1-Oxo-2-ethoxy-2-(2-diethylphosphonoethyl)-1,2-diphenylethane (25 g, prepared from benzoin ethyl ether and diethyl vinyl phosphonate as described in Example 3 of U.S. Pat. No. 4,082,821) is dissolved in ethanol (200 ml). Sodium hydroxide (20 g) is added to the solution and the mixture is heated under reflux for 3 hours. The ethanol is removed by rotary evaporation, water is added, and the mixture is washed with diethyl ether (4×100 ml) to remove unreacted starting material. Concentrated hydrochloric acid is added to the aqueous mixture until the pH is zero. The resulting mixture is washed with ether, water is evaporated and the residue is recrystallised from a mixture of acetone and ether to give 12.5 g of product, a compound of formula XVIII where R1 is ethoxy, R3 and Ar1 are each phenyl, R7 is --CH2CH2 -- and R8 is --CH2CH3.
