The invention provides novel compounds of formula (I), wherein: R1 is a radical derived from one of the known ring systems; R2 is a phenyl radical optionally substituted; Xn represents a birradical selected from the group consisting of: —(CH2)1-4—, (C2-C4)-alkenyl, (C2-C4)alkynyl, —S—(CH2)1-3—#, and —(CH2)1-3—O—#; wherein the symbol # indicates the position at which Xn is attached to R1; Yn is a birradical selected from the group consisting of: —(CH2)2-4—, —S—(CH2)1-3#, and —O—(CH2)1-3—#; wherein the symbol # indicates the position at which Yn is attached to R2; and R3 is a radical selected from the group consisting of: —OR4. The compounds of formula (I) are useful in the treatment of cancer.