Conformations, pyranose rings conformational stability

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... 

The transition state for the substitution step requires that, for maximum stability, the incoming nucleophile be coplanar with the three-membered oxide ring, and, for an epoxide on a pyranose ring (for which a half-chair conformation is expected, by analogy with... 

The chemical transformations that lead to the conversion of cellulose to mixed polysaccharides differing from cellulose in the conformation of the pyranose ring and the number and configuration of the hydroxyl groups of the repeating unit of the macromolecule, may exert a considerable effect on the structure of the material as well as on its important chemical properties (rate of acetylation and O-alkylation of OH groups, stability of the acetal linkage) and physicochemical indices (solubility of modified preparations of cellulose and cellulose ethers and esters). 

Empirical force field calculations on the simplest acyclic diols show that there is a tendency for 1,2-diols to adopt the gauche O-C-C-0 conformations. In protic media, these conformations are stabilized by specific activation. The stereochemical properties of the glycosidic linkages in flexible pyranose rings have been studied by PCILO quantum chemical methods. The energies of thirty six conformers were calculated, as were the influences of solvent on the equilibrium and hence the magnitude of the exo-anomeric effect. 

On the basis of conformational studies, Hassel and Ottar 68) have advanced a theory to explain the greater stability of one or the other of the two anomeric 0-acetyl-D-glycopyranosyl halides. It is considered that the pyranose ring is theoretically capable of eight strainless ring conformations, six boat and two staggered or chair forms. (See also Chapter I.) The evi-... 

At the temperatures lower than Tg several secondary low-scale physical transitions were registered in cellulose. For example, in [36, 41] the transition at 180—200 K is related to the methyl group stability and polarization of the primary hydroxyl in the electric fieldAs far as the transition at 273-310 K in cellulose and its derivatives are concerned, the researchers [32, 38, 42] attribute them to libration of pyranose rings around a glucoside bond or to conformational alteration of an armchair-bath type at the glycopyranose group level. 

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