the atom marked with the symbol * is linked to the atom marked with the same symbol in the compound of formula (I); R3 and R4 have the meaning previously described; R8 is a hydrogen atom or a linear or branched, saturated or unsaturated C1-C20-alkyl, C3-C20-cycloalkyl, C6-C20-aryl, C7-C20-alkylaryl, or C7-C20-arylalkyl radical, optional containing one or more heteroatoms belonging to groups 13-17