New process for producing cephalosporin antibiotics, and novel intermediates for use in such process and their production

In the process for the production of a syn-isomer of the formula Ia, ##STR14## in which R.sub.1 is hydrogen, alkyl, phenalkyl, carbalkoxyalkyl, acyl or carboxyalkyl,

• R.sub.2 is hydrogen, pivaloyloxymethyl or a carboxy protecting group, and
• R.sub.4 is hydrogen, acetoxy, carbamoyloxy or --S--Y, in which Y is a heterocyclic ring which may be substituted;
• or a pharmaceutically acceptable salt form thereof, which process comprises acylating a compound of formula III, ##STR15## in which R.sub.2 and R.sub.4 are as defined above, and
• R.sub.5 is hydrogen or an amino protecting group, with a reactive derivative of the acid of the formula, ##STR16## wherein R.sub.1 is as defined above;
• and effecting any subsequent disprotecting steps and interconversion of the free acid in which R.sub.2 is hydrogen and salts thereof required to obtain a syn-isomer of formula Ia or a pharmaceutically acceptable salt form thereof, the improvement which comprises employing as said reactive derivative a syn-isomer of the formula IIa:
• ##STR17## where R.sub.1 is as defined above, and ##STR18## is 2-benzthiazolyl.