O,O-Dimethyl-1,4-oxoazetin-2-ylphosphonate (I) was prepd. by reaction of (MeO)(3)P with 4-acetoxy-acetidin-2-on at 110 degrees C for 5 h (m.p. 96 degrees C, yield 79%), converted either to BuOOCCH(2)CH(NH(2))PO(OH)(2) by hydrolysis with HCl in BuOH/SOCl soln. or to its Me, ester in BuOH/H(2)SO(4)/CH(2)Cl(2) (yields 80% or 62%, resp.). The hydrolysis of phosphonate I in HCl/MeOH resulted in formation of MeOOCCH(2)CH(NH(2)) PO(OH)(OMe) and MeOOCCH(2)CH(NH(2))PO(OMe)(2) mixt. when the CF(3)COOH or HO medium was used, HOOCCH(2)CH(NH) PO(OH)(MeOH) and HOOCCH(2)CH(NH(2))PO(OMe)(2) were performed. The hydrolysis of phosphonate I by using lipases and beta-lactamase was, however, unefficient because of low enantiomeric excess of the phosphonic analogues of aspartic acid produced (25% ee).