Reversal of Diastereoselectivity of the Reaction of Chiral Boron and Titantium Enolates with Nitrones via N-Acyloxyiminium Intermediates. Asymmetric Synthesis of Diastereomeric (alpha)-Substituted (beta)-Amino Acids

Reaction of nitrones and acyl halides gives N-acyloxyiminium species, which are more reactive toward soft carbon nucleophiles than nitrones. Addition of chiral enolates to the N-acyloxyiminium species gave N-hydroxy-(beta)-amino acid derivatives highly diastereoselectively. Reversal of diastereoselectivity was observed between the boron enolates and titanium enolates. Using this method all of the four stereoisomers of (alpha)-methyl-(beta)phenylalanines can be prepared as enantiomerically pure forms.