C-13-H-1 and C-13-C-13 spin-coupling constants in methyl beta-D-ribofuranoside and methyl 2-deoxy-beta-D-erythro-pentofuranoside: Correlations with molecular structure and conformation

Methyl beta-D-ribofuranoside 5 and methyl 2-deoxy-beta-D-erythro-pentofuranoside (methyl 2-deoxy-beta-D-ribofuranoside) 6 were synthesized with single sites of C-13-enrichment at each carbon, and a complete set of C-13-H-1 and C-13-C-13 spin-coupling constants (37 couplings in 5, 41 couplings in 6) in these molecules were obtained by 1D and 2D NMR spectroscopy. (2)J(CH) coupling signs were determined from the observation of relative cross-peak displacements in 2D TOCSY data. The C-13-H-1 couplings (one-, two-, and three-bond) were interpreted in structural and conformational terms with assistance from conformational models of 5 and 6 based on conventional two-state pseudorotational analysis of (3)J(HH) values (PSEUROT) and on theoretical predictions of conformational energies and J(CH) values obtained from ab initio molecular orbital calculations on the ten envelope and planar conformers of beta-D-ribofuranose 4 and 2-deoxy-beta-D-erythro-pentofuranose (2-deoxy-beta-D-ribofuranose) 3. A comparison of theoretical J(CH) values in 3 and 4 allowed an assessment of the effect of C2 structure (C2 oxy vs C2 deoxy) on coupling magnitudes and signs. Results show that the behavior of related J(CH) values in 5 and 6 may differ, especially when C2 is involved as a coupled nucleus. In addition, several (1)J(CH), (2)J(CH), and (3)J(CH) values in beta-D-ribo and 2-deoxy-beta-D-ribo rings were found to be sensitive to furanose ring and hydroxymethyl group conformation; in the deoxyribo ring, the presence of diastereotopic protons at C2 produces paired C-13-H-1 coupling pathways involving H2R and H2S that are complementary, and differences in the resulting paired J(CH) values are, in some instances, sensitive to ring conformation. J(CC) values in beta-D-ribo and 2-deoxy-beta-D-ribo rings also reflect differences in ring structure and geometry, although fewer (e.g., (2)J(C3,C5), (3)J(C1,C5), (3)J(C2,C5)) appear useful as conformational probes. The correlations drawn between ring structure/conformation and J(CH)/J(CC) magnitude and sign in 5 and 6 will be useful in anticipated applications of these couplings to assess furanose ring conformation/dynamic in DNA and RNA oligomers and in other biomolecules containing beta-D-ribo and 2-deoxy-beta-D-ribo rings.