The electron ionization mass spectra of five substituted, partly saturated 3,1-benzoxazino-1,3-benzoxazines and four substituted 1,3-benzoxazino-l,3-benzoxazines were measured and analysed. The fragmentation pathways were elucidated by metastable ion analysis and exact mass measurement. Although the principal fragmentations were similar for all these compounds, there was an important difference between the two groups as concerns the fragmentation routes. Both routes led to the same fragment ion, but in different ways depending on the ring structure. On the basis of the different fragmentation mechanisms, the 3,1-benzoxazino-1,3-benzoxazines and the 1,3-benzoxazino-l,3-benzoxazines can be distinguished mass spectrometrically. The saturation, the stereochemistry and the substituents all affected the fragmentations, mainly the peak intensities.

The 70 eV electron ionization mass spectra of 35 differently 2-phenyl-and 1-nitrogen-substituted imidazolidines were studied. Tautomerism was observed between the open-chain and ring forms in the gas phase. The fragment ions detected in the low-energy mass spectra showed that in the gas phase the compounds mostly existed in the ring form; except those having a nitro substituent on the 2-phenyl group, for which only open-chain fragment ions were found. The most important fragment ion relating to the ring form for Nmethyl-substituted compounds was the Received 8 March 1998; Revised 6 May 1998; Accepted 11 May 1998Ring-chain tautomerism involving the reversible addition of a heteroatom to a heteropolar double bond is a well known process for heterocycles. 1 Imidazolidines, which are 5-membered 1,3-N,N-heterocycles, are most often prepared by condensation of a diamine with an aldehyde.2,3 They have been studied in the liquid phase, and equilibrium has been demonstrated between the open-chain and ring forms. 4,5 Mass spectrometry can be used to study gas-phase ring-chain tautomerism by observing the ratio of the relative abundances of peaks relating to one or another tautomeric form.6-9 The validity of the method greatly depends on the reliability with which a fragment ion can be attributed to only one tautomeric form. Differences in the ionization efficiencies of the ring and open-chain forms, the absence of intense primary fragment ions, difficulties in the measurement of their decomposition efficiencies and the lack of thermal equilibrium before ionization are further factors which may cause problems. At best, the method is qualitative only. For example, the ring-chain equilibria of fiveand six-membered 1,3-O,N-heterocycles have been studied extensively by mass spectrometry, with varying results. [6][7][8][9][10][11][12] In the present work, 35 2-aryl-substituted imidazolidines, with either methyl, ethyl, n-propyl, isopropyl or phenyl on one nitrogen, were studied mass spectrometrically under electron ionization.The main question of interest was to examine whether it was possible to determine the position of the ring-chain equilibrium for the imidazolidines in the gas phase by mass spectrometry. The effect of the electron-donating or withdrawing substituent X on the 2-phenyl group on the ringchain tautomerism was carefully studied. The possible *Correspondence to: P. Vainiotalo,

No abstract

The 70 eV electron ionization mass spectra of fourteen differently N-substituted 2,5-dimethylpyrroles were studied. In accord with the aromatic character of the pyrrole ring, all the compounds studied gave rise to an intense molecular ion peak. Regardless of the functional group at the 1-substituent, for all the compounds the principal fragmentation products were practically the same, although the relative peak intensities varied depending on the structure of the substituent. In addition, an amino group substituent prompted intense radical site initiated cleavages which were minor or totally absent with other compounds. The data showed that extensive rearrangement reactions took place. Some of these were seen directly from the mass spectra, and some only from closer examination of the ion structures. The structures of the ions at m/z 108 and 94, formally representing the [MÀ1] ions of 1,2,5-trimethylpyrrole and 2,5-dimethylpyrrole, respectively, were carefully studied using the collision induced dissociation (CID) technique. Measurements showed that ring expansion reactions took place, protonated 2,5-lutidine and protonated 2-picoline being the most likely isomers formed for the m/z 108 and 94 ions, respectively. The results obtained from the CID experiments were supported by semi-empirical calculations of heats of formation. #

The mass spectra of ten substituted oxazino[4,3-a]isoquinolines, two isoquino[2,1-c][ 1,3]benzoxazines and seven substituted oxazino[4,3-a]isoquinolin-4-ones were recorded under electron impact ionization. Fragmentations were examined by metastable ion analysis, collision induced dissociation and exact mass measurement. The oxazinoisoquinolines and oxazinoisoquinolin-4-ones behaved similarly, although there were a few differences in the fragmentation and especially in the peak intensities. The most important fragmentation began with the opening of the oxazole ring. The substituents affected the fragmentation of the isomeric compounds, whereas the spectra of the stereoisomeric compounds were identical. The two isoquinobenzoxazines differed noticeably from the other compounds in their fragmentation behaviour; the additional phenyl ring prompted entirely new fragmentations unique to this structure.