Mass spectral fragmentations of two cyclopentane, four cyclohexanelene and four norbornanelene fi-amino acids were studied under electron ionization by low-resolution, high-resolution, metastable ion analysis and collisioninduced 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 a-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 The mass spectrometric differentiation of 1,2-disubstituted stereoisomeric cyclopentane,1.2 cyclohexane/ene2-6 and norbornane/ene (bicyclo[2.2. llheptane/ene)2-4. 7,8 compounds by electron ionization (EI) has proved to be difficult. The greatest differences between stereoisomers have been observed in the mass spectra of 1,2-cyclohexanedicarboxylic acids, where the spatial arrangement of the carboxyl groups allows the decarboxylation reaction to take place only in the case of the trans isomer.6 Under chemical ionization (CI) many acyclic difunctional compounds, such as amino alcohols9 and diamines, lo have high proton affinity, due to the formation of cyclic structures through a stable intramolecular hydrogen bond. When functional groups are connected to a cyclic framework, on the other hand, hydrogen bonding between the groups is not always possible, and then only one of the functional groups is involved in protonation. Studies on the mass spectrometric behaviour of cyclic diols,', l1 diethers,12 dia~etates,'~ amino alcohols'. l4 and carboxylic acid derivatives15 have demonstrated that the proton transferred to these molecules upon ionization coordinates between two functional groups only where a linear hydrogen bridge is able to form. Thus, the CI mass spectra of cyclic stereoisomeric compounds will sometimes show marked stereochemical effects.Dissociation reactions of protonated amino alcohols and amino acids have been examined under a variety of reaction conditions.2'16 Under the thermodynamic control encountered in the chemical ionization source, the fragmentation of the protonated molecules results in thermochemically favoured dehydration, whereas under collision-induced dissociation (CID) conditions, kinetically more stable deamination products predominate.16 Furthermore, the extent of the dehydration and deamination of protonated a,o-amino acids largely depends on the length of the carbon chain.16As a continuation of our mass spectral studies on norbornane derivative^,**^*'^ l7 we now examine the mass spectrometric fragmentations of some norb...