound
Received 11 October 2009
Revised 26 October 2009
Accepted 27 October 2009
Available online 30 October 2009
(21) had an excellent level of potency against
a
2d and was profiled in in vivo models of pain and anxiety.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Gababutin
Gabapentin
Pregabalin
Pain
Anxiety
Gabapentin (NeurontinÒ) (1)1 was launched as an add-on ther-
apy for epilepsy in 1994. Utility against neuropathic pain2,3 and
anxiety4,5 have been reported preclinically and efficacy against
neuropathic pain has been demonstrated clinically in humans.6
Pregabalin7,8 (LyricaÒ) (2), has superior potency and pharmacoki-
netics9 to gabapentin and has been approved for the management
of neuropathic pain associated with diabetic peripheral neuropa-
thy, post-herpetic neuralgia, adjunctive treatment of partial sei-
zures, and fibromyalgia in the US.
a range of 3-substituted gababutin analogues and identified the
3-(R)-methyl gababutins (3) and (4). Both (3) and (4) bind to the
gabapentin binding site with high affinity but have different
in vivo profiles, with (3) being effective on oral dosing in models
of anxiety and (4) being effective on oral dosing in models of neu-
ropathic pain. We now wish to report an extension to this study
which has focused on the synthesis and biological evaluation of
3,4-disubstituted gababutins.
The synthesis of 3,4-dialkyl gababutins presents a different set
of challenges to the 3-alkyl gababutins.
HO2C
NH2
H2N
CO2H H2N
CO2H
HO2C
NH2
CO2H
O
Control of key
quaternary centre
unnecessary
NH2
NH2
(pseudo-asymmetric centre)
(1)
(2)
(3)
IC50=30nM
(4)
IC50=71nM
Single enantiomer
(C2 symmetric)
IC50=140nM
Gabapentin
IC50=80nM
Pregabalin
CO2H
O
Control of key
quaternary centre
provided by cis-
dimethyl groups
Gabapentin and pregabalin are thought to mediate their phar-
macological actions through binding to the 2d subunit of a voltage
gated calcium channel10,11 and it has been shown that gabapentin
and pregabalin bind to this 2d subunit with IC50 values of 140 nM
a
Meso ketone
a
and 80 nM, respectively. We have recently disclosed our initial SAR
investigations around five-membered ring gabapentin analogues,
which we have termed gababutins.12 In that Letter, we investigated
The two alkyl groups can be cis or trans to one another. In the
case of the 3,4-dimethyl gababutin, if the methyl groups are trans,
then the stereochemistry of the quaternary centre carrying the
aminomethyl group is irrelevant as this centre is pseudo-asymmet-
ric; the key problem is synthesis of the C2 symmetric chiral ketone
precursor. If the methyl groups are cis, then the starting ketone is
* Corresponding author. Tel.: +44 1304 644573.
E-mail address: david.blakemore@pfizer.com (D.C. Blakemore).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.10.121

D. C. Blakemore et al. / Bioorg. Med. Chem. Lett. 20 (2010) 248–251
249
not chiral but introduction of the stereogenic quaternary centre
carrying the aminomethyl group must be controlled (by making
use of the steric impact of the two cis-dimethyl groups).
A number of different methods were utilised to synthesise the
starting cyclopentanones. The synthesis of 3,4-trans-dimethyl
cyclopentanone (14), is detailed in Scheme 1.
An alternative approach towards the 3,4-trans-cyclopentanones
made use of 3-acetoxy cyclopent-2-en-1-one. Conjugate addition
of an alkyl Grignard reagent to the acetoxy-cyclopentenone17–19
(made with enantiomeric excess of >99%) followed by elimination
and a further conjugate addition gave the 3,4-trans-dialkyl cyclo-
pentanone without loss of stereochemical integrity (Scheme 2).
For example, conjugate addition of the n-propyl zincate (gener-
ated from n-propyl magnesium chloride and dimethylzinc)
occurred exclusively on the least hindered face of the
cyclopentenone (the acetoxy group forcing the n-propyl group
onto the opposite face) to generate propyl cyclopentanone (15).
Low temperature elimination with DBU followed by conjugate
addition of the methyl zincate occurred without any epimerisation
to give the 3,4-dialkyl cyclopentanone (17) in >98% enantiomeric
excess (as determined by chiral GC).
Fumaryl chloride (5) was converted to the diester (6) by reac-
tion with (À)-menthol in good yield. The ratio of diastereoisomers
of (7) obtained in the Diels–Alder reaction with butadiene varied
according to the choice of Lewis acid with the auxiliary setting
the facial selectivity.13–15 Diethylaluminium chloride at À60 °C
gave an excellent ratio of diastereoisomers (9:1), with diisobutylal-
uminium chloride giving an even better ratio (94:6) (as measured
by 1H NMR).16 Titanium tetrachloride at À10 °C gave a poorer dia-
stereoselectivity (83:17) but was the preferred Lewis acid for this
reaction because of its ease of handling in large scale (>1 mol) reac-
tions. Attempts to increase the diastereoselectivity by lowering the
reaction temperature were unsuccessful as the dienophile–TiCl4
complex would precipitate from solution. The lower diastereose-
lectivity did not prove to be a hindrance, as the diol (8) is highly
crystalline. Therefore, reduction of diester (7) with lithium alumin-
ium hydride afforded diol (8) as a 83:17 mixture of enantiomers
which could be recrystallised to give the (3S,4S)-diol as a single
enantiomer (the optical purity was confirmed by chiral HPLC and
chiral GC at >99.5%). The diol was then mesylated and reduced
with lithium aluminium hydride/NaH to give the volatile (bp
110 °C) alkene (10). The alkene was oxidatively cleaved with
KMnO4 under phase transfer conditions to give diacid (11) as a
white solid. This was then esterified and a Dieckmann cyclisation
with potassium tert-butoxide gave cyclopentanone ester (13).
Hydrolysis and decarboxylation was carried out using water in
DMSO to give the desired (3S,4S)-3,4-dimethyl cyclopentanone
(14). The optical integrity was confirmed by chiral GC. The pure
(3R,4R)-3,4-dimethyl cyclopentanone could be produced in a sim-
ilar manner by utilising (+)-menthol as the chiral auxiliary. The
3,4-cis-dimethyl cyclopentanone was also synthesised in a similar
manner starting from the Diels–Alder adduct, cis-1,2,3,6-tetrahydr-
ophthalic anhydride.
Conversion of the ketones through to the gababutin analogues
is detailed in Scheme 3. Ketone (14) was converted to a,b-unsatu-
rated ester (18) via a Horner–Wadsworth–Emmons reaction. Sub-
sequent nitromethane anion addition gave the nitroester (19).
Hydrogenation to the lactam (20) and then acid hydrolysis gave
the final amino acid (21).
For 3,4-cis-dimethyl cyclopentanone, two possible diastereoiso-
mers of the final gababutin are possible and control of the stereo-
chemistry was obtained by making use of the steric impact of the
two methyl groups.
For synthesis of (28),
a nitroalkene route was employed
(Scheme 4). Here, ketone (22) was reacted with the dianion of
nitromethane utilising 2 equiv of butyl lithium to generate alcohol
(23). Acetylation and base-catalysed elimination gave the nitroalk-
ene (25). Low temperature conjugate addition of the ethyl acetate
anion gave nitro ester (26) as a 9:1 mixture of diastereoisomers,
addition occurring from the face opposite to the two methyl groups
O
O
O
(i)
(ii)
AcO
AcO
(15)
(16)
(iii)
O
CO2R*
CO2R*
COCl
COR*
(i)
(ii)
ClOC
R*OC
(17)
(5)
(6)
(7)
(iii)
Scheme 2. Reagents and conditions: (i) n-PrMgCl, Me2Zn, THF, À78 °C; (ii) DBU,
Et2O, À40 °C (68% over two steps); (iii) MeMgCl, Me2Zn, THF, À78 °C (63%).
(v)
(iv)
OMs
OMs
OH
OH
(10)
(9)
(8)
CO2Et
(vi)
NO2 CO2Et
O
(i)
(ii)
MeO2C
O
(viii)
(vii)
HO2C
HO2C
MeO2C
MeO2C
(14)
(18)
(19)
(iii)
(11)
(12)
(13)
(ix)
O
R* =
H
N
O
O
NH2 CO2H
(iv)
(14)
Scheme 1. Reagents and conditions: (i) (À)-menthol, pyridine, CH2Cl2; (ii) buta-
diene, TiCl4, toluene, À10 °C (100% yield, 65% de) or butadiene, Et2AlCl, toluene,
À60 °C (64% yield, 95% de); (iii) LiAlH4, THF; recrystallisation from acetone; (iv)
pyridine, MsCl, 0 °C, 18h (82%); (v) LiAlH4, diethyl ether, 40 °C, 2h (98%); (vi)
KMnO4, nBu4NBr, H2O–CH2Cl2, rt, 18h; then SO2, 0 °C (82%); (vii) methanol, cH2SO4,
rt, 18h (90%) (viii) KOtBu, THF, 75 °C, 3h (100%); (ix) DMSO, H2O, 140 °C, 4 h (86%).
(21)
(20)
Scheme 3. Reagents and conditions: (i) triethylphosphonoacetate, NaH, THF, 0 °C
to rt (95%); (ii) MeNO2, TBAF, THF, reflux (65%); (iii) H2, Ni, MeOH; (iv) 6 N HCl, 1,4-
dioxane, reflux (69% from nitroester).

250
D. C. Blakemore et al. / Bioorg. Med. Chem. Lett. 20 (2010) 248–251
NO2
NO2
NO2
O
CO2H
CO2H
CO2H
Et
H2N
H2N
H2N
Et
HO
AcO
(i)
(ii)
(iii)
(22)
(23)
(24)
(25)
(iv)
(28)
IC50=107nM
(31)
IC50=153nM
(36)
IC50=4200nM
O
H
N
H2N
CO2H
O2N
CO2Et
Compound (21) was evaluated in in vivo models of pain and
anxiety. The compound proved to have a similar profile to pregab-
alin in anxiety models as illustrated for the water-lick (Vogel) con-
flict model of anxiety (Fig. 1). In this model, a group of rats were
dosed orally with compound and were then placed in a cage con-
taining a drinking tube from which they received a shock each time
they drank. The mean number of shock episodes received per rat
over 10 min was measured at different doses. The more anxiety
that the animals feel, the less they drink and the less shocks they
receive. For an effective compound, as the dose of compound in-
creases then the number of shocks received by the animals should
also increase.
(vi)
(v)
(28)
(27)
(26)
Scheme 4. Reagents and conditions: (i) BuLi (2 equiv), MeNO2, THF, HMPA, À78 °C
(40%); (ii) Ac2O, H2SO4, reflux (98%); (iii) KOH, MeOH, 0 °C (53%); (iv) AcOEt,
LHMDS, THF, À78 °C (45%); (v) H2, Raney Ni, MeOH; (vi) 6 N HCl, 1,4-dioxane, reflux
(45% from nitroester).
The minimum effective dose (MED) for activity for compound
(21) in this model was identical to that of pregabalin at 10 mg/
kg. It was noted, however, that at higher doses of compound
(21), sedation of the animals was occurring.
CO2Et
NO2 CO2Et
CO2H
NH2
Compound (21) was also evaluated in the carageenan induced
thermal hyperalgesia model of pain (Fig. 2). In this model, the
(29)
(30)
(31)
10:1 mixture
of diastereoisomers
Scheme 5.
(1H NMR, 2D NMR and HPLC confirmed ratio and relative stereo-
chemistry). Reduction and spontaneous cyclisation gave the lactam
(27) which was hydrolysed without purification in 6 N HCl to give
the amino acid (28). A single recrystallisation from ethyl acetate/
methanol/heptane gave (28) as a single diastereoisomer as con-
firmed by 1H NMR and HPLC analysis.
Utilising a nitromethane conjugate addition route from (29)
gave the alternative diastereoisomer (31) (Scheme 5).
As with the 3-alkyl gababutins, chirality proved to have a signif-
icant impact on binding affinity at
a
2d (a radioligand binding assay
incorporating [3H]gabapentin at the
a
2d subunit of a calcium chan-
nel was utilised in this work, as previously described10), with one
enantiomer of 3,4-trans-dimethyl gababutin, (21), having excellent
levels of potency while the other enantiomer, (32), bound weakly.
The racemic 3,4-trans-diethyl gababutin, (33) also had weak bind-
Figure 1. * = P <0.05, significantly different compared to vehicle.
ing affinity at
a2d proving consistent with our 3-alkyl gababutin
hypothesis that space in the binding pocket is limited; however,
replacement of one methyl group of (21) by a propyl group to give
(34) (as a mixture of diastereoisomers) was not detrimental to
binding affinity.
H2N
CO2H
H2N
CO2H
H2N
CO2H
H2N
CO2H
(+/-)
Et
(33)
IC50=1100nM
Et
(34)
IC50=21nM
(21)
IC50=22nM
(32)
IC50=882nM
1:1 mixture of
diastereoisomers
The 3,4-cis-dimethyl gababutins (28) and (31) both had reason-
able levels of potency against 2d but were considerably less po-
tent than the trans-dimethyl analogue (21). As with the trans
analogues, the diethyl analogue (36) bound weakly to 2d.
a
a
Figure 2. * = P <0.05, ** = P <0.01, significantly different compared to vehicle.

D. C. Blakemore et al. / Bioorg. Med. Chem. Lett. 20 (2010) 248–251
251
of 1.2 h makes the compound ideal as a fast-acting agent that is re-
moved from the system rapidly. Compound (21) has been pro-
gressed for further studies.
pro-inflammatory agent carrageenan was administered via intrapl-
antar injection into the paw of a rat. Within 2 h of injection, the
paw became inflamed and hyperalgesia had been induced. An
infra-red light source was shone on the paw and the time it took
the animal to withdraw its paw from the heat-source was mea-
sured. Before treatment, baseline measurements were taken giving
a reading of 10 s to paw withdrawal. After 2 h, once the hyperalge-
sia had fully formed the time to paw withdrawal had reduced to
2.5 s. At this point the compound to be assessed was dosed orally
and any reversal of the hyperalgesia was assessed by measuring
the time to paw withdrawal.
Compound (21) showed excellent levels of efficacy in this mod-
el with a 30 mg/kg dose fully reversing the hyperalgesia and giving
a similar effect to that of pregabalin. Again, sedation of the animals
was noted at higher dosing.
The pharmacokinetic profile of compound (21) in the rat is
shown below:
Acknowledgements
We would like to thank Jane McGuffog for GC/HPLC analysis
and Giles Ratcliffe for NMR studies.
References and notes
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(%)
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(21)
87
2.4
4
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Compound (21) is a highly polar (log D7.4 = À1.0), low molecular
weight zwitterionic molecule; it has a pharmacokinetic profile that
is similar to pregabalin with high bioavailability (driven by active
transport from the gut combined with negligible first pass metabo-
lism) and a short half-life in rat. The compound is renally cleared at
just under glomerular filtration rate with no noticeable reabsorp-
tion in the proximal tubule. The short half-life coupled with a tmax