Preferably in the above formula (I) X is hydrogen or halogen, i.e. iodine, bromine, chlorine or fluorine, especially iodine or bromine; Y is an oxo group; R1 is halogen, preferably bromine or iodine, azido or a group ##STR7## as defined above, especially a group --NH2 ; R2 is hydrogen or, when R1 is halogen, R2 is the same halogen as R1.