A new 2-isoxazoline ring-opening pathway leading to β-(acylaminooxy) ketones 9 is described. It occurred through covalent hydration of the C−N double bond giving stable 3-isoxazolidinol 5, followed by acylation with anhydrides. The ring-chain tautomerism of 3-isoxazolidinol 5 in comparison with its N-acyl(aroyl) derivatives 9, 10 was studied by NMR and chiral liquid chromatography. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)