Continuous Flow Photochemical Benzylic Bromination of a Key Intermediate in the Synthesis of a 2‐Oxazolidinone

A continuous flow procedure for the benzylic photobromination of methyl N,N ‐bis ( tert ‐butoxycarbonyl) phenylalaninate is presented. This photochemical transformation generating the brominated intermediate is the critical step in the synthesis of the oxazolidinone methyl (4 S ,5 R )‐3‐ N ‐ tert ‐Butoxycarbonyl‐5‐phenyl‐1,3‐oxazolidin‐2‐oxo‐4‐carboxylate, a key intermediate for the preparation of active pharmaceutical ingredients. The reaction was optimized in three continuous flow photoreactors: A self‐made reactor based on a T5 8 W black light lamp, as well as the commercially available VapourTec® UV‐150 and Corning® Advanced‐Flow™ reactors, both equipped with LED light sources. Under optimal conditions, concentrated solutions (95 g/L) of the starting material could be processed on a multi‐gram scale, providing high conversion (96 %) within 7 min at 30 °C, using 2 equivalents of N ‐bromosuccinimide (NBS).