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Preparation of and nuclear magnetic resonance studies on some specifically fluorinated carbohydrates Manville, John Fieve

Abstract

The ¹H and ¹⁹F nuclear magnetic resonance (n.m.r.) spectra of a series of hexopyranosyl and pentopyranosyl fluorides were examined in some detail. The ¹H n.m.r. analyses (first order) enabled the gross molecular conformations to be determined and at the same time provided an opportunity to check the anomeric assignments previously made through application of other techniques. The¹⁹F n.m.r. parameters were then considered and several steric dependencies were observed. The ¹⁹F chemical shifts are controlled, in part, by the spatial orientation of the fluorine, but also, by the orientation of remote acetate groups or other substituents. Rules were subsequently developed for predicting the ¹⁹F chemical shifts for axial fluorines attached to C₁ of a pyranose carbohydrate. These rules were used to predict the ¹⁹F chemical shifts of the remaining "rare" giycopyranosyl fluorides. It was also noted that the ¹⁹F-¹H couplings resemble the ¹H-¹H couplings in that J(trans) > J(gauche). Moreover these ¹⁹F-¹H couplings are several times larger than the corresponding ¹H-¹H couplings and are also more sensitive to their precise steric environment. This study led to the observation that the β-D-anomers of the pentopyranosyl fluorides exist in the alternate chair form (¹C₄), which allows the fluorine atom to adopt an axial orientation. Subsequent to these initial n.m.r. investigations, a new reaction sequence was developed for the addition of "XF" to alkenes, Whereas this reaction sequence (AgF and X₂) results only in trans addition to cyclohexene (courtesy of D.L. Jones); it results in both cis and trans additions to cyclic vinyl ethers, although the trans adduct(s) always predominate: a possible reaction mechanism is discussed. This reaction sequence (X = I, Br,CI) was first applied successfully to D-glucal triacetate and then extended to other α,β-unsaturated cyclic ethers. The reaction products to D-glucal triacetate were for the most part crystalline. These 2-deoxy-2-halo-D-hexo-pyranosyl fluorides proved to be excellent models for extending previously observed electronegative dependencies. Moreover, when related to the 2-deoxy-D-glycopyranosyl fluorides, these compounds revealed that the orientation of remote substituents must play an important role in - determining ¹⁹F chemical shifts. Finally, a brief introduction is given to the utilization of both the "selective irradiation" and the "spin-tickling" methods for determining the relative signs of proton-proton coupling constants of carbohydrate derivatives. For most saturated carbohydrates the vicinal ¹H-¹H couplings have been found to be positive and the geminal couplings are negative in sign. For unsaturated derivatives both vicinal and geminal couplings are positive. The signs of the couplings of an episulphide derivative are the same as those of the saturated carbohydrates, whereas, the signs for an epoxide resemble those of unsaturated carbohydrates. These findings arc in accord with previous experimental and theoretical results. It has been demonstrated that the signs of long-range ( ⁴J) couplings of saturated derivatives exhibit a regular stereospecificity: those between two equatorially oriented protons (⁴ J, ₑ,ₑ) being positive and those between an axial and an equatorial proton (⁴Jₑ,ₐ ) being negative in sign. These findings are contrasted with the previous theoretical predictions and their potential as an adjunct to stereochemical studies is discussed. For unsaturated carbohydrate derivatives the long-range couplings are all negative in sign. The "selective irradiation" and "spin-tickling" homonuclear magnetic double resonance (n.m.d.r.) techniques have been used to determine the relative signs of the vicinal and geminal ¹⁹F- ¹H couplings of a series of glycopyranosyl fluorides. In all cases these two couplings have the same relative signs. Heteronuclear decoupling experiments on 2-bromo-2-deoxy-α-D-mannopyranosyl fluoride triacetate showed that for this derivative the vicinal coupling between H₁ and H₂ and the geminal coupling between H₁ and F₁ have the same relative signs. On this basis an absolute positive sign was assigned to both vicinal and geminal ¹⁹F- ¹H couplings.

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