Synthesis and ¹⁵N- and ¹⁷O-NMR Spectroscopy of 5-Methyl-(¹⁵N₂)[O²,O⁴-¹⁷O₂]uridine   5-Methyl-(¹⁵N₂)[O²,O⁴-¹⁷O₂]uridine was synthesized and analyzed by ¹⁵N- and ¹⁷O-NMR spectroscopy. (¹⁵N₂)Urea was condensed with 2,3-dibromo-2-methylpropanoyl chloride and cyclized to form (¹⁵N₂)thymine. After glycosidation, the ¹⁷O isotopes were introduced in two separate steps: hydrolytic ring opening of 2,5'-anhydro derivative and hydrolysis of 4-(3-nitro-1,2,4)-triazole derivative with labelled water in the presence of a strong base. The ¹⁵N- and ¹⁷O-NMR spectra of the doubly-labelled product in phosphate-buffered water serve as references for heteronuclear NMR spectra of labelled RNA fragments.