Azetidinone N-phosphonomethyl esters

The invention provides azetidinones of the formula ##STR1## where R.sub.4 is hydrogen, amino or protected amino; R.sup.0 is H when R.sub.4 is amino or protected amino, or C.sub.1 -C.sub.4 alkyl when R.sub.4 is H; R.sub.1 is H, CH.sub.3, --CH.sub.2 --.sub.n Y where Y is OH, protected OH, --CH.sub.2 OH, protected --CH.sub.2 OH, halogen, COOH or protected COOH; n is 1 or 2; --CH.sub.2 --C(O)SR.sub.1 ' where R.sub.1 ' is e.g., C.sub.1 -C.sub.4 alkyl; R.sub.2 is H or protecting group and R.sub.3 is e.g., alkyl or phenyl. The azetidinones obtained are useful intermediates to carbapenems and carbacephems and monocyclic antibacterials, e.g., .alpha.-(dialkylphosphono)-[[3.beta.-[2-(2-aminothiazol-4-yl)-2-(syn)metho xyiminoacetylamino]azetidin-2-one-1-yl]]acetic acid and pharmaceutically acceptable salts thereof.