Iron and Ruthenium Lewis Acid Catalyzed Asymmetric 1,3-Dipolar Cycloaddition Reactions between Nitrones and Enals

The single coordination-site transition metal Lewis acids [CpM(BIPHOP-F)][SbF6] (M = Fe, Ru) catalyze the [3+2] dipolar cycloaddition reaction between reactive nitrones and α,β-unsaturated aldehydes to give chiral isoxazolidines with ee values of 75 to >96%. The stereochemistry of the major enantiomer is consistent with an endo approach of the nitrone to the Cα-Si-face of the enal in the s-trans conformation in the (R,R)-catalyst site. The absolute configuration is based on an X-ray structure determination.