Stereochemical applications of Mass Spectrometry. Part 5—fragmentation of protonated and methylated maleic and fumaric acid and derivatives

Abstract

The unimolecular metastable ion and collision-induced dissociation (CID) fragmentation reactions of protonated and methylated monoamides, monomethyl esters and methyl esters of the monoamides of maleic and fumaric acids were studied. In addition, some studies of the fragmentation of protonated and methylated maleic and fumaric acids were carried out. The [MH - H 2 O] + ions derived from protonated maleic and fumaric acids show distinctly different CID mass spectra, that for the [MH - H 2 O] + ion from the maleic acid being the same as that of protonated maleic anhydride; the results show that the stereochemistry about the double bond is retained in the [MH - H 2 O] + ions. Fragmentation of specifically deuterium-labelled and protonated or deuterated maleic acids show that the added proton becomes scrambled with the carboxylic hydrogens prior to loss of H 2 O. The fragmentation of similarly labelled fumaric acids show that a 1,3-H + migration followed by elimination of H 2 O is not the only pathway to water elimination; the results implicate proton migration from one carboxyl group to the other as well as some involvement of the C-bonded hydrogens in the water-loss reaction. A major fragmentation reaction of protonated maleamic acid forms NH 4 + ; this reaction is of only minor importance for protonated fumamic acid. Other primary fragmentation reactions involve elimination of NH 3 and H 2 O from the protonated species, The protonated monomethyl esters fragment initially by loss of H 2 O or loss of CH 3 OH; the former is more prominent for the maleate whereas the latter dominates for the fumarate. Protonation of methyl maleamate and methyl fumamate results in loss of NH 3 or CH 3 OH as primary fragmentation reactions; these primary fragment ions undergo less facile further fragmentation for the maleamate than for the fumamate. The CH 3 + adducts of the monoamides fragment by loss of NH 3 , H 2 O and CH 3 OH; the CID spectra of the adducts are distinctly different from those of the protonated methyl esters of the monoamides, indicating predominant addition of the methyl to the amide oxygen. The CH 3 + adduct of monomethyl maleate fragments primarily by loss of methanol, the two methyl groups having become equivalent prior to fragmentation. A minor fragmentation route involves loss of dimethyl ether, a reaction not observed for protonated dimethyl maleate. Elimination of dimethyl ether is a major fragmentation channel for the CH 3 + adduct of monomethyl fumarate. Since this reaction channel is not observed for protonated dimethyl fumarate, the results indicate predominant CH 3 + addition to the carbomethoxy group of the monoester.