Asymmetric syntheses of the naturally occurring β-amino acids, β-lysine, β-leucine and β-phenyl-β-alanine via nitrone cycloaddition

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Abstract
A general asymmetric synthesis of β-amino acids is based on the dipolar cycloaddition of nitrones 7(R* chiral) with vinyl acetate 8a, ketene acetals 8b or α-chloroacrylonitrile 8c. The cycloadducts 9 are converted either directly (9b) or via the isoxazolidinones 10(9a, 9c) into the free β-amino acids 11. Diastereoselectivity at C-3 in the adducts 9 ranges between 2:1 and 11 :1. The natural β-amino acids, β-lysine, β-leucine and β-phenyl-β-alanine, have been prepared in this way.
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