Substrates which can be used for the detection of enzymes include compounds of the general formula: ##STR12## in which R1, R2, R3 and R4, which can be the same or different, are hydrogen or halogen atoms, alkyl and alkoxy radicals containing up to 6 carbon atoms, carboxyl or sulphonyl groups or R1 and R2 or R3 and R4 can also form an alkyl or alkylene bridge, X' is a hydroxyl group or an a