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-doubl y labelled pyrimidine nucleosides is described. The N-15 isotopes in u ridine and the O-17 isotope in the urea-derived carbonyl group of urid ine and cytidine originate from (N-15(2))[O-17]urea (5) which was synt hesized 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-posi tion of uridine. The N-15- and O-17-NMR spectra of the target compound s 12 and 14 in phosphate-buffered H2O serve as references for heteronu clear NMR spectra of labelled RNA fragments.