Expedient Synthesis of threo-β-Hydroxy-α-amino Acid Derivatives: Phenylalanine, Tyrosine, Histidine, and Tryptophan

An expedient synthesis of enantiomerically pure threo-beta-hydroxy-alpha-amino acid derivatives of phenylalanine, tyrosine, histidine, and tryptophan is described. The NBS-mediated radical bromination of the N,N-di-tert-butoxycarbonyl protected alpha-amino acids and subsequent treatment with silver nitrate in acetone provided the trans-oxazolidinones predominantly. Cesium carbonate catalyzed hydrolysis then generated the beta-hydroxy amino acid derivatives in excellent overall yield.