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The reaction of asymmetrically substituted β‐diketones with hydroxylamine to give 3,5‐diarylisoxazoles has been re‐investigated, and has been found to occur with a low degree of site‐selectivity unless steric effects are operating. The isoxazole that has the more electron‐deficient aryl group in position 3 is formed preferentially when the reactions are run with hydroxylamine hydrochloride. When the reactions are performed in a neutral medium, a reversed site‐selectivity is observed. Steric effects have a much stronger influence on the selectivity of the reaction: when a substituent is present in the ortho position(s) of the phenyl ring, the heterocycle bearing the ortho‐substituted phenyl group in position 5 is obtained as the sole product both in acidic and in neutral medium.

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