Characteristic 1 H NMR spectra of β-d-ribofuranosides and ribonucleosides: factors driving furanose ring conformations.

A series of β-d-ribofuranosides and ribonucleosides fused with 2,3- O -isopropylidene ring was synthesized and studied in terms of their conformational preferences. Based on the 1 H NMR spectra, DFT calculations, and X-ray analysis the E 0 -like and E 4 -like conformations adopted by these furanosides are identified. The 3 E -like and 2 E -like conformations are assigned to ribonucleosides without the 2,3- O -isopropylidene group. The studies are supported by analysis of the structural data of β-d-ribofuranosides and ribonucleosides deposited in the Cambridge Crystallographic Data Center (CCDC) database. Finally, the factors influencing the conformational preferences of the furanose ring with the β-d-ribo configuration are indicated. These are the unfavorable ecliptic orientation of the 2-OH and 3-OH groups, the 1,3-pseudodiaxial interaction of the aglycone and terminal hydroxymethyl group and the endo -anomeric effect. It is also proved that the exo -anomeric effect acts in β-d-ribofuranosides.