STUDY OF 2 DIASTEREOMERIC PAIRS OF REGIOSOMERIC 2-ISOXAZOLINES BY TANDEM MASS-SPECTROMETRY WITH ELECTRON-IMPACT IONIZATION

On electron impact (EI) ionization, two cis/trans pairs of 4-methyl-5- phenyl and 4-phenyl-5-methyl regioisomeric 3-carbethoxy-2-isoxazolines showed normal mass spectra and mass-analysed ion kinetic energy (MIKE ) spectra of metastable (MI) and collision-activated (CA) molecule ion s, allowing unequivocal differentiation of the regioisomers. The cis/t rans stereoisomers of each regioisomer showed very similar normal mass spectra. Very interestingly, the cis- and trans-4-phenyl-5-methyl ste reoisomers appeared reasonably differentiated by the molecule ion MIKE spectra, whereas the 4-methyl-5-phenyl regioisomeric pair of stereois omers did not. The influence of the phenyl substituent to the fragment ation processes was notable. Some fragments of interest were studied b y comparison of their MIKE spectra with those of model ions, generated by EI from suitable substrates, including (i) the isomeric alpha,beta -unsaturated oxime, namely ethyl (Z)-2-(hydroxymino)-3-methyl-4-phenyl but-3-enoate, a by-product of importance for the mechanism(s) of the a ddition/cycloaddition reactions of nitrile oxides to alkenes and (ii) trans-beta-methylstyrene, a dipolarophilic reactant in the same reacti ons. The favoured heterocyclic C(5)-O(1) bond cleavage occurred only f or the ionized 4-methyl-5-phenyl 2-isoxazoline pair, leading to a dist onic ion of relevance, as it can represent either a reasonable precurs or for both the isomerization to the ionized alpha,beta-unsaturated ox ime and the EI-induced cycloreversion yielding ionized beta-methylstyr ene, or the ionized form of a zwitterionic intermediate, which had bee n proposed previously for the addition/cycloaddition mechanism(s) in t he solution phase, currently under study.