The invention provides a process for preparing a compound of formula (I) and pharmaceutically acceptable derivatives thereof wherein: R0 and R1 are independently selected from H, halogen, C1-6alkyl, C1-6alkoxy, or C1-6alkoxy substituted by one or more fluorine atoms; R2 is H, C1-6alkyl, C1-6alkyl substituted by one or more fluorine atoms, C1-6alkoxy, C1-6hydroxyalkyl, SC1-6alkyl, C(O)H, C(O)C1-6alkyl, C1-6alkylsulfonyl, C1-6alkoxy substituted by one or more fluorine atoms, halogen, CN, CONR4R5, CO2H, CO2C1-6alkyl, or NHSO2R4; R3 is H or phenyl substituted by SO2C1-6alkyl or SO2NH2; R4 and R5 are independently selected from H, C1-6alkyl, phenyl, phenyl substituted by one or more atoms or groups R6, or together with the nitrogen atom to which they are attached form a saturated 4 to 8 membered ring R6 is halogen, C1-6alkyl, C1-6alkoxy or C1-6alkoxy substituted by one or more fluorine atoms; which comprises rearrangement of an azirine of formula (II)  wherein R0 to R3 are as defined for formula (I), or a protected derivative thereof, in the presence of a catalyst and a solvent.