Mass-spectrometric differentiation of diexo- and diendo-fused isomers of norbornane/ene-condensed 2-thiouracil and 1,3-thiazino[3,2-a]pyrimidine derivatives: Stereoselectivity of retro-diels-alder fragmentations under EI and CI conditions

Vladimir V. Ovcharenko, Rousten A. Shaikhutdinov, Kalevi Pihlaja, Géza Stájer

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Abstract

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.

Original languageEnglish
Pages (from-to)1011-1019
Number of pages9
JournalJournal of the American Society for Mass Spectrometry
Volume12
Issue number9
DOIs
Publication statusPublished - dec. 1 2001

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ASJC Scopus subject areas

  • Structural Biology
  • Spectroscopy

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