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Conformational studies of furanosyl fluorides by proton and fluorine nuclear magnetic resonance spectroscopy Steiner, Paul Robert

Abstract

Proton and fluorine nuclear magnetic resonance spectroscopy was used to investigate the favoured forms of several furanosyl fluorides. The low energy barrier to pseudo-rotation in these systems precludes the assignment of only one favoured conformation for each molecule. A systematic study of five membered ring sugar conformations was undertaken by examining three types of systems, varying from a relatively rigid molecule involving three fused rings, to more flexible molecules involving a "free" furanosyl fluoride ring. The spectral assignment was greatly facilitated by heteronuclear fluorine decoupling and computer analysis. 5-fluoro-3,6-anhydro-α-L-idofuranose was synthesized in good yield from several precursors and the molecule's furanose ring conformation of ₂T³was assigned on the basis of vicinal ¹H-¹H coupling constants. Using the values for vicinal ¹H- ¹⁹F couplings obtained from this molecule, together with data from other workers, a partial Karplus type curve was constructed relating vicinal ¹H- ¹⁹F couplings to dihedral angles. This curve - which was found to be more asymmetric than the Karplus relationship for vicinal ¹H-¹H couplings - was then used, together with the known Karplus curve for vicinal ¹H-¹H couplings, to determine the conformations of the triester furanosyl fluorides of ribose, arabinose and xylose. Assuming pure sp³ hybridization and maximum ring puckering, the conformations for most of the sugars studied were found to involve the displacement of C₂ and/or C₃ positions out of the plane of the ring formed by the other atoms. The conformation for α-fluoro-ribo-furanosyl triester ( ₃T⁴ ) was found to be different than that of the β-fluoro-ribofuranosyl triester (₃T⁴). A significant and apparently stereospecific long-range ⁴J[subscript H].₄,F coupling through oxygen of ca. 6.5 Hz, is observed when H₄ and F are in a trans arrangement, while a much smaller coupling for ⁴J[subscript H].₄,F of ca. 1.0 Hz. is observed when these two nuclei are in a cis or approximately "planar M" arrangement. On the other hand, ⁴J[subscript H].₃,F couplings were found to have a value of ca. 2.2 Hz. in the cis or "planar M" geometry, while in the trans geometry, ⁴J[subscript H].₄,F is ca. 0.5 Hz.

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