NaNO2/K2S2O8-mediated selective transformation of 3-formylchromones to 2-hydroxyiminobenzofuran-3-ones and 2-alkoxy-3-(hydroxyimino)chromanones

NaNO2/K2S2O8-mediated deformylative oximation ring contraction (DORC) or deformylative alkoxyoximation (DAO) bifunctionalization reactions of 3-formylchromones under benign conditions are described. The DORC of 3-formylchromones takes place in the presence of K2S2O8, NaNO2 and H2O to give 2-hydroxyiminobenzofuran-3-ones, while the DAO features the bifunctionalization of 3-formylchromones in the presence of K2S2O8, NaNO2 and ROH (alcohol) to afford 2-alkoxy-3-(hydroxyimino)chromanones. The preliminary mechanistic studies indicate that NO + NO3− (one form of NO2 radical dimer) initially adds to the electron-rich C3-site of 3-formylchromones to form a cationic intermediate, followed by attack of a nucleophile such as H2O or ROH to form a hemiacetal or acetal intermediate. Subsequently, the hemiacetal undergoes a tandem deformylation, ring-opening and ring-closing process to give 2-hydroxyiminobenzofuran-3-ones, while the acetal undergoes deformylation to give 2-alkoxy-3-(hydroxyimino)chromanones.