Reactions of 3-(polyfluoroacyl)chromones with hydroxylamine: synthesis of novel RF-containing isoxazole and chromone derivatives

Reaction of 3-(polyfluoroacyl)chromones with hydroxylamine free base proceeds via nucleophilic 1,4-addition followed by opening of the pyrone ring and subsequent cyclization to 4-(polyfluoroalkyl)-4H-chromeno[3,4-d]isoxazol-4-ols in good yields. On treatment with trifluoroacetic acid, the isoxazole ring of this annulated heterocyclic system opens to give 3-cyano-2-(polyfluoroalkyl)chromones, which were successfully hydrolyzed with concentrated H2SO4 to afford 3-carbamoyl-2-(polyfluoroalkyl)chromones. On the other hand, oximation of 3-(polyfluoroacyl)chromones with hydroxylamine hydrochloride occurs either at the carbonyl carbon atom connected to the RF group or at the C-2 atom to give 3-RFC(NOH)-chromones and 5-RF-4-salicyloylisoxazole oximes, respectively. The former were easily converted to 3-RF-4-salicyloylisoxazoles by simple heating in dimethyl sulfoxide.