There is provided a process for preparing a compound of the formula (I): ##STR1## wherein R.sub.1 is an organic residue, R.sub.2 is a hydrogen or a chlorine atom, a methoxy, ethoxy or an acetoxy group, and n represents 0, 1 or 2.The process comprises reacting a compound of the formula (II): ##STR2## wherein R.sub.2 and n are as defined above, with an inorganic or organic nitrite, in an alcohol R.sub.1 OH, wherein R.sub.1 is as defined above, or in a mixture R.sub.1 OH-organic solvent in the presence of an inorganic or organic acid.The compounds of the formula (I) are known intermediates in the synthesis of human leucocyte elastase inhibitors (HLE).