Compounds of formula I are obtained according to process variant b) by reacting amines of formula III with compounds of formula IV. Halogen or aliphatic or aromatic sulphonyloxy groups are suitable as the leaving group X in formula IV. Conveniently, the reaction is carried out in a solvent, for example in an alcohol such as methanol or ethanol or in an aprotic solvent such as acetonitrile, N,N-dim