A compound selected from these of formula (I):wherein:R1 represents hydrogen, alkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl, alkoxycarbonyl, aryloxycarbonyl, arylalkoxycarbonyl, heterocycloalkoxycarbonyl, alkylsulfonyl, arylsulfonyl, arylalkylsulfonyl or phosphono,R2 represents oxygen or sulphur,R3 represents hydrogen or alkyl, cycloalkyl or arylalkyl,X represents linear or branched (C1-C6)alkylene,R4 represents a group selected from amino optionally substituted by one or two groups, -N(R3)-X1-OR5 wherein R3, X1 and R5 are as defined in the description, -N(R3)-X1-NR6R7 wherein R3, X1, R6 and R7 are as defined in the description, hydroxy, alkoxy, aryloxy, -O-X1-OR5 wherein R5 and X1 are as defined in the description, and -O-X1-NR6R7 wherein R6, R7 and X1 are as defined in the description,its isomers, and also addition salts thereof with a pharmaceutically acceptable acid or base, and medical products containing the same which are useful in the treatment of cancer.