Preferably, R.sub.8 represents a hydrogen atom.Where X represents a group --NR.sub.9 R.sub.10, suitably R.sub.9 represents a hydrogen atom or an alkyl group, for example a methyl group and RIO represents a substituted alkyl group.Where R.sub.10 represents a substituted alkyl group, said group preferably represents a group of general formula --CHR.sub.21 COOH wherein R.sub.21 represents an optional