Investigations into the function and pharmacology of cys-loop
GABA receptors in both mammalian and invertebrates systems
have relied, in part, on their sensitivity to various agonists and
synthesized GABA derivatives.1-3 The conformationally
restricted analogue of GABA, trans-4-aminocrotonic acid
(TACA) (Fig. 1) has been studied extensively and used to
distinguish different GABA receptor subtypes.4 While now
available commercially at a high cost (ca. $1000/100mg), TACA
was originally prepared through the dehydration of 4-amino-3-
hydroxybutyric acid (GABOB).5 Since then there have been no
published reports of alternative approaches for the synthesis of
TACA from GABOB. However, numerous reports continue to
utilize TACA as a pharmacological tool in various receptor
studies.6-9 As such, this study presents a new and reliable method
to synthesize TACA from GABOB at an overall cost of
approximately $25/100mg.
Given that both stereoisomers of 4-amino-3-hydroxybutyric
acid (R(–)-GABOB and S(+)-GABOB) are ligands for the
receptor, we sought to also examine whether other closely related
electronic isostere derivatives would make suitable substrates.
The hydroxyl group in both stereoisomers of GABOB is
electronegative. Therefore, we wondered whether other
electronegative derivatives would serve as suitable substrates.
As such, we decided to synthesize halogenated GABA
derivatives with chlorine or bromine substitution at the C3
position.
Commercially available racemic GABOB was converted to a
methyl ester derivative 1 by reaction with trimethylsilyl chloride
and anhydrous methanol in quantitative yield.10 Subsequently,
the amino group was protected with the tert-butoxycarbonyl
group, by reacting 1 with di-tert-butyl dicarbonate (BOC2O) in
basic solution to afford 2 in good yield. Substitution of the
hydroxyl group of 2 with trichloroacetonitrile and
triphenylphosphine afforded the chlorinated derivative 3 in 65%
yield, whereas reaction of 2 with triphenylphosphine with carbon
tetrabromide afforded the brominated derivative 4 in 75% yield.
Finally, deprotection of the BOC group, first with trifluoroacetic
acid, followed by ester hydrolysis with a 2M HCl solution under
reflux, afforded the halogenated derivatives 7 and 8 in excellent
yield (Scheme 1).
Conditions: (a) 2 equiv of trimethylsilyl chloride (TMSCl), methanol, r.t.,
100%; (b) di-tert-butyl dicarbonate (BOC2O), NaHCO3, THF, water, 24 hr,
76%; (c) 3 equiv of triphenylphosphine, 1.5 equiv of trichloroacetonitrile,
toluene, 70 °C, 15 min, 65% of 3; (d) 2.0 equiv of triphenylphosphine, 2.0
equiv of carbon tetrabromide, toluene, r.t., 1 hr, 75% (e) TFA, DCM, r.t.,
82% (5) or 98% (6); (f) 2M HCl, 3 hr, 84% (7) or 100% (8).
Scheme 1. Synthesis of chlorinated and brominated GABOB
derivatives.
TACA is a well characterized GABA receptor agonist.4 The
report describing its synthesis is several decades old and is poorly
characterized.5 Therefore, based on the successful syntheses of
the halogenated derivatives 3 and 4, we sought to expand the
synthesis of TACA by subjecting compound 4 to basic
conditions, in order to determine if elimination of the chloride
group would afford the alkene TACA.
We investigated a variety of bases (potassium tert-butoxide,
potassium carbonate, triethylamine, and sodium hydride) in the
presence of compound 4 to determine which base would be most
appropriate to form the TACA derivative. We identified that
with three equivalents of K2CO3, the allyl compound 9 formed in
good yield. Deprotection of both the BOC group, and the methyl
ester in refluxing 2M HCl afforded the TACA salt in excellent
yield (Scheme 2).
Figure 1. Structure of TACA
Conditions: (a) 3 equiv of K2CO3, DMF:THF = 1:1, r.t., 70%; (b) 2M HCl, 3
h, reflux, 86%.
Scheme 2. Synthesis of TACA
Forrester, Sean G.
