This compound has been obtained by three related ways: 1) The reaction of 4-phthalimido butyryl chloride (I) with phosphorous acid at 130 C, followed by hydrolysis with refluxing 6N HCl gives the target phosphonic acid. 2) The reaction of 4-phthalimido butyric acid (II) with phosphorous acid and phosphorous trichloride at 80 C, followed by hydrolysis with refluxing 48% HBr gives the target phosphonic acid. 3) The reaction of 4-maleimido butyric acid (III) with phosphorous acid and phosphorous trichloride at 80 C, followed by hydrolysis with refluxing 48% HBr gives the target phosphonic acid.