A suspension of 8.2 g. 7β-aminocephalosporanic acid in 100 ml. of tetrahydrofuran and 24 ml. of N,O-bistrimethylsilylacetamide is heated to reflux and stirred for two hours to yield a limpid solution. 6 grams of 2,2-dimethyl-4,6-dioxo-5-propanoyl-1,3-dioxane are added and the mixture is stirred for 3 hours at reflux and then cooled to room temperature and evaporated to dryness. The residue is dissolved in 300 ml. of ethyl acetate and 200 ml. of water. The insoluble phase is filtered off and separated. The organic phase is washed with 100 ml. of water, dried, decolourised over charcoal and evaporated to dryness. The residue is taken up with diisopropyl ether and a gummy product is obtained, which is triturated with diethyl ether to give 2 g. of 7β-(2-oxovaleramido)-3-acetoxymethylceph-3-em-4carboxylic acid melting at 127°-134° C. (with decomposition). To a solution of 1.6 g. of 7β-(3-oxovaleramido)-3-acetoxymethylceph-3-em-4-carboxylic acid in 25 ml of glacial acetic acid are added dropwise 0.6 g of sodium nitrite dissolved in 6 ml. of water. The mixture is stirred for one hour and a half, then 100 ml. of ethyl acetate and 100 ml. of a saturated solution of sodium chloride are added thereto. The organic phase is separated, washed twice with 50 ml. of water, made anhydrous and concentrated to small volume. The precipitate is filtered off and gives 0.9 g. of 7β-(2-hydroxyimido-3-oxovaleramido)-3-acetoxymethylceph-3-em-4-carboxylic acid melting at 190°-204° C. (with decomposition); [α]D =+97° C. (c=0.2 in tetrahydrofuran/water 1:1).
