ELECTRON-IMPACT MASS-SPECTRAL FRAGMENTATION PATTERNS OF ISOXAZOLINES

Electron-impact mass spectra of 32 isoxazolines, viz. monocyclic, bicyclic and tricyclic isoxazolines are reported. The major fragmentation pattern for the isoxazolines with alkyl, aryl, methoxycarbonyl, (methylthio)methyl substituents at C-5 is alpha-cleavage while there is a competition between the groups at C-5 for elimination in the case of 5,5-disubstituted isoxazolines. Bicyclic and tricyclic isoxazolines undergo ring fission to afford (isoxazole+allyl radical cation) and (isoxazole + cyclopentane radical cation) respectively, to reduce the ring strain present in the parent isoxazolines. In the case of 5-(2-methylphenyl)-isoxazolines, the isoxazole cation is observed as the base peak arising out or CH3 loss followed by H-migration.