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.