Pyranoses conformational free energies

In the light of revised values for the conformational free energy of methyl and hydroxymethyl substituents at position 2 in tetrahydro-pyran (now believed to be 12.1 kJ mol rather than the 7.5 kJ mol reported by E.L.Eliel (J. Am. Chem. Soc., 1982, 104, 3635)), the conformations of ido- and altro-pyranose derivatives have been re-examined it was shown that C (D) conformations make a greater contrib-... 

Dunfield, L.G. and Whittington, S.G., 1977, A Monte Carlo investigation of the conformational free energies of the aldohexo-pyranoses, J. Chem. Soc. Perkin II, 654. 

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

Noviose (6-deoxy-5-C-methyl-4-0-methyl-L-h/xo-hexose) differs from the preceding compounds only in its configuration at C-2 its solution composition at 40° is 26% of a- and 74% of/J-pyranose.61 The y -pyranose is somewhat less stable than that of the xylo isomer, owing to the presence of an axial hydroxyl group on C-2 the a-form, on the other hand, is somewhat more stable, because it is a conformational mixture (— 7 3) of the two chair forms (44 and 45), which are of almost equal free-energy. 

The preferred conformation of pyranoses in solution can be predicted by empirical approaches.14 For example, free energies have been successfully estimated by summing quantitative free-energy terms for unfavourable interactions and accounting for the anomeric effects, which are individually depicted in Figure 1.9. The estimated free energies for both chair conformers can be calculated by summation of the various... 

At that time, unfortunately, very little was known about the conformations of the sugars, and the attempts to correlate conformation, reactivity, and optical rotation were necessarily limited. It has recently been suggested by Isbell that the addition of bromine to the pyranose anomers provides the intermediate bicyclic structures IV and VI. The difference in the rate of reaction is related to the free energy of these intermediates and to the relative position of the two rings —with the glycosidic group... 

The calculated free-energies given in Table XV are relative to an imaginary hexopyranose that has no non-bonded interactions. The work provides a basis for predicting the stabilities of aldohexo-pyranoses in conformations that may apply to certain reaction intermediates, as well as to sugars in solution. 

Fig. 15 The six detected confonnations of free Ribose are six-membered pyranose rings, adopting C4- or" Ci-chair skeletons of both p- and a-anomers. The experimental conformational energies in red are estimations derived from the relative intensities of the microwave transitions. The blue horizontal lines represent the theoretical (MP2) AG298 predictions...

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