Synthesis of enantiomerically enriched β,γ-unsaturated-α-amino acids

A variety of enantiomerically enriched β,γ-unsaturated-α-amino acids are synthesized by olefination of a Cbz-protected serine aldehyde equivalent, readily prepared from serine. A cyclic ortho ester protecting group is employed to minimize racemization. The deprotected amino acids are obtained in good yield, ranging from 70–95% ee, with double-bond geometry determined by the type of Wittig reagent used. Isotopically labeled side chains are readily introduced by this procedure, and free γ-13C-vinylglycine was prepared in 44% yield from the protected serine aldehyde synthon.