or more heteroatoms belonging to groups 13-17 of the Periodic Table of the Elements; preferably R2 is a hydrogen atom; preferably R3 is a hydrogen atom or a linear or branched, saturated or unsaturated C1-C20-alkyl radical optionally containing one or more halogen atom; preferably R4 is a hydrogen atom or a linear or branched, saturated or unsaturated C1-C20-alkyl radical, more preferably R4 is h