Ruthenium-Catalyzed Asymmetric Reduction of Isoxazolium Salts: Access to Optically Active Δ4-Isoxazolines.

A tethered MsDPEN-ruthenium catalyst reduces a series of isoxazolium salts, affording optically active Δ4-isoxazolines in moderate to good yields and enantioenrichment. The redundancy of heating or high pressures allowed for chemoselective reduction with no subsequent heterocyclic ring opening. Our results reinforce our understanding of the workings of these Noyori-class catalysts.