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, G. 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 - 2001

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Norbornanes
Thiouracil
Stereoselectivity
Butanes
Isomers
Ionization
Stereoisomerism
Methane
Electrons
Derivatives
Gases
Pyrimidinones
Hydrocarbons
Ions
pyrimidine
2-norbornene
Chemical activation
Decomposition

ASJC Scopus subject areas

  • Structural Biology
  • Spectroscopy

Cite this

@article{87e955b3735440158af5039642e761ff,
title = "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",
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.",
author = "Ovcharenko, {Vladimir V.} and Shaikhutdinov, {Rousten A.} and Kalevi Pihlaja and G. St{\'a}jer",
year = "2001",
doi = "10.1016/S1044-0305(01)00280-X",
language = "English",
volume = "12",
pages = "1011--1019",
journal = "Journal of the American Society for Mass Spectrometry",
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number = "9",

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TY - JOUR

T1 - Mass-spectrometric differentiation of diexo- and diendo-fused isomers of norbornane/ene-condensed 2-thiouracil and 1,3-thiazino[3,2-a]pyrimidine derivatives

T2 - Stereoselectivity of retro-diels-alder fragmentations under EI and CI conditions

AU - Ovcharenko, Vladimir V.

AU - Shaikhutdinov, Rousten A.

AU - Pihlaja, Kalevi

AU - Stájer, G.

PY - 2001

Y1 - 2001

N2 - 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.

AB - 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.

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U2 - 10.1016/S1044-0305(01)00280-X

DO - 10.1016/S1044-0305(01)00280-X

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JO - Journal of the American Society for Mass Spectrometry

JF - Journal of the American Society for Mass Spectrometry

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