Abstract: The invention relates to a process for preparing a compound of formula (I) ##STR1## in which R.sub.1 is H, an alkyl radical or an alkoxy, hydroxyalkyl or halogen radical, and R.sub.3 is N.sub.3 or a CN radical, characterized in that a compound of formula (II): ##STR2## is reacted with a phosphine derivative and an azodicarboxylic acid diester and a carboxylic acid R.sub.2 --COOH in a solvent compatible with the reaction conditions, to form the compound of formula (III): ##STR3## which, after separation if required, is opened in the presence of an azide or a cyanide in a solvent compatible with the reaction conditions, and the compound of formula (I) is then isolated from the reaction medium after deprotection of the 5'-position.