fluorine or chlorine atom, a saturated, straight-chain or branched-chain C1-C6 alkyl group, a group ???X???R18, in which X is a sulfur atom and R18 means a saturated, straight-chain or branched-chain C1-C6 alkyl group, a chloromethyl or chloroethyl group or an optionally-substituted aryl or heteroaryl radical, R14 means a hydrogen atom in ??-position, and R15 means a hydrogen atom or R14 and R15 t