X is an sulfur atom, R18 is a saturated, straight-chain or branched-chain C1-C6 alkyl group, R11 is a hydrogen atom, a fluorine or chlorine atom, a saturated, straight-chain or branched-chain C1-C6 alkyl group, a group ???X???R18, a chloromethyl or chloroethyl group or an optionally substituted aryl or heteroaryl radical. 6.