Fragmentation-rearrangement of Δ3-oxadiazolin-5- and 3-ones

Abstract

2-Phenylbenzimidazole (6)(major product) and 2-phenylbenzoxazole (10) were obtained from 2,3-diphenyl-Δ³-1,2,4-oxadiazolin-5-one (2) by thermolysis (250 °C, diphenyl ether), and by photolysis (254 nm, dioxan). Photolysis also isomerised the oxadiazolinone (2) into 2,5-diphenyl-Δ⁴-1,2,4-oxadiazolin-3-one (8), and 2,4-diphenyl-Δ²-1,3,4-oxadiazolin-5-one (9). Similar thermolysis and photolysis of 2-benzyl-3-phenyl-Δ³-1,2,4-oxadiazolin-5-one (3) gave cyaphenine (11) and lophine (12). The diphenylazomethine nitrene (14) can be the precursor to the imidazole (6) and the transformations (2)→(10) and (3)→(11)+(12) can proceed from the appropriate carbamic acid derivative [(16) or (23)], which are available from (2) or (3) by a 1,3-migration of oxygen from nitrogen to carbon.