Synthesis of ( 15 N 2 )[ 17 O]Urea, ( 15 N 2 )[ O 2 , O 4 ‐ 17 O 2 ]Uridine, and ( 15 N 3 )[ O 2 ‐ 17 O]Cytidine

A general synthetic approach for the synthesis of 15 N‐ and 17 O‐doubly labelled pyrimidine nucleosides is described. The 15 N isotopes in uridine and the 17 O isotope in the urea‐derived carbonyl group of uridine and cytidine originate from ( 15 N 2 )[ 17 O]urea ( 5 ) which was synthesized from 15 NH 4 Cl, thiophosgene ( 1 ), and H 2 [ 17 O]. The third 15 N isotope of cytidine in 4‐position stems from the substitution of the 1,2,4‐triazole moiety of ( 15 N 2 )[ O 2 ‐ 17 O]uridine derivative 8a/b with 15 NH 4 OH. Hydrolysis of the same key intermediate 8a/b with Na[ 17 O]H/H 2 [ 17 O] introduced the second 17 O isotope into the 4‐position of uridine. The 15 N‐ and 17 O‐NMR spectra of the target compounds 12 and 14 in phosphate‐buffered H 2 O serve as references for heteronuclear NMR spectra of labelled RNA fragments.