The invention concerns anti-tumour agents of formula (I) wherein each of R.sup.1, R.sup.2 and R.sup.3 has the meanings defined in the specification including hydrogen, (1-4C)alkyl, (3-4)alkenyl, (3-4C)alkynyl, amino, (1-4C)alkylamino and (1-4C)alkoxy; each of R.sup.4 and R.sup.5 is (1-4C)alkyl; each of R.sup.6 and R.sup.7 is hydrogen or (1-4C)alkyl; X is oxygen; m is 1 or 2 and each R.sup.8 is as defined in the specification; each of Y.sup.1 and Y.sup.2 is halogeno, (1-4C)alkanesulphonyloxy, benzenesulphonyloxy or phenyl-(1-4C)akanesulphonyloxy; or a pharmaceutically-acceptable salt thereof; provided that at least one of R.sup.1, R.sup.2 and R.sup.3 is other than hydrogen; a process for preparation, pharmaceutical compositions containing them and their use in the production of an anti-proliferative effect. ##STR1##