3-Halo-5,6-dihydro-4-1,2-oxazine -oxides as synthetic equivalents of unsaturated nitrile oxides in the [3 + 2]-cycloaddition with arynes: synthesis of substituted 3-vinyl-1,2-benzisoxazoles

The reaction of 3-halo-5,6-dihydro-4 H -1,2-oxazine N -oxides with arynes was studied. Arynes were generated from o -silylaryl triflates and underwent consecutive [3 + 2]-cycloaddition/[4 + 2]-cycloreversion with N -oxides leading to substituted 3-vinyl-benzisoxazoles in high yields. In the presented sequence, 1,2-oxazine N -oxides act as surrogates of rarely employed unsaturated nitrile oxides. A broad substrate scope was demonstrated. The influence of the substitution pattern of an aryne on the reaction outcome was determined. In the presence of bulky substituents, polycyclic 4,4a-dihydro-3 H -benzofuro[3,2- c ][1,2]oxazines were selectively formed. Mechanistic schemes for the observed reaction pathways were proposed. The synthetic utility of the products was demonstrated by their follow-up modifications. 3-Halo-1,2-oxazine N -oxide derivatives act as surrogates of vinyl nitrile oxides in tandem [3 + 2]-cycloaddition/[3 + 2]-cyclofragmentation with arynes leading to benzisoxazoles.