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Determination of the Karplus relationships for the C‐2, H‐1′ and C‐6, H‐1′ vicinal coupling paths of uridine derivatives

A number of uracil cyclo‐nucleosides with known glycosidic bond conformations have been synthesized as model compounds for evaluating the Karplus parameters of vicinal carbon‐proton coupling across the glycosidic bond. The magnitudes of 3J(C‐2, H‐1′) and 3J(C‐6, H‐1′) were determined from proton‐coupled 13C NMR measurements, and both coupling constants showed an approximate Karplus dependence with dihedral angle. Careful analysis of the results on model compounds reveals that the C‐2, H‐1′ and C‐6, H‐1′ coupling paths are not equivalent, and so the magnitudes of Karplus parameters have been determined for each coupling path for the Karplus relationship in the form 3J=A cos2 θ + B cos θ + C, i.e. A6 = 6.2, B6 = −2.4 and C6 = 0.1 Hz and A2 = 5.0, B2 = 2.1 and C2 = 0.1 Hz. Comparison is made between proton‐carbon coupling magnitudes in nucleoside and peptide coupling paths.