When R6 and R7 in formula (I) are combined such that --NR6 R7 represents the residue of a saturated mono cyclic secondary amine, such monocycles preferably have 5 to 7 ring atoms optionally containing another hetero atom (--O--, --S-- or --N--) in addition to the indicated nitrogen atom, and optionally bear one or more, preferably 1 or 2, substituents such as phenyl, benzyl and methyl.