Electron Impact Induced Fragmentation of 4-Aryl-4,6,7,8-tetrahydro-1H,3H-quinazoline-2,5-diones

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, OCH,, CH,; 2,3-, 2,4-, 2,6-, 3,4-dichloro) giving rise to prominent (M - X-.)(divided by) 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, exclusively a chlorine atom. As corroborated by H-2-labelling, reversible Ii-migration from C-4 to the phenyl group takes place (1, la 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 the other cases loss of X-. is explained as a consequence of 4-H migration to the phenyl group.