Process for the preparation of compounds of formula (IV): wherein R1, R2, R3 and R4, which may be identical or different, each represent a hydrogen atom, a linear or branched (C1-C6)alkyl group, a linear or branched (C1-C6)alkoxy group, a fluorine atom, a chlorine atom, a protected amine group, a protected hydroxyl group, an alkoxycarbonyl group in which the alkoxy group is linear or branched (C1-C6), or a CF3 group, or R1═R4═H and R2 and R3 together with the carbon atoms carrying them form a 1,3-dioxolane group, R5 represents a saturated or unsaturated, linear or branched (C1-C6)alkyl group, a linear or branched (C1-C6)hydroxyalkyl group in which the hydroxyl function is protected, or a CO2R7 group in which R7 is a linear or branched (C1-C6)alkyl group, R6 represents a cyano group or a CO2R8 group in which R8 is a linear or branched (C1-C6)-alkyl group. Application in the synthesis of ivabradine, addition salts thereof with a pharmaceutically acceptable acid and hydrates thereof.