A process for the preparation of a compound of formula III of claim 2 which comprises condensing an unsymmetrical or symmetrical dialkyl or diaryl succinate, or monoalkyl monoaryl succinate or dicycloalkyl succinate of formula V wherein each R3 and R4 independently of the other is an alkyl or a cycloalkyl or an aryl radical, with two moles of nitrile of formula VI R1???CN???(VI) or of formula VII