The invention relates to the compounds of the formula: ##STR1## in which R.sub.1 and R.sub.2 denote, independently of one another, a hydrogen atom, a C.sub.1 -C.sub.4 alkyl radical or a C.sub.2 -C.sub.3 mono- or polyhydroxyalkyl radical,Z and Z' denote, independently of one another, a C.sub.1 -C.sub.4 alkyl or C.sub.2 -C.sub.4 hydroxyalkyl radical, with the proviso that, when R.sub.1 and R.sub.2 simultaneously denote a hydrogen atom, Z and Z' do not simultaneously denote a methyl radical, and to their addition salts with an acid, to a process for preparation thereof and also to their use, by way of couplers, in an aqueous vehicle, in combination with at least one oxidation dye percursor of the para type, for the dyeing of human hair.The invention also relates to the intermediate compounds of the formula: ##STR2## in which R' denotes a hydrogen atom, a C.sub.1 -C.sub.4 alkyl radical, a C.sub.2 or C.sub.3 mono- or polyhydroxyalkyl radical, or an acetyl or .beta.-chloethoxycarbonyl radical, and z and z' denote, independently of one another, a C.sub.1 -C.sub.4 alkyl radical.