Ozonolysis of 3-bromomethyl-2-cephem-4-carboxylic acid esters provides 1-(1-protected carboxy-2-hydroxy-3-bromo-1-propenyl)-3-amido(or imido)-4-formylthio-azetidin-2-ones, useful as intermediates in the preparation of cephalosporin antibiotics.
A monocyclic .beta.-lactam antibiotic having a formamido substituent at the 3S-position, including compounds of the formula (I): ##STR1## and salts thereof wherein R is --SO.sub.3 H; --PO(OH)Y wherein Y is C.sub.1-6 alkoxy, hydroxy, aryloxy, C.sub.1-6 alkyl or aryl; ##STR2## wherein Z represents C.sub.1-6 alkyl or aryl; ##STR3## wherein Z.sup.1 and Z.sup.2 may be the same or different and each represents hydrogen, C.sub.1-6 alkyl, aryl, amino or C.sub.1-6 alkoxy, or Z.sup.1 and Z.sup.2 together form the residue of a heterocyclic ring; --OSO.sub.3 H; or ##STR4## where X is hydrogen or hydroxy; R.sup.1 is an amino, protected amino or carboxylic acylamino group, and R.sup.2 and R.sup.3 are independently selected from hydrogen or a hydrocarbon group of 1 to 18 carbon atoms.
