HETEROCYCLIC PHOTOREARRANGEMENTS - SOME INVESTIGATIONS OF THE PHOTOCHEMICAL BEHAVIOR OF 3-ACYLAMINO-1,2,4-OXADIAZOLES - SYNTHESIS OF THE QUINAZOLIN-4-ONE SYSTEM

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 lambda = 254 nm in methanol involves excitation of the benzoylamino group chromophore to a triplet state species and induces a 6-pi electrocyclic closure to a tricyclic intermediate. On the other hand, irradiation of 3-acetylamino-5-phenyl-1,2,4-oxadiazole at lambda = 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.