The reduction of 3,4-dinitrobenzoic acid amide (I) with H2 over Pd/C in methanol gives 3,4-diaminobenzoic acid (II), which is cyclized with 3-ethoxy-3-iminopropionic acid ethyl ester (IV) (obtained by reaction of cyanacetic acid ethyl ester (III) with ethanolic HCl) in hot acetic acid to yield 2-(5-carbamoyl-1H-benzimidazol-2-yl)acetic acid ethyl ester (V). The reaction of 3-methoxy-4-nitrobenzoic acid (VII) with aqueous methylamine in a sealed tube at 100 C gives 3-(methylamino)-4-nitrobenzoic acid (VIII), which is reduced with H2 over Pd/C in methanol/THF to yield 4-amino-3-(methylamino)benzoic acid (VI). The cyclization of (VI) with (V) by means of 1,3-dimethylperhydropyrimidin-2-one (DMPU) at 190 C affords the substituted bis-benzimidazolylmethyl (IX). The reaction of (IX) with 2-amino-3-phosponopropionic acid tris-trimethylsilyl ester (X) (prepared by esterification of 2-amino-3-phosphonopropionic acid (XI) with N-methyl-N-(trimethylsilyl)trifluoroacetamide (XII)) by means of (PyBroP) in DMF provides the carboxamide (XIII). Finally, the cleavage of the trimethylsilyl ester groups by means of TFA furnishes the target phosphonic acid.