According to the present invention, among the compounds of formula (I) as defined above, the ones that are particularly preferred are those that have one or a combination of the following characteristics: R1 represents a methyl, ethyl or isopropyl group,R2 represents a chlorine or bromine atom or a methyl, ethyl, isopropyl or tert-butyl group,R3 represents a hydrogen atom,R4 and R4??? are either i