a-Phosphono-(3p-amino or 3a-alkylazetidin-2-one-1-yl)acetic acids and esters thereof are provided via cyclization process comprising the reaction of a β-hydroxy phosphonomethyl acid amide with triphenylphosphine-di-(C1-C3 alkyl)azodicarboxylate in an anhydrous aprotic solvent. The azetidinones obtained are useful intermediates to carbapenems and carbacephems and monocyclic antibacterials, e.g., a-(dial- kylphosphono)-[[3β-[2-(2-aminothiazol-4-yl)-2-(syn)methoxy- iminoacetylamino]azetidin-2-one-1-yl]]acetic acid and pharmaceutically acceptable salts thereof.