Electron impact mass spectra of δ-keto enol esters

Abstract

Enol esters with a remote keto function in the δ-position produce upon electron impact (EI) (70 eV) molecular ions of very low abundance (< 1%) due to two fast main fragmentations. The first one involves loss of a carboxylic acid fragment from the α-carbon and subsequent cyclisation to a stable furan ion. A second, more abundant, fragmentation leads, by loss of ketene and an acyl moiety, to a protonated α,β-enone. The stability of this ion is governed by the substituents in the βand γ-positions, which thus determine the order in which a ketene or an acyl radical is lost. Mass-analysed ion kinetic energy (MIKE) spectra of the most abundant ions were also recorded. In one case, a perfect match was found between the collisional activation (CA) mass spectrum of the most abundant ion of one enol-ester under EI conditions and the CA mass spectrum of the corresponding protonated α,β-enone obtained under chemical ionisation (CI) conditions. The fragmentation mechanisms were also inferred from the spectra of two deuterium-labelled δ-keto enol esters.