Efficient synthesis of the GABAA receptor agonist trans-4-aminocrotonic acid (TACA)

Article abstract of DOI:10.1016/j.bmcl.2017.07.050

Investigations into the pharmacology of different types of cys-loop GABA receptor have relied for years on the chemical modification of GABA-like compounds. The GABA metabolite GABOB is an attractive molecule to modify due to its convenient chemical structure. In the process of developing new GABA-mimic compounds from GABOB as a starting compound three small molecule GABA derivatives were synthesized using a variety of chemical transformations. Amongst these, a new and reliable method to synthesize TACA (trans-4-aminocrotonic acid) is reported.

Full text of DOI:10.1016/j.bmcl.2017.07.050

Accepted Manuscript
Efficient synthesis of the GABAA receptor agonist trans-4-aminocrotonic acid
(TACA)
Sean Forrester, Joshua Foster, Sebastien Robert, Lidya Salim, Jean-Paul
Desaulniers
PII:
S0960-894X(17)30757-6
DOI:
http://dx.doi.org/10.1016/j.bmcl.2017.07.050
BMCL 25163
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
14 June 2017
17 July 2017
19 July 2017
Please cite this article as: Forrester, S., Foster, J., Robert, S., Salim, L., Desaulniers, J-P., Efficient synthesis of the
GABAA receptor agonist trans-4-aminocrotonic acid (TACA), Bioorganic & Medicinal Chemistry Letters (2017),
doi: http://dx.doi.org/10.1016/j.bmcl.2017.07.050
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Efficient synthesis of the GABAA receptor agonist trans-4-aminocrotonic acid
(TACA)
Sean Forrester, Joshua Foster, Sebastien Robert, Lidya Salim, and Jean-Paul Desaulniers
O
OH
O
5 steps
H2N
H2N
OH
OH
34.3% overall yield
- reproducible
- low cost
TACA
EC50 = 290 µM
should not be changed or altered.

Bioorganic & Medicinal Chemistry Letters
journal homepage: www.els evier.com
Efficient synthesis of the GABAA receptor agonist trans-4-aminocrotonic acid
(TACA)
Sean Forrester,a Joshua Foster,a Sebastien Robert,a Lidya Salim,a and Jean-Paul Desaulniersa,∗
aFaculty of Science, University of Ontario Institute of Technology, Oshawa, ON Canada L1H 7K4
ARTICLE INFO
ABSTRACT
Article history:
Received
Investigations into the pharmacology of different types of cys-loop GABA receptor have relied
for years on the chemical modification of GABA-like compounds. The GABA metabolite
GABOB is an attractive molecule to modify due to its convenient chemical structure. In the
process of developing new GABA-mimic compounds from GABOB as a starting compound
three small molecule GABA derivatives were synthesized using a variety of chemical
transformations. Amongst these, a new and reliable method to synthesize TACA (trans-4-
aminocrotonic acid) is reported.
Revised
Accepted
Available online
Keywords:
H. contortus
GABA
2009 Elsevier Ltd. All rights reserved
.
TACA
GABAA receptor
GABOB
———
∗ Corresponding author. Tel.: +1-905-721-8668 ext. 3621; fax: +1-905-721-3304; e-mail: jean-paul.desaulniers@uoit.ca

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

Therefore, TACA can be reliably synthesized in five steps from
racemic GABOB in 34.3% overall yield. Only two of the five
steps requires column chromatography, so it is feasible to scale-
up and synthesize TACA from GABOB in several days.
Although this synthesis is more steps compared to the reports
from Musashi in 1954,5 this is a relatively efficient approach
providing TACA in a reliably good yield.
that appears to interact with the electronegative hydroxyl group
of GABOB.9 The derivatives 7 and 8 which contain
electronegative atoms Cl and Br might not be capable of
hydrogen bonding due to their larger size and low electron
distribution and may explain why these compounds failed to
activate the channel.
In conclusion we report a relatively straightforward
method of synthesizing TACA from the starting compound
GABOB. This method, while requiring more synthetic steps
compared to the traditional method, does result in good
reproducible yield. This method can also be further modified
leading to the synthesis of additional GABA derivatives.
To test the activity of compounds TACA, 7 and 8, we
expressed the UNC-49B/C GABA receptor from the parasitic
nematode Haemonchus contortus in Xenopus laevis oocytes and
characterized the channel using 2-electrode voltage clamp
electrophysiology and procedures outlined in Kaji et al 2015.9
Compounds 7 and 8 which were tested at 500 µM did not activate
the channel. However, TACA did successfully activate the
channel with an EC50 (290 15 µM; n = 9) which is in a similar
range as previously reported for this receptor (Figure 2).9
Acknowledgments
We acknowledge UOIT for support for this project.
Supplementary Material
Experimental procedures and copies of NMRs are found here
References and notes
1. Olsen, R.W. Neurochem Res. 2014, 39,1924.
2. Ng, C.K.; Kim, H.L.; Gavande, N.; Yamamoto, I.; Kumar, R.J.;
Mewett, K.N.; Johnston, G.A.; Hanrahan, J.R.; Chebib, M. Future
Med. Chem. 2011, 3, 197.
3. Hosie, A.M.; Sattelle, D. B. Br. J. Pharmacol. 1996, 119, 1577.
4. Johnston, G.A. Neurochem Res. 2016, 41, 476.
5. Musashi, A.; Hoppe-Seyler's, Z. Physiol. Chem. 1954, 297, 71.
6. Allan, R.; Curtis, D.; Headley, P.; Johnston, G.; Lodge, D.;
Twitchin, B., J. Neurochem. 1980, 34, 652.
7. Johnston, G.; Curtis, D.; Beart, P.; Game, C.; McCulloch, R.;
Twitchin, B. J. Neurochem. 1975, 24, 157.
8. Vale, C.; Vilaro, M. T.; Rodriguez-Farre, E.; Sunol, C. J.
Neuroscience Res. 1999, 57, 95.
9. Kaji, M. D.; Kwaka, A.; Callanan, M.K.; Nusrat, H.; Desaulniers,
J.-P.; Forrester, S.G. Br. J. Pharmacol., 2015, 172, 3737.
10. Li, J.; Sha, Y. Molecules, 2008, 13, 1111.
Figure 2: Dose-response analysis of TACA activation at the
H. contortus UNC-49 receptor
It is interesting to note that the binding site of the UNC-
49B/C GABA receptor contains a serine residue at position 215
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