The invention relates to a compound of the formula (I) in which: R.sub.1, R.sub.2 and R.sub.3 are independently a methoxy group optionally substituted by one or more fluorine atoms; R.sub.4 is a hydrogen atom; R.sub.5 and R.sub.6 are identical and each represent a hydrogen or fluorine atom; A is a cycle selected from the group including aryl and heteroaryl groups, wherein said groups can be substituted.