Mass spectrometric study of some cycloalkane/ene-condensed 2-thioxo-2935556-pyrimidin-4(1H)-ones and cycloalkane/ene-condensed [l,3] thwzino [3,2-a]-pyrimidinones under electron impact

The mass spectra of four cycloalkane/ene cis- and trans-condensed 2-thioxo-2,3,5,6-tetrahydropyrimidin-4(1H)-ones and the corresponding cycloalkane/ene cis- and trans-condensed [l,3]thiazino[3,2-a]pyrimidinones were measured. Fragmentation pathways were confirmed by metastable ion analyses and exact mass measurements. The bicyclic isomers were easy to distinguish from each other, whereas the corresponding tricyclic isomeric adducts gave fairly similar mass spectra, owing to their favoured reiro-Diels- Alder fragmentations accompanied by the migration of one or two hydrogen atoms.