Synthesis of [3H]muscimol

Abstract

Muscimol, a potent GABA_A receptor agonist and psychoactive alkaloid found in Amanita mushrooms, is widely used as a tool compound in neurochemical research. Despite its importance, synthetic access to [³H]muscimol of high specific activity has remained limited due to the challenges associated with conventional labeling strategies. Herein, we report a novel synthetic approach for the preparation of [³H]muscimol based on the reduction of a suitably protected amide precursor using in situ generated tritioborane (BT₃·THF). The precursor was synthesized in four steps from dimethyl acetylenedicarboxylate, and subsequent electrophilic reduction afforded [³H]benzyl-protected muscimol in a radiochemical yield of 44 mCi (1.63 GBq) and a molar activity of 48.3 Ci/mmol (1.79 TBq/mmol). Final deprotection with HBr in acetic acid yielded [³H]muscimol·HBr in > 95% radiochemical purity. The method avoids the use of bulk tritiated water employed in established synthetic protocols and enables safe, reliable, and efficient access to this valuable radioligand for applications in GABA receptor studies.

Keywords: 3H NMR; B3H3; GABA; [3H]muscimol; tritioborane; tritium.

FIGURE 1

GABA receptor binding ligands.

SCHEME 1

Synthetic pathway to the key intermediate 8.

SCHEME 2

Generation of B²H₃ and reduction of the amide 8.

SCHEME 3

[³H]muscimol synthesis via in situ formation of BT₃·THF and reduction of amide 8.

FIGURE 2

HPLC‐UV chromatogram (255 nm) of crude reaction mixture of [³H]‐9 (top); radio‐HPLC chromatogram of the reaction mixture (bottom), the arrows at 17.2 and 17.5 min show peaks of [³H]‐9.

FIGURE 3

³H‐NMR spectrum of tritiated precursor [³H]‐9 in CD₄OD (top) and of [³H]muscimol in D₂O (bottom).