Photoinduced single electron transfer on 5-aryl-1,2,4-oxadiazoles: Some mechanistic investigations in the synthesis of quinazolin-4-ones

The photochemistry of some 5-aryl-3-methoxy- (or 5-aryl-3-phenyl-) 1,2,4-oxadiazoles irradiated in the presence of different sensitizers [such as diphenylacetylene (DAC), 9,10-diphenylanthracene (DAN), or triphenylene (TPH)] or ground-state donors such as triethylamine (TEA) has been investigated. Intermediates arising from breaking of the ring O-N bond develop both into quinazolin-4-ones (by a heterocyclization reaction involving the aryl at the C-5 of the oxadiazole nucleus) and into open-chain products (corresponding to a reduction at the ring O-N bond), in different ratios depending on their structures and photoreaction conditions. A reasonable explanation considers sensitization by photoinduced electron transfer either from the sensitizer in its excited state to the oxadiazole in its ground state or from the electron donor reagent (TEA) to the excited oxadiazole; in both cases an oxadiazole radical anion is formed as a key species from which breaking of the ring O-N bond takes place. Reduction potentials of representative oxadiazoles confirm this hypothesis. Possible applications in the synthesis of variously substituted quinazolin-4-ones are recognized.