MASS-SPECTRAL FRAGMENTATION REACTIONS OF SOME STEREOISOMERIC CYCLIC BETA-AMINO ACIDS UNDER ELECTRON AND CHEMICAL-IONIZATION

Mass spectral fragmentations of two cyclopentane, four cyclohexane/ene and four norbornane/ene beta-amino acids were studied under electron ionization by low-resolution, high-resolution, metastable ion analysis and collision-induced dissociation techniques. All stereoisomeric com pounds gave rise to identical 70 eV electron-ionization mass spectra. The major fragmentation pathway for the saturated compounds began as a n alpha-cleavage reaction with respect to the nitrogen atom. For the u nsaturated compounds, a retro-Diels-Alder reaction was favoured. In ad dition to a normal retro-Diels-Alder fragmentation reaction, the norbo rnene compounds underwent a retro-Diels-Alder fragmentation reaction a ccompanied by the migration of one hydrogen atom. In the case of the c hemical ionization mass spectra, differences between stereoisomers wer e observed only under methane chemical ionization, where monocyclic tr ans isomers decomposed more readily than the corresponding cis isomers . All the [M+H]+ ions decomposed mainly by dehydration and deamination reactions. The fragmentation in the chemical-ionization ion source re sulted in the same elimination ratio [M+H-H2O]/{[M+H-H2O]+ [M+H-NH3]} as observed under kinetic control in the collision region.