ELECTRON IONIZATION MASS-SPECTRA OF SOME DIEXO NORBORNANE-FUSED AND NORBORNENE-FUSED PHENYL-SUBSTITUTED 1,3-OXAZINES AND RELATED SYSTEMS - COMPETITIVE RETRO-DIELS-ALDER FRAGMENTATIONS IN ,8A-TETRAHYDRO-5,8-METHANO-4H-BENZO[E]1,3-OXAZINES

The electron ionization spectra of seven diexo norbornane- and norborn ene-fused six- and seven-membered heterocycles containing O and N in p ositions 1 and 3 are reported. Molecular ions of norbornane-fused oxaz ines, especially the 2-anilino derivative, underwent retro-Diels-Alder fragmentation leading to a cleavage of the heterocyclic ring whereas, in norbornene-fused compounds, two concurrent retro-Diels-Alder fragm entations occurred. Fragmentation patterns of norbornane-fused oxazin- 2-ones(or -thiones) are closely related to each other and, in some cas es, provide evidence for the presence of minor amounts of the ionized open-chain tautomers in the gas phase. The ionized molecular ions of t he 2-anilino derivatives appear to exist as mixtures of 2-phenylimino and 2-anilino isomers with exo- and endocyclic C=N bonds, respectively . (C) 1997 by John Whey & Sons, Ltd.