ON THE OXIMATION OF DIARYL-BETA-DIKETONES

The reaction of asymmetrically substituted beta-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.