Reversal of Diastereoselectivity of the Reaction of Chiral Boron and Titanium Enolates with Nitrones via N-Acyloxyiminium Intermediates. Asymmetric Synthesis of Diastereomeric α-Substituted β-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-β-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 α-methyl-β-phenylalanines can be prepared as enantiomerically pure forms.