Synthesis of (N-15(2))[O-17]urea, (N-15(2))[O-2,O-4-O-17(2)]uridine, and (N-15(3))[O-2-O-17]cytidine

A general synthetic approach for the synthesis of N-15- and O-17-doubly labelled pyrimidine nucleosides is described. The N-15 isotopes in uridine and the O-17 isotope in the urea-derived carbonyl group of uridine and cytidine originate from (N-15(2))[O-17]urea (5) which was synthesized from (NH4Cl)-N-15, thiophosgene (1), and H-2[O-17]. The third N-15 isotope of cytidine in 4-position stems from the substitution of the 1,2,4-triazole moiety of (N-15(2)[O-2-O-17]uridine derivative 8a/b with (NH4OH)-N-15. Hydrolysis of the same key intermediate 8a/b with Na[O-17]H/H-2[O-17] introduced the second O-17 isotope into the 4-position of uridine. The N-15- and O-17-NMR spectra of the target compounds 12 and 14 in phosphate-buffered H2O serve as references for heteronuclear NMR spectra of labelled RNA fragments.