New synthesis of 5,5-dimethyl-3-oxo-4,5-dihydro-3H-pyrrole 1-oxide

A procedure has been developed for the synthesis of 5,5-dimethyl-3-oxo-4,5-dihydro-3H-pyrrole 1-oxide via oxidation of 2,2,6,6-tetramethyl-4-oxopiperidine 1-oxyl with chlorine in aqueous sodium perchlorate and subsequent ring contraction in the resulting 2,2,6,6-tetramethyl-1,4-dioxopiperidinium perchlorate in dilute perchloric acid. The relative rates of proton exchange between the CH₂ and CH groups in the title compound were determined by ¹H NMR spectroscopy in D₂O, and the acidity constant was estimated on the basis of the dependence of the UV spectrum upon pH (pK _a = 13.2).