To a solution of 13.2 g (44.72 mmol) of carbonic acid 5-methyl-2-oxo-[1,3]dioxol4-ylmethyl ester 4-nitro-phenyl ester in 200 ml of dimethylsulfoxide are added 20.0 g of (6R,7R)-7-[(Z)-2-(5-amino-[1,2,4]thiadiazol-3-yl)-2-hydroxyimino-acetylamino]-8-oxo-3-[(E)-(R)-2-oxo-[1,3′]bipyrrolidinyl-3-ylidenemethyl]-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid. The mixture is stirred under argon at room temperature for 4 hours and 1000 ml of acetone are added. The slightly turbid solution is clarified by filtration over a fluted filter. To the clear motherliquour are added 34.0 ml (34 mmol) of a 1N solution of sodium-2-ethylcaproate in acetone at room temperature during 20 minutes. The slightly yellow suspension is stirred for 10 minutes at room temperature, the solid is collected by filtration, washed with 1000 ml of acetone and 1000 ml of n-pentane and dried under high vacuum. The product is suspended in 600 ml of acetone and stirred at room temperature for 2 hours. The product is collected by filtration and dried under high vacuum to yield 23.94 g of the title compound as an off-white powder.
