R6 have the meanings given hereinafter, R3 is a hydrogen or halogen atom or a carboxyl group, R4 and R'4 are hydrogen or halogen atoms, carboxyl groups, lower alkoxy radicals or lower alkyl radicals which are preferably in the m-position and R1 and R4 or R2 and R4 and R4 and R'4, when the two substituents are ortho to one another, can together form a saturated or unsaturated hydrocarbon chain cont