| Abstract: | A reaction of 2-(2-nitrobenzoylmethyl)-1,3-dioxolane ( 3 ) with hydroxylamine, followed by acid catalyzed cyclization, produced 5-(2-nitrophenyl)isoxazole ( 5 ) as the only isolable product, whereas 2-(benzoylmethyl)-1,3-dioxolane ( 9 ) under identical conditions produced a 2.5:1 mixture of 3-phenyl and 5-phenylisoxazoles 10 and 11 . These findings contradict the literature report that β-keto ethyleneacetals on treatment with hydroxylamine produce exclusively 3-substituted isoxazoles. As an additional proof, 3-(2-nitrophenyl)isoxazole ( 8 ) was prepared by an unambiguous method via the nitrile oxide route for comparison. The intermediate obtained on treatment of 2-(2-nitrobenzoylmethyl)-1,3-dioxolane ( 3 ) with hydroxylamine was found to be an isomeric mixture of 5-hydroxy-5-(2-nitrophenyl)-2-isoxazoline ( 4 ) and the syn and anti mono-oximes 19 (at least in solution), either of which could give 5-(2-nitrophenyl)isoxazole ( 5 ) on acid treatment. A mechanistic rationale is provided to explain the anomalous results. |
