Study of reactions induced by hydroxylamine treatment of esters of organic acids and of 3-ketoacids: application to the study of urines from patients under valproate therapy

Hydroxylamine used at alkaline pH as oximating agent in the search for organic aciduria by gas chromatography/mass spectrometry (GC/MS) induces other chemical reactions. Esters are partially transformed in their corresponding hydroxamic acids. GC/MS characteristics of the trimethylsilylated derivatives of the hydroxamic acids arising from alpha-unsaturated esters are here reported. Their mass spectral fragmentation helps in the recognition of peaks arising from the glucuronides of 2-ene- and probably 2,3'-diene-valproic acid. By heating in the injection port of the gas chromatograph, part of some trimethylsilylated hydroxamic acids are transformed to the corresponding isocyanates by a Lossen-like rearrangement. In addition to the corresponding hydroxamic acids, hydroxylamine treatment of alpha-unsaturated esters forms 2-isoxazolidin-3-ones by intramolecular Michael addition. GC/MS characteristics of the trimethylsilylated derivatives of these compounds are reported. Submitted to hydroxylamine, 3-ketoacids forms 2-isoxazolin-5-ones by cyclization of the oximes after acidification. This explains the existence of two GC peaks observed from urine extracts of patients under valproate therapy, which correspond to two tautomers of 2-isoxazolin-5-one originating from the oximes of the 3-keto-valproic acid.