Electron ionization mass spectra of some norbornane/ene-fused 2-N-phenyliminoperhydro-1,3-oxazines

The 70 eV electron ionization (EI) mass spectra were recorded for eight norbornane/ene-fused 2-N-phenyl-iminoperhydro-1,3-oxazines, and the fragmentation patterns were studied by metastable ion analysis and exact mass measurement. Whereas the stereoisomeric unsaturated compounds could not be distinguished on the basis of their EI mass spectra, the stereoisomeric saturated compounds gave rise to clearly different spectra. The ionized unsaturated compounds decomposed mainly by two consecutive retro-Diels-Alder (RDA) reactions. A methyl substituent on the ring nitrogen strongly influenced the charge distribution on the RDA fragments. The ionized saturated compounds fragmented through several pathways. Loss of cyclopentadiene from the molecular ion was the energetically favoured fragmentation reaction for the saturated di-endo-fused compounds but was unimportant for the di-exo-fused compounds.