3‐aryl‐2‐isoxazolines: Electron‐impact fragmentation and substituent effects

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Abstract
The mass spectra were investigate of six 3‐aryl‐2‐isoxazolines, and three main fragmentation pathways were found to occur through loss of 28 and 30 a.m.u. The losses of 28 (C2H4)and 30 (CH2O) are seen as competitive mass spectrometric retro‐1,3‐dipolar cycloaddition processes. These reactions are also repeated successively, and take place in different ions either following the degradation of the substituent or the attack of the benzene ring. The substituents in the para position influence both these reactions and also the attack upon the rings. A quantitative study has been carried out on the effect of the substituent on the [M‐28] reaction, which can be interpreted in terms of the first ionisation energies. These terms are linearly intercorrelated, for the compounds under examination, with the Hammett type σ‐constants.