Electron impact induced fragmentation of 4-aryl-4,6,7,8-tetrahydro-1H,3H-quinazolinone-2,5-diones

Abstract

The mol. ions (M+×) of 4-substituted aryl-4,6,7,8-tetrahydro-1H,3H-quinazoline-2,5-diones (Biginelli compds.) (2 - 18) decomp. by loss of the substituents X of the Ph 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, resp. 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 2H-labeling, reversible H-migration from C-4 to the Ph group takes place. 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 calcns. (MOPAC 6.0, PM 3 Hamiltonian) of the M+× of all ortho-substituted derivs. 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 Ph group.