The economical synthesis of [2′-13C, 1,3-15N2]uridine; preliminary conformational studies by solid state NMR

The synthesis of [2′-13C, 1,3-15N2]uridine 11 was achieved as follows. An epimeric mixture of D-[1-13C]ribose 3 and D-[1-13C]arabinose 4 was obtained in excellent yield by condensation of K13CN with D-erythrose 2 using a modification of the Kiliani-Fischer synthesis. Efficient separation of the two aldose epimers was pivotally achieved by a novel ion-exchange (Sm3+) chromatography method. D-[2-13C]Ribose 5 was obtained from D-[1-13C]arabinose 4 using a Ni(II) diamine complex (nickel chloride plus TEMED). Combination of these procedures in a general cycling manner can lead to the very efficient preparation of specifically labelled 13C-monosaccharides of particular chirality. 15N-labelling was introduced in the preparation of [2′-13C, 1,3-15N2]uridine 11 via [15N2]urea. Cross polarisation magic angle spinning (CP-MAS) solid-state NMR experiments using rotational echo double resonance (REDOR) were carried out on crystals of the labelled uridine to show that the inter-atomic distance between C-2′ and N-1 is closely similar to that calculated from X-ray crystallographic data. The REDOR method will be used now to determine the conformation of bound substrates in the bacterial nucleoside transporters NupC and NupG.