Photorearrangements of phenyloxazoles

Abstract

Various aryl oxazoles have been subjected to photolysis. The resulting photorearrangements have been classified as type A or B. Type A involves formal interchange of two adjacent ring atoms (exchange of positions 2 and 3, or 4 and 5). Type B involves formal exchange of positions 2 and 4, or 3 and 5. Irradiation of 2-phenyloxazole (18) in benzene or cyclohexane gives 3-phenyl-2H-azirine-2-carbaldehyde (19)[thermal rearrangement leads to 3-phenylisoxazole (21)] and 4-phenyloxazole (20). Similarly, irradiation of 5-methyl-2-phenyloxazole (23) results in 2-acetyl-3-phenyl-2H-azirine (24). The correlation between photolysis of the oxazole (18) and its electronic structure, as deduced from semiempirical SCF-MO and SCF-Cl calculations, is discussed. The type A rearrangement can be rationalised in terms of a ring-contraction–ring-expansion sequence. A pathway involving a bicyclic intermediate is proposed for the type B rearrangement.