Catalytic aza-Wittig Reaction of Acid Anhydride for the Synthesis of 4<i>H</i>‑Benzo[<i>d</i>][1,3]oxazin-4-ones and 4‑Benzylidene-2-aryloxazol-5(4<i>H</i>)‑ones

Compared to the aza-Wittig reaction of aldehydes, ketones, amides, and esters, the aza-Wittig reaction of acid anhydride was always overlooked, which should be an important part of Wittig reactions. Here, the aza-Wittig reaction of anhydride and catalytic aza-Wittig reaction of anhydride were both developed with high yields, which provides an efficient method to synthesize of 4<i>H</i>-benzo­[<i>d</i>]­[1,3]­oxazin-4-ones and 4-benzylidene-2-aryloxazol-5­(4<i>H</i>)-ones. The strategy of copper-catalyzed reduction of phosphine oxide was used and found effective for this transformation. Additionally, the one-pot catalytic aza-Wittig reaction of carboxylic acids was achieved. Furthermore, NMR experiments and Hammett plot recorded the process of catalytic aza-Wittig reaction of anhydride, which provides direct proof that the copper-catalyzed reduction of waste phosphine oxide is the key step in this transformation