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-ox adiazoles irradiated in the presence of different sensitizers [such as diph enylacetylene (DAC), 9,10-diphenylanthracene (DAN), or triphenylene (TPH)] or ground-state donors such as triethylamine (TEA) has been investigated. I ntermediates arising from breaking of the ring O-N bond develop both into q uinazolin-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 the ir structures and photoreaction conditions. A reasonable explanation consid ers sensitization by photoinduced electron transfer either from the sensiti zer 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 ox adiazole radical anion is formed as a key species from which breaking of th e ring O-N bond takes place. Reduction potentials of representative oxadiaz oles confirm this hypothesis. Possible applications in the synthesis of var iously substituted quinazolin-4-ones are recognized.