The invention provides a process for preparing a compound of formula (I) ##STR1## and pharmaceutically acceptable derivatives thereof wherein: R.sup.0 and R.sup.1 are independently selected from H, halogen, C.sub.1-6 alkyl, C.sub.1-6 alkoxy, or C.sub.1-6 alkoxy substituted by one or more fluorine atoms; R.sup.2 is H, C.sub.1-6 alkyl, C.sub.1-6 alkyl substituted by one or more fluorine atoms, C.sub.1-6 alkoxy, C.sub.1-6 hydroxyalkyl, SC.sub.1-6 alkyl, C(O)H, C(O)C.sub.1-6 alkyl, C.sub.1-6 alkylsulfonyl, C.sub.1-6 alkoxy substituted by one or more fluorine atoms, halogen, CN, CONR.sup.4 R.sup.5, CO.sub.2 H, CO.sub.2 C.sub.1-6 alkyl, or NHSO.sub.2 R.sup.4 ; R.sup.3 is H or phenyl substituted by SO.sub.2 C.sub.1-6 alkyl or SO.sub.2 NH.sub.2 ; R.sup.4 and R.sup.5 are independently selected from H, C.sub.1-6 alkyl, phenyl, phenyl substituted by one or more atoms or groups R.sup.6, or together with the nitrogen atom to which they are attached form a saturated 4 to 8 membered ring R.sup.6 is halogen, C.sub.1-6 alkyl, C.sub.1-6 alkoxy or C.sub.1-6 alkoxy substituted by one or more fluorine atoms; which comprises rearrangement of an azirine of formula (II) ##STR2## wherein R.sup.0 to R.sup.3 are as defined for formula (I), or a protected derivative thereof, in the presence of a catalyst and a solvent.