Acylation of amino acids using benzoyl chloride in aqueous alkali leads to benzoylamino acids containing one percent of benzoyldipeptide. Use of diisopropylethylamine instead of sodium hydroxide as base eliminates the side reaction responsible for the contaminant. Ethoxycarbonylamino acids are advantageously prepared in the same manner using ethyl chloroformate or diethyl dicarbonate. The latter gives rise to some N-substituted dipeptide when used in aqueous alkali. The method is unsatisfactory for the benzyloxycarbonylation of amino acids. Use of 9-fluorenylmethyl chloroformate and diisopropylethylamine gives the pure derivative of leucine in moderate yield.

Methoxycarbonylation at the urethane nitrogen of N-benzyloxycarbonyl- and N-methoxycarbonylvaline has been identified as a side reaction accompanying the aminolysis of the mixed anhydride formed from the acid and methyl chloroformate when the anhydride is coupled in aqueous dimethylformamide and the nucleophile is an amino acid anion. The side reaction occurs to a lesser but still significant extent when the activated residue is leucyl.