Acylation of lithium enolates of ethyl acetate and ethyl thioacetate with derivatives of (S)-2- hydroxy-3-methylbutanoic acid, activated at the carboxyl function as the acid chloride, unsymmetrical carbonic anhydride, or active ester, furnished the corresponding@-keto esters 5 or 8-keto thio esters 6. Coupling of ester derivatives of a-amino acids to the@-keto thio ester was promoted by copper (1) iodide to yield amino acid derivatives acylated at ...
