Cis/trans stereochemical effects in the negative chemical ionization/OH− mass spectra of strained-ring azabicycloalkanes using MIKE and CA/MIKE spectrometry

Stereospecific decomposition reactions of isomeric (cis and trans) deprotonated molecules from azabicycloalkane derivatives as azetidinols generated under negative chemical ionization (NCI)/OH^? have been examined using mass-analysed ion kinetic energy (MIKE) and collisional activation (CA)/MIKE spectra. These measurements together with the ones obtained on specifically labelled compounds enabled us to determine the origin of the stereochemical effects. The results indicate that the hydroxylic proton constitutes the preferential (?90%) site for the deprotonation process. Subsequent fragmentations of the deprotonated species observed in the second field-free region of a reversed geometry instrument are affected by the stereochemistry of the hydroxylic group. The isomer with the hydroxyl group in the cis position relative to the hydrogen at the ring junction mainly loses H₂O, while the trans isomer eliminates CH₃˙, both processes occurring with high specificity. Labelling studies indicate that two major pathways exist for the elimination of H₂O from the cis isomer and the loss of CH₃˙ from the trans isomer. The course of the reaction is determined by the ability of the stereoisomers to transfer a proton during the first decomposition step. When the size of the lactam ring is increased from a five-membered ring to a six- or seven-membered ring, these stereochemical effects tend to become less pronounced.