The invention relates to compounds of formula (I) wherein R represents aryl or heteroaryl, X is oxygen, a carbonyl group, an oxime derivative of a carbonyl group or an α,β-unsaturated carbonyl group, and the substituents R1 to R6 have the meanings given in the specification, for use as medicaments, to novel compounds of formula (I), to methods of synthesis of such compounds, to pharmaceutical compositions containing compounds of formula (I), to the use of a compounds of formula (I) for the preparation of a pharmaceutical composition for the treatment of neoplastic and autoimmune diseases, and to methods of treatment of neoplastic and autoimmune diseases using such compounds of formula (I) or of pharmaceutical compositions containing same