Stereochemical Effects in the Mass Spectra ofcis-andtrans-2-Aryl-4a,5,6,7,8,8a-hexahydroquinazolin-4(3H)-ones

Abstract

The electron-impact (EI) and collision-induced dissociation (CID) spectra of the title compounds were studied. The main fragmentation routes of M+· ions included losses of hydrocarbon species and the formation of ArCNH+ cations. The geometry of ring annelation predictably affected the stabilities of the molecular ions and the relative abundances of some characteristic fragment ions. The abundance ratios [M−H]+/M+·, which were higher for trans-annelated isomers, correlated with the electronic effects of aromatic ring substituents. Molecular ions and [M−H]+ ions of stereoisomers gave different CID spectra, suggesting that stereochemical differences are still preserved after the loss of hydrogen from M+· ions. The stereochemical effects in the EI spectra were more pronounced at lower impact energies (16–24 eV). Several criteria for differentiating cis- and trans-annelated isomers based on the relative abundances of characteristic fragment ions were established.