Photosensitizer-Free Benzo[1,4]oxazin-2-one Synthesis by Continuous-Flow Photochemistry.

We report a photosynthetic method for producing 2H-benzo[b][1,4]oxazin-2-ones from aryl azides and α-ketoacids. This method is highly sustainable, requiring only visible light irradiation of the substrates and no external additives. Furthermore, we implemented a continuous-flow system to achieve efficient light irradiation and rapid mixing, significantly improving reaction efficiency and reducing reaction time compared to the batch process. The flow system enabled gram-scale synthesis. We also demonstrated the utility of the products, by employing the benzo[1,4]oxazin-2-one moiety as a directing group for C-H activation on the 3-aryl substituent. This green approach highlights the potential for developing environmentally friendly synthetic processes.