Some amino acid esters - An improved preparative method

This paper describes an improvement to an earlier method for preparing some esters of amino acids. Amino acids react with ethyl acetoacetate and the strong base N,N,N',N'-tetramethylguanidine in dimethylformamide to form the salt of the corresponding enamine. Addition of a substituted benzyl or picolyl halide to this solution results in rapid esterification. The amino-protection can then be removed by cold methanolic hydrogen chloride to give the amino acid ester hydrochloride. For asparagine, protection of the amide group is also necessary. With tryptophan, ethyl acetoacetate cannot be used but can be replaced by acetylacetone. No racemization has been detected.