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Paper | Regular issue | Vol 48, No. 6, 1998, pp.1169-1183
Published online, 1st January, 1970
DOI: 10.3987/COM-98-8133
Electron Impact Induced Fragmentation of 4-Aryl-4,6,7,8-tetrahydro-1H,3H-quinazoline-2,5-diones

Klaus K. Mayer,* Stefan Dove,* Herwig Pongratz, Mevlüt Ertan, and Wolfgang Wiegrebe

*Pharmazeutische Chemie I, Institut für Pharmazie, Universität Regensburg, Universitätsstraße 31, D-93040 Regensburg, Germany

Abstract

The molecular ions (M+·) of 4-substituted aryl-4,6,7,8-tetrahydro-1H,3H-quinazoline-2,5-diones (Biginelli compounds) (2-18) decompose by loss of the substituents X of the phenyl group (X = o-F; o-, m-, p-Cl, Br, OCH3,CH3; 2,3-, 2,4-, 2,6-, 3,4-dichloro) giving rise to prominent (M - ·X)+ ions at 70 and 12 eV, respectively. In the cases of o-Cl and o-Br substitution, the M is extremely unstable. In general, metastable M (1st ffr) eliminates preferably H·, that of 15 (2,6-dichloro), however, exclusibely a chlorine atom. As corroborated by 2H-labelling, reversible H-migration from C-4 to the phenyl group takes place (1, 1a-1c). The collisional activation spectra of the (M· - X)+ ions of 3 (o-Cl) and 6 (o-Br) are identical but different from the indistinguishable spectra of the (M - ·X)+ ions of 4 (m-Cl), 5 (p-Cl), 9 (o-OCH3), 11 (p-OCH3), and 14 (p-CH3). Semiempirical MO calculations (MOPAC 6.0, PM 3 Hamiltonian) of the M of all ortho-substituted derivatives support a close interaction of o-Cl and o-Br with the carbonyl oxygen, leading to elimination of these substituents and affording cyclic oxonium ions. In th other cases loss of X· is explained as a consequence of 4-H migration to the phenyl group.