Asymmetric Induction in the [2,3] Sigmatropic Rearrangement via Chiral Ammonium Ylides

The asymmetric [2,3] sigmatropic rearrangements of the chiral ammonium chlorides (E)-(3a,b) were achieved upon treatment with t-BuOK followed by acidic hydrolysis of the aminonitriles (5a,b) obtained to give (R)-(+)-2-methyl-2-phenyl-3-butenal (6). The rearrangement of (E)-(3a) at −78° resulted in the highest optical yield (90%) of (R)-(+)-(6). The geometrical isomer (Z)-(3a) underwent the same [2,3] shift under the same conditions to afford (S)-(−)-(6). The mechanism of this asymmetric induction is discussed based on these results.