Electron impact studies. XXXVII. Skeletal-rearrangement fragments in the mass spectra of alkyl and aryl isoxazoles

Abstract

The mass spectra of substituted isoxazoles are reported and discussed. The spectra of isoxazoles are strikingly different from those of oxazoles because the initial fragmentations of isoxazoles involve N-O bond fission. Specific skeletal-rearrangement processes are observed in many spectra, and it is proposed that they proceed through azirine and oxazole intermediates. Certain fragmentations have been studied by deuterium labelling. The hydrogens attached to an isoxazole ring do not randomize with the hydrogens of the methyl and phenyl substituents.

Calcium-mediated one-pot preparation of isoxazoles with deuterium incorporation

“…The other way to yield 4-or 5-deuteroisoxazoles is the sequential treatment of 4-and 5-halogenated isoxazoles with butyl lithium and deuterium oxide (Scheme 1, path b). 8 Additionally, 4-or 5-deuteroisoxazoles can be obtained by lithium-mediated H-D exchange (Scheme 1, path c). 1a,b The application of BuLi as a reagent imposes even more difficulties with regard to the functional group tolerance.…”

The mass spectra of some 3,5‐diarylisoxazoles

“…Diaroylmethanes (1) with different aryl groups give rise to either or both of the isomeric isoxazoles (2 and 3). 5 The structure of the isoxazole obtained R'COCH,COR* -NH,OH R 2 e R , + "-(O;y R L (1) (2)…”

The mechanism of water loss from protonated cathinones

“…To the best of our knowledge, only Qian et al have proposed the formation of azirine structure for the [M + H − H 2 O] + ion of one cathinone, namely mexedrone . On the contrary, the formation of azirine‐ring‐containing ions has been proposed for fragmentation processes of N‐heterocyclic compounds that occurred under EI conditions, under fast atom bombardment (FAB) conditions, and under ESI conditions …”

“…[15][16][17][18][19] This mechanism (scheme 2) is disputable because (a) it indicates that the loss of H 2 O occurs also for those compounds that contain Table S1 (supporting information). In the experimental part, it is explained 21 On the contrary, the formation of azirine-ring-containing ions has been proposed for fragmentation processes of N-heterocyclic compounds that occurred under EI conditions, [22][23][24][25] under fast atom bombardment (FAB) conditions, 26 and under ESI conditions. 27 We also obtained the product ion spectra of Figure 1), and the calculated exact mass is 147.0810, so the mass accuracy is −6.1 ppm.…”