Abstract: The invention relates to compounds of the formula ##STR1## in which R.sup.1 is absent or denotes hydrogen, alkyl or acyl, A denotes an acyl radical or an amino acid residue, B denotes an amino acid residue, and R.sup.2, R.sup.3 and R.sup.4 are as defined in the specification, and to their salts, to a process for their preparation, to pharmaceutical products containing them, and to their use as medicaments, and intermediates for the preparation of these compounds.

Abstract: Compounds which themselves are antibacterially active and/or which are useful as intermediates for producing antibacterially active materials such as penicillins and cephalosporins are obtained by the following syntheses:Benzyl-2-(2,2-dimethyl-1,3-dioxan-5-ylidene)-2(N-formyl-1)-aminoacetate to benzyl-2,2-dimethylspiro-[1,3-dioxane-5,5'-1',3-thiazoline]-4'-carboxylate to benzyl 2-(2,2-dimethyl-1,3-dioxan-5-ylidene)-2[N-(methylthiomethylene)-amino] acetate to benzyl 2-(trans-3-azido-4-methylthio-2-oxo-1-azetidinyl)-2-(2,2-dimethyl-1,3-diox an-5-ylidene) acetate to benzyl 2(trans-3-benzoylamino-4-methylthio-1-azetidinyl)-2-(2,2-dimethyl-1,3-diox an-5-ylidene) acetate to either (1) benzyl 2-(7-oxo-3-phenyl-4-oxa-2,6-diazabicyclo[3.2.0]hept-2-en-6-yl)-2-(2,2-dime thyl-1.3-dioxan-5-ylidene)acetate or (2) benzyl 2-(trans-3-benzoylamino-4-chloro-1-azetidinyl)-2-(2,2-dimethyl-1,3-dioxan- 5-ylidene) acetate and then to the product benzyl-7-benzoylamino-1-dethia-1-oxa-3-hydroxymethyl-cephem-4-carboxylate.