Ibotenic acid: on the mechanism of its conversion to [3H] muscimol

Muscimol is a small but structurally unique isoxazole alkaloid isolated from the mushroom Amanita muscaria . Because of its potent gamma aminobutyric acid (GABA) agonist activity, muscimol was the elusive target of tritiation attempts for many years. Our laboratory was the first to solve this radiolabelling challenge by a robust and straightforward high specific activity synthesis of [ 3 H] muscimol involving treatment of related amino acid ibotenic acid with tritiated water in dimethyl sulfoxide at ambient temperature. The surprising simplicity of this method prompts the question of exactly how, when and where tritium is installed in the muscimol framework. Using tritium NMR and mass spectrometry analysis of [ 3 H] muscimol, we now propose a likely answer and sequence of events to explain this mechanistic question.