Synthesis and photooxygenation of 3-(p-substituted phenyl)-3a,8a-dihydro-4H-cyclohepta[d]isoxazoles: Facial selectivity

Two 3-(<i>p</i>-substituted phenyl)-3a,8a-dihydro-4<i>H</i>-cyclohepta[d]isoxazoles were synthesized by 1,3-dipolar cycloaddition of the corresponding nitrile oxides with cycloheptatriene. Two endoperoxides were synthesized as facially selective and single products in high yields (93%-95%) from the reactions of isoxazole derivatives with singlet oxygen. The exact configurations of the endoperoxide with a methyl group in the phenyl ring and the diol synthesized from it were confirmed by X-ray analysis. To elucidate the mechanism, the formation energy of the endoperoxide was investigated by simulations using the software package Gaussian 09 and density functional theory calculations via the M06-2X/6-311+G(d,p) level method in dichloromethane. The results were consistent with experimental findings showing the formation of isoxazole products.