## wherein R3 is a hydrogen atom or a saturated hydrocarbon group having not more than 5 carbon atoms, R4 is a hydrogen atom or a saturated hydrocarbon group having not more than 20 carbon atoms, a saturated hydrocarbon group having at least one hydroxyl group and not more than 20 carbon atoms, a benzyl group, a substituted benzyl group, a phenyl group or a substituted phenyl group,which contains