Mass spectral fragmentations of four norbornaneleoe di-em-and di-endo-fused 1,3-oxazin-2( lH)-ones and four 1,3-oxazine-2(1H)-thiones were examined by means of metastable ion analysis, the collision-induced dissociation technique and exact mass measurement. Under electron impact (EI) conditions all the saturated compounds gave rise to complicated fragmentations, including several rearrangements. For the unsaturated compounds, a retro-Diels-Alder (RDA) process was the most favoured fragmentation pathway, which took place with hydrogen rearrangement to yield RDA + H fragments. The EI mass spectra did not permit isomeric differentiation. Under chemical ionization conditions, stereochemical effects were more perceptible, but isomeric differentiation was still difficult.
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RESULTS AND DISCUSSION
EI spectraThe 70 eV EI mass spectra of the compounds studied are presented in Table 1. Comparison of the spectra of saturated and unsaturated compounds revealed striking differences. Compounds 1 and 2 gave rise to the hardly visible molecular ion peaks. With these compounds the presence of the double bond in the carbobicyclic moiety makes a retro-Diels-Alder (RDA) reaction possible. Since this is known to be a very favoured process for