The stereoisomers of the title compounds produce nearly identical electron ionization (EI) mass spectra, which are dominated in the case of the norbornene-condensed derivatives by retro-Diels-Alder (RDA) fragmentation of the hydrocarbon ring. The RDA fragmentation mainly occurs with H transfer and gives rise to [M-C5H5]+. For the norbornane-condensed derivatives, the main fragmentation routes include the formation of [M-C5H7]+ (protonated thiouracil) and [M-C7H9]+ (only from thiazinopyrimidines). The latter species are formed via RDA decomposition of the pyrimidone subunit of the heterocyclic system, a process previously observed for cyclohexane-condensed analogs of these compounds. Only minor differences could be detected between the EI spectra of the diexo and diendo isomers. Under chemical ionization (CI) conditions, the norbornane-condensed compounds produced no significant fragment peaks with either isobutane or methane as reagent gas. In contrast, the isobutane and methane CI spectra of the norbornene-condensed compounds exhibited prominent peaks of [MH-C5H6]+ and [(M+CxHy)-C5H6]+ originating from moderately stereoselective RDA fragmentations. The relative abundances of the RDA ions obtained from the respective stereoisomers with the same reagent gas were consistently different over a range of experimental conditions. The non-occurrence of RDA fragmentation of the thiazinopyrimidine ring under CI conditions suggested that its energy of activation is higher than that for either of the norbornene-ring RDA fragmentations (with or without H transfer) observed under EI and CI conditions.
|Number of pages||9|
|Journal||Journal of the American Society for Mass Spectrometry|
|Publication status||Published - dec. 1 2001|
ASJC Scopus subject areas
- Structural Biology