Acylation of 2-amino-4-chlorobenzaldehyde (I) with acid chloride (II) provides the corresponding amide (III). Intramolecular cyclization of the aldehyde-amide (III) in the presence of piperidine in refluxing toluene gives rise to the quinolinone (IV). The electrophillic nitration of (IV) using HNO3/H2SO4 affords the 6-nitroquinoline (V), which is further reduced to amine (VI) by reaction with iron powder and NH4Cl. Diazotization of amine (VI), followed by reaction with SO2 and CuCl2 leads to the sulfonyl chloride (VII). This is converted into the sulfonamide (VIII) upon treatment with NH4OH. The diethyl phosphonate ester of (VIII) is finally hydrolyzed to the title phosphonic acid using bromotrimethylsilane in refluxing acetonitrile.