Electron ionization mass spectrometry of spiroepoxides generated from E‐3‐arylideneflavanones

Abstract

The electron-impact induced decomposition of some spiroepoxides prepared from E-3-arylideneflavanones has been examined by means of low-and high-resolution mass spectrometry, mass-analysed ion kinetic energy (MIKE) spectrometry and collision-induced dissociation (CID) experiments. The compounds studied were found to undergo a rearrangement reaction resulting in a very intense odd-electron ion. The MIKE and CID-MIKE spectra of this type of ion generated from the unsubstituted epoxide proved to be identical to those of the molecular ion of trans-stilbene. The isomerization of epoxides to the corresponding carbonyl compounds prior to fragmentation was not observed.The electron ionization (EI) mass spectrometry of aromatic epoxides has been widely studied and some important features of their spectra have been established. Audier and co-workers' described an unexpected behaviour of aromatic epoxides, namely the migration of aryl and alkyl groups between the two carbons of the epoxy ring resulting in the isomeric carbonyl radical cation. Another competing mechanism which involves the direct elimination of a neutral aldehyde or ketone was found to play an important role as well. On the basis of Pritchard's results2 the isomerization of epoxide to aldehyde can be ruled out in the case of aliphatic derivatives. Fetizon el ~1 .~ studied the mass spectra of a number of aromatic epoxides and those of the corresponding carbonyl compounds. They suggested that phenyl, alkyl or hydrogen can be shifted from one carbon atom of the epoxy ring to the other, accompanied by ring opening, resulting in isomeric ketone or aldehyde structures. The fragmentation can also proceed directly through loss of a molecule of the appropriate carbonyl compound. In a later report, Torroni et ~1 .~ demonstrated-on the basis of metastable-ion studies-that the molecular ion of stilbene oxide does not rearrange to either diphenylacetaldehyde or phenyl benzyl ketone. The 70 eV mass spectra of cis and trans isomers of stilbene oxide were found to be identical. In the case of nitro-substituted aromatic epoxides isomerization to aldehyde or ketone was not observed.In our present study, the 70 eV mass spectra of nine spiroepoxides generated from E-3-arylideneflavanones are presented. High-resolution measurements, massanalysed ion kinetic energy (MIKE) and collisioninduced dissociation (CID-MIKE) studies have been conducted to elucidate the fragmentation pathways of these compounds.tPaper delivered at the 11th Informal Meeting on Mass Spectrometry, Budapest, 1993. Author to whom correspondence should be addressed. EXPERIMENTALLow-resolution mass spectra were recorded on a VG 7035 (VG Analytical Ltd, Manchester, UK) doublefocusing instrument. The accelerating voltage was 4kV, the source temperature 150°C and the spectra were acquired at a rate of 1 sidecade. The samples were introduced via the direct-insertion inlet. For the accurate mass measurements, an AEI MS 902 (Kratos, Manchester, UK) mass spectrometer was used. The MIKE and CID-MIKE experiments were p...