The present invention relates to a compound of formula I wherein R1 is a heterocyclic radical or an unsubstituted or substituted aromatic radical; G is C1-C7-alkylene, -C(-O)-, or C1-C6-alkylene-C(-O)- wherein the carbonyl group is attached to the piperazine moiety; Q is -NH- or -O-, with the proviso that Q is -O- if G is -C(-O)- or C1-C6-alkylene-C(-O)-; and X is either not present or C1-C7-alkylene, with the proviso that a heterocyclic radical R1 is bonded via a ring carbon atom if X is not present; or a salt thereof, which is useful for treating anti-proliferative diseases.