Rapid Synthesis of the <i>epi</i>-Biotin Sulfone via Tandem <i>S</i>,<i>N</i>‑Carbonyl Migration/aza-Michael/Spirocyclization and Haller–Bauer Reaction

A synthesis of 2-epi-biotin sulfone was accomplished from commercially available l-cysteine. The synthesis features an unprecedented tandem S,N-carbonyl migration/aza-Michael/spirocyclization reaction from an l-cysteine-derived enone with aq. ammonia, in which three new sigma bonds and two rings are formed. In addition, the synthesis includes a highly diastereoselective late-stage Haller–Bauer reaction of sulfone for direct introduction of the carbon side chain.