A new simple and efficient metal-free 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) promoted regioselective synthesis of 3,5-disubstituted isoxazoles and isoxazolines from aldoximes has been described. This method allows the reaction to proceed efficiently on aldoximes containing unprotected phenolic hydroxyl groups. Furthermore, with the use of higher equivalents of N-chlorosuccinimide, chloro-substituted isoxazoles and isoxazolines were obtained as the only products via tandem one-pot 1,3-dipolar cycloaddition followed by regioselective chlorination.