Ethenylidenebisphosphonic acid tetraethyl ester (I, 3.00 g), ethyl acetoacetate (II, 1.4 ml), and DBU (0.25 ml) are heated to 50° in THF (20 ml) for 1.5 hours. The reaction is cooled, diluted with ethyl acetate, filtered through magnesium sulfate and concentrated under reduced pressure. The concentrate is chromatographed eluting with ethyl acetate, ethyl acetate/acetone (1/1). The appropriate fractions are pooled and concentrated to give the title compound, MS (m/e) 430 (M+), 388, 339, 301 and 288; IR (neat) 2984, 1740, 1716, 1478, 1444, 1392, 1368 and 1250 cm-1 ; NMR (CDCl3) 4.20, 2.43, 2.29, 1.35 and 1.28 δ.
