R1 is a halogen atom or is an alkyl, cyclic alkyl, or aryl group having from 1 to 10 carbon atoms, and preferably it is either a halogen atom or a group having 1 to 2 carbon atoms; R1 groups may contain one or more heteroatoms such as nitrogen or oxygen and may be partially-, fully- or non-halogenated, provided, however, that whenever X is a hydrogen atom, R1 contains an allylic carbon atom relati