Chemoselective N-ethylation of Boc amino acids without racemization

The Boc derivatives of amino acids such as phenylalanine, methionine, and tyrosine benzyl ethers have been selectively ethylated to give, respectively, the enantiomerically pure Boc-N-ethyl amino acids 12, 20, and 23. The benzyl, trimethylsilyl, and tert-butyldimethylsilyl groups were employed as transient protecting groups for the phenolic hydroxyl in the synthesis of Boc-Et-Tyr.