Cycloreversal reaction and ortho interactions in 2-aryl-4H-3, 1-benzoxazin-4-ones on electron impact

A cycloreversal reaction, leading to aroyl cations, is the major process in 2‐aryl‐4H‐3,1‐benzoxazin‐4‐ones under electron impact conditions. The ortho interaction of the methoxy and the nitro groups in the 2‐phenyl moieties in these compounds present the most abundant ions at m/z 119 and 134, respectively, in their mass spectra as a result of the transfer of a hydrogen atom from the former and an oxygen atom from the latter to the imine nitrogen of the heterocycle. The ion structures and the mechanisms for the proposed fragmentations are based on high‐resolution data, B/E and B2/E linked‐scan spectra, collision‐activated decomposition–B‐/E linked‐scan spectra and deuterium labelling.