The synthesis of [14C]-labeled KT1-32 has been reported: The acylation of 7-isopropylazulene-1-carboxylic acid ethyl ester (I) with 1-[14C]-acetyl chloride-SnCl4 in dichloromethane gives the corresponding 3-acetyl derivative (II), which is reduced with sodium cyanoborohydride and boron trifluoride in ether to the 3-ethyl derivative (III). The decarboxylation of (III) with polyphosphoric acid at 100 C affords 1-[14C]-ethyl-5-isopropylazulene (IV), which is finally sulfonated with the pyridine-SO3 complex in refluxing benzene.