To a solution of diphenylmethyl 2-[(3R,4R)-4-[(benzothiazol-2-yl)dithio]-3-phenoxyacetamido-2-oxo-azetidin-1-yl]-3-methyl-3-butenoate (XIIIa. 12.12 g, 0.0182 mole) in acetone (325 mL) and water (37 mL), silver nitrate (3.86 g, 0.0227 mole) was added all at once. A freshly prepared solution of sodium p-toluenesulfinate hydrate (4.05 g; 0.0227 mole) in acetone (255 mL) and water (37 mL) was added dropwise over 1 h, while the reaction mixture was protected from light. After an additional 1h period at room temperature, Celite (5 g) was added and the mixture was diluted with acetone (500 mL) and then was filtered. The filtrate was evaporated to a small volume, and the crude product was extracted into ether (3×250 mL). The ether phase was dried with magnesium sulfate and concentrated to afford a crude product. Flash chromatography (SiO2, eluting with 40% ethyl acetate in hexane) gave 9.05 g (73.5%) of the title compound as a white solid.
