Cyclic carbonylation of hydroxyureas 3 with 1,1'-carbonyldiimidazole gave 3-(3,5-dioxo-[1,2,4]oxadiazolidin-2yl)propylphosphonic acid diethyl esters 4 which were converted into the corresponding phosphonic acids 5 with bromotrimethylsilane.
Benzyloxyureas (4) have been prepared by reactions of diethyl 3-benzyloxyamino-propyl-phosphonate (3) with isocyanates, potassium cyanate or 1,1’-carbonyldiimidazole / methylamine. Conversion of phosphonic esters 4 into phosphonic acids 6 by means of bromotrimethylsilane and catalytic hydrogenation of 4, 6 afforded the target compounds 5, 7.