Compounds of formula (I):in which:R1 and R2 represent, independently of one another, a hydrogen atom or a (C1-C6)alkyl group,R3 represents one or more hydrogen or halogen atoms,X represents from 1 to 4 substituents, identical to or different from one another, chosen from hydrogen, halogen or (C1-C6)alkyl,in the form of the base or of an addition salt with an acid.Therapeutic use and synthetic process.