Synthesis of 1‐(2‐ethoxyethyl)‐2‐(4‐methyl‐1‐homopiperazinyl)‐[2‐ 14 C]benzimidazole difumarate([ 14 C]KB‐2413)

1‐(2‐Ethoxyethyl)‐2‐(4‐methyl‐1‐homopiperazinyl)‐[2‐ 14 C]‐benzimidazole difumarate ([ 14 C]KB‐2413), a new antihistaminic agent, was prepared from labelled urea. The synthetic intermediate, 1‐(2‐Ethoxyethyl)benzimidazolone, obtained in good yield by the condensation of N‐(2‐ethoxyethyl)‐o‐pheynylenediamine with [ 14 C]urea, was chlorinated with phosphorus oxychloride and followed by the reaction with N‐methylhomopiperazine to give the base of [ 14 C]KB‐2413. It was then converted to the difumarate, which was obtained in an overall radiochemical yield of 74% starting from [ 14 C] urea. The specific activity was 54.4 mCi/mmol and its radiochemical purity was 98.0% in reverse isotope dilution analysis.