The cyclization of 3-[2-(carboxymethyl)phenyl]propionic acid (I) in the presence of conc. H2SO4 at 100 C gives 2-(1-oxoindan-4-yl)acetic acid (II), which is esterified with (COCl)2 and EtOH to yield the ethyl ester (III). The bromination of (III) with Br2 in HOAc affords 2-(2-bromo-1-oxoindan-4-yl)acetic acid ethyl ester (IV). The condensation of (IV) with 4-(diethoxyphosphoryl)-1H-imidazole-2-carboxylic acid ethyl ester (V) (obtained by cyclization of hydroxyamidinocarboxylic acid ethyl ester (VI) with ethynylphosphonic acid diethyl ester (VII) in the presence of TEA) provides the adduct (VIII). The cyclization of (VIII) with ammonium acetate in refluxing acetic acid gives the tetracyclic phosphonate (IX), which is finally hydrolyzed with refluxing aqueous HCl to yield the target phosphonic acid.