. . . . "In compounds of formula (1a) and in general in compounds of the invention the optional substituents on R13 groups include one or more substituents which may be the same or different selected from halogen atoms, especially fluorine, chlorine or bromine atoms, C1-6alkyl groups, specially methyl, ethyl and i-propyl groups, carboxyl (???CO2H) or esterified carboxyl (???CO2Alk4) groups, amino (???NH2)" .