A process for the preparation of a compound of formula (I) ##STR00001## comprising the steps: (a) reacting a compound of formula (II) ##STR00002## wherein L and L' are suitable leaving groups, with a compound of formula (III) UNH.sub.2 (III) to prepare a compound of formula (IV) ##STR00003## and subsequently (b) substituting the group R.sup.1 by replacement of the leaving group L'.