R 5 is for example hydrogen, CrC 4 alkyl, phenyl which optionally is substituted by one or more CrC 4 alkyl, CrC 4 alkoxy or halogen; or is (C 2 -C 6 alkylene0) n -R6; in particular is hydrogen, CrC 4 alkyl or phenyl, especially hydrogen or CrC 4 alkyl, preferably CrC 4 alkyl.