Photocatalytic selective reduction by TiO2 of 5-nitrosalicylic acid ethyl ester: a mild route to mesalazine

Photoexcited TiO2 dispersed in a de-aerated acetonitrile/2-propanol (CH3CN/2-PrOH) reaction mixture catalyzes the quick and selective reduction of 5-nitrosalicylic acid methyl ester to the corresponding aniline, an immediate precursor of the drug mesalazine. The transformation is selective also when the starting concentration of nitro compound is increased by orders of magnitude and occurs at room temperature and atmospheric pressure. The photocatalyst can be reused. The photocatalytic reaction can be carried out also under aerated conditions without any loss in selectivity and efficiency. All these factors point out the feasibility of this important synthesis under mild conditions.