A novel synthesis of carbon-labelled quinolone-3-carboxylic acid antibacterials

3‐lodoquinolones were prepared from the corresponding quinolone‐3‐carboxylic acids by Hunsdiecker‐type iododecarboxylation reactions with lead tetraacetate and iodine. Cyanation of the iodo compounds with mixtures of potassium [13C]cyanide and copper (I) iodide, gave [3‐13C]cyanoquinolones which on acidic hydrolysis afforded quinolone‐[3‐13C]carboxylic acids. In this way, nalidixic acid, an immediate precursor of norfloxacin, and quinolone WIN57273 were labelled with carbon‐13 in the metabolically stable carboxylic acid fragment.