Particularly useful R1 and R2 substituents include halogen atoms, especially fluorine or chlorine atoms, or methyl, halomethyl, especially ???CF3, ???CHF2 or ???CH2F, methoxy or halomethoxy, especially ???OCF3, ???OCHF2 or ???OCH2F groups. [0080] R3 in compounds of the invention is in particular a hydrogen atom. [0081] R in the compounds of formulae (1) and (2) is pr