or cn c 1 3 alkyl c 1 3 alkoxy hydroxyl c 1 3 fluoroalkyl or c 1 3 fluoroalkoxy groups r 3 represents a hydrogen atom or a c 1 6 alkyl group optionally substituted with a c 3 7 cycloalkyl group r 4 represents a hydrogen atom or a c 1 6 alkyl group r 5 and r 5 u2032 represent independently of each other a hydrogen or halogen atom a hydroxyl or c 1 3 alkyl group or r 5 and r 5 u2032 form together an oxo or oxime group such as where r 7 represents a hydrogen atom or a c 1 3 alkyl n represents an integer ranging from 0 to 3 and r 6 represents independently of each other when n 2 or 3 a hydrogen or halogen atom a hydroxyl c 1 3 alkyl c 1 3 alkoxy c 1 3 fluoroalkyl or c 1 3 fluoroalkoxy group among the compounds of general formula i a sub group of preferred compounds consists of the compounds for which x represents an oxygen or sulfur atom and or r 1 represents a c 1 5 alkyl group preferably a methyl ethyl 1 methylethyl 1 1 dimethylethyl propyl 1 methylpropyl 2 methylpropyl 1 ethylpropyl optionally substituted with a phenyl a thienyl or r 1 represents a c 3 7 cycloalkyl group preferably a cyclohexyl a thienyl or pyridinyl group the thienyl groups being optionally substituted with one or two c 1 3 alkyl groups preferably a methyl.