Synthesis of 3′‐CTNU radiolabelled with 14 C in the alkylating moiety and 3 H in the carbamoylating moiety

[6‐ 3 H]3′‐[3‐(2‐Chloroethyl)‐3‐nitrosoureido]‐3′‐deoxythymidine ([ 3 H]3′‐CTNU) radiolabelled with 3 H in the carbamoylating moiety has been synthesized in high yield by conversion of tritiated 3′‐amino‐3′‐deoxythymidine([ 3 H]3′‐ATdR) to [6‐ 3 H]3′‐[3‐(2‐chloroethylureido)]‐3′‐deoxythymidine ([ 3 H]3′‐UTdR), followed by nitrosation with N 2 O 3 gas. The title compound radiolabelled with 14 C in the alkylating moiety was prepared by conversion of [2‐ 14 C]ethanolamine hydrochloride to [1‐ 14 C]chloroethylamine hydrochloride, followed by reaction with phosgene to yield [1‐ 14 C] chloroethyl isocyanate. This was converted to 3′‐3‐([1‐ 14 C] 2‐chloroethylureido)‐3′‐deoxythymidine([ 14 C]3′‐UTdR), and nitrosated to afford 3′‐[3‐([1‐ 14 C] 2‐chloroethyl) ‐3‐nitrosoureido]‐3′‐deoxythymidine ([ 14 C]3′‐CTNU). HPLC methodology was developed for the purification of intermediate and final products.