Alkylation of 2,5-dichlorothiophenol (I) with bromoacetaldehyde diethyl acetal (II) produced thioether (III). Benzothiophene (IV) was then obtained by cyclization of (III) under Friedel-Crafts conditions in the presence of polyphosphoric acid. Metalation of benzothiophene (IV) with BuLi at -78 C, followed by addition of sulfur dioxide gave rise to the sulfinic acid (V). This was converted to the sulfonyl chloride (VI) by chlorination with N-chlorosuccinimide, and further treatment of (VI) with ammonium hydroxide provided the sulfonamide (VII). Addition of formaldehyde to the benzothiophene-2-sulfonamide (VII), followed by condensation with trimethyl phosphite, furnished the dimethyl (sulfonamidomethyl)phosphonate (VIII). The phosphonate ester function of (VIII) was then hydrolyzed by means of bromotrimethylsilane, producing phosphonic acid (IX). This was finally coupled to 4-nitrophenol (X) using trichloroacetonitrile in hot pyridine to afford the title mono-phosphonate ester.

Addition of formaldehyde to 5,7-dichlorobenzothiophene-2-sulfonamide (I), followed by condensation with trimethyl phosphite, furnished the dimethyl (sulfonamidomethyl) phosphonate (II). The phosphonate ester function of (II) was then hydrolyzed by means of bromotrimethylsilane, producing phosphonic acid (III). This was finally coupled to 4-nitrophenol (IV) using trichloroacetonitrile in hot pyridine to afford the title mono-phosphonate ester.