30 ml of ethyl acetate were added at room temperature 2 96 ml of bistrimethylsilylacetamide 0 012 mol after stirring for 20 minutes a clear solution was formed the solution was cooled to 0 u00b0 c and added within five minutes to the reaction mixture a the temperature was kept between 30 u00b0 c and 25 u00b0 c then the reaction mixture was stirred at 10 u00b0 c for 11 2 hours and at 0 u00b0 c for 1 hour after evaporation of the solvent in vacuo the remaining syrup was treated with water to yield 3 1 g of crude product 3 1 grams of crude 3s z 2 1 2 amino 4 thiazolyl 2 1 3 1 4 dihydro 5 hydroxy 4 oxo 2 pyridinyl carbonyl amino 2 4 dioxo imidazolidinyl sulfonyl amino carbonyl 2 oxo 3 azetidinyl amino 2 oxoethylidene amino oxy 2 methylpropanoic acid diphenylmethyl ester was added at 10 u00b0 c to a mixture of 31 ml of trifluoroacetic acid and 6 2 ml of anisole after stirring for 1 hour at 10 u00b0 c the crude trifluoroacetic acid salt of 3s z 2 1 2 amino 4 thiazolyl 2 1 3 1 4 dihydro 5 hydroxy 4 oxo 2 pyridinyl carbonyl amino 2 4 dioxo 1 imidazolidinyl sulfonyl amino carbonyl 2 oxo 3 azetidinyl amino 2 oxoethylidene amino oxy 2 methylpropanoic acid was precipitated by the addition of ether 2 7 g the crude product was dissolved in a mixture of 10 ml of water and 10 ml of acetone and the ph was adjusted to 5 5 6 by the addition of 2n sodium hydroxide.