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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...

The mass spectral fragmentations of eight substituted pyrimido[6,1-a]isoquinolin-2-ones and four pyrimido[6,1-a]isoquinoline-2,4-diones under electron ionization were examined by metastable ion analysis, a collision-induced dissociation technique and exact mass measurements. The major fragmentation pathways began as radical site-initiated cleavage, and ionization of the nitrogen atom in the isoquinoline ring seemed to prevail. Only the isoquinolinediones gave fragments resulting from ionization of the phenyl ring. Substituents on N3 and C4 had larger effects on the peak intensities than substituents on C6. 2-6 The present work describes a mass spectrometric study of twelve pyrimido[6,1-a]isoquinolines bearing substituents in different positions (Table 1); it forms part of a project relating to investigation of the synthesis, stereochemistry, pharmacology and mass spectrometry of 1,3-oxazino[4,3-a]-and pyrimido[6,1-a]isoquinolines. [4][5][6][7][8][9] The compounds were prepared according to described methods. [8][9][10][11][12][13][14][15] . The fragmentations observed in the 70 eV electron ionization (EI) mass spectra were established by using metastable ion analysis and a collision-induced dissociation (CID) technique. Exact mass measurements were used to confirm the elemental compositions of the ions, but it should be borne in mind that the indicated ion structures are speculative, and merely meant to aid in the visualization of the fragmentation pathways. Our main interest, besides the basic fragmentations of this specific ring system, was focused on the effects of substituents on the fragmentation pathways. A comparison was also made with an earlier study on the mass spectrometric behaviour of 1,3-oxazino[4,3-a]isoquinolines. 7 EXPERIMENTALThe mass spectra were recorded using a Jeol JMS-D300 mass spectrometer (Jeol, Tokyo, Japan) equipped with a combined EI/CI ion source and connected to a Jeol JMA-2000H data system. For all measurements, samples were introduced through a direct inlet at temperatures of 428 to 493 K. Typical source conditions were temperature 433 K, electron energy 70 eV, accelerating voltage 3 kV and ionization current 300 µA. Accurate mass measurements were made for all the compounds by scanning the magnetic field using scan speed 20 s/decade at a resolution of 5000. Perfluorokerosene (PFK) was used as internal reference compound. For some of the compounds, the results were checked by using a Fourier transform ion cyclotron resonance mass spectrometer, Bruker BioApex 47e (Bruker Daltonics, Billerica, USA). The measurements were made in broad band mode (mass range 100-600) with resolution ≥ 30 000. The instrument was calibrated using perfluorotributylamine (PFTBA) as external calibrant. The average precision of the measurements was better than 3 ppm for six measurements. Fragmentation pathways were verified via metastable transitions and/ or CID spectra generated by linked scans at constant B/E. In the CID experiments, helium was added to the first field-free region so that the transmission ...

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