Stereochemical effects in the mass spectra of endo- and exo-2,3-bismethoxycarbonylbicyclo[2.2.1]heptane and endo- and exo-2-methoxycarbonylbicyclo[2.2.1]heptane

Abstract

The endo- and exo-isomers of the title compounds show markedly different behaviour upon electron impact. The molecular ion of the endo-isomer of 2,3-bismethoxycarbonylbicyclo[2.2.1]heptane (M^+·212) eliminates cyclopentadiene leading to the base peak at m/e 146; this decomposition occurs to a negligible extent for the molecular ion of the exo-isomer [m/e 146 (4%; after correction for natural ¹³C contribution)], but expulsion of a resonance-stabilized cyclopentenyl radical is found instead, giving the base peak at m/e 145. A successive loss of methanol and carbon monoxide or keten from the molecular ion of the endo-isomer of 2-methoxycarbonylbicyclo[2.2.1]heptane (M^+·154) leading to peaks at m/e 122, 94, and 80 is found. These reactions appear to be largely suppressed in the case of the corresponding exo-isomer. All these observations have been explained in terms of stereochemically controlled hydrogen migrations to the ester functions.