A process for the preparation of a compound of formula (I), or a salt thereof wherein R is H, C1-C8 alkyl, aryl or aryl-C1-C8 alkyl; comprising the reaction between a compound of formula (II) wherein R1 and R2 are independently hydrogen, C1-C8 alkyl, aryl, aryl-C1-C8 alkyl, or R1 and R2, taken together, form a —(CH2)m—V—(CH2)n— group, in which V is NR3 or C(R3)2 wherein R3 is hydrogen, C1-C8 alkyl, aryl or aryl-C1-C8 alkyl; and m and n, which can be the same or different, are 1 or 2; with a compound of formula (III) in which R4 and R5 are independently C1-C8 alkyl, aryl or aryl-C1-C8 alkyl; in the presence of a Ni (II) salt, an organic ligand and a basic agent, to obtain a compound of formula (I) wherein R is C1-C8 alkyl, aryl or aryl-C1-C8 alkyl and, if desired, its conversion to a compound of formula (I) wherein R is hydrogen or to a salt thereof.