According to a feature of the present invention, the pyridine derivatives of general formula I, wherein B is other than oxygen and sulphur atoms, Z represents a grouping of the formula: ##STR8## R1 represents a grouping of the formula: --COOR4, in which R4 represents a hydrogen atom, or an alkyl group containing from 1 to 12 carbon atoms, and the other symbols are as hereinbefore defined, i.e. com