The invention relates to compounds of formula (I) Wherein the substituents R1, R2 and R4 have the meaning as set forth and explained in the description of the invention, to processes for the preparation of these compounds, pharmaceutical compositions containing same, the use thereof optionally in combination with one or more other pharmaceutically active compounds for the therapy of a disease which responds to an inhibition of protein kinase activity, especially a neoplastic disease, in particular leukaemia, and a method for the treatment of such disease.