Process for preparing cephalosporin derivatives

A 7-acylamino (or 7-amino)-3-(substituted thiomethyl)cephalosporin derivative or a carboxyl derivative thereof is prepared by heating the corresponding 7-acylamino (or 7-amino)-3-carbamoyloxymethylcephalosporanic acid or a carboxyl derivative thereof with a thiol corresponding to the group which it is desired to introduce at the 3-position under conditions in which the water present is minimized, specifically, either no water is present or the amount of water is limited to less than 5 times the weight of the 7-acylamino (or 7-amino)-3-carbamoyloxymethylcephalosporanic acid or carboxyl derivative thereof. The compounds thus prepared are useful as pharmaceuticals or as intermediates in the production of pharmaceuticals. One of the compounds thus prepared, namely 7 alpha -methoxy-3-(1-methyl-1H-tetrazol-5-yl)thiomethyl-7 beta -(O-5-p-nitrobenzoylamino-5-carboxyvaleramido-3-cephem-4-carboxylic acid, is a new compound and it and its salts and esters also form part of the present invention.