(3a) and (3b) and the 4-acetoxyazetidin-2-ones (4c–e) in the presence of acetic acid at 21–40 °C and without solvent at 100–110 °C were carried out to elucidate the rearrangements. The major products from (3a) and (3b) in acidic solutions at 40 °C were N-acetyl-3-acetamido-2,4,4,N-tetramethylpent-2-enamide (6a), N-methylacetamide (9a), 2,5-dimethyl-4-t-butyl-1,3-oxazin-6-one (10a), 2-piperidone (9b)