Abstract: This invention relates to a process for the preparation of compounds having the formula: ##STR1## in which R.sub.1 is H or lower alkyl and R.sub.2 is H or lower alkyl, or phenyl or benzyl optionally substituted with at least one halogen or a lower alkyl, lower alkoxy, trifluoromethyl or nitro group.According to this method, a compound of the formula: ##STR2## in which R.sub.1 and R.sub.2 are as defined above and R.sub.3 is H or optionally substituted alkyl or benzyl, is reacted with a derivative Hal--SO.sub.2 --R (III) in which Hal is halogen and R is optionally substituted alkyl, aryl or aralkyl, to give a derivative of the formula: ##STR3## in which the various symbols have the above-defined meanings, and the sulfonated derivative of the formula (IV) is heated in the presence of an acid, to give the compound of the formula (I).