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.