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 CH(2) and CH groups in the title compound were determined by (1)H NMR spectroscopy in D(2)O, and the acidity constant was estimated on the basis of the dependence of the UV spectrum upon pH (pK (a) = 13.2).