1,8-Diazabicyclo[5.4.0]undec-7-ene, or DBU for short, as a recognized sterically forbidden non-nucleophilic base, has been widely used in organic synthesis. In recent years, it has been found that DBU can also participate in the reaction as an effective nucleophilic base.

The mixed anhydride method was developed in the early 1950s. Beginning with mixed anhydrides of acylamino acids with diphenylphosphoric acid, followed by mixed anhydrides with dibenzylphosphoric acid and mixed anhydrides with benzoic acid. When the mixed anhydride formed by benzyloxycarbonyl amino acid and benzoic acid reacts with the amino component, a by-product of benzoylation of the amino component will be formed. In order to suppress this side reaction, mixed anhydrides formed with other carboxylic acids with higher electron density or greater steric hindrance on the carboxyl carbon atom were used later.