1-[.alpha.-(Carboxy)-4-hydroxybenzyl]-3.beta.-aminoazetidin-2-one esters are prepared by converting 2-acyl-3,3-dialkyl-7-oxo-.alpha.-[4-(benzyloxy)phenyl]-4-thia-2,6-diazabicyclo[3.2.0]heptane-6-acetic acid esters with mercuric acetate in an aqueous organic solvent mixture, e.g., in aqueous methanol, to 7-oxo-3-phenyl-.alpha.-[4-(benzyloxy)phenyl]-4-oxa-2,6-diazabicyclo[3.2.0]hept-2-ene-1-acetic acid esters and the latter are reacted with PCl.sub.5 and pyridine to provide the monocyclic 1-[.alpha.-(carboxy)-4-benzyloxybenzyl]-3.beta.-(.alpha.-chlorobenzylideneamino)-4-chloroazetidin-2-one esters. Reduction of the dichloro azetidin-2-one with an organo tin hydride and azobisisobutyronitrile affords the deschloro, 1-[.alpha.-(carboxy)-4-benzyloxybenzyl]-3.beta.-benzylideneaminoazetidin-2-one ester. The latter is hydrolyzed and the benzyloxy group is cleaved via catalytic hydrogenolysis to yield an ester of 1-[.alpha.-(carboxy)-4-hydroxybenzyl]-3.beta.-aminoazetidin-2-one. The 3.beta.-amino ester is useful for the preparation of the antibiotic FR 1923 (nocardicin).