One-Bond ~(13)C—~1H Spin-Coupling Constants in Aldofuranosyl Rings: Effect of Conformation on Coupling Magnitude

摘要

One-bond ~(13)C—~1H spin-coupling constants (~1J_(CH)) have been studied in the model furanose 2-deoxy-β-D-glycero-tetrofuranose (1) as a function of ring geometry in order to assess their utility as conformational probes. Ab initio molecular orbital (MO) calculations were conducted on the ten envelope forms and the planar form of 1 using the second-order Moller—Plesset (MP2) electron correlation treatment with a polarized split-valence (6-31G~*) basis set. The derived structures were used to compute ~1J_(CH) values in conformers of 1 at the Hartree-Fock and MP2 levels of theory which were subsequently scaled using a factor derived from accurate quadratic configuration interaction (QCISD) calculations. MO results indicate that C-H bond lengths in 1 vary with ring geometry, with a given C-H bond longest when quasi-axial and shortest when quasi-equatorial. Computed ~1J_(CH) values were found to be sensitive to C—H bond orientation, with greater couplings observed when a C—H bond is quasi-equatorial. Computations conducted on β-D-ribofuranose (3) and 2-deoxy-β-D-erythro-pentofuranose (4) (HF/6-31G~* level) show a C-H bond length/orientation dependence similar to that observed in 1. Experimental (NMR) data are presented which support the proposed correlation. These results suggest a role for ~1J_(CH) in the conformational analysis of furanose rings which may complement current methods based on ~3J_(HH), ~3J_(CH), and ~2J_(CH) values.