Electron ionization (EI) mass spectra of some 3,4-disubstituted-1,2,4-oxadiazin-5-ones and -thiones and 3,5-disubstituted 1,2,4-oxadiazin-6-ones

The EI mass spectra of 3,4-disubstituted-1,2,4-oxadiazin-5-ones 1 − 6 and –thiones 7 , 8 and 3,5-disubstituted 1,2,4-oxadiazin-6-ones 9 , 10 were recorded and their fragmentation pathways solved and compared with each other. The fragmentation routes of 5-ones and 5-thiones do not differ very much from each other but compounds 9 and 10 behave differently as could be expected based on their lactone type structures. Only compounds 4 − 6 exhibit a loss of CO and compound 7 a loss of NO. The loss of a benzyl group dominates the behaviour of compounds 1 and 2 which showed only few additional fragmentations. Some earlier data on some 3,4-disubstituted-1,2,4-diazin-5-ones 11 − 13 and –thiones 14 , 15 have been reanalyzed and discussed in further detail.


Introduction
The title compounds possess a diversity of pharmacological activities, [1][2][3][4][5] e.g.antibacterial against some micro-organisms and yeast cultures. 5Only some low resolution EI data are available for a few 1,2,4-oxadiazin-5-ones 6,7 and two 1,2,4-oxadiazin-5-thiones. 6 No published mass spectrometric data was found for 1,2,4-oxadiazin-6-ones.Therefore we thought it interesting to discuss the mass spectrometric behaviour of the title compounds 1−10 (Figure 1) under electron ionization (EI) in detail.The syntheses of the studied compounds have been published earlier. 5,8he low resolution EI mass spectra of some 3,4-disubstituted-1,2,4-oxadiazin-5-ones (cf. Figure 1: 11−13) and -5-thiones (cf. Figure 1: 14 and 15) have been reported. 6In another report some EI mass spectrometric data have been given for seven 2-(2-thienyl)-4-substituted-1,2,4-oxadiazin-5-ones but the results do not appear to agree with the present data or with those given in Ref. 6. 7 We have also reanalyzed the data for compounds 11−15.1) which forms the base peak in both cases (Scheme 1).This also explains why their molecular ions are only moderately abundant as compared with those of compounds 3−6 (Tables 1 and 2) for which they form the base peaks.The loss of C 2 H 2 NO 2 • leads to medium strong ions a at m/z 195 both for 1 and 2. It is interesting that 2 gives a relatively abundant ion (RA 20.5%) c+1, C 7 H 8 N + , at m/z 106 the parent ion of which is e+1 at m/z 210 (Table 1) which corresponds to loss of C 2 HO 2 • from the molecular ion.Both of the former ions are missing from 1 which obviously must be due to the influence of the 2-pyridyl substituent when compared to the 4-pyridyl substituent in 2. Both compounds also show ions b+1 (2-or 4-PyNH + ) and c−1, [(Bzl−H)N] + .The few further low mass ions are listed in Table 3.
Scheme 1.Typical fragmentations of compounds 1 and 2. The sites of fragmentations are also applicable for 3−8 and 11−15.  where y is equal to 5−7 from the ions e+1 and e (Tables 1 and 2 (Tables 1 and 3).The latter ion is formed directly from the molecular ion, i.e. through a cyclization between the ortho position and the C=O oxygen assisted by the p-methoxy substituent (Scheme 3) and the consequent fragmentation gives the ion  3).

5-Isopropyl-(9) and 5-benzyl-1,2,4-oxadiazin-6-one (10)
The fragmentations of these two compounds (Figure 1, Table 4), possessing a lactone function, is very simple and differs completely from those of 5-ones 1−6 and 5-thiones 7 and 8 which in turn resemble fairly much each other.In both cases (Table 4) the base peak (ion A) corresponds to loss of the 5-substituent (i-Pr or Bzl).A weak ion [M−CO 2 ] +• (B) is also present both for 9 and 10.The ion C at m/z 148 in turn corresponds to [A−CO] + .Both compounds exhibit also the ion D, 2-PyCNH + , at m/z 105 and the ion E, 2-Py + , at m/z 78.We have reanalyzed the data for these compounds (Schemes 1 and 2 and Table 5) based on the present results and they appear to be in general agreement with our observations for 1−8.The brief comments given in the original paper are mainly correct although there are also some shortcomings.First of all, the reported [M+1] + and [M+2] + peaks for 11−15 are due to the carbon and sulfur isotopes.The authors 6 state that compounds 11−15 also exhibit peaks due to the nitrile oxide ions, R 1 CNO + .This appears to be true only for 11 (m/z 57 (4%), 13 (m/z 119(5%) and 15 (m/z 119(8%) since for 12 and 14 the peak at m/z 119 is due to the 13 C equivalent of the ion a.However, the compounds studied in our work did not give nitrile oxide ions at all.The ions R 1 CN +• (b) and/or R 1 CNH + (b+1) instead are relatively abundant for all compounds 11−15.
Only compound 15 appears to give the ion c.As to the ion m/z 42 one should emphasize that in the case of 11 it can be either a or b+1 (Table 5).Also ions e−1, e, e+1, i and i−1 appear in most of 11−15, like in 1−8.The diaziridine ion, [R 1 CNR 2 ] +• , is most abundant for 13, not for the thio analogs 14 and 15 as stated in Ref. 6. Based on its mass spectrum, 6 compound 15 still appears to contain a substantial amount of 13 from which it was prepared 9 as may be deduced from the appearance of the ion m/z 252 (RA 21%) which can be explained only by being due to the molecular ion of 13.Based on this observation we have corrected the relative abundances of those ions for 15 which also appear in the mass spectrum of 13.
The EI mass spectra were recorded on a VG ZABSpec mass spectrometer (VG Analytical, Division of Fisons, Manchester, UK), that was equipped with Opus V3.3X program package (Fisons Instruments, Manchester, UK).The ionization energy was 70 eV, accelerating voltage 8 kV and source temperature 160 °C.Direct insertion probe was used.Perfluorokerosene (PFK) was used for calibration of the mass scale.The fragmentation pathways were confirmed by B/E-linked scans (1st FFR) for metastable ions.Also B 2 /E-linked scans were used to clarify these pathways.The low resolution, B/E and B 2 /E spectra were measured using resolution of 3000.To solve the ion structures the accurate masses (Tables 1 -3) were determined practically for all ions by voltage scanning (10% valley definition) using 6,000-10,000 resolution.

Table 5 .
Some This is most probably a combination of R 1 and R 2 since also 12 and 14 exhibit the ion C 6 H 5 a a and b+1 have the same elemental composition.b c