R3 is a phenyl or cyclohexyl group which can be substituted with 1 to 5 identical or different substituents independently selected from halogen atoms, alkyl, alkoxy or hydroxyl groups and R4 is a hydrogen atom, phenyl or cyclohexyl group which can be substituted with 1 to 5 identical or different substituents independently selected from halogen atoms, alkyl, alkoxy or hydroxyl groups,