Heterocyclic photorearrangements. Some investigations of the photochemical behaviour of 3-acylamino-1,2,4-oxadiazoles. Synthesis of the quinazolin-4-one system

Abstract

Some mechanistic investigations on the photochemical behaviour of 1,2,4-oxadiazoles are reported. Irradiation of 3-benzoylamino-5-methyl-1,2,4-oxadiazole at λ= 254 nm in methanol involves excitation of the benzoylamino group chromophore to a triplet state species and induces a 6π electrocyclic closure to a tricyclic intermediate. On the other hand, irradiation of 3-acetylamino-5-phenyl-1,2,4-oxadiazole at λ= 254 nm involves excitation of a 5-phenyl-substituted oxadiazole chromophore which leads to the photolysis of the ring O–N bond in a singlet state species. Furthermore, irradiation of some selected 5-phenyl substituted oxadiazoles in the presence of a triplet sensitizer produces an excited triplet state species which collapses in a ring closure reaction involving the C₅-phenyl moiety.