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Protonkatalysierte Wasser-Abspaltung aus 1-Dekalon und 8α-Methyl-1-dekalon in the Gasphase. Verlust der stereochemischen Integrität der cis/traras-Ringverknüpfung bei [MH]+ -Ionen
Wrolfschütz, Roland; Halim, Hermann; Schwarz, Helmut
Water loss from protonated gaseous 1-decalon and 8α-methyl-1-decalon under the condition of chemical ionisation mass spectrometry proceeds via at least three distinct pathways, all of which commences with a [1.2]-hydrogen or [1.2]-methyl migration, thus giving rise to the formation of the central intermediate 10. H2O loss from 10 proceeds either directly, involving a hydrogen from the position (C-4a) (Scheme 3; pathway 10 →15 →16), or from the rearranged ions 17 and/or 20. The fact that eis-and trans-8α-methyl-1-decalones and their deuterated isotopomers behave essentially identical is explained by loss of the stereochemical integrity of the ring conjunction. The mechanistic features of this unusual process (Scheme 2) have been explored by semi-empirical MINDO/3 calculations for the model system 23 (Scheme 4). The calculations reveal that there are two competing pathways for CH3 migration, one involving a sequence of [1.2]-shifts (23 →24 →25 →26→27), the other -being energetically less favoured -corresponds to a direct [1.3]-methyl migration (Scheme 6; 23 →33 →27), in which the methyl group migrates with retention of configuration. This process is symmetry forbidden. For the corresponding symmetry allowed [1.3]-methyl migration with inversion of configuration of the migrating group no pathway could be detected computationally. Both the experi-mental and theoretical investigations allow one to draw a detailed picture of H2O loss from protonated cyclic ketones in the gas phase. -The syntheses of the labelled substrates are described in detail.
