The synthesis of 7-β-phenoxyacetamido-4′-hydroxybenzo[3,4]-O-2-isocephem 14 is described. Condensation of 5 with cinnamaldehyde gave 6 which was converted to the 3-azido-4-styrrylazetidinone 7. Ozonolysis and reduction of the resulting aldehyde 8 gave 9. Compound 9 was converted to the mesylate 12 by reduction of the azide to the amine 10, coupling of 10 with phenoxyacetic acid to give 11, and mesylation of 11. Removal of the benzyl groups of 12 by hydrogenolysis and base catalyzed ring closure of the phenol gave 14.