PHOTOINDUCED MOLECULAR-REARRANGEMENTS - THE PHOTOCHEMISTRY OF SOME 1,2,4-OXADIAZOLES IN THE PRESENCE OF NITROGEN NUCLEOPHILES - FORMATION OF 1,2,4-TRIAZOLES, INDAZOLES, AND BENZIMIDAZOLES

The photochemistry of some 3,5-disubstituted 1,2,4-oxadiazoles in the presence of nitrogen nucleophiles [external, such as added amines or h ydrazines, 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 nucleophili c reagent to give open-chain intermediates, which develop into 1,2,4-t riazolin-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 th e presence of hydrazines have been also investigated. In the irradiati on of 3-(o-aminophenyl)-5-methyl-, 3-[o-(methylamino)phenyl]-5-methyl- , and 3-(o-aminophenyl)-5-phenyl-1,2,4-oxadiazoles, concomitant format ion of indazoles and benzimidazoles, presumably arising from a common photolytic species, has been observed. Some mechanistic aspects have b een considered, and possible applications in synthesis have been point ed out.