Diisopropylethylamine eliminates dipeptide formation during the acylation of amino acids using benzoyl chloride and some alkyl chloroformates

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.