Furanoid rings, conformations

It has been shown that stereoelectronic effects play an important role in many enzyme-catalyzed biochemical reactions, as, for example, in ribozymes [2-4], serine proteases [5], lysozymes [6], and ribonucleases [3, 7], While most of the studies on stereoelectronic effects were conducted on pyranoid forms of sugars, the relationship between stereoelectronic effects and furanoid ring conformations, to the best of our knowledge, is almost totally neglected. This is a big void since the conformations of furanoid forms of sugars are of great importance for overall conformations of ribo- and deoxyribonucleic acids (RNA and DNA). 

The early development of the Karplus relationship has been outlined in Section 1.4 (see p. 60). The significance of this method was soon realized by Jardetzky and by Lemieux, who applied it to the determination of the conformation of the o-ribofuranose and 2-deoxy-D-eri/first time, that furanoid rings are nonplanar in solution. However, it was not realized at that time that the Karplus parameters are subject to a substituent effect, so the original parameters were used hence, the particular values obtained in these investigations may require some slight modification. All of the following empirical modifications of the Karplus parameters were based on the intuitive assumption that allowance must be made for a substituent effect, and this step has only recently been justified by Williamson, and by Laszlo and Schleyer. ... 

Five-membered furanoid rings occur in envelope (E) or twist (T) conformations, which are in a rapid equilibrium at room temperature. Both conformers help to avoid energy-rich eclipsed orientation of neighboring substituents, which would occur in the planar conformations. The preferred conformation in methyl furanosides is E in which Cl is above the oxygen-C(2,3,4) plane and the hydroxymethyl group is in axial position. This conformer is enforced by the... 

NMR spectroscopy provides a sensitive method for assaying the ring size and substitution of acetonides in the carbohydrate series. The chemical shifts of the acetal carbon in dioxolanes appear at 5 108.1-111.4 for monocyclic or ci.v-fused dioxolanes to a pyranoid or cyclohexane ring and 6 111.8-112.3 for the trans f xsed series 6 111.3-115.7 when fused to a furanoid ring. The corresponding acetal carbon in the chair form of a 1.3-dioxane ring appears at b 97.1-99.9 whereas the skew conformation appears at 6 100.6-101.1. 

The twist conformations were chosen on the basis of reports - that stated that tetrahydrofuran is thought to exist in a twist conformation having the oxygen atom located on the axis of symmetry, instead of in an envelope conformation. Thus, alternative twist conformations and envelope conformations for the furanoid ring were not rigorously excluded by this study. 

The crystal structure of d-deoxy-ct-L-sorbofuranose has been determined the furanoid ring had a C-i -exo-CA -endoCTf) conformation. The paper contains a discussion of the conformations adopted by ketofuranosides. Some Vh-2,h-5 couplings of 0.8-0.95 Hz have been observed in 3,6-anhydro-D-idofuranose... 

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