-tolylamino)benzoic acid.Preferred examples of R.sub.1 and R.sub.3 in the compound of formula (I) are lower alkyl groups having 1 to 3 carbon atoms, especially a methyl group, and examples of R.sub.2 are lower alkoxy-carbonyl groups with the alkyl moiety having 1 to 3carbon atoms, such as an ethoxycarbonyl group.