PHOTOINDUCED MOLECULAR-REARRANGEMENTS - THE PHOTOCHEMISTRY OF 1,2,4-OXADIAZOLES IN THE PRESENCE OF SULFUR NUCLEOPHILES - SYNTHESIS OF 1,2,4-THIADIAZOLES

The photochemistry of some 1,2,4-oxadiazoles in the presence of sulphu r nucleophiles has been investigated. Irradiation of the 5-amino-3-phe nyl- and 3,5-dipbenyl-1,2,4-oxadiazole at lambda = 254 nm in methanol in the presence of sodium hydrogen sulphide or thiols gave a photo-ind uced redox reaction at the ring O-N bond, leading to the corresponding N-substituted benzamidines. By contrast, irradiation of the 5-amino-3 -phenyl-1,2,4-oxadiazole in the presence of thioureas or thiocarbamate s, essentially gave 3-phenyl-5-substituted 1,2,4-thiadiazoles, which p resume an N-S bond formation between the ring-photolytic species and t he sulphur nucleophile. In turn, irradiation of the same 5-amino-3-phe nyl-1,2,4-oxadiazole in the presence of thioamides again afforded the redox reaction; in addition, amounts of 3-phenyl-5-substituted-1,2,4-t hiadiazoles were also formed. Some mechanistic considerations are repo rted and synthetic methodologies leading to 1,2,4-thiadiazoles are emp hasized. (C) 1997 Elsevier Science Ltd.