Mass spectral fragmentation reactions of some stereoisomeric cyclic β-amino acids under electron and chemical ionization

Mass spectral fragmentations of two cyclopentane, four cyclohexane/ene and four norbornane/ene ß‐amino acids were studied under electron ionization by low‐resolution, high‐resolution, metastable ion analysis and collision‐induced dissociation techniques. All stereoisomeric compounds gave rise to identical 70 eV electron‐ionization mass spectra. The major fragmentation pathway for the saturated compounds began as an α‐cleavage reaction with respect to the nitrogen atom. For the unsaturated compounds, a retro‐Diels–Alder reaction was favoured. In addition to a normal retro‐Diels‐Alder fragmentation reaction, the norbornene compounds underwent a retro‐Diels‐Alder fragmentation reaction accompanied by the migration of one hydrogen atom. In the case of the chemical ionization mass spectra, differences between stereoisomers were observed only under methane chemical ionization, where monocyclic trans 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 resulted in the same elimination ratio [M + H  H2O]/{[M + H  H2O] + [M + H  NH3]} as observed under kinetic control in the collision region.