3- Amino-5-phenyl-1,2,4-oxadiazole, irradiation

A particular case of a photochemical strategy towards functionalized benzimidazoles regards the synthesis of 2-acylamino derivatives 70, which can be obtained from irradiations in methanol of 3-(o-amino)phenyl-l,2,4-oxadiazoles 68. In this reaction, a photolytic cleavage of the ring O—N bond, followed by migration of the aryl substituent, leads to the carbodiimide intermediate 69, a precursor of the benzimidazole nucleus (Scheme 12.20) [47]. 

The irradiation of 3-amino-5-phenyl-l,2,4-oxadiazole (98) gave the corresponding 1,3,4-oxadiazole (99) (Scheme 41) [88JCS(P1)1313 88JHC931 95FI(41)2095). The formation of 99 has been explained by assuming a RCRE mechanism. 

In analogy to the reactivity described above, 1,2,4-oxadiazoles have also been used as photochemical precursors of 1,2,4-thiadiazoles. When 5-amino-3-phenyl-l,2,4-oxa-diazole 116 is irradiated at 254nm in methanol, in the presence of a sulfur nucleophile such as thioureas, significant yields of thiadiazoles 121 were obtained, most likely through the open-chain species 120 (Scheme 12.33) [73],... 

Buscemi, S., Vivona, N. and Caronna, T. (1995) A generalized and efficient synthesis of 3-amino-, 3-(N-alkylamino)-, 3-(N, N-dialkylamino)-5-alkyl-l, 2,4-oxadiazoles by irradiation of 3-alkanoylamino-4-phenyl-l, 2,5-oxadiazoles (furazans). Synthesis, (8), 917-919. 

Amino-5-phenyl-l,2,4-oxadiazole and 1 gave enamino-ketone 188 that on irradiation with a low-pressure mercury lamp gave benzimidazole 189 as a result of a photoinduced rearrangement of the 1,2,4-oxadiazole ring (88JHC1551) (Scheme 32). 

Bifunctional systems form heterocyclic rings for example (194) yields (195). Photolysis of N-(triphenylmethyl)anilines results exclusively in C-N bond homolysis to give the trityl (< ) = 0.6-0.8) and anilino radicals." Reduction of the N-O bond cleaved intermediate (196) (formed on irradiation of 5-amino or 5-phenyl-1,2,4-oxadiazoles) by sodium hydrogen sulfide, thiols or thioamides yields the corresponding benzamidines (197). In contrast, in the presence of nucleophilic thioureas (199) or thiocarbamates (200), N-S bond formation occurs and subsequent elimination from the possible intermediate (201) yields the thiadia-zoles (198) or (202) respectively. ... 

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