BOP-Cl is a phosphoric acid derivative used as a coupling reagent for amide and ester formation. BOP-Cl is effective in the acylation of N-methylated amino acids and in the introduction of thiopeptide bonds on imino acids. BOP-Cl requires mild conditions, gives good yields and suppresses epimerisation. BOP-Cl and other phosphinic acids activate the carboxylic acid by forming a carboxylic-phosphinic mixed anhydride, in which stereochemistry is conveniently retained. BOP-Cl gives good performances in the acylation of N-methylated amino acids due to intramolecular base catalysis.