Yp. Tu et Ag. Harrison, STEREOCHEMICAL APPLICATIONS OF MASS-SPECTROMETRY - PART 5 - FRAGMENTATION OF PROTONATED AND METHYLATED MALEIC AND FUMARIC-ACID AND DERIVATIVES, Journal of mass spectrometry, 33(9), 1998, pp. 858-871
The unimolecular metastable ion and collision-induced dissociation (CI
D) fragmentation reactions of protonated and methylated monoamides, mo
nomethyl esters and methyl esters of the monoamides of maleic and fuma
ric acids were studied. In addition, some studies of the fragmentation
of protonated and methylated maleic and fumaric acids were carried ou
t. The [MH - H2O](+) ions derived from protonated maleic and fumaric a
cids show distinctly different CID mass spectra, that for the [MH - H2
O](+) ion from the maleic acid being the same as that of protonated ma
leic anhydride; the results show that the stereochemistry about the do
uble bond is retained in the [MH - H2O](+) ions. Fragmentation of spec
ifically deuterium-labelled and protonated or deuterated maleic acids
show that the added proton becomes scrambled with the carboxylic hydro
gens prior to loss of H2O, The fragmentation of similarly labelled fum
aric acids show that a 1,3-H+ migration followed by elimination of H2O
is not the only pathway to water elimination; the results implicate p
roton migration from one carboxyl group to the other as well as some i
nvolvement of the C-bonded hydrogens in the water-loss reaction, A maj
or fragmentation reaction of protonated maleamic acid forms NH4+; this
reaction is of only minor importance for protonated fumamic acid, Oth
er primary fragmentation reactions involve elimination of NH3 and H2O
from the protonated species. The protonated monomethyl esters fragment
initially by loss of H2O or loss of CH3OH; the former is more promine
nt for the maleate whereas the latter dominates for the fumarate, Prot
onation of methyl maleamate and methyl fumamate results in loss of NH3
or CH3OH as primary fragmentation reactions; these primary fragment i
ons undergo less facile further fragmentation for the maleamate than f
or the fumamate, The CH3+ adducts of the monoamides fragment by loss o
f NH3, H2O and CH3OH; the CID spectra of the adducts are distinctly di
fferent from those of the protonated methyl esters of the monoamides,
indicating predominant addition of the methyl to the amide oxygen. The
CH3+ adduct of monomethyl maleate fragments primarily by loss of meth
anol, the two methyl groups having become equivalent prior to fragment
ation. 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 CH3+ adduct of
monomethyl fumarate, Since this reaction channel is not observed for
protonated dimethyl fumarate, the results indicate predominant CH3+ ad
dition to the carbomethoxy group of the monoester, (C) 1998 John Wiley
& Sons, Ltd.