0.5 mole of methylene diphosphonic acid tetraethyl ester is heated for 6 h to
110°C with 0.5 mole of maleic acid diethyl ester in the presence of 25 ml of a
saturated sodium ethylate solution. The sodium ethylate is then neutralized
with acetic acid and the reaction mixture is quickly distilled invacuo in order to
avoid decomposition. The crude distillate is redistillated, the main fraction
accumulating in the range from 190-213°C at a pressure of 0.05 Torr, yield
50%.
The ester is hydrolyzed by boiling with concentrated hydrochloric acid. 1,1-
Diphosphonopropane-2,3-dicarboxylic acid crystallizes out after purification
with active carbon and concentration of the hydrolysis product, yield 100%.
