Abstract-An [M -31]+ ion was a prominent fragment in the mass spectra of three orrlzo-methoxyphenyl-Zpropanone oximes and is shown to be due to the expulsion of a methoxyl radical from the molecular ion as a result of an ortho-effect. In contrast, an [M -31]+ ion was absent from the spectra of a structurally related ketone and a hydroxylamine, and was not observed in the spectra of meta-and para-methoxyphenyl-2-propanone oximes.CABLE et a1.l recently reported that the mass spectra of the phenylhydrazones of ortho-substituted benzaldehydes and acetophenones showed characteristic [ M -X]+ ions when the ortho-substituent X, was I, Br, C1, OH and OCH,. We have observed a similar ortho-effect with three oximes (la to Ic), two of which were prepared as possible in vitro metabolites of the hallucinogenic compounds 1-(2,5-dimethoxypheny1)isopropylamine and 1 -(2,5-dimethoxy-4-methylphenyl)isopropylamine (DOM).Whereas the [M -31]+ ions in the mass spectra of the o-methoxyphenylhydrazones studied by Cable et al. were of low abundance, this ion was prominent in the mass spectra of Ia to Ic (see Table 1). Subsequent fragmentation of the [M -31]+ ion was interesting. Coutts and Mukherjee2 have observed that aromatic hydroxylamines fragment by expelling an oxygen atom and a hydroxyl radical. The cyclic nitronium ion (a) behaved similarly on electron-impact, producing ions b and c. It may be argued that the formation of an odd-electron ion (c) from an even-electron precursor (a) is energetically unlikely. A metastable ion was present in the mass spectra of both Ia and Ib, however, to support an a -t c transition. In addition, accurate mass measurements of all fragment ions a to c in the spectrum of Ic were made and were in agreement with the structures given in Scheme 1.An additional diagnostic fragment, an [M -33]+ ion, was present in the three spectra Ia to Ic. An accurate mass measurement of this fragment in the spectrum of Ic confirmed its elemental composition (C,,H,,NO,).The initial assumption was that this [M -33]+ ion originated from [M -31]+ by the expulsion of a hydrogen molecule from the latter. When the spectra of benzylmethylketoxime (Ira), p-methoxybenzylmethylketoxime (IIb) and 3,4,5-trimethoxybenzylmethylketoxime (Ilc) were examined, however, all three contained an [M -33]+ fragment, and the presence of appropriate metastable ions indicated that this fragment resulted from the expulsion of a methyl radical and a molecule of water from the molecular ion.It is noteworthy that the spectra of Ira, IIb and IIc lacked an [M -31]+ ion. The absence of this ion, especially in the spectra of IIb and IIc, gives credence to the claim that the [M -31]+ ion in the spectra of Ia to Ic is formed as a result of an orfho-effect.
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