The photochemistry of some 3,5-disubstituted 1,2,4-oxadiazoles in
the presence of nitrogen
nucleophiles [external, such as added amines or hydrazines,
or internal, such as an
o-aminophenyl
moiety at C(3) of the oxadiazole ring] has been investigated. In
the irradiation of 5-amino-(or 5-N-substituted amino) 3-phenyl-1,2,4-oxadiazoles in the presence of
aliphatic primary amines (or
ammonia), photolytic species arising from heterolytic cleavage of the
ring O−N bond capture the
nucleophilic reagent to give open-chain intermediates, which develop
into 1,2,4-triazolin-5-ones.
Similarly, irradiations of 3,5-diphenyl-, 3-methoxy-5-phenyl-, and
5-methyl-3-phenyl-1,2,4-oxadiazoles gave 1,2,4-triazoles. In the same context, irradiations of
representative substrates in the
presence of hydrazines have been also investigated. In the
irradiation of 3-(o-aminophenyl)-5-methyl-, 3-[o-(methylamino)phenyl]-5-methyl-, and
3-(o-aminophenyl)-5-phenyl-1,2,4-oxadiazoles,
concomitant formation of indazoles and benzimidazoles, presumably
arising from a common
photolytic species, has been observed. Some mechanistic aspects
have been considered, and possible
applications in synthesis have been pointed out.