The present invention provides the compounds of the formula (II): ##STR4## wherein R1 is a phenyl group optionally substituted by a fluorine, chlorine or bromine atom or an alkyl or alkoxyl group of up to 3 carbon atoms; R2 is a hydrogen atom or an alkyl group of up to 4 carbon atoms; R3 is a hydrogen atom or an amino group, a nitro group or a group of the formula NHCOR4 or NHCO2 R4 where R4 is an