Catalytic <i>N</i> ‐Bromo Succinimide Enables Achmatowicz Rearrangement with Oxone

Achmatowicz rearrangement (AchR) has been widely used for the synthesis of various heterocyclic building blocks and natural products. N ‐bromo succinimide (NBS) was often used as a stochiometric oxidation reagent to promote AchR with many synthetic advantages; however, the generation of stochiometric succinimide as the byproduct is undesirable from the perspective of green chemistry and occasionally complicates the purification process in a large scale. Herein, we report the development of catalytic NBS (5 mol%) for AchR with oxone as the terminal oxidant, which generates only small amount (5%) of the succinimide byproduct and substochiometric K 2 SO 4 (95%). The 24‐times reduction of NBS (from 1.2 equivalent to catalytic 5 mol%) substantially improves the greenness of this protocol and greatly eases the purification of the AchR product. The efficiency and utility of this protocol are consistently superior to the NBS condition and demonstrated with the broad substrate scope (21 examples including steroid substrates), excellent yields, and scalability. Moreover, mechanistic studies suggested that NBS initiated the AchR reaction and the concomitant bromide byproduct could be oxidized by oxone into reactive brominating species in the form of NBS and/or HOBr for subsequent AchR reaction. We believe this new catalytic NBS protocol will find wide applications in organic synthesis.