¹³C−¹H Spin-Coupling Constants in the β-d-Ribofuranosyl Ring: Effect of Ring Conformation on Coupling Magnitudes

Experimental and computational methods have been used to examine the behavior of one-, two-, and three-bond 13C−1H spin-coupling constants (1JCH, 2JCH and 3JCH, respectively) within the β-d-ribofuranosyl ring 1 that may be potentially affected by ring conformation. Ab initio molecular orbital (MO) calculations at the HF/6-31G* and MP2/6-31G* levels of theory were employed to assess the effect of ring conformation on molecular parameters (i.e., bond lengths, angles, and torsions) of β-d-ribofuranose (2) and methyl β-d-ribofuranoside (3), and these data were validated through comparison to corresponding parameters obtained by X-ray crystallography. The MO-derived structural data were subsequently used to compute 1JCH, 2JCH and 3JCH values in 2 as a function of ring conformation. This predicted behavior was then tested experimentally through the measurement of JCH values in conformationally-rigid model compounds (aldopyranosides) containing 13C−1H coupling pathways similar to those found in specific conformers of 2 and was examined for consistency with previously-derived empirical rules correlating JCH with structure in carbohydrates. Available JCH data obtained on several biologically-important compounds containing β-d-ribofuranosyl rings have been interpreted in light of the new correlations with ring conformation.