A group of interesting compounds are those compounds of formula (I) wherein R6 and R7 are taken together to form a bivalent radical of formula --R6 --R7 --, in particular a bivalent radical of formula (a-1), (a-5), (a-6) or (a-8) wherein one of the hydrogen atoms may be replaced by C1-6 alkyl, in particular methyl; and R5 is phenyl or C1-6 alkyl and preferably is methyl.