The condensation between benzonitrile oxide and 1-acetyl- or 1-benzoyl-cyclopentene gave the 3-phenyl-5 (or 4) -acyl-4, 5-trimethylene-2-isoxazolines. The Schmidt reaction on both ketones furnished, together with the corresponding isoxazoline amides, variable quantities of 3-phenyl-5, 6-dihydrocyclopent[d]isoxazole, whose structure has been confirmed through ring opening hydrogenation and through an independent synthesis, starting from 3-phenyl-4,5-trimethylene-2-isoxazoline and N-bromosuccinimide with subsequent dehydrobromination of the intermediate 4-bromoderivative.