Asymmetric Synthesis of Fmoc-Protected β-Hydroxy and β-Methoxy Amino Acids via a Sharpless Aminohydroxylation Reaction Using FmocNHCl

An efficient asymmetric synthesis of l-threo-β-hydroxyasparagine and l-threo-β-methoxyaspartate that are suitably protected for Fmoc solid phase peptide synthesis is described. The key step in these syntheses was a Sharpless asymmetric aminohydroxylation reaction under basic conditions using N-chlorofluorenyl carbamate (FmocNHCl), a readily prepared and storable nitrogen source.