Herein　we　report　the　highly　regioselective　synthesis　of　5-　and　4-(2,1-azaborine)　substituted　isoxazoles　via　[3　+　2]　cycloaddition　reactions　between　nitrile　oxides　and　B-ethynyl-1,2-azaborines.　5-(2,1-Azaborine)　substituted　isoxazoles　are　favored　in　the　absence　of　a　catalyst,　while　4-(2,1-azaborine)　substituted　isoxazoles　are　favored　in　the　presence　of　a　ruthenium　catalyst.　The　reaction　exhibits　excellent　regioselectivity,　mild　reaction　conditions,　high　functional　group　tolerance　and　a　broad　substrate　scope.