Mass-spectrometric differentiation of di exo- and di endo-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–C 5H 5] +. For the norbornane-condensed derivatives, the main fragmentation routes include the formation of [M–C 5H 7] + (protonated thiouracil) and [M–C 7H 9] + (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 di exo and di endo 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–C 5H 6] + and [(M+C xH y)–C 5H 6] + 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.